Pr Jean-Marc Chovelon

Pr Villenave Eric

CR Bedos Carole

ECOSYS UMR1402, examiner FERRONATO Inrae Agroparistech

MCF Corinne

DR Mailhot Gilles

Richard Claire

Table of figures

Table of tables

Table 1 Air quality guidelines and standards of specific pollutants depending on the exposure time, source: [START_REF]Air quality guidelines. Global update[END_REF]

Abstract

The green roof system is an environmentally friendly approach to help mitigate air pollution. Many studies estimate its removal capacity towards several air pollutants such as particulate matter (PM), nitrogen oxides (NOx), ozone (O3), etc. using dry-deposition models. In our studies, our aim was to quantitatively determine the depolluting potential of green roof plant species towards two prevalent air pollutants: NO2 and O3, and to examine the photochemical interactions possibly taking place on the surface of leaves. In chapter one, we built an experimental setup that enabled us to measure the NO2/NO concentration produced by Imidacloprid (IMD), an insecticide. This served as the base of the main setup used to study the depollution ability of plants and helped to find a stable and continuous source of NO2 using a chemical rather than pressurized gas cylinders. In chapter 2, screening of 13 plants species was done. Their capacity to remove NO2 and O3 was measured which further led to the selection of the 3 most performant species: sedum sexangulare, thymus vulgaris, and heuchera Americana L. The uptake/removal mechanisms were investigated using scanning electron microscopy (SEM), and Automated thermal desorption GC-MS (ATD-GC-MS) to inspect possible interactions between the pollutants and volatile organic compounds (VOCs). In chapter 3, photochemical processes occurring between volatiles emitted by green roof plant species and pesticides sprayed were studied. Two pesticides were selected: IMD, a nitrating/nitrosating agent, and chlorothalonil (CT), a strong oxidant. Possible photoproducts were detected with their acute/chronic toxicities predicted using an ECOlogical Structure-Activity Relationship approach (ECOSAR). This work is a step forward towards the development of experimental measurements of green roof depollution performance. By understanding the uptake mechanisms and the photochemical interactions taking place, the choice of appropriate green roof plant species/pesticides can become possible. Ultimately, this will facilitate the deployment of green roofs in urban environments.

La végétalisation des toits est une approche respectueuse de l'environnement pour aider à atténuer la pollution de l'air. De nombreuses études ont démontré, sur la base de modèles, la capacité de ce système à éliminer les polluants atmosphériques tels que les particules en suspension (PM), les oxydes d'azote (NOx), l'ozone (O3), etc. Dans cette thèse, l'objectif était de déterminer quantitativement le potentiel dépolluant de différentes espèces végétales utilisées sur les toits verts à l'égard de deux polluants atmosphériques prédominants : NO2 et O3 et d'examiner les réactions photochimiques entre les métabolites secondaires des plantes et les pesticides. Dans le premier chapitre, nous avons développé un dispositif expérimental qui nous a permis de mesurer les quantités de NO2/NO produites par imidaclopride (IMD), un insecticide. Ce premier dispositif a servi de base à un deuxième système utilisé pour étudier la capacité de dépollution des plantes.

IMD a pu être utilisé comme source stable et continue de NO2. Dans le chapitre 2, le criblage de 13 espèces végétales a été effectué. Leur capacité à éliminer NO2 et O3 a été mesurée, ce qui a conduit à la sélection des 3 espèces les plus performantes : sedum sexangulare, thymus vulgaris et heuchera americana L. Les mécanismes d'absorption/élimination et les réactions possibles entre les polluants et les composés organiques volatils (COVs) ont été étudiés par microscopie électronique à balayage (MEB), piège à HONO, et par thermodésorption GC-MS (ATD-GC-MS).

Dans le chapitre 3, les réactions photochimiques se produisant entre les substances volatiles émises par les espèces végétales utilisées sur les toits verts et les pesticides pulvérisés sur ces plantes ont été étudiés. Deux pesticides ont été sélectionnés : IMD, un agent nitrant/nitrosant et chlorothalonil (CT), un oxydant fort sous irradiation. Les principaux photoproduits ont été détectés et leurs toxicités aiguës/chroniques prédites à l'aide du logiciel ECOlogical Structure-Activity Relationship approach (ECOSAR). Ce travail constitue une avancée vers le développement de mesures expérimentales des performances de dépollution par les toits verts. La compréhension des mécanismes d'absorption et les réactions photochimiques doit permettre le choix d'espèces végétales/pesticides les plus appropriés pour les toits verts. À terme, cela facilitera le déploiement de toitures vertes en milieu urbain.

Introduction

This PhD fellowship was supported by the University Clermont Auvergne/French Ministry of Higher Education and Research and undertaken in the Institute of Chemistry of Clermont-Ferrand (Photochemistry group/ Université Clermont Auvergne/ France) with Dr. Claire Richard and Dr. Mohamad Sleiman as supervisors.

Our goal was to understand the role of certain green roof plants in depollution; especially those implemented in urban environments. The atmospheric composition is in a constant change due to the continuous release of hazardous air pollutants as carbon dioxide (CO2), nitrogen oxides (NOx), ozone (O3), particulate matter (PM), etc. The excessive emission of pollutants is due to anthropogenic activities in urban areas as fossil fuels combustion, industries, incomplete fuel combustion in all transportation forms, etc. leading to the popular environmental problem: air pollution. This imposes a great threat to humans and the environment. For instance, millions of people die each year due to heart failures, central nervous system disorders, or cardiovascular diseases. This is all linked to the ambient air pollution which increases mortality and morbidity. In addition to that, the latter causes O3 depletion, climate change, urban heat island, etc. which are then again related to the human health deterioration. Therefore, an approach using vegetation to decrease the concentration of harmful pollutants can be promising. Green roofs make use of the numerous impermeable roofs found in urban areas where they can uptake air pollutants.

Green roofs are roofs with plants in their final layer. They are a good compensation to the vegetation that has been cut or destroyed during the building's construction. Their benefits include lessening the urban heat island (UHI), improving the stormwater runoff's quality, reducing noise pollution, and most importantly decreasing air pollution. In general, extensive green roofs are more preferred where certain types of plants are implemented as herbaceous perennials, sedum species, as well as grass. Numerous studies deal with the depollution ability of vegetation but not of green roofs specifically. Published articles on green roofs reveal their capacity to reduce NO2, CO2, O3, and PM. However, they were generally based on deposition models taking into consideration neither the possible interactions in the gaseous phase involving VOCs nor the latter's potential to interact with pesticides sprayed on the plants. This renders experimental data a must to fully understand the green roof system.

In this research, we first constructed an experimental setup that allowed us to screen and quantify the uptakes of NOx and O3, two detrimental pollutants to the health and environment by different plant species. Besides of having adverse health effects, these two pollutants are regulated and contribute to the oxidative capacity of the atmosphere. It was comprised of flow controllers, pollutants sources, reactors, and analyzers. Special attention was given to find sources of the pollutants. O3 was produced simply by the aid of an O3 generator equipped with a UV pen-ray lamp while NOx was produced via an Imidacloprid (IMD) solution. This insecticide was reported to have the ability of releasing the nitro group under irradiation. In our study, IMD dissolved in water with a small percentage of ACN was able to significantly produce NO2 with a small concentration of NO without irradiation. This setup enabled us to determine quantitatively the uptake capacity of the 3 chosen plants often used on extensive green roofs: sedum sexangulare, thymus vulgaris, and heuchera Americana L. The effect of plants volatile organic compounds on the uptake was elucidated and the analysis of the products formed between the plants volatiles and pollutants was conducted. Although many studies show the important role of VOCs in the life of plants or crops, the information about the interaction that could happen between vegetation and the urban environment is still lacking.

Plants are sprayed with pesticides to protect them from harmful pests. Although several studies report the ability of certain pollutants -acting as oxidants-to degrade pesticides or their photolysis in solution or on supports, none or few studies -to our knowledge-focused on the probable interaction between certain pesticides and VOC-emitting plants. Different pesticides are usually sprayed on green roofs or greenery systems; hence, the possibility of an interaction to happen between the former and VOCs produced is high. In this regard, we investigated the interactions of two pesticides: the insecticide Imidacloprid and the fungicide Chlorothalonil with Thymus Vulgaris VOCs (a plant frequently used on green roofs) in solution and directly on its leaves. Different photoproducts were detected using different techniques such as HS-GC-MS, GC-MS, and LC-HR-MS.

In summary, our work intended to focus on experimental studies poorly available in the literature. We started by investigating the capacity of green roofs plants to lessen the concentration of target pollutants. We took into account the different processes occurring responsible for the depollution ability. Afterwards, attention was given to the interaction of certain pesticides with the VOCs emitted by plants implemented on green roofs with the identification of possible formed photoproducts with their toxicities.

Bibliography 1.1 Air pollution increase

Economic developments and modernization have created a greater need of energy which in turn increased the use and combustion of fossil fuels. This is of a great concern since tremendous amounts of carbon dioxide (CO2), nitrogen oxides (NOx) particulate matter (PM), and ozone (O3) are being produced, contributing to major health risks and climate changes. Ambient air pollution is a topic vastly studied due to the lack in the homogeneity of its environmental and health effects.

In other words, the health effects inflicted in developing countries as China, India, or the Middle Eastern region vary from those in developed countries as United States of America or Europe. This is explained by the difference in the combustion sources used, energy use, lifestyle, etc. between countries which consequently affects the concentrations of the pollutants released [1]. Pollution in China is in general a result of coal combustion or coal burning [2] [START_REF] Cai | Impacts of coal burning on ambient PM2.5 pollution in China[END_REF]. However, heating and cooking, in other words-'residential energy demands', are one of the main causes of air pollution in India or Bangladesh [1]. In 2013, almost 90 % of the world's population lived in polluted zones that don't meet the World Health Organization (WHO) guidelines [START_REF] Brauer | Ambient Air Pollution Exposure Estimation for the Global Burden of Disease[END_REF].

Vegetation itself can affect the air quality by emitting Volatile Organic Compounds VOCs) which can react with NOx producing secondary pollutants thus changing the microclimate around it [START_REF] Weber | Ecosystem Services Provided by Urban Vegetation: A Literature Review[END_REF]. However, and fortunately, vegetation can also help reduce and remove gaseous air pollutants or PM. This topic has been subject of numerous studies [5] [6]where vegetation can directly uptake the pollutants by the absorption into their stomata or adsorption on their leaves where they might be further broken down (Fig. 1). Vegetation can as well remove pollutants indirectly by cooling the surrounding air via transpiration and shading, hence limiting photochemical interactions that form secondary pollutants as O3 [START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF]. Out of the numerous studies, a study in the USA showed that trees can eliminate up to 711,000 tons/year of air pollutants [START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF]. Another in London manifested that 2000 m 2 of uncut grass can remove about 4000 kg of PM [START_REF] Johnston | A guide to using plants on roofs, walls and pavements[END_REF]. Hence, vegetation can be a good sink to pollutants and a promising solution to air pollution.

Air pollutants of vast importance

1.2.1 Ground-level Ozone (O3)
O3 is a trioxygen gas with a pale blue color which is beneficial in the stratosphere forming a protective layer towards harmful UV light. However, it is considered as a pollutant in the troposphere with high health risks and a cause to air pollution (ground-level ozone) [START_REF] Tan | Exploring ozone pollution in Chengdu, southwestern China: A case study from radical chemistry to O3-VOC-NOx sensitivity[END_REF]. The low level O3 is not emitted directly from vehicle engines or industries, but it is rather a reaction between NOx, VOC, and sunlight [START_REF] Mauzerall | NOx emissions from large point sources: Variability in ozone production, resulting health damages and economic costs[END_REF]. Its formation is totally dependent on the season; in summer for example, it shows the highest concentrations with huge traffic jam, slow wind movement, and high UV index. Sources of the tropospheric O3 are fossil production, industries, transportation, smoking, chemical processes, etc. [START_REF] Mauzerall | NOx emissions from large point sources: Variability in ozone production, resulting health damages and economic costs[END_REF]. Nevertheless, a small amount of O3 in the troposphere is considered essential; it undergo photolysis in the presence of UV light with a wavelength ≥ 290 nm to produce the excited oxygen where different reactions can further take place [11][12]. it is the key to initiate tropospheric reactions (Fig. 2) [START_REF] Atkinson | Atmospheric chemistry of VOCs and NOx[END_REF].

Although it has been reported that plants remove pollutants, several studies show the threat that O3 imposes on certain plants which can vary from visible foliar injuries to the negative effects regarding photosynthesis and stomatal conductance [START_REF] Fredericksen | Size-mediated foliar response to ozone in black cherry trees[END_REF] [START_REF] Hoshika | Whole-tree water use efficiency is decreased by ambient ozone and not affected by O3-induced stomatal sluggishness[END_REF]. O3 can enter the leaves through stomata and change the membrane properties leading to phototoxicity in the leaf tissues [START_REF] Calatayud | Effects of 2-month ozone exposure in spinach leaves on photosynthesis, antioxidant systems and lipid peroxidation[END_REF] [START_REF] Torsethaugen | Ozone inhibits guard cell K+ channels implicated in stomatal opening[END_REF].

Since stomata are one of the ways that plants utilize to degrade or remove pollutants, the precision in choosing specific plants for a target pollutant becomes essential. Out of 473 plant species studied, 378 showed sensitivity towards O3. However, environmental stress can induce the production of VOCs which may contribute to air pollution through their interactions with NOx [START_REF] Heiden | Emission of volatile organic compounds from ozone-exposed plants[END_REF][19] or may react with O3 thus decreasing its ambient concentration .

Nitrogen oxides (NOx)

The symbol 'NOx' mainly marks NO and NO2 since they reach the troposphere in those forms-called as the fresh nitrogen oxides. Modernization created a high demand for fossil fuels; their combustion generates high amounts of pollutants especially nitrogen oxides. Their sources, particularly anthropogenic, are vast; they include incineration [START_REF] Gómez-García | Pollution by nitrogen oxides: An approach to NOx abatement by using sorbing catalytic materials[END_REF], the use of nitric acid [START_REF] Skalska | Effectiveness of nitric oxide ozonation[END_REF], industries. Out of the nitrogen oxides, NO and NO2 are considered toxic [START_REF] Skalska | Trends in NOx abatement: A review[END_REF]. NOx can react with VOCs or sulfur dioxide (SO2) producing tropospheric ozone, smog, or acid rain. Fig. 3 shows the photolysis of NO2 giving NO and O( 3 P) where the latter further reacts with O2 to produce O3. But, NO can react with O3 regenerating NO2 and O2 creating an equilibrium between the formation and destruction of O3 (①in figure 3). However, this photo-equilibrium is disturbed by the presence of VOCs. The substituted alkyl peroxy radicals (RO2 • ) and HO2 • (hydroperoxyl) radicals formed due the degradation of VOCs can react with NO generating NO2 which can undergo photolysis again and hence form O3 (② in figure 3). If no NO or low concentrations of NO are present, the radicals might attack O3 leading to O3 loss. So, in this case, the formation or loss of O3 is driven by the NO concentrations as well as the rate of the reaction between the radicals and NO. Nevertheless, the quantity of O3 formed is limited and stopped by radical sinks, NOx sinks, and other possible reaction pathways involving NOx. They can influence the amount of NO converted to NO2 and • OH concentrations; and since • OH initiates photochemical reactions, the system or cycle is fully interrupted. In addition to that, NO and NO2 can directly react with O3 forming NO2 and NO3 respectively where the latter can react together giving N2O5 (dinitrogen pentoxide) or with • OH radicals leading to HONO (nitrous acid) and HNO3 (nitric acid) respectively. Vegetation or green roofs can help decrease the concentration of NOx. For example, a study in Chicago using deposition models revealed that with a 19.8 ha of green roofs, around 1700 kg of pollutants was removed with the NO2 percentage uptake being 27% [START_REF] Yang | Quantifying air pollution removal by green roofs in Chicago[END_REF]. Branches, twigs, and leaves provide a high surface area which is available as a sink for pollutants; and since NO2 is soluble in water, it is expected that the NO2 absorbed into the stomata is metabolized [START_REF] Hill | Vegetation: A Sink for Atmospheric Pollutants[END_REF]. Hence, vegetation can rather degrade NO2 than act as a temporary sink where pollutants are washed off by rain entering the soil and water system.

Particulate matter (PM)

PM consists of tiny particles or liquid droplets of soil, dust particles, chemicals, etc. [START_REF]Particulate Matter| Air & Radiation[END_REF] and categorized based on their diameter (size); those having a diameter of 10 µm or less, 2.5 µm or less, and 0.1 µm or less (PM10 coarse, PM2.5 fine, and PM0.1 ultrafine respectively). The PM's source and their behavior in human airways helps as well with this categorization. Sources include combustion, vehicle emissions, tobacco smoke, volcanoes, dust storms, etc. PM are considered dangerous since certain gases like O3 can adsorb on their surface reaching the lungs after inhalation. In the USA, the major PM2.5 source is due to the coal burning to produce electricity (fuel combustion) followed by residential wood burning (Fig. 4). This makes it difficult to avoid the exposure to PM2.5; and since PM can be carried over large distances and secondary particles can be formed in the atmosphere (primary particles are those emitted directly from sources), having policies for reducing PM levels is tricky. However, there was a 23 % decrease in the PM2.5 national average from 2010 to 2019 [START_REF]Particulate Matter (PM2.5) Trends[END_REF]. A study in the winter of 1993-1994 showed low PM10 concentration in the north of Europe (20 µg/m 3 mean urban value) whereas higher concentrations were found in Amsterdam, Berlin, and central Europe (40-50 µg/m 3 ). Concentrations were even more elevated in southern Europe. However, little to no difference was detected within the countries considering rural areas [START_REF]Air quality guidelines. Global update[END_REF].

Vegetation is a promising approach to attenuate PM's health negative effects, in particular, that related to the decrease in the life expectancy especially concerning vulnerable people [START_REF]Health aspects of air pollution with particulate matter, ozone and nitrogen dioxide[END_REF]. The size and structure of leaves is an important factor in determining the removal ability [START_REF] Fowler | Deposition of atmospheric pollutants on forests[END_REF]. For example, trees are very efficient in offering a space for PM deposition due to the large leaf area. A study in the USA showed that urban trees and shrubs remove almost 220,000 tons of PM10 each year [START_REF] Nowak | Air pollution removal by urban trees and shrubs in the United States[END_REF] while another in China indicated the removal of 772 tons of PM10 in the city center of Beijing [START_REF] Yang | The urban forest in Beijing and its role in air pollution reduction[END_REF]. Accumulation of PM is driven also by the leaf's surface hair density and wax quantity [START_REF] Saebø | Plant species differences in particulate matter accumulation on leaf surfaces[END_REF]. Vegetation targeting PM is able to remove as well polycyclic aromatic hydrocarbons (PAH) and heavy metals.

