GLOBAL DISTRIBUTION OF TRITIUM
673
because the land stations are affected by return of tritium to the at-
mosphere through evapotranspiration. Two curves were prepared, one
to represent the high fallout season (March through August) and the
other to represent the low fallout season (September through February),
The precipitation data of Brooks and Hunt® were used for precipitation
over the continents and those of Wust' for precipitation over the oceans.
Precipitation over the continents and the oceans within a given latitude
band may be significantly different. Hence it is not satisfactory to take
an average value for precipitation within a given latitude band.
The results of the computation are given in Table 8.
Table 8— PRECIPITATION OF TRITIUMIN 1963
Latitude
band
90°
70°
60°
50°
40°
to
to
to
to
to
70°N
60°N
50°N
40°N
30°N
Mec
Land
4.0*
21.0*
37.2
35.8
20.5
Ocean
Kg
Mc
Kg
0.4*
2.1*
3.8
3.6
2.1
4.0*
14.0*
46.5
62.6
35.7
0.4*
1.4*
4.7
6.3
3.6
1.3
30° to 20°N
10.2
4.6
2.9
2.0
0.5
0.3
0.2
1.0
12.6
11.4
10.8
10.0
1.2
1.1
1,0
Totals
138.2
14.0
201.6
21.0
20° to 10°N
10° to O°N
0° to 90°S
*Estimated values: In the 90 to 70°N latitude band,
one-half
the area is considered ‘‘land’’ because of permanent ice.
The computation indicates that 14 kg of tritium were brought down
by rain or snow over the continents in 1963 and that 21 kg was brought
down over the oceans. This corresponds to a total activity of approximately 340 Mc of tritium.
The computation considers only the direct precipitation of tritium
by rain or snow and does not introduce the factor of vapor exchange
of tritium with the oceans. Bolin® and Eriksson’ have shownthatthis
may be a major effect and may increase the tritium deposition over the
oceans by a factor with a possible maximum value of 3. Whenthis factor is applied to the data in Table 8, the total tritium deposition for
1963 is increased to 77 kg.
On the basis of these values for deposition, the tritium inventory
in the stratosphere at the beginning of 1964 may be computed. In the
computation it is assumed that the tritium deposition in 1962 was
25% of that in 1963. This factor was obtained from the continuousfallout record for several stations which indicate approximately a 1 to 4
ratio for the two years. Hence tritium direct precipitation for the two
years is 9 and 35 kg, respectively, which totals 44 kg, This would