Large reductions in O3 observed in recent years over Antarctica in spr
ing are the consequence of catalytic reactions involving industrially
related radicals of chlorine and bromine. About 75% of the loss observ
ed in 1987 was due to the ClO dimer scheme proposed by Molina and Moli
na (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 denitrificat
ion, the efficiency with which HNO3 is removed from the stratosphere b
y precipitation in particulate form. It depends also, according to pre
sent 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 st
ratosphere during January and early February of 1989. It is estimated
that about 10% of O3 contained in the Arctic stratosphere between abou
t 16 and 20 km was lost during the winter of 1989. The extent of denit
rification and the persistence of the vortex are the key factors influ
encing the magnitude of O3 loss in the north. It is shown, based on an
alysis of data from the Atmospheric Trace Molecule Spectroscopy experi
ment for early May 1985 at 47-degrees-S (Farmer et al., 1987, JPL Publ
ication 87-32, JPL, Pasadena, CA), that heterogeneous chemistry (speci
fically the reaction of N2O5 with H2O on sulfuric acid droplets) can h
ave an influence also on the composition of the mid-latitude stratosph
ere. Implications for midlatitude O3 of consequent changes in the conc
entrations of nitrogen, hydrogen and halogen radicals are discussed. I
t is suggested that changes in the abundance of O3 in the lower strato
sphere in the tropics can have implications also for climate. The rela
tively warm climates of the Eocene and Cretaceous and the cold climate
s of recent glacial epochs may be associated with shallower and deeper
stratospheres, respectively, with related expansion and contraction o
f the symmetric (Hadley) circulation. Changes in lower stratospheric O
3 may be related to variations in stratospheric circulation correspond
ing to differing levels of CO2, with additional contributions for the
contemporary environment due to elevated levels of industrial chlorine
and bromine.