- phan ne
INTERNATIONAL SYMPOSIUM ON TRACE GASES AND RADIOACTIVITY
les IWN(----), WNN{——-——)
NNN(-———), SSN(——)
dicates a rather constant Pb™ concentration in
the range of | to 20 meters. For a constant Rn™
exhalation, changes of the turbulent mixing rate
will cause a variation of the Pb” concentration
in this laver by a fuctor of ubout 10.
The ealculated profiles shown in Figure 10 are
standardized to a Rn™ exhalation rate of 1
atom/cm’* see. Measurements of the exhalation
rate are not available, but on the basis of the
diffusion theory and the known Rn™ exhalation
rate, a rough estimation is possible. Since the
average Ra™ and Th™ contents of normal soil
material are about equal and amount to 1.10"
e/g [Rankama and Sahama, 1950], the Rn™
exhalation rate must be lower by a factor of
about \/Ace/Ae» =» 1/80 than the Rn*™ exhalation rate, if both emanations escape from the
-oil material with the same efficiency into the
eround air (Israel, 1958]. Consequently, a Rn™
Tkm
10km 30km
ulated with # = 1 atom/
al profiles of Rn™, Pb7, and
‘or the typical profiles of the
n coefficient shown in Figure 1.
ort half-life, the concentration
s rapidly with height. In the
age turbulence) about 80 per
atoms will decay within 20
und level, For a strong inver-
lary layer (case IWN) about
n™” is concentrated in a layer
y" rather quickly approaches
ium with Rn™.
Pb™ (tj: = 10.6 hr) greatly
™. Therefore, the Pb™ atoms,
on, can diffuse to greater altir Po”. As in the relations be»™”, this difference in residence
Pb**/Rn™ ratio in the boundound level and an excess of
higher altitudes. Radioactive
mn the two is reached at only
ude of which varies with mixge from 1 to 100 meters. For
at mixing rate the theory in-
exhalation rate of 107% atom/em? see will be a
rough mean value for uncovered, dry ground of
normal Th content.
For comparison with natural conditions, the
verlicaul profiles shown in Figure 10 must therefore be lowered by a factor of about 0.01. For
the mean turbulence profile NV¥N the theory
predicts a mean Pb? concentration of about
3710" ¢/m?® and ao omeaun Pb’*/Rn™ ratio of
about 0.03 in surface air (sce Figure 10). Mea-
surements in this Inver xt several places [sce
compilation of Multyrist, 1956] indicate a mean
Pb™®concentration of 10°? to 10e/m® and a
mean ratio of 0.01 to 0.05, which is in rather
good agreement with the theoretical values. The
assumed A profile AVN and the mean Rn™
exhalation rate thus seem to be reasonable.
It follows from Figure 10 that it is impossible
to determine Itn* coneentration from Pb** measurements under the assumption of radioactive
equilibrium. Sinee no direct methods for the de-
termination of low Rn™ concentrations in the
presence of Rnare yet available, the Rn™ concentration in the atmosphere is not known. The
agreement between theory and observation,
stated above, for Pb” allows us to conclude
that the calculated mean Rn™ profile for the
cease NNW is quite correct under normal conditions. Therefore, a mean Rn™ concentration of
about 10°c/m? should be expeeted near ground
level, which is of the same order of magnitude
as the mean Rn™ concentration in this layer.
Conclusion,
The theorctical calculations give
38138
a general survey of the vertical distribution of
Rn™, Rn™, and their decay products in the at-
mosphere, and their dependence on the vertical
profile of turbulent mixing. The vertical distrputions calculated for the average turbulence
profile NNN agree rather well with the observed distribution of natural radioactivity in
the atmosphere. On this basis a reasonable pre-
diction of the Rn™ concentration in groundlevel air and its variation with altitude can be
given. On the basis of the theoretical results presented here it is necessary to revise the previously used method of estimating the mean
residence time of acrosols in the troposphere
from Pb™°/Rn™ or Po™*/Pb™ ratios.
The computed profiles show new aspects for
a successful use of Rn™, Rn™, and their decay
products as natural tracers in the study of mixing processes in the troposphere and lower
stratosphere.
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