Claude Universite

Bernard -Lyon

M Le Professeur

Frédéric Fleury

Hamda Ben

Hadid M Le Professeur Didier

Revel M Le Professeur

Philippe Chevalier

M Fabrice

Vallée Mme

Dominique Marchand

Composantes Sante

Prof Marcello Brigante

Paola Dra

Formenti

Liselotte, Monica, Raluca, Yoan, Stéphanie Nathalie Peter

Chunlin, Xinke, David, Ibrahim Antoine Martin

Clément Bastien

Sonia Diego

Alvaro Géraldine

Antoinette Boréave

Corinne Emmelin

Philippe Vernoux

Chantal Guillard

Barbara D'anna

Frédéric Dapozze

Nicolas Charbonnel

Sébastien Perrier

Laurence Retailailleau-Mevel

Stephane Dumas

Anne-Marie Grézaud

Sébastien Albert

Narayman El-Jerrari

Marie-Laure Venditti

Carol, David, Magda Bruno

Lilian

Keywords: mineral dust, photochemistry, surface chemistry, photocatalysis, volatile organic compounds, UV-A light, oxygenated organic compounds, dicarboxylic acids, uptake coefficients, aerosol aging

I could write a lengthy article about all the people I would like to acknowledge. In my opinion, no one arrives in a Ph.D. and even more finish it, by accident. A doctoral thesis is the result of a long process that starts many years before the graduate program.

In my case, the beginning of everything was the beautiful sea and beaches of my hometown.

Over my childhood, I spent almost all my free time on the beach: swimming, diving, walking along the wide strips of sand and coconut trees of paradisiac sunny beaches on the coast of Bahia. From those memories, I remember it starts my curiosity about nature and life. And even more, my deep respect for how amazing and perfect nature is. This respect and admiration have led me to want to understand natural phenomena better so that we can preserve all the wealth we have on Earth.

The first and most special thanks are to my parents, for the unconditional love and encouraging in different ways. This work is dedicated to you two. My father, Roberto, who was my partner for most of those sunny moments described before and who encouraged my curiosity and interest in physics, philosophy, and sciences in general. My mother, Alcina, has always been my bedrock foundation supporting me in all my decisions. She was responsible for my independence

COMPOSANTES ET DEPARTEMENTS DE SCIENCES ET TECHNOLOGIE

Tudo no mundo começou com um sim. Uma molécula disse sim a outra molécula e nasceu a vida.

Everything in the world began with a 'yes'. One molecule said yes to another molecule and life was born."

Clarice Lispector, Brazilian writer, in "A Hora da Estrela" vi vii

Resumé

Les émissions de particules de poussière transportées par le vent sont réparties de manière importante à travers le monde alors que ces particules traversent de grandes distances jusqu'à des milliers de kilomètres. Chaque année, la quantité de poussière minérale liberée dans l'atmosphere est estimée entre 1 000 et 3 000 Tg. Les impacts de la poussière minérale sur les écosystèmes terrestres sont multiples, influençant, par exemple, l'équilibre radiatif, la formation de gouttelettes de nuages et la distribution de la pluie ainsi que la disponibilité de nutriments biochimiques. Chimiquement, une variété de processus hétérogènes peut se produire à la surface de ces minéraux, modifiant leur composition et, par conséquent, leurs propriétés physico-chimiques et leurs interactions dans l'atmosphère. L'absorption de gaz à l'état de traces transforme la poussière minérale en puits pour certaines espèces et en source pour d'autres. La présence de composés absorbant la lumière dans ces particules tels que TiO2, Fe2O3 peut favoriser des processus photosensibilisés, notamment des réactions photocatalytiques qui pourraient être déclenchées naturellement dans la troposphère par les rayonnements solaires dans la gamme des longueurs d'onde UV-Vis.

La photochimie de la poussière minérale est donc un domaine de recherche très innovant qui est en train d'émerger et d'évoluer. Par conséquent, le nombre d'études portant sur ce sujet a récemment augmenté. Cependant, la compréhension du processus global ainsi que des processus spécifiques est encore naissante, de sorte qu'il existe un large éventail de questions devant être étudiées plus en détail. Pour cette raison, ce travail aborde différents sujets concernant l'interaction des poussières minérales et la réactivité vis-à-vis des composés organiques lors de l'irradiation UV-A.

Cette thèse est composée de six chapitres dont les résultats ont été divisés en trois chapitres redigés sous la forme de quatre articles scientifiques. Ce résumé présentera brièvement le contenu de chaque section.

Le Chapitre I est une introduction générale sur ce sujet qui présente une synthèse bibliographique ainsi que le contexte scientifique sur lequel ce travail est basé. En partant des sujets plus généraux, de la définition de l'atmosphère et de l'aérosol, tout au long du texte, nous approfondissons les concepts plus spécifiques détaillant les impacts de la poussière minérale sur l'atmosphère.

Des études en laboratoire ont été effectuées dans le but de simuler des processus hétérogènes qui se produisent naturellement dans la troposphère. Par conséquent, le Chapitre II présente la méthodologie et les principaux éléments du protocole expérimental utilisé pendant ces travaux. Des modifications spécifiques ont été détaillées dans chaque chapitre présentant les résultats. La viii spectrométrie de masse par réaction de transfert de protons utilisée comme principale technique d'analyse est aussi présentée en détail.

Le Chapitre III est consacré à l'étude de la capture (uptake) de composés organiques volatils oxygénés (OVOC) sur différentes composés minérales modèle comme la « poussière d'essai d'Arizona » (Arizona test dust -ATD), TiO2 et SiO2 dans des conditions opérationnelles proches de l'environnement atmosphérique. L'effet de l'humidité est particulièrement discuté. Le Chapitre III a.

présente les résultats de la capture de butanol sur la poussière d'essai de l'Arizona, un véritable échantillon du désert, en se concentrant sur la capture réactive déclenchée par la lumière UV-A.

L'influence des conditions externes telles que la concentration initiale de butanol, l'intensité lumineuse, la température et l'humidité ont été évaluées. Il a été observé que l'intensité lumineuse et l'humidité jouent un rôle significatif sur la cinétique de capture (uptake coefficients) du butanol. Les produits en phase gazeuse ont été identifiés, montrant que les réactions de surface du butanol adsorbé ont conduit à l'émission de divers composés carbonylés, comme, par exemple, le formaldehyde et Au Chapitre IV, cinq acides dicarboxyliques à chaîne linéaire saturée, de C4 (acide succinique) à C8 (acide subérique) ont été choisis pour évaluer le vieillissement photochimique se produisant à la surface de la poussière minerale. La photo-oxydation de ces acides dicarboxyliques à la surface de l'ATD a été étudiée en surveillant la composition en phase gazeuse et les produits sorbés en surface.

Nous avons montré que la photochimie des acides dicarboxyliques favorisée par la surface libère de nombreux OVOC dans la phase gazeuse et produit d'autres composés hautement oxygénés à faible volatilité qui restent dans la phase particulaire. Nous avons proposé un mécanisme réactionnel pour la formation photoinduite des produits oxygénés observés. Les implications atmosphériques de ces réactions sur la surface des aérosols sont discutées.

Le Chapitre V est dédié à l'influence exercée par la présence de nitrates co-sorbés sur la capture et la réactivité des composés organiques sur la surface des poussières minérales. Le nitrate de sodium a été utilisé comme source d'anions de nitrate et l'ATD comme substrat minéral. L'effet de la teneur en anions nitrates a été évalué sur deux aspects: la capture d'acétone et la réactivité de l'acide ix glutarique. La présence de nitrate a causé une forte réduction du coefficient de capture de l'acétone par l'ATD. D'autre part, la réactivité de l'acide glutarique a présenté une tendance parabolique, d'abord en augmentant, puis en diminuant la totalité des produits libérés en phase gazeuse en fonction de la teneur en nitrate.

Enfin, le Chapitre VI fournit une conclusion générale, qui résume les principales conclusions et les implications atmosphériques communes à tous les résultats. Nous exposerons également les perspectives de cette étude pour d'éventuels travaux futurs qui pourraient répondre aux interrogations n'ayant pas encore d'explication dans ce domaine de recherche aussi riche qu'est la photochimie de la poussière minérale.

Mots-clés: poussière minérale, photochimie, chimie de surface, photocatalyse, composés organiques volatils, lumière UV-A, composés organiques oxygénés, acides dicarboxyliques, coefficients de capture, vieillissement d'aérosol.

General Introduction

"When I watched the horizon, I saw the abrupt, contrasting transition from the earth's light-colored surface to the absolutely black sky. I enjoyed the rich color spectrum of the earth. It is surrounded by a light blue aureole that gradually darkens, becoming turquoise, dark blue, violet, and finally coal black."

Yuri Gagarin, as quoted in Earth's Aura (1977) by Louise B. Young.

In April 1961, Yuri Gagarin was the first human to go into the outer space aboard a spacecraft.

After his successful revolutionary mission, he said this statement that was the first description of Earth's atmosphere and its layers, seen from outside the planet. Some years later, the notorious photography of the Earth, the "Blue Marble" was taken by the crew from the Apollo 17 mission, on December 7, 1972. The picture was massively spread out worldwide; it was the first time our planet could be seen this way, complete. The "big blue sphere" revealed all the splendor and fragility of our planet and had drawn people's attention to the fact that we only have a single Earth, a limited space to live.

Air is a vital medium for the life of virtually every living thing on the planet, and small changes in its composition can have disastrous consequences on ecosystems. Acute air pollution episodes are one of the most noticeable effects of man's presence and interference in the environment. Smog (fog) events in London were already reported in the 16 th century [START_REF] Goldemberg | Energia, meio ambiente e desenvolvimento[END_REF] and, in the twentieth century, became recurrent in several large cities over the world like Los Angeles, Mexico City, Tokyo, etc. The effects of the air pollution that in the first events were local, now occur in regional and even global scales.

Air pollution has diverse and dramatic effects. Currently, it is estimated that 4.2 million deaths per year are related to external air pollution, and 91% of the world's population lives in places where air quality exceeds World Health Organization (WHO) guideline limits (World Health Organization, 2018). WHO considers air pollution -both ambient (outdoor) and household (indoor) -a major environmental risk to health and it is, now, a priority in their global agenda. Furthermore, there is another major challenge to be faced by humanity: warming of the climate system is unequivocal. Concerning the atmosphere, the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) states that each of the last three decades has been successively warmer at the Earth's surface than any preceding decade since 1850 (IPCC, 2013). Global warming, in turn, has disastrous consequences, such as the change in the circulation pattern of air masses leading to extreme droughts and rainfall precipitation; increased acidity and sea level; the decrease in mass of polar ice sheets and shrinking of glaciers worldwide. Since the First Assessment Report (AR1) of the IPCC in 1990, climate sciences have been progressing, minimizing the uncertainties, advancing in measurement, and modeling capabilities, however, we do not know the feedbacks and the adaptive responses from ecosystems to these changes.

Air pollution and climate change are highly correlated issues; they have common sources and solutions. Both are associated with the demand for energy and the chain of fossil fuels that supports the basic needs of the population as food, habitation, transportation, and production of consumer goods. Besides CO2, which is the main, and best-recognized greenhouse gas released by fuels combustion, several other substances influence Earth's climate. Aerosols, for instance, have multiples direct and indirect effects on radiative forcing and in the distribution of rainfall.

Chemistry is closely involved with these issues because part of the solution lies in studying and understanding processes that are naturally and anthropogenically induced. The atmosphere is a large "open reactor" where most of the substances injected into it are not inert and chemically evolve due to its oxidative capacity. Additionally, Earth systems are highly interconnected and are constantly in exchange. For example, the nitrogen cycle includes multiple physical and chemical transformations of species that circulate between different ecosystems. In other words, an action in a particular ecosystem can trigger impacts on another one, sometimes kilometers away.

In this context, a number of scientific questions remains open and can only be resolved through further research. There is, therefore, an actual need for investment in research and technology coupled with a paradigm shift by policymakers in order to ensure the correct application of scientific advances. This is the only way to achieve the goal of development that meets the needs of the present without compromising the ability of future generations to meet their own needs (from 'Our Common Future,' also known as the 'Brundtland Report'). This doctoral work is strongly motivated by this need to understand some of the chemical processes that are still a gap in our knowledge about the atmosphere. Particularly, we aim to address the heterogeneous processes that take place on the surface of airborne solid particles and trace gases; both are minor but concurrently important components in the atmospheric systems.

In the next section, we will present the definitions and the scientific context that underlie this work. We start with the more general topics, and throughout the text, we will delve into the more specific concepts. Finally, at the end of the Chapter I, we introduce the scientific questions that we intend to assess during this thesis.

I.1 -The atmosphere

Earth's atmosphere consists mainly of N2 (78%), O2 (21%), and Ar (1%). This gaseous mixture appears to be stable and non-reactive in the lower atmosphere even at high temperatures and solar intensities, however, many reactions take place in this environment, regardless of whether the air is clean or polluted. Water vapor is the next most abundant constituent; its concentration is very variable and is controlled by physical phenomena such as evaporation and precipitation, altitude and latitude.

Furthermore, other components called trace gases, represent less than 1% of the total composition yet, they play a fundamental role in the chemical properties of the atmosphere and the radiative balance of Earth. In addition to gaseous constituents, the atmosphere comprises airborne particles, generally called aerosols. All these atmospheric components interact with each other and with other terrestrial systems such as hydrosphere, lithosphere, cryosphere.

Typically, the atmosphere is an oxidizing medium, where chemical compounds are usually converted into higher chemical oxidation state substances than their parent molecules. Often these oxidative transformations lead to an increase in polarity (and hence water solubility) and a decrease in volatility [START_REF] Seinfeld | Atmospheric chemistry and physics : from air pollution to climate change[END_REF]. Atmospheric sciences, in this way, study and try to understand how these transformations occur, what are their drivers and subsequent impacts.

I.2 -Atmospheric Aerosols

Aerosols are broadly defined as fine liquid or solid particles (in size range from a few nanometers 'nm' to tens of micrometers 'μm') suspended in the air. The importance of aerosols arises from the multiple impacts they cause in their surroundings by interacting physically, chemically and biologically with environmental components. Their effects on terrestrial radiative balance are crucial in Earth's climate as well as their relation to air quality issues and human health damages. Chemically speaking, aerosols are an active surface on which reactions can occur through pathways that are distinct from those in the gas phase, thereby altering the atmospheric composition.

Atmospheric aerosols can be directly emitted from natural sources, such as windborne dust, sea spray, volcanoes, biomass fires, microorganisms (fungal spores, bacteria cells, pollens), and from anthropogenic activities, such as combustion of fuels, industrial processes. Alternatively, they can be generated in the atmosphere by gas-to-particle conversion processes from organic and inorganic precursors.

Particle size and eventually composition will determine their lifetime in the atmosphere; fine particles (d <2,5 μm) residence times may vary from a few days to a few weeks while coarse particles can, therefore, have very short atmospheric lifetimes (minutes to hours). When suspended in air, particles alter their size and composition by different process such as condensation and/or evaporation of gaseous species, by coagulating with other particles, chemical reactions, activation by water to become fog and cloud droplets [START_REF] Seinfeld | Atmospheric chemistry and physics : from air pollution to climate change[END_REF]. The ensemble of transformations that aerosols undergo during their lifetime is called aerosol aging.

The level of understanding concerning aerosols and their myriad of atmospheric interactions is still very low. Therefore, each of these possible aging processes is of scientific interest and is the subject of research by atmospheric scientists. Specifically, in this work, mineral aerosols and their interactions with organic compounds were chosen as the object of study, in this way, both concepts in their scientific context will be detailed hereafter.

I.3 -Atmospheric Mineral Dust

Wind-blown dust is emitted in large quantities around the world, and it is the most abundant atmospheric aerosol regarding aerosol dry mass (Choobari et al., 2014). It is estimated that every year a range between 1000 to 3000 Tg of mineral dust is released into the atmosphere and accounts for approximately 50% of the global aerosol load [START_REF] Formenti | Recent progress in understanding physical and chemical properties of African and Asian mineral dust[END_REF][START_REF] Karydis | Effects of mineral dust on global atmospheric nitrate concentrations[END_REF].

I I.3.1 Sources, transportation, climate impact

The main sources of mineral dust are deserts, semi-arid and their adjacent areas, usually covered by sand dunes, arid soils, and dried lakes beds. Figure I. 2 presents the location of the greatest global deserts in the world [START_REF] Karydis | Effects of mineral dust on global atmospheric nitrate concentrations[END_REF]. Even though, deserts are not homogeneous dust sources, and emissions take place from preferential source regions also called 'hotspots' [START_REF] Grini | Model simulations of dust sources and transport in the global atmosphere: Effects of soil erodibility and wind speed variability[END_REF]. For instance, the Bodélé depression located at the Sahara Desert, northeast of the Lake Chad was responsible for more than 58 8 Tg of dust during the winter-springtime of 2003-2004[START_REF] Koren | The Bodélé depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest[END_REF]. Understanding why and how these regions become hotspots is important for understanding global dust production in general, as well as for improving the accuracy of models [START_REF] Tegen | Modeling of mineral dust in the atmosphere: Sources, transport, and optical thickness[END_REF]. The so-called anthropogenic sources of dust are related to human occupation and land use that leave soils exposed and susceptible to the action of wind erosion. The atmospheric dust load attributed to human land-use impacts are still under discussion but it is estimated that they contribute to 20 to 50% of total present-day dust emissions (Choobari et al., 2014). However, meanwhile natural dust sources have been maintained constant over the past several hundred years [START_REF] Prospero | African Droughts and Dust Transport to the Caribbean: Climate Change Implications[END_REF], the decrease in vegetation cover due to disturbance of the land use, deforestation, and climate variability may increase desertification and consequently, the significance of human-induced dust sources [START_REF] Huang | Accelerated dryland expansion under climate change[END_REF][START_REF] Tegen | Relative importance of climate and land use in determining present and future global soil dust emission[END_REF].

Upon emission, suspended dust particles are subjected to long-range transport across the planet being able to travel for several kilometers including intercontinental scales. For instance, Saharan dust is transported across the Caribbean, the Mediterranean, Americas, and Europe (Kaufman, 2005;Sa'id, 2012); while dust from Asia affects eastern Asia and crosses the Pacific Ocean to arrive in the western part of North America (Uno et al., 2009). Although there are preferred locations for deposition, (Figure I. 3) almost any place on the Earth's surface has already received dust at least in trace levels [START_REF] Schepanski | Transport of Mineral Dust and Its Impact on Climate[END_REF].

It is interesting to note that there are substantial seasonal variations of atmospheric dust concentrations, dispersion plumes altitudes, transport pathways and even composition [START_REF] Ben-Ami | Patterns of North African dust transport over the Atlantic: winter vs. summer, based on CALIPSO first year data[END_REF][START_REF] Tegen | Modeling of mineral dust in the atmosphere: Sources, transport, and optical thickness[END_REF]. Dust activity in the Gobi Desert mainly occurs in spring, especially in April, while dust storms are least frequent in summer (Choobari et al., 2014). Regarding the Sahara, from a total of 240 80 Tg of dust transported annually from Africa to the Atlantic Ocean, 140 Tg are transported in the summer and 100 Tg in the winter [START_REF] Koren | The Bodélé depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest[END_REF].

I.3.1.2 Climate Impacts

The IPCC Fourth Assessment Report [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF] classified aerosol radiative forcing into direct and indirect effects. The first and direct effect of dust aerosol (as much as other aerosols) on climate is the absorption and scattering of light, shortwave and longwave radiation, influencing radiative forcing (RF) and global albedo. The optical properties of the aerosol are the key parameter which determines its direct RF. Dust aerosol exerts a combined positive and negative effect on RF. From one hand, the absorption of shortwave radiation reduces the incoming solar radiation reaching the surface, resulting in a decrease on the heating of the atmosphere below the dust layer. On the other hand, by capturing the longwave output radiation, thermal radiative forcing of dust at the surface is positive, like greenhouse gases [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF]. Both positive and negative RF effects reduce shortwave irradiance on the surface of the atmosphere (Sa'id, 2012).

Indirect effects result from microphysical influence on the amount and lifetime of clouds. The unperturbed clouds contain large cloud droplets, but an increase in (dust) aerosol concentration will form a greater number of smaller cloud droplets. This phenomenon has two consequences. First, the so-called cloud albedo effect or Twomey effect, as for a fixed liquid water content (LWC) the smaller drops leads to an enhanced cloud reflectance. Second, the cloud lifetime effect or Albrecht effect, as the decrease in droplet size makes coalescence and precipitation difficult, so there is an important association between mineral dust and the occurrence of rain (Choobari et al., 2014;[START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF]. The overall radiative impact of mineral dust on the atmosphere remains uncertain, and considerable effort has been made by the IPCC to better account for it. For instance, global mean radiative forcing of mineral dust was estimated as -0.60 to +0.40 W m -2 as reported in the Third Assessment Report . At the AR4 (Fourth Assessment Report) , it was reevaluated to account for smaller dust aerosol anthropogenic fraction adjusting the values to -0.30 to +0.10 W m -2 [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF]. [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF].

Like any other atmospheric aerosols, mineral dust impacts depend on its physical and chemical properties, such as size, shape, composition, surface state, and mixing state of particles. Formenti and coworkers [START_REF] Formenti | Recent progress in understanding physical and chemical properties of African and Asian mineral dust[END_REF] gave an overview of the current knowledge about these physicochemical properties highlighting the great heterogeneity and the challenges in characterizing dust particles. Table I. 1, adapted from this reference, summarizes the role of dust physicochemical properties in their climatic and environmental impacts.

Table I. 1: The role of dust physicochemical properties in their climatic and environmental impact. Adapted from [START_REF] Formenti | Recent progress in understanding physical and chemical properties of African and Asian mineral dust[END_REF]. Roughness: Surface area, gas adsorption Some factors like shape and size will determine the surface area that is available for interaction, for instance, with solar radiation. For example, the deviation from spheroid shaped particles implies a greater light scattering. The diversity concerning mineralogical and crystallographic properties of each material, noticeably, also influences its ability to scatter and absorb radiation (both in the solar and terrestrial spectrum). Moreover, these same parameters exert a significant influence on particles heterogeneous process and reactivity with trace gases and water vapor, a subject that will be discussed later.

I.3.1.3 Health Effects

Besides all the atmospheric impacts, dust particles have strong effects on human health. Once inhaled, they can depose on the respiratory tract and migrate, especially fine particles (PM2.5), to the alveoli in the deep lung. Adverse health effects associated with particle exposure and inhalation include both acute and chronic diseases, particularly respiratory and cardiovascular.

The concern about the long-term exposure to particulate matter air pollution has been the subject of several studies [START_REF] Brook | Particulate matter air pollution and cardiovascular disease: An update to the scientific statement from the american heart association[END_REF]. The degrading conditions and the increase in hospitalizations during and after a severe sand/dust event have also been documented [START_REF] Ebrahimi | Effects of dust storm events on emergency admissions for cardiovascular and respiratory diseases in Sanandaj, Iran[END_REF][START_REF] Kanatani | Desert Dust Exposure Is Associated with Increased Risk of Asthma Hospitalization in Children[END_REF][START_REF] Zhang | A Systematic Review of Global Desert Dust and Associated Human Health Effects[END_REF].

I I.3.2 Composition and reactions on dust particles surface

The chemical behavior of mineral aerosols, their optical properties, interaction and reactivity with trace gases and water vapor are closely related to their composition. Airborne particles are composed by crustal elements like Si, Al, Mg, Ca, Fe, Ti although the relative abundance of each element is variable and often related to the source soil [START_REF] Krueger | Heterogeneous chemistry of individual mineral dust particles from different dust source regions: the importance of particle mineralogy[END_REF]Usher et al., 2003). Although presenting similar elemental composition, studies have indicated a diversified distribution regarding mineralogy: clays, quartz, potassic and sodic feldspars, calcium carbonates, among other minerals have been reported [START_REF] Formenti | Mapping the physico-chemical properties of mineral dust in western Africa: mineralogical composition[END_REF]Usher et al., 2003). Usually, there is a size-dependence related to mineralogy; smaller particles are prone to exhibit a wider diversity of species than coarse ones, comprehending clay minerals and micas. Kaolinite and illite are generally the major clay components of dust derived from deserts [START_REF] Gomes | A comparison of characteristics of aerosol from dust storms in Central Asia with soil-derived dust from other regions[END_REF]. Given their composition, the surface of dust particles is not inert. Oxides such as CaO, TiO2, Al2O3, Fe2O3 have either basic or acidic properties (or both for TiO2) (Li et al., 2001) that can promote reversible or irreversible adsorption of atmospheric gases. A variety of heterogeneous process can take place on the surface of those minerals, altering their composition and consequently, their physicochemical properties and their interactions in the atmosphere. The uptake of trace gases turns mineral dust into a sink for some species and a source for others. Furthermore, surface-promoted reactions differ from those in the gas-phase, providing, for instance, faster kinetics and alternative pathways (Crowley et al., 2010;George et al., 2015;Usher et al., 2003).

Additionally, as it can be seen in Figure I. 6 a, as in other studies [START_REF] Formenti | Chemical composition of mineral dust aerosol during the Saharan Dust Experiment (SHADE) airborne campaign in the Cape Verde region, September 2000[END_REF][START_REF] Klaver | Physico-chemical and optical properties of Sahelian and Saharan mineral dust: in situ measurements during the GERBILS campaign[END_REF], dust particles contain Fe2O3 and TiO2, semiconductor metal oxides that show photocatalytic activity. A recent study on Saharan and Sahelian dust [START_REF] Formenti | Mapping the physico-chemical properties of mineral dust in western Africa: mineralogical composition[END_REF] reported that average TiO2 contents for eroded Sahelian dust and advected Saharan region were, respectively, of 1.2% ± 0.1 and 0.71% ± 0.01. The impact of a mineral on the atmosphere is not proportional to its relative abundance and even being present in minor mass fractions, a small percentage of TiO2 (0 -5%) can be effective in the removal or formation of atmospheric trace gases (Chen et al., 2012).

In 1996, Dentener and co-workers [START_REF] Dentener | Role of mineral aerosol as a reactive surface in the global troposphere[END_REF] had used three-dimensional model calculations to support the importance of surface reactions on mineral aerosol. They analyzed the consequences of SOX, NOX and O3 heterogeneous reactions on dust in a global scale and at that point, uncertainties on the calculations relied mostly on the lack of information about sorption and reaction rate constants, i.e., uptake coefficients, of those gases onto mineral surfaces.

Since then, heterogeneous reactions of atmospheric relevant trace gases on mineral aerosols have been receiving more attention, and the uptake of a few reactive gases by mineral aerosols has been studied using a variety of experimental approaches. The uptake kinetics of inorganic compounds, such as NO2, HNO3, N2O5, O3, and SO2 (Adams et al., 2005;Hanisch and Crowley, 2003;Seisel et al., 2005;Underwood et al., 2000), as well as some organics (Carlos-Cuellar et al., 2003;Li et al., 2001;Romanias et al., 2016) were investigated for different mineral proxies. Interestingly, Umann et al. [START_REF] Umann | Interaction of mineral dust with gas phase nitric acid and sulfur dioxide during the MINATROC II field campaign: First estimate of the uptake coefficient γ HNO 3 from atmospheric data[END_REF] performed one of the first in situ field measurements of uptake coefficients for African dust that have encountered polluted air masses from Europe (loaded with HNO3 and SO2) and traveled over the Sahara where they mix and react with uplifted dust, Figure I. 7. In addition, they found out that HNO3 reacts very efficiently with Saharan dust being depleted or even totally removed from the atmosphere for dust loads higher than 30 μm²/cm³.

Figure I. 7: ''The atmospheric flow reactor"

Pollutants such as HNO3 (blue spots) or SO2 (green spots) are injected into the atmosphere over south Europe or North Africa. Over northwest Africa, these air masses mix with uplifted mineral dust (yellow spots), when heterogeneous reactions may take place. From [START_REF] Umann | Interaction of mineral dust with gas phase nitric acid and sulfur dioxide during the MINATROC II field campaign: First estimate of the uptake coefficient γ HNO 3 from atmospheric data[END_REF].

Adsorbed water can also change dust surface characteristics, reaction pathways and the partitioning of products. Two recent review articles discussed the interactions of water and mineral dust. The first one (Rubasinghege and Grassian, 2013) detailed the roles of adsorbed water in the surface chemistry that can, for example, enhance or suppress reactivity, hydrolyze reactants and products, solvate ions, among other effects. In the most recent one (Tang et al., 2016), the authors concentrated their attention in reviewing hygroscopicity measurements techniques and water adsorption on solid particles discussing the results of studies done in the past years.

Dust hygroscopic properties are very important parameters since, as previously commented, these particles have an important role in ice nucleation (IN) and as cloud condensation nuclei (CCN)

that is, presumably linked to their water adsorption capacities. Aging is also a relevant related factor because the degree and chemical characterization of processed dust interfere in their CCN and IN activities, refractive index and lifetime (Keskinen et al., 2014;Niedermeier et al., 2010;Tang et al., 2016).

Besides the adsorption of traces gases, the surface of mineral aerosols is susceptible to the condensation of low vapor pressure organic compounds. A complex mixture of coated carbonyl compounds was detected on dry particles collected in the Caribbean marine boundary layer (Russell et al., 2002) and Falkovich and coworkers (Falkovich et al., 2004) identified oxalic, succinic, malic, glutaric and adipic acids adsorbed on mineral dust particles collected in Israel. The adsorption of low vapor pressure compounds creates an organic coating that, subsequently, can undergo heterogeneous chemical reactions enriching dust surface with oxygenated and polar organics. The oxidation of these adsorbed organic compounds is a key point in the fate of mineral dust and its overall impact on the atmosphere.

