Letter to the Editor

Humus: Dark side of life or intractable "aether"? Dear Editor, Johannes Lehmann and Markus Kleber, in a controversial paper entitled "The contentious nature of soil organic matter" [START_REF] Lehmann | The contentious nature of soil organic matter[END_REF], suggested the abandonment of the term "humus" and other associated terms such as "humic", "humin", "humified", "humification". The first reason they cited was that these labels are only instrumental in outdated soil analytical methods, which are based on alkaline extraction of soil organic matter (SOM). The second reason was that a wide array of soil analytical methods suggests a continuum from macromolecules synthesized by soil-dwelling organisms (plants, animals, and microbes) and released from their living and dead parts to small organic molecules excreted by organisms or issued from the enzymatic degradation of macromolecules. The third reason was that too many "humus" chemists still rely only on extraction procedures, without due attention to the decomposer community. These reasons prompted the authors to propose a model for the fate of organic debris, called the "soil continuum model (SCM)", which considers biopolymers of various sizes, monomers, processes of aggregate formation and destruction, transformation of residues to carbon dioxide, adsorption and desorption of organic matter to mineral surfaces. I believe that their updated view of SOM, rejecting the chemical sense appropriated to the word "humus", is received amiably since soil biology has currently become an unavoidable component of "soil science". However, a further step should be taken towards clarifying our understanding of the humus concept before rejecting it from the field of science, as has been suggested by [START_REF] Ohno | Current understanding of the use of alkaline extractions of soils to investigate soil organic matter and environmental processes[END_REF] in their conclusion to an overview of the most recent developments in SOM chemistry. [START_REF] Piccolo | The supramolecular structure of humic substances: A novel understanding of humus chemistry and implications in soil science[END_REF] described humic substances as "supramolecular associations of self-assembling heterogeneous and relatively small molecules deriving from the degradation and decomposition of dead biological material". This definition is independent of previous ones and opens up new avenues in SOM chemistry. The notion of "supramolecular association", already highlighted based on previous experiments by [START_REF] Piccolo | Molecular size of humic substances. Supramolecular associations versus macromolecular polymers[END_REF], explains why humic compounds extracted by classical analytical methods elude chemical description. Even if unit components of humus have been explored for a long time by soil chemists, no one can describe a humus molecule with certainty. However, the supramolecular concept (small molecules linked by van der Waals forces and hydrogen bonds) allows explaining that organic matter, not only in soils but also in sediments and even in atmospheric aerosols [START_REF] Kiss | Estimation of the average molecular weight of humic-like substances isolated from fine atmospheric aerosol[END_REF], may upon degradation become self-reassembled in molecular clouds doted of original properties. Among them, the ability to rapidly incorporate organic molecules (e.g., pesticides, proteins, sugars) and to intimately associate with minerals (e.g., clays, metal (oxyhydr)oxides) is remarkable [START_REF] Livens | Chemical reactions of metals with humic material[END_REF][START_REF] Senesi | Binding mechanisms of pesticides to soil humic substances[END_REF][START_REF] Varadachari | Clay-humus complexation: Effect of pH and the nature of bonding[END_REF][START_REF] Lichtfouse | Isotope evidence for soil organic carbon pools with distinct turnover rates-II. Humic substances[END_REF][START_REF] Zang | Encapsulation of protein in humic acid from a histosol as an explanation for the occurrence of organic nitrogen in soil and sediment[END_REF]. Molecular disorder operating during humus formation is opposed to molecular ordering in synthesis of proteins and other macromolecules by organisms. Of interest is that the disordered regions of proteins allow them to interact with their binding partners, like humus does [START_REF] Turoverov | The protein kingdom extended: Ordered and intrinsically disordered proteins, their folding, supramolecular complex formation, and aggregation[END_REF]. The strong capacity of humic substances to incorporate and sequester extraneous organic molecules [START_REF] Spaccini | Increased soil organic carbon sequestration through hydrophobic protection by humic substances[END_REF] points to the postulated ability of humus to store recently added atmospheric carbon and thus help mitigate climate change [START_REF] Lal | Soil carbon sequestration to mitigate climate change and advance food security[END_REF]. [START_REF] Gerke | Concepts and misconceptions of humic substances as the stable part of soil organic matter: A review[END_REF] criticized the SCM model proposed by [START_REF] Lehmann | The contentious nature of soil organic matter[END_REF], arguing that they did not take into consideration the polymerizing and further reacting ability of humic substances, restricting their model to decomposition processes and to the shift of plant and animal residues to biopolymers and then to monomers and finally to carbon dioxide. However, this criticism concerns only the processing chain displayed in the central part of the SCM model, overlooking that on both sides of their scheme, [START_REF] Lehmann | The contentious nature of soil organic matter[END_REF] showed that the molecules derived from the degradation of organic debris and the organic debris itself contribute to the formation and destruction of aggregates and the adsorption and desorption processes involving mineral surfaces. The notion of "black carbon" [START_REF] Goldberg | Black Carbon in the Environment[END_REF] was also discussed in detail in Gerke's review, claiming that humic aromatic structures are derived from fire-affected organic matter and thus should not be termed humus. Gerke explained why the methods used to quantify black carbon (molecular marker or UV methods) overestimated it and showed that black carbon and humic substances interacted strongly through covalent and non-covalent linkages.

Baveye and Wander (2019) also responded to [START_REF] Lehmann | The contentious nature of soil organic matter[END_REF]. They showed that despite the turmoil in the soil scientific community caused by Lehmann and Kleber's proposal to reject "humus", this term continues to be largely used by scientists, with an ever-increasing number of publications citing it routinely. They also showed that the "new" SCM model was not new and had been elaborated more than 80 years ago by [START_REF] Waksman | Humus: Origin, Chemical Composition and Importance in Nature[END_REF]. This author defined humus as consisting "of certain constituents of the original plant material resistant to further decomposition, of substances undergoing decomposition, either by processes of hydrolysis or by oxidation and reduction, and of various compounds synthesized by microorganisms". Following Waksman's idea that a purely chemical assessment of humus was a dead end, [START_REF] Baveye | The (bio)chemistry of soil humus and humic substances: Why is the "new view" still considered novel after more than 80 years?[END_REF] pleaded for multidisciplinary research on humus, indicating that this notion was not a prerogative of chemists. From their point of view, microbiology and agronomy have their say, too. We also acknowledge and highly recommend involving zoology as well.

Commonly, non-chemists use the term "humus" to designate any kind of organic matter which cannot be assigned by the naked eye to recognizable plant or animal debris, either in the form of dark-colored deposits of fine organic matter (in superficial humus layers, below the litter) or mixed with mineral matter deeper in the soil [START_REF] Zanella | A European morpho-functional classification of humus forms[END_REF]. This highly transformed organic substrate is the target of well-managed composting processes [START_REF] Sugahara | Composition analysis of humus and characterization of humic acid obtained from city refuse compost[END_REF] and is used to amend the soil for agricultural or horticultural purposes under the name of "compost" (e.g., vermicompost). The application of humified matter to soil is known to improve water retention [START_REF] Giusquiani | Urban waste compost: Effects on physical, chemical, and biochemical soil properties[END_REF], nutrient retention and exchange [START_REF] Steiner | Nitrogen retention and plant uptake on a highly weathered central Amazonian Ferralsol amended with compost and charcoal[END_REF], and heat capture [START_REF] Pinamonti | Compost mulch effects on soil fertility, nutritional status and performance of grapevine[END_REF] and protect soil from erosion [START_REF] Bazzoffi | The effect of urban refuse compost and different tractors tyres on soil physical properties, soil erosion and maize yield[END_REF], among other ecosystem services. Humic substances have been recently shown to be biologically active from a nutritional or physiological point of view. They can be taken up by plants to be assimilated as extra carbon and nitrogen sources [START_REF] Näsholm | Uptake of organic nitrogen by plants[END_REF] and display nutrient-capture and growth-promoting hormone-like properties [START_REF] Nardi | Physiological effects of humic substances on higher plants[END_REF], soil and roots being involved in a win-win feedback mediated by positive interactions [START_REF] Nardi | Soil-root cross-talking: The role of humic substances[END_REF].

However, what is humus to a biologist? When observation moves from the naked eye to the microscopic level of organic and mineral-organic horizons, the biological nature of humus is revealed. [START_REF] Ponge | Ecological study of a forest humus by observing a small volume, first results. I. The L 1 layer of a moder under Scots pine[END_REF][START_REF] Ponge | Ecological study of a forest humus by observing a small volume. II. The L 2 layer of a moder under Scots pine[END_REF][START_REF] Ponge | Ecological study of a forest humus by observing a small volume. III. The F 1 layer of a moder under Scots pine[END_REF][START_REF] Ponge | The Soil Under the Microscope: The Optical Examination of a Small Area of Scots Pine Litter (Pinus sylvestris L.)[END_REF] showed that, by scrutinizing a small volume of pine litter at varying stages of decomposition, most plant (pine and moss) remains were processed by microbes and animals, turning to "black matter" composed of faecal pellets, in which minute plant, fungal, and bacterial remains were clearly visible under a light microscope. The most minute arthropods (e.g., springtails, mites) and annelids (e.g., earthworms and enchytraeids) comminute plant and fungal remains to such an extent that they can only be identified using the maximum magnification of a light microscope. In contrast, larger litterconsuming arthropods (e.g., millipedes, woodlice, fly larvae) accumulate gross fragments, visible under a dissecting microscope, in their faeces. Similar observations were made in mineral-organic horizons, where the intimate association of organic matter with minerals can be unravelled. A large amount of debris, either of plant or microbial origin, can be easily identified in organic-mineral assemblages by transmitted electron microscopy [START_REF] Foster | Microenvironments of soil microorganisms[END_REF][START_REF] Saur | Alimentary studies on the collembolan Paratullbergia callipygos using transmission electron microscopy[END_REF]. Previously, [START_REF] Tisdall | Organic matter and water-stable aggregates in soils[END_REF] showed that in ultrathin sections, the so-called soil microaggregates are quiescent microbial colonies embedded in clay sheets. [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF] showed that the links between the amorphous (non-recognizable using a light microscope) part of SOM and the silt-and clay-sized mineral particles are controlled by the dynamics of earthworm populations. [START_REF] Topoliantz | Burrowing activity of the geophagous earthworm Pontoscolex corethrurus (Oligochaeta: Glossoscolecidae) in the presence of charcoal[END_REF] showed that in tropical slashand-burn cultivated fields, charcoal pieces are ingested and ground into tiny particles in the muscular gizzard and mixed with mineral matter by earthworms. Such observations of biological contributions to humus formation are not new, being a long-standing aim of soil micromorphologists [START_REF] Kubiëna | Micropedology[END_REF][START_REF] Zachariae | Traces of animal activity in the beech forest floor[END_REF][START_REF] Zaiets | Micromorphology of organic matter and humus in Mediterranean mountain soils[END_REF][START_REF] Colombini | Techno-moder: A proposal for a new morpho-functional humus form developing on Technosols revealed by micromorphology[END_REF]. Knowledge of the feeding and behavioral habits of soil organisms, together with plant anatomy, allows much more plant and microbial material to be observed and identified and much more structures (e.g., aggregates, coatings) to be assigned to the activity of animals and microbes, in particular when soil organisms can be observed and identified near traces of their activity [START_REF] Ponge | Ecological study of a forest humus by observing a small volume. I. Penetration of pine litter by mycorrhizal fungi[END_REF][START_REF] Ponge | Food resources and diets of soil animals in a small area of Scots pine litter[END_REF]. Using total DNA as a tracer of biotic (mostly microbial) origin, Zaccone et al. (2018a) showed that most SOM located between aggregates (free or associated with minerals) originates from soil organisms, either as excreta or living or dead bodies.

