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

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.

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

Novel Broadband Cavity-Enhanced Absorption Spectrometer for Simultaneous Measurements of NO2 and Particulate Matter

International audienceA novel instrument based on broadband cavity-enhanced absorption spectroscopy has been developed using a supercontinuum broadband light source, which showcases its ability in simultaneous measurements of the concentration of NO2 and the extinction of particulate matter. Side-by-side intercomparison was carried out with the reference NOx analyzer for NO2 and OPC-N2 particle counter for particulate matter, which shows a good linear correlation with r2 > 0.90. The measurement limits (1σ) of the developed instrument were experimentally determined to be 230 pptv in 40 s for NO2 and 1.24 Mm−1 for the extinction of particulate matter in 15 s. This work provides a promising method in simultaneously monitoring atmospheric gaseous compounds and particulate matter, which would further advance our understanding on gas−particle heterogeneous interactions in the context of climate change and air quality

Absolute determination of chemical kinetic rate constants by optical tracking the reaction on the second timescale using cavity-enhanced absorption spectroscopy

International audienceWe report a new spectroscopic platform coupled to an atmospheric simulation chamber for the direct determination of chemical rate constants with high accuracy at a second time-scale resolution. These developed analytical instruments consist of an incoherent broadband cavity enhanced absorption spectrometer using a red light emitting diode (LED) emitting at ∼662 nm (LED-IBBCEAS) associated with a multipass cell direct absorption spectrometer (MPC-DAS) coupled to an external cavity quantum cascade laser (EC-QCL) operating in the mid-infrared region at approximately 8 μm (EC-QCL-MPC-DAS). Spectrometers were employed to investigate the NO3-initiated oxidation of four selected volatile organic compounds (VOCs) for the determination of the corresponding rate constants with a dynamic range of 5 orders of magnitude (from 10−11 to 10−16 cm3 molecule−1 s−1). Rate constants of (6.5 ± 0.5) × 10−15, (7.0 ± 0.4) × 10−13, and (5.8 ± 0.5) × 10−16 cm3 molecule−1 s−1 for propanal, isoprene and formaldehyde, respectively, were directly determined by fitting the measured concentration–time profiles of NO3 and VOCs (measured using a proton transfer reaction time-of-flight mass spectrometer, PTR-ToF-MS) to chemical models based on the FACSIMILE simulation software (version 4.2.50) at 760 torr and 293 ± 2 K. The obtained rate constants are in good agreement with the most recent recommendations of the IUPAC (International Union of Pure and Applied Chemistry). In addition, a rate constant of (2.60 ± 0.30) × 10−11 cm3 molecule−1 s−1 for the oxidation of 2-methoxyphenol by NO3 radicals was first determined using the absolute kinetic method. Compared to the mostly used indirect relative rate method, the rate constant uncertainty is reduced from ∼20% to ∼12%. The results demonstrated the high potential of using modern spectroscopic techniques to directly determine the chemical reaction rate constants