Impact of air pollution

On the well-being

Air pollution causes millions of deaths each year where 3.7 million people pass away due to ambient air pollution, and 4.3 million die from household pollution [START_REF]Air pollution: Consequences and actions for the UK, and beyond[END_REF]. Its harmful effects are vast varying from the long-term exposure effects to the short-term ones; but it the long-term effects that matter the most [START_REF] Ghorani-Azam | Effects of air pollution on human health and practical measures for prevention in Iran[END_REF]. Short-term exposure effects are manifested by headaches, nose or throat inflammation, skin irritation, etc. while those of long-term are associated with heart failures, cardiovascular diseases, central nervous system disorders, hospital admission increase, and reproductive system failure (Fig. 5). For example, a study showed the direct link between air pollution and male infertility [START_REF] Zhou | Air pollution and decreased semen quality: A comparative study of Chongqing urban and rural areas[END_REF].

Each pollutant, nevertheless, can undesirably affect the body in different ways. For instance, PM is known to be associated with cardiovascular morbidity and mortality which is responsible for almost 3 million deaths each year [START_REF] Brook | Particulate matter air pollution and cardiovascular disease: An update to the scientific statement from the american heart association[END_REF]. O3, on the other hand, mostly causes lung impairments which affects the overall health, respiratory and cardiovascular diseases and acute exposures lead to headaches, wheezing, and breathing problems [START_REF] Zhong | Sensitivity of projected PM2.5 and O3-related health impacts to model inputs: A case study in mainland China[END_REF]. Also, short or long term exposure to NO2 leads to problems such as respiratory/cardiovascular diseases and increased mortality [START_REF] Chaloulakou | Compliance with the annual NO2 air quality standard in Athens. Required NOx levels and expected health implications[END_REF]. NOx is also a precursor to tropospheric ozone which has great impact on health. Air pollution, in general, increases mortality and morbidity.

Climate change

A noticeable change in the global and regional climates is present starting from mid to late 20th century onwards. This is connected to the increased emissions of greenhouse gases compared to pre-industrial period or in other words, 'enhanced greenhouse effect'. Major greenhouse gases are CO2 and water alongside with CH4, O3, and N2O. Increasing the amounts of these gases due to industrialization and human activities increases their role in the greenhouse effect thus leading to an enhanced greenhouse effect. Global average surface temperature is elevating which contribute to the melting of the Antarctic Sea ice and hence affecting sea levels [START_REF] Chao | Scientific basis of climate change and its response[END_REF]. Extreme weather/climate events (hurricanes, floods, droughts, etc.) have been happening changing precipitation. This all affects biological cycles, food production, health and economy. In addition to the global climate change, urban heat island (UHI) is an important consequence of air pollution. UHI is an urban area with a temperature higher than usual, or higher than rural areas due to human activities [START_REF] Kim | Urban heat island[END_REF] as shown in Fig. 7. During the day and by the mid-morning, heating starts to take place and the temperature can be warmer by 10⁰C compared to rural areas. This phenomenon is increasing and it is expected that by 2050, 2/3 of the world population will be living in urban areas [START_REF] Santos | Urbanization and climate change: buildings and GHG[END_REF]. Increased temperatures will also lead to higher energy demands thus elevating more the already present air pollution. This transformation of the natural surfaces into construction spaces has influenced the thermal balance.

One solution to this is the use of roofs with high albedos like white roofs. Certain studies showed that increasing the albedo can mitigate the UHI. For example, the peak summertime temperatures were decreased by 1.5⁰C in Los Angeles [START_REF] Sailor | Simulated urban climate response to modifications in surface albedo and vegetative cover[END_REF] and by 1-2⁰C in Athens, Greece [START_REF] Synnefa | On the use of cool materials as a heat island mitigation strategy[END_REF]. However, reflective surfaces may have some drawbacks manifested by the decrease in efficiency in terms of UHI mitigation in winter. Hence, an alternative solution can be vegetation or green facades. It doesn't only compensate the lost or destroyed vegetation when a certain building was being constructed but also by uptaking pollutants through their stomata, deposition on their surfaces of leaves, or by breaking down some contaminants in their plant tissues [START_REF] Baker | Terrestrial higher plants which hyperaccumulate metallic elements, A Review of their distribution, ecology, and Phytochemistry[END_REF]. They can also indirectly decrease the production of secondary air pollutants by lowering the microclimate around them due to transpiration and shading properties. Green roofs, specifically, can reduce the heat transfer through the roof thus reducing the energy demands for cooling the building [START_REF] Palomo | Analysis of the green roofs cooling potential in buildings[END_REF]. A study in Madrid showed the reduction of 6 % in the cooling demand in a big residential building [START_REF] Saiz | Comparative life cycle assessment of standard and green roofs[END_REF].

Ozone depletion

In the stratosphere, O3 forms a protective layer against harmful UV light coming from the sun. However, studies have shown that ODSs or ozone depleting substances have been getting emitted into the atmosphere the last couple of decades [START_REF] Singh | Atmospheric burden of ozone depleting substances (ODSs) and forecasting ozone layer recovery[END_REF]. Compounds that have chlorine, fluorine, and carbons (CFCs) or those produced by industries or human activities lie in the ODSs category [START_REF] Singh | Atmospheric burden of ozone depleting substances (ODSs) and forecasting ozone layer recovery[END_REF] [START_REF] Kellmann | Global CFC-11 (CCl3F) and CFC-12 (CCl2F2) measurements with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS): Retrieval, climatologies and trends[END_REF]. They are contained in the troposphere and then slowly carried to the stratosphere by air movements. Naturally, the ozone production and destruction rates in the stratosphere are equal but, when the destruction is favored over production, depletion arises [START_REF] Angell | Quasi-biennial and long-term fluctuations in total ozone[END_REF]. This leads to a serious threat to human health indirectly due to the diminishing of the ozone layer leading to an increase in the UV radiation (cancer, weakened immune system, cataracts), or directly if some substances have accumulated in the troposphere [START_REF] Singh | Atmospheric burden of ozone depleting substances (ODSs) and forecasting ozone layer recovery[END_REF]. It also affects plants and animals.

Catalytic processes of O3 depletion:

Overall: O3 + O → 2O2 (middle and upper stratosphere) Overall: 2O3 → 3O2 (lower stratosphere)

Acid rain

Pollutants involved in this process are NOx and (SOx) sulfur oxides; they undergo a series of reactions yielding harmful amounts of nitric and sulfuric acids. Escalated anthropogenic production of the acids responsible for acid rain is due mainly to industrialization and urbanization [START_REF] Du | Direct effect of acid rain on leaf chlorophyll content of terrestrial plants in China[END_REF]. Pollutants principally react with oxygen and water-reactants found abundantly in the atmosphere-producing protons and lowering the pH [START_REF] Singh | Acid rain and its ecological consequences[END_REF]. Europe and north America are the biggest regions to receive acid rain [START_REF] Wei | Effects of simulated acid rain on soil fauna community composition and their ecological niches[END_REF], this is related to the economic and technological developments.

Acid rain can influence plants' photosynthesis and transpiration [START_REF] Du | Direct effect of acid rain on leaf chlorophyll content of terrestrial plants in China[END_REF]. It may speed the destruction of buildings, statues, and sculptures; or damage lakes, ponds, and soils; as well as decrease or cause the loss of biodiversity, or even affect humans indirectly because of the heavy metals liberated from soils and leached into underground waters consumed and used by humans [START_REF] Singh | Acid rain and its ecological consequences[END_REF].

Control strategies

Different strategies have been adopted to try to decrease air pollution or improve the air quality varying from source reduction strategies to depollution approaches. However, having regulations and air quality managements is an important step to understand the technique to be implemented based on the target pollutant and its accepted ambient concentration (Table 1). The purpose of air quality management is to preserve the human health as well as the environment from the unfavorable effects of air pollution. Air quality standards-the basis of air quality managementdefines the limits on a pollutant atmospheric concentration or the allowable air pollution level in a specific country or region which are adopted by regulatory authorities [START_REF]Air quality guidelines. Global update[END_REF]. Guidelines of a specific pollutant depend on diverse factors as the pollutants properties, its health effects, its acceptable risk level, the feasibility of compliance with the standards, and the decision uncertainty regarding the standards. In general, standards vary in their averaging times.

For example, a 1 h average time is created to protect the population from peak concentrations (acute exposures) while the longer average times are suitable for chronic exposures and their irreversible adverse health effects [START_REF]Air quality guidelines. Global update[END_REF]. PM2.5 is known to cause health effects after short or long exposures; hence, guidelines were formed to protect from acute and chronic exposures. A study showed that in China and after implementing a policy or an air pollution regulation in 1998 that imposes strict regulations on emissions from power plants, infant mortality decreased by 20 % especially during the neonatal period [START_REF] Tanaka | Environmental regulations on air pollution in China and their impact on infant mortality[END_REF]. Implementing guidelines is beneficial for the public health; however, inconsistency between them is an issue. For instance, the maximum exposure to NO2 during 1 year is 40 µg/m 3 according to the WHO standards but this concentration can be exceeded up to 100 µg/m 3 in the USA; likewise, O3 and PM2.5. Those differences are due to the geographical locations, meteorological parameters, air pollution extent, the concentration of the target air pollutant, the health of a specific nation, etc. For that, some counties impose specific regulations based on their situation to protect their people.

Reduction strategies

Several interventions have been implemented to decrease ambient air pollution varying from source reduction strategies as decreasing the number of vehicles to depollution approaches. These strategies have been evaluated to assess their effectiveness. Some approaches are explained in detail in the following paragraphs: selective catalytic oxidation and photocatalysis.

Selective catalytic reduction

The gas exiting the exhaust of pipes in industries or power plants contains mainly water vapor, oxygen, NOx, etc. The reaction to happen to abate NOx is the transformation of NO into O2 and N2. However, in an oxidizing medium, NO reacts with O2 producing NO2, and hence this reaction is marked unsuccessful [START_REF] Guan | Review of state of the art technologies of selective catalytic reduction of NOx from diesel engine exhaust[END_REF]. A reductant such as anhydrous ammonia (NH3) with a catalyst is required. The reduction catalyst should be active and selective towards N2 and stable when unfavorable compounds are present. Some drawbacks of this technique as limited catalyst life duration, high costs of catalysts, and high temperature required are present; but its good efficiency and relatively simple installation makes it a good method for removing NOx [START_REF] Guan | Review of state of the art technologies of selective catalytic reduction of NOx from diesel engine exhaust[END_REF].

Photocatalysis

Building surfaces or walls can be covered with a photocatalytically active material which oxidizes/reduces pollutants in the presence of sunlight [START_REF] Chen | Photocatalytic construction and building materials: From fundamentals to applications[END_REF]. The covered surfaces, however, should be as close to the source of pollutants as possible. Titanium dioxide (TiO2) is used as a catalyst which can transform NOx to NO3 -for example after being activated by UV light [START_REF] Cassar | Photocatalysis of cementitious materials[END_REF]. Recent studies show the possibility of using visible light which makes its application indoors or in tunnels more viable [START_REF] Fujishima | Titanium dioxide photocatalysis: present situation and future approaches[END_REF]. This heterogeneous photocatalysis is widely used and its efficiency is related to the catalyst ability to create free radicals which further reacts with the pollutants. The latter can be removed by washing the surface or by rain. Fig. 8 shows the photocatalysis mechanism. The process starts after TiO2 absorbs an energy greater than its band-gap energy and gets excited. This generates electrons and holes pairs that move to the TiO2 surface and undergo redox reactions with a probable formation of superoxide radical anions and hydroxyl radical ( • O2 -and • OH respectively). The latter are the ones responsible for the degradation of organic pollutants in water which can further produce CO2 and H2O [START_REF] Feng | One-dimensional nanostructured TiO2 for photocatalytic degradation of organic pollutants in wastewater[END_REF].

Other simple ways to reduce air pollution are to favor use of bikes or walking, taking buses or carpooling (BlaBlaCar in France which is an online marketplace for carpooling), turning off appliances when not in use, and most importantly raising awareness towards the deleterious effects of air pollution [START_REF]Reducing Air Pollution[END_REF]. Nevertheless, it is worth to mention the emergence of the COVID-19 virus which froze all economic activities and social events during the past two years. Although the pandemic led to a fall in the economy [START_REF] Barua | COVID-19 implications for banks: evidence from an emerging economy[END_REF][68], environmentalists call this pandemic a 'blessing in disguise'. Lockdowns and strict regulations have given rise to a reduction in all pollutants emissions [START_REF] Muhammad | COVID-19 pandemic and environmental pollution: A blessing in disguise?[END_REF] since public mobility ceased and airlines shut down. This caused a significant decrease in greenhouse gases (GHGs) concentration and elucidated as well that outdoor air pollution and the cause of premature deaths each year are mainly due to human activities [70][71].

As an example, NO2 emissions were reduced by 30 % in Wuhan comparing the periods of before and after the lockdown. Similarly, France encountered a 20-30 % decrease especially in Paris and other major cities [START_REF] Muhammad | COVID-19 pandemic and environmental pollution: A blessing in disguise?[END_REF]. In general, air pollution decreased worldwide. The pandemic has given nature a chance to recover, but this constructive effect is temporary since governments can't force people to be in an endless lockdown. So, strategies to decrease air pollution should remain in progress varying from source-reduction to depollution approaches.

Vegetation as a depollution strategy

Since source reduction strategies can be expensive or inefficient, depollution methods can be beneficial. An interesting ecological depollution approach can be the use of vegetation.

Vegetation is a general broad term targeting all plant species without referring to a specific type or characteristic. Numerous studies reflect the ability of plants or trees to remove pollutants through various mechanisms which are described in the following paragraphs [START_REF] Baker | Terrestrial higher plants which hyperaccumulate metallic elements, A Review of their distribution, ecology, and Phytochemistry[END_REF].

Mechanisms of pollutant uptake by vegetation

Absorption into stomata

Stomata are small openings found in plant tissues surrounded by certain cells which control the opening and closure of the pore. The latter regulates gas exchange between the plant and surrounding environment thus controlling the atmospheric environment [START_REF] Gray | Plant Development: Three Steps for Stomata[END_REF]. Stomatal pores absorb CO2 and transpire water, a process called photosynthesis [START_REF] Cowan | The Relative Role of Stomata in Transpiration and Assimilation[END_REF]; they are necessary plant organs which help plants evolve according to the environment around or environmental stresses [START_REF] Mansfield | Chapter 10: The role of stomata in resistance mechanisms[END_REF]. Since they control the absorbance and quantity of CO2 entering leaf tissues, they can play a role in uptaking or removing air pollutants. Studies show that when pollutants as NO2, O3, or SO2 are present, uptake takes place (Fig. 9) but followed by a plant damage manifested by the closure of stomata [START_REF] Robinson | Disturbances In stomatal behaviour caused by air pollutants[END_REF]. It has been concluded that NO2 uptake is determined by the stomatal aperture and that cuticular deposition contributes not more than 5 % [76] [START_REF] Geßler | NH3 and NO2 fluxes between beech trees and the atmosphere-correlation with climatic and physiological parameters[END_REF]. One study revealed than at high NO2 concentrations, cuticular deposition was 2 times less than that into the stomata [START_REF] Wellburn | Why are atmospheric oxides of nitrogen usually phytotoxic and not alternative fertilizers ?[END_REF]; and another regarding tomato showed that it is 4 times less [START_REF] Kerstiens | Dry deposition and cuticular uptake of pollutant gases[END_REF]. However, at high humidity, water films can form on the leaf surfaces acting as sinks to NO2 [START_REF] Theone | Absorption of atmospheric NO2 by spruce (Picea abies L. Karst) trees. I. NO2 influx and its correlation with nitrate reduction[END_REF][80] [START_REF] Burkhardt | Thin water films on coniferous needles (With an Appendix 'A new device for the study of water vapour condensation and gaseous deposition to plant surfaces and particle samples[END_REF] . NO2, in general is known to be less toxic to plants than O3 or SO2 [START_REF] Addison | Effect of Sulfur Dioxide on Woody Boreal Forest Species Grown on Native Soils and Tailings[END_REF]. O3 can decline photosynthesis due to its direct effect on stomata [START_REF] Reichenauer | The influence of ambient and elevated ozone concentrations on photosynthesis in Popuius nigra[END_REF]. A research conducted on seedlings where the latter was exposed to 0.3 ppm of NO2 and a mixture of 0.3 ppm NO2 and 0.1 ppm O3, showed that the grown trees that were exposed to NO2 were healthy. However, those that were exposed to the mixture had visible injuries and smaller leaves [START_REF] Okano | Differences in Ability of NO2 Absorption in Various Broad-leaved Tree Species[END_REF].

Adsorption on leaf surface

Adsorption is the adhesion or deposition of certain molecules to a surface. Unlike absorption, adsorption is a surface phenomenon which creates an adsorbate film on the adsorbent.

It can be either due to Van der Waals forces or covalent bonding. Plants or vegetation (adsorbent) intercept/adsorb pollutants (adsorbate) by their leaves, branches, barks, etc. due to the large surface area available. PM can be retained by vegetation surfaces for the moment before being washed out by rain or carried by wind to the soil or the atmosphere [START_REF] Przybysz | Accumulation of particulate matter and trace elements on vegetation as affected by pollution level, rainfall and the passage of time[END_REF]. The deposited PM can be as well absorbed through the plant's external layer or penetrate the stomata [START_REF] Burkhardt | Particulate pollutants are capable to 'degrade' epicuticular waxes and to decrease the drought tolerance of Scots pine (Pinus sylvestris L.)[END_REF]. Particles can be removed from the atmosphere by wet deposition (by precipitation), or dry deposition (settling of particles to be collected/intercepted). Since trees have large surface areas, they are capable of collecting particles more than any other form of vegetation [START_REF] Gallagher | Measurements of aerosol fluxes to speulder forest using a micrometeorological technique[END_REF]. It has been indicated that the deposition velocity of aerosols in the woods is 3 times higher than that in a grassland [START_REF] Fowler | Measuring aerosol and heavy element deposition on urban woodland and grass using inventories of 210Pb and element concentrations in soil[END_REF]. A study in Chicago revealed that by increasing the tree cover from 19% to 24%, a 21% increase of PM10 removal from the atmosphere can be attained [START_REF] Mcpherson | Chicago's Urban Forest Ecosystem: Results of the Chicago Urban Forest Climate Project[END_REF]. Another in Philadelphia, using a model, displayed that the present tree cover can reduce PM10 by 0.72%; however, it might reach 12% in woody areas [START_REF] Nowak | Modeling the effects of urban vegetation on air pollution[END_REF].

In addition to that, O3 can be deposited on the surfaces of plants. A study considering cuticles and O3 showed that it can undergo deposition to be further destroyed or might enter the latter [START_REF] Kerstiens | Interactions between ozone and plant cuticle I. Ozone deposition and permeability[END_REF]. Hair present or plants with irregular surface features can augment the deposition velocity by providing a greater surface area. It is important to understand the dry deposition process since it is a way to remove pollutants from the air thus improving the air quality but at the same time, some plants can be harmed especially if the pollutant enters the stomata affecting photosynthesis [START_REF] Clifton | Removal of Ozone Air Pollution by Terrestrial Ecosystems[END_REF]. So, vegetation can remove pollutants from polluted air by absorption or adsorption which might be followed by a series of reactions.

