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Sunlight Cloud Chemistry Atmospheric Oxidants

Sunlight in Clouds Creates Hidden Oxidants, Scientists Reveal

The TROPOS photoreactor, designed for laboratory research into liquid-phase atmospheric processes occurring in water-bearing particles. The experiments were carried out using a similar setup. Credit: Thomas Schäfer, TROPOS

Atmospheric Chemistry Breakthrough with Global Implications

Hydroperoxides are powerful oxidizing agents that play a crucial role in atmospheric chemistry. An international research team, including scientists from the Leibniz Institute for Tropospheric Research (TROPOS), has now revealed that these compounds can also form from α-keto acids such as pyruvic acid in clouds, rain and aerosol water when exposed to sunlight.

The researchers estimate that these reactions could account for between 5 and 15% of hydrogen peroxide (H₂O₂) observed in the atmosphere's aqueous phase. Published in Science Advances, the findings identify the photolysis of α-keto acids as a previously unrecognized source of atmospheric oxidants. Because such oxidation processes affect both the formation and breakdown of airborne particles and pollutants, the discovery has important implications for air-quality and climate modelling.

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The Role and Sources of α-keto Acids

At the heart of the discovery are α-keto acids, a class of carboxylic acids distinguished by an additional keto group made up of a carbon atom double-bonded to oxygen. These compounds enter the atmosphere through a range of chemical reactions involving precursor gases such as isoprene, aromatic compounds and acetylene, which can originate from both natural sources like vegetation and human activities including industry.

α-keto acids are widespread in nature and are essential to life on Earth, playing key roles in biochemistry and amino acid metabolism within cells. Until now, however, their influence on atmospheric chemistry and the global climate has been largely underestimated.

Environmental chemistry and human impact insights

Sunlight-Driven Reactions in Atmospheric Water

By studying three α-keto acids—glyoxylic acid, pyruvic acid and 2-ketobutyric acid—researchers demonstrated through laboratory experiments and modelling that, in the presence of sunlight, these substances contribute to the formation of hydroperoxides, which subsequently generate hydrogen peroxide.

These reactions occur in the atmospheric liquid phase, specifically within water-containing airborne particles.

Global Collaboration Behind the Discovery

The study brought together researchers from a wide range of international institutions, including:

  • The Chinese Academy of Sciences in Guangzhou
  • The Weizmann Institute of Science
  • Fudan University in Shanghai
  • The University of the Chinese Academy of Sciences in Beijing
  • Kunming University of Science and Technology
  • The University of Turin
  • Shandong University in Qingdao
  • The Leibniz Institute for Tropospheric Research (TROPOS)

Leading Experts in Atmospheric Photochemistry

Three leading experts in photochemical processes within atmospheric liquids played a central role in the collaboration.

Among them was Sasho Gligorovski, who completed his doctoral studies at TROPOS in Leipzig two decades ago, went on to conduct research in France, later became a professor at the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences and has been working at the Chinese-Israeli joint institute Guangdong Technion—Israel Institute of Technology since 2025.

They were joined by Davide Vione, Professor at the University of Turin, and Prof Hartmut Herrmann, who has been studying the tropospheric multiphase system at TROPOS and the University of Leipzig since 1998 and more recently at Shandong University since 2018 and Fudan University since 2019.

Human health, air quality and environmental systems

Modelling and Implications for Atmospheric Science

At TROPOS in Leipzig, the atmospheric chemistry department applied laboratory data generated in Shanghai and Turin to its liquid-phase CAPRAM model (Chemical Aqueous Phase Radical Mechanism) to assess the wider atmospheric implications of the findings produce forward-looking projections.

Developed and refined over many years, the CAPRAM model is capable of representing highly complex reaction pathways, and the newly identified processes have now been integrated as additional feedback mechanisms.

Key Findings for Climate and Air-Quality Models

"This research delivers the first quantitative framework describing hydroperoxide formation from  α-keto acids and clarifies the critical roles of pH and concentration for atmospheric modelling," said Prof Hartmut Herrmann of TROPOS and Shandong University in Qingdao.

"Through international collaboration, we have added an important missing piece to the intricate puzzle of multiphase atmospheric chemistry."

Gaps in Knowledge and the Need for Field Data

The study offers important first insights but also exposes clear gaps in current understanding. Notably, systematic field measurements of α -keto acids concentrations in aerosols and cloud water across diverse environments are still lacking—data that are essential for integrating these processes into climate models.

Addressing these gaps would allow scientists to better quantify the global atmospheric budget of hydroperoxides and clarify their influence on aqueous-phase particle formation and sulphate production.

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