OPTICAL-PROPERTIES OF COAGULATED PARTICLES

For particles grown in the two limiting cases of coagulation (particle -cluster agglomeration and cluster-cluster agglomeration), lower and u pper limits of the extinction at wavelengths from 1 mu m to 1 mm are d erived. The particle sizes are in the Rayleigh limit for the considere d wavelength range and the number of constituent grains in each partic le is constant. Effective medium theories, the discrete dipole approxi mation and the discrete multipole method are applied to compute the op tical behaviour of the coagulated particles. The spectral representati on for inhomogeneous media is employed to investigate topology effects systematically. We found that the extinction limits strongly depend o n the used refractive indices and on the topology of the aggregates an d that the porosity of the particles is not the most important paramet er. For particles composed of amorphous carbon, the enhancement of the extinction with respect to the extinction value for compact spheres o f the same mass is between 2 and 1000 at 1 mm wavelength. For silicate particles, the enhancement of the extinction is in the range 1.5-3.5 at 1 mm wavelength.