The polarimetric and spectrophotometric data of observations, the results of laboratory simulations, and theoretical calculations are considered as evidence in favor of the presence of large irregular particles in cometary atmospheres. The attempt is made to define more precisely the particle parameters. In particular, observations of some comets at small phase angles can be interpreted by light scattering on large icy grains. The results of laboratory experiments with ice at low temperatures and pressures are adduced; this can be explain the formation of a large icy grain cloud near the cometary nucleus. Changes of these particles under the effects of solar radiation are considered.