JACOBI AND ANDRE
3812
K-profiles IWN{-----), WNN(————)
Specific activity (dps/cm’)
NNN(——), SSN——)
lem
10cm
im
0m
100m
Tkm
km 30km
Altitude
Fig. 10.
Vertical profiles of Rn™ and its decay products, calculated with Z = 1 atom/
cm? sec.
expected from theory because of the lack of
precipitation in the stratosphere. This agreement leads to the conclusion that the observed
profile ean be explained by vertical mixing without assuming a horizontal advection of Pb”°enriched air in the lower stratosphere. Preliminary measurements of the profile over the
United States, which were mentioned by Machta
[1960], indicate no significant increase in the
concentration above the tropopause. If this discrepancy is confirmed by further investigations,
a dependence of the Pb™ profile on longitude
must be assumed.
Burton and Stewart [1960] observed an increase of the Po™/Pb™ratio in air with increasing altitude as expected from theory. This is
also consistent with the observation of Lehmann
and Sittkus [1959] that the ratio in rain is
higher by a factor of about 2 than that in
ground-level air.
Rnand its decay products, Owing to the
rather short half-life of Rn” (é,. = 54 sec) and
its decay products, their vertical distribution is
restricted to the lower troposphere. Figure 10
shows the vertical profiles of Rn™, Pb”, and
Bi™ calculated for the typical profiles of the
turbulent diffusion coefficient shown in Figure 1.
Because of its short half-life, the concentration
of Rn™ decreases rapidly with height. In the
ease NNN (average turbulence) about 80 per
cent of all Rn™ atoms will decay within 20
meters above ground level. For a strong inversion in the boundary layer (case JWN) about
90 per cent of Rn™ is concentrated in a layer
1 meter thick. Po™ rather quickly approaches
radioactive equilibrium with Rn™.
The half-life of Pb™ (é,2 = 10.6 hr) greatly
exceeds that of Rn™. Therefore, the Pb™ atoms,
after their formation, can diffuse to greater altitudes than Rn™ or Po™. As in the relations between Rn™ and Pb”, this difference in residence
time causes a low Pb™/Rn™ ratio in the bound-
ary layer near ground level and an excess of
Pb” over Rn™ at higher altitudes. Radioactive
equilibrium between the two is reached at only
one point, the altitude of which varies with mix-
ing rate in the range from 1 to 100 meters. For
a constant turbulent mixing rate the theory in-