MEASUREMENTS OF AIRBORNE RADIONUCLIDES 209 Table 2——RATIO OF 33py TO ics IN GROUND-LEVEL AIR AT RICHLAND, WASH., FROM APRIL 1962 THROUGH JUNE 1964 Ratio, x 103 Month January February March April. May June duly August September October November December 1962 1963 1964 5.8 7.6 8.2 5.5 6.2 4.6 2.6 3.9 3.7 4.3 4.0 3.2 3.5 3.6 3.0 8.2 10.0 * 8.9 13.0 9.0 4.5 3.0 5.2 4.1 3.0 9.6 2.4 from air filters has shown that the ratio of *®Pu to **Pu is less than 0.02 at the present time. It is both interesting and significant that the activation products 88y (105 days) and !4sbh (60 days) reached concentrations this year (1964) which are comparable with those of last year, whereas the fission product *Zr—*Nb (65 days) is lower by about a factor of 20. Also, the fission product '87Cs (30 years) reached a concentration comparable with that of last year, and !°*Ru (1 year) and '4Ce (285 days) reached a concentration about one-half that of last year. Since there have been no nuclear detonations aboveground!” during 1963 and through September 1964, all of these radionuclides entered the atmosphere prior to 1963, and their reservoir in the atmosphere has decreased with their respective half-lives since that time. These con- centrations point to an obviously different source of ®y and 'Sb relative to most of the *Zr—*%Nb and the other fission products. In Fig. 7 the air concentrations of **y and ‘Sb have been corrected for decay to the time of their probable injection into the atmosphere’ and are compared with '*"Cs, The large increasein the fallout rates of *y and }4Sb relative to '*"Cs clearly points to a different origin. AIR PROFILES OF RADIONUCLIDES During the 1962 nuclear-weapons testing period, a few sampling flights were made. The flights were planned with the object of measuring air profiles under specific meteorological conditions, Air was drawn by a vacuum pump through a membranefilter of 5p pore size