Dissertation Abstract

The oxygen triple-isotopic composition (D17O) of sulfate or nitrate provides insight into the relative importance of the different pathways that lead to their formation in the atmosphere, with implications for their radiative impact, lifetime, and aerosol chemistry. Measurements of sulfate and nitrate D17O from ice cores can constrain long-term changes in the formation of sulfate and nitrate and potentially constrain changes in oxidant abundances. Quantitatively connecting changes in sulfate and nitrate D17O to changes in atmospheric conditions remains a key challenge in applying these measurements to paleo-chemistry. In this work, a new ice core record of sulfate and nitrate isotopes from WAIS Divide, Antarctica is presented, and global chemical transport models and box models of sulfate and nitrate formation are used to quantitatively interpret ice core records of nitrate and sulfate D17O over a variety of time scales. In Ch. 2, a global chemical transport model is used to simulate preindustrial to present-day changes in tropospheric oxidants and the D17O of sulfate, constrained by previously published Greenland and Antarctic ice core records. The sulfate D17O record in Greenland demonstrates the increasing importance of metal-catalyzed sulfate production by O3 due to increases in anthropogenic emissions of transition metals. Antarctic sulfate D17O indicates extratropical Southern Hemisphere increases in both H2O2 (51%) and O3 (27%), constraining the relative increases in their abundances in the remote Southern Hemisphere due to anthropogenic activity. In Ch. 3, a new ice core record of both nitrate and sulfate isotopes from the West Antarctic Ice Sheet (WAIS) Divide ice core spanning the past 2400 years is presented. There is a large (1.1‰) step increase in sulfate D17O in the early 19th century, while nitrate D17O shows a more gradual downward trend of 5.6‰ between the mid-19th century and the present-day. Using other chemical measurements from the WAIS Divide ice core, global chemical transport models, and box models, we investigate the possible explanations for the variability observed in the new ice core record. The increase in sulfate D17O suggests an increase in aqueous-phase sulfate production by O3 that is difficult to reconcile with our understanding of sulfate chemistry and suggests that other oxidants (e.g. hypohalous acids, HOCl and HOBr) may play an important role in extratropical Southern Hemisphere marine boundary layer sulfate formation. The decrease in nitrate D17O is consistent with an increase in the importance of RO2 relative to O3, suggesting a 50-80% decrease in the O3/RO2 ratio in extratropical Southern Hemisphere NOx-source regions resulting from anthropogenic activity. In Ch. 4 the ICE age Chemistry And Proxies (ICECAP) model is used to interpret existing measurements of Antarctic sulfate D17O spanning the last glacial-interglacial cycle. It is demonstrated that the increase in the concentration of OH in the extratropical Southern Hemisphere during the last glacial period is can explain up to 2/3 of the increase in gas-phase sulfate production. A reduction in the glacial cloud fraction leads to a further increase in marine boundary layer sulfate formation in the gas phase. The reduction in clouds has a secondary effect of allowing a greater fraction of SO2 to reach the free troposphere, where it is oxidized by OH...