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 δ3C with a precision of better than 0.03 %. The amplitudes of the seasonal cycles of δ3C 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 δ3C, it was found that the rate of change in δ3C with respect to the CO₂ concentration is about –0.05 %/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. δ3C decreased secularly at an average rate of about –0.04 %/year, due mainly to combustion of fossil fuel with isotopically light CO₂. Interannual variations of δ3C, 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.