Sources of error and variability in particulate matter sensor network measurements

Abstract

The quality of mass concentration estimates from increasingly popular networks of low-cost particulate matter sensors depends on accurate conversion of sensor output (e.g., voltage) into gravimetric-equivalent mass concentration, typically using a calibration procedure. This study evaluates two important sources of variability that lead to error in estimating gravimetric-equivalent mass concentration: the temporal changes in sensor calibration and the spatial and temporal variability in gravimetric correction factors. A 40-node sensor network was deployed in a heavy vehicle manufacturing facility for 8 months. At a central location in the facility, particulate matter was continuously measured with three sensors of the network and a traditional, higher-cost photometer, determining the calibration slope and intercept needed to translate sensor output to photometric-equivalent mass concentration. Throughout the facility, during three intensive sampling campaigns, respirable mass concentrations were measured with gravimetric samplers and photometers to determine correction factors needed to adjust photometric-equivalent to gravimetric-equivalent mass concentration. Both field-determined sensor calibration slopes and intercepts were statistically different than those estimated in the laboratory (α = 0.05), emphasizing the importance of aerosol properties when converting voltage to photometric-equivalent mass concentration and the need for field calibration to determine slope. Evidence suggested the sensors’ weekly field calibration slope decreased and intercept increased, indicating the sensors were deteriorating over time. The mean correction factor in the cutting and shot blasting area (2.9) was substantially and statistically lower than that in the machining and welding area (4.6; p = 0.01). Therefore, different correction factors should be determined near different occupational processes to accurately estimate particle mass concentrations.

Keywords:

• Correction factor
• field calibration
• low-cost sensors
• particle composition
• particulate matter concentration; photometer

Additional information

Funding

This work was supported by the U.S. National Institute for Occupational Safety and Health under Grant R01 OH010533. C. Zuidema was supported by the Johns Hopkins University Education and Research Center for Occupational Safety and Health (ERC), which is funded by NIOSH under grant number T42 OH 008428, and the University of Washington’s Biostatistics, Epidemiology, and Bioinformatics Training in Environmental Heath (BEBTEH), grant number T32ES015459, from the National Institute for Environmental Health Science (NIEHS).