You should use fog, both atmospheric and height fog. It’s just a challenge to get the exact settings for a particular effect, such as an ambient night. I think you’ll need some form of volumetric scattering to enhance the ambience, possibly with a lower height atmospheric fog or sky atmosphere effect. A second atmospheric fog might do it, too, that is set distant from the camera and comprises a much shorter range of the atmosphere (ground to top of sky) than the first atmospheric fog. However, the Mie and Raleigh Scattering in the Sky Atmosphere could be modified to get the enhanced ambience with a night sky. The first step, I think, is getting the correct color of the night sky and horizon.