EPA-650/2-74-128
NOVEMBER 1974 Environmental Protection Technology Series
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EPA-650/2-74-128
MEASUREMENT OF THE OPACITY
AND MASS CONCENTRATION
OF PARTICULATE EMISSIONS
BY TRANSMISSOMETRY
by
William D. Conner
Chemistry and Physics Laboratory
ROAP No. 26AAM
Program Element No. 1AA010
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, N. C. 27711
November 1974
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EPA REVIEW NOTICE
This report lias been reviewed by (he National Knviromnenlul Research
Center - Research Triangle Park, Office of Uesearch anil Development,
liPA , and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Environmental
Protection Agency , nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environ-
mental Protection Agency, have been grouped into series. These broad
categories were established to facilitate further development and applica-
tion of environmental technology. Elimination of traditional grouping was
consciously planned to foster technology transfer and maximum interface
in related fields. These series are:
L. ENVIRONMENTAL HEALTH EFFECTS RESEARCH
2. ENVIRONMENTAL PROTECTION TECHNOLOGY
3. ECOLOGICAL RESEARCH
4. ENVIRONMENTAL MONITORING
5. SOCIOECONOMIC ENVIRONMENTAL STUDIES
6. SCIENTIFIC AND TECHNICAL ASSESSMENT REPORTS
9. MISCELLANEOUS
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to
develop and demonstrate instrumentation, equipment and methodology
to repair or prevent environmental degradation from point and non-
point sources of pollution. This work provides the new or improved
technology required for the control and treatment of pollution sources
to meet environmental quality standards.
This document is available to the public for sale through the National
Technical Information Service, Springfield, Virginia 22161.
Publication No. EPA-650/2-74-128
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CONTiNTS
Page
LIST OF FIClMliS iv
INTRODUCTION 1
OPACITY AND VISUAL EFFECTS OF SMOKE PLUMES 3
Visibility Obscuration by Plumes 3
Visibility of Smoke Plumes 7
Observer Evaluation of Opacity 8
MEASUREMENT OF SMOKE OPACITY BY TRANSMISSOMETRY 10
Light Transmittance of Aerosols 10
Wavelength Consideration 11
Collimation Considerations 14
MEASUREMENT OF MASS CONCENTRATION BY TRANSMISSOMETRY 17
REMOTE MEASUREMENT OF PLUME OPACITY 24
Sun Photometry 24
Contrasting Target Telephotometry 25
Lidar 25
SUMMARY AND CONCLUSIONS 27
REFERENCES 31
TECHNICAL REPORT DATA AND ABSTRACT 33
111
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LIST OF FIGURES
Figure Page
1. Observer Estimates of the Opacity of Black and White Experimental
Plumes with Equivalent Vision Obscuration 7
2. Particle Extinction Coefficients for Various Aerosols Calculated
from the Mie Theory 12
3. Opacity of an Experimental White (Oil) Plume to Light of Various
Colors 13
4. Opacity of an Experimental Black (Carbon) Plume to Light of Various
Colors 13
5. Schematic of Transmissometer with Collimating Optics ]4
6. Opacity of an Aerosol as Measured with Transmissomctcrs with
Different Light Detection Angles as a Function of the True Opacity
of the Aerosol 15
7. Opacity of Aerosols Containing Particles of Different Sizes as
Measured with a 20 Degree Light Detection Angle as a Function of
the True Opacity of the Aerosol 15
8. Opacity of Smoke Plumes Containing Particles of Different Sizes
and Refractive Indexes as a Function of Their Mass Concentration _ .19
9. Opacity-Mass Concentration Relationship of Laboratory Generated
Coal - Fired Power Plant Emissions with Different Particle Sizes . .20
10. Opacity-Mass Concentration Relationship for Particulate Emissions
from a Kraft Recovery Furnace 22
11. Opacity-Mass Concentration Relationship for Particulate Emissions
from a Cement Plant Kiln 23
12. Opacity-Mass Concentration Relationship for Particulate Emissions
from a Bituminous-Coal-Fired Boiler 23
IV
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MEASUREMENT
OF THE OPACITY AND MASS CONCENTRATION
OF PARTICULATE EMISSIONS BY TRANSMISSOMETRY
INTRODUCTION
Evaluation of smoke emissions on the basis of appearance began near
the end of the nineteenth century when a number of different methods and
scales for visual estimation of smoke were proposed. It was during this
period that Maximilian Ringelmann developed a series of charts of gradu-
ated shades of gray for use as visual comparators for the evaluation of
gray or black smoke. Ringelmann Charts are still considered the refer-
ence standard of smoke regulations for visible emissions in most industrial
nations.
