Modelling optical properties of atmospheric black carbon aerosols

The optical properties of atmospheric black carbon (BC) aerosols are needed to model the direct radiative forcing of the climate system, as well as for interpreting and assimilating remote sensing observations from satellites. Modelling efforts during the past decade have predominantly been based on using morphologically highly realistic representations of the particle geometry in conjunction with numerically exact methods for solving the light-scattering problem. We review (i) the present state of knowledge about the morphological, dielectric, and compositional properties of BC aerosols, (ii) the state-of-the-art in numerical light-scattering methods frequently applied to black carbon, and (iii) the recent literature on modelling optical properties of BC aggregates, both bare and internally mixed with liquid-phase material. From this review we formulate some key lessons learned regarding those morphological properties that have a dominant impact on the optical properties. These morphological key features can form the basis for devising simplified model particles that can be used in large-scale applications. We illustrate this approach with one example appropriate for climate modelling, and one example relevant to the interpretation of lidar remote sensing data. (C) 2020 The Authors. Published by Elsevier Ltd.