The density of the atmosphere (and hence, the relative number of molecules of gases available for chemical reactions) decreases as altitude increases because:

Boyle's Law, an inverse relationship exists between pressure and volume

Explanation:

We know that atmospheric preassure drops 1 mm Hg for every 10 m high.

And according to Boyle's Law, if preassure decreases volume will be increased.

Having in mind that density is the mass of anything (air included) divided by the volume it occupies; if volume increases density will be lower.

So, lower pressure lower density.