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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/acp-2020-778
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-778
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  13 Aug 2020

13 Aug 2020

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This preprint is currently under review for the journal ACP.

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

Amina Khaled, Minghui Zhang, Pierre Amato, Anne-Marie Delort, and Barbara Ervens Amina Khaled et al.
  • Université Clermont Auvergne, CNRS, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand, 63000 Clermont-Ferrand, France

Abstract. Water-soluble organic compounds represent a significant fraction of total atmospheric carbon. The main oxidants towards them in the gas and aqueous phases are OH and NO3 radicals. In addition to chemical solutes, a great variety of microorganisms (e.g. bacteria, viruses, fungi) has been identified in cloud water. Previous lab studies suggested that for some organics, biodegradation by bacteria in water is comparable to their loss by chemical processes. We perform model sensitivity studies over large ranges of biological and chemical process parameters using a box model with a detailed atmospheric multiphase chemical mechanism and biodegradation processes to explore the importance of biodegradation of organics in the aqueous phase. Accounting for the fact that only a small number fraction of cloud droplets (~ 0.0001–0.001) contains active bacteria cells, we consider only a few bacteria-containing droplets in the model cloud. We demonstrate that biodegradation might be most efficient for volatile organic compounds (VOC) 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 evaporation of organics from bacteria-free droplets to the gas phase, followed by the dissolution into bacteria-containing droplets. For non-volatile organics (NVOC), such as dicarboxylic acids, the upper limit of organic loss by biodegradation can be approximated by the amount of organics dissolved in the bacteria-containing droplets (< 0.01 %). We compare results from this detailed drop-resolved model to simplified model approaches, in which either (i) all cloud droplets are assumed to contain the same cell concentration (0.0001–0.001 cell droplet−1) or (ii) only droplets with intact bacteria cells are considered in the cloud (liquid water content ~ 10−11 vol/vol). Conclusions based on these approaches generally overestimate of the role of biodegradation, in particular, for highly soluble VOC. Our model sensitivity studies suggest that current atmospheric multiphase chemistry models are incomplete for organics with intermediate solubility and high bacterial activity.

Amina Khaled et al.

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Amina Khaled et al.

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