RADIATIVE FORCING OF CLIMATE BY SULFATE AEROSOLS AS DETERMINED BY A REGIONAL CIRCULATION CHEMISTRY TRANSPORT MODEL

H2SO4/SO42- aerosols have the potential to modify the radiation budget of the atmosphere. Under clear-sky condition they scatter solar radia tion back to space, reducing absorption of solar irradiance (direct ef fect). The capability of sulfate particles to act as cloud condensatio n nuclei, thus influencing cloud droplet number concentration, cloud a lbedo and the development of precipitation is referred to as indirect effect. Evidence has been presented that sulfate aerosol climate forci ng is sufficiently large to reduce significantly the positive forcing by anthropogenic greenhouse gases regionally, especially in the Northe rn Hemisphere. Until now, only coarse grid global models with rather s imple chemistry modules have been applied to estimate the radiative fo rcing of sulfate aerosols. In this paper we would like to ascertain th e short wave sulfate forcing over Europe, one of the main anthropogeni c source regions. For this purpose the three-dimensional European sulf ate distribution was generated by a regional climate model in combinat ion with a complex chemistry transport model. Then a computationally e fficient radiation transfer model was applied. It estimates the direct and indirect short wave forcing of sulfate aerosols on the basis of t he variable sulfate mass distribution and meteorological input data. F or comparison coarse grid global model results will be shown. Regional and global model results are comparable concerning the distribution a nd amount of sulfate burden and radiative forcing over Europe. Hence, for the estimation of the European budgets, the global model produces sufficiently precise information. As expected, the advantage of the hi gher resolution regional model is to show smaller scale phenomena, whi ch cannot be resolved by the global model. Different predictions of th e amount of clouds by the regional and global model modify the forcing significantly, emphazising the role of clouds in estimating the direc t (and, of course, the indirect) short wave forcing of sulfate aerosol s. It is further interesting to note that in the global model results about 30% of the sulfate burden and its short wave radiative forcing o ver Europe, one of the main anthropogenic source regions, is caused by sulfate from natural sources (DMS and volcanoes). Thus, the long-rang e transport of primary and secondary pollutants from outside the regio nal model domain contributes significantly to the limited area model's atmospheric load. Therefore, the initial and boundary chemical compos ition of the atmosphere For the limited area model should be investiga ted in more detail in the future. (C) 1998 Elsevier Science Ltd. All r ights reserved.