Balloon-borne air-filter equipment has been employed in the U.S. Atomic Energy Commission Ash Can Program for sampling up to levels of about 95,000 feet. Until recent improvements in balloon filter-sampling techniques, the balloon data have been unreliable as well as limited in amount. Since the tops of Ivy and Castle test clouds reportedly rose to heights of 25 miles, ‘8 the altitude range of past balloon sampling was inadequate to assess the subsequent distribution from these early tests. Unless sedimentation, circulation or mixing are effective in the rapid downward transport of debris from all levels above 95,000, the need for a higher altitude sampling capability is clear. Particle Size of Stratospheric Debris The importance of gravitational sedimentation as a mechanism for the removal of stratospheric fallout has not been fully assessed due to lack of experimental data on the size distribution of radioactive bomb debris in the stratosphere. The long storage time exhibited by stratospheric debris !* provided the first indication that the particles must be in the sub-micron size range. The apparent large altitude and latitude influence on stratospheric storage times® indicate that circulation or mixing, not settlement, govern stratospheric fallout, Recent evidence from tritium and strontium-90 concentration of Bedford rains)? show a similar pattern and thus a similar mechanism of removal for stratospheric water vapor and particulate debris from the October 1958 Soviet tests at Novaya Zemla. Direct experimental evidence obtained by my associates !® on the size distribution of radioactive particles in the stratosphere is presented in Figures 1 and 2, The balloon-borne instrument used was a large-volume two-stage impactor, based on a design of Junge! backed up by a polystyrene microfiber filter. Particles larger than 0.15 radius and between 0.02 and 0.15 uw radius were collected on the first and second stage of the impactor, respectively. very high efficiency. Particles of smaller radius were retained on the backup filter with presumably The instrumentation was built, calibrated, and flown by General Mills, Inc., under Contract No, AF19(604)-6202. The vertical profile temperature and total B-activity data are shown in Figure 1 for the two 1960 flights over Minneapolis. The vertical lines indicate the altitude interval over which each sample was taken. The vertical distribution is distorted by the procedure of continuous sampling during balloon rise over each altitude increment. The 30,000 to 50,000 foot value is most affected since essentially all of the activity over this in- terval was collected above the tropopause. The size distribution data, Figure 2, show a marked trend with altitude. in the lower stratosphere is below 0.15 4 radius. total is below 0.02 » radius. About 90 percent of the activity Between 90,000 and 100, 000 feet about 60 percent of the In view of the small sample volumes and low total and fractional activities, the Similarity in results for the two flights, Figures 1 and 2, is reassuring. Preliminary comparison with the size distribution observed by Junge? for natural sulfate particles in the stratosphere indicates a smaller mean size for the radioactive particles. The results indicate that sedi- mentation must not play a significant role in the removal of stratospheric debris, at least at relatively late times following injection, Sampling instruments for use at levels above 100, 000 feet must be designed for collection of particles from a few hundredths micron radius to very much smaller sizes. 79