We have included detailed microphysical processes accounting for the formation of polar stratospheric clouds (PSC) into a global chemical/dynamical two-dimensional model to study the effect of heterogeneous reactions occurring on the surface of PSCs on stratospheric ozone. The model explicitly calculates the formation of the PSC particles (50 bin sizes) in terms of heterogeneous nucleation, condensation, and sedimentation. The transport of the particles and heterogeneous reactions on the particles are also represented in the model. The calculated PSC particles show that the distributions of PSCs in the Arctic and in Antarctica are very different. Over Antarctica, nitric acid trihydrate particles (type I PSCs) are formed from early June to late September with a maximum surface area of 15 μm2/cm3, while in the Arctic, type I PSCs are formed only in January with a maximum surface area of 5 μm2/cm3. Ice crystal clouds (type II PSCs) are present over Antarctica in August, but are not seen in the Arctic. ¿ American Geophysical Union 1996