Interaction of plant volatile metabolites with atmospheric pollutants

VOCs are principally present in the gas phase including a vast variety of compounds with boiling points ranging between less than 0 ⁰C to 400 ⁰C [START_REF] Clifton | Removal of Ozone Air Pollution by Terrestrial Ecosystems[END_REF]. Plants use VOCs as a sort of communication among each other to transmit information in case of the presence of stress factors [95][96]. The latter are not only scents but rather warning signals to other plants or a source of attraction to pollinators [97][98]. Since vegetation emit VOCs and is known to remove pollutants, a certain interaction between VOCs and pollutants can occur. Thyme is a perennial plant used on green roofs and emits various VOCs such as thymol [START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF]. A study involving the catalytic ozonation of thymol reported the degradation of the latter where O3 attacked the para and ortho positions of thymol followed by several reactions [START_REF] Wang | Catalytic ozonation of thymol in reverse osmosis concentrate with core/shell Fe3O4@SiO2@Yb2O3 catalyst: Parameter optimization and degradation pathway[END_REF]. O3 can react with terpenes indoors producing aldehydes and PM [START_REF] Weschler | Production of the Hydroxyl Radical in Indoor Air[END_REF][101], this was in line with a study which indicated an increase in PM when O3 and terpenes were concurrently present [START_REF] Weschler | Indoor ozone/terpene reactions as a source of indoor particles[END_REF]. Cleaning events as well using a detergent containing VOCs, can form PM [START_REF] Chen | Source strength of ultrafine and fine particle due to Chinese cooking[END_REF]. This reaction can form many radical species including • OH which might then react with other compounds or pollutants [104][105]. Hence, O3 reacts with VOCs; this reaction leads to the formation of PM or other health-depleting pollutants. PM however, would deposit on the surface of leaves to be further followed by different reactions (Fig. 11) [86][87]. As previously mentioned, plants emit VOCs due to environmental stress. An example is isoprene; its emission is widespread in plants. Its role is still unclear, but studies suggest its ability to increase the thermotolerance of plants and hence, protects plants from environmental stress.

Isoprene plays a role in atmospheric chemistry where it can either breakdown in the presence of NOx and sunlight and produce O3 or can directly react with O3 [START_REF] Chameides | The role of biogenic hydrocarbons in urban photochemical smog: Atlanta as a case study[END_REF]. This reaction can form formaldehydes or other oxidative species. Nevertheless, isoprene can contribute to the O3 resistance in plants; it can scavenge O3 in leaves yielding less toxic products [START_REF] Sauer | Formation of hydrogen peroxide in the ozonolysis of isoprene and simple alkenes under humid conditions[END_REF]. To elucidate this, leaves of two ozone-sensitive genotypes of tobacco were exposed to O3 with and without the presence of isoprene (these plants don't emit isoprene). Results showed that when no isoprene was present, acute and short exposures (300 ppb, 3h) to O3 decreased photosynthesis which was worse 12 h after the treatment ended [START_REF] Haggestad | Origin of Bel-W3, Bel-C and Bel-B tobacco varieties and their use as indicators of ozone[END_REF]. However, no important decrease in photosynthesis was reported when isoprene (3 ppm) was present at the end of exposure; but, a decrease was observed 12 h after ending the treatment but to a lesser extent. Hence, this study proves the protective effect isoprene has towards plants exposed to O3. In order to fully understand the role of BVOCs, other types should be considered regarding their interactions with O3.

A study involving the interaction between O3 and selected terpenes (d-limonene, αterpinene, and α-pinene) in an indoor environment showed that that the particle concentrations were the highest with d-limonene [START_REF] Weschler | Indoor ozone/terpene reactions as a source of indoor particles[END_REF] followed by α-terpinene and α-pinene. D-limonene is often found indoors since it is used in cleaning products giving a citrus scent [START_REF] Brown | Concentrations of Volatile Organic Compounds in Indoor Air-A Review[END_REF][110], and its interaction with O3 was more significant than the one with α-terpinene even if it reacts 100 times slower than the latter [START_REF] Shu | Rate Constants for the Gas-Phase Reactions of O3 with a Series of Terpenes and OH Radical Formation from the O3 Reactions with Sesquiterpenes at 296 ± 2 K[END_REF]. This might be explained by the production of many condensable products in the case of limonene. In their experiments and in simulated O3-terpene conditions, the formation of particles was as high as 95 µg/m 3 . However, under typical indoor conditions, the production was between 2.5 and 5.5 µg/m 3 . Nonetheless, it was necessary to understand the course of the interaction since terpenes are emitted abundantly indoors, especially limonene which was found in more than 50 % of monitored buildings according to the EPA's BASE study [START_REF] Kwtham | A detailed study of Volatile Organic compounds of 5-air conditioned building in Singapore[END_REF]. O3, as well, can be present in the buildings due to the air exchange and other sources such as photocopiers, air purifiers, and electrostatic air cleaners [START_REF] Sheldon | Measurement of Indoor Air Emissions from Dry-Process Photocopy Machines[END_REF]. Hence, all chemicals needed to form particles can be found indoors [START_REF] Weschler | Potential interactions among indoor plants[END_REF], the place where people spend more than 90 % of their time [START_REF]Human Exposure Assessment for Airborne Pollutants: Advances and Opportunities[END_REF].

Nevertheless, another study confirmed this interaction which used the new state-of-the-art Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) in the French Landes to identify gas-phase molecules emitted by the Landes forest [START_REF] Li | Terpenes and their oxidation products in the French Landes forest: Insights from Vocus PTR-TOF measurements[END_REF]. The latter produces large quantities of monoterpenes and hence, can be a good site to study the Biogenic volatile organic compounds (BVOC). The high precision of the instrument compared to the conventional one detected many hydrocarbons (monoterpenes) and VOC oxidation products. Reaction products having a broad daytime peak were the more oxidized terpene reaction products which undergo a strong photochemical reaction during the day. However, the less oxidized products had peaks in the early morning or the evening. The main oxidants were O3, OH, and NO3 radicals. An ozonide is formed following the reaction of O3 with unsaturated BVOCs which further decomposes to a carbonyl as seen in Fig. 12 by pathways 1 and 2 [START_REF] Atkinson | Gas-phase tropospheric chemistry of biogenic volatile organic compounds: A review[END_REF]. The reaction involving the formation of an epoxide occurs with a low yield especially with α-pinene and isoprene [118][119].

Nevertheless, reactions of O3 with alkenes produce • OH [120][121]; the • OH has a great oxidizing capacity and can assist in the formation of NOx for example which is considered as a precursor to PM formation.

Literature review on the pollutant removal ability of vegetation

Despite the strategies implemented to decrease air pollution, human activities especially in urban areas continue to produce enormous amounts of air pollutants which causes humans to live in a poor air quality zone. For example, in 2012, 60-80 % European urban residents were living in a region with the pollutants concentration exceeding the European Union (EU) and WHO limitvalues [START_REF]Air quality in Europe -2014 report[END_REF]. In 2011, PM2.5 concentrations triggered around 460,000 premature deaths in Europe.

In France and in 2012, 5 % of its population living in urban areas were subject to high PM10 concentrations. In Paris particularly, an increase in NO2 and PM increased the number of hospital admissions and consultations linked to asthma attacks [START_REF] Chatignoux | Expositions à la pollution atmosphérique et recours aux urgences pour pathologies respiratoires chez les enfants en Île-de-France[END_REF]. Some research studies display that using new materials in constructing a building can reduce air pollution [START_REF] Chen | Photocatalytic construction and building materials: From fundamentals to applications[END_REF][124] [START_REF] Ângelo | An overview of photocatalysis phenomena applied to NOx abatement[END_REF] while others reflect the importance of using vegetation to mitigate the negative air pollution effects [31][126].

Naturally, urban areas still have an 'urban' forest including all vegetation and soils already integrated into the roads, power lines, and infrastructure maintaining -to a certain extent-the quality of life [START_REF] Mcpherson | Chicago's Urban Forest Ecosystem: Results of the Chicago Urban Forest Climate Project[END_REF]. However, planting more trees is beneficial due to the various advantages vegetation imposes varying from carbon sequestration, air pollution reduction (can remove SO2, NOx, O3, etc.), temperature regulation (due to shading and evapotranspiration thus decreasing energy demands), and climate change diminution [31][32]. Vegetation can also serve recreational, cultural, and aesthetic needs.

Countless studies deal with the ability of vegetation, specifically trees, to remove air pollutants as O3, NO2, PM10, and SO2. A study in the USA approximated that urban trees remove 711,000 tons of pollutants with an annual economic benefit of 3.8 billion $. They were able to remove 305,000 tons of O3, 98,000 tons of NO2, and 215,000 tons of PM10 [START_REF] Nowak | Air pollution removal by urban trees and shrubs in the United States[END_REF]. Their results were based on a modelling study consisting of meteorological and air pollution concentration data obtained. However, the pollution removal rates vary among different cities; this is due to the difference in the tree cover, pollution degree, meteorological variables, and leaf-off/leaf-on seasons which all affect deposition velocities. They, in general, uptake pollutants through their stomata, or by the pollutants deposition on the leaf surface. Once on the leaf surface, pollutants might enter the intercellular spaces reacting with the water present inside or might just be retained on the surface to be discharged again to the atmosphere [START_REF] Smith | Air pollution and forests: interactions between air contaminants and forest ecosystems[END_REF]. Another research in Beijing [START_REF] Yang | The urban forest in Beijing and its role in air pollution reduction[END_REF], the capital of China and one of the top 10 most air polluted cities in the world, reflected that the urban forest can store around 225,000 tons of carbon in biomass thus decreasing the amount of CO2 in the atmosphere which might reduce the global climate change. Older trees were bigger and able to store more carbon than younger ones; thus old 'unchanged' cities can remove more carbon as an overall. Trees were able to eliminate 1260 tons of pollutants from the atmosphere in one year with the PM10 removal being 61 %. This might be due to the fact that PM10 is a main pollutant in Beijing.

Nevertheless, they also removed significant amount of O3, NO2, ad SO2. It is important to state that removal rates depend on the season; seasonal variations affect leaf expansion which in turn influences the total tree area available. Seasonal variations also mean a change in precipitation patterns and the climate generally. The study also compared the pollution removal rates with those of different American cities and showed that the urban forest in Beijing had a higher rate which is explained by the higher concentration of air pollutants present. Nevertheless, trees can indirectly decrease air pollution by decreasing the temperatures leading to less air conditioning and energy demands; 16.4 % cover of trees/shrubs can decrease the temperature by 1.6⁰C.

In Europe, particularly in Strasbourg-France, a study highlighted that trees in public green spaces have the ability to remove around 88 tons/year (between 2012 and 2013) of pollutants varying from CO, NO2, O3, SO2, and PM. The maximum uptake was for O3 followed by PM (PM10 and PM2.5) and then NO2 being 55.8 tons/year, 16.2 tons/year, and 13.8 tons/year respectively [START_REF] Selmi | Air pollution removal by trees in public green spaces in Strasbourg city, France[END_REF].

Hence, big polluted cities started integrating and planting more trees as seen in Fig. 13. Pollutants mitigation was more efficient during the leaf-on season recording an 81 % of pollutant removal. This is explained by the greater surface area available, the warmer temperatures, the lower wind speed, as well as the more considerable stomatal conductance which all increase the concentrations and deposition velocities of pollutants. The tree species and their land use affect the removal potential. For example, in Strasbourg, forest lands were the most efficient in removing pollutants, followed by parks, industrial areas, and agricultural lands. On the other hand, the leaf area of a specific species can lead their depollution capacity. According to the study, the European beech was the best performant. They used a model called i-Tree model to assess their results which takes into consideration the forest structure and ecosystem services. This model was already used by other researchers in Spain and the United Kingdom (UK) [129][130]. In both Spain and the UK, the urban forest removed PM10, NO2, O3, CO, and SO2. The former eliminated 305.6 tons/year and 50 t/year of pollutants with PM10 and O3 having the highest removals respectively. The differences in tree cover, pollutants concentrations, leaf area, species present, and climate clarify the discrepancies obtained in all studies.

Trees then directly ameliorate air quality by decreasing the air temperature and by removing pollutants; and indirectly by reducing energy demands and consumption. However, at the same time, trees can be a source of pollutants. VOCs emitted can act as a precursor to O3 and particles through their reaction with NOx [START_REF] Benjamin | Estimating the ozone-forming potential of urban trees and shrubs[END_REF]. Also, the pollen and the particles produced can inflict serious health problems on allergic people [START_REF] Beckett | Urban woodlands: their role in reducing the effects of particulate pollution[END_REF].In addition to that, toxic pollutants will eventually accumulate in the soil resulting in a contaminated one which affects water quality and future land use [START_REF] Fowler | Measuring aerosol and heavy element deposition on urban woodland and grass using inventories of 210Pb and element concentrations in soil[END_REF]. In urban sites and densely populated cities, there is little space to plant trees. For example, in Midtown Manhattan, a district in New York, 94 % of the land is impervious surfaces like streets and rooftops. This makes it impossible to find a place to plant trees which also in turn take several years to grow to be able to mitigate air pollution. So, green roofs or greenery systems can make use of the available rooftops (40-50 % of the impermeable area in a city) and be a more appropriate solution to this perplexity as well as air pollution [START_REF] Lepp | Planting Green Roofs and Living Walls[END_REF].

Greenery systems

Urbanization has created a pronounced construction demands where buildings, factories, and residential sectors are being constructed at the expense of green areas; turning our world into a densely populated place suffering from pollution [START_REF] Berndtsson | Green roof performance towards management of runoff water quantity and quality: A review[END_REF]. By changing the already-present unused rooftops and by introducing different plant species to those rooftops, a compensation to the lost greens while the building was being constructed can be an outcome [START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF]. So, greenery systems are a way of 'greening' and at the same time mitigating air pollution. They are sorted as vertical greenery systems and green roofs. Green roofs or roofs with plants in their final layer are known as eco-roofs and are coated with green plants and growing media. Green roofs consist in general of vegetation with its substrate, filter fabric, root barrier, and the insulation layer (Fig. 14). The layer holding the vegetation is the uppermost layer that adds life to the green roof and which can act as the air pollution mitigator. Those properties make the choice of plants a delicate factor. Since many limitations exist as water scarcity and building load restrictions, and since green roofs are not a favorable or a normal environment for plants to grow, the latter used should have specific properties to ease their implementation [7][136]. Plants should be drought-tolerant with little maintenance needs, have short roots, and be capable of removing contaminants from the soil or air. Preferable green roof vegetation is of the succulent type as the sedum species because they are able to limit transpiration and store water [137][138]. For instance, a study highlighted that sedum species were able to stay photosynthetically active even after lacking water for 4 months [START_REF] Brevis | Flower Age Affects Fruit Set and Stigmatic Receptivity in Rabbiteye Blueberry[END_REF]. Another study indicated that sedum species survived 2 years without water [START_REF] Teeri | The response of leaf water potential and crassulacean acid metabolism to prolonged drought in Sedum rubrotinctum[END_REF]. This is due to their characteristic of storing water in their stems and leaves enabling them to survive drought conditions [START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF]. In addition to that, their shallow roots give them the advantage to be used excessively on extensive green roofs (with a small substrate depth) while also being healthy [START_REF] Getter | Media depth influences Sedum green roof establishment[END_REF] Consequently, those properties decrease the cost of green roofs. Nonetheless, as sedum species are non-native for all countries in the world, research has been focusing on the use of native species since they are already evolved to adapt to the present weather conditions, thus resistant to local pests. Many studies dealt with native plant species to find the most suitable for green roofs and the closest to the qualities of sedum [START_REF] Vijayaraghavan | Can green roof act as a sink for contaminants? A methodological study to evaluate runoff quality from green roofs[END_REF][144] [START_REF] Monterusso | Establishment and Persistence of Sedum spp. and Native Taxa for Green Roof Applications[END_REF]. Nevertheless, using multiple types of plant species can yield a positive effect on the effectiveness of green roofs [START_REF] Berardi | State-of-the-art analysis of the environmental benefits of green roofs[END_REF]. Some types of plants used on green roofs are: thymus vulgaris, sedum species, oregano, grasses, etc.

Components of green roofs

The growth substrate affects the performance of green roofs. It is related also to the thermal and sound insulation hence, it must have some unique properties as a low bulk density, high sorption capacity, good water holding capacity, and stable. On the other hand, the filter layer which separates the growth substrate and the drainage element should have high tensile strength, small pores to prevent particles from entering the drainage layer, and should act as a root barrier specially to plants that have soft or short roots. Those properties can improve the water retention capacity of green roofs. In general, all components should have specific compatible properties to ensure the most effectiveness possible [START_REF] Vijayaraghavan | Green roofs: A critical review on the role of components, benefits, limitations and trends[END_REF].

Types of green roofs

Minimal Low

Table 2 shows that among the 3 types, extensive green roofs are the most common due to their low cost, maintenance demands, and weight meeting the building weight restrictions.

History of green roofs

The oldest example of a greenery system goes back to the city of Babylon 500 years BC where 'hanging gardens' were employed, as well as the Roman and Greek empires. Also, in the old Mediterranean area, vines were grown on pergolas providing shade or directly on the walls of the building lowering the indoor temperatures and providing more comfortable indoor conditions [START_REF]Green Roof Guidelines for the Planning, construction and maintenance of green roofing -green roofing guideline[END_REF]. Years after, the use of 'climbers' increased as it became popular particularly in UK and Central Europe. In the 20 th century, various studies especially in the botanical domain [START_REF] Köhler | Green facades-a view back and some visions[END_REF] were being done where they understood the significance of green facades towards ecology. Afterwards, green roofs were implemented in Germany. With passing years, more and more areas of green roofs were seen in Berlin where 10% of the buildings in Germany use green roof technology now [START_REF] Oberndorfer | Green roofs as urban ecosystems ecological structures, functions, and services[END_REF] [151]. Lately, developed countries as USA, Canada, Japan, etc. started integrating new laws in the favor of green roofs and facades. Consequently, 15 % of roofs in Basel, a city in Switzerland, have been greened [START_REF] Townshend | Study on Green Roof Application in Hong Kong[END_REF]; in Tokyo-Japan, a regulation was implemented that states that all newly built and enlarged buildings with land area ≥ 1000 m 2 must have green roofs [START_REF] Chen | Performance evaluation and development strategies for green roofs in Taiwan: A review[END_REF]. Similarly, in Canada where a law declares that if the floor area exceeds 2000 m 2 , 20-60 % of the roof area should include green roofs [START_REF] Chen | Performance evaluation and development strategies for green roofs in Taiwan: A review[END_REF]. It was approved in 2015 that in France, the new commercial constructions must at least partially cover their rooftops with plants or solar panels. This is a great step towards more sustainable cities [START_REF] Gloede | France Mandates Green Roofs For All New Commercial Construction[END_REF].