It is worth pointing out that the uptake of gases and condensation of low vapor pressure compounds on the surface of mineral particles turn them into reservoirs and vectors in the transport of these adsorbed substances. Later changes in environmental conditions, such as humidity or the incidence of light, can trigger the reactivity of dust surface releasing products into the gas phase and altering the local atmospheric composition. The transformations that mineral aerosols undergo and cause during their transport in the atmosphere are multiple. These processes are complex and interrelated what make their study incredibly interesting and challenging. The need to investigate through laboratory, field observations and modeling approaches, is unquestionable so that we can gain insight into Earth's climate.

I.4 -Volatile Organic Compounds (VOCs) I I.4.1-Sources of volatile organic compounds

Any biological activity releases somehow, organic compounds to the atmosphere but sources are usually divided into biogenic and anthropogenic ones. Biogenic VOCs are the compounds emitted from natural environments and encompasses emissions from vegetation, soils, oceans [START_REF] Steiner | Biogenic VOCs[END_REF]. The so-called anthropogenic VOCs are specifically related to human activities.

Overall, there is a wider range of sources of VOCs than for the most of other common trace gases and air pollutants what makes the sources identification and the inventory of emissions especially complex [START_REF] Pandis | Source inventories and control strategies for VOCs[END_REF]. Sources, sinks, atmospheric mechanisms and residence times of VOCs in the atmosphere are still under discussion in the scientific community [START_REF] Koppmann | Volatile organic compounds in the atmosphere[END_REF].

Isoprene and terpenoids are the most abundant biogenic compounds and have recognized atmospheric significance due to their impact on atmospheric photochemical mechanisms and ozone production (Fuentes et al., 2000;[START_REF] Steiner | Biogenic VOCs[END_REF]. Thereby, a great part of atmospheric sciences researchers turned their attention to characterizing the reactional pathways and the multiphase processes involving the BVOCs [START_REF] Andreae | Atmospheric Aerosols: Biogeochemical Sources and Role in Atmospheric Chemistry[END_REF][START_REF] Carlton | A review of Secondary Organic Aerosol (SOA) formation from isoprene[END_REF]Lee Ng et al., 2017). Biogenic hydrocarbons oxidation contributes to secondary organic aerosol (SOA) formation, a topic of large interest regarding the role of aerosols in atmospheric chemistry and their impacts on climate whose quantification still has a high level of uncertainty [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF][START_REF] Hallquist | The formation, properties and impact of secondary organic aerosol: current and emerging issues[END_REF][START_REF] Singh | Distribution and fate of selected oxygenated organic species in the troposphere and lower stratosphere over the Atlantic[END_REF][START_REF] Stocker | Technical Summary[END_REF].

Among the anthropic sources of VOCs, the use of fossil fuels for energy, transport and industrial processes stands out in the overall emission. Other domestic activities such as the use of paints, cleaning products, cosmetics, cooking are particularly relevant for indoor air quality [START_REF] Crump | Volatile organic compounds in indoor air[END_REF]. Various organic compounds have biogenic and anthropogenic sources in common meanwhile, human activities have introduced new exogenous substances into the environment. Many of them are toxic or persistent, characteristics that attract especial attention concerning public health problems and air pollution.

Worldwide, environmental protection agencies such as the US Environmental Protection Agency (US EPA) and the European Environment Agency (EEA) produce annual reports and inventories of anthropogenic VOCs emissions. In Figure I. 8 a, we can note that nowadays, the principal emitting countries are in Asia and Africa that account for about 65% of global NMVOCs. North America and Europe contribute only to 14% (an expressive reduction compared to the 70's, when they used to produce 37% of global VOC emissions) [START_REF] Huang | Speciation of anthropogenic emissions of nonmethane volatile organic compounds: A global gridded data set for 1970-2012[END_REF]. According to the European Environmental Agency, the EU-28 member states have been reducing their NMVOCs emissions which have decreased 61% from 1990 to 2015 (European Environment Agency, 2016). It is interesting to note in Figure I. 8 b that the relevance of each sector to total emissions is unevenly distributed according to the region: while in Asia agriculture is the largest emitting source, in Europe and North America land transport and energy are the main sources, despite the technological advances and the more severe legislation for the control of emissions of these regions. In this way, it is remarkable that an anthropogenic VOCs emissions inventory reflects the division of labor and the economic functions played by each geopolitical region of the planet.

Inventories of biogenic emissions are more difficult and less systematized because the emissions of VOCs by plants is closely linked to their photosynthetic activities, which in turn are related to meteorological conditions [START_REF] Peñuelas | BVOCs and global change[END_REF]. Several articles have developed assessments, usually by modeling and/or estimating plants emission fluxes adjusting them to environmental factors (such as temperature, humidity, light intensity, etc) [START_REF] Guenther | The model of emissions of gases and aerosols from nature version 2.1 (MEGAN2.1): An extended and updated framework for modeling biogenic emissions[END_REF][START_REF] Sindelarova | Global data set of biogenic VOC emissions calculated by the MEGAN model over the last 30 years[END_REF][START_REF] Zemankova | Emissions of biogenic VOC from forest ecosystems in central Europe: Estimation and comparison with anthropogenic emission inventory[END_REF].

I I.4.2-VOCs degradation in the atmosphere

VOCs are removed and transformed in the troposphere by physical processes such as wet and dry deposition, and chemical processes such as photolysis, reaction with hydroxyl (OH) radicals, reaction with nitrate (NO3) radicals and reaction with ozone (O3), and eventually reactions with chlorine atoms (Cl) (Atkinson and Arey, 2003). In the troposphere, VOCs are rapidly oxidized into oxygenated organic compounds (OVOCs). For instance, the atmospheric oxidation of all hydrocarbons leads to OVOCs, for n-butane, approximately 60 aldehydes, alcohols, organic nitrates, carboxylic acids, etc. are reported as products (Calvert, 2011;Mellouki et al., 2015). In addition, OVOCs are also directly emitted into the atmosphere by anthropogenic and biogenic sources (Fall, 2003;[START_REF] Koppmann | Oxygenated Volatile Organic Compounds[END_REF]. In this work, we concentrated our attention on oxygenated organic species heterogeneous reactions, therefore, in this review, we will focus specifically on them.

Gas-phase loss process for the majority of VOCs, notably, oxygenated saturated species is, generally dominated by OH radical reactions. OH-radical reactions proceed by H-atom abstraction from C-H and, in minor proportions, from O-H bonds and by the addition of OH to the double bond, for unsaturated OVOCs (Mellouki et al., 2015). Reactions with NO3 radical are representative normally only for unsaturated compounds, whereas H-atom abstractions by NO3 radicals from C-H bonds are slower and minimal compared to the corresponding OH radical reaction. The same occurs concerning O3 reactions which occur preferentially by addition to the C = C bonds (Atkinson and Arey 2003). In the case they are relevant, NO3 initiated reactions are important at night because during the day these radicals photolyze rapidly and hence their diurnal concentration is diminished.

Reaction rates of Cl atoms with OVOCs (and with VOCs in general) are comparable to OH radical rates, however, to be significant in the troposphere, it should exist a mechanism that can produce large amounts of chlorine atoms [START_REF] Ravishankara | Are chlorine atoms significant tropospheric free radicals?[END_REF]. Recent studies have discussed new pathways for Cl precursors formation at polluted areas [START_REF] Young | Chlorine as a primary radical: evaluation of methods to understand its role in initiation of oxidative cycles[END_REF] and also via surface-mediated reactions suitable to occur indoors [START_REF] Raff | Chlorine activation indoors and outdoors via surface-mediated reactions of nitrogen oxides with hydrogen chloride[END_REF]. Either way, chlorine (Cl atoms) concentrations are generally very low, and its reactivity could be expressive only under specific locations and conditions such as in the coastal marine boundary layer or the Arctic troposphere during springtime. Therefore, it is considered a minor VOC loss process (Atkinson and Arey, 2003;[START_REF] Chang | Atmospheric concentrations of the Cl atom, ClO radical, and HO radical in the coastal marine boundary layer[END_REF].

Table I. 3 shows the reaction rate constants and the calculated low-troposphere lifetime (for T = 298 K) for some VOCs with respect to reaction with OH radicals, NO3 radicals, O3 and Cl. The selected VOCs in the table were studied in this work, such as butanol and acetone, or were found as products, such as aldehydes, alcohols, and low molecular weight monocarboxylic acids. By way of comparison, the table included some hydrocarbons such as simple alkanes, benzene, and isoprene.

OVOCs play a crucial role in secondary chemistry. Both homogeneous and heterogeneous OVOCs chemistry is related to a number of atmospheric phenomena such as the oxidizing capacity of the atmosphere, upper tropospheric NOx and HOx cycles (Lary and Shallcross, 2000;Monks, 2005;[START_REF] Singh | Distribution and fate of selected oxygenated organic species in the troposphere and lower stratosphere over the Atlantic[END_REF], the regional formation of ozone [START_REF] Paulot | Impact of the isoprene photochemical cascade on tropical ozone[END_REF], indirect global greenhouse effect [START_REF] Derwent | Sources, distributions, and fates of VOCs in the atmosphere[END_REF], and the formation of secondary organic aerosol [START_REF] Donahue | Volatility and Aging of Atmospheric Organic Aerosol[END_REF]. Additionally, reactive oxygen-containing functional groups (as COH, C=O, C(O)OH) are polar and prone to heterogeneous reactions [START_REF] Shen | Heterogeneous reactions of volatile organic compounds in the atmosphere[END_REF] and to interact with aerosol particles. Despite the recognized significance in atmospheric cycles, OVOCs removal processes are clearly understood just for some few compounds, and consequently, their global budgets still have major gaps [START_REF] Koppmann | Oxygenated Volatile Organic Compounds[END_REF]. g -for all VOCs shown in the table (except for isoprene), kO3 is too small and the lifetime concerning this reaction is so slow that could be considered negligible.

Table I. 3: Reaction

I.4.2.1 -Photolysis

Photolysis is an important removal process for some compounds whose show strong absorption for ultraviolet and visible regions of the spectrum [START_REF] Derwent | Sources, distributions, and fates of VOCs in the atmosphere[END_REF]. To undergo photolysis in the troposphere, compounds must adsorb under wavelengths greater than 290 nm.

The energy of photons of wavelengths near the ultra-violet is comparable to covalent bond energies. Photolysis can, thus, occur when the photon energy exceeds the binding energy of the specific chemical bond resulting in its cleavage, and fragmenting the excited molecule. The rate of photolysis depends on the actinic flux which varies with the altitude, the latitude, the day of the year and time [START_REF] Seinfeld | Atmospheric chemistry and physics : from air pollution to climate change[END_REF].

Regarding the compounds discussed in this work, formaldehyde, acetaldehyde, acetone and possibly other higher aldehydes may undergo photolysis under tropospheric conditions where they also act as an important source of free radical species. The photolysis pathways and corresponding HOx radical yields are shown in Table I. 4. Photolysis rates and lifetimes are also presented in Table I.

3 for the above-mentioned compounds.

Table I. 4: Photolysis channels for formaldehyde, acetaldehyde, and acetone.

Respective yield of HOx radicals in the troposphere, wavelenght thresholds and quantum yields. Adapted from [START_REF] Koppmann | Oxygenated Volatile Organic Compounds[END_REF]. For acetone, specifically, in the lower troposphere, OH-radical reaction is the predominant remove process photolysis being important at higher altitudes.

Reaction

I I.4.3 -Semi-volatile organic compounds (SVOCs): Mono and dicarboxylic acids

Originally, VOCs are defined as organic compounds that boiling point is up to 250°C at atmospheric pressure (or that vapor pressure is greater than 10 Pa at 25°C), whereas semi-volatile organic compounds (SVOCs) have boiling points up to 380-400°C [START_REF] Epa | European Union emission inventory report 1990-2014 under the UNECE Convention on Long-range Transboundary Air Pollution (LRTAP)[END_REF]. This rigid definition has become blurred since many organics, especially those generated from atmospheric chemical reactions, are partitioned between the gas and the condensed phases. Lately, there is a new approach in the designation of organic compounds considering their partition into phases rather than exclusively their volatility; organic compounds associated with the condensed phase should, therefore, be regarded as semi-volatile. SVOCs include, thereby, compounds that more than 1% of their mass encounters in both condensed and gaseous phases [START_REF] Donahue | Coupled Partitioning, Dilution, and Chemical Aging of Semivolatile Organics[END_REF]).

The fundamental thermodynamic properties that determine the equilibrium gas-to-particle of organic compounds are the saturation vapor pressures and the enthalpies of vaporization and sublimation (Bilde et al., 2015). Gas-phase oxidation reactions can lead to lower volatility products by addition of polar functional groups or isomerization [START_REF] Kroll | Chemistry of secondary organic aerosol: Formation and evolution of low-volatility organics in the atmosphere[END_REF]. Those reduced volatility compounds may, therefore, condense onto existing ambient particles. Changes in the distribution of products proceed in the condensed phase that is also suitable for photochemical oxidation reactions and may lead to other semi-volatile as well as nonvolatile organics.

The interest in compounds of reduced volatility lies, notably, in their participation in the mechanisms of Secondary Organic Aerosols (SOA) production. There is considerable effort and focus in laboratory and modeling studies on the quantification of SOA formation, from a diversity of precursors [START_REF] Carlton | A review of Secondary Organic Aerosol (SOA) formation from isoprene[END_REF][START_REF] Hallquist | The formation, properties and impact of secondary organic aerosol: current and emerging issues[END_REF][START_REF] Nozière | The Molecular Identification of Organic Compounds in the Atmosphere: State of the Art and Challenges[END_REF][START_REF] Pandis | Source inventories and control strategies for VOCs[END_REF], since the nucleation process leading to new particle formation is not yet fully understood [START_REF] Ciccioli | High-Molecular-Weight Carbonyls and Carboxylic Acids[END_REF][START_REF] Donahue | Volatility and Aging of Atmospheric Organic Aerosol[END_REF][START_REF] Hoffmann | Organic Aerosols[END_REF][START_REF] Kroll | Chemistry of secondary organic aerosol: Formation and evolution of low-volatility organics in the atmosphere[END_REF]. The semi-volatile components could close a gap and explain some discrepancies between measured and modeled amounts of SOA [START_REF] Von Schneidemesser | Chemistry and the Linkages between Air Quality and Climate Change[END_REF].

Organic acids are generally less volatile and more water soluble when compared to their oxygenated analogs such as alcohols, ketones, aldehydes, with the same carbon number. The addition of an oxygenated functional group has a greater effect on vapor pressure than changes to the size of the carbon skeleton (Table I. 5), whereby acid groups may lower vapor pressure by over two orders of magnitude [START_REF] Kroll | Chemistry of secondary organic aerosol: Formation and evolution of low-volatility organics in the atmosphere[END_REF]. Furthermore, organic acids are fairly unreactive in the gas phase, partly because of the O-H bond which is quite strong and, also, inhibits, somehow, the reactivity of the neighboring sites (Calvert, 2011). For aliphatic acids, except for ketoacids, tropospheric photolysis is also of negligible importance. Thus, the gas-phase lifetime for saturated organic acids is determined by their partition and solubility coefficients (Henry law constants), and their fates are closely correlated with atmospheric particles (Chebbi andCarlier, 1996, Mellouki et al., 2015;[START_REF] Shen | Heterogeneous reactions of volatile organic compounds in the atmosphere[END_REF].

Table I. 5: Changes in the vapor pressure of an organic compound due to the addition of a functional group.

Adapted from [START_REF] Kroll | Chemistry of secondary organic aerosol: Formation and evolution of low-volatility organics in the atmosphere[END_REF].

Functional group Structure Multiplicative factor in vapor pressure K Ketone -C(O)- 0.10 A Aldehyde -C(O)H 8.50E-02 H Hydroxyl -OH 5.70E-03 H Hydroperoxyl -OOH 2.50E-03 C Carboxylic acid -C(O)OH 3.10E-04 P Peroxyacid -C(O)OOH 3.20E-04 N Nitrate -ONO2 6.80E-03 e extra carbon -CH2- 0.35
Dicarboxylic acids (DCA) are even less volatile and more soluble in water when compared to the respective monoacids (MCA) and are mainly present in the particulate phase (Chebbi and Carlier, 1996;Kawamura et al., 1996). Water-soluble C2-C9 DCAs are ubiquitous in the atmosphere and have been widely identified in atmospheric aerosols in the most diverse environments such as urban, rural, marine and remote locations as polar regions and over high mountains [START_REF] Boreddy | Long-term (2001-2013) observations of watersoluble dicarboxylic acids and related compounds over the western North Pacific: trends, seasonality and source apportionment[END_REF]Kawamura and Bikkina, 2016;Kawamura and Yasui, 2005). Besides, they are produced as oxidation products of gaseous aliphatic monocarboxylic acids, which are, in turn, produced by photo-induced oxidations of biogenic unsaturated fatty acids and other precursors such as midchain n-alkanes, aldehydes and ketocarboxylic acids (Kawamura et al., 1996). Longer-chain DCAs may also be precursors of smaller DCAs, and oxidation of ambient DCAs contribute to the formation of oxo-DCAs and hydroxy-DCAs, etc. Figure I. 10 illustrates possible pathways for the formation of dicarboxylic acids from different sources [START_REF] Pavuluri | Laboratory photochemical processing of aqueous aerosols: formation and degradation of dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls[END_REF]. Additionally, field studies have found dicarboxylic acids and other carbonyl compounds internally (and externally) mixed on mineral dust (Falkovich et al., 2004;Russell et al., 2002;Sullivan and Prather, 2007;Wang et al., 2015). Therefore, in this work, SVOCs were then represented by dicarboxylic acids in order to study their reactive interaction with surfaces of mineral aerosols and possible photochemical oxidation pathways that could contribute to dust chemical aging.

I.5 Heterogeneous Photochemistry

I I.5.1 -How do small amounts of airborne particles change the composition of the atmosphere?

Condensed media can reduce energy barriers or increase the availability of reactive species and thus provide reactional pathways that would be unlikely or slower in the gas phase. The study of surfacemediated reactions has been widely developed in the field of applied sciences, notably in heterogeneous catalysis. The traditional field of surface sciences has merged with the atmospheric sciences from the need to understand how surface structure and functionalities of aerosols affect interfacial chemistry [START_REF] Chapleski | Heterogeneous chemistry and reaction dynamics of the atmospheric oxidants, O 3 , NO 3 , and OH, on organic surfaces[END_REF].

As mentioned previously, aerosol's role in the climatic systems, particularly in the troposphere is now absolutely recognized, but not completely understood, what boosted the interest in heterogeneous and multiphase chemistry from an atmospheric point of view. The so-called heterogeneous reactions occur in more than one phase, or at the interface between two phases, as a gas reacting with a solid or with a liquid. Ravishankara in his review [START_REF] Ravishankara | Heterogeneous and Multiphase Chemistry in the Troposphere[END_REF] used the terminology of 'heterogeneous chemistry' to designate reactions that take place on a solid and 'multiphase chemistry' for those occurring in liquids. The major difference between the two concepts lies in the fact that solid surfaces prevent (or hinder) the diffusion of the reactants restricting the reactions to the surface. Conversely, in the case of liquid particles, chemical reactions can occur in the bulk, depending on the diffusional characteristics of the system and, therefore, the whole volume of the particle should be considered.

Solar radiation is the primordial driver of chemical reactions on Earth's atmosphere.

Consequently, the effects of solar radiation were included within the heterogeneous atmospheric science "package of interest", giving rise to a series of studies on photochemical heterogeneous process. The presence of light absorbing compounds in aerosol particles may promote photosensitized processes on all types of atmospheric surfaces such as organic aerosols, particulate matter, soot, mineral dust and even at the air-water interface. Finally, the integration of irradiation as a parameter for reaction activation on solid surfaces of mineral oxides resulted in a photocatalytic approach that will be next presented.

I I.5.2 Heterogeneous Photocatalysis

Semiconductor-mediated heterogeneous photocatalysis has been reported in gas and liquid phases referring to fundamental, integration and application studies [START_REF] Hoffmann | Environmental Applications of Semiconductor Photocatalysis[END_REF][START_REF] Ngo | Kinetics and mechanism of the photocatalytic degradation of acetic acid in absence or presence of O 2[END_REF][START_REF] Teoh | Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors[END_REF][START_REF] Yalçin | Fe+3-doped TiO2: A combined experimental and computational approach to the evaluation of visible light activity[END_REF][START_REF] Zorn | Effect of molecular functionality on the photocatalytic oxidation of gas-phase mixtures[END_REF]. Generally speaking, photocatalyst is a material that can induce chemical reactions upon efficient light absorption [START_REF] Gaya | Heterogeneous photocatalysis using inorganic semiconductor solids[END_REF]. However, the most notable one is TiO2 and TiO2-based materials.

In 2010, Paz [START_REF] Paz | Application of TiO2 photocatalysis for air treatment: Patents' overview[END_REF] carried out a survey of patents on the application of TiO2 to photocatalytic technologies showing an expressive increase in numbers, especially for indoor air treatment. Besides that, there is practical interest in TiO2 application for air and water remediation, self-cleaning and self-sterilizing surfaces, solar cells, water splitting, hydrogen generation [START_REF] Chen | Semiconductor-based Photocatalytic Hydrogen Generation[END_REF][START_REF] Friedmann | TiO2for water treatment: Parameters affecting the kinetics and mechanisms of photocatalysis[END_REF][START_REF] Fujimoto | Photocatalytic oxidation of selected gas-phase VOCs using UV light, TiO2, and TiO2/Pd[END_REF][START_REF] Longo | Solid-State and Flexible Dye-Sensitized TiO2 Solar Cells: a Study by Electrochemical Impedance Spectroscopy[END_REF][START_REF] Ollis | Kinetics of photocatalyzed film removal on self-cleaning surfaces: Simple configurations and useful models[END_REF][START_REF] Pichat | Some views about indoor air photocatalytic treatment using TiO2: Conceptualization of humidity effects, active oxygen species, problem of C1-C3 carbonyl pollutants[END_REF][START_REF] Shen | An overview of photocatalytic materials[END_REF]. Some other examples of semiconductor solids used in heterogeneous environmental photocatalysis include ZnO, CdS, WO3, Bi2WO6, SnO2, ZnS, CdTe, CdTe, AgNbO3 and SrTiO3 [START_REF] Elango | Efficacy of SnO2 nanoparticles toward photocatalytic degradation of methylene blue dye[END_REF][START_REF] Nascimento | Biosilica from diatomaceous earth as support to CdS-mediated photocatalysis in dry and aqueous phase[END_REF][START_REF] Oliveira | Photoelectrochemical and Photocatalytic Properties of TiO 2 , WO 3 and WO 3 -TiO 2 Porous Films in the Photodegradation of Rhodamine 6G in Aqueous Solution[END_REF][START_REF] Sakthivel | Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2[END_REF]. As mentioned in section I.3.2, mineral dust particles contain varying amounts of semiconducting metal oxides, such as TiO2 and Fe2O3, conferring to them photocatalytic properties (Usher et al., 2003).

As for any other heterogeneous process (such as the uptake of a gas by a liquid droplet or traditional heterogeneous catalysis), the overall heterogeneous photocatalytic process can be decomposed into five independent steps (Herrmann, 2010) The difference from other processes resides particularly in step 3, because photocatalysis implies the activation of the solid by photons, instead of, for instance, catalytic thermal activation.

Upon illumination with photons carrying energy equal to or greater than its band gap, an electron in the semiconductor's valence band is excited to the conduction band, leaving a positive charge called "hole" (h + ) and creating an electron-hole pair, reaction R I. 1. The holes (h + ) may accept an electron from an adsorbed donor species (Dads) which undergoes oxidation, R I. 2. The free electron (e -), in turn, can be transferred to an adsorbed electron acceptor (Aads), thus promoting its reduction, R I. 3.

Redox potentials play an important role in photocatalysis since: for the oxidation of an adsorbed e -donor, Dads, the valence-band holes must be more positive than its redox potential; while for the reduction of an e -acceptor to take place, the conduction-band must be more negative than the corresponding Aads redox potential. The holes usually present very positive oxidation potentials, in the range of +2.0 V to +3.5 V depending on the semiconductor and the pH [START_REF] Nogueira | A fotocatálise heterogênea e sua aplicação ambiental[END_REF] being enough to generate radicals.

R I. 1 R I. 2 R I. 3
Alternatively, in the absence of suitable electron and hole scavengers, excited conductionband electrons and valence-band holes may recombine to dissipate energy, the charge distribution on the semiconductor surface returns to its initial state as before the excitation. Figure I. 12 represents a more realistic model of energy band diagram for TiO2 (Herrmann, 2010). This representation avoids misconceptions normally induced by the traditional and generally adopted figure that mixes an allegoric spherical catalyst particle with an energy band diagram. Besides, instead of the round arrows normally used to indicate the charge transfer reactions giving the false impression that the photoexcited species formed on the surface escape from it to react in bulk, angled arrows were used to make it clear that these reactions take place on the surface of the photocatalyst. VOCs can be directly oxidized by the holes leading to lower molecular weight products (Chen et al., 2012) and free radicals [START_REF] Hoffmann | Environmental Applications of Semiconductor Photocatalysis[END_REF].

Adsorbed oxygen (O2) acts as the dominant electron acceptor, especially on particles exposed to ambient air, and produces the superoxide radical anion (O2 -• ) that triggers a series of chain reactions generating more hydroxyl radicals and other reactive oxygen species (ROS) (Hirakawa et al., 2007). O2 in the air is considered the main oxidizing agent for mild oxidation in anhydrous systems (Herrmann, 2001).

In light of the scientific concepts and topics covered in this chapter we would like to remark that, mineral dust photochemistry combines knowledge of three broad areas: atmospheric sciences, catalysis and photochemistry, as evidenced in Figure I. 13.

Figure I. 13: Schematic representation of the multidisciplinarity of mineral dust photochemistry.

This subject is "situated" in the intersection of knowledge of three broad areas: atmospheric sciences, catalysis and photochemistry.

Photocatalysis emerged from the encounter between (heterogeneous) catalysis and photochemistry and, as mentioned earlier, is a broad and fruitful field in environmental applications and technologies.

The uptake of reactive species on the surface of aerosol particles is a subject combining catalysis surface sciences with atmospheric sciences, as briefly discussed in section I.5.1.

The study of atmosphere's oxidative capacity involves gas-phase chemistry, photolysis rates and photosensitized reactions and lies in the intersection between atmospheric sciences and photochemistry.

The investigation of photo-enhanced reactions on mineral dust stands at the intersection of all of these large areas and require the constant exchange of skills between them. It is, therefore, extremely multidisciplinary and a very innovative research field that is currently emerging and evolving.

I.6 -Thesis Overview

As it was previously introduced, dust chemistry has environmental impacts at different levels hence, the number of studies interested in this topic has been rising lately. However, the understanding of the global process as well as specific processes is still incipient, so there is a wide range of issues that need to be further investigated. Thereupon emission, mineral dust particles will be in contact with different atmospheric components; organic and inorganic compounds issued from natural and anthropogenic activities.

During dust lifetime, those interactions are responsible for their atmospheric processing and raise the following questions. This question is closely linked to aging of mineral aerosols. The combined effect of mineral and organic aerosols has been poorly explored, and usually, both are studied separately. Low and medium volatility organic compounds, characteristic of biogenic and biomass burning emissions, are known to condense on particle's surface forming organic coatings. How these compounds interact with semiconductive materials present on dust aerosols when illuminated, the reaction pathways and the gas-phase products partition are some of the aspects discussed.

Chapter IV is dedicated to addressing this question. Five saturated linear chain dicarboxylic acids, from C4 (succinic acid) to C8 (suberic acid) were chosen to evaluate the photochemical aging occurring on dust surface. Gas-phase composition and surface sorbed products were monitored. The surface-mediated photochemistry of dicarboxylic acids was shown to release many OVOCs to the gas-phase as well as to produce other low volatility highly oxygenated compounds.

Question 3: How inorganic coatings interact with dust and co-sorbed organics?

This question is also related to the aging of mineral aerosols in a further level. Dust plumes often cross polluted regions where they can interact with inorganic compounds such as nitrates and sulfates, typical of anthropogenic emissions. How inorganic and organic compounds simultaneously adsorbed on the surface interact with particles photochemical properties is the topic addressed with this question.

Chapter V is focused on the influence that the presence of co-sorbed nitrates exerts on the uptake and reactivity of organic compounds on mineral dust surface. Sodium nitrate was used as the source of nitrate anions and ATD as the mineral substrate. The effect of nitrate was evaluated concerning two aspects: the uptake of acetone and the reactivity of glutaric acid.

Finally, Chapter VI provides a general closure, conclusions, and atmospheric implications common to this work. We also make an outlook with perspectives for possible future works that may address unanswered questions in such a rich research field that is mineral dust photochemistry.

II.1 General Methodology

All experiments reported in this thesis were performed in a flow tube set-up. Starting from the execution of the experiments, the obtention of data and its subsequent treatment, the methodology encompasses, generally, three steps, depicted in Step 1 consists of the sample preparation, that is, coating the tube insert with the respective substrate of interest, Arizona test dust (ATD), titanium dioxide (TiO2) or silica (SiO2). The coating method varied depending on the mineral in use, and it will be detailed separately in each chapter.

Step 2 consists of the flow tube experiments where the uptake and reactivity of different organic compounds were studied for different mineral substrates. The experimental set-up is described in detail hereinafter.

Step 3 is the post-experiment stage and comprises all the mass spectra analysis, product identification, kinetic calculations to obtain the uptake coefficients. A general presentation is given in the following sections and, details are discussed individually in each chapter.

