HR: 0800h
AN: A21C-0893    [Abstracts]
TI: Atmospheric Histories of Methane and Carbon-13 Inferred From Polar Firn Air
AU: * Trudeau, M E
EM: Michael.Trudeau@noaa.gov
AF: Mauna Loa Observatory, 154 Waianuenue Ave., Suite 203, Hilo, HI 96720 United States
AU: Tans, P P
EM: Pieter.Tans@noaa.gov
AF: NOAA/CMDL, 325 Broadway, Boulder, CO 80305 United States
AB: A new firn gas transport model has been developed to reconstruct histories of atmospheric methane and methane isotopic composition using firn air measurements from expeditions at North GRIP, Greenland and Siple Dome, Antarctica. CO₂ is used as a reference tracer to reconstruct gas diffusivities in the porous snow, and a Monte Carlo modeling approach is used to quantify reconstruction uncertainties. Methane mixing ratio histories derived from Siple Dome firn air agree well with direct southern hemisphere measurements and the reconstruction from Law Dome (Etheridge et al., 1998). The methane history inferred from North GRIP measurements demonstrates growth rates between 1.0--1.1%yr^-1 from 1950 to 1990, and also shows good agreement with available direct measurements. The polar southern hemisphere methane δ ¹³C reconstruction using carbon isotope measurements from Siple Dome firn shows growth rates that range between 0.02--0.03±0.03‰, values which are lower and contain a larger uncertainty than those published in previous firn air studies. Siple Dome methane δ ¹³C measurements used for this model study include the uncertainty due to sampling procedures and flask storage effects, a combined contribution which is approximately equal to the instrument uncertainty alone. With this added uncertainty, the Siple Dome results suggest that the evolution of the atmospheric methane δ ¹³C signal in the recent past is too small to be resolved with historic reconstructions using isotope ratio measurements of methane from air contained in polar firn.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0322 Constituent sources and sinks
DE: 0325 Evolution of the atmosphere (1610, 8125)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005