Three-dimensional simulation of stratospheric background aerosol: First results of a multiannual general circulation model simulation

A sulfuric acid aerosol model has been implemented in the global general ci rculation model ECHAM4. This model treats the formation, the development, a nd the transport of stratospheric sulfuric acid aerosol. The aerosol size d istribution and the sulfuric acid mass fraction are calculated as a functio n of the H2SO4/H2O concentration, temperature, and air pressure in a size r ange between 0.001 mum and 2.58 mum. Binary homogeneous nucleation of H2SO4 /H2O, condensation and evaporation of H2SO4 and H2O, Brownian coagulation a nd gravitational sedimentation are included. The microphysical model for st ratospheric sulfate aerosol and a stratospheric sulfur chemistry are combin ed with a representation of the tropospheric sulfur chemistry. This troposp heric scheme accounts for the natural and anthropogenic emissions, chemistr y, and dry and wet deposition of DMS, SO2, and SO42-. Globally and seasonal ly different SO2- and SO42- sources for stratospheric aerosol can therefore be taken into account. Results of a multiannual simulation show that the s imulated 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 troposp here and in polar spring. The aerosol surface area density and the aerosol mass concentration reproduce lower stratospheric background conditions quit e well. Effective radius and aerosol mixing ratio agree also with satellite and in situ measurements at Northern Hemisphere midlatitudes.