PurpleAir vs. government sensors?
August 26, 2020 12:58 PM   Subscribe

I am noticing that the hourly numbers from the Bay Area Air Quality Management District have deviated from the equivalent numbers from the consumer sensors at PurpleAir. Any explanations?

For reference, here's a screenshot of the PurpleAir numbers, which I checked are the PM2.5 EPA Air Quality Index (so scaled to 100) and averaged over an hour. Obviously there are differences in terrain, wind, etc., between sensors, but these seem wildly different. Pretty much the whole Bay Area is in the green as far the the BAAQMD goes, but PurpleAir is twice that. It seemed like when things were really bad earlier in the week that they were tracking much more closely. My nose is telling me that things are pretty nice outside, though I understand you can't smell PM2.5 if it comes from far away.
posted by wnissen to Science & Nature (4 answers total) 6 users marked this as a favorite
Best answer: Here's an explanation. It has to do with the lighter weight of wood smoke particles. Try toggling to the LRAPA conversion. The monitors are pretty accurate, but they're measuring particles. The smoke levels can also change quickly, so PurpleAir's real time readings are more useful.
posted by pinochiette at 1:01 PM on August 26, 2020 [7 favorites]

Response by poster: Oh. my. god. That is exactly what I was looking for. The LRAPA conversion at PurpleAir doesn't quite bring them into agreement, but at least they're in the ballpark. The combined map at https://fire.airnow.gov/ is very helpful as well. Thanks so, so much.
posted by wnissen at 1:10 PM on August 26, 2020

Response by poster: I notice Why PurpleAir and AirNow show different AQI scores during wildfires is one of the top trending articles on SFGate.
posted by wnissen at 8:57 AM on August 28, 2020

Best answer: Just as a heads up, I was about to link that article from pinochiette to someone else when I noticed this update:
Edit: 9/1/2020: PurpleAir will soon feature a 3rd correction factor specifically created by the EPA. In the meantime, EPA recommends that you use AQandU instead of LRAPA, especially if the AQI is higher than 150. From the EPA: “The US EPA correction will be available on PurpleAir.com as a selection shortly. Until then, the AQ&U correction more closely matches our correction throughout a wide range of concentrations while LRAPA shows significant underestimation above 65 ug/m3 [150 AQI].” (bracketed AQI added by me) If you’re curious about the EPA formula, see note #5 at the end of this article.
posted by btfreek at 4:49 PM on September 2, 2020 [1 favorite]

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