Step 4 was introduced for the study of dicarboxylic acids (DCA) heterogeneous photochemistry (Chapter IV). DCA have relatively low volatility, as well as their oxidation products and therefore tend to remain adsorbed in the condensed phase. For this reason, the extraction of the dust powder and its subsequent analysis by ultra-high performance liquid chromatography-heated electrospray mass spectrometry (UHPLC/HESI-MS) provided additional information about products and reaction intermediates to support and clarify the reaction mechanism. where the dust were coated in the inner surface, and which fits within the larger tube, providing good contact to the jacket. The temperature was maintained constant during each experiment by circulating thermostatically controlled water around the outer jacket using a recirculating chiller (Huber CC 130).

II.2 Experimental

The gases were introduced into the reactor through a Pyrex tube injector of 25 cm length and 0.3 cm internal diameter. The injector is movable and allows varying the surface of interaction between the gas and the solid immobilized on the inner walls of the sample insert. In addition, the flow rates used throughout this work (200 -300 ml/min) were chosen to assure laminar flows inside the coated tube. The reactor dimensions and the fluid dynamics data for the operating conditions used are summarized in Table II. 1. Acquired using a Spectrometer AvaSpec-2048 (Avantes), using a calibration lamp AvaLight-DH-CAL and FC-UV400 fiber.

One of the main advantages of flow tube reactors is that they can operate at atmospheric pressure and thus enable the study of reactions as a function of relative humidity that are not suitable, for instance, for Knudsen cell reactors [START_REF] Styler | Heterogeneous photochemistry of atmospheric dusts and organic films by and organic films[END_REF]. On the other hand, one disadvantage is the requirement to immobilize the mineral samples coating them on the insert surface. Although simple, the coating method for flow tube experiments with solid substrates has some challenges. First, the procedure is difficult to replicate, and two films prepared following the same protocol will hardly be identical. The surface of the film even homogeneous to the naked eye is never precisely reproducible and indeed contains heterogeneities at a microscopical level along the reactor. Underwood et al.

remarked the importance of having evenly distributed coverages of the powder that should also cover the entire geometrical area of the sample holder (Underwood et al., 2000). Otherwise, kinetics measurements may reflect the amount of uncovered/unreactive surface in the sample holder as well as the sample mass.

A second point therefore arises, although the geometric surface area of a coated wall flow tube is simply calculated, the surface area of a heterogeneous and multilayer solid layer has a large uncertainty. Figure II. 5 depicts the microscopic view of a dust coating emphasizing heterogeneities in particle packing and dimensions. Therefore, the real total surface accessible and prone for heterogeneous reactions is uncertain and impossible to determine accurately. Very recently, a new study by Li and coworkers evinced the effect of the surface roughness in coated-wall flow tube setups used for gas uptake and kinetic measurements [START_REF] Li | Technical note: Influence of surface roughness and local turbulence on coated-wall flow tube experiments for gas uptake and kinetic studies[END_REF]. They discuss how coating surface roughness may potentially distort the laminar flow inducing local turbulence and introducing some uncertainties in the calculated uptake coefficient. They propose, though, a correction method to account for roughness-induced local turbulence effects.

Lastly, the need to pull back and forward the mobile injector also induces mechanical disturbances in the flow resulting in significant noise for some uptake profiles. [START_REF] Evonik | Fumed Silica Technical Overview[END_REF].

The film preparation consisted, essentially in preparing suspensions of the selected mineral in ethanol or ethanol and water (depending on the experiment). These suspensions were sonicated for 10 minutes and then dripped into the Pyrex insert, which was rotated manually to obtain a uniform coated film. Once achieved, the excess solution, which had not adhered to the walls, was allowed to flow out, and the tube was dried flowing nitrogen through it. This step was repeated few times until obtaining a homogeneous film (visual inspection). Finally, the insert was placed in an oven for drying and evaporating the solvent. Temperature and time varied depending on the experiment, and it is specified in each chapter. After drying, all tubes were weighed, the mass of films ranged from 15 to 150 mg. and Na (C, N, and O were not included in this analysis). Titanium was detected in trace quantities [START_REF] Cwiertny | Characterization and acid-mobilization study of iron-containing mineral dust source materials[END_REF]. Comparison between XPS and SEM/EDX revealed a reasonable agreement between the surface and bulk compositions. The authors emphasize that correlations between surface and bulk elemental concentrations are not evident, because the surface of source materials are exposed to different conditions and can undergo chemical weathering during transportation.

In addition, SEM/EDX revealed that iron is unevenly distributed in ATD particles, some grains exhibited high or more moderate iron content, while others were entirely free of iron. Besides, iron was identified at different mineralogy states. In some particles, iron was in regions free of Si and Al, an indication of iron oxide, whereas in other cases, the iron was located within grains containing also Si and Al (consistent with Fe substitution into an alluminosilicate mineral). Further analysis showed that smaller particles, which were subjected to longer-range atmospheric transport, were enriched with Fe form such as (oxyhydr) oxides (e.g., goethite and hematite). Additionally, Mossbauer Spectroscopy provided qualitative information indicating the presence of iron II and III substituted into alluminosilicates but also some Fe(III) as ferric oxide.

I II.2.4 Permeation System

A VICI Metronics Dynacalibrator®, Model 150 permeation oven system was used to generate the organic compound of interest in the gas phase. This device provides accurate and repeatable gaseous streams with precise concentrations of volatile compounds (in the ppm or ppb range) for a wide range of substances. The permeation tubes are sealed PTFE (Polytetrafluoroethylene) membranes filled with the selected analyte, usually in the liquid phase, which permeates through the walls of the membrane at a constant rate (permeation rate). By heating the furnace chamber, the volatilization of the compound occurs, as well as the gas exchanges and diffusion with the external medium through the entire length of the tube until the seals, impermeable. An inert carrier gas is used for sweeping the calibration gas/vapor from the chamber. A wide range of rates can be achieved by varying the length and thickness of the tube, with typical rates varying from 5 ng/min to 50,000 ng/min. A temperature controller maintains the internal chamber temperature at the selected value with an accuracy of ± 0.01 ° C, based on NIST standards (VICI Metronics Inc., 2013).

The desired volumetric concentration can be easily modified by varying the flow of the carrier gas into the chamber. The outlet stream of the oven can be blended with a make-up flow and the concentration of the analyte is inversely proportional to the total gas flow in the system and can be readily determined using the following equation,

(II.1)
where K = 24.45 / molecular weight of gas; P = permeation rate in ng/min (information provided in the permeation device documentation); QT = chamber carrier flow (ml/min).

For a permeation rate known at a reference temperature, the rate at a second temperature can be estimated as

(II.2)
P0 is the permeation rate at reference temperature T0; P is the new permeation rate at temperature T.

The permeation tubes are calibrated and certified by the manufacturer, and the permeation rate specifications are given at a reference temperature. For this study, VICI Metronics Dynacal® Permeation tubes were used with the characteristics shown in Table II. 3. I II.2.5 Moisture Generator

Water vapor was introduced into the system using a sintered glass bubbler saturator. The bubbling system is submerged in water at constant temperature, thereby maintaining the same moisture conditions throughout the experiment. The output stream is saturated at 100% relative humidity. In this way, the final relative humidity of the system can be adjusted by diluting with a make-up flow. A

II.3 How heterogeneous reactions are studied in our experimental set-up

One of the key points of this experimental work was kinetic measurements of VOC uptake coefficients for different mineral proxies and conditions. The definition of the uptake coefficient (γ) derives from the Kinetic Theory of Gases, specifically from the theory of collisions. In this way, the uptake coefficient is defined as (Crowley et al., 2010): "The probability that a molecule A will undergo a collision with a surface is retained on the surface." And expressed by:

(II.3)
The coefficient is the kinetic parameter that express the efficiency of heterogeneous reactions and it is generally used for both for gas-particle and gas-liquid reactivity.

The experiment consisted in measuring the system response (inlet concentration x outlet concentration) for each position of the mobile injector, which corresponds to a given residence time.

All the uptake measurements reported herein were performed by moving the injector stepwise (steps of 2 cm), exposing the coated tube surface for 10 minutes. The procedure was repeated until the entire length of the tube was exposed, which is equivalent to 10 positions of 2 cm each. A typical profile for butanol uptake is presented in For the determination of the uptake coefficient, the decay of the analyte in the gas phase is supposed to obey the first-order kinetics as follows

(II.4)
kobs is the observed first-order rate constant; C(0) is the inlet and C(t) the outlet concentration of the trace gas after a reaction time (t); ‹v› is average molecular velocity, V is the total volume (of the reactor), and SSurf is the surface area exposed to the gas. Figure II. 9 c illustrates how kobs is generally obtained from the slope of the curve ln(C(0)/C(t)) x Residence time (t).

For a cylindrical flow tube, the uptake coefficient is given by

(II.5)
where r is the radius.

When calculated using Equation (II.5), the uptake coefficient is called the 'geometric coefficient' γgeo, because it takes into account the geometric surface of the sample holder to consider the occurrence of collisions that will result in adsorption/reaction.

In many cases, a dependence of the uptake coefficient, γgeo, calculated using the geometric surface area is observed as a function of the mass of the solid sample. The linear dependence of γgeo with the mass is generally considered as an indication that the entire surface area of the solid is accessible to the gaseous reagent (Underwood et al., 2000).

The mass-dependent γgeo can be converted into a mass-independent coefficient, considering, for example, the Brunauer-Emmett-Teller surface area (N2-BET) of the sample. The coefficient γBET is obtained by the ratio of the geometric surface area to the total BET area,

(II.6)
where SBET is the BET surface area, and mfilm is the mass of the solid sample applied for the respective measurement.

Using whether the geometric surface area whether the total BET area produces a divergence in the calculated uptake coefficients by approximately a few orders of magnitude. The geometric uptake coefficient could be regarded as an upper limit for the realistic uptake coefficient whereas the BET uptake may be considered a lower limit (Crowley et al., 2010). In fact, the surface area veritably available for heterogeneous uptake should lie between the two extreme cases and varies particularly for each setup and mineral in question.

II.4 Proton Transfer Reaction -Time of Flight Mass Spectrometer

During this work, we utilized a high-resolution Proton Transfer Reaction Time of Flight Mass Spectrometer (PRT-ToF-MS, Ionicon 8000, Austria) as the main analytical tool to follow online the mixing ratios of reactants and products in the gas-phase. The instrument was coupled to the outlet of the experimental set-up, and the operational parameters changed throughout the evolution of this work but remained constant for each series of experiments, and so are specified in each chapter of results. A brief description of the major characteristics of this technique will be discussed as follows.

The proton-transfer reaction mass spectrometry is a relatively new MS technique that was first developed in the late 1990s by Lindinger and co-workers at the University of Innsbruck, Austria [START_REF] Hansel | Proton transfer reaction mass spectrometry: on-line trace gas analysis at the ppb level[END_REF][START_REF] Lindinger | Proton-transfer-reaction mass spectrometry[END_REF]. It was especially designed to enable soft ionization, i.e., non-dissociative proton-transfer and detect atmospheric VOCs at few ppbv levels without any preconcentration treatment. It is, therefore a very powerful tool for online monitoring of trace compounds in the air. Despite its initial scope for applications to the atmospheric sciences, the PTR-MS has been widely used in several other research fields, such as food and flavor analysis, biotechnology, clean-room monitoring, medical sciences, botanical applications, etc, [START_REF] Acton | Headspace analysis of new psychoactive substances using a Selective Reagent Ionisation-Time of Flight-Mass Spectrometer[END_REF][START_REF] Erickson | Measuring long chain alkanes in diesel engine exhaust by thermal desorption PTR-MS[END_REF][START_REF] Hayeck | Monitoring of organic contamination in the ambient air of microelectronic clean room by proton-transfer reaction/time-of-flight/mass spectrometry (PTR-ToF-MS)[END_REF][START_REF] Jardine | Green leaf volatiles and oxygenated metabolite emission bursts from mesquite branches following light-dark transitions[END_REF][START_REF] Kuś | Discrimination of Polish unifloral honeys using overall PTR-MS and HPLC fingerprints combined with chemometrics[END_REF].

I II.4.1 Operational principle

The PTR-MS connects chemical ionization and flow-drift tube techniques using the ionization of water to form hydronium ions (H3O + ) which will subsequently ionize the analytes. The design of the instrument allows ionizing VOCs with very little fragmentation and without ionizing simultaneously inorganic trace gases common in the air like CO2, CO, N2, and O2. Basically, the instrument consists of four main parts: the ion source, the drift tube, the transfer lenses and the mass analyzer (Figure II. 10). The time-of-flight mass analyzer provided a significant improvement in the mass resolution and in the acquisition time of PTR spectrometers. In this analyzer, the ions mass-to-charge ratios are determined from their flight times and a complete mass spectrum is generated for each scan interval [START_REF] Jordan | A high resolution and high sensitivity proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS)[END_REF]. The operation of our instrument provides a mass resolution of over 5000 m/Δm allowing the differentiation of compounds into approximately ± 0.02 amu (for m/z = 100).

I II.4.2 Ion Chemistry

Proton-transfer reactions of H3O + ions can ionize VOC species (M) as follows, R II 1 R II 1

The reaction will be exothermic and take place in the drift tube if the proton affinities (PA) of species M are higher than water proton affinity (166.5 kcal/mol).

For VOCs that have a PA slightly higher than water, the reverse reaction is non-negligible and the proton-transfer reaction will be less efficient and more dependent on the humidity condition of the system. An example of VOC that has the proton affinity close to H2O is formaldehyde, and for this reason some studies specifically discuss this compound in attempt to better characterize its detection by PTR-MS (Inomata et al., 2008;Schripp et al., 2010;[START_REF] Vlasenko | Formaldehyde measurements by Proton transfer reaction -Mass Spectrometry (PTR-MS): correction for humidity effects[END_REF].

The concentration of the MH + ions generated from R II 1 can be calculated by:

(II. 7)

[M] is the concentration of trace gas M; [H3O + ]0 is the concentration of reagent ions injected from the ion source; k is the rate coefficient for the proton-transfer reaction of H3O + with M; Δt is the reaction time in the drift-tube (≈ 10E-4 s).

The rate coefficient k has typical values between 1.5 to 4 x 10E-9 cm³ s -1 [START_REF] Lindinger | Proton-transfer-reaction mass spectrometry[END_REF]. For the data treatment in this study, we adopted the recommended mean value of 2.0 E-9 cm³ s -1 [START_REF] Cappellin | On quantitative determination of volatile organic compound concentrations using proton transfer reaction time-of-flight mass spectrometry[END_REF]. Room-temperature rate constants between H3O + and various VOCs were measured or theoretically calculated in many studies [START_REF] Cappellin | On quantitative determination of volatile organic compound concentrations using proton transfer reaction time-of-flight mass spectrometry[END_REF][START_REF] Zhao | Proton transfer reaction rate constants between hydronium ion (H3O+) and volatile organic compounds[END_REF] and typical uncertainties are ± 20% [START_REF] Hansel | Proton transfer reaction mass spectrometry: on-line trace gas analysis at the ppb level[END_REF].

The approximation in Equation (II. 7) is valid only if a small fraction of H3O + is consumed. In other words, if [H3O + ]0 ≈ [H3O + ], and for this reason the instrument should always work in excess of hydronium ions, or the respective primary ion if another ionization mode is in use.

In this case, the measured values and in counts per second (cps) in the mass analyzer are related to MH + concentration by

(II. 8)
The factors TMH+ and TH3O+ are the transmission efficiencies for MH + and H3O + ions, respectively, and are determined experimentally.

I II.4.3 Fragmentation and spectra analysis

The proton-transfer reaction theoretically should provide soft ionization, and VOCs should be detected at the protonated molecular mass, M+1 amu, but in practice, fragmentation is observed.

Typically, the fragmentation follows a pattern, and each class of organic compounds has more or less representative channels compared to MH + protonation; such as the loss of a water molecule for alcohols, Figure II. 11. Fragmentation patterns for atmospherically relevant OVOCs have already been studied (Buhr et al., 2002;[START_REF] Smith | Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis[END_REF][START_REF] Spanel | A selected ion flow tube study of the reactions of H3O+, NO+, and O2+ with saturated and unsaturated aldehydes and subsequent hydration of the product ions[END_REF][START_REF] Ravishankara | Heterogeneous and Multiphase Chemistry in the Troposphere[END_REF], Spanel and Smith, 1998a, 1998b[START_REF] Ravishankara | Heterogeneous and Multiphase Chemistry in the Troposphere[END_REF] and recently reviewed by Yuan et al. (Yuan et al., 2017). However they could be altered depending on the actual experimental conditions (for example, humidity) and instrument settings (de Gouw and Warneke, 2007) especially, regarding the relative importance of each channel. At 300 K determined using a SIFT (Soft Ionization flow tube) mass spectrometer apparatus. From [START_REF] Smith | Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis[END_REF].

Additionally, not only humidity but also an altered sample gas matrix, e.g., with elevated levels of CO2, may change the ion distribution in the reaction chamber and thereby influence quantitation using the theoretical approach [START_REF] Beauchamp | On the performance of protontransfer-reaction mass spectrometry for breath-relevant gas matrices[END_REF].

I II.4.4 Data analysis

The data treatment and processing from the raw signals recorded by the PTR-ToF software involved several steps: (1) mass calibration, (2) generation of the peak list, (3) peak fitting and peak integration, (4) product identification and molecular assignment. We used the PTR-ToF-MS Viewer Software v.

3.1. 0.31 (Ionicon Analytik GmbH). This software provides different custom-built tools that facilitate the data analysis.

(1) Mass calibration or m/z calibration convert and correct the ion time of flight to mass-to-charge ratio. We used the isotope of reagent ion H3 18 O + m/z = 21.0220 and some impurities ubiquitous in all measurements, C3H6OH + m/z = 59.0491, NO + m/z 29.9974.

(2) The peak lists for each series of experiments was initially defined using the tool 'Auto search' from PTR Viewer. Then, spectra were further explored to search for other products and fragments that might be missing from the initial list. In this way, the so-called user-defined peak lists were created for each type of experiment based on the characteristics of the system to be analyzed.

(3) Using the user-defined peak lists, each peak shape was manually checked to define the fitting amplitude of each ion. The software offers three types of adjustment: normal, Gauss and multipeak. The gauss and multipeak modes are particularly used when there are overlapping peaks of very close masses, Figure II. 12. In this way, it is possible to determine the exact limits of each one, make the deconvolution of the peaks and, therefore, minimize the influence of one mass on the other during the peak integration. (4) Product identification and molecular assignment are the last and most time-consuming step.

The interpretation of mass spectra in our experiments is complicated by fragmentation, with many fragments common to several VOCs.

Overall, each of the steps being manual and in a certain way, visual the data processing is prone to biases. Nevertheless, all quantification was based on the relative values of concentration differences between the inlet and the outlet of the reactor or before and after the lights were switched on during the same experiment. In this way, intrinsic variation for VOCs concentrations and systematic instrumental errors can be considered equivalent during each run.

To improve our product identification, fragmentation patterns of some reagents and some At our instrument, under our operational conditions, for pure butanol standard.

In our instrument, butanol was found at m/z 57.069 and 41.039, which corresponds to the loose of a water molecule, [M-H2O] + and the C3H5 + fragment, respectively. As it can be seen, the fragmentation channels found experimentally are not the same reported by Smith and Spanel [START_REF] Smith | Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis[END_REF], Figure II. 11: Fragmentation patterns (or product ion distributions) for the reactions of H3O + with some alcohols and aldehydes., where butanol is expected to be detected mostly in its protonated form. Our results, meanwhile, are in accordance with other alcohol fragmentation measurements (Buhr et al., 2002;Yao and Feilberg, 2015) that detected butanol in the same m/z as us.

Details about m/z to molecular assignment and standards procedures are discussed in each chapter. In Table II. 4, it is presented the results for the detected ions due to the fragmentation of standards performed at our PTR-Tof-MS instrument operating on H3O+ under our operational conditions.

Table II. 4: Fragmentation ions of performed standard compounds on PTR-Tof-MS operating on H3O + , at our instrument, under our operational conditions.

For further details see Chapter III a and Chapter IV.

line monitoring of volatile organic compounds at pptv levels. Chem. Soc. Rev. 27, 347.

III. Reactive uptake of VOCs on mineral dust surface III a.1 -Introduction

Every year, large amounts of mineral dust are emitted into the atmosphere and transported over several kilometers before being deposited. For instance, Kaufman and coworkers estimated that 240 ± 80 Tg of dust are transported annually from Africa to the Atlantic Ocean, and of these, 50 Tg arrives in the Amazon Basin (Kaufman, 2005). During their lifetime in the troposphere, dust particles undergo physical interactions, including absorbing and scattering light, which influences radiative forcing and global albedo, (Sa'id, 2012) and inducing chemical reactions when acting as a substrate for the conversion or scavenging of various trace gases (Usher et al., 2003). Heterogeneous reactions of atmospheric relevant trace gases on mineral aerosols have received more attention lately, and the uptake of a few reactive gases by mineral aerosols has been studied using a variety of experimental approaches. For instance, the uptake kinetics of inorganic compounds such as NO2, HNO3, N2O5, O3, SO2, (Adams et al., 2005;Hanisch and Crowley, 2003;Seisel et al., 2005;Underwood et al., 2000) as well as some organics (Carlos-Cuellar et al., 2003;Li et al., 2001) were investigated on mineral dust and various proxies. However, few studies have explored the effect of irradiation on the uptake.

Suspended dust particle composition varies depending on its sources location and may contain variable amounts of semiconductor metal oxides that show photocatalytic activity, such as TiO2 and Fe2O3, whose application in wastewater and air treatment have been extensively studied (Alberici and Jardim, 1997;[START_REF] Herrmann | Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants[END_REF][START_REF] Litter | Heterogeneous photocatalysis Transition metal ions in photocatalytic systems[END_REF]Wang et al., 2016). Recently, the photocatalytic nature of dust particles has been suggested and discussed (Chen et al., 2012;George et al., 2015). However, up to now, the most investigations focused on inorganic compounds such as ozone, (Nicolas et al., 2009) nitrogen dioxide (Ndour et al., 2008), sulfur dioxide [START_REF] Dupart | Mineral dust photochemistry induces nucleation events in the presence of SO2[END_REF][START_REF] Huang | Kinetics of Heterogeneous Reaction of Sulfur Dioxide on Authentic Mineral Dust: Effects of Relative Humidity and Hydrogen Peroxide[END_REF], and hydrogen peroxide (Zein et al., 2014), and little is known about mineral dust photoreactivity towards volatile organic compounds (VOC) [START_REF] Styler | Heterogeneous photochemistry of atmospheric dusts and organic films by and organic films[END_REF]Donaldson, 2012, 2011).

To assess the importance of photocatalytic transformations of volatile organic compounds on mineral dust, we investigated the uptake kinetics of butanol on Arizona test dust (ATD). We performed experiments under dark and illuminated conditions and measured uptake coefficients as a function of several experimental parameters, including initial VOC concentration, relative humidity, external temperature, and light intensity. Additionally, we analyzed the gas phase products, evaluating the role of dust and the influence of light, whose effects are quantified and discussed.

Butanol was selected as a proxy for small alcohols (and/or oxygenated VOCs; OVOC) that are abundant in the atmosphere. OVOCs are generally emitted directly by anthropogenic sources, biogenic sources, or produced from the degradation of other VOCs (hydrocarbon oxidation). Butanol, specifically, is an alcohol that does not absorb light in the UV or visible regions, but its incomplete oxidation products can generate photochemically active radicals.

III a.2 -Experimental Section

I III a.2.

Coated wall flow tube

Experiments were performed in a horizontal coated wall flow tube reactor described in detail previously (Ndour et al., 2009). The temperature was maintained constant during each experiment by circulating thermostatically controlled water around an outer jacket. Dust was coated on the inner surface of a removable Pyrex insert (id 0.55 cm, length 20 cm), which fit within a larger tube, providing good contact with the jacket. Seven actinic lamps (Phillips 20 W) surrounded the reactor and could be switched on or off to modify the light intensity between 0 and 1.4 mW/cm² (see Supplementary

Information, Figure III a. S 1) measured at the outer surface of the reactor. It is important to highlight that in this layout, UV light was irradiated from the back of the coatings, which were relatively thin (μm) with a maximum absorption of 15% i.e., the film is mostly homogeneously illuminated. Also, our approach does not consider light scattering in the dust film. However, Dupart et al [START_REF] Dupart | Heterogeneous uptake of NO2 on Arizona Test Dust under UV-A irradiation: An aerosol flow tube study[END_REF] showed that this approach led, nevertheless, to similar uptake kinetics measured in more complex aerosol flow tube experiments where such limitations do not exist. Therefore, we believe that the light intensity measured is appropriate for the experiments described herein.

All experiments were carried out at atmospheric pressure using filtered dry air as a carrier gas at a total flow of 200 mL min -1 , except when stated otherwise. For humid experiments, water vapor was introduced into the reactor by bubbling air through a thermostated gas bubbler filled with deionized water. Humidity and temperature were monitored by a SP UFT75 sensor. A schematic overview of the experimental setup is given in Figure III a. S 2. Gas phase butanol was generated using a calibrated permeation tube (Dynacal® -permeation rate 100 ng/min at 60 ºC) placed in a temperature-controlled permeation device (VICI Metronics Dynacalibrator, Model 150).

III a.2.2 Gas-phase analysis

The coated wall flow tube was coupled to a high-resolution proton transfer reaction time of flight mass spectrometer (SRI-PTR-TOF-MS 8000 from Ionicon Analytik GmbH -Innsbruck, Austria) enabling online monitoring of the disappearance of butanol and the identification and quantification of products.

The instrument continuously sampled 50 sccm at the flow tube outlet and its inlet temperature was 60 °C. Spectra were collected at a time resolution of 3.2 s. All measurements were performed using the following parameters: source current of 4.6 mA, drift voltage of 600 V, drift temperature of 60 °C, and a drift pressure of 2.42 mbar. E/N was about 125 Td (1 Td = 10E-17 V cm²). All results presented here were acquired using the H3O + ionization mode as the reaction rates and fragmentation patterns for OVOCs such as alcohols, aldehydes, ketones, etc., have been extensively studied and documented [START_REF] Smith | Reactions of the selected ion flow tube mass spectrometry reagent ions H3O+ and NO+ with a series of volatile aldehydes of biogenic significance[END_REF]Spanel et al., 1997;Spanel andSmith, 1998, 1997).

Ideally, the proton transfer reaction should provide soft ionization resulting in very little or no fragmentation of compounds. However, in practice, fragmentation is observed. Fragmentation patterns for small OVOCs have already been studied, but it may be altered depending on the actual experimental conditions (for example, humidity) and instrument settings (de Gouw and Warneke, 2007). Therefore, fragmentation patterns of the main products were examined by measuring the signals of vaporized liquid standards under the same experimental conditions. Details about this procedure are available in the Supplementary Information.

I III a.2.3 Film Preparation

Arizona Test Dust (ATD) was used as provided by Powder Technology Inc. without further pretreatment.

The ATD Brunauer-Emmett-Teller surface area (BET) is 4.2 ± 0.012 m² g -1 as measured with a Micromeritics Tristar 3000, with particles having an average grain size of 4.08 ± 2.91 μm.

ATD suspensions in ethanol were prepared to facilitate coating the inner surface of the Pyrex tube insert. These suspensions were sonicated for 10 minutes and then dripped into the insert, which was rotated to produce a uniform film. Once coated, the excess solution was allowed to flow out and the tube was dried by flowing nitrogen through it. This step was repeated few times until a visually homogeneous film was obtained. Finally, the insert was placed in an oven to dry at 60 °C for 24 h. After drying, all tubes were weighed, and the mass of ATD films varied from 20 to 150 mg.

III a.2.4 Uptake Experiments

Butanol loss was measured as a function of the movable injector position, that is, interaction time between the VOC and dust surface. In a typical experiment, a freshly coated tube was placed in the reactor and the injector was set to bypass the reactor, so that no surface was exposed to the gas flow (position 0). The flow of butanol was then established, and its concentration monitored. When it was constant, lights were switched on. After 60 minutes, the uptake measurements started by moving the injector stepwise (steps of 2 cm), exposing the ATD surface for 10 minutes. The injector was then reset to position 0 for another 10 minutes. The procedure was repeated until the entire length of the coated tube was exposed, which is equivalent to 10 positions of 2 cm each.

The decay of gas phase butanol obeyed first order kinetics according to (III a.1) C( 0) and C(t) are the inlet and outlet concentrations of butanol after a reaction time (t), and kobs is the measured first order rate constant. The uptake coefficient can then be calculated according to

(III a.2)
where V is the total volume of the reactor, Ssurf is the surface area exposed to the gas, and ‹ν› is average molecular velocity. For a cylindrical flow tube, the geometric uptake coefficient is given by

(III a.3)
where r is the radius (rtube = 0.55 cm).

It is worth noting that the uptake coefficients reported here are at steady state, reached after 10 minutes of exposure, and not the initial uptake coefficients usually reported in the literature (Underwood et al., 2000).

I III a.2.5 Diffusion correction

Uptake was corrected for diffusion effects that may form axial and radial concentration profiles along the reactor, limiting the reaction rate. The method used, proposed by Cooney-Kim-Davis and derived by Murphy and co-workers (Murphy and Fahey, 1987), assumes that in the boundary condition the analyte is adsorbed or reacts with some characteristic probability on each wall collision, which is the corrected uptake coefficient. We solved the differential equation numerically, as proposed by Li et al., (Li et al., 2016) where the corrected uptake coefficient is given as function of the transmittance (C/Co).

