14-CO in Glacial Ice from Law Dome, Antarctica as a Tracer of Changes in Atmospheric OH Abundance from 1870 AD to Present
Dr Peter Neff1, Vasilii Petrenko2, David Etheridge3, Andrew Smith4, Edward Crosier2, Benjamin Hmiel2, David Thornton3, Lenneke Jong5, Ross Beaudette6, Christina Harth6, Ray Langenfelds3, Blagoj Mitrevski3, Mark Curran5, Christo Buizert7, Lee Murray2, Cathy Trudinger3, Michael Dyonisius2, Jessica Ng6, Jeff Severinghaus6, Ray Weiss6
1University Of Minnesota, Saint Paul, United States, 2University of Rochester, Rochester, United States, 3CSIRO, Aspendale, Australia, 4ANSTO, Kirrawee, Australia, 5Australian Antarctic Division, Kingston, Australia, 6Scripps Institution of Oceanography, La Jolla, United States, 7Oregon State University, Corvallis, United States
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. Corrections for in-situ 14CO production, validated against direct atmospheric measurements for the more recent samples, have allowed us to develop a preliminary 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.
Peter is a glaciologist and climate scientist working primarily to develop glacier ice core records of past climate, environmental conditions, and atmospheric chemistry. His current research focuses on better understanding recent climate of dynamic coastal regions from high-resolution ice core archives, spanning both West and East Antarctica as well as the Coast Mountains of British Columbia, Canada.