A study of the global distribution of sulfate aerosols with a 2D model including microphysics

Summary The global distribution of sulfate aerosols in the troposphere and lower stratosphere has been calculated using a two-dimensional model. The chemistry includes the main families (NO _x , NO _y , HO _x ) plus the sulfur compounds, while the heterogeneous processes are modelled with a microphysics code which takes into account nucleation, condensation and coagulation. The results are compared with experimental data wherever available. A too low concentration of sulfuric acid is predicted by the model in the troposphere and this is explained by too large a flux of condensation nuclei and partly by the absence of sulfate production from methane sulfuric acid (MSA) in our scheme. Aerosol concentration and size distribution are comparable to observations; however the calculations show a more complex meridional structure than observed, while the size distribution is somewhat shifted toward smaller sizes. This fact is also attributed to the presence of a large number of nuclei and to the absence of any mechanism for evaporation of aerosol particles back to the core. We have also perturbed the anthropogenic fluxes of SO₂, CS₂ and the OCS ground mixing ratios to asses the sensitivity of the aerosol concentration: we have found very little changes in the aerosol distribution because H₂SO₄ chemistry is dominated by heterogeneous processes.