Bacteria in clouds biodegrade atmospheric formic and acetic acids

Abstract. Formic and acetic acids are major organic species in cloud water and affect precipitation acidity. In current atmospheric models, their losses are limited to chemical oxidation in the gas and aqueous phases and deposition processes. Previous lab studies suggest that these acids can be efficiently biodegraded in water by atmospherically relevant bacteria. However, the importance of biodegradation as a loss process in the atmospheric multiphase system has not been fully assessed. We implemented biodegradation as an additional sink of formic and acetic acids in a detailed atmospheric multiphase chemistry model. In our model, biodegradation is considered in a small subset of cloud droplets according to atmospheric bacteria concentrations of 0.1 cm⁻³. We predict that up to 20 ppt h⁻¹ formic acid and 5 ppt h⁻¹ acetic acid are biodegraded, affecting the total change of acid concentrations by 20 % and 3 %, respectively. Our model sensitivity studies suggest that acetic acid is most efficiently biodegraded at high cloud water pH (> 5) whereas biodegradation is least efficient for formic acid under such conditions. This trend is explained by the higher solubility of formic acid (high effective Henry’s law constant) that results in less evaporation from bacteria-free and subsequent uptake into bacteria-containing droplets. Our analysis demonstrates that previous estimates of the importance of atmospheric biodegradation were often biased high as they did not correctly account for such diffusion limitation of phase transfer processes between droplets. The results suggest that under specific conditions, biological processes can significantly affect atmospheric composition and concentrations in particular of volatile, moderately soluble organics.