Orientation-averaged optical properties of natural aerosol aggregates were analyzed by using discrete dipole approximation (DDA) for the effective radius in the range of 0.01 to 2 m with the corresponding size parameter from 0.1 to 23 for the wavelength of 0.55 m. Effects of the composition and morphology on the optical properties were also investigated. The composition show small influence on the extinction-efficiency factor in Mie scattering region, scattering-and backscattering-efficiency factors, and asymmetry factor, but great on the extinction efficiency factor in Rayleigh scattering region and absorption-efficiency factor. The extinction-efficiency factor with the size parameter from 9 to 23 and asymmetry factor with the size parameter below 2.3 are almost independent of the natural aerosol composition. The extinction-, absorption-, scattering-, and backscattering-efficiency factors with the size parameter below 0.7 are irrespective of the aggregate morphologly. The intrinsic symmetry and discontinuity of the normal direction of the particle surface have obvious effects on the scattering properties for the size parameter above 4.6. Furthermore, the scattering phase functions of natural aerosol aggregates are enhanced at the backscattering direction (opposition effect) for large size parameters in the range of Mie scattering.
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