Abstract
Abstract. Reliable and reproducible measurements of atmospheric aerosol particle number size distributions below 10 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 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 1.3 nm in Millikan–Fuchs mobility equivalent diameter, Dp (corresponding to 1.2 × 10−4 m2 V−1 s−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 × 10−6 m2 V−1 s−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 = 1.47 nm) 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 = 1.5 nm) and maintained a good total transmission efficiency (6.3 % at Dp = 1.5 nm) at moderate inlet and sheath airflows (2.5 and 30 L min−1, respectively). In this paper, by measuring size distributions at high size resolution down to 1.3 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 nm to 1 µm.
Highlights
Aerosol particles are an active subject of research because of their impact on climate (Paasonen et al, 2013; IPCC, 2013) and human health (Mauderly and Chow, 2008; Rohr and Wyzga, 2012), and because of their potential in the synthesis of new materials (Banin et al, 2013; Schauermann et al, 2012)
We present a successful application of the nano-radial differential mobility analyzer (DMA) (nRDMA), combined with a particle size magnifier (PSM) (Airmodus A09), to classify and measure ions, as used during the CLOUD (Cosmics Leaving Outdoor Droplets) 7 campaign in 2012
The nRDMA–PSM system was first deployed during the CLOUD 4 campaign for chamber experiments at CERN (Kirkby et al, 2011)
Summary
Aerosol particles are an active subject of research because of their impact on climate (Paasonen et al, 2013; IPCC, 2013) and human health (Mauderly and Chow, 2008; Rohr and Wyzga, 2012), and because of their potential in the synthesis of new materials (Banin et al, 2013; Schauermann et al, 2012). To tackle the low transmission, the University of Tartu developed low-resolution DMAs that have high inlet flow rates They have successfully used the Air Ion Spectrometer (AIS; Asmi et al, 2009; Gagn’e et al, 2011; Mirme and Mirme, 2013), the Balanced Mobility Scanning Analyzer (BSMA; Tammet, 2006) and the Symmetric Inclined Grid Mobility Analyzer (SIGMA; Tammet, 2011) to measure concentrations and size distributions of ambient ions. Jiang et al (2011b) developed a scanning mobility particle spectrometer (DEG SMPS) for measuring number size distributions of particles down to ∼ 1 nm mobility diameter Their DEG SMPS included an aerosol charger, a TSI 3085 nanoDMA, an ultrafine condensation particle counter (UCPC) using diethylene glycol (DEG) as the working fluid and a conventional butanol CPC to detect the small droplets leaving the DEG UCPC. During the CLOUD 7 campaign, a-pinene and sulfuric acid nucleation studies were carried out
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