ABSTRACT

In September 1982 and June 1983, diurnal variations in the ¹³C/¹²C ratio and concentration of atmospheric CO₂ were studied. The concentration and δ¹³C value of CO₂ turn to be maximum just before dawn and minimum during daytime. This dirnal variations are caused through the simple mixing of two different isotopic CO₂ species, which are CO₂ in the ambient air and CO₂ from biosphere. The carbon isotopic ratio of CO₂ derived from biosphere is estimated to be -22 to -23‰ with respect to PDB standard. The estimated value is a little higher than the δ¹³C value attributed to the plant respiration. According to the observed δ¹³C value in soil air ranging from -11 to -16‰, the little higher value may be explainable as a consequence of addition of soil air CO₂ into the surface air.