Abstract.We have designed and characterized a new inlet and aerodynamic lens for the Aerodyne aerosol mass spectrometer (AMS) that transmits particles between 80 nm and more than 3 µm in vacuum aerodynamic diameter. The design of the inlet and lens was optimized with computational fluid dynamics (CFD) modeling of particle trajectories. Major changes include a redesigned critical orifice holder and valve assembly, addition of a relaxation chamber behind the critical orifice, and a higher lens operating pressure. The transmission efficiency of the new inlet and lens was characterized experimentally with size-selected particles. Experimental measurements are in good agreement with the calculated transmission efficiency.

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The experimental data and numerical modeling were utilized to investigate the effects of exhaust sampling parameters on the measurements of particulate matter (PM) emitted at the exit plane of gas-turbine engines. The results provide guidance for sampling system design and operation. Engine power level is the most critical factor that influences the size and quantity of black carbon soot particles emitted from gas-turbine engines and must be considered in sampling system design. The results of this investigation indicate that the available soot surface area significantly affects the amount of volatile gases that can condense onto soot particles. During exhaust particle measurements, a dilution gas is typically added to the sampled exhaust stream to suppress volatile particle formation in the sampling line. Modeling results indicate that the dilution gas should be introduced upstream before a critical location in the sampling line that corresponds to the onset of particle formation microphysics. Also, the dilution gas should be dry for maximum nucleation suppression. In most aircraft PM emissions measurements, the probe-rake systems are water cooled and the sampling line may be heated. Modeling results suggest that the water cooling of the probe tip should be limited to avoid overcooling the sampling line wall temperature and, thus, minimize additional particle formation in the sampling line. The experimental data show that heating the sampling lines will decrease black carbon and sulfate PM mass and increase organic PM mass reaching the instruments. Sampling line transmission losses may prevent some of the particles emitted at the engine exit plane from reaching the instruments, especially particles that are smaller in size. Modeling results suggest that homogeneous nucleation can occur in the engine exit plane sampling line. If newly nucleated particles, typically smaller than 10 nm, are indeed formed in the sampling line, sampling line particle losses provide a possible explanation, in addition to the application of dry diluent, that they are generally not observed in the PM emissions measurements.

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