strontium-90 data. The average ratio of plutonium-239 to strontium-90 for the period May 1965 through February 1968 was 0.017 with a 2orange of + 0.009. ‘here is a gradual rise in this ratio following this period, the average value veing 0.028 from ifarch 1968 to March 1969. This increase in the plutonium-239 to strontium-90 ratio is attributed to the sixth, seventh and eighth Chinese tests, which were reported to be high yield events. the ratio of plutonium-238 to plutonium-239 remained relatively constant at v.04 + 0.02 from May 1965 to Sply 1966. A change in the ratio was fairly evident at NERHL in mid-1966 63 This change in ratio indicated a new source of plutonium~238 other than that which was previously present. Other investi-~- gators announced this change in plutonium-238 to plutonium-239 ratio somewhat earlier in June and July of 1966 6 , the conclusion being that SNAP--9A plutonium-233 had reached ground level in the Northern Hemisnvhere. Figure 3 is a plot of the SYAP-9A plutonium-238 concentrations in ground level air based on the above ratio of plutonium isotopes prior to influx of the satellite debris. The dip in levels in May of 1968 appears to be an anomaly. tie levels of SNAP-9A plutonium-238 are slightly elevated over the 1967 levels. This elevation is expreted, based on prior predictions of increasing plutonium-236 levels in 1968. A comparison of the predicted results, the reported results at ISPRA, and NERHL results is presented in Table 4. The wwkiL results (3-month averages) are somewhat lower than the predictions. aieensmes “a es ARR PRAMSNR CMT 7 é 2 “. SUMMARY Jata have been presented on the concentrations of strontium—-90, plutonium-239, and plutonium-238 in ground level air particulates. From these data, the following observations may be made: l. The Chinese atmospheric nuclear tests appear to have contributed the majority of the influx of fresh strontium-90 in ground level air narticulates. 2. It is estimated that in 1968 approximately 60 percent of the strontium-90 present in ground level air is of post-1963 origin. 3. The ratio of plutonium-239 to strontium-90 is somewhat elevated since early 1968 due to recent Chinese atmospheric tests. 4. The levels of SNAP-9A plutonium-238 debris in ground level air are somewhat lower than had been predicted.