The hydroxyl radical (OH radical) is the most important oxidant in the atmosphere, determining the self-cleaning capacity of the troposphere. However, the "hypothesis of non-traditional recycling sources of OH radicals" proposed for urban and forest areas by Peking University and the Jülich Research Center in 2009 (Science 2009) and by the Max Planck Institute in 2008 (Nature 2008) has remained an unresolved challenge in the field of international atmospheric chemistry.

Identifying the "non-traditional recycling sources of OH radicals" is of great significance for implementing precise prevention and control of secondary atmospheric pollution and addressing climate change. Although previous studies have indicated that the isoprene autoxidation mechanism contributes to OH radicals under ultra-low NOx conditions (such as in forests), its contribution is relatively small in medium to high NOx regions like urban areas, and thus it does not provide a theoretical explanation for the "non-traditional recycling sources of OH radicals."

Professor Lu Keding's team constructed a comprehensive field observation dataset from seven large-scale summer and autumn campaigns focused on radical chemistry, conducted since 2006 in China's typical city clusters. They discovered a strong linear correlation between oxygenated volatile organic compound (OVOC) reactivity and the intensity of "non-traditional recycling sources" of OH radicals, suggesting OVOCs as key species involved in these non-traditional recycling reactions. To further validate this hypothesis, the team conducted in-depth investigations into OH radical sources using multidimensional research approaches including quantum chemical calculations and numerical simulation closure experiments. They revealed for the first time that the autoxidation mechanism of higher aldehydes (HAM) within OVOCs constitutes the key reaction pathway for "non-traditional recycling sources" of OH radicals. The specific reaction steps involve: carbonyl-containing peroxy radicals (R(CO)O₂) generated from the oxidation of higher aldehydes, which undergo hydrogen transfer to form carbonyl-containing hydroperoxy compounds (HPCs). These HPCs subsequently undergo photolysis to rapidly regenerate OH radicals (Figure 1).

Figure 1 (a) Autoxidation Mechanism of Higher Aldehydes (HAM), (b) Energy Level Diagram of the Hydrogen Transfer Reaction of RC(O)O₂ Radicals

Based on this, the article further proposes a Generalized Aldehyde Autoxidation Mechanism (RAM), in which all carbonyl-containing peroxy radicals (R(CO)O₂) can regenerate OH radicals to varying degrees. Analysis of radical observations from 14 global sites reveals that the aldehyde autoxidation mechanism is a key component of the "non-traditional recycling sources" of OH radicals. Its contribution exceeds that of the isoprene autoxidation mechanism in urban and some forest areas (Figure 2).

Figure 2. Effects of Isoprene Autoxidation Mechanism (IAM) and Aldehyde Autoxidation Mechanism (RAM) on Radical Recycling Across Different NO Concentration Ranges

The paper ultimately proposes a conceptual model of OH radical recycling sources (Figure 3). The model indicates that the isoprene autoxidation mechanism (IAM) predominantly contributes to OH radical recycling under extremely low NOx conditions (e.g., forests), while the aldehyde autoxidation mechanism (RAM) contributes significantly in low NOx concentration ranges (such as transitional zones between forests and urban areas). The contribution of NOx to OH radical recycling becomes dominant in high NOx ranges. With the rapid advancement of global carbon neutrality strategies, atmospheric nitrogen oxide concentrations are expected to decrease substantially. Consequently, the role of the aldehyde autoxidation mechanism in OH radical recycling and atmospheric oxidizing capacity will become increasingly prominent.

Figure 3. Conceptual Model of OH Radical Recycling Source Contributions Under Different NOx Conditions (where "Base" represents the classical atmospheric chemistry mechanism, primarily regenerating through reactions with NO; "LIM1" denotes the isoprene autoxidation recycling mechanism; and "RAM" indicates the aldehyde autoxidation recycling mechanism)

This work provides a key theoretical explanation for the conceptual model of non-traditional OH radical recycling mechanisms, representing another original achievement by our team in atmospheric OH radical chemistry research. It holds critical significance for improving our understanding of tropospheric atmospheric chemical reaction mechanisms.

The above research findings were published online in Nature Communications on February 22, 2024, under the title “Reactive aldehyde chemistry explains the missing source of hydroxyl radicals”. Professor Lu Keding and Academician Zhang Yuanhang from the College of Environmental Sciences and Engineering at Peking University served as co-corresponding authors. The co-first authors were Assistant Researcher Yang Xinping (Chinese Research Academy of Environmental Sciences, Ph.D. graduate of Peking University in 2021) and Associate Professor Wang Haichao (Sun Yat-sen University, Ph.D. graduate of Peking University in 2018, Boya Postdoctoral Fellow). Professor Long Bo's team from Guizhou Minzu University provided crucial support for the quantum chemical calculations in this study. The research was funded by the National Natural Science Foundation of China's Distinguished Young Scholar Fund and the National Key R&D Program of the Ministry of Science and Technology, among other projects.

Yang, X., Wang, H., Lu, K. et al. Reactive aldehyde chemistry explains the missing source of hydroxyl radicals. Nat Commun 15, 1648 (2024).https://doi.org/10.1038/s41467-024-45885-w