9 for the argument that the Sr surface air concentration data relate to stratos. pheric debris since the total fission yield represented by this source will generally mask the Sr90 produced in tests of small atomic weanons. The low concentrations observed at Kodiak during June and July 1952 provide the most ~ convine ing argument that Nevada tests, and thus small weapons tests generally, contribute negligibly to the sr9 concentration observed on surface air filters. Thus, it appears that the masurezent of Sr7° on surface air filters pro~ vides a direct measure of the stratospheric burden and that the apparent seasonal variation in the Sr90 surface air concentration nay relate to seasonal variation in mixing through the tropopause and/or tropospheric washout rate.