INCREASING CONCENTRATIONS OF ATMOSPHERIC METHANE, 1979-1983 (LIFETIME MODEL)

The concentration of methane in the atmosphere was measured from 1979 through 1983 to establish the worldwide background mixing ratios. Sampling sites were located from Alaska (71 N) to Chile (54 S). An increase in the average worldwide methane mixing ratio from about 1.55 ppmv to about 1.62 ppmv during the four year study was observed. The samples were compared to a standard with an absolute accuracy of (+OR-) 1% provided by the National Bureau of Standards. The precision of each individual measurement was (+OR-) 0.0006 ppmv. A consistent increase of 0.018 (+OR-) 0.002 ppmv per year was observed over the four year period.;Two methods were used in determining the atmospheric lifetime of methane. The first approach compared the reaction rate constants of CH(,4) with CH(,3)CCl(,3), yielding a lifetime range between 9 and 10 years. The second method utilized a one-dimensional (latitude) computer model of the atmosphere to determine the lifetime of CH(,4). Latitudinal mixing coefficients and OH distributions adjusted to fit latitudinal gradients in halocarbon concentrations were used in conjunction with an estimated latitudinal distribution for CH(,4) sources. The most consistent fit to the methane latitude distribution was again found for an atmospheric lifetime between 9 and 10 years.