“It is quite paradox that one of the most popular outdoor air quality sensor for citizen science projects has actually in their T&Cs that you do NOT own the very data you collect.”
The sensor modules inside monitors up to $1,000 cost $2 to $15. Some makers wrap them in proprietary cartridges sold at ten times that and nag you to replace them early. One of the largest manufacturers crippled its free API until customers paid up. You buy the hardware, then you rent your own air.