The global distribution of methane in the troposphere |
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| Authors: | L P Steele P J Fraser R A Rasmussen M A K Khalil T J Conway A J Crawford R H Gammon K A Masarie K W Thoning |
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| Institution: | (1) NRC-NOAA Research Associate, Geophysical Monitoring for Climatic Change, Air Resources Laboratory/NOAA, 325 Broadway, 80303 Boulder, CO, U.S.A.;(2) Cooperative Institute for Research in Environmental Sciences, University of Colorado/NOAA, 80309 Boulder, CO, U.S.A.;(3) Department of Chemical, Biological, and Environmental Sciences, Oregon Graduate Center, 19600 NW Von Neumann Drive, 97006-1999 Beaverton, OR, U.S.A.;(4) Geophysical Monitoring for Climatic Change, Air Resources Laboratory/NOAA, 325 Broadway, 80303 Boulder, CO, U.S.A.;(5) Present address: Cooperative Institute for Research in Environmental Sciences, University of Colorado/NOAA, 80309 Boulder, CO, U.S.A.;(6) Present address: Division of Atmospheric Research, CSIRO, Station Street, 3195 Aspendale, Victoria, Australia;(7) Present address: Pacific Marine Environmental Laboratory/NOAA, 7600 Sand Point Way NE, 98115 Seattle, WA, U.S.A. |
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| Abstract: | Methane has been measured in air samples collected at approximately weekly intervals at 23 globally distributed sites in the NOAA/GMCC cooperative flask sampling network. Sites range in latitude from 90° S to 76° N, and at most of these we report 2 years of data beginning in early 1983. All measurements have been made by gas chromatography with a flame ionization detector at the NOAA/GMCC laboratory in Boulder, Colorado. All air samples have been referenced to a single secondary standard of methane-in-air, ensuring a high degree of internal consistency in the data. The precision of measurements is estimated from replicate determinations on each sample as 0.2%. The latitudinal distribution of methane and the seasonal variation of this distribution in the marine boundary layer has been defined in great detail, including a remarkable uniformity in background levels of methane in the Southern Hemisphere. We report for the first time the observation of a complete seasonal cycle of methane at the South Pole. A significant vertical gradient is observed between a sea level and a high altitude site in Hawaii. Globally averaged background concentrations in the marine boundary layer have been calculated for the 2 year-period May 1983–April 1985 inclusive, from which we find an average increase of 12.8 ppb per year, or 0.78% per year when referenced to the globally averaged concentration (1625 ppb) at the mid-point of this period. We present evidence that there has been a slowing down in the methane growth rate.Presented at the Conference on the Scientific Application of Baseline Observations of Atmospheric Composition (SABOAC), Aspendale, Australia, 7–9 November 1984. |
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| Keywords: | Methane troposphere global distribution trend seasonal cycle latitudinal gradient vertical gradient |
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