These settings provide control over the background sky and atmosphere (fog or volume scattering).
There are two types of atmosphere in Arnold, fog, and atmosphere_volume. Fog simulates the effect of light scattering, which causes more distant objects to appear lower in contrast, especially in outdoor environments. Atmosphere_volume simulates light scattered by a thin, uniform atmosphere. It produces shafts of light and volumetric shadows cast from geometric objects.
It is not possible to render both fog and atmosphere_volume in the same scene.