https://docs.arnoldrenderer.com/display/A5AFMUG/Fog
Related to atmospheric volume
https://docs.arnoldrenderer.com/display/A5AFMUG/Atmosphere+Volume
Found in the Arnold render settings.
This atmospheric shader node simulates the effect of light scattering, which causes more distant objects to appear lower in contrast, especially in outdoor environments.
The defaultFog node can be selected from the outliner (Show Dag Objects Only must be disabled).