The Bay Area Just Turned Orange. All Eyes Are on PurpleAir

When I woke up on Wednesday morning, I thought my phone was wrong when it showed the time as 7:45. So I checked my watch and my microwave—both 7:45. It was nearly dark out and not because it was dawn, but because the San Francisco sky was choked with wildfire smoke, turning the landscape a Blade Runner dark orange. As I’ve done first thing every morning for the past several weeks during California’s unprecedented outbreak of wildfires , I checked the website PurpleAir—my rock in this summer’s churning Sea of General Awfulness.Drawing crowdsourced data from air quality sensors it sells to the public—ranging between $200 and $300—PurpleAir builds maps that show in real time just how bad a neighborhood like mine is suffering from particulate matter (PM) 2.5 pollution, the particles that make up wildfire smoke. (PM 2.5 means 2.5 micrometers in diameter, a size the Environmental Protection Agency deems “fine inhalable particles.”) This summer I’ve been checking it obsessively, watching the air quality index, or AQI, swing wildly from the green-is-good indicator that indicates a PM 2.5 score in the low teens up past the alarming purple of a 200 score. I do this knowing that PurpleAir’s sensors are necessarily imperfect, and because they are deployed at home by customers, not by an environmental agency, they are not meant to serve as official measurements of air quality. Still, they’re an important source of data even for government bodies, like the Bay Area Air Quality Management District, whose own sensors feed the EPA’s AirNow.gov tool, which also uses a green-to-purple color scale.
Apparently I’m not alone in my obsession. As AQI scores have soared in the Bay Area over the past month and a half, so has PurpleAir’s business. “We’ve seen 1,000 or more percent increase in traffic on the site,” says Adrian Dybwad, the company’s founder and CEO. “We have a whole heap of emails—people calling. We have a lot of activity going on right now. We've had to increase our staff to make more sensors, because we make them ourselves.” Nine thousand of these sensors are now distributed all around the world, which you can see on this map. If you zoom on the West Coast, you’ll see atrocious AQI numbers—some over 700—as wildfires rage across Washington, Oregon, Colorado, and California. (Yes, 700 is higher than PurpleAir’s own map legend, which stops at 500.)Those sensors work quite differently from the ones the Bay Area Air Quality Management District is using to monitor AQI. Inside a PurpleAir sensor shines a laser beam, which illuminates particulate matter floating in the air, a technique known as light scattering. Think of it like shining a flashlight through the desert—you’ll see all kinds of dust particles moving about. “The intensity of the reflection will give you an idea of the size of the particle, and then the number of reflections gives you an idea of the number of particles,” says Dybwad. A PurpleAir sensor converts these counts into an estimated mass of the particulate matter, which is then converted into the AQI you see on its maps.
By contrast, the BAAQMD’s devices—some 30 of them scattered around the Bay Area—more directly measure mass by collecting particulate matter in filters. These very sophisticated, very precise air quality monitors do that for 52 minutes at a time, then analyze the samples for another 8 minutes, giving the AirNow.gov website an hourly calculation of PM 2.5 pollution. Compare that to PurpleAir, which gives essentially a real-time measure of air quality because it can quickly count particles with that laser, then calculate the mass of pollution from there.