Biological Activity in Clouds: From the Laboratory to the Model

Microorganisms are present in the atmosphere and can survive in cloud droplets. They are able to transform organic compounds and can consequently compete with radical chemistry. Because the cloud system is a complex multiphase medium, the efficiency of biodegradation by cloud microorganisms has to be evaluated by numerical models of different complexity simulating multiphase chemical processes in clouds. However, only abiotic processes are taken into account in these numerical tools. The objective of this work was thus to integrate biological data in an atmospheric cloud chemistry model and to evaluate the effect of microorganisms in the transformation of chemical compounds. For this, experimental biodegradation rates of acetic and formic acids, formaldehyde and hydrogen peroxide by various bacterial representative of cloud biodiversity were experimentally measured. These values have been implemented in a cloud chemistry model describing aqueous phase chemistry with the explicit CLEPS (Cloud Explicit Physico-chemical Scheme) mechanism. Several simulations with and without biodegradation processes have been performed changing temperature (5 °C or 17 ℃) and actinic flux to simulate summer or winter conditions. The chemical scenario (gas concentrations and emission/deposition) is representative of low-NOx emission with significant isoprene emissions.