27 Because of differences among the sensors, they knew combining the two streams of data would be complicated. Researchers noticed that PurpleAir devices were problematic on smoky days, at times dramatically overestimating readings of PM2.5. Woodsmoke particles have a different density from gravel dust or other pollutants, which can confuse PurpleAir’s sensors. By contrast, the government’s more delicate sensors are costly, state regulated, regularly calibrated by scientists, and can more accurately measure fire pollution. Unlike the PurpleAir machines, which use a laser to estimate the concentration of PM2.5 in an air sample, the official devices measure particulate matter by drawing air through a filter and then weighing the filter once an hour. Placement is another problem. It’s hard to know, for instance, if a PurpleAir sensor has been placed next to a park or inside a chimney. AirNow sensors are installed according to more stringent and transparent rules to not bias the measurements. Climate Action (Of course, even official assurances should be checked: in the runup to the Beijing Olympics in 2008, officials dramatically improved the city’s air quality readings simply by moving the pollution sensors to areas outside the city limits; in response, the US Embassy began publicizing its own AQI numbers.) To start, researchers at EPA and dozens of state, local, and tribal partners began installing PurpleNow sensors nationwide, side by side with the official monitors. Armed with months Users of the AirNow app can see smoke and air quality across North America and zoom in on health recommendations based on current conditions.
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