III a.3 -Results and Discussion

I III a.3.1 Butanol Uptake Kinetics

For a solid film, the reactive surface area may be larger than the geometric surface area due to the presence of multiple layers of particles and/or because of the existence of pores. In the case of a fully accessible internal surface, the specific surface area can be used for the uptake calculations. Therefore, butanol uptake coefficients were measured as a function of dust film mass (Figure III a. S 3a). For these measurements, different fresh films of ATD were tested while other conditions were kept constant.

The uptake coefficient increased linearly with the mass of the film, indicating that the entire inner surface is participating in the uptake process (Underwood et al., 2000). Therefore, mass-dependent γGeo uptake coefficients were corrected by the BET surface area of the samples according to

(III a.4)
where SBET is the BET surface area and mfilm is the mass coated in the sample holder tube. The BETcorrected uptake coefficients, reported below, were observed to be independent of the actual mass of the film (Figure III a. S 3b).

Uptake rates were measured for different initial (inlet) concentrations of butanol and were observed to decrease from 5.3 x 10 -7 to 2.2 x 10 -7 when the concentration increases from 20 to 55 ppb (Figure III a. S 4). For an initial concentration of 800 ppb (± 160 ppb) of butanol, uptake could not be detected, due to very rapid surface saturation. Initial uptake coefficients tend to be independent of concentration when all active sites on the fresh surface are available for adsorption/reaction (Adams et al., 2005;Romanias et al., 2016). El Zein et al. (Zein et al., 2014) reported no concentration dependence for the initial uptake coefficient, whereas an inverse dependence on initial H2O2 concentration was observed for steady state uptake. Nicolas et al., (Nicolas et al., 2009) using a similar experimental apparatus, also reported that both dark and irradiated uptake kinetics on titanium dioxide decreased inversely to initial ozone mixing ratio.

When the temperature increased from 283 to 298 K, the uptake coefficient decreased from 1.2

x 10 -6 to 1.8 x 10 -7 , which is approximately one order of magnitude (Figure III a. S 5). The reactive uptake coefficient reported herein represents the overall kinetics of the heterogeneous process, and therefore encompasses both chemical reactions and physical adsorption. Adsorption is an exothermic phenomenon favored by a decrease in temperature (Herrmann, 2001). Thereby, the temperature dependence suggests the importance of adsorption/desorption in the butanol uptake mechanism, even under reactive conditions. A similar dependence was previously reported for other VOCs [START_REF] Liu | Temperature dependence of the heterogeneous uptake of acrylic acid on Arizona test dust[END_REF]Romanias et al., 2016).

Humidity also has a strong influence on the uptake kinetics of butanol onto ATD (Figure III a. S 6). At 10% relative humidity, the uptake coefficient is 6% of the value measured under dry conditions, suggesting that water and butanol compete for available sites on the surface of ATD. To better understand the influence of relative humidity on butanol uptake, an ATD film was successively exposed to different relative humidity levels under dark conditions. First, the tube was exposed to air and butanol (20 ppb) under dry conditions, and the disappearance of butanol was recorded. Then, butanol flow was turned off and the tube was left under air until butanol was not detected. Thereafter, butanol flow was resumed, and the sample was exposed to the VOC with 2%, 5%, and 10% moisture. Between each relative humidity level, the sample was left under dry air flow for evacuation. The presence of water vapor in the feed stream significantly reduced butanol surface adsorption capacities (Figure III a. S 7). Surface saturation was faster at higher water vapor contents, up to 10% relative humidity, no change in butanol concentration was observed.

The uptake of water onto mineral particles has been studied by several authors and recently reviewed (Tang et al., 2016). However, few studies have focused on the competitive uptake of water and other trace gases, particularly under irradiation. For pure TiO2, relative humidity has a positive effect up to about 30% relative humidity, as observed by Sassine et al., (Sassine et al., 2010) who reported that the uptake of formaldehyde increased from 6% to 30% relative humidity, and decreased at higher humidities. The influence of humidity seems to vary depending on the combination of trace gas and mineral studied, and results are often controversial. Under dark conditions using ATD and other realistic dust, regardless of the trace gas studied, relative humidity has been found to have a negative influence on gas uptake (Bedjanian et al., 2013;[START_REF] Huang | Kinetics of Heterogeneous Reaction of Sulfur Dioxide on Authentic Mineral Dust: Effects of Relative Humidity and Hydrogen Peroxide[END_REF]Romanias et al., 2016;[START_REF] Zein | Kinetics and products of heterogeneous reaction of HONO with Fe2O3 and arizona test dust[END_REF]. Our results show that the presence of light did not change this negative trend for butanol uptake.

In terms of competition between butanol and water for available surface sites, water molecules are smaller and faster, giving them an advantage over butanol molecules. Regarding the affinity between the mineral surface and water molecules, Gustafson and coworkers (Gustafsson et al., 2005) noted a difference between ATD and CaCO3 isotherms in the sub-monolayer range, demonstrating stronger water-ATD interactions compared to CaCO3. They proposed that the difference might be due to the higher electronegativity of Si which increases the polarity of the OH group, resulting in a stronger H-bond in the ATD. Furthermore, water adsorption capacity is related to other characteristics such as the mineral composition, cation hydration energies, presence of swellable clay minerals, porosity, etc [START_REF] Navea | A comparative evaluation of water uptake on several mineral dust sources[END_REF].

Interestingly, the system shows a hysteresis. When the ATD film was exposed to decreasing levels of relative humidity under irradiation, butanol uptake increased more rapidly than was observed for a freshly humidified surface (Figure III a. 1). That is, water condensation may first reduce heterogeneous processing, but its evaporation may afterwards change the surface characteristics to enhance its reactivity. Water uptake measurements on ATD have shown that one monolayer of water is developed at about 11% relative humidity (Gustafsson et al., 2005). In going from dry to slightly humid conditions, adsorbed water molecules dissociate leaving OH groups on the surface. By increasing relative humidity, once surface is fully hydroxylated, extra water molecules will be accommodated in outer layers via hydrogen bonds (Rubasinghege and Grassian, 2013). Those surface OH groups can form OH radicals under dry conditions and illumination, providing an additional pathway for butanol oxidation.

Figure III a. 1: Decreasing relative humidity (RH) and successively exposing the sample surface to butanol and H2O vapor.

For butanol initial concentration of 20 ppb, temperature = 20 °C, 7 lamps, mass of ATD = 69 mg.

Our experimental setup allowed switching between 0 and 7 lamps in different geometrically symmetrical configurations. Each lamp emits UV-A radiation in the wavelength range from 300 to 420 nm, with a maximum around 350 nm. Figure III a. S 8 shows the systematic increase of the uptake coefficient as a function of UV light irradiance. The dark uptake coefficient is 5 times less than the light uptake coefficient for irradiation with seven lamps. This gives clear evidence of the photochemical nature of the interactions between butanol (which does not absorb within this wavelength region) and ATD (which semiconductor content is activated by light). This is in agreement with previous studies which also observed photo-enhanced uptake for inorganic compounds over different mineral dust proxies (Bedjanian et al., 2013;Nicolas et al., 2009;Zein et al., 2014[START_REF] Zein | Kinetics and products of heterogeneous reaction of HONO with Fe2O3 and arizona test dust[END_REF]. However, few similar studies on organic compounds are available (Styler and Donaldson, 2011).

I III a.3.2 Photochemistry of butanol on ATD surface

The uptake (loss) of butanol was accompanied by the formation of a variety of gas phase products (Inomata et al., 2008;Schripp et al., 2010).

No product formation was observed in the dark. For initial butanol concentration of 20 ppb, dry conditions, T = 20 °C, 7 lamps, mass of ATD = 110 mg.

To explore whether these products are arising from "freshly" up taken butanol or from adsorbed molecules, a film was exposed to gas phase butanol in the dark until surface saturation (i.e., constant butanol signal). Maintaining the butanol flow throughout the reactor, the lights were then switched on and off in the following sequence: 1) 30 minutes two lamps on; 2) 30 minutes four lamps on; 3) 1 hour all lamps off; 4) 30 minutes 6 lamps on. We observed (see Figure III a. 3) production of different compounds (CH3O + , C2H5O + , C3H7O + , C5H9 + , and C7H15O + ) immediately after the lights were turned on, with increasing intensity at higher light fluxes. Nevertheless, there was no drop in the gas phase butanol concentration, suggesting that products were formed from butanol already adsorbed on the surface. Product formation was quantified (subtracting the background value from the concentration during irradiation) for each round of illumination (number of lamps switched on; Figure Concerning the study of alcohols under atmospheric simulated conditions onto different mineral proxies, mainly small aldehydes, ketones, and carboxylic acids were reported as products.

Styler and Donaldson (Styler and Donaldson, 2011) studied the photooxidation of isopropanol and npropanol over TiO2 films in a Knudsen cell, operated at much lower pressure than in the current investigation, and identified acetone and propionaldehyde, respectively, as main products. The same authors, (Styler et al., 2013) studied the photooxidation of fluorotelomer alcohols onto Mauritanian sand and volcanic ashes and found aldehydic products in the gas phase and perfluorinated carboxylic acids as sorbed products. Recently, Romanias and coworkers (Romanias et al., 2016) also studied the photochemistry of isopropanol on Gobi natural dust under pure air, reporting the formation of acetone, formaldehyde, acetic acid, and acetaldehyde as products when the dust was illuminated. Most of the available literature for alcohols, and specifically butanol, investigates photocatalytic oxidation (PCO) on TiO2, with the goal of optimizing conditions to mineralize the organic compound.

Yao and Feilberg (Yao and Feilberg, 2015) quantified product formation for the photocatalytic degradation of 52 ppb of n-butanol through TiO2-coated ceramic filters, reporting acetaldehyde, butyraldehyde, propionaldehyde/acetone, ethanol, formic acid, acetic acid, and formaldehyde as potential byproducts. They pointed out, however, that the observed products accounted for only a fraction of the reactant consumption, suggesting that butanol could be directly degraded producing CO2 and water.

In our case, the total yield (sum of the yield of all detected products) is around 30 -40%, implying that either all of the adsorbed butanol is not converted into products, or some butanol is converted into non-volatile products that accumulate on the surface or yet is mineralized to CO2 and water.

Butanal can be expected to be one of the first intermediates from the butanol oxidation, it is therefore interesting to note that we did not detect it. It was however detected in studies investigating the oxidation on pure TiO2 at much higher concentrations than ours [START_REF] Benoit-Marquié | VOC photodegradation at the gas-solid interface of a TiO2 photocatalyst[END_REF][START_REF] Blake | Selectivity control during the photoassisted oxidation of 1-butanol on titanium dioxide[END_REF][START_REF] Kirchnerova | Photocatalytic oxidation of n-butanol under fluorescent visible light lamp over commercial TiO2 (Hombicat UV100 and Degussa P25)[END_REF]Peral and Ollis, 1992;Yao and Feilberg, 2015). In addition to butanal, propionaldehyde, acetaldehyde, 1-propanol, and ethanol were also found by those authors.

It is worthwhile to note that all of these studies investigated oxidation on pure TiO2 at much higher concentrations than used in this study. Nevertheless, this study generally agrees with previous reports that, mineral dust (ATD containing 2.0 -5.0 wt. % Fe2O3 and 0.5 -1 wt. % TiO2) can promote photooxidation of an organic compound on its surface similar to pure semiconductors such as TiO2.

Gas-phase products give some insight into the butanol degradation mechanism under our experimental conditions. We propose that during photooxidation adsorbed butanol is first directly oxidized to an aldehyde by photo-generated holes or OH radicals from residual water on the surface.

R III a. 1 • R III a. 2 • R III a. 3
The process then continues via a series of complex reactions, which are expected to be similar to those proposed by other authors [START_REF] Benoit-Marquié | VOC photodegradation at the gas-solid interface of a TiO2 photocatalyst[END_REF][START_REF] Nimlos | Gas-Phase Heterogeneous Photocatalytic Oxidation of Ethanol: Pathways and Kinetic Modeling[END_REF][START_REF] Ye | Photocatalytic oxidation of aldehydes: Byproduct identification and reaction pathway[END_REF], with aldehydes being converted to shorter chain aldehydes and/or alcohols.

As presented by Ye et al., [START_REF] Ye | Photocatalytic oxidation of aldehydes: Byproduct identification and reaction pathway[END_REF] aldehydes are oxidized via two pathways, either through the C(n) acid, or by direct conversion to the C(n-1) aldehyde. No acids were detected here, which may be due to their lower volatility or their high affinity for the dust surface (and tubing).

Alcohols such as propanol and ethanol can also remain adsorbed on dust surface [START_REF] Chapuis | Photocatalytic Oxidation of Volatile Organic Compounds Using Fluorescent Visible Light[END_REF]. Some studies highlight the contribution of superoxide radical and other radical species derived from oxygen (Hirakawa et al., 2007;[START_REF] Nimlos | Gas-Phase Heterogeneous Photocatalytic Oxidation of Ethanol: Pathways and Kinetic Modeling[END_REF]. The superoxide radical may contribute, for instance, to charge separation (between and ). Molecular O2 is also very important, as it can directly oxidize the aldehyde to an acid [START_REF] Benoit-Marquié | VOC photodegradation at the gas-solid interface of a TiO2 photocatalyst[END_REF] or react with alkyl radicals, [START_REF] Nimlos | Gas-Phase Heterogeneous Photocatalytic Oxidation of Ethanol: Pathways and Kinetic Modeling[END_REF] forming a hydroxylated peroxyl radical and HOO• radicals. In anhydrous systems, O2 is the active oxidizing species, and its ionosorbates are the only negatively charged species able to react with holes (Herrmann, 2001).

A
R III a. 5 R III a. 6 R III a. 7
Additionally, intermediary reactions in the oxidation mechanism may lead to the formation of radicals, resulting in the propagation of chain reactions [START_REF] Nimlos | Gas-Phase Heterogeneous Photocatalytic Oxidation of Ethanol: Pathways and Kinetic Modeling[END_REF] involving O2 (Hirakawa et al., 2007). Recombination of radicals is then likely to occur, resulting in products of higher molar mass than the initial compound.

Figure III a. 5: Suggested mechanism for butanol photooxidation on ATD surface.

Carbonyl compounds were the predominant products observed in the gas phase. Aldehydes and ketones absorb at UV-Vis wavelengths (>290 nm), acting as radical precursors for further photochemical reactions. On the other hand, alcohols, including butanol, do not absorb light at these wavelengths and therefore generally have longer lifetimes in the troposphere (Mellouki et al., 2015).

We emphasize that mineral dust plays an important role not only acting as a scavenger for alcohol molecules, but also in converting them into more reactive species.

I III a.3.3 Atmospheric Implications

We presented the effect of ambient conditions such as temperature, relative humidity, and light intensity on an oxygenated organic compound uptake on realistic mineral dust particles. Uptake was maximized at low temperature and low humidity.

We estimated the lifetime of butanol due to its heterogeneous photooxidation on mineral dust surfaces. The calculation used the first order rate constant under atmospheric conditions i.e., with the uptake coefficient measured at 20 ppbv of butanol, for dry conditions, at 20°C under a real UV-A (315-400 nm) irradiance, determined using the Quick TUV model, for latitude of 40° North at noon during equinox day, or solar zenith angle of 40°. For very high dust loadings (surface area density of 0.052 m²m -³), the butanol lifetime was found to be 1.48 days whereas for regular clean days (surface area density of 1x10-4 m²m -³) it was 770 days. By comparison, the butanol lifetime due to OH radical reaction is estimated as 1.4 days (Mellouki et al., 2015). As a consequence, this butanol removal pathway can only be effective in a dust plume.

More importantly, the photochemistry described here occurs on illuminated mineral dust, with adsorbed organic compounds typically being oxidized to shorter and more volatile products. Butanol, for instance, will produce aldehydes such as formaldehyde, acetaldehyde, and propionaldehyde which are potential sources of atmospheric radicals. More complex products (e.g., dimers of reaction intermediates) were also formed in small amounts. In other words, mineral dust can be regarded as a reservoir for such compounds, transporting pollution for source regions to remote places and may transport, where tens of pptv of aldehydes may be a significant contribution to their actual concentrations. Overall, mineral dust is not inert and opens new photochemical pathways that so far have not been considered in current atmospheric models.

III a. 4 -Supplementary Information

L Lamps Irradiance

The experimental setup allows to switch on different number of lamps (from zero to 7) in different geometric configurations. Each lamp emits UV-A radiation, in wavelengths range from 300 to 420 nm, with a maximum around 350 nm. Lamps irradiance was measured with a Spectrometer AvaSpec-2048 (Avantes), using a calibration lamp AvaLight-DH-CAL and FC-UV400 fiber with cosine corrector.

For the study of the influence of light intensity, the number of lamps was varied, always maintaining a symmetrical configuration. The total irradiance for different configuration is shown in For initial concentration of 20 ppb of butanol, dry conditions, temperature 293 K.

Product identification and PTR-MS Standards

The fragmentation patterns of the main products were examined by measuring the signals of vaporized liquid standards under the same experimental conditions (i.e. the compounds passed on the same pathway, using the same setup and air as carrier gas). The standards were performed using the (Buhr et al., 2002;Spanel et al., 1997) we can assign fragments of a determined m/z to compounds that originated them, results are shown in the Table III a. 1.

III b. 1 -Introduction

Acetone is one of the most relevant atmospheric ketones, it is both directly emitted and produced in situ from several anthropogenic and natural sources. Plants, as conifers emit acetone from their buds (Fuentes et al., 2000), as well as cyanogenic plants that have acetone and other ketones as by-products of their protecting mechanisms (Fall, 2003). Among anthropogenic sources, solvent usage and vehicular emissions are significant contributors. Additionally, acetone is a secondary product of the oxidation of many volatile organic compounds (VOCs) namely, C3-C5 isoalkanes and terpenes (Calvert, 2011).

The primary loss processes of ketones from the atmosphere are photolysis and reaction with OH radicals. Removal by O3 and NO3 with saturated ketones are negligible under atmospheric conditions. For acetone, OH reactions are predominant close to the surface, with photolysis prevailing at higher altitudes (Mellouki et al., 2015). The rate constant kOH is 1.8 × 10 13 cm 3 molecule -1 s -1 implying lifetimes with respect to OH of approximately 60 days 1 . Khan et al. used a 3-D global model estimating the acetone burden to be 3.5 Tg [START_REF] Khan | A study of global atmospheric budget and distribution of acetone using global atmospheric model STOCHEM-CRI[END_REF]. The incorporation of photochemical mechanisms to this model led to a better agreement with measured concentrations compared with previous ones that did not consider these mechanisms. The authors reinforced, thereby, the need to well characterize photooxidative pathways that could alter the acetone burden.

Heterogeneous processes are among those pathways that can alter the concentration of organic and inorganic compounds, photochemical oxidants, acid gases, free radicals and others in the troposphere. [START_REF] Kolb | An overview of current issues in the uptake of atmospheric trace gases by aerosols and clouds[END_REF]. The uptake of trace gases on solid and condensed phase particles has created rising interest since it can change the distribution of reactive atmospheric gases acting as a sink for some species and as a source for others. Among atmospheric particulate matter, mineral particles are the most abundant regarding aerosol dry mass (Choobari et al., 2014), which makes them a large surface potentially available for heterogeneous reactions to take place. In addition to participating in photochemical processes, the impacts of mineral dust on Earth's ecosystems are multiple, influencing, for example, the radiative balance, the formation of clouds droplets and the distribution of rain, the availability of biochemical nutrients [START_REF] Tang | Heterogeneous reactions of mineral dust aerosol: implications for tropospheric oxidation capacity[END_REF]Usher et al., 2003).

Acetone is a recognized indoor pollutant, and thereby, the study of its photocatalytic degradation received some attention in the 1990s (Alberici and Jardim, 1997;Peral and Ollis, 1992;[START_REF] Sauer | Acetone Oxidation in a Photocatalytic Monolith Reactor[END_REF][START_REF] Vorontsov | Study of TiO2 Deactivation during Gaseous Acetone Photocatalytic Oxidation[END_REF]. Meanwhile, the atmospheric removal of acetone by heterogeneous processes on the surface of particles has been very little explored. Since now, besides OH and photolysis, atmospheric models only consider dry deposition to land and oceanic uptake [START_REF] Jacob | Atmospheric budget of acetone[END_REF][START_REF] Khan | A study of global atmospheric budget and distribution of acetone using global atmospheric model STOCHEM-CRI[END_REF] as sinks for acetone.

In the present study, the heterogeneous interactions of acetone with mineral dust were briefly addressed. We measured the uptake kinetics of acetone on the surface of three different mineral proxies: Arizona test dust (ATD), TiO2, and SiO2 investigating the influence of light irradiation and humidity discussing the role of these two parameters on the uptake coefficients.

III b. 2-Experimental Section

The uptake kinetics of acetone on different mineral substrates were measured using a coatedwall flow tube reactor set-up. The experiments followed the same procedure of our previous study (Chapter III.a), consisting of evaluating the acetone loss in the flow tube as a function of the movable injector position; uptake coefficients determination is detailed in the Supplementary Information. The method proposed by Cooney-Kim-Davis (CKD) (Murphy and Fahey, 1987) was used to correct the uptake coefficients for diffusion effects of acetone along the reactor.

Synthetic air (79:21 N2/O2) was used as carrier gas at a total flow of 300 mL/min which correspond to mean residence times from 0 to 3.5 s. The temperature was kept constant at 293 K by circulating water from a cryostat bath (Huber CC 130) through the reactors outer jacket. The relative humidity (RH) and the temperature of the gas flow were measured by an SP UFT75 sensor coupled to the reactor outlet. The irradiation intensity was varied switching on and off the UV-A lamps that are symmetrically distributed around the flow tube.

Acetone was generated in the gas-phase by means of a permeation system consisting of a calibrated permeation tube (Dynacal® -permeation rate 98 ± 25 ng/min at 40 ºC) and a temperaturecontrolled permeation device (VICI Metronics Dynacalibrator, Model 150). The gas-phase composition was monitored by a Proton-Transfer Reaction Time of Flight Mass Spectrometer (PTR-ToF-MS, Ionicon 8000) using H3O + as ionization mode.

ATD (Ultrafine A1, Powder Technology Inc), TiO2 (P25 Degussa), and SiO2 (Evonik Aerosil® 130) were chosen as proxies for mineral dust substrates being deposited in the internal wall of a Pyrex tube insert. Slurries were prepared individually, for each mineral, using ethanol as solvent; the coating process, however, was the same. The suspension was sonicated for 10 minutes, and then aliquots were transferred to the tube insert forming a thin layer of dust coating its internal walls. After each layer, the tube was dried with a gaseous nitrogen flow. The process was repeated a few times until obtaining a homogeneous film (visual inspection). Ultimately, the tube was placed in an oven to dry at 373 K overnight, and then weighed.

III b. 3-Results and Discussion

I III b. 3.1 Arizona test dust

The uptake of acetone on ATD was studied for dark and irradiated conditions (0, 3 and 7 lamps, corresponding to 0, 0.56 and 1.4 mW/cm², respectively); dry and for 5% of relative humidity. In Overall, the BET uptake coefficients of acetone in ATD presented relatively high values, varying between 1.4 to 2.2 x 10 -6 . For a matter of comparison, it is approximately an order of magnitude higher than for butanol under the same conditions (for butanol, the initial concentration was 55 ppb, 0% RH, 1.4 mW/cm² of irradiation, 293 K).

The relative humidity, on the other hand, exhibited a strong influence on acetone uptake onto ATD. The uptake coefficient for 5% relative humidity was 1.13 ± 0.21 x 10 -7 i.e., an order of magnitude below the values obtained for dry conditions.

An effect similar to that reported for butanol was also observed in this case, as can be seen in been observed for different trace gases on ATD (Bedjanian et al., 2013a(Bedjanian et al., , 2013b;;Zein et al., 2014).

However, for organic compounds, the negative effect of water vapor seems to be more pronounced as already reported in Chapter III. a, for ATD and butanol, and also by other authors as Romanias et al. for Gobi dust and isopropanol (Romanias et al., 2016). The strong affinity between ATD and water (Gustafsson et al., 2005) favors its adsorption to the detriment of organic molecules.

I III b. 3.2 TiO2 and SiO2

Under dry and irradiated conditions, TiO2 presented a fast and reactive uptake that was sustained for a long period. It can be seen in For 5% of relative humidity, reactive uptake was also observed even for extended periods.

However, there is a significant difference, as shown in For other trace gases, the uptake on TiO2 was also competitively influenced by water vapor usually exhibiting a parabolic dependence on RH (Ndour et al., 2009;Nicolas et al., 2009;Sassine et al., 2010). Other authors studying photocatalytic degradation of acetone on TiO2 have also reported adverse effects of water vapor [START_REF] Choi | Investigation on TiO2-coated optical fibers for gas-phase photocatalytic oxidation of acetone[END_REF]Peral and Ollis, 1992;[START_REF] Raillard | Kinetic study of ketones photocatalytic oxidation in gas phase using TiO2-containing paper: Effect of water vapor[END_REF]. Raillard et al. have found higher degradation rates for dry conditions, and lower acetone adsorption onto TiO2 under humid (30%) atmosphere [START_REF] Raillard | Kinetic study of ketones photocatalytic oxidation in gas phase using TiO2-containing paper: Effect of water vapor[END_REF]. They suggested that water formed a physisorbed layer that hindered acetone molecules access to the surface sites. Peral and Ollis reported an inhibitory effect of H2O vapor on acetone photooxidation (Peral and Ollis, 1992).

Both One can observe the distinct effects of light and humidity regarding the various minerals. On the one hand, for ATD, moisture had a notable influence on acetone uptake whereas light in spite of influencing it led only to a slight increase. For TiO2, on the other hand, light perceptibly plays an important role, which is explained by the recognized photocatalytic properties of titanium dioxide; while 5% of RH decreased the uptake coefficient in 40% compared to dry conditions.

SiO2 was used as a proxy for insulating material that did not have photochemical activity. Li and coworkers have shown that acetone weakly adsorbs on SiO2 surface, under dark conditions (Li et al., 2001), moreover, compared to other minerals (Al203, Fe2O3, TiO2, and CaO), silica presented the lowest initial uptake measured with a Knudsen cell for acetone and also for other organic compounds such as acetaldehyde, propionaldehyde (Li et al., 2001), acetic acid, methanol and formaldehyde (Carlos-Cuellar et al., 2003). Consistently, the uptake of acetone was perceptibly smaller on silica, and the irradiation did not influence its performance.

Despite its high reactivity, TiO2 exhibited lower uptake coefficients compared to ATD when the total BET surface is taken into account. At the same time, SiO2 has very low uptakes, and they become even smaller when corrected by the total BET area. This occurs because TiO2 has an intermediate surface area (SBET is 50 m²/g) while ATD is not a porous material, and its specific surface area is about 10 times smaller than that of TiO2 (ATD SBET = 4.5 ± 0.5 m²/g). The silica used is even more porous (the BET surface area is 130 ± 25 m² / g).

Therefore, if we consider that the entire sample area is available (multiplying the BET specific area by the mass of the film coating the tube) to uptake the VOC molecules, the TiO2 performance is comparable to that of the ATD, meanwhile reducing, even more, the result for SiO2.

I III b. 3.4 Photochemical Products

Under dark conditions, Li et al. 2001 performed FTIR measurements and found adsorbed unsaturated carbonyl compound attributed to aldol condensation of acetone on the surface sites that have basic or acidic properties (CaO, TiO2, α-Al2O3, α-Fe2O3) (Li et al., 2001). From Knudsen cell experiments the same study reported trace amounts of aldol products in the gas-phase. Even though the authors emphasized that gas-phase products represented less than 1% of the amount of carbonyl taken up by the surface.

For acetone adsorption on TiO2, El-Maazawi et al have shown that the higher the surface coverage, mesityl oxide generation (from acetone aldol condensation) is favored. [START_REF] El-Maazawi | Adsorption and photocatalytic oxidation of acetone on TiO2: An in situ transmission FT-IR study[END_REF]). Schmidt et al. [START_REF] Schmidt | Photochemistry of the indoor air pollutant acetone on Degussa P25 TiO2 studied by chemical ionization mass spectrometry[END_REF] that also studied the interaction of acetone with TiO2 without light reported the presence of irreversibly adsorbed acetone molecules and mesityl oxide.

However, no products were detected in the gas-phase.

El-Maazawi et al. also investigated the photooxidation of acetone upon illumination reporting gaseous CO2 and H2O as products. They observed formaldehyde at FTIR spectrum for high acetone surface coverages. Other studies of acetone degradation onto TiO2 mentioned no intermediate reaction products (Alberici and Jardim, 1997;[START_REF] Choi | Investigation on TiO2-coated optical fibers for gas-phase photocatalytic oxidation of acetone[END_REF]Peral and Ollis, 1992;[START_REF] Sauer | Acetone Oxidation in a Photocatalytic Monolith Reactor[END_REF]. We did not find any study on photo-induced heterogeneous reactions of acetone in materials other than TiO2. Besides, studies on TiO2 concern photocatalytic degradation of VOCs, and are therefore not necessarily performed under atmospheric conditions.

For ATD, only very few products were detected in the gas-phase such as formaldehyde and acetaldehyde. Their concentrations were very low and close to background levels being difficult to assert that they were meaningfull photochemical reactions products. For TiO2, no products were found in the gas-phase.

The results suggest that, under our experimental conditions, acetone may be mainly physically adsorbed without expressive chemical reactivity or possibly, some products stay adsorbed on the surface or even that acetone is directly mineralized forming CO2 and H2O. However, it is not possible to clarify which of these hypotheses is the most probable one.

