Well, in the day's sun = higher O2 and at night progressively higher CO2 and lower O2 as plant respiration/decomposition. I am sure this has been measured and is online - where?
There is also a chance that the intangible 'scent' of freshness, similar to after a shower might sway your senses?
I feel like I can sense the "stuffiness" (CO2) in a room apart from the scent, but I do think it would be worth an experiment to make sure. I agree it's a potential confounding(?) variable.
