SSCI-VIDE+ATARI:CARE+BNO:BDAInternational audienc

Glyoxal, a common atmospheric gas, has been reported to be depleted in some regions of the atmosphere. The corresponding sink could be accounted for by reactions in or at the surface of atmospheric particles, but these reactions were not identified. Recently, we showed that inorganic ammonium ions, NH(4)(+), are efficient catalysts for reactions of carbonyl compounds, including glyoxal, in the liquid phase. To determine whether ammonium-catalyzed reactions can contribute to depletion of glyoxal in the atmosphere, the reactivity of this compound in aqueous solutions containing ammonium salts (ammonium sulfate, chloride, fluoride, and phosphate) at 298 K has been studied. The products identified by LC-HRMS and UV absorption revealed a mechanism involving two distinct pathways: a Bronsted acid pathway and an iminium pathway. The kinetics of the iminium pathway was studied by monitoring formation of a specific product. This pathway was second order in glyoxal in most of the solutions studied and should therefore be second order in most ammonium-containing aerosols in the atmosphere. The corresponding rate constant, k(II) (M(-1) s(-1)), increased strongly with ammonium ion activity, a(NH(4)(+)), and pH: k(II) (M(-1) s(-1)) = (2 +/- 1) x 10(-10) exp((1.5 +/- 0.8)aNH(4)(+)) exp((2.5 +/- 0.2)pH). This iminium pathway is a lower limit for the ammonium-catalyzed consumption of glyoxal, but the contribution of the acid pathway is expected to be small in tropospheric aerosols. With these results the reactive uptake of glyoxal on ammonium-containing aerosols was estimated and shown to be a possible explanation for depletion of this compound in Mexico City.

Many atmospheric aerosols contain both organic compounds and inorganic material, such as sulfate salts. In this work, we show that these sulfates could trigger some chemical transformations of the organic compounds by producing sulfate radicals, SO4−, when exposed to UV light (280–320 nm). In particular, we show by mass spectrometry (LC/ESI‐MSMS) that isoprene, methyl vinyl ketone, methacrolein, and α‐pinene in irradiated sulfate solutions (ammonium and sodium sulfate) produce the same organosulfates as previously identified in aerosols, and even some that had remained unidentified until now. With a typical time constant of 9 h instead of 4600 days for esterifications, these radical reactions would be a plausible origin for the atmospheric organosulfates. These reactions also produced efficient surfactants, possibly resembling the long‐chain organosulfates found in the experiments. Thus, photochemistry in mixed sulfate/organic aerosols could increase cloud condensation nuclei (CCN) numbers, which would be supported by previous atmospheric observations.

Abstract. The reactions of a-pinene and of its main oxidation product, pinonaldehyde (3-acetyl-2,2-dimethyl-cyclobutyl-ethanal), with OH radicals have been studied in the laboratory using Fourier transform infrared spectroscopy for real-time monitoring of the gas-phase chemical species and a Scanning Mobility Particle Sizer system (3071 A, TSI) for the study of the secondary aerosol formation. All gas-phase molar yields were quantified using calibrated reference of the pure compound, except for the nitrates products. The results were: for the a-pinene experiments in the presence of NOx, pinonaldehyde, (87 _ 20)%; total nitrates (18 _+ 9)%; formaldehyde, (23 +_ 9)%; acetone (9 _ 6)%; for the a-pinene experiments in the absence of NOx: pinonaldehyde, (37 _+ 7)%; formaldehyde, (8 _ 1)%; acetone, (7 _+ 2)%; for the pinonaldehyde experiments in the presence of NO, formaldehyde (152 _ 56)% and acetone (15 _+ 7)%. The aerosol measurements showed that the condensed products accounted for the missing carbon in the gas-phase balance. The partitioning of the products into the condensed phase was found to be potentially .significant under experimental conditions but less than 10% for initial a-pinene concentrations lower than 1013 molecule cm -3 and hence negligible under atmospheric conditions in the absence of aerosol seeds. On the basis of these results a comprehensive mechanism for the gas-phase reaction of a-pinene with OH in the presence of NOx has been proposed, including quantitative values for all the involved branching ratios. al., 1998], have been studied in laboratory. These studies have generally focused on the first oxidation step and only a few products have been quantified. Pinonaldehyde (3-acetyl-2,2-dimethyl-cyclobutyl-ethanal) was found to be the main product but has never been quantified on an absolute basis by means of a calibrated sample of the pure compound. The only other reaction product identified and quantified is acetone. Moreover, most of the studies have not taken into account the condensed products formed by these reactions, which may represent a significant fraction of the carbon balance under laboratory conditions as it will be shown in the present work. As a result, the understanding of the mechanism for the reaction of a-pinene with OH radicals remained difficult. Furthermore, secondary oxidation steps such as the further reactions of pinonaldehyde, which are important to assess the overall effect of a-pinene on tropospheric chemistry, have never been studied.In the present work, the mechanism of the reaction of a-pinene with OH radicals has been studied. Several gas-phase products such as pinonaldehyde, alkyl nitrates, acetone, and formaldehyde, have been identified and, except for the alkyInitrates, quantified using calibrated reference samples. This work also gives a first insight into the oxidation mechanism beyond the initial oxidation step by studying the reaction of pinonaldehyde with OH radicals. A quantification of the condensed products formed in these reactions has been performe...