III b. 4 Conclusions and Atmospheric Implications

The heterogeneous properties of ATD are different from those of silica, its main component, and also of TiO2 to which the photocatalytic properties are generally attributed. Our results indicate that there is a synergistic effect among the constituents of Arizona dust so that its heterogeneous photocatalytic interaction cannot be fully represented by the pure oxides. The presence of iron oxides in ATD (2-5%) must also contribute to its general photocatalytic behavior. The influence of ironbearing oxides should be better investigated to improve the understanding of ATD as an atmospheric photocatalyst.

For TiO2, water vapor had both inhibitory and promoting effects, occuring simultaneously. For 5% relative humidity, the production of hydroxyl radicals exceeds the surface blocking by water, the uptake being yet significant, but smaller than that for dry conditions. For ATD, on the other hand, the inhibitory effect of water was much more pronounced, therefore, at 5% RH, the acetone uptake strongly decreased.

Acetone uptake on Arizona dust was fast, even for dark conditions. Considering that the γBET reported herein is a lower limit where the uptake is underestimated, we can wonder that acetone removal due to heterogeneous processes may be significant under specific environments (especially for dry atmosphere) and may be compared to homogeneous gas-phase losses.

III b. 5 -Supplementary Information

I III b. 5.1 Uptake Coefficients Determination

The loss of acetone in the gas phase was monitored as a function of the position of the movable which corresponds to different residence times. The acetone decay obeys the first-order kinetics and the uptake coefficient can be derived as follows

(III.1)
where kobs is the measured first-order rate constant; C(0) is the inlet and C(t) the outlet concentration of acetone after a reaction time (t); is average molecular velocity, and r is the reactor radius.

The so-called geometric uptake coefficients (γgeo) calculated by Equation (III.1) were further corrected by the Brunauer-Emmett-Teller (BET) surface area. By using the BET area, we consider that the entire porous surface is participating in the uptake process

(III.2)
where SBET is the BET surface area of the substrate; Ssurf is the geometric surface area, and mfilm is the mass coated in the tube.

I III b. 5.2 Acetone uptake profile on ATD

IV. Heterogeneous photochemistry of dicarboxylic acids on mineral dust IV. 1 Introduction

Dicarboxylic acids (DCA) are ubiquitous in the atmosphere and are especially abundant in aerosols (Kawamura and Bikkina, 2016). Due to the presence of two carboxylic groups, they are less volatile and more soluble in water when compared to the respective monoacids and are mainly present in the particulate phase (Kawamura et al., 1996). Oxalic acid (C2) is the most abundant diacid species, followed by malonic (C3) and succinic (C4) acids (Kawamura et al., 1996).

Dicarboxylic acids and related compounds are emitted from anthropogenic and natural primary sources such as fossil fuel combustion, biomass burning, cooking, and oceans. Besides, they are widely used in hydraulic fluids, agricultural chemicals, pharmaceuticals, dyes, complexing agents, and as intermediates in the manufacture of esters and polyamides [START_REF] Helmi Rozaini | The Chemistry of Dicarboxylic Acids in the Atmospheric Aerosols[END_REF], increasing the chances of fugitive emissions during transportation or storage.

Although direct emissions contribute to the diacids abundance, several studies have shown that low molecular weight (LMW) diacids are produced in the atmosphere by photochemical chain reactions of unsaturated hydrocarbons and fatty acids as well as their oxidation products (Kawamura et al., 1996;[START_REF] Kawamura | Seasonal Changes in the Distribution of Dicarboxylic Acids in the Urban Atmosphere[END_REF]Kawamura and Yasui, 2005). These results are supported by the observation of diurnal and seasonal variation when an increase of DCA concentration in the afternoon and summer times coincides with higher levels of atmospheric oxidants (Kawamura and Bikkina, 2016).

Except for the ketoacids, carboxylic acids do not absorb significantly at wavelengths greater than 250 nm, and generally, do not undergo photolysis in the troposphere (Mellouki et al., 2015).

Additionally, they have weak reactivity in the gaseous phase, thus, their removal from the atmosphere is mainly by heterogeneous processes (dry and wet depositions) (Chebbi and Carlier, 1996). Watersoluble C2-C9 DCAs have low vapor pressure which favors their condensation onto airborne particles [START_REF] Yang | Photooxidation of dicarboxylic acids-Part II: Kinetics, intermediates and field observations[END_REF]. An additional and possible deposition pathway for dicarboxylic acids may be at the surface of mineral aerosols, particularly over Asia were desert dust, volcanic, and marine air crosses massive anthropogenic pollution (Pan et al., 2017;Sullivan et al., 2007).

The condensation of low vapor pressure acids on the surface of dust creates an organic coating that, subsequently, can undergo heterogeneous chemical reactions enriching dust surface with oxygenated and polar organics. Russell and coworkers revealed the presence of complex mixtures of carbonyl compounds on dry particles from the Caribbean marine bondary layer (Russell et al., 2002).

Corroborating this result, oxalic acid was found in Asian mineral dust particles for both sub and supermicron diameters (Sullivan and Prather, 2007) and Falkovich and coworkers identified oxalic, succinic, malic, glutaric and adipic acids adsorbed on mineral dust particles collected in Israel (Falkovich et al., 2004). Moreover, the deposition of DCA alters mineral dust hygroscopic properties (Russell et al., 2002) and consequently their action as ice nucleation and cloud condensation nuclei (Keskinen et al., 2014;Niedermeier et al., 2010;Tang et al., 2016).

Mineral dust is the most abundant emitted aerosol accounting for approximately 1500-2000 Tg every year (George et al., 2015). Recently, the photo-induced heterogeneous chemistry initiated by near-UV/Vis light-absorbing species (such as TiO2, Fe2O3) present in mineral aerosol was evinced (Ndour et al., 2009;Ponczek and George, 2018;Romanias et al., 2016;[START_REF] Styler | Heterogeneous photochemistry of atmospheric dusts and organic films by and organic films[END_REF]Donaldson, 2012, 2011) and reviewed (George et al., 2015;Usher et al., 2003). All these studies reveal a new and potentially important pathway of organic compounds processing in the atmosphere.

There are few studies of the kinetics and products identification for the atmospheric photooxidation of aqueous phase chemistry of dicarboxylic acids (Charbouillot et al., 2012;[START_REF] Pavuluri | Laboratory photochemical processing of aqueous aerosols: formation and degradation of dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls[END_REF][START_REF] Yang | Photooxidation of dicarboxylic acids-Part II: Kinetics, intermediates and field observations[END_REF]. However, knowledge on the mechanism and kinetics for heterogeneous reactions on typical mineral dust is still very limited. In this study, we assessed the photoreactivity of five dicarboxylic acids: succinic (butanedioic) acid, glutaric (pentanedioic) acid, and adipic (hexanedioic) acid, pimelic (heptanodioic) acid and suberic (octanodioic) acid on Arizona test dust (ATD) particles. The heterogeneous processing on the surface of dust particles was investigated under UV-A irradiation. We discuss the similarities and discrepancies among the homologous species and propose a general mechanism for DCA reactions on ATD.

IV. 2 Experimental Section

Experiments were carried out in a horizontal coated wall flow-tube reactor, previously described (Jammoul et al., 2008;Ponczek and George, 2018), where the dust samples are immobilized in the internal walls of a Pyrex tube insert.. The reactor has an outer jacket and was maintained at a constant temperature of 293 K by circulating thermostated controlled water from a chiller bath (Huber CC 130).

Seven lamps (Phillips 20W) surround the reactor and could be switched on and off modifying the light intensity (from 0 to 1.4 mW cm -2 for wavelengths in the range of 300-400 nm). A schematic of the experimental set-up is depicted in Figure IV. S 1.

I IV.2.1 Tube coating

Acids were added as internal mixtures of 10% w/w in ATD. An aqueous solution of the respective acid was mixed with an ATD suspension of ethanol. The suspension was then transferred to the tube inserts coating their inner walls, similar to the coating process described in the previous chapter. The tubes were left in an oven at 373 K for 1h to evaporate the solvents (ethanol and water) but not the DCA which boiling points are between 473 and 673 K.

IV.2.2 Experiment procedure

After coating, the insert was placed in the flow tube reactor, under a flow of 300 ml/min of synthetic air (N2 and O2 mixture in the ratio 79:21) as a carrier gas. To investigate photo-oxidation reactions, each sample followed the same illumination sequence: 30 minutes off (for background measurement); 1h under irradiation, 1h lights off, 1h under irradiation.

The gas-phase products were monitored online by coupling the reactor outlet to a highresolution proton-transfer-reaction mass spectrometer (PTR-ToF-MS 8000, Ionicon Analytik GmbH).

The instrument settings used were: drift voltage of 600 V, drift temperature of 333 K and drift pressure of 2.42 mbar, resulting in an E/N ratio of about 125 Td (1 Td = 10 -17 V cm²). The reactor's outlet and the PTR sample inlet tubing were constantly heated to 333 K to minimize adsorption and condensation of products. Spectra were acquired at a time resolution of 5 s for regular experiments and 10 s for the long-term experiments. VOC mixing ratios were calculated from the H3O + mode using the built-in feature of PTR-ToF-MS Viewer Software v. 3.1.0.31 (Ionicon Analytik GmbH). Transmission curves were obtained using a calibrated TO-14 aromatic mix (Restek). It should be noted that uncertainties in the absolute mixing ratios can be estimated to be about 30%.

IV.2.3 Surface Products -Sample Extraction and LC-MS Analysis

After irradiation, surface sorbed products were extracted from the dust as follows. First, 10 ml of solvent (9:1 v:v of acetonitrile/water -Optima® LC/MS, Fischer Scientific, USA) was added to the coated tube and sonicated for 10 min. Then, the suspension in the tube was left in a rotating device overnight (for approx. 15h) providing good contact solvent/dust. After that, the suspension was transferred to a 30 mL amber vial and left to decant. The supernatant was, next, transferred to another amber vial and filtered using 0.2 μm GHP membrane (13 mm, Pall Corporation, USA), and evaporated almost to dryness under a mild nitrogen flow. The extracts were then reconstituted in 1mL of a 50:50 volume mixture of water and acetonitrile.

For the analysis, the appropriate amount of the final reconstituted extract sample (different sample volume for each acid, see Table IV. S 1) was diluted by adding 500 μL of acetonitrile and 500 μL of water. 5 μL of diluted solutions were analyzed by reversed phase ultra-high performance liquid chromatography UHPLC (Dionex 3000, Thermo Scientific, USA) coupled to an Orbitrap high-resolution mass spectrometer (Thermo Scientific, USA) using heated negative electrospray ionization ( -HESI).

Analytes were separated using a Waters Acquity HSS T3 column (1.8 μm, 100 × 2.1 mm) with mobile phases consisting of (A) 0.1% formic acid in water (Optima® LC/MS, Fischer Scientific, USA) and (B) 0.1% formic acid in acetonitrile (Optima® LC/MS, Fischer Scientific, USA). Gradient elution was performed by the A/B mixture at a total flow rate of 300 μL/min as detailed elsewhere (Wang et al., 2016). Further information regarding the chromatographic and mass spectrometric methods is available in the Supplementary Information.

For a preliminary analysis, the obtained chromatograms were analyzed with MZMine 2.21 for a non-target screening of potential products. The software algorithm identifies the products assuming that the extracted ions in the range of 50-750 m/z were formed by the loss of a proton from the analytes. To refine the results, the major products identified by MZMine were reevaluated and each corresponding peak was manually checked using the software Xcalibur 2.2 (Thermo, USA), thus excluding possible peaks detected erroneously by the automatic screening.

Chromatograms of each sample (extracted from the irradiated tube after the experiment) were compared to the results of a replicate tube prepared under the same conditions but left in the dark.

Additionally, two procedural blanks consisting of coated tubes with only Arizona test dust (and no acid) one left in the dark and the other irradiated were run in parallel to the samples.

Standards solutions of the studied acids were also analyzed to facilitate their identification.

Experiments using succinic acid have not been analyzed by LC-MS since this acid is very small and it is poorly retained in the column, as probably are its oxidation products. During the same experiment, when the lights were switched on for the second time, we could confirm the increasing of the same products. It is also worth noting that the rise and decay profile is different for each product, some of them easily reach steady state such as C4H7 + for adipic acid (C6) and C3H7O + for suberic acid (C8), whereas others exhibit almost linear increasing during one-hour irradiation such as C4H9O2 + for glutaric acid (C5) and C5H13O2 + for pimelic acid (C7). Similarly, some of them drop-off quite fast after switching off lights such as C4H7O2 + for adipic acid and C5H9 + for pimelic acid, while others take considerable time to decay back to background levels likewise C3H7 + for glutaric acid and C2H5O + for adipic acid.

IV.3 Results and Discussion

Control experiments were performed to verify that the OVOCs detected were emitted from the photoinduced process and not as a result of: i) evaporation from the dicarboxylic acid or; ii) due to the presence of previous impurities on the dust. For the first case, dust/acid coated tubes were placed in the flow reactor subjected to the same experimental conditions and left overnight in the dark while the concentration of VOC was monitored. For the second case, dust films without any acid were submitted to the same irradiation schedule. In both cases, no significant difference in OVOCs concentrations was detected.

We carried out a thorough evaluation of the products, comparing the results before and after 1h of irradiation. About 50 different m/z ions were found in the record of PTR-MS spectra, which were characteristically distributed for each of the five diacids studied. The complete list of all detected VOCs is given in the SI (Table IV. S 2). We focused our attention on the main products that may provide important information about the photo-oxidation mechanism. Although the PTR-ToF-MS has a high mass resolution (more than 4000 m/Δm), allowing to differentiate compounds with very close m/z, e.g., ethanol (C2H7O + , m/z 47.049) and formic acid (CH3O2 + , m/z 47.013), it should be mentioned that for the interpretation of the mass spectra, each peak (m/z) may be characteristic of its parent molecule (the protonated compound MH + ) but also fragments from larger molecules. Especially, small ions present in our results such as C3H5 + (m/z 41.039), C2H3O + (m/z 43.018), C3H7 + (m/z 43.054), C4H9 + (m/z 57.069), and others, may arise from different original compounds that unequally contribute for their total peak intensity. For a complex mixture (as in our case), it is difficult to quantify the influence of each compound on each fragment's peak intensity. For that, it would be necessary to use pure substances to obtain the fragmentation patterns of each compound checking their fragments, to afterward consider their relative percentages in each result spectrum.

Furthermore, an ion with a certain m/z mass to charge ratio can denote several isomers and correspond to different compounds in different experiments. An example of the latter case is the ion m/z 71.048 (C4H7O + ) which is the parent ion from the isomers (C4H6O) methyl vinyl ketone, methacrolein or crotonaldehyde (Yuan et al., 2017) or may be derived from the fragmentation (loss of a water molecule) of butanoic acid (C4H8O2). In this case, if the m/z 71.048 (C4H7O + ) ion represents the butanoic acid fragment, it should be accompanied by the m/z 89.059 (C4H9O2 + ) ion, parent ion of the butanoic acid molecule.

Fragmentation patterns for atmospherically relevant OVOCs have already been studied (Yuan et al., 2017), but it may be altered depending on the actual experimental conditions (for example, humidity) and instrument settings (de Gouw and Warneke, 2007). In an attempt to improve product identification, we measured pure standards for some monocarboxylic acids (butanoic, pentanoic, heptanoic and octanoic). We found out that under our operational conditions, the parent molecule MH + is approximately, 77% of the relative intensity. Fragments related to the loss of one molecule of water ([MH + ] -H2O) and one molecule of water and one group CO ([MH + ] -H2O -CO) were also found, whereas these two alternative fragmentation channels were more variable, between 5-10% for ([MH + ] -H2O) and between 5 and 20% for ([MH + ] -H2O -CO) depending on the acid. where ΔCA is the mixing ratio of the respective VOC emitted (subtracting the values of the baseline signal), and ΔCTotal is the sum of all VOC emitted ( ).

As evidenced in

The relative abundance, as well as the formula assignment and identification of the three foremost gas phase products for each dicarboxylic acid, is shown in Table IV. 1. It can be observed that monocarboxylic acids and aldehydes are the most common products of diacids photooxidation.

Formic acid, the simplest MCA appeared for succinic (C4), glutaric (C5) and pimelic (C7) acids with similar relative abundances, while acetaldehyde appeared as a product for succinic (C4), adipic (C6)

and suberic (C8) acids although its relative abundance for succinic acid was much higher than in the other two diacids.

It is worth pointing out that the detection of products in the gas phase depends not only on their formation reaction rates but also on their desorption/absorption equilibrium at the surface of the substrate. More volatile compounds, such as small aldehydes should preferentially evaporate, for example. The affinity between the organic compound and the surface may also play a role in its gasto-particle partition, and OVOCs interact differently with oxides components of ATD (Carlos-Cuellar et al., 2003;Li et al., 2001). For instance, molecules liable to hydrogen bonds may become more strongly bounded to dust surface hydroxyl groups (Chen et al., 2012;Styler et al., 2013). Therefore, some products may desorb more easily than others migrating into the gas phase, while some of then must remain adsorbed to further oxidation. This point is further discussed in the discussion of the mechanism (Sec. IV.3.5). The effect of light intensity on the formation and yield of the products was investigated. While all the other experimental parameters were kept constant, the number of lamps switched on was gradually decreased from 7 to 4 and then to 2, therefore decreasing the radiant flux (300-400 nm) As the light intensity decreased, the concentration of products perceptibly declined, the same behavior was observed for the other DCA. This observation highlights the photo-induced nature of the reaction occurring on ATDs surface since photocatalytic reaction rates are usually proportional to the radiant flux for low/moderate illumination intensities; above a certain value, which is characteristic of each system they become proportional to the flux square root for higher intensities (Herrmann, 2010).

For the region where the reaction rate is linearly proportional to the radiant flux, the formation of photogenerated holes (h + ) is the limiting step, and once formed, they must react with an electron donor molecule.

IV.3.2 Long-term Irradiation Experiments

Long-term experiments were carried out leaving the coated tube surface under illumination for 14h.

We aimed at evaluating the influence of irradiation time, that is, how long light exposure changes the distribution of products and how the concentration of these products evolves over this period. ATR-IR spectra indicated that malonic acid adsorbed on TiO2 from an aqueous solution in the dark exhibited two different carboxylate groups adsorption geometry [START_REF] Dolamic | Photoassisted Decomposition of Malonic Acid on TiO 2 Studied by in Situ Attenuated Total Reflection Infrared Spectroscopy[END_REF].

Later, the same authors [START_REF] Dolamic | Photocatalysis of dicarboxylic acids over TiO2: An in situ ATR-IR study[END_REF] compared ATR-IR vibration bands of malonic (C3)

and succinic (C4) and proposed different adsorption modes for each acid since for succinic acid both carboxylic groups were equivalent. Sun and coworkers [START_REF] Sun | An Unexpected Fluctuating Reactivity for Odd and Even Carbon Numbers in the TiO 2 -Based Photocatalytic Decarboxylation of C2-C6 Dicarboxylic Acids[END_REF] In Table IV. 1, it can be seen that the odd-numbered diacids mostly produced the C(n-1) monocarboxylic acid and formic acid whereas even-DCAs mainly produced aldehydes. Succinic acid is an exception, since even having four carbons, had as primary products propanoic and formic acids.

Another remarkable observation was the fact that glutaric (C5) and pimelic (C7) acid tubes showed a very strong odor after irradiation indicating the production of odorous compounds in concentrations perceptible to the human nose. The even-carbon DCAs tubes did not exhibit any strong odor.

I IV.3.4 S Surface products

To understand better the mechanism and possible differences between the odd and even compounds, The results indicate that the photo-oxidation mechanism of dicarboxylic acids results in the diacid C(n-1) suggesting their decarboxylation is an important step. Also, other highly oxygenated products such as oxo-acids and/or keto-acids (CnH2n-2O3), hydroxy acids, and dihydroxy acids, etc were also found, Table IV After decarboxylation, the alkyl radical can react either abstracting an H (from another organic molecule or surface species) or with O2. The H-abstraction (step 2) will lead to the C(n-1) MCA observed mostly for the odd-DCA, and the reaction with oxygen (step 2) gives the C(n-1) DCA [START_REF] Dolamic | Photocatalysis of dicarboxylic acids over TiO2: An in situ ATR-IR study[END_REF]. The adsorption/desorption equilibrium determines the partition of the product in the gas/surface phases. For odd-DCA, the decarboxylation is faster than for even-DCA, and the C(n-1) MCA is abundant enough to be released and detected in the gas-phase (step 4). For the even-DCA, the C(n-1) MCA is scarce, and once it is formed, it proceeds to next oxidation steps.

CmH2m-2O2 dialdehyde C6H10O2 C6H10O2 CmH2m
The monocarboxylic acid, in turn, continues the oxidation process, undergoing decarboxylation and, once again, forming alkyl radicals (step 5) which can react with O2 resulting in peroxyl radicals (step 6) that will lead to carbonylated compounds. The arrow in step 7 represents this pathway which can occur successively with each of the intermediates leading to the formation of (n-1) products at each stage. The higher volatility of aldehydes and formic acid favor their desorption from the surface and explains why they are overwhelmingly detected in the gas-phase.

For a didactic question, the mechanism was depicted in Figure IV. 8 by separating two reactional pathways, one that leads to the gas-phase products and the other that results in surface-sorbed compounds. However, the C(n-1) DCAs observed on the dust surface are also liable to undergo the same kind of reactions for the original DCA and contribute to the products detected on the gas-phase.

Moreover, other pathways lead to other sorbed species. Similar to reaction shown in step 6, after the first decarboxylation, the C-centered alkyl peroxy radicals (R-O-O•) can lead to oxo-acids, as R, in this case, still has another carboxylic group, Reaction R IV.1 -R IV.4).

R• + O2 → R-O-O• R IV.1 R-O-O• + RH → ROOH + R• R IV.2 ROOH → R-O• + •OH R IV.3 R-O• + O2 → RCHO + HO2• R IV.4
If R• is formed from the DCA, RCHO is an oxo-acid: HOOC-CH2(n)-CHO If R• is formed from the MCA, RCHO is an aldehyde.

As was discussed by Styler and Donaldson in their study of oxalic acid decomposition on different mineral proxies (Styler and Donaldson, 2012), the heterogeneous reactivity of realistic dust has a contribution of iron and titanium dioxide photochemistry. According to their observations, the presence of O2 reduced iron-mediated process whereas enhanced the reactivity for pure TiO2. The different behavior of each of the semiconductors punctuates the complexity of the process and emphasizes the importance of the investigation of natural and complex samples where synergy effects among constituent materials can modify the expected results for single components systems.

The same comment holds when we compare our results with the studies by [START_REF] Sun | An Unexpected Fluctuating Reactivity for Odd and Even Carbon Numbers in the TiO 2 -Based Photocatalytic Decarboxylation of C2-C6 Dicarboxylic Acids[END_REF]by Dolamic andBurgui 2007 who have investigated the photocatalytic degradation of dicarboxylic acids aqueous solutions on TiO2 using a depollution approach. Although these studies bring similar and useful mechanistic insights, regarding the products, both reported only acids (mono and dicarboxylic); whereas, in this study, we observed a greater variety of products, notably carbonyl compounds (aldehydes / ketones) in the gas-phase and oxygenated and hydroxylated substituted products in the particulate phase.

From the atmospheric point of view, the emission of carbonyl compounds is relevant since they are photoactive and contribute to the formation of radicals that will participate in the bulk gas-phase chemistry (Mellouki et al., 2015). For instance, carbonyls are fundamental in for the formation of HOx species; additionally, reactions of carbonyls with Br and Cl are also major sources of HBr and HCl in the upper troposphere and lower stratosphere (Lary and Shallcross, 2000).

About the products detected on the solid phase, it is emphasized that the presence of highly oxygenated and substituted products is especially important because of their higher polarity and affinity with water. Keskinen et al. measured the hygroscopicity of ATD seed particles exposed to SOA from the ozonolysis of α-pinene and found an increase by a factor of four for 100-150 nm particles (Keskinen et al., 2014). Their results highlight the effect of organic matter heterogeneous coating on solid particles hygroscopic properties and cloud droplet activation. Sullivan and Pradher, 2007, by contrast, commented that the association of DCA acids with mineral dust or sea salt could form insoluble complexes possibly diminishing cloud activation properties of those particles (Sullivan and Prather, 2007).

In addition, studies of ice nucleation (IN) have shown that particle coatings decreased IN ability compared to pure ATD (Niedermeier et al., 2010). Möhler and coworkers have investigated the ice nucleation efficiency of ATD particles coated by condensation of semi-volatile products from the reaction of α-pinene with ozone and found out that these SOA compounds lowered or even suppressed, in certain conditions, the heterogeneous ice nucleation potential of dust. The influence of organic matter oxidation state on CCN and IN activity is not yet a consensus in the scientific community, and more mechanistic understanding is needed. However, it is broadly recognized that organic compounds coating insoluble dust cores affect their CCN and IN potential.

Water is known to play an important role in photocatalytic reactions generating efficiently OHradicals and changing selectivity towards total oxidation (Herrmann, 2001) but also competing for reactive sites. We did not investigate the direct influence of water or relative humidity, although in our procedure no effort was made to eliminate all the water used in the coating process; and therefore, water molecules must have remained on the surface, especially those strongly bonded (by hydrogen bonds). However, a systematic evaluation of the role of relative humidity might be interesting and indeed, recommended for future studies.

Overall, the results obtained herein agree with other studies (Kawamura and Bikkina, 2016), [START_REF] Helmi Rozaini | The Chemistry of Dicarboxylic Acids in the Atmospheric Aerosols[END_REF], [START_REF] Kawamura | Molecular distributions of water soluble dicarboxylic acids in marine aerosols over the Pacific Ocean including tropics[END_REF] suggesting that longer chain DCA are precursors of shorter diacids. We could wonder that heterogeneous photochemical processing might be an important pathway for small DCA formation.

I IV.3.6 Conclusions and Atmospheric Implications

In this study, we have shown that photo-induced reactions can take place on realistic dust surfaces coated with organic compounds. Our results show that when starting with a single dicarboxylic acid, the particles produced a complex mixture of organic compounds upon irradiation. We have characterized the products of this photo-oxidation process presenting the partition between the volatile compounds that are emitted in the gas-phase and the low volatility ones which stay adsorbed on dust. Moreover, it was proposed a mechanism that agrees and summarizes experimental observations.

We did not have the opportunity to investigate the role of water vapor and possible influences in the surface-mediated photochemistry of DCA on dust. Considering the important role of water in the mechanism of photocatalysis and furthermore, emphasizing that in real atmospheric conditions water molecules are ubiquitous, its effect on the reaction's pathway is one point to be discussed in further studies. Yet, this work shows that at dry environments heterogeneous photo-oxidation occurs and may be significant.

In conclusion, this study revealed the role of irradiation in the aging of organic coated mineral surfaces. The chemical and physical properties of mineral aerosols can be substantially influenced by the photochemical production of small diacids and the evolution of their intermediates. Furthermore, the dust potential to induce reactions that lead to the oxidation of organic acids and the release of oxygenated volatile organic compounds to the gas-phase could alter atmospheric composition in the surroundings of a dust plume.

IV.4.3.2 Additional Results -Dicarboxylic acids gas-phase products

V.1 Introduction

Mineral dust is recognized as an important class of atmospheric aerosol. In fact, it is a key component of the atmospheric system that affects the radiative balance, nutrient availability in oceans, biogeochemical cycles, cloud condensation, among others. Mineral dust is typically composed by crustal elements such as Si, Al, Mg, Ca, Fe, Ti although the relative abundance of each element is variable and often related to the source soil (Usher et al., 2003). Uplifted dust particles are subjected to long-range transport across the planet being able to travel for many kilometers, for instance, over the Pacific Ocean from Asia to North America (Uno et al., 2009) or over the Atlantic Ocean from Africa to South America (Kaufman, 2005).

The surface of dust particles is not inert. A variety of heterogeneous processes can take place altering their composition and, more importantly, their physicochemical properties and their interactions in the atmosphere. The uptake of trace gases turns mineral dust into a sink for some species and a source for others. Some laboratory studies have shown that the uptake of NO2, HNO3, N2O5, for instance, on different mineral proxies led to the formation of nitrate anions on the surface (Hanisch andCrowley, 2001a, 2001b;[START_REF] Tang | Heterogeneous Interaction of SiO 2 with N 2 O 5 : Aerosol Flow Tube and Single Particle Optical Levitation-Raman Spectroscopy Studies[END_REF]Underwood et al., 2001b).

Concomitantly, field studies have found nitrate coatings on mineral dust [START_REF] Li | Observation of nitrate coatings on atmospheric mineral dust particles[END_REF][START_REF] Matsumoto | Scavenging of pollutant acid substances by Asian mineral dust particles[END_REF]Pan et al., 2017;Sullivan et al., 2007). Most of those studies were located in Asia where dust storms reach very polluted areas. The presence or absence of nitrates (and sulfates) is, indeed, regularly used as an indicator of the degree of aerosol chemical aging (Sullivan and Prather, 2007). In addition, recently, Zhan and co-workers [START_REF] Zhan | Changes in oxidative potential of soil and fly ash after reaction with gaseous nitric acid[END_REF] demonstrated the toxic effects of nitric acid aged soil particles and fly ashes that led to a substantial increase in their oxidative potential.

It is also remarkable that nitrate is regularly associated with other compounds; organic and inorganic species were found internally (and externally) mixed with samples collected all over the world. For instance, Falkovich and co-workers (Falkovich et al., 2004) evidenced the adsorption of low volatility organic compounds (i.e., dicarboxylic acids) and the copresence of inorganic and organic species on mineral dust particles. Wang et al., 2015 reported an effective correlation between NO3 - and oxalic acid in coarse particles (>2.1μm). Also, they proposed that nitric acid reactions on dust surface increase its hygroscopic properties and facilitate the condensation of dicarboxylic acids and related compounds and their further heterogeneous processing.

