EPA-650/2-74-034
June 1974
Environmental Protection Technology Series
• » • • • • • «
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EPA-650/2-74-034
OPTICAL MEASUREMENTS
OF SMOKE PARTICLE SIZE
GENERATED BY ELECTRIC ARCS
by
P. W. Chan
Colorado State University
Fort Collins, Colorado 80521
Grant No. R-800150
ROAP No. 21ADL-21
Program Element No. 1AB012
EPA Project Officer: D. C. Drehmel
Control Systems Laboratory
National Environmental Research Center
Research Triangle Park, N. C. 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D. C. 20460
June 1974
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This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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ABSTRACT
Light extinction and photometric scattering measurements employing
laser sources have been used for sizing polydispersed metallic particulate
systems of unknown index of refraction. Two time-averaged intensity ratio
measurements including (i) intensity ratio at two angles and (ii) polari-
zation ratio at one angle, were carried out at several small forward-
scattering angles so that the measured values are essentially independent
of the index of refraction. Results were presented for smoke generated
by electric arc discharges between carbon, copper, aluminum and tungsten
electrodes. In the two-angle ratio method, the average size is obtained
by using the theory of Hodkinson. In the polarization method which is
more suitable for sub-micron particles, the experimental curve of polar-
ization ratio at several scattering angles is fitted by a theoretical
best fit using a logarithmic normal size distribution similar to that
used by Kerker et al., from which the modal value of the radius and the
logarithmic deviations are obtained. The optical system was calibrated
with standard latex spheres. The fit was found to be rather good for the
arc smokes (polydisperse systems) and good for latex (monodisperse)
spheres. The light scattering results agree with those obtained by light
extinction method. Combination of the two schemes in one instrument may
provide a device for sizing polydisperse particulate systems from .Olyia
to lOym in diameter, independent of refractive index. The possibility
of measuring size distribution in real-time using instantaneous measure-
ments based on the above schemes, similar to that used by Gravatt, are
considered. With some improvements, the device will be able to size
7 3
particulate systems up to 10 /cc or higher at a flow rate of 10 cm/sec
using commercially available electronics. The same device will also
count the number density to determine the particle concentration.
iii
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ACKNOWLEDGEMENTS
The technical assistance of Mr. Opas Apichatanon in the laboratory
throughout the course of investigation is greatly appreciated. Discus-
sions with Dr. C. Y. She of the Physics Department, Colorado State
University and with Drs. D. C. Drehmel, J. Abbott and W. Kuykendal of
the National Environmental Research Laboratory, Research Triangle Park
are most helpful.
This work is supported by a grant sponsored by the U. S. Environ-
mental Protection Agency.
IV
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CONTENTS
Page
Abstract .......................... iii
Acknowledgements .................... iv
List of Figures ...................... vi
List of Tables ...................... vi
Sections
I Conclusions ...................... 1
II Recommendations .................... 2
III Introduction ..................... 3
IV Theoretical Background - .Mie Scattering Theory
and Applications .................... 5
V Optical Instrumentations for Particle
Size Measurements ................... 14
VI Experimental Methods, Results and Analysis ...... 19
VII Instantaneous Forward Scattering Methods ....... 32
VIII Discussion ...................... 39
IX References ...................... 41
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FIGURES
No. Page
1. Light Scattering Geometry 6
2. Essentials of Extinction (Transmission)
Measurements 15
3. Optical System for Scattering Measurements 17
4. Experimental Set-up for Observation of
Scattered and Transmitted Light 18
5. Theoretical Curve of E(m,a) for DOP Smoke (m = 1.486) . 21
6. Experimental Results of Three-wavelength
Transmission Measurements 22
7. Variation of Intensity Ratios versus Size Parameter
a According to Fraunhofer Diffraction 26
8. Viewing Particles in a Flowing System 37
9. A Proposed Real-time Device for the Determination
of Particle Size Distribution 38
TABLES
No. Page
I. Theoretical Scattering Intensity - ratios 13
II a. Three Wavelength Transmission Measurements
of DOP Smokes 20
b. Analysis of Three Wavelength Transmission
Measurements of DOP Smokes 23
III Average Particle Size of Smokes Particles as Measured
by the Two-Angle Ratio Method 27
IV Summary of Experimental Results of Parameters of Arc
Smokers as Measured by the Two-Angle Ratio Method and
by the Polarization Method 31
Vl
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I. Conclusions
Two main conclusions can be drawn as a result of Che present inves-
tigation dealing with optical methods for measuring sub-micron polydispersed
smoke systems of unknown refractive indices:
(i) The two-angle ratio forward scattering technique has been found to
be a reliable and accurate method for obtaining average sizes of several
smoke systems generated by an electric arc between carbon, copper, aluminum
and tungsten electrodes. The results can almost be obtained in real-time
by comparing experimental results with available universal graphs of
Hodkinson. However, in order to obtain particle size distribution (i.e.
mean radius as well as standard deviation) for sub-micron particulate
systems, the inversion procedure is rather lengthy and inaccurate since
the two-angle intensity ratio is insensitive to particle size smaller
than 0.2 ym.
(ii) The polarization ratio method appears to be a simpler amd more
practical method for obtaining size distribution of sub-micron polydisperse
smoke systems. The tecnique involves in measuring the polarization ratio
of the scattered light at several scattering angles and fitting the
experimental data with the best theoretical fit. For monodisperse latex
particles, the fit is extremely good. However for polydispersed systems
the fit is only fair. Performing the measurements of polarization ratio
in small forward scattering angles also renders the technique to be
applicable to particulate systems with unknown refractive indices.
(iii) In addition, theoretical calculations indicate that these techniques
have great potentials in real-time applications for sizing sub-micron poly-
7 3
dispersed particulate systems with concentrations 10 /cc and flow rate 10
cm/sec with megahertz electronics, the number concentration that can be handled
increases with the bandpass of electronics and decreases with flow rate.
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II. Recommendations
The results of this investigation establish the feasibility of a
practical technique for measurement of size distributions of polydis-
persed, absorptive particulate systems of unknown index of refraction
throughout the range of particle size ranging from O.Olym to several
microns.
The next phase of this investigation will be to extend the present
time-average measurements to instantaneous intensity-ratio and polari-
zation measurements.
Theoretical estimates (Section VII) show that with commercially
available fast electronics, these techniques can be used to monitor
smoke size in real-time for particulate systems with concentrations up
7 3
to 10 particles/cc at flow rates of 10 cm/sec, typical for many smoke
stack and effluents from power plants. Thus it is strongly recommended
that a self-contained unit suitable for field operation be built for
measuring particulate size distribution of fine particles.
