Atmospheric chemistry of carboxylic acids: microbial implication versus photochemistry
1Clermont Université, Université Blaise Pascal, Laboratoire de Synthèse Et Etude de Systèmes à Intérêt Biologique (SEESIB), BP 10448, 63000 Clermont-Ferrand, France
2CNRS, UMR 6504, 63177 Aubière, France
3Clermont Université, Université Blaise Pascal, OPGC, Laboratoire de Météorologie Physique (LaMP), BP 10448, 63000 Clermont-Ferrand, France
4CNRS, UMR 6016, 63177 Aubière, France
5Clermont Université, Université Blaise Pascal, Laboratoire de Photochimie Moléculaire et Macromoléculaire (LPMM), BP 10448, 63000 Clermont-Ferrand, France
6CNRS, UMR 6505, 63177 Aubière, France
Abstract. Clouds are multiphasic atmospheric systems in which the dissolved organic compounds, dominated by carboxylic acids, are subject to multiple chemical transformations in the aqueous phase. Among them, solar radiation, by generating hydroxyl radicals (•OH), is considered as the main catalyzer of the reactivity of organic species in clouds. We investigated to which extent the active biomass existing in cloud water represents an alternative route to the chemical reactivity of carboxylic acids. Pure cultures of seventeen bacterial strains (Arthrobacter, Bacillus, Clavibacter, Frigoribacterium, Pseudomonas, Sphingomonas and Rhodococcus), previously isolated from cloud water and representative of the viable community of clouds were first individually incubated in two artificial bulk cloud water solutions at 17 °C and 5 °C. These solutions mimicked the chemical composition of cloud water from "marine" and "continental" air masses, and contained the major carboxylic acids existing in the cloud water (i.e. acetate, formate, succinate and oxalate). The concentrations of these carboxylic compounds were monitored over time and biodegradation rates were determined. In average, they ranged from 2 ×10−19 for succinate to 1 × 10−18 mol cell−1 s−1 for formate at 17 °C and from 4 × 10−20 for succinate to 6 × 10−19 mol cell−1 s−1 for formate at 5 °C, with no significant difference between "marine" and "continental" media. In parallel, irradiation experiments were also conducted in these two artificial media to compare biodegradation and photodegradation of carboxylic compounds. To complete this comparison, the photodegradation rates of carboxylic acids by •OH radicals were calculated from literature data. Inferred estimations suggested a significant participation of microbes to the transformation of carboxylic acids in cloud water, particularly for acetate and succinate (up to 90%). Furthermore, a natural cloud water sample was incubated (including its indigenous microflora); the rates of biodegradation were determined and compared to the photodegradation rates involving •OH radicals. The biodegradation rates in "natural" and "artificial" cloud water were in the same order of magnitude; this confirms the significant role of the active biomass in the aqueous reactivity of clouds.