Unraveling the Differentiated Roles of Reactive Chlorine in Regulating Atmospheric Oxidation and Secondary Formation

Reactive chlorine species significantly perturb atmospheric oxidation, yet systematic observations of multiple chlorine species and comprehensive assessments of their distinct photochemical roles remain limited. Here, we present concurrent field measurements of four reactive chlorine species (ClNO2, Cl2, HOCl, and NCl3) in a coastal urban atmosphere. Cl2 photolysis dominated Cl• production (40.2%), whereas HOCl photolysis served as a dual source of Cl• and OH•, a mechanism previously overlooked, and most effectively elevated ROx• levels. Temporally, ClNO2 photolysis enhanced early morning oxidation, Cl2 prevailed around noon, and HOCl dominated the afternoon radical budgets, while NCl3 photolysis contributed minimally throughout the diurnal cycle. Incorporating chlorine-induced volatile organic compounds (VOC) consumption in the box model amplified O3 and PAN production by up to 43.4 and 43.8%, respectively, without altering the VOC-limited regime. Our findings underscored that regulating coastal atmospheric oxidation required a species-specific, time-resolved treatment of chlorine chemistry, with direct implications for air quality management.