Trajectory hunting: A case study of rapid chlorine activation in December 1992 as seen by UARS

Trajectory hunting (i.e., a technique to find air parcels sampled at least twice over the course of a few days) is applied to analyze Upper Atmosphere Research Satellite (UARS) measurements in conjunction with the Atmospheric and Environmental Research, Inc. (AER) photochemical box model. As a case study, we investigate rapid chlorine activation in the Arctic lower stratos phere on December 29, 1992 associated with a polar stratospheric cloud (PSC ) event. Eleven air parcels that have been sampled several times along 5-da y trajectories at the 465 K (similar to 46 hPa), 520 K (similar to 31 hPa), and 585 K (similar to 22 hPa) levels were investigated. For the first time , the latest versions of the Cryogenic Limb Array Etalon Spectrometer (CLAE S, version 9) and Microwave Limb Sounder (MLS, version 5) data sets are ana lyzed, and their consistency is assessed. A detailed sensitivity study with the AER photochemical box model along these trajectories leads to the conc lusion that for the December 24-29, 1992 episode (1) the individual CLAES v ersion 9 ClONO2 and MLS version 5 ClO measurements are self-consistent with in their uncertainties; and (2) most of the time, UARS measurements of ClO, ClONO2, HNO3, and aerosol extinction at 780 cm(-1) agree within the range of their uncertainties with the model calculations. It appears that the HNO 3 and aerosol extinction measurements for four parcels at 520 K look more s upportive for the nitric acid trihydrate (NAT) scheme. However, the uncerta inties in the individual UARS measurements and U.K. Meteorological Office t emperature do not allow a definite discrimination between the NAT and super cooled ternary solution (STS) PSC schemes for this chlorine activation epis ode in December 1992.