Photosensitizers Regulate Nitrate Photoproduct Yields in Bulk Aqueous Matrices

Photolysis of nitrate (NO₃^-) in the presence of photosensitizers is thought to promote the release of atmospherically important reactive nitrogen species (NO), such as HONO, via mechanisms that are poorly understood. To address this knowledge gap, we conducted photochemistry studies on mixtures of NO₃^-, a model photosensitizer [4-benzoylbenzoic acid (4-BBA)], and various aliphatic organic matter proxies. We found that aliphatic organic matter enhances the production of NO and superoxide (O₂^-) from bulk aqueous nitrate photolysis, while the addition of 4-BBA decreases NO and O₂^- yields in most cases. This effect was not observed in heterogeneous systems, where 4-BBA enhanced photochemical NO production when coadsorbed with NO₃^- on silica surfaces. This demonstrates that photosensitizers act as both oxidants and reductants, regulating the yields of NO and reactive oxygen species from nitrate photochemistry. Given sufficient residence time in the bulk aqueous phase, nitrate photoproducts can be scavenged via secondary reactions with photosensitizers. In heterogeneous and aerosol systems with high surface area-to-volume ratios, however, volatilization of NO photoproducts is favored, and NO production is less affected by chromophoric organic matter sinks in the bulk phase. This work demonstrates the intricate role of aliphatic and chromophoric moieties in natural organic matter in the photochemical conversion of NO₃^- to NO (i.e., renoxification) in the atmosphere.