Scattering directionality parameters of fractal black carbon aerosols and comparison with the Henyey-Greenstein approximation.

Current radiation transfer schemes employ the Henyey-Greenstein (HG) phase function to connect three single parameter representations of aerosol scattering directionality-the hemispherical upscatter fraction (β), the backscatter fraction (b), and the asymmetry parameter (g). The HG phase function does not account for particle morphology, which could lead to significant errors. In this Letter, we compute these single parameters for fractal black carbon (BC) aerosols using the numerically exact superposition T-matrix method. The variations in β, g, and b as a function of aerosol morphology are examined. Corrected empirical relationships connecting these parameters are proposed. We find that the HG phase function could introduce up to a 35% error in β and g estimates. Interestingly, these errors are suppressed by the large mass absorption cross-sections of BC aerosols in radiative transfer calculations and contribute to ≤8% error in direct forcing efficiencies.