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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.

Recently, a particle number (PN) limit was introduced in the European light-duty vehicles legislation. The legislation requires measurement of PN, and particulate mass (PM), from the full dilution tunnel with constant volume sampling (CVS). Furthermore, PN measurements will be introduced in the next stage of the European Heavy-Duty regulation. Heavy-duty engine certification can be done either from the CVS or from a partial flow dilution system (PFDS). For research and development purposes, though, measurements are often conducted from the raw exhaust, thereby avoiding the high installation costs of CVS and PFDS. Although for legislative measurements requirements exist regarding sampling and transport of the aerosol sample, such requirements do not necessarily apply for raw exhaust measurements. Thus, measurement differences are often observed depending on where in the experimental set up sampling occurs.