734
PELLETIER, WHIPPLE, AND WEDLICK
H varied from 0.7 x 10° m for the Norwegian data of 1959 to 2.0 x 10° m
for the British data of 1962 and 1963. Differences in sampling techniques could easily cause a difference of a factor of 3 between two
locations. However, the measurements in the western Lake Erie area
were made by the same group using the same sampling techniques, and
the value of H changed from 0.8 x 10° to 1.4 x 10° m. It seems certain
that, for the western Lake Erie measurements at least, the changes in
H are real and will have to be explained.
At the present time the authors think the most likely explanation
_for the sudden changes in H lies in the distribution of radioactivity with
height in the atmosphere. Obviously an air sample taken at ground level
does not necessarily represent the air concentration at the raingenerating level. However, if the distribution of radioactivity with
depth, whatever it may be, remains fairly constantfor a period of time,
then the ground-level air concentration will be proportional to the air
concentration aloft over this same period of time. Now, if the radioac-
tivity in rain is a function ofthe air concentration at the rain-generating
level, it would also be a function of the surface-air concentration. If
then the depth distribution changed to some other distribution, the relation between the surface-air concentration and the rain concentration
would also change. In Fig. 8 we haveplotted the air concentration in the
western Lake Erie area as a function of time. The first change in H
occurred at a time when the air concentration began an exponential
decrease. This probably coincides with the time when mixing between
the stratosphere and the troposphere decreased or stopped altogether.
The next change in H occurred near the bottom of the exponential decrease, one month before the activity began to increase again. Since the
only source of radioactivity during this period was the stratosphere,
this change indicates that there was an increase in the exchange be-
tween the stratosphere and the troposphere. The next change in H occurred after 8 weeks and continued for 12 weeks. There were no
Changes in the air concentration that would set this period apart from
the others. It may have been that there was a change in therate of
mixing between the stratosphere and the troposphere that could not be
seen at the ground level during this 12-week period.
In the final analysis the model will have limited use in the predicting of deposition until we canfind the reason for and predict the changes
in H or until enough data have been accumulated to allow derivation of
some empirical relation for H.
There is one more point that should be brought out. It was not
possible with our data to show a correlation between the surface-air
concentration and the amount of rainfall. When 143 weeksof airconcentration measurements taken at Tecumseh were plotted against
weekly amounts of rain during the period from November 1961 to July
1964, a slightly negative slope was found by linear regression analysis,