Microplastics (MPs) are presumed to be inert during aging under ambient conditions. In this study, four types of virgin MPs, including polystyrene (PS), phenolformaldehyde resin (PF), polyethylene (PE), and polyvinyl chloride (PVC), were aged under simulated solar light irradiation. Surprisingly, several environmentally persistent free radicals (EPFRs), which are considered to be a type of emerging contaminant, were detected on the irradiated PS and PF, rather than PE and PVC, by electron paramagnetic resonance (EPR) spectroscopy. Depending on the photoaging duration time, the characteristic g-factors of the EPFRs produced on PS and PF were 2.0044−2.0049 and 2.0043− 2.0044, respectively. The generated EPFRs on PS and PF decayed rapidly at the initial stage and then slowly disappeared with the elapsed aging time. Analyses by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC) suggested that MPs might experience chemical chain scission, O 2 /H 2 O addition, and EPFR formation under the light irradiation. Accompanying with the formation of EPFRs, reactive oxygen species, such as O 2•− and •OH, were also observed. The findings provide a novel insight to evaluate the potential hazards of MPs to organisms and ecosystems.

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