A Framework for Representing Biodegradation in Atmospheric Multiphase Models

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Abstract. Biodegradation is a key process in natural ecosystems through which microorganisms recycle organic matter. Cloud water can host metabolically active microorganisms, opening the possibility that biodegradation contributes to atmospheric processing of organics. However, the extent to which microbial activity occurs in the atmosphere and modifies carbon budgets, and the parameters that control the rates, remain poorly constrained. We present a framework to quantify the key parameters of biodegradation of six major cloud-water organics (methanol, ethanol, formaldehyde, acetaldehyde, formic and acetic acids) and for unspecified organics. Using a multiphase box model, we perform sensitivity analyses to evaluate the dependence of biodegradation rates on bacterial abundance (Nbact), Henry's law constants (KH(eff)) and biodegradation rate constants (kbact). Biodegradation scales proportionally with Nbact, while fits indicate weaker sensitivities to KH(eff) and kbact with slopes of 0.89 and 0.39 in log–log space, respectively. Biodegradation of compounds with KH > 105 M atm−1 and/or kbact > 2 × 10−13 L cell−1 s−1 is limited by insufficient substrate replenishment in bacteria-containing droplets. Biodegradation rates normalized by cell concentration are consistent in magnitude with values reported for other aqueous environments, resulting in an effective rate constant of kbact,DOM ~10−13 L cell−1 s−1 for dissolved organic matter (DOM). Comparison of predicted biodegradation rates with atmospheric chemical loss processes and biodegradation in other aquatic environments indicates that atmospheric biodegradation may be a significant sink for some individual compounds (C2H5OH, HCOOH, CH3COOH) but generally represents a minor sink (< 1 %) under the conditions explored. Overall, the empirical relationships and sensitivities derived here provide a framework for implementing biodegradation in atmospheric multiphase models of different complexity, enabling assessment not only of its contribution to atmospheric carbon cycling but also of how environmental conditions may affect microbial functioning of the airborne portion of Earth' microbiome.

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