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

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 O_x (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.