Caractérisation d'une nouvelle voie de formation des aérosols organiques secondaires (AOS) dans l'atmosphère (rôle des précurseurs polyaromatiques)

Ce travail a pour objectif d'étudier la formation des aérosols organiques secondaires (AOS) formés dans l'atmosphère à partir de l'oxydation en phase gazeuse de composés organiques volatils en présence d oxydants atmosphériques (ozone, radicaux hydroxyle, chlore et nitrate). Parmi eux, les hydrocarbures aromatiques polycycliques (HAP) ont été proposés comme étant une source potentiellement importante d AOS d origine anthropique. Ainsi, l oxydation de quatre HAP gazeux majoritaires (naphtalène, acénaphtylène, acénaphtène et phénanthrène) en présence des principaux oxydants atmosphériques a été menée afin de déterminer la formation d AOS. La caractérisation des phases gazeuse et particulaire par spectrométrie de masse et spectroscopie optique a permis d identifier les principaux produits d oxydation afin de proposer des mécanismes réactionnels conduisant à la formation d AOS. Les différents rendements de formation ont également été déterminés dans le but d'évaluer l'impact de l'oxydation des HAP en phase gazeuse comme source d aérosols. Les expériences ont été conduites en chambres de simulation atmosphérique ainsi qu'en réacteur à écoulement. L'évolution de l'AOS au cours de son vieillissement a également été étudiée pour identifier les différents processus oxydatifs mis en jeu au sein de l'aérosol organique.This work deals with the secondary organic aerosol (SOA) formation from gas phase oxidation of volatile organic compounds in the presence of atmospheric oxidants (ozone, hydroxyl radical, chlorine and nitrate radical). Among them, polycyclic aromatic hydrocarbons (PAHs) have been proposed as an important potential source of anthropogenic SOA. The oxidation of 4 main gaseous PAHs (naphthalene, acenaphthylene, acenaphthene and phenanthrene) in the presence of main atmospheric oxidants has been performed in order to investigate the SOA formation. Characterization of both gas and particulate phases has been carried out using mass spectrometry and optical spectroscopy allowing the identification of products in both phases. Then, chemical mechanisms have been proposed in order to explain SOA formation. SOA yields have been also determined to evaluate the impact of the gas phase oxidation of PAHs in SOA formation. Experiments have been carried out using flow tube and atmospheric simulation chambers. SOA fate has been investigated to determine the different oxidation processes involved in SOA aging.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Caractérisation des dérives du benzo[a]pyrène (B[a]P) en atmosphère réelle et en laboratoire

National audiencePolycyclic Aromatic Hydrocarbons (PAH) are emitted by all the combustion sources and are present in both the particulate and the gas phases. Since they are potentially carcinogenic and/or mutagenic they have been widely studied. Benzo[a]pyrene (B[a]P) have been chosen as a tracer for PAH because of his toxicity. It is mostly present in the particular phase (90%). Very reactive, it can be oxidized to form nitrated and oxygenated by-products which can be more toxic than B[a]P itself. These reactions can occur in the air and/or on the sampling support. The purpose of this work is to determine the major place of reaction, to identify the oxidizing species responsible for these degradations and by-products formation, for a better B[a]P quantification. Field campaign and reactor tests will allow to identify the reaction products and oxidants, to determine the major place of degradation, to improve B[a]P monitoring. Analytical and sampling methods will be described. The analysis of filters sampled with an impactor will be presented.Les Hydrocarbures Aromatiques Polycycliques (HAP) sont des composés émis sous forme gazeuse et particulaire, lors de combustions incomplètes. Ces composés potentiellement cancérigènes et/ou mutagènes sont réglementés. C'est le benzo[a]pyrène (B[a]P) qui a été choisi comme traceur pour cette famille de composé par la IVème directive fille européenne (2004/107/CE), en raison de sa forte toxicité. Ce composé qui est présent à plus de 90% en phase particulaire est très réactif. Il peut réagir avec la plupart des oxydants gazeux pour former des composés nitrés et oxygénés qui peuvent potentiellement être plus toxiques que le B[a]P lui-même. Ces réactions peuvent avoir lieu dans l'air (en amont du prélèvement) et/ou sur le support de prélèvement. Le but de l'étude est de déterminer le lieu de réaction majoritaire, puis d'identifier les espèces oxydantes responsables de ces dégradations, ainsi que les produits formés, afin de proposer des solutions permettant de mieux quantifier le B[a]P. Pour cela, une étude de terrain couplée à des tests en réacteur permettront dans un premier temps, d'identifier les produits de réactions et les oxydants, puis de déterminer le lieu majoritaire de dégradation. La méthode analytique, ainsi que les méthodes de prélèvements seront décrits. Les résultats présentés sont issus de filtres prélevés avec un impacteur en cascade

