The Key Role of Sulfate in the Photochemical Renoxification on Real PM2.5

The active nitrogen species (HONO, NO, and NO₂) have important impacts on the atmospheric oxidative capacity and the transformation of many atmospheric species. In this study, a fast photochemical renoxification rate of adsorbed HNO₃/NO₃⁻ to active nitrogen species (HONO, NO, and NO₂) was detected on real urban PM2.5, and sulfate was found to play a key role in this process. Different from the reported direct photolysis pathway, the photochemical reaction of HNO₃/NO₃⁻ on PM2.5 is dominated by a photosensitizing mechanism. Acidic protons are proved to be essential for this pathway. The role of sulfate, because of the nonvolatility of its conjugated acid, is to conserve the necessary acidic protons when interacting with HNO₃ and thus maintain its photoreactivity. This work implies that sulfate will have important implications in atmospheric nitrogen cycling by accelerating the release of nitrogen oxides from photochemical renoxification of HNO₃/NO₃⁻ adsorbed on ambient particulates and thus can cause major environmental problems.