PurpleAir-II sensor correction factors and its use in measuring neighborhood-scale temperature and particulate matter anomalies

The PurpleAir-II sensor provides low-cost in situ measurements of meteorological variables including temperature and humidity, as well as fine particulate matter (PM_2.5) in real time.  The sensors have been used in several studies investigating intracity differences in temperature and PM_2.5.  While the adoption and use of low-cost sensors has many benefits, care must be taken to ensure proper calibration and testing.  We have shown previously that the PurpleAir-II sensor has a high bias of 4-5 degrees Fahrenheit and a relative humidity bias of about -17%.  These results were obtained during a continuous measurement period from December 2022 to August 2023 in Asheville, North Carolina.  PurpleAir-II values were compared to temperature measurements with a Campbell Scientific 107 temperature probe and Vaisala HMP45C relative humidity probe.  We now have a longer data record (December 2022 – Summer 2024) spanning multiple seasonal changes and have updated our bias and error calculations.  We have also determined and applied correction factors for PurpleAir-II temperature and relative humidity measurements.  PurpleAir-II PM_2.5 concentrations were previously compared to a nearby regulatory PM_2.5 monitor maintained by the Asheville Buncombe Air Quality Agency during the same December 2022 – August 2023 measurement period.  PurpleAir-II bias exhibited concentration dependence with low bias at low concentrations and high bias at high concentrations.  We have updated bias and error calculations for PM_2.5 concentrations using the longer measurement period, and we have also determined and applied correction factors.  Finally, we describe an upcoming investigation using the PurpleAir-II sensor to measure neighborhood-scale temperature, relative humidity, and PM_2.5 in Asheville, North Carolina.  Such measurements will enable us to determine if certain populations are at greater risk for heat stress and PM_2.5 exposure.