Measurement of Atmospheric Aerosols using the Wide-Range Particle
Spectrometer (WPSTM)
Keung S. Woo, Francisco J. Romay, William D. Dick, and Benjamin Y. H. Liu
MSP Corporation, Shoreview, MN 55126
Abstract
Increases in ambient aerosol concentrations are associated with adverse health effects
such as respiratory irritation and changes in pulmonary function. To test hypotheses regarding
effects of atmospheric aerosols, it is essential to measure the atmospheric aerosol properties
accurately and efficiently. Since atmospheric aerosols are contributed by multiple sources of
aerosol with a wide range of particle diameters, it is important to measure atmospheric aerosols
from a few nanometers to several microns in particle diameter.
The Wide-Range Particle Spectrometer (WPS™) is a recently introduced commercial
aerosol instrument with the unique capability to measure size distributions of aerosols over a
diameter range of 0.01 to 10 µm (Liu et al. 2005). A Scanning Mobility Spectrometer (SMS)
comprised of a Differential Mobility Analyzer (DMA) and a Condensation Particle Counter
(CPC) is used to measure particles from 0.01 to 0.5 µm and a Laser Particle Spectrometer (LPS)
is used to measure particles from 0.35 to 10 µm. These components are small enough to fit
within a single portable cabinet (~25 kg) with all accompanying control hardware and
electronics. No external pumps are required and power consumption is only 150 W.
The WPS was tested in the field for semi-continuous atmospheric sampling during
extended periods of time as a way to measure the time-evolution of the atmospheric aerosol size
distribution in an urban setting. Results from these measurements show that the WPS works
well in various ambient conditions with temperature ranging from 11 oC to 35 oC and relative
humidity ranging from 10% to 70%. Detailed diurnal profiles and nucleation events of
atmospheric aerosol size distributions were observed using the WPS in these measurements.
With its relatively small size, light weight, low power consumption, and user
friendliness, the WPS can perform unprecedented aerosol size distribution measurements in a
wide particle diameter range (10 nm – 10,000 nm). The use of algorithms in the operation of
the instrument (e.g., pressure and temperature compensation) and in the data analysis (e.g
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particle refractive index correction), allows the user to easily interpret and display the
experimental data, making the WPS a truly an invaluable tool for atmospheric aerosol studies.
Introduction
Size-resolved measurement of the atmospheric particulate matter (PM) is important as
PM effects on health and human welfare can strongly depend on particle size. Particle size
information can also provide clues on source attribution. Several studies have reported
associations between particles and morbidity and mortality (Schwartz et al., 1996; Oberdörster et
al., 1995; Seaton et al., 1995). Current air quality standards for atmospheric particulate matter in
the United States are based on mass concentrations of particles with aerodynamic diameters of
zero to 10 µm, or from zero to 2.5 µm (e.g. known as PM10 or PM2.5 mass fractions). The
aerodynamic diameter is defined as the diameter of a spherical particle of unit density (i.e. 1
g/cm3 ) with the same settling velocity as the spherical particle in question. These particles are
known to penetrate beyond the upper airways and are regulated. Particles smaller than 2.5 µm
are referred to as “fine,” while particles larger than this are referred to as “coarse.” These two
modes of particulate matter differ in sources, formation mechanisms, chemical composition and
exposure relationships. Coarse mode particulate matter is formed by grinding, crushing and road
traffic, as well as from natural sources such as dust, pollen and sea spray. Major components of
coarse particles include crustal material such as iron, calcium and silicon; aluminosilicates in soil
dust; sodium in sea spray and organic particulate matter from plant and animal materials. Coarse
particles are removed from the atmosphere mainly by gravitational settling. Their lifetimes are
short (in the range of minutes to hours) and they travel only short distances (< 10’s km). Thus,
coarse particles tend to be unevenly distributed across urban areas and their effects are more
localized.
PM2.5 originates from both natural and anthropogenic sources. Important primary
sources of PM2.5 include soot or semivolatile organic compounds emitted from combustion
sources (e.g. fly ash, etc). Secondary PM2.5 is produced by atmospheric chemical
transformations of precursor gases emitted by combustion or biogenic sources. Major
components of fine particles include sulfates, nitrates, organic compounds and trace elements.
