HR: 0800h
AN: A21A-0824    [Abstracts]
TI: 2000 Year Atmospheric History of Methyl Bromide and Methyl Chloride from an Antarctic Ice Core
AU: * Williams, M B
EM: mbwillia@uci.edu
AF: University of California, Irvine, Department of Earth System Science 1212 Croul Hall University of California, Irvine, Irvine, CA 92697-0001 United States
AU: Aydin, M
EM: maydin@uci.edu
AF: University of California, Irvine, Department of Earth System Science 1212 Croul Hall University of California, Irvine, Irvine, CA 92697-0001 United States
AU: Saltzman, E S
EM: esaltzma@uci.edu
AF: University of California, Irvine, Department of Earth System Science 1212 Croul Hall University of California, Irvine, Irvine, CA 92697-0001 United States
AB: Methyl chloride (CH₃Cl) and methyl bromide (CH₃Br) mixing ratios have been measured in air extracted from 50 Antarctic ice core samples from South Pole (SPRESSO-2002), extending the atmospheric history for these trace gases to roughly 2000 years before present. Ice core samples (~ 500g) were shredded at -40°C to -50°C to extract ~ 40 cm3 of air that was analyzed by a gas chromatography/mass spectrometry system. CH₃Br mixing ratios vary between 4.5 ppt to 6.5 ppt with a mean of 5.6±0.8 ppt. These results are in good agreement with the 350-year record of CH₃Br previously measured in ice cores from Siple Dome, Antarctica. The ice core data provide evidence that, for over 2000 years, CH₃Br levels were 2-4 ppt lower that the modern ambient levels of 7-8 ppt (Southern Hemisphere) that peaked in late 1990s and have been declining since regulation of its use as part of Montreal protocol. The mean mixing ratio of CH₃Cl is 463.0±16.0 ppt with values ranging from 418.0 ppt to 543.0 ppt. These levels are also consistent with the range of values measured in the Siple Dome core. Modern ambient mixing ratios in the Southern Hemisphere are on the order of 500-550ppt, indicating that anthropogenic contributions to the modern CH₃Cl budget are relatively small. Measured CH₃Cl levels vary by as much as 50ppt over 50 year periods. Periodicity in CH₃Cl observed in Siple Dome is not obvious in this record, but cannot be precluded due to the lower time resolution of the SPRESSO core. These measurements constitute the longest atmospheric record for both gases and demonstrate the fidelity of the ice core gas record for these trace gases over millennial time scales. There is potential for constructing records extending beyond the Holocene, and over glacial/interglacial transitions for these trace gases. It is unclear how either of these gases responds to changing climate. Shifts in plant emissions, soil uptake, ocean production, and atmospheric oxidation capacity due to past climate changes may all manifest in CH₃Cl and CH₃Br records. Establishing a paleoatmospheric record for these and other trace gases should improve the understanding of these feedbacks.
DE: 0490 Trace gases
DE: 0724 Ice cores (4932)
DE: 4932 Ice cores (0724)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005