Grouping and adding method for calculating light scattering by large fluffy aggregates

We present a method to derive the light scattering properties of very porous fractal aggregates composed of a large number of monomers where the size parameter of monomer is larger than unity. Our new method is based on the grouping of the aggregate: The aggregate is divided into groups, where each group is located along a line of the incident light, and the scattering properties of the group are calculated taking into account multiple scattering with monomers located inside the group, as well as those in a buffer region around the group. The scattering and absorption efficiencies are obtained by adding the resultant scattering properties for all the groups. We have shown that the method effectively works when the monomer scatters the incident lights predominantly in the forward direction, which is the case if the monomer size is large compared to the wavelength of the incident light.The errors in resulting scattering and absorption efficiencies for porous aggregates are investigated for various refractive indices and sizes of monomers. We found that the errors are larger for low absorbing materials and they can be reduced by expanding the buffer region. In the case of the buffer region for each group consisting of 1/8 of the total number of monomers, the results show errors less than 15% and 10% for absorption and scattering, respectively. It is also shown that the errors have a small standard deviation (i.e., 2%) for different directions of the incident light.