THE CHANGING STRATOSPHERE

Large reductions in O3 observed in recent years over Antarctica in spring are the consequence of catalytic reactions involving industrially related radicals of chlorine and bromine. About 75% of the loss observed in 1987 was due to the ClO dimer scheme proposed by Molina and Molina (1987, J. phys. Chem. 91, 433) with the balance associated with the ClO-BrO mechanism introduced by McElroy et al. (1986, Nature 321, 759). The magnitude of O3 loss is sensitive to the extent of denitrification, the efficiency with which HNO3 is removed from the stratosphere by precipitation in particulate form. It depends also, according to present understanding, on the relative abundances of ClNO3 and HCl in air trapped originally in the polar vortex in late fall or early winter. High concentrations of ClO and BrO were observed also in the Arctic stratosphere during January and early February of 1989. It is estimated that about 10% of O3 contained in the Arctic stratosphere between about 16 and 20 km was lost during the winter of 1989. The extent of denitrification and the persistence of the vortex are the key factors influencing the magnitude of O3 loss in the north. It is shown, based on analysis of data from the Atmospheric Trace Molecule Spectroscopy experiment for early May 1985 at 47-degrees-S (Farmer et al., 1987, JPL Publication 87-32, JPL, Pasadena, CA), that heterogeneous chemistry (specifically the reaction of N2O5 with H2O on sulfuric acid droplets) can have an influence also on the composition of the mid-latitude stratosphere. Implications for midlatitude O3 of consequent changes in the concentrations of nitrogen, hydrogen and halogen radicals are discussed. It is suggested that changes in the abundance of O3 in the lower stratosphere in the tropics can have implications also for climate. The relatively warm climates of the Eocene and Cretaceous and the cold climates of recent glacial epochs may be associated with shallower and deeper stratospheres, respectively, with related expansion and contraction of the symmetric (Hadley) circulation. Changes in lower stratospheric O3 may be related to variations in stratospheric circulation corresponding to differing levels of CO2, with additional contributions for the contemporary environment due to elevated levels of industrial chlorine and bromine.