A lot of these cheap, portable sensors "cheat" in one way or another. E.g CO2 sensors that assume they'll be subject to open air once every few days, using that

A lot of these cheap, portable sensors "cheat" in one way or another. E.g CO2 sensors that assume they'll be subject to open air once every few days, using that minimal CO2 value as a baseline. So while they give numbers which can be useful if the user is aware of its limitations, they're far from being analytical tools. In contrast PM is pretty easy to get right. Shine laser, count particles.