A DMA-Train for precision quantification of early nanoparticle growth

Description

New particle formation from ambient precursor gases is the largest source of cloud condensation nuclei (CCN) in the atmosphere (Spracklen et al., 2008). The probability of freshly nucleated particles to grow to CCN sizes strongly depends on the particle growth rates and the condensation sink. However, measurements of growth rates in the sub-10nm range are difficult to perform due to high particle losses and low detection efficiencies, especially below 3nm. Also time resolution of conventional SMPS limits the quantitative evaluation of growth rates (Winkler et al., 2012). Here we present the development of a Differential Mobility Analyzer - Train (DMA-Train) operating six DMAs (Grimm SDMA) in parallel for high time resolution quantification of nanoparticle growth rates down to 1.5 nm. To this end, each DMA channel is operated at a fixed voltage allowing precise measurement of the evolution of individual particle sizes. For the detection of classified particles we use five butanol based condensation particle counters (CPC) (TSI3776) and one water based CPC (TSI3788). For two sub-2 nm channels two Airmodus A10 particle size magnifiers (PSM) are used. Minimization of sampling losses is achieved by one total sampling line providing a high sampling flow for all channels. Subsequently, two X-ray chargers (TSI3088) bring the sample aerosol to a defined charging state. Afterwards the flow is split up into the six DMA channels and is then analyzed by either a PSM-CPC combination or by a CPC alone. This setup follows the classical scanning mobility particle sizer (SMPS) design but with six distinct channels. Therefore no voltage adjustment at the DMA is necessary during standard operation. This provides a much higher time resolution by avoiding voltage scanning and signal retention due to voltage changes. Furthermore, the data inversion procedure for the extraction of the spectral data is simplified and a full statistical approach is used to determine the growth rates, significantly reducing the measurement uncertainties. In our experimental approach, the SDMAs were first characterized with electrosprayed tetrahepthylammoniumbromide (THABr) (Ude and Fernández de la Mora, 2005) in order to determine penetration efficiencies as well as transfer functions following the principle of Jiang et al. (2011). For the THABr monomer at a mobility diameter of 1.47 nm we measured a transmission efficiency of ~ 9 % for the SDMA (see Figure 1) providing sufficient particle counts for statistical evaluation in the sub-3 nm size range. Second, a full instrument test was performed at the CLOUD Experiment at CERN (Kirkby et. al., 2011) during an instrument test campaign. First data of this test-run and data from an earlier prototype experiment show that the instrument is capable of determining growth rates in the desired range down to 1.5 nm. It should be noted that for the regular response time of the CPCs size distribution information can theoretically be retrieved at time resolutions of ~ 1 second if the particle concentration is high enough. However, at low concentrations the signal can still be interpreted statistically by averaging over time periods of several seconds. This allows us to gain information on particle evolution from individual counts although the total concentration in a certain channel might be less than one particle per cc. Remarkably, the lower size limit of 1.5 nm already overlaps with mass spectrometry measurements and therefore closes the gap between conventional particle counter measurements and mass spectrometry. This work was supported by the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement No. 616075.

Period	11 Sept 2015
Event title	European Aerosol Conference (EAC) 2015
Event type	Conference
Location	Mailand, ItalyShow on map