I know this question has been asked in similar places, but I'm after a specific part of the problem. Assuming the following:

* Perfectly flat, infinite world
* Earth's atmosphere
* Sun directly overhead of the viewer, no matter the time or location
* No obstacles

How far could a human see in all directions, before atmospheric refraction or other phenomena washed out any visible objects (i.e., mountains)?

All the answers I've seen so far have been set on Earth, and that is the very thing I'm trying to avoid. I'm tagging this with hard science, because the numbers matter to me.