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

• Published in: Atmospheric chemistry and physics Vol. 21; no. 4; pp. 3123 - 3141
• Main Authors: Khaled, Amina, Zhang, Minghui, Amato, Pierre, Delort, Anne-Marie, Ervens, Barbara
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 02-03-2021; European Geosciences Union; Copernicus Publications
• Subjects: Acetic acid; Adenosine; Analysis; Atmospheric models; Bacteria; Biodegradation; Biological activity; Cells; Chemical reactions; Chemical Sciences; Chemistry; Cloud condensation nuclei; Cloud droplets; Clouds; Condensation nuclei; Dicarboxylic acids; Droplets; Evaporation; Free radicals; Fungi; Gases; Metabolism; Microorganisms; Moisture content; Multiphase; Organic acids; Organic compounds; Oxidants; Oxidation; Oxidizing agents; Parameter sensitivity; Process parameters; Sensitivity; Solubility; Solutes; Vapor phases; Viruses; Water; Water chemistry; Water content
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-21-3123-2021

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)
have 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 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. For
cloud condensation nuclei (CCN)-derived compounds, 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 (<0.1 %). We
compare results from our detailed drop-resolved model to simplified model
approaches, in which (i) either all cloud droplets are assumed to contain
the 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.