Important new source of oxidation in the atmosphere found

Shantou/Turin/Leipzig. Hydroperoxides are strong oxidants that have a significant influence on chemical processes in the atmosphere. Now, an international research team involving the Leibniz Institute for Tropospheric Research (TROPOS) has shown that these substances also form from α‑keto acids such as pyruvic acid in clouds, rain and aerosol water when exposed to sunlight. These reactions could be responsible for 5 to 15 percent of the observed atmospheric hydrogen peroxide (H₂O₂) in the aqueous phase. This means that the photolysis of α-keto acids has now been identified as another important source of atmospheric oxidants, the researchers write in Science Advances, the open-access journal of the renowned scientific journal SCIENCE. Since these oxidation processes influence both the formation and degradation of particles and air pollutants, the newly discovered reaction pathway is of great importance for air quality and climate forecasts.

The key to this discovery are α-keto acids. These carboxylic acids  contain an additional so-called keto group with a carbon atom and a double-bonded oxygen atom. The α-keto acids get into the atmosphere through different reactions from a number of precursor gases such as isoprene, aromatics, or acetylene, which can be biogenic or anthropogenic – originating from both vegetation and industry. They are widespread and play a fundamental role in life on Earth, for example in biochemistry in amino acid metabolism in cells. However, their importance for the atmosphere and the global climate has been rather underestimated until now. Using three α-keto acids (glyoxylic acid, pyruvic acid and 2-ketobutyric acid), the researchers were able to demonstrate in laboratory experiments and model calculations that these substances, together with light, are involved in the formation of hydroperoxides, which in turn produce hydrogen peroxide. These processes take place in the atmospheric liquid phase – in other words, in water-containing particles.

The study involved researchers from the Chinese Academy of Sciences (Guangzhou), the Guangdong Technion - Israel Institute of Technology, the Weizmann Institute of Science, Fudan University (Shanghai), the University of Chinese Academy of Sciences (Beijing), Kunming University of Science and Technology, the University of Turin, Shandong University (Qingdao) and the Leibniz Institute for Tropospheric Research (TROPOS). Three experts in photochemical processes in atmospheric liquids played an important role in the collaboration: Sasho Gligorovski, who wrote his doctoral thesis at TROPOS in Leipzig 20 years ago, then conducted research in France, became a professor at the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences, and has been conducting research at the Chinese-Israeli joint venture Guangdong Technion - Israel Institute of Technology (GTIIT) since 2025. Davide Vione, who works as a professor at the University of Turin. And Prof. Hartmut Herrmann, who has been researching the tropospheric multiphase system at TROPOS and the University of Leipzig since 1998, as well as at Shandong University since 2018 and Fudan University in Shanghai since 2019.

The atmospheric chemistry department at TROPOS in Leipzig used the laboratory data from Shanghai and Turin in its liquid phase model CAPRAM (Chemical Aqueous Phase Radical Mechanism) to evaluate the atmospheric effects of the laboratory results and make projections. The CAPRAM model has been refined over many years of work to the point where it can map highly complex reaction chains, and such new findings have now be incorporated as new feedback channels.

"This work provides the first quantitative framework for the formation of hydroperoxides from α‑keto acids and clarifies the pH and concentration dependencies that are crucial for atmospheric models. Through international cooperation, we have succeeded in finding another piece of the puzzle in the highly complex field of multiphase atmospheric chemistry," explains Prof. Hartmut Herrmann from TROPOS and Shandong University Qingdao.

The study now published provides initial approaches, but also highlights gaps in knowledge: for example, there is a lack of systematic field measurements of the concentrations of α-keto acids in aerosols and cloud water in different environments, which are needed to incorporate these mechanisms into climate models. Such studies would help to better estimate the global budget of hydroperoxides in the atmosphere and their role in particle formation in the aqueous phase and sulfate production. Tilo Arnhold

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