Assessment of two condensation particle counters (CPCs) in photometric mode for high concentration exhaust emission measurements

Abstract

Condensation particle counters (CPCs) use light scattering to count particles after they have grown to micron size in a supersaturated environment. In single counting mode each particle is counted depending on whether the scattered light exceeds a threshold value or not. In photometric mode the total scattered light is converted in particle number concentration. While for laboratory grade particle number systems, CPCs are allowed to operate only in single counting mode, there is no such requirements for portable emissions measurements systems (PEMS) for real-driving emissions (RDE) testing or for instruments for periodic technical inspection (PTI) of vehicles. In this study two CPCs of the same model were assessed in single counting and photometric modes with silver and graphite particles with sizes ranging from 10 nm to 100 nm. The results showed that the concentration was measured accurately enough for particles in the 25 nm to 50 nm size range, but was underestimated and overestimated for smaller and larger particles, respectively. The key message is that the photometric mode should be avoided or calibrated in function of concentration and particle size.