  Equipartition magnetic fields can dramatically affect the polarization of
radiation emerging from accretion disk atmospheres in active galactic nuclei.
We extend our previous work on this subject by exploring the interaction
between Faraday rotation and absorption opacity in local, plane-parallel
atmospheres with parameters appropriate for accretion discs. Faraday rotation
in pure scattering atmospheres acts to depolarize the radiation field by
rotating the polarization planes of photons after last scattering. Absorption
opacity in an unmagnetized atmosphere can increase or decrease the polarization
compared to the pure scattering case, depending on the thermal source function
gradient. Combining both Faraday rotation and absorption opacity, we find the
following results. If absorption opacity is much larger than scattering opacity
throughout the atmosphere, then Faraday rotation generally has only a small
effect on the emerging polarization because of the small electron column
density along a photon mean free path. However, if the absorption opacity is
not too large and it acts alone to increase the polarization, then the effects
of Faraday rotation can be enhanced over those in a pure scattering atmosphere.
Finally, while Faraday rotation often depolarizes the radiation field, it can
in some cases increase the polarization when the thermal source function does
not rise too steeply with optical depth. We confirm the correctness of the
Silant'ev (1979) analytic calculation of the high magnetic field limit of the
pure scattering atmosphere, which we incorrectly disputed in our previous
paper.
