Introduction
Aerosol particles are an active subject of research because of
their impact on climate and human health , and because of their potential in the synthesis of
new materials .
Recent technological developments have allowed measurements of smaller and
smaller particles e.g.,. One of the driving forces for these developments
has been the need to understand atmospheric aerosol formation
, a
process that has been identified as one of the most important sources of
aerosol particles in the atmosphere .
The crucial size range in understanding the details of aerosol formation is
from 1 to 3 nm. This is the size range of the new clusters, either neutral or
charged, that are formed by the collision of precursor vapor molecules such
as sulfuric acid , ammonia ,
amines and organic
precursors . The small
size of these clusters poses a challenge for their measurement and detection,
as they are easily lost by diffusion during sampling and are difficult to
charge for electrical classification. On the one hand, their low
concentrations hinder the use of an electrometer as the detector. On the
other hand, when using condensational techniques for detection, a very high
supersaturation is required to grow the particles large enough for optical
detection.
A combination of condensation techniques and electrical mobility
classification has proven to be an effective method for atmospheric aerosol
number distribution measurements . In practice, the number size distribution can be measured
using a differential mobility analyzer (DMA), which classifies the aerosol by
mobility, and a condensation particle counter (CPC), which determines the
number concentration. The combination of DMA and CPC is termed differential
mobility particle sizer (DMPS) or scanning mobility particle sizer (SMPS),
depending on whether the classifying high voltages are stepped or ramped,
respectively. When sampling sub-3 nm atmospheric particles, ultrafine CPCs do
not have high enough detection efficiency to activate and count such small
particles, and conventional DMAs do not have adequate transmission efficiency
nor the high enough size resolution required to analyze the very first steps
of nucleation in the atmosphere.
The DMA measurement range has recently been extended down to 1 nm through
design of new high-resolution DMAs
. By coupling these
high-resolution DMAs with highly sensitive detectors (particle counters or mass
spectrometers), even nanometer-sized molecular clusters can be classified and
characterized. Unfortunately, only a tiny fraction of the ions or charged
particles entering these instruments reach the detector. This low
transmission efficiency limits their use for direct measurement of
atmospheric ions. 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;, the Balanced Mobility
Scanning Analyzer BSMA; and the Symmetric
Inclined Grid Mobility Analyzer SIGMA; to
measure concentrations and size distributions of ambient ions.
These instruments have extremely high inlet flow rates (from 54 to thousands of liters per
minute, L min-1), which enable high ion transmission efficiencies (greater than 80 %) but limit
their resolution to R < 2, where R is defined as the ratio of the mobility of the ion/particle that
is transmitted with the greatest efficiency to the range of mobilities for which the transmission efficiency
is at least half of that value. Unfortunately, such extreme flow rates also limit the usefulness of these
instruments in the laboratory or in chamber studies.
With DMA measurements extended into the sub-3 nm size range, commercially available CPCs were no
longer capable of detecting the size classified particles selected by the DMA. overcame the
inability of the CPC to detect particles smaller than 2.5 nm by introducing a two-stage CPC. Particles are first
activated using a high-surface-tension, low-vapor-pressure working fluid that minimizes the risk of homogeneous
nucleation at the high supersaturations required to activate 1 nm particles. The activated droplets are then
grown to detectable size using a second, “booster” CPC. modified a commercial, ultrafine
CPC for the first stage, removing the optical detector and feeding the activated particles to a conventional,
butanol CPC. Several groups have followed that design , producing instruments
that have probed new particle formation to sizes close to that of the critical cluster. However, the low
sample flow rates typical of this design lead to poor counting statistics when measuring low concentrations
of particles using the CPC as a detector behind a DMA. – inspired by the work
of , and – developed a two-stage
CPC that counts particles in a much larger flow by replacing the laminar flow design of the traditional instruments
with a mixing-type activation stage. Moreover, this particle size magnifier (PSM) can rapidly step through a number
of supersaturated states and thereby measure the activated particles. These data are used to estimate the size
distribution of particles in the 1 to 3 nm size range, albeit with significant sensitivity to particle
composition . A considerable step forward would
be a combination of a PSM with a high-transmission DMA. Such an instrument would enable reliable sizing
that is able to count single particles.
