Effects of Mixing States on the Multiple-Scattering Properties of Black Carbon Aerosols

The incident solar spectral radiative field can be modified due to scattering or absorption by soot aerosol particles. The radiative properties of soot aerosols are highly sensitive to the mixing states of black carbon particles and other aerosol components. Light absorption properties are enhanced by the mixing state of soot aerosols. Quantification of the effects of mixing states on the scattering properties of soot aerosol are still not completely resolved, especially for multiple-scattering properties. To investigate and quantify the effects of mixing states on the multiple scattering of soot aerosols, the angular dependence of multiple-scattering effects with the top-of-the-atmosphere upward radiances/polarization and upwelling hemispheric flux are studied with variable soot aerosol loadings for clear and haze scenarios.

The single-scattering properties of soot aerosols with different mixing states were accurately calculated using the Multiple Sphere T-Matrix model, which used the numerically exact solution methods of Maxwell’s equations. The multiple-scattering properties due to soot aerosols with different mixing states are considered by using the vector radiative transfer model. Specifically, the surface is assumed to be Lambertian with albedo, and the atmosphere is assumed to be plane-parallel and cloud-free.

Two types of soot aerosols with different mixing states such as external mixture soot aerosols and internal mixture soot aerosols are studied. The extinction properties and single-scattering albedo of the external mixture soot aerosol model differ from those calculated for the internal mixture aerosol model. The extinction cross-section of internal mixture aerosol model is higher than that of the external mixture aerosol model. The different mixing states of soot aerosols resulted in dramatic changes in single-scattering albedo. The single-scattering albedo was enlarged from 0.72 to 0.83 at 440 nm, which corresponds to an enhancement factor of 1.15.

Our study showed dramatic changes in upward radiance/polarization due to the effects of the mixing state on the multiple scattering of soot aerosols. The relative difference in upward radiance due to different mixing states can reach 16%, while the relative difference of upward polarization can reach 200%. The effects of the mixing state on the multiple-scattering properties of soot aerosols show angular dependence. The angular dependence of the effects of the mixing state on polarization is quite different from that of the effects of the mixing state on radiance.

The effects of soot aerosol mixing on upward polarization decrease with increasing soot aerosol loading (aerosol optical depth), which is contrary to the magnitude of the angular variations of the radiance. The reason is that multiple scattering tends to diminish the polarization feature of the reflected light while enhancing the intensity of the backscattering.

The effects of the soot aerosol mixing state on upwelling hemispheric flux are much smaller than on TOA bidirectional radiance and polarization, which increase with increasing solar zenith angle. The relative difference in upwelling hemispheric flux due to the different soot aerosol mixing state can reach 18% when the solar zenith angle is 75°. The dependence of the effects of soot aerosol mixing state on the solar zenith angle is weakened due to surface contribution. The effects of the mixing state on the upwelling hemispheric flux depend on soot aerosol loading. When the soot aerosol loading increases, the multiple scattering of soot aerosols is enhanced by an increase in path lengths or particle concentration or both. The dependence of the effects of soot aerosol mixing state on soot aerosol loading is enhanced due to surface contribution.

The findings should improve our understanding of the effects of mixing states on optical properties of soot aerosols and their effects on climate. Mixing mechanism of soot aerosols is of critical importance in evaluating the climate effects of soot aerosols, which should be explicitly included in radiative forcing and aerosol remote sensing.