Laboratory evaluation of the Alphasense OPC-N3, and the Plantower PMS5003 and PMS6003 sensors

Ambient levels of particulate matter (PM) are linked to numerous adverse health effects. How-ever, fewer studies have evaluated the effects of coarse PM (larger than 2.5 mu m in diameter), and ambient measurements of coarse PM are particularly sparse. Although low-cost PM sensors have been used to complement regulatory measurements of PM2.5, many of these sensors, such as the Plantower PMS5003, are ineffective in measuring coarse PM. The Alphasense OPCs have shown promise in detecting coarse PM and have been used in the field to measure PM10 concentrations. These field evaluations have identified inter-sensor variability. Although field evaluation is critical for understanding sensor performance under environmentally relevant conditions, it provides limited information about sensor response characteristics, which are essential for determining the factors that may affect sensor measurements and contribute to inter-sensor variability. This study aims to understand these factors by conducting a size-selectivity study using monodisperse particles, and evaluating the effect of instrument-specific properties, like flow rate and laser strength, on the sensor-reported sizes and number counts. This study also evaluates a common low-cost sensor, the Plantower PMS5003, and a newer version, the PMS6003, for size selectivity. Monodisperse dioctyl sebacate particles of various diameters (2, 3, 5, 6, 9, and 10 mu m) were generated using a flow-focusing monodisperse aerosol generator, and the performance of nine different OPCs was evaluated. For all sizes tested, the nine OPC-N3s detected the particles, showed a peak near the target diameter, and exhibited some inter-sensor variability. The four PMS5003s and four PMS6003s detected all particle sizes but assigned all particles to the smallest size bin i.e., 0.35 mu m-1 mu m. With an aerodynamic particle sizer (APS) as a reference, the OPCs showed a positive bias for mean particle diameter and a coefficient of variance (CV) of less than 10%. For number concentration, the OPCs showed a negative bias, compared to the APS, and inter-sensor variability increased with the particle diameter. The laser wavelength of the OPC-N3s varied between 600 and 650 nm and appeared to have some effect on inter-sensor variability of the mean size. The flow rate reported by the OPC appeared to affect the inter-sensor variability in the number concentration.