Long-term variation of 14C concentration in atmospheric CO2 in Japan from 1991 to 2000

The long-term variation of ¹⁴C concentrations in atmospheric CO₂ samples collected every 10 days was measured in the Ohkuwa area of Kanazawa, Ishikawa prefecture, Japan (36.3°N, 136.4°E), from April 1991 to December 2000. The ¹⁴C concentration decreased gradually by about 5‰ each year from 1991 to 2000, with a pronounced seasonal cycle which began to increase from spring and reached the maximum in late July or early August, followed by a decrease to the minimum from December to February. The appearance of large seasonal cycles and the extreme decrease of ¹⁴C concentration in winter seemed to be caused mainly by the air containing a large amount of ¹⁴C-free fossil fuel CO₂ transported across the Sea of Japan from the Asian continent, not by the influence of local fossil fuel consumption.