For those like me wondering why oxygen emission depends on altitude, it's that the excited lifetime is very long for red, so it tends to decay via collisions at lower altitudes.
There is hardly anything. However, there's never nothing. The atmosphere is a thin and continuously decreasing thing, but there's no point where it stops. Space is nearly a vacuum, but technically not totally. It's close enough that we normally say it is, but there are particles bouncing around.