Transition metal‐catalyzed oxidation of atmospheric sulfur: Global implications for the sulfur budget

Abstract

We use observations of the oxygen‐17 excess (Δ17O) of sulfate in the Arctic to quantify the sulfate source from aqueous SO2 (S(IV)) oxidation by O2 catalyzed by transition metals. Due to the lack of photochemically produced OH and H2O2 in high latitudes during winter, combined with high anthropogenic SO2 emissions in the Northern Hemisphere, oxidation by O3 is predicted to dominate sulfate formation during winter in this region. However, Δ17O measurements of sulfate aerosol collected in Alert, Canada, are not consistent with O3 as the dominant oxidant and indicate that a S(IV) oxidant with near‐zero Δ17O values (O2) 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 O2. 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 Δ17O 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.