Spontaneous Oxygen-Carbonate Synergistic Photoactivation as a Novel Pathway for Atmospheric Radicals

Carbonate is a common component of soil and coarse atmospheric particulate matter, while few studies have focused on its photochemical reactivity. We present a novel photoactivation mechanism of carbonate and O₂ in the absence of additional potent oxidants. Under light conditions, carbonate ions (CO₃^2-), O₂, and H₃O⁺/OH^- could spontaneously form a complex and generate CO₃^- and HO₂/O₂^- radicals through an electron transfer process. Under these conditions, the H₂O₂ production in 1 mM Na₂CO₃ solution was 0.37 ± 0.10 μg·L^-1 and the cumulative concentration of OH radicals in 4 h was 34.6 nM. The proposed mechanism could be a potential source of atmospheric oxidizing capacity, and the reactive species produced by the CO₃^2--O₂ photoactivation process could promote secondary sulfate production. Our findings underscore the importance of carbonate atmospheric reactivity, especially at special reaction surfaces such as air-water interfaces and microdroplets.