Chlorine Radical Cycling Amplifies Photolytic NOx Release

The atmospheric chlorine radical (Cl•) is a highly reactive oxidant. However, its formation on particulate matter (PM) surfaces and its role in nitrogen chemistry remain insufficiently understood. This study investigated the Cl• chemistry on photoactive PM and evaluated its potential influence on atmospheric nitrogen cycling. Experimental evidence confirms that the chloride ion (Cl-) on the photoactive surface undergoes oxidation by photogenerated hole (h+) to form Cl•, which subsequently accelerates nitrite intermediate conversion to gaseous nitrogen oxides (NOx). Under the model experimental conditions, this process enhanced NOx formation by approximately 40-fold, while Cl- was regenerated during the reaction. Additional experiments with different chloride salts and acidity controls indicate that the enhancement is mainly associated with Cl--involved photochemistry. This mechanism maintains efficacy across various humidity conditions and can be extended to diverse photoreactive PM (e.g., soot) and different nitrogen-containing components (e.g., ammonium, 4-nitrophenol). Experiments with reduced photoactive matter loadings and simulated atmospheric PM further reveal that the reaction rate is regulated by PM composition and the abundance of photoactive matter. Environmental simulations suggest that this surface Cl• cycling process may contribute 0.7%-6.2% to NO2 variability in the study area. Overall, this study identifies a previously overlooked Cl•-mediated surface process on photoactive PM and highlights its potential contribution to tropospheric nitrogen transformations.