Spontaneous Chlorine Generation via Interfacial Oxidation of Chloride by O₂ on Acidic Microdroplets

Reactive chlorine species significantly influence atmospheric chemistry, yet their sources remain inadequately defined. We have discovered a spontaneous formation pathway for substantial molecular chlorine (Cl₂) from the oxidation of chloride (Cl^–) through interfacial oxidation by O₂ on acidic microdroplets. This efficient interfacial reaction is facilitated by protons (H⁺), which enrich Cl^– at the gas–liquid interface and mediate the electron-transfer process, concurrently releasing gaseous Cl₂ and hydroxyl radicals (OH). Given the prevalence of acidic, chloride-containing droplets in marine and polluted urban environments, this formation mechanism constitutes a substantial and previously unaccounted tropospheric chlorine source. Unlike established mechanisms, this pathway generates active halogens without consuming gaseous oxidants like O₃ or OH, instead producing particulate reactive oxygen species (ROS). Consequently, current atmospheric models likely underestimate the contribution of reactive halogens to atmospheric oxidation capacity.