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73 Instrument Considerations Given that nvPM mass emissions from modern combustion engines are now often near the limit of detection for practical instrumentation that measures mass, particle number concentration is being used to provide a more sensitive measurement of engine emissions. The AVL Advanced Particle Counter (APC) for aircraft applications is a real-time non-volatile particle number count- ing instrument that reports particle number concentration values in units of particles per cubic centimeter. By measuring solid particles, the dependence on the sampling system is reduced and the sample is more stable over time. To eliminate contributions of volatile particles, the device employs a two-stage dilution process coupled with a volatile particle remover (VPR). During first- stage dilution, dilution air heated to 150°C is added to the exhaust sample with a chopper diluter. Then the sample is transported to a catalytic stripper or VPR, maintained at 350°C. Material pre- sent as homogeneous volatile particles or volatile coatings on particles are eliminated by vaporiza- tion and subsequently oxidation, leaving only solid particles. After volatile removal in the catalytic stripper, second-stage dilution cools the sample before it enters a condensation particle counter (CPC). In the CPC, butanol is condensed on the particles, which causes them to grow, thereby enabling light scattering of a laser beam to be used to count particles. Several factors affect the value reported by the APC. Measurement of particles is always sub- ject to losses in the sample line. In the APC, significant losses also occur in the VPR, which is a catalytic stripper containing many small-diameter passages. The losses are measured in the laboratory as a function of particle size and reported for each individual instrument by the vendor. Typical losses in an AVL APC are ~50%. In addition to the losses, the reported number concentration depends on the counting efficiency curve of the CPC. The APC has a counting efficiency of â¥50% at 10 nm and â¥90% at 15 nm. A CPC with a different counting efficiency curve would report a different concentration. If particle concentrations are to be compared, it is important to understand the instrument cut offs, counting efficiency curve, and line losses associated with the measurements. Inlet/Sample Line Losses The particle instruments that perform measurements on samples extracted from a flow are dependent on the sampling system used to make the measurement. Particles, by definition, have a minimum size of nanometers and, therefore, do not have molecular properties. Unlike mol- ecules, particles do not necessarily follow the streamlines of a flow. Particle transport is suscep- tible to physical mechanisms that result in particle loss by changing the trajectory of the particle such that the particle deposits on the wall of the transport tubing. Except for thermophoresis, the particle losses are size dependent. Thermophoretic loss occurs when the temperature of the PM Line Losses A P P E N D I X I
