C046-0001
14CO in Glacial Ice from Law Dome, Antarctica as a Tracer of Changes in Atmospheric OH Abundance from 1870 AD to Present

Monday, 14 December 2020
Poster
Peter D Neff1, Vasilii V Petrenko2, David M Etheridge3, Andrew Milford Smith4, Edward Crosier2, Benjamin Hmiel2,5, David Thornton6, Lenneke M Jong7, Ross Beaudette8, Christina M Harth8, Ray Leonard Langenfelds3, Blagoj Mitrevski3, Mark Curran7, Christo Buizert9, Lee T Murray2, Cathy M Trudinger3, Michael Dyonisius10,11, Jessica Yi-Jun Ng12, Jeffrey P Severinghaus12 and Ray F Weiss13, (1)University of Minnesota Twin Cities, Department of Soil, Water, and Climate, Minneapolis, MN, United States, (2)University of Rochester, Department of Earth and Environmental Sciences, Rochester, NY, United States, (3)CSIRO, Oceans and Atmosphere, Aspendale, VIC, Australia, (4)Australian Nuclear Science and Technology Organisation (ANSTO), Kirrawee DC, Australia, (5)Environmental Defense Fund, Austin, TX, United States, (6)CSIRO, Aspendale, VIC, Australia, (7)Australian Antarctic Division, Kingston, TAS, Australia, (8)University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States, (9)Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States, (10)University of Rochester, Department of Earth and Environmental Sciences, Rochester, United States, (11)Niels Bohr Institute, University of Copenhagen, Physics of Ice Climate and Earth, Copenhagen, Denmark, (12)Scripps Institution of Oceanography, La Jolla, CA, United States, (13)Univ California San Diego, La Jolla, CA, United States
Abstract:
Hydroxyl, OH, is the main tropospheric oxidant and determines the lifetime of methane and most other trace gases in the atmosphere, thereby controlling the amount of greenhouse warming produced by these gases. Changes in OH concentration ([OH]) in response to large changes in reactive trace gas emissions (which may occur in the future) are uncertain. Measurements of 14C-containing carbon monoxide (14CO) and other tracers such as methyl chloroform over the last ≈25 years have been successfully used to monitor changes in average [OH], but there are no observational constraints on [OH] further back in time. Reconstructions of 14CO from ice cores could in principle provide such constraints but are complicated by in-situ production of 14CO by cosmic rays directly in the ice. Recent work in Antarctica and Greenland shows that this in-situ component would be relatively small and can be accurately corrected for at sites with very high snow accumulation rates. A joint US and Australian team sampled and measured firn air and ice at Law Dome, Antarctica (2018-19 season, site DE08-OH, 1.2 m a-1 ice-equivalent snow accumulation), to a maximum depth of 240 m. Trapped air was extracted from the ice using an on-site large-volume ice melting system. Preliminary comparisons of methane measured in the samples to existing ice core records and atmospheric measurements suggest ice core air sample ages spanning from the 1870s to the early 2000s. Firn-air samples from the snow surface to 81 m depth capture air from the early 2000s to present. Analyses of [CO] and halocarbons in the samples show a relatively low and stable procedural CO blank and demonstrate that the samples are unaffected by ambient air inclusion. 14CO analyses in these firn and ice core air samples have been successfully completed, and corrections and firn-air modeling are in progress to develop a 14CO history. This history will be interpreted with the aid of the GEOS-Chem chemistry-transport model to place the first observational constraints on the variability of Southern Hemisphere [OH] since ≈1870 AD.
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