The sensitivity of the oxygen isotopes of ice core sulfate to changing oxidant concentrations since the preindustrial

Abstract

Abstract. Changes in tropospheric oxidant concentrations since preindustrial times have implications for the ozone radiative forcing, lifetimes of reduced trace gases, aerosol formation, and human health but are highly uncertain. Measurements of the triple oxygen isotopes of sulfate in ice cores (described by Δ17OSO4 = δ17O − 0.52 × δ18O) provide one of the few constraints on paleo-oxidants. We use the GEOS-Chem global atmospheric chemical transport model to simulate changes in oxidant concentrations and the Δ17OSO4 between 1850 and 1990 to assess the sensitivity of Δ17OSO4 measurements in Greenland and Antarctic ice cores to changing tropospheric oxidant concentrations. The model indicates 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 and present. Modeled Δ17OSO4 is consistent with measurements from ice core and aerosol samples. Model results indicate that the observed decrease in the Arctic Δ17OSO4 in spite of increasing O3 is due to the combined effects of increased sulfate formation by O2 catalyzed by anthropogenic transition metals and increased cloud water acidity. In Antarctica, the Δ17OSO4 is sensitive to relative changes of oxidant concentrations, but in a nonlinear fashion. Sensitivity studies explore the uncertainties in preindustrial emissions of oxidant precursors.