The impact of anthropogenic emissions on atmospheric sulfate production pathways, oxidants, and ice core Delta super(17)O(SO sub(4) super(2-))

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 ( Delta super(17)O = delta super(17)O-0.52 delta super(18)O) of sulfate in Greenland and Antarctic ice cores and aerosols. The oxygen isotopic composition of non-sea salt sulfate ( Delta super(17)O(SO sub(4) super(2-))) is a function of the relative importance of each oxidant (e.g. O sub(3), OH, H sub(2)O sub(2), and O sub(2)) during sulfate formation, and can be used to quantify sulfate production pathways. Due to its dependence on oxidant concentrations, Delta super(17)O(SO sub(4) super(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 sub(3) since the preindustrial period. The model simulates a significant increase in the fraction of sulfate formed via oxidation by O sub(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 sub(3) and H sub(2)O sub(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 Delta super(17)O(SO sub(4) super(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 sub(3), a 10% decrease in OH, and a 58% increase in H sub(2)O sub(2) between the preindustrial period and present. Model results indicate that the observed decrease in the Arctic Delta super(17)O(SO sub(4) super(2-)) - in spite of increasing tropospheric O sub(3) concentrations - can be explained by the combined effects of increased sulfate formation by O sub(2) catalyzed by anthropogenic transition metals and increased cloud water acidity, rendering Delta super(17)O(SO sub(4) super(2-)) insensitive to changing oxidant concentrations in the Arctic on this timescale. In Antarctica, the Delta super(17)O(SO sub(4) super(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 timesc