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The impact of anthropogenic emissions on atmospheric sulfate production pathways, oxidants, and ice core Δ17O(SO42-)

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(SO42-)) 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(SO42-) 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(SO42-) 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(SO42-) - 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(SO42-) insensitive to changing oxidant concentrations in the Arctic on this time scale.  In Antarctica, the Δ17O(SO42-) 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(SO42-) to the modeled changes in oxidants is small.  There is little net change in Δ17O(SO42-) 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(SO42-) 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(SO42-).  The measurement locations are shown in (a).

 

People:

 
Eric Sofen, Shelley Kunasek and Becky Alexander, University of Washington
Publications:  
Sofen, E.D., B. Alexander, and S.A. Kunasek, The impact of anthropogenic emissions on atmospheric sulfate production pathways, oxidants, and ice core Δ17O(SO42-), Atmos. Chem. Phys., 11, 3565-3578, doi:10.5194/acp-11-3565-2100 (2011).

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"

Funding:  
NSF-AGS 0704169 and NSF-ANT 0538049
Data for colleagues:  
Dust and air density (GEOS3, 2x2.5 horizontal resolution, 30 vertical layers, year 2001)