AIR CLEANSING BY CONVECTIVE STORMS
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579
DISCUSSION OF RESULTS
A consistent pattern was observed in the time variation of the
concentration of atmospheric contaminants in rain during the early
portions of the well-organized storms. Concentrations of plant pollens
were determined in two of these storms; fallout radioactivity was determined in all three, In the cases for which data are available on both
contaminants, concentrations of radioactivity and pollens exhibit similar
patterns of temporal change during the early parts of the rain, both
showing a rapid decrease in concentration,
The similar trends of two contaminants that are known to be highly
dissimilar in physical properties and behavior suggest that both were
removed from the same air, Since the pollens are low-level air tracers,
one infers from this that low-level air is the carrier of the radioactivity and pollens that are found at the beginning of the heavy rains.
It is of interest to consider the possible modes and locations of
input of contaminants to the convective system. With regard to the
pollens, the bulk of which are contained in the lowest few thousand feet
of the atmosphere, it is pertinent to refer to a previous study‘ of
ragweed-pollen concentrations in the rain ofSept. 1, 1961. Computations
of washout which were made in that study and which were based upon
the hypothesis of a stagnant atmosphere beneath the rain-generating
level are plotted in Fig. 3. Comparison of the curves of computed and
observed pollen concentrations led to the conclusion thata more or less
continuous flux of contamination into the storm was necessary for
pollen to be found in rain that fell more than a few minutes after the
beginning of rainfall. This requirementis fulfilled as long as low-level
pollen-bearing air can be entrained into the storm. Most such input
must be associated with the convective updraft that also supplies the
water vapor.
Circulation models for persistent convective storms (e.¢g., Newton®)
usually show a strong updraft to be located at the leading edge of the
storm, Thus the first rain tofallata station is expected to fall partially
through the updraft or in close proximity to it. It therefore has good
opportunity to mix with contaminated updraft air and to become highly
contaminated itself. The rapid decrease in the contamination level
occurs as the system moves across the station. The rain that falls
from the interior of the storm has little chance of contact with the
contaminated updraft.
In contrast to that of the pollens, the vertical distribution of artificial radioactivity is such that the highest concentrations are found in
the upper troposphere and especially in the stratosphere. This is true
except for those cases where stable laminae of highly radioactive
stratospheric air are present at low levels of the troposphere.* Suppose
*See paper by E. F. Danielsen, this volume.