A new study has deciphered the dominant mechanism in the formation of formic acid in the atmosphere. The findings will help scientists to further refine atmospheric models and improve our understanding of weather and climate.
The study was undertaken by an international team of researchers, led by Forschungszentrum Jülich in Germany and co-authored by scientists at the National Center for Atmospheric Research (NCAR).
The acidity of the atmosphere is increasingly determined by carbon dioxide and organic acids such as formic acid. This acid contributes to the formation of airborne particles that lead to raindrops, and it influences the acidity of precipitation.
The chemical processes behind the formation of formic acid, however, have not previously been well understood, and it tended to play a small role in previous atmospheric chemistry models.
In the new study, the researchers determined the chemical processes that lead to most atmospheric formic acid, and they confirmed their findings through computer models and observations.
Dr Bruno Franco and Dr Domenico Taraborrelli from Jülich’s Institute of Energy and Climate Research – Troposphere found that formaldehyde is formed naturally by photo-oxidation of volatile organic compounds. Formaldehyde reacts in cloud droplets with water molecules to form methanediol. The majority of this is outgassed and reacts with OH radicals, sometimes called the “detergent of the atmosphere”, in a photochemical process to form formic acid. A smaller portion reacts with the liquid phase of the water droplets to also form formic acid that is spread by rain.
“According to our calculations, the oxidation of methanediol in the gas phase produces up to four times as much formic acid as what is produced in other known chemical processes in the atmosphere,” said Dr Taraborrelli. This amount reduces the pH of clouds and rainwater by up to 0.3, which highlights the contribution of organic carbon to the natural acidity in the atmosphere.
As a first step, the two scientists tested their theory using MESSy, a global atmospheric chemistry model, and compared the results with remote sensing data. To carry out the modeling, they used the Jülich supercomputer JURECA. Subsequent experiments in Jülich’s SAPHIR atmosphere simulation chamber confirmed the results.
“We assume that the mechanism demonstrated is also active in aqueous aerosols and applies to other organic acids such as oxalic acid, which are not adequately accounted for in atmospheric chemistry models to date,” noted Taraborrelli. One of the effects of this could be an improved understanding of the growth of aerosol particles and the development of clouds.