United States
Environmental Protection
Agency
Environmental Monitoring Systems
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-82-040 Sept. 1982
Project Summary
Wide Range
Aerosol Classifier
Dale A. Lundgren and David C. Rovell-Rixx
The purpose of this project was to
design, construct, calibrate, and field
test a mobile ambient particulate
matter sampler (Wide Range Aerosol
Classifier) to collect size-classified
samples of large aerosol particles. The
sampler design was based on a similar
stationary sampling system previously
constructed by the Principal Investi-
gator, Dr. Dale Lundgren.
The sampler is fitted into a trailer
and consists of a large, high flow rate
inlet from which five isokinetic samples
are withdrawn. Four of the samples
are passed through single-stage im-
pactors with different cutpoints and
the fifth is passed through a total
particulate matter filter. The four
impactors were designed to collect
particles greater than 7.5 mm, 15
mm, 30 mm and 60 mm diameter.
Aerosol particles smaller than 7.5//m
are sized by using separate lower flow
rate cascade impactors following the
last single stage impactor.
An accompanying analysis lab was
set up in a mobile van. Analysis
equipment includes an analytical
balance and a sample equilibration
chamber.
The mobile sampler was briefly field
tested in Gainesville, Florida.
The full report was submitted by the
Department of Environmental Engin-
eering, University of Florida, in fulfill-
ment of U.S. Environmental Protection
Agency grant number R-806714-
010. The report covers the total
project period from October 1, 1979
to November 30, 1980.
This Project Summary was devel-
oped by EPA's Environmental Moni-
toring Systems Laboratory, Research
Triangle Park, NC. to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
The present air quality standard for
particulate matter is based upon the
total amount of suspended particulate
matter collected by the reference
method, a high volume air sampler. This
sampler has been assumed to collect
particles less than —100 /urn diameter
(Stokes equivalent) Tests conducted by
Wedding and co-workers indicate,
however, that the sampling efficiency of
the high volume air sampler may be as
low as 7 percent for 50fjm particles and
18 percent for 30 fjm particles in
moderate winds The high volume air
sampler is also sensitive to orientation,
showing a 20 percent drop in collection
efficiency for particles 1 5/urn in diameter
and larger with a 45° shift in wind
direction. Because the mass of a particle
increases as the cube of its diameters,
the mass concentrations measured by
the high volume air sampler can also
vary widely.
The U.S. Environmental Protection
Agency (EPA) in 1979 defined Inhalable
Particles (IP) as particles less than 15
fjm in diameter and is currently con-
sidering an upper size limit of 10 /urn.
The EPA has also considered establish-
ing a fine particle standard consisting of
particles less than ~2 to 3/jm diameter.
Consideration of an IP standard has
generated considerable interest in
defining the total atmospheric particle
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size distribution. Only then will it be
possible to determine what fraction
of the total atmospheric aerosol is being
collected and what fraction would be
desirable to collect by existing or pro-
posed sampling devices for mhalable or
other paniculate measurements
Lundgren and Paulus1 previously
described a stationary sampling system
which effectively sampled large atmo-
spheric particles ( up to —100 /urn), and
determined the large particle size
distribution and total atmospheric mass
concentration; they compared results
with collections by dust fall plates and
with a standard high volume air sampler.
That study provided the background for
the present project to design, construct,
and field test a mobile large particle
sampler which determines the mass
distribution of large atmospheric part-
icles. With this sampler it is possible to
characterize the total particle mass size
distribution of ambient aerosol and to
compare the results with particle mass
data collected simultaneously by the
Total Suspended Particulate (TSP) Hi-
Vol, the Size Selective Inlet Hi-Vol (SSI),
small particle cascade impactors, and
the dichotomous samplers. The sam-
pling system will be especially useful for
evaluating areas with high fugitive
particle concentration. Data showing
the relationship between TSP, IP, FP and
other measures of Particle Concentration
can be obtained while the total atmo-
spheric particle mass distribution is
being measured. Differences in the
quantity of particulate mass measured
by various devices canthen be rationally
explained and properly related to the
aerosol size distribution in the atmo-
sphere.
The objective of this project was to
design and construct a mobile particle
sampler capable of collecting size-
separated mass samples providing
mass distribution of the total atmos-
pheric particle size range up to 200/jm
aerodynamic diameter.
Procedure
The sampler is fitted into a trailer and
consists of a large, high flow rate (1380
CFM) inlet from which five isokinetic
samples are withdrawn. Four of the
samples are passed through single-
stage impactors with different cutpoints
and the fifth is passed through a total
particulate matter filter. The four
impactors are designed to collect
particles greater than 7.5 mm, 15 mm,
30 mm, and 60 mm diameter, respec-
tively. Smaller aerosol particles are
sized by attaching cascade impactors
following the single-stage impactors.
