Chlorine deactivation in the lower stratospheric polar regions during late winter: Results from UARS

Abstract

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 (MLS) measurements of ClO within the polar vortices are used to infer ClOx (ClO + 2Cl2O2) abundances that are then correlated with simultaneous Cryogenic Limb Array Etalon Spectrometer (CLAES) measurements of ClONO2 and Halogen Occultation Experiment (HALOE) measurements of HCl obtained starting within 5 days of the end of the MLS and CLAES high‐latitude observing periods in each hemisphere. Time series of vortex‐averaged mixing ratios are calculated on two potential temperature surfaces (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 northern hemisphere. The observed mixing ratios are adjusted for the effects of vertical transport using diabatic vertical velocities estimated from CLAES tracer data. In the northern hemisphere, the decrease in ClOx is balanced on both surfaces by an increase in ClONO2. In the southern hemisphere, continuing polar stratospheric cloud activity prevents ClO from undergoing sustained decline until about September 3. In contrast to the northern hemisphere, there is no significant chemical change in vortex‐averaged ClONO2 at 465 K, and there is an apparent decrease in ClONO2 at 585 K, even after the enhanced ClO abundances have started to recede. Results from the SLIMCAT chemical transport model [Chipperfield et al., this issue] initialized with UARS data and run with OH + ClO → HCl + O2 as an 8% channel suggest that the primary recovery 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 by 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 contribution from extrapolated HCl is not sufficient to offset the loss in ClOx, and there is a slight imbalance between the decrease in reactive chlorine and the change in chlorine reservoirs. The difficulty in closing the chlorine budget in the southern hemisphere may arise from complications caused by ongoing activation, incomplete photochemical assumptions, and/or inadequate data quality.