More rapid polar ozone depletion through the reaction of HOCI with HCI on polar stratospheric clouds

Abstract

THE direct reaction of HOC1 with HC1, known to occur in liquid water¹ and on glass surfaces², 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 HC1 into more reactive species denoted by ClO_x(refs 8–12) and, two, the discovery of the ClO-dimer (C1₂O₂) 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 HC1 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 HOC1 with HC1 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 HOC1 with HC1 on polar stratospheric clouds becomes the most important pathway for releasing that stratospheric chlorine which goes into polar night as HC1.