RADIOACTIVITY ALONG THE 80TH MERIDIAN
485
northern-hemisphere activity levels are highest. Some crossover does
occur, as has been observed in the past during the Operations Greenhouse and Hardtack tests and the French Sahara tests and following the
renewal of testing of large-yield devices by the U.S.S.R. in September
1961. Several months after the northern-hemisphere air had become
heavily contaminated by the U.S.S.R. 1961 tests, short-lived fission
products were detected at several sites in South America.
From the relative activity levels in the two hemispheres in early
1962, shown in Fig. 3, it can be estimated that only a few percentof the
radioactivity from the northern hemisphere could have migrated south
of the equator. Because of the small background there of much older,
long-lived radioactivity, the new debris contributed heavily to the
gross-activity levels at the lower latitudes but addedlittle to the con-
centrations of such long-lived fission products as "sr and '3"cs.
The Operation Dominic I tests held in the Christmas Island area
(2°N) beginning in late April 1962 caused large increases in radioactivity in the lower latitudes of the southern hemisphere during May
but contributed surprisingly little activity to the ground-level air in the
northern hemisphere. At both Miami and Miraflores, where the major
impact of tropospheric debris from these tests was expected, the activity concentrations during the period April through June decreased
with a half-period of less than one month, similar to the rate observed
in the spring of 1959. Radiochemical analyses confirmed the small
influence of these tests on activity levels in the northern hemisphere
during 1962; the situation may be similar to that existing following the
Operation Hardtack tests of 1958 when ‘®w results showed the major
part of the long-lived radioactive debris to be held in the stratosphere
until the next spring.'3.45
90Sr IN THE AIR
The concentrations of *Sr in the ground-level air at a number of
sites along the 80th meridian are shown in Figs. 5 and 6; calculated
tropospheric burdens of §0Sr are shown in Fig. 7. These results are
more significant in delineating the magnitude of the seasonal changes
in activity levels than are gross-activity measurements since they are
.
less sensitive to radioactive decay.
A definite spring maximum of *Sr is observed in the northern
hemisphere every year regardless of the past history of nuclear testing.
The occurrence of maximums each spring during the moratorium on
nuclear testing provides direct evidence that there is a seasonal factor
involved in the downward mixing of radioactive debris from the stratoSpheric source. The seasonal cycle has its highest amplitude in the
subtropical latitudes; moreover, the time of arrival of the radioac-