RE-FORMATION OF CHLORINE RESERVOIRS IN SOUTHERN-HEMISPHERE POLAR SPRING

This paper focuses on the recovery of chlorine reservoir species in th e lower stratosphere in late Antarctic spring. The investigations are based on measurements from the Halogen Occultation Experiment (HALOE) on board the Upper Atmosphere Research Satellite (UARS) and calculatio ns by the Mainz photochemical box model and the NASA Langley Research Center trajectory model. During late Antarctic spring 1994, HALOE obse rved high HCl mixing ratios up to 2.7 ppbv at 20 km altitude in the oz one-depleted air inside the polar vortex. These values correspond appr oximately to the sum of all available inorganic chlorine species. In t he preceding period of chlorine activation on polar stratospheric clou ds (PSCs), the observed HCl mixing ratios in some cases were below 0.3 ppbv. This indicates a fast conversion of active chlorine species int o the form of HCl after PSCs disappear with increasing stratospheric t emperatures. Box model calculations are presented that assess the rate of HCl increase in late spring when heterogeneous chemistry on polar stratospheric clouds becomes insignificant. The calculations were perf ormed along Lagrangian trajectories starting from HALOE measurements i n September 1994. Sensitivity calculations are presented regarding unc ertainties in input parameters of the calculations. In the vortex edge region, calculated HCl increase rates are significantly lower compare d with HALOE observations. Introducing additional HCl-yielding branche s of the reactions of ClO with OH and HO2 helps to reduce this discrep ancy.