The concept of apparent polarizability for calculating the extinction of electromagnetic radiation by porous aerosol particles

In the current study, the electromagnetic properties of porous aerosol particles are calculated in two ways. In the first, a porous target input file is generated by carving out voids in an otherwise homogeneous particle, and the discrete dipole approximation (DDA) is used to compute the extinction efficiency of the particle assuming that the voids are near vacuum dielectrics and assuming random particle orientation. In the second, an effective medium approximation (EMA) style approach is employed in which an apparent polarizability of the voids is defined based on the well-known solution to the problem in classical electrostatics of a spherical cavity within a dielectric. It is found that for porous particles with smaller overall diameter with respect to the wavelength of incident radiation, describing the voids as near vacuum dielectrics within the DDA sufficiently reproduces measured values of extinction efficiency, whereas for porous particles with moderate to larger overall diameters with respect to the wavelength of the radiation, the apparent polarizability EMA approach better reproduces the measured values of extinction efficiency.