Atmosphere doesn't mean high altitude, it's all the gas around our planet, including what you are breathing right now. Also, the CO2 concentration is pretty mu

Atmosphere doesn't mean high altitude, it's all the gas around our planet, including what you are breathing right now. Also, the CO2 concentration is pretty much constant (as a function of height) below the turbopause, because of mixing, so it doesn't matter at which altitude you extract it.