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 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; here, H₃O⁺ facilitates the activation of Br⁻ by O₂, serves as a significant source of atmospheric reactive bromine and exerts a profound impact on the atmospheric oxidation capacity.