ANALYSIS OF UARS DATA IN THE SOUTHERN POLAR VORTEX IN SEPTEMBER 1992 USING A CHEMICAL-TRANSPORT MODEL

We have used a new, isentropic-coordinate three-dimensional chemical t ransport model to investigate the decay of ClO and evolution of other species in the Antarctic polar vortex during September 1992. The model simulations cover the same southern hemisphere period studied in a co mpanion data paper by Santee et al. [this issue]. The model is initial ized using the available data from the Microwave Limb Sounder (MLS) an d Cryogenic Limb Array Etalon Spectrometer (CLAES) on the Upper Atmosp here Research Satellite (UARS). During the model initialization chemic al inconsistencies in the UARS data became evident. Fields of odd nitr ogen (NOy) derived from CLAES N2O underestimated the sum of the direct observations of the major NOy species. Results from the model integra tions at 465 K and 585 K are sampled in the same way as the various UA RS instruments and compared to the observations both directly and by c onsidering average quantities in the inner and edge vortex regions. Sa mpling the observed species ill the same way as the UARS instruments i s important in removing any spurious trends due, for example, to chang ing solar zenith angle. While the model can reproduce the magnitude of the MLS ClO observations at 585 K, this is not possible at 465 K. The model partitions too much ClO into Cl2O2 to reproduce the observed Cl O which is around 2.0 parts per billion by volume (ppbv) averaged with in the polar vortex. The model also underestimates CLAES ClONO2 in the inner vortex at 465 K due to heterogeneous processing. The observatio ns require that effectively all of the inorganic chlorine is in the fo rm of ClO and C1ONO2 in the inner vortex at this altitude. In the basi c model run, the decay of ClO produces ClONO2 which is not observed by CLAES. Our results indicate the potential importance of the speculati ve reaction between OH and ClO producing HCl for the recovery of HCl i n the Antarctic spring. By including this reaction, the decay of model ClO into HCl is enhanced, yielding better agreement with HCl data fro m the Halogen Occultation Experiment (HALOE) data. Similar results can also be obtained by including the reaction between HO2 and ClO to pro duce HCl with a 3% channel. The model generally reproduces the observe d O-3 destruction during September. The most significant discrepancy f or O-3 is in the inner vortex at 465 K where the model underestimates the observed O-3 loss rate, especially when the effects of vertical mo tion are included.