Measurements of atmospheric methane at the Japanese Antarctic Station, Syowa

Precise and continuous measurements of the atmospheric CH 4 concentration were initiated at Syowa Station, Antarctica in February 1988, using a gas chromatograph equipped with a flame ionization detector. The measurement precision was typically ± 0.07% in a concentration range of 0.5 to 2.5 ppmv. The standard gases were CH 4 -in-air mixtures and their CH 4 concentrations were determined gravimetrically with uncertainties of ± 0.2%. The CH 4 concentrations measured at the station were extremely stable, and outliers due to local contamination such as station activities were not found. The diurnal CH 4 variation was hardly observable throughout the year. The average seasonal CH 4 variation showed minimum and maximum concentrations in early March and late September, respectively, and a peak-to-peak amplitude of 30 ppbv. Annual mean values of the CH 4 concentration were 1640 and 1651 ppbv in 1988 and 1989, respectively. A concentration difference of 11 ppbv between the two years may suggest that recent annual increase rate of the CH 4 concentration is smaller than previously reported values of about 1% year -1 .