Te
The sparsity of stations end the variability of the data neure
it
impracticable to draw an instantancous pieture of the world-vide distribution
of Sr-90.
However,
it
is possible to make a rough estimate of the average
concentration and distribution of debris in the stratosphere over an extended
period of the order of six months.
Figure 6 shows the averase concentration
of debris in the stratosphere as a function of altitude and latitude for two
six-month periods, Januery - June, 1957, and January-June, 1958.
The data at
each sampling altitude and station were averaged over the period and corrected
for the filter efficiencies given in Table 2.
It is evident that the sixteen data points, each based on a mean of only
a few observations with large variability cannot serve to define the
stratospheric Sr-90 content of the world.
distribution can be extracted.
tHowever, some information as to the
A series of isolines showing what is felt to
be the most probable distribution based on the available observations are
also shown, although there are admittedly many other logical patterns which
may be drawn.
A comparison of the two cross-sections shows that between the first half
of 1957 and the first half of 1958, there appeared to be a general decrease in
the Intensity of the gradients observed, principally due to a decrease in the
enncentration of Sr-90 at the two northernmost stations at 65,000 feet.
Another evident change is the increase at all levels over the Canal Zone,
possibly as a result of the British thermonuclear tests in the Christmas
Island area which occurred in May, June, and November of 1957.
These tests
may @lso be responsible for the increases at 90,000 feet over Texas and
Minnesota.
NaS
The patterns shown in Figure 6 may be integrated to estimate the total
stratospheric burden of Sr-90.
This computation gives the equivalent of 7.8 MI
of fission products in 1957 and 5.5 MP in 1958.
(A computation based on 0.25
filter efficiency factor at all altitudes gives 12 MP in 1957 and 8 Mr
in 1958).
These figures can be compared with estimates of the stratos-
pheric content made by budgeting the debris.
Table 1 shows that about 39 MI
had been injected into the stratosphere as of the end of 1957.
of this debris has been removed from the stratosphere.
However, much
Libby estimates the
mean removal rate to be about 10% per year, Machta and List about 20% per
year.
These estimates would result in a stratospheric content of 20 to 24 Mr
8
Revue La ou
Sob TONS
OF TH ARCriyo> OF THE
NATIONAL ACADEMY OF SCIENCES