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. Volumetric Scattering 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 volume scattering in the same scene.