Although the Ringelmann Chart generally remains the reference standard
for regulation of visible emissions, its method of application has changed,
and other measurement methods have been applied. " One of the most common
methods is transmissometry, which has become an accepted instrumental
method for measuring the opacity of visible emissions. An equivalency has
been accepted between the opacity readings and Ringelmann number values of
"black" plumes. By the use of opacity measurements, visible emissions
2
regulations have been extended to plumes that are not gray or black, have
limited the effect of the environment on the method, and have made the
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regulation better represent the concentration of the particulate emissions.
Today, smoke inspectors are commonly trained to read visible emissions at a
plume evaluation school; they qualify when they learn to read black and
white training plumes with acceptable accuracies relative to transmissometer
measurements of the opacity of the emissions.
Opacity and/or mass standards are, or will be, placed on the
particulate emissions from new and existing sources. Emission controls
at these sources are required for implementation, maintenance, and
enforcement of emission standards and ambient air quality standards
promulgated by the Environmental Protection Agency as required by the
Clean Air Act of 1970. Standards of performance for new stationary
sources also require the monitoring of smoke emissions from a number
of sources, and the requirement is expected to be extended to additional
new and existing sources.
This paper discusses the relationship between the opacity, light
transmittance, visual effects, and mass concentration of particulate
emissions. It also discusses some optical-design factors of transmis-
someters that can affect their measurement of the opacity of visible
emissions.
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OPACITY AND VISUAL EFFECTS OF SMOKE PLUMES
Two different visual effects may be attributed to plumes containing
particles that scatter and/or absorb light: (1) the plumes become
visible, and (2) they obscure the visibility of objects behind them.
These visual effects have been analyzed in this laboratory in terms of
the luminance contrast between objects (or plumes) and their surroundings.
The work indicated that both effects are dependent not only on opacity but
on the amount of light scattered by plumes into the viewer's eyes and,
consequently, are dependent upon the environmental lighting conditions of
the plumes. In the case of obscuration of visibility by plumes, however,
the ambient lighting conditions have been shown to be less important.
It is the obscuration of visibility by plumes that represents the
basis for regulating plumes by a visual effect. The term opacity has been
defined as "the degree to which emissions reduce the transmission of light
and obscure the view of an object in the background." A theoretical
hypothesis of vision obscuration by a plume in terms of the reduction in
luminance contrast of an object viewed through the plume can be presented
to illustrate the opacity concept.
VISIBILITY OBSCURATION BY PLUMES
The luminance contrast (C ) between an object of luminance (B ) viewed
against an extended background such as the sky with luminance (B ) is:
:-Bs
Bs
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If the object is viewed through a plume, its apparent luminance (B )
and the apparent luminance of the sky behind the plume (B ) will depend
on the amount of light that the plume scatters from the surroundings and
sun into the viewer's eyes (B ), and on the amount of light transmitted
3.
through the plume from the background and object. The apparent contrast
of the object (C ) may be written as:
R
. .
c' .
The denominator of equation (2) remains, as in equation (1), the
intrinsic luminance of the sky (B ) since it remains the dominant lumi-
nance of the scene. Dividing by the intrinsic luminance of the sky
simplifies the analysis by removing the light scattering term (B ) from
3.
the final results. This differs from a more complicated earlier analysis
from this laboratory in which the reduction in contrast between two con-
trasting targets located behind the plume was discussed. Dividing by
the intrinsic luminance of the sky would not be realistic for atmospheric
pollution or for extended sources where the scatter and absorption of light
by the particulates would affect the luminance of the entire scene. For
these cases, the denominator of equation (2) should be the apparent lumi-
nance of the sky (B ) .
If the plume has a transmittance (T) and the amount of light scattered
by the plume toward the observer is B , then B_ = B T + B and B = B T + B
/r a'ttasss
and equation (2) may be written:
ct> - 1^
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The reduction in object to background contrast due to the plume may
be defined as:
C - C 1
t t
CR ' \-^ <4>
which, after substitution of equations (1) and (3) and rearrangement,
becomes:
CR = 1 - T (5)
Equation (5) indicates that the reduction in luminance contrast and
visibility of an object behind a plume viewed against an extended back-
ground is equivalent to one minus the transmittance (1-T) of the plume.