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III. Introduction
Many optical methods exist for measuring particle size (average),
size distribution, concentration and shape. Broadly speaking they
can be classified as light transmission (extinction) measurements,
photometric (multi-particle) scattering measurements and single part-
icle scattering (counting) measurements. Except under special laboratory
controlled conditions where particles under study are sufficiently mono-
dispersed and the flow is non-turbulent, very few satisfactory methods
have been found to size particles in the size range of .Olum to several
microns. Since particles of this size range are of utmost concern to
air pollution control and public health, development of accurate, real-
time methods to monitor particulate size and concentration in this size
range are of urgent importance for legislating environmental standards
for air pollution control. Indeed, accurate characterization of particle
size, distribution and concentration is important for developing tech-
niques for conglomeration of fine particles. This report describes sev-
eral optical particle sizing techniques suitable for in situ measurements
and remote sensing for micron and sub-micron particles generated by an
electric arc, which are highly dispersive systems similar to those en-
countered in smoke stacks and power plants. The advantages and limita-
tions of each of the methods will be discussed. It should be noted that
since the goal of the investigation is to result in a real-time particle
in situ instrument for particulate monitoring, the emphasis of the in-
vestigations are on the reliability of the Instrumentation and in the
simplicity of extracting information from experimental data.
Reliable and rugged lasers such as the He-Ne or Argon ion lasers
are used for the light source which, because of the monochromaticity, also
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greatly increases the signal to ambient noise if narrow -interference filter
is used in the detector system. In Section IV, the Mie scattering theory
which forms the theoretical background of the experimental methods used
in this investigation is discussed. Section V describes the optical and
electronic instrumentation for both transmission and scattering measure-
ments. Section VI describes experiments on transmission methods from
which particle concentration and average size is obtained as well as two
scattering methods: one based on measuring the scattered ratio of inten-
sities measured at two forward scattering angles, and the other based on
measuring the polariziatlon artio at a particular angle, (measured at
several scattering angles) from which the average size as well as the
size distribution are obtained. The possibilities and capabilities of
real-time devices based on these optical techniques are analyzed in detail
in Section VII. A summary of the present investigation as well as some
suggestions for future work in developing the techniques to a real-time
system for air pollution is presented in the discussion in Section VIII.
Detailed literature references are given in Section IX.
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IV. Theoretical Background - Mie Scattering Theory and Applications
The Mie equations provide a general description of light scattered
by uniform spherical particles illuminated by a plane wavefront. The
results can be expressed in terms of the following parameters:
m = relative index of refraction of particles to the background medium,
can be a complex quantity if the particle is absorptive: m - m1 - im"
A = wavelength of incident radiation
r = radius of spherical particles
a = 2irr/A = ird/A = size parameter
o
I = irradiance of incident light (Watts/m )
6 = scattering angle (defined with respect to forward direction)
Plane of observation - plane containing the direction of propagation
of incident wave (k ) and the direction of observation (k. ) .
Plane of Polarization - plane containing the direction of propagation
of incident wave (k ) and the direction of its electric field in
linearly polarized incident beam (E).
= angle between the plane of observation and polarization (see Fig. 1)
For a linearly polarized incident beam, a scattered irradiance I
s
2 1
(Watts/m ) at a distance R from the scattering center is given by
A2I
I {a,m,6,<|>} = - — (i-{a,m,0} sin 2 + i, {a,m,6} cos2) (1)
S 4ir2R2 i 2
and for an unpolarized incident beam,
\2I
I {a,m,9,<|>} = - — (i..{a,m,e} + i {a,m,9» = I, + I- (2)
S 8ir2R2 * 2 X 2
The scattered intensity function i, is proportional to the scattered
light perpendicular to the plane of observation and i~ is proportional
to the scattered light parallel to the plane of observation. The functions
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Plane of Polarization
Plane of
Observation
Fig. 1. Scattering geometry shewing the incident wave vector k , scattered wa_ve vector kj,
electric field vector E of a linearly polarized incident wave, (k , E) is the plane
of polarization and (lc k^) is the plane of observation. $ is the* angle between the
planes of polarization and observation, (fl = 90° is a common scattering geometry).
6 is the scattering angle between k^ and kj , 9 =0 define the forward direction
of the beam.
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i..(a,m,0) and i~(a,m,9) each have the form of an infinite series of half
2
order Bessel functions and Legendre polynomials. In addition, express-
ions for light scattering for non-spherical particles have been worked
out in simple cases such as cylinder, disks and ellipsoids. Light scat-
tering from irregular shape particles such as fragments of coal and
3 4
quartz has been studied by Hodkinson ' experimentally and the same gross
light scattering characteristics similar to those of spherical particles
are found, except for some fine structures. Thus, Mie scattering can be
applied to non-spherical particles for obtaining information on size
distribution and concentration or number density.
Simplified theoretical solutions exist for very small particles,
particles with relative index close to one, and for large particles.
2 i 2
For example, for small particles (a«l,ma«l), i. = a6 ( T~O ) and
L m —/
i. = i. cos29. For large particles (a»l), all but the finest details
can be described by an incoherent superposition of expressions for
diffraction, refraction, reflection and absorption. This is particular-
ly useful for o»50 since in this case the series contained in the Mie
equations converge slowly and Involve small differences of large terms,
requiring relatively lengthy computer calculations for precision.
For the size range from O.Olym - lOym employing visible light source
(0.4 - 0.7ym) no simplified solutions exist for the Mie equations although
computer calculations are fairly readily available.
In real situations where the particles are highly polydispersed and
in some cases where the index of refraction is complex and not known, special
methods aiming at both simplicity and elegance and real-time practical
application are desirable. Several techniques aiming at these goals are
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-8-
investigated, the theory of the methods, one based on transmission
or extinction measurement, and the other on two-angle forward scattering
ratio and polarization ratio measurements will now be discussed.
A. Light Transmission Method
The method of light extinction or transmission measurement is useful
for obtaining average size. It is based on the Bouguer or Lambert-Beer
law which relates the transmitted flux f and the incident flux f
to the thickness t of the suspension
f = f exp ( -naEt) (3)
o
where n is the number density, a is the projected area of the
particle (against the beam), and E(ct,m) is the particle extinction
coefficient. For spherical particles of a diameter d and of known
refractive index m (relative to the suspending medium), illuminated
in monochromatic light of wavelength X in that medium, the variation
of E against the particle-size parameter a = ird/X can be calculated
from the Mie theory. For diotyl phthalate (DOP) n = 1.486, the E
versus a curve has been calculated by R. B. Penndorf and shows
the usual oscillatory features. To make use of the theoreticel curve
for deriving the average particle size, the transmittances of a partic-
ulate suspension at three wavelengths Xp A , Ag are measured and are
equal to (f/f0)lt (f/fo)2» and (f/fo)3 respectively. From Bouger's
Law E : E2 : E3 = ln(f/fQ) : ln(f/fQ)2 : ln(f/fQ)3 and the ratios
existing between the corresponding particle-size parameters are
cij^c^a-j = Xj^ :^2~ :^3 .
