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

The active nitrogen species (HONO, NO, and NO) 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 HNO/NO to active nitrogen species (HONO, NO, and NO) was detected on real urban PM, and sulfate was found to play a key role in this process. Different from the reported direct photolysis pathway, the photochemical reaction of HNO/NO on PM 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 HNO 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 HNO/NO adsorbed on ambient particulates and thus can cause major environmental problems.