Source apportionment of environmentally persistent free radicals in size-resolved PM: Implications for driving...

Environmentally persistent free radicals (EPFRs) have emerged as pollutants capable of persisting for days to weeks in the atmosphere, posing substantial environmental and health risks. However, the spatiotemporal distribution and health implications of EPFRs from long-range transport remain poorly understood. Here, this study examines the temporal dynamics of EPFRs and their contributions to oxidative potential (OP) and reactive oxygen species (ROS) using comparative multi-probe analysis, including oxidative potential measured by electron paramagnetic resonance (OP^EPR), dithiothreitol assay (OP^DTT), simulated lung fluid assay (OP^SLF), and hydroxyl radical-quenched dithiothreitol assay (OP^OH), integrated with meteorological analysis, chemical speciation, and source apportionment analyses. Results showed that EPFRs in clean periods were predominantly oxygen-centered or semiquinone-type radicals, while those in polluted periods were characterized by carbon-centered and proximal oxygen-centered species. These transitions corresponded with threefold increases in OP_m^DTT, OP_m^OH and OP_m^SLF by 300 %, 300 %, and 150 %, respectively. Organic radicals (•R) showed moderate correlations with transition metals (Cu, Fe, Zn, Mn, Tl) in clean periods, likely linked to incomplete combustion emissions, while hydroxyl radicals (•OH) were mainly associated with Fe, Zn, and K (r > 0.4) in polluted periods, possibly related to traffic emissions. Ocean emissions and biomass burning account for 63.06-85.39 % of ROS production, reflecting significant long-range transport contributions. Potential source contribution function (PSCF) analysis identified southeastern China as a principal source region, influenced by mixed traffic, marine, and construction emissions. These findings highlight biomass burning and marine emissions as key drivers of EPFR formation and OP/ROS generation under combined long-range and local influences.