For a biologist, humus comprises plant, fungal, and bacterial remains varying from micrometers to millimeters in size and "amorphous" matter where partly degraded plant and microbial cell pieces varying from nanometers to micrometers in size can be discerned by transmission electron microscopy [START_REF] Foster | The ultrastructure and histochemistry of the rhizosphere[END_REF]. An increase in nanometer-sized electron-dense particles can be observed during plant cell wall degradation (Messner et al., 1985, Saur and[START_REF] Ponge | Ecological study of a forest humus by observing a small volume. III. The F 1 layer of a moder under Scots pine[END_REF]. These particles could be considered, with caution, as "true" humic substances, the existence of which is still debated [START_REF] Schmidt M W I, Torn M S, Abiven | Persistence of soil organic matter as an ecosystem property[END_REF]. In this regard, unfortunately, over the last 30 years, "modern" techniques of organic matter analysis, such as stable isotopes [START_REF] Briones | Use of δ 13 C to determine food selection in collembolan species[END_REF][START_REF] Tu | Early degradation of plant alkanes in soils: A litterbag experiment using 13 C-labelled leaves[END_REF], high-resolution molecular techniques [START_REF] Lynch | Microbial diversity in soil: Ecological theories, the contribution of molecular techniques and the impact of transgenic plants and transgenic microorganisms[END_REF], and more recently metabolomics [START_REF] Swenson | Untargeted soil metabolomics methods for analysis of extractable organic matter[END_REF], have taken precedence over soil imaging, while adapting the scale of observation to the studied process should be a basic requirement of the search for causal relationships in complex systems [START_REF] Coleman | Soil biology, soil ecology, and global change[END_REF][START_REF] Chapura | Scale, causality, complexity and emergence: Rethinking scale's ontological significance[END_REF].

How to reconcile the view of the biologist with the most recent developments in humus chemistry? The transformation of organic matter in soil, as viewed by the biologist, is mainly a physical process, including comminution [START_REF] Mori | Tree influence on soil biological activity: What can be inferred from the optical examination of humus profiles?[END_REF], leaching of decomposition products [START_REF] Nykvist | Leaching and decomposition of water-soluble organic substances from different types of leaf and needle litter[END_REF], compaction [START_REF] Chauvel | Pasture damage by an Amazonian earthworm[END_REF], physicochemical protection [START_REF] Balesdent | Relationship of soil organic matter dynamics to physical protection and tillage[END_REF][START_REF] Giannetta | Distribution and thermal stability of physically and chemically protected organic matter fractions in soils across different ecosystems[END_REF], mechanical displacement along the soil profile, and mixing (or not) with mineral matter [START_REF] Lavelle | Ecosystem engineers in a self-organized soil: A review of concepts and future research questions[END_REF]. The net result of these transformations, occurring over a large range of scales, is exemplified in the concept of humus form [START_REF] Bal | Morphological investigation in two moder-humus profiles and the role of the soil fauna in their genesis[END_REF]Zanella et al., 2018b). This physical transformation of organic matter is mainly affected by saprophagous animals [START_REF] Wolters | Invertebrate control of soil organic matter stability[END_REF] and, to a more limited extent, by microbial [START_REF] Tisdall | Organic matter and water-stable aggregates in soils[END_REF] and abiotic processes [START_REF] Denef | Importance of macroaggregate dynamics in controlling soil carbon stabilization: Short-term effects of physical disturbance induced by dry-wet cycles[END_REF]. To these physical transformations, visible to the naked eye in the formation of humus horizons (Zanella et al., 2018b), are superimposed microbial [START_REF] Keeler | Effects of long-term nitrogen addition on microbial enzyme activity in eight forested and grassland sites: Implications for litter and soil organic matter decomposition[END_REF] and, to a lesser extent, faunal (Garvı?n et al., 2000) enzymatic degradations, resulting in the formation of easily leached [START_REF] Allison | Extracellular enzyme activities and carbon chemistry as drivers of tropical plant litter decomposition[END_REF] or metabolized small molecules [START_REF] Tian | Chemical composition of dissolved organic matter in agroecosystems: Correlations with soil enzyme activity and carbon and nitrogen mineralization[END_REF]. At the same time, every soil-dwelling organism elaborates its own biomass [START_REF] Powlson | Measurement of soil microbial biomass provides an early indication of changes in total soil organic matter due to straw incorporation[END_REF], which is in turn processed along soil trophic networks [START_REF] Lueders | Identification of bacterial micropredators distinctively active in a soil microbial food web[END_REF] or accumulates as more or less degraded dead bodies [START_REF] Kallenbach | Microbial physiology and necromass regulate agricultural soil carbon accumulation[END_REF]. All those are humus. Most properties humus confers to the soil ecosystem [START_REF] Ponge | The soil as an ecosystem[END_REF] are linked to its high surface area for nutrient exchange and water retention [START_REF] Chiou | The surface area of soil organic matter[END_REF] and its strong affinity to mineral surfaces [START_REF] Vermeer | Adsorption of humic acid to mineral particles. 1. Specific and electrostatic interactions[END_REF]. Some of the abovementioned biological processes contribute to the degradation of organic matter (until respired as carbon dioxide), whereas others stabilize it in various forms, e.g., deep carbon by roots [START_REF] Kell | Breeding crop plants with deep roots: Their role in sustainable carbon, nutrient and water sequestration[END_REF] or earthworms [START_REF] Shuster | Deep-burrowing earthworm additions changed the distribution of soil organic carbon in a chisel-tilled soil[END_REF], clay-humus assemblages by earthworms [START_REF] Scullion | Earthworm activity affecting organic matter, aggregation and microbial activity in soils restored after opencast mining for coal[END_REF] or bacteria (Zaccone et al., 2018a). However, humus-forming processes where biological activity does not play an active role should be mentioned, too. This is the case for peat soils, where humification progresses at a very low rate due to the inherent recalcitrance of organic inputs and the anoxic environment (Zaccone et al., 2018b), and for mor humus (also called "raw humus") accumulating in nutrient-poor terrestrial environments in the absence of notable faunal activity [START_REF] Hempfling | Litter decomposition and humification in acidic forest soils studied by chemical degradation, IR and NMR spectroscopy and pyrolysis field ionization mass spectrometry[END_REF].

All models proposed by soil chemists cope with this view as far as they do not give precedence to a pure chemical formulation of humus which does not fit to SOM complexity even at the smallest scale [START_REF] Lehmann | Spatial complexity of soil organic matter forms at nanometre scales[END_REF]. It has been claimed that most properties given to the soil by organic matter cannot be deduced from its molecular composition [START_REF] Schmidt M W I, Torn M S, Abiven | Persistence of soil organic matter as an ecosystem property[END_REF], and thus that a better knowledge of the environment and of the organisms which contribute to SOM dynamics is urgently needed if we want to dispose of reliable models of carbon cycling and storage [START_REF] Hedges | The molecularly-uncharacterized component of nonliving organic matter in natural environments[END_REF]. Models of SOM dynamics proposed by [START_REF] Komarov | Romul_Hum model of soil organic matter formation coupled with soil biota activity. I. Problem formulation, model description, and testing[END_REF] and [START_REF] Blankinship | Improving understanding of soil organic matter dynamics by triangulating theories, measurements, and models[END_REF], including measurements of microbial and animal effect traits, indicative of their activity, are pivotal steps in this direction. We suggest speaking of humus as the "dark side" of life and not as an abiotic SOM component, as most authors suggest it to be [START_REF] Gerke | Concepts and misconceptions of humic substances as the stable part of soil organic matter: A review[END_REF]. The recognition of the biological nature of humus would allow better assessment of its origin, dynamics, and emergent properties [START_REF] Ponge | Emergent properties from organisms to ecosystems: Towards a realistic approach[END_REF], like a step has been taken in soil science when the direct role of soil organisms in mineral weathering has been universally acknowledged [START_REF] Neilands | Siderophores: Structure and function of microbial iron transport compounds[END_REF][START_REF] Jongmans | Rock-eating fungi[END_REF].

Our knowledge of the large array of soil organic and mineral-organic components collectively called 'humus' could benefit from a close cooperation between chemists and biologists. The concept of soil as a collection of embedded aggregates, the basic unit being the microaggregate [START_REF] Totsche | Microaggregates in soils[END_REF], is highly promising, because it allows processes (e.g., stability, respiration, organic-mineral interactions, nutrient exchange) to be studied at the scale at which they occur in the soil ecosystem [START_REF] Ponge | The soil as an ecosystem[END_REF]. Another promising aspect of strong cooperation between chemists and biologists is the inclusion of biological processes in modelling SOM formation (Chertov et al., 2017a, b;[START_REF] Blankinship | Improving understanding of soil organic matter dynamics by triangulating theories, measurements, and models[END_REF]. Some burning questions such as the use of soil for mitigating climate warming by sequestering more atmospheric carbon [START_REF] Lal | Beyond Copenhagen: Mitigating climate change and achieving food security through soil carbon sequestration[END_REF] could be resolved by focusing our research effort on the interplay between biotic and abiotic soil compartments, with humus at the interface (Zanella et al., 2018a).

