WORLDWIDE FALLOUT 71 



0.26 



0.20 



0.16 



0.020 — 



0.015 



I- 



K 0.010 



240pu/239pu - 



241PU/239PU _ 



I960 



1962 



1964 

 YEAR 



1966 



1968 



1970 



Fig. 6 Atom ratios of plutonium isotopes from an air column (15,000 to 17,000 ft 

 high) at 70° N latitude. 



and minimum concentrations occur in the winter. The rate of decrease in the 2 3 9,2 40pjj 

 concentrations from 1963 through 1967 corresponded to a stratospheric half-residence 

 time of 10 to 11 months, which is similar to the half-residence times calculated from 

 measurements of other radionuclides of stratospheric origin. The 2 39,240pjj Qoncentra- 

 tions remained fairly constant from 1967 to 1972, primarily because of yearly injections 

 of plutonium by thermonuclear tests conducted by the Chinese at Lop Nor (44°N);the 

 contribution from the French tests in the South Pacific (23°S) may also have significance. 

 From 1962 througli 1965 the ^^^Puand '^^ '^'^^Pu in surface air at Richland, Wash., 

 came primarily from the 1961 and 1962 U.S. and U.S.S.R. series. The ^^^Pu/^^^-^'^^Pu 

 activity ratio averaged about 0.020 in 1964. The activity ratio stayed almost the same in 

 1965, but, by the spring of 1966, it had increased to 0.042, which suggests that ^^^Pu 

 from the SNAP-9A burnup was present. The amount of SNAP-9A '^^^Pu present was 

 determined from the '■'^Pu concentrations and the ^^^pxi/^^^ •^'^^Pu activity ratios;the 

 activity ratio in debris from nuclear weapons tests was assumed to be 0.020. These 

 considerations indicate that the ^^^Pu in Richland air t>om 1967 to 1971 came largely 

 from SNAP-9A. From 1967 through 1969, the concentrations of SNAP-9A plutonium at 

 Richland remained fairly constant, which indicates that the '"'^Pu was being transferred 

 into the northern hemispheric lower stratosphere at a rate comparable to the rate at 

 which ^^^Pu was being deposited on the earth's surface. This suggests that a substantial 

 amount of ■^ '^^Pu was retained in the upper stratosphere, and its slow movement into the 

 lower stratosphere maintained a nearly constant level for about 2 yr. The fact that the 

 ^''^Pu concentrations showed the usual seasonal variations typical of radionuclides of 



