10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Calibration of Particle Number Concentration by Mobility-based Particle Size Distribution Instruments with Metrological Traceability
YOSHIKO MURASHIMA, Hiromu Sakurai, AIST
Abstract Number: 1166 Working Group: Instrumentation
Abstract Particle size distribution (PSD) measurements based on electrical mobility classification can accurately measure particle size and number concentration of fine particles and are used for various applications. Performance evaluation methods for them, however, have not been established, especially for particle number concentration. For conventional scanning mobility particle sizer (SMPS) spectrometers that consist of separable components accessible to the user, the performance can be evaluated to some extent by individually inspecting and/or calibrating the components such as the charger, classifier, and detector. On the other hand, the fast-response instruments such as TSI’s fast mobility particle sizer (FMPS) are packaged so that the integrated components are not separable for individual testing. The same can be said for recent compact, battery-powered portable instruments. For those instruments, techniques to evaluate the overall performance of a complete system as a whole are needed by treating each instrument as a “black box”. Such techniques would be also useful for testing conventional SMPS's in addition to the tests/calibrations of individual components. This method also allows evaluation of the performance of a charger in a conventional SMPS in terms of the extrinsic charging probability.
We propose a method that evaluates the overall performance of a particle size distribution measurement based on checking the accuracy of the total number concentration using monodisperse particles such as polystyrene latex (PSL). In this method, the total number concentration of monodisperse particles of the same aerosol is measured in parallel by a test PSD instrument (e.g., SMPS) and a reference CPC. The total number concentration that is determined by integrating the size distribution density function measured by the test PSD instrument (CN,test) is compared with the number concentration measured by the reference CPC (CN,ref). Monodisperse particles are generated by passing particles from an aerosol generator (e.g., electrospray) through a neutralizer and subsequently classifying them with a differential mobility analyzer (DMA). Then, after mixed well with a dilution air, particles are delivered to the test PSD instrument and reference CPC simultaneously. By using a reference CPC that has been calibrated with traceability to a national or international standard (e.g., AIST's primary standard), the test PSD instrument calibrated by this method will have metrological traceability to the standards with respect to particle number concentration.
We applied this method to two commercial instruments, which were a TSI SMPS model 3936L75 with an Am-241 neutralizer separately installed and without an impactor, and a TSI engine exhaust particle sizer (EEPS) spectrometer model 3090. Monodisperse PSL particles of 4 sizes (particle diameter: 30, 60, 100, and 200 nm) were used, and a total number concentration measured by the test PSD instruments was compared to the value by calibrated CPCs in this evaluation. We will report the results and discuss the uncertainties of the evaluation. We found that, while the information obtained in the tests was somewhat limited, the method was simple to perform and allowed size-resolved and quantitative evaluations of the performance of the test instruments, which would be useful for example in determining size-dependent correction factors and identifying malfunctioning components, even for a black-box instrument.