An accompanying analysis lab is set
up in a mobile van. Analysis equipment
includes a precision balance, optical
sizing microscope, and a sample equi-
libration chamber.
Results and Discussion
Inlet Design.
The major task in measuring large
atmospheric particles is transporting a
true or representative sample of the
particles into a measurement device.
This difficulty is particularly serious for
the collection of particles larger than
—30 /um diameter because of their great
inertia and high settling rate.
Particle settling velocity (Vs) is
determined by a balance between the
force of gravity acting on a particle and
the fluid drag force exerted by the
medium through which the particle
falls. Simply put, settling velocity
increases rapidly with increasing particle
size. Sampling tube inlets which are
pointed upward will capture a greater
porportion of large particles than are in
the actual distribution due to the
settling of large particles into the inlet.
The increase in the relative number of
large particles sampled is equal to 1 +
[Vs/Vo], where Vo equals the sampling
tube inlet velocity. The error can be kept
reasonably small if the inlet velocity (Vo)
is made several times greater than the
settling velocity (Vs) of the largest
particle desired. Therefore, a Vo of 25
times Vs would give a permissible error
of only 4 percent overestimation for a
tube pointed upward. The Wide Range
Aerosol Classifier meets this criterion
for a 55 /L/m diameter particle, and an 11
percent overestimation of a 100-yum
diameter particle is predicted.
Particle inertia is a function of particle
mass. From Newton's first law, where
force equals mass times acceleration, a
larger force is required to accelerate (or
decelerate) a larger (heavier) particle as
quickly as a smaller (lighter) particle.
Because the size of a particle also
effects the drag it experiences from the
air, the relaxation time (T) is used as an
indication of its ability to accelerate or
decelerate. Particle relaxation time (T)
has been defined as: T = Dp2p/18r;,
where Dp is the diameter of the
particles, p is its density, 77 is the viscos-
ity of air, and 7 has units of time.
It is also convenient to define particle
stopping distance /, as the distance a
particle will travel when decelerated in
a fluid medium (air) from an initial
velocity (V) to rest. It has been shown
that/=Vr.
Davies2 uses/to evaluate the effect of
particle inertia on sampling efficiency
for two situations. When sampling at a
known inlet flow rate, Q, a suction
velocity (Vx) will be produced; Vx ;s a
function of the distance from the inlet
orifice To examine the case where Vx is
evaluated at a distance /from the center
of the orifice, Davies uses the equation
Vx = Q/477-/2, where Q is the flow rate of
the sampler inlet and 4nP is the surface
area of a sphere of radius /
More appropriate might be an equation
determined by Dalle Valle3from measured
velocity contours of exhaust hoods For
a point at a distance / along the center
line from the hood face, Vx =Q/10/2+A,
where A equals the inlet face area
Davies reasons that if the radius of the
inlet (R) is several times larger than /,
the effects of inertia will be negligible.
Davies also examined the situation of
sampling in a crosswind with some
velocity (Vs) and reasons that the inlet
radius (R) should be much greater than
the stopping distance associated with
the wind (i.e., /= Vw T). He suggeststhat
R should be at least 5/. For the mobile
sampler, the condition that R = 51
requires that / be less than 6 cm. At a
distance of 6 cm from the inlet orifice,
the maximum velocity using the latter
equation would be 2 m/s (~ 4.5 mph).
The stopping distance for a 100-/um
diameter particle at this velocity is 6 15
cm. This nearly meets Davies' criteria
and suggests that the mobile sampler
inlet is dimensionally adequate to
efficiently sample 100 //m diameter
particles in winds up to ~2 m/s (4.5
mph).
Several researchers have noted that
Davies' theoretical criteria seem overly
restrictive for efficient sampling and are
not met by several commercially avail-
able sampling instruments. A theoret-
ical study of sampling efficiencies by
Agarwal and Liu4 took into account both
particle inertia and settling. They
determined the flow field around a
vertical thin-walled inlet from the
Navier-Stokes equations and then cal-
culated particle trajectories. They deter-
mined a critical particle trajectory which
is a distance, Re, from the inlet axis,
such that those particles inside this
radius enter the inlet. Sampling ef-
ficiency was calculated by comparing
the concentration of particles entering
the inlet versus the actual concentra-
tion of the air sampled.
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Agarwal and Liu noted that the
sampling efficiency was a function
independently of both the Stokes
number (STK =pCDp2Vo/9^D) and the
relative settling velocity (Vs' = Vs/Vo).
