Insight into the Overlooked Photochemical Decomposition of Atmospheric Surface Nitrates Triggered by Visible Light

The nitrogen oxides (NOx) formed by photochemical reaction of surface nitrates raise significant concerns. However, little is known about the effect of visible light (>380 nm) on nitrate decomposition and the reaction mechanism. Herein, the decomposition of surface nitrates is investigated under visible light. The results indicate that visible light photocatalysis contributes significantly to nitrate decomposition. Monodentate nitrate (m-NO₃ ^- ) can be decomposed into NOx by photogenerated electrons starting from the weakly coordinated N-O bond. Water vapor promotes NOx generation because more stable bidentate nitrate (b-NO₃ ^- ) will be converted into m-NO₃ ^- by surface hydroxyl groups through hydrogen bonding interactions. Alternatively, b-NO₃ ^- can be directly decomposed to NO₂ ^- by NO attack, but this process is subject to photocatalytic oxidation. This work brings a new focus on the atmospheric NOx sources and provides a more nuanced understanding of nitrates decomposition processes.