Sampling precautions for the measurement of nitrated polycyclic aromatic hydrocarbons in ambient air

International audienceBecause of the toxicity of polycyclic aromatic hydrocarbons (PAHs) and of their oxidation products, such as nitrated and oxygenated PAHs (NPAHs and OPAHs), the determination of their concentrations is of great interest in terms of atmospheric pollution control. Then, normalisation of sampling procedures appears essential. In this context, this paper presents a comparison of particulate PAH, OPAH and NPAH concentrations determined with two different samplers (cascade impactor and conventional high volume sampler) installed in parallel during several field sampling campaigns carried out under different environmental conditions. For winter and summer periods, the PAH and OPAH concentrations determined with both sampling systems were considered as equivalent. In the summer period, NPAH concentrations quantified with both sampling devices were similar whereas in the winter period, the conventional high volume sampler underestimated their concentrations by a factor of 3-4. This underestimation was observed in the same proportion for all the 17 quantified NPAHs. Analytical error, NPAH formation during the sampling and NPAH degradation by reaction with gaseous oxidants associated to sampling methodology were unable to explain such differences between both samplers used in parallel. A probable hypothesis is that the heating of the PM10 head of the high volume sampler in the winter period generates an increase of the internal sampler temperature that could intensify the chemical degradation of the NPAHs inducing the underestimation of their concentrations in the atmosphere. Further investigations will be necessary to confirm the importance of the temperature on the chemical degradation of these compounds and to understand the different behaviour of PAHs and OPAHs. Consequently, we suggest using oxidant scrubber to prevent chemical degradation of PAHs and derivatives during their sampling. Moreover, we advise against the heating of the sampling head which could induce an increase of these reactions of degradation especially for NPAHs

Ecological importance of soil bacterivores for ecosystem functions

BackgroundBacterivores, mostly represented by protists and nematodes, are a key component of soil biodiversity involved in soil fertility and plant productivity. In the current context of global change and soil biodiversity erosion, it becomes urgent to suitably recognize and quantify their ecological importance in ecosystem functioning.ScopeUsing meta-analysis tools, we aimed at providing a quantitative synthesis of the ecological importance of soil bacterivores on ecosystem functions. We also intended to produce an overview of the ecological factors that are expected to drive the magnitude of bacterivore effects on ecosystem functions.ConclusionsBacterivores in soil contributed significantly to numerous key ecosystem functions. We propose a new theoretical framework based on ecological stoichiometry stressing the role of C:N:P ratios in soil, microbial and plant biomass as important parameters driving bacterivore-effects on soil N and P availability for plants, immobilization of N and P in the bacterial biomass, and plant responses in nutrition and growth

Polycyclic aromatic hydrocarbons (PAHs), nitrated PAHs and oxygenated PAHs in ambient air of the Marseilles area (South of France) : concentrations and sources

