Scanning Electrical Mobility Methods for Aerosol Characterization

Abstract

The scanning electrical mobility measurement is the most common tool used to characterize the size distribution of fine particles in the atmosphere. This thesis develops the methods for retrieving the particle size distribution from scanning electrical mobility measurement data for two systems: (1) the scanning electrical mobility spectrometer (SEMS; also known as the scanning mobility particle sizer, SMPS), which measures particle size distribution ranging from 15 - 1000 nm; (2) the scanning radial opposed migration ion and aerosol classifier (ROMIAC) system, which uses a two-stage condensation particle counter as particle detector to complete the 1 - 20 nm particle size distribution measurements. SEMS / SMPS data have traditionally been inverted to determine the particle size distribution by solving a Fredholm integral equation in which the kernel function is based upon constant-voltage operation of the mobility classifier. The viscous boundary layer within the classifier renders that model invalid. This thesis determines, for the first time, the transfer function for a real differential mobility analyzer (DMA) that is operated in the scanning mode. The flow and electric fields within the instrument were obtained by finite element simulations taking into account its detailed geometry. Brownian dynamics simulations were then used to simulate diffusive particle trajectories within the instrument as the voltage was scanned. There results were coupled with empirically-derived response-time functions for the condensation particle counter that serves as a detector in the SEMS / SMPS to obtain integrated system transfer function that substantially improve the fidelity of the SEMS / SMPS data inversion. This approach was also applied to adaptation of the radial opposed migration ion/aerosol classifier (ROMIAC) for scanning-mode operation. The transfer function obtained through simulation of the scanning ROMIAC was used in the experimental validation of this new measurement method. This new instrument was then used to measure wall loss rates for 1.6 nm to 20 nm particles in the Caltech environmental chamber.