A sulfuric acid aerosol model has been implemented in the global general circulation model ECHAM4. This model treats the formation, the development, and the transport of stratospheric sulfuric acid aerosol. The aerosol size distribution and the sulfuric acid mass fraction are calculated as a function of the H2SO4/H2O concentration, temperature, and air pressure in a size range between 0.001 μm and 2.58 μm. Binary homogeneous nucleation of H2SO4/H2O, condensation and evaporation of H2SO4 and H2O, Brownian coagulation and gravitational sedimentation are included. The microphysical model for stratospheric sulfate aerosol and a stratospheric sulfur chemistry are combined with a representation of the tropospheric sulfur chemistry. This tropospheric scheme accounts for the natural and anthropogenic emissions, chemistry, and dry and wet deposition of DMS, SO2, and SO42-. Globally and seasonally different SO2- and SO42- sources for stratospheric aerosol can therefore be taken into account. Results of a multiannual simulation show that the simulated SO2 and H2SO4 concentrations are generally in good agreement with available observations. The formation of new particles through homogeneous nucleation takes place in the tropical lower stratosphere and upper troposphere and in polar spring. The aerosol surface area density and the aerosol mass concentration reproduce lower stratospheric background conditions quite well. Effective radius and aerosol mixing ratio agree also with satellite and in situ measurements at Northern Hemisphere midlatitudes. ¿ 2001 American Geophysical Union