Shipboard determinations of the distribution of 13C in atmospheric methane in the Pacific

Measurements of the mixing ratio and delta(13)C methane (delta(13)CH(4)) are reported from large, clean air samples collected every 2.5 degrees to 5 degrees 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 delta(13)CH(4) was highly dependent on season and varied from < 0.1 parts per thousand in June 1996 to > 0.5 parts per thousand in November 1996 with an estimated annual mean of 0.2-0.3 parts per thousand. The Seasonal cycles in delta(13)CH(4) 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 delta(13)CH(4) 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 delta(13)CH(4) data in the extratropical Southern Hemisphere are used to estimate a value for the net fractionation in the CH4 sink of 12-15 parts per thousand, 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.