In conclusion, this short focus on humus and its various significances (for the gardener, for the chemist, and for the biologist) was intended to show that this complex matter (in both literal and figurative senses) could benefit from a better cooperation between all scientific disciplines devoted to soil studies. Rather than abandoning the term "humus", as provocatively suggested by [START_REF] Lehmann | The contentious nature of soil organic matter[END_REF], I propose considering humus as a prominent agent of measurable soil ecosystem services, including plant growth and fixation of atmospheric carbon, requiring as much protection as life, from which it might be considered as the "dark side".

ACKNOWLEDGEMENT

The author greatly acknowledges the editorial board of Pedosphere and three anonymous reviewers for their major contribution to the improvement of the original manuscript.


Sébastien Thomas

Nicole Blin-Simiand

Michel Héninger

Pascal Jeanney

Joel Lemaire

Lionel Magne

Hélène Mestdagh

Stéphane Pasquiers

Essyllt Louarn

Insights into non-thermal plasma chemistry of acetone diluted in N2/O2 mixtures: a real-time MS experiment

Keywords: Non-thermal plasma, real-time analysis, plasma chemistry, photo-triggered discharge, chemical ionization, acetone

ABSTRACT. Understanding non-thermal plasma reactivity is a complicated task as many reactions take place due to a large energy spectrum. In this work, we used a well-defined phototriggered non-filamentous discharge to study acetone decomposition in N2/O2 gas mixtures. The plasma reactor is associated to a compact chemical ionization FTICR mass spectrometer (BTrap) in order to identify and quantify in real-time acetone and by-products in the plasma. Presence of oxygen (1 to 5%) decreased notably acetone degradation. A tremendous change is observed in the by-product's distribution concomitantly to a global decrease of their total concentration. While main products observed in oxygen-free gas mix are nitrile compounds, in oxygenated media they are replaced by formaldehyde, methanol and ketene. Methanol is maximum for 1% of O2 whereas formaldehyde and ketene concentration reach their maximum value at the highest oxygen concentration tested (5%). A number of nitrate, nitrite and isocyanate organic compounds (C1 and C2) are observed as well along with HNO2, HNO3 and HNCO. These results allow to describe qualitatively the possible reactions occurring in the plasma mixture. It will be used for comparison to a 0D kinetic model.

1-Introduction

Plasma is a partially or totally ionized gas with specific properties. Generally, two types of plasma are considered for industrial processes: thermal plasma, having all the species at the same temperature (local thermal equilibrium), and non-thermal plasma (NTP) where temperature equilibrium is not reached and ions and neutrals are at low temperature whereas electrons are much hotter. In NTP produced in air using pulsed electrical discharges, electrons interact very efficiently with nitrogen, oxygen, and water molecules forming high energy species such as atoms and radicals, metastable and radiative electronic excited states. These highly reactive species can then react with other molecules composing the mixture. They provide an efficient fragmentation method for many molecules, being precursors of a complex chemistry involving all the neutral species. As a result, NTP are often considered in decontamination and depollution technologies, in particular removal of Volatile Organic Compounds (VOCs) diluted in air at a low concentration level (typically less than 1%) [START_REF] Mizuno | Industrial Applications of Atmospheric Non-Thermal Plasma in Environmental Remediation[END_REF][START_REF] Vandenbroucke | Non-Thermal Plasmas for Non-Catalytic and Catalytic VOC Abatement[END_REF][START_REF] Schiavon | Non-Thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: A Review[END_REF][START_REF] Veerapandian | Abatement of VOCs Using Packed Bed Non-Thermal Plasma Reactors: A Review[END_REF] .

VOCs are a large family of organic compounds present in gaseous form at usual pressure and temperature. In ambient air, they are found at trace levels. Nevertheless, they can cause some dramatic health effects due to their toxicity or to their tendency to form aerosol in urban areas.

Contrary to the other techniques, NTP are efficient in a wide range of VOC concentration from below 100ppm to more than 1000ppm [START_REF] Schiavon | Non-Thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: A Review[END_REF]

. It makes it an interesting technique for depollution of low

VOCs emission such as in indoor air depollution 5 or odor removal [START_REF] Preis | Potential of Electric Discharge Plasma Methods in Abatement of Volatile Organic Compounds Originating from the Food Industry[END_REF] . It can operate at room temperature, and allows for rapid on/off switching at moderate energy consumption. However, understanding the processes involved and identifying the species in action are required to improve targeted pollutant degradation, so as to optimize plasma depollution technologies. Mass spectrometry can bring valuable answers for this issue [START_REF] Giardina | Atmospheric Pressure Non-Thermal Plasma for Air Purification: Ions and Ionic Reactions Induced by Dc+ Corona Discharges in Air Contaminated with Acetone and Methanol[END_REF] .

Discharge plasma reactivity starts with formation of electrons and ions. They transfer energy to the background gas molecules present in the dry mix composed of nitrogen and oxygen. Neglecting the role of ions for electrical discharges performed with moderate energy deposition in the plasma, main reactive species are excited states of nitrogen, atomic nitrogen and atomic oxygen [START_REF] Kossyi | Kinetic Scheme of the Non-Equilibrium Discharge in Nitrogen-Oxygen Mixtures[END_REF][START_REF] Guerra | Modelling N2-O2 Plasmas: Volume and Surface Kinetics[END_REF] :

e -+ N2 → N2* + e - (1a)
e -+ N2 → 2 N + e - (1b)

e -+ O2 → 2 O + e - (2)
In reaction (2), the ground state, O( 3 P), and the first excited state, O( 1 D), of atomic oxygen are formed first. In the mix, the latter is rapidly quenched down to O( 3 P) by collisions with N2 and O2.

Then, thereafter in the text, O is considered to be only at its ground state O( 3 P).

Dissociation of oxygen is induced as well by N2:

N2* + O2 → 2 O( 3 P) + N2 (3) i
This reaction participates to the quenching of metastable states of nitrogen [START_REF] Kossyi | Kinetic Scheme of the Non-Equilibrium Discharge in Nitrogen-Oxygen Mixtures[END_REF][START_REF] Guerra | Modelling N2-O2 Plasmas: Volume and Surface Kinetics[END_REF] , that are predominant for the conversion of VOCs in pure nitrogen plasma [START_REF] Pasquiers | Dissociation against Oxidation Kinetics for the Conversion of VOCs in Non-Thermal Plasmas of Atmospheric Gases[END_REF] , and modifies greatly the distribution of reactive species in the plasma. Change in this distribution is essential as it affects the reactivity of the studied pollutants in the NTP environment [START_REF] Penetrante | Identification of Mechanisms for Decomposition of Air Pollutants by Non-Thermal Plasma Processing[END_REF] . The effect of oxygen on the targeted VOC and on types and concentrations of compounds following the decomposition of this molecule has to be studied in detail. However, the great majority of the studied VOC depollution technologies by plasma are operated using dielectric barrier discharges, or moderate applied voltage corona discharges [START_REF] Vandenbroucke | Non-Thermal Plasmas for Non-Catalytic and Catalytic VOC Abatement[END_REF][START_REF] Schiavon | Non-Thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: A Review[END_REF][START_REF] Veerapandian | Abatement of VOCs Using Packed Bed Non-Thermal Plasma Reactors: A Review[END_REF][START_REF] Giardina | Atmospheric Pressure Non-Thermal Plasma for Air Purification: Ions and Ionic Reactions Induced by Dc+ Corona Discharges in Air Contaminated with Acetone and Methanol[END_REF] . These discharges are characterized by highly non-homogeneous transient plasmas.

It makes experimental reactivity studies very difficult, and all the more kinetic modeling. As another way of study, we developed a pre-ionized photo-triggered discharge (PTD) characterized by a homogeneous plasma, giving access to repeatable chemical reactivity in well controlled density and temperature conditions [START_REF] Rozoy | NO Removal in a Photo-Triggered Discharge Reactor[END_REF][START_REF] Magne | OH Kinetics in Photo-Triggered Discharges Used for VOCs Conversion[END_REF][START_REF] Magne | Production and Reactivity of the Hydroxyl Radical in Homogeneous High Pressure Plasmas of Atmospheric Gases Containing Traces of Light Olefins[END_REF][START_REF] Faider | Effect of Oxygen on the Conversion of Acetaldehyde in Homogeneous Plasmas of N2/O2/CH3CHO Mixtures[END_REF][START_REF] Faider | Role of Quenching of Metastable States in Acetaldehyde Decomposition by a Non-Equilibrium Nitrogen Plasma at Sub-Atmospheric Pressure[END_REF][START_REF] Moreau | Propane Dissociation in a Non-Thermal High-Pressure Nitrogen Plasma[END_REF][START_REF] Pasquiers | Role of Nitrogen Metastable States in Non-Thermal Plasma Conversion of Volatile Organic Compounds[END_REF] . In addition, duration of the electrical energy deposition in the gas mixture is limited to several tens of nanoseconds, followed by a long post-discharge period of several hundreds of microseconds without excitation of the reactive medium. As for other discharges, presence of oxygen in the plasma modifies greatly the distribution of species in the mixture [START_REF] Pasquiers | Dissociation against Oxidation Kinetics for the Conversion of VOCs in Non-Thermal Plasmas of Atmospheric Gases[END_REF][START_REF] Faider | Effect of Oxygen on the Conversion of Acetaldehyde in Homogeneous Plasmas of N2/O2/CH3CHO Mixtures[END_REF] . This necessarily has an impact on the degradation of the pollutant molecule added to N2/O2, and on by-products coming from the plasma reactivity.

