Metal catalyzed sulfur oxidation in the atmosphere
| We use observations of the oxygen-17 excess (D17O) 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, D17O measurement so f sulfate aerosol collected in Alert, Canada are not consistent with O3 as the dominant oxidant, and indicate that a S(IV) oxidatn with near zero D17O 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 D17O magnitudes in the Alert date 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. | |
|
|
| Figure 4. Monthly mean observations (open diamonds with gray error bars) and model calculations at Alert, Canada from the model simulation with no metal catalyzed S(IV) oxidation (gray squares) and the simulation with metal catalyzed S(IV) oxidation (black squares). (Top) nssSO42- D17O [‰]. (Middle) nssSO42- concentrations [ng m-3]. (Bottom) fraction of total sulfate from the metal catalyzed S(IV) + O2 oxidation pathway. | |
|
|
|
| Figure 5. Annual mean fraction of total sulfate at the surface from the metal catalyzed S(IV) + O2 oxidation pathway. | |
 |
|
|
Figure 6. Percent decrease (increase) in SO2 (sulfate) concentrations in the boundary layer (< 2 km altitude) after adding metal catalyzed S(IV) oxidation to the model. |
|
| Data: SO4D | |
| Collaborators: | |
|
Rokjin Park, Seoul National University Daniel Jacob, Harvard University Sunling Gong, Environment Canada | |
| References: | |
| Alexander,
B., R. J. Park, D. J. Jacob, and S. L. Gong (2008), Transition metal
catalyzed oxidation of atmospheric sulfur: Global implications for the
sulfur budget, in press J. Geophys. Res.
(.pdf) McCabe, J.R., J. Savarino, B. Alexander, and M.H. Thiemens, Isotopic constraints on non-photochemical sulfate production in the Arctic winter, Geophys. Res. Lett., 33, L05810 (2006). (.pdf) | |
| Feichter, J., E. Kjellstrom, H. Rodhe, F. Dentener, J. Lelieveld, and G. J. Roelofs, Simulation of the tropospheric sulfur cycle in a global climate model, Atmospheric Environment 30, 1693-1707 (1996). | |
| Funding: | |
| NSF-AGS 0607846 and NSF-AGS 0704169 | |
