These settings provide control over the background and atmosphere (Fog or Atmosphere Volume).
Atmosphere
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.
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You can choose None (no atmospheric effects), Fog, or Atmosphere Volume. If you select fog or volume scattering, the Attribute Editor will display the controls for each node.
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.
Background
Use this setting to create a Background shader. Click and hold on the create button 
to create an Environment shader.
Background is considered deprecated and will be removed in a future release.