Take a look at this thread. I like the 2D Fog plane trick for this kind of thing, if you don't want to mess with a Z pass and composite in post. You can place the fog plane as far away as you like, to define where the effect begins.

https://render.otoy.com/forum/viewtopic.php?f=9&t=72603&p=369014 ...

Hi! I generated a landscape and now I'm truying to add an atmosphere to it

http://photoshare.ru/data/110/110586/1/9nteg2-ggf.jpg

Like there, far objects are blue

http://photoshare.ru/data/110/110586/1/9ntee5-8y7.jpg


I tried to use medium, but it cuts off too many light, and I guess I can ...