Développement d'un système de mesure de radicaux hydroxyles par spectroscopie d'absorption en cavité résonante hors axe

Le radical hydroxyle OH est un oxydant puissant qui intervient, entre autre,dans de nombreux processus photochimiques atmosphériques. L'objectif de cette thèse était de développerun système de mesure desradicaux OH dédié aux études de laboratoire. Celui-ci est basé sur la spectroscopie d'absorption en cavité résonante hors d'axe (OA-ICOS,Off-Axis integrated Cavity Output Spectroscopy).Notre système repose sur le couplaged'une diode laser fibrée (DFB) émettant dans le proche infrarouge ( 1435 nm) au sein d'une cavitéoptique àhaute finesse de 50 cm. Le parcours d'interaction du faisceau avec le milieu atteint 1263 m. Le couplage hors de l'axe permet d'atteindre une limite de détection à 2,12 x 10 OH / cm . La spectroscopie par modulation de la longueur d'onde (WMS) en modulant l'intensité de la diode laser à 10 kHz est utilisée conjointement à la cavité OA-ICOS. Cette technique permet de s'affranchir du bruit en 1/f. La WM-OA-ICOS atteint alors une limite de détection de 5,7 x 10 OH/cm . Les effets de la Modulation Résiduelle d'Amplitude (RAM) occasionnés par la modulation de l'intensité laser ont ensuite été supprimés. L'implémentation d'une boucle de contrôle de l'intensité laser en amont de la cavité optique a permis de réduire les variations d'intensité de la source et les effets de RAM. Ceci se traduit par une limite de détection plus basse à 5,7 x 10 OH/cm pour 100 secondes d'intégration. La suppression des effets de RAM par moyen optique est utilisée pour la première fois pour réduire la limite de détection de systèmes WM-OA-ICOS. Ceci nous permet d'atteindre une valeur de NEAS qui est parmi les meilleures au monde et offre des perspectives intéressantes.The hydroxyl free radical (OH) plays a central role in atmospheric chemistry due to its high reactivity with VOCs and other trace species. In this thesis, we demonstrate the feasibility of OH radical detection by means of a compact sensor based on off-axis integrated cavity outpout spectroscopy (OA-ICOS) dedicated to laboratory studies. The developed system requires the coupling of a distributed feedback diode (DFB) emitting in near infrared ( 1435 nm) to a 50 cm long spherical high finesse cavity. The effective interaction path length reaches 1263 m. The off-axis injection of the laser beam allows a detection limit of 2,12 x 10 OH / cm . The OA-ICOS is used in combination with wavelength modulation spectroscopy (WMS) by modulating the diode current at 10 kHz. This technique is efficient to remove 1/f noise in the signal. The developed WM-OA-ICOS setup achieves alower detection limit at 5,7 x 10 OH/cm . While modulating DFB current, Residual Amplitude Modulation occurs. This background contribution was removed at the optical level by the implementation of a control-loop on the laser intensity before its onjection to the cavity. This stabilisation loop on WM-OA-ICOS achieves a detection limit 5,7 x 10 OH/cm for an integration time of 100 s. RAM suppression at the optical level was first used to enhance the detection limit of WM-OA-ICOS setup. It makes our OA-ICOS system one of the most efficient in Noise Equivalent Absorption Sensitivity in the world and provides great opportunities for future development.DUNKERQUE-SCD-Bib.electronique (591839901) / SudocSudocFranceF

Theory of heterogeneous / homogeneous catalytic reaction with presaturation

In this communication we present a theoretical study of chemical reactions in a system of solid particles, immersed in a fluid environment. Our primary concern here is investigation of the systems of catalytic particles in a liquid media; however with some slight modifications our results are applicable to aerosol systems as well. We consider a second-order reaction, which proceeds both homogeneously in the environment, and heterogeneously at surfaces of catalyst particles. The Langmuir adsorption-desorption kinetics is considered explicitly. For definiteness, we are considering the so-called "three-step mechanism'' of the heterogeneous reaction.Мы рассматриваем одну реакцию второго порядка, которая протекает как гомогенно в окружающей среде, так и гетерогенно на поверхностях каталитических частиц. Адсорбционно-десорбционная ленгмюровская кинетика учитывается явно. Для определённости мы рассматриваем так называемый "трёхступенчатый механизм'' гетерогенной реакции.Ми розглядаємо одну реакцію другого порядку, що протікає як гомогенно в оточуючому середовищі, так і гетерогенно на поверхнях каталітичних частин. Адсорбційно-дисорбційна ленгмюрівська кінетика враховується явно. Для визначеності ми розглядаємо так званий "триступінчатий механізм'' гетерогенної реакції