Benefits of green roofs

Green roofs could promote biodiversity, reduce temperatures, mitigate the Urban Heat Island (UHI) effect, improve air quality, ameliorate the city image, and has many social and economic advantages as well [START_REF] Lundholm | Green Roofs and Facades: A Habitat Template Approach[END_REF][156][157] [START_REF] Theodoridou | Assessment of retrofitting measures and solar systems' potential in urban areas using Geographical Information Systems: Application to a Mediterranean city[END_REF]. Some benefits will be addressed in detail herein below.

Thermal performance enhancement

Enhancement in the thermal performances of a building positively affects and reduces its energy demands. This is due to the thermal insulation and shading properties as well as the thermal mass present [START_REF] Köhler | Green facades-a view back and some visions[END_REF]. A study in Greece showed that green roofs decrease the cooling energy demands by 2 to 48% (the percentage being related to the area covered by plants) with an indoor temperature decrease [START_REF] Niachou | Analysis of the green roof thermal properties and investigation of its energy performance[END_REF]. The growth medium helps in insulating the roof as well as the water content which can trap the heat; the vegetation layer plays a role in absorbing or reflecting the incoming solar radiation which limit the thermal loads. From the total incoming solar radiation, 27% was reflected, 60% absorbed by the plants, and 13% transferred to the growing medium [START_REF] Eumorfopoulou | The contribution of a planted roof to the thermal protection of buildings in Greece[END_REF]. According to the types of plants present and the thickness of the soil, the annual energy consumption savings can vary between 1-15% in a five-floor commercial building in Singapore [START_REF] Wong | Investigation of thermal benefits of rooftop garden in the tropical environment[END_REF]. Some authors claim that green roofs also have positive insulating properties during winter and hence on energy demands; however, others believe that it actually increases the latter [162][163]. Generally, green roofs decrease the ambient air temperature due to an increase in the albedo (fraction of light radiation reflected by a surface) of urban areas; for instance, the albedo of a green roof was shown to vary between 0.7 to 0.85 comparable to that of white roofs (0.8) while that of bitumen, tar, and gravel roofs range between 0.1 and 0.2 [START_REF] Berardi | State-of-the-art analysis of the environmental benefits of green roofs[END_REF] which hence reduces the urban heat island effect.

Stormwater management

While raining, a fraction of the water will get absorbed by the growing medium; this water can be used by plants to be stored in their tissues/transpired back or retained in the pore spaces.

However, the 'unabsorbed or untrapped' remaining water will percolate and reach the drainage system where it will be retained. Draining will happen with the saturation of the drainage space.

The retained water will either be absorbed again by the plant where part of it will be transpired or evaporated [START_REF] Berndtsson | Green roof performance towards management of runoff water quantity and quality: A review[END_REF][164] [START_REF] Nagase | Amount of water runoff from different vegetation types on extensive green roofs: Effects of plant species, diversity and plant structure[END_REF]. A study revealed that the runoff retention can reach 66

% on an intensive green roof which is much higher than that on a normal roof (34 %) [START_REF] Speak | Urban particulate pollution reduction by four species of green roof vegetation in a UK city[END_REF]. Green roofs are recognized as a way to decrease the chances of flooding by retaining stormwater.

Vegetation, the thickness and type of the growing medium, the drainage system present, slope of the roof, storage capacity, etc., all specify the retention potential of the roof with the growing medium playing the biggest role [START_REF] Vijayaraghavan | Can green roof act as a sink for contaminants? A methodological study to evaluate runoff quality from green roofs[END_REF]. Some studies reflect that the runoff delay from green roofs is due to the high water holding capacity of the growing substrate [START_REF] Graceson | The water retention capabilities of growing media for green roofs[END_REF]. Vegetation also plays a significant role in the runoff retention. Grasses were reported as being efficient for runoff control followed by sedum [START_REF] Nagase | Amount of water runoff from different vegetation types on extensive green roofs: Effects of plant species, diversity and plant structure[END_REF]. Sedum species transpire available water quickly and can retain rainwater by contributing to 40 % of the green roof capacity to retain stormwater [START_REF] Berghage | Quantifying evaporation and transpirational water losses from green roofs and green roof media capacity for neutralizing acid rain Report, National Decentralized Water Resources (NDWRCP) Research Project[END_REF].

Noise attenuation

Green roofs act as a cover separating or creating a boundary between indoors and outdoors.

They can attenuate noise coming from the road or from traffic by absorbing sound waves or by increasing the road insulation [169] [170]. A study showed that the noise transmission loss of nonvegetative roofs is less than that of vegetative roofs by 10 to 20 dB [START_REF] Connelly | Experimental investigation of the sound transmission of vegetated roofs[END_REF]. The comparison of sound insulation between an extensive and an intensive green roof showed that an extensive roof is rather effective in reducing sound transmission however, an intensive roof showed no positive effects [START_REF] Van Renterghem | Numerical evaluation of sound propagating over green roofs[END_REF]. Not a lot of research focus on acoustical benefits but in general, noise pollution in urban spaces is reduced.

Green roofs can be viewed as a design for boring flat buildings. They enhance aesthetic values and act as a tool to restore biodiversity. For instance, Fig. 15 shows a green-roofed school in France which is more psychologically and aesthetically pleasing than a normal asphalt one. They, as well, protect roofs from heat and ultra-violet radiation which decreases the daily expansion and contraction of conventional roofs [START_REF] Vijayaraghavan | Green roofs: A critical review on the role of components, benefits, limitations and trends[END_REF]. The latter usually reaches 70ºC of temperature compared to a 25ºC of that of green roofs.

Air pollution mitigation: A literature review

Carbon sequestration

Green roofs also sequester carbon. The global warming and the enhanced greenhouse effect are facts that we have been living ever since the industrial revolution took place [173]. The increased demand for fossil fuels generated a great amount of CO2 which is a by-product of combustion. CO2 is one of the main gases that keeps our planet warm by trapping the received energy from the sun. Then, logically, an increase in CO2 leads to an increase in the greenhouse effect. CO2 concentrations increased by 32 % since 1750 and it is expected to escalate more since it has been proposed in 2010 to construct 100 new powerplants to support the expected energy demands [174] [175]. Green roofs can trap carbon naturally -a process called photosynthesishence sequestering carbon in their tissues. The latter can then reach the soil substrate due to the fall of dead organic material (plant litter) or due to the release of carbon by the roots. However, this phenomenon always reaches an equilibrium so, green roofs also decrease the emission of CO2 by mitigating the UHI and by decreasing the energy demands as seen in Fig. 13. A study in Michigan quantified the sequestered carbon by sedum species on an extensive green roof and revealed that sedum and the substrate sequestered around 380 g/m 2 [START_REF] Getter | Carbon sequestration potential of extensive green roofs[END_REF].

Modelling studies-Removal of adverse air pollutants

Vegetation as trees and shrubs is very efficient in mitigating the UHI and lowering pollution levels. But, due to limited availability of spaces especially in urban areas, the use of a vegetated roof or a green roof can be a good solution. They make use of impervious areas and at the same time, lower the pollutants concentration. Numerous researches tried to estimate the depollution capacities of green roofs. Starting with Toronto-Canada, a study revealed the ability of green roofs to reduce NO2, SO2, PM10, and O3 levels [START_REF] Currie | Estimates of air pollution mitigation with green plants and green roofs using the UFORE model[END_REF]. It dealt with a dry deposition model called Urban Forest Effects (UFORE) which quantifies the reduction in concentrations of the targeted pollutants by calculating the vegetation cover, weather data, and the pollutants concentrations. Their model estimated that intensive green roofs can play a big role in improving the air quality while an extensive roof plays a smaller one. However, due to the higher cost of intensive green roofs owing to the added structural loading requirements, an extensive roof is preferable. Another in Chicago-USA revealed the removal of 1675 kg of pollutants in one year by 19.8 ha of green roofs [START_REF] Yang | Quantifying air pollution removal by green roofs in Chicago[END_REF]. The latter comprised short grass, herbaceous plants, trees and shrubs, and other plants; which is the ensemble of all 71 green roofs studied varying from extensive, semi-intensive, and intensive green roofs. The O3 uptake was the largest contributing to 52 % of the total uptake followed by NO2, PM10, and then SO2. This is explained by the high concentration of O3 present at that year which peaked in summer. This study used a dry-deposition model and indicated that if all green roofs were of the intensive type in Chicago, pollutants removal can reach up to 2047 metric tons.

However, the installation cost is a great obstacle. As with normal vegetation, the most efficient pollutant removal was when the leaves were fully open and expanded. The annual removal rate per hectare of canopy cover was greater in the study in Chicago than in Toronto. This is due to the difference in plant cover, weather, and pollution severity. Also, the methods used in the modeling play a role. Similarly, a study on a 50 ha extensive green roof in UK holding 2 different grass types, a flowering plant, and sedum showed the ability of those species to capture PM10 [START_REF] Speak | Urban particulate pollution reduction by four species of green roof vegetation in a UK city[END_REF]. The grasses were more efficient than sedum in removing the particulate matter where both grass species combined were able to mitigate around 2.5 metric tons/year of PM10. Sedum removed 0.21 metric tons; this difference is due to the surfaces provided by the plant species. Tan and Sia [START_REF] Rosenfeld | Cool communities: strategies for heat island mitigation and smog reduction[END_REF] -SO2

Table 3 shows the amount of pollutants removed by several studies. In all the previous studies [START_REF] Yang | Quantifying air pollution removal by green roofs in Chicago[END_REF] [126] [START_REF] Speak | Urban particulate pollution reduction by four species of green roof vegetation in a UK city[END_REF], the plant species studied were able to remove PM10, O3, NO2, and SO2.

For example, Currie & Bass [START_REF] Currie | Estimates of air pollution mitigation with green plants and green roofs using the UFORE model[END_REF] showed that grass can abolish 1270 kg of O3 and 650 kg of NO2;

while shrubs can remove 1270 kg of the former and 1240 kg of the latter. Speak et al. [START_REF] Speak | Urban particulate pollution reduction by four species of green roof vegetation in a UK city[END_REF] reported that sedum can capture 210 kg of PM10 when Red Fescue (a grass species) trap 1605 kg. According to Yang et al. [START_REF] Yang | Quantifying air pollution removal by green roofs in Chicago[END_REF], a mixture of 63 % grass/14 % herbaceous plants/11% shrubs/12 % hard surfaces mitigated 871 kg of O3 and 452 kg of NO2. The dissimilarities show the different effect each plant species has and the necessity to understand the mechanisms of each to help set the goals concerning air pollution mitigation.

Other studies showed that 2000 m 2 of grass on a green roof can abolish 4000 kg of PM [START_REF] Johnston | A guide to using plants on roofs, walls and pavements[END_REF].

So, if 0.1 kg of PM was released into the atmosphere in one year by one car, a 1 m 2 of uncut grass is sufficient to remove this pollutant. Similarly, it was reported that if only 20 % of industrial and commercial buildings in Detroit-USA were covered with extensive sedum green roofs, around 800,000 kg of NO2 can be offset [START_REF] Clark | Optimization of green roofs for air pollution mitigation[END_REF]; their estimation was achieved by assuming constant NO2 removal rates by green roofs. In Singapore, the concentration of SO2 was compared before and the installation of a 4000 m 2 green roof. Results show that a decrease of 37 % of SO2 was attained above the roof [START_REF] Tan | A pilot green roof research project in Singapore[END_REF].

In addition to the direct removal of pollutants and decreasing the pollution extremity, green roofs can as well indirectly remove pollutants by decreasing the emissions from powerplants. In Los Angeles, it was estimated that 350 tons/day of NO2 can be eliminated by lessening the need for air conditioning which leads to a 10 % reduction in the formation of smog and a saving of 1 million dollars/day [6][179]. Due to evapotranspiration and shading properties, a green roof helps in bringing down indoor temperatures during summer which reduces cooling needs and insulates the building in winter hence decreases heating requirements [START_REF] Santamouris | Investigating and analysing the energy and environmental performance of an experimental green roof system installed in a nursery school building in Athens, Greece[END_REF][181]; therefore, mitigating the UHI (Fig. 16).

Experimental studies

All previous studies show that green roofs are effective in removing pollutants, improving air quality, and mitigating the UHI. In addition to that, they can increase biodiversity, improve the building insulation, and increase the aesthetic value of a building with a boring standard roof.

However, all those researches were based on modelling and few studies deal with laboratory or field measurements of green roofs depollution ability. A very recently published article compared the efficiencies of different ventilation filters (ventilation system) in removing O3 [START_REF] Abbass | Ozone removal efficiency and surface analysis of green and white roof HVAC filters[END_REF]. One of the filters was installed on a white roof while the other on a vegetated one involving mostly succulents like sedum species. The ventilation system affects the quality of indoor air since it is usually installed on buildings and outdoor air can interact with the green or white roof prior to entering the building ventilation system. In their studies, they built a setup which enabled them to test the ozone removal efficiency (Fig. 17). An air supply system was used followed by different filters and drying media to purify and condition the air. Purification and the removal of VOCs from the supplied air was done by 2 different filters and a drying medium while the control of humidity was achieved by a glass impinger filled with distilled water. The temperature was also monitored and recorded using a temperature sensor. This process was then followed by a flow meter controlling the flow of the air entering the O3 generator which produces O3 to be fed to the filters under study. O3 analyzers were found in the end of the setup monitoring the O3 concentrations.

The results showed that green roof filters had a greater removal performance than white roof filters or even than an unused filter ranging from 10-25 % for green roof filters and from 5-15 % for white roof filters after 5 h of O3 exposure (with a temperature of 21 ⁰C and relative humidity being between 30 % and 70 %). This was explained by the help of the Scanning Electron Microscope (SEM) images that showed that non-ozonated green roof filters had an accumulation of deposits such as pollen or other biotic coatings. Contrary to that of white roof filters which had fewer deposits. Deposits offer a greater surfaces area which increases the amount of particles captured by the filter. The same group investigated the O3 removal efficiency of the same filters present on a green roof as well as on a white roof with the variation of temperature and relative humidity [START_REF] Ramasubramanian | Impact of green and white roofs on air handler filters and indoor ventilation air[END_REF]. They selected 3 temperatures and 3 relative humidity values: 15 ⁰C, 23 ⁰C, and 31 ⁰C, and 20 %, 50%, and 80 % to cover all realistic conditions. The removal ability of all filters increased with the increase of relative humidity; however, the trend is uncertain regarding the temperature. Overall, they found that the best conditions were 23 ⁰C temperature and 80 % relative humidity which suggests that O3 removal by both roofs is relatively better in the winter season. This is in line with their previously reported work [START_REF] Abbass | Ozone removal efficiency and surface analysis of green and white roof HVAC filters[END_REF]. Green roofs hence directly improve the indoor air quality by removing O3 or indirectly by affecting the effectiveness of ventilation/heating/air conditioning filters placed in the building ventilation system.

Research questions/gaps

Uncertainty of models

Since air pollution mitigation ability of green roofs was estimated or predicted using models rather than quantitatively calculated using field or laboratory research, many limitations and uncertainties can be an outcome. The study using UFORE model [START_REF] Currie | Estimates of air pollution mitigation with green plants and green roofs using the UFORE model[END_REF] held some unreliability since in their data, grass was the plant species studied instead of sedum because data for other vegetation -usually used on extensive green roofs-were not available. The model predicted the leaf area index and evapotranspiration rates of grass in order to calculate the air pollution removal values and then compare the latter with those of trees and shrubs. The only model that was accessible in North America to estimate the vegetation effect on air pollutants was this one where its precision is unknown. Hence, plant selection to be studied was constrained. The economic benefits and analysis were also limited since they were related to values derived from a work done in New York [START_REF] Gryning | Air pollution modelling and its application XII[END_REF]. Their data were based on the deduced cost of air pollution regarding its effects on the health and environment. This might pose a problem due to the probable differences in values between countries. The model acts mostly as a program where grass was chosen as a proxy unit.

Similarly, studies considering dry deposition/big-leaf models report various uncertainties.

For example, the estimation of the air pollution removal by green roofs obtained from [START_REF] Yang | Quantifying air pollution removal by green roofs in Chicago[END_REF] is more an approximation than an accurate estimation. This is because the generalization of green roofs as continuous surfaces containing mostly grass, herbaceous plants, and deciduous trees with uniform heights was an obligation for implementing the big-leaf model at a big scale. Also, the air pollutants concentrations were deemed constant. However, the heights of green roofs, type of vegetation present, and the distances to the sources of emission (distance between the building having the green roof and the emission source) largely influence pollutants removal. Green roofs, for instance, located in a highly polluted area would uptake more pollutants that a roof situated in a clean area.

Calibrating the dry deposition model and verifying the reported results is mandatory since the way that the deposition velocity on plants was modelled is also erroneous; they were taken from the literature without considering the differences among species.

Accurate laboratory and field studies are necessary to precisely measure and understand the depollution effect of green roofs. Starting from a small setup-to be extrapolated-, one can measure the uptake capacity of many plant species usually implemented on green roofs. Based on the uptake capacity of a certain plant species towards a target pollutant, the choice of green roofs, and plants to be installed in a region with an elevated concentration of this target pollutant would become easier.

Lack of studies concerning the interaction of VOCs and pollutants

VOCs emitted by plants play an important role in the plants life as in the chemical interaction occurring in the atmospheric compartment [START_REF] Dicke | Induced plant volatiles: from genes to climate change[END_REF]. Being released by any type of vegetation, VOCs are usually a result of different stimuli. They are involved in the plant communication, pollinators attraction, stress adaptation, and defense from predators [START_REF] Baldwin | Volatile Signaling in Plant-Plant Interactions[END_REF][187][188] [START_REF] War | Mechanisms of plant defense against insect herbivores[END_REF]. Nevertheless, the emission of VOCs is directly related to the plant species [START_REF] Llusi | Seasonal and species-specific response of VOC emissions by Mediterranean woody plant to elevated ozone concentrations[END_REF] and different plant species have different solutions to the same problem. For example, dissimilar flowers produce various odorous volatiles to face the problem of attracting the same pollinator [START_REF] Pichersky | Genetics and biochemistry of secondary metabolites in plants: an evolutionary perspective[END_REF]. Those biogenic volatile organic compounds (BVOCs) include isoprene, monoterpenes, and sesquiterpenes with alkanes, alkenes, alcohols, etc. as well [START_REF] Kesselmeier | Biogenic Volatile Organic Compounds (VOC): An Overview on Emission, Physiology and Ecology[END_REF].

The interactions between the urban vegetation and urban environment are rarely studied and since urban vegetation is always exposed to elevated pollutants concentrations and temperatures, it is important to understand the reactions taking place (Fig. 18) [START_REF] Calfapietra | Urban plant physiology: Adaptation-mitigation strategies under permanent stress[END_REF].

For example, when vegetation is exposed to a change in the abiotic components (change in temperature, presence of high concentrations of pollutants, etc.), its ability to remove pollutants as well as its volatile emissions change. An augmentation of the latter leads to an increase in interactions between the VOCs and pollutants; this might provoke a boost in air pollution [START_REF] Churkina | Natural selection? Picking the right trees for urban greening[END_REF].

