MATERIALS AND MEFHODS Table 1. In 1975 and 1976, the McClellan Central Laboratory determined the activity levels and isotopic compositions in plutonium samples from Edwards Air Force Base in California. A second sampling station was located at Ramey Air Force Base in Puerto Rico, The atmospheric plutonium samples were collected on IPC-1478 filter papers (DASA-1168; Rimberg, 1968), which were exposed by high-volume ground-based filtration units. The papers were dissolved in HNO3, HF, HC10,, and then spiked and equilibrated with 242pu tracer. Plutonium was extracted by the thenolytrifluoracetone (TTA) method (Cuninghame and Miles, 1956; Coe, 1965) and analyzed on high-sensitivity thermal ionization mass spectrometers. RESULTS AND DISCUSSION If samples are collected in regions where resuspension of fallout debris is negligible, the plutonium isotopic ratios measured by ground-based filtration units are representative of both the troposphere and stratosphere. This method of sample collection was used to update the 1959-1970 background study. The plutonium ground filter Force Base in variations in isotopic ratios and activity levels of the sample obtained by units at Edwards Air Force Base in California and Ramey Air Puerto Rico are shown in Table 1. The locations display daily both the isotopic ratios and activity levels. The average 240pu/239pu ratio at Edwardes Air Force Base was 0.146 + 0.017. 240py/239py ratio at Ramey Air Force Base was 0.159 + 0.011. The average Similar results were obtained from samples collected at other locations. These data seem to indicate that the 24°py/239py isetope ratios in the northern hemisphere have continued to decrease since the last measurements were made in the 1959-1970 background study. The decrease in 24°py/233py isotopic ‘ratios is the result of the atmosphere injections that occurred during and after 1967, the date when atmospheric testing was resumed. Additional samples were collected at each location and analyzed for plutonium activity levels. The results are reported in Table 2. Edwards Air Force Base had an average plutonium concentration of 0.021 fCi/SCM. Ramey Air Force Base had an average atmospheric concentration of 0.004 £C1/SCM. The difference in the average concentrations of the two locations may possibly be explained by the fact that the rate of deposition from the stratosphere increases with distance from the equator to a maximum at 40 -50° latitude. Plutonium Isotopic Ratios From Ground-Based Filtration Units EDWARDS AIR FORCE BASE, CALIFORNIA Day (1975) Volume Sampled (KSCM) 189 190 195 197 199 204 206 711 213 273 288 289 303 305 307 317 324 329 336 70.4 12.2 36.7 28.2 24.3 25.2 23.6 24.9 24.3 37.0 73.4 11.8 12,0 26.5 35.3 24.3 25.2 34.8 38.2 29 (1976) 31 34 37 39 43 45 48 5? 64 65 69 72 76 77 84 89 91 240py/239Py 0.141 0.154 0,137 0.144 0.139 0.141 0.121 0.102 0.132 0.138 0.138 0.163 0.157 0.139 0.135 0.159 0.113 0.140 0.155 + .002 + .003 + .001 + .015 + ,004 + .O11 + .004 + .009 + .004 + .008 + .003 + .008 + .0L3 + .003 + .006 + .006 +:.004 + .003 + .008 0.147 0.176 0.142 0.186 0.158 0.171 0.163 0.152 0.138 0.158 0.152 O.15k 0.142 0.110 0.145 0.143 0.142 0.161 24.4 25.0 36.3 23.8 37.5 24.5 24.4 44.6 24.8 28.8 24,0 37.1 23.7 37.7 23.7 24.6 35.7 25.5 255 + + + + + + + + + + + + + + + + + + .004 .007 .005 .005 .014 .012 .005 .002 .002 .005 .008 .009 .003 .002 .003 .006 .002 .004 aci/SCM 5.5 53.1 41.4 17.9 161.7 11.9 15.2 18,2 25.5 6.7 4.3 3.5 2.1 5.3 5.9 9.5 10.2 10.5 4.9 9.3 410.1 8.1 5.6 3.9 6.1 16.4 6.0 13.8 7.7 5.5 14.9 29,5 11.3 15.6 16.8 10.6 9.8