MORE RAPID POLAR OZONE DEPLETION THROUGH THE REACTION OF HOCL WITH HCL ON POLAR STRATOSPHERIC CLOUDS

THE direct reaction of HOCl with HCl, known to occur in liquid water 1 and on glass surfaces 2, has now been measured on surfaces similar to polar stratospheric clouds 3,4 and is shown here to play a critical part in polar ozone loss. Two keys to understanding the chemistry of the Antarctic ozone hole 5-7 are, one, the recognition that reactions on polar stratospheric clouds transform HCl into more reactive species denoted by ClO(x) (refs 8-12) and, two, the discovery of the ClO-dimer (Cl2O2) mechanism that rapidly catalyses destruction of O3 (refs 13-15). Observations of high levels of OClO and ClO in the springtime Antarctic stratosphere 16-19 confirm that most of the available chlorine is in the form of ClO(x) (refs 20, 21). But current photochemical models 22,23 have difficulty converting HCl to ClO(x) rapidly enough in early spring to account fully for the observations 5-7,20,21. Here I show, using a chemical model, that the direct reaction of HOCl with HCl provides the missing mechanism. As alternative sources of nitrogen-containing oxidants, such as N2O5 and ClONO2, have been converted in the late autumn to inactive HNO3 by known reactions on the sulphate-layer aerosols 24-27, the reaction of HOCl with HCl on polar stratospheric clouds becomes the most important pathway for releasing that stratospheric chlorine which goes into polar night as HCl.