In the present paper, we expose how we boosted the performance of a commercial condensation particle counter (CPC) model TSI 3010 to detect particles as small as 1.5 nm while preserving the robustness and reliability of the original instrument. The TSI 3010 was selected because of our deep knowledge of its internal workings and its large incorporated butanol reservoir that allows continuous operation for several hours without refill, which is well suited to airborne operation. Aside from this, it is still pretty easy to buy instruments from the TSI 3010 family from companies that specialize in used scientific instrument retail. The CPC described in this study is called B3010 hereafter, where the “B” stands for boosted. We provide an evaluation of its performances down to 1 nm using standard calibration methods and comparisons with ultrafine CPCs (TSI 3025 and TSI 3776), as well as with its original version. One important application of the B3010 is for high-altitude measurement stations and airborne studies, the instrument's detection efficiency was quantified for various inlet flow rates and pressures.
It is now widely acknowledged that atmospheric particles have an impact on
climate and health. Their concentration in the atmosphere is largely
determined by their sources, which can be primary (mechanically emitted) or
secondary (from a gas-to-particle conversion process within the atmosphere).
Among the formation pathways of secondary aerosols, nucleation is the process
responsible for the formation of new nanoparticle clusters (as opposed to the
process of condensation onto preexisting particles). Triggered by
photochemical processes, oxidized lower-volatility products are formed, of
which some have the properties of nucleating into new particulate clusters.
Once particle clusters are formed by nucleation, they may be lost on
preexisting particles via coagulation if they do not rapidly grow to larger
sizes by condensation of less volatile but more abundant species. The
processes of nucleation and early growth lead to the occurrence of new
particle formation (NPF) in the atmosphere. NPF occurs over several hours
and is considered responsible for generation of a large number of aerosols at
the global scale (
In particular, at high altitudes, NPF events have been recorded with a high
frequency in the French Massif Central (
Diurnal conditions are necessary for the study of nucleation because they
determine the presence of photochemical processes at the origin of
nanoparticle precursor gases. High-altitude stations, however, are frequently
influenced by uplifted air masses during the day, due to forced convection
on mountainous slopes or natural heat convection. Therefore, it is
relatively rare to meet the appropriate conditions for the study of the
nucleation process taking place above the atmospheric boundary layer from
ground measurement stations. Airborne measurements offer a much higher
potential, not only for overcoming artifacts related to the topography of ground
stations but also for evaluating the spatial (horizontal and vertical)
extension of the process and reaching specific aerosol plumes (e.g.,
desert dust or volcanic ash in which the nucleation process could be
favored). In the past, instrumentation embedded in an aircraft has been able
to detect newly formed particles in a size range between 5 and 10 nm
(
The TSI 3010 condensation particle counter (CPC) is a later version of the TSI 3760, which was designed by
The TSI 3760 controls the sample flow with a critical orifice and needs a vacuum pump to operate. In order to further reduce the risk of contamination, a second critical orifice is used to flush the air from the inner volume of the CPC housing. This is called the purge flow. The slight under-pressure in the housing causes any particle to be evacuated to the vacuum pump. In addition, the purge flow helps cool the electronics.
Back in 1988, all butanol-fueled CPCs had a sample flow rate of
0.3 L min
Keady's TSI 3760 does not have such a bypass. It operates with a sample flow
rate of 1.415 L min
The minimum size of the particles that can act as condensation nuclei depends
on the supersaturation ratio of the vapor of the working fluid in the cooled
condenser. The smaller the particle, the higher the supersaturation ratio
required to initiate the vapor-to-droplet conversion (nucleation). The
supersaturation profile in the condenser depends on the flow rate, the
vapor-saturated air thermodynamic properties and condenser temperature
The higher the flow rate, the farther the supersaturation peak from the
entrance of the condenser. Keady's design (
The optical detector features a 180
The optical detector was made to count single particles. When a particle
crosses a laser beam, light is scattered and sensed by the photodetector, which
in turn generates an electrical pulse. The pulse is conditioned, then
captured by a digital counter. As the flow rate is constant, it is easy to
calculate the particle number concentration. At high concentrations, the
probability for two particles or more to overlap as they cross the beam
increases. Then, only one pulse is generated as several particles traverse
the detector. This phenomenon, known as coincidence, results in
undercounting. A correction method based on Poisson's equation
(
Although the TSI 3760 was designed for the clean rooms market, the good
performance and affordable price helped make it popular in a wide range of
applications, including atmospheric research (
Thanks to its 10 nm cutoff diameter and low price compared to an ultrafine
sheathed CPC, the TSI 3010 was soon widely adopted in SMPS systems, covering
many fields, including laboratory experiments and field measurements
(
The TSI 3010 is marketed with a cutoff diameter of 10 nm, when operated at
the default temperature gradient
CPCs can be separated in two main categories: non-sheathed sample flow
(
The goal of this development, encouraged by a recent study by
As we target airborne measurements, aircraft safety rules and specific constraints made the design process somewhat more complex. Airborne requirements include the ban of external butanol fill bottles and the need for all instruments to connect to common inlets and exhaust lines in order to avoid a critical cabin pressure drop. In other words, the inner flow paths of the instruments are at the outside ambient pressure, while the rest of the instruments are at cabin pressure. Aside from this, the power supplies found in aircrafts can produce large voltage transients, possibly causing permanent damage to electronic devices.