The characteristic length and velocity
are the inlet diameters, D, and the inlet
velocity, Vo. Their research indicates
that if the product (STK) (Vs') is less than
0.10, then sampling efficiency will be
greater than 90 percent. Agarwal and
Liu caution, however, that their criterion
for efficiency regards all particles which
enter the inlet as sampled, whether or
not they impact a nd possibly stick on the
inside wall of the inlet. The value of (STK)
(Vs')for sampling 100/urn particles with
the mobile sampler is 0.025. Solving for
particle diameter with (Stk) (Vs'( equal to
0.10 indicates that the mobile sampler
can sample particles as large as 130^m
diameter with an accuracy of 90
percent These values assume, however,
that Stokes law holds for particles this
size
The shroud of the Wide Range
Aerosol Classifier was designed to
provide a calm air space around the inlet
orifice so that the sampler would be less
sensitive to cross winds. To be effective,
it was made large enough so that
particles would not impact on it but would
flaw around and over the shroud To
insure this, it was designed large in
comparison to /. Using the same ratio of
1:5, this criterion was met for a particle
with a stopping distance less than or
equal to 1 /5 the diameter of the shroud,
or 30 cm. This condition was met for
100-/um diameter particle in a 9 m/s (20
mph) wind.
A rain shield designed for use when
the sampler is operating consists of a
90-cm flat disk supported 30 cm above
the top of the shroud and centered
above the inlet. The shield prevents rain
from falling directly into the inlet under
light wind conditions. A winddriven ram
should fall at an angle and impact on the
side of the tube From there it should run
down the side and into the air plenum
box and not into the samplers
Selective Sampler Design
The size-selective sampler inlets
were designed to extract an isokinetic
sample from the air flowing through the
inlet tube. The samplers were con-
structed of 0.10 cm aluminum. The
sampler inlets are of equal area so that
an equal volume of air is sampled
through each sampler. Each impactor
inlet measures 17.8 cm * 6.55 cm. The
inlet area was determined by the slot
width required for the first impactor;
therefore, the Number 1 impactor is a
straight nozzle (neither converging nor
diverging).
To minimize losses and facilitate
construction, a rectangular jet design
was used. The impactors were designed
with the jets 17.8 cm long in order to
leave more room for the airflow over the
impactor plate to turn down into the
filter. This reduces the particle losses on
the walls of the impactor.
The fifth sampler, which collects a
total particle sample, was designed with
a square inlet so that it would fit better
between the impactor inlets. The sides
of all sampler inlets are straight to
reduce any loss of particles onto the
walls. Inlet area of the fifth sampler is
the same as for the other samplers.
An additional advantage of using high
volume air sampler blowers is that flow
controllers are readily available on the
market. These flow controllers insure a
constant flow rate through the sample
so that the total air volume for the
sampling period is known and that a
representative sample is drawn for each
hour over the normal 24-hr period.
Therefore, if the total sampling time is
known and the flow rate is known and
constant, the total air volume sampled is
known Aerosol mass concentrations
can then be computed. A constant flow
rate is additionally critical for impactors
because the aerosol impaction efficiency
is a function of the flow rate. Field
sampling and evaluation at selected
sites has shown that the sampler can
provide reliable information on the size
distribution of particle mass including
particles up to 100 /urn aerodynamic
diameter. The system is self sufficient
with the accompanying mobile labora-
tory for processing and analyzing the
collected samples.
Conclusions and
Recommendations
The results of limited field testing
indicate that the Wide Range Aerosol
Classifier (WRAC) can be easily set up at
a suitable field location and operated to
collect valid 24-hr samples of size-
separated atmospheric aerosols. The
accompanying analysis van can be used
to prepare, condition, and weigh the
collected aerosol samples. The samples
can be used to determine an aero-
dynamic particle size distribution based
upon the mass of the ambient aerosol.
Calibration tests indicate that the
classification is adequate to provide a
good characterization of the atmospheric
aerosol.
References
1. Lundgren, D.A. andH.J. Paulus. The
mass distribution of large atmo-
spheric particles. J. Air Poll. Cont.
Assoc., 25:1227-1231, 1975.
2. Davies, C. N. The entry of aerosols
into sampling tubes and heads. Brit.
J. Appl. Phys., Ser 2, 1:921-932,
1968.
3. Dalle Valle, J. M. Exhaust Hoods.
New York: Industrial Press. 1952.
4. Agarwal, J K., and B. Y. H. Liu. A
criterion for accurate aerosol sam-
pling in calm air. Am. Indust. Hyg.
Assoc. J., 41:191-197 1980.
Dale A. Lundgren and David C. Rovell-Rixx are with the University of Florida,
Gainesville, FL 32601.
Robert M. Burton is the EPA Project Officer (see below).
The complete report, entitled "Wide Range Aerosol Classifier," (Order No. PB
82-256 264; Cost: $9.00. subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
* U.S GOVERNMENT PRINTING OFFICE 198Z-559-017/0837
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