The Caltech nano-radial DMA nRDMA; is a nano-DMA known for its compact design and for its
high resolution at relatively low sheath gas flows. Previous studies
showed that the nRDMA has a higher resolving
power and a better transmission than other DMA designs operating in the
range of 1 to 100 nm. Accordingly, the device shows promise to extend the
accessible measurement range for DMAs into the sub-3 nm region. DMAs could
provide a measurement technique that would allow a continuous, high-resolution
overview of the size distributions, from molecular clusters to
aerosol particles.
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 nano-DMA, 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. successfully
deployed their DEG SMPS in the field in Atlanta,
Georgia, USA, where the concentrations of sub-3 nm particles exceeded 104 cm-3.
This paper builds upon that foundation in order to measure both charged and
neutral aerosol particles in the same size range, but at concentrations lower
than 104 cm-3. Measurements of neutral aerosol clusters are not
trivial, as standard chargers (for example, radioactive bipolar charger or
corona chargers) provide a spectrum of charger ions that goes up to
almost 5 × 10-5 m2 V-1 s-1, ∼ 2 nm in Millikan–Fuchs
mobility equivalent diameter . The charger ions change with air composition, and their
size distribution overlaps with that of the aerosol particles that we want to
measure. For this reason, our study is limited to ions.
This work continues the work done in the past on preventing the edge
disturbances in air ion measurements . Here, we introduce and validate
a new inlet design that increases the transmission efficiency from a few
percent to about 12 % for particles with a mobility diameter of 1.5 nm (9.7 × 10-5 m2 V-1 s-1 in mobility). The high transmission
efficiency achieved makes it possible to use the instrument to measure ions
at ambient concentration down to 1.3 nm
(1.2 × 10-4 m2 V-1 s-1 in mobility). All diameters
reported here are Millikan–Fuchs equivalent mobility diameters
. As the diameter is not a well-defined concept
at very small sizes, we choose to indicate next to the diameter the particle
electrical mobility, which is the measured quantity. We also present a
successful application of the nRDMA, combined with a PSM (Airmodus A09),
to classify and measure ions, as used during the CLOUD (Cosmics Leaving Outdoor Droplets) 7 campaign in 2012.
The CLOUD aerosol chamber is located at
CERN, in Switzerland. It has been used for several aerosol nucleation studies
.
During the CLOUD 7 campaign, a-pinene and sulfuric acid nucleation studies
were carried out.
Experimental setup
nRDMA description
The nRDMA design extends the RDMA design of
to allow classification of particles in the low
nanometer regime . The version of the nRDMA used
in this study is the new version of the DMA used in
. The nRDMA used in this study is a refined version
of the first nRDMA. The main difference was an increased diameter in the
exhaust pipe line to allow larger sheath flow rates to be used without
causing turbulence. Here, we give just a brief description. To visualize the
radial design, start from a vertical section of a planar DMA (Fig. a). If we rotate the section of the planar DMA around the axis
along and parallel to the lower electrode, we obtain a classical cylindrical
DMA (Fig. b). If we rotate the section along the vertical axis,
perpendicular to the electrodes, we obtain a radial DMA (Fig. c). The
nRDMA consists of two parallel circular plates, one at the top and one at the
bottom. These two plates form the electrodes of the classification region.
The sheath airflow moves radially from the outside to the center, through
the space between the two plates, and it exits from a central hole in the
circular plate at the top. The polydisperse aerosol sample enters the top
plate tangentially to its outer circumference and reaches the classification
region through a circular peripheral slit. After the aerosol enters the
classification region, it is driven toward the bottom plate by an electric
field. Two factors determine the particle's trajectory: the radial
velocity, determined by sheath flow, and the perpendicular velocity,
determined by the electric field on the charged particle of mobility Z.
Classified particles (those within a narrow range of mobilities around the
target mobility of Z) are extracted through an outlet port, which is located
at the center of the bottom plate. The classified aerosol is then counted by
an electrometer or a CPC. In order to produce the electric field within the
nRDMA, the top inlet plate is maintained at high voltage (positive for
positively charged particles, negative for negative ones), while the bottom
plate is electrically grounded. The inlet flow plumbing that brings the
aerosol sample into the DMA is also grounded, introducing an adverse electric
field that charged particles must traverse as they enter the DMA. This
adverse gradient limits the transmission efficiency. This paper describes the
development of a new, high-transmission-efficiency inlet to reduce the impact
of this adverse gradient.
