These settings provide control over the background and atmosphere (Fog or Atmosphere Volume).
Use this field to define a background shader for the scene. Create an Arnold material with a background shader (e.g., Physical Sky or Ray Switch shader) and drag and drop it onto the Background shader slot. Once connected you can double click the shader ball icon to the right of the slot, in order to select the shader and show its parameters in the attribute manager.
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.