DISTRIBUTION OF AIRBORNE RADIOACTIVITY WITH PARTICLE SIZE LUTHER B, LOCKHART, JR., ROBERT L. PATTERSON, JR., and ALLEN W, SAUNDERS, JR. U. S. Naval Research Laboratory, Washington, D. C. ABSTRACT Particle-size resolution of radioactive particles has been obtained by ~ use of a filter-pack technique wherein atmospheric air containing ra- dioactive aerosol particles successively transits a series of filters of increasing retentivity for Small particles. From the relative amounts of radioactivity collected on the various filters and a knowledgeof the retentivity characteristics of the filters as a function of particle size, information can be obtained on the size distribution of the radioactive particles. Following the termination of atmospheric testing of nuclear de- vices in December 1962, fission-product radioactivity was observed to become associated with particles of smaller average size. Within a matter of months, however, this trend ended, but large short-term differences continue to be observed. Radiochemical analysis of mate- rial collected by three-filter packs during 1963 and 1964 showed no gross evidence of isotopic fractionation with size; however, some dit_ ferences in composition were noted between the largest and the smallest size fractions. It is concluded that nuclear debris from a stratospheric source becomes associated with the relatively large nonradioactive aerosols characteristic of the tropospheric air by the time the debris arrives at ground level. The resulting particles have an average size between 0.3 and 1.0 u in diameter (assuming spherical particles of 1.8 g/cm® density) and a rather restricted particle-size distribution. 119