THE TEMPORAL VARIATION OF STRATOSPHERIC FALLOUT DERIVED FROM THE CHERNOBYL ACCIDENT

The monthly depositions of Sr-90 and caesium isotopes at Tsukuba through December 1988 are reported and the temporal variation of Chernobyl fallout is discussed. The measurements indicate that the radioactive fallout derived from the stratospheric Chernobyl radioactivity continued until the end of 1988. The annual Cs-137 deposits were 135, 0.95 and 0.57 Bq m-2 in 1986, 1987 and 1988, respectively. The annual Sr-90 deposits were 1.8 and 0.15 Bq m-2 in 1986 and 1987, respectively. In 1987, about 80% of the Cs-137 deposition and about 40% of the Sr-90 deposition were from Chernobyl radioactivity transported into the stratosphere. It is estimated that 5.1 x 10(-3) of the released caesium isotopes and 1.9 x 10(-3) of the released Sr-90 have been transported to the stratosphere. With regard to the altitude distribution of the stratospheric Chernobyl radioactivity, 46% was in the active mixing and exchange layer near the tropopause, while 54% was transported into the lower stratosphere. The Cs-137/Sr-90 activity ratio of the stratospheric Chernobyl radioactivity is estimated to be 12. The contribution of resuspension to annual deposition in 1987 was less than 1% for Cs-134 and 2% for Cs-137. In 1988, it was less than 14% for Cs-134 and 34% for Cs-137.