Effects of the test aerosol on the performance of periodic technical inspection particle counters

Abstract

A new scheme for periodical technical inspection (PTI) of diesel-engine exhaust based on the measurement of particle number (PN) concentration was recently introduced in several European countries. PN concentration is measured directly at the tailpipe using portable particle counters known as PN-PTI. The technical specifications of these instruments are defined at a national level, and the type-examination and verification procedures differ considerably among European countries. In this study, we examined how the choice of test aerosol can affect the counting efficiency profile of the PN-PTI sensors, the majority of which are based on diffusion charging (DC). Carbonaceous aerosols were produced by combustion and spark discharge generators, sodium chloride (NaCl) particles were generated from aqueous solutions using several commercially available nebulizers and silver particles were produced by a silver particle generator. In most cases, the response of DC-based sensors depended strongly on the morphology of the test particles. Comparison with a Euro 3 diesel engine without a diesel particle filter (DPF) indicated that the combustion aerosols generated by combustion generators were the best proxy for soot emitted by diesel vehicle engines. On the contrary, the use of NaCl and spark discharge particles as test aerosols led in some cases to an over- or underestimation of the PN concentration by up to about a factor of two. Moreover, our results showed that the response of each type of DC-based sensors is individual and, as a result, it is impossible to determine a generally applicable factor to correct for the impact of the test aerosol properties. We recommend that the verification of PN-PTI instruments be preferably performed with soot particles produced by a combustion generator (identical or similar to the one used during the type-examination) in order to avoid corrections for the impact of the material on the response of the instruments.