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United States
Environmental Protection
Agency
Environmental Sciences Researcrr •
Laboratory "nv~ >>
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-82-009 May 1982
Project Summary
In-Situ Aerodynamic Sizing of
Aerosol Particles with the
SPART Analyzer
M. K. Mazumder, R. E. Ware, J. D. Wilson, R. G. Renninger, P. D. McLeod, and
R. A. Sims
A single-particle aerodynamic relaxa-
tion time (SPART) analyzer was devel-
oped to measure the aerodynamic size
distribution of aerosol particulates in the
range of 0.1 to 10.0 m In diameter.
The analyzer sizes and counts individual
suspended particles and droplets from
sampled aerosols, determines their
aerodynamic diameter, and generates
the size distribution in real time. The
measurement is independent of the
electrical and optical properties of the
particles.
The size range and resolution of the
SPART analyzer depend on the acoustic
frequency. Operating at 27 kHz, the
instrument's effective sampling rate
was calibrated in the range of 0.3 to
6.0 /Ltm using test aerosols containing
mortodisperse polystyrene latex spheres
(PLS) and olive oil droplets. The instru-
ment counts and sizes aerosol particu-
lates at a maximum rate of 200 particles/
sec. The sensing volume of the instru-
ment is approximately 10~5 cc; this
allows the size distribution of an aerosol
containing a relatively high paniculate
concentration to be measured without
an appreciable coincidence loss.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Aerodynamic diameter is the diameter
of a spherical particle of unit density
having the same aerodynamic proper-
ties as the particle in question. The
deposition of inhaled aerosol particu-
lates in the lung depends primarily on
their aerodynamic diameter. The site
and quantity of lung deposition and, con-
sequently, the possible health hazard
resulting from inhaled particles can be
determined from measurement of the
aerodynamic size distribution of the
aerosol. There are'numerous methods
of sizing airborne particulates; however,
none of the commercially available in-
struments can measure the aerodynamic
diameter of individual, suspended parti-
cles and droplets in real time in the size
range of 0.1 to 10.0 jum in diameter.
This size range contains the major mass
fraction of respirable aerosols.
The purpose of this study was to
develop a laser-based instrument that
can be used to monitor the size distribu-
tion of atmospheric aerosols within the
respirable fraction of 0.1 to 10.0 /Ltm in
aerodynamic diameter. Measurements
must be made in real time and on a
single particle basis. The process of
measurement must not be influenced
by the properties of aerosols not related
to size such as refractive index, electrical
charge, and chemical composition.
The instrument developed under this
study, a single particle aerodynamic
relaxation time (SPART) analyzer, meets
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these criteria. The SPART analyzer sizes
and counts individual particles from
sampled aerosol, determines their aero-
dynamic diameters and generates the
size distribution of the aerosol. Utilizing
a frequency-biased, differential laser
Doppler velocimeter (LDV) and a micro-
phone, the analyzer determines the
aerodynamic relaxation time, rp, of in-
dividual particles suspended in air. The
quantity rp is the time in which a parti-
cle, when suspended in still air, reaches
63.2 percent of its terminal velocity.
The aerodynamic diameter (da) is ob-
tained from the value of rp. Thus, the
measurement of size distribution can be
made depending upon the aerodynamic
properties of the particutates.
Procedure and Results
Two prototype SPART analyzers were
constructed and tested for laboratory
and field applications. A block diagram
of the SPART analyzer is shown in Figure
1. The analyzers use a laser Doppler vel-
ocimeter (LDV) with its sensing volume
formed by the intersection of the two
laser beams. The sensing volume is lo-
cated at an antinode of an acoustic
standing-wave pattern, generated by a
pair of acoustic transducers located in-
side a relaxation cell. The aerosol to be
sampled is drawn into the relaxation cell
and across the sensing volume. The
LDV measures vp(t), the velocity of a
particle as a function of time, by detect-
ing the Doppler shift of light scattered
by an oscillating particle passing through
the sensing volume; a microphone mea-
sures ug(t), the fluid velocity as a func-
tion of time. The relative phase lag 4>
between ug(t) and vp(t) is calculated by
a data-processing circuit. A micropro-
cessor determines da as a function of 4>-
The process takes approximately 2.5
msec for each particle measurement.
Figure 2 shows the relationship between
4> and da for different frequencies of
acoustic drive. Figure 3 shows a typical
particle size distribution, as measured
by the SPART analyzer, obtained from
an aerosol containing a mixture of three
different sizes of polystyrene latex
spheres (PLS).
