MODEL STUDY OF POLAR STRATOSPHERIC CLOUDS AND THEIR EFFECT ON STRATOSPHERIC OZONE .1. MODEL DESCRIPTION

We have included detailed microphysical processes accounting for the f ormation of polar stratospheric clouds (PSC) into a global chemical/dy namical two-dimensional model to study the effect of heterogeneous rea ctions occurring on the surface of PSCs on stratospheric ozone. The mo del explicitly calculates the formation of the PSC particles (50 bin s izes) in terms of heterogeneous nucleation, condensation, and sediment ation. The transport of the particles and heterogeneous reactions on t he particles are also represented in the model. The calculated PSC par ticles show that the distributions of PSCs in the Arctic and in Antarc tica are very different. Over Antarctica, nitric acid trihydrate parti cles (type I PSCs) are formed from early June to late September with a maximum surface area of 15 mu m(2)/cm(3), while in the Arctic, type I PSCs are formed only in January with a maximum surface area of 5 mu m (2)/cm(3). Ice crystal clouds (type II PSCs) are present over Antarcti ca in August, but ar not seen in the Arctic.