AIR CLEANSING BY CONVECTIVE STORMS
967
within a single rainfall. Variations within rains have been reported by
others;’’? variations also occurred in the five rains discussed in this
paper. Because the variability within a system is pertinent to the
understanding of that between systems, this paper is especially concerned with the study of the variations within individual rainfalls. The
principal focus concerns variations within convective rains.
OBSERVATIONAL PROGRAM
A basic premise of our research has been that any information
bearing on rain scavenging, whether of radioactive or of other material, is pertinent to the problem and probably useful in its solution.
Therefore we have paid specific attention to the rain scavenging of
airborne pollens whose source, distribution, and physical properties
can be defined very well.
Sampling Pans
Rain was collected in two large pans, whichtogether total 5.2 m? in
area, The sampling pans were located on the flat roof of the structure
shown in Fig. 1, in which sample bottling
out. The pans were tilted slightly to provide
was conveyed from the pans to the bottling
flexible plastic tubing. It is emphasized that
were carried out at a single manned station.
operations were carried
drainage. The rainwater
station directly below by
the sampling operations
Sampling Procedure
Before each rain the pans were scrubbed to remove all dry depo-
sition. This technique was found sufficient to remove nearly all the
radioactive fallout from the pans, and it can be assumedto have removed pollen grains. For rains in which both radionuclides and pollens
were determined, the usual procedure was to take alternate samples
for radiochemical and pollen analysis, at least during the early and
middle parts of each rain, In most cases, l-gal samples were taken
for radiochemical analysis and l-qt samples were taken for pollen
analysis. When radionuclides alone were to be determined, successive
1-gal samples were taken.
Complementary Data
In addition to data obtained from the radiochemical analyses of
rain samples, supporting data on the nature of the precipitation system
that produced the rain were taken at the collection station and acquired
elsewhere. These additional data include conventional synoptic weather