(a) Section of a schematic design of a planar DMA. The
sheath gas flow (Qsh) enters from the left. The polydisperse
aerosol flow (Qae) enters the classification region from a slit at
the top left. The classified aerosol (Qcl) exits from a slit at the
bottom right, and the excess flow (Qexc) is discarded at the right,
along with the unclassified aerosol. (b) Section of a schematic
design of a cylindrical DMA, obtained by turning the section of the planar
DMA along its horizontal axis, along the bottom electrode.
(c) Section of a schematic design of a radial DMA, obtained by
turning the section of the planar DMA along its vertical axis. Qsh
flows radially from the periphery of the classification region, towards the
center. Qae enters the classification region from a tangential slit
at the top. Qcl exits from the outlet located at the bottom plate.
Qexc is discarded, with the unclassified aerosol, through the
outlet at the center of the top electrode.
High-transmission inlet description
As noted above, the need to transition from the grounded sampling line to the
high voltage of the inlet creates an adverse gradient that causes charged
particles to migrate along the electric field lines and deposit before they
enter the classification region. This problem, also known as edge effect
, is not unique to the
nRDMA; described electrostatic losses in the
outlet of the TSI DMA and reduced those losses by lining the Teflon
insulator at the outlet of the instrument with brass.
applied scaling rules to characterize similar
losses at the outlet of the original RDMA. The adverse gradient is a problem
that remains inherent to DMAs. While we cannot eliminate it, the present
paper describes a novel inlet design that dramatically weakens it. This high-transmission
inlet design consists of two parts: a voltage divider and a core
sampling probe (Fig. b). The voltage divider consists of a set of
electrostatic lenses (made of stainless steel) with an inner diameter of
6.35 mm (1/4 in.), an outer diameter of 25.4 mm (1 in.)
and a width of 2 mm. The lenses are alternatively stacked with electrically
insulating spacer discs (Delrin®), to form a tube with a total
length of 50 mm. Each of the electrostatic lenses is connected to its
neighboring lens by a 1M Ω resistor. The lens that is furthest away
from the nRDMA is connected to ground potential, while the one next to the
nRDMA inlet is at the same potential as the top plate of the nRDMA.
Sketch of the nRDMA inlet. (a) shows the original inlet;
(b) shows the high-transmission inlet. Moving from right to left,
inlet flow enters from the voltage divider. Thanks to the inlet design, the
voltage increases gradually, from ground potential to the potential of the
top plate of the nRDMA. In this way, the gradient of the electric potential
is decreased so that the drift velocity of the ions, due to the electric
field, is small compared to the flow velocity. Before reaching the top plate
of the nRDMA, the portion of the inlet flow closer to the walls is discarded,
and only the core of the total inlet flow enters the classification region.
The efficacy of the reduced gradient is enhanced by increasing the velocity
of the flow that enters the inlet assembly, increasing the velocity such that
the induced migration away from the nRDMA is overwhelmed.
The voltage divider is used to minimize the gradient of the electric potential so that the
drift velocity of the ions, due to the electric field, is small compared to the flow velocity,
resulting in an improved overall ion transmission efficiency. A core sampling probe withdraws
the sample from the center of the voltage divider, away from inhomogeneities of the electric
field at the walls. The probe consists of a small tube at the end of the voltage divider,
sampling a flow of 2.5 L min-1 from the center of the 6.35 mm inlet tube, which carries the
total inlet flow (9 L min-1). The remaining airflow is discarded.