To derive an estimate of particle-size, the theoretical curve of E
versus a is plotted on a log-log graph paper. On a second, identical
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-9-
sheet of graph paper are plotted the points
[ln(f/fo)1/ln(f/fo)l = 1, 1r/X1]
[In(f/f0)2/ln(f/fo)1 § */X2]
[In(f/f0)3/ln(f/f0)1 f Ti/X3].
The second sheet is then slid over the first, keeping the corresponding
axes on both sheets parallel, until the plotted points best fit the
theoretical curve. The correspondence of the logarithmic abscissae
nd/X and ir/X gives the value of d; the value of E for any wavelength
may be read also, and the particle number density (n) can be derived
from Bouguer Law.
The results could be checked by a direct measurement of the mass
concentration c. If p is the bulk density of the particles, and n
the number density, then — irr3pn = c and from Bouguer's Law
D - log,Qf/f = naEt/2.303 where a = irr2 for spherical particles.
Hence c = 4prD(2.303)/3 tE and r and n may be calculated. It should be
noted the mass concentration of smoke particles is difficult to be
measured accurately for particulate systems in real situations.
B. Light Scattering Methods
In particle size analysis it is usually assumed that light is scat-
tered incoherently by the particles so that the scattered intensity is
proportional to the number density. It is also assumed that the system
is so dilute that no neutral polarization effect or multiple scattering
occurs. While mutual polarization effect occurs only when particles
are very close together (distance smaller than several particle diameters),
multiple scattering occurs quite commonly. As a rule of thumb, trans-
mission should not be less than 90% to reduce the error due to multiple
scattering.
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10
The basic theory of forward light scattering is based on the fact,
g
first noted by Hodkinson, that the angular distribution of the scattered
light within the forward lobe is primarily due to Fraunhoffer diffraction,
and as such, is independent of the index of refraction. Since the forward
scattering arises from the light passing near the particle rather than
from rays undergoing reflection and refraction (which depends on the re-
fractive index), the scattering intensity function is given by
i (8, a) = i|(0,a) + ii, (9, a) = 2a [J^ (a sin 8) / sin26]
- 2a2 (J±2 (a 9) /02) (4)
The scatter flux (per unit angle per unit area) for small 6 is
1(9, a) = — — 2a2 (J 2 (a 9)/92)
X
= a2(J 2 (a 8)/92) where a E -^ , a = a particle radius (5)
JL A
For a polydisperse system of spheres
1(9) = / a2^2 ( / O2]
is a function of any set of small angles, 0, and 6_. A set of
Q
graphs is thus obtained by Hodkinson from which a or the radius of the
particles can be calculated. In order to obtain the size distribution,
one can assume a size distribution in Eq. (6), or the integral can be
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11
9 10
inverted as done by Shifrin and Perlman * to give the size distribution
as
OB
p(o) = -(2/a2) / 06.^(0 9) N^a 8) *(9) d6 (8)
o
where
*(e) =~ {I 2ym)
particles and narrow size distributions. One of the limitations of the
two-angle ratio method is that it becomes rather insensitive for particles
with o < 1 (d<0.2ym).
The polarization ratio method first proposed by Kerker et al. is
more sensitive for submicron particles. Based on Hie theory, for a
monochromatically illuminated sphere which scatters incoherently, the
two polarization components of scattered light are given by
p(a) da (10)
I||(8) = c/ i|j(6) p(a) da (11)
The polarization ratio for each of the angles 6 is defined by
p (6) 4 Ijj (6) I ^(8) (12)
and is a function of the refractive index m (relative to the surrounding
medium), a = 2ira/X, with X=X /m where X is the wavelength in vacuum.
o o
The procedure here is to assume a size distribution. A zero-order log-
arithmic distribution (ZOLD) is used and is defined by
(log a - log a^2 .
p(a) = exp 5^2 (2ir) OQ a^
where a = log standard deviation
« = modal value of the radius.
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12
12
For narrow distributions, the standard deviation a = a« a . The
M o
theoretical curve is then fitted to experimental data. Excellent agree-
ment has been obtained by E. Matijevic et al. Note that the index of
refraction must be known and the method fails if a > 0.3. (broad size
o —
distribution). In order to apply the polarization method for submicron
particles, without knowing the index of refraction, a natural thing to
do is to perform the polarization ratio measurements at forward scat-
14
tering angles. Using the values of Mie scattering functions, Table 1
shows some value of ii(6) / i.i (6). One sees that this ratio does not
depend on the index of refraction especially for small angles. Thus,
by combining the features of the forward scattering and the polarization
ratio method, a system could result to provide a practical method of
measuring size distribution of particles from .Olpm to lOum or larger.
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13
Table I. Calculated Scattering Intensity-ratios for Different
Size Parameter a and Refractive Index m.
*
CM
0
II
0
1)
2)
3)
4)
5)
6)
0
•
f-H
II
a
:D
:2>
:3)
:4)
:s)
[6)
0
•
CM
II
a
(i)
:2)
[3)
:4)
:s)
(6)
o
•
U1
II
a
CD
(2)
(3)
(4)
(5)
(6)
m = 1.05
1.0076
1.1324
1.0001
1.0234
1.0116
1.0468
1.0075
1.1312
1.0046
1.0277
1.0160
1.0649
1.0073
1.1267
1.0187
1.0415
1.0299
1.1247
1.0066
1.1115
1.1271
1.1496
1.1382
1.7083
m = 1.15
1.0076
1.1323
1.0001
1.0234
1.0116
1.0468
1.0074
1.1286
1.0046
1.0273
1.0158
1.0647
1.0063
1.1134
1.0196
1.0401
1.0297
1.1237
1.0046
1.0720
1.1379
1.1535
1.1456
1.7601
m = 1.30
1.0076
1.1321
1.0002
1.0233
1.0116
1.0467
1.0072
1.1234
1.0047
1.0267
1.0156
1.0634
1.0053
1.0895
1.0210
1.0374
1.0291
1.1213
1.0011
0.9996
1.1605
1.1635
1.1624
1.8746
Intensity-ratios :
(1) 1 (5-)/i|, (5°)
(2)
(3)
(4)
(5)
(6)
iM(200)]
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-14-
V. Optical Instrumentations for Particle Size Measurements
Optical instrumentations for both transmission and angular scattering
are basically simple. With the advent of lasers as well-collimated light
sources, the usual condenser-pinhole-collimator combination for producing
a collimated light beam can be eliminated especially if only urn or sub-
micron particles are of interest.