The production of isoprene is involved in the formation of ground-level O3 while monoterpenes or sesquiterpenes are rather related to the increase in PM [195] [196]. In general, in the areas rich in NOx, VOCs go through chemical interactions forming O3 and PM [START_REF] Steinfeld | Atmospheric Chemistry and Physics: From Air Pollution to Climate Change[END_REF]. This was in line with a study done in China showing that BVOCs affect the formation of O3 and PM [START_REF] Ghirardo | Urban stress-induced biogenic VOC emissions and SOA-forming potentials in Beijing[END_REF]. Out of the hundreds VOCs emitted by plants, only few impact the air quality [START_REF] Park | Active atmosphereecosystem exchange of the vast majority of detected Volatile Organic Compounds[END_REF]. However, some studies state a decrease in O3 concentrations in the presence of plants. O3, after being formed, can be captured by the vegetation (leaves, stomata, stems, etc.) or can react with NO and VOCs as monoterpenes and sesquiterpenes thus lowering its concentration [START_REF] Fares | Ozone fluxes in a Pinus ponderosa ecosystem are dominated by non-stomatal processes: Evidence from long-term continuous measurements[END_REF] [201 [START_REF] Gerosa | Ozone uptake by an evergreen Mediterranean Forest (Quercus ilex) in Italy. Part I: Micrometeorological flux measurements and flux partitioning[END_REF]. In order to solve this complexity, laboratory or field studies should investigate the influence of a plant species emitting VOCs on O3 concentrations. Also, to avoid the uncertainties carried by models, quantitative experimental work is needed.

Lack of studies regarding pesticides + VOCs/ pesticides + pollutants

Pesticides + pollutants

Pesticides are chemical agents that control pests and protect plants or crops; they include herbicides, insecticides, fungicides, etc., each having a role. Once on leaves, they can react with urban pollutants. Hence, understanding the fate of pesticides is necessary since their degradation can lead to an increase in doses applied or number of treatments. The formation of degradation products might lead sometimes to contamination of the environment as well. Numerous researches studied the interactions of pesticides and pollutants as O3 in water, while others deal with their interaction on supports in the presence of O3 (heterogeneous reactions).

In solution

Due to the industrial wastes, surface run-off owing to agricultural applications, domestic employments, etc., pesticides reach surface waters and pollute them [START_REF] Bedding | Organic Contaminants in the Aquatic Environment. I. Sources and Occurrence[END_REF]. According to a survey, surface and groundwaters were contaminated by different types of pesticides where concentrations up to 0.6 µg/L were detected in surface waters [START_REF] Croll | Pesticides in Surface Waters and Groundwaters[END_REF]. A review considering the ozonolysis of various types of pesticides in aqueous solutions showed that almost all pesticides were degraded in the presence of O3. For example, the insecticide called Aldrin-which is now banned -showed a consumption of around 90-100% when O3 of a concentration between 1 and 4 mg/L was present [205] [206]. O3 usually attacks the carbon-carbon double bond producing even more harmful and more toxic compounds. Dichlorodiphenyltrichloroethane (DDT) on the other hand, showed a low reactivity towards O3 [START_REF] Kuehn | Oxidation techniques in Drinking water treatment. Drinking water pilot project[END_REF]. However, 94 % conversion was reported after prolonged ozonation [START_REF] Beltrán | Advanced oxidation of atrazine in water-II. Ozonation combined with ultraviolet radiation[END_REF]. Many factors affect this reaction such as the presence of humic substances, the solubility of pesticides in water, and the pH. This is important since pesticides in potable water poses a threat to the human health, thus ozonolysis can be applied only if complete mineralization is achieved or if the intermediate products are less toxic than the starting chemical.

On solid

Out of the various studies done on the heterogeneous phase, one involving 8 commonly used pesticides adsorbed on silica model particles and ozonolysis showed degradation concerning 4 pesticides while no effect was reported related to the others [START_REF] Socorro | Heterogeneous reactions of ozone with commonly used pesticides adsorbed on silica particles[END_REF]. Ozonolysis was done for 26 h for 6 O3 concentrations (0 to 997 ppb). The decay was proportional to the O3 concentrations. In the absence of O3, no effect was noted. However, the degradation increased with the O3 concentration.

In their experiments, they built a setup involving O3 generator, O3 analyzer, chambers, and a humidity system. This suggests a possible interaction taking place between pesticides and O3 yielding products. Their results were in line with other studies that performed ozonolysis with many pesticides [START_REF] Pflieger | Kinetic study of heterogeneous ozonolysis of alachlor, trifluralin and terbuthylazine adsorbed on silica particles under atmospheric conditions[END_REF][210] [START_REF] Pflieger | The heterogeneous ozonation of pesticides adsorbed on mineral particles: Validation of the experimental setup with trifluralin[END_REF]. The same group also studied the heterogeneous degradation of the 8 commonly utilized pesticides by • OH radicals [START_REF] Mattei | Heterogeneous degradation of pesticides by OH radicals in the atmosphere: Influence of humidity and particle type on the kinetics[END_REF]. The latter was chosen since pesticides, in general, undergo photolysis and react with atmospheric oxidants like • OH, O3, or nitrate radicals (NO3). Pesticides were adsorbed on silica particles where a source of • OH was present. Similarly, 4 out of the 8 pesticides were degraded. The results concerning the 4 unreactive pesticides were unexpected since they are degraded in the gas phase according to some software calculations [START_REF] Meylan | Computer estimation of the Atmospheric gas-phase reaction rate of organic compounds with hydroxyl radicals and ozone[END_REF].

Hence, • OH can degrade the pesticides in the gaseous phase but the reactivity significantly decreases in the heterogeneous phase due to the probable steric hindrance and the absence of water on the solid surfaces rendering the diffusion of OH • or O3 through solid the substrate limited. This reflects the complexity of this phase.

Pesticides photolysis

Pesticides, principally, undergo direct and indirect photolysis. Direct photolysis takes place via excited states when it has the ability to absorb a wavelength > 290 nm (UV-visible absorption) (Fig. 19). Photoproducts different than the starting molecule are possibly formed; whereas indirect photolysis happens when the pesticide reacts with a photogenerated reactive oxygen species due to the presence of dissolved organic matter especially in water bodies. The singlet oxygen and • OH are important reactants in this process [START_REF] Burrows | Reaction pathways and mechanisms of photodegradation of pesticides[END_REF][215] [START_REF] Katagi | Photodegradation of Pesticides on Plant and Soil Surfaces Toshiyuki Katagi Contents[END_REF].

In solution

Leaching and run-off processes, for example, help pesticides reach waterbodies thus posing a threat to humans and the environment [218] [219]. Subsequently, they can be phototransformed in sunlit waters which can be the major transformation pathway for many pesticides besides hydrolysis and other biological reactions [START_REF] Fenner | Evaluating Pesticide Degradation in the Environment: Blind Spots and Emerging Opportunities[END_REF]. Bentazon didn't undergo any degradation in the dark but was phototransformed when irradiated by UV light in the collected lake/paddy water [START_REF] Carena | The role of direct photolysis in the photodegradation of the herbicide bentazone in natural surface waters[END_REF]. In general, photodegradation is mostly due to direct photolysis. Dissolved organic matter (DOM) that is ubiquitous in surface waters has a dual effect. It can inhibit the photolysis due to a competition for light between the pesticide itself and the chromophoric groups found in DOMs.

Conversely, it can enhance the photodegradation because it generates reactive species such as excited states, • OH, singlet oxygen radicals that are able to react with pesticides [START_REF] Ter Halle | The effect of mixing two herbicides mesotrione and nicosulfuron on their photochemical reactivity on cuticular wax film[END_REF]. Many studies have been and are still undertaken to investigate these reactions.

On solid/leaves surfaces

Studies on solid surfaces mostly investigate the pesticides photodegradation on glass (simplest model) and silica gel to compare their photoreactivity. Pesticides degrade more slowly on glass since they form a thicker layer which causes light attenuation [START_REF] Chen | Comparative study of thin film photodegradative rates for 36 pesticides[END_REF]. Usually, pesticides deposited on silica gel experience photoinduced homolytic cleavage that produces radicals, and photodegradation further takes place. A study involving benzyl phenylacetate concluded that radicals separated very well on silica gel surface via a translational motion [START_REF] David Avnir | Surface Photochemistry: Radical Pair Combination on a Silica Gel Surface and in Micelles[END_REF]. Similarly, azobis(isobutyronitrile) under UV light and on silica surfaces was degraded with the formation of radicals according to another study [START_REF] Johnston | Surface Photochemistry: Decomposition of Azobis(isobutyronitrile) on Dry Silica Gel[END_REF]. These experiments demonstrate the degradation of pesticides on solid surfaces; however, they do not exactly represent the possible processes taking place on the surface of plants since the reaction environment is totally different from cuticular surface. Plant surfaces themselves are complex media influenced by several factors.

One study took into consideration the photodegradation of 2 herbicides (mesotrione and nicosulfuron) on thin wax films [START_REF] Ter Halle | The effect of mixing two herbicides mesotrione and nicosulfuron on their photochemical reactivity on cuticular wax film[END_REF]. Previous studies on mesotrione show its photolysis in aquatic media as well as on wax films [226] [227]. Photoproducts of mesotrione were due to photohydrolysis since water molecules in the air/film interface can be present and involved in this reaction, photocyclisation, or photoreduction; while those of nitrosulfuron were mainly because of hydroxylation, photoextrusion (loss of SO2), and photohydrolysis [START_REF] Ter Halle | The effect of mixing two herbicides mesotrione and nicosulfuron on their photochemical reactivity on cuticular wax film[END_REF]. These photochemical reactions were also reported with other herbicides on wax films [228][229]. The photoreactivity of both herbicides was faster when present as formulated molecules. This owes to the additives which facilitates their spreading and hence the absorption of light. In addition to that, the mixture of these 2 compounds leads to a further increase in photoreactivity due to the additive effect of additives or the photosensitizing effect. Experiments on wax films can be more representative of the cuticular surfaces than glass/silica; but the wax films still lack some characteristics or components naturally found in leaves. The structures of pesticides are present in Fig. 21.

Figure 17 Chemical structures of the pesticides.

To have a better approach to cuticular surfaces, a study used the solution of soaked corn leaves as a deposit on Petri dishes. The solution was left to evaporate until a layer of plant wax remained on the surface. Dicamba, a herbicide, was then applied on the dishes and left to evaporate which thus created a solid layer [START_REF] Ter Halle | Phototransformation of the herbicide sulcotrione on maize cuticular wax[END_REF]. Irradiating the dishes revealed dicamba photodegradation with the formation of photoproducts. The photochemical reaction is however slower on the solid/inert surfaces than in aqueous solutions; this is probably since dicamba is in the solid state.

Nevertheless, when the solution of dicamba is applied as a formulation, faster photodegradation takes place. This is related to the presence of certain surfactants lowering the surface tension and other chemicals which helps the herbicide to spread on the surfaces. The matrix environment influences the photodegradation rates and photoproducts produced [START_REF] Sleiman | Heterogeneous Photochemistry of Agrochemicals at the Leaf Surface: A Case Study of Plant Activator Acibenzolar-S-methyl[END_REF][232] [START_REF] Trivella | Perinaphthenone phototransformation in a model of leaf epicuticular waxes[END_REF]. For instance, an experiment done with the same herbicide and irradiation conditions directly on glass Petri dishes showed that no degradation takes place. Epicuticular wax surfaces are a complex medium; pesticides deposited on them can undergo slow photolysis (screening effect) or a fast one (photosensitization). They can be good candidates to represent the wax surfaces of plants but still lack some properties like the effect of volatiles or the real cuticular structure.

Plant surfaces, in other words leaves surfaces, are the first reaction site for pesticides.

Sunlight can reach those surfaces provoking photodegradation [START_REF] Katagi | Photodegradation of Pesticides on Plant and Soil Surfaces Toshiyuki Katagi Contents[END_REF]. For example, a study where bifenazate, an insecticide, was deposited on green pepper showed that bifenazate underwent photodegradation in dark and under irradiation [START_REF] Hamdache | Unravelling the reactivity of bifenazate in water and on vegetables: Kinetics and byproducts[END_REF]. The autoxidation was linked to the presence of water in the pepper or to transpiration. Various degradation products were detected with a major one formed by the elimination of 2H atoms. The photodegradation on green pepper skin needed water, oxygen, and a protic solvent. Another, demonstrated the phototransformation of Acibenzolar-S-methyl (BTH) when present on apple leaves and under irradiation (Fig. 22) [START_REF] Sleiman | Heterogeneous Photochemistry of Agrochemicals at the Leaf Surface: A Case Study of Plant Activator Acibenzolar-S-methyl[END_REF]. Around 75 % of BTH was degraded after 4 h; contrastingly to wax films which showed that only 10 % degradation was reported when BTH was deposited on them. The nature of the apple leaves was responsible for this difference i.e. the metabolism in the leaf, chemistry of the cuticle, formation of reactive species due to stress, and the emission of VOCs. Interestingly, volatile photoproducts arising from BTH were detected showing that pesticides can as well produce VOCs.

Contrary to inert surfaces, 'plants leaves' is an 'alive' medium influenced by many factors such the anatomy of the leaf, wax, weather, etc. [START_REF] Katagi | Photodegradation of Pesticides on Plant and Soil Surfaces Toshiyuki Katagi Contents[END_REF]. Fig. 23 shows the structures of studied pesticides.

Pesticides + VOCs

In addition to the previous interactions, it is necessary to investigate how pesticides would react with the VOCs naturally emitted by plants or crops. A research considered the interaction between an insecticide and BVOCs produced by orange trees [START_REF] Borrás | Ozone and secondary organic aerosol production by interaction between and organophosphorous pesticide and biogenic VOCs mixture[END_REF]. The study was done under sunlight where results showed that this combination of BVOCs and pesticide formed O3 and secondary organic aerosols (SOA). Those two highly contribute to the formation of smog. Having enough knowledge about atmospheric interactions involving pesticides facilitates the estimation of their impact on the environment as well as the selection of 'environmentally friendly' pesticides.

Since green roofs are ecofriendly, attention should be given to the interaction of the volatile metabolites of plants implemented and pesticides sprayed so that they don't lose this beneficial characteristic. Hence, the necessity to understand this interaction is crucial especially with VOCemitting green roof plants (since not all produce significant levels of VOCs).

Conclusion/scope of work

Holding many uncertainties, models should be carefully considered and more attention should be given to experimental/laboratory research in order to obtain quantitative results. In the first part of our study, we developed an experimental setup including flow meters, stainless steel reactors, an irradiation system, and analyzers to measure the uptake performances of certain plant species implemented on extensive green roofs. Two air pollutants with deleterious health and environmental effects were chosen to be studied: NOx and O3. Special attention was devoted to finding a long-lasting stable source of these 2 pollutants. An O3 generator equipped with a UV penray lamp was used as a source of O3. However, the NOx source used was based on a study done in our team involving a solution of irradiated Imidacloprid which produced NOx (NO + NO2) with NO2 being the major product [START_REF] Palma | New Route to Toxic Nitro and Nitroso Products upon Irradiation of Micropollutant Mixtures Containing Imidacloprid: Role of NOx and Effect of Natural Organic Matter[END_REF].

In the second part, 13 different plants were selected for preliminary experiments. Their uptakes were noted by calculating the difference in the initial concentrations (coming from the source) and the ones read on the analyzers after passing through the reactors containing the plants.

Following screening, the best performant species were selected: Sedum sexangulare, Thymus vulgaris, and Heuchera Americana L. Scanning electron microscopy was performed to characterize cuticular surfaces, headspace gas chromatography coupled with mass-selective detection (HS-GC-MS) was undertaken for the VOC analysis, Automated Thermal Desorption-Gas Chromatography coupled to Mass Spectrometry (ATD-GC-MS) analysis was done to identify possible reactions taking place in the gas phase, and a nitrous acid (HONO) trap was employed to detect the potential reaction between water and NO2. All these factors were taken into consideration to deeply understand the uptake mechanisms. Nonetheless, experiments were done on both detached leaves and full size plant to test the feasibility of extrapolation; effect of soil was also evaluated.

The third part was dedicated to study the interactions between two abundantly used pesticides (Imidacloprid and Chlorothalonil, Fig. 24) and Thymus Vulgaris volatiles after irradiation. Those pesticides have the potential to react with volatiles. Chlorothalonil, for example, has photooxidant properties due to the formation of its triplet excited state or singlet oxygen after irradiation; while Imidacloprid is a good nitrosating/nitrating chemicals through the release of NO and NO2. Experiments were conducted in solution to study the potential of the interaction, and then on Thymus Vulgaris leaves to approach realistic conditions. HS-GC-MS was performed to detect gaseous photoproducts, and GC-MS and liquid chromatography-high resolution mass spectrometry

Materials and Methods

In this section, the different chemicals, plants, instruments, and the process of sample preparation are described. Experiments related to the uptakes of plants are marked with a green color while those concerning the interactions of pesticides with the plants' volatiles are distinguished by a blue color (in solution) and a brown color (on thyme leaves).

Chemicals

The chemical reagents used are listed in Table 4 below. All chemicals were used without further purification. Water was produced using a reverse osmosis RIOS 5 and Synergy (Millipore) device (resistivity 18 MΩ cm, DOC < 0.1 mg L -1 ).

Prepared solutions were stored in a fridge at 4 ⁰C.

Plants

Thirteen different plant species were selected and purchased for screening experiments (Table 5). They vary in shape, leaf size, emission of volatile organic carbons (VOCs), tolerance, rooting depth, etc. Plants were purchased from a plant nursery (Botanic®, Beaumont, France) and stored in an indoor greenhouse. They were stored indoors in the laboratory near a large window where they received sufficient sunlight and were watered every 2 days. The plants leaves were cut prior to the experiment using scissors and weighed (0.9-1.2 g) or were used directly as a full-size plant having a pot size varying between 9-15 cm and containing soil.

Irradiation systems

Irradiation of plants a. For visible light irradiation, 2x55W Starlite tubes (λ= 400-800 nm) were used (Fig. 26).

Usually, these lamps emit light necessary for photosynthesis. They were used for irradiation in the experiments concerning plants with their uptakes. Two air-tight cylindrical stainless steel reactors were used. Their volume ranged from 0.6 L (small reactor) to 12 L (big reactor). The distance between the leaves/ full plant and the lamps was 30 cm. Irradiation of solutions b. For calculating the quantum yields of solutions (CT + volatiles), a high-pressure mercury lamp equipped with an Oriel monochromator was used (Fig. 27). A radiometer QE65000 from Ocean optics was utilized to measure the photon flux. Since the photon beam is homogeneous and parallel, Io measurement becomes possible as well as the absorbed light intensity (Ia). The lamp is connected to a cooling system; both the monochromator and the lamp are switched on 20 min before their use. This ensures a stable and homogeneous photon flow for the solution to be irradiated. Solutions of CT + volatiles were placed in a 1 cm thick cuvette equipped with Teflon caps and irradiated at 313 nm. c. For monitoring the disappearance of molecules, 3 polychromatic tubes (Philips, TL20W/01-RS, λmax = 310 nm) installed inside a custom-made cylindrical irradiation device were used (Fig. 28).

