Atmospheric circulation from observations of sodium 22 and other short-lived natural radioactivities

Abstract

The radioactivities of the cosmic-ray-produced nuclides Be 7 , S 35 , and Na 22 have been measured in samples of dust filtered at a number of points in the atmosphere. It is found that the short-lived activities of Be 7 and S 35 are in approximate secular equilibrium with their local production at altitudes of 18 to 20 km. The degree of saturation exceeds 50 per cent of the equilibrium value even in regions just above the tropopause. The observed activities of the longer-lived isotope Na 23 are, however, found to be considerably undersaturated throughout the lower stratosphere (up to altitudes of about 20 km). The departure from the equilibrium values is found to be strongly altitude dependent; in regions close to the tropopause the activities are observed to be an order of magnitude lower than those expected, whereas at altitudes of 18 to 20 km they reach about 50 per cent of the expected equilibrium values. These data clearly show that the measurements of natural activities whose half-lives are shorter than 3 months are not adequate by themselves for the study of the nature of transport and mixing in the stratosphere. Na 22 (half-life 2.6 years) is ideally suited for the studies of stratospheric circulation, and studying it along with any of the short-lived isotopes, e.g. Be 7 or P 32 , should prove very useful. The present data are not very extensive; nevertheless, they are sufficient to test the validity of certain important aspects of the various models of circulations currently discussed in the literature.