I think the main difference is that airnow mainly relies on expensive sensors with government-approved calibration. There are only a few such sensors per count

I think the main difference is that airnow mainly relies on expensive sensors with government-approved calibration. There are only a few such sensors per county. Airnow's map shows both the government sensors and the Purple Air sensors, but that seems to be a recent "pilot project". I doubt they use the low-cost sensor data anywhere else.