Electrometer and particle size magnifier
Two detectors were used with the nRDMA to measure the concentration of the
classified aerosol: an electrometer (TSI 3068) and a PSM (Airmodus A09)
together with a booster CPC. We used the electrometer in the electrospray
experiments, as the generated ion concentrations were high enough (> 104 cm-3) to generate a clear signal. In fact, for the electrometer in use,
the minimum detectable current I is around 1 fA. If the electrometer is
operated at 1 L min-1 inlet flow rate (F), and the ions carry only one single
charge e, then the corresponding concentration of ions N=IFe= 374 cm-3. Accounting for the nRDMA's ion transmission efficiency of
about 15 %, the ion (or charged particle) concentration of a given mobility
must be larger than 6000 cm-3. Thus, the electrometer is not suitable
for ambient measurements where the total ion concentration is on the order
of 1000 cm-3. However, the PSM has the required sensitivity, because of
its high counting efficiency ([45–65] % at 1.3 nm).
For measurements at low concentrations, it is important to carefully tune the
operation temperatures of the PSM to achieve the lowest possible background
counts and to reach the highest particle detection efficiencies. The
supersaturation inside the PSM needs to be set close to that at which
homogeneous nucleation would occur, in order to push the limit of detection
to the lowest possible size. Nonetheless, when operated at optimal
conditions, the PSM's background counts are low (only 1–2 counts min-1), corresponding to a background concentration of about 10 cm-3.
The PSM counting efficiency and low background signal allow measurement of
concentrations as low as 60 ions cm-3. When neutral particles are
measured, an aerosol charger must be added to the setup, which, in the case of a
bipolar charger being used, reduces the signal intensity by about 99.5 %, due to
the low charging probability at particle sizes < 3 nm
. Thus, even when using a PSM, these
measurements remain extremely challenging, unless particle concentrations are
on the order of several tens of thousands cm-3.
Spectrum of negative ions in the CLOUD aerosol chamber during a
nucleation event. The spectrum was measured during the CLOUD 4 campaign,
using the nRDMA–PSM system with the old inlet (Fig. a). It should
be noted that it was not possible to detect particles below about 2.5 nm
(dashed magenta line). The gap in the color plot between 05:30 and 06:00 UTC is due
to the electric field inside the CLOUD chamber being switched on, causing all
the ions in the chamber to be swept out.
Results and discussion
The nRDMA–PSM system was first deployed during the CLOUD 4 campaign for
chamber experiments at CERN . During the CLOUD 4 campaign, the nRDMA–PSM system was not able to detect particles below 2.5 nm
(Fig. ). To address this unexpected lack of transmission, a series
of laboratory experiments was carried out: electrospray experiments, transfer
function determination and field measurements. The electrospray experiments
were carried out to characterize the inlet region and were used to derive an
inlet design to maximize ion transmission. The transfer function
determination was carried out to characterize the DMA's transmission
efficiency and resolving power. The field measurements were carried out to
prove that it is possible to measure ion-mobility spectra at ambient
concentrations. The nRDMA–PSM system with the new inlet design was used
during the CLOUD 7 campaign, improving the detection efficiency of ions from
2.5 to 1.3 nm.
Electrospray experiments
Electrospray experiments were carried out to investigate the performance of
the inlet region of the nRDMA. These results were used to collect information
for the design of the high-transmission-efficiency inlet. For These tests, we
used an electrometer (TSI 3068A) to detect the classified ions and a
custom-built electrospray source (ES) to generate Tetraheptylammonium ions
(THAB) as mobility standards . The nRDMA was
operated with an aerosol inlet flow (Qae) of 2.4 L min-1 and a
sheath gas flow (Qsh) of 15.7 L min-1. When the ES was used with its
counter electrode grounded, we observed no signal from the THAB monomer
(mobility: 9.71 × 10-5 m2 V-1 s-1; equivalent
diameter: 1.47 nm) and only a small signal from the THAB dimer (mobility: 6.54 × 10-5 m2 V-1 s-1; equivalent diameter: 1.78 nm). This
seems to be in disagreement with the results shown in
; however, when the potential of the counter
electrode was allowed to float, the THAB monomer was detected and the THAB
dimer showed a signal increase of more than one order of magnitude
(Fig. ), achieving a signal comparable to the one reported by
. We related the low transmission efficiency
obtained with the grounded ES counter electrode to the low transmission
efficiency obtained in the chamber measurements. We attributed this change in
transmission to a change in electrostatic losses in the region between the
inlet and the top electrode of the nRDMA. In that space, the ions experience
a steep adverse potential gradient when the counter electrode of the
electrospray is grounded in the same way as in the chamber measurements,
where the inlet line is grounded (Fig. ). The design of the new
inlet confirmed our hypothesis. In fact, the new design decreases the slope
of the potential gradient, keeping the ES counter electrode grounded and
allowing an increase in transmission from 0 to about 12 % for the THAB
monomer and from 10 to 50 % for the dimer. This phenomenon can be understood
by analyzing the magnitude of vE, the drift velocity of the ions due
to the electric field, and vf, the velocity of the ions due to the
carrier flow. vE=ZE, where Z is the electrical mobility and
E is the electric field. Since the direction of vE is
opposed to vf, small ions, which have a high mobility, are more
easily lost than larger charged particles (Fig. ).
