Observation of the tight correlation between greenhouse gas concentrations and indicators of temperature from the ice core record has had a profound impact on our understanding of the relationship between greenhouse gases and climate. However, despite many decades of research, we still do not have a firm understanding of the causes of the variability in greenhouse gas concentrations. The hydroxyl radical (OH) plays a major role in determining the lifetimes of atmospheric trace gases important to both climate change (e.g. CH4) and human health (e.g. CO), and largely determines the oxidative capacity of the atmosphere. The oxygen isotopic composition (∆17O) of nitrate and sulfate aerosols extracted from ice cores provides information about the chemistry of past atmospheres, and can serve as a tool for evaluating model simulations of paleo atmospheric chemistry. Initial measurements of sulfate and nitrate ∆17O from the Vostok, Antarctica ice core suggest higher tropospheric O3 concentrations in polar-regions and higher OH concentrations in the mid- to high-latitudes during the last glacial period relative to the preindustrial Holocene. Quantitative interpretation of these data sets is ongoing using a global chemistry-climate model containing the isotopic tracers, and demonstrates the sensitivity of nitrate and sulfate ∆17O to oxidant concentrations. Currently, no measurements of sulfate and nitrate ∆17O exist on the glacial-interglacial timescale in the Northern Hemisphere. Our goal here is to expand the existing observational data set on the glacial-interglacial timescale to the Northern Hemisphere in order to test the hypothesis that variability in methane concentrations on the on this timescale is largely (50%) due to changes in tropospheric OH concentrations, as has been suggested by terrestrial vegetation modeling. We also will provide a first look into whether changes in OH may partially explain variability of methane during two abrupt climate events. Quantitative information about the sink strength of methane will transform our ability to quantitatively interpret the methane record as a proxy for terrestrial climate conditions, and as such improve our understanding of the mechanistic feedbacks between climate change and the sources and sinks of methane. Understanding the affects of a changing climate on atmospheric chemistry and the oxidizing capacity of the atmosphere is crucial for predicting the impacts of future climate change on local and regional air pollution and human health. The project will involve training two graduate students (one US and one French) in the tools of geochemical analysis in an interdisciplinary and international context.