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
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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