Global Impact of Particulate Nitrate Photolysis on Fine Sulfate Aerosol

Recent studies have suggested that particulate nitrate (NO3–) photolysis could be an important source of atmospheric oxidants, and one of its main products, N(III), contributes to the heterogeneous formation of sulfate (SO42–) in aerosol water─a potential missing source of SO42– in polluted environments. However, its effects on SO42–and air quality in different regions remain unexplored. In this study, we implement a detailed model representation of SO42– formation via NO3– photolysis into the global chemical transport model GEOS-Chem. Our results find considerable impacts of NO3– photolysis on surface SO42–, especially over India and other coastal regions (up to 15% increment of annual average concentrations), which were previously unaccounted for in most models. The effects are mainly due to S(IV) oxidation by OH, H2O2, and O3 following renoxification, which outcompetes aqueous oxidation by N(III), contrary to previous laboratory and modeling studies. Further analysis suggests that past studies might have underestimated the particle-to-gas transfer of N(III) in ambient aerosols. We suggest that future modeling works should use experiment-derived parameters with caution and a thorough understanding of the mechanism before implementing them into models, especially when dealing with those from large particles or bulk solutions involving mass transfer.