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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 9, issue 6 | Copyright
Atmos. Meas. Tech., 9, 2709-2720, 2016
https://doi.org/10.5194/amt-9-2709-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 29 Jun 2016

Research article | 29 Jun 2016

A new high-transmission inlet for the Caltech nano-RDMA for size distribution measurements of sub-3 nm ions at ambient concentrations

Alessandro Franchin1, Andy Downard2, Juha Kangasluoma1, Tuomo Nieminen1,3,7, Katrianne Lehtipalo1,4,a, Gerhard Steiner5,6, Hanna E. Manninen1, Tuukka Petäjä1, Richard C. Flagan2, and Markku Kulmala1 Alessandro Franchin et al.
  • 1Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
  • 2Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA, USA
  • 3Helsinki Institute of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
  • 4Airmodus Ltd, Gustaf Hällströmin katu 2, 00560 Helsinki, Finland
  • 5Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria
  • 6Institute of Ion Physics and Applied Physics, University of Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria
  • 7Department of Applied Physics, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland
  • acurrently at: Paul Scherrer Institute, 5232 Villigen PSI, Switzerland

Abstract. Reliable and reproducible measurements of atmospheric aerosol particle number size distributions below 10nm require optimized classification instruments with high particle transmission efficiency. Almost all differential mobility analyzers (DMAs) have an unfavorable potential gradient at the outlet (e.g., long column, Vienna type) or at the inlet (nano-radial DMA), preventing them from achieving a good transmission efficiency for the smallest nanoparticles. We developed a new high-transmission inlet for the Caltech nano-radial DMA (nRDMA) that increases the transmission efficiency to 12% for ions as small as 1.3nm in Millikan–Fuchs mobility equivalent diameter, Dp (corresponding to 1.2 × 10−4m2V−1s−1 in electrical mobility). We successfully deployed the nRDMA, equipped with the new inlet, in chamber measurements, using a particle size magnifier (PSM) and as a booster a condensation particle counter (CPC). With this setup, we were able to measure size distributions of ions within a mobility range from 1.2 × 10−4 to 5.8 × 10−6m2V−1s−1. The system was modeled, tested in the laboratory and used to measure negative ions at ambient concentrations in the CLOUD (Cosmics Leaving Outdoor Droplets) 7 measurement campaign at CERN. We achieved a higher size resolution (R = 5.5 at Dp = 1.47nm) than techniques currently used in field measurements (e.g., Neutral cluster and Air Ion Spectrometer (NAIS), which has a R  ∼ 2 at largest sizes, and R  ∼ 1.8 at Dp = 1.5nm) and maintained a good total transmission efficiency (6.3% at Dp = 1.5nm) at moderate inlet and sheath airflows (2.5 and 30Lmin−1, respectively). In this paper, by measuring size distributions at high size resolution down to 1.3nm, we extend the limit of the current technology. The current setup is limited to ion measurements. However, we envision that future research focused on the charging mechanisms could extend the technique to measure neutral aerosol particles as well, so that it will be possible to measure size distributions of ambient aerosols from 1nm to 1µm.

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High transmission efficiency is key for classifying and counting atmospheric aerosol below 10 nm. We developed a new high­-transmission inlet for the Caltech nano-­radial DMA (nRDMA) and successfully deployed the nRDMA, equipped with the new inlet, in chamber measurements, using a particle size magnifier (PSM) and a booster CPC as a counter. With this setup, we were able to measure size distributions of ions between 1.3 and 6 nm in mobility diameter.
High transmission efficiency is key for classifying and counting atmospheric aerosol below...
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