Cosmic-ray produced radioactive isotopes as tracers for studying large-scale atmospheric circulation

Abstract

In the interval between two consecutive condensations of moisture, atmospheric air accumulates radioactive isotopes produced by cosmic-ray particles in collisions with nitrogen, oxygen and argon. The production rates of these isotopes per gramme of air depends strongly on altitude and latitude; the rate of removal depends, between rains, essentially on the half-life of each isotope. These facts can be used to label air masses and to trace their trajectories. The isotopes which seem to be most useful for meteorological investigations are: ³²P (half-life 14 days); ³³P (25 days); ⁷Be (53 days); and ³⁵S (87 days). Their concentrations have been studied in rains in India during 1956-1957 and approximate values have been obtained for the annual fall-out. These experimental fall-out values have been compared with the values which one calculates from the expected isotope production by cosmic radiation in various parts of the atmosphere. From such a comparison one can draw the following conclusions: 1. (1) The fall-out based on the annual production of the isotope ⁷Be agrees well with the measured annual fall-out. The measured fall-out of ³²P is somewhat higher and that of ³³P and ³⁵S between two and five times higher than the calculated values. The discrepancy is probably significant, although the fall-out data are not very accurate; it is presumably due to an underestimate of the contribution which neutrons with energy below ˜40 MeV are making to the number of nuclear disintegrations in argon. 2. (2) The data indicate that condensation of water vapour removes cosmic-ray induced radioactivity from the air very efficiently. One can, therefore, conclude that different isotopes appear in rain water in the same proportion in which they existed in the air before the moisture was removed. 3. (3) Stratosphere air differs markedly from air which has remained in the troposphere throughout the period between successive condensations and precipitations; apart from the fact that its level of activity will in general be higher, it contains a much larger proportion of long-lived compared to short-lived isotopes. Air which has descended from the stratosphere and moves into lower regions, remains clearly distinguishable for very long periods until it has been strongly diluted or else cleansed by condensation. The simultaneous determination of radioactivity due to two or more isotopes in an individual rain leads, therefore, to information on the past history of the air mass from which the water was condensed. 4. (4) A detailed analysis of twenty-one rain samples from various parts of India, taken between July and December 1957, showed that none of the corresponding air masses had the characteristics of air which had descended from the stratosphere.