How Well Can mPEMS Measure Particulate Matter Motor Vehicle Exhaust Emissions? 2020-01-0391

Real world emissions are increasingly the standard of comparison for motor vehicle exhaust impact on the environment. The ability to collect such data has thus far relied primarily on full portable emissions measurement systems (PEMS) that are bulky, expensive, and time consuming to set up. The present work examines four compact, low cost, miniature PEMS (mPEMS) that offer the potential to expand our ability to record real world exhaust emissions over a larger number of operating conditions and combustion engine applications than currently possible within laboratory testing. It specifically addresses the particulate matter (PM) capabilities of these mPEMS, which employ three different methodologies for particle measurement: diffusion charger, optical scattering, and a multi-sensor approach that combines scattering, opacity, and ionization. Their performance is evaluated against solid particle number and PM mass with both vehicle tests and flame generated soot. These mPEMS detect PM emissions at < 1 mg/m³ and 10⁶ particles/cm³, levels that are consistent with modern vehicles meeting < 3 mg/mi and 6 x10¹¹ particles/km emissions standards. Difficulties remain in quantifying the emissions in terms of number or mass. The diffusion charger mPEMS measure charged particle electrical currents in very good agreement with more sophisticated laboratory instruments but lack the particle sizing capability necessary for accurate conversion to number or mass. Instead, they and the optical approaches, rely on calibration to a suitable engine PM or surrogate. Since this depends on the reference PM’s composition and morphology, the mPEMS are semi-quantitative in absolute accuracy. Nevertheless, they can play a valuable role in many PM emissions applications requiring good relative accuracy, high portability, low cost, and ease of use.