On the Performance of a Bipolar Multi-Outlet Differential Mobility Analyzer (BiMoDMA)
SHIPENG KANG, Tongzhu Yu, Yixin Yang, Huaqiao Gui, Jianguo Liu, Da-Ren Chen, Virginia Commonwealth University
Abstract Number: 256
Working Group: Instrumentation and Methods
Abstract
A typical DMA with a bipolar particle charger measures particles charged in one polarity. Recent studies however revealed the charge fraction (in one polarity) of particles is subjected to the variation of relative humidity of carrier air. The same studies further observed that the sum of positive and negative charge fractions of particles is much less affected by the relative humidity (Li et al., 2022). To measure particles in both polarities at the same time, a Bipolar Multi-outlet Differential Mobility Analyzer (BiMoDMA) was designed, and its performance was experimentally evaluated in this study.
The plate-to-plate configuration is selected for BiMoDMA. It has one polydisperse particle inlet (located in the middle of spacing defined by two parallel plates) and four pairs of particle sampling outlets (located at both plates). The particle classification zone is defined by the space between two plates. The electric field in the classification zone is established by separately applying positive and negative high voltages on both plates. Once entering the classification zone, particles charged in one polarity are deflected toward the plate in high voltage with the opposite polarity. Four pairs of sampling outlets are used to extract classified particles out from the classification zone.
For the performance evaluation, the BiMoDMA transfer function was evaluated using a Tandem Differential Mobility Analyzer (TDMA) technique under the condition of difference mobility particle sizes, sheath flowrate and sampling schemes. A general method of piecewise transfer function inversion is introduced to retrieve the transfer function of BiMoDMA from the measured TDMA data. Two sampling strategies were studied: (1) equal sampling mode, i.e., the sampling flowrate for each port in a pair is the same as the polydisperse aerosol flowrate; and (2) cycling mode, i.e., the polydisperse aerosol flowrate is the sum of sampling flowrates of two ports in a pair. The detail design of BiMoDMA and results of this evaluation will be presented in this talk.