Radiative forcing of climate by sulfate aerosols as determined by a regional circulation chemistry transport model

Abstract

H2SO4/SO42- aerosols have the potential to modify the radiation budget of the atmosphere. Under clear-sky condition they scatter solar radiation back to space, reducing absorption of solar irradiance (direct effect). The capability of sulfate particles to act as cloud condensation nuclei, thus influencing cloud droplet number concentration, cloud albedo and the development of precipitation is referred to as indirect effect. Evidence has been presented that sulfate aerosol climate forcing is sufficiently large to reduce significantly the positive forcing by anthropogenic greenhouse gases regionally, especially in the Northern Hemisphere. Until now, only coarse grid global models with rather simple chemistry modules have been applied to estimate the radiative forcing of sulfate aerosols. In this paper we would like to ascertain the short wave sulfate forcing over Europe, one of the main anthropogenic source regions. For this purpose the three-dimensional European sulfate distribution was generated by a regional climate model in combination with a complex chemistry transport model. Then a computationally efficient radiation transfer model was applied. It estimates the direct and indirect short wave forcing of sulfate aerosols on the basis of the variable sulfate mass distribution and meteorological input data. For comparison coarse grid global model results will be shown. Regional and global model results are comparable concerning the distribution and 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 higher resolution regional model is to show smaller scale phenomena, which cannot be resolved by the global model. Different predictions of the amount of clouds by the regional and global model modify the forcing significantly, emphazising the role of clouds in estimating the direct (and, of course, the indirect) short wave forcing of sulfate aerosols. It is further interesting to note that in the global model results about 30% of the sulfate burden and its short wave radiative forcing over Europe, one of the main anthropogenic source regions, is caused by sulfate from natural sources (DMS and volcanoes). Thus, the long-range transport of primary and secondary pollutants from outside the regional model domain contributes significantly to the limited area model's atmospheric load. Therefore, the initial and boundary chemical composition of the atmosphere for the limited area model should be investigated in more detail in the future.