Utrafine (< 50 nm) particles are removed by coagulation with accumulation mode (0.1 – 1.0 µm)
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particles, which are removed from the atmosphere by forming cloud droplets or by dry
deposition. Fine particles have long lifetimes (in the range of days to weeks) in the atmosphere
and travel hundreds to thousands of kilometers. Thus, fine particles tend to be evenly distributed
across urban areas. In addition, fine particles have a higher probability of being deposited in the
alveoli and respiratory bronchioles of the lung, where removal mechanisms are inefficient.
Several studies suggested that fine particles are more likely to cause adverse health effects (e.g.
Oberdörster, 1996; Peters et al., 1997). Due to the differences in composition and respiratory
deposition patterns of fine and coarse particles, it is necessary to measure these atmospheric
particles in a wide size range.
With the implementation of PM10 (1987 Particulate Matter NAAQS) and PM2.5 (1997
Particulate Matter NAAQS) particulate air quality standards based on particles smaller than 10
µm and smaller than 2.5 µm aerodynamic diameter respectively, interests in wide-range
atmospheric aerosol measurement is now generally limited to particles of 10 µm and less in
diameter for environmental research, health effects, air pollution, and air quality studies. Aerosol
measurement in this more limited, though still wide size range, is still difficult for most
experimental researchers due to the lack of a suitable instrument covering the entire size range of
interest.
Measurements of atmospheric aerosol size distribution have been conducted in many
cities around the world. Li et al. (1993) measured particle size distributions in the size range of
17-866 nm in Taipei, China. Buzorius et al. (1999) measured particle size distributions in the
size range of 10-500 nm in Helsinki, Finland. Hughes et al. (1998) measured particle size
distributions in the size range of 17-3000 nm in Los Angeles, CA . Shi et al. (1999) studied the
particle size distributions (10-400 nm) in Birmingham, UK. Also, particle size distributions in
the size range of 65-900 nm in Santiago de Chile were measured by Trier (1997). All these
researchers tried to cover the widest possible aerosol size range, so that they could have as much
information as possible about the aerosols they were measuring. To cover the broad aerosol size
range, these researchers had to combine several aerosol measurement instruments. This is
because there was no instrument that could measure the aerosol size distribution in a wide size
range. However, using several different instruments to measure atmospheric aerosol in a wide
size range is difficult. The user has to: (1) design a flow system to bring the sample to each
instrument, (2) design custom control software to communicate with each instrument, (3) use
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several extra hardware components (e.g., pumps, valves and computers, etc), and (4) deal with
specific instrument details from each instrument manufacturer, It is not uncommon that it takes
researcher a year to incorporate several instruments as part of a single rack system before making
aerosol size distribution measurements in a wide size range.
This paper describes the use of a recently introduced commercial aerosol instrument with
the unique capability to measure size distributions of aerosols over a diameter range of 0.01 to 10
µm (Liu et al. 2005) in atmospheric aerosol measurements. The overall design of the WPS™ as
well as the experimentally obtained atmospheric size distribution measurements at several
locations are described and presented in this paper.
Wide-Range Particle Spectrometer (WPS™)
The Wide-Range Particle Spectrometer (WPS™) is a new aerosol measuring instrument
that combines several aerosol sizing and sensing techniques to measure aerosols over a wide size
range. The individual sizing or sensing techniques used are well known and have been available
for some years as stand-alone instruments. Taking advantage of recent advances in
microelectronic and computer technology, and of the increased understanding of the underlying
physical principles of aerosol measurement, MSP Corporation has succeeded in developing the
WPS™ into a small compact device with advanced measurement capabilities.
The techniques used in the WPS™ include laser light scattering, differential mobility
analysis and condensation particle counting. Light scattering instruments are generally referred
to as optical particle counters, laser particle counters, or as laser aerosol spectrometers (Liu et al.,
1985; Barnard and Harrison, 1988). These instruments sense individual particles by light
scattering and measure the scattered-light amplitude to determine particle size. Sizing and
counting of particles by the differential mobility analyzer (DMA) and the condensation particle
counter (CPC) involve classifying aerosol particles by electrical mobility with the DMA and then
counting the classified particles by condensation and droplet growth with the CPC. The
combination is usually referred to as a differential mobility particle sizer (known as DMPS) (Liu
et al., 1978) or as a scanning mobility particle sizer (known as SMPS) (Wang and Flagan, 1990),
depending on whether the voltage on the DMA electrode is changed in steps (DMPS) or varied
continuously and scanned. In the latter case, the instrument is then referred to as a SMPS.
