The use of natural radioactive tracers in a study of atmospheric residence time

The global values of the activity ratios of 7Be/32P, 7Be/35S, and 7Be/22Na are presented and discussed. The wide range of variations of these observed ratios and residence times derived from them indicate a varying input of a stratospheric component of these isotopes. This shows that the troposphere is not a closed system and residence times calculated from the above ratios are likely to be larger than the actual values. More consistent results are obtained for tropospheric residence times based on 210Bi/210Pb activity ratios from various locations. The 7Be/35S ratios are lower than the expected ratios by a factor of 2 to 3, calculated from cosmic ray production values, indicating other sources like bomb production. Since the ratios are less even during periods of low fallout activity, another source, such as graphite reactors is a possibility. (authors) 31 refs