(A) Consider first the optical requirements for transmission measurements.
It is clear that from the definition of transmission or extinction that
its requirement demands all of the undeviated light, and virtually none
of the scattered light, to reach the measuring device. This is accom-
plished in the simple apparatus as shown in Fig. 2. Consider the case,
if the first pin-hole can be assumed as a point aperture then the only
light reaching the photocell, besides the transmitted beam, would be
light scattered through angles less than the exact angle subtended at
the pole of the collector lens by the radius of pin-hole (2). The
scattered light is concentrated closer to the forward direction, the
greater the particle size, and with visible light ( X- 0.5 ym) a pin-
hole is needed only for particles with d>10pm. It is sufficient in
practice if the angle subtended by each pin-hole at its lens is not
more than one-tenth of the angle of the first angular minimum in the
Fraunhofer diffraction pattern of a disc equal to the particle's pro-
jected area, i.e., not more than 1/10 of 3.84/a radians. For example,
if the largest particle is 5 ym, the pin-hole angle should be limited
to 0.7° and for a lens of 10 cm, 1.2 mm pin-hole would be small enough.
If the largest particles are d = 1 urn, the angle is 3.5° and the pin-
hole would be 5 mm in diameter. Thus for a He-Ne laser of 1-2 mm diameter,
apertures of 1mm placed fin the beam and in front of the photodetector
are sufficient for the present experimental conditions where only
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Pin-hole I
Pin-hole 2
Light
Source
Particles
Condenser
Lens
Collimator
Lens
Collector
Lens
Photocell
Ln
I
Fig. 2. Essentials of Extinction (Transmission) Measurements
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-16-
partides of 0.01-2vim are of interest provided the detector is far away
(more than 10 cm) from the scattering centers. For spectral rejection,
a narrow band interference filter (A = 6328 A° = A, ), should
center laser"
also be placed in front of the detector to reduce room light noise level.
(B) Next consider light scattering requirements. An idealized optical
system is shown in Fig. 3. With reference to Fig. 3, the illuminated
3
volume is W /sin6, the solid angle within which light is received is
wh/J,2 ster. and if there are n particles per unit volume of projected
2
area irda /4, and 1(6) is the scattered intensity dejrined as _flux_scattered
nird 2 19
per unit solid angle per particle, then fs(6) - —r-2-W3(sin 6 ) x whi 1(8).
Since all values in the formula are derived from measurements, the projected
2
area concentration C_ = nird /4 is found if 1(6) is known from theory.
** Si
In most light scattering sizing methods, only relative intensity measure-
ments are important. From the above formula we see that, for angular
scattering measurements, f (8)/ f (62) = (sin62/sin 8^ ' (I^O)/^^))
and f2(8)/f..(6) = I_(9)/I (9) = p(8) so that all but the obliquity factor
drops out when measurements involve only the reading of the photodetector.
The experimental set-up for both extinction and scattering measurements
for the present investigation is summarized as shown in Fig. 4. The light
source is a low-power (l-2mw) He-Ne laser. The scattering cell is of
cylindrical geometry to permit angular scattering observation. The
angular observation is carried out by rotating a rigid optical bench
located at the center of the scattering cell. A polarizer at the collection
-*•
optical arm selects either the perpendicular (E i plane of observation)
or the horizontal Intensity. Aqueous solutions of latex spheres of known
size distribution and DOP smokes are used for checking the accuracy of
the experimental results.
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Slit h
(_L to Paper)
Light
Source
Scattering
Chamber
Aperture =hxW
Photomultipiier
Fig. 3. Optical System for Scattering Measurements.
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•Interference Filter
•Slit Aperture
'Rotatoble Optical Bench
•Cylindrical Lens
Pin-hole
Aperture^*,
L
Photomultiplier
Pin-hole
Aperture
He-Ne Laser
(X0=6328 A)
oo
i
Interference
Filter
(X0=6328 A)
Cylindrical Smoke
Chamber (Plexiglas)
Fig. 4. Experimental Set-up for Observation of Scattered and Transmitted Light
from Smoke Particles Generated by an Electric Arc.
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19
VI. Experimental Methods, Results and Analysis
This Section describes the experimental methods of transmission
measurement and angular scattering measurements for various kinds of
smoke particulate systems. Results are presented for smokes generated
by carbon, copper, aluminum and tungsten arcs. Standard latex spheres
and DOP smokes are used as calibration standards.
A. Transmission Measurements of DOP Smoke Particulates
DOP smoke is used as a calibration standard since the index of re-
fraction is well known, and the extinction coefficients E(a,m) are
available in the literature, and is reproduced here in Fig. 5. The
apparatus was essentially the same as that given in Fig. 3 and Fig. 4 of
Section V. Two schemes were used, one employing the measurement of the
o o
transmission at three different wavelengths at 6328 A (red), 5460 A (green)
o
and 4358 A (blue), and the other by measuring transmission at one wave-
o
length (6328 A) as well as weighing the smoke. In the transmission method,
the light sources used were a He-Ne laser which provides the red line at
o
A = 6328 A and a mercury arc lamp with a green (Corning 4-74) filter
o
and blue (Corning 5-56) filter, providing the green line at 5460 A and
e
the blue line at 4258 A respectively. In the case of the mercury arc
light source, the light was first passed through a pinhole, then con-
densed and collimated by a lens system. Dioctyl phthalate (DOP) smoke
was dispersed in a clean smoke Plexiglas chamber for 15 seconds with the
fan on to ensure smoke homogeneity. The initial (without smoke) and the
final (with smoke) readings of the photomultiplier tube (RCA 1 P22) were
taken for each of the three light sources used.
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20
Experimental Results and Analysis
The experimental results are tabulated as follows:
Table Ha.—Three-Wavelength Transmission Measurements of OOP Smoke
Wavelength
Flux
(yA meter reading)
f
o
f
XL = 0.6328 y
500 yA
165 yA
X2 - 0.546 y
80 yA
30 yA •
*3 = 0.438 y
200 yA
80 yA
The theoretical curve of E versus a for dioctyl phthalate smoke
(m = 1.486) is shown in Fig. 5. On a second identical sheet of paper
were plotted the three points located at
= 1, 4.9646]
/ ln(f/fo)1 , ir
[In(f/f0)2 /
[0.8846, 5.7531]
[ln(f/fo)3 / ln(f/fQ)1 , ir/X3] = [0.827, 7.21]
as shown in Fig. 6. A second sheet is then slid over the first with the
axes parallel until the plotted points best fit the theoretical curve.
From the best fit the extinction coefficient E can be found, d is then
read off from the graph and n can be calculated from Bonguer law. The
results are summarized in Table lib.