International audienceAmbient measurements (gas + particle phases) of 15 polycyclic aromatic hydrocarbons (PAHs), 17 nitrated PAHs (NPAHs) and 9 oxygenated PAHs (OPAHs) were carried out during July 2004 on three different sites (urban, sub-urban and rural) in the region of Marseilles (South of France). Atmospheric concentrations of these classes of polyaromatics are great of interest because of their high potential mutagenicity and carcinogenicity. OPAH concentrations were of the same order of magnitude as those of PAHs while NPAH concentrations were one to two orders lower. 9-Fluorenone and 9,10-anthraquinone were the most abundant OPAHs, accounting for about 60% and 20% of the total OPAH concentration. Respectively 1-and 2-nitronaphthalene were the most abundant NPAHs and were accounting for about 30-50% and 15-30% of the total NPAH concentration. NPAHs and OPAHs concentration levels were consistent with the characteristics of the sampling sites. Study of source specific ratios (2-nitrofluoranthene/1-nitropyrene) clearly showed those primary NPAH sources influence the urban and sub-urban sites whereas production of secondary NPAHs by gas phase reactions was prevalent at the rural site. The study of NPAH and OPAH sources suggested that gasoline engines were an important source of such compounds Whereas the dominant source of 1-nitropyrene, 2-nitrofluorene, 6-nitrochrysene and benz[a]anthracene-7,12-dione seems to be diesel vehicles. Finally, 9,10-anthraquinone presents a double origin: primary diesel emission and photochemical processes. Formation of 9,10-anthraquinone from anthracene ozonation was shown at the rural site. Further investigations will be necessary in order to discriminate when (before or during the sampling) the OPAHs are formed

Effect of a legume cover crop (Mucuna pruriens var. utilis) on soil carbon in an ultisol under maize cultivation in southern Benin

Long term fallow is no longer possible in densely populated tropical areas, but legume cover crops can help maintain soil fertility. Our work aimed to study changes in soil carbon in a sandy loam Ultisol in Benin, which involved a 12-year experiment on three maize cropping systems under manual tillage: traditional no-input cultivation (T), mineral fertilized cultivation (NPK), and association with Mucuna pruriens (M). The origin of soil carbon was also determined through the natural abundance of soil and biomass C-13. In T, NPK and M changes in soil carbon at 0-40 cm were -0.2, +0.2 and +1.3 t C ha(-1) yr(-1), with residue carbon amounting to 3.5, 6.4 and 10.0 t C ha(-1) yr(-1), respectively. After 12 years of experimentation, carbon originating from maize in litter-plus-soil (0-40 cm) represented less than 4% of both total carbon and overall maize residue carbon. In contrast, carbon originating from mucuna in litter-plus-soil represented more than 50% of both total carbon and overall mucuna residue carbon in M, possibly due to accelerated mineralization of native soil carbon (priming effect) and slow mulch decomposition. Carbon originating from weeds in litter-plus-soil represented c. 10% of both total carbon and overall weed residue carbon in T and NPK. Thus mucuna mulch was very effective in promoting carbon sequestration in the soil studied

Gas- and particle-phase products from the photooxidation of acenaphthene and acenaphthylene by OH radicals

This work is focused on the gas-phase oxidation of acenaphthylene and acenaphthene by OH radicals and associated secondary organic aerosol (SOA) formation under low and high-NOx conditions. Experiments were carried out in an atmospheric simulation chamber using a proton transfer reaction time-of-flight-mass spectrometer (PTR-TOF-MS) and an aerosol time-of-flight-mass spectrometer (ATOFMS) to chemically characterize the gas- and particle-phase products, respectively. Due to the structures of these two aromatic compounds, the proposed chemical mechanisms exhibit some differences. In the case of acenaphthene, H-atom abstraction from the saturated cyclopenta-fused ring was found to be competitive with the OH-addition to the aromatic rings. During the photooxidation of acenaphthene using nitrous acid (HONO), aromatic ring-opening products such as indanone and indanone carbaldehyde, generated through OH addition to the aromatic ring, were formed in higher yields compared to low-NOx conditions. In the case of acenaphthylene, OH addition to the unsaturated cyclopenta-fused ring was strongly favored. Hence, ring-retaining species such as acenaphthenone and acenaphthenequinone, were identified as the main reaction products in both gas- and particle-phases, especially under high-NOx conditions. Subsequent SOA formation was observed in all experiments and SOA yields were determined under low/high-NOx conditions to be 0.61/0.46 and 0.68/0.55 from the OH-initiated oxidation of acenaphthylene and acenaphthene, respectively