A DMA-Train for precision quantification of early nanoparticle growth

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.