Stratospheric circulation studies based on natural and artificial radioactive tracer elements

Abstract

The nature of large scale stratospheric circulation is studied using the cosmic ray produced isotopes P³², Be⁷, S³⁵ and Na²² as tracers. Supplementary information obtained from observations of the distribution of the bomb-produced Na²² and radongenic Pb²¹⁰ is taken into account. The activities of these tracer elements have been measured in the stratospheric air, up to altitudes of 20 km, during 1960-64. Data are fairly extensive for studying the characteristics of the mean circulation in the stratosphere as well as seasonal changes in patterns of mixing/transport of air in certain regions of the stratosphere. The interpretation of the data on cosmic ray tracers is based on a comparison of their observed activities with the expected production rates due to cosmic rays. For this purpose, the work of Lal & Peters is extended to evaluate the variations in the relative production rates of the isotopes P³², Be⁷, S³⁵ and Na²² in the atmosphere. These have to be taken into account when isotope data are compared for different altitudes and latitudes in the stratosphere where relative isotope production rates are different because of the markedly different prevailing energy spectrum of nucleons. The analysis allows us to distinguish threé zones in the lower stratosphere (below 20 km), well separated from the tropopause, having distinct circulation patterns. These regions are separately well mixed either vertically or horizontally; the mean time of residence of aerosols in these regions differs appreciably too. The most stable region in the stratosphere is found to be 18-20 km region at 0-30° latitude, where apparent residence times are of the order of twenty months. Polar regions are observed to exhibit an enhanced vertical mixing during November-February. Combining these results with the observations of dispersion of bomb-produced Na²², which appeared in significant amounts from early 1962 onwards all over the stratosphere, we deduce that in the polar regions, vertical mixing occurs rapidly during November-February so that any activity injected in this region at 20 km or so mixes downwards at the rate of about 1.5 km month^-1. It is concluded that the observed spring peaks in the troposphere are merely the consequence of this phenomena which is triggered in upper levels (above 20 km) of the stratosphere during October-November. The observations of concentrations of Pb²¹⁰ in the stratosphere are discussed. The analysis reveals that an appreciable gravitational settling of Pb²¹⁰ seems to have occurred, at least during the period over which data were collected, from the stratospheric air between 55°-75° latitude. These observations imply that the residence time of air in this region of the atmosphere is appreciably higher than that deduced from tracers which attach themselves to aerosols. Lastly, the Pb²¹⁰ data indicate that appreciable amounts of tropospheric radon presumably enter the equatorial stratosphere; this conelusion rests on the observation that Pb²¹⁰ concentrations are higher in this region compared to that in the surrounding air.