10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Differential Diffusion Analyzer
ANSSI ARFFMAN, Paxton Juuti, Juha Harra, Jorma Keskinen, Tampere University of Technology, Tampere, Finland
Abstract Number: 1330 Working Group: Instrumentation
Abstract In many areas of aerosol research the interesting particle size range, but at the same time one of the most challenging to study, is from molecules to a few nanometers. For example, atmospheric and anthropogenic nucleation studies investigate particle formation in the sub-3-nanometer size range (Kirkby, 2016; Alanen, 2015). Aerosol measurement instruments that are capable to operate in this size range include high flow rate DMAs, CPCs, and diffusion batteries. A high flow rate DMA connected in series with a low cut-off size CPC offers possibility to measure the particle size distribution down to 2 nm and even below. The major drawback of the DMA based size classification is that particles must be charged prior to the electrical size classification. The charging efficiency of sub-3-nm particles is very low in neutralizers or chargers and depends on many factors as shown by Leppä et al. (2017). This results large uncertainty in the concentration measurement, and furthermore, the charger ions lie in the same size range as the measured particles producing a measurement artefact. The CPC’s cut-off size can be varied in order to measure the cumulative size distribution (Vanhanen, 2011) but the detection efficiency is particle material dependent. Diffusion batteries are well-suited instruments to study the sub-10-nm particles and have been developed for in real-time measurement with electrical detection (Fierz, 2002). However, the diffusion stages heavily overlap in size, and thus, the data needs to be treated with advanced inversion methods.
We introduce a new size classifier called a differential diffusion analyzer (DDA; Arffman et al., 2017). This is the first instrument that uses diffusion to differentially size classify aerosol particles, keeping them airborne. The instrument has a flow arrangement similar to a DMA with a sheath and a sample flow, but in the size classification regime there is no electrical field, and particles drift across the sheath flow based on diffusion motion. Therefore, the particles entering the DDA sample outlet have been size selected based on their diffusion coefficient that has an unambiguous and a well-known connection to the mobility particle size via the Stokes-Einstein equation. The main advantage of the DDA is that there is no need for particle charging prior to the DDA, and in addition, it can be used to collect size classified particle samples down to molecular size.
We show theoretically and experimentally that the differential size classification based on the particle diffusion is possible. We studied theoretically a simple DDA geometry where the sample flow is sandwiched between sheath flows, and the sample out flow is drawn from the edges of the two sheath flows. Theoretical investigation results in a simple equation that can be used to roughly estimate the DDA penetrating particle size. With a numerical model we studied how the transmission efficiency function changes with the sheath an sample flow rate ratio and with the total flow rate. It was found that the maximum transmission efficiency of 20% could be achieved while still having a differential transmission function. Experimentally we studied the feasibility of the DDA concept with a cylindrical DMA (TSI model 3071A). With evaporation-condensation generated and monodisperse silver particles, we measured the transmission function of the DMA without electric field. Results verified that with proper sheath and sample flow rates, the transmission function has a differential shape. To further understand the operation, the particle transport inside the DMA with zero electric field was simulated with a CFD model of the DMA.
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