The activity levels of atmospheric plutonium are well documented by collections
from rockets, high-altitude balloons, aircraft, and ground-based filtration
units. The most extensive survey is contained in the HASL Quarterly Reports
in conjunction with Profect Airstream (Krey and Toonkel, 1975; Krey, 1967).
The 24°py/?2 39py {sotopic ratios in the atmosphere have also been measured.
The most extensive survey of 2"9pu/239Pu ratios was obtained from the atmospheric
sampling and radiochemical analysis program that was conducted from 1959-1970
by the U.S. Department cf Defense, Atomic Energy Commission, and the National
Oceanic and Atmospheric Administration. Fission products in many of the
samples were measured (Anonymous, 1961-1970).
The plutonium isotopic ratios
were reported
in HASL-273
(Anonymous,
1000
1973).
A review of the preceding data shows an excellent correlation with known
atmospheric injections (Glasstone, 1964; Jackson, 1976).
These data were
combined with other measurements and led to the development of various models
of atmospheric transport including intrastratospheric transport, effects of
seasonal variations, and transfer mechanisms from the stratosphere to the
ground (Stewart et al., 1958; Dingle, 1965; Pierson, 1969; Krey and Krajewskti,
1970), They also provide a means for predicting mean atmospheric residence
times, backcalculating the total atmospheric burden of Pu and other nuclides,
and forecasting general global fallout patterns (Brewer, 1949; Rand Corporation,
1953; Haxel and Schumamm, 1955; Dobson, 1956; Libby, 1956; Kuroda, 1958;
Machta and List, 1958; Martell, 1959; Machta and List, 1959; Prawitz, 1964;
Beck, 1966; Davidson et al., 1966; Feeley et al., 1966; Karol, 1966; Machta,
100
STRATOSPHERE
The average yearly concentrations of plutonium in the stratosphere and the
troposphere of the northern hemisphere are shown in Fig. 1. Yearly stratospheric
plutonium levels were calculated from the data reported in HASL-273.
Samples
collected near ground level in New York (Bennett, 1976) were used to illustrate
the trends in tropospheric plutonium.
It is interesting to note from Fig. 1
that the ratio of plutonium in the stratosphere to that in the troposphere
changes from year to year. This variation is explained by the fact that
high-altitude injections must settle to near the tropopause before they are
available for transfer to the troposphere.
Today, samples collected at ground level are representative of the tsotopic
composition of plutonium in beth the troposphere and stratosphere.
The tropo~
sphere has a mean residence time 30 days--a value which is 12-15 times shorter
than the mean residence time of the stratosphere (Libby, 1956; Burton and
Stewart, 1960; Kuroda et al., 1962; Volchok, 1967; Krey and Krajewski, 1970).
Therefore, tropospheric plutonium must be continuously replenished by the
transfer of submicron sized fallout particles from the stratosphere (Lockhart
et al., 1965: Shlefen et a?., 1966).
Resuspension of previously deposited
debris is secondary and extremely negligible source of tropospheric plutonium.
Typical values for resuspension are 107 -10-!% -1 (Volchok, 1971; Anspaugh
et al., 1974; Bennett, 1976).
The plutonium associated with fallout is a complex mixture originating from
two primary sources. The plutonium may be residual fuel from the nuclear
device or it can be made by neutron capture reactions during the explosion.
Therefore, the **%pu/?79pu isotope ratios can fluctuate over a large range.
239+ 240 Py, fci / sem
1966; Bhandari et ai,, 1966; Swindle and Kuroda, 1969).
ww
0.01
FIG. 4
_
i
L
1960
ATMOSPHERIC
I
1965
PLUTONIUM
257
1970
LEVELS
1