Wildfires in LA

Air Quality Events
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We have received and seen a number of comments about the air quality in LA. Generally, people are asking why the PurpleAir map is green when the fires are still ongoing. There are a few things to consider, which we’ll talk about below:

PurpleAir and the AirNow Fire and Smoke Map

PurpleAir data is displayed on the EPA’s fire and smoke map, available at https://fire.airnow.gov/. If you’re ever wondering if PurpleAir sensors are incorrect, you can look at the Fire and Smoke map to see if they agree with EPA sensors. PurpleAir sensors will appear as small dots, while the larger dots are federal sensors.

PurpleAir sensors and the federal sensors on the Fire and Smoke map have displayed similar readings through the whole of the wildfires.

Check out the Fire and Smoke map

Particulate Matter and Other Air Pollutants

PurpleAir sensors primarily measure particulate matter (PM), which is a term for solid and liquid particles that float around in the air. When you look at the PurpleAir map, the numbers and colors you’re seeing are only reflecting particulate levels. There are other pollutants that could be in the air affecting air quality.

Check out the article below to learn more about what other pollutants could be in the air:

Wind Direction

Wind direction has played a large part in directing where smoke from the LA fires has gone. A lot of the time, smoke has been sent out over the ocean, instead of back inland. Take a look at the following article to see how the different fire’s smoke plumes have developed.

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Not to dismiss the utility of PA for wildfires, but comparing smoke PM from PA to EPA data does not mean the PA is correct when they are similar. Most of the EPA realtime PM FEM monitors are now TAPI-640 optical methods. They are known to overmeasure wildfire PM by about a factor of 2, just about the same error for smoke as the PA. So yes they may agree, but they are both wrong for wildfire smoke. Divide the reported PA PM2.5 (with the EPA correction) by about 1.8 and you get a value that agrees with either BAM FEM or the EPA reference method that uses gravimetric filter measurements (FRM). This smoke bias is well known in the research community.