Aerosol Absorption and Radiative Forcing

Direct, semi-direct and indirect aerosol effects are recognized to affect global and regional climate. Aerosol absorption, predominantly by black carbon, plays a crucial role for the direct aerosol radiative effects as it co- determines the aerosol single scattering albedo that, together with the albedo of the underlying surface, is the key factor in controlling the sign of the top-of-the-atmosphere forcing. As no direct measurements of the mid- visible aerosol absorption with global coverage are available, current estimates rely on global aerosol models. To improve the understanding about aerosol absorption and its role in aerosol forcing, we investigate the aerosol absorption and its effect on the aerosol forcing as simulated by the models participating in the AeroCom (Aerosol Model Intercomparison Initiative) forcing experiment, that indicate a considerable diversity. Based on these findings, we employ the ECHAM5-HAM aerosol-climate model to scrutinize key uncertainties in the simulation of aerosol absorption and the consequences for the aerosol forcing, such as aerosol aging, aerosol refractive indices, the aerosol mixing state and effective refractive index mixing rules for internally mixed aerosol. The results help to explain the diversity in the simulations of aerosol forcing as well as to prioritize areas of improvement for the next generation of global aerosol models.