Seasonal and long-term variations in atmospheric CO2 and 85Kr in Tsukuba, central Japan

Since February 1992, measurements of atmospheric CO2 at 200 m above the ground have been taken from a meteorological tower (lat 36.1 degrees N, long 140.1 degrees E, 25 m a.s.l.) in Tsukuba, central Japan. We have examined seasonal/long-term variations in atmospheric CO2, based on atmospheric Kr-85, a noble gas with well-known source and sink terms, monitored continuously at weekly intervals in Tsukuba since May 1995. During the annual cycle, the daily mean values of atmospheric CO2 in the afternoons (13001600 JST) showed maximum in late April, except in 2002, and minimum in late August or early September. The seasonal variations in atmospheric Kr-85, which is low in summer and high in winter, were caused by the seasonal variations in the large-scale atmospheric circulation, and latitudinal distribution of atmospheric Kr-85. Compared with seasonal variations in atmospheric Kr-85, the draw-down (DD) and buildup (BU) for atmospheric CO2 occurred about 1 week and 3 weeks later, respectively. The peak-to-trough amplitude of de-trended seasonal variations in atmospheric CO2 concentrations ranged from 11 to 14 ppm. A decrease in the seasonal CO2 minimum occurred with a decrease in the Kr-85 concentration, showing the effect of air mass transport from different origins. Unlike the minimum, the maximum CO2 did not vary as expected from the air mass transport. The daily mean values of atmospheric CO2 increased at an average rate of 2.0 ppm yr(-1), with a range from -0.5 to 4 ppm yr(-1). While both CO2 and Kr-85 are emitted into the atmosphere, due to energy consumption, the growth rate of atmospheric CO2 showed a pattern different from that of atmospheric Kr-85 (0.03 Bq m(-3) yr(-1)).