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.