On-Site Checks of the Particle Number Measurement Systems with Polydisperse Aerosol

Since 2011 a particle number (PN) limit was introduced in theEuropean light-duty diesel vehicles legislation. The PN measurementsystems consist of i) a hot diluter and an evaporation tube at300-400°C for the removal of the volatiles (Volatile ParticleRemover, VPR) and ii) a particle number counter (PNC) with a 50%cut-point (cut-off) at 23 nm. The PN measurement systems arecalibrated and validated annually with monodisperse aerosol: TheVPR for the particle concentration reduction factor (PCRF) and thePNC for the linearity and the cut-off size. However, there areconcerns that the PN measurement systems can drift significantlyover this period of time, raising concerns regarding the validityof the previous measurements, especially if the yearly validationfails. In this paper we describe some relatively fast and simpleon-site checks for the evaluation of the proper operation of the PNmeasurement systems with polydisperse aerosol, thus avoiding theneed of electrostatic classifiers and radioactive sources. Thenecessary equipment are i) a particle generator that producespolydisperse aerosol with thermally stable particles of known countmedian diameter (CMD) at different concentrations and ii) acalibrated reference PNC with a nominal 50% cut-point at 23 nm andlinear response at the whole measurement range. The recommended checks for the PNC under evaluation (test PNC)are: 1) Linearity check: The test PNC measures in parallel with thereference PNC polydisperse aerosol with CMD ≻50 nm at differentconcentrations. The differences, similarly with the legislationrequirements, have to be within 10%. 2) Cut-off size check: Onlythe test PNC measures polydisperse aerosol with CMD=10 nm. Theconcentration measured by the test PNC should be ≺100 p/cm₃. If aPNC with a 50% cut-point at 10 nm or lower is available anotherpossibility is to produce an aerosol with CMD=23 nm and compare theratio of the test PNC to the reference PNC, which should be withinthe legislation requirements (0.38-0.62). The recommended checks for the VPR are: 3) PCRF check of theVPR: The reference PNC measures upstream and downstream of the VPR(set to a low PCRF ≺1000) thermally stable aerosol with CMD around50 nm. Similarly with the legislation requirements, theupstream-to-downstream ratio has to be within 10% of the PCRFsetting. 4) Relative check of the rest PCRFs: The reference PNCmeasures downstream of the VPR and different PCRFs are measured.The PCRF corrected emissions for each PCRF measured should bewithin 10% of the previously checked low PCRF. Theoretical calculations showed that these checks can identify adrift of the VPR or PNC and experimental data confirmed that theywork in practice.