In previous works, Magne et al. [START_REF] Magne | OH Kinetics in Photo-Triggered Discharges Used for VOCs Conversion[END_REF][START_REF] Magne | Production and Reactivity of the Hydroxyl Radical in Homogeneous High Pressure Plasmas of Atmospheric Gases Containing Traces of Light Olefins[END_REF] studied in the PTD the reactivity of atomic oxygen and hydroxyl radical in the presence of a light olefin, ethane, in N2/O2 mixtures. They observed:

C2H6 + O → • OH + C2H5 • (4)
followed by:

C2H6 + • OH → H2O + C2H5 • (5)
These two reactions compete with the quenching of N2* leading to the dissociation of the hydrocarbon. Maximum concentration of • OH represented circa 2% of the atomic oxygen i All reactions listed in the text are gathered in Table S1 in the supplementary information.

concentration produced by the discharge, for O2 initial quantity of less than 10%. Though, • OH is lost after the discharge due to its reactivity with atomic O to form H • and O2 and to a lower extent with N • to form NO and H • . In the mix, nitric oxide is formed as well [START_REF] Kossyi | Kinetic Scheme of the Non-Equilibrium Discharge in Nitrogen-Oxygen Mixtures[END_REF] mainly by the reactions:

N2* + O → NO + N at short timescale (6)
N + O + N2 → NO + N2 at longer timescale (7) Nitrogen oxides, NO2, N2O, and N2O5, are also formed. Finally, atomic oxygen reacts with oxygen to form ozone:

O + O2 + N2 → O3 + N2 (8)
For the N2/acetone mixture, previous studies using the same reactor [START_REF] Magne | OH Kinetics in Photo-Triggered Discharges Used for VOCs Conversion[END_REF][START_REF] Magne | Production and Reactivity of the Hydroxyl Radical in Homogeneous High Pressure Plasmas of Atmospheric Gases Containing Traces of Light Olefins[END_REF] have suggested that quenching of N2* by CH3COCH3 leads to the production of H • , CH3 • , and CH3CO •10 :

N2* + CH3COCH3 → CH3 • + CH3CO • + N2 (9a) N2* + CH3COCH3 → H • + • CH2COCH3 + N2 (9b)
Recently, we proposed a unique coupling of a chemical ionization FTICR mass spectrometer (CI-FTICR-MS) to the PTD reactor in order to discriminate the products formed in a N2/acetone mix [START_REF] Thomas | Direct and Real-Time Analysis in a Plasma Reactor Using a Compact FT-ICR MS: Degradation of Acetone in Nitrogen and Byproduct Formation[END_REF] .

We could identify many degradation products formed in real-time. Main observed products were HCN and other nitrile compounds and to a lesser extent formaldehyde, acetaldehyde and ketene.

Thanks to real-time detection capacity, an intriguing two step depletion reactivity was observed for high acetone concentrations (>100 ppm).

In this new work, we study acetone removal in nitrogen/oxygen plasma for 500ppm acetone initial concentration and increasing oxygen concentration up to 5% at a total pressure of 0.46 bar. We present in this work a new extensive experimental dataset of byproduct formation and behavior in the plasma during the degradation thanks to CI-FTICR-MS technology. We discuss the dataset in the light of the existing literature about elementary reactions between neutral species which should be created by the plasma reactivity.

2-Material and methods

2.1-Photo-triggered discharge reactor

The operating principle of the photo-triggered discharge 'UV510' has been extensively described elsewhere [START_REF] Magne | Production and Reactivity of the Hydroxyl Radical in Homogeneous High Pressure Plasmas of Atmospheric Gases Containing Traces of Light Olefins[END_REF][START_REF] Faider | Effect of Oxygen on the Conversion of Acetaldehyde in Homogeneous Plasmas of N2/O2/CH3CHO Mixtures[END_REF] . It is a kind of pre-ionized discharge allowing the production of a perfectly homogeneous non-thermal plasma between two planar metallic electrodes. The plasma reactivity proceeds during a short current pulse of 60 ns (electron collisions on molecules to produce atoms, radicals, excited states, ions and various reactions between these species). The discharge volume is VD=50 cm [START_REF] Schiavon | Non-Thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: A Review[END_REF] , enclosed in a stainless-steel cell with a volume VC=500 cm [START_REF] Schiavon | Non-Thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: A Review[END_REF] . A compressor is used to produce a gas flow (7 L.s -1 at 0.46 bar) through the inter-electrode space, in a closed loop with total volume VT = 8.85 L much higher than VD, and the pulse repetition frequency (1 Hz) is chosen such that VC is completely renewed between two consecutive pulses. After each current pulse follows a post-discharge period during which the chemical reactivity between neutral species develops. The primary organic molecules progressively disappear in the gas mixture whereas stable by-products accumulate in VT as the mixture undergoes an increasing number of current pulses, each for equal deposited electrical energy in VD. Concentrations of molecules are measured in VT as a function of the number of pulses, noted ND (discharge number) in the text and figures.

For the present experiment the deposited energy in the plasma volume, per current pulse, is 4.6 J/pulse (92 J.L -1 ). Since the pulse repetition frequency is 1Hz, the number ND is equivalent to the time in seconds.

2.2-Compact FTICR mass spectrometer

BTrap is a compact FTICR-MS instrument (AlyXan, Juvisy-sur-Orge, France) [START_REF] Heninger | Gas Analysis by Electron Ionization Combined with Chemical Ionization in a Compact FTICR Mass Spectrometer[END_REF] . The instrument and its principles have been described in detail in previous instrumental works [START_REF] Heninger | Gas Analysis by Electron Ionization Combined with Chemical Ionization in a Compact FTICR Mass Spectrometer[END_REF][START_REF] Mauclaire | A Compact Permanent Magnet Fourier Transform Ion Cyclotron Resonance Mass Spectrometer[END_REF] . Briefly, the instrument is based on a Halbach permanent magnet, and allows for chemical ionization (CI) in the ICR cell as it makes use of a sequential introduction system. Typical analysis is obtained in 1 to 4 s. Different precursor ions may be used in positive mode (H3O + , O2 + , CF3 + , C6H4F2.H + , etc) [START_REF] Thomas | Direct and Real-Time Analysis in a Plasma Reactor Using a Compact FT-ICR MS: Degradation of Acetone in Nitrogen and Byproduct Formation[END_REF][START_REF] Dehon | Quantitative Analysis of a Complex Mixture Using Proton Transfer Reaction in an FTICR Mass Spectrometer[END_REF][START_REF] Dehon | Chemical Ionization Using CF3+: Efficient Detection of Small Alkanes and Fluorocarbons[END_REF][START_REF] Latappy | Protonated 1,4-Difluorobenzene C6H5F2+: A Promising Precursor for Proton-Transfer Chemical Ionization[END_REF] and negative mode (HO -, O -, etc) [START_REF] Vot | Oxygen Anion (O-) and Hydroxide Anion (HO-) Reactivity with a Series of Old and New Refrigerants[END_REF] . For this study, as many ions formed are oxygenated, H3O + was chosen as a precursor. H3O + is a generalist precursor whose reactivity allows for identification of a large number of oxygenated or nitrogenated VOCs. It reacts by proton transfer and forms a MH + daughter ion whose mass is 1.007 amu above the mass of the neutral molecule.

This technique is often called Proton Transfer Reaction Mass Spectrometry (PTR-MS) [START_REF] Ellis | Chemical Ionization: Chemistry, Thermodynamics and Kinetics[END_REF] and is widely used in the environmental field.

Unless stated otherwise, the data presented here are summed up on 10 spectra, with FT transformation calculated after adding several raw signals in order to decrease the detection limit.

The obtained signal is then an average over 10 datapoints, representing circa 30 discharges.

2.3-Chemical Ionization and ion formation

H3O + is formed by reaction of 40 eV accelerated electrons with a pulse of water vapor (P=10 - 6 mbar). After circa 300 ms, no H2O + is observed and only H3O + is detected. H3O + reacts by proton transfer with species having a higher proton affinity (PA) than H2O (691 kJ.mol -1 ) forming the protonated species:

M + H3O + → MH + + H2O (10)
In some cases, fragmentation of the protonated ion occurs. We identified two products formed by fragmentation of protonated molecules: HONO and CH3NO3 (see supplementary S1). In selectedion flow-tube (SIFT) and drift tube (DT) apparatuses, due to the experimental conditions, HONO.H + is nearly entirely fragmented into NO +27-30 . In our experiments, we observe both HONO.H + (63%) and NO + (37%) as we tend to find lesser fragmentation compared to SIFT or DT [START_REF] Dehon | Quantitative Analysis of a Complex Mixture Using Proton Transfer Reaction in an FTICR Mass Spectrometer[END_REF][START_REF] Vot | Oxygen Anion (O-) and Hydroxide Anion (HO-) Reactivity with a Series of Old and New Refrigerants[END_REF] (Supplementary Figure S1-a andc). Fragmentation of alkyl-nitrite in NO + is limited as well in SIFT and DT instruments [START_REF] Anicich | Evaluated Bimolecular Ion-Molecule Gas Phase Kinetics of Positive Ions for Use in Modeling Planetary Atmospheres, Cometary Comae, and Interstellar Clouds[END_REF] , we then considered that NO + is only due to HONO.

After CH3NO3 protonation, two products are related (Supplementary Figure S1-b andd):

CH3NO3.H + (76%) and CH3OH + • (24%). CH3NO3.H + is probably due to methyl nitrate CH3ONO2 as it is the most stable compound [START_REF] Arenas | Approach to the Atmospheric Chemistry of Methyl Nitrate and Methylperoxy Nitrite. Chemical Mechanisms of Their Formation and Decomposition Reactions in the Gas Phase[END_REF] . Measurement of alkyl nitrate is possible by chemical ionization mass spectrometry (CIMS) as demonstrated by other authors [START_REF] Duncianu | Measurement of Alkyl and Multifunctional Organic Nitrates by Proton-Transfer-Reaction Mass Spectrometry[END_REF][START_REF] Aoki | Detection of C1-C5 Alkyl Nitrates by Proton Transfer Reaction Time-of-Flight Mass Spectrometry[END_REF] . The route to the formation of CH3OH + • is not straightforward and, to our knowledge, has not been reported yet in the scarce literature on the protonation of methyl nitrate or its isomers. Understanding its formation path would need a complete proton reactivity study. This is far beyond the dataset we have and the goal of the present study. We then considered that the signal is due to methyl nitrate whose formation is the most probable. However, the important issue being to understand the plasma reaction routes and to obtain more exact quantification, this question is currently under study.