Recently, the photochemical properties of dust started to be explored revealing the role of light activating or enhancing new reactional pathways (Chen et al., 2012;George et al., 2015).

Semiconductor oxides such as TiO2 and Fe2O3 that have been applied for remediation process both for wastewater and air treatment [START_REF] Serpone | Semiconductor Photocatalysis -Past, Present, and Future Outlook[END_REF]Wang et al., 2016 and references therein) are also present in small amounts in natural minerals and may trigger photo-induced reactions. A number of studies investigate the photochemistry of adsorbed nitrate on different mineral proxies comparing the effect of light on semi-conductive materials as TiO2-SiO2 (Ndour et al., 2009a), Fe2O3 (Lesko et al., 2015;[START_REF] Nanayakkara | Surface Photochemistry of Adsorbed Nitrate: The Role of Adsorbed Water in the Formation of Reduced Nitrogen Species on α-Fe2O3 Particle Surfaces[END_REF] and light-inert ones like Al2O3, [START_REF] Rubasinghege | Photochemistry of Adsorbed Nitrate on Aluminum Oxide Particle Surfaces[END_REF][START_REF] Schuttlefield | Photochemistry of Adsorbed Nitrate[END_REF], SiO2 (Lesko et al., 2015) and exploring the surface-mediation effects. Nitrate anions, in turn, are well-known chromophores in natural waters, whose UVA and UVB radiation absorption results in photolysis [START_REF] Vione | Reactions Induced in Natural Waters by Irradiation of Nitrate and Nitrite Ions[END_REF]. Concerning atmospheric interfaces, their reactions have been investigated on ice [START_REF] Jacobi | Investigation of the photochemical decomposition of nitrate, hydrogen peroxide, and formaldehyde in artificial snow[END_REF], organic films [START_REF] Handley | Photochemical Loss of Nitric Acid on Organic Films: a Possible Recycling Mechanism for NO x[END_REF], deliquescent organic aerosols [START_REF] Yu | Nitrate ion photochemistry at interfaces: a new mechanism for oxidation of α-pinene[END_REF] and urban grime [START_REF] Baergen | Photochemical Renoxification of Nitric Acid on Real Urban Grime[END_REF] all evidencing the role of the surface in mediating these reactions.

Nitrate anions adsorbed on the surface of minerals can cause a synergistic effect by modifying the reaction rates of photooxidation of organic compounds. A positive influence was observed by Styler and Donaldson (2011) for isopropanol oxidation on TiO2 reporting that both the uptake and the emission of its main oxidation product, acetone, has increased for films containing NO3 -. More recently, Shang and co-workers [START_REF] Shang | Synergistic effect of nitrate-doped TiO2 aerosols on the fast photochemical oxidation of formaldehyde[END_REF] presented a positive influence for small nitrate contents followed by a reduced reactivity for formaldehyde photooxidation also on TiO2.

Up to now, there are only two studies (mentioned above) investigating the influence of cosorbed nitrates on oxygenated organic compounds uptake and heterogeneous photooxidation while field observations recurrently reported organic-inorganic mixtures on mineral dust. In this study, we investigated the effect of co-sorbed nitrate by two different approaches: kinetic measurements of uptake coefficients of acetone and the product formation of co-sorbed NO3 -salts and glutaric acid. In both, the content of nitrate was varied and the effects of the different amounts were discussed.

V.2 Experimental

All experiments were carried out in a horizontal coated wall flow-tube reactor, previously described (Ndour et al., 2008). The reactor was maintained at 293 K by circulating thermostatically controlled water in the outer jacket. Nitrogen/oxygen 79:21 mixture was used as the carrier gas accounting a total flow of 300 ml/min. Acetone was generated in the gas-phase at ppb levels using a calibrated permeation tube (Dynacal® -permeation rate 98 ± 25 ng/min at 40 ºC) and a temperaturecontrolled permeation device (VICI Metronics Dynacalibrator, Model 150).

The gas-phase composition was monitored online by coupling the coated-wall flow tube to a High-Resolution Proton Transfer Reaction Time of Flight Mass Spectrometer (SRI-PTR-ToF-MS 8000 from Ionicon Analytik GmbH -Innsbruck, Austria). The instrument operated using H3O + ionization mode and the following parameters: source current of 4.6 mA, drift voltage of 600 V, drift temperature of 333 K and a drift pressure of 2.42 mbar. E/N was about 125 Td (1 Td = 10 -17 V cm²). The reactor outlet and the PTR-MS sample inlet were constantly heated to 333 K to minimize adsorption and condensation in the tubing.

V V.2.1 Uptake Experiments

For uptake experiments, sodium nitrate (NaNO3) was added as internal mixtures in Arizona test dust (ATD) (Ultrafine A1, Powder Technology Inc) or SiO2 (Evonik Aerosil® 130). For both minerals, slurries were prepared using ethanol as solvent and mixing NaNO3 and the dust powder directly. After the coating process detailed in our previous study (Ponczek and George, 2018), the tubes were left to dry in an oven at 373 K overnight.

The uptake coefficient (γgeo) is related to the first order reaction rate constant (kobs) and can be calculated according to

(V.1)
kobs is the measured first-order rate constant; C(0) is the inlet and C(t) the outlet concentration of acetone after a reaction time (t); is average molecular velocity, and r is the reactor radius.

The decay of acetone concentration, C(t), is followed for different residence times by moving the injector position along the reactor and therefore successively exposing the dust-coated surface for 10 minutes. In between each intermediate position, the injector is reset to 'bypass position', where no surface is exposed to the gas flow [C(0)] also for 10 minutes. A typical time profile for acetone uptake on ATD is show in Figure V. S 1.

Radial and axial diffusion limitation effects were taken into account by correcting the observed uptake coefficients using a CKD (Cooney-Kim-Davis) based method derived by Murphy and Fahey (Murphy and Fahey, 1987). As proposed by Li and co-workers (Li et al., 2016) we numerically solved the differential equations obtaining the corrected uptake coefficient as a function of transmittance (C/Co).

Geometric uptake coefficients (γgeo) were further corrected by the Brunauer-Emmett-Teller (BET) surface area that considers the entire porous surface is participating in the uptake process (Underwood et al., 2000). Therefore, all uptake coefficients reported herein were obtained according to

(V.2)
where SBET is the BET surface area of the dust substrate; Ssurf is the geometric surface area, and mfilm is the mass coated in the sample holder tube and γgeo is the geometric uptake coefficient calculated from eq. V.1 and corrected by the CKD method.

V V.2.2 Co-sorbed glutaric acid

For co-sorbed glutaric acid experiments, the acid and sodium nitrate (NaNO3) were added to ATD as internal mixtures of 10% w/w for acids and different contents (0, 1%, 5%, 10% w/w) for nitrate.

Suspensions of ATD, ethanol and an aqueous solution of the acid and the respective amount of NaNO3

were prepared and coated on the internal walls of the Pyrex tube sample insert. The drying was carried out in an oven for 1h at 373 K.

The experiment procedure is the same as reported in our previous study (Chapter IV). After coating, the sample insert was placed in the flow tube reactor leaving its entire surface under the carrier gas flow (Qtotal= 300 ml/min) and subjected to the following illumination sequence: 30 minutes off (for background measurement); 1h under irradiation; 1h lights off; 1h under irradiation; lights off.

The net product formation (ΔC) in ppb was quantified always considering the product concentration after one hour of irradiation and subtracting the values of the baseline signal.

V.3 Results and Discussion

V V.3.1 Uptake experiments: acetone onto ATD

The BET uptake coefficients for acetone onto ATD with different contents of nitrate is shown in Acetone presented a relatively high uptake coefficient on pure ATD. Compared to butanol, at the same setup and experimental conditions (0% relative humidity, 1.4 mW/cm² of irradiation, 293 K), the uptake coefficient (for initial butanol concentration of 55 ppb) was,approximately, an order of magnitude lower.

We can observe in Figure V. 1 that the presence of a small content of nitrate (1%) drastically reduces the acetone uptake. Furthermore, comparing 1% and 5% nitrate content there is no significant difference. The ATD-NO3 -uptake coefficient is approximately five times lower than for pure ATD. As a matter of comparison, time profiles for acetone uptake experiments on ATD films with different NO3 - contents are presented in Figure V. S 2.

It is worth to point out that the kinetics of acetone uptake is quite rapid and is influenced by mass diffusion. For instance, the uptake coefficient for pure ATD is twice as high after the CKD correction than the measured values. As far as the uptake coefficients decrease by the presence of nitrate ions, the influence of the diffusion correction also decreases being reduced up to 25 -30%.

Regarding the photochemical products from acetone oxidation, only very few products were detected in the gas-phase both for ATD and ATD-NO3 -. These were formaldehyde and acetaldehyde at very low concentrations and very close to background levels, indicating that despite the relatively high uptake coefficients, under our experimental conditions, acetone is rather directly mineralized with very little generation of oxidation intermediates, or it is mainly physical adsorption without chemical reactivity.

In order to compare the influence of the nitrate ions, experiments were carried out, under the same conditions, with amorphous SiO2. The comparison between the uptake coefficients of acetone on the different substrates is illustrated in Figure V. 2.

Figure V. 2: Comparison between all uptake coefficients of acetone onto ATD and SiO2

with and without NO3 -, irradiated and dark. For an initial concentration of 70 ppb of acetone, dry conditions (0% relative humidity), irradiation of 0 (dark) or 7 lamps (1.4 mW/cm²).

It is important to mention that the silica used in these experiments is highly porous; its BET surface area is 130 ± 25 m²/g, whereas ATD is a very little porous material, SBET = 4.5 ± 0.5 m²/g. The contrast in the BET surface area explains the difference of approximately an order of magnitude between the uptake coefficients of ATD and SiO2. However, as can be observed in Figure V. 2, the presence of 1% of nitrate did not change the uptake coefficients of acetone on silica. For the three cases studied, pure SiO2, SiO2 1% NO3 -and SiO2 1% NO3 -dark, γBET values are very close. We can also remark that irradiation did not alter the uptake coefficient for either SiO2 or ATD containing 1% of NO3 - .

The results suggest that nitrates may hinder the uptake on ATD surface, probably reducing the availability of adsorption surface sites on which organic compounds could bind and react. For this reason, in this case, the illumination did not show an enhancement in the uptake capacity of acetone.

For SiO2, however, the same effect was not observed. First, silica is not a semi-conductive material and, thus, do not show photocatalytic activity, so the presence of light should not change its adsorption capacity (see Chapter III.b) nor promote the generation of nitrate radicals. Secondly, despite its high surface area, SiO2 exhibited weak chemical interaction and reactivity with both organic compounds (as mentioned previously) and nitrogenous inorganic compounds such as HNO3, NO2 and NO3 - [START_REF] Goodman | Heterogeneous Reaction of NO 2 : Characterization of Gas-Phase and Adsorbed Products from the Reaction, 2NO 2 (g) + H 2 O(a) → HONO(g) + HNO 3 (a) on Hydrated Silica Particles[END_REF]Underwood et al., 2001bUnderwood et al., , 2001a)). In fact, no evidence was found for the dissociative adsorption of nitric acid on SiO2; the adsorption was completely reversible [START_REF] Goodman | Heterogeneous Reaction of NO 2 : Characterization of Gas-Phase and Adsorbed Products from the Reaction, 2NO 2 (g) + H 2 O(a) → HONO(g) + HNO 3 (a) on Hydrated Silica Particles[END_REF]. It is not surprising, therefore that nitrates did not influence acetone uptake on SiO2.

V V.3.2 Influence of nitrate on photochemical products -Glutaric Acid Experiments

Experiments investigating acetone photochemistry presented very few gas-phase products.

Therefore, in order to study a possible influence of nitrate in the oxidation mechanism of organic compounds mediated by mineral dust, we performed experiments with glutaric acid co-sorbed on ATD.

Dicarboxylic acids (DCA) was proved to exhibit rich photochemistry on ATD surface as shown in our previous study, (Chapter IV), with the emission of several oxygenated compounds to gas-phase as well as the generation of low volatility products that stay on the surface. Styler and Donaldson (Styler and Donaldson, 2011) irradiated TiO2 films containing 9 to 14% of co-sorbed NO3 -(AgNO3 and KNO3) and reported the enhancement of isopropanol uptake associated to higher acetone formation compared to pure TiO2. The authors indicated that NO3 -photochemistry enhancement was not due to its direct photolysis, but it is surface-mediated by TiO2. Ndour et al. gave further evidence for semiconductor-mediated process (Ndour et al. 2009) showing that NOx production from adsorbed nitrate was higher for pure TiO2 than TiO2/SiO2 mixtures whereas pure SiO2 produced negligible NOx.

Two possible phenomena are attributed to nitrate ions that could influence the reactivity on the surface of semiconductors. First, positive holes (hvb + ) generated by light absorption can react with surface-adsorbed nitrate to form nitrate radical (NO3 • ); R V. 1

Nitrate radicals have a very short lifetime [START_REF] Gankanda | Nitrate photochemistry on laboratory proxies of mineral dust aerosol: Wavelength dependence and action spectra[END_REF] and photolyze leading to NO or NO2 (R V. 2 and R V. 3).

(for λ ≤ 640 nm) R V. 2

for 585 ≤ ≤ 640 nm R V. 3

Reaction R V. 2 is approximately ten times faster than R V. 3 [START_REF] Gankanda | Nitrate photochemistry on laboratory proxies of mineral dust aerosol: Wavelength dependence and action spectra[END_REF].

Additionally, the wavelength range for reaction R V. 3 is not overlapping with the emission spectrum of our lamps. Thus, reaction R V. 3 is unlikely to occur in our system, although it may occur in the troposphere. It has been suggested that at dry conditions, NO3 -is most likely converted to adsorbed species (NO2 and potentially NO + , NO , and N + ), whereas when humidity was present, gas-phase products were observed [START_REF] Rosseler | Chemistry of NO x on TiO 2 Surfaces Studied by Ambient Pressure XPS: Products, Effect of UV Irradiation, Water, and Coadsorbed K +[END_REF].

As a second effect, NO3 -ions could limit the electron-hole recombination by trapping photogenerated electrons (Styler and Donaldson, 2011).

R V. 4

Adsorbed NO2 formed on reaction R V. 2 can also trap an electron [START_REF] Nanayakkara | Surface Photochemistry of Adsorbed Nitrate: The Role of Adsorbed Water in the Formation of Reduced Nitrogen Species on α-Fe2O3 Particle Surfaces[END_REF] R V. 5

When nitrate anions are present on ATD surface, we can say that the overall reactivity regarding glutaric acid (and other organic compounds) is a result of the better compromise between the availability of adsorption sites for photooxidation of the organic compound and the formation of NO3 radicals. For small nitrate contents, there is less electron-hole recombination due to nitrate reactions with photogenerated holes and the electron trapping effect (Reactions R V. 1 to R V. 5). For higher nitrate contents, nitrate ions may occupy most of the available sites and their hole /electron trapping effect would compete and prevail over the oxidation of organic compounds.

The competition is particularly evidenced by the decrease in the yield of butanoic acid (Table V.

1) which is the first intermediate product of glutaric acid photooxidation. Photo-Kolbe decarboxylation is the first step in the transformation of dicarboxylic acids (see Chapter IV). If nitrate ions scavenge most of photogenerated holes, decarboxylation does not occur, and the C-centered radical is not formed. Afterwards, once a hole is trapped by a glutaric acid molecule, the reaction may proceed as proposed, generating smaller monocarboxylic acids and aldehydes. The relative abundance (RA) was defined as

(V.3)
Alternatively, the availability of nitrate radicals could give rise to another reaction pathway, oxidizing organic compounds or leading to organic nitrate compounds. In the gas phase, reactions between saturated organic acids and nitrate radicals are not very favorable, albeit hydrogen abstraction in the carbonyl group may occur, more propitiously for aldehydic species [START_REF] Kerdouci | Structure-activity relationship for the gas-phase reactions of NO3 radical with organic compounds: Update and extension to aldehydes[END_REF]. In the aqueous phase, the NO3 radical reactivity is much lower in comparison to OH and SO4 - [START_REF] Herrmann | Tropospheric Aqueous-Phase Chemistry: Kinetics, Mechanisms, and Its Coupling to a Changing Gas Phase[END_REF]. Typical rate constants are approximately 2 orders of magnitude smaller than OH rate constants for aliphatic mono-and dicarboxylic acids and carbonyl compounds (Lee Ng et al., 2017).

The significance of the pathway involving NO3 radical will depend on how the acid is adsorbed on dust, protonated or deprotonated. For aqueous solutions, de Sémainville et al. have shown that the reactivity and mechanisms for NO3 radical reactions with dicarboxylic acids are strongly pH dependent [START_REF] De Sémainville | Aqueous Phase Reactivity of Nitrate Radicals (NO 3 ) Toward Dicarboxylic Acids[END_REF]. For undissociated DCA, rate constants are extremely slow, on the other hand, reactions were much faster at higher pH (above the second pKa).

In our case, the pH of the solutions of glutaric acid and NaNO3 was 3.2 independent of the content of sodium nitrate. When ATD was added, completing the suspension used for the coating, the pH was 4.6. The pKa of glutaric acid are respectively, 4.32 (18 ° C) and 5.42 (25 ° C) which means that the diacid when adsorbed on ATD should be in its mono-ionic form, between the first and second pKa.

Therefore, at neutral and alkaline conditions, nitrate radical may gain importance favoring charge transfer reactions on the carboxylate group that may initiate decarboxylation. Surface can however, play a role mediating this reaction differently than in the bulk medium. From [START_REF] Gross | Products and kinetics of the reactions of an alkane monolayer and a terminal alkene monolayer with NO 3 radicals[END_REF].

The hypothesis we can form organic nitrates is not completely excluded and we could have two possibilities to explain why we did not detected them:

1) The organic nitrates formed are not enough volatile to desorb and remain in the particulate phase. Lee Ng and co-workers (Lee Ng et al., 2017) even emphasize that organonitrates are organic compounds with extremely low volatility (ELVOC) and hence, they play a key role in the nucleation of new particles.

2) Some organic nitrates can desorb; however, they fragment leading to the same OVOCs fragments already observed without nitrates.

Concerning organonitrates detection by PTR-ToF-MS, considerable fragmentation takes place.

Protonation occurs at the RO group of RONO2 and the main channel is the loose of a HNO3 molecule.

Aoki et al [START_REF] Aoki | Detection of C1-C5alkyl nitrates by proton transfer reaction time-of-flight mass spectrometry[END_REF] found as major product NO2 + , RO + , R + , and ROH•H + ions for C1-C5 alkyl nitrates. The protonated ion was barely detected accounting for a very few percentage of relative intensity. The fragments detected by PTR-MS were at same m/z that those for common VOCs.

An effort to detect surface-products, and potentially organonitrated ones, should be done in future experiments.

V.4 Conclusions and Atmospheric significance

Co-sorbed nitrate salts have a significant influence both in the uptake and in the reactivity of heterogeneous processes between organic compounds and mineral dust. The uptake of acetone on ATD decreased from 2.26 ± 0.17 x 10 -6 to approximatly 4.14 ± 0.70 x 10 -7 when 1% or 5% of NO3 was adsorbed on ATD surface.

The removal of acetone from the atmosphere occurs mainly by reactions with OH, photolysis being important at high altitudes (Mellouki et al., 2015). The lifetime of acetone with respect to OHradicals reactions is approximately 68 days (considering kOH 1.7 x 10E -13 cm³ molecule -¹ s -¹ (Atkinson and Arey, 2003) and [OH] = 1 x 10E +6 molecules cm -3 ). The lifetime for acetone due to heterogeneous mineral dust uptake was estimated using the uptake coefficients reported herein. For the calculations, we extrapolated the uptake coefficients measured at 70 ppbv of acetone, at dry conditions, 20°C for a real UV-A (315-400 nm) irradiance, determined using the Quick TUV model, for Beijing area (latitude of 40° North, longitude 116° East, at noon during equinox day). For medium dust loadings (surface area density of 2E-3 m²/m³), the lifetime regarding to pure ATD is 9.44 days and for ATD coated with 1% of nitrate is 47.32 days. It is worth to underline that the γBET used for those calculations is a lower limit where the uptake is probably underestimated. Acetone depletion due to heterogeneous processes may be, therefore, significant compared to homogeneous gas-phase reactions in certain conditions.

The reactivity of glutaric acid presented a parabolic trend, first increasing and then, decreasing total gas-phase products released as a function of nitrate content. Nitrate ions adsorbed on the surface of minerals were presented a synergistic effect by modifying the photooxidation reactivity of organic acids.

We could not determine if organo-nitrates were formed due to a new reaction pathway induced by NO3 radicals. Identify and characterize this potential pathway is extremely important to assess if under certain atmospheric conditions, heterogeneous mineral surface-NO3 radical-organic reactions may be a source of organo-nitrates. This issue should be clarified in future studies. Furthermore, under specific atmospheric conditions, when NO2 concentrations are high, organo-nitrates may also form by a similar mechanism that starts with NO2 uptake on dust and subsequently nitrate radical formation.

As have already been demonstrated, NO2 is efficiently taken up by mineral dust (Ndour et al., 2009b(Ndour et al., , 2008) ) and could also present a synergistic interaction with organic compounds leading to new reactional routes.

The hygroscopic properties of mineral dust can also be altered by nitrate and organic coatings.

Particles interactions with water vapor form droplets and ice crystals and hygroscopicity is an important parameter that will affect these mechanisms. Several laboratory and field studies have investigated the effect of atmospheric processing on the CCN activity of mineral aerosols (Tang et al., 2016) where it is shown that nitrate and organic coatings favor water condensation. Shi and co-workers clearly observed that nitrate coated dust particles enhanced the hydrophilicity of coarse particles [START_REF] Shi | Influences of sulfate and nitrate on the hygroscopic behaviour of coarse dust particles[END_REF]. The effect of water vapor is, therefore, an important parameter to be evaluated in future studies.

In Chapter IV, we concentrated our attention on the photochemical reactivity of dust towards semi-volatile organic compounds. Dicarboxylic acids that have been identified by field studies coating mineral dust particles were selected to mimic the semi-volatile organic compounds. Organic coated mineral particles exhibited rich photochemistry that resulted in a complex mixture of products upon irradiation. We have conclusively shown that higher dicarboxylic acids are precursors for smaller ones.

Also, several volatile compounds were emitted to the gas-phase, and highly oxygenated low volatility products remained adsorbed on dust surface.

The extraction of the adsorbed products on the surface of the dust particles proved to be useful to characterize these products and their partitioning between the phases. In this work, however, the method only began to be developed, without the intention of being a quantitative analysis. This procedure can therefore be improved to take into account quantitative aspects, such as determining the efficiency of the extraction and quantifying the products formed and their yields.

In Chapter V, we proceeded with our study simulating aged mineral aerosol by coating inorganic nitrate salts on dust. In the light of the previous results for the uptake of OVOCs and the photochemistry of dicarboxylic acids, we evaluated synergistic effects caused by the co-presence of NO3 -by means of two case studies: the uptake of acetone and the reactivity of glutaric acid onto Arizona test dust. The presence of nitrate ions had strong consequences on ATD reducing the uptake of acetone and altering the reactivity of glutaric acid, first increasing and then, decreasing the total of released gas-phase products as a function of nitrate content. The results of this work give rise to explore this theme in several fronts, for example to investigate: how the nitrate ions are placed on the surface of the substrates and how they can interact in situ; the effect of different nitrate sources and the possible influence of cations (Na, K, Ag, etc).

In summary, the results reported in this thesis represent a gain in knowledge in the emerging research on the reactivity of mineral aerosols in the atmospheric sciences. The rising interest in this research topic is notable. Since the beginning of the activities of this project (end of 2014) and the publication of the review by George and coworkers, in 2015 (George et al., 2015), several other scientific articles have appeared on this topic, such as (Lesko et al., 2015;[START_REF] Park | Heterogeneous Photooxidation of SO2 in the Presence of Two Different Mineral Dust Particles: Gobi and Arizona Dust[END_REF]Romanias et al., 2016;[START_REF] Zhao | Interactions between Heterogeneous Uptake and Adsorption of Sulfur Dioxide and Acetaldehyde on Hematite[END_REF].

Overall, we demonstrated that surface-mediated heterogeneous photochemistry substantially modifies gas-phase composition, transforming non-absorbing compounds as alcohols and carboxylic acids in absorbing ones as aldehydes and ketones. These reactions can have meaningful repercussions on tropospheric chemistry because they potentially alter the atmosphere oxidative capacity, since some carbonyl compounds, found as products, such as formaldehyde, acetaldehyde, propionaldehyde are sources of HOx in the atmosphere (Monks, 2005).

Like any ongoing research topic still under development, there are many challenges to be overcome and a number of perspectives to consider, for instance:

The representativeness of the material used as a substrate in its ability to mimic the aerosol found in nature: realistic dust x pure oxides.

No material can completely represent authentic atmospheric particles. In general, the vast mixture of different oxides in real soil/dust causes a synergistic effect between their constituents, so that the heterogeneous reactivity does not correspond precisely to the behavior of the pure minerals components.

Even real particles collected in the field can exclusively represent the region from which they were collected. As presented by Ndour et al, samples from different regions of the Sahara showed some variability in composition, reflecting in their results concerning NO2 uptake (Ndour et al., 2009). In addition, exposed soil and uplifted particles undergo intense weathering thus modifying their elemental composition and/or mineralogical structure during transport even when coming from the same origin. The choice of the substrate is therefore tricky and must reconcile numerous criteria such as stability, uniformity, ease of access, convenience, representativeness, among others.

Further explore the effect of relative humidity on reactivity and product distribution.

Humidity dependence was shown to be critical: water and OVOCs molecules presented a strong competition toward active surface-sites on mineral dust (Chapter III). We recommend the further investigation of H2O vapor on heterogeneous photocatalytic reactions. For instance, evaluate the role of water in enhancing/reducing reactivity or even altering reactions pathways during dust aging.

Investigate possible temperature effects.

Temperature can have opposite effects on heterogeneous processes. The increase in temperature accelerates the kinetics of bulk chemical reactions. However, in heterogeneous reactions, the increase in temperature may decrease the adsorption rate on the substrate but increases the convection and diffusion of gases. Considering the formation of particles and SOA, temperature affects volatility, Henry's and partition coefficients which may be a critical factor for semi-volatile compounds such as carboxylic acids. Exploring the effect of temperature on the partition of the gas phase / condensed phase for products of medium volatility (as in Chapter IV) can lead to exciting results.

The challenge of extrapolating laboratory controlled experimental work to the real world and to incorporate the results on current atmospheric models.

One of the biggest challenges in this research topic is how to extend the results of laboratory studies to the phenomenology of the real atmosphere. There are still some missing links between models and fundamental experimental studies and closer collaboration among laboratory, field, and modeling studies is needed.

Besides, among lab studies, crucial discrepancies in the uptake kinetics have often been observed even when using the same gas/substrate samples. The divergencies are attributed mainly to the different measurement principles and operating conditions required by different techniques such as Knudsen cells, DRIFTS cells, ATR-FTIR, coated wall flow tubes, etc. which vary, e.g. in operating pressure and experiments time-scales (steady state x initial uptake coefficients). The scientific community should work on finding ways to standardize the results, which will also facilitate usability and incorporation of such results into simulation models.

Studies are currently performed in unitary systems, i.e., a single trace gas.

We understand that in the beginning, it is necessary to simplify the system and avoid interference of parameters, however in the real atmosphere several components interact at the same time, and several oxidants are present, O2, O3, NOx, etc. Once understood the interactions of dust + inorganic gases and/or dust + organics and evaluated the role of mineral aerosols as a natural atmospheric (photo) catalyst, the direction should lead to gradually approach the conditions of the Earth's atmosphere where we have multicomponent interaction: VOCs + N2, NO2, SO2, O3, NH3…

Lastly, we hope that the present work and the results herein can contribute to the recognition of the role of the photocatalytic properties of airborne particles (not only dust but in general) in atmospheric chemistry and further develop this research field.

Faculté

  Pharmaceutiques et Biologiques Institut des Sciences et Techniques de la Réadaptation Département de formation et Centre de Recherche en Biologie Humaine Directeur : M. le Professeur G.RODE Directeur : Mme la Professeure C. BURILLON Directeur : M. le Professeur D. BOURGEOIS Directeur : Mme la Professeure C. VINCIGUERRA Directeur : M. X. PERROT Directeur : Mme la Professeure A-M. SCHOTT

Faculté

  Techniques des Activités Physiques et Sportives Observatoire des Sciences de l'Univers de Lyon Polytech Lyon Ecole Supérieure de Chimie Physique Electronique Institut Universitaire de Technologie de Lyon 1 Ecole Supérieure du Professorat et de l'Education Institut de Science Financière et d'Assurances Directeur : M. F. DE MARCHI Directeur : M. le Professeur F. THEVENARD Directeur : Mme C. FELIX Directeur : M. Hassan HAMMOURI Directeur : M. le Professeur S. AKKOUCHE Directeur : M. le Professeur G. TOMANOV Directeur : M. le Professeur H. BEN HADID Directeur : M. le Professeur J-C PLENET Directeur : M. Y.VANPOULLE Directeur : M. B. GUIDERDONI Directeur : M. le Professeur E.PERRIN Directeur : M. G. PIGNAULT Directeur : M. le Professeur C. VITON Directeur : M. le Professeur A. MOUGNIOTTE Directeur : M. N. LEBOISNE i Statue of Clarice Lispector and her dog 'Ulisses' at Leme Beach in Rio de Janeiro, Brazil

  l'acetaldehyde. Le Chapitre III b. est axé sur la capture d'acétone sur différents substrats, ATD, TiO2 et SiO2. Les résultats des coefficients de capture pour chaque matériau sont comparés et l'effet de la lumière et de la vapeur d'eau est discuté. La capture d'acétone par la poussière était rapide dans les conditions d' « obscurité » et d'« irradiation », mais aucun produit n'a été identifié, suggérant qu'en dépit des coefficients de capture relativement élevés, soit l'acétone est minéralisée sans production d'intermédiaires, soit il s'agit d'une pure adsorption physique sans réactivité chimique.

