The impact of anthropogenic emissions on atmospheric sulfate production pathways, oxidants, and ice core Δ¹⁷O(SO₄^2–)

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 (Δ¹⁷O = &delta¹⁷O–0.52 × &delta¹⁸O) 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 (e.g. 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 fractions 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 CE) 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 indicate that 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 timescale. In Antarctica, the Δ¹⁷O(SO₄^2–) 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 Δ¹⁷O(SO₄^2–) to the modeled changes in oxidants is small. There is little net change in the 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.