Chlorine (Cl) radicals can profoundly affect the atmospheric oxidation capacity and the fates of pollutants. Hypochlorous acid (HOCl) is a potentially crucial Cl precursor, yet the understanding of its formation mechanisms and atmospheric impacts is still limited. Here, we observed high concentrations of HOCl in a coastal city of Southeast China during the autumn of 2022, with an average daytime peak of 181 ppt. Machine learning analysis identified Cl₂, O₃, nitrate, temperature, and iron as the primary factors affecting HOCl distribution. Beyond Cl₂ photolysis, both nitrate photolysis and aerosol iron photochemistry also contributed to Cl radical production, which drove daytime HOCl production through reactions involving ClO and HO₂ radicals in the presence of O₃. Both OH and Cl radicals released via HOCl photolysis increased the levels of RO_x radicals by ∼10%, thereby enhancing the daytime O₃ generation and atmospheric oxidation capacity. Our findings emphasize the significant role of HOCl in atmospheric chemistry and suggest that controlling O₃ levels could alleviate Cl radical production and its adverse impacts on air quality.