Solutions of CT/IMD alone or CT/IMD + volatiles were placed in a cylindrical Pyrex reactor (1.4 cm internal diameter) sealed with an air-tight silicon cap and surrounded by these tubes. Cooling was provided by a ventilator maintaining the temperature at around 23⁰C. Aliquots were sampled at chosen irradiation times. Irradiation of leaves d. For irradiating thyme twigs alone or covered with CT/IMD, a solar simulator CPS Atlas equipped with a xenon arc lamp and a filter cutting off wavelengths shorter than 290 nm was used with its power set at 500 W m -2 in our experiments (Fig. 29). This light source has a spectral distribution very close to that of solar radiation. It included a cooling system maintaining the bottom of the device at 10 °C. The twigs were put in headspace vials which were placed inside the device. This system uses ventilation to ensure the cooling of lamps; hence, a cover made of quartz (plate) was used to protect the vials from the air flow caused by ventilation during irradiation.

Actinometry

e. The photon flux, I0, was measured using a radiometer QE65000 from Ocean optics. It measures the number of photons emitted by the light used. Io=number of photons emitted by each light was measured in photons cm -2 sec -1 which is directly related to the degradation capacity. The higher the number of photons emitted, the higher will be the number of photons absorbed (Ia) and hence, a higher rate of photolysis.

Spectroscopic techniques

UV-Visible absorbance of solutions was measured by a Varian Cary 3 UV-Visible spectrophotometer in a 1 cm-thick quartz cell (Fig. 30). The absorbance spectra of thymol, linalool, 3-carene, α-pinene, chlorothalonil, imidacloprid, thymoquinone, and thymohydroquinone were recorded, all having a concentration of 10 -4 M. The absorbance precision is of ± 0.002.

A baseline correction was necessary before every analysis. For this, the solvent used, which was ACN in our experiments, was put in a cuvette and its absorption reported. The latter serves as a baseline. Baseline correction is a pre-processing technique that enhances the precision of the spectroscopic signal by separating it from interference and background effects.

Chromatographic techniques

a. High performance liquid chromatography coupled to a diode array detector (HPLC-DAD, Fig. 31)

Disappearance of thymol

To monitor the disappearance of thymol after bubbling O3, HPLC analyses were carried out. They were performed using a NEXERA XR HPLC equipped with an auto-sampler, a pump, and coupled to a UV/Vis DAD apparatus. The analysis was done in the reverse phase using a phenomenex -kinetex C18 column (100 mm × 2.1 mm, 2.6 μm particle size) analytical column. An identical pre-column was added. The flow rate was set at 0.2 mL/min and the eluent was a mixture of ACN and water. It was an isocratic elution with 10 % ACN maintained for 10 min. The retention time of thymol was 1.3 min (λ= 274 nm). The injection volume was 10 µL.

Degradation of the pesticides (CT and IMD)

HPLC analyses were as well performed to monitor the degradation profiles of CT and IMD in solution with and without the presence of volatiles. Regarding CT experiments, the analytical column was a Phenomenex-kinetex C18 (100 mm × 2.1 mm, 2.6 μm particle size); the flow rate was set at 0.2 mL min -1 ; while for the IMD experiments, the analytical column was EC 150/4.6 NUCLEODUR C18 endcapped column with a flow rate of 1 mL min -1 (an identical pre-column was also present). The injection volume was 10 µL in all the analyses.

CT IMD

Figure 26 HPLC-DAD used.

Purging was performed prior to every experiment. In the HPLC system, a certain percentage of ACN with water was used depending on the method. Its aim is to flush through the lines any remaining solvent or air bubbles from previous analysis to be replaced by the new mobile phase.

b. High performance liquid chromatography coupled to mass spectrometry (LC-MS, Fig. 32)

Photoproducts of the reaction between pesticides and volatiles in solution and on leaves

Photoproducts obtained in solution and on thyme leaves due to the interaction between volatiles and pesticides under irradiation were identified by high resolution mass spectrometer (HRMS) on an Orbitrap Q-Exactive (Thermoscientific) coupled to an ultra-high performance liquid chromatography (UHPLC) instrument Ultimate 3000 RSLC (Thermoscientific) equipped with a UV/Vis detector. The sample to be analyzed is introduced into the mass spectrometer from the HPLC by direct vaporization in a nebulization chamber. In this chamber, the molecules are ionized by electrospray (or elecyrospray ionization ESI). The column used in our experiments was a Kinetec EVO C18 (100 × 2.1 mm), particule size of 1.7 μm (Phenomenex). The elution gradient started with 15% acetonitrile maintained for 5 min, then the percentage of acetonitrile was linearly increased to reach 55% after 5 min. The flow was set at 0.45 mL min -1 . Analyses were made in positive (M+H + /M+Na + ) and negative (M-H + ) electrospray modes (ESI + /ESI -). Mass accuracy was less than 5 ppm. c. Gas chromatography coupled to mass spectrometry (GC-MS, Fig. 33)

Photoproducts of the reaction between pesticides and volatiles in solution and on leaves

Photoproducts were also analyzed using an Agilent 6890 gas chromatograph coupled to an Agilent 5973 mass spectrometry detector. The separation was carried out using a HP-5 µs column (25 m × 0.25 mm × 0.25 μm) operating initially at 80°C over 1 min, followed by a 10°C min -1 ramp to reach to reach 200°C and then a 30⁰C min -1 ramp to reach 260°C maintained for 1 min (to calculate the degradation of volatiles) in the case of CT + volatiles experiments, while the analysis was operated initially at 70°C followed by a 10°C min -1 ramp to reach 160°C for 5 min and then a 30°C min -1 ramp to reach 250°C maintained for 2 min in the case of IMD + volatiles. The flow rate was 1 mL min -1 with an injection volume of 1 µL in both cases. In the GC system, purging with the carrier gas removes traces of O2 and moisture from the injection port and hence column. Pressure stability in the column before running the analysis should be ensured in all cases.

d. Headspace gas chromatography coupled to mass spectrometry (HS-GCMS)

Volatile metabolites of the reaction between pesticides and volatiles on leaves

The volatile metabolites produced by thyme when found alone or after being dipped in CT before and after irradiation were detected using HS-GC/MS (Shimadzu HS-20 coupled with QP2010SE). Twigs (0.2 g) were transferred into a 20 mL headspace glass vial (Fig. 34) and incubated for 5 min at 80⁰C. The analytical column (Mega 5-MS 30 m × 0.25 mm) was operated initially at 50°C for 1 min, followed by an 8°C min -1 ramp to reach 170°C and held for 4 min, and then a 15°C min -1 ramp to reach 275°C maintained for 1 min. The mass spectrometer source was heated to 200°C, and signals were detected between mass to charge ratios (m/z) of 50 and 350.

Identification of the major constituents was carried out using the NIST 17 database and when necessary using authentic standards. e. Automated thermal desorption gas chromatography couple to mass spectrometry (ATD-GC-MS, Fig. 35)

Products formed between O3 and thyme's volatiles

were investigated with a field-emission scanning electron microscope (SH 4000 M) using a 20 kV acceleration voltage and a 10 mm working distance. Both analyzers (Fig. 37) were turned on 30 min prior to uptake experiments to allow their lamps to heat up. The background concentrations of NOx and O3 were always noted to correct any interference coming from the laboratory room air. Each analyzer needed at least a flow rate of 0.6 L min -1 ; hence when the flow was not sufficient, a T shaped tube fitting was installed to allow the analyzer to suck air from the room. This was taken into consideration in our calculations.

On thyme leaves

Two thyme twigs were cut using scissors, their leaves counted, and then dipped into a 7.5

x 10 -4 M solution of either CT or IMD for 30 s. They were then left freely to dry for 1h and transferred into a hermetically sealed head space vials to be left in the dark or irradiated (Fig. 40).

Possible products were directly analyzed by HS-GCMS or extracted by 2 ml ACN (immersed and agitated) to be analyzed by GC-MS, LC-MS, or HPLC-DAD.

Toxicity estimation

ECOSAR (ECOlogical Structure-Activity Relationship) computer program

The toxicity of certain compounds was estimated using this software by using a structureactivity approach. LC50 (Acute toxicity, the concentration that is lethal to half of fish after 96 h of exposure to a certain molecule), ChV (Chronic toxicity), and logKow were obtained. The reported toxicities are those of the compounds on fish in fresh waters. The program (v 2.0) is available on the website: epa.gov/tsca-screening-tools/ecological-structure-activity-relationships-ecosarpredictive-model.

Results and discussion

Summary of the main results

The objectives of this PhD thesis were to study the depollution mechanism and performances of plant species implemented on green roofs towards two prevalent air pollutants:

94

To achieve the experiments with air pollutants, we needed to get an O3 and a NOx source.

Our laboratory was equipped with an UVP ozone generator having a UV-pen ray lamp; however, a stable and continuous NOx source was necessary. NO2 cylinders could be used for this purpose but they are potentially harmful and hazardous due to the high pressure stored inside. Moreover, they do not last long which makes them costly due to the need of using often a new one. Then, we investigated the ability of VOCs emitted by plants to possibly react with the pesticides sprayed on them (Fig. 42,C). Pesticides are used to protect the plants from pests.

However, they can be toxic to humans or other living species, and it is therefore important to get Figure 39 A diagram of the steps followed to study the interactions between volatiles and pesticides.

Manuscript 1:

In this manuscript, we studied the ability of IMD to generate NOx (gaseous phase) and NO2 - /NO3 -ions (aqueous phase) as well as the latter's ability to produce nitro/nitroso compounds (Fig. 45). Aqueous solutions of IMD were irradiated within the wavelength range of 300-450 nm.

Results showed that IMD underwent photodegradation. Although other photoproducts were detected, HPLC and LC-MS analyses showed the main formation of desnitro-IMD in accordance with the release of NO2. This formation is due to the cleavage of the N-NO2 bond present in IMD (Fig. 46). Both NO2 and NO were detected with NO2 production much higher than that of NO; 1 h of IMD irradiation was enough to produce around 800 ppbv and 350 ppbv of NO2 and NO respectively. Quantum calculations showed that IMD photolysis takes place in the triplet state and the dissociation of N-NO2 is mainly homolytic. NO, on the other hand, could be produced by the photodecomposition of the nitroso derivative (IMD-O). In addition to that, NO2 -and NO3 -ions were detected in solution. Their photolysis should also form NO2 and NO but their contribution to the total amount of NOx detected was found to be minor. An experiment where IMD was deposited on glass petri dishes and irradiated was performed. In this case, lower concentrations of NO ( 23ppbv) and NO2 (297 ppbv) compared to the ones produced by IMD solution were detected in solid probably because IMD is much less dispersed than in water, and the absorption of light is lower.

We also showed that the irradiation of IMD photoinduced the nitration and nitrosation of phenolic probes representing water NOM moieties. It is supposed that phenols were oxidized by NO2 into phenoxyl radicals which could further react with NO2 or NO to yield nitro or nitroso compounds (Fig. 46). The reactor's NOx/O3 uptakes were also taken into consideration (Fig. 48). Thyme showed the highest uptake followed by sedum and then heuchera (Fig. 48). To understand the reasons behind the uptakes and their mechanisms, additional experiments were performed. Using SEM, we observed that thyme exhibited the largest stomatal area and the second highest stomatal density. This explains thyme's good performance towards NO2 and its removal mechanism.

In addition to that, the surface reaction between NO2 and water potentially released by plants was investigated by using a nitrous acid (HONO) trap. Although the uptake is directly related to stomata, a surface reaction between NO2 and H2O potentially released by the plant due to the irradiation heat is possible. This reaction forms HONO. Among the 3 plants tested, sedum showed that more than half of the NO2 uptake is due to this reaction. Hence, its uptake is because of stomata and the fact that it is a succulent plant that stores water in its leaves which might be released and react with NO2 (Fig. 49). The same calculation was done as with NOx to find out the most effective plants in removing O3. The order was the same as for NOx with thyme being the most performant followed by sedum and then heuchera. Thyme emits volatile compounds that might react with O3. An experiment was performed in solution by bubbling O3 into thymol (a major volatile emitted by thyme leaves) and directly on thyme leaves + O3. Results showed that an interaction between volatiles and the pollutant takes place. Therefore, NOx and O3 are removed by the plants through different mechanisms; thyme removes O3 by a reaction involving its VOCs, sedum removes NO2 through a surface reaction involving water, while heuchera adsorbs both on its big hairy leaves making them a good adsorbent in addition to stomatal absorption which occur for all three studied plants.

The uptake coefficients and deposition velocities were calculated to understand the performance and to confirm our obtained results. The uptake coefficient shows the reactive uptake after a gas collides with a surface while the deposition velocity reflects the deposition rate of the gas on the surface. Values were in this order thyme > sedum > heuchera.

Extrapolation was tested by performing experiments on whole plants. The values obtained in both cases (NOx and O3 uptakes) were similar to those with detached leaves. Therefore, extrapolation is possible, and the uptake depends on the area exposed, as it can be expected.

General conclusions

CO2, NOx, O3, and other air pollutants are being enormously emitted into the atmosphere leading to health problems, urban heat island effect, and climate change. Fortunately, different solutions to decrease the impact of air pollution on the environment exist. Among them, green roofs have shown promising results in removing prevalent air pollutants besides being aesthetically pleasing, enhancing the building thermal performance, and managing stormwater. Research concerning green roofs and their air pollution removal capacity were predicted or estimated using models holding many uncertainties. This renders quantitative data a necessity to exactly measure their depollution performance towards specific pollutants. Our objective was to get a better insight into the depolluting potential of green roof plant species, in particular, their uptake capacity towards air pollutants, and their behavior in the presence of pesticides and solar light. To answer those questions, a setup was first built -after carefully considering the sourcesconsisting of different components that enabled us to monitor the change in concentrations of NOx and O3. Then, we started with our experimentation by calculating, measuring quantitatively, as well as understanding the depollution performances and mechanisms of green roof plant species. In addition, interactions between a volatiles-emitting plant and two pesticides were investigated while noting the toxicity of the formed photoproducts.

Our setup was modular that allowed us to measure both NOx and O3 uptakes of plants. An O3 generator produced O3 while IMD was the source of NO2. Both sources were stable, continuous, and the amount of O3 and NO2 generated (90 ppbv and 50 ppbv respectively) are close to the recommended exposure limit. The insecticide, IMD, showed a production of NO2 in solution and on solid (petri dishes) with different yields. Hence, this setup permits the measurement of NO2/O3

As a final conclusion, this thesis revealed the depollution capacity of green roofs towards NO2 and O3. Thyme significantly removed both pollutants, sedum uptook NO2, while heuchera retained the pollutants due to its hairy leaf structure. In an urban environment extremely polluted with O3 and NOx, choosing sedum and thyme to be implemented on green roofs would be very efficient and beneficial. However, interactions happening on the roof between the sprayed pesticides and volatiles emitted can lead to toxic photoproducts. In our case, IMD produced nitrated/nitrosated compounds while CT acted as a strong oxidant in the presence of volatiles.

Hence, the careful choice of pesticides to be sprayed on a specific plant is compulsory. Nonetheless, even if IMD, for example, wasn't directly sprayed on an odorous plant, its presence in the which has adverse health effects or SO2. This should be done by considering other possible mechanisms, the presence of microorganisms on leaves, or the secondary pollutants formed as SOA. Since the leaves of plants are a reaction site, further research must examine deeply this complex system. An example is the effect of plant cuticles on the uptake and possible stress factors affecting the opening of stomata. Naturally, plants respond to stress by emitting VOCs, our results revealed the possible interactions between sprayed pesticides and the latter; nevertheless, the fate of formed products should be studied more systematically and these interactions on other plant species can be investigated. This might help optimize the choice of plants and pesticides in a specific urban environment.

Figure 1 8 Figure 2 9 Figure 3 11 Figure 5 13 Figure 6 14 Figure 15 Figure 8 19 Figure 9 21 Figure 10 22 Figure 11 23 Figure 12 25 Figure 13 28 Figure 14 A 42 Figure 16 42 Figure 17 44 Figure 18 45 Figure 19 46 Figure 20 47 Figure 21 75 Figure 22 76 Figure 23 76 Figure 24

 182931151361415819921102211231225132814421642174418451946204721752276237624 Figure 1 Removal of air pollutants and the release of VOCs by plants. .......................................... 8 Figure 2 O3 photolysis in the troposphere, source: [9][10]. ............................................................. 9 Figure 3 NO2 photolysis in the troposphere, source: [13]. ............................................................. 10 Figure PM2.5 anthropogenic sources (according to the national emission inventory for 2001), source: [26]. .................................................................................................................................... 11 Figure 5 Short-term and long-term air pollution exposure effects on humans. ............................. 13 Figure 6 Polluted air in Clermont-Ferrand and Riom (France), source: [41]................................. 14 Figure Temperature differences between urban and other areas due to the Urban Heat Island (UHI), source: [43]. ........................................................................................................................ 15 Figure 8 The Photocatalysis mechanism, source: [64]. .................................................................. 19 Figure 9 Absorption of air pollutants into a stoma, source of the leaf picture: [85]. ..................... 21 Figure 10 The retention of particles on the abaxial and adaxial leaves surfaces of 8 different plants. Images obtained by the Scanning electron microscopy, source: [94]. ........................................... 22 Figure 11 Reaction of VOCs with O3. ............................................................................................ 23 Figure 12 Reaction pathways of O3 with BVOCs, source: [114]. .................................................. 25 Figure 13 The different components of green roofs, source: [133]................................................ 28 Figure 14 A green-roofed school in France, source: [170]. ........................................................... 33 Figure A simplified diagram showing a photosensitized reaction where the photosensitizer produces a singlet oxygen species. ................................................................................................. 42 Figure 16 Simplified state energy diagram, source: [216], [217]. ................................................. 42 Figure 17 Chemical structures of the pesticides. ............................................................................ 44 Figure 18 The reaction of a pesticide under sunlight [231]. .......................................................... 45 Figure 19 Chemical structures of the studied pesticides ................................................................ 46 Figure 20 Chemical structures of Chlorothalonil and Imidacloprid. ............................................. 47 Figure 21 Plants chosen for further experiments, from left to right: Sedum sexangulare, Thymus vulgaris, Heuchera Americana L. .................................................................................................. 75 Figure 22 Starlite tubes and their light spectrum. .......................................................................... 76 Figure 23 Monochromator used with the spectral distribution of its Hg lamp. ............................. 76

Figure 25

 25 Figure 25 Suntest device used and the spectral distribution of its lamp. .......................................

Figure 26

 26 Figure 26 HPLC-DAD used. ..........................................................................................................

Figure 27

 27 Figure 27 LC-MS apparatus. ..........................................................................................................

