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

• Main Authors: Becky Alexander, Rokjin J. Park, Daniel J. Jacob, Sunling Gong
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Arctic; Canada
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.142.7518; http://www-as.harvard.edu/chemistry/trop/publications/alexander2008.pdf

We use observations of the oxygen-17 excess (Δ 17 O) 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 [Feichter et al., 1996]. However, Δ 17 O 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 Δ 17 O values (O2) is important during winter [McCabe et al., 2006]. 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 Δ 17 O magnitudes in the Alert data during winter. Globally, we find that this mechanism contributes 16 % to sulfate production. 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.