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. The traditional stand-alone laser-light-scattering instrument typically has a lower size
limit of 0.1 µm. The upper size limit is typically 10µm. For the DMPS or SMPS, the typical
measurement range is 0.01 µm to 1.0 µm. In the WPS™, these measurement principles are
combined to allow the measurement of particles from 10nm to 10,000nm (0.01µm to 10µm). In
the WPS™, a Scanning Mobility Spectrometer (SMS) comprised of a Differential Mobility
Analyzer (DMA) and a Condensation Particle Counter (CPC) is used to measure particles from
0.01 to 0.5 µm and a Laser Particle Spectrometer (LPS) is used to measure particles from 0.35 to
10 µm. These components are small enough to fit within a single portable cabinet (~55 lbs) with
all accompanying control hardware and electronics. No external pumps are required and power
consumption is only 150 W.
WPS™ System Overview
Figure 1 is a simplified system diagram for the WPS™. The sampled air flow enters the
instrument through a common inlet at the sample flow rate of 1.0 liter per minute (L/min). Of
this total flow, 0.70 L/min is sampled into the laser particle spectrometer (LPS) and the
remaining 0.30 L/min is sampled for electrical mobility classification by the DMA followed by
the CPC to count the single, individual particles. At the inlet of the DMA there is a single-stage
impactor with a cutpoint of 0.5 µm and a radioactive Polonium 210 ionizer for aerosol charge
neutralization (Liu et al., 1986). The impactor provides a known upper limit on the particle size
that can be sampled by the DMA and the charge neutralizer imparts an equilibrium bipolar
charge distribution on the aerosol particles sampled by the DMA.
Flow Measurement and Control—An aerosol counting and sizing instrument such
as the WPS™ requires accurate flow rate measurement and control because both the particle size
and concentration are dependent on the flow rate. An important consideration in the design of
the WPS™ was to insure that all flow rates were controlled and measured accurately by the
system.
A critical part of the flow control and management system is the use of separate
miniature DC pumps to control the individual flow rates to the desired set-point values. Four
miniature variable speed DC pumps are used: two for establishing the sample flow rates for the
LPS, and for the DMA and CPC, and two for providing the clean, re-circulating sheath air flows
needed by the LPS and by the DMA. Three flow loops use laminar-flow elements connected to a
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differential pressure transducer for sensing the pressure drop and providing an electrical output
for flow measurement and control. The pumps are protected at the inlet and outlet by high
efficiency air filters to minimize particle contamination and to prolong pump life. The DMA and
CPC flow loop uses an orifice as the pressure-drop element. In the case of the sample flow
pumps, the high efficiency filters at the pump outlet serve to filter the exhaust gas before it is
discharged to the ambient.
For flow control, the pump speed is adjusted with pulse-width modulation, or PWM, so
that just the right amount of electrical power is supplied to each pump to keep the DC pumps
running at the required, but greatly reduced speed to provide the set-point flow and to overcome
the flow resistance in each particular flow circuit. This reduces both the energy consumption and
prolongs pump life. The nominal flow rates and control accuracies for the WPS™ are
summarized below in Table 1.
Table 1 WPS Sample and Sheath Flows
Flow Loop
Flow Rate
(L/min)
LPS sample flow
0.700
LPS sheath flow
3.00
DMA and CPC sample flow
0.300
DMA sheath flow
3.00
Accuracy
(L/min)
± 0.02
± 0.06
± 0.006
± 0.06
Differential Mobility Analyzer—The differential mobility analyzer in the WPS™ has
a cylindrical geometry with an annular space for the laminar aerosol and sheath air flows (see
Figure 2). The key dimensions of the DMA are given in Table 2. These critical dimensions
were optimized to obtain size classification of particles between 5 and 500 nm with a maximum
voltage smaller than 10,000 volts and a mobility-classifying length of only 12.25 cm. A
precision absolute pressure transducer is connected to the sheath flow inlet on the DMA to
measure the internal DMA pressure. A temperature transducer is mounted near the mobility tube
to measure the DMA temperature. These transducers provide the necessary output to provide
automatic temperature and pressure compensation needed for accurate DMA mobility and
particle size analysis. The DMA has all the flow ports (with the exception of the aerosol inlet
port) on the base. The high-voltage connector is also located on the DMA base. Without
temperature and pressure compensation, the DMA measurement is subject to error that can vary
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greatly due to elevation and temperature changes. For instance, if the flow rate of a DMA is
measured with a thermal mass flow transducer and given in standard liters per minute, the
difference in volumetric flow between sea level, and an elevation of 5,000 ft, for instance, can
cause a volumetric flow difference and measurement error of as much as 20%. For high
accuracy aerosol measurement, such errors are not acceptable even for routine measurements.
