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Environmentally persistent free radical (EPFR) formation by visible-light illumination of the organic matter in atmospheric particles.

A secondary process may be an important source of environmentally persistent free radicals (EPFRs) in atmospheric particulates, yet this process has remained to be elucidated. This study demonstrated that secondary EPFRs could be generated by visible-light illumination of atmospheric particulate matter (PM), and their lifetimes were only 30 min to 1 day, which were much shorter than the lifetimes of the original EPFRs in PM. The yields of secondary EPFRs produced by PM could reach 15% - 60% of those of the original EPFRs. The extractable organic matter contributed to the formation of secondary EPFRs (~55%), and a humic-like substance was the main precursor of the secondary EPFRs and was also the most productive precursor compared to the other aerosol components. The results of simulation experiments showed that the secondary EPFRs generated by the extractable and nonextractable PM components were similar to those produced by phenolic compounds and polycyclic aromatic hydrocarbons, respectively. We have found that oxygen molecules play an important role in the photochemical generation and decay of EPFRs. Reactive oxygen capture experiments showed that the original EPFRs may contribute to singlet oxygen generation, while the secondary EPFRs generated by photoexcitation may not produce singlet oxygen or hydroxyl radicals.