Want to know what’s going on with air quality closer to home or decide where to go to enjoy the outdoors today? Clean Air Methow has placed dozens of low-cost air quality sensors manufactured by “Purple Air” ™ with Clean Air Ambassadors spanning over 60 miles of our valley – from Lost River Airport to Pateros.
We don’t have a ton of cars or industry around here, but we do generate our fair share of smoke. Our local pollutant of concern, PM2.5, is shorthand for particulate matter 2.5 microns or smaller in diameter. For reference, a human hair is about 60-80 microns in diameter. These really small particles suspended in the air come from incomplete combustion, or burning. Some are generated from car exhaust, but most are from smoke, such as woodsmoke from home heating in the winter, or forest fires in the summer. Visit Health page for more information.
The air quality monitors suck air into the instrument and bounce a laser beam off the particulate matter in the air. The instrument then counts how many particles are present in a certain volume of air depending on how the light is refracted, or bent. A concentration of PM2.5 (by weight) in a cubic meter of air can then be calculated.
The numbers are associated with colors that change from green to yellow to red (and beyond) as air quality gets worse, as a way to visually comprehend how bad the air is for you to breathe. Washington Department of Ecology has a great description of what actions you can take at the different levels to protect your health. Visit PDF.
Each monitor detects how many particles are in the air at the exact location of the instrument, and air quality conditions may be different not that far away. However, if you noticed that our Purple Air monitor numbers are slightly different than the WA Ecology and AirNow.gov monitors in the same location (co-located at the Winthrop library and Twisp town hall), good for you to pay such close attention!
In this case, the numbers are different for a couple of reasons.
Fancier instrument. The WA Ecology and AirNow.gov monitors are much more expensive instruments called “nephelometers,” and are more accurate and precise than the Purple Air monitors.
Real-time versus 24h average. When you look at online data from Purple Air monitors, the default display is the concentration of particles they are measuring every 80 seconds and represent real-time conditions. In contrast, the WA Ecology monitors calculate a 24-hour rolling average of particle concentrations. This is because the studies done looking at serious health risks of air pollution focused on what would happen if you breathed those levels for 24 hours. The method WA Ecology and AirNow.gov uses to find the 24-hour average is called the “Now-Cast” method.
It’s not just you! This unfortunately has caused no small amount of confusion, but here’s what you should know:
The Purple Air website shows the AQI, the EPA’s color-coding system. This means that the WA Ecology monitor and our Purple Air monitor right next to each other at the Winthrop library could be measuring the same concentration of PM2.5 and be displayed as yellow (moderate) on the Purple Air website, but unhealthy for sensitive groups (orange) on the WA Ecology website.
We’re so glad you noticed and asked! The purple air sensors default to displaying PM2.5 levels as the EPA Air Quality Index, but also measure temperature, humidity, and PM10.
If you look in the right-hand, upper corner of this box, there is also the ability to apply a “conversion” factor. Especially at very high concentrations of PM2.5 such as we’ve seen during forest fires, the purple air monitors tend to overestimate concentrations. We recommend applying the US EPA conversion factor, which will apply a factor that “corrects” the purple air numbers; you will typically see the numbers decrease after checking this box. If you really want to get into it, a scientific poster describing the development of this conversion factor can be found here.
