Investigation of solid particle number measurement: Existence and nature of sub 23 nm particles under PMP methodology

A Particle Measurement Programme (PMP)-compliant system, an AVL advanced particle counter (APC) and an alternative volatile particle removal system, a catalytic stripper (CS) were evaluated and compared for measuring solid particle number (PN) emissions. The evaluations and comparisons were conducted using diluted exhaust from a diesel particle filter (DPF)-equipped heavy-duty diesel vehicle operated on a heavy-duty chassis dynamometer under steady speed conditions at two different engine loads. PN emissions between 3 and 10 nm downstream the APC were ∼ 2 and 7 times higher than the PN emissions of particles above 10 nm at the 74 and 26% engine load, respectively. At the 26% engine load, PN level of the 3 to 10 nm particles downstream the APC were significantly higher than that in the dilution tunnel, demonstrating that the APC was making 3 to 10 nm particles. The PN emission of 3 to 10 nm particles downstream the APC was related to the heating temperature of the APC evaporation tube, suggesting these particles are artifacts formed by renucleation of semivolatiles. Considerably fewer particles between 3 to 10 nm were seen downstream of the CS for both engine loads due mainly to removal of semivolatile material by the catalytic substrates, although some of this difference could be attributed to diffusion and thermophoretic losses. The findings of this study imply that improvement of the current PMP protocol would be necessary if the PMP were to be used in other applications where the PN emissions of particles below 23 nm are important.