The acidity of the atmosphere is increasingly determined by carbon dioxide and organic acids such as formic acid. The second of these contributes to the formation of aerosol particles as a precursor to raindrops and therefore has an impact on cloud growth and the pH of rainwater. In previous atmospheric chemistry models of acid formation, formic acid tended to play a small role. The chemical processes behind its formation were not well understood. An international team of researchers under the aegis of the Forschungszentrum Jülich has now succeeded in filling this gap and deciphering the dominant mechanism of formic acid formation. This makes it possible to further refine the atmosphere and climate models. The results of the study have now been published in the peer-reviewed journal Nature.
In Germany, we are familiar with acid rain, especially from our experience in the 1980s. The cause was that the nitrogen oxides and sulfur oxides released into the atmosphere by humans reacted with the droplets of water in the clouds to form sulfuric acid and nitric acid. Acid rain has a pH of about 4.2-4.8, lower than that of pure rainwater (5.5-5.7), which results from the natural carbon dioxide content of the atmosphere.
However, the chemical process that forms most of the formic acid in the atmosphere has so far been unknown. Dr Bruno Franco and Dr Domenico Taraborrelli from the Jülich Energy and Climate Research Institute – Troposphere have now deciphered it: formaldehyde is formed naturally by photo-oxidation of volatile organic compounds. Formaldehyde reacts in cloud droplets with water molecules to form methanediol. Most of it is degassed and reacts with OH radicals, sometimes referred to as “atmospheric detergent”, in a photochemical process to form formic acid. A smaller part reacts with the liquid phase of the water droplets to also form formic acid which is shed by the rain.
“According to our calculations, the oxidation of methanediol in the gas phase produces up to four times more formic acid than what is produced in other known chemical processes in the atmosphere,” explains Domenico Taraborrelli. This amount reduces the pH of clouds and rainwater down to 0.3, highlighting the contribution of organic carbon to the natural acidity of the atmosphere.
First, the two scientists tested their theory using MESSy, a model of global atmospheric chemistry, and compared the results with remote sensing data. To perform the modeling, they used the Jülich JURECA supercomputer. Subsequent experiments in Jülich’s SAPHIR atmosphere simulation chamber confirmed the results. “We assume that the demonstrated mechanism is also active in aqueous aerosols and applies to other organic acids such as oxalic acid, which are not sufficiently taken into account in atmospheric chemistry models to date”, explains Taraborrelli. One effect of this could be a better understanding of aerosol particle growth and cloud development.
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