Conclusions
A new instrument was developed for
measuring in real time the aerodynamic
size distribution of aerosol particulates
in the respirable range of 0.1 to 10.0 /im
in diameter. The instrument, a single
particle aerodynamic relaxation time
(SPART) analyzer, measures the aero-
dynamic relaxation time of individual
particles and droplets without removing
the particulates from their aerosol phase.
Two prototype SPART analyzers, one
for laboratory use and another for field
applications, were developed under this
study. Measurements can be made at a
maximum count rate of 200 particles/
sec, although coincidence error restricts
the count rate to a lower limit. The size
resolution is better than 0.1 /Ltm in the
range of 0.3 to 3.0 Mm in aerodynamic
diameter. The size range and resolution
depend upon the frequency of acoustic
excitation used in the analyzers. One
analyzer was operated at 27.0 kHz and
the other at 5.0 kHz.
The SPART analyzer samples aerosol
at a rate of 550 cc/min. Approximately
500 cc/min flows as sheath air. The
effective sampling rate, the flow rate of
the aerosol passing through the sensing
volume, is approximately 0.6 cc/min for
particles with aerodynamic diameters in
the range of 0.3 to 6 /xm. The effective
sampling rate is reduced outside this
range.
The SPART analyzer is currently used
to measure: (1) size distribution of
atmospheric aerosols, (2) size distribu-
tion of therapeutic aerosol at a tempera-
ture of 37 °C and 90 percent relative
Microphone
Aerosol inlet
Amplifier
H
Zero crossing
detector
RF amplifier
K
Demodulator
Analog gate
— Phase shift
Relaxation cell
Drive
»
Acoustic Transducer
Zero crossing
detector
Phase detector
Microcomputer
Digital printer
Teletype output
Figure 1. Block diagram of the SPART analyzer.
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50°
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| 70°
60°
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| 50°
2
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• 40°
8
30°
20°
10°
0°
0.1
1.0 10
Aerodynamic diameter, da (/jm) - *•
humidity to simulate the ambient condi-
tion of the lung, and (3) fractional effi-
ciency of air pollution control devices as
a function of aerodynamic diameter.
Studies performed to date on the SPART
analyzer indicate many potential uses
for this instrument; however, several
modifications and improvements are
necessary before the instrument can
receive routine usage.
Recommendations
The SPART analyzer was developed
to measure the aerodynamic size distri-
bution of aerosol particulates in the
respirable range. Its unique feature of
measuring the aerodynamic diameter of
single particles and droplets in real time
complements existing instruments used
to measure the size distribution of
aerosols.
Although the prototype SPART ana-
lyzer has been operating satisfactorily
for a period of approximately two years,
several possibilities remain to be ex-
plored: (1) increasing the range of the
SPART analyzer from 0.05 to 20.0 jtzm
in aerodynamic diameter by operating
the analyzer at two acoustic frequencies
Figure 2. Calculated relationship between da and o lor different
acoustic excitation frequencies.
50,000 0.6, 1.101, and 2.02 micron pis.
in sequential modes with improvement
of the LDV optics and detection sensi-
tivity; (2) increasing the effective sam-
pling rate; (3) developing the automatic
correction of the effective sampling rate
as a function of aerodynamic diameter;
(4) developing the modulation of the
sensing volume so that the smaller par-
ticles can be counted with a small sens-
ing volume for a minimum coincidence
loss while larger particles are counted
with a large sensing volume for a high
sampling rate; and (5) modifying the
SPART analyzer to measure both the
effective shape factor and aerodynamic
sizes of the particulates.
This report was submitted in fulfill-
ment of Grant No. R 804429 by the
Department of Electronics and Instru-
mentation, Graduate Institute of Tech-
nology, University of Arkansas, Little
Rock, Arkansas, under the sponsorship
of the U.S. Environmental Protection
Agency. This report covers the period
June 1, 1976, to February 29, 1980,
and the work was completed February
29, 1980.
0.10 0.20 0.50 1.0 2.0 5.0
Aerodynamic diameter (Micrometers)
10.
Figure 3. Number distributions of aerosol particles containing a
mixture of three sizes of PLS as measured by the SPART analyzer after
calibration with 0.804 iim PLS aerosol.
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M. K. Mazumder, R. E. Ware, J. D. Wilson, R. G. Renninger. P. C. McLeod. andR.
A. Sims are with the Department of Electronics and Instrumentation,
University of Arkansas Graduate Institute of Technology, Little Rock, AR
72203.
Charles W. Lewis is the EPA Project Officer (see below).
The complete report, entitled "In-Situ Aerodynamic Sizing of Aerosol Particles
with the SPARTAnalyzer," (Order No. PB 82-197 195; Cost: $7.50, 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 Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
- U S GOVERNMENT PRINTING OFFICE. 1982 — 559-017/0730
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Environmental Protection
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Center for Environmental Research
Information
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