Synoptic-scale transport drives wintertime secondary organic aerosol variability and high-PM episodes in Seoul

Secondary organic aerosol (SOA) constitutes a major fraction of fine particulate matter and poses substantial risks to human health, yet its sources and variability remain difficult to resolve in urban environments. Here, we investigate the processes driving episodic wintertime PM pollution in Seoul using winter 2022 measurements of non-refractory submicron aerosols (NR-PM₁) from a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). Positive matrix factorization (PMF) resolved six organic aerosol (OA) factors, comprising four primary-like OA (POA) components-hydrocarbon-like OA (HOA), oxygenated hydrocarbon-like OA (OHOA), cooking OA (COA), biomass burning OA (BBOA)-and two oxygenated OAs (OOA1 and OOA2). Coupled analysis of OA factors and co-measured gaseous precursors indicates that POA accumulates under stagnant conditions, consistent with predominantly local emissions, whereas OOA is associated with aged aerosol in continental outflow, highlighting the importance of regional transport. In particular, OOA2 exhibits pronounced event-scale enhancements during high-PM episodes, while OOA1 maintains a persistent and baseline contribution under lower-PM conditions. Synoptic meteorological analysis further demonstrates that elevated OOA2 contributions occur under warm and humid air masses arriving via low-altitude southwest transport pathways. Concurrent increases in toluene and O₃ during high-OOA2 periods support the co-transport of SOA precursors and enhanced chemical processing. Overall, these results show that wintertime SOA variability in Seoul is strongly modulated by synoptic circulation and transport pathways, underscoring the value of integrating aerosol composition, gas-phase chemistry, and meteorology to diagnose PM pollution in urban downwind regions.