I’ve also been wondering this lately. In particular if I’m feeling slightly ill, I feel much better outdoors. At least in my experience, it isn’t necessarily sunlight as I think it’s also better at night. And it isn’t super “fresh” air or hearing birdsong or the wind rustling through trees (positive things I associate with being outdoors) as I live in a city. I hadn’t thought about humidity. Currently my best guesses are:

- a correlation between being outdoors and stood up and moving

- colder temperature (but I guess in the summer when it’s not stifling it can be preferable to be out so maybe this is wrong)

- some psychological/placebo effect

- less CO2 in the air. I can’t really put any numbers to this hypothesis as I don’t have a meter.