Radiative forcing by mineral dust is one of the major uncertainties in assessing the impact of aerosols on the climate. Coagulation and condensation of pollutants on dust particles can further complicate the problem by altering their optical properties. During four Asian dust events in the spring 2002, concentration and size distribution of aerosols were measured in Taipei, Taiwan. Black carbon (BC), organic carbon (OC) and optical properties of the dust aerosols were measured. We found that both the mass specific absorption efficiency and imaginary refractive index of the aerosols decreased significantly during the dust events because of reduced black carbon concentrations. In addition the black carbon in aerosols was found to have significantly higher absorption efficiency than that adopted in current climate models. We suggest that the higher absorption efficiency is due to the formation of an internal mixture of BC and other particulate matter.