Biodegradation of formic and acetic acids is a significant atmospheric sink

Bioaerosols include bacteria that comprise about <1% of the total aerosol number concentration in the atmosphere. In current atmospheric models, bacteria are considered as not being metabolically active during their atmospheric residence time. This assumption is contradicting laboratory studies that have revealed that small organic compounds, such as formic and acetic acids, can be efficiently biodegraded by bacteria found in the atmosphere.

Formic and acetic acids are ubiquitous in the atmosphere, constituting main organic acids in the gas and aqueous phases. Their sources are usually dominated by direct emissions from biomass burning, fossil fuel combustion, vegetation, and soil, besides secondary production from gas and aqueous phase photochemistry. Their sinks are usually considered to be limited to wet and dry deposition, and oxidation by radicals (OH, NO₃).

To explore the potential role of biodegradation as an additional sink of formic and acetic acids, we implemented their biodegradation in cloud droplets in a detailed atmospheric multiphase chemistry model. As opposed to aqueous phase chemical reactions that occur in all droplets, biodegradation only occurs in a small fraction of droplets (~0.1%) taking into account the small number concentration of bacteria cells in the atmosphere.

We perform model sensitivity studies to identify atmospheric conditions (e.g., pH, cloud droplet size, processing time), under which biodegradation represents a significant sink of the two acids. Our model results show that the concentration of formic and acetic acids may be overestimated by up to 5% (~20 ppt) and 3% (~8 ppt), respectively if biodegradation is not included. The net formation or loss rates are predicted to be reduced by up to 20%. These contributions exceed by far the number concentration of bacteria-containing droplets, which implies that the acids evaporate from bacteria-free droplets and are efficiently taken up and biodegraded in the small portion of droplets. We show that the assumption of an average biodegradation rate in all droplets leads to an overestimate of the biodegradation rate, particularly at pH > 5. Generally, the highest importance of biodegradation is identified for large droplets and at pH ~5, which may be considered representative for remote locations. The results are highly sensitive to the pH value, as it not only increases the partitioning of the acids to the aqueous phase (effective Henry’s law constant) but also the rate constants of the OH reactions which compete with the biodegradation as acid sinks.

We conclude that current atmospheric chemistry models may be incomplete to assess the loss of organics in the atmospheric multiphase system as biodegradation might be a significant loss of formic and acetic acids and possibly of related organics.