? for the ereument that the Sr?° surface air ecncentration data relate to stratosnheric debris since tho total fission vield revresented by this sourco vill cenerally mask the Sr90 produced in tests of small atomic weanons. The low concentrations observed at Kodiak during June and July 1952 provide the most convincing argument that Nevida tests, ad thus small woanons tests generally, contribute negligibly to the Sr90 concentration observed on surface air filters. Thus, it aorears that the ~easurenent of Sr?° on surface air filters vrovides a dircct measure of the stratospheric burden and thet the anparent seasonal variation in the S$r°0 surface air ccacentration mar relate to seasonal variation in mixinre throuch the trovoepanse and/or tropospheric ‘washout rate.