Table 2 Critical Dimensions of Classifying Region in the DMA
Description
Dimension Dimension
(inches)
(cm)
Outer diameter of annular classifying region
1.750
4.444
Inner diameter of annular classifying region
1.250
3.175
Longitudinal distance between aerosol inlet and outlet
4.820
12.24
The high voltage power supply generates the DMA operating voltage that can vary from
10 VDC to 10,000 VDC. This power supply has two output voltage monitors, one for the low
voltage range (0 to 1000 VDC) and the other for the high voltage range (1,000 to 10,000 VDC)
to achieve high accuracy over the wide voltage range needed for accurate aerosol size
measurement by the DMA.
The DMA has been evaluated experimentally using the TDMA (Tandem Differential
Mobility Analysis) technique reported by Birmili et al. (1997) and more recently by Martinsson
et al. (2001). TDMA measurements were made for 18 particle diameters from 15 nm to 450 nm
while operating the DMAs at 3.10 L/min sheath air (i.e. Qs = 3.10 L/min) and 0.310 L/min
aerosol flow (i.e. Qa = 0.310 L/min) (Liu et al. 2005). By incorporating the measured DMA
transfer function as a function of particle diameter in the data inversion algorithm, it is possible
for the WPS™ to obtain accurate aerosol size distribution measurements down to 10 nm without
systematic errors in the measurement. Automatic data analysis is important in modern aerosol
measurement. It allows for automatic correction of any known systemic error, (i.e. diffusion,
time response, temperature, pressure, etc. ) provided the magnitude of the error can be calculated
and/or measured for incorporation into the data analysis software.
A NIST traceable calibration using the Standard Reference Materials (SRM) from the
National Institute of Standards and Technology is performed for the DMA to ensure accurate
size measurement. This involves generating a known sized PSL-sphere aerosol with the SRM
and performing a DMA scan with a CPC. The results are then used to determine an empirical
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factor (the ratio of scanned peak voltage to the calculated peak voltage based on the diameter of
the SRM, the DMA flow rates and dimensions) that is subsequently included as a calibration
factor to further eliminate any known or unknown systematic error in the DMA (Liu et al. 2005).
Condensation Particle Counter— Figure 3 is a schematic diagram of the
condensation particle counter (CPC). The CPC counts the individual particles and together with
the measured flow, determines the number concentration of particles in the aerosol coming from
the DMA.
The CPC has a dual reservoir design, one for the working fluid (1-butanol referred from
now on as butanol) and the other for the condensate. The CPC is of the thermal diffusion type,
with a saturator maintained at 35 ºC and the condenser at 10 ºC. The aerosol is pre-heated by
passing through a tubular passageway in the saturator heating block and thus at the saturator
temperature upon entering the saturator tube.
The saturator is made of porous metal. One end of the porous metal tube is dipped into
the butyl alcohol reservoir. Through capillary surface tension, the interstitial pore space of the
porous metal is filled with butanol. As aerosol flows along the inside surface of the porous metal
tube, butanol is evaporated from the surface of the tube, causing the aerosol stream to become
saturated with butanol vapor. In the condenser, the aerosol stream cools and becomes
supersaturated. The vapor then condenses on the particles to form droplets. The droplets are
detected by optical light scattering and counted individually as pulses by the pulse counting
circuitry.
Both the saturator and condenser temperatures are adjustable allowing different saturation
to be achieved for vapor condensation on the particles. The saturator and condenser
temperatures are adjusted to 35 and 10 ºC, respectively. The optics block is located at the
condenser exit and is maintained at 40 ºC. The light scattering optics is of the conventional
design. It includes a diode laser light source and a solid-state photo-detector. The detector circuit
board is mounted on the optics block and provides pulses to be counted by the signal processing
electronics and through the digital system board (DSB) communicate with the single board
computer for data storage and analysis. A feedback flow control system maintains the CPC flow
rate at a constant value of 0.300 L/min. The aerosol leaving the CPC is mixed with a dilution
clean air flow of about 3.0 L/min before it exits the exhaust port on the back panel of the
instrument.
