Comparison of mass-based and non-mass-based particle measurement systems for ultra-low emissions from automotive sources.

Drastic reduction in particle emissions of diesel-powered vehicles and new findings on the health impact of particles raise the question of a more sensitive measurement procedure. In this paper, 16 different particle mass measurement systems are compared on a diesel heavy-duty engine equipped with a particle filter to investigate their feasibility for particle characterization for future ultra-low concentration levels. The group of instruments comprises mass-related methods (filter methods, laser-induced incandescence, photoacoustic detection, photoelectric charging, combined inertial and mobility sizing, opacity) as well as non-mass-related methods (CPC, diffusion battery, diffusion charger, ELPI, light scattering). The instruments are compared on the basis of repeatability, limit of detection, sensitivity, time resolution and correlation with the regulated gravimetric filter method, and elemental carbon fraction (EC). Several time-resolved methods show good performance and give reliable results. Opacimeters and light scattering, however, reveal shortcomings at these low concentrations. For all time-resolved advanced methods, poor correlation with the regulated filter method is observed, but most of them show good correlation with the EC fraction of the particles. This outcome demonstrates the crucial role of the sampling conditions for measurement methods that do not exclude volatile material from detection. A clear improvement in sensitivity is observed when non-mass-based instruments are applied (e.g., number or surface-related methods). The results reveal that reliable measurement methods exist for future measurement procedures. However, a change in the measurement method will lead to a discontinuity in the inventories, making it difficult to compare the particle emissions from future and past vehicle generations.