VARIATION OF RADIOACTIVITY IN SURFACE AIR
503
%7r/8ICg ratio observed in samples taken at 65,000 ft by the Ash Can
balloon flights’: are also shown in Fig. 4. These latter data indicate a
somewhat
more
recent time of production for debris
collected at
65,000_ft during 1963-1964 than is observed in surface air. The simi-
larity between the *Zr/"’Cs ratios at Chilton and Argonne, as well as
the constancy of the “Mn/!%"Cs ratio in surface air at Argonne for the
past year or more, point to a well-mixed stratospheric source for the
bulk of surface activity.
Further information regarding the cause of the change in the relative magnitude of the activity levels seen at Chilton and Argonne may
be obtained from "Be measurements in surface air, the results of which
are shown in Fig. 5. Reliable ‘Be concentration values were not obtained
from October 1961 through mid-1963. The concentrations observed in
late 1963 and into 1964 are some two to three times greater than those
seen during 1959 through mid-1961. This might be due in part to decreased precipitation over the area in question. (Lower than average
annual rainfall has been observed in the North Central United States for
the past two or three years.) However, the major cause appears to be a
difference in circulation resulting in a more rapid transport of stratospheric air to the surface. The apparent production of 'Be calculated
according to the method of Cruikshank ef al.'’ in 1963-1964 was
some three times greater than that during 1959--1961
Wt» «ne use or
the equilibrium "Be production rates in a Stationary atmosphere pre-
sented by Murayama,” the apparent age of ‘Be in surface air was ap-
proximately 65 days in 1963-1964 compared with the 170 days seen
previously. Hence it appears that meteorological processes have caused
a more rapid transport of stratospheric debris to ground level over the
North Central United States during the last few years than has previously been the case.
The ™mn/%"cs ratio and activity ratios between various fission
products have been used to partition the total '°"Cs observed in surface
T
TUTTI TTT TT PITTI TTT TTT TTT eT PTT TTT TTT TTT rer ery rer Tyre |
x
3 107
=
ee
q
a
mJ
oe
g
7
4
=
1072
t
edi
Jon. Dec.
Jan.— Dec.
Jan.— Dec.
Jan.— Dec.
1959
1960
1961
1962
Jan. — Dec.
Jan. — Dec.
1963
1964
Fig. 5— “Be concentration in surface aiy at Argonne, Ill,