Transition Metal-Catalyzed Oxidation of Atmospheric Sulfur: Global Implications for the Sulfur Budget

Abstract

We use observations of the oxygen-17 excess ((Δ^{17}O)) of sulfate in the Arctic to quantify the sulfate source from aqueous (SO_2 (S(IV))) oxidation by (O_2) catalyzed by transition metals. Due to the lack of photochemically produced OH and (H_2O_2) in high latitudes during winter, combined with high anthropogenic (SO_2) emissions in the Northern Hemisphere, oxidation by (O_3) is predicted to dominate sulfate formation during winter in this region. However, (Δ^{17}O) measurements of sulfate aerosol collected in Alert, Canada, are not consistent with (O_3) as the dominant oxidant and indicate that a S(IV) oxidant with near-zero (Δ^{17}O) values ((O_2)) is important during winter. We use a global chemical transport model to interpret quantitatively the Alert observations and assess the global importance of sulfate production by Fe(III)- and Mn(II)-catalyzed oxidation of S(IV) by (O_2). We scale anthropogenic and natural atmospheric metal concentrations to primary anthropogenic sulfate and dust concentrations, respectively. The solubility and oxidation state of these metals is determined by cloud liquid water content, source, and sunlight. By including metal-catalyzed S(IV) oxidation, the model is consistent with the (Δ^{17}O) magnitudes in the Alert data during winter. Globally, we find that this mechanism contributes 9–17% to sulfate production. The inclusion of metal-catalyzed oxidation does not resolve model discrepancies with surface SO2 and sulfate observations in Europe. Oxygen isotope measurements of sulfate aerosols collected near anthropogenic and dust sources of metals would help to verify the importance of this sulfur oxidation pathway.