  We investigate the scattering and absorption of light by random ballistic
aggregates of spherical monomers. We present a general measure for the porosity
of an irregular particle. Three different classes of ballistic aggregates are
considered, with different degrees of porosity. Scattering and absorption cross
sections are calculated, using the discrete dipole approximation (DDA), for
grains of three compositions (50% silicate and 50% graphite; 50% silicate and
50% amorphous carbon; and 100% silicate), for wavelengths from 0.1 micron to 4
micron. For fixed particle mass, increased porosity increases the extinction at
short wavelengths, but decreases the extinction at wavelengths long compared to
the overall aggregate size. Scattering and absorption cross sections are
insensitive to monomer size as long as the constituent monomers are small
compared with the incident wavelength. We compare our accurate DDA results with
two other approximations: the analytical multi-layer sphere (MLS) model and
effective medium theory (EMT). For high porosity and/or absorptive materials,
the MLS model does not provide a good approximation for scattering and
absorption by ballistic aggregates. The EMT method provides a much better
approximation than the MLS model for these aggregates, with a typical
difference less than 20% in extinction and scattering cross sections compared
with DDA results, for all types, compositions and wavelengths probed in this
study.
