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. 2022 Jun 23;22(13):4741.
doi: 10.3390/s22134741.

Calibration of PurpleAir PA-I and PA-II Monitors Using Daily Mean PM2.5 Concentrations Measured in California, Washington, and Oregon from 2017 to 2021

Lance Wallace  1 , Tongke Zhao  2 , Neil E Klepeis  3   4
Affiliations

Calibration of PurpleAir PA-I and PA-II Monitors Using Daily Mean PM2.5 Concentrations Measured in California, Washington, and Oregon from 2017 to 2021

Lance Wallace et al. Sensors (Basel). .

Abstract

Large quantities of real-time particle data are becoming available from low-cost particle monitors. However, it is crucial to determine the quality of these measurements. The largest network of monitors in the United States is maintained by the PurpleAir company, which offers two monitors: PA-I and PA-II. PA-I monitors have a single sensor (PMS1003) and PA-II monitors employ two independent PMS5003 sensors. We determine a new calibration factor for the PA-I monitor and revise a previously published calibration algorithm for PA-II monitors (ALT-CF3). From the PurpleAir API site, we downloaded 83 million hourly average PM2.5 values in the PurpleAir database from Washington, Oregon, and California between 1 January 2017 and 8 September 2021. Daily outdoor PM2.5 means from 194 PA-II monitors were compared to daily means from 47 nearby Federal regulatory sites using gravimetric Federal Reference Methods (FRM). We find a revised calibration factor of 3.4 for the PA-II monitors. For the PA-I monitors, we determined a new calibration factor (also 3.4) by comparing 26 outdoor PA-I sites to 117 nearby outdoor PA-II sites. These results show that PurpleAir PM2.5 measurements can agree well with regulatory monitors when an optimum calibration factor is found.

Keywords: ALT-CF3; PM2.5; PMS1003; PMS5003; PurpleAir; algorithm; calibration factor; low-cost particle monitors; particles; sensors.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Comparison (using revised CF of 3.4) of 39,474 daily average PM2.5 measurements by 182 PurpleAir PA-II outdoor monitors within 0.5 km of 47 Federal and State Agency regulatory monitors in three states. Data labels are shown for the PurpleAir sites. The associated regulatory monitor values are not displayed but can be read from the graph. The x-axis shows the normal probability standard deviations (Z-score). A Z-score of zero corresponds to the median value.
Figure 2
Figure 2
Overall mean PM2.5 PurpleAir ALT-CF3.4 vs. mean PM2.5 at matched FRM sites within 0.5 km. All 182 matched pairs of sites were required to have ≥30 full days (≥18 h per day).
Figure 3
Figure 3
PA-I outdoor overall mean PM2.5 averages are regressed against matched mean PA-II outdoor averages at 117 sites less than 0.5 km distance apart. All matched pairs of sites were required to have ≥30 full days (≥18 h per day).
See this image and copyright information in PMC

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