Spectra of electrosprayed THAB solution measured using the nRDMA as
a classifier and an electrometer as a counter. The circular markers indicate a
spectrum obtained by keeping the potential of the electrospray counter
electrode floating. The triangles show a spectrum obtained by grounding the
electrospray counter electrode. The spectrum indicated by squares was
obtained using the new inlet.
Illustration of the sources and nature of the adverse potential
gradient that occurs when the counter electrode of the electrospray is
grounded, as in chamber measurements, where the sampling inlet is grounded.
The schematics show the setup used for measuring the spectra of THAB ions,
along with sketches of the potential gradient present in the nRDMA inlet
region. (a) shows the apparatus when the potential of the electrospray
counter electrode is grounded and the resulting potential variation along the
flow path (red line). The black arrows show the direction of the electric
field experienced by the ions. (b) shows the change in electrical
potential when the counter electrode of the electrospray is allowed to float.
(c) shows the setup with the new inlet. The adverse gradient is
drastically reduced by the use of the voltage divider.
Saturation effect
During the experiments using the ES, we noticed that the signal for the THAB
dimer was low and that, if the counter electrode of the electrospray source
was kept grounded, no signal corresponding to the THAB monomer was detected.
When the counter electrode of the ES was allowed to float, a clear signal for
the monomer was observed and the dimer signal increased sixfold. We
interpreted this change in transmission as a purely electrostatic effect. The
simulation of the electric potential at the nRDMA's inlet shows that, when
the ES counter electrode potential is floating, the electric field attracts
the ions towards the nRDMA. When the ES counter electrode is grounded, the
ions are repelled from the nRDMA's inlet (Fig. ). We relate the
absence of signal below 2.5 nm during the measurements at the CLOUD chamber
to this effect. An interesting question that arises is why, in the work by
, ions below 3 nm were detected with a comparable
transmission efficiency to that observed with the floating electrospray
counter electrode potential. This apparent discrepancy could be the result of
a saturation effect (Fig. ). When the nRDMA inlet is at ground
potential (as in the CLOUD experiments), the high potential gradient between
the inlet and the top plate repels the ions, pushing them back against the
flow and against the walls, thus increasing the losses. When unipolar ions
are used, as in , we hypothesize that the ions
quickly deposit on the walls of the electrical insulator that separates the
inlet from the top plate of the nRDMA. This effect is shown in Fig. ,
where unipolar ions (mobility: 8.0 × 10 -5 m2 V-1 s-1; equivalent diameter: 1.6 nm) are sent through a small
piece of insulator (inner diameter 4 mm, length 2.5 cm), comparable to the
one used inside the nRDMA. The concentration of ions was kept constant and
was monitored by an electrometer downstream from the insulator and a PSM
downstream from the insulator. The fraction of ions seen by the PSM, and
therefore the transmission of the insulator piece, changes with time. At the
beginning of the experiments no ions are detected by the PSM. After 5 min, the ion concentration saturates at its equilibrium value, in this
case around 10 %, resulting from a combination of the PSM detection
efficiency and the transmission of the insulator that we tested. We speculate
that the ions on the walls may create a weak electric field that focuses the
ions towards the center of the insulator and reduces ion loss. In the case of
a bipolar environment, the insulator's surface is constantly neutralized by
the deposition of positive and negative ions, resulting in higher ion losses.
Based on the measurements carried out so far, this effect seems to be
negligible for mobility diameters bigger than about 3 nm.
