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 δ¹³C with a precision of better than 0.03 ‰. The amplitudes of the seasonal cycles of δ¹³C 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 δ¹³C, it was found that the rate of change in δ¹³C 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. δ¹³C 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 δ¹³C, 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.