Differences between tailpipe and dilution tunnel sub-23 nm nonvolatile (solid) particle number measurements

European Union vehicle and engine regulations require measurement of nonvolatile (solid) particles with diameter >23 nm at the dilution tunnel. In 2019 it was decided to include particles >10 nm in the post Euro 6/VI regulations. Recent studies showed that sub-23 nm measurements are not only susceptible to volatile artifacts (i.e., re-nucleation downstream of the evaporation tube of the particle number system) but also to nonvolatile artifacts (i.e., nonvolatile particles formed in the tubing between the vehicle and the particle number system or in the particle number system itself). In order to investigate the origin of the nonvolatile particle formation, steady-state tests with a moped, a compressed natural gas (CNG), and a diesel vehicle while regenerating were conducted. Systems at the tailpipe and the dilution tunnel with evaporation tubes or catalytic strippers and condensation particle counters (CPCs) with 50% detection efficiencies at 2.5 nm, 4 nm, 10 nm, and 23 nm were used. The results showed higher concentrations of sub-23 nm particles at the dilution tunnel than at the tailpipe when facility preconditioning was not appropriate, the exhaust gas temperature exceeded 300 degrees C, and high concentrations of semi-volatile material were emitted (e.g., regenerations, lubricant oil).