Atmospheric black carbon (BC) or soot — formed by the incomplete combustion of fossil fuels, biofuel and biomass — causes warming by absorbing sunlight and enhancing the direct radiative forcing of the climate. As BC ages, it is coated with material due to gas condensation and collisions with other particles. These processes lead to variation in the composition of BC-containing particles and in the arrangement of their internal components — a mixture of BC and other material — though global climate models do not fully account for these heterogeneities. Instead, BC-containing particles are typically modelled as uniformly coated spheres with identical aerosol composition, and these simplifications lead to overestimated absorption.
Laura Fierce of Brookhaven National Laboratory, USA, and colleagues exploit laboratory and field measurements to empirically update this common model approximation. By accounting for how material is arranged within BC-containing particles and, more importantly, the variability in their internal composition, the authors resolve these persistent model discrepancies. These results offer a framework for improving climate models and providing better estimates of the historical and future climate impacts of BC.