B3010
In order to reduce the development time, our design reuses the saturator, condenser and optics of an original TSI 3010. Everything else was redesigned, involving 3-D CAD modeling, electronics and software design. The saturator taken from the TSI 3010 is also a reservoir that can hold more working fluid (butanol) than needed for a standard flight of 4 to 6 h, thus eliminating the need for an external fill bottle.
The key parameters that govern the cutoff diameter of a CPC are the volume flow rate and the temperature gradient between the saturator and the condenser. In order to gain full control over the supersaturation process, it was necessary to control each of them separately. Indeed, the original TSI 3010 uses a thermoelectric cooler (TEC) to pump the heat out of the condenser and into the saturator, thus acting as a cooler and a heater at the same time. The temperature gradient is kept constant but without control on the condenser absolute temperature. Aside from this, the flow rate is set by a critical orifice and is not measured. The flow rate cannot be changed unless the orifice is replaced.
For our purpose, we had to break the thermal bond between the hot side of the
TEC and the saturator block. The tall heat sink was replaced by a smaller
one but with forced convection. The TEC was replaced by two TECs connected
in series. Resistive heaters were stuck on the saturator block. In addition,
in order to prevent the butanol from condensing on the optics, a heater was added
on the optical block. The optical block temperature is kept above
ca. 40
We measure the flow rate with a laminar flow element corrected for absolute
pressure. The absolute pressure is measured by a miniature sensor connected
to the optical chamber with a capillary tube. The pressure intake is
centrally located, between the saturator–condenser block and the laminar flow
element. The volume flow rate is calculated from the differential pressure
measured by a 50 Pa miniature sensor across a laminar flow element and
compensated for absolute pressure. The volume flow rate was calibrated with a
DryCal Gilibrator bubble volume flowmeter for a number of absolute
pressures. The volume flow rate is a key measurement, since it is used to
calculate the particle number concentration. The concentration
The electronic boards of the CPC were redesigned from scratch. The power
supply board was designed to operate off aircrafts' 28 V DC board with special care
taken to stand reverse polarity and load dumps. The power supply can stand
The system is controlled by a credit card sized computer board powered by an
ARM processor. The operating system is a custom-made Linux system, built from
scratch with the Buildroot framework. The computer runs advanced software
algorithms (
Experimental setup.
The experimental setup given in Fig.
The differential mobility analyzer (DMA) used in this study is called a
Herrmann-type DMA and has been described in detail in
However, this high resolution comes at a price: the small particle size
range. Indeed, at such a high sheath flow rate, the voltage required for
selecting particles bigger than 5–6 nm produces electric arcs in the DMA and
thus sets the upper limit. Aside from this, the principle of linking the DMA voltage
and actual particle size is based on the DMA voltage at which the peaks of a
molecular standard of known size are resolved (
Two different types of aerosols are used in this study to test the response
of the B3010 in the sub-3 nm range. Mobility standard ions generated with an
electrospray source for organic and metal oxides produced with a glowing
wire generator for hydrophobic particles are used sequentially as the
sources of polydisperse aerosols in front of the high-resolution Herrmann-type
DMA (
Nitrogen is used as a carrier gas for the wire generator and electrospray
source to push the particles into the Herrmann DMA at a flow rate of 6 L min
The monodisperse aerosol flow exiting the DMA is distributed via a three-port flow splitter to the device being tested (B3010), the Keithley 6517B reference aerosol electrometer (AEM) and an exhaust line for the excess air. Conductive soft tubes of equal lengths are used to connect the splitter to the B3010 and the AEM in order to level off the deposition losses in both lines.
