Calibration of low-cost PurpleAir outdoor monitors using an improved method of calculating PM2.5

Abstract PM2.5 hourly average measurements from 33 outdoor PurpleAir particle monitors were compared with hourly measurements from 27 nearby US EPA Air Quality System (AQS) stations employing Federal Equivalent Method (FEM) monitors in California over an 18-month (77-week) period. A transparent and reproducible alternative method (ALT) of calculating PM2.5 from the particle numbers in three size categories was used in place of the estimates provided by Plantower, the manufacturer of the sensors used in PurpleAir monitors. The ALT method was superior in several ways (better precision, lower limit of detection, improved size distribution) compared to Plantower’s CF1 or ATM data series. PurpleAir monitors were strongly correlated with the nearby US EPA Air Quality System AQS stations. A calibration factor (CF) ranging between 2.9 and 3.1 was empirically derived for the PurpleAir estimates using the ALT method. This value was based on comparing the average value of 177,329 PurpleAir measurements to the value calculated from the FEM stations. The monitoring period included about 13 weeks showing very high outdoor values due to several major fires covering several hundred thousand acres. The CF during these 13 weeks averaged 2.39, whereas the CF for the remaining 64 weeks averaged about 3.21, suggesting a different response to the smoke from wildfires compared with normal ambient fine particulate matter (PM2.5). The standard Plantower CF1 data series overestimated the FEM values by about 40%, in agreement with several other studies.