MEASUREMENTS OF AMERICIUM AND PLUTONIUM IN THE ATMOSPHERE* D, W. Efurd, G. L. Merrill, Jr., J. T. Phelps, and K. D. Rosenlof McClellan Central Laboratory McClellan Air Force Base, CA ABSTRACT Alpha spectrometry and high sensitivity mass spectrometry have provided complementary data which have proven useful in understanding fluctuations in the atmosphere, particularly the relationship between plutonium activity levels and variations in the 2"%pu/239py isotope ratios. A summary of past plutonium data plus some recent measurements are presented. Previously unpublished 24lam data collected from 1963-1970 are also presented. These data indicate that the 2414m/235py atom ratio in the atmosphere has been decreasing since 1967. Explanations for this anomaly are postulated. INTRODUCTION Virtually ail the transuranic activity on the surface of the earth has come Therefore, for any predictive mode! of plutonium buildup via the atmosphere. in the biosphere to be accurate, it is critically important to assess the element's magnitude, distribution, physical form, and chemical form in both the stratosphere and troposphere. The primary sources of activity in the past were 325 KCi of 239°240p, fallout debris from nuclear testing (Hardy et al., 1973) and 1.7 KCi of 238pu from the burnup of the SNAP-9A capsule in 1964 (Harley, 1964). Accidents involving aircraft carrying nuclear weapons resulted in localized releases 1000 times less (Langham, 1968). The largest accidental releases from land-based facilities have been 50 times smaller still (Hammond, 1971; Krey, 1976). 4This work was conducted under the auspices of the Air Force Technical Applications Center. 249