Preprints
https://doi.org/10.5194/egusphere-2024-3618
https://doi.org/10.5194/egusphere-2024-3618
09 Dec 2024
 | 09 Dec 2024

Retrieval of Bulk Hygroscopicity From PurpleAir PM2.5 Sensor Measurements

Jillian Psotka, Emily Tracey, and Robert Sica

Abstract. PurpleAir sensors offer a unique opportunity for a large-scale and densely populated array of sensors to study surface air quality. While the PurpleAir sensors are inexpensive and abundant, they require calibration against a validated coincident measurement to ensure the quality of the measurement. Traditionally, this calibration is performed using statistical (empirical) methods. We propose a method to extend the aerosol properties determined by PurpleAir sensors to include estimates of the hygroscopic growth of aerosols using a novel calibration method based on the optimal estimation method (OEM). The hygroscopic growth can be estimated during calibration due to the calibration’s sensitivity to relative humidity, which influences the measured size distribution of the aerosols. Our OEM-based retrieval for calibration includes an estimation of the bulk hygroscopicity. By employing the physically-based calibration with the London’s Ministry of the Environment, Conservation and Parks site calibrated measurements, the average daily Mean Absolute Error (MAE) of the PurpleAir PM2.5 measurements decreased from 5.58 μg/m3 to 1.68 μg/m3, and the average daily bias from 4.75 μg/m3 to -0.23 μg/m3. This improvement is comparable to the improvement seen using conventional statistical methodologies. In addition to calibration, using our OEM retrieved allowed us to estimate seasonal bulk hygroscopicity values ranging from 0.33 to 0.40. These values are consistent with the accepted ranges of bulk hygroscopicity values determined in previous studies using sophisticated air quality measurement instruments.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
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PurpleAir sensors provide a low-cost way to monitor air quality, with over 30,000 sensors...
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