A novel pathway of atmospheric sulfate formation through carbonate radicals

Carbon dioxide is considered an inert gas that rarely participates in atmospheric chemical reactions. Nonetheless, we show here that CO2 is involved in some important photo-oxidation reactions in the atmosphere through the formation of carbonate radicals (CO3-). This potentially active intermediate CO3- is routinely overlooked in atmospheric chemistry concerning its effect on sulfate formation. The present work demonstrates that the SO2 uptake coefficient is enhanced by 17 times on mineral dust particles driven by CO3-. Importantly, upon irradiation, mineral dust particles are speculated to produce gas-phase carbonate radical ions when the atmospherically relevant concentration of CO2 presents, thereby potentially promoting external sulfate aerosol formation and oxidative potential in the atmosphere. Employing a suite of laboratory investigations of sulfate formation in the presence of carbonate radicals on the model and authentic dust particles, ground-based field measurements of sulfate and (bi)carbonate ions within ambient PM, together with density functional theory (DFT) calculations for single electron transfer processes in terms of CO3 - initiated S(IV) oxidation, a novel role of carbonate radical in atmospheric chemistry is elucidated.