124 LOCKHART, PATTERSON, AND SAUNDERS in Fig. 4, where the percentageof the total activity retained by the top (poorest) filter in a series of identical filter packs under the same conditions of flow is plotted against the sampling date, This graph demonstrates that initially there is a general decrease in particle size as tropospheric debris is deposited and also that large, short-term er 7 \ \ gor xe 5= z \ TTT tT TT PT PTT TP 7 tT 7 _ \ \ \ \ 50 -— — 40;-—- — ee] wo ow 3t-- ___vue-- -4 20 -~ _ FACE VELOCITY, 280 TO 290 FT/MIN 10 oh > = t 0¥Y | | it a! 264% | > « z8 SPs 2s 1942 py Ye Zo tt tt G6 EK & 9 ES 3. O Y a e& 4 g € >& 322246285653 2 Ft 5 1963 1944 Fig. 4—-Temporal changes in the retention of gross fission-product vadioactivity by IPC 1478 filters. variations in particle size occur even when the sole source of fissionproduct radioactivity is located within the stratosphere, Meteorological phenomena must therefore influence the size distribution of atmospheric radioactivity as well as the concentration of radioactivity in the lower atmosphere. The variations shown in Fig. 4 ares real; they have been verified both by duplicate collections and by others made at the same time but at different flow rates. Attempts have been made to obtain a more quantitative representation of the size distribution of the particulate radioactivity by a mathematical analysis of the radioactivity data.2 Some preliminary re- a4