A comparison of the PurpleAir PA-II sensor to both regulatory particulate matter monitors and meteorological instrumentation in Asheville, North Carolina

Routine ambient air quality monitoring is mandated by U.S. federal law, but many regions of the U.S. are sparsely monitored due in part to the high cost of regulatory monitors.  Low-cost sensors can fill data gaps when it is neither feasible nor cost-effective to expand regulatory monitoring networks.  Initial comparisons of ambient measurements from the PurpleAir PA-II sensor – a low-cost (<$300 USD) and commercially available sensor that measures fine particulate matter (PM_2.5) using a laser particle counter – with nearby (~2 km) regulatory PM_2.5 monitors maintained by the Asheville Buncombe Air Quality Agency in Asheville, North Carolina, indicate general agreement.  Continuous data collection began in January 2022, thus capturing a range of atmospheric conditions and seasonal changes in PM_2.5 concentrations.  High water vapor content is known to affect the performance of optical particle counters such as the PA-II, so additional summertime measurements may reveal persistent biases in air quality measurements in these conditions.

The PA-II sensor also measures temperature, relative humidity, and barometric pressure.  Preliminary analyses compare the sensor’s measurements of these environmental variables to co-located research-grade meteorological instrumentation, including Campbell Scientific, Inc. 107 and 109 temperature probes, a Vaisala HMP45C temperature and relative humidity probe, and a Vaisala PTB101B barometer. Results reveal an expected high temperature bias of several degrees Fahrenheit caused by the small amount of heat generated by the electronics contained in the PA-II’s housing. A low relative humidity bias is a consequence of the temperature performance. Barometric pressure measurements from the PA-II sensor are consistently high by about 2-3 hPa.