10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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High Resolution Online Measurement of Aerodynamic Diameters Using the Differential Aerodynamic Particle Sizer (DAPS)
DENNIS KIESLER, Thore Rosenberger, Frank Einar Kruis, University Duisburg-Essen
Abstract Number: 1440 Working Group: Instrumentation
Abstract Introduction: The aerodynamic diameter is an important aerosol property to describe, among others, particle deposition as well as particle trajectories in aerosol instruments. The common method to determine this property for nanoaerosols is the measurement with cascade impactors (e.g.: ELPI+, Dekati Ltd; MOUDI-II, TSI Inc.), which have a fast response time but a limited size resolution due to their stage-based design. Instruments with a differential transfer characteristic, where the selected size can be shifted continuously, can obtain a similar high size resolution as the Differential Mobility Analyzer (DMA) for mobility measurements. This comes at the cost of time resolution due to the longer time needed to complete a scan. First instruments offering these for an aerodynamic characterization are the Aerodynamic Aerosol Classifier (AAC, Cambustion Ltd.) (Tavakoli et al., 2014) and the here presented Differential Aerodynamic Particle Sizer (DAPS) (Kiesler and Kruis, 2016, Huber et.al., 2016). Both have the advantage, that the classification is independent of particle charge. The Differential Aerodynamic Particle Sizer is based on a single-orifice aerodynamic lens. The obtainable aerodynamic diameter range is 25 nm to 5 µm. Due to the fast response of the aerodynamic lens, a measurement time of 60s for a full scan with 60 data points is possible.
Setup: The central part of the DAPS consists of an aerodynamic lens (Liu et al., 1995) with a single orifice. By using sheath gas, a well-defined radial starting position for the particles as well as a particle free central axis are established at the inlet. The orifice focuses a single particle size onto the central axis. These focused particles are then sampled. This gives the DAPS a differential transfer function (Kiesler and Kruis, 2012). The sheath to aerosol ratio a well as the size of the sampler determine the shape of the transfer function. By scanning the pressure in the system, the gas velocity is changed and thus the focused diameter.
This central particle classification part is combined with an unipolar corona charger in front of the inlet and a faraday cup electrometer after the sampling outlet to count the size selected particles. In combination with a pressure sweep, this allows the measurement of an aerodynamic size distribution.
Results: By using monodisperse particles, it is shown that the DAPS has a differential transfer function. A smaller aerosol to sheath ratio leads to a narrower transfer function. This can be shown in simulation and experiments. A resolution comparable to DMAs is possible. Also the importance of the correct sampling flow to obtain an optimized transfer behavior is shown in simulation and experiments.
Results are presented for two applications. One application is the study of particle behavior at a thin plate orifice at different pressures and flow rates as well as gas compositions. This helps to validate simulations of particle movement with high accelerations and can lead to optimized multi-orifice aerodynamic lens system used in vacuum transfer systems. Another application is the combined measurement of the aerodynamic diameter and the mobility diameter. This combination is a good tool to study agglomerate structure properties (Stein et al, 2013). The high resolution and fast scan time of the DAPS system are well suited for this task. This can be shown by measurements of agglomerates with different fractal dimension.
Acknowledgement: The research leading to these results has received funding from the Deutsche Forschungsgemeinschaft (DFG) in the framework of the joint research program “multi-parameter characterization of particle-based functional materials by innovative online measurement technology” (PAK688).
References: [1] Huber, F., Pitz, M., Kiesler, D. (2016), Partec 2016 - International Congress on Particle Technology (Proceedings), Nuremberg, 19.-21.04.2016, paper 4.34. [2] Kiesler, D. and Kruis, F.E. (2012) European Aerosol Confererence, Poster, Granada Spain. [3] Liu, P. Ziermann, P.J., Kittelson, D.B. and McMurry, P.H. (1995) Aerosol Sci. Technol. 22, 293. [4] Stein, M., Kiesler, D. and Kruis, F.E. (2013) Aerosol Sci. Technol., 47 (11), 276. [5] Tavakoli F., Symonds J.P.R., Olfert J.S. (2014) Aerosol Sci. Technol., 48(3),i.