If you’re using reference grade instruments, I would assume they’re calibrated in labs using well controlled environments.

I.e. put sensor in vacuum chamber, remove all gases, then introduce know amount of gases to produce a known target environment.

Given their reference instruments, I assume they’re also capable of maintaining their calibration for a long period of known time, before they need to be re-calibrated. They would never rely on something as inaccurate as “20 mins outdoors” to calibrate themselves.