The carbon isotopic ratio of atmospheric carbon dioxide at Tsukuba, Japan

Abstract

To find out the secular and seasonal trends of the δ13C value and CO2 concentration in the surface air and the determination of the δ13C in the atmospheric CO2 collected at Tsukuba Science City was carried out during the period from July 1981 to October 1983. The monthly average of the δ13C value of CO2 in the surface air collected at 1400 LMT ranged from -7.52 to \s-8.45‰ with an average of -7.96±0.25‰ and the CO2 concentration in the air varied from 334.5 μl 1-1 to 359 μl 1-1 with an average of 347.2±6.3 μl 1-1. The δ13C value is high in summer and low in winter and is negatively correlated with the CO2 concentration. In general, the relationship between the δ13C and the CO2 concentration is explainable by a simple mixing model of two different constant carbon isotopic species but the relationship does not always follow the model. The correlation between the δ13C value and the CO2 concentration is low during the plant growth season and high at other times. The observed negative deviation of the δ13C value from the simple mixing model in the plant growth season is partly due to the isotopic fractionation process which takes place in the land biota.