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Making Sense of Purple Air vs. AirNow, and a New Map to Rule Them All

KQED
Published September 4, 2020 at 4:20 PM PDT

Smoke from wildfires around the Bay Area has led air regulators to extend a “Spare the Air” alert through Saturday, which will be the 19th consecutive day of poor air quality in the region.

The streak represents the longest continuous period of alerts ever issued by the Bay Area Air Quality Management District. The previous record of two weeks was linked to smoke from the 2018 Camp Fire. 

As containment of the fires has increased, the air has improved. But because the fires are still burning, the BAAQMD is still expecting smoke to create pockets of unhealthy air.

In the Bay Area, residents have become used to the ritual of checking air quality maps for a possible escape from the smoke, if there is one, when fires burn nearby forests and chaparral. 

During the Camp Fire, AirNow, an air quality website created by a partnership of states, the federal government, Canada and Mexico, crashed because it could not handle so many people flocking online to put numbers to what their noses were telling them: The air smells really bad.

The failure sent web users rushing to alternative, unofficial websites like the interactive, crowdsourced map maintained by PurpleAir, a manufacturer of low-cost air monitors.

But this year, users are faced with a different problem. Official sites, maintained by air regulators, and crowdsourced maps like PurpleAir show different air quality readings.

So, What’s the Deal?

Broadly put, PurpleAir provides more localized, current and less accurate readings than AirNow,

A couple of weeks ago, the EPA and the U.S. Forest Service launched a pilot project of what some may consider to be the Holy Grail of air quality maps: combined readings taken from PurpleAir’s low-cost sensors and those from official government monitoring devices, all in a single map. Click here or on the image below to access the map; the circles represent official government monitors, and the squares indicate PurpleAir sensors.

Screenshot from AirNow Fire and Smoke map, a pilot project, on Sept. 4, 2020

“While these [unofficial] sensors don’t meet the rigorous standards required for regulatory monitors, they can help you get a picture of air quality nearest you, especially when wildfire smoke is in your area,” the website states.

PurpleAir readings and those from government sensors like the ones maintained by the BAAQMD differ in several key ways: speed, accuracy and placement.

Users of PurpleAir can toggle between real-time data and readings averaged over the last 10 or 30 minutes. The data comes from the commercial sensors the company sells, which members of the public install on porches, yards and other neighborhood sites. The readings can be helpful for people deciding whether to go for a walk or engage in other outdoor activity. (Remember to deselect “indoor sensors” to see outdoors-only readings.)

The AirNow site displays hourly, not real-time readings. The government sensors that send data to AirNow are very expensive, state-regulated and regularly calibrated by scientists to accurately measure the density of wildfire ash or other particles in the air. But they are more sparsely located than PurpleAir’s network of hundreds of monitors in the region. In contrast, PurpleAir’s devices rely on a laser to count the particles in the air,  using their average density to determine air quality at the monitor’s location. The particle count is an estimate, however, especially during fire season, as wood-smoke particles have a different density from gravel dust or other pollutants.

“To try to remedy the situation, we’ve had a whole bunch of different groups, different scientists, different universities, different agencies look at the data and convert it into a calibrated reading that more accurately compares to the EPA’s data,” PurpleAir founder Adrian Dybwad said.

PurpleAir users can now toggle between two conversions — one listed as “LRAPA,” developed by the Lane Regional Air Pollution Agency in Oregon, and “AQandU,” developed by University of Utah, which align more closely with EPA readings.

Smoke to Continue

Meanwhile, it is not time for Bay Area residents to put away their air purifiers just yet.

“Depending on wind conditions and the level of containment, you’re either getting more or less smoke,” said Ralph Borrmann, a spokesperson for BAAQMD, Thursday. “We’ve had smoke readings that have been very high in isolated areas.”

Meteorologists forecast a Labor Day weekend heat wave, which could exacerbate the ongoing fire-containment effort.

“That’s a concern because we expect triple-digit temperatures in parts of the Bay Area, and hot temperatures don’t typically bode well for fighting fires,” Borrmann said. “It may be a couple of days still that we will be suffering from smoke.”

Smoke from the Woodward fire continues to drift into the North Bay, he says, while smoke from the CZU Lightning Complex around Santa Cruz is affecting the Peninsula.

Copyright 2020 KQED
Regional Interests