Aerosol modeling over Europe:: 2.: Interannual variability of aerosol shortwave direct radiative forcing

[1] Aerosol distribution over Europe and its direct radiative forcing have been simulated with a regional atmosphere-chemistry model and an off-line radiation transfer model. Primary and secondary organic and inorganic aerosols have been considered. The simulation was conducted for meteorologically different years 2002 and 2003 to analyze the spatial and temporal variability of the aerosol distribution and the direct forcing. The accompanying paper focuses on the aerosol distribution, while radiative forcing is discussed in this paper. The mixing state of aerosols, externally or internally, is shown to influence the strength, regional distribution and sign of radiative forcing, thereby regulating the forcing efficiency. Positive top-of-the-atmosphere forcing was simulated over eastern and southeastern Europe in spring and winter because of contribution of black carbon. Its strength varies from + 0.2 to + 1 W m(-2), depending on aerosol mixing assumptions. Sensitivity studies show a mean European direct forcing of -0.3 W m(-2) in winter and -2.5 W m(-2) in summer, regionally ranging from -5 to + 4 W m(-2).