Spontaneous molecular bromine production in sea salt aerosols

Bromine chemistry is responsible for the catalytic ozone destruction in the atmosphere. The heterogeneous reactions of sea‐salt aerosols are the main abiotic sources of reactive bromine in the atmosphere. Here, we present a novel mechanism for the activation of bromide ions (Br‐) by O2 and H2O in the absence of additional oxidants. The laboratory and theoretical calculation results demonstrated that under dark conditions, Br‐, O2 and H3O+ could spontaneously generate Br and HO2 radicals through a proton‐electron transfer process at the air‒water interface and in the liquid phase. Our results also showed that light and acidity could significantly promote the activation of Br‐ and the production of Br2. The estimated gaseous Br2 production rate under light conditions was up to 1.55×1010 molecules·cm‐2·s‐1 under light and acidic conditions; these results showed a significant contribution to the atmospheric reactive bromine budget. The reactive oxygen species (ROS) generated during Br‐ activation could promote the multiphase oxidation of SO2 to produce sulfuric acid, while the increase in acidity had a positive feedback effect on Br‐ activation. Our findings highlight the crucial role of the proton‐electron transfer process in Br2 production.