As chemical ionization is performed in controlled pressure and temperature conditions, direct quantification is possible when knowing the reaction rate coefficient and the signal height of the precursor ion and daughter ions [START_REF] Dehon | Quantitative Analysis of a Complex Mixture Using Proton Transfer Reaction in an FTICR Mass Spectrometer[END_REF] . Proton reaction rate coefficients are usually taken from Anicich [START_REF] Anicich | Evaluated Bimolecular Ion-Molecule Gas Phase Kinetics of Positive Ions for Use in Modeling Planetary Atmospheres, Cometary Comae, and Interstellar Clouds[END_REF] and, when not available, estimated to be equal to capture rate constant as calculated by Su and Chesnavich [START_REF] Su | Parametrization of the Ion-Polar Molecule Collision Rate Constant by Trajectory Calculations[END_REF] . When multiple isomers are possible for the same formula, the most stable isomer is considered first, such as for HNCO compared to HOCN or HCNO. In the case of CH3NO2, the capture rate constants used to determine concentration are different for the two isomers CH3NO2 and CH3ONO as their dipole moments are different. The capture rate constants are then calculated to be kC = 2.9 10 -9 cm 3 .s -1 and 4.4 10 -9 cm 3 .s -1 for methyl nitrite and nitromethane respectively. The latter value is used for concentration determination. Parameters and rate coefficients used for concentration calculation are available in Table S2 in supplementary information.

Moreover, due to collisions of precursor ions with the molecules of the matrix gas, the product ion concentrations obtained have to be corrected by a multiplying factor. This factor is the same for all compounds and has been determined to be 1.50 using calibrated gas [START_REF] Thomas | Direct and Real-Time Analysis in a Plasma Reactor Using a Compact FT-ICR MS: Degradation of Acetone in Nitrogen and Byproduct Formation[END_REF] .

The experimental setup

Acetone is available as a gas diluted in nitrogen at 1000 ppm (Crystal mix, B50 Alphagaz, AirLiquide). Pure nitrogen and air (20% O2) bottles (B50, Alphagaz 1) are purchased from Air

Liquide. Different concentrations of oxygen and acetone are operated in the reactor by pressure adjustment for each gas addition. The total pressure is set to 0.46 bar, value chosen in order to ensure the homogeneity of the plasma [START_REF] Magne | Production and Reactivity of the Hydroxyl Radical in Homogeneous High Pressure Plasmas of Atmospheric Gases Containing Traces of Light Olefins[END_REF][START_REF] Faider | Effect of Oxygen on the Conversion of Acetaldehyde in Homogeneous Plasmas of N2/O2/CH3CHO Mixtures[END_REF] . Before each mix formation, the volume VT is emptied using a turbo-molecular pump at a residual pressure well below 0.1 mbar. First, gas is introduced from the 1000 ppm acetone bottle at a specific pressure depending on the final required acetone partial pressure. Afterwards, oxygen is added from the air bottle and finally pure nitrogen gas is added up to 0.46 bar.

BTrap was connected to the reactor through its "sniffer" inlet. This inlet enables direct introduction of a gas at a pressure close to atmospheric pressure into the apparatus [START_REF] Heninger | Gas Analysis by Electron Ionization Combined with Chemical Ionization in a Compact FTICR Mass Spectrometer[END_REF] . The pressure drop is produced in two steps. The first drop is from the pressure of the reactor (0.46 bar) down to a pressure of a few millibars using a needle valve and a membrane primary pump with a flow of 10 to 50 mL/min connected to a waste. The second drop uses a stainless-steel capillary (internal diameter 130 μm; length 50 mm) placed before the three-way valve. To connect the inlet to the plasma reactor, a fast sampling loop was added to the main gas loop as an intermediate between the reactor and the instrument. A PTFE-coated mini-diaphragm pump (Laboport N86KT, KNF Neuberger SAS, Village-Neuf, France) provided a circa 6 L/min flow in the 1/8-inch PFA tubing.

A 1/8-inch tee connected this loop to the entrance of the sniffer on the instrument. The response time was a few seconds.

3-Results and discussions

A great amount of work has been published about plasma produced in mixtures of atmospheric gases, and many kinetic processes arising in those plasmas are now well-known [START_REF] Kossyi | Kinetic Scheme of the Non-Equilibrium Discharge in Nitrogen-Oxygen Mixtures[END_REF][START_REF] Guerra | Modelling N2-O2 Plasmas: Volume and Surface Kinetics[END_REF] . Plasma reactivity starts with formation of atoms, radicals, electronic excited states, and ions following collisions of electrons on N2 and O2. In conditions of the present experiment, role of ions is negligible and main important species are nitrogen and oxygen atoms, N and O, as well as metastable states of the nitrogen molecule, referred to as N2* [START_REF] Magne | OH Kinetics in Photo-Triggered Discharges Used for VOCs Conversion[END_REF][START_REF] Magne | Production and Reactivity of the Hydroxyl Radical in Homogeneous High Pressure Plasmas of Atmospheric Gases Containing Traces of Light Olefins[END_REF][START_REF] Faider | Effect of Oxygen on the Conversion of Acetaldehyde in Homogeneous Plasmas of N2/O2/CH3CHO Mixtures[END_REF] . Presence of oxygen greatly modifies the distribution of species in the mixture. This necessarily has an impact on the degradation of the pollutant molecule added to N2/O2, and on types and concentrations of byproducts coming from the plasma reactivity. It is expected that the amount of oxygen molecules plays an important role on these issues, discussed in the following for acetone degraded by the homogeneous plasma of the photo-triggered discharge.

3.1-Acetone degradation

With increasing oxygen content, the degradation efficiency of acetone drops drastically as shown on Figure 1, as expected from previous results obtained for the UV510 reactor at 2000 ppm and at 5000 ppm of acetone [START_REF] Pasquiers | Dissociation against Oxidation Kinetics for the Conversion of VOCs in Non-Thermal Plasmas of Atmospheric Gases[END_REF] , and also from other non-thermal plasma results [START_REF] Li | Reaction Kinetic Study of Nonthermal Plasma Continuous Degradation of Acetone in a Closed-Loop Reactor[END_REF][START_REF] Zheng | Experimental Study of Acetone Removal by Packed-Bed Dielectric Barrier Discharge Reactor[END_REF] . In pure nitrogen, the degradation presents a two-step process for initial concentration higher than 100 ppm as observed in our previous work [START_REF] Thomas | Direct and Real-Time Analysis in a Plasma Reactor Using a Compact FT-ICR MS: Degradation of Acetone in Nitrogen and Byproduct Formation[END_REF] . This feature disappears in the presence of oxygen, no marked breakpoints are observed on the corresponding lines in Figure 1. The absence of the breakpoint may also be due to the small value of the slope which would hinder the detection of a second small slope.

Figure 2 presents the evolution of the characteristic number of discharge NC, and of the initial converted quantity per pulse CD, with increasing oxygen content. NC represents the inverse of the initial slope of concentration versus number of discharges ND. It is defined so that:

𝐶𝐶 = 𝐶𝐶 0 𝑒𝑒𝑒𝑒𝑒𝑒 � -𝑁𝑁 𝐷𝐷 𝑁𝑁 𝐶𝐶 � (E1)
Where C is the acetone concentration in the total volume VT.

CD is the converted quantity for a single current pulse performed in the initial (i.e. non-treated) gas mixture in the discharge volume VD, for the different oxygen contents. CD is defined as [START_REF] Moreau | Propane Dissociation in a Non-Thermal High-Pressure Nitrogen Plasma[END_REF] :

𝐶𝐶 𝐷𝐷 = 𝐶𝐶 0 (1 -exp �- 1 𝑁𝑁 𝐶𝐶 �) 𝑉𝑉 𝑇𝑇 𝑉𝑉 𝐷𝐷 (E2)
conversion efficiency. CD shrinks from 290 ppm in pure nitrogen to 140 ppm for 5% oxygen. on the target molecule. In the case of acetone, quenching of N2* is efficient (k=5.5 10 -11 cm 3 .s - 1 ) [START_REF] Clark | Energy Transfer Reactions of N2(A3.SIGMA.U+)[END_REF][START_REF] Herron | Evaluated Chemical Kinetics Data for Reactions of N(2D), N2)), and N2(*3*u+) in the Gas Phase[END_REF] , while oxidation by O reactions (reaction 11) is slow (k=7.5 10 -16 cm 3 .s -1 ) [START_REF] Manion | NIST Chemical Kinetics Database[END_REF] :

O + CH3COCH3 → CH3COC • H2 + • OH ( 11
)
On the contrary, oxidation processes are more important for acetaldehyde and propane [START_REF] Pasquiers | Dissociation against Oxidation Kinetics for the Conversion of VOCs in Non-Thermal Plasmas of Atmospheric Gases[END_REF] . Hence the measured acetone degradation efficiency decreases in the presence of oxygen. In oxygen-free studies, quenching of N2* by CH3COCH3 leads to the production of H • , CH3 • , and CH3CO • 10 according to reactions (9a-b). Thus, for the N2/CH3COCH3 mixture, numerous compounds are formed following radicals' reactivity [START_REF] Thomas | Direct and Real-Time Analysis in a Plasma Reactor Using a Compact FT-ICR MS: Degradation of Acetone in Nitrogen and Byproduct Formation[END_REF] . However, in the presence of a slight amount of oxygen, a tremendous change is observed in by-products distribution (Figure 3). On the one hand, the total quantity of by-products is decreased due to the subsequent change in acetone degradation efficiency in presence of oxygen. In particular, HCN (HCN.H + : m/z 28.0182) maximum concentration measured in VT drops from 257 ppm in pure nitrogen to 73 ppm in 1% O2, 42 ppm in 3% O2 and 31 ppm in 5% O2 (see Figure 4). On the other hand, there is a relative increase in oxygenated compounds such as formaldehyde (CH2O.H + : m/z 31.0178). Nitrite and nitrate compounds are observed as well.

3.2-By-products formation

In this section we will describe the occurrence of specific families. For all detected compounds in the volume VT, evolution of the measured concentration discussed below, studied as a function of the discharge number ND, can result from a complex competition between several gain and loss kinetic processes. Gains and losses compete firstly in the plasma volume VD for each current pulse.

