In September 1982 and June 1983, diurnal variations in the
13C/
12C ratio and concentration of atmospheric CO
2 were studied. The concentration and δ
13C value of CO
2 turn to be maximum just before dawn and minimum during daytime. This dirnal variations are caused through the simple mixing of two different isotopic CO
2 species, which are CO
2 in the ambient air and CO
2 from biosphere. The carbon isotopic ratio of CO
2 derived from biosphere is estimated to be -22 to -23‰ with respect to PDB standard. The estimated value is a little higher than the δ
13C value attributed to the plant respiration. According to the observed δ
13C value in soil air ranging from -11 to -16‰, the little higher value may be explainable as a consequence of addition of soil air CO
2 into the surface air.
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