Sensitivity of atmospheric oxidation chemistry to climate inferred from Greenland ice core records of Δ17O(NO3-)

Friday, 19 December 2014: 12:05 PM
Lei Geng1, Becky Alexander1 and Lee T Murray2, (1)University of Washington, Seattle, WA, United States, (2)NASA Goddard Institute for Space Studies, New York, NY, United States
The oxygen-17 excess of atmospheric nitrate (Δ17O(NO3-)) is indicative of the relative abundances of oxidants (O3/HOx, where HOx = OH + HO2 + RO2) in the troposphere at the time of nitrate formation, with larger O3/HOx ratios leading to larger values of Δ17O(NO3-). The preservation of nitrate in polar ice sheets can thus be used to retrieve information related to past changes in local and/or regional tropospheric oxidation chemistry, contributing to the understanding of atmospheric chemistry and climate interactions. We measured Δ17O(NO3-) in ice core samples from the Greenland Ice Sheet Project 2 (GISP2), covering the Holocene and the last glacial periods, as well as two Dansgaard–Oeschger (DO) events in the last glacial period. The values of Δ17O(NO3-) are higher in the glacial period compared to during the Holocene. In addition, Δ17O(NO3-) changes abruptly by as much as 3 ‰ in ~100 years during these two DO events, suggesting a high sensitivity of O3/HOx to climate in the mid- to high northern latitudes. The observations suggest a general relationship between tropospheric O3/HOx ratios and climate, with larger O3/HOx ratios in colder climates compared to during warmer climates. High resolution measurements during the two DO events further indicate a second regime that in relatively warm climates, the O3/HOxratios decrease with decreasing temperature.

We hypothesize that the switch between the two regimes is due to changes in the relative importance of 1) surface emissions and chemistry, and 2) stratosphere-troposphere exchange (STE) for tropospheric O3/HOx in the mid- to high northern latitudes. Surface emissions and chemistry dominate variability in tropospheric O3/HOx in relatively warm climates. In colder climates, changes in STE become dominant for the observed variability in tropospheric O3/HOx in the mid- to high northern latitudes. Prior calculations from global chemistry-climate models are qualitatively consistent with this hypothesis.