Table 4.

Summary of expected performance and recommendations for calibration materials in the use of low-cost optical particle sensors to measure the particulate mass loadings of different aerosol types.

			Sensor performance by OPS type*
Aerosol type	Aerosol properties	Suggested calibrant	Low-cost OPC	High-end OPC	Nephelometer
Fossil-fuel combustion	Very small GMD, mostly non-hygroscopic, moderate absorbing RI (Bond et al., 2002; Bond and Bergstrom, 2006; Chang et al., 2004)	Calibrate with aerosols closer in RI, such as from combustion sources	Will perform poorly due to the small GMD and absorption component of the aerosol	Will perform moderately well, though it will miss ultrafine particles	Can perform moderately well if calibrated using appropriate materials
Wildfire	Varying PSD, moderate absorbing component of RI (Laing et al., 2016; McMeeking, 2004)	Calibrate with aerosol of similar optical properties and GMD (ideally biomass smoke)	Will likely undersize and underestimate mass due to the absorbing component of the aerosol; the GMD will change with proximity to the source, leading to changes in accuracy	Will perform moderately well, though it may mis-size the particles as the properties of the aerosol change as the plume evolves	Can perform well under certain circumstances; moderate error should be expected as the GMD of the wildfire plume evolves
Urban	Varying GMD, moderate hygroscopicity (Hussein et al., 2004; Wehner and Wieden-sohler, 2003)	Calibrate with NIST urban aerosol or collected urban aerosol	Performance depends on uniformity of sources; large errors will occur as aerosol source (and GMD) changes	Will perform moderately well to well, though it will miss ultrafine particles	Will perform moderately well if averaged over a long period of time to normalize the GMD
Dust	Large GMD, non-hygroscopic (Petzold et al., 2009; Rocha-Lima et al., 2018)	Calibrate with Arizona road dust or collected dust	Likely to perform well given the large particle sizes	Likely to perform well given the large particle sizes	Likely to perform well

^*Based on properties of the aerosol only and not external environmental parameters (i.e., RH).