Environment - Arnold for Maya

See Render Settings in the Arnold User Guide

This setting provides control over the atmosphere (fog or atmosphere_volume). You can choose None (no atmospheric effects), fog, or atmosphere_volume. If you select fog or atmosphere_volume, 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).}