Site-Specific Calibration of Low-Cost Particulate Matter (PM) 2.5 Monitors in the United States: A Comparison of Industrial and Non-Industrial Communities

Abstract Particulate matter (PM) poses a significant public health concern, particularly in industrialized communities. While modern sophisticated air monitors like those deployed by the United States Environmental Protection Agency (US EPA) provide accurate 24-h PM2.5 (PM ≤ 2.5 µm) data, limited spatial and temporal coverage may create monitoring gaps. Low-cost monitors, such as a PurpleAir monitor (PA-II), offer an accessible alternative; however, their accuracy can be influenced by local environmental conditions and PM characteristics. This study evaluated the site-specific performance of PA-II in an industrial region within northern Lake County (N = 6) and a suburban region of Tippecanoe County (N = 4), both in Indiana, United States. As a reference measurement, a deployable particulate sampler (DPS) was co-located with each PA-II. For microscopic analysis, PM was collected on a glass substrate. PA-II data were assessed in raw form and following calibration using two methods, with performance assessed by root mean square error (RMSE) and the coefficient of determination (R2). Microscopic analysis revealed the presence of dark, irregular particles near industrial sites in northern Lake County, which may affect sensor readings by altering light-scattering efficiency. Raw PA-II data showed weak absolute agreement with DPS measurements in northern Lake County (RMSE: 2.04 µg/m3, R2: 0.25), while the US-wide calibration exhibited the best performance (RMSE: 1.40 µg/m3; R2: 0.67). In contrast, raw data showed the best performance in Tippecanoe County (RMSE: 0.62 µg/m3; R2: 0.94). These findings highlight the relevance of site-specific calibration approaches to enhance the accuracy of low-cost PM2.5 monitors, especially in environmentally impacted communities. Graphical Abstract