-------
21
10
1 1 I I I I I 1 1 1 I I I I I
UJ
-------
1.0
22
10
I I
O.I
10
too
Fig. 6. Experimental Results of Three-Wavelength Transmission
Measurements.
-------
23
Table lib.—Analysis of Three Wavelength Transmission
Wavelength
\x = .6328 u
X2 = .546 u
\3 = .438 p
o = ttd/X
3.8
4.4
5.4
d (ym)
.76 ± .06
.76 ± .06
.75 ± .06
E n = D(2.303 / aEt
4.0 3.3x10 particles/cc
3.4 3.5x10 particles/cc
3.2 3.6x10 particles/cc
The accuracy of the measurements was about 10%. Hence, the average
diameter of the particles is .76 ± .06 m, and the average number den-
sity is 3.47 ± 0.4 x 10 particles/cc; the values are quite consistent
for all the three wavelengths. To check the accuracy of these figures,
we repeated the extinction measurement using the He-Ne laser light source,
and weighed the smoke by the usual practice of passing the smoke and
weighing the asbestos before and after. Since the volume of the smoke
3
chamber is known (18.35 x 25.4 x 33.4 cm ), the mass concentration C can
be calculated; it has already been shown that C - 47rrD(2.303) / 3tE where
p = 0.8692 gm/cc is the bulk density of the OOP smoke, where D is the
optical density, and t = 18.35 cm in our experiment. From the value E
for the three wavelength extinction measurements, a mass concentration
C was obtained 9.33 mg/cc which agrees well with the direct measured
value.
Although this transmission method is quite accurate, a knowledge of
the index of refraction is necessary. The theoretical curve of E(m,a)
versus a is also needed for obtaining particle size. It appears that
the technique has very limited potential for sizing highly polydispersed
arc smoke particles with absorptive and often unknown refractive index.
-------
24
Thus, forward scattering methods which do not require a knowledge of
the refractive indices were carried out. However the transmission tech-
nique gives a convenient method of checking the particle size against
values obtained by light scattering methods. As a general rule, at
least in laboratory work, light transmission measurements should be
carried out in addition to light scattering measurements. Transmission
data also can be used for monitoring the reproducibility of the smoke
system, and to ensure that smoke density is not too high as to create
multiple scattering, which could render light scattering measurements
inaccurate.
Theoretically, one could obtain the size distribution n(r) from the
transmission measurement by Mellin's transformation. However, except
in special cases where the particles are very large or very small or
have narrow size distributions, the inversion has been found to be highly
inaccurate especially for sub-micron particles. Hence the inversion pro-
cedure of transmission method will not be pursued here.
B. Angular Scattering Measurements
The two-angle ratio and the polarization ration method, both meas-
ured at small scattering angles are used. These methods are both inde-
pendent of the refractive indices because scattering is primarily due to
Fraunhofer Diffraction.
1. Two-Angle Intensity Ratio Method
In this method the ratios of the scattered intensities at selected
pairs of angles are measured. The intensity ratios vary with the par-
ticle size parameter a according to the Fraunhofer diffraction- formula
Eq. (4). The two main types of scattering geometries are the goniometer
set-up, and the axially symmetric anular set-up. In the goniometer
-------
25
set-up, a telescope is rotated around the scattering cell whereas in the
axially symmetric anular set-up, light is collected by an anulus ring
(lens with the center blocked) or fiber optics. Because of the simplicity
of the goniometer set-up, this is employed in the present study. The
photomultiplier is mounted on a rigid bar rotable around the center of
the scattering volume. Accurate and reproducible angular scattering
geometry is achieved in this way. The reproducibility of the smoke sys-
tem generated each time is checked by monotoring the transmitted light.
Although there are slight deviations, the reproducibility is good to 10%
provided the arc spacing and electrode surfaces are reset and repolished
each time. DOP smoke is also used as a reference as its properties have
been measured by the transmission technique.
Experimental Results and Analysis
All the smokes (except those of DOP) are generated in an electric
arc for a specific time period and subsequently dispersed with a fan in
the smoke chamber for a specified period. The experiment was run at
least two times to check the reliability of the method. The results are
summarized in Table II for each smoke particulate system and the particle
size parameter is read from the theoretical curve as shown in Fig. 7 as
calculated by Hodkinson.
The results of the DOP smokes with d = (.66 ± .06) m agrees quite
well with the results obtained by the transmission method (d = 0.76 ±
.06) ym. The results of the arc smokes are the average of two separate
runs and the agreements between runs are good.
-------
26
0)
x
1.0
0.9
0.8
0.7
0.6
•o
-------
27
Table III—Two-Angle Forward Scattering Measurements of Latex, DOP,
Carbon, Tungsten, Copper and Aluminum Arc Particles
Type of Smokes
or Particles
Latex
DOP
Carbon
Tungsten
Copper
Aluminum
2. Polarization
Average Particle Size Diameter of Particle
Parameter (a) (micron)
2.0
3.3
2.97
3.81
2.52
5.2
Ratio Method
0.34 ±
0.66 ±
0.60 ±
0.77 ±
0.51 ±
1.05 ±
.05
.06
.10
.16
.13
.10
Essentially the same experimental arrangement as in the two-angle
forward scattering experiment was used with an additional polarizer
placed in front of the photomultiplier to select either the vertically
(Ij_(6)) or horizontally (I/;(9)) polarized components of the scattered
light. The experiment was performed with standard latex spheres in
dilute aqueous solution for scattering angles from 10° to 40° at 2.5°
intervals. Background light for each polarization is measured at each
angle and is subtracted from the total scattered light to obtain I//(9)
(horizontal polarization) and 1^(9) (vertical polarization). The
polarization ratio p(8) defined as the ratio of the intensitied of the
horizontally polarized (I//(6)) to the vertically polarized components
of the light scattered at each angle was measured at several scattering
angles. He-Ne laser (X = .6328y) light source was used in the experi-
ment. All particles investigated were assumed to follow a Zeroth Order
Logarithmic Distribution (ZOLD) function characterized by a modal value
-------
28
of size parameter, CL. or cL., and a logarithmic standard deviation, o .
The computer program, which compared the experimental values of p(6)
with theoretical values of p(9) X for m = 1.332 (index of refraction of
12
latex relative to water) at X = 0.6328 ym or A = .63/1.333 - .474 urn .
The values of OL. and a will be the solutions which fit the data. The
experiments and calculations were repeated with other smoke systems.
