MODEL SENSITIVITY STUDIES OF ARCTIC OZONE DEPLETION

We have used an off-line chemical transport model (CTM) to diagnose th e expected chemical ozone destruction in the Arctic winters of 1993/19 94 to 1996/1997, and to investigate the sensitivity of the model-calcu lated loss to meteorological variability, chlorine and bromine loading s, denitrification, dehydration, and increased stratospheric H2O. The model was integrated on a single isentropic surface at 475 K (about 18 km or 50 hPa altitude) using analyses from the European Centre for Me dium-Range Weather Forecasts. The CTM produces local depletions of up to 45% in the polar vortex in the cold winter of 1995/1996. In winter 1996/1997 the large-scale temperatures cold enough for polar stratosph eric clouds did not start until early January which delayed chlorine a ctivation. However. because the cold temperatures persisted well into March, the local O-3 depletion in the model vortex was around 40% in l ate March. The sensitivity experiments using 1994/1995 meteorology sho w that the chlorine loading is much more important than bromine for co ntrolling the polar ozone loss given the expected abundances, but the ClO + BrO ozone loss cycles are calculated to be more important than C lO + ClO. Accordingly, the local relative efficiency factor we calcula te from the model (cr) is large, especially in the polar region where it is around 60 for current day halogen loadings. Denitrification can increase Arctic ozone depletion through a delay in chlorine deactivati on. However, very strong denitrification early in the winter causes le ss O-3 depletion in the model, with the current heterogeneous chemistr y scheme, due to enhanced recovery into HCl and inefficient chlorine r eactivation. The additional inclusion of dehydration reduces the model ed Arctic O-3 depletion, due to decreased heterogeneous processing rat es, and the reduced occurrence of equilibrium nitric acid trihydrate p articles. The modeled Arctic O-3 depletion increases slightly (by 2-4% of the initial O-3) using 1994/1995 meteorology when the stratospheri c H2O loading is increased. This is due to slightly more polar stratos pheric cloud (PSC) activity in this cold winter, although this effect would be potentially more important in a warmer winter where PSC proce ssing was marginal.