Scanning DMA data analysis II. Integrated DMA-CPC instrument response and data inversion

Abstract

Analysis of scanning electrical mobility spectrometer (SEMS) or SMPS data requires coupling the scanning differential mobility analyzer (DMA) transfer function with the response functions for the instrument plumbing and the detector. In the limit of plug flow (uniform velocity) within the DMA, the scanning DMA transfer function has the same form as that for constant voltage. Most SEMS/SMPS data analysis uses this model, though previous studies have shown that boundary layers distort the transfer function during scanning DMA measurements. Part I determined the instantaneous transfer function during scanning of the TSI Model 3081 A long column DMA by modeling the flows, fields, and particle trajectories within the actual DMA geometry. This study (Part II) combines that transfer function with empirical data on the efficiencies and delay time distributions of the plumbing and detector of the SEMS/SMPS to determine the instantaneous rate at which particles are counted, and integrates the count rate over the finite counting time interval to obtain the integrated SEMS/SMPS response function. Simulations using this geometrical model are compared with those obtained using traditional, idealized DMA models for scan rates ranging from slow (240 s) to very fast (10 s), and with measurements of monodisperse calibration aerosols. Data inversion studies show that both increasing and decreasing voltage scans can be used to determine the particle size distribution, even with fast scans.