DISTRIBUTION OF AIRBORNE RADIOACTIVITY 133 particles between 0.3 and 1.1 in diameter with only minor components of very small (<0.3 u) particles. On the other hand, the short-lived natural radon daughter products (RaB + C) are associated with much smaller particles with the major part of the activity attached to aerosol particlés of less than 0.3 u in diameter. The longer-lived natural radioisotope ?!°Pb, because of its longer average residence time in the atmosphere, behaves more like the fission products. The size distribution of the fission products found at ground level is quite unlike that expected for the stratospheric source from which these fission debris are derived. The long residence time of this material in the stratosphere precludes the presence there of any considerable quantity of larger particles; it might be supposed that the particlesize range would be similar to that found by Junge and Manson for nonradioactive stratospheric aerosols.’ Indeed, it would appearlikely that the smaller particles from nuclear explosions would soon become attached to the inactive aerosols and would partake in the growth processes involving those aerosols. The filter-pack technique shows considerable promise as a means of obtaining information on the size distribution of aerosols. Its utility is not limited to the study of radioactive aerosols but is applicable to the sizing of any kind of airborne particulate matter for which suitable analytical methods are available. The techniques of radioactivity measurement, chemical and activation analyses, etc., may be equally suit- able provided the filters employed have sufficiently low backgrounds of interfering substances. REFERENCES 1. U. 8S. Naval Research Laboratory, Final Report; Greenhouse, Project B-29 and Project B-28, Report NRL-3963, Mar. 3, 1952. (Classified) 2. L. B. Lockhart, Jr., and R. L. Patterson, Jr., Filter Pack Technique for Classifying Radioactive Aerosols by Particle Size. Part 1. Preliminary Re- port and Evaluation, Report NRL-5970, U. S. Naval Research Laboratory, Aug. 16, 1963. 3. L. B. Lockhart, Jr., R. L. Patterson, Jr., and W. L. Anderson, Characteristics of Air Filter Media Used for Monitoring Airborne Radioactivity, Report NRL-6054, U. S. Naval Research Laboratory, Mar. 20, 1964. 4. M.H. Wilkening, Natural Radioactivity as a Tracer in the Sorting of Aerosols According to Mobility, Rev. Sci. Instr., 23: 13 (1952). 5. R. A. Baus, P. R. Gustafson, R. L. Patterson, Jr., and A. W. Saunders, Jr., Procedure for the Sequential Radiochemical Analysis of Strontium, Yttrium, Cesium, Cerium, and Bismuth in Air-filter Collections, Report NRL-Memo758, U. 8. Naval Research Laboratory, November 1957. 6. L. B. Lockhart, Jr., R. L. Patterson, Jr., and A. W. Saunders, Jr., Filter Pack Technique for Classifying Radioactive Aerosols by Particle Size. Part 2. Isotopic Fractionation with Particle Size, Report NRL-6164, U.S. Naval Research Laboratory, October 1964. 7. C. E. Junge and J. E. Manson, Stratospheric Aerosol Studies, J. Geophys. Res., 66: 2163-2182 (1961).