Modifications in the optical properties of black carbon (BC) due to ageing are presented and quantified in this study using a state-of-the-art description scheme of BC fractal aggregates. It is shown that the relative change in BC radiative forcing can be larger than 50 % as a function of changing fractal dimension and organic content. A comprehensive parameterization scheme for coated BC optical properties is developed with applications for modelling, ambient, and laboratory-based BC studies.