WHAT ROLE DO TYPE-I POLAR STRATOSPHERIC CLOUD AND AEROSOL PARAMETERIZATIONS PLAY IN MODELED LOWER STRATOSPHERIC CHLORINE ACTIVATION AND OZONE LOSS

The chlorine activation and subsequent ozone loss of the northern wint er lower stratosphere have been modelled using different schemes for t ype I polar stratospheric clouds (PSCs) and sulphate aerosols. Type I PSCs were assumed to consist of either nitric acid trihydrate (NAT) at equilibrium, supercooled ternary solutions (STS) at equilibrium, or t o follow a hysteresis cycle between frozen and liquid particles depend ing on the temperature history. The sulphate aerosol was assumed to be present as either liquid binary H2SO4/H2O aerosol (LBA) or as solid s ulphuric acid tetrahydrate (SAT). Our box model integrations show that NAT and STS, representing the upper and lower limits of lower stratos pheric chlorine activation, respectively, appear to destroy ozone equa lly efficiently after a cold PSC event (T-min less than or equal to 19 0K at 50 mbar). For higher minimum temperatures, up to the equilibrium NAT point, there is significantly more ozone loss in the NAT scheme t han in the STS scheme. On NAT, chlorine is activated directly by ClONO 2 + HCl --> 2Cl + HNO3, whereas on STS, indirect activation by ClONO2 + H2O --> HOCl + HNO3 followed by HOCl + HCl --> 2Cl + H2O, dominates. During the processing period, the indirect activation on STS will pro duce a temporary peak in HOCl. Box model integrations also show that d irect chlorine activation is faster on SAT than on LBA, yielding signi ficantly more ozone loss in air parcels which remain below the SAT mel ting point (215-220 K). Our single-layer chemical transport model simu lations (theta = 465K) of the lower stratospheric chlorine activation during Arctic winter 1994/1995 show that chlorine is activated more qu ickly on NAT than on STS. However, in mid December 1994, when temperat ures are low enough for substantial STS particle growth, maximum activ e chlorine becomes similar in both schemes and remains similar until t he end of January 1995. A model integration which includes SAT produce s up to 200 parts per trillion by volume more ClOx, inside the polar v ortex during Arctic winter 1994/1995, than a model integration which i ncludes LBA. The high melting point of SAT means that it may contribut e to midlatitude ozone loss when filaments of processed air are shed b y the vortex. For example, a model integration shows that air peeling off the Arctic vortex on February 14, 1995, contains 10% more ClOx at middle latitudes in an integration that includes SAT formation in a hy steresis scheme, than in an integration that includes LBA. The major d ifferences in ozone loss predicted by the model PSC schemes occur insi de the polar vortex. The largest differences in ozone at 465 K inside the vortex at the end of March 1995, up to 500 parts per billion by vo lume, are found between the equilibrium NAT and STS schemes. Ozone val ues in the hysteresis schemes are intermediate between those of the NA T and STS schemes. The inclusion of SAT in a hysteresis scheme does no t have a major global effect.