Does the gravitational pull of the moon affect the composition of Earth's atmosphere? In particular, does the pull of the moon affect the amount of water vapour in the Earth's atmosphere? In other words, if the moon were larger, or orbiting more closely to the earth, would the increased gravity cause the composition of the atmosphere to change?
The atmosphere is continuously mixed by turbulence, so no sorting of its  ordinary components can occur easily and rapidly. Twice a day change, anyway, is far too fast for anything but some tidal effect on pressure and thus on temperature. Ok, these might indirectly slightly modify some thresholds and balance e.g. between states change (water) and some atmospheric chemistry, by a fraction of what day/night change does. I wouldn't call that "change of composition", though.