Hey !
That's a nice one, I can see that you understand a lot of things already.
But things can always be better so I'll try to give you some advices.
First of all, with the atmospheric perspective : I can see that you know the concept, so that's a good start. On your image we have some problems to figure out the distances. To resolve that you need to make your foreground darker, as well as a smoother progression in the distance into the light. But not too much, your city in the background is maybe a bit too light and don't detach itself enough from the sky.
As well, you have a big rock at the front left which seems far on the distance, but in the same time the grass at the bottom is huge, so there's kind of a contradiction.
On the same note, you have your middle rock which is the same shape as your right rock. And that's a really good way to mark the distance. When we see shapes that are the same like this, our brain interpret that as it is the same object or at least a similar one.
But your middle rock is a lot smaller that your right one, and in the same time he is not that lighter and the river is still very large so there's a contradiction.
Then your building is on the same ground line that the middle rock, which induce our brain to make a size comparison with the right rock and then the character and that's a good way to mark the distance.
And, in desert environment like that, there can be a lot of floating dust which can be a great tool to mark the shapes and the distances.
Also when we are on a sunset like environment (which seems to me to be the case here), the top of the structures are always lighter than the bottom, and taller is your structure, lighter it will become.
On a really positive note, I think the way you've rendered the river is really cool and make a lot of sense.
So that's it for me, I hope this comment has been useful for your. Keep going like this !
