Mechanistic study on photochemical generation of I^•/I₂^•− radicals in coastal atmospheric aqueous aerosol

The reactive iodine species may exhibit significant impacts on many global atmospheric issues and the I^•/I₂^•− radicals play key roles for inducing the formation of these reactive iodine species. However, the current understanding on the formation of I^•/I₂^•− radicals in atmospheric aqueous aerosol is still quite limited. The results reported herein suggest that I^•/I₂^•− can be produced simultaneously in aqueous aerosol by several sunlight-driven photochemical pathways including direct photo-dissociation of soluble organic iodine (SOI) at rates of 0.10–1.34 × 10⁻⁹ M ns⁻¹ and 0.99–5.68 × 10⁻⁷ M μs⁻¹, ^•OH-mediated oxidation of I⁻ at 0.03–1.41 × 10⁻⁸ M ns⁻¹ and 0.05–4.10 × 10⁻⁶ M μs⁻¹, and ³DOM^⁎-induced oxidation of I⁻ at 1.57–1.65 × 10⁻⁹ M ns⁻¹ and 0.99–5.68 × 10⁻⁷ M μs⁻¹ for generation of I^• and I₂^•−, respectively. Meanwhile, the pathway of e_aq⁻-initiated stepwise reduction of IO₃⁻ to I_2(aq) and further photolyzed into I^• plays negligible role in formation of I^•/I₂^•− due to the low reaction rates and severe quenching effect of e_aq⁻ by dissolved O₂. Our work presented the new data on mechanism and kinetics for comprehensive elucidation of I^•/I₂^•− formation in coastal atmospheric aqueous aerosol and would help to better understand the transformation mechanism of iodine species, pathways of iodine cycling and the associated environmental impacts involving atmospheric reactive iodine radicals.