The Atmospheric and Environmental Research, Inc., photochemical model has been used to simulate the concentrations and time development of key trace gases in the Antarctic stratosphere before, during, and after the Airborne Antarctic Ozone Experiment (AAOE). The model includes complete gas phase photochemistry and heterogeneous reactions of ClNO3 (g) and N2O5 (g) with HCl (s) and H2O (s). Observations of long-lived species by the AAOE instruments have been used to constrain the intial conditions in our calculations. We present results from four cases illustrating the evolution of the trace gases for a range of possible initial conditions and duration of heterogeneous activity. The amount of ClO produced by heterogeneous conversion of HCl is determined not only by the initial concentrations of NOx (NO+NO2+NO3), N2O5, and ClNO3 during winter, but also by the rate at which NOx is resupplied by photolysis of N2O5 and HNO3, or by transport. Results from the four cases presented bracket column measurements of HCl, CLNO3, and HNO3 by the Jet Propulsion Laboratory and National Center for Atmospheric Research infrared spectrometers onboard the NASA DC-8, and in situ measurements of ClO and NOy by instruments aboard the NASA ER-2. Comparison of results and measurements of HCl and ClO suggests that heterogeneous chemistry was maintained throughout the month of September in 1987. We suggest field observations and kinetic data which would further constrain the photochemistry of the spring Antarctic stratosphere. The behavior of ozone is discussed in a companion paper (Ko et al., this issue). ¿ American Geophysical Union 1989