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One feature of the current CPC—a feature that is absent in other conventional CPC’s—is
the use of a separate reservoir to collect condensate from the condenser. In ambient
measurements, if the dew point is higher than the set-point temperature of the condenser, water
vapor will also condense in the condenser tube. For instance, at a condenser temperature of
10ºC, moisture will condense if the ambient dew point is higher than 10 oC. For an ambient
temperature of 30 ºC and a dew point of 10 oC, the relative humidity is 28%. Under such
conditions, water will condense along with the butanol vapor in the condenser when the ambient
relative humidity is higher than 28%, which is a common occurrence during the summer.
In a conventional CPC, the condensed water as well as the condensed butanol is returned
to the alcohol reservoir. Because of the high surface tension of water, water in the porous
saturator will displace alcohol from the interstitial pore space, leading to incomplete saturation of
the aerosol stream, causing incorrect data to be generated.
The new CPC design makes sure that the condensate, including any water, if present, is
returned to a separate reservoir to avoid possible inaccurate aerosol measurement due to water
vapor condensation and the presence of water in the alcohol reservoir. A liquid level sensor in
the condensate reservoir warns the user when the reservoir is full and needs to be drained. There
is a small drainage hole at the bottom of the condenser tube to allow the condensate to drip into
the reservoir below. This hole is normally filled with a few microliters of condensate with the
weight of the condensate being supported by capillary surface tension, thus providing an
effective seal between the condenser tube and the reservoir air space below. This liquid seal is
effective in preventing air exchange between the aerosol stream above and the stagnant gas
volume below.
The condensation particle counter described above has been evaluated experimentally to
determine its relative counting efficiency as a function of particle size (Liu et al. 2005). This
was done by comparing the particle concentration of the CPC with the particle concentration
measured with another CPC (TSI Model 3025 CPC). For these measurements monodisperse
aerosols of silver were generated and classified by a DMA. The ultrafine silver particles were
generated using the technique of Scheibel and Porstendorfer (1983). Figure 4 shows the relative
counting efficiency of the WPS CPC from which we can determine that the 50% cutoff size is
about 5 nm.
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Laser Particle Spectrometer—The Laser Particle Spectrometer (LPS) is shown in
Figure 5. The LPS includes a single-particle, wide-angle optical sensor used for sensing and
detecting particle from 350nm to 10,000 nm in diameter. Particles are drawn into the aerosol
inlet at a rate of 0.70 L/min and focused aerodynamically with a 3.0 L/min flow of sheath air to
provide a narrow aerosol stream that passes through the center of a ribbon-shaped laser beam
generated by a laser diode (785 nm, 5 mW). Light scattered by each particle is collected with a
spherical mirror over a 20 to 100° range at a mean scattering (polar) angle 60o in the forward
direction of the laser. The scattered light is projected onto a red-enhanced photomultiplier tube
(PMT). In response, the PMT generates a pulse of electrical current which is then converted into
voltage by the LPS Preamplifier board. The LPS Amplifier board conditions each lightscattering pulse from the Preamplifier board for input into the next stage of electronics in which
pulses are sorted and binned according to their amplitude.
Due to the wide-angle collection optics of the LPS, its response is monotonic with respect
to particle size for a moderate range of particle refractive indexes. The response of the LPS
described before has been modeled using Mie theory. Figure 6 shows the theoretical response
versus particle diameter as a function of the particle refractive index. Due to the wide-angle
collecting optics of the sensor, the LPS response in nearly independent of refractive index for
particles from 1 to 10 microns. Between 0.3 and 1 microns there is some dependence on the
particle refractive index but the response is monotonic with respect to particle diameter. For n =
1.7 there is flat are on the response curve between 0.8 and 1.1 microns. This characteristic
allows particle diameter measurements made using the assumption of a particular refractive
index value to be converted to a diameter scale based upon a different refractive index value, as
may be done with the WPS Commander software. Conversely, in the embedded software, real
refractive index n may be selected from a range of 1.30 to 1.60 in 0.05 intervals (including
n=1.585 for PSL) for setting thresholds that are used in binning pulses with respect to diameter.
