Assessing the efficiency of water-soluble organic compound biodegradation in clouds under various environmental conditions

The objective of this research was to evaluate the role of atmospheric microorganisms in chemical transformations occurring within clouds. To achieve this objective, we measured in the laboratory the rates of biodegradation for four chemical compounds of interest in atmospheric chemistry (formic and acetic acids, formaldehyde and hydrogen peroxide). We implemented them in an explicit model of cloud chemistry simulating the exchange processes between air and droplets and chemical reactivity in both phases. The biodegradation rates were not kept constant; rather, they depended on the concentration of the four targeted species. For this purpose, a series of incubation experiments were performed in the laboratory with microbial strains isolated from cloud water by modulating the initial concentration of the substrate and ambient temperature (5 degrees C and 17 degrees C). Different simulations were carried out to investigate the role of biological activity in contrasting environmental conditions (season, day/night). Sensitivity tests were also performed on parameters controlling the chemical reactivity and exchanges between phases such as the cloud liquid water content or acidity. Analysis revealed that biodegradation in the aqueous phase was competitive with the abiotic processes of transformations for the four compounds, especially in summer (up to 94% in terms of relative contribution). The concentration of formic acid exerted the most significant impact in both the aqueous phase and in terms of total concentration in the cloud. In summer, biodegradation had a strong impact, during both day and night, on formic acid concentration, inducing a loss of 23 and 65%, respectively. At night, in summer, biodegradation was the main sink for formic acid, acetic acid and formaldehyde (81, 56 and 98%, respectively). Sensitivity tests (pH and liquid water content - LWC) further demonstrated the added value of modulating the biodegradation efficiency according to the concentrations of each compound. For instance, the decrease of formic acid aqueous concentration due to biodegradation is between 50 and 70% more important when LWC is reduced by a factor of 3.