The coupling of temperature, aerosol area density, and chlorine activation is studied by comparing values from a three-dimensional chemical transport model with observed temperature, aerosol area density, and ClO data in the Arctic for the time period December 1, 1995, to January 9, 1996. The three-dimensional model uses United Kingdom Meteorological Office (UKMO) winds and temperatures, run on pressure surfaces between 316 and 0.31 hPa, and the model results are examined at 100, 68, 46, 31, and 21 hPa. Radiosonde values are compared to individual UKMO temperature values to assess the impact of model temperature errors upon heterogeneous chemistry reaction probabilities. Polar Ozone and Aerosol Measurement (POAM II) aerosol extinction values are transformed into area density values and compared to model daily averaged time trends. The model is run with gas phase chemistry only, gas phase plus sulfate aerosol chemistry, and the case where gas phase, sulfate aerosol, and polar stratospheric cloud reactions are active. Results from these calculations are used to examine the sensitivity of chlorine activation to temperature biases, area density uncertainties, and the effects of the reaction paths OH+ClO→HCl+O2 (4% yield) and HO2+ClO→HCl+O3 (4% yield). Sulfate aerosol is demonstrated to be very effective in activating chlorine in the Arctic polar vortex. Time trends of five-day averages of model and Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) version 4 ClO over extended regions inside and outside the vortex at 21 and 46 hPa agree within the experimental error. ¿ 2000 American Geophysical Union