Preprints
https://doi.org/10.5194/egusphere-2025-4624
https://doi.org/10.5194/egusphere-2025-4624
05 Oct 2025
 | 05 Oct 2025
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Driving Factors of Oxalic Acid and Enhanced Role of Gas-Phase Oxidation under Cleaner Conditions: Insights from 2007–2018 Field Observations in the Pearl River Delta

Yunfeng He, Xiang Ding, Quanfu He, Yuqing Zhang, Metin Baykara, Duohong Chen, Tao Zhang, Kong Yang, Junqi Wang, Qian Cheng, Hao Jiang, Zirui Wang, Ping Liu, Xinming Wang, and Michael Boy

Abstract. Secondary organic aerosol (SOA) is a dominant constituent of fine particulate matter, exerting significant impacts on both climate and human health. Oxalic acid (C2), a key end-product formed from the oxidation of volatile organic compounds, can provide insights into the formation mechanism of SOA. Thus, long-term measurements of C2 and related compounds help understand the changes in SOA formation with decreasing pollutant levels. In this study, C2 and its homologs, along with five primary anthropogenic source markers and three SOA markers, were measured in the Pearl River Delta (PRD) during 2007–2018. The concentrations of C2 and its homologs did not exhibit significant downward trends, despite substantial reductions in anthropogenic emissions, for example, biomass burning (−11 % yr−1), vehicle emissions (−17 % yr−1), and cooking emissions (−7 % yr−1). Correlation analysis revealed that aerosol liquid water content (ALWC) and Ox (O3 + NO2) were the main drivers of C2 variation. Moreover, the relative contribution of biogenic SOA increased under cleaner conditions. A machine learning model was applied to quantify the contributions of anthropogenic precursors emission, biogenic precursors emission, aqueous-phase oxidation processes, and gas-phase oxidation processes to C2 variability. As pollution levels declined, the contribution of gas-phase oxidation increased from 24 % to 48 %, whereas that of aqueous-phase oxidation declined from 35 % to 20 %. This shift indicated a transition from aqueous-phase to gas-phase pathways in C2 and SOA formation. Our findings highlight the increasing importance of gas-phase oxidation and underscore the need for effective ozone control strategies to further reduce SOA in the future.

Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
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Yunfeng He, Xiang Ding, Quanfu He, Yuqing Zhang, Metin Baykara, Duohong Chen, Tao Zhang, Kong Yang, Junqi Wang, Qian Cheng, Hao Jiang, Zirui Wang, Ping Liu, Xinming Wang, and Michael Boy

Status: open (until 16 Nov 2025)

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Yunfeng He, Xiang Ding, Quanfu He, Yuqing Zhang, Metin Baykara, Duohong Chen, Tao Zhang, Kong Yang, Junqi Wang, Qian Cheng, Hao Jiang, Zirui Wang, Ping Liu, Xinming Wang, and Michael Boy
Yunfeng He, Xiang Ding, Quanfu He, Yuqing Zhang, Metin Baykara, Duohong Chen, Tao Zhang, Kong Yang, Junqi Wang, Qian Cheng, Hao Jiang, Zirui Wang, Ping Liu, Xinming Wang, and Michael Boy
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Latest update: 05 Oct 2025
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Short summary
This study conducted long-term measurements for oxalic acid and several molecular markers of primary anthropogenic emissions in the Pearl River Delta. We found that the impact of reduction in anthropogenic precursors on SOA formation was limited. In addition, our results highlight the increasing importance of gas-phase oxidation in SOA formation under low-pollution conditions, underscoring the need for effective ozone control strategies to further reduce SOA in the future.
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