We note, in applying this method, the index of refraction must be
known; and therefore, the method may not be suitable for complicated
polluted smoke particles. However, polarization ratio measurements
limited to small scattering angles (6 < 40°) do not depend on the index
of refraction (Fraunhofer approximation) and can thus be used under
conditions where the refractive index is not accurately known. In
particular since p(6) is sensitive for small a (small particles), it
therefore complements the two-angle forward scattering scheme in extend-
ing the lower limit of particle size to the sub-micron range down to
0.01 ym. The polarization ratios were measured at 2.5° intervals from
scattering angles of 10°-40° for the smoke particulates. The computer
program was used to fit the experimental data. The average particle
size and standard deviation are obtained from the best fit.
Experimental Results and Analysis
(i) Latex Spheres
The theoretical polarization ratio values p(6) were computed by the
computer CDC 6400 for an extensive array of combinations of CL,, o , m
and 6; viz
OL. = 0.50 to 2.400 in step of ActM = 0.10
a = 0.100 to 0.145 in step of Aa = 0.005
o o
-------
29
9 - 10° to 40° in step of A8 - 2.5°
m = 1.322 = index., of refraction of latex relative to
water
The acceptable results are the best theoretical polarization which fit
with the experimental value at different scattering angles.
The best ten fits for CL. and o are values for which agreement was
obtained between the experimental and theoretical values of p(6) at 10
angles. The final best solution of a and a was selected from o - o
values at different scattering angles for which the mean square deviation
between the experimental and theoretical p(0) values was a minimum [11].
The final best solution of modal a in log-normal distribution is found
to be o^ = 2.400, and a = .115. The diameter is 0.36 ± .06 y and the
standard deviation .04 y.
(ii) Carbon Smokes
The parameters were used in computation
CL^ = 4.000 to 5.900 in step of ACL^ = 0.10
o =0.16 to 0.25 in step of Ao = 0.01
o o
9 - 10° to 40° in step of A9 = 2.5°
m » 2.000 - 0.6600 i17
The best ten fits for OL^ and o values are for which agreement was ob-
tained between the experimental and theoretical values of p(9) at ten
angles. The final best solution is CL. = 4.00, o = 0.16, diameter -
0.80 ± .10 y and the standard deviation 0.08 u.
(iii) Tungsten Arc Smokes
The parameters were used in computation
OL^ - 4.000 to 5.900 in step of Aa = 0.10
o = 0.160 to 0.250 in step of Ao = 0.01
o o
-------
30
6 = 10° to 40° in step of A9 = 2.5°
m - 3.4000 - 2.7000 i at 6330 A 18
The best ten fits for a and a are values for which agreement was ob-
tained between the experimental and theoretical values of p(e) at ten
angles. The final best solution is o^ = 4.00, a = 0.169, diameter =
0.80 ± .08 u and the standard deviation 0.13 u.
(iv) Copper Arc Smokes
The parameters were used in computation
o^ = 4.000 to 5.900 in step of Ao^ = 0.10
o = 0.160 to 0.250 in step of Ac = 0.01
o o
6 = 10° to 40° in step of A9 = 2.5°
m = 0.5600 - 3.0100 i at 6330 A1
The best ten fits for OL. and o are values for which agreement was ob-
tained between the experimental and theoretical values of p(0) at ten
angles. The final best solution is oLj = 3.9, o = .250, diameter =
0.78 ± .15 u and the standard deviation 0.19 p.
(v) Aluminum Arc Smokes
The parameters were used in computation
c^ = 4.000 to 5.900 in step of A
-------
31
Su/nmo'ur o
The experimental results of the polarization ratio measurement are
summarized in Table IV together with those measured by the two-angle
ratio method for the five particulate systems investigated in this work.
Table IV.—Summary of Experimental Results of Parameter of
Particles Generated by Electric Arcs
Smokes or
Particles
Latex
Carbon
Tungsten
Copper
Aluminum
Two- Angle Ratio
Methods (d) pm
0.34 ± 0.06
0.60 ± 0.10
0.77 ± 0.16
0.51 ± 0.13
1.05 ± 0.10
Polarization Ratio Method
d (ym)
0.36 ± .06
0.80 ± .10
0.80 ± .08
0.78 ± .15
1.18 ± .10
o
o
0.11
0.16
0.16
0.25
0.25
It should be noted that the size of latex as measured by the two-
angle ratio method and by the polarization method agree well with each
other. The theoretical fit of the experimental data (polarization
method) was very good in the case of latex but not as good for all other
arc smokes. It is believed that the poor fit is due to the broad size
distribution of the arc smokes as explained in Section IV. Thus these
forward scattering techniques are only partially successful in solving
the problem of sizing sub-micron polydisperse particles with unknown
refractive indices. Instantaneous intensity ratio methods are suggested
in the next chapter on a possible solution to this problem.
-------
32
VII Instantaneous Intensity-Ratio Measurements
The fact that the intensity-ratio measurements In the forward scat-
tering lobe yield results insensitive to refractive index, makes the
size measurement of highly dispersive systems (with unknown index m)
similar to those encountered in smoke stacks and power plants possible.
Besides its limitation in handling complex and broad size distributions,
the methods of average intensity-ratio measurements described above are
time consuming; they cannot be considered as real-time methods to monitor
particulate size distribution in the size range of urgent importance
(O.Olym to lOvnu). The difficulties encountered in determining the
particle distribution from angular-dependent average intensity-measure-
ments can be avoided if the particles are viewed one at a time. If the
instantaneous two-angle intensity ratios, 1(0 )/i(8_) and the polariza-
tion ratio, i|i(6)/i|(9) can be measured with sufficient resolution, not
only can theoretical analysis be much simplified, a real-time determina-
19
tion of complex size distributions should be possible. Gravatt was
the first to devise a system to measure size distribution in real-time
20
with two-angle scattering. Chan et al. have considered the possibil-
ities of real-time measurements in more details.
If there is at most one particle scattering light at any instant,
the measured intensity-ratio is a function of time,
1(6,, a)
= P[e9'al = p[a(t)1 = p(t)
When there is no particle scattering light at a particular instant,
p[a(t)] gives no signal (only noisy fluctuations). Whenever a particle
passes through the scattering volume, light will be scattered to yield
an intensity-ratio current pulse corresponding to a particular particle
size. The pulse width depends on the response of the photomultlplier
-------
33
and amplifiers used. Processing of the measured signal, p(t), by means
of either a probability analyzer or a pulse-height analyzer would give
the frequency of occurrence of the intensity-ratio current pulses as a
function of pulse height (thus particle size). When the pulse height is
transformed to the size parameter by Mie scattering theory, a direct
determination of size distribtuion f(a) can be made. The discussion
here holds for either the instantaneous two-angle intensity-ratio or
the instantaneous polarization-ratio measurements. The technique can
handle nearly any type of particle distributions, and gives real-time
measurements.
