MODEL STUDIES OF CHLORINE DEACTIVATION AND FORMATION OF CLONO2 COLLARIN THE ARCTIC POLAR VORTEX

We have investigated the processes which lead to the deactivation of a ctive chlorine (ClO + 2Cl(2)O(2)) and the formation of ClONO2 in and a round the Arctic polar vortex. By using a range of three-dimensional a nd trajectory models, forced by meteorological analyses for the 1991/1 992 winter, we have been able to separate processes associated with th e model formulation and resolution, from true chemical processes. The rate of ClO deactivation can be overestimated in a three-dimensional m odel due to numerical diffusion from a poor transport scheme or to low resolution. In our experiments we found that vertical resolution is a t least as important as horizontal resolution. The use of isentropic l evels reduced the spurious vertical mixing by providing a true separat ion between horizontal and vertical motion. Increasing the horizontal resolution of the model reduced mixing and produced a sharper, narrowe r ClONO2 collar region with slightly higher maximum values. The domain -filling trajectories produce a ClONO2 collar structure with no mixing between parcels. These results show that it is in situ chemical deact ivation of ClO and not mixing which produces the ClONO2 collar. Result s from the high-resolution isentropic simulations and domain-filling t rajectories have been compared with Cryogenic Limb Array Etalon Spectr ometer data and diagnosed for the chemical cause of ClO deactivation. The models generally reproduce the observed features of the collar reg ion but underestimate the highest ClONO2 mixing ratios. As the polar l ower stratosphere is heavily denoxified the cause of the ClONO2 recove ry is release of NO2 from HNO3. For this process, reaction of HNO3 wit h OH can be as important as photolysis at high solar zenith angles.