A series of coordinated investigations of polar stratospheric chemistry, radiation, and dynamics has resolved and generated many questions regarding ozone depletion in the Earth's atmosphere. Photodissociation plays a key role, and the radiation field models utilized have often been singled out as possible sources of uncertainty. Mean instabilities (source functions) are compared for three independent models: discrete-ordinate, integral equation, and Monte Carlo simulations. It was found that the results differ by <10% for clear, cloudy, and aerosol conditions over an altitude range of 0--50 km, wavelength interval of 175--800 nm and a wide range of solar zenith angles. It is shown that in the presence of Pinatubolike aerosol concentrations, twilight photodissociation rates within an aerosol layer centered at 20 km can be reduced by more than an order of magnitude, whereas for solar zenith angles <60¿, photodissociation rate changes are a few percent or less. Assessment of the effects of the aerosols on twilight stratospheric chemistry within the aerosol layer shows that radiation field perturbations on the homogeneous photochemistry are small and that heterogeneous processes are the major source of the altered photochemistry. ¿ American Geophysical Union 1995