A 3D TRANSPORT MODEL STUDY OF CHLORINE ACTIVATION DURING EASOE

A 3D chemical transport model (CTM) has been used to study the behaviour of stratospheric trace species during the EASOE campaign. The model contains a comprehensive gas phase chemistry scheme as well as a treatment of heterogeneous reactions on PSCs and sulphate aerosols. The CTM is forced using the ECMWF analyses providing realistic meteorological conditions throughout the model simulations. Experiments have been performed to examine the evolution of chlorine species throughout the winter and to estimate the magnitude of chemical ozone loss. Heterogeneous reactions on PSCs lead to large (over 1.5ppbv) abundances of active chlorine in the model polar lower stratosphere in early January. The level of active chlorine then decreases from mid January as PSCs become less frequent. In the model PSCs are more efficient at activating chlorine than the sulphate aerosols although the latter cause extensive denoxification which maintains the high ClO(x) levels after processing. Despite the high abundances of ClO and BrO the model results show that the opportunity for chemical ozone destruction during this time was limited by the lack of sunlight.