The impact of anthropogenic emissions on atmospheric sulfate production pathways, oxidants, and ice core Δ 17 O(SO 4 2– )

Abstract. 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 (Δ 17 O = &delta 17 O–0.52 × &delta 18 O) of sulfate in Greenland and Antarctic ice cores and aerosols. The oxygen isotopic composition of non-sea salt sulfate (Δ 17 O(SO 4 2– )) is a function of the relative importance of each oxidant (e.g. O 3 , OH, H 2 O 2 , and O 2 ) during sulfate formation, and can be used to quantify sulfate production pathways. Due to its dependence on oxidant concentrations, Δ 17 O(SO 4 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 O 3 since the preindustrial period. The model simulates a significant increase in the fraction of sulfate formed via oxidation by O 2 catalyzed by transition metals in the present-day Northern Hemisphere troposphere (from 11% to 22%), offset by decreases in the fractions of sulfate formed by O 3 and H 2 O 2 . 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 Δ 17 O(SO 4 2– ) since preindustrial times (1850 CE) is consistent with Arctic and Antarctic observations. The model simulates a 42% increase in the concentration of global mean tropospheric O 3 , a 10% decrease in OH, and a 58% increase in H 2 O 2 between the preindustrial period and present. Model results indicate that the observed decrease in the Arctic Δ 17 O(SO 4 2– ) – in spite of increasing tropospheric O 3 concentrations – can be explained by the combined effects of increased sulfate formation by O 2 catalyzed by anthropogenic transition metals and increased cloud water acidity, rendering Δ 17 O(SO 4 2– ) insensitive to changing oxidant concentrations in the Arctic on this timescale. In Antarctica, the Δ 17 O(SO 4 2– ) is sensitive to relative changes of oxidant concentrations because cloud pH and metal emissions have not varied significantly in the Southern Hemisphere on this timescale, although the response of Δ 17 O(SO 4 2– ) to the modeled changes in oxidants is small. There is little net change in the Δ 17 O(SO 4 2– ) in Antarctica, in spite of increased O 3 , which can be explained by a compensatory effect from an even larger increase in H 2 O 2 . In the model, decreased oxidation by OH (due to lower OH concentrations) and O 3 (due to higher H 2 O 2 concentrations) results in little net change in Δ 17 O(SO 4 2– ) due to offsetting effects of Δ 17 O(OH) and Δ 17 O(O 3 ). 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.