1-T is commonly called the opacity of the plume. The relationship also
indicates that the visibility reduction is independent of the environmental
illumination, and that there is an equivalency between the opacity of
plumes and their reduction of visibility.
The equivalent opacity concept was studied by the Bay Area Air Pollu-
tion Control District in the mid-1960's. In that study, a group of
observers was used to determine the light transmittances (opacities) of
black (carbon) and white (oil) experimental plumes with equivalent vision
obscurations. Because the observers had no previous experience in reading
smoke, they were screened to establish a statistically consistent group.
After instruction in the use of the Ringelmann Charts for evaluating black
plumes, the observers were asked to assign Ringelmann number values to
black plumes of various densities and to observe how much the black plumes
obscured the visibility of a target located behind them. They were then
asked to view a similar target behind whi£e plumes of various densities and
assign "equivalent Ringelmann numbers" to the plumes by equating their
5
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visibility reduction to the black plumes. During the tests, the opacity
of both plumes was monitored by identical photoelectric transmissometers.
The Bay Area APCD used the results of the tests to calibrate the trans-
missometers on the black and white smoke generators in terms of Ringelmann
numbers and "equivalent Ringelmann numbers," respectively. These curves
represented the average of 14,400 observer estimates of the Ringelmann
number values of the black plumes (9 observers, 80 series of 20 readings),
and 8,120 observer estimates of the "equivalent Ringelmann number" values
of white plumes (7 observers, 58 series of 20 readings).
Since the two Ringelmann number scales reported by the Bay Area APCD
are related through the equivalency in the vision obscuration produced by
the plumes, the two curves may be combined to show a general equivalency
in the transmissometer-measured opacity and the reduction in visibility
by the two plumes (Figure 1). The averaged data shown in Figure 1 indicate
that, in the important opacity region below 35 percent, black plumes have
slightly greater vision obscuration than white plumes with equivalent
opacities. In the opacity region above 35 percent, white plumes were
judged to have greater vision obscuration than black plumes with equivalent
opacities. Since standard deviations of up to 12 percent opacity are
reported for the data, the averaged curve shown in Figure 1 is within one
standard deviation of the theoretical one-to-one relationship.
Although the opacity of plumes represents a measure of how much they
obscure visibility, the plumes are generally located where they are observ-
able only against the sky, and their obscuration of visibility (obscuration
of objects behind them) is not observable. Consequently, it is the visual
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100
90
80
!"
of 60
u 50
> 40
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£ 30
20
10
I I
I I I I I I
10 20 30 40 SO 60 70
OPACITY BLACK PLUME, percent
90 100
Figure 1. Observer estimates of the opacity of black and white
experimental plumes with equivalent vision obscuration.
appearance of plumes that is usually observed by both the casual observer
and by the trained observer assessing their opacity.
VISIBILITY OF SMOKE PLUMES
6 .
Analysis of the visibility of plumes in terms of their luminance
contrast with their background (C ) has shown that:
S = r- -
(6)
where: B = the amount of light scattered by the plume toward the
observer
B = the luminance of the background (usually the sky)
T = the light transmittance of the plume
(1-T) = the opacity of the plume
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Equation (6) indicates that the visibility of a plume depends on the
amount of light scattered by the plume from the sun and its surroundings
into the viewers eyes (B ) relative to the luminance of the plume's back-
a
ground (B ) and the opacity of the plume (1 - T).
The dependency of the appearance of plumes on their environmental
lighting, background, and general viewing conditions makes plume visibility
or simple telephotometric contrast measurements undesirable for regulating
particulate emissions. Consequently, visible emissions regulations are
based on the opacity (visibility obscuration) of the plumes, which is an
intrinsic property of the plume and a better indicator of the particulate
emissions. It may be measured instrumentally by transmissometry or esti-
mated by trained observers.
OBSERVER EVALUATION OF OPACITY
The smoke inspector or trained observer receives training at a smoke
inspector training school where he is taught to evaluate the opacity of
black and white training plumes with prescribed accuracies relative to
transmissometer measurement of their opacities. Upon passing the course,
he becomes certified by the school as capable of evaluating the opacity of
smoke plumes from their appearance. This procedