OZONE DEPLETION IN THE LATE WINTER LOWER ARCTIC STRATOSPHERE - OBSERVATIONS AND MODEL RESULTS

Ozone loss rates in the lowermost part of the Arctic stratosphere (at potential temperature levels less than or equal to 375 K) in the perio d January and February 1993 are calculated using a chemistry-trajector y model and 30-day back trajectories. The results were compared with o bservations carried out during the first Stratosphere Troposphere Expe riment by Aircraft Measurements (STREAM) in February 1993 in the Arcti c lower stratosphere. Relatively low N2O and low O-3 concentrations we re measured during STREAM, and O-3 loss rates of 8.0 (+/-3.6) ppbv d(- 1) were calculated from O-3-N2O STREAM data in the vortex area. The av erage O-3 loss rate calculated by the model is 8.6 ppbv d(-1) (1.3% d( -1)), in agreement with observations. However, the calculated O-3 loss rate decreases to the lower value of the observed loss rates when tak ing into account N2O-Cl-y interrelations from different studies. Heter ogeneous reactions on liquid sulfuric acid aerosols, in particular tho se involving the chlorine reservoir species ClONO2 and HCl, must be co nsidered to explain the observed O-3 loss rates. Complete conversion o f ClONO2 and HCl to active chlorine by heterogeneous reactions in the model occurs at temperatures less than or equal to 205 K under conditi ons with enhanced aerosol loading, and at temperatures less than or eq ual to 200 K with background aerosol levels. Since the trajectory temp eratures were frequently below 205 K and occasionally below 200 K, the model results are (1) sensitive to the Cl-y level but relatively inse nsitive to the initial chlorine partitioning within Cl-y and (2) show significant O-3 loss at background aerosol levels, being only 1-2 ppbv d(-1) less compared to conditions with enhanced aerosol loading. We c onclude that future O-3 loss in the Arctic lower stratosphere is quite sensitive to temperature changes, while it appears to be less sensiti ve to enhanced aerosol loading (e.g., by volcanic sulfate particles).