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Atmospheric microplastics amplify sulfate formation via heterogeneous SO2 oxidation
Summary
Sunlight breaks down tiny plastic particles in the air in a way that makes them much better at triggering chemical reactions—turning common air pollution (sulfur dioxide, from things like burning fossil fuels) into sulfate particles up to 2.3 times faster than fresh plastic would. This matters because sulfate particles are a major ingredient in harmful fine-particle air pollution, meaning sunlight-aged microplastics floating in our air could be quietly making smog and haze worse, with humid conditions amplifying the effect even further.
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 (O), thereby promoting SO uptake and catalytic oxidation. Consequently, photochemically aged PS particles significantly accelerate the heterogeneous conversion of SO 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 SO oxidation. This work reveals the synergistic mechanism between EPFRs-mediated oxygen activation and heterogeneous SO oxidation on photochemically aged microplastic surfaces, providing new insights into interfacial catalytic processes driving atmospheric oxidation and secondary aerosol formation.