Figure 28

 28 Figure 28 GC-MS instrument. ........................................................................................................

Figure 29 HS

 29 Figure 29 HS vial with thyme. .......................................................................................................

Figure 30 ATD

 30 Figure 30 ATD (automated thermal desorption) system. ...............................................................

Figure 31

 31 Figure 31 Thyme's adaxial surface seen by SEM. .........................................................................

Figure 32

 32 Figure 32 The NOx (bottom) and the O3 (top) analyzers used in our experiments. ......................

Figure 33 IMD

 33 Figure 33 IMD solution in the glass reactor. ..................................................................................

Figure 34 3 -

 3 Figure 34 3-carene stock solution. .................................................................................................

Figure 35

 35 Figure 35 Thyme twigs after extraction. ........................................................................................

Figure 36 A

 36 Figure 36 A scheme summarizing the PhD steps. ..........................................................................

Figure 37 A

 37 Figure 37 A diagram of the setup used to demonstrate the formation of NO2/NO from IMD. .....

Figure 38 A

 38 Figure 38 A simplified diagram of the setup used to study the NOx or O3 uptake by plants. .......

Figure 39 A

 39 Figure 39 A diagram of the steps followed to study the interactions between volatiles and pesticides. .......................................................................................................................................

Figure 40 IMD

 40 Figure 40 IMD degradation pathway and its possible nitration/nitrosation ability. .......................

Figure 41

 41 Figure 41 Structures of IMD, nitrophenol, and nitrosophenol. ......................................................

Figure 42 A

 42 Figure 42 A diagram of the steps followed to monitor NOx and O3 uptakes of different plants. ..

Figure 43 The

 43 Figure 43 The NOx uptake of the 3 plants in the mall reactor. ......................................................

Figure 44 A

 44 Figure 44 A succulent plant releasing water. .................................................................................

Figure 45 A

 45 Figure 45 A diagram of the steps followed in manuscript 3. .......................................................

Figure 46

 46 Figure 46 Thyme oxidation products. ..........................................................................................

Figure 47

 47 Figure 47 Photoproducts obtained in the mixture of thymol + IMD. ..........................................

Figure A 1 .

 1 Figure A 1. Setup used to monitor NOx formation upon irradiation of IMD solutions. ..............

Figure A 2 .

 2 Figure A 2. The NOx (red) and O3 (blue) setups. .........................................................................

  [START_REF]Council directive 1999/30/EC of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air[END_REF] [58]. .................................................................................................................... Table 2 Differences between the 3 green roof types, source: [145]. .............................................. Table 3 Amounts of pollutants mitigated by different studies. ...................................................... Table 4 Chemical reagents used in this study. ............................................................................... Table 5 Plants chosen to be studied. .............................................................................................. Table 6 HPLC conditions used for the CT/IMD experiments........................................................ 1

Figure 1

 1 Figure 1 Removal of air pollutants and the release of VOCs by plants.

Figure 2 O3

 2 Figure 2 O3 photolysis in the troposphere, source: [9][10].

Figure 3 NO2

 3 Figure 3 NO2 photolysis in the troposphere, source: [13].

Figure 4

 4 Figure 4 PM2.5 anthropogenic sources (according to the national emission inventory for 2001), source: [26].

Figure 5

 5 Figure 5 Short-term and long-term air pollution exposure effects on humans.

1. 3 . 2

 32 On the environment Numerous environmental effects exist due to air pollution. Some examples are climate change, ozone depletion, and acid rain.

Figure 6

 6 Figure 6 Polluted air in Clermont-Ferrand and Riom (France), source: [41].

Fig. 6

 6 Fig.6 demonstrates the bad air quality in Clermont-Ferrand and Riom (2 cities in France, Puy-de Dôme department-Rhône-Alpes region) on a polluted day.

Figure 7

 7 Figure 7 Temperature differences between urban and other areas due to the Urban Heat Island (UHI), source: [43].

Figure 8 The

 8 Figure 8 The Photocatalysis mechanism, source: [64].

Figure 9

 9 Figure 9 Absorption of air pollutants into a stoma, source of the leaf picture: [85].

Figure 10

 10 Figure 10 The retention of particles on the abaxial and adaxial leaves surfaces of 8 different plants. Images obtained by the Scanning electron microscopy, source: [94].

Figure 11

 11 Figure 11 Reaction of VOCs with O3.

Figure 12

 12 Figure 12 Reaction pathways of O3 with BVOCs, source: [117].

Figure 13

 13 Figure 13 The different components of green roofs, source: [135].

Figure 14 A

 14 Figure 14 A green-roofed school in France, source: [172].

Figure 15 A

 15 Figure 15 A simplified diagram showing a photosensitized reaction where the photosensitizer produces a singlet oxygen species.Fig.20 shows that when a pesticide absorbs light, this molecule in the ground state (S0) gets excited to the first excited singlet state (S1) where several processes can further take place. S1 can be deactivated through the nonradiative internal conversion (IC) to S0, through the emission of fluorescence (Fl), via an intersystem crossing (ISC) from S1 to T1, or by undergoing further reactions forming photoproducts. T1 is much long-lived than S1 and has thus more time to undergo reactions with other reactants (O2, starting chemical, etc…).

Figure 16

 16 Figure 16 Simplified state energy diagram, source: [216], [217].

Figure 18

 18 Figure 18 The reaction of a pesticide under sunlight [231].

Figure 19

 19 Figure 19 Chemical structures of the studied pesticides

(

  LC-MS) to determine other formed photoproducts. The photochemical reactivity of these two pesticides in the presence of the volatiles was studied with the possible photooxidation or nitrosating/nitrating properties in case of Chlorothalonil and Imidacloprid respectively.

Figure 20

 20 Figure 20 Chemical structures of Chlorothalonil and Imidacloprid.

Figure 21

 21 Figure 21 Plants chosen for further experiments, from left to right: Sedum sexangulare, Thymus vulgaris, Heuchera Americana L.

Figure 22

 22 Figure 22 Starlite tubes and their light spectrum.

Figure 23

 23 Figure 23 Monochromator used with the spectral distribution of its Hg lamp.

Figure 24

 24 Figure 24 The irradiation device used.

Figure 25

 25 Figure 25 Suntest device used and the spectral distribution of its lamp.

Figure 27 LC

 27 Figure 27 LC-MS apparatus.

Figure 28 GC

 28 Figure 28 GC-MS instrument.

Figure 29 HS

 29 Figure 29 HS vial with thyme.

Figure 31

 31 Figure 31 Thyme's adaxial surface seen by SEM.

Figure 32

 32 Figure 32 The NOx (bottom) and the O3 (top) analyzers used in our experiments.2.8 Preparation of samples/experiments

Figure 34 3 -

 3 Figure 34 3-carene stock solution.

Figure 35

 35 Figure 35 Thyme twigs after extraction.

  NOx and O3 (Fig41, B), as well as to investigate the possible photochemical interactions taking place between secondary metabolites and pesticides sprayed on plants (Fig41, C). In both cases, the leaves surface is the site of reactions involving the plants constituents and chemicals.

Figure 36 A

 36 Figure 36 A scheme summarizing the PhD steps.

  Hence, the idea was to use a chemical as a source of NO2. Our choice fell on Imidacloprid (IMD) as a candidate that possibly produces NO2/NO (Fig 41, A). A setup was first developed to monitor and evidence the NOx production from aqueous solutions of IMD (Manuscript 1). In this manuscript, I performed experiments on IMD by irradiating it first to monitor NO2/NO values via the developed setup as well as experiments regarding its nitrating/nitrosating effect. The setup consisted of flow controllers, a reactor, an irradiation system, and an NOx analyzer as seen in Fig.42. It is fully described in Annex 1. This first set-up served as a base to the more complicated setup used to monitor the ability of plants to retain pollutants.

Figure 37 A 43 .

 3743 Figure 37 A diagram of the setup used to demonstrate the formation of NO2/NO from IMD.

Figure 38 A

 38 Figure 38 A simplified diagram of the setup used to study the NOx or O3 uptake by plants.

  insight into their fate to assess and estimate their (or their degradation products) risks. Solar light can induce reactions involving pesticides deposited on leaves, and the presence of VOCs in the close vicinity of pesticides could provoke cross reactions. The last part of the thesis was thus dedicated to study the photochemical interactions between VOCs emitted by a fragrant plant and two pesticides: Chlorothalonil (CT) and IMD (Manuscript 3). The diagram of the different steps followed in this case is given in Fig.44.

Figure 40 IMD

 40 Figure 40 IMD degradation pathway and its possible nitration/nitrosation ability.

Figure 41

 41 Figure 41 Structures of IMD, nitrophenol, and nitrosophenol.

Figure 42 A

 42 Figure 42 A diagram of the steps followed to monitor NOx and O3 uptakes of different plants.

Figure 43 The

 43 Figure 43 The NOx uptake of the 3 plants in the mall reactor.

Figure 44 A

 44 Figure 44 A succulent plant releasing water.

Figure SI- 1 .Figure SI- 2 .Figure SI- 4 .

 124 Figure SI-1 . Irradiance of the fluorescent tubes Sylvania F15W/BL 368S used in devices 1 and 2.

Figure SI- 5 .

 5 Figure SI-5. Structures (a)-(k) of the minimum energy structures and TSs at the MN12SX/6-311++G(d,p),PCM//MN12SX/6-31+G(d,p),PCM level. Energies are given relatively to the global minimum (see Figure4). Specific bond distances (Å) are also indicated.

Figure SI- 6 .Figure SI- 7 .Figure SI- 8 :

 678 Figure SI-6. Structures of RN + (a), RN + ,H2O (b) and RN,H2O (c) obtained at the MN12SX/6-31+G(d,p),PCM level and calculation of RN + energy:

4 :Figure SI- 1 .Figure SI- 4 .

 414 Figure SI-1. VOCs of thyme obtained by headspace-GC-MS.

Figure SI- 5 .Figure SI- 6 .Figure SI- 1 : 155 Figure SI- 2 :Figure SI- 3 :Figure SI- 4 :

 561155234 Figure SI-5. NO production by dry and wet soils after 1 h of visible light irradiation.

Figure SI- 5 :Figure SI- 6 :

 56 Figure SI-5: MS spectrum of TQO (m/z = 179.0702 in ES -)

Figure SI- 7 :Figure SI- 8 :

 78 Figure SI-7: MS spectrum of TO-TO-2H (m/z = 329.1753 in ES -)

Figure SI- 9 :Figure SI- 10 :

 910 Figure SI-9: MS spectrum of IMD-NO2+H (m/z = 211.0742 in ES + )

  Air pollutants chosen were NOx and O3 being chief ones emitted abundantly while pesticides were CT and IMD since they are reactive upon solar irradiation. This led to different questions to be solved: How can we quantitatively measure the depollution capacity of plant species used on green roofs? Which plants are efficient in removing pollutants and what are the mechanisms taking place? Do plants treated with pesticides undergo interactions with the latter through their volatiles under irradiation? What are the photoproducts formed and are they toxic?

  environment will lead to the spontaneous formation of NO2. The latter can further react with the thymol naturally emitted by thyme or undergo other photochemical reaction worsening the alreadypresent air pollution problem. Future experiments should consider the validation of results on real green roofs to understand the uptake capacity of selected plants towards target pollutants on the larger scale. Simulations can be first done to have an insight before directly moving to experiments on green roofs. Nevertheless, seasonality, plant physiology, concentration of pollutants in the specific urban area, and temperature are factors to be noted. In this PhD, we studied the ability of green roof plants to remove NOx and O3; nonetheless, other pollutants should be studied as PM2.5

Table 1

 1 Air quality guidelines and standards of specific pollutants depending on the exposure time, source:[START_REF]Air quality guidelines. Global update[END_REF] [57][START_REF]Review of the National Ambient Air Quality Standards for Particulate Matter[END_REF].

	Source

Nitrogen dioxide (µg/m 3 ) PM2.5 (µg/m 3 ) O3 (µg/m 3 ) 1 year 24 h 1 h 1 year 24 h 8 h 1 h WHO

		40	-	200	10	25	100	-
	European Union	40	-	200	-	-	120	-
	United States	100	-		15	65	157	-

Table 2 Differences between the 3 green roof types, source: [147]. Green roof type Substrate layer thickness Vegetation types Maintenance recurrence

 2

	Cost

Table 3 Amounts of pollutants mitigated by different studies.

 3

	Study	Plant species studied	Pollutants abolished
			(kg/year)
	Currie & Bass [126]	Grass	1270 O3
			650 NO2
		Shrubs	1270 O3
			1240 NO2
	Speak et al. [166]	Sedum	210 PM10
		Red Fescue	1605 PM10
	Yang et al. [23]	Grass/ Herbaceous plants/	871 O3
		Shrubs	452 NO2
	Johnson and Newton [8]	Grass	4000 PM
	Clark et al. [177]	Sedum	800,000 NO2

Table 4 Chemical reagents used in this study.

 4

	Name	Structure	Purity (%)	Supplier
	Imidacloprid (IMD)	≥ 98.0	Sigma Aldrich
	C9H10ClN5O2
	Chlorothalonil (CT)	≥ 98.0	Sigma Aldrich
	C8Cl4N2
	Sodium	carbonate	≥ 99.5	Fluka
	anhydrous
	Na₂CO₃
	Thymol		≥ 98.5	Sigma Aldrich
	C10H14O
	α-pinene		98	Sigma Aldrich
	C10H16

Table 6 HPLC conditions used for the CT/IMD experiments.

 6

	Time (min)	% ACN	Time (min)	% ACN
	0.01	30	0	30
	2	50
	4	60
	8	60
	15	80	10	30
	15.1	30
	21	30

Table SI - 2 :

 SI2 UPLC-HMRS of IMD and photoproducts.

	Name	m/z exp.	m/z theor.	ppm	Formula of the
		in ES +	in ES +		neutral molecule
	IMD	256.0603/	256.0596/	2.82	C9H10ClN5O2
		258.0571	258.0566
	IMD+O	272.0549/	272.0545/	1.38	C9H10ClN5O3
		274.0517	274.0515
	IMD-O	240.0650/	240.0647/	1.52	C9H10ClN5O
		242.0620	242.0617
	Desnitro-IMD	211.0746/	211.0745/	0.33	C9H11ClN4
		213.0717	213.0716
	Desnitro-	243.0650/	243.0643/	2.84	C9H11ClN4O2
	IMD+2O	245.0619	245.0614

Table SI - 3 :

 SI3 Cartesian coordinates of the minimum energy structures and TSs at the MN12SX/6-

