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. Water-soluble organic compounds represent a significant fractionof total atmospheric carbon. The main oxidants towards them in the gas andaqueous phases are OH and NO3 radicals. In addition to chemicalsolutes, a great variety of microorganisms (e.g., bacteria, viruses, fungi)have been identified in cloud water. Previous lab studies suggested that forsome organics, biodegradation by bacteria in water is comparable to theirloss by chemical processes. We perform model sensitivity studies over largeranges of biological and chemical process parameters using a box model witha detailed atmospheric multiphase chemical mechanism and biodegradationprocesses to explore the importance of biodegradation of organics in theaqueous phase. Accounting for the fact that only a small number fraction ofcloud droplets (∼0.0001–0.001) contains active bacterial cells, we consider only a few bacteria-containing droplets in the model cloud. We demonstrate that biodegradation might be most efficient for water-soluble organic gases with intermediate solubility (∼104≤KH(eff) [M atm−1] ≤106, e.g., formic and acetic acids). This can be explained by the transport limitation due to evaporation of organics from bacteria-free droplets to the gas phase, followed by the dissolution into bacteria-containing droplets. Forcloud condensation nuclei (CCN)-derived compounds, such as dicarboxylic acids, the upper limit oforganic loss by biodegradation can be approximated by the amount of organics dissolved in the bacteria-containing droplets (&lt;0.1 %). Wecompare results from our detailed drop-resolved model to simplified modelapproaches, in which (i) either all cloud droplets are assumed to containthe same cell concentration (0.0001–0.001 cell per droplet), or (ii) only droplets with intact bacterial cells are considered in the cloud (liquid water content ∼10-11 vol / vol). Conclusions based on these approaches generally overestimate the role of biodegradation, particularly for highly water-soluble organic gases. Our model sensitivity studies suggest that current atmospheric multiphase chemistry models are incomplete for organics with intermediate solubility and high bacterial activity.

Biodegradation by bacteria in clouds: an underestimated sink for some organics in the atmospheric multiphase system