moa t re tog t wy I. ’ a Fi are a Tnaia | An upper and lower limit of the average monthly tropospheric inventory of the excess Pu-239 was then estimated from the above result, depending upon the assumed distribution with altitude. If the excess assumed an altitude profile similar to that of stratospheric debris, an upper limit of 70 curies of excess Pu~239 in the trcepeo- sphere was indicated. If the excess concentration is essentially constant to an elevation of 1000 meters, with nothing above, a lower limit of about 1 curie was obtained. There were no reported atmospheric nuclear tests in the Northern Hemisphere from December 1967 to December 1968 (11); hence the mid-tclate-1968 anomalies cannot be explained as due to fresh Gekris. Other possible surface releases of Pu-239, such as airplane crashes with less of nuclear material, leaks from processing plants or other similar sources cannot be evaluated with information presently available. ever, since virtually all of the Pu-238 in surface air, the anomaly came out of the stratosphere from the burn-up of SNAP-9A) How— in the period ct (either from nuclear tests ox a sharp decrease in the ratio Pu-238/Pu-734 would have occurred in the event of significant tropospheric relesses of Pu-239. As Figure 4 indicates, this sharp decline cannot be seen, Some decrease did occur in 1968, much of it reflecting fallout from I - 20