Aerial perspective is a setting found in the Sky fog & haze tab of the atmo editor, when using a spectral atmosphere. An AP of 1 corresponds to the atmosphere of our world, but you can notice lack of distant haze with an AP of 1 when your scene scale doesn't match the real world ( a standard 256m terrain for example., so you can cheat by using an Ap of 10 or more to simulate the distant haze. The slider only goes up to 10, but you can enter any value in the field.