TIME AND SPACE VARIATIONS OF THE CARBON ISOTOPIC RATIO OF TROPOSPHERIC CARBON-DIOXIDE OVER JAPAN

CO2 samples collected in the troposphere over Japan during the period from April 1984 to November 1990 were analyzed for deltaC-13 with a precision of better than 0.03 parts per thousand. The amplitudes of the seasonal cycles of deltaC-13 and CO2 concentration decreased with increasing height, with a phase delay of about 1 month between the lower and upper troposphere. From a comparison of the observed seasonal variation of the CO2 with that of deltaC-13, it was found that the rate of change in deltaC-13 with respect to the CO2 concentration is about -0.05 parts per thousand/ppmv. This suggests that the seasonal CO2 cycle over Japan is mainly produced by seasonally-dependent biospheric activities. However, air transport from different latitudes is also important for the seasonal CO2 cycle, particularly in the upper troposphere. deltaC-13 decreased secularly at an average rate of about -0.04 parts per thousand/year, due mainly to combustion of fossil fuel with isotopically light CO2. Interannual variations of deltaC-13, superimposed on the long-term trend, were also observed, which were approximately opposite in phase with those of the CO2 concentration. The comparison of both interannual variations suggested that the cause could be primarily attributed to an imbalance in the CO2 exchange between the atmosphere and the biosphere rather than between the atmosphere and the oceans.