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 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 H 2 SO 4 /H 2 O concentration, temperature, and air pressure in a size range between 0.001 μ and 2.58 μ. Binary homogeneous nucleation of H 2 SO 4 /H 2 O, condensation and evaporation of H 2 SO 4 and H 2 O, 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, SO 2 , and SO 4 2− . Globally and seasonally different SO 2 − and SO 4 2− sources for stratospheric aerosol can therefore be taken into account. Results of a multiannual simulation show that the simulated SO 2 and H 2 SO 4 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.