We have developed a fast two-dimensional residual circulation stratospheric model. In order to calculate possible effects of long-term changes for trace gases for a large number of scenarios and to examine the model sensitivities to dynamical and photochemical assumptions and inputs, the model is designed to minimize computer requirements. The species continuity equations are solved using process splitting, that is, by successively applying the operators associated with advective changes with photochemical and diffusive forcing. The first study undertaken with this model concerns family chemistry approximations, in which groups of species are related by photochemical equilibrium assumptions and transported as one species. These assumptions are tested by comparing results for the family transport model (FTM), in which odd nitrogen (NOy=N+NO+NO2+NO3+2N2O5 +HO2NO2+ClONO2+HNO3) is transported as a family, with the results for a separate transport model (STM) in which HNO3, HO2NO2, ClONO2 and N2O5 are transported separately from NOx=N+NO+NO2+NO3. Two cases are considered: (1) a current atmosphere annual cycle; and (2) a typical scenario for increased fluorocarbons, methane, and nitrous oxide. Although there are differences in odd nitrogen species partitioning, especially at high latitudes, the calculated O3 distributions are nearly identical. For the perturbation scenario the annual average column ozone change and its temporal and spatial characteristics are nearly the same for the FTM and the STM. ¿ American Geophysical Union 1989