The technique of detecting the individual particles is achieved by
focusing the viewing laser radiation onto a small scattering volume V;
the particulate matter under study is flowing across this scattering
volume with speed v as shown in Fig. 8(a). For clarity, the scatter-
ing volume is assumed to be orthorhombic with volume V ° abfc. The total
number of particles in the scattering volume at any time would on the
average be N - nV where n is particle concentration. The scattered
radiation is detected by a photomultiplier-amplifier system whose
response time or resolution time it T_.
K
One way'to view individual particles is to allow one particle at a
time into the scattering volume, i.e., N = 1. This would put an upper
limit on the detectable concentration, i.e., n <^V = (ab£) . Since
there is only one particle traversing the scattering volume at any
time, the photomultiplier-amplifier system records non-overlapping cur-
rent pulses whose width equals particle transit time T . For the
arrangement considered T = b/v. Fig. 8(b) shows current pulses de-
tected for a modest flow speed v; the magnitude of an individual
-------
34
pulse depends on the particle size under viewing at the instant. As
flow speed increases, T decreases and the current pulses come closer
together in time as shown in Fig. 8(c). Because It takes a finite time
for the electronics in the detection system to respond, the transit
time T must be larger than the resolution time T_ if the current
C R
pulses are to maintain their individual identities. This limits the
flow speed to be less than b/T , i.e., v = b/T < b/T . For example,
K t J\
_3
if we choose a scattering volume with a = b = 2 x 10 cm (20ym) and
_3
I = 25 x 10 cm (250ym), which is typical and practical, the maximum
concentration which can be handled would be n = (ab£) - 10 /c.c. for
—(\ —"\ —6
TD = 10 sec, the maximum flow speed manageable is v = 2 x 10 /10 =
K
2 x 10 cm/sec. Although not commonly recognized, it is quite clear
that by committing oneself to this case of N = 1, the electronic system
is not efficiently used for slower flows. One should be able to increase
the manageable concentration by improving the detection efficiency.
This can be done by allowing the total number of scatterers to be greater
than unity.
When there are several particles in the scattering volume at a
time, the detecting system will record several overlapping pulses in a
transit time T . The number of ripples during a transit time T
shown in Fig. 8(d) corresponds to the number of particles N in the
scattering volume. Due to the fact that individual particles are
still seen individually as long as the ripples in Fig. 8(d) are distinc-
tive. As flow speed increases, or T decreases, the ripples come
closer together as shown in Fig. 8(e). Because the time between two
successive ripples, T , called particle arrival interval, is much less
A
-------
35
than the transit time T (T = — T for N = 3 as shown in Figs. 8 (a)
t *\ J t
and 8(e), one must make sure that individual particles arrive at an
interval T. longer than the electronic response time of the system T
if the particles are to be individually identified. Since on the average
there are N particles arrived for viewing in a transit time, T. = T /N.
The only criterion for "single" particle detection is then,
b ___ b_ _ 1 T_ (15)
N " N vN vnabA ~ vna£ -
Or, equivalently ,
n < (valT" (16)
-3 -3
Using the same example of a = b = 2 x 10 cm, I = 25 x 10 cm,
3 -6
v = 2 x 10 cm/sec and T = 10 sec, one obtains again the maximum
K
detectable particle concentration of n = 10 /c.c. However, when the
flow speed is made slower by two orders of magnitude, the maximum
g
detectable concentration can go up to 10 /c.c. for the same scattering
— 7 ^ —ft
volume V = 10 cm and resolution time T = 10~ sec. The detectable
concentration can be made even higher if faster electronics are used.
In short, the capability of the detection system is fully utilized when
the total number of scatterers in the scattering volume is allowed to
be larger than one. The flow speed v and the size of scattering volume
(more precisely the cross-sectional area, ai) together determine the
detectable concentration. For readily available electronic and optical
set up, instantaneous intensity-ratio measurements can be easily carried
out for particle concentrations of 10 /c.c. under a typical flow con-
dition with speed around 10 cm/sec.
o
Continuous-Wave He-Ne laser (X= 6328A) and argon-ion laser
o o
(A = 4880A and 5140A) may be used for the experiments. Different
-------
36
wavelengths not only provide a check for the size distribution measure-
ment but allow an extension of the range of particle size measurable by
the proposed techniques. For a given measurable parameter a = ird/A,
the particle diameter measured depends on the incident wavelength.
A self-contained real-time device for determination of particle
size distribution is shown in Fig. 9. The set up is schematically
19
similar to that used by Gravatt, except the conversion function
p[6 ,6.,a] resulted from Mie theory is included in the present scheme.
The Inclusion of this conversion function should not only broaden the
range of detectable size parameter a, but increase the accuracy of
particle sizing. Because the particles are viewed "one at a time In
the set up, the system of Fig. 9 permits automatic determination of
particle concentration as well.
-------
37
Sample
Volume
Lens Aperture
Laser
I(t)4
PMT
AMP
1(1)
Light/
Block
(a)
Large
Particle
Small
Particle
J I
j I
(b)
(0
H h-TA
(d)
(e)
a.
b.
c.
d.
e.
Fig. 8 Viewing Particles in a Flowing System
Experimental arrangement.
Current pulses for a modest flow speed.
Current pulses for an increased flow speed.
Viewing several flowing particles in the scattering volume.
Viewing several flowing particles in the scattering volume
at increased speed.
-------
Optical
Chamber
Electronic Unit
PMT
Movable on
Track
Intake
Probability
or
Pulse-Height
Analyzer
-[AMP j-
I2(0
x= Pin-hole
Fig. 9. A Proposed Real Time Device for the Determination of Particle Size Distribution
-------
39
VIII Discussion
In this chapter we summarize the research results obtained so far,
the investigations which are being continued but could not be reported
at this time and suggest some extended work for real-time application
which should be done in the future.
A. Summary of Results and Critique of the Experimental Methods
The transmission method investigated in this work gives both the
number density and the average size particles. However, the method
involves more than one light source or one set of optical set-up and
only gives average size information. The application of the method
also depends on a knowledge of the refractive index and an available
E (a,m) theoretical curve. In air pollution monitoring, this does
not seem to be promising as having great potentials for developing
into real-time techniques.
Light scattering measurements, in spite of its complications, offer
many more variations and possibilities and in special circumstances,
they can be greatly simplified. Two special scattering methods have
been investigated. The two-angle forward scattering ratio method has
been found to give reliable and accurate results of size measurement
and most important, the method does not require a knowledge of the
refractive index and it would seem to have a practical and real-time
potential application. The polarization ration method is found to
give both average size and size distribution. Although in general the
method depends on the refractive index, measurements limited to the
forward direction could make its dependence on refractive index
unimportant.
