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Introduction

During last decade, after the discovery of the Antarctic stratospheric ozone hole, the behavior of the stratospheric ozone at middle latitudes has been deeply investigated. Recent analyses (Solomon, 1999 for a complete review) have shown an ozone reduction at middle latitudes of 4 % per decade in the Southern hemisphere and of 3 % per decade in the northern hemisphere. Several works were devoted to the estimate of the relative influence on mid-latitude stratospheric ozone decrease of the processes in the Polar regions and the in-situ processes.

Solomon et al. (1998) have shown that the decrease can be attributed mainly to the heterogeneous chemistry on the in-situ stratospheric aerosols. Polar air transport simulations during 1991 to 1993 winters indicates that inter-annual variability of transport is not correlated with the interannual variability of the middle latitude ozone total column (Norton and Chipperfield, 1995).

Nevertheless, simulations presented by Hadjinicolau et al. indicates that the influence of the chemically activated at high latitudes contributes up to 50 % to the ozone budget at the middle latitudes.

In this work we quantify the amount of

Aerosol measurements

In this work we estimate the role of the Polar Stratospheric Clouds outside or at the edge of the Northern hemisphere polar stratospheric vortex . ALOMAR R/M/R lidar measurements show the presence of PSC outside the core of the polar vortex using an equivalent latitude definition of the surf zone.

We used a theoretical approach in order to evaluate the transport of air masses containing active chlorine toward middle latitudes. Horizontal resolution currently used in CTM's simulations do not allow the description of sub-grid features due to the numerical horizontal diffusion. Spurious diffusive effects reduce the concentration of chemical species. This effect is considered as responsible for an underestimate of the efficiency of the chemical processes involved in ozone depletion (Edouard et al. 1999). Our approach is based on an high resolution transport model that allow to reduce numerical diffusion, coupled with a simplified chemical scheme in order to perform long time simulations.