The role of bacterial biodegradation for atmospheric budgets of formic and acetic acids

Formic and acetic acids are ubiquitous components in the atmospheric gas and condensed (clouds, particles, fogs) phases. They originate from various anthropogenic or biogenic sources.

Their production and loss processes in the atmosphere are usually assumed to occur by chemical oxidation processes only. In atmospheric models, their chemical formation and loss processes are described by oxidation reactions with abundant oxidants (e.g., OH, NO3 radicals).

However, lab and model studies suggest that bacteria can efficiently biodegrade these acids and similar organic compounds. Their highest metabolic activity of bacteria is thought to be limited to their time in warm clouds due to the presence of liquid water.

 

We use a process model with detailed descriptions of cloud microphysics, multiphase (gas/cloud) chemistry and biodegradation processes in individual cloud droplets. The model is initialized with data from the Puy de Dome observatory (Auvergne, France), where long-term data sets of chemical, microphysical and biological cloud data in a variety of air masses were collected.

The model description of the multiphase chemistry and cloud microphysics is based on well-established models. Bacterial processes are implemented using lab-derived biodegradation rates for various atmospherically relevant bacteria strains and conditions.

 

We perform model studies for a variety of cloud chemical, biological and microphysical parameter ranges to identify atmospheric conditions, under which biodegradation represents a major loss process of formic and acetic acids. Since the number of bacteria cells is much smaller than that of cloud droplets, we will discuss the importance of the accurate model representation of cloud droplet properties (number concentration, diameter, lifetime) for model results. 

Our study demonstrates that microbiota in the atmosphere interact with chemical compounds and affect their budgets. It shows the need to (i) extend current atmospheric chemistry models and (ii) provide information on microbiota distribution and activity.  Thus, our work represents a study at the interface of atmospheric sciences and biogeochemistry and gives new research perspectives for interdisciplinary efforts in these fields.