Orientation-averaged light-extinction characteristics of compound sulfate–carbon-soot particles have been analyzed with a discrete-dipole algorithm (ddscat code) for (ratio of primary-particle radius to secondary-particle radius) in the range 7 to 1 and for wavelengths from 0.4 to 0.8 μm. It was found that compound particles above a particle radius of approximately 0.2 μm exhibit light-extinction characteristics that closely match those of a pure sulfate particle. The shielding of the carbon particle by the primary particle apparently reduces the absorption effect of the soot particle over the range of all possible orientations. In light of the fact that soot particles tend to be small in comparison with host sulfate particles, the light-extinction characteristics of compound particles are dictated by the optical properties of the host particles. This result has been applied for aerosol aggregates with log-normal size distributions. For the aggregate extinction coefficient of a group of compound particles remains within 12% of that of a group consisting only of sulfate particles. This allows for effective calculation of the overall aerosol light extinction on the basis of the optical and geometrical properties of the constituent particles without having to include a compound-geometry effect.
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