  FIGURE I. 1: SCHEMATIC OF MINERAL DUST LIFE CYCLE AND IMPACTS ON THE BIOSPHERE. ..................................................... 6 FIGURE I. 2: LOCATION OF THE MAIN DESERTS OF THE WORLD AND POTENTIAL SOURCES OF MINERAL DUST. ....................... 7 FIGURE I. 3: ANNUAL TOTAL DUST DEPOSITION FLUXES (G M-2 YEAR-1). .................................................................................. 8 FIGURE I. 4: SCHEMATIC DIAGRAM SHOWING AEROSOLS RADIATIVE MECHANISMS ASSOCIATED WITH CLOUD EFFECTS. .... 10 FIGURE I. 5: PERCENTAGE OF DEPOSITION OF PARTICULATE MATTER OF A SPECIFIED PARTICLE DIAMETER .......................... 11 FIGURE I. 6: RELATIVE COMPOSITION OF DUST FROM DIFFERENT SOURCES. .......................................................................... 12 FIGURE I. 7: ''THE ATMOSPHERIC FLOW REACTOR" ................................................................................................................. 14 FIGURE I. 8: ANTHROPOGENIC VOCS EMISSIONS INVENTORY. ................................................................................................ 17 FIGURE I. 9: PHYSICAL AND CHEMICAL PROCESSES OF VOCS IN THE ATMOSPHERE. ................................................................ 18 FIGURE I. 10: POSSIBLE ATMOSPHERIC PHOTOCHEMICAL PRODUCTION AND /OR DEGRADATION PATHWAYS FOR DICARBOXYLIC ACIDS AND OTHER INTERMEDIATES CARBOXYLIC/ CARBONYLIC PRODUCTS. .............................. 25 FIGURE I. 11: THE FIVE BASIC STEPS OF HETEROGENEOUS PHOTOCATALYTIC PROCESSES. ..................................................... 27 FIGURE I. 13: ELECTRON ENERGY BAND DIAGRAM FOR TIO2. .................................................................................................. 29 FIGURE I. 14: SCHEMATIC REPRESENTATION OF THE MULTIDISCIPLINARITY OF MINERAL DUST PHOTOCHEMISTRY. ............. 30 FIGURE I. 15: GRAPHICAL ABSTRACT FOR THIS THESIS. ............................................................................................................ 31TABLE I. 2: CHEMICAL COMPOSITION OF THE MINERALS ENCOUNTERED IN AIRBORNE DUST PARTICLES ............................... 13 TABLE I. 3: REACTION RATE CONSTANTS AND THE CALCULATED LOW TROPOSPHERIC LIFETIMES TO OH RADICALS, NO3 RADICALS, O3 AND CL FOR SELECTED VOCS. ............................................................................................................. 21 TABLE I. 4: PHOTOLYSIS CHANNELS FOR FORMALDEHYDE, ACETALDEHYDE, AND ACETONE. .................................................. 22 TABLE I. 5: CHANGES IN THE VAPOR PRESSURE OF AN ORGANIC COMPOUND DUE TO THE ADDITION OF A FUNCTIONAL GROUP. ...................................................................................................................................................................................... 24

FIGURE

  FIGURE II. 1 SCHEMATIC REPRESENTATION OF THE EXPERIMENTAL METHODOLOGY. ............................................................. 49 FIGURE II. 2: SCHEMATIC REPRESENTATION OF COATED WALL FLOW-TUBE REACTOR ............................................................ 51 FIGURE II. 3: THE "GREEN BOX" PROTECTION HOUSING FOR THE FLOW TUBE REACTOR AND UV LAMPS. ............................... 52 FIGURE II. 4: EMISSION SPECTRUM OF UV-A LAMPS USED IN THIS WORK. ............................................................................... 52 FIGURE II. 5: MICROSCOPIC ARBITRARY VIEW OF A DUST COATING EMPHASIZING HETEROGENEITIES IN PARTICLE PACKING AND DIMENSIONS. ................................................................................................................................................................. 53 FIGURE II. 6: TUBE INSERTS COATED WITH DIFFERENT MINERAL SAMPLES. ............................................................................. 55 FIGURE II. 7: ARIZONA TEST DUST COMPOSITION (%WT) PROVIDED BY POWDER TECHNOLOGY INC. (PTI). ............................ 55 FIGURE II. 8: WATER BUBBLER FOR RELATIVE HUMIDITY (RH) GENERATION. ........................................................................... 58 FIGURE II. 9: TYPICAL PROFILE OF BUTANOL UPTAKE ................................................................................................................ 60 FIGURE II. 10: SCHEMATIC DRAWING OF THE PRT-TOF-MS, IONICON 8000. ............................................................................. 63 FIGURE II. 11: FRAGMENTATION PATTERNS (OR PRODUCT ION DISTRIBUTIONS) FOR THE REACTIONS OF H3O+ WITH SOME ALCOHOLS AND ALDEHYDES. ......................................................................................................................................... 65 FIGURE II. 12: GAUSS AND MULTIPEAK FITTINGS AT PTR VIEWER SOFTWARE V. 3.1.0.31. ....................................................... 67 FIGURE II. 13: SCHEMATIC REPRESENTATION OF BUTANOL FRAGMENTATION. ....................................................................... 68TABLE II. 1: REACTOR DIMENSIONS AND THE FLUID DYNAMICS PARAMETERS FOR THE OPERATING CONDITIONS USED DURING OUR EXPERIMENTAL WORK. .......................................................................................................................................... 51 TABLE II. 2: COMPARISON OF THE CHEMICAL COMPOSITION OF ATD. ..................................................................................... 56 TABLE II. 3: PERMEATION TUBES SPECIFICATIONS. ................................................................................................................... 58 TABLE II. 4: FRAGMENTATION IONS OF PERFORMED STANDARD COMPOUNDS ON PTR-TOF-MS OPERATING ON H3O+, AT OUR INSTRUMENT, UNDER OUR OPERATIONAL CONDITIONS. .............................................................................................. 69

  FIGURE III A. 1: DECREASING RELATIVE HUMIDITY (RH) AND SUCCESSIVELY EXPOSING THE SAMPLE SURFACE TO BUTANOL AND H2O VAPOR..................................................................................................................................................................... 85 FIGURE III A. 2: PRODUCT CONCENTRATION PROFILE FOR EACH POSITION/RESIDENCE TIME ALONG THE REACTOR. ............. 87 FIGURE III A. 3: SATURATION EXPERIMENT: .............................................................................................................................. 88 FIGURE III A. 4: NET PRODUCT FORMATION OF MAJOR PRODUCTS; ......................................................................................... 89 FIGURE III A. 5: SUGGESTED MECHANISM FOR BUTANOL PHOTOOXIDATION ON ATD SURFACE. ............................................. 91 TABLE III A. 1: PRODUCT IDENTIFICATION AND M/Z TO MOLECULES ASSIGNMENTS. ............................................................... 86 FIGURE III A. S 1: TOTAL IRRADIANCE FOR DIFFERENT CONFIGURATION OF LAMPS. ................................................................ 93 FIGURE III A. S 2: A SCHEMATIC OVERVIEW OF THE EXPERIMENTAL SET-UP ............................................................................. 94 FIGURE III A. S 3: DEPENDENCE OF GEOMETRIC UPTAKE COEFFICIENT OF BUTANOL ............................................................... 95 FIGURE III A. S 4: γBET AS FUNCTION OF BUTANOL CONCENTRATION. ..................................................................................... 96

  FIGURE IV. 1: GAS PHASE PRODUCTS OBSERVED BY PTR-TOF-MS FOR IRRADIATED ATD FILMS ............................................. FIGURE IV. 2: NET PRODUCT FORMATION FOR A) GLUTARIC ACID AND B) ADIPIC ACID AFTER 1H IRRADIATION.................... FIGURE IV. 3: LIGHT INFLUENCE ON PRODUCT MIXING RATIOS FOR SUCCINIC ACID: ............................................................. FIGURE IV. 4: GAS PHASE PRODUCTS -LONG-TERM IRRADIATION FOR PIMELIC ACID. .......................................................... FIGURE IV. 5: TOTAL GAS-PHASE PRODUCTS MIXING RATIO FOR ALL FIVE DICARBOXYLIC ACIDS STUDIED, ........................... FIGURE IV. 6: CHROMATOGRAMS OBTAINED FOR DIACIDS SURFACE PRODUCTS, ................................................................. FIGURE IV. 7: SORBED-PHASE PRODUCTS DETECTED BY UHPLC-MS ANALYSIS OF .................................................................. FIGURE IV. 8: PROPOSED MECHANISM FOR DICARBOXYLIC ACIDS PHOTO-OXIDATION ON ATD SURFACE UNDER UV-A RADIATION. .................................................................................................................................................................. TABLE IV. 1: MAIN GAS-PHASE PRODUCTS FOR EACH DICARBOXYLIC ACID. ........................................................................... TABLE IV. 2: LIST OF THE MAIN CONDENSED PHASE COMPOUND CLASSES OF PRODUCTS ..................................................... FIGURE IV. S 1: SCHEMATIC OF EXPERIMENTAL SET-UP .......................................................................................................... FIGURE IV. S 2: SELECTED GAS PHASE PRODUCTS OBSERVED BY PTR-TOF-MS FOR IRRADIATED ATD FILMS COATED WITH ... FIGURE IV. S 3: TOTAL MIXING RATIOS AS A FUNCTION OF DUST FILM MASS FOR EACH DCA. ................................................ TABLE IV. S 1: VOLUME OF RECONSTITUTED EXTRACT SAMPLE USED FOR EACH ACID LC-MS ANALYSIS. ............................... TABLE IV. S 2: LIST OF ALL GAS PHASE M/Z FOUND FOR THE PTR-TOF OPERATING ON H3O+ MODE, ...................................... CHAPTER V FIGURE V. 1: UPTAKE COEFFICIENT, ΓBET, FOR ACETONE AS A FUNCTION OF NITRATE CONTENT ON ATD FILMS. ................. FIGURE V. 2: COMPARISON BETWEEN ALL UPTAKE COEFFICIENTS OF ACETONE ONTO ATD AND SIO2 ................................... FIGURE V. 3: SELECTED GAS PHASE PRODUCTS FOR GLUTARIC ACID ON ATD. ........................................................................ FIGURE V. 4: PRODUCTS DISTRIBUTION AND NET TOTAL PRODUCT FORMATION OF GAS PHASE PRODUCTS FOR GLUTARIC ACID ON ATD WITH DIFFERENT NO3 -CONTENTS. .................................................................................................................. FIGURE V. 5: PROPOSED MECHANISM FOR ADDITION OF NO3 TO AN ALKANE SURFACE IN THE PRESENCE OF NO2 AND O2. TABLE V. 1: SELECTIVITY OF MAIN GAS PHASE PRODUCTS FOR GLUTARIC ACID PHOTOOXIDATION ON ATD WITH DIFFERENT NO3 -CONTENTS............................................................................................................................................................ FIGURE V. S 1: TIME PROFILE FOR AN UPTAKE EXPERIMENT RUN FOR ACETONE ON A PURE ATD FILM.................................. FIGURE V. S 2: TIME PROFILES FOR ACETONE UPTAKE EXPERIMENTS ON ATD FILMS WITH DIFFERENT NO3 -CONTENTS ....... FIGURE V. S 3: SELECTED GAS PHASE PRODUCTS,FOR GLUATRIC ACID ON ATD, ..................................................................... FIGURE V. S 4: CONCENTRATION OF BUTANOIC ACID (REPRESENTED AS FRAGMENT C4H9O2 + ), ............................................. TABLE V. S 1: LISTS ALL THE FRAGMENTS AND POSSIBLE MOLECULE / COMPOUND ASSIGNMENTS .......................................

  Depending on their source, formation, transport, pathway and aging under diverse atmospheric conditions those particles will exhibit a variety of different physical, chemical, surface and structural properties and consequently will present distinct fates and impacts. The aspects concerning mineral dust life cycle are schematically represented in Figure I. 1, each of these topics will be briefly discussed.

Figure I. 1 :

 1 Figure I. 1: Schematic of mineral dust life cycle and impacts on the biosphere.

Figure I. 2 :

 2 Figure I. 2: Location of the main deserts of the world and potential sources of mineral dust.FromKarydis et al., 2006.

Figure I. 3 :

 3 Figure I. 3: Annual total dust deposition fluxes (g m -2 year -1 ). Averaged over the 10-year period 2003-2012; shown deposition rates include dry deposition, sedimentation, wet deposition due to large-scale precipitation and wet deposition due to convective precipitation. From Schepanski, 2018.

  Figure I. 4 summarizes these mechanisms associated with cloud effects caused by aerosols in general, including mineral dust.

Figure I. 4 :

 4 Figure I. 4: Schematic diagram showing aerosols radiative mechanisms associated with cloud effects. The small black dots represent aerosol particles; the larger open circles cloud droplets. Straight lines represent the incident and reflected solar radiation, and wavy lines represent terrestrial radiation. The filled white circles indicate cloud droplet number concentration (CDNC). The vertical grey dashes represent rainfall, and LWC refers to the liquid water content. From IPCC AR4,[START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF].

  Figure I. 5 illustrates the deposition of distinguished size particles in different segments of the respiratory system.

Figure I. 5 :

 5 Figure I. 5: Percentage of deposition of particulate matter of a specified particle diameter that reaches different segments of the respiratory system. From ICAO Information Paper (ICAO, 2008).

  Figure I. 6: Relative composition of dust from different sources. a) Major element average composition of dust from different sources. Data from (Goudie and Middleton, 2001; Jeong, 2008); b) Mineralogical dust composition for western Saharan dust. Boxes indicate the 25, 50 and 75% percentiles. Error bars indicate the minimum and the maximum values, and the open squares indicate the mean value of the distribution. Adapted from Formenti et al., 2014.

  Figure I. 6 b presents the average mineralogical composition for 51 samples collected at Western Sahara (Formenti et al., 2014). Table I. 2 presents the chemical composition of the minerals encountered in airborne dust particles, represented in Figure I. 6 b.

  Figure I. 8: Anthropogenic VOCs emissions inventory. a) Total non-methane volatile organic compounds (NMVOC) emissions for the period 2000-2005 per geo-economic region. b) NMVOC emissions per sector and per region, comparing the EDGAR-HTAP V1 inventory for 2000 with the Representative Concentration Pathways (RCP) 2000 base inventory. From (Janssens-Maenhout et al., 2012).

  Figure I. 9 summarizes VOCs processing in the troposphere.

Figure I. 9 :

 9 Figure I. 9: Physical and chemical processes of VOCs in the atmosphere.Adapted from[START_REF] Williams | Volatile Organic Compounds in the Atmosphere: An Overview[END_REF]

  rate constants and the calculated low tropospheric lifetimes to OH radicals, in day, except if stated otherwise. c -from Atikinson and Arey, 2003, except if stated otherwise. d -from IUPAC vol II (Atkinson et al., 2006), except if stated otherwise. e -from Calvert et al. 2011. f-from Calvert, 2011, lifetimes calculated for clear skies at 40 °N latitude, ozone column = 350 DU, in the Spring equinox (March 22).

Figure I. 10 :

 10 Figure I. 10: Possible atmospheric photochemical production and /or degradation pathways for dicarboxylic acids and other intermediates carboxylic/ carbonylic products. From Pavuluri et al., 2015.

  , Figure I. 11.

Figure I. 11 :

 11 Figure I. 11: The five basic steps of heterogeneous photocatalytic processes.

Figure I. 12 :

 12 Figure I. 12: Electron energy band diagram for TiO2. Electron energy E plotted upwards as a function of the distance from the surface to the bulk of the solid. From Hermann 2010.

For

  this reason, this thesis addresses different topics concerning mineral dust interaction and reactivity towards organic compounds upon illumination. The primary scientific questions that this work aims to clarify are depicted in Figure I. 14. Laboratory studies were performed in the attempt to simulate heterogeneous process that occur naturally in the free troposphere.

Figure I. 14 :

 14 Figure I. 14: Graphical abstract for this thesis.The figure represents some of the possible interactions between mineral aerosols and other atmospheric components and the questions that have been raised and we intend to assess.

Question 1 :

 1 photo-reactive uptake, water dependence may be even more critical, since H2O molecules are important drivers of photocatalytic reactions. Chapter III a. presents the results of the uptake of butanol onto Arizona test dust, a real desert sample, focusing on the reactive uptake trigged by UV-A light. The influence of external conditions as initial butanol concentration, light intensity, temperature, moisture in evaluated. Kinetic data (uptake coefficients), gas phase reaction products, reaction mechanisms and the role of water vapor are discussed. Chapter III b. focus on acetone uptake on different substrates, ATD, TiO2, and SiO2. The results of uptake coefficients for each material are compared, discussing the effect of light and water vapor.

  Figure II. 1. A fourth step was included for the study of dicarboxylic acids heterogeneous photochemistry (Chapter IV).

Figure II. 1

 1 Figure II. 1 Schematic representation of the experimental methodology.

  set-up I II.2.1 The reactor Experiments were carried out in a horizontal coated wall flow-tube reactor, schematically represented in Figure II. 2. The set-up consists of, in addition to the reactor, other auxiliary components such as mass flow controllers (Bronkhorst El-flow), humidifier, UV lamps, humidity sensor, cooling water circulation system that were adapted according to the type of experiment. Details and modifications on the set-up are specifically described in each chapter.The tubular reactor is entirely made of Pyrex, 24 cm long and 1.44 cm inside diameter with a double wall allowing the circulation of fluids for the temperature control. The useful volume of the flow tube correspond to the dimensions of the removable Pyrex insert (radius 0.55 cm, length 20 cm),

Figure II. 2 :Figure

 2 Figure II. 2: Schematic representation of coated wall flow-tube reactor

Figure II. 4 :

 4 Figure II. 4: Emission spectrum of UV-A lamps used in this work.Acquired using a Spectrometer AvaSpec-2048 (Avantes), using a calibration lamp AvaLight-DH-CAL and FC-UV400 fiber.

Figure II. 5 :

 5 Figure II. 5: Microscopic arbitrary view of a dust coating emphasizing heterogeneities in particle packing and dimensions.

Figure II. 6 :

 6 Figure II. 6: Tube inserts coated with different mineral samples.

Figure

  Figure II. 7: Arizona Test Dust Composition (%wt) provided by Powder Technology Inc. (PTI).

  picture of the device is shown in Figure II. 8.

Figure II. 8 :

 8 Figure II. 8: Water bubbler for relative humidity (RH) generation.

  Figure II. 9: Typical profile of butanol uptake a) "raw-data" time profile obtained by moving forward and backawards the inlet injector which was moved in steps of 2 cm length. Butanol concentration was online monitored using a PTR-ToF-MS operating in H3O+ mode; b) concentration as a function of residence time corresponding to the respective movable injector positions show infigure 'a'; c) First-order kinetics logarithmic linearization for the concentration profile shown in 'b'.

Figure II. 10 :

 10 Figure II. 10: Schematic drawing of the PRT-ToF-MS, Ionicon 8000.The instrument used in this work. It consists of the hollow cathode discharge ion source, the drift tube reaction chamber, the transfer lens system and the reflectron time-of-flight mass spectrometer. From[START_REF] Jordan | A high resolution and high sensitivity proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS)[END_REF]

Figure II. 11 :

 11 Figure II. 11: Fragmentation patterns (or product ion distributions) for the reactions of H3O + with some alcohols and aldehydes.At 300 K determined using a SIFT (Soft Ionization flow tube) mass spectrometer apparatus. From[START_REF] Smith | Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis[END_REF].

Figure

  Figure II. 12: Gauss and multipeak fittings at PTR Viewer Software v. 3.1.0.31. First, a) Gauss fitting for the peak m/z 61.0277 and secondly, b) multipeak fitting for a triplet peak m/z 43.018; 43.030 and 43.054.

  expected products were examined by measuring the signals of vaporized liquid standards under the same experimental operating conditions. The fragmentation of butanol is shown in Figure II. 13.

Figure II. 13 :

 13 Figure II. 13: Schematic representation of butanol fragmentation.At our instrument, under our operational conditions, for pure butanol standard.

Figure III a. 2 :

 2 Figure III a. 2: Product concentration profile for each position/residence time along the reactor. For initial butanol concentration of 20 ppb, dry conditions, T = 20 °C, 7 lamps, mass of ATD = 110 mg.

  III a. 4). Although only major products are shown in Figure III a. 3 and Figure III a. 4, the same minor products listed in Table III a. 1 were also present. Figure III a. 4 clearly demonstrates that irradiation enhanced product formation. It is known that photocatalytic reaction rates are proportional to the radiant flux for low/moderate illumination intensities(Herrmann, 2010).[START_REF] Chapuis | Photocatalytic Oxidation of Volatile Organic Compounds Using Fluorescent Visible Light[END_REF]) also observed lower product concentrations at lower light intensities, other factors being equal, for the photocatalytic oxidation (PCO) of n-butanol.

Figure

  Figure III a. 3: Saturation experiment: Concentration profiles for butanol (right scale) and main products over butanol-saturated ATD surface for different irradiation intensities (2, 4, and 6 lamps, indicated by bars above the graph). Experimental conditions were: air flow rate 200 ml/min, dry conditions, T = 20 °C, mass of ATD = 35 mg.

Figure

  Figure III a. 4: Net product formation of major products; for different light intensities during the saturation experiment (Figure III a. 3). The number of lamps is represented by bar color as indicated in the legend.

  global mechanism is suggested in Figure III a. 5. In general, we can say • R III a. 4

Figure

  Figure III a. S 1 and Table III a. S 1.

Figure

  Figure III a. S 1: Total irradiance for different configuration of lamps.

SFigure

  Figure III a. S 2: A schematic overview of the experimental set-up

Figure

  Figure III a. S 4: γBET as function of butanol concentration.For butanol initial concentration from 20 to 55 ppb, dry conditions, temperature 293 K, for all 7 lamps irradiation (1.4 mWatt/cm²).

Figure

  Figure III a. S 5 : Uptake coefficient, γBET, as a function of reactor temperature.For initial concentration of 20 ppb of butanol, dry conditions (0% relative humidity), irradiation of 7 lamps (1.4 mWatt/cm²).

Figure

  Figure III a. S 6: Uptake coefficient, γBET, as a function of relative humidity. For initial concentration of 20 ppb of butanol, temperature 293 K and illumination of 7 lamps (1.4 mWatt/cm²).

Figure

  Figure III a. S 7: Butanol loss, exposing an ATD film to different relative humidity levels under dark conditions.For initial concentration of 20 ppb of butanol, temperature 293 K.

Figure

  Figure III a. S 8: Uptake coefficient, BET as a function of number of lamps.For initial concentration of 20 ppb of butanol, dry conditions, temperature 293 K.

  Figure III a. S 9 and results are summarized on Table III a. S 2.

Figure

  Figure III a. S 9: Setup for fragmentation patterns for standards compounds on PTR-ToF-MS operating on H3O + Mode.

  contrast to what was observed for butanol kinetics, the presence of light irradiation just slightly affected the uptake of acetone in ATD, Figure III b. 1.

Figure III b. 1 :

 1 Figure III b. 1: Acetone uptake coefficients, γBET, for ATD as a function of the number of lamps. For an initial concentration of 70 ppb of acetone, dry conditions.

Figure III b. 2 .

 2 Figure III b. 2. The surface becomes rapidly saturated, and the acetone concentration returns to its initial level in a short time. When water vapor was suppressed from the inlet stream, the concentration of acetone plummeted to a very low value. By comparison, the uptake profile for dry conditions is depicted in Figure III b. S 1; it took approximately 16 h to saturate the ATD surface, i.e., for the acetone to reach its initial concentration value. Figure III b. S 1 overlaps acetone uptake profiles under dry and 5% RH conditions for different surface exposures (for whole runs for uptake coefficient experiments) evidencing the expressive effect of the presence of moisture.

Figure III b. 2 :

 2 Figure III b. 2: ATD film exposed to acetone at two levels of relative humidity. First 5% and then, 0% RH both upon irradiation of 7 lamps.

Figure

  III b. S 3 for the long-term experiment, during 16h of exposure to acetone, TiO2 surface presented steady state and constant uptake. When the lights were turned off, one by one, even with just one lamp switched on, we still observe reactive uptake. Finally, when all the lamps were turned off, acetone concentration starts to increase gradually towards surface saturation, as is illustrated in Figure III b. 3.

Figure III b. 3 :

 3 Figure III b. 3: Concentration profiles for acetone uptake on TiO2, for dry conditions. Lamps were turned off gradually leading to different irradiation intensities (as indicated by the bars above the graph).

  Figure III b. 4: when the lights were turned off, acetone returned to its initial concentration (Cin) instantaneously. Next, when the lights were turned on again, acetone concentration dropped and reached the same value as before (in Figure III b. 4 for time about 60 min). Continuing the experiment, water vapor was suppressed (at approx. 72 min), and we do not observe the same sharp drop seen for ATD; conversely, acetone remains at the same level, while the lights were on. When the lights were turned off, however, (for RH = 0%), the acetone concentration progressively increased, similar to the result shown in Figure III b. 3.

Figure

  Figure III b. 4: Concentration profile for acetone uptake on TiO2 at two levels of RH. First for 5% relative humidity and then suppressing water vapor, as indicated in the right 'Y' scale. Lamps were turned on and off as indicated by bars above the graph.

  Figure III b. 5 illustrates the acetone uptake coefficients for the tree substrates under study, as a matter of comparison. The determination of uptake coefficients was described in the Supplementary Material.

Figure III b. 5 :

 5 Figure III b. 5: Comparison of acetone uptake coefficients for ATD, TiO2, and SiO2 Under irradiation (of 1.4 mW/cm²) or dark (black diagonal hatched), dry or 5% RH conditions (blue horizontal hatched). For initial concentration of 70 ppb of acetone.

Figure

  Figure III b. S 1: Acetone long-term uptake profile on ATD film at dry conditions.Under irradiation of 1.4 mW/cm 2 , during 15h of exposure of ATD surface. We can observe that the surface becomes saturated. For acetone initial concentration of 70 ppb.

Figure III b. S 2 :

 2 Figure III b. S 2: Acetone uptake profiles under dry and 5% RH conditions for different surface exposures (a whole run for an uptake coefficient experiment).

For

  Figure IV. 1.

Figure IV. 1 :

 1 Figure IV. 1: Gas phase products observed by PTR-ToF-MS for irradiated ATD films coated with a) Glutaric acid(C5) and b) Adipic acid (C6).

  photooxidation products (Figure IV. 2 and Figure IV. S 2) facilitates the evaluation of what compounds were formed in each experiment. In both figures, error bars were calculated from thee replicates. The mass variability of each sample was not an issue for the reproducibility of the results as it can be observed in Figure IV. S 23.

Figure IV. 2 :

 2 Figure IV. 2: Net Product formation for a) Glutaric acid and b) Adipic acid after 1h irradiation. The error bars represent the standard deviation of three replicates, evidencing the reproducibility of the results.

  Figure IV. 2 (and Figure IV. S 2) the results were reproducible concerning both the total emission of VOCs and the distribution of products. Therefore, the relative abundance (RA)of each product was calculated with respect to the total of VOCs emitted (IV.1)

  from 1.4 to 0.65 and 0.40 mW/cm². Figure IV. 3 illustrates the concentration profiles and the quantification of products of a succinic acid/ATD coated film at different light intensities.

Figure IV. 3 :

 3 Figure IV. 3: Light Influence on product mixing ratios for succinic acid: a) temporal profile and b) net product quantification. The number of lamps (2, 4 or 7) is indicated by the colors in the legend.

Figure IV. 4 :

 4 Figure IV. 4: Gas phase Products -Long-term Irradiation for Pimelic Acid. a) Mixing ratio during time and b) net product distribution for 1h, 6 h and 13h of illumination.

  confirmed the difference on the surface adsorption geometry of glutaric and succinic acids, extrapolating this pattern: for odd DCA one carboxylic group adopt monodentate coordination and the other, bidentate coordination with TiO2 surface; for even DCA the two carboxylic groups exhibit the same bidentate bridging coordination. These observations can explain what could be different about the bulk and heterogeneous photo-oxidation mechanisms of dicarboxylic acids to produce such different results.

  we analyzed by UHPLC/(-)HESI-MS the extracts of products that remained adsorbed on the mineral surface. In Figure IV. 6 we compare the irradiated sample chromatogram of pimelic acid, adipic acid, and glutaric acid experiments to tree respective blanks: the pure solvent mixture, a pure ATD irradiated tube, a sample from dust containing the same acid but kept in the dark. Succinic acid (C4) (m/z 117.0190) was found as major surface-product from glutaric acid (C5) reaction. Glutaric acid (m/z = 131.0349) was found in the adipic acid (C6) experiment sample, and the ion m/z = 145.0503 corresponding to adipic acid was observed in the pimelic acid (C7) sample. Diacids identification was made by comparison with standard solutions in concentrations equivalents to those found in the samples. As observed in Figure IV. 6, the mentioned products were only found in the irradiated sample containing the corresponding DCA.

