10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Study of HR-ELPI+ Data Inversion with Porous Collection Substrates
ANSSI ARFFMAN, Ari Ukkonen, Anssi Järvinen, Sampo Saari, Esa Luntta, Ville Niemelä, Dekati Ltd., Kangasala, Finland
Abstract Number: 1186 Working Group: Instrumentation
Abstract In this study, the effect of porous collection substrates on the data inversion algorithm of the HR-ELPI+ (Dekati Ltd.) was explored. The operation and verification of the inversion algorithm has been described and evaluated earlier for smooth collection plates (Saari, 2018). The porous collection substrates have been used in impactors to avoid particle bounce and to increase the loading capacity (e.g. Reischl & John, 1978; Van Gulijk, 2003; Tsai, 1995). The probability of particle bounce decreases and the loading capacity increases especially when oil is added to the porous collection substrate. The oil creates a sticky surface preventing the particle bounce, and the porous surface structure provides micro reservoirs for the oil, thus, keeping the impaction surface sticky even after several layers of particles has formed on a top of the collection substrate. The porous collection substrate also reshapes the impactor collection efficiency curve compared to a smooth or a plain surface. The cut size shifts to smaller size, the collection efficiency curve is less steep, and the fine particle losses will increase (see, e.g., Marjamäki, 2004). The increased losses influence to the ‘tail’ of the collection efficiency curve and also introduce the so-called excess collection efficiency in the sub-cut point particle size range as described in detail by Huang et al. (2001).
As stated earlier, the porous collection substrates decrease the cut points and reduce steepness of collection efficiency curves. Thus, it is important to evaluate how these changes affects to operation of inversion algorithm and results obtained when smooth collection plates are replaced with porous ones. The performance of HR-ELPI+ inversion was studied by numerical simulations and experimentally in lab conditions. The measured cut-sizes, collection efficiency curves, and fine particle losses for porous substrates were used in both studies.
In simulations, first, the theoretical current response of the ELPI+ for a log-normal size distribution was calculated and then the HR-ELPI+ inversion was used to calculate backwards the initial input size distribution. The results were then compared to the initial size distribution. In the experimental tests, the test aerosol generated from DEHS oil was measured with the HR-ELPI+ equipped with sintered metal substrates and the results were compared to the measurement results of the HR-ELPI+ with the normal smooth collection plates.
The simulation results showed that in the size range of 20 to 1000 nm the difference in number concentration between the inverted and the input concentration was 6% in maximum. For size distributions with GSD 1.3 or larger, the differences were smallest and the maximum differences were observed for narrow size distributions. The experimental tests showed that the similar accuracy can be achieved also in practice for evaporation-condensation method generated DEHS aerosols. Overall, results showed that the HR-ELPI+ inversion can be successfully used to retrieve the high resolution input size distribution although the porous metal collection substrates change the collection efficiency curves of the ELPI+ impactor in large extent.
[1] Chuen-Jinn Tsai & Yu-Hsiang Cheng (1995) Solid Particle Collection Characteristics on Impaction Surfaces of Different Designs, Aerosol Science & Technology 23:96-106. [2] Huang, C.-H., Tsai, C.-J., Shih, T.-S. (2001) Particle collection efficiency of an inertial impactor with porous metal substrates. Journal of Aerosol Science 32:1035-1044. [3] Marjamäki, M., Keskinen, J. (2004) Effect of impaction plate roughness and porosity on collection efficiency, Journal of Aerosol Science 35(3):301-308. [4] Reischl, G. P., W., John (1978) The collection efficiency of impaction surfaces: A new impaction surface. Staub Reinhalt Luft 38:55. [5] Saari S., Arffman, A., Harra, J., Rönkkö, T., Keskinen, J (2018) Performance evaluation of the HR-ELPI+ inversion. submitted to Aerosol Science and Technology. [6] van Gulijk, C., Marijnissen, J.C.M, Makkee, M., Moulijn, J.A (2003) Oil-soaked sintered impactors for the ELPI in diesel particulate measurements. Journal of Aerosol Science 34(5): 635-640.