Design, simulation, and characterization of a radial opposed migration ion and aerosol classifier (ROMIAC)

Abstract

We present the design, simulation, and characterization of the radial opposed migration ion and aerosol classifier (ROMIAC), a compact differential electrical mobility classifier. We evaluate the performance of the ROMIAC using a combination of finite element modeling and experimental validation of two nearly identical instruments using tetra-alkyl ammonium halide mass standards and sodium chloride particles. Mobility and efficiency calibrations were performed over a wide range of particle diameters and flow rates to characterize ROMIAC performance under the range of anticipated operating conditions. The ROMIAC performs as designed, though performance deviates from that predicted using simplistic models of the instrument. The underlying causes of this nonideal behavior are found through finite element simulations that predict the performance of the ROMIAC with greater accuracy than the simplistic models. It is concluded that analytical performance models based on idealized geometries, flows, and fields should not be relied on to make accurate a priori predictions about instrumental behavior if the actual geometry or fields deviate from the ideal assumptions. However, if such deviations are accurately captured, finite element simulations have the potential to predict instrumental performance. The present prototype of the ROMIAC maintains its resolution over nearly three orders of magnitude in particle mobility, obtaining sub-20 nm particle size distributions in a compact package with relatively low flow rate operation requirements.

A review of transfer theory and characterization of measured performance for differential mobility analyzers

“…Building on the Knutson-Whitby analysis, Stolzenburg (1988) produced an approximate analytical model of the effect of particle diffusion within the classification region of the DMA. His model has been applied in a number of studies to characterize the performance of a DMA (Stolzenburg 1988;Zhang and Flagan 1996;Jiang et al 2011;Cai et al 2017;Stolzenburg et al 2018) as well as to the recently developed ROMIAC (Mui et al 2017). With the addition of empirically-determined adjustment factors for each of the above three key characteristics, these efforts have been significantly successful.…”

Characterization of a high-resolution supercritical differential mobility analyzer at reduced flow rates

“…However, the penetration efficiency of the Grimm S-DMA is much lower than the TSI nanoDMA 3085, the TSI DMA 3086, and the mini-cyDMA, e.g., 0.066 when classifying 1.48 nm molecular ions (Jiang et al 2011c). Other DMAs such as the Caltech nano-RDMA (Brunelli et al 2009) and other instruments such as the radial opposed migration ion and aerosol classifier (ROMIAC, Mui et al 2013;Mui et al 2017) are also capable of sizing sub-3 nm particles, yet their applications in atmospheric measurements have not been reported.…”

Charge distribution uncertainty in differential mobility analysis of aerosols

“…Mui et al (2017) also measured the transmission efficiency of high mobility ions in the ROMIAC; for the flow rate ratio in this work, the transmission efficiency of 1-2 nm ions ranges from 6-23 %.…”

“…The derivation of the OMAC transfer function was described in detail by Mai (2016) and is applied by Mui et al (2017) in a comprehensive characterization of the instrument using high mobility electrosprayed tetralkyl ammonium halide ions as small as 1.16 nm in mobility diameter. The ROMIAC transfer function, with explicit correction factors for the ROMIAC, is Ω = [S1]…”

“…HEPA-filtered, particle-free, room air was directed into the charger and the ions exiting the charger were segregated according to their electrical mobility using a Radial Opposed Migration Ion and Aerosol Classifier, ROMIAC (Mui et al 2013;Mui et al 2017), with a TSI 3068 electrometer used for detecting the ions. The measurement covered the mobility range from 0.1 cm 2 V -1 s -1 to 5.9 cm 2 V -1 s -1 .…”