30 percent: however, estimates of an- deposition measurements could not be ular this relationship between the concen- nual deposition are not affected. made. The estimated values are in good The two constants, Xy and Xg, are agreement with the measured values probably not the same value, be- from 1967 and 1968. cause their averages. each based upon It appears. therefore. that the annual 60 individual monthly determinations, deposition of a nuclide can he accurateare not within one standard deviation. ly calculated from average monthly Thesignificance of the difference is not hemispheric concentrations of that nuimmediately attributable to any partic- clide in surface air. The significance of meteorological phenomenon, al- though it is intriguing to speculate on the possible contributions from the relative areas of sea in the two hemispheres and from interhemispheric transfer in the troposphere. Using the derived hemispheric constants and the measured concentrations of Srin surface air for the period from 1963 through 1968, we calculated the annual Sr” deposition. These estimated deposition values are compared with the measured values in Table 1. The agreement is reasonably good. especially in the most recent years. Althoughtherelatively large deviation for 1963 and 1964 might suggest that X varies with the age of debris, there is no evidence of such a variation in 1967 and 1968 when the French and Chinese atmospheric tests occurred and most of the deposited radionuclides were “fresh.” This method of computing fallout from measured concentrations of Sr®° in surface air should be applicable to all materials dispersed like Sr® in the atmosphere. The only other radionuclide whose fallout we could verify with deposition measurements was Pu738 which was disseminated during the reentry and burnup of a SNAP-9A (Sys- tems for Nuclear Auxiliary Power) power source on a weathersatellite (5). The estimates of plutonium-238 deposition for 1967 and 1968 are shown in Table 2, along with the measured val- ues (6) and values derived from the annual depletion of the stratospheric reservoir extrapolated to the end of 1968 (7). Unfortunately. the data on Pu?" in surface air prior to 1967 are not reliable, and comparisons with early Reprinted from SCIENCE, 17 October 1969, volume 166, pages 376-377 tration of a radionuclide in surface air and its corresponding fallout lies in the relative simplicity of surface air sampling and the sensitivity of the existing system. If we can make worldwide estimates of fallout with acceptable accuracy, based simply upon a limited number of measurements of radionuclides in surface air, we can effect substantial savings in both time and funds. Further refinement of this relationship to smaller specific geographic areas should make possible estimates of local deposition and may lead to a greater understanding of hemispheric differences. In addition, the possibility of estimating the world- wide contamination from pollutants dispersed in a mannersimilar to ‘that of global fallout is of great value. MICHAEL T. KLEINMAN HERBERT L, VOLCHOK Health and Safety Laboratory, U.S. Atomic Energy Commission, New York, New York 10014 References 1. H. L. Volchok. Nature 206, 4988 (1965); H. L. Volchok and M. T. Kleinman, U.S. At. Energy Comm. Rep. HASL 204 (January 1969), p. I-17, U.S. At. Energy Comm. Rep. HASL 207 (April 1969), p. C-1; H. L. Volchok, U.S, At. Energy Comm. Rep. HASL 210 (July 1969), p. 1-2. 2. H. L. Volchok and P. W. Krey, J. Geophys. Res. 72, 20 (1967). 3. H. L. Volchok, U.S. At. Energy Comm. Rep. HASL 200 (October 1968), p. 1-2. 4. D. H. Pierson and R. S. Cambray, Nature 216, 5117 (1967). 5. J. H. Harley, U.S. At. Energy Comm. Rep. HASL 149 (October 1964), pp. 138-141. 6. H. L. Volchok, U.S. At. Energy Comm. Rep. HASL 207 (April 1969). 7. K. Telegadas, U.S. At, Energy Comm. Rep. HASC 204 (January 1969), p. I-2; personal communication. 16 June 1969 Copyright © 1969 by the American Association for the Advancement of Science