Why is EPA PM2.5 AQI so far off from the official airnow measurement?

I noticed that in my area (Northeast US) often the purpleair PM2.5 AQI is very different than the official airnow PM2.5 measurement. For example right now in my area it’s 60+ (yellow or orange) whereas on airnow it’s less than 30 (good).

Are the official EPA sensors accurate and the ones from purple air not accurate or incorrectly calibrated? Are they using a different technology, perhaps purpleair sensors have a systematic error due to fog or other particle in the air that don’t show up in the EPA measurents?

See below for comparison


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Anyone from purpleair monitoring and answering support question?

Or is there a different way to get support for customers?

I personally find vast differences between my sensor’s readings and the official AQI readings in my region. There’s lots of reasons for that.
The air quality can vary immensely over quite short distances and also the way data is presented. Often my readings are often more than 20 times worse than the official readings. This is basically due to the high proliferation of poorly operated wood heaters in my local area. The official readings here are averaged of a one hour period. That essentially masks low occurrences of individual or occasional high readings. Our official air quality meter is also situated where air quality isn’t affected so much from residential wood heaters.
That is the main reason why I have my own sensor. The official readings just do not reflect what is going on in my local area.

I don’t work for Purple Air but I suspect the answer is simply differences in the scale used. For an analogy, consider that 32 degrees Fahrenheit is the same thing as 0 degrees Celsius. They are both the same temperature, just using a different scale.

Hello @Andrei

This is a great question.

PurpleAir Sensors on the EPA Map

The readings between the PurpleAir Map and the AirNow Fire and Smoke Map can be different for a few reasons. After collecting the data from our sensors and before putting it on the AirNow Fire and Smoke map, they do a quality control screening; the data is averaged hourly, and the EPA conversion formula is applied to the data. If you apply the US EPA conversion and hourly averages on the PurpleAir map, you will see a closer resemblance between the data on our map and the Fire and Smoke Map.

PurpleAir Sensors and Regulatory Monitors

Air quality can also be highly variable over short and long distances, and could vary throughout a city. This could also be seen in the difference between AirNow regulatory monitors and PurpleAir’s monitors.

The technology difference between PurpleAir monitors and the regulatory monitors is different, and there is a huge cost difference between them. PurpleAir monitors are considered low-cost sensors. The EPA monitors are highly accurate and highly precise whereas PurpleAir monitors are highly precise but not as accurate. With that being said, the EPA is then able to apply their conversion equation for wildfire conditions to more closely align our data with theirs.