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

Alessandro Franchin, Andy Downard, Juha Kangasluoma, Tuomo Nieminen, Katrianne Lehtipalo, Gerhard Steiner, Hanna E. Manninen, Tuukka Petäjä, Richard C. Flagan, Markku Kulmala
2016 Atmospheric Measurement Techniques  
<p><strong>Abstract.</strong> Reliable and reproducible measurements of atmospheric aerosol particle number size distributions below 10<span class="thinspace"></span>nm 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
more » ... 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<span class="thinspace"></span>% for ions as small as 1.3<span class="thinspace"></span>nm in Millikan–Fuchs mobility equivalent diameter, <i>D</i><sub>p</sub> (corresponding to 1.2<span class="thinspace"></span> × <span class="thinspace"></span>10<sup>−4</sup><span class="thinspace"></span>m<sup>2</sup><span class="thinspace"></span>V<sup>−1</sup><span class="thinspace"></span>s<sup>−1</sup> 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<span class="thinspace"></span> × <span class="thinspace"></span>10<sup>−4</sup> to 5.8<span class="thinspace"></span> × <span class="thinspace"></span>10<sup>−6</sup><span class="thinspace"></span>m<sup>2</sup><span class="thinspace"></span>V<sup>−1</sup><span class="thinspace"></span>s<sup>−1</sup>. 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 (<i>R</i><span class="thinspace"></span> = <span class="thinspace"></span>5.5 at <i>D</i><sub>p</sub><span class="thinspace"></span> = <span class="thinspace"></span>1.47<span class="thinspace"></span>nm) than techniques currently used in field measurements (e.g., Neutral cluster and Air Ion Spectrometer (NAIS), which has a <i>R</i> ∼ <span class="thinspace"></span>2 at largest sizes, and <i>R</i> ∼ <span class="thinspace"></span>1.8 at <i>D</i><sub>p</sub><span class="thinspace"></span> = <span class="thinspace"></span>1.5<span class="thinspace"></span>nm) and maintained a good total transmission efficiency (6.3<span class="thinspace"></span>% at <i>D</i><sub>p</sub><span class="thinspace"></span> = <span class="thinspace"></span>1.5<span class="thinspace"></span>nm) at moderate inlet and sheath airflows (2.5 and 30<span class="thinspace"></span>L<span class="thinspace"></span>min<sup>−1</sup>, respectively). In this paper, by measuring size distributions at high size resolution down to 1.3<span class="thinspace"></span>nm, 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 1<span class="thinspace"></span>nm to 1<span class="thinspace"></span>µm.</p>
doi:10.5194/amt-9-2709-2016 fatcat:3znxa4t73ja3fpixfyxkhdlbf4