Figure IV. 6 :

 6 Figure IV. 6: Chromatograms obtained for diacids surface products, following extracted ions as indicated, comparing the irradiated samples [ATD_DCA_IRR] to the blanks (pure solvent [Blc Solv], pure ATD irradiated [Pure ATD_IRR] and the corresponding diacid/ATD sample kept in the dark [ATD_DCA_D]); for (a) pimelic acid (C7) sample, ion m/z = 145.0503; (b) adipic acid (C6) sample, ion m/z = 131.0349; (c) glutaric acid (C5) sample, ion m/z = 117.019.

Figure IV. 7 :

 7 Figure IV. 7: Sorbed-Phase Products detected by UHPLC-MS analysis of a) adipic acid (C6) and b) pimelic acid (C7) and (c) suberic acid (C8). The results for the irradiated samples are compared to the dark ones prepared the same day.

  photo-oxidation(Figure IV. 8). It is worthwhile to mention that the reactivity of mineral dust is attributed to the content of semiconductor materials generally present in variable amounts. ATD, specifically, contains 2.0 -5.0 wt% Fe2O3 and 0.5 -1 wt% TiO2.

Figure IV. 8 :

 8 Figure IV. 8: Proposed mechanism for dicarboxylic acids photo-oxidation on ATD surface under UV-A radiation. Species in solid rectangles were observed by our analytical techniques.

Figure IV. S 2 :

 2 Figure IV. S 2: Selected gas phase products observed by PTR-ToF-MS for irradiated ATD films coated with a) Succinic acid b) Pimelic acid and c) Suberic acid. On the left, mixing ratios profiles, on the right, the net product formation. Error bars represent the standard deviation of three replicates.

Figure

  Figure V. 1.

Figure V. 1 :

 1 Figure V. 1: Uptake coefficient, γBET, for acetone as a function of nitrate content on ATD films. For an initial concentration of 70 ppb of acetone, dry conditions, irradiation of 7 lamps (1.4 mW/cm²).

Figure V. 3 :

 3 Figure V. 3: Selected gas phase products for glutaric acid on ATD. Observed by PTR-ToF-MS for irradiated films coated with a) 1% and b) 10% wt NO3 -.

Figure V. 4 :

 4 Figure V. 4: Products distribution and net total product formation of gas phase products for glutaric acid on ATD with different NO3 -contents. a) Products distribution (after 1h of irradiation) for glutaric acid coated in ATD and b) Net total product formation, for different NO3 -contents. In both figures, error bars represent the standard deviation for three replicates.

  [START_REF] Gross | Products and kinetics of the reactions of an alkane monolayer and a terminal alkene monolayer with NO 3 radicals[END_REF] proposed a mechanism for oxidation of an immobilized alkane monolayer by high concentrations of gas-phase NO3 radicals (1 x 10E+11 molecule/cm³) (Figure V. 5).After the exposure of a film of 1-octadecanethiol for 12.6 days they detected the formation of organonitrates by different surface analytical techniques. XPS showed minimal volatilization (i.e., loss of the carbon chain), and the formation of carbonyl groups (ketones or aldehydes) and carboxylic groups. The authors conclude that organic nitrates were a significant product of the heterogeneous oxidation reaction.

Figure V. 5 :

 5 Figure V. 5: Proposed mechanism for addition of NO3 to an alkane surface in the presence of NO2 and O2.From[START_REF] Gross | Products and kinetics of the reactions of an alkane monolayer and a terminal alkene monolayer with NO 3 radicals[END_REF].

TABLE I .

 I 2: CHEMICAL COMPOSITION OF THE MINERALS ENCOUNTERED IN AIRBORNE DUST PARTICLES ............................... 13 TABLE I. 3: REACTION RATE CONSTANTS AND THE CALCULATED LOW TROPOSPHERIC LIFETIMES TO OH RADICALS, NO3 RADICALS, O3 AND CL FOR SELECTED VOCS. ............................................................................................................. 21 TABLE I. 4: PHOTOLYSIS CHANNELS FOR FORMALDEHYDE, ACETALDEHYDE, AND ACETONE. .................................................. 22 TABLE I. 5: CHANGES IN THE VAPOR PRESSURE OF AN ORGANIC COMPOUND DUE TO THE ADDITION OF A FUNCTIONAL GROUP.

...................................................................................................................................................................................... 24

CHAPTER II

TABLE II .

 II

1: REACTOR DIMENSIONS AND THE FLUID DYNAMICS PARAMETERS FOR THE OPERATING CONDITIONS USED DURING OUR EXPERIMENTAL WORK. .......................................................................................................................................... 51

TABLE II .

 II 2: COMPARISON OF THE CHEMICAL COMPOSITION OF ATD. ..................................................................................... 56 TABLE II. 3: PERMEATION TUBES SPECIFICATIONS. ................................................................................................................... 58 TABLE II. 4: FRAGMENTATION IONS OF PERFORMED STANDARD COMPOUNDS ON PTR-TOF-MS OPERATING ON H3O+, AT OUR

INSTRUMENT, UNDER OUR OPERATIONAL CONDITIONS. .............................................................................................. 69 CHAPTER III -A FIGURE III A. 1: DECREASING RELATIVE HUMIDITY (RH) AND SUCCESSIVELY EXPOSING THE SAMPLE SURFACE TO BUTANOL AND H2O VAPOR..................................................................................................................................................................... 85 FIGURE III A.

  FIGURE III A. S 5 : UPTAKE COEFFICIENT, γBET, AS A FUNCTION OF REACTOR TEMPERATURE. ................................................. FIGURE III A. S 6: UPTAKE COEFFICIENT, γBET, AS A FUNCTION OF RELATIVE HUMIDITY........................................................... FIGURE III A. S 7: BUTANOL LOSS, EXPOSING AN ATD FILM TO DIFFERENT RELATIVE HUMIDITY LEVELS UNDER DARK CONDITIONS. ................................................................................................................................................................. FIGURE III A. S 8: UPTAKE COEFFICIENT, γBET AS A FUNCTION OF NUMBER OF LAMPS. ........................................................... FIGURE III A. S 9: SETUP FOR FRAGMENTATION PATTERNS FOR STANDARDS COMPOUNDS ON PTR-TOF-MS OPERATING ON H3O+ MODE.................................................................................................................................................................... TABLE III A. S 1: TOTAL IRRADIANCE FOR DIFFERENT CONFIGURATION OF LAMPS FOR DIFFERENT RANGES OF WAVELENGTH: 300 TO 400 NM, 400 TO 600 NM AND 300 TO 600 NM. .................................................................................................. TABLE III A. S 2: RESULTS FOR FRAGMENTATION PATTERNS OF STANDARDS COMPOUNDS ON PTR-TOF-MS OPERATING ON H3O+. .............................................................................................................................................................................. FIGURE III B. 1: ACETONE UPTAKE COEFFICIENTS, ΓBET, FOR ATD AS A FUNCTION OF THE NUMBER OF LAMPS. .................... FIGURE III B. 2: ATD FILM EXPOSED TO ACETONE AT TWO LEVELS OF RELATIVE HUMIDITY. ................................................... FIGURE III B. 3: CONCENTRATION PROFILES FOR ACETONE UPTAKE ON TIO2, FOR DRY CONDITIONS. .................................... FIGURE III B. 4: CONCENTRATION PROFILE FOR ACETONE UPTAKE ON TIO2 AT TWO LEVELS OF RH. ...................................... FIGURE III B. 5: COMPARISON OF ACETONE UPTAKE COEFFICIENTS FOR ATD, TIO2, AND SIO2 ................................................ FIGURE III B. S 1: ACETONE LONG-TERM UPTAKE PROFILE ON ATD FILM AT DRY CONDITIONS. ..............................................

	CHAPTER III -B

FIGURE III B. S 2: ACETONE UPTAKE PROFILES UNDER DRY AND 5% RH CONDITIONS ............................................................. FIGURE III B. S 3: ACETONE CONCENTRATION PROFILE ON TIO2 FOR A LONG PERIOD OF IRRADIATION. ................................ CHAPTER IV FIGURE IV. 1: GAS PHASE PRODUCTS OBSERVED BY PTR-TOF-MS FOR IRRADIATED ATD FILMS ............................................. FIGURE IV. 2: NET PRODUCT FORMATION FOR A) GLUTARIC ACID AND B) ADIPIC ACID AFTER 1H IRRADIATION.................... FIGURE IV. 3: LIGHT INFLUENCE ON PRODUCT MIXING RATIOS FOR SUCCINIC ACID: ............................................................. FIGURE IV. 4: GAS PHASE PRODUCTS -LONG-TERM IRRADIATION FOR PIMELIC ACID. .......................................................... FIGURE IV. 5: TOTAL GAS-PHASE PRODUCTS MIXING RATIO FOR ALL FIVE DICARBOXYLIC ACIDS STUDIED, ........................... FIGURE IV. 6: CHROMATOGRAMS OBTAINED FOR DIACIDS SURFACE PRODUCTS, ................................................................. FIGURE IV. 7: SORBED-PHASE PRODUCTS DETECTED BY UHPLC-MS ANALYSIS OF .................................................................. FIGURE IV. 8: PROPOSED MECHANISM FOR DICARBOXYLIC ACIDS PHOTO-OXIDATION ON ATD SURFACE UNDER UV-A RADIATION. ..................................................................................................................................................................

TABLE IV.

Table I . 2: Chemical composition of the minerals encountered in airborne dust particles

 I

	(see Figure I. 6 b)
	Mineral	Chemical Composition	Characteristics
	Quartz	SiO2	tectosilicate mineral
	Calcite	CaCO3	carbonate mineral
	Dolomite	CaMg(CO3)2	carbonate mineral
	Gypsum	CaSO4 2 H2O	Sulphate mineral
	Orthoclase	KAlSi3O8	tectosilicate mineral
	Albite	NaAlSi3O8	feldspar, tectosilicate mineral
	Kaolinite	Al₂Si₂O₅(OH)₄	Clay mineral, layered silicate
	Illite	K0.6(H3O)0.4Al1.3Mg0.3Fe 2+ 0.1Si3.5 O10(OH)2•(H2O) a	Clay mineral; phyllosilicate, or layered alumino-silicate
	a -Chemical composition can vary depending on weathering.

  I II.2.2 Tube CoatingDuring this thesis, three different minerals were used as a substrate for heterogeneous gas/solid interface: Arizona test dust (ATD), titanium dioxide (TiO2) and silicon dioxide (SiO2). A photo of tubes with different mineral coatings is shown in FigureII. 6.

	Arizona Test Dust was used as provided by Powder Technology Inc without any further
	pretreatment. More information about ATD characterization is presented hereupon.
	TiO2 was provided by Evonik Aeroxide® P25. It is a fine white powder that exhibits a high
	hydrophilic character due to hydroxyl groups on the surface (Evonik, 2013). P25 Evonik (before,
	Degussa) is widely applied in photocatalytic studies worldwide. It contains 80-90% (wt.) of anatase
	and specific surface area (BET -Brunauer-Emmett-Teller) is 50 ± 15m²/g.

SiO2 was provided by

Evonik, Silica Aerosil® 130 (Hydrophilic Fumed Silica)

. It is an amorphous, highly porous and well-known and characterized material, used in a variety of processes, the BET surface area is 130 ± 25 m²/g. It is hydrophilic caused by silanol groups present on its surface

Table II . 2: Comparison of the Chemical composition of ATD.

 II The information provided by the manufacturer (PTI) and analysis by[START_REF] Dupart | Impact de la chimie des poussieres minerales sur la photochimie atmospherique[END_REF][START_REF] Vlasenko | Generation of Submicron Arizona Test Dust Aerosol: Chemical and Hygroscopic Properties[END_REF]

	Chemical composition of major oxides (%wt.)	Elemental composition in % of atoms
		ATD Manufacturer	Dupart, 2012		Vlasenko, 2005
	SiO2	68.0 -76.0	77.2	Si	79.1
	Al2O3	10.0 -15.0	8.13	Al	8.2
	Fe2O3	2.0 -5.0	2	Fe	2.2
	CaO	2.0 -5.0	2.38	Ca	4
	K2O	2.0 -5.0	2.41	K	1.7
	Na2O	2.0 -4.0	1.4	Na	2.3
	MgO	1.0 -2.0	0.66	Mg	2.1
	TiO2	0.5 -1.0	0.33

Average particle size is 4.08 ± 2.91 μm, and specific gravity (H2O = 1) is 2.65 as informed by the manufacturer. The Brunauer-Emmett-Teller (BET) surface area is 4.5 ± 0.5 m²g -1 as measured with a Micromeritics Tristar 3000.

[START_REF] Cwiertny | Characterization and acid-mobilization study of iron-containing mineral dust source materials[END_REF]

performed X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) characterization of ATD particles. While XPS is specific to the near-surface region, SEM/EDX provides information concerning the bulk composition. Generally, Si and Al were most abundant elements; followed by Fe, Ca, Mg, K,

Table II . 3: Permeation tubes specifications.

 II

	Compound	n-butanol (C4H10O)	acetone (C3H6O)
	Length (cm)	5.9	6.1
	Permeation Rate (ng/min)	100	98
	Reference Temperature ( °C)	60	40

Table III a. 1: Product identification and m/z to molecules assignments.

 III

		Major Products		Minor Products
	m/z	Formula	Compound	m/z	Formula	Compound
	45.033	C2H5O +	Acetaldehyde	83.085	C6H11 +	fragment Hexanal
	31.017	CH3O +	Formaldehyde /	87.080	C5H11O +	Pentanal
			fragment Propanal
	59.049	C3H7O +	Propanal/ Acetone	97.101	C7H13 +	fragment Heptanal
	33.033	CH4O +	Metanol	111.116	C8H15 +	fragment Octanal
	69.069	C5H9 +	fragment Octanal /	129.127	C8H17O +	Octanal
			Pentanal
	115.111	C7H15O +	Heptanal

Table III a. S 1: Total irradiance for different configuration of lamps for different ranges of wavelength: 300 to 400 nm, 400 to 600 nm and 300 to 600 nm.

 III

	Irradiance			Number of lamps
	(μWatt/cm²)	2	3	4	6	7
	300 to 400 nm	349.4	484.16	553.04	1003.8	1226.1
	400 to 600 nm	54.6	81.82	92.74	162.3	201.1
	300 to 600 nm	404.0	565.98	645.78	1166.1	1427.2

Table III a. S 2: Results for fragmentation patterns of standards compounds on PTR-Tof-MS operating on H3O + .

 III

	Compound	Formula	Mw	Proposed Ions	m/z detected	Ratio (%)
	Acetaldehyde	C2H4O	44.05	[MH + ]	45.033	100%
				[MH + ]	59.049	88.2%
	Propanal	C3H6O	58.08	[MH + ] + (C2H4)	31.017	11.8%
				[MH + ] -H2O	55.054	80.0%
	Butanal	C4H8O	72.11	[MH + ]	73.063	15.6%
				[MH + ] -H2O isotope	56.057	4.4%
				[MH + ] -H2O	97.096	50.6%
				[MH + ] + (C3H8O)	55.054	28.5%
	Heptanal	C7H14O	114.18	[MH + ]	115.11	14.9%
				[MH + ] -H2O isotope	98.10	5.9%
				C5H9+ (+ C3H5+)	69.069	29.3%
				[MH + ] -H2O	111.11	24.0%
				[MH + ]	129.12	21.5%
	Octanal	C8H16O	128.21	C3H5 + (+ C5H9+ )	41.038	10.1%
				[MH + ] isotope	112.12	8.0%
				[MH + ] + (C4H10O)	55.054	7.1%
	Acetone	C3H6O	58.08	[MH + ]	59.049	100%
				[MH + ] -H2O	57.069	90%
	Butanol	C4H10O	74.12	[MH + ] -H2O-CH4	41.039	10%
	Summarizing the results from the standards procedure and what has been reported on
	literature,

Table IV . 1: Main gas-phase products for each dicarboxylic acid.

 IV

	I IV.3.1 Light Intensity Influence
	The standard deviation was calculated for tree replicates.
					Products
	Starting
	DCA	m/z	Formula	Suggested product	ΔC (ppb) Std Dev	Selectivity (%)	St error Selectivity
		45.033	C2H5O+	Acetaldehyde	9.44	1.99	59.5%	8.4%
	C4	75.042	C3H7O2+	Propanoic acid	1.60	1.00	10.0%	5.7%
		47.011	CH3O2+	Formic acid	1.12	0.35	7.1%	2.3%
		89.059	C4H9O2+	Butanoic Acid	52.82	2.37	50.1%	0.7%
	C5	47.011	CH3O2+	Formic acid	6.39	1.09	6.1%	1.0%
		59.049	C3H7O+	Propanal	6.02	1.00	5.7%	1.0%
		55.054	C4H7+	Butyraldehyde	1.25	0.19	14.7%	0.7%
	C6	45.033	C2H5O+	Acetaldehyde	1.07	0.48	12.3%	4.1%
		87.042	C4H7O2+	Crotonic Ac/ Succialdehyde	0.78	0.09	9.2%	1.4%
		69.069	C5H9+	Pentanal	3.74	0.65	16.6%	0.4%
	C7	117.091	C6H13O2+ Hexanoic Acid	3.37	1.26	14.6%	3.3%
		47.011	CH3O2+	Formic Acid	1.68	0.30	7.5%	1.0%
		83.084	C6H11+	Hexanal	1.58	0.59	20.6%	4.7%
	C8	45.033	C2H5O+	Acetaldehyde	0.85	0.12	11.4%	2.3%
		55.053	C4H7+	Butyraldehyde	0.84	0.27	10.9%	1.9%

Table IV . 2: List of the main condensed phase compound classes of products

 IV . 2. It should be pointing out the appearance of molecules of greater molecular weight than the original acid, suggesting dimerization or recombination of radicals throughout photooxidation. when glutaric, adipic, pimelic or suberic acid ATD films were exposed to light. Most abundant products are highlighted in bold.

	General	Proposed	Glutaric (C5) Adipic (C6)	Pimelic (C7)	Suberic (C8)
	Formula	Function

Table V . 1: Relative abundance of main gas phase products for glutaric acid photooxidation on ATD with different NO3 -contents.

 V

		0% NO3 -	1% NO3 -	5% NO3 -	10% NO3 -
	CH3O +	2.5%	1.0%	1.3%	3.5%
	C3H5 +	4.6%	4.0%	4.1%	5.3%
	C3H7 +	7.9%	9.8%	9.5%	6.7%
	C2H5O +	5.2%	3.3%	3.5%	11.9%
	CH3O2 +	7.3%	5.5%	7.0%	11.3%
	C3H7O +	7.0%	4.0%	4.5%	13.1%
	C2H5O2 +	3.1%	1.8%	2.0%	4.4%
	C4H7O +	2.1%	3.9%	3.5%	1.5%
	C3H7O2 +	5.0%	4.6%	5.4%	2.8%
	C4H7O2 +	2.6%	2.6%	2.8%	4.4%
	C4H9O2 +	48.8%	55.0%	51.1%	26.3%
	C4H5O3 +	3.9%	4.4%	5.4%	8.8%

Considering, [OH] = 1 x 10E +6 molecules cm -3(Mellouki et al.,

2015)

Acknowledgements

thank my big brother, Vlady, for all the advice, you are my academic counselor! Finally, I gratefully acknowledge the Brazilian National Council for Scientific and Technological Development (CNPq) and the Science without Borders (Ciência Sem Fronteiras) program for the financial support.

III a. Kinetics and Product Formation during the Photooxidation of Butanol on

Atmospheric Mineral Dust

Published on Environmental Science & Technology 2018 52 (9), 5191-5198 DOI: 10.1021/acs.est.7b06306

Abstract

Mineral dust particles have photochemical properties that can promote heterogeneous reactions on their surfaces and therefore alter atmospheric composition. Even though dust photocatalytic nature has received significant attention recently, most studies have focused on inorganic trace gases. Here we investigated how light changes the chemical interactions between butanol and Arizona test dust, a proxy for mineral dust, under atmospheric conditions. Butanol uptake kinetics were measured, exploring the effects of UV light irradiation intensity (0 -1.4 mW/cm²), relative humidity (0 -10%), temperature (283 -298 K), and butanol initial concentration (20 -55 ppb). The composition of the gas phase was monitored by a high-resolution proton-transfer-reaction mass spectrometer (PTR-ToF-MS) operating in H3O + mode. Water was observed to play a significant role, initially reducing heterogeneous processing of butanol, but enhancing reaction rates once it evaporated. Gas phase products were identified, showing that surface reactions of adsorbed butanol led to the emission of a variety of carbonyl containing compounds. Under actinic light these compounds will photolyze and produce hydroxyl radicals, changing dust processing from a sink of VOC into a source of reactive compounds.

III b. Uptake of acetone on different substrates: ATD, TiO2 and SiO2

Abstract

Acetone is one of the most important atmospheric oxygenated organic compound which is directly emitted and produced in situ from several anthropogenic and natural sources. Global acetone burden is estimated to be 3.5 Tg and its atmospheric lifetime can vary between 15 days to 2 months.

In this study, the heterogeneous interactions and photochemistry of acetone on different minerals such as Arizona Test Dust (ATD), TiO2, and SiO2 were investigated under simulated atmospheric conditions using a coated wall flow tube reactor. The uptake kinetics were evaluated with respect to UV-A light irradiation intensity (0 -1.4 mW/cm 2 ) and relative humidity (0 and 5%) for acetone gas phase concentration of 70 ppb. The gas-phase composition was monitored on-line by a PTR-ToF-MS operating in H3O + mode. Acetone dark and irradiated uptake on dust were fast, and for ATD, the relative humidity seems to have more influence on the uptake coefficient than the luminous intensity, while for TiO2 the opposite occurred. No products were identified, suggesting that despite relatively high uptake coefficients, under our experimental conditions, whether acetone is mineralized without the generation of intermediates or it is pure physical adsorption without chemical reactivity.

Abstract

Dicarboxylic acids are an important fraction of organic aerosols. They have low volatility and hence are present mostly in the particulate phase, including the surface of dust particles, as have been identified by previous field studies. Mineral dust, globally the most emitted aerosol, has photocatalytic properties that can initiate photo-induced heterogeneous chemistry of organic compounds which is presently still poorly characterized. We investigated the chemistry of five dicarboxylic acids: succinic (butanedioic) acid, glutaric (pentanedioic) acid, adipic (hexanedioic) acid, pimelic (heptanodioc) acid and suberic (octanodioic) acid on Arizona test dust (ATD) particles upon UV-A light irradiation (0 -1.4 mW cm -2 ). Gas-phase composition was monitored by a high-resolution proton-transfer-reaction mass spectrometer (PTR-ToF-MS), and surface sorbed products were extracted and analysed by ultra-high performance liquid chromatography coupled to an electrospray ionization mass spectrometer (UHPLC-HESI-MS). We identified and quantified reaction gas-phase products whose monoacids and aldehydes were the major compounds. LC-MS indicated the appearance of shorter chain DCA and highly oxygenated products. Interestingly, DCA presented an even-odd alternation concerning their heterogeneous chemical reactivity, with odd-numbered carbon diacids being more reactive than their even-numbered homologous ones. We present and discuss a reaction mechanism for the C4-C8 DCA heterogeneous photooxidation catalysed on TiO2/Fe2O3-rich dust particles. Our results suggest that photochemical process on dust aerosols surface should be regarded as an important aging process that can alter their action in ice nucleation and as cloud condensation nuclei, as well as a source of reactive gas-phase compounds. ) for each dicarboxylic acid studied as a function of the number of carbons where the alternation of reactivity between even and odd compounds is evidenced. Odd-carbon DCA exhibit higher reactivity than the even-carbon analogous ones. Several studies report odd-even alternation of solid-state physical properties of diacids, namely, saturation vapor pressure (Bilde et al., 2015), melting point [START_REF] Thalladi | The Melting Point Alternation in α , ω -Alkanedicarboxylic Acids †[END_REF], solubility in water [START_REF] Zhang | Identification and Molecular Understanding of the Odd-Even Effect of Dicarboxylic Acids Aqueous Solubility[END_REF] and in organic solvents [START_REF] Zhang | An odd-even effect on solubility of dicarboxylic acids in organic solvents[END_REF]. Odd-carbon compounds are more soluble, have higher vapor pressure and lower melting points and sublimation enthalpies. Recently, [START_REF] Fairhurst | Kinetics, mechanisms and ionic liquids in the uptake of n-butylamine onto low molecular weight dicarboxylic acids[END_REF][START_REF] Fairhurst | Kinetics, mechanisms and ionic liquids in the uptake of n-butylamine onto low molecular weight dicarboxylic acids[END_REF] and Zhao et al 2017 [START_REF] Zhao | New insights into atmospherically relevant reaction systems using direct analysis in real-time mass spectrometry (DART-MS)[END_REF] reported odd/even alternation, respectively, in the reactive uptake on solid C3-C8 diacids films and chemical reactivity of submicron C3-C7 dicarboxylic acid particles with gas-phase amines.

I IV.3.3 Even-Odd alternation of diacids reactivity

Concerning photo-oxidation reactions, Yang and coworkers [START_REF] Yang | Photooxidation of dicarboxylic acids-Part II: Kinetics, intermediates and field observations[END_REF]

Abstract

Mineral dust is a crucial component of the atmospheric system; its rich and diverse crustal elements composition raises interest in its reactivity toward trace gases. In fact, the surface of suspended particles is nowadays recognized as a reactive interface, and therefore, the study of these heterogeneous reactions has gained importance in recent years. Field studies have confirmed that mineral aerosols are often highly mixed and coated with organic compounds such as low volatility organic acids, and inorganic compounds such as nitrates and sulfates. This work aims at investigating the physical-chemical interactions of mineral aerosols, here mimicked by Arizona test dust (ATD) and SiO2, pure or covered with inorganic material (sodium nitrate) in different contents. The first part discusses the influence of nitrate anions on the uptake (dark and irradiated) of acetone on ATD and SiO2. For ATD, the presence nitrate anions strongly reduced the BET corrected uptake coefficient from 2.2 ± 0.17 x 10 -6 to 4.4 ± 0.60 x 10 -7 ; whereas for SiO2 the same effect was not observed, and the uptake coefficient was maintained in the same order of magnitude (2.2 ± 0.60 x 10 -8 ). In the second part, the role of mineral dust as a photocatalyst in the oxidation of low molecular weight organic compounds was analyzed by discussing the influence of nitrate anions on the reactivity of glutaric acid on ATD. The reactivity of glutaric acid was strongly reduced when 10% of NO3 -was present, decreasing total gas-phase products released.

V.5 Supplementary Information

V V.5.1 Uptake profiles Figure V. S 1 shows a typical acetone uptake time profile on pure ATD.

Figure V. S 1: Time profile for an uptake experiment run for acetone on a pure ATD film

For Acetone initial concentration Cin ≈70 ppb, 7 lamps, 0% RH, T= 293 K, mATD = 0.028 g. This data is used to calculate the uptake coefficient.

Figure V. S 2: Time profiles for acetone uptake experiments on ATD films with different NO3 -contents

As indicated in the legend, black, pure ATD, light brown 1% of NO3 -, and dark brown 5% of NO3 -. For acetone initial concentration Cin ≈70 ppb, 7 lamps, 0% RH, T=293 K.

V V.5.2 Gas phase products for ATD/glutaric acid films co-coated with nitrate.

VI -Conclusions and Outlook

The main objective of this work was to address different questions concerning surface-mediated heterogeneous reactions on mineral aerosols. Although heterogeneous reactions of atmospheric trace gases, inorganic and organic compounds, on aerosols under dark conditions have been studied for the last two decades, there is still considerable uncertainty regarding uptake coefficients, the effect of ambient conditions and the reactional mechanisms. Besides, the concerning about the effect of solar radiation on these heterogeneous reactions is even more recent and has not been sufficiently explored. We aimed to improve the understanding of the photocatalytic nature of mineral dust exploring, particularly, the still devoid topic of the reactivity of organic compounds on these surfaces.

Chapter I presented the background to support the research and the methodology used in the following sections, also it underlined the atmospheric significance of the combination of the three objects of study of this work: mineral dust, organic compounds, and light. Chapter II briefly described the methodological approaches needed to conduct the experiments.

We initially demonstrated, in Chapter III, that mineral surfaces can uptake trace amounts of oxygenated volatile organic compounds (OVOCs). In Chapter III.a, we have chosen butanol as a proxy for an OVOC. The effects of environmental conditions have been explored: the uptake was maximized at lower temperatures and low humidity. Even though the heterogeneous removal efficiency due to the uptake onto dust is supposed to be significant only under high dust mass loadings, our study has shown that the incidence of light has a significant influence: the system goes from non-reactive to highly reactive. The release of carbonyl compounds to the gas-phase was revealing.

Based on the positive influence that irradiation presented for butanol uptake on Arizona test dust in the previous section, in Chapter III.b, we have studied another OVOC, acetone, using the same experimental procedure and conditions. Acetone, however, presented a very different uptake profile compared to butanol. For 0% relative humidity, 1.4 mW/cm² of irradiation, at 293 K, the butanol uptake coefficient (for initial concentration of 55 ppb) was 2.62 ± 0.87 x 10 -7 , whereas for acetone (at 70 ppb of initial concentration) it was 2.26 ± 0.17 x 10 -6 , that is, a difference of approximately an order of magnitude. The study with acetone also showed another meaningful difference: the influence of irradiation was less significant for the uptake, neither was found products released to the gas phase.

Nevertheless, acetone was efficiently removed from the gas phase mainly for ATD and TiO2 as substrates. For both VOCs studied, moisture represented a key parameter strongly interfering in the uptake kinetics on ATD. For this mineral, water has a preference for surface adsorption reducing drastically the capacity of interaction with the OVOCs.