METEOROLOGICAL INFLUENCES ON Sr CONCENTRATIONS
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It is postulated? that the Sr concentration from a simple convective cell moving directly over a collector is related to the life cycle of
the cell and reaches a maximum value during the mature stage. Therefore the maximum value should be related tothe height of the convective
cell and the concentration of *Sr-bearing particles in the environment.
Air is incorporated into convective cloud systems not only through
the warm—moist updraft at the base of the cloud but also by entrainment of drier air by mixing processes with the environment’ at all
levels. Dry particulate debris containing higher Sr concentration of
stratospheric origin thus may be used as condensation nuclei at these
higher levels. Amplification of Sr availability to the precipitation also
may result by the penetration of the tropopause by the cloud tops’ or by
mixing of the clouds in the middle troposphere with parcels of dry air
of recent stratospheric residence, This last phenomenon has been
described by Danielsen® and Staley.’
Furthermore,
the
moisture
content of the air supporting the
precipitation varies during the life cycle of the cell. During the downdraft period of the mature stage, some of the water moving downward
must evaporate to maintain the saturation of the air. The extent of the
downdraft evaporation is related to the height of the storm and the
intensity of the downdraft. This evaporation and the subsequent evaporation of the resulting smaller drops below the cloud base enhance the
already increasing Sr concentration with increasing height of the
cloud tops.
The postulation that the Sr concentration in precipitation from the
cumulus stage of a cell increases as the height of the cloud develops®
also requires the condition that the *Sr-bearing particulates be in-
volved in the condensation and accretion process and that scavenging
of particulate matter below the cloud by the falling raindrops is of small
consequence. This requirement is supported by several observations.
For example, Greenfield,° Langmuir,’° and Vaughan and Perkins! have
each shown that for aerosols smaller than a few microns the collection
efficiency of falling drops is extremely low. Furthermore, Landsberg”
has pointed out that the number of condensation nuclei decreases
rapidly with height. More recently, Durbin and Murgatroyd’® have shown
that at cloud levels the number of nuclei is less than the concentrations
of cloud particles ordinarily observed. In addition, the work of Junge"
indicates that the layer of sulfates in the atmosphere is likely to collect radionuclides such as “Sr. Since (NH,),SO, is hygroscopic, it may
serve aS a source of excellent condensation nuclei. Thus each of the
smaller number of particles in the higher levels of the cloud has a
higher probability of being utilized as a condensationnucleus. Since the
Sr-bearing particles are known to be of stratospheric origin after a
month or so following nuclear detonation, it is believed that removal of
Sr-bearing debris
results primarily from condensation processes