We use a global three-dimensional chemical transport model to quantify the influence of anthropogenic emissions on atmospheric sulfate production mechanisms and oxidant concentrations constrained by observations of the oxygen isotopic composition (Δ¹⁷O = δ¹⁷O - 0.52 x δ¹⁸O) of sulfate in Greenland and Antarctic ice cores and aerosols.  The oxygen isotopic composition of non-sea salt sulfate (Δ¹⁷O(SO₄^2-)) is a function of the relative importance of each oxidant (O₃, OH, H₂O₂, and O₂) during sulfate formation, and can be used to quantify sulfate production pathways.  Due to its dependence on oxidant concentrations, Δ¹⁷O(SO₄^2-) has been suggested as a proxy for paleo-oxidant levels.  However, the oxygen isotopic composition of sulfate from both Greenland and Antarctic ice cores shows a trend opposite to that expected from the known increase in the concentration of tropospheric O₃ since the preindustrial period.  The model simulates a significant increase in the fraction of sulfate formed via oxidation by O₂ catalyzed by transition metals in the present-day Northern Hemisphere troposphere (from 11% to 22%), offset by decreases in the fraction of sulfate formed by O₃ and H₂O₂.  There is little change, globally, in the fraction of tropospheric sulfate produced by gas-phase oxidation (from 23% to 27%).  The model calculated change in Δ¹⁷O(SO₄^2-) since preindustrial times (1850 C.E.) is consistent with Arctic and Antarctic observations.  The model simulates a 42% increase in the concentration of global mean tropospheric O₃, a 10% decrease in OH, and a 58% increase in H₂O₂ between the preindustrial period and present.  Model results indicated the the observed decrease in the Arctic Δ¹⁷O(SO₄^2-) - in spite of increasing tropospheric O₃ concentrations - can be explained by the combined effects of increased sulfate formation by O₂ catalyzed by anthropogenic transition metals and increased cloud water acidity, rendering Δ¹⁷O(SO₄^2-) insensitive to changing oxidant concentrations in the Arctic on this time scale.  In Antarctica, the Δ¹⁷O(SO₄^2-) is sensitive to relative changes in oxidant concentrations because cloud pH and metal emissions have not varied significantly in the Southern Hemisphere on this time scale, although the response of Δ¹⁷O(SO₄^2-) to the modeled changes in oxidants is small.  There is little net change in Δ¹⁷O(SO₄^2-) in Antarctica, in spite of increased O₃, which can be explained by a compensatory effect from an even larger increase in H₂O₂.  In the model, decreased oxidation by OH (due to lower OH concentrations) and O₃ (due to higher H₂O₂ concentrations) results in little net change in Δ¹⁷O(SO₄^2-) due to offsetting effects of Δ¹⁷O(OH) and Δ¹⁷O(O₃).  Additional model simulations are conducted to explore the sensitivity of the oxygen isotopic composition of sulfate to uncertainties in the preindustrial emissions of oxidant precursors.

PD - PI change in global, tropospheric mean concentrations of (a) O₃, (b) OH, (c) H₂O₂ and (d) the surface Δ¹⁷O(SO₄^2-).  The measurement locations are shown in (a).

People:

Kunasek, S.A., B. Alexander, E.J. Steig, E.D. Sofen, T.L. Jackson, M.H. Thiemens, J.R. McConnel, D.J. Gleason, H.M. Amos, Sulfate sources and oxidation chemistry over the past ~230 years from sulfur and oxygen isotopes of sulfate in a West Antarctic ice core, J. Geophys. Res., 115, D18313, doi:10.1029/2010JD013846 (2010) (.pdf). Highlighted in Eos Vol. 91, No. 49, 7 December 2010 "Research Spotlight"