Key Model Parameters for Constraining Biodegradation as an Atmospheric Sink for Organic Compounds

Biodegradation by airborne bacteria in clouds represents a potential sink for C₁ and C₂ monofunctional organic compounds in the atmosphere [1,2], yet this sink remains largely unaccounted for in atmospheric chemistry models. Several factors contribute to this gap: (i) Biodegradation rates are available for only a limited number of atmospheric bacteria strains and organic substrates, and (ii) systematic measurements of ambient bacteria concentration and diversity, which are needed for model initialization, are sparse.  

To identify compounds for which biodegradation may represent an efficient sink in the atmosphere, we performed model sensitivity studies to identify key parameters that most significantly influence the biodegradation rates for organics in the atmospheric multiphase system, where biodegradation occurs in a small subset (0.1%) of cloud droplets. These parameters include bacterial cell concentration (N_bact) and diversity, and biodegradation rate constants (k_bact), as well as the physicochemical properties of the biodegraded substrates, such as Henry’s law constants (K_H) and chemical reactivity.

Our findings indicate that the amount of biodegraded material (ΔC) scales approximately with the number of active bacteria cells (ΔC ∝ N_bact). Sensitivity of ΔC to the Henry’s law constants of the organic substrate and to biodegradation rate constants are lower, with ΔC ∝  0.9 K_H and ΔC ∝  0.4 k_bact, respectively. However, we find that biodegradation is unlikely to be a significant sink for highly soluble and/or quickly biodegraded compounds that exceed K_H ~ 10⁵ M atm^-1 and k_bact ~ 2·10^-13 L cell^-1 s^-1. For these compounds, biodegradation in individual cloud droplets proceeds so efficiently that the substrate replenishment from the gas phase is not sufficiently fast. Comparing biodegradation rate constants for major organics in cloud water to those derived from measurements in other aquatic environments, such as rivers [3], shows good agreement. Based on this, we suggest that a general rate constant (k_bact = 2·10^-13 L cell^-1 s^-1) can be used to estimate the loss of total water-soluble organic carbon in clouds.   

In conclusion, our model sensitivity studies provide guidance for future lab and field measurements to constrain the data needed to assess the role of biodegradation as a sink for organics in the atmosphere. The identified sensitivities across wide parameter ranges will help to identify conditions and substrates for which atmospheric biodegradation may be significant.

 

[1] Nuñez López, L., Amato, P., and Ervens, B.: Bacteria in clouds biodegrade atmospheric formic and acetic acids, Atmos. Chem. Phys., 24, 5181–5198, https://doi.org/10.5194/acp-24-5181-2024, 2024.

[2] Khaled, A., Zhang, M., Amato, P., Delort, A.-M., and Ervens, B.: Biodegradation by bacteria in clouds: An underestimated sink for some organics in the atmospheric multiphase system, Atmos. Chem. Phys. 21, 3123–3141, https://doi.org/10.5194/acp-21-3123-2021, 2021.

[3] Catalán, N., J. P. Casas-Ruiz, D. von Schiller, L. Proia, B. Obrador, E. Zwirnmann and R. Marcé, Biodegradation kinetics of dissolved organic matter chromatographic fractions in an intermittent river, J. Geophys. Res. Biogeosci., 122, 131– 144, https://doi.org/10.1002/2016JG003512, 2017.