Optical properties and radiative forcing of mixed black carbon aerosols

Black carbon (BC), produced by combustions of fossil fuel, biomass, and so on, is one of the strongest light-absorbing aerosols, and has critical impacts on the global climate by altering the radiative forcing of the atmosphere. In the atmosphere, BC particles are frequently mixed with aerosols of different kinds, and both numerical and observational studies showed that BC optical properties largely depend on their morphology and mixing states. However, there are significant uncertainties about the effects of mixing state on its optical properties, especially in terms of the absorption. Numerical modeling provides a unique aspect to understand the relationship between BC microphysical and optical properties. To capture the complicated mixing states of BC particles in the ambient atmosphere, this study numerically builds thousands of non-spherical and inhomogeneous BC particles, which are randomly mixed with non-absorbing or weakly absorbing aerosols, and the discrete dipole approximation can fully capture the inhomogeneous structures of those particles during the optical property simulations. Our results indicate that the overall mixing structures, especially the coating mass and the degree of mixing, affect the optical properties most significantly, whereas detailed minor structures as well as BC geometries have less influences. Furthermore, we find that the mixing states would further show noticeable influences on the radiative forcing of BC, and the BC vertical distributions have to be more carefully treated to accurately obtain atmospheric heating rate caused by BC absorption. Furthermore, we suggest that comprehensive experiments and numerical studies should be combined more closely to better understand BC microphysical and optical properties.