The user may select the particle refractive index most appropriate for the aerosol sample to
obtain more accurate measurement of the geometric diameters based on light scattering. Each
LPS response curve has been subdivided in 24 particle diameter channels.
The effect of particle refractive index is demonstrated in Figure 7. Figure 7 shows the
average surface area distribution of 200 room air samples. The darker color shows the
distribution data obtained by the LPS and the light color shows the distribution data obtained by
the DMA and CPC. Note that the difference in the overlapping region (0.35 µm to 0.5 µm) is
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quite large in Figure 7a. Since the LPS is typically calibrated by PSL, which has a particle
refractive index of 1.585, the size response will be different when the LPS is used to measure
particles (e.g., room air) other than PSL. If we use a more representative particle refractive index
value (e.g. 1.45) for room air to analyze the LPS data, the difference in the overlapping region
will be reduced. This is shown in Figure 7b.
To confirm the theoretical response of the LPS experiments were performed with NIST
traceable PSL spheres classified by a DMA.
Figure 8 shows the measured voltage outputs as a
function of particle diameter and also shows the scaled theoretical response for PSL spheres (i.e.
n = 1.585). The agreement between the theoretical and experimental responses is very good,
confirming the validity of the LPS predicted response.
Signal Processing and Control Electronics—The WPS hardware system is
controlled with MSP proprietary software executed by a low-power Pentium® single-board
computer operating on a dedicated Windows® XP platform. The computer has an LCD display,
a 60 GB hard drive, a serial communication card and a mouse.
Two controller boards were used to communicate with and to control all system
components in the WPS. One board, the Analog System Board (ASB) is used primarily for
analog I/O. Two digital boards, or the Digital System Boards (DSB) are dedicated to digital I/O.
The computer interfaces with the controller boards using one RS-485 serial communication port.
Control functions include operating flow controllers, monitoring flow rates, pressures,
temperatures, relative humidity, flow levels, setting and operating temperature controllers,
setting and monitoring high voltage levels, operating solenoid valves, and receiving/processing
particle counts from the CPC and the LPS.
One low-voltage switching power supply and one high-voltage DC power supply provide
all the power required by the instrument. The low-voltage power supply has several DC output
voltages (i.e. +12 VDC, +5VDC, -12VDC, -5 VDC) that supply power to the different circuit
boards of the system.
Table 3 lists the main circuit boards along with a short description of the function that
each performs on the WPS.
Table 3 Circuit Boards on the WPS
Circuit Board
Function
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CPC Detector
CPC Counter
CPC/LPS Master
Laser Driver
PSB Board
AH&T Board
LPS Slave
LPS Detector and
Preamp
LPS HVPS
LPS Amplifier
CPC photo-detector board
CPC counting board (slave)
Communicates with CPC Counter and LPS Slave
Provides controlled power to the CPC laser diode and
LPS laser diode
Board with absolute and differential pressure
transducers
Powers and signal conditions the ambient temperature
and relative humidity sensor
LPS counting board (slave)
LPS PMT and pre-amplifier board
High voltage power supply for LPS PMT
LPS amplifier board
Atmospheric Aerosol Size Distributions Measured with the WPS
One of the most common applications for the WPS is the monitoring of atmospheric
aerosol size distributions, both in urban polluted environments and in pristine environments (e.g.
protected National Parks). For this application the WPS is particularly useful as it can be used
with semi-continuous sampling schedules to measure the time-evolution of the aerosol size
distribution. This type of monitoring allows us to detect the presence of nucleation events,
whether they are of natural or anthropogenic origin (Woo et al., 2001).
