Why at the horizon line? Atmospheric Haze has a direction?
Oh, you are saying it is perspective?
Yes I think perhaps the lower denser atmosphere would appear at a distance because of perspective to be essentially a solid wall that the sun would disappear behind, as if you were viewing an object moving away from just below the edge of the surface, like the table and coin demonstration.
Or perhaps think of it like this.
Imagine the atmosphere is perfectly clear and you could see all the way to the edge of the plane.
Now imagine in the very far distance there is a 10km high solid wall, which at a great distance would appear to be only a few millimeters in height because of perspective.
As the sun moves across the sky you would have full visibility until the bottom edge reached the wall, then the sun would begin to disappear behind the wall.
I think possibly the dense lower atmosphere is like that wall.