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

Particle transfer theory for steady-state differential mobility analyzers (DMAs) with and without diffusion is reviewed in detail with a particular focus on the assumptions and approximations made in the analysis. Impacts of the approximations are discussed and, where available, methods to reduce the errors of these approximations are suggested. The nondiffusing theory uses just one approximation, affecting the centroid calculation, which can be readily addressed via numerical modeling of the electric field. The diffusing theory makes numerous approximations to achieve an analytical expression. One of the most serious of these, neglecting secondary flows in the vicinities of the aerosol entrance and exit slits, could be improved upon using a numerical model of the flow field. Losses in the aerosol entrance plumbing can perturb the inlet profile to the classification region. The maximum effects on the transfer function are estimated to be a 1% increase in the mean mobility and a 14% reduction of the nondiffusing contribution to the variance. Methods of fitting transfer theory to measurements are also reviewed. Tandem differential mobility analyzer measurements generally do not have the resolving power to distinguish different shapes of the transfer function but newer measurements using truly monomobile ions have the potential to more rigorously test the diffusive transfer model. In adjusting the width of the theoretical transfer function to fit measurements from a real DMA demonstrating nonideal performance, it is physically more meaningful and accurate to use an additive adjustment to the variance as opposed to a multiplicative adjustment to the width. Copyright (c) 2018 American Association for Aerosol Research

本文详细综述了考虑与不考虑扩散效应的稳态差分迁移率分析仪（DMA, Differential Mobility Analyzer）颗粒传输理论，重点梳理了分析过程中所采用的各项假设与近似处理手段。本文还讨论了各类近似处理手段带来的影响，并在可行的前提下提出了降低此类近似误差的改进方法。无扩散传输理论仅采用了一项近似，该近似会影响颗粒迁移质心的计算，但可通过电场数值建模的方式便捷修正。扩散传输理论为获取解析表达式，采用了多项近似处理手段，其中最为关键的一项近似是忽略气溶胶进出口狭缝附近的二次流，该问题可通过流场数值建模得到改善。气溶胶入口管路的颗粒损耗会干扰分类区域的入口流场分布，该效应对传输函数的最大影响估算为：平均迁移率提升1%，且无扩散项对方差的贡献降低14%。本文同时综述了将传输理论与实测数据拟合的相关方法。串联差分迁移率分析仪（TDMA, Tandem Differential Mobility Analyzer）的测量结果通常无法区分传输函数的不同形态，但采用纯单迁移率离子的新型测量手段则具备更严谨地验证扩散传输模型的潜力。当针对实际非理想工况的DMA实测数据调整理论传输函数宽度时，相较于对宽度进行乘性修正，对传输方差进行加性修正在物理意义上更为合理且结果更准确。© 2018 美国气溶胶研究协会