Schematic of the voltage transition at the inlet of the nRDMA. The
inlet flow goes from right to left with velocity vf. The drift
velocity of the ions vE=ZE. Z is the electrical mobility
of the ion, and E the electric field generated by the potential
difference between the inlet and the top plate of the nRMDA. vf is
the velocity of the ions due to the carrier flow. The direction of
vE is opposed to vf. Therefore, small, high-mobility ions
are more easily lost than larger ions or charged particles.
Saturation effect due to the deposition of ions on the inner surface
of a Teflon tube. The ions are size-selected and sent through the tube
insulator. The PSM is used as a counter downstream of the tube. The
transmission for small ions is zero at the beginning and reaches equilibrium
after about 10 min. After that, the tube is grounded and the transmission
goes back to zero.
Interestingly, we did not observe a saturation effect during our electrospray
experiments. Although we did not carry out a dedicated experiment to
investigate this effect using the electrospray source, an increase of the
transmitted signal would be expected, similarly to how it is observed when using
a monodisperse aerosol. Two key differences between the electrospray
experiments and other tests performed are (1) the high concentrations of ions
(charged particles) produced by the electrospray and (2) the presence of
solvent molecules in vaporized form in the aerosol flow. The observed high
transmission efficiency and the lack of an observed saturation effect might
both result from rapid saturation at high concentrations of charged particles
and/or from the effect of the deposition of the solvent molecules onto the
walls, which, even though present in trace amount, could change the surface
properties of the insulator in the inlet.
Transfer function determination
To determine the transfer functions, we carried out a set of experiments
using an ammonium sulfate test aerosol, classified with a high-resolution
differential mobility analyzer
HR-DMA;. By setting the HR-DMA to
a fixed voltage, we selected a narrow mobility band and simultaneously
scanned the nRDMA voltage through its accessible mobility range. The detector
used after the HR-DMA was a TSI 3068B electrometer. The detector used
downstream of the nRDMA was a PSM (Airmodus A09), previously calibrated with
the same setup. The PSM was operated at Tgt = 4 ∘C,
Tinlet = 5 ∘C and Tsat = [72–78] ∘C.
We measured the particle detection efficiency as a function of size for the
PSM at different saturator temperatures (Fig. ), the transfer
functions of the nRDMA (Fig. ) and the total transmission efficiency
of the nRDMA–PSM system. The resolution was determined as
ZpeakΔZFWHM , where Zpeak is the mobility
of the peak and ΔZFWHM is the full width at half maximum of
the peak . The resolution measured with our
method (monodisperse test aerosol and scanning voltage for the DMA to
characterize) is also function of the DMA voltage
. However, this effect is considered to be small
with respect to the measurement uncertainties. The transmission efficiency of
the nRDMA presented in this work was calculated as the ratio between the
concentration of the test aerosol measured with the electrometer and the
concentration measured with the PSM after the nRDMA. The total transmission
efficiency at 1.47 nm was 6.3 %. After correcting for the PSM detection
efficiency the transmission for the nRDMA was 12 %. The resolution for the
same size was 5.5 (Fig. ). The transfer function measurements were
used to estimate the number size distribution measured during the CLOUD
experiments. The total transmission at 1.47 nm was 6.3 %, corresponding to a
transmission for the nRDMA of 12 %. The resolution for the same size was 5.5
(Fig. ). The transfer function measurements are fundamental to
retrieving quantitative ion measurements (i.e., number size distribution) and
are used to invert the data recorded during the CLOUD experiments.
PSM detection efficiency curve as a function of size for different
saturator temperatures, obtained using ammonium sulfate aerosol.
Transfer functions of the nRDMA–PSM system as a function of size.
The total transmission at 1.47 nm was 6.3 %, corresponding to a transmission
for the nRDMA of 12 %. The resolution for the same size was 6. The
experiments were carried out using ammonium sulfate. The temperature of the
PSM saturator Tsat = 78 ∘C; sheath airflow Qsh = 29 L min-1; aerosol flow Qae = 2.5 L min-1; total inlet flow was 10 L min-1.
