Measurements of atmospheric CO₂ from a meteorological tower in Tsukuba, Japan

Since February 1992, measurements of atmospheric CO₂ at 200 m above the ground have been taken from a meteorological tower (lat 36°04′N, long 140°07′E, 25 m a.s.l.) in Tsukuba, central Japan. A diurnal variation of atmospheric CO₂ at 200 m was clearly observed in summer and less clearly in winter. In summer, the maximum occurred a few hours after dawn and the minimum in the late afternoon; in winter, the CO₂ mixing ratio was fairly constant because of temperature inversion below 200 m. The daily mean values of atmospheric CO₂ in the afternoon (1300–1600 JST), selected as they were close to the representative values in the planetary boundary layer, were as high as 4 to 6 ppm greater than at stations in remote areas of the same latitudinal zone. This was mainly due to CO₂ emissions from anthropogenic sources in central Japan. The peak-to-trough amplitude of the seasonal variation, caused by the CO₂ exchange between the air and the land biosphere, ranged from 11 to 14 ppm. Year-to-year changes in the seasonal variation of atmospheric CO₂ during the period from May to July were correlated well with the surface air temperature among climate variables that affect the photosynthesis/respiration of the land biosphere. The change in CO₂ mixing ratio of the detrended seasonal variation from May to July increased with the surface air temperature, which was suggestive of the larger temperature dependence of respiration rate as compared with that of photosynthesis rate. Over the period from 1993 to 1999, the annual mean of the CO₂ mixing ratio increased from 363.0 ppm to 374.6 ppm. The growth rate from February 1992 to January 2000 showed a large interannual variability ranging from −0.8 to 4.6 ppm yr⁻¹ with a mean rate of 2.0 ppm yr⁻¹.