FISSION-PRODUCT CONCENTRATIONS IN THE TROPOSPHERE AND LOWER STRATOSPHERE OVER THE PACIFIC NORTHWEST SINCE 1962 P, H. GUDIKSEN, G, L. JONES, W. R. SCHELL, C. A. L. SWANSON, N. ERICKSON, and A. W. FAIRHALL University of Washington, Seattle, Washington ABSTRACT The concentration of fission products, injected into the atmosphere by nuclear-weapons detonations, has been studied by aircraft sampling in the lower stratosphere and upper troposphere over the Pacific North- west since 1962. The individual activities of Mn, *zr—Nb, '"Cs, 106Ru— Rh, 18Sb, and 4Cce were determined by gamma-ray spectrom- etry and subsequent computer analysis. In general, a direct correlation between stratospheric and tropospheric activity levels at any given time is observed. This correlation indicates that rapid diffusion may be taking place across the tropopause. The 7 r— =Nb/ "Cs activity ratios for stratospheric and tropospheric samples at any given time are the same, thus demonstrating that no fractionation processes are operating across the tropopause. The *Mn/'"Cs ratios display a maxi- mum during April and May 1963; thereafter the decrease in the ratios is consistent with the 300-day half-life of *“Mn. The maximum is probably due to a nonuniformly mixed stratosphere with respect to 54Min. INTRODUCTION In our fallout studies the atmospheric structure is being studied over a particular geographic area located in the region of the tropopause gap. Samples are collected by using a U. S. Navy TA-3B twin-jet trainer through the cooperation of the Whidbey Island Naval Air Station. 428