Radionuclide Concentrations in
therefore,
Surface Air: Direct Relationship
to Global Fallout
Abstract. By relating the average
monthly hemispheric concentrations of
radionuclides in the surface air to the
monthly hemispheric fallout of these
nuclides, we have derived proportionality constants. For every disintegration
per minute of a radioactive nuclide per
1000 cubic meters of the surface air
in the Northern Hemisphere. 3.3 kilocurtes of that nuclide will be deposited
on the surface; in the Southern Hemisphere, 5.0 kilocuries will deposit for
every disintegration per minute per 1000
cubic meters. On the basis of these
factors, the global deposition of radionuclides can be estimated from a rela-
tively small number of measurements
of the radioactivity in the surface air.
Using data obtained in two U.S. AEC
Health and Safety Laboratory (HASL)
sampling programs, we report here on
an attempt to relate the concentration
of strontium-90 in samples of surface
air to the deposition of Sr" on a worldwide scale. Since the beginning of
1963, HASL has been conducting mea-
surements of the monthly concentrations of selected radionuclides in surface air (J) at about 25 stations, mostly along the 80th meridian ranging in
latitude from Thule,
Greenland.
at
about 76°N to the South Pole. Like-
wise, the monthly deposition of Sr
has been measured since 1959 at more
than 100 sites (2, 3).
If we assume that the concentration
of a nuclide in surface air is directly
proportional to the amount of that
nuclide deposited on the surface of the
earth, the following equation may be
written:
D= XC,
standard cubic meters (disintegrations
per minute per 1000 m*) during month
n (calculated by weighting the concen-
trations measured in each [0° band of
latitude according to the surface area
of that latitude band).
This equation also assumes that.
within each hemisphere. the troposphere
is
rapidly
mixed
longitudinally.
and,
stations
spheres to be completely insulated from
each other, and separate equations are
written for cach.
If we substitute the measured month-
ly hemispheric depositions of Sr°? and
the average monthly hemispheric con-
centrations of Sr’ in surface air into
Eq. 1, the value of the constants in
both the Northern and Southern hemispheres can be computed for each
month
in
the
period
1963
through
1967, The constants average 3.3 + 0.7
ke per disintegration per minute per
1000 m? in the Northern Hemisphere
and 5.0+0.7 ke per disintegration
per minute per 1000 m? in the Southern Hemisphere. Thus, for every disintegration per minute of Sr® per 1000
m* of surface air in the Northern
Hemisphere, 3.3 ke will be deposited
on the surface of that hemisphere; similarly,-for every disintegration per min-
ute of Sr°® per 1000 m?* of surface
air in the Southern Hemisphere, 5.0 kc
will be deposited.
The monthly values of X in each
hemisphere exhibit seasonal variability.
In general, it appears that the ratio of
the deposition of a radionuclide to its
concentration in surface air is lower in
both hemispheres during the seasons
of maximum falfout, the spring and
winter. Because of this seasonal variability, deposition estimates for individual months maybe in error by about
Table 1. Measured {M) and estimated (E)
deposition of Sr® (in kilocuries).
Northern
Southern
Hemisphere
Hemisphere
MM
E
1963
2620
3714
325
44d
1964
1660
2083
436
476
1965
780
820
3454
334
1966
330
376
208
282
1967
164
. 146
114
115
[S68
190
181
50
120
(1)
curtes) of a nuclide on the earth’s surface during month , X is a constant
which relates the concentration of that
nuclide in the surface air to its deposition, and C,, is the average concentration of that nuclide in surface air in
disintegrations per minute per t000
sampling
tively slow (4), we consider the hemi-
Year
where D, is the deposition (in kilo-
that the
for the surface air are representative
of their respective bands of latitude.
Because the rate of interhemispheric
exchange of tropospheric air is rela-
ME
Table 2. Worldwide deposition of Pu** from
a SNAP-9A power source (in kilocuries).
Source of data
Y
ear
1967
1968
Measured
deposition
4.2
0
Stratospheric
depletion
(3, 4)
5.1
2.7
Surface
air
5.7
3.3