Assessing the accuracy of low-cost optical particle sensors using a physics-based approach

Abstract

Abstract. Low-cost sensors for measuring particulate matter (PM) offer the ability to understand human exposure to air pollution at spatiotemporal scales that have previously been impractical. However, such low-cost PM sensors tend to be poorly characterized, and their measurements of mass concentration can be subject to considerable error. Recent studies have investigated how individual factors can contribute to this error, but these studies are largely based on empirical comparisons and generally do not examine the role of multiple factors simultaneously. Here, we present a new physics-based framework and open-source software package (opcsim) for evaluating the ability of low-cost optical particle sensors (optical particle counters and nephelometers) to accurately characterize the size distribution and/or mass loading of aerosol particles. This framework, which uses Mie theory to calculate the response of a given sensor to a given particle population, is used to estimate the fractional error in mass loading for different sensor types given variations in relative humidity, aerosol optical properties, and the underlying particle size distribution. Results indicate that such error, which can be substantial, is dependent on the sensor technology (nephelometer vs. optical particle counter), the specific parameters of the individual sensor, and differences between the aerosol used to calibrate the sensor and the aerosol being measured. We conclude with a summary of likely sources of error for different sensor types, environmental conditions, and particle classes and offer general recommendations for the choice of calibrant under different measurement scenarios.

First Measurements of Ambient PM2.5 in Kinshasa, Democratic Republic of Congo and Brazzaville, Republic of Congo Using Field-calibrated Low-cost Sensors

“…PurpleAir PM2.5 is known to strongly correlate (r > 0.9) with reference grade monitors but are subject to biases at high relative humidity in particular (Jayaratne et al, 2018;Magi et al, 2020;Malings et al, 2019;Tryner et al, 2020). Sensitivity to relative humidity has also been identified in other low-cost air pollution monitoring devices (Di Antonio et al, 2018;Hagan and Kroll, 2020;He et al, 2020;Jayaratne et al, 2018;Kelleher et al, 2018). PA-II sensors cost approximately $250 USD per unit which are about 100 times cheaper than reference PM 2.5 monitors, making them attractive for multi-sensor networks.…”

“…understanding local conditions is vital. Calibration factors and sensor technical performance vary strongly with conditions such as temperature, relative humidity, particle size distribution, and particle loading (Hagan and Kroll, 2020;Levy Zamora et al, 2019;Tryner et al, 2020). Other factors such as sensor aging can also influence sensor performance and have been studied in both a laboratory and field setting (Malings et al, 2019;Tryner et al, 2020).…”

“…Careful co-location, or side-by-side placement of LCS with reference monitors is an essential step to getting accurate data out of LCS. Several recent studies have performed either a field calibration or a laboratory calibration for a variety of different sensors in a variety of different environmental conditions, paving the way for additional studies in diverse environments (Hagan and Kroll, 2020;Holstius et al, 2014;Jayaratne et al, 2018;Jiao et al, 2016;Johnson et al, 2016;Kelly et al, 2017;Tryner et al, 2020Tryner et al, , 2019. However, to our knowledge, published LCS co-location and performance evaluation studies within sub-Saharan Africa are limited (R Subramanian, 2020).…”

Influence of Particle Composition and Size on the Accuracy of Low Cost PM Sensors: Findings From Field Campaigns

“…However, most measurements are usually conducted at one angle with a large field of view and assuming that the particle size is directly proportional to the pulse height (Grimm and Eatough, 2009;Morpurgo et al, 2012;Gao et al, 2013). Most low cost sensors (Hagan and Kroll, 2020;Molaie and Lino, 2021 and references therein) consist of a dark chamber, a laser or LED light source and a photodiode. The sampled air flows inside the chamber and particle is illuminated by the laser light.…”

“…The sampled air flows inside the chamber and particle is illuminated by the laser light. The wavelength of the light varies from about 400 to 980 nm (Hagan and Kroll, 2020). The light scattered by the particle is detected by a photodiode that generates a voltage pulse whose height is approximately proportional to the size of the particle.…”