The WPS was used to measure ambient aerosol size distributions in different countries
around the world, such as Hong Kong, Japan, and the United States. These measurements were
performed under different ambient conditions. For instance, ambient temperature ranged from
11 oC to 35 oC, ambient relative humidity ranged from 10% to 70%. The WPS worked well in
these environments. Also, the WPS was shown to be very easy to transport. In a recent
sampling campaign, the WPS was used to measure atmospheric aerosol size distributions in
Hong Kong and Japan. The WPS was shipped in a manufacturer provided hard traveling case as
a regular airline-checked luggage. After the WPS arrived at the sampling site, the operator spent
less than 15 minutes to unpack and setup the WPS. Setting up the WPS includes supplying the
WPS with an AC power source (90 – 264 VAC, 50/60 Hz), and after the WPS is turned on,
filling the CPC working fluid (the CPC working fluid must be completely drained before
shipping). The WPS needed only ~ 15 minutes warm up time for the CPC temperatures to reach
their set points. After this warm up time, the operator just needed to load a recipe (i.e., to tell the
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WPS what measurement mode, sampling time, and measurement size range, etc…) and start
sampling. In this particular sampling campaign, the WPS was set to measure continuously for
sixty one-minute samples. Thus, the total sampling time was one hour. After the operator
started the measurement, he just left the WPS unattended and came back an hour later when the
measurement was completed. To pack the WPS for shipment to another sampling site was also
very easy. It just included draining the CPC working fluid, installing a shipping plug inside the
WPS (to isolate the CPC optics) and putting the WPS inside the hard traveling case. Typically, it
takes less than 15 minutes to pack the WPS for shipment.
Atmospheric Aerosol Size Distributions in Hong Kong
The WPS was used to sample aerosol size distributions on a weekday (Monday) and on a
weekend day (Saturday). For each day, the WPS was installed in two sampling locations in
Hong Kong, one in Tsuen Wan and one in Yuen Long. Both sampling locations are the air
sampling stations of the Hong Kong Environmental Protection Department.
Tsuen Wan is a mixed residential/commercial/industrial city. We would expect that the
aerosol size distribution will be different for weekdays and weekends. Figure 9 shows the
average of 60 1-minute scan samples measured at the Tsuen Wan sampling station. Figure 9a
shows the average aerosol size distribution obtained on a weekday and Figure 9b shows the
average aerosol size distribution obtained on a weekend. Both average aerosol size distributions
show a bi-modal distribution function. Note that the aerosol number size distribution on a
weekday (Figure 9a) has a higher concentration than that of on a weekend (Figure 9b). The
geometric mean diameter is larger on a weekday. This suggests that the smaller particles
agglomerated to bigger particles, increasing the mean of the accumulation mode.
Figure 10 shows the average of 60 1-minute scan samples measured at the Yuen Long
sampling station. Figure 10a shows the average aerosol size distribution obtained in a weekday
and Figure 10b shows the average aerosol size distribution obtained in a weekend. Yuen Long is
mainly a residential city that is growing. As seen from the figures, the difference in
concentration between the average distribution on a weekday (Figure 10a) and on a weekend
(Figure 10b) is not significant. Note also that both average aerosol size distributions show a bimodal distribution function. However, in addition to these two modes, there is a number
concentration peak at around 10 nm. It is believed that particles in the 10 nm peak are
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contributed by nucleation events. One of such nucleation events was observed at the Yuen Long
sampling station is shown in Figure 11.
Figure 11 shows a sequence of four 1-minute long size distribution measurements taken
at around noon time. Frame (a) shows the size distribution of the typical atmospheric aerosol
during that part of the day. In frame (b) a nucleation event takes place giving a high aerosol
concentration at 10 nm. In frame (c) the freshly nucleated aerosol agglomerates to larger sizes.
In frame (d) the aerosol size distribution returns to the original one shown in frame (a). This
demonstrates that the WPS is able to measure the time-evolution of atmospheric aerosol size
distributions, as it can detect the presence of nucleation events, whether they are of natural or
anthropogenic origin.
Atmospheric Aerosol Size Distributions in Japan
After the measurements in Hong Kong, the WPS was shipped to Japan. Similar to the
Hong Kong measurement, the WPS was used to sample aerosol size distributions on a weekday
and a weekend day. For each day, the WPS was installed in two sampling locations in Tokyo:
Noge Park and Kawasaki. Both sampling locations are considered to be urban areas and are
located at the intersections of major roadways. Kawasaki has a similar volume of passenger
vehicular traffic on weekdays and weekends, but has several times more light to heavy duty
vehicular traffic on a weekday than on a weekend. On the other hand, Noge Park has two times
higher volume of passenger vehicular traffic on a weekend than on a weekday, but three times
lower volume of light to heavy duty vehicular traffic on weekends.
The average size distributions of 60 1-minute scan samples measured at Kawasaki on a
weekday and on a weekend are shown in Figure 12. While the size distributions show similar
peak diameters, the weekday distribution (Figure 12a) shows about 2.5 times higher number
concentration than that of weekend (Figure 12b). This is consistent with the higher volume of
light to heavy duty vehicular traffic on weekday.
