Airborne microplastics: An unexpected source of atmospheric brown carbon

Microplastics are increasingly recognized as emergent atmospheric contaminants1,2, yet they are generally presumed to be inert constituents that rarely participate in chemical reactions of importance due to their strong resistance to reactive species in the atmosphere under mild conditions3. Here, we show that common airborne microplastics transform into atmospheric brown carbon (BrC) in the presence of nitrate aerosols following solar irradiation. Strong interfacial electric fields at the nitrate aerosol air-water interface promote production of reactive species that accelerate microplastic aging and produce strongly adsorbing nitroaromatic and nitrogenous heterocyclic products with 365 nm mass absorption coefficients approaching ~1 m2 g-1. We estimate that microplastic-sourced BrC contributes up to 21% of light absorption in Asian metropolises and lower, yet significant, levels of light absorption worldwide. These results illustrate the overlooked radiative effects of ever-increasing levels of airborne microplastics and their impacts on local, regional, and global air quality.