SIZE DISTRIBUTIONS OF RADIOACTIVE AEROSOLS the activity ratio shows very little altitudinal variation, 179 having an average value of about 35. In Fig. 8 the “4ce/*Sr activity-ratio data are plotted for the August 1963 collection when At is about 240 days. If one disregards the three ratios with upper limits and the abnormally high activity ratio for r< 0.02 particles at 21 to 27 km, an average activity ratio of about 25 is obtained for this collection. When this value is corrected for radioactive decay to the May 1963 collection, the result is consistent with the average M4 oe /%Sr activity ratio observed for that collection. Again, less than 5% of the total 4Ce and “Sr collected within each altitude band in August 1963 is on particles with r> 0.15 yu. It is concluded that these data also show that the two nuclides are unfractionated. The need for larger volume samples beeam¢Sppar this flight, because of the very low “Sr comment “ye. 9577/*¢r Activity Ratios. - sat articularly in hr 6 = In Figs. 7 and 8, 9577/5, activity ratios iré pitted as a function of size and altitude for the May and August 1963 collections, respectively. The genera pattern appears to be less systematic than that shown by the agedtOee activgty-ratigsdata with the possible exception of some of the August’Soogercouta. e °Zy and“@r distributions among the particle size ranges 0.02 Prec 0.15 p and r < 0.02 u ap- pear to be identical in thré®of four cases for this collection. One can gay little with respect to the question of chemical fractionation of %7y relative to “Sr on the basis of *Zr/*Sr activity ratios alone except that there are large fluctuations in the size distribution as a function of altitude. For example, see Fig. 7, which depicts the May 1963 data. if“Ce and “Sr are not fractionated in this collection, then the variable 7 r/Sr activity suggests the occurrence of fractionation between %2r and Sr. In Fig. 8 ‘4Ce/*Sr activity ratios for the three uppermost sampling intervals for August 1963 suggest an identical distribution of these nuclides between the size ranges 0.02 p<r<0.15 wandr< 0.02 nu. The same. result is obtained for Zr and "Sr for this collection. As observed for “Sr, at least 90% of the *Zr is found on particles with r< 0.15 un. The most striking result derived from the May and August 1963 data for *Zr is that, for particles withr < 0.15 y, the smaller particles (r < 0.02 4) appear to be the more highly enriched in “Zr relative to Sr. This is the case throughout the 9- to 30-km region for both flights. On an activity basis, this effect has its greatest influence on the 21- to 30-km altitude region within which these smallest particles carry from about 40 to 70% of the totalZr and 20 to 40% of the total Sr, Data for the 9- to 21-km altitude region show that only about 5 to 15% of the 82r and “Sr is associated with the smallest particles.