HR: 0800h
AN: A11C-0594    [Abstracts]
TI: Changes In Atmospheric Methane And Its Stable Isotope Signatures Between 1978-2005 Deduced From Interhemispheric Differences and Trends
AU: * Kai, F
EM: fmkai@uci.edu
AF: Earth System Science Dept. , Univ. of Calif. , Irvine, 3200 Croul Hall, Irvine, CA 92697-3100, United States
AU: Tyler, S C
EM: styler@uci.edu
AF: Earth System Science Dept. , Univ. of Calif. , Irvine, 3200 Croul Hall, Irvine, CA 92697-3100, United States
AU: Randerson, J T
EM: jranders@uci.edu
AF: Earth System Science Dept. , Univ. of Calif. , Irvine, 3200 Croul Hall, Irvine, CA 92697-3100, United States
AU: Blake, D R
EM: drblake@uci.edu
AF: Dept. of Chemistry, Univ. of Calif. , Irvine, 570 Rawland Hall, Irvine, CA 92697-2025, United States
AB: During the past 2 decades a slow-down in CH₄ concentration has been observed. The causes for this slow- down trend are still under debate. Meanwhile, carbon and hydrogen isotopic measurements of atmospheric CH₄ and its sources as well as measurements of the isotopic effect of CH₄ sink processes have provided useful information on determining CH₄ source and sink strengths and distributions. Here we present the synchronous relationships of CH₄ mixing ratio and stable isotopes (¹³C/¹²C and D/H) between the Northern and Southern Hemispheres over the past 20 years. Our results show that the seasonally detrended interhemispheric difference (IHD) of mixing ratio has increased from about 60 to 100 ppbv over the period around 1980 to 1990, and then decreased to ca. 80 ppbv in 2005. Furthermore, the seasonally detrended IHD of carbon isotope signature has decreased from about 0.3 permil to 0.1 permil from 1990 to 2005. However, the change of the seasonally detrended IHD of hydrogen isotope signature was negligible over roughly the same period. We apply a 2-box model (N/S) to deduce the source and sink processes governing the variations of the trends with the help of the dual constraint offered by simultaneous CH₄ carbon and hydrogen stable isotope data. To explain the decreasing IHD of carbon isotope signature and CH₄ mixing ratio, a reduction in the NH flux of isotopically light sources (likely rice and livestock) is required. This obviates the hypothesis that a decrease in fossil fuel flux has contributed much to the leveling off of CH₄ over this time period.
DE: 0365 Troposphere: composition and chemistry
DE: 0368 Troposphere: constituent transport and chemistry
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0490 Trace gases
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting