VARIATION OF RADIOACTIVITY IN SURFACE AIR
501
Through mid-1962 the surface-air measurements indicated that 10% or
less of the total '°’Cs present was of high-altitude origin, as shown by
a 4mn/!8'cs ratio of essentially unity. From July 1962 until July 1963,
the ratio in surface air rose Steadily, reaching a peak value of 8 to 1.
The ratio then dropped during late summer to a value of 6 to 1, which
has been maintained through mid-1964. Taken at face value, this would
imply that the bulk of the TCs present in surface air during 1963 and
into 1964 was associated with **Mn and may have originated at con-
siderable altitude. The peak value of the ratio seen during July 1963
may have resulted from the downward movement of debris during the
preceding winter and spring months. A second decrease in ratio seen
in the Ash Can results during early 1964 does not seem to have caused
a Similar decrease in the ratio in surface air during the spring and the
summer of 1964. This may indicate that somewhat different sources or
mixtures of debris were sampled at Argonne and at San Angelo. There
did appear to have been an appreciable component of '’Cs in surface
air during 1961-1964 that was of high-altitude origin. The delay in de-
scent might have been due to particle size, or the length of the pathway
traversed may in itself have been sufficient to delay the occurrence of
spring maximumsin surface air relative to pastobservations. The con-
centration of airborne radioactivity in surface air is influenced by the
precipitation history of the air mass in which it is contained. A change
in the amount of precipitation as wellasin its time of occurrence would
affect the concentration of activity found in tropospheric air, particularly at the surface. Thus a decrease in precipitation over a wide area
during the spring would lead to a spring maximum of increased magnitude and, conceivably, of longer duration.
The concentration of !°Cs in surface air measured at Argonne and
at Chilton, England,’’!? is shown in Fig. 3 for the period 1959 through
mid-1964 and for Washington, D. C.,':° for the period 1959 through mid-
1962. The times at which the spring maximumsoccurred at each location were almost the same. However, the amplitudes differed between
the three sites during any given year. In 1959, 1960, and 1961, the con-
centrations seen at Washington and Chilton were quite comparable,
whereas the concentrations observed at Argonne were lower by essentially a factor of 2. In 1962 the situation was partially reversed, with
comparable values at Washington and Argonne but with only about onehalf of these concentrations at Chilton. The reversal between Argonne
and Chilton was maintained through mid-1964. This reversal in relative amplitude could be due to differences in the source of debris,
changes in circulation, changes in precipitation, or a combination of
the three. The source of’debris seen at both Chilton and Argonne ap-
pears to be the same as judged from the ®Zr/'"Cs activity ratio plotted
as a function of time as shown in Fig. 4. The ratio observed month by
month is almost identical during 1962 to mid-1964. The values of the
nO