Index Geophysics

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 = &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.

Sofen, E. D.

Alexander, B.

Kunasek, S. A.

2018-01-15

Text

application/pdf

10.5194/acp-11-3565-2011

https://acp.copernicus.org/articles/11/3565/2011/

eISSN: 1680-7324

eng