A new model improves detection of acids that promote cloud formation

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Atmospheric acids play a role in cloud formation. (Unsplash/Marc Wieland)

Researchers have detected the chemical process that formaldehyde goes through to become formic acid, one of the most common acids found in the atmosphere, marking an important step in understanding the function of atmospheric acids.

Much weaker than their nonorganic counterparts, such organic acids may help govern cloud formation, but scientists have struggled to understand how they are produced, and existing chemical-climate models have underestimated the prevalence of formic acid. A paper published Wednesday in Nature describes a means of formic acid creation that could apply to other acids as well.

While the acid rain of the 20th century has largely vanished because of pollution-control strategies, many organic acids still exist in the atmosphere. 

"Understanding these formation pathways over organic acids is an important advance to understand how the biosphere can affect the formation of clouds," said co-lead author Domenico Taraborrelli, an atmospheric chemistry researcher at Forschungszentrum Jülich, an interdisciplinary research center in Europe. 

Acids can help form clouds through simple chemistry. When an acid dissolves in water in the atmosphere, it creates hydrogen ions in a solution that attracts counter ions in water vapor. As more vapor is incorporated into the solution, it condenses into cloud droplets.

Figuring out how acids function in the atmosphere involves chemistry-climate models. These models help represent all of the exchanges of trace gases, whether anthropogenic pollutants or those from the biosphere, between the earth's surface and the atmosphere, as well as their chemical transformation. 

But these models have underestimated the amount of organic acids in the atmosphere, sometimes by a factor of five or 10, making them difficult to study and accurately predict climate and air quality based on atmospheric composition, according to Taraborrelli. 

"As a modeler, I am constantly looking for new mechanisms to explain what we cannot explain yet in atmospheric chemistry," Taraborrelli said in an interview with The Academic Times. "For example, what does this formic acid come from? And then I got focused on the chemistry of formaldehyde in the cloud droplets."

Formaldehyde manifests in the atmosphere when plants release volatile organic compounds. These carbon compounds enter the atmosphere and experience photochemical reactions, which can create formaldehyde. After studying atmospheric data and reading prior research, Taraborrelli intuited that methanediol, the hydrated form of formaldehyde, could return to a gas phase as clouds dissipate, and become formic acid, whereas prior models accounted only for formaldehyde being washed away by clouds without becoming formic acid.  

Taraborrelli turned this hunch into a model that accounted for methanediol changing into formic acid. To check the accuracy of this model, Taraborrelli and his colleagues weighed their results against satellite data, which could accurately detect how much formic acid was present in the atmosphere. They found that the surface density of formic acid in the model showed improvement over past models when compared with the accuracy of the satellite readings. 

Taraborrelli and his colleagues confirmed their findings further in experiments requested by Nature to publish their research. In one, they sprayed a solution of formaldehyde and water into an atmospheric chamber and found that the formaldehyde quickly turned into methanediol, 100% of which quickly oxidized into formic acid.

"By understanding where these organic acids come from, we have a better knowledge of where all the carbon from the biosphere in the atmosphere goes," Taraborrelli said. "There might be a slight negative feedback in the sense that when it's warm, the biosphere is quite active, and it can release organic carbon, which could enhance how many clouds are up there."

The addition of clouds could cool the atmosphere, according to Taraborrelli.

This new pathway can also help explain why the tropics tend to hold so much formic acid. And though Taraborrelli and his colleagues have made progress creating an accurate climate-chemistry model, it still does not detect high levels of formic acid above boreal forests, though these areas are home to thawing peat, which is partially decomposed plant matter in acidic environments. Peat contains high levels of volatile organic compounds. 

In future research, Taraborrelli wants to study how pollutants such as ammonia react with larger organic acids, especially those that help form atmospheric brown carbon, the thick, hazy aerosols that form brown clouds across parts of Asia and Central Africa. Current models do not represent this globally. 

The study, "Ubiquitous atmospheric production of organic acids mediated by cloud droplets," published May 12 in Nature, was authored by B. Franco, Forschungszentrum Jülich and Université libre de Bruxelles; T. Blumenstock, F. Hase and M. Schneider, Karlsruhe Institute of Technology; C. Cho, H.-P. Dorn, T. Emmerichs, H. Fuchs, G. Gkatzelis, T. Hohaus, A. Kerkweg, A. Kiendler-Scharr, A. Novelli, D. Reimer, S. Rosanka, R. Tillmann, L. Vereecken, A. Wahner and D. Taraborrelli, Forschungszentrum Jülich; L. Clarisse and P.-F. Coheur, Université libre de Bruxelles; C. Clerbaux, Université libre de Bruxelles and Sorbonne Université; M. De Mazière, I. De Smedt, M. Van Roozendael and C. Vigouroux, Royal Belgian Institute for Space Aeronomy; D.W.T. Griffith, N. Jones and C. Paton-Walsh, University of Wollongong; S.Gromov, Max Planck Institute for Chemistry and Institute of Global Climate and Ecology; J.W. Hannigan and I. Ortega, National Center for Atmospheric Research; E. Lutsch and K. Strong, University of Toronto; E. Mahieu, University of Liège; M. Pommier, Sorbonne Université and Ricardo Energy and Environment; and A. Pozzer and R. Sander, Max Planck Institute for Chemistry. 

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