Q: What is the PurpleAir map?

A: This map is populated exclusively with data from PurpleAir sensors from the PurpleAir network. Each icon on the map represents a public PurpleAir sensor, and the color indicates the real time PM2.5 reading on the US EPA Air Quality Index scale. The default display shows the US EPA PM2.5 AQI of both outdoor and indoor sensors. You can customize the data displayed and sensor types visible by using the map legend.

When clicking a sensor icon, two informative boxes display. One shows the sensor information and averages over the past week, while the other shows a graphical chart of readings from Channels A and B over the past week.

Question: Why does one channel on a sensor, on the map, have a higher reading than the other channel? For example, why does a B channel get higher than the A channel, and Visa versa, and which of those readings would be more accurate? I also read the fine print about these readings; that the unit may overestimate the concentrations of PM2.5 in wildfire smoke as opposed to PM2.5 blown from another source like a gravel pit. Many of these readings are a lot higher than the U.S. government hourly update readings. Why should anyone trust PurpleAir readings, if this is the case?

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Hi @Bumchamp7,

More information on channels A and B is found here: What are Channel A and Channel B?.

The laser counters in PurpleAir sensors measure the air quality independently of each other. This means that the readings on one channel may be different from another.

The data from PurpleAir laser counters correlate very well with regulatory monitors. The US EPA created the “US EPA” conversion factor to match PurpleAir sensor data even closer to that of regulatory monitors. Evaluations done on the sensors can be found here: PurpleAir Sensor Evaluations and Studies.

So if you say the PurpleAir readings correlate very well with the “regulatory”(government, or AirNow monitors that give 1-hour updates, are you saying that the PurpleAir readings may SEEM higher than the government monitors? Well, for example, now, this past weekend, if I saw an AirNow reading of 75, updated hourly, and a real-time Purple-Air reading of 100, are you saying that, by a conversion factor, the readings correlate? Our local TV station has also had reports that the numbers on AirNow do not seem to “match” with different numbers from different apps, and PurpleAir certainly is one of those whose numbers don’t “match” with AirNow; our state D.E.C. official even recommends people use AirNow. So, he’s implying that AirNow is the best to use. So, he probably does not trust PurpleAir, he being a state government worker. Anyway, those are my thoughts.

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AirNow displays PurpleAir sensor data on their Fire and Smoke map.

There are many different ways to display air quality data. To match the readings from AirNow much closer, we recommend you use hourly averages on the PurpleAir map, use the “US EPA PM2.5 AQI” scale, and enable the “US EPA” conversion factor.

OK, thanks for the tip.

OK, so by the Fire and Smoke Map, for example, suppose I see the hourly reading in Liverpool, New York, listed as 42. On the real-time Purple Air Map, the device is registering 59 and the hourly average is what you suggest i use; it’s calculated as 60, not 42. If Purple Air’s QI is registering 60 for an hourly average, what is this “conversion factor” so I could enter it into a calculator, to see if the conversion closely matches the government reading of 42?

Not long after I posted my last set of questions, I did figure it out; how to access the Conversion Factor, so thanks for the info about the conversion factor!!

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Our apologies for the delay, but we are glad you were able to get it working!