DIFFERENTIAL ELECTRICAL MOBILITY ANALYSIS - A THEORETICAL-STUDY

The subject of this work is the theoretical investigation of slowly sc anning differential mobility analyzers (DMAs) which are, e.g., utilize d to determine I)MA transfer functions and to measure particle mobilit y distributions. A model to describe such systems is introduced and ap plied to investigate three different regimes of input mobility distrib utions: 1) a mobility distribution much narrower than the DMA transfer function, 2) a mobility distribution of about the same width as the D MA transfer function, and 3) a mobility distribution much wider than t he DMA transfer function. Cases 1) and 2) are relevant for DMA transfe r function measurements utilizing tandem differential mobility analyze r (TDMA) systems. Far either regime, it is not possible to determine D MA transfer functions directly from the concentration distributions me asured at the outlet of a DMA. For these cases, a deconvolution proced ure is needed. Therefore, an iterative deconvolution procedure was dev eloped. Determining DMA transfer functions utilizing the developed dec onvolution procedure, different shapes of transfer function (triangula r, Gaussian) are discussed. Case 3) is relevant for particle size dist ribution measurements. Here, the mobility distribution upstream of the DMA can be obtained by dividing the concentration distribution measur ed downstream of a DMA by the DMA transfer function area. The DMA tran sfer function area is influenced by diffusional losses inside the DMA, and therefore is size;dependent. Neglecting this size dependence resu lts in an under prediction of particle number concentrations in the ul trafine particle size range. (C) 1997 American Association for Aerosol Research.