Atmospheric microplastics amplify sulfate formation via heterogeneous SO₂ oxidation

Atmospheric microplastics (MPs) are widespread, and previous studies have shown that photochemical aging can generate environmentally persistent free radicals (EPFRs) on microplastic surfaces, enabling them to catalyze environmental redox reactions. However, their roles in complex atmospheric chemical processes remain insufficiently understood. Here, we demonstrate that ultraviolet-aged polystyrene (PS) acts as a highly reactive interfacial medium. UV-induced polymer chain scission produces surface oxygen-containing functional groups and EPFRs, which facilitate molecular oxygen activation to form superoxide radicals (O₂^•⁻), thereby promoting SO₂ uptake and catalytic oxidation. Consequently, photochemically aged PS particles significantly accelerate the heterogeneous conversion of SO₂ to sulfate via EPFRs-mediated redox processes, resulting in sulfate yields up to 2.3 times higher than pristine particles. Moreover, under high relative humidity, the interfacial water layer further promotes the generation of reactive oxygen species, enhancing SO₂ oxidation. This work reveals the synergistic mechanism between EPFRs-mediated oxygen activation and heterogeneous SO₂ oxidation on photochemically aged microplastic surfaces, providing new insights into interfacial catalytic processes driving atmospheric oxidation and secondary aerosol formation.