A microphysical model for simulation of stratospheric aerosol in a climatemodel

As one part of a chemical-microphysical model, a stratospheric aerosol mode l has been developed for the implementation in the Hamburg climate model EC HAM. This model treats the formation, the development and the transport of stratospheric sulfuric acid aerosol. The aerosol size distribution and the chemical composition of the droplets are calculated dependent on the atmosp heric conditions (temperature, air pressure) and the partial pressure of H2 SO4 and H2O. Binary homogeneous nucleation of H2SO3/H2O condensation and ev aporation of H2SO4 and H2O, Brownian coagulation and gravitational sediment ation are included. Sensitivity studies with the microphysical model were performed in order to study the influence of the nucleation and condensation parameterization on the aerosol size distribution. The impact of the initial conditions (tempe rature, water vapor, SO2, OH) on the aerosol size distribution is also inve stigated, Box simulations of the stratospheric background aerosol are in good agreeme nt with in situ measurements above 73 hPa in middle and high latitudes. The model is able to reproduce important features of the stratospheric aerosol , like the observed strong increase of the aerosol size ratio above 50 hPa and the decrease of the aerosol mixing ratio. Due to the neglect of advecti on and diffusion processes the microphysical model alone is not able to rep roduce observations from regions with strong vertical exchange processes. T he simulated temporal development of the aerosol spectrum in a volcanically perturbed atmosphere is close to observations. After four to five years th e background level is reached again.