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.
The aiFog and aiAtmosphereVolume nodes can also be found in the Outliner by switching to 'Show Dag Objects Only':
Fog and Atmosphere Volume selected in the Outliner (Show Dag Objects Only).
It is not possible to render both fog and Atmosphere Volume in the same scene.
Use this setting to create a Background shader. Click and hold on the create buttonto create an Environment shader.
Background is considered deprecated and will be removed in a future release.