Fe ghee EM ek jo air is enriched in Pu-239 in this particular period, somewhere in the atmosphere or on the ground we should cbserve data showing depletion in that isotope. sparse Fallout data do not support this idea. {only two deposition Sites in ples routinely analyzed for Pu-239), Although the Northern Hemisphere had sam- the fallout results indicate tha: Pu-239/Sr-90 ratios in precipitation are about the same as in the sur~ face air (9, 15, 16, 17, 18, 19, 20, 21, 22, 23}. On the ccher hard it certainly would not be unreasonable to expect fractionation between twa such chemically different elements as Pu and Sr. their origins differ markedly, on particles of different size. Furthermore, since they may very likely enter the troposphe-rWe do not prcpose to speculate further on the possible mechanisms of fractionation; however, it does seem cles: that an acceptable case for fractionation cf these radionuclides. cc:ld be presented. Conclusions The anomalous ratios of Pu-239/Sr-90 in Northern Hemisphere surface air in 1968-1969 remain unexplained. explanations advanced in this report, Neither of the most pleusitle tropospheric releases of Pu-=239 and tropospheric fractionation of the radionuclides, mentally substantiated. have been experi- In fact both explanations seem to be contra- indicated by other observations and data. Sampling and analysis for Pu-239 and Sr-90 will be continued in both the stratospheric and surface air programs, and other studies which may bear on this subject will be considered to help finally understand this paradox. ES