[krat0sprakhar]

Didn't know about airnow.gov - thanks!

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?

[karlding]

Someone posted a Medium article [0] on the /r/bayarea subreddit where they did some digging into this. Apparently the sensors used by Purple Air rely on a constant that represents the average density of the particles it detects. Because wood smoke particles are less dense than typical PM 2.5 particles, the resulting AQI values are too high.

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...

[aaroninsf]

^^^ this

That article is well worth a read. I now am using PA in this mode.

[tjohns]

AirNow uses official EPA sensor sites, which are professionally maintained but fewer in number. These will have the highest quality data.

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.

[gnutun]

A useful explanation of the differences: https://medium.com/@16fcali/562923a55226

[tmpz22]

Purple air has a mix of indoor sensors, try using the filter to hide indoor sensors and you should see more of a parity between the two.

[unsrsly]

In addition to what other commenters have said, I'd guess that some purpleair sensors are placed in garages or other places with a local concern for air quality. That may explain some of the local variation you see on purpleair.

[ilyagr]

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.

[gonehome]

I think purple air is better.

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?