Variation of SMPS data in the Unsteady Particle Concentration Condition

Abstract

Scanning Mobility Particle Sizer (SMPS) is a useful instrument to measure the size distribution of aerosol and nano-particles. It is composed of a differential Mobility Analyzer (DMA) to classify particle sizes according to an exponential voltage change and a Condensation Particle Counter (CPC) to measure particle concentrations for the fractionated particle sizes. The SMPS usually requires about 120 s for a measurement. However, an uncertain particle-size distribution will be measured by the SMPS if the aerosol concentration is noticeably changed with time during scanning. Therefore, in the present study, we numerically analyze the SMPS measurement results for an unsteady aerosol concentration, and compare them with the experimental data. In order to predict the size distribution from the SMPS measurements in the unsteady particle concentration condition, we assume the followings. A. Geometric Mean Diameter (GMD) and Geometric Standard Deviation (GSD) of a virtual size distribution inputted into the SMPS program are constant during scanning. B. Only its overall concentration is changed, i.e. increased or decreased with time. C. The concentration change begins as soon as the SMPS scanning starts. D. The concentration is exponentially increased or decreased with time The SMPS data were obtained by putting the virtual distribution into the SMPS program. The SMPS measurement resulted in a false size distribution with a smaller GMD than the real GMD. This phenomenon is caused by the continuous decrement of concentration as the scanning time reaches its end. It implies that the GMD measured by the SMPS could differ a lot from the GMD of the real particle size distribution, if the concentration varies significantly during scanning.