A DMA-Train for precision quantification of early nanoparticle growth

Authors/others:Stolzenburg, Dominik; Winkler, Paul
Abstract:New particle formation from ambient precursor gases isthe largest source of cloud condensation nuclei (CCN) inthe atmosphere (Spracklen et al., 2008). The probabilityof freshly nucleated particles to grow to CCN sizesstrongly depends on the particle growth rates and thecondensation sink. However, measurements of growthrates in the sub-10nm range are difficult to perform dueto high particle losses and low detection efficiencies,especially below 3nm. Also time resolution ofconventional SMPS limits the quantitative evaluation ofgrowth rates (Winkler et al., 2012).Here we present the development of a DifferentialMobility Analyzer - Train (DMA-Train) operating sixDMAs (Grimm SDMA) in parallel for high timeresolution quantification of nanoparticle growth ratesdown to 1.5 nm. To this end, each DMA channel isoperated at a fixed voltage allowing precisemeasurement of the evolution of individual particlesizes. For the detection of classified particles we use fivebutanol based condensation particle counters (CPC)(TSI3776) and one water based CPC (TSI3788). For twosub-2 nm channels two Airmodus A10 particle sizemagnifiers (PSM) are used.Minimization of sampling losses is achieved byone total sampling line providing a high sampling flowfor all channels. Subsequently, two X-ray chargers(TSI3088) bring the sample aerosol to a defined chargingstate. Afterwards the flow is split up into the six DMAchannels and is then analyzed by either a PSM-CPCcombination or by a CPC alone. This setup follows theclassical scanning mobility particle sizer (SMPS) designbut with six distinct channels. Therefore no voltageadjustment at the DMA is necessary during standardoperation. This provides a much higher time resolutionby avoiding voltage scanning and signal retention due tovoltage changes. Furthermore, the data inversionprocedure for the extraction of the spectral data issimplified and a full statistical approach is used todetermine the growth rates, significantly reducing themeasurement uncertainties.In our experimental approach, the SDMAs werefirst characterized with electrosprayedtetrahepthylammoniumbromide (THABr) (Ude andFernández de la Mora, 2005) in order to determinepenetration efficiencies as well as transfer functionsfollowing the principle of Jiang et al. (2011). For theTHABr monomer at a mobility diameter of 1.47 nm wemeasured a transmission efficiency of ~ 9 % for theSDMA (see Figure 1) providing sufficient particlecounts for statistical evaluation in the sub-3 nm sizerange. Second, a full instrument test was performed atthe CLOUD Experiment at CERN (Kirkby et. al., 2011)during an instrument test campaign.First data of this test-run and data from an earlierprototype experiment show that the instrument is capableof determining growth rates in the desired range down to1.5 nm. It should be noted that for the regular responsetime of the CPCs size distribution information cantheoretically be retrieved at time resolutions of ~ 1second if the particle concentration is high enough.However, at low concentrations the signal can still beinterpreted statistically by averaging over time periods ofseveral seconds. This allows us to gain information onparticle evolution from individual counts although thetotal concentration in a certain channel might be lessthan one particle per cc. Remarkably, the lower size limitof 1.5 nm already overlaps with mass spectrometrymeasurements and therefore closes the gap betweenconventional particle counter measurements and massspectrometry.This work was supported by the European ResearchCouncil under the European Community's SeventhFramework Programme (FP7/2007-2013) / ERC grantagreement No. 616075.
Date of publication:11.9.2015
Publication Type:Working paper