HR: 17:10h
AN: A44B-05    [Abstracts]
TI: Measurements of light alkanes (C₂-C₄) in firn air at Summit, Greenland and West Antarctic Ice Sheet Divide, Antarctica: Is there evidence for a recent decline in polar tropospheric levels?
AU: Saltzman, E S
EM: esaltzma@uci.edu
AF: UC Irvine, Dept. of Earth System Science, Irvine, CA 92697, United States
AU: * Aydin, M
EM: maydin@uci.edu
AF: UC Irvine, Dept. of Earth System Science, Irvine, CA 92697, United States
AU: Williams, M B
EM: mbwilliams@arc.nasa.gov
AF: Bay Area Env. Res. Inst./NASA Ames, MOffett Field, Moffett Field, CA 94035, United States
AU: Sofen, E
EM: esofen@u.washington.edu
AF: University of Washington, Dept. of Atmospheric Sciences, Seattle, WA 98195, United States
AU: Battle, M
EM: mbattle@bowdoin.edu
AF: Bowdoin College, Dept. of Physics and Astronomy 8800 College Station, Brunswick, ME 04011-8488, United States
AB: Light alkanes are an important part of the tropospheric photochemical system, acting as precursors for ozone (O₃) and carbon monoxide (CO) and as a removal mechanism for the hydroxyl radical (OH). In this study, we report measurements of ethane (C₂H₆), propane (C₃H₈), and n-butane (n- C₄H₁₀) in firn air collected at Summit, Greenland (May-June 2006) and West Antarctic Ice Sheet Divide (WAIS-D) (Dec-Jan 2005-2006). C₂H₆, C₃H₈, and n-C₄H₁₀ levels in Summit firn were in the 1.5-2.0 ppb, 400-600 ppt, and 150-250 ppt range, respectively. These levels are within the range of modern mean annual levels in surface air. C₂H₆, C₃H₈, and n-C₄H₁₀ mixing ratios measured in the WAIS-D firn were much lower than the Summit values, ranging from 200-300 ppt, 20-40 ppt, and 10-20 ppt, respectively. This is consistent with expectations from the interhemispheric differences in the distribution of sources for these short-lived gases. The reliability of firn air as an archive for tropospheric levels of light alkanes was assessed by comparison of firn air records to surface air flask measurements. If the firn air alkane data are interpreted as atmospheric histories, the depth profiles suggest that there has been a decline on the order of 20-40 % in annual mean levels over Greenland during the last two decades of the 20^th century. Interestingly, WAIS-D data also suggest a modest decline in C₂H₆ levels over the West Antarctic Ice Sheet around the same time period. Trends in C₃H₈ and n-C₄H₁₀ data from WAIS-D are harder to interpret due to higher noise in the measurements resulting from lower background levels.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0325 Evolution of the atmosphere (1610, 8125)
DE: 0365 Troposphere: composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting