Seasonal source shifts drive oxidative potential of size-resolved particulate matter in coastal southeastern China.

Ambient particle oxidative toxicity in coastal monsoon transition regions has been deemed a growing concern because long-range transport and atmospheric aging can rapidly reshape particle composition and reactivity. Thereby, we investigated size-resolved particulate matter (PM0.4-10) collected at a peri-urban hilly site (IHS) and a coastal lowland site (CLS) in southeastern China, throughout 2024-2025 to elucidate the seasonal variability of oxidative potential (OP) and environmentally persistent free radicals (EPFRs). The results revealed that seasonal shifts in particulate chemical composition and dominant source contributions lead to pronounced changes in OP activity and radical generation, with consistently higher OPDTT and EPR-derived radicals at the IHS than at the CLS, indicating enhanced oxidative toxicity under more inland-influenced conditions. Furthermore, distinct EPFR signatures were observed between the two sites, as coastal samples were enriched in semiquinone and oxygen radicals while inland samples tended toward more carbon- and oxygen-centered radicals, suggesting that EPFR speciation and redox cycling can modulate OP and ROS-relevant signals beyond bulk composition alone. Moreover, source apportionment resolved six major factors, including ship emissions, aged sea salt, biomass burning influence, traffic, industrial activities, and dust, demonstrating that temporal source switching rather than a single pollutant precursor largely controlled OP dynamics. Potential source region analysis further identified the Guangdong-Fujian coastal corridor and adjacent marginal seas as common upwind sectors, highlighting a coupled land-sea transport regime that governs seasonal source transitions and oxidative toxicity in coastal Fujian. This improved understanding of seasonally evolving PM oxidative behavior and its source transport drivers will assist in developing targeted mitigation strategies to reduce health-relevant particle toxicity in coastal monsoon regions.