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 did not exhibit significant downward trends, despite substantial reductions in anthropogenic emissions, such as 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 variations. Moreover, the relative contribution of biogenic SOA increased under cleaner conditions. A machine learning model was applied to quantify the impacts of changes in anthropogenic precursor emissions, biogenic precursor emissions, aqueous-phase oxidation processes, and gas-phase oxidation processes on C2 variability. As pollution levels declined, the impacts of gas-phase oxidation increased from 37 % to 55 %, whereas that of aqueous-phase oxidation declined from 42 % to 30 %. 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 under low-pollution conditions and underscore the need for effective ozone control strategies to further reduce SOA in the future.
He et al. (2026) studied this question.