Calibration and Validation of Various Commercial Particle Number Measurement Systems 2009-01-1115

Measurement of particle number was introduced in the Euro 5/6 light duty vehicle emissions regulation. Although particle number measurement systems have to be calibrated by the manufacturers, labs have to validate the proper operation of their systems within one year of the emissions test. The systems must achieve a >99% reduction of an aerosol containing 30 nm tetracontane (CH₃(CH₂)₃₈CH₃) particles (C40) with an inlet concentration >10⁴ #/cm³. They must also include an initial heated dilution stage with dilution of at least 10 which outputs a diluted sample at a temperature of 150°C–400°C. The system as a whole must achieve a particle number concentration reduction factor for particles of 30 nm and 50 nm electrical mobility diameters, that is no more than 30% and 20% respectively higher, and no more than 5% lower than that for particles of 100 nm.

In this paper the terms dilution factor, penetration, losses, particle number concentration reduction factor and volatile removal efficiency are defined. Then, various commercial instruments, like the Solid Particle Counter (SPCS) from Horiba, the NanoMet-C from Matter Eng., the ejector diluter and the thermodenuder from Dekati are characterized. The penetrations, the particle number concentration reduction factors and the volatile removal efficiency of the above systems are discussed and compared with the legislation requirements. Finally, some calibration/validation issues are discussed, like the effect of the under-pressure downstream of the electrostatic classifier and the use of one or two particle number counters for the calibration. The importance of a stable at high temperatures aerosol is also pointed out.

It is also shown that the systems can be checked using polydisperse aerosol but still many parameters need to be considered (e.g. concentration range, pressure upstream and downstream of the systems). Till more experience is gained, labs are highly recommended to use simple gas dilution factors to check their systems.