Discrepancies with AirNow

General
1

Has anyone looked into discrepancies between AirNow’s AQI readings and Purple Air’s readings? I have yet to purchase a Purple Air device but there are two near where I live and they both had AQI’s just over 90 at the same time Air Now reported 65.

Is it just that Purple Air is measuring something different? Purple Air devices are very affordable, how do they stack up against what I assume are more costly and more accurate/reliable devices that feed AirNow. Or is it something else?

I would appreciate any insights. Thank you.

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The discrepancies between PurpleAir’s map and AirNow’s map are created and affected by a few things.

  • ​Data reporting frequency
  • Conversion rate of PM2.5 AQI

Data Reporting Frequency

​The data from our sensors on AirNow’s map is reported once an hour. The number they report is based on the one-hour average we collected over the past 60 minutes.
​The data on the PurpleAir map is reported on an average of 10 minutes by default.
​For every single report that is shown from a PurpleAir sensor on AirNow, we show a minimum of 6 reports in the same time.
​The data will differ during this period, but we should show similar figures when the averaging period is set to one hour.

​

​Conversion Rate

​The conversion rate on our map is a report of US EPA PM2.5.
​The conversion rate applied to the data on the AirNow map is a conversion of PM2.5 through the US EPA conversion found here: The "Apply Conversion" Field.
​You can replicate this conversion in our map by navigating to the “Apply Conversion” field and selecting “US EPA.”

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3

Hi Nick

Thanks for your note. Can we please schedule a call tomorrow?

I am on the east coast USA, Eastern time and can be flexible.

Thanks

Joel

973-650-4162.

4

I have been wondering about the discrepancies myself between what our local area shows on Purple Air and what AirNow does. Here’s the thing. AirNow only measures HOURLY data. It is NOT real-time like Purple Air is. AirNow is owned by the government. It’s run by the EPA. The EPA is in charge of all the government sensors. The AQI is also an index that’s measured according to whatever the dominant pollutant is that day. A lot of times it’s a lot lower than Purple Air if it’s measuring ozone as the dominant pollutant for your area, and if it’s measuring non-woodsmoke PM2.5 and that ozone is the higher reading than non-woodsmoke PM2.5. At the same time, there’s SOME ozone, the WOOD smoke PM2.5 may be a lot higher than ozone. Sometimes AirNow could be measuring PM10 pollution, and it MAY be dominant over ozone. So, these are possible reasons for discrepancies between AirNow and the real-time Purple Air readings.

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Hi

Thanks for your note.

I had asked for a phone call - 973-650-4162 — but perhaps you don’t do this as policy. Ok, we can go back and forth a bit via email.

If a Purple Air sensor was placed directly next to an EPA sensor, and each delivered their results at the same moment, would they get the same results?

Purple Air retails for ~$220 and my wild guess is that the EPA sensors are much more costly. I get leapfrog technology and all that, but is there any comparative takeaway? Are the readings equally accurate?

I’m starting to get the AQI calculation and can I ask, does Purple Air deliver an AQI, or something else? "The AQI is also an index that’s measured according to whatever the dominant pollutant is that day. A lot of times it’s a lot lower than Purple Air if it’s measuring ozone as the dominant pollutant for your area,…” Or does Purple Air ONLY measure non-woodsmoke PM2.5? What about the recent Canadian wildfires, which obviously were woodsmoke — is this different?

Thanks again.

Joel

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When you use the Purple Air website, on the left-hand side of the page you can set the readings such that they display non-woodsmoke US EPA AQI, or if you toggle the WOODSMOKE button, the readings are slightly different but it is the wood smoke calculation based on the US EPA reading.

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