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

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 MillikanFuchs mobility equivalent diameter, D p (corresponding to 1.2 × 10 −4 m 2 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 m 2 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 D p = 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 D p = 1.5 nm) and maintained a good total transmission efficiency (6.3 % at D p = 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.

Size-resolved online chemical analysis of nanoaerosol particles: a thermal desorption differential mobility analyzer coupled to a chemical ionization time-of-flight mass spectrometer

“…In addition, charging probability decreases with size. At 15 nm, the fraction of singly negatively charged particles is less than 0.07 (Fuchs, 1963;Tigges et al, 2015) for the soft X-ray charger used here. For an aerosol of multi component composition, it applies that the more particle mass is collected, the more substances overcome the lower detection limit and the more detailed the observed mass spectrum will be.…”

A miniature cylindrical differential mobility analyzer for sub-3 nm particle sizing

“…The classified aerosol flow channel consists of two sections (i.e., one at high voltage and the other at electrical ground) separated by an insulated section. Since the effective way to minimize particle electrostatic loss when transporting from high voltage to ground sections of the flow channel (Bezantakos et al, 2015;Tammet, 2015;Franchin et al, 2016) is not available, we simply reduce the length of the insulation section to reduce the time needed for classified particles to pass through.…”

A new balance formula to estimate new particle formation rate: reevaluating the effect of coagulation scavenging

“…Recent developments in diethylene glycol (DEG) condensation particle counters (CPC, Iida et al, 2009;Vanhanen et al, 2011) made it feasible to develop new scanning mobility particle spectrometers (SMPS) for extending aerosol size distribution measurement from ~3 nm down to ~1 nm (Jiang, et al, 2011a;Franchin et al, 2016). These new spectrometers were deployed in atmospheric observations (Jiang, et al, 2011b) and in chamber measurements (Franchin et al, 2016) to study NPF. A miniature cylindrical differential mobility analyser (mini-cyDMA, Cai et al, 2017) was developed to improve the performance of the DEG SMPS.…”