The primary sources of ROx 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 ROx radicals play a very important role in O3 photochemistry. However, as major precursors of ROx 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 ROx 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 ROx radical production around noon, while HONO photolysis (∼47 %) dominated ROx 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 ROx sources should be valued to inhibit the intensification of O3 pollution. Our study stressed the importance of primary ROx sources for O3 photochemistry in coastal regions, provided new insights into elucidating the self-purifying effect of the atmospheric environment.