Performance evaluation of a bipolar multi-outlet differential mobility analyzer (BiMoDMA)

Abstract

It has been evidenced that the accuracy of fine particle size distribution data collected by the DMA technique would be less prone to the uncertainty of particle charging ions by measuring electrical mobility of particles in both polarities instead of only measuring particles charged in one polarity (in the current practice). Unipolar DMAs are required to scan the DMA voltage from one polarity limit to the opposite polarity limit to fulfill the above measurement task. Bipolar DMAs are thus desired for reducing the voltage scanning time of unipolar DMAs. The measuring cycle of DMAs can be further reduced with the feature of multiple outlets (having different particle classification distances) when all the outlets are connected to individual particle concentration detectors. A Bipolar multi-outlet DMA (i.e., BiMoDMA) is thus designed. Although featured with multiple pairs of outlets, this study focused on the performance evaluation of BiMoDMA with only one outlet pair open. The prototype is in the plate-to-plate (or parallel-plate) configuration with a single aerosol inlet slit and four paired aerosol sampling outlet slits (labeled as A1B1-A4B4 from the pair at the nearest classification distance to that at the farthest distance from the aerosol inlet). For the voltage in the range of $50\sim 9,000V$, and sheath flowrate of $36L/\min$, this BiMoDMA can classify particles in the size ranges of $2\sim 30\text{nm}$, $4\sim 55\text{nm}$, $7\sim 104\text{nm}$, and $10\sim 155\text{nm}$ for the outlet pairs of A1B1, A2B2, A3B3 and A4B4, respectively. TDMA setup was applied to calibrate the sizing voltage of this BiMoDMA for a given particle electrical mobility, and to evaluate the DMA transfer functions at different sheath flowrates and particle sizes. It is found that the measured size-voltage relationship is in reasonable agreement with the general trend calculated by the 2D DMA model. The correction factor was introduced to better correlate calculated voltage with measured data. The half-height width and area of BiMoDMA transfer functions decreased with the increase of sheath flowrate, while the height and area of transfer functions increased with the increase of particle size.