Advances in reconstructing past atmospheric oxidation chemistry using the oxygen isotope composition of nitrate and sulfate in ice cores

by Kunasek, Shelley A., Ph.D., UNIVERSITY OF WASHINGTON, 2009, 137 pages; 3393959


Atmospheric oxidants determine the lifetimes of reduced trace gases that act as greenhouse gases and pollutants. There has thus been great interest in deducing past variability in oxidant concentrations from ice cores. Ice core measurements of Δ17O of nitrate and sulfate (Δ 17O(NO3) and Δ17O(SO 42−)(Δ17O ≈ δ 17O − 0.52× (δ18O)) provide a means of reconstructing past changes in the oxidation chemistry of nitrate and sulfate production. This dissertation describes improvements to methods for Δ 17O(NO3) and Δ17O(SO 42−) analysis and contributes new snow and ice core records of Δ17O(NO3) and Δ17O(SO42−) in Greenland and Antarctica, respectively. Measurements of sulfur isotopes of sulfate (δ 34S, Δ33S, Δ36S) provide complementary information on sources of atmospheric sulfate.

Methods are presented for automated, simultaneous analysis of Δ 17O(NO3) and Δ17O(SO 42−) at micromole levels and for Δ 17O(NO3) analysis at submicromole levels. The automation of simultaneous Δ17O(NO3 ) and Δ17O(SO4 2−) analysis will expand environmental applications of these complementary isotopic measurements. Reduced sample size requirements will improve the temporal resolution of ice core analysis.

We present measurements of seasonal changes in Δ17O(NO 3) from a snowpit at Summit, Greenland, and compare them with calculations from an atmospheric chemical box model. The box model underestimates summer Δ17O(NO3), suggesting several important influences on nitrate isotopic composition that are not accounted for in our box model: Non-zero Δ 17O of OH over polar regions, stratospheric influence on surface O 3 at Summit, participation of BrO in nitrate production, and tropospheric transport of nitrate. A box model sensitivity study shows that annual mean Δ 17O(NO3) at Summit is most sensitive to changes in the ratio of [O3]/([HO2]+[RO2]) in summer.

Ice core measurements of Δ17O, δ34 S, Δ33S, and Δ36S of sulfate over the past 230 years from the West Antarctic Ice Sheet (WAIS) Divide are also presented. Sulfur isotope measurements suggest stronger influence of volcanogenic and/or stratospheric sulfate in West relative to East Antarctica. The lack of change in Δ17O of non-sea salt sulfate from the mid-1800s to early 2000s (2.4–2.6±0.2‰) is consistent with atmospheric chemistry model estimates indicating preindustrial to industrial increases in O3 as high as 50% and decreases in OH of 20% in the southern polar troposphere, as long as H2O2 concentrations also increase by over 50%.

AdviserEric J. Steig
Source TypeDissertation
SubjectsAtmospheric chemistry; Paleoclimate science; Geochemistry
Publication Number3393959

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