Variation in 14C concentration of the atmosphere since 1964 in Japan

The variation in the 14C concentration with time has been experimentally studied both in the upper atomosphere (at an altitude of about 10.6 km above sea level) and on the ground at various localities since 1964. During the first several years, the activity of 14C in the upper troposphere was lower than that in the lower stratosphere but higher than that at the localities on the ground. But, the difference in the 14C activity among the lower stratosphere, the upper troposphere and the localities on the ground as well as in the 14C concentration has decreased with time. The samples in the upper atmosphere have been classified by the Siberian and Ogasawara air masses which are originated in the polar continental zones(Pc) and in the subtropical zones(Tm), respectively. At first, the 14C concentration in the Siberian air mass was higher than that of the Ogasawara air mass and decreased more rapidly than that of the Ogasawara air mass. In 1965, the 14C concentration in both air masses became the same value and then the concentration in the Ogasawara air mass seems to become much higher. The variation in the 14C concentration with time at the localities on the ground has been compared with the report of United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and our results show slightly lower 14C activity than that of the UNSCEAR report. (orig.)