Unveiling the key drivers and formation pathways for secondary organic aerosols in an eastern China megacity

Secondary organic aerosol (SOA) significantly contributes to atmospheric fine particles, yet its formation mechanisms remain insufficiently understood. In this study, we investigated the formation pathways of organic aerosol (OA) and their key drivers in Hangzhou through an intense two-month field campaign. Meteorological conditions generally hinder SOA dispersion during pollution episodes, with various effects on SOA. Machine learning analysis using XGBoost combined with SHAP interpretation highlighted temperature and relative humidity as the most influential meteorological factors affecting SOA formation. Furthermore, biogenic volatile organic compounds, particularly isoprene oxidation products (methacrolein), were identified as the dominant precursors for less-oxidized oxygenated OA(LO-OOA) under high atmospheric oxidation capacity. In contrast, anthropogenic VOCs (AVOCs), including methylethylketone and toluene, contributed significantly to the formation of more-oxidized oxygenated OA (MO-OOA) via aqueous-phase processes likely enhanced by Fe-catalyzed Fenton reactions in aerosol liquid water. These findings suggest that effective SOA mitigation in Hangzhou requires controlling both anthropogenic VOC emissions as well as biogenic VOCs.