Secondly, molecules created in VD, later diluted in VT, are treated by the discharge similarly with acetone but with specific reaction paths. Thus, it is expected that, for each by-product coming from the decomposition of acetone, concentration increases when ND increases up to a value above which treatment of the by-product becomes effective enough to induce a decrease of its concentration. This decrease can also be caused by large losses, or lower gains, of molecules or radicals responsible for the formation of the considered by-product. As a consequence, kinetic discussions given below should be understood as qualitative. The development of a detailed kinetic scheme for N2/O2/CH3COCH3 mixtures, coupled to the physical model describing the electrical energy transfer to the homogeneous plasma and the effect of current pulse accumulation [START_REF] Faider | Role of Quenching of Metastable States in Acetaldehyde Decomposition by a Non-Equilibrium Nitrogen Plasma at Sub-Atmospheric Pressure[END_REF] , is underway to get more insight into the importance of each process. Results for major species are presented in Table 1. The concentration, CMD, of a molecule formed in the discharge volume for a single current pulse in the initial mixture is simply given by [START_REF] Moreau | Propane Dissociation in a Non-Thermal High-Pressure Nitrogen Plasma[END_REF] :

𝐶𝐶 𝑀𝑀𝐷𝐷 = η 𝑉𝑉 𝑇𝑇 𝑉𝑉 𝐷𝐷 (E3)
where η is the slope of the linear increase in concentration measured in VT at low discharge numbers. As a typical example, Figure 4 presents evolution of the concentration of the hydrogen cyanide molecule as a function of ND: in the absence of oxygen, a linear growth is measured up to about 160 ppm for ND=180. For higher ND values, the treatment by the discharge of HCN molecules produced and diluted in the mixture becomes efficient, and/or formation of this molecule becomes less important, so that the concentration reaches a maximum followed by a slow decrease due to the treatment by the discharge.

Nitrile compounds

Three nitriles are detected throughout the experiments: HCN, CH3CN and C2H5CN. The other nitriles are detected only in pure N2/acetone plasma, among them: C2H3CN, HC3N and C3H7CN [START_REF] Thomas | Direct and Real-Time Analysis in a Plasma Reactor Using a Compact FT-ICR MS: Degradation of Acetone in Nitrogen and Byproduct Formation[END_REF] .

They all present drastically lower concentrations when oxygen is introduced. For instance, HCN maximal concentration is divided by more than 3.5 when 1% oxygen is introduced, and it is increasingly reduced as oxygen amount increases (Figure 4a). At 5% of oxygen, a concentration of 31 ppm is reached in VT at a rate of 11 ppm of HCN formed in the discharge volume for the first 150 pulses.

Nitriles are the main products coming from conversion of hydrocarbons diluted in nitrogen, see for example Torokova et al. [START_REF] Torokova | Atmospheric Pressure Glow Discharge Generated in Nitrogen-Methane Gas Mixture: PTR-MS Analyzes of the Exhaust Gas[END_REF] who studied methane in a mono-filamentary glow discharge at atmospheric pressure. Such compounds have been detected as well in the treatment of diluted acetaldehyde in nitrogen by the filamentary plasma of a pulsed dielectric barrier discharge (DBD) [START_REF] Koeta | Decomposition of Acetaldehyde in Atmospheric Pressure Filamentary Nitrogen Plasma[END_REF] .

Formation of HCN can be explained in particular by the following set of reactions involving radicals from acetone dissociation processes (9a-b), firstly:

N • + • CH3 → HCN + H2 (12a)
but most importantly:

N • + • CH3 → H2CN • + H • (12b)
followed by the three processes:

N • + H2CN • → HCN + NH • (13)
H • + H2CN • → HCN + H2 (14)
H2CN • + H2CN • → HCN + CH2NH (15)
Secondly, the following can also occur:

H • + CH3CO • → • CH3 + HCO • (16)
followed by:

N • + HCO • → HCN + O (17)
On the other hand, routes for the formation of methyl cyanide, CH3CN, are more difficult to identify. As discussed in Koeta et al. [START_REF] Koeta | Decomposition of Acetaldehyde in Atmospheric Pressure Filamentary Nitrogen Plasma[END_REF] about degradation of acetaldehyde, CH3CN can result from a complex scheme involving hydrocarbon radicals and molecules, CH2, C2H3 • , C2H2, C2H4. In particular:

N • + C2H3 • → CH3CN (18)
where C2H3 • is coming from the degradation kinetics of the ketene molecule, CH2CO (see further section 3.2.3) and from C2H4 formation and reactivity. Also, presence of propyl cyanide, C2H5CN, could be explained by [START_REF] Koeta | Decomposition of Acetaldehyde in Atmospheric Pressure Filamentary Nitrogen Plasma[END_REF] :

CN • + CH3CN → HCN + • CH2CN (19)
followed by:

• CH3 + • CH2CN → C2H5CN (20)
and where the cyano radical, CN • , in (19) can come from [START_REF] Moreau | Propane Dissociation in a Non-Thermal High-Pressure Nitrogen Plasma[END_REF][START_REF] Koeta | Decomposition of Acetaldehyde in Atmospheric Pressure Filamentary Nitrogen Plasma[END_REF][START_REF] Tsai | Gas-Phase Atom-Radical Kinetics of Atomic Hydrogen, Nitrogen, and Oxygen Reactions with Fluoromethylene Radicals[END_REF] :

N • + CH2 → • CN + H2 (21)
or from the quenching of N2* by hydrogen cyanide [START_REF] Herron | Evaluated Chemical Kinetics Data for Reactions of N(2D), N2)), and N2(*3*u+) in the Gas Phase[END_REF] :

N2* + HCN → H • + CN • + N2 (22)
CH3CN is also an effective quencher of the nitrogen metastable states [START_REF] Herron | Evaluated Chemical Kinetics Data for Reactions of N(2D), N2)), and N2(*3*u+) in the Gas Phase[END_REF] , as is probably also

C2H5CN: N2* + CH3CN → • CH3 + CN • + N2 (23a) N2* + CH3CN → H • + • CH2CN (23b)
Reactions ( 22)-( 23) can reasonably be considered as the main loss processes inducing a decrease in the concentration of these molecules for ND higher than 400, for the mixture without oxygen.

As emphasized in Figure 4 and Table 1 for both HCN and CH3CN, CD is drastically decreased with addition of only 1% oxygen: presence of O2 in the mix inhibits nitrile formation instead of increase their losses. Indeed, the reactivity of nitriles towards atomic oxygen is limited: the corresponding rate constants are 1.5 10 -17 cm 3 .s -1 for HCN at 298 K [START_REF] Burkholder | Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies[END_REF] and at higher temperature, as no data at 298 K were found in the literature, 3.09 10 -15 cm 3 .s -1 at 321 K for CH3CN and 1.0 10 - 14 cm 3 .s -1 at 354 K for C2H5CN [START_REF] Budge | The Reactions of O(3P) with Acetonitrile and Propionitrile[END_REF] .

Hydroxyl reactivity is limited as well: 3.1 10 -14 cm 3 .s -1 for HCN, 2.3 10 -14 cm 3 .s -1 for CH3CN [START_REF] Burkholder | Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies[END_REF] and 1.8 10 -14 cm 3 .s -1 for C2H5CN [START_REF] Harris | Rate Constants for the Reaction of OH Radicals with CH3CN, C2H5CN AND CH2=CH-CN in the Temperature Range 298-424[END_REF] . This low reactivity with O and OH as well as the decrease of the N2* concentration, owing to quenching by O2 (Reaction 3) and consequently the less importance of reactions ( 22) and ( 23), should explain the long tail of HCN and CH3CN at high discharge number. The other nitrile compounds likely react similarly, but their concentrations go rapidly under detection limit.

Concurrently, reaction of the oxygen atom with the methyl radical coming from the dissociation of acetone is efficient. As a result, addition of O2 in the mixture downplay reactions (12 a-b) in the production of HCN.

To this, it can be added the consumption of N through reaction ( 7) with atomic oxygen. It leads to hinder the formation of methyl and propyl cyanides, as discussed above. This is consistent with results displayed in Figure 4c.

Formaldehyde and methanol

In pure nitrogen mixtures, they both have been detected despite absence of oxygen [START_REF] Thomas | Direct and Real-Time Analysis in a Plasma Reactor Using a Compact FT-ICR MS: Degradation of Acetone in Nitrogen and Byproduct Formation[END_REF] . As soon as oxygen is introduced, and unlike nitrile compounds, CH2O and CH3OH concentrations increase: by a factor of four for formaldehyde, and by a factor of two for methanol at 1% oxygen (Figures 4b-4d). Nevertheless, concentrations do not increase steadily with oxygen content, as the maximum concentration is obtained at 3% oxygen for formaldehyde and 1% for methanol.

Examining the literature about gas phase chemistry at low temperature 41 reveals that, in the presence of oxygen, formaldehyde and methanol can be produced by many reactions, that would be tedious to describe in details here. Considering the value of their rate constants, the most important ones are given in the Annex 1.

In absence of oxygen in the mixture, formaldehyde formation can be basically explained by:

HCO • + HCO • → CH2O + CO ( 24
)
where HCO • comes from (16) and various other reactions. Addition of oxygen causes many other reactions to take place, involving radicals ( • CH3, CH3O Methanol may be produced thanks to the reaction:

• OH + • CH3 + N2 → CH3OH + N2 ( 25
)
where the hydroxyl radical should come in particular from H • recombination with O through the three-body reaction [START_REF] Magne | Production and Reactivity of the Hydroxyl Radical in Homogeneous High Pressure Plasmas of Atmospheric Gases Containing Traces of Light Olefins[END_REF] :

O + H • + N2 → OH • + N2 ( 26
)
and to a lesser extent from oxidation reactions of organic radicals (for instance in reaction 30) or molecules (for example reaction 35).

Oxygen addition in the mixture has a tremendous effect on the formaldehyde concentration produced for one discharge in the volume VD: CMD increases from 7.3 ppm in nitrogen up to 28.1 ppm for 1% O2 (Table 1). For higher oxygen contents, dependence of CMD with respect to the concentration of O2 is much less important, loss processes hindering the increase of the formaldehyde. For the low oxygen percentage values of the present experiment (< 5%), main loss process for CH2O is probably the quenching of N2 * : N2* + CH2O → products + N2 (27) for which a high rate constant equal to 5.0 10 -10 cm 3 .s -1 has been reported [START_REF] Pasquiers | Role of Nitrogen Metastable States in Non-Thermal Plasma Conversion of Volatile Organic Compounds[END_REF] .

