On-Site Checks of the Particle Number Measurement Systems with Polydisperse Aerosol 2012-01-0873

Since 2011 a particle number (PN) limit was introduced in the European light-duty diesel vehicles legislation. The PN measurement systems consist of i) a hot diluter and an evaporation tube at 300-400°C for the removal of the volatiles (Volatile Particle Remover, VPR) and ii) a particle number counter (PNC) with a 50% cut-point (cut-off) at 23 nm. The PN measurement systems are calibrated and validated annually with monodisperse aerosol: The VPR for the particle concentration reduction factor (PCRF) and the PNC for the linearity and the cut-off size. However, there are concerns that the PN measurement systems can drift significantly over this period of time, raising concerns regarding the validity of the previous measurements, especially if the yearly validation fails.

In this paper we describe some relatively fast and simple on-site checks for the evaluation of the proper operation of the PN measurement systems with polydisperse aerosol, thus avoiding the need of electrostatic classifiers and radioactive sources. The necessary equipment are i) a particle generator that produces polydisperse aerosol with thermally stable particles of known count median diameter (CMD) at different concentrations and ii) a calibrated reference PNC with a nominal 50% cut-point at 23 nm and linear 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 the reference PNC polydisperse aerosol with CMD ≻50 nm at different concentrations. The differences, similarly with the legislation requirements, have to be within 10%. 2) Cut-off size check: Only the test PNC measures polydisperse aerosol with CMD=10 nm. The concentration measured by the test PNC should be ≺100 p/cm₃. If a PNC with a 50% cut-point at 10 nm or lower is available another possibility is to produce an aerosol with CMD=23 nm and compare the ratio of the test PNC to the reference PNC, which should be within the legislation requirements (0.38-0.62).

The recommended checks for the VPR are: 3) PCRF check of the VPR: The reference PNC measures upstream and downstream of the VPR (set to a low PCRF ≺1000) thermally stable aerosol with CMD around 50 nm. Similarly with the legislation requirements, the upstream-to-downstream ratio has to be within 10% of the PCRF setting. 4) Relative check of the rest PCRFs: The reference PNC measures downstream of the VPR and different PCRFs are measured. The PCRF corrected emissions for each PCRF measured should be within 10% of the previously checked low PCRF.

Theoretical calculations showed that these checks can identify a drift of the VPR or PNC and experimental data confirmed that they work in practice.