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 (SO₄^2–) in aerosol water─a potential missing source of SO₄^2– in polluted environments. However, its effects on SO₄^2–and air quality in different regions remain unexplored. In this study, we implement a detailed model representation of SO₄^2– formation via NO₃^– photolysis into the global chemical transport model GEOS-Chem. Our results find considerable impacts of NO₃^– photolysis on surface SO₄^2–, 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.