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Shipboard determinations of 13C in atmospheric methane in the Pacific

Lowe, D.C. and Bromley, A. and Allan, W. and Brailssford, G. and Manning, M.R. and Ferretti, D. and Gomez., A. and Knobben, R. and Martin, R. and Mei, Z. and Moss, R. and Koshy, Kanayathu and Maata, Matakite (1999) Shipboard determinations of 13C in atmospheric methane in the Pacific. Journal of Geophysical Research: Atmospheres, 104 (D21). pp. 26125-26135. ISSN 0148-0227

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    Measurements of the mixing ratio and δ13C in methane (δ13CH4) are reported from large, clean air samples collected every 2.5° to 5° of latitude on four voyages across the Pacific between New Zealand and the West Coast of the United States in 1996 and 1997. The data show that the interhemispheric gradient for δ13CH4 was highly dependent on season and varied from <0.1‰ in June 1996 to >0.5‰ in November 1996 with an estimated annual mean of 0.2–0.3‰. The seasonal cycles in δ13CH4 reveal three distinct latitude bands differentiated by phase. Maxima occur in January-February for the extratropical Southern Hemisphere, in September-October for the tropics, and in June-July for the extratropical Northern Hemisphere. The data are compared with results from a three-dimensional transport and atmospheric chemistry model that simulates the observed latitudinal structure of either δ13CH4 or the methane mixing ratio well, but not both simultaneously. The requirement that a methane source-sink budget be consistent with both types of data clearly imposes stricter constraints than arise from either mixing ratio or isotopic data alone. The seasonal δ13CH4 data in the extratropical Southern Hemisphere are used to estimate a value for the net fractionation in the CH4 sink of 12–15‰, which is larger than can be explained by current laboratory measurements of a kinetic isotope effect for the OH + CH4 reaction and soil sink processes. The hypothesis that the discrepancy is caused by competitive reaction of active chlorine with methane in the marine boundary layer is discussed.

    Item Type: Journal Article
    Subjects: Q Science > QD Chemistry
    Divisions: Faculty of Science, Technology and Environment (FSTE) > School of Biological and Chemical Sciences
    Depositing User: Matakite Maata
    Date Deposited: 09 Jan 2018 09:39
    Last Modified: 09 Jan 2018 09:39

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