SIZE DISTRIBUTIONS OF RADIOACTIVE AEROSOLS 15 tems for the meaSurement of absolute concentrations of atmospheric radioactivity. Previous impactor—filter studies have provided some results on the total beta-activity distribution'* and limited data or the “Sr and “‘Ce distributions’ as a function of particle size. Balloor flights were made in 1960 over Minneapolis, Minn. (45°N 93°W), anc in 1961 over Hyderabad, India (17°N 79°E), and stratospheric aerosols were collected over specific altitude intervals up to about 30 km, At least 60% of the total beta activity was found to be associated witt particles of 0.02 to 0.15 uw in radius up to about 27 km, andat leas! 60% was found on particles smaller than 0.02 u in radius in the altitude range of 27 to 30 km. From a number of constant-'=vel balloon flights. with each balloon having a pair of filter samplers with one operating at a different velocity from the other, and a few flights with a twostage cascade impactor’ at altitudes ranging from 18 to 27 km during 1959, mean particle sizes of radioactive debris were calculated or the basis of total beta activity and found to be roughly in the range oi 0.02 to 0.05 Hin radius. There were, however, large variations from this range, and these values should be regarded as tentative. Our report presents the results of the four balloon flights conducted over Minneapolis, Minn., during 1962 and 1963 for the collection of radioactive particulates specifically for the determination ol size and vertical distributions of selected individual fission products over the altitude range of 9 to 30 km. This study is a continuation oi the impactor—filter studies initiated earlier, which are described by Drevinsky and Martell.’ All samples were analyzed for total betz activity, Sr, and ‘4Ce. The 1963 samples were also analyzed fox total gamma activity, “Mn, and Zr. Some data were obtained for 40R, in 1962 samples. Individual radionuclide concentrations anc activity ratios are reported. A discussion is given of the significance of our findings with respect to the question of chemical fractionatior of measured fission products in stratospheric aerosols that were ‘collected from severa! days to about 240 days after a megaton-~range atmospheric nuclear test. Further analysis of our datais in progress. EXPERIMENTAL TECHNIQUES Sampling The balloon-borne sampling system consisted of four largevolume impactor—filter samplers, which had been designed and cali- brated in cooperation with General Mills, Inc. (GMI).** The system *Now Atmospheric and Aerosol Physics Research Section, Applied Science Division, Litton Systems, Inc., St. Paul. Minn.