DISTRIBUTION OF AIRBORNE RADIOACTIVITY 127 spheric testing (January through April 1963). It is apparent, too, that the fission debris, even from a Stratospheric source, is associated with particles in the 0.3- to 1.1- range on arrival at ground level. This strongly suggests that the assumed small fission-product conglomerates existing in the stratosphere coalesce with or become attached to the larger nonradioactive tropospheric aerosols during Slow migration of the aerosols to ground level. Thus meteorological phenomena that affect the size distribution of normal atmospheric dusts should have a corresponding effect on the fission-product conglomerate seen at ground level. Although as yet no attempt has been made to correlate the particle-size distribution with weather, the data for March and April 1964 suggest a progressive decrease in the size of the radioactive aerosol particles during a prolonged rainy period. A comparison of the size distribution of radon daughter products (RaB + C) with the extremes in the fission-product distributions observed through use of this method is shownin Table 3. It is obvious that Table 3—-COMPARISON OF THE SIZE DISTRIBUTION OF RaB + C ACTIVITY WITH THE OBSERVED EXTREMES FOR FISSION PRODUCTS Collection dates Total Activity Activity contribution of activity, on initial arious s1ze groups, counts/min filter, % llp Weather ne Cloud Rainfall, cover, % in. O6p O.8p O13 pn RaB+C Mar. 15, 1963 (0810-0840) 136.4 9.0 + 1.2* 4.8 0.5 30.0 64.7 G 0.0 270.5 9.9 + 0.5 3.6 6.8 33.2 56,4 100 0.0 151.8 13.1 + 0.6 aed 16.4 20.1 98.2 0 0.0 Apr. 19 to 22, 1963 3098.8 27.8 + 0.1 21.1 9.0 30 Trace July 23 to 27, 1964 41.6 o9.6 + 1.4 46.6 2.3 70 Trace Mar. 27, 1963 (0815—~ 0845) Mar. 18, 1964 (0830-0900) . Fission products Oct, 4 to 7, 1963 467.8 26.3 + 0.2 13.1 2.7 27.0 51.0 67.2 56.8 0.1 3.1 10 0.0 *Standard deviation, co, based on counting statistics. RaB+C activity is associated primarily with particles smaller than 0.3 # in diameter. The smallest observed fission-product distribution peaked near 0.3 4, whereas the largest distribution peaked between 0.6 and 1.1 4. In most cases the contribution of fission products to the <0.3-u grouping was about 2 to 3% of the total, although on one occasion (Apr. 19 to 22, 1963) 9% of the fission-product radioactivity fell into this grouping. Radiochemical Results Radiochemical analyses for some long-lived fission products and for *!°Pb (RaD) have been carried out on seven three-filter collections® made during 1963. Similar analyses are being performed on one combined month-long collection during each quarter of 1964.