Seasonality and aerosol microenvironment-associated processing of organic aerosol in a humid inland city.

Organic aerosol (OA) constitutes a major fraction of atmospheric fine particulate matter, yet quantifying its sources and transformation pathways remains a significant scientific challenge. To address this, field measurements were conducted in January, March, June, and September-October 2023 in a medium-sized inland city, and the resulting dataset was analyzed by integrating molecular-level organic markers with Positive Matrix Factorization (PMF), the tracer-based method (TBM), and an explainable machine learning framework (XGBoost-SHAP). Organic matter was a major constituent of PM2.5, and OA composition showed campaign-based seasonal contrasts, shifting from stronger primary-source influence during the cold-season campaigns to stronger secondary contributions during the warm-season campaigns. Biogenic SOA tracers showed stronger associations with aerosol acidity and liquid water content (ALWC), indicating that biogenic SOA processing was linked to aerosol microenvironmental conditions under ambient conditions. Complementary analyses further indicated that 2-methyltetrols were associated with temperature, aerosol acidity, and ALWC under relatively lower-pH conditions, whereas first-generation monoterpene oxidation products were associated mainly with O3 and temperature. By contrast, 3-hydroxyglutaric acid (3-HGA), a more oxidized/later-generation monoterpene tracer, was more strongly associated with aerosol acidity and temperature. These observations are consistent with progressive oxidation of monoterpene-derived SOA under ambient conditions, although the relative contributions of continued gas-phase oxidation and aerosol-phase processing could not be uniquely resolved. PMF further showed that more-oxidized OA increased not only in warm season but also during humid cold season pollution episodes.