. Bu 1058 ane ee ow 12 Orisin of ost in PMN milk. A nw 4% = — 4 - < AS snown in figure 1, there are ten veriods with I reaver than 30 oci/1 in the L321 concentrations PMN from April 1963 through December 19658. The Jonuary through March 1963 is a continuation of the elevated values Bfu Cc Ne —] Ne WS Ne Ww Ne kK Pin Ne i. Oo fu o KH wo te ck ry @ Oo a) re w oy cht in fig. 1) followed six of the eight atmospheric tests conducted at Lop Nor in weatern China (40°N, 90°Z). These six nuclear tests were reported to have total yields ranging from less than 20 kilotons to a few hundred kilotons equivalent TNT (3). Nuclear explosions near the ground with yields in this range inject radioactivity mainly into the troposphere, the active weather layer of the | atmosphere (4). The movements of the leading edges of these six nuclear cloucs appear in Figures 2 through 7. The behavior of the clouds from the : Mey 1955 and Mey 1966 tests has already been discussed by Machta (5) in conection , With preferential thunderstorm scavenging in the mid-western United States. Two other atmospheric nuclear detonations were reported each having @ total yield of about 3 megatons of equivalent TNT (6). Fallout Sy or all bomb produced radioisotopes from the first megaton test of 17 June 1967 was virtually non-existent.in ground Level air or rainwater for months after +. ‘This may be due to the fact that an explosion with the yield of - the event. eee about 3 megatons will inject most of its radioactivity into the stratosphere. The second megaton test took place on 27 December 1968 so that its fallout, if axy, would not cccur in the pericd covered by this study. Tnixrteen atmospheric nuclear tests have been conducted by the Republic “4 of France in the South Pacific near Muraroa (22°S, 140°W), (7, 8). Unfortunately, the sparsity of weather deta over the equatorial oceans eee ee prevents the construction of reliable meteorological trajectories of the aa ae Icdine-13l1 concentrations in excess of 30 pci/1 occurred during period 10 following tests in the South Pacific. Hot clouds from these tests. One milk sample, 32 pCi/l, in the Canal Zone in the lest week of August 1968 probably cerived its ys from these tests. was Yound ther short-lived radioactivity in air during the same period av stations in South America and the tropics, which fits a pattern expected of a South Pacific source (9). .