09 Nov 2023
 | 09 Nov 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Bacteria in clouds biodegrade atmospheric formic and acetic acids

Leslie Nuñez López, Pierre Amato, and Barbara Ervens

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.

Leslie Nuñez López et al.

Status: open (until 21 Dec 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Leslie Nuñez López et al.

Leslie Nuñez López et al.


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Short summary
Living bacteria comprise a small particle fraction in the atmosphere. Our model study shows that atmospheric bacteria in clouds may efficiently biodegrade formic and acetic acids that affect acidity of rain. We conclude that current atmospheric models underestimate losses of these acids as they only consider chemical processes. We suggest that biodegradation can affect atmospheric concentration not only of formic and acetic acids but also of other volatile, moderately soluble organics.