The flow rate in the loop of the DMA is constant but is neither measured nor
known. The flow control uses the actual speed output of the high-flow blower
to control the flow rate. Tetraheptylammonium bromide is used as a standard
to calibrate the flow rate of the DMA at the beginning of the experiments
and to check the stability of the system afterwards. Then, a different type
of aerosol can be injected into the DMA because the parameters of the DMA
don't change as long as the flow rates are kept constant. The calibration
factor
As
Equation (
The mobility diameter of the measured mobility distribution is then converted
to mobility diameter using the Stokes–Cunningham equation
(
We use the molecular standards listed in Table
Molecular standards dissolved in ethanol
The monomer and dimer of the molecular standards are so small that they can only bear a single electric charge. But most particles larger than about 1.8 nm carry multiple charges and hence generate a much higher current in the electrometer than if they all had borne a single charge. Thus, the electrometer overestimates the particle number concentration by at least an order of magnitude. As a result, the CPC detection efficiency is underestimated for these particles.
Traditionally, molecular standards have mostly been used up to about
2 nm. But in order to measure the ability of the B3010 to detect molecular
standard ions at sizes larger than 2 nm, we insert a small corona
discharge device, operated at a DC voltage of about 3 kV. The sign of the
voltage is the opposite of that of the charge of the particles selected in
the DMA. The benefit of the corona discharge can be seen in
Fig.
Effect of the corona discharge. When the corona discharge is off
Detection efficiency curves measured for various laboratory-generated aerosols.
In the second set of experiments, we use hydrophobic oxide particles produced
by a glowing wire generator, as described in
In this study, we use a tungsten oxide alloy (WOX)
The detection efficiency is the most representative characteristic of a CPC
and is what we focus on in this section. The detection efficiency
We measured the detection efficiency of the B3010 for three molecular
standards, including positively and negatively charged particles for a
However, we can see in Fig.
The efficiency should only increase with increasing particle diameter.
However, the curves in Fig.
B3010 detection efficiency as a function of flow rate in the sub-2 nm range.
B3010 detection efficiency as a function of inlet pressure for two fixed-sized standards: TXAB monomer and dimer.
Figure
B3010 detection efficiency as a function of temperature gradient for TXAB monomer.
Influence of temperatures on detection efficiency of tungsten oxide
particles. The black lines are the typical curves of ultrafine CPCs TSI 3025
(
In order to study the effect of pressure on the detection efficiency, we
inserted a pinched tube section in the setup of Fig.
Gain in performance. Tungsten oxide particles. The data from TSI
CPCs are taken from product brochures
Comparing B3010 with TSI 3025 and TSI 3010 during a high-concentration episode
As the pressure decreases in the condenser, the mean free path of the particles increases as well. The probability for them to hit the walls is greater. This leads to an increase in diffusion losses, especially for the smaller, more mobile particles.
Figure
The B3010 in a CPC battery.
The increase in performance achieved in this development is shown in
Fig.
The cutoff diameter is defined as the particle size at 50 % efficiency and
is abbreviated
The response time of our CPC was measured in
Finally, we had the B3010 measure ambient air in a suburban location close to
Clermont-Ferrand (France), alongside a TSI 3025 and a TSI 3010 for 3 d. The settings were
The maximum particle counting rate is limited by the coincidence in the
optics. The higher the number of particles flowing through the optics, the
higher the probability of coincidence. In the TSI 3025, only a fraction of
the intake air is sampled, the rest being used to make filtered sheath air.
Thus, this dilution reduces the number of particles flowing through the
optics and hence the coincidence phenomenon. According to the manual, it can
count up to
In Fig. Small – B3010 minus TSI 3025, 2.5 < Medium – TSI 3025 minus TSI 3010, 3.0 < Large – TSI 3010,
In this example, one can see that particles in the range 2.5–3.0 nm account
for about a third of the total particle concentration.
In this project, we demonstrate that a CPC with a simple, proven design such as the TSI 3010's can be slightly modified to bring the detection efficiency close to that of ultrafine, sheathed CPCs. Both laboratory and ambient measurements confirm this result. However, as emphasized in the ambient measurements section, the maximum measurable concentration is definitely limited by the design of the flow path and the optics.
In light of the results presented here, a few guidelines for CPC designers and users can be drawn. Still today, designers could reasonably imagine non-sheathed, yet performing CPC geometries, as long as the total concentration in the targeted application is not too high. Aside from this, owners of non-sheathed CPCs can dramatically reduce the cutoff diameter of their devices by simply adjusting the saturator and condenser temperatures. This simple tweak, when allowed by commercial CPC firmware, can potentially help save the extra cost of an ultrafine CPC.
Data are available upon request.
DP wrote the first draft and ran the ambient measurements. MA provided the laboratory calibration facility. DP, MA and KS participated in laboratory tests, data analysis and editing of the paper.
The authors declare that they have no conflict of interest.
This work was funded by the ClerVolc project – Program 1 “Detection and characterization of volcanic plumes and ash clouds” funded by the French government's “Laboratory of Excellence” initiative.
This paper was edited by Szymon Malinowski and reviewed by four anonymous referees.