Atmospheric chemistry of carboxylic acids: microbial implication versus photochemistry

The objective of this work was to compare experimentally the contribution of photochemistry vs. microbial activity to the degradation of carboxylic acids present in cloud water. For this, we selected 17 strains representative of the microflora existing in real clouds and worked on two distinct artificial cloud media that reproduce marine and continental cloud chemical composition. Photodegradation experiments with hydrogen peroxide (H2O2) as a source of hydroxyl radicals were performed under the same microcosm conditions using two irradiation systems. Biodegradation and photodegradation rates of acetate, formate, oxalate and succinate were measured on both media at 5 degrees C and 17 degrees C and were shown to be on the same order of magnitude (around 10(-10) - 10(-11) Ms-1). The chemical composition (marine or continental origin) had little influence on photodegradation and biodegradation rates while the temperature shift from 17 degrees C to 5 degrees C decreased biodegradation rates of a factor 2 to 5. In order to test other photochemical scenarios, theoretical photodegradation rates were calculated considering hydroxyl (OH) radical concentration values in cloud water estimated by cloud chemistry modelling studies and available reaction rate constants of carboxylic compounds with both hydroxyl and nitrate radicals. Considering high OH concentration ([OH]=1x10(-12) M) led to no significant contribution of microbial activity in the destruction of carboxylic acids. On the contrary, for lower OH concentration (at noon, [OH]=1x10(-14) M), microorganisms could efficiently compete with photochemistry and in similar contributions than the ones estimated by our experimental approach. Combining these two approaches (experimental and theoretical), our results led to the following conclusions: oxalate was only photodegraded; the photodegradation of formate was usually more efficient than its biodegradation; the biodegradation of acetate and succinate seemed to exceed their photodegradation.