Methane: Interhemispheric concentration gradient and atmospheric residence time.

The ground level concentrations of methane in the atmosphere have been measured to be in the range from 1.45 to 1.62 parts per million by volume (ppmv) of dry air in remote locations between 62 degrees N and 54 degrees S latitudes during the time period from November 1977 to July 1979. The average (+/-rms) concentration for the northern hemisphere was 1.57 +/- 0.02 ppmv in January 1978 and 1.59 +/- 0.02 in July 1979. The average concentration in the southern hemisphere was lower-1.47 +/- 0.02 in January 1978 and 1.51 +/- 0.01 in July 1979. The ratio of concentrations between the two hemispheres was 1.068 +/- 0.016 in January 1978 and 1.055 +/- 0.013 in July 1979, for an average of 1.06 +/- 0.01. The higher concentrations in the northern hemisphere require either that the sources of methane lie preferentially in the northern hemisphere or that the removal processes operate more rapidly in the southern hemisphere or both. The primary removal process for CH(4) is reaction with tropospheric OH radicals and its estimated atmospheric lifetime is 10.5 +/- 1.8 yr. The observed interhemispheric gradient is consistent with this lifetime and preferential release of methane in the northern hemisphere. Measurements taken in the Amazon basin region indicate the presence of a substantial source of methane in that area.