Levoglucosan and phenols in Antarctic marine, coastal and plateau aerosols

Due to its isolated location, Antarctica is a natural laboratory for studying atmospheric aerosols and pollution in remote areas. Here, we determined levoglucosan and phenolic compounds (PCs) at diverse Antarctic sites: on the plateau, a coastal station and during an oceanographic cruise. Levoglucosan and PCs reached the Antarctic plateau where they were observed in accumulation mode aerosols (with median levoglucosan concentrations of 6.4 pgm(-3) and 4.1 pgm(-3), and median PC concentrations of 15.0 pgm(-3) and 7.3 pgm(-3)). Aged aerosols arrived at the coastal site through katabatic circulation with the majority of the levoglucosan mass distributed on larger particulates (24.8 pgm(-3)), while PCs were present in fine particles (34.0 pgm(-3)). The low levoglucosan/PC ratios in Antarctic aerosols suggest that biomass burning aerosols only had regional, rather than local, sources. General acid/aldehyde ratios were lower at the coastal site than on the plateau. Levoglucosan and PCs determined during the oceanographic cruise were 37.6 pgm(-3) and 58.5 pgm(-3) respectively. Unlike levoglucosan, which can only be produced by biomass burning, PCs have both biomass burning and other sources. Our comparisons of these two types of compounds across a range of Antarctic marine, coastal, and plateau sites demonstrate that local marine sources dominate Antarctic PC concentrations. (C) 2015 Elsevier B.V. All rights reserved

Secondary organic aerosol formation from the gas-phase reaction of hydroxyl radicals with m-, o- and p-cresol.

International audienc

Secondary organic aerosol formation from the gas-phase reaction of hydroxyl radicals with m-, o- and p-cresol.

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Atmospheric reactivity of hydroxyl radicals with guaiacol (2-methoxyphenol), a biomass burning emitted compound: Secondary organic aerosol formation and gas-phase oxidation products

Methoxyphenols are low molecular weight semi-volatile polar aromatic compounds produced from the pyrolysis of wood lignin. The reaction of guaiacol (2-methoxyphenol) with hydroxyl radicals has been studied in the LPCA simulation chamber at (294±2) K, atmospheric pressure, low relative humidity (RHx conditions using CH3ONO as OH source. The aerosol production was monitored using a SMPS (Scanning Mobility Particle Sizer); the SOA yields were in the range from 0.003 to 0.87 and the organic aerosol formation can be expressed by a one-product gas/particle partitioning absorption model. Transmission (TEM) and Scanning (SEM) Electron Microscopy observations were performed to characterize the physical state of SOA produced from the OH reaction with guaiacol; they display both liquid and solid particles (in an amorphous state).GC-FID (Gas Chromatography - Flame Ionization Detection) and GC-MS (Gas Chromatography - Mass Spectrometry) analysis show the formation of nitroguaiacol isomers as main oxidation products in the gas- and aerosol-phases. In the gas-phase, the formation yields were (10±2) % for 4-nitroguaiacol (1-hydroxy-2-methoxy-4-nitrobenzene; 4-NG) and (6±2) % for 3- or 6-nitroguaiacol (1-hydroxy-2-methoxy-3-nitrobenzene or 1-hydroxy-2-methoxy-6-nitrobenzene; 3/6-NG; the standards are not commercially available so both isomers cannot be distinguished) whereas in SOA their yield were much lower (≤0.1%).To our knowledge, this work represents the first identification of nitroguaiacols as gaseous oxidation products of the OH reaction with guaiacol. As the reactivity of nitroguaiacols with atmospheric oxidants is probably low, we suggest using them as biomass burning emission gas tracers. The atmospheric implications of the guaiacol+OH reaction are also discussed

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International audienc

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International audienc

Secondary organic aerosol formation from the gas-phase reaction of guaiacol (2-methoxyphenol) with NO3 radicals

International audienceMethoxyphenols are oxygenated aromatic compounds emitted by wood combustion (consequently to the pyrolysis of lignin). The atmospheric reaction of nitrate radical (NO3) with guaiacol (2-methoxyphenol), one of the principal representatives of this class of compounds has been investigated in the dark at (294 ± 2) K, atmospheric pressure and low relative humidity (RH < 2%). The formation of secondary organic aerosols (SOAs) has been studied in two simulation chambers. The concentrations time profiles of guaiacol were followed with a PTR-ToF-MS (Proton Transfer Mass Reaction – Time of Flight – Mass Spectrometer) and those of SOAs by an SMPS (Scanning Mobility Particle Sizer). Aerosol yields (Y) were calculated from the ratio of the suspended aerosol mass concentration corrected for wall losses (M0), to the total reacted guaiacol concentration assuming a particle density of 1.4 g cm−3. The aerosol yield increases as the initial guaiacol concentration rises, leading to yield values ranging from 0.01 to 0.21. A very good agreement was observed between the experiments performed in both chambers which gives confidence in the data obtained in this study. The organic aerosol formation can be represented by a one-product gas/particle partitioning absorption model with a stoichiometric coefficient α = 0.32 ± 0.04 and an equilibrium constant K = (4.2 ± 1.0) × 10−3 m3 μg−1. The chemical composition of the aerosols formed was studied after sampling on quartz fiber filter, ultrasonic extraction and analysis by ESI-LC-QToF-MS-MS (ElectroSpray Ionization - Liquid Chromatography - Quadrupole - Time of Flight – Tandem Mass Spectrometry). The oxidation products observed in the condensed phase are mostly nitro-aromatics; they display chemical structures with one, two and three aromatic rings. A reaction mechanism leading to these products has been proposed. To our knowledge, this work represents the first study on the SOAs formation from the reaction of guaiacol with NO3 radicals