Ubiquitous atmospheric production of organic acids mediated by cloud droplets

The common assumption in global atmospheric chemistry models is illustrated in black: aqueous-phase methanediol (HOCH2OH) is neglected and aqueous-phase formic acid (HCOOH) is assumed to form directly from formaldehyde (HCHO) on reaction with OH. The implementation of HOCH2OH multiphase equilibria is illustrated in red: the explicit representation of the slow dehydration of aqueous-phase HOCH2OH, of its fast outgassing from cloud droplets and of its OH-initiated oxidation in the gas phase leads to a pervasive production of gaseous HCOOH. Under typical daytime conditions with average [OH](g) = 1 × 106 molecules cm−3 and [OH](aq) = 1 × 10−13 mol l−1, the lifetimes of HOCH2OH against OH are about 1 × 105 s and 3 × 104 s, respectively. Under typical midday conditions with [OH](g) = 5 × 106 molecules cm−3, the gas-phase sink is five times stronger. Thus, gas-phase oxidation sustains the chemical gradient that drives HOCH2OH from the aqueous to the gas phase.

Carbon dioxide and organic acids are rising in importance concerning the acidity of the atmosphere. One of these organic acids, formic acid, is presently heavily underestimated in chemistry-climate models because key processes regarding sources and sinks are yet to be completely understood. Formic acid contributes to the clouds’ and rainwaters’ acidity and to cloud droplet nucleation. Therefore, this knowledge gap must be filled as soon as possible.

A new research article published in Nature by Franco et al. at the Jülich Supercomputing Center titled “Ubiquitous atmospheric production of organic acids mediated by cloud droplets” concentrated on the chemical processes of the formation of formic acid in the atmosphere and its impact on other processes.

The simulations were made with the numerical global atmosphere-climate model ECHAM5/MESSy (EMAC) which is a modular global climate and chemistry simulation system with sub-models for the calculation of processes in the troposphere and middle atmosphere and ocean-land surface interactions. The simulations were performed on the Jülich Research on Exascale Cluster Architectures (JURECA) supercomputer. For determining the burden of formic acid, data from the Infrared Atmospheric Sounding Interferometer (IASI) onboard a MetOp-A satellite was used. The model predictions were also tested against experiments at the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich.

The study showed that gaseous formic acid is formed by formaldehyde via a multiphase pathway involving its hydrated form, methanediol. Methanediol outgasses fast but dehydrates slowly in warm cloud droplets. The authors estimate that methanediol’s gas-phase oxidation generates up to four times more formic acid than all other known chemical sources combined. The pH of clouds and rainwater is reduced by up to 0.3 due to the additional formic acid burden which leads to higher atmospheric acidity.

The scientists add that the knowledge gained through this study could also be applied to other aldehydes and may be beneficial for explaining the high levels of organic acids affecting aerosol growth and cloud evolution. 

To read the full publication: Franco, B., Blumenstock, T., Cho, C. et al. (2021). Ubiquitous atmospheric production of organic acids mediated by cloud droplets. Nature 593, 233–237. https://doi.org/10.1038/s41586-021-03462-x