The primary sources of RO_x radicals in atmosphere: Oxidative capacity and self-purifying effect

Ambient ozone (O₃) is generated through the reactions of nitrogen oxides and volatile organic compounds (VOCs) in sunlight, and the primary sources of RO_x radicals play a very important role in O₃ photochemistry. However, as major precursors of RO_x radicals, the systematical evaluation of ClNO₂, HCHO, and HONO impacts on O₃ photochemistry remains limited. Here, we utilized the observations of ClNO₂, HCHO, and HONO conducted in a coastal city of Southeast China during a photochemical O₃ pollution episode, combined with model simulations to elucidate their impacts on RO_x radicals and atmospheric oxidation capacity (AOC), as well as O₃ formation. Decreased concentrations of ClNO₂ and HONO were observed after sunrise, while HCHO concentrations peaked in the daytime. HCHO photolysis contributed the largest (∼25 %) to RO_x radical production around noon, while HONO photolysis (∼47 %) dominated RO_x radical production in the morning and late afternoon, and VOCs consumed by Cl radical released via ClNO₂ photolysis was more important (∼10 %) in the early morning, similar to their effects on the AOC levels. The results of model simulations indicated that HCHO photolysis greatly enhanced the photochemical formation of O₃, followed by HONO and ClNO₂ photolysis. Except for reducing VOCs due to a VOC-limited regime, the impacts of HCHO photolysis as primary RO_x sources should be valued to inhibit the intensification of O₃ pollution. Our study stressed the importance of primary RO_x sources for O₃ photochemistry in coastal regions, provided new insights into elucidating the self-purifying effect of the atmospheric environment.