However, for higher oxygen content, formaldehyde degradation should be enhanced by its oxidation reaction with atomic oxygen (1.2 10 -13 cm 3 .s -1 at 298K [START_REF] Burkholder | Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies[END_REF] ) as well as by the reaction of • OH (1.0 10 -11 cm 3 .s -1 ) [START_REF] Burkholder | Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies[END_REF] . This behavior would explain the inversion observed in observed for CMD in Table 1 between 3% and 5% of O2.

The situation is quite different for methanol, for which the measured concentration is about one order of magnitude lower than that of formaldehyde. The input of 1% of oxygen in the mixture does not induce an important increase of CMD, and furthermore it decreases when the oxygen percentage further increases up to 5% (Table 1). Thus, the radical kinetics for the production of CH2O and CH3OH (see Annex 1) should be rather in favor of CH2O than CH3OH production. The rate constants for main losses of methanol, i.e. quenching of N2* [START_REF] Herron | Evaluated Chemical Kinetics Data for Reactions of N(2D), N2)), and N2(*3*u+) in the Gas Phase[END_REF] and oxidation by O and by OH [START_REF] Manion | NIST Chemical Kinetics Database[END_REF] are lower than those of formaldehyde by a factor of 10 or 100, quenching of N2* remaining the dominant. However, interestingly, oxygen amount in the plasma plays an amazing role on methanol degradation efficiency. In Figure 4, we observe that for a number of discharges higher than 500, the curves cross, suggesting that more discharges are needed to remove the same quantity of methanol in higher oxygen content compared to 1% oxygen. Then, oxygen at concentrations higher than 1% tends to decrease both formation and depletion rates of methanol. The lower rate of methanol reaction with O and OH may be the reason for this observation. Methanol is then inefficiently depleted in higher oxygen amount as observed for acetone.

Carbonyl compounds

Three main carbonyl compounds are found: C2H4O, C2H2O and C3H6O2. Acetaldehyde (CH3CHO) has a decreasing trend with increasing oxygen content (Figure 5a). On the contrary, ketene (CH2CO) concentration increases with oxygen (Figure 5b). C3H6O2 is an intermediate case as the maximum is obtained for 1% oxygen.

CH3CHO and CH2CO behave quite differently as acetaldehyde concentration is maximum in the oxygen-free mixture, whereas ketene concentration reaches its highest for maximum oxygen percentage. It may also be noticed that ketene and formaldehyde concentrations behave quite similarly as functions of the oxygen content (but with one order of magnitude difference). Both acetaldehyde and ketene may be formed from CH3CO • radical which is directly formed from acetone splitting (reaction 9). The radical can undergo many reactions (see Annex 1), in particular it forms acetaldehyde (reaction 28a) and ketene (reactions 28b, 29-30) along the reactions:

CH3O • + CH3CO • → CH2O + CH3CHO (28a) CH3O • + CH3CO • → CH3OH + CH2CO (28b)
where the radical CH3O • is coming from the kinetics of the methyl, • CH3, in mixtures containing oxygen (see Annex 1), and:

H • + CH3CO • → CH2CO + H2 (29)
O + CH3CO • → CH2CO + • OH (30)
More indirect paths for acetaldehyde production involves the HCO radical:

• CH3 + HCO • → CH3CHO (31)
HCO • + CH3CO • → CO + CH3CHO (32)
where the HCO • radical can come from (16) and various other reactions (see Annex 1), in particular oxidation of ketene (S28 and S29).

Acetaldehyde is also lost by oxidation processes, producing the radical CH3CO • as well in the process:

O + CH3CHO → • OH + CH3CO • (33)
• OH + CH3CHO → H2O + CH3CO • (34)
Reactions ( 28), ( 29), ( 31) and ( 32) are important processes in the oxygen free mixtures, but as for other compounds quenching of N2* breaks down molecules [START_REF] Herron | Evaluated Chemical Kinetics Data for Reactions of N(2D), N2)), and N2(*3*u+) in the Gas Phase[END_REF] :

N2* + CH2CO → products + N2 (35)
N2* + CH3CHO → products + N2

Previous studies about acetaldehyde [START_REF] Faider | Effect of Oxygen on the Conversion of Acetaldehyde in Homogeneous Plasmas of N2/O2/CH3CHO Mixtures[END_REF] suggest that ketene is a part of products in acetaldehyde dissociation (Reaction 36) together with radicals • CH3, HCO • , and CH3CO • .

It is clear from results displayed in Figure 5 that oxygen favored ketene at the expense of acetaldehyde. It results from a complex balance between all reactions cited above, (28) to (36).

Many molecules with C3H6O2 formula are bound to appear such as propenoic acid (CH3CH2COOH), methyl acetate (CH3C(=O)OCH3), ethyl formate (H2C(=O)OCH2CH3) and hydroxyacetone (CH3C(O)CH2OH). Oxidation processes are responsible for the formation of acids, then propenoic acid could be formed. Yet, profiles of C3H6O2 are similar to the one obtained for methanol (see Figure 4), where the maximum of the products are obtained at 1% of oxygen (1.5 ppm). The production of the C3H6O2 could then be due to the common radical CH3CO • and • CH2OH forming hydroxyacetone [START_REF] Tsang | Chemical Kinetic Data Base for Combustion Chemistry. Part 2. Methanol[END_REF] according to:

• CH2OH + CH3CO • → CH3C(O)CH2OH (37)
Where • CH2OH comes from the reactivity of CH3OH and CH2CO.

Formation of methyl acetate may be promoted by [START_REF] Hassinen | Kinetics of Reactions between Methoxycarbonyl, Methyl, and Methoxy Radicals Formed in Flash Photolysis of Dimethyl Oxalate in Gas Phase[END_REF] :

• COOCH3 + CH3 • → CH3C(O)OCH3 (38)

Nitrite and nitrate compounds

Reactivity of N2/O2 plasma promotes the formation of nitrous acid (HONO) in the mix and to a lesser extent the formation of nitric acid (HNO3) (Figure 6). HONO quantity increases both in function of discharge number and oxygen amount. This molecule may be produced by addition reaction with the hydroxyl radical:

OH + NO + N2 → HONO + N2 (39)
whereas HNO3 should come from addition with the hydroperoxyl radical:

HO2 + NO + N2 → HNO3 + N2 (40) with HO2 following from addition of H to the oxygen molecule by three body collision.

Concurrently, depletion of HONO is limited and, at 5% of O2, concentrations appear to reach a steady state at high discharge number. Indeed, HONO reaction with atomic oxygen is slow (9.1 10 - 16 cm 3 .s - 1 50 ). Then it mainly accumulates in the mix and increases with oxygen content for the tested concentrations. Contrary to HONO that presents concentrations up to 50 ppm, the concentration of nitric acid is limited to trace level (< 1 ppm), that is to say more than one order of magnitude lower than the concentration of HONO. A compound of formula CH3NO2 is produced as well during the experiment. The product appears at a constant rate independent on the oxygen content (Figure 6). Two CH3NO2 species are likely to appear: methyl nitrite CH3ONO or its isomer nitromethane CH3NO2. The latter is more stable than CH3ONO by 12.1 kJ.mol -1 51 . Their formation may involve the following reactions:

• CH3 + NO2 + N2 → CH3NO2 + N2 (41a) • CH3 + NO2 + N2 → CH3ONO + N2 (41b) CH3O • + NO • + N2 → CH3ONO + N2 (42)
According to photolysis studies of NO2-hydrocarbons mixtures, CH3NO2 is formed (Reaction 41a) in large excess over CH3ONO (Reaction 41b) [START_REF] Canosa | Product Quantum Yields from the Photolysis of NO2 at 366 Nm in the Presence of Ethylene. -The Role of NO3*[END_REF] . According to high level calculations by Nguyen et al. [START_REF] Nguyen | Nitromethane-Methyl Nitrite Rearrangement: A Persistent Discrepancy between Theory and Experiment[END_REF] , recombination reactions such as (41a) and ( 42) are barrier-less processes. Moreover, isomerization between CH3NO2 and CH3ONO goes through a transition state at 223 kJ.mol -1 51 .

Then nitromethane is probably the most prominent form.

C3H5NO2 is detected at trace level (<1ppm) when oxygen content is increased. As CH3NO2, its formation rate is independent on oxygen concentration. Supposing that C3H5NO2 formation mechanism involves the recombination of an alkyl radical and NO2 similarly to CH3NO2, a possible structure for C3H5NO2 is 3-nitropropene. The compound may as well be due to a nitrite compound formed by the combination of the product of O reaction on acetone and NO according to the reaction [START_REF] Delbos | Pressure and Temperature Dependence of the Rate Constants for the Association Reactions of Vinoxy and 1-Methylvinoxy Radicals with Nitric Oxide[END_REF] :

CH2=C(O • )CH3 + NO → C3H5ONO (43)
The isomerization barrier from nitrite-to nitro-form is high: 235 kJ.mol -1 54 .

A small amount of CH3NO is detected in the mix signaling the direct formation of a C-N bond from NO by:

CH3 • + NO + N2 → CH3NO + N2 (44)
However due to the small quantity observed, NO recombination is probably a minor source of reactivity and probably a minor path to the formation of C3H5NO2. This compound disappears rapidly and comes down to 0 after 1200 discharges.

Another nitrate compound, CH3NO3, is also observed (Figure 6). It presents a concentration up to 2 ppm increasing with O2 content. As we discussed previously, this molecule is probably methyl nitrate CH3ONO2. As for CH3NO2, the initial formation rate appears to be independent of oxygen content. Formation of the compound may be due to the reaction with NO2:

CH3O • + NO2 + N2 → CH3ONO2 + N2 (45)
whose termolecular rate constant equals to 2.3 10 -29 cm 6 .s -2 45 . The temporal evolution curves of CH3ONO2 concentration (Figure 6) are similar to those of ketene: as the initial rate appears independent of O2 content, the maximum concentration corresponds to the highest oxygen content.

