  Study of cosmic dust and planetary aerosols indicate that some of them
contain a large number of aggregates of the size that significantly exceeds the
wavelengths of the visible light. In some cases such large aggregates may
dominate in formation of the light scattering characteristics of the dust. In
this paper we present the results of computer modelling of light scattering by
aggregates that contain more than 1000 monomers of submicron size and study how
their light scattering characteristics, specifically polarization, change with
phase angle and wavelength. Such a modeling became possible due to development
of a new version of MSTM (Multi Sphere T-Matrix) code for parallel computing.
The results of the modeling are applied to the results of comet polarimetric
observations to check if large aggregates dominate in formation of light
scattering by comet dust. We compare aggregates of different structure and
porosity. We show that large aggregates of more than 98% porosity (e.g.
ballistic cluster-cluster aggregates) have angular dependence of polarization
almost identical to the Rayleigh one. Large compact aggregates (less than 80%
porosity) demonstrate the curves typical for solid particles. This rules out
too porous and too compact aggregates as typical comet dust particles. We show
that large aggregates not only can explain phase angle dependence of comet
polarization in the near infrared but also may be responsible for the
wavelength dependence of polarization, which can be related to their porosity.
