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.