-------
40
B. Future Work
The ultimate goal of this work is to develop, at least in principle,
a rugged, practical, outdoor and real-time air-pollution monitoring
system for the enforcement of pollution standards by the EPA. Hence,
the future work will involve in refining, extending and combining the
two-angle forward scattering ratio method as well as the polarization
method so that they will yield size distribution measurement in real-
time or almost real-time conditions. Although the 'average* scattered
light measurements described in this investigation does rely on the
computer to obtain particle size, preliminary investigations suggest
with the fast electronics now commercially available, 'instantaneous1
scattered signal can be analyzed and a real-time device based on this
approach is indeed possible. The details of this investigation as well
as design specifications of the device are described in a forthcoming
proposal to the EPA.
-------
IX. References;
1. H. C. Van de Hulst, Light Scattering by Small Particles, (Wiley
& Sons, Inc., New York, 1957).
2. S. Ramo, J. R. Whinnery and T. Van Duzer, Fields and Waves in
Communication in Electronics, (John Wiley, New York, 1967).
3. J. R. Hoskinson, Proceedings of the First National Conference
on Aerosols, edited by K. Spuony (Gordon & Reach Science
Publishers, New York, 1965).
4. J. R. Hodkinson, I.C.E.S., Electromagnetic Scattering, edited
by Milton Kerker (Pergammon Press, London, 1963).
5. J. R. Hodkinson and I. Greensleaves, J. Opt. Soc. Am.^3, 577
(1963).
6. R. B. Porndorf, J. Opt. Soc. Am. 47, 1010 (1957).
7. J. R. Hodkinson, The Optical Measurement of Aerosols in Aerosol
Science, edited by C. N. Davies (Academic Press, N. Y., 1966).
8. J. R. Hodkinson, Appl. Optics .5, 839 (1966).
9. K. S. Shifrin and A. Y. Perlman, Tellus 1£, 566 (1966).
10. K. S. Shifrin and A. Y. Perlman, Optical Spect. (Eng. Trans.)
20, 385 (1966).
11. M. Kerker, E. Matijevic, W. F. Espenscheid, W. A. Farone and
S. Kitani, J. Coll. Sci., 19_, 213 (1964).
12. M. Kerker, The Scattering of Light, (Academic Press, New York,
1919).
13. E. Matijeori, S. Kitani, and M. Kerker, "Aerosol Studies by
Light Scattering II. Preparation of Octanoic Acid Aerosols of
Narrow and Reproducible Size Distributions," Jour. Coll. Sci.
19, 233 (1964).
14. H. H. Denman, W. Heller, and W. J. Pangonis, Angular Scattering
Functions for Spheres, Wayne State Univ. Press, Detroit (1966).
15. K. S. Shifrin and A. Y. Perlman, Electromagnetic Scattering,
edited by Rowell and Stein (Gordon and Breach Science Pub-
lishers, New York, 1965).
16. 0. Apichatanon, "Measuring Aersol Distributions," M.S. Thesis,
Colorado State University (1974).
17. J. E. Me Donald, "Visibility Reduction Due to Jet-Exhaust Carbon
Particles," J. Appl. Met., ],, 391 (1962).
-------
18. G. Hass and L. Hadley in American Institute of Physics Handbook,
edited by Daniel N. Fischel, Harold B. Crawford, Don A. Douglas,
and Winifred C. Eisler (McGraw-Hill Book Company, New York, Third
Edition, 1972).
19. C. C. Gravatt, Jr., "Real Time Measurement of the Size Distribution
of Farticulate Matter in Air by a Light Scattering Method," presented
at Annual Meeting of the Air Pollution Control Association at Miami
Beach, Florida, June 18-22, 1972.
20. P. W. Chan, 0. Apichatanon, and C. Y. She, "Particle Size Measure-
ments of Arc Smokes by Forward Scattered Intensity Ratio Techniques,"
Presented at the Annual Meeting of Air Pollution Control Association
at Denver, Colorado, June 10-13, 197A.
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43
TECHNICAL REPORT DATA
(Please read luanii tuna on llir rerene before completing)
I REPORT NO.
EPA-650/2-74-034
J TITLE AND SUBTITLE
2.
Optical Measurements of Smoke Particle Size
Generated by Electric Arcs
5. REPORT DATE
June 1974
6. PERFORMING ORGANIZATION CODE
3. RECIPIENT'S ACCESSION-NO.
7 AUTHOR(S)
P.W. Chan
8 PERFORMING ORGANIZATION REPORT NO.
9 PERFORMING ORGANIZATION NAME AND ADDRESS
Colorado State University
Fort Collins , Colorado 80521
10 PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-21
11 CONTRACT/GRANT NO.
Grant R-800150
12 SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
NERC-RTP, Control Systems Laboratory
Research Triangle Park, NC 27711
13 TYPE OF REPORT AND PERIOD COVERED
• Final
14 SPONSORING AGENCY CODE
15 SUPPLEMENTARY NOTES
16 ABSTRACT ft\e report gives results of a study of the use of lipt transmission ana
photometric scattering measurements using lasers for sizing polydispersed metal-
lic particulate systems (often with unknown index of refraction) produced by a dc elec-
tric arc smoke generator. The study emphasized the reliability and accuracy of these
techniques, the information obtainable, and potentials for developing these methods
into real-time techniques. Transmission (extinction) methods are simple to apply and
give average particle size and number density if the refractive index is known: accu-
racy is good but the size distribution cannot be obtained. The two-angle forward scat-
tering intensity ratio method gives by far the most accurate results- most important,
it does not depend on a knowledge of the refractive index. Average size measurements
have been obtained with this technique on several metallic and non-metallic particulate
systems; the size distribution can also be obtained by computer calculation. The
polarization ratio method gives better sensitivity for smaller sub-micron particles
(though dependent on refractive index unless forward scattering is used) and has been
used successfully to measure size distribution of sub-micron latex spheres. Also
discussed is combining the intensity ratio method (at two forward scattering angles)
and the polarization ratio method (at one forward angle) for sizing polydispersed
particulate systems. and developing it into real-time devices.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b IDENTIFIERS/OPEN ENDED TERMS
C. COSATI 1 Icld/Group
Air Pollution
Optical Measurement
Smoke
Particle Size
Distribution
Electric Arcs
Lasers
Light Transmission
Smoke Generators
Reliability
Accuracy
Forward Scattering
Rexractivity
Extinction
Air Pollution Control
Photometric Scattering
Particulates
Polarization Ratio
Two-Angle Forward
Scattering Intensity
Ratio
13B, 20F,
14B , 15B
21B , 14D
20N
20H
20C
20E
'1 OlSmiBUTION STATEMENT
Unlimited
19 SECURITY CLASS (Hut Report)
Unclassified
21 NO. OF PAGES
49
20 SECURITY CLASS (Tinspage)
Unclassified
22 PRICE
CPA Form 2220-1 (9-73)
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