Dof? I don't see any dof effect here, but I do think it lacks some haze. Hard to see if you are using a spectral atmosphere, but if so, in the sky fog & haze tab, change the aerial perspective parameter to 30 ( manual input). This parameter "shrinks" the skydome to fit the scale of your scene. The higher the value, the smaller the skydome, so the distant haze becomes more visible.
If non spectral, add more haze.