I'm surprised though why the data is so different between airnow and purpleair. On AirNow, SF currently shows unhealthy air (150~) whereas purpleair shows 88 (moderate). Which one of these is more reliable and real-time?
The Lane Regional Air Protection Agency has developed a conversion formula that is built into Purple Air (if you apply the LRAPA conversion factor in the UI), so you can have a better comparison.
[0] https://medium.com/@16fcali/understanding-purpleair-vs-airno...
That article is well worth a read. I now am using PA in this mode.
That said, the PurpleAir devices use high quality laser particulate sensors, so they're still significantly more accurate than most consumer devices.
Both are real time, but during a wildfire where smoke is shifting between neighborhoods, I'd go with the PurpleAir site to get a score for your particular neighborhood. Just be sure to turn on the LRAPA scoring model, as others have mentioned.
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.
Purple air relies on a bunch of devices people have bought and installed all over. I think airnow is just a handful of government ones?