Chamber measurements
In the CLOUD 7 measurement campaign at CERN, the nRDMA–PSM setup, equipped
with the new high-transmission inlet and previously characterized in the
laboratory, was used to measure the fraction of charged clusters and ions in
the range from 1.3 to 6 nm. The nRDMA was operated in a closed loop. The
recirculated sheath flow was cooled to guarantee a sheath gas flow
temperature as close as possible to the temperature of the aerosol chamber
(278 K). We calculated the size distribution for the nRDMA–PSM system
correcting the raw data for the total transmission. The total transmission
and the width of the transfer functions were calculated, making an interpolated
curve of the nRDMA transmission and FWHM as a function of voltage, based on the
values given in Fig. . The data in Fig. show the size
distribution dNdDp calculated by inverting nRDMA data
as follow:
dN(Z)dZ=Ncounter(Z∗)×Ω(Z,Z∗)-1×ϵcounter(Z),
where Ncounter(Z∗) are the raw counts from the PSM + CPC for the centroid
mobility Z∗, Ω(Z,Z∗)-1 is the inverse of the transfer function
matrix and ϵcounter is the detection efficiency of the PSM + CPC.
A nucleation experiment during the CLOUD 7 campaign where the
high-transmission inlet was used in front of the nRDMA. (a) Measurement of
negative ions between 1 and 7 nm using the nRDMA–PSM system. (c) NAIS
negative ion spectrum between 0.8 and 42 nm. Comparison between the size
distributions measured with nRDMA–PSM and NAIS (b) in logarithmic scale and
(d) with magnified x axis and the y axis in linear scale. Based on the
figures, the nRDMA–PSM setup can be used to measure the ions down to 1.3 nm
(dashed line in d). The gaps in the color plot around 00:00 and
02:00 UTC are due to the electric field inside the CLOUD chamber being switched on,
causing all the ions in the chamber to be swept out.
dN(Z)dZ was then converted into
dN(Dp)dlog10(Dp) by
multiplying it by dZdlog10(Dp).
As shown in Fig. , the new inlet allows the measurement of
ambient atmospheric ions down to 1.3 nm with a nice agreement with the
Neutral cluster and Air Ion Spectrometer (NAIS) negative ion size
distribution.
Conclusions
We reported the successful application of the newly developed high-transmission
inlet for the nRDMA. The nRDMA–PSM system was tested in the
laboratory and in chamber experiments. In this paper we gave an explanation
for why it is possible to measure high transmission in laboratory
experiments, using the old inlet . We pointed out
that the characterization of the old inlet with unipolar ions might not be
representative for bipolar conditions during field measurements. We showed
experimentally that the transmission increases with time until saturation
(Fig. ), when unipolar, monodisperse ions are used, due to the
charge deposition in the insulator. The improvement of the transmission of
the nRDMA, using our new inlet, was achieved via the mitigation of the adverse
electric field in the inlet region.
The performance of the nRDMA–PSM system, equipped with the new high-transmission
inlet, increases the detection of ions with diameters as small as
1.3 nm, achieving a total transmission efficiency of 12 % and a resolution R
of 5.5 for ions of mobility 0.97 × 10-4 m2 V-1 s-1,
extending the limit of the current technology, which so far has allowed
measuring in the same size range, e.g., with the NAIS, with higher total
transmission efficiency (∼ 80 %) but with lower size resolution
(R < 2), or with the DEG SMPS, with comparable resolution (R = 3.9)
but with lower total transmission efficiency (∼ 3 %).
The new inlet now allows the nRDMA–PSM to be used in chamber measurements and
in field studies to investigate the onset of new particle formation and to
determine formation and growth rates of freshly formed particles with a
higher resolution. Recently, two interesting studies showed via experiments
and theoretically
how, using the same working principle presented
in this work, it is possible to create a cost-effective electrostatic filter.
To create an ion filter, the magnitudes of the ratio between the flow velocity
and the electrostatic velocity have to be much smaller than 1
(vfvE≪1), whereas in our high-transmission inlet
vfvE≫1 (Fig. ). In their study,
used an electrostatic dissipative
material with surface resistivity ranging from 105 to
1012 Ω sq-1. In a future design of the inlet, a tube made of the same material could
replace our segmented tube, making it possible to avoid the use of the
resistors and improving the robustness and portability of the inlet.