			C	-0.389542	-1.299857	H	-3.745817	1.032188	-	C	-0.908128		-1.653261	H	-3.826097	0.923517	-
			1.516466				1.193510				1.387958			0.890056
			H	-0.200563	-2.300278	C	-2.262716	0.219044	-	H	-0.902819		-2.744665	C	-2.262662	0.235823
			1.925345 311++G(d,p),PCM//MN12SX/6-31+G(d,p),PCM level. 0.023736 1.506130 H -0.778435 -0.671310 N -2.607783 -1.069180 H -1.139081		-1.213097	0.242243 N -2.592191	-1.049179
			2.328589				0.109505				2.368362			0.519810
			C	-1.356164	-2.374792	-	N	-3.779593	-1.321407	-	C	-2.131166	-1.961989	-	N	-3.771921	-1.362575
	Min S0, 1 RNNO2 (a) 0.593330 C -2.639341 -2.062339	Min S1 (b) 0.472256 -O -4.195045	Min T1, 3 RNNO2 (c) 0.816481 -2.471112 -C -3.316312	TS S0 (d) 0.037006 -1.199517 -O -4.181012	1 RNNO-O (e) -2.488907
			1.370850				0.504790				1.397255			0.271856
			H	-1.313314	-3.403713	-	O	-5.077016		-0.351132	H	-2.317457	-3.035770	-	O	-5.042584	-0.382344	-
	C -3.904635 0.220838 0.765039	C -3.398081 0.178466 0.341802	C -3.362675 -0.347631 -0.700481	C -0.203027 -0.349078 1.673348	C -0.154117	0.191187
	0.032387	H	-0.452574	0.079196 -2.169155 -	H	-0.599483	0.074888 -1.080291	H	-1.226612	1.121773 -1.815491	-	H	1.381299 -0.503267	-0.945154
	C -2.651975 1.190489 1.265986 -	C -2.294923 1.416376 1.176317	C -2.232460 -1.117072 -1.433860	H -0.281192 -0.106962 1.687434	H -0.106941	0.737387
	0.317873	H	-3.477316	0.255215 -2.694880 -	C	1.214956	0.347477 -0.100755	H	-4.270264	2.195510 -1.631624	-	C	2.336454 1.260847	-0.028331
	C -1.771163 1.044078 0.388254 -	C -1.082502 0.803031 0.731770 -	C -1.023506 -0.662472 1.055222	C -0.927867	1.995016 0.862984	C -0.739471	2.341866
	0.933223	H	-2.515145	0.248795 -2.155929 -	C	1.507667	0.156625 -0.062886	-	H	-3.306916	0.518132 -1.162074	-	C	0.147460 1.450728 -0.180374	-
	C -2.166764 -0.931261 -2.453515	C -1.017011 -0.505420 -0.559029	C -0.987178	0.524124 2.491041	C -2.038247	2.499264 -0.509285	C -1.809222	2.629975 -
	1.177332	N	-1.441694	0.896764 -1.426608	C	2.288934	0.897419 -0.215521	N	-1.997941	0.402607 -1.317254	C	0.911387 2.399265	0.002449
	C -3.448748 -1.301054 -0.518570	C -2.191008 -1.246366 -1.688497	C -2.187175	1.202458 0.485603	H	0.065289	2.357516 1.670371	H	0.281189	2.492397 -
	0.779059	N	-2.871707	0.998624 -0.672843 -	H	0.708298	1.087676 0.036117	-	N	-3.085288	0.229170 0.122779	-	H	0.221553 0.597108 -0.199956	-
	N -4.312703 -0.469965 -0.985360	N -3.370853 -0.830912 -1.296650	N -3.364975	0.775416 0.824530	H -1.113855	2.266057 1.190479	H -0.881746	2.940803
	0.181967	H	-3.677759	0.522588 -0.121158 -	C	3.583197	0.614761 -0.300980	H	-3.789700	1.571397 0.848562	-	C	1.060570 3.655784	-0.128373
	H -0.776932 1.268011 0.727778 -	H -0.190402 1.195184 1.353466 -	H -0.110339 -1.235334 -0.910140	H -1.666108	2.679980 -1.096183	H -1.412985	2.518630 -
	1.229275	C	-2.250335	0.141601 -0.412582	H	2.116853	0.023647 -0.238813	C	-2.443143	1.422076 -0.018085	H	1.930749 2.307632	0.129040
	H -2.387350 0.167004 2.301614 -	H -2.391683 2.765821 2.132733	H -2.306763 -2.033495 -0.379887	H -2.521117	3.407213 -2.750445	H -2.254452	3.623066 -
	0.116328	N	-2.366870	0.763368 0.699289	C	3.736657	0.928024 -0.256583	-	N	-2.271254	0.029650 0.910898	C	0.803036 3.706095 -0.278963	-
	H -3.809069 -2.319038 -0.925083	H -2.192201 -2.220737 -0.188209	H -2.212382	2.137059 1.266655	N -1.031695	0.548155 0.287035	N -0.982752	0.931296
	0.948182	N	-2.480190	1.492300 2.017349 -	H	4.444850	1.652683 -0.392440	N	-0.783212	0.348818 2.567466	-	H	0.453497 4.565384	-0.108351
	Cl -5.046843 0.129050 1.828429	Cl -4.947854 1.852223 0.860345	Cl -4.909736 -0.885948 -0.175823	N -2.954805	1.363624 -1.691679	N -2.783146	1.578648 -
	0.817110	O	-3.713558	0.690667 2.017662 -	Cl	5.345353	0.680589 -0.362644	-	O	-1.580424	0.384090 2.968442	-	Cl	5.264756	0.630835 -0.448675	-
	C -1.232405 -1.933964 -0.435706	C	0.284558 -1.025771 -0.862800	C	0.307430	1.054822 0.977686	H -3.762834	1.318289 -1.059718	H -3.601560	1.412056 -
	1.814625	O	-1.882135	1.465310 3.011505	N	2.749246	1.468931 -0.137062	-	O	-0.042379	0.997301 3.151509	N	1.206440 2.654169 -0.302675	-
	H -1.798679 -2.809084 -0.314613	H	0.121930 -1.973713 -1.053440	H	0.132063	1.999553 0.565595	C -2.283558	0.237601 -1.082295	C -2.199555	0.582978
	2.160279			1.994034			1.999723				0.063034	0.040735
	H -0.725483 -1.498193 -	H	0.717191 -0.308883 -	H	0.737372	0.344379	N -2.734621 -1.004600 -	N -2.702912 -0.649266
	2.682400			2.173030			2.185474				0.084846	0.324821
	C -0.574092 -3.204168	C	1.131843 -2.341972	C	1.135361	2.343184 -	N -3.959398 -1.067285 -	N -3.874881 -0.779031 -
	0.277229			0.556493			0.572120				0.660242	0.195123
	C	0.789058 -3.429027	C	2.423456 -2.204268	C	2.415193	2.194510 -	O -4.611643 -2.141095 -	O -4.346713 -1.939414
	0.931785			1.370553			1.400977				0.408242	0.072304
	H -1.068425 -4.135321 -	H	1.028070 -3.311187	H	1.036222	3.327626 -	O -5.110203 -0.465414	O -5.621750 -2.141845 -
	0.020573			0.056855			0.102311				0.134532	0.462369
	H -1.253750 -2.632790	H	0.232750 -2.158848	H	0.230410	2.138675 -	H -0.594941 -1.366250	H -0.644501 -0.770714
	0.931988			1.167751			1.167309				0.989337	1.576793
	H	1.249157 -4.371466	H	3.217372 -2.862363	H	3.203420	2.879828 -	C	1.242504 -0.290799	C	1.240908 -0.022062
	0.600354			0.989606			1.057836				0.691388	0.843308
	H	0.746251 -3.415572	H	2.272044 -2.399108	H	2.245955	2.346043 -	C	1.593661 -0.164240 -	C	1.469772 -0.258065 -
	2.024795			2.435751			2.470799				0.652341	0.511323
	N -0.214848 -2.410322 -	N	1.268626 -1.268798 -	N	1.301086	1.300108	C	2.279394 -0.394872	C	2.348739 -0.015140
	0.893377			0.425800			0.439606				1.619394	1.691979
	N	1.544992 -2.291783	N	2.767602 -0.804712	N	2.786811	0.811780 -	H	0.821427 -0.066346 -	H	0.640957 -0.266019 -
	0.421331			1.131045			1.113668				1.418580	1.222977
	H	2.526684 -2.126330	H	3.643477 -0.393806	H	3.658194	0.398663 -	C	3.598640 -0.382619	C	3.615398 -0.245686
	0.609672			1.439213			1.430265				1.185131	1.172933
	C	0.977749 -1.810901 -	C	2.172605 -0.379626	C	2.210014	0.409620	H	2.059851 -0.483432	H	2.226772	0.173523
	0.690410			0.008438			0.025425				2.684555	2.759720
	N	1.360434 -0.869752 -	N	2.361218	0.770849 -	N	2.440532 -0.712142	C	3.810872 -0.256142 -	C	3.706360 -0.466007 -
	1.568602			0.659494			0.738625				0.184742	0.198603
	N	2.579242 -0.334902 -	N	3.279295	1.593014	N	3.347063 -1.548856	H	4.433881 -0.460282	H	4.503037 -0.245919
	1.426154			0.010847			0.067730				1.877288	1.801116
	O	2.875910	0.546229 -	O	4.420065	1.561650 -	O	4.205043 -2.174075	Cl 5.450699 -0.230562 -	Cl 5.279412 -0.743932 -
	2.242636			0.596315			0.755423				0.787984	0.906833
	O	3.377159 -0.691010 -	O	2.747453	2.691061	O	2.498420 -2.230334 -	N	2.857413 -0.145456 -	N	2.682984 -0.474873 -
	0.533892			0.409641			0.725516				1.090040	1.031162
				TS1, T1 (f)		TS2, T1 (g)			3 RN---NO2	3 RNNO-O in
														(h)		T1 (i)
			C	3.187194	0.227055	-	C	-0.205293		-0.060599	C	2.814838	-0.330504	-	C	-0.119348	0.059714
			0.124355				1.314892				0.104796			1.469669
			C	2.151026	1.138337	H	-0.225749		0.396895	C	2.114040		0.556284	H	-0.069546	0.605839
			0.074237				2.317390				0.708377			2.425420
			C	0.975015	0.646553	C	-1.037432		2.128770	C	0.901175		0.115662	C	-1.011961	2.155277
			0.621106				0.331767				1.220359			0.347300
			C	0.879107	-0.711259	C	-2.129584	2.411617	-	C	0.436348		-1.163967	C	-2.178567	2.355329	-
			0.943283				0.703288				0.903215			0.625482
			C	1.988973	-1.514046	H	-0.045765		2.465962	C	1.243134		-1.952450	H	-0.044390	2.445562	-
			0.692219				0.008335				0.083372			0.078388
			N	3.132100	-1.057506	H	-1.264221		2.578975	N	2.416805	-1.548618	-	H	-1.158771	2.696668
			0.166573				1.311582				0.417693			1.295427
			H	0.138042	1.327863	H	-1.716339	2.479670	-	H	0.313353		0.770053	H	-1.840656	2.350273	-
			0.800686				1.720069				1.867321			1.672011
			H	2.270553	2.187294	-	H	-2.695331	3.321754	-	H	2.507230		1.545953	H	-2.733398	3.278748	-
			0.186483				0.483212				0.930126			0.434443
			H	1.966680	-2.580645	N	-1.088372		0.671828	H	0.939235	-2.968736	-	N	-1.065274	0.715845
			0.929145				0.437138				0.181882			0.595987
			Cl	4.689901	0.800493	-	N	-2.966371	1.222997	-	Cl	4.350410	0.173373	-	N	-2.997235	1.179441	-
			0.804530				0.571628				0.772346			0.350447
																	122

  I-T-6h #111 RT: 0.87 AV: 1 NL: 1.01E9 T: FTMS + p ESI Full ms [50.0000-750.0000] MS spectrum of IMD-IMD-N2O3 (m/z = 432.0744/434.0715/436.0689 in ES -) Figure SI-12: MS spectrum of TNO2 (m/z = 194.0812 in ES -) IMD-30min_2104 #440 RT: 3.45 AV: 1 NL: 8.24E6 T: FTMS -p ESI Full ms [50.0000-750.0000] MS spectrum of TONO2 (m/z = 210.0761 in ES -)

	Relative Abundance Relative Abundance	Relative Abundance	211.0744 227.0691 200 250 186.9566 158.9614 150 207.9303 235.9256 300 Figure SI-13: I-T-6h #572 RT: 4.63 AV: 1 NL: 1.10E7 126.0097 50 100 20 30 40 50 60 70 80 90 100 116.9271 T: FTMS -p ESI Full ms [50.0000-750.0000] 40 50 60 70 80 90 100 116.9270 Figure SI-11: 100 150 200 250 350 10 125.8717 190.9275 30 210.0761 174.9548 248.9599 20 0 0 10 235.9256 388.1399 421.1417 m/z 400 450 300 m/z 285.0402 146.9370 61.9866 404.1346 190.9273 301.1804 75.0070 374.1601 417.1294	484.1379 500 329.0667 513.0030 566.2004 516.6428 550 600 350 400 631.6309 650 432.0744 700 450 389.0689 448.0703 470.0482 750 352.8531 396.8912
				100	150	200	250	300	350		400	450	500	550	600	650	700	750
											m/z
			I-T-6h #624 RT: 5.04 AV: 1 NL: 1.37E8
			T: FTMS -p ESI Full ms [50.0000-750.0000]
			IMD-30min_2104 #70 RT: 0.55 AV: 1 NL: 1.31E7 194.0806
			T: FTMS -p ESI Full ms [50.0000-750.0000]
						223.0385
			100
			90
			80
			70
	Relative Abundance	Relative Abundance	40 50 60	116.9270	239.0336				477.0959
			30
							257.0443
								306.0390
			103.0244
					211.0383
			20
					197.0224
											432.0733
			10
			59.0234
					180.8979	205.8386		316.9474	389.0689		494.0739		642.1230	688.1291
															596.1437
			0
				116.9269
			50	100	150	200	250	300	350		400	450	500	550	600	650	700	750
			99.9238							m/z
			61.9866	146.9370	235.9255	294.0078	364.9521	403.1501	472.0932	524.0633
				100	150	200	250	300	350		400	450	500	550	600	650	700	750
											m/z
																160 161 162 163

Sedum floriferum jaune

Sedum reflexum jaune Sedum sexangulare Sedum spurium

Thymus vulgaris

The preliminary experiments studying the plants' uptakes helped us to choose 3 plants among [START_REF] Atkinson | Atmospheric chemistry of VOCs and NOx[END_REF] for further experiments: Sedum sexangulare, Thymus vulgaris, and Heuchera Americana L (Fig. 25). One of the sedum species was selected since the latter are known to be drought tolerant and suitable to be implemented on green roofs. Sedum sexangulare, specifically, had the best removal capacity of NOx among the sedum species in our preliminary experiments; it also has a good performance under severe climate conditions according to literature. The second chosen plant was Thymus vulgaris which is a perennial plant emitting various VOCs having the possibility to remove pollutants such as O3 through possible reactions with volatile metabolites; while Heuchera Americana L. is an aesthetically-pleasing plant often used on green roofs due to its beautiful colors, and can in turn retain particulate matter (PM) due to its hairy leaves.

To detect the volatile compounds collected in a cartridge after air sampling formed due to the reaction between thyme leaves (put in a reactor) and O3, ATD-GC-MS was used. The reactor's gas phase was sampled for 3 min at a sampling rate of 100 mL min -1 into the Tenax sorbent tubes.

After sampling, the tubes were thermally desorbed using TurboMatrix thermal desorption unit

Microscopic techniques

Scanning electron microscopy (SEM)

Thyme's surface

Thyme's abaxial and adaxial cuticular surfaces were examined by SEM (Fig. 36). Leaves were mounted on aluminum stubs using double-sided adhesive tape and sputter-coated with 10-15 nm gold-palladium (20 s, 25 mA, partial argon pressure 60 mTorr, Denton Desk V. The samples noted; if a decrease in values was noticed, IMD powder was added to the already prepared solution in order to attain the needed NO2 concentration. A previous study helped us with the choice of the source which showed that the irradiation of IMD solution produced significant amount of NO2 due to the homolytic cleavage of RN-NO2 [1].

Figure 33 IMD solution in the glass reactor.

Nitrous acid (HONO) trap

The trap was prepared by placing Na2CO3 in a 6 cm Teflon tube sealed with glass wool on both sides and put before the NOx analyzer. Sodium carbonate covered only about ⅙ of the tube.

It trapped the HONO that might have been generated. The trap acts as a denuder [2]. The concentration of HONO was calculated by subtracting the concentration of NO2 from IMD with the trap from that without the trap.

Calculation of the compounds' concentrations

Quantification of thymol after bubbling O3, of CT/IMD, and of the volatiles analyzed by HPLC-DAD was done by the calibration method. In this method, a sample of a known concentration is analyzed (using the same method) serving as a reference. This helps us calculate the concentration of our target molecule by using the following equations:

Where A0 and At are the absorbance of the reference and that of the molecule at any time t respectively, ε is the molar absorptivity coefficient (L mol -1 cm -1 ), ℓ is the path length (1 cm), and Co and Ct are the molar concentrations of the reference molecule and the studied molecule at any time t respectively. By dividing Eq1 by Eq2, we get:

Ao and At are obtained after the HPLC analysis, Co is given; hence, Ct can be calculated using Eq3 by cross multiplication. Note that ε and ℓ in both equations cancel each other.

Same calculations (Eq3) were followed after LC-MS analysis by using the surface area.

Manuscript 3 (C):

Pesticides are regularly sprayed on plants to protect them from harmful pests and their presence on the leaves makes possible interactions, in particular photochemical ones, with plant constituents. For instance, an odorous plant like thyme can emit volatiles which might react with the sprayed pesticides. In this manuscript, the interaction of thyme's volatiles and 2 pesticides (the fungicide CT, and the insecticide IMD) was studied as well as the toxicity of photoproducts (Fig. 50). We first determined what are the products of thyme emitted in the dark and under irradiation.

The HSGC-MS analyses showed the production of thymol, linalool, 3-carene, and α-pinene confirming previous analyses of ATD-GCMS (Manuscript 2). Non-volatile compounds were also detected by LC-MS and GC-MS: several oxidation products, TO, the most concentrated, TQ, TQO, and dimers (Fig. 51). Under irradiation, their production increased; for example, that of TO was 20fold higher. The photochemical interactions between thymol (a phenolic derivative), linalool, 3-carene, and α-pinene (terpenes) with CT was first studied in acetonitrile solutions. They all significantly enhanced the rate of phototransformation of CT that was photoreduced. Laser flash photolysis experiments confirmed the ability of the triplet excited state of CT to react with thyme's volatiles and allowed to determine the reaction rate constants. Then, CT was irradiated on thyme's leaves directly. Again, CT underwent photoreduction. This result is in line with those obtained in solution and suggests the involvement of thyme's volatiles in the process.

Similarly, experiments were conducted with IMD and thymol in solution. In this case, thymol partly inhibited the photolysis of IMD and new photoproducts derived from thymol were detected (Fig. 52). Some of them were nitrated in accordance with the capacity of IMD to release NO2 under irradiation (Manuscript 1). It is supposed that NO2 oxidizes thymol and TO leading to TNO2 and TONO2 respectively. The inhibiting effect of thymol on the IMD photolysis strongly suggests that NO2 also oxidizes IMD. Thyme's twigs dipped in IMD, and either left in the dark or irradiated in the solar simulator produced these nitrated compounds. However, in the presence of light, the formation of TO and TONO2 was drastically increased. These experiments demonstrate that that nitrated compounds can be formed on plants treated with this insecticide. The ECOSAR (ECOlogical Structure-Activity Relationship) computer program was used to estimate the toxicity of the starting and formed compounds. Thymol is moderately toxic; however, its oxidation products (TQ, T-TO-2H, TO-TO-2H) are more toxic. Also, the nitrated oxidation products of thymol (TNO2 and TONO2) are considered toxic especially TNO2. Therefore, formed photoproducts are in general more toxic than the starting one rendering the choice of pesticides on plants important and the study of chemicals in real conditions necessary for a better assessment of their fates and toxicities.

Manuscripts

In this part, the 3 manuscripts will be presented. Mohamad.sleiman@sigma-clermont.fr Claire.richard@uca.fr uptakes of both leaves and full-size plant, the study of interactions between VOCs and O3, the production of HONO through the HONO trap present before the NOx analyzer, and can enable future research to study the possible reaction between NOx and VOCs.

Screening of 13 plants -using their leaves-utilizing the setup led to the choice of the 3 best performant plant species: sedum sexangulare (sedum), thymus vulgaris (thyme), and heuchera Americana L (heuchera). Experiments showed that thyme had the best NO2 uptake followed by sedum and then heuchera. SEM analysis revealed that thyme had the largest stomatal area which explained its good performance in removing NO2 whereas the HONO trap installed before the analyzer confirmed the surface reaction between NO2 and water, case of sedum. Heuchera retained NO2 due to its big hairy leaves. Moreover, the same trend was found with the O3 uptake where thyme had the highest one followed by sedum and then heuchera. ATD-GC-MS explained thyme's performance which showed that its VOCs have the ability to react with O3. Consequently, thyme can be a good solution for urban environments polluted with O3 and NO2. It can be used on all types of green roofs (intensive, semi-intensive, and extensive). However, with pesticides being often sprayed on these roofs, thyme's emission of volatiles, and the presence of light, a photochemical interaction can take place probably producing toxic compounds.

Further experiments dealt with two pesticides: IMD and CT, and thyme: an odorous plant under irradiation (mimicking real irradiation conditions). Manuscript 1 revealed that IMD releases NO2 under irradiation that photoinduces the nitration/nitrosation of phenol and resorcinol. This suggests the possible nitration/nitrosation of phenolic metabolites released by plants. In our experiments, we confirmed this by irradiating thymol and IMD in solution as well as real thyme leaves dipped in IMD. The LC-MS and GC-MS analyses showed the formation of nitrated thymol, nitrated TO, and nitrated dimers. Nonetheless, IMD alone underwent degradation faster than in the presence of thymol which inhibited the process due a competition between the two. In addition to that, photoproducts detected in solution of volatiles (thymol, α-pinene, 3-carene, and linalool) + CT and thyme leaves dipped in CT showed the dechlorination of CT and chlorination of volatiles especially thymol which had the highest disappearance yield. Nevertheless, in the case of CT, this pesticide didn't undergo degradation when present alone but disappeared in the presence of the volatiles especially thymol. In general, thymol was oxidized and CT reduced. The formed photoproducts in both cases showed an important toxicity.

Annex 1 Description of the setup used for the IMD experiments

To investigate the IMD behavior under irradiation (with two fluorescent tubes emitting light with a wavelength range of 300-450 nm with λmax= 365 nm) and identify the possible products generated, IMD aqueous solutions were put in a cylindrical Pyrex gas flow reactor (Fig. A1). Set up configuration for experiments with O3. This setup resembles the previous one in mostly all parts. The difference was the presence of an ozone generator rather than a solution of IMD. Path 1 and path 2 were also present. O3 was generated by turning on the UV pen-ray lamp found inside the generator. Dioxygen bond is broken due to UVC light (185 nm) producing 2 radicals which will react with the O2 and produce a continuous flow of O3. The setup was assembled in a way enabling us to measure both NOx and O3 concentrations by just changing some tube connections.