CHLORINE DEACTIVATION IN THE LOWER STRATOSPHERIC POLAR-REGIONS DURINGLATE WINTER - RESULTS FROM UARS

Recovery from enhanced chlorine conditions in the lower stratospheric polar regions of both hemispheres is investigated using data from the Upper Atmosphere Research Satellite (UARS). Microwave Limb Sounder (ML S) measurements of ClO within the polar vortices are used to infer ClO x (ClO+2Cl(2)O(2)) abundances that are then correlated with simultaneo us Cryogenic Limb Array Etalon Spectrometer (CLAES) measurements of Cl ONO2 and Halogen Occultation Experiment (HALOE) measurements of HCl ob tained starting within 5 days of the end of the MLS and CLAES high-lat itude observing periods in each hemisphere. Time series of vortex-aver aged mixing ratios are calculated on two potential temperature surface s (585 K and 465 K) in the lower stratosphere for approximately month- long intervals during late winter: August 17 - September 17, 1992, in the southern hemisphere and February 12 - March 16, 1993, in the north ern hemisphere. The observed mixing ratios are adjusted for the effect s of vertical transport using diabatic vertical velocities estimated f rom CLAES tracer data. In the northern hemisphere, the decrease in ClO x is balanced on both surfaces by an increase in ClONO2. In the southe rn hemisphere, continuing polar stratospheric cloud activity prevents ClO from undergoing sustained decline until about September 3. In cont rast to the northern hemisphere, there is no significant chemical chan ge in vortex-averaged ClONO2 at 465 K, and there is an apparent decrea se in ClONO2 at 585 K, even after the enhanced ClO abundances have sta rted to recede. Results from the SLIMCAT chemical transport model [Chi pperfield et al., this issue] initialized with UARS data and run with OH + ClO --> HCl O-2 as an 8% channel suggest that the primary recover y product in the south during this time period is not ClONO2, but HCl. HALOE HCl mixing ratios are extrapolated back to the time of the MLS and CLAES data. At 585 K, the chlorine budget can be made to balance b y extrapolating HCl back to a value of 0.6 parts per billion by volume (ppbv) at the beginning of the study period; at 465 K, the contributi on from extrapolated HCl is not sufficient to offset the loss in ClOx, and there is a slight imbalance between the decrease in reactive chlo rine and the change in chlorine reservoirs. The difficulty in closing the chlorine budget in the southern hemisphere may arise from complica tions caused by ongoing activation, incomplete photochemical assumptio ns, and/or inadequate data quality.