SIZE DISTRIBUTIONS OF RADIOACTIVE AEROSOLS 181 SUMMARY AND CONCLUSIONS From this study altitude and size distributions have been obtained primarily for total beta activity and the fission products "Sr, *Zr, and “44Ce, The 9- to 30-km altitude range over Minneapolis, Minn. (45°N 93°W), has been sampled with a balloon-borne inpactor—filter sampler that collected particles in three size ranges with radius, r, of ap- proximately r >0.15y, 0.02 u<r<0.15u, and r< 0.02 u. Collections were made on Mar. 16 and Sept. 25, 1962, and on May 16 and Aug. 15, 1963; these samplings contained primarily nuclear debris that was from several days to about 240 days old. The altitude distributions of the total activity (beta activity, Sr, Zr, or 40) within each altitude interval sampled in all four flights show a peak concentration in the 15- to 21-km band. This is consistent with the broad maximum at 18 to 23 km in the number concentration of worldwide natural aerosols in the stratosphere and the peak in the number distribution of the natural sulfate aerosol.? In the majority of cases at least 90% of the measured radioactivity was carried by particles with r < 0.15 . Withinthis general size range, most of the radioactivity was found on particles with 0.02 u< r< 0.15 yn, a range which is at the lower end of particle sizes observed for stratospheric natural aerosols.’ The influence of relatively fresh debris was evidenced in the September 1962 results by the presence of a higher percentage of radioactivity on particles with r >0.15u compared to the March 1962 data. The general trends shown by the vertical profiles of total beta activity, Sr,Zr, and “*Ce within each given size range are quite similar to each other and to the total radioactivity. In the May and August 1963 collections, a surprisingly large amount (about 50 to 70%) of the %57r was found on particles with r< 0.02 yin the 21- to 30-km altitude region. These were the only collections which were analyzed for Zr. Our findings from activity-ratio data for the three particle size ranges studied may be summarized as follows: First, there is no chemical fractionation between Sr and ‘Ce in debris of several days to 240 days old; i.e., the “4ce/"Sr activity ratios are constant as a function of particle size range. Second, on the basis of the *Zr, Sr, and '““Ce results, it is impossible to make a conclusive statement about the possible occurrence of chemical fractionation of Zr relative to %Sr and “4Ce in debris of this age. The ®Zr/*Sr activity ratios in May 1963 are extremely variable with particle size range; but for August 1963 the size ranges that contain most of these nuclides exhibit constant ratios in the 15- to 30-km region. The ™Zr/'‘Ce activity ratios observed in both May and August fluctuate widely as a function of size range.