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Portable emission measurement systems (PEMS) for particle number (PN) counting are under development in Europe, along with the vehicle testing protocol. A PN PEMS was developed by using a non-heated exhaust diluter, and applying a diffusion charger sensor (DCS) as the PN detector which is fitted with diffusion screens in order to selectively remove all particles, including volatiles, below 30 nm. Detection efficiencies of the DCS could be successfully adjusted by the number of diffusion screens installed before it. Equivalent results of the PN PEMS to a conventional system were observed by vehicle tests. However, variations were observed under specific vehicle operating conditions. Also, as part of the same program, a commercially available hand-held condensation particle counter (CPC) was compared with the standard CPC by vehicle tests as one of candidates to PEMS. Differences in PN concentrations were observed depending on the engine conditions