As for CH3NO2, CH3ONO2 depletion occurs efficiently, in a parallel manner whatever O2 content.

Isocyanate compounds

At m/z 44.0131, a (H2CNO) + ion attributed to a "HCNO" compound is detected as well in pure nitrogen and in the presence of oxygen. A maximum is obtained, as for methanol, for 1% oxygen.

From exact mass, three isomers are possible: i) cyanic acid HOCN, that would result from the reactivity of • OH and • CN radicals; ii) fulminic acid HCNO; iii) isocyanic acid HNCO.

Isocyanic acid has been detected in the atmosphere by CIMS, either by PTR-MS or using anionic precursors such as acetate ion [START_REF] Leslie | Isocyanic Acid (HNCO) and Its Fate in the Atmosphere: A Review[END_REF][START_REF] Jankowski | The Stability and Generation Pattern of Thermally Formed Isocyanic Acid (ICA) in Air -Potential and Limitations of Proton Transfer Reaction-Mass Spectrometry (PTR-MS) for Real-Time Workroom Atmosphere Measurements[END_REF] . It is emitted by incomplete combustion in oxygen-poor media.

It is thought to be formed from is close in energy [START_REF] Fourré | Relative Thermodynamic Stability of the [C,N,O] Linkages as an Indication of the Most Abundant Structures in the ISM[END_REF] , and may be present as well. Both molecules, CH3NCO and HOCH2CN, are known to be prebiotic molecules [START_REF] Ligterink | The Prebiotic Molecular Inventory of Serpens SMM1. I. An Investigation of the Isomers CH3NCO and HOCH2CN[END_REF] . However, to our better knowledge, no gas phase formation route has been reported yet for HOCH2CN.

Methyl isocyanate may be the product of:

• CH3 + • NCO → CH3NCO (46)
CH3NCO oxidation by • OH proceeds by H-abstraction leading to HNCO, CO2 and CO as main products and small quantity of N2O [START_REF] Papanastasiou | Atmospheric Fate of Methyl Isocyanate, CH3NCO: OH and Cl Reaction Kinetics and Identification of Formyl Isocyanate, HC(O)NCO[END_REF] .

Reactions (45) and ( 46) imply the presence of • NCO radical formation during plasma reactivity.

Formation of • NCO radical is due to reactivity of • CN radical in oxygenated atmosphere according to [START_REF] Perry | Kinetics of the Reactions of NCO Radicals with H2 and NO Using Laser Photolysis-Laser Induced Fluorescence[END_REF] :

CN • + O2 → • NCO + O • (47) CN • + • OH → • NCO + H • (48)

4-Conclusions

We proposed in this work an innovative study of acetone conversion by a transient non-thermal plasma generated in mixtures of nitrogen and oxygen (for a maximum concentration of 5%) with addition of 500 ppm of the organic molecule at a total pressure of 0. Our results show that introduction of oxygen in the carrier gas mix, even at low percentage, greatly modifies the reactivity of the plasma. Firstly, acetone degradation rate is drastically reduced with increasing oxygen content in the mix, and, secondly, this parameter has a tremendous effect on concentrations of some by-products.

When O2 is added in the mix, the decrease of the acetone degradation efficiency by the plasma can be related to the competition between the quenching of metastable electronic excited states of nitrogen N2* by the organic molecule, leading to its dissociation and producing H, CH3 and CH3CO radicals, and the much less efficient reaction of acetone with the oxygen atom. Quenching Isocyanate compounds (HNCO, CH3NCO,...) were identified as well. Furthermore, this study showed that nitrile compounds previously found in the plasma of nitrogen/acetone mixture [START_REF] Thomas | Direct and Real-Time Analysis in a Plasma Reactor Using a Compact FT-ICR MS: Degradation of Acetone in Nitrogen and Byproduct Formation[END_REF] , in particular hydrogen cyanide, are still present when oxygen is added although their concentration values drastically decrease.

Species typical of an oxygen-free media, such as nitriles, are replaced by oxygenated species such as formaldehyde, methanol, and ketene. Formaldehyde is the most abundant product containing carbon. It is increasingly correlated to oxygen content, but methanol concentration is the highest for very low oxygen content (1%). Also, we could observe that acetaldehyde and ketene presented a correlation as increasing oxygen concentration had an inverse effect on this species: the first being decreased, whereas the second increases. Interestingly, the sum of the concentrations of these two compounds is independent of O2 content, as shown in Supplementary S2. This result suggests that there is probably a competition for the same intermediate (CH3CO • ) that is changed in the presence of oxygen, acetaldehyde being formed in pure nitrogen media, whereas ketene is preferably formed in oxygenated media.

Finally, these results allow to describe qualitatively the possible reactions occurring in the plasma mixture. They will be compared to a kinetic model associating Statistical Molecular Fragmentation (SMF) approach of fast bond cleavages occurring initially in the plasma [START_REF] Désesquelles | The Statistical Molecular Fragmentation Model Compared to Experimental Plasma Induced Hydrocarbon Decays[END_REF] and a 0D chemical kinetics model simulating the complex radical chemistry involved at longer time and in the postdischarge zone [START_REF] Pasquiers | Dissociation against Oxidation Kinetics for the Conversion of VOCs in Non-Thermal Plasmas of Atmospheric Gases[END_REF] . This global approach has recently been validated by comparison with experimental plasma analysis results, in the case of small hydrocarbons such as propene [START_REF] Désesquelles | The Statistical Molecular Fragmentation Model Compared to Experimental Plasma Induced Hydrocarbon Decays[END_REF] . Then, the present results will be compared to the predictions of the kinetic model in order to validate and optimize the model for the case of nitrogen and oxygen containing compounds.

Figure 1 .

 1 Figure 1. Evolution of acetone concentration (in ppm) in different N2/O2 mixes at 500 ppm initial concentration in log scale. Each point corresponds to 10 acquisitions, summed and averaged before Fourier transformation using FTICR software.

Figure 2 .

 2 Figure 2. Variation of (a) treated concentration CD in the discharge volume VD and (b) characteristic number of discharges NC for oxygen content varying from 0 to 5%.

Figure 3 .

 3 Figure 3. Mass spectra for 375 discharges in presence (blue spectrum in front) or not (magenta spectrum behind) of oxygen. Signal is cut at 5 for clarity, but H3O + , HCN.H + (in the case of no oxygen) and Acetone.H + have a signal larger than 5.

Figure 4 .

 4 Figure 4. Evolution of the concentrations of HCN, CH2O, CH3CN, CH3OH for different O2 amounts.

Figure 5 .

 5 Figure 5. Evolution of the concentrations of a) acetaldehyde CH3CHO and b) ketene CH2CO.

Figure 6 .

 6 Figure 6. Evolution of the concentrations of HONO, CH3NO2, HONO2 and CH3NO3 for different O2 contents.

19 . 1 .

 191 Scheme 1. Simplified scheme giving main reactivity routes towards the principal VOC subproducts (in bold) observed in a pure nitrogen plasma (black schemes) and in a N2/O2 plasma (black, blue and pink schemes). Number in parentheses are the rate constants of the reaction in 10 - 11 cm 3 .s -1 (the number in brackets are values for three-body reaction evaluated for a total pressure of 0.46bar).

  of N2 metastable states by O2, producing O, decreases their availability to dissociate acetone, whereas reaction of O with CH3COCH3 is not able to compensate for the removal of the organic molecule. Acetone depletion by N2 * is faster than it is by O.On top of that, BTrap has allowed identification and quantification of many molecules (HCN, CH3CN, CH2O, CH3CHO, CH3OH, CH2CO, …) coming from the reactivity of various atoms andradicals (H • , • CH3, HCO • , CH3CO • , N, • CN, O, • OH, HO2 • , CH3O • , CH3O2 • , CH3C(O)O2 • ). Scheme1 summarizes some of the main reaction paths that could explain the observed by-products. Also, nitrite and nitrate compounds (HNO2, HNO3, CH3NO2, CH3ONO2, …) were detected, highlighting the role of nitrogen oxides (NO, NO2) which are inevitably produced in the plasma of N2/O2 mixture.

Table 1 .

 1 CMD, quantity formed by the discharge in VD for a single current pulse in the initial mixture (in ppm) and Cmax, maximal concentration obtained in the total volume VT (in ppm) for HCN, CH3CN, formaldehyde and methanol for different oxygen concentrations.

			0%	1%	2%	3%	5%
	HCN	CMD Cmax	105.4 257.1	31.2 72.9	22.8 53.0	15.6 41.7	11.1 31.4
	CH3CN	CMD Cmax	21.8 7.7	6.1 10.8	4.6 8.7	3.2 7.2	2.2 5.3
	CH2O	CMD Cmax	7.3 7.7	28.1 32.5	31.0 38.8	34.1 45.0	31.4 43.0
	CH3OH	CMD Cmax	2.1 2.9	3.9 6.1	3.0 5.5	2.3 5.2	1.9 4.3

  • NCO radical, which may react either with H • by association or with a hydrogen-containing species by H • atom abstraction[START_REF] Rosanka | Atmospheric Chemical Loss Processes of Isocyanic Acid (HNCO): A Combined Theoretical Kinetic and Global Modelling Study[END_REF] , such as with HCO• 43 :Presence of C2H3NO is detected as well. Methyl isocyanate CH3NCO is then probably formed in the mix, as this molecule is the most stable of the C2H3NO isomers. Yet, glyconitrile HOCH2CN

	• NCO + HCO • → HNCO + CO	(45)

ACKNOWLEDGMENT

The authors would like to thank AlyXan company (Juvisy-sur-Orge, France) for the use of the BTrap-instruments for the experiments.

ABBREVIATIONS

NTP non-thermal plasma; VOC volatile organic compound; CI chemical ionization; FTICR/MS Fourier transform ion cyclotron resonance mass spectrometry; INC ion-neutral complex

ASSOCIATED CONTENT Supporting Information. A Supporting Information file is provided (pdf format). It contains literature data used for our calculation and discussion and reactivity schemes.

AUTHOR INFORMATION

Corresponding Author

* email address: essyllt.louarn@universite-paris-saclay.fr Present Addresses † Université de Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON, 69160

Villeurbanne, France

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources

The PhD thesis of Sebastien Thomas was funded by IDEX Paris Saclay.