Figure 13 shows the average of 60 1-minute scan samples measured at Noge Park.
Figure 13a shows the average aerosol size distribution obtained on a weekday and Figure 13b
shows the average aerosol size distribution obtained on a weekend. Although Noge Park has
higher volume of passenger vehicular traffic on a weekend, the expected increase in number
concentration is offset by the lower volume of light to heavy duty vehicular traffic on a weekend.
14
Overall, the average size distribution obtained on a weekday still has a higher number
concentration than on a weekend. Note that both sampling locations show similar peak
diameters. This suggests that the sources of particles for these two locations are similar. It is
believed that particulate matter from vehicle emissions is the major source for these two
sampling locations.
Conclusions
Advances in aerosol instrumentation, together with advances in microelectronics and
computer technology, have led to reduced size, weight and power requirements of aerosol
measuring instruments. The WPS can perform unprecedented aerosol size distribution
measurements in a wide particle diameter range (10 nm – 10,000 nm). The use of algorithms
in the operation of the instrument (e.g., pressure and temperature compensation) and in the data
analysis (e.g particle refractive index correction), allows the user to easily interpret and display
the experimental data, making the WPS a truly an invaluable tool for atmospheric aerosol
studies.
The Wide-Range Particle Spectrometer (WPS) has been used to measure atmospheric aerosol
size distributions in the range of 10 nm to 10,000 nm. Measurements were made in several
urban locations in Hong Kong and Japan, with ambient temperature ranging from 11 oC to 35 oC
and relative humidity ranging from 10% to 70%. The experimental results show that the WPS
worked well in these environments. It has been shown that the WPS can measure detailed timeseries atmospheric aerosol size distributions, allowing the user to detect diurnal profiles of the
aerosol size distribution and the detection of nucleation events.
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17
Figure 1. Schematic Flow Diagram of the WPS™
Figure 2 Differential Mobility Analyzer
18
Figure 3 Condensation Particle Counter
1.20
Relative Counting Efficiency
1.00
0.80
0.60
0.40
0.20
Condenser Temperature: 10 °C,
Saturator Temperature: 35 °C
0.00
0
5
10
15
20
Particle Diameter (nm)
Figure 4 Relative Counting Efficiency of CPC
19
25
Figure 5 Laser Particle Spectrometer
LPS Theoretical Threshold Values
1.E+03
n=1.30
n=1.40
1.E+02
n=1.50
2
1E+9*Csca(cm )
n=1.60
n=1.70
1.E+01
1.E+00
1.E-01
1.E-02
0.1
1.0
Sphere Diameter (µm)
Figure 6 Theoretical and Experimental LPS Response
20
10.0
(a)
Refractive
Index
= 1.585
(b)
Refractive
Index
= 1.45
Figure 7 Average of 100 Room Air Surface Area Distributions Analyzed with Different
Particle Refractive Indexes: (a) 1.585 (b) 1.45
21
LPS Amplifier Output (Volts)
1.E+01
Hi Gain Theory
1.E+00
Hi Gain Meas
Lo Gain
Theory
1.E-01
Lo Gain Meas
1.E-02
1.E-03
0.1
1.0
10.0
PSL Sphere Diameter (µm)
Figure 8 Theoretical and Experimental LPS Response
22
(a)
(b)
Figure 9 Average of 60 1-minute Atmospheric Size Distributions in Tsuen Wan: (a)
Weekday (b) Weekend
23
(a)
(b)
Figure 10 Average of 60 1-minute Atmospheric Size Distributions in Yuen Long: (a)
Weekday (b) Weekend
24
(a)
(b)
(c)
(d)
Figure 11 Example of an atmospheric nucleation event observed at Yuen Long: (a) Before
nucleation (b) During nucleation event (c) Agglomeration of freshly nucleated aerosol (d)
Back to initial sizze distribution before nucleation took place
25
(a)
(b)
Figure 12 Average of 60 1-minute Atmospheric Size Distributions in Kawasaki: (a)
Weekday (b) Weekend
26
(a)
(b)
Figure 13 Average of 60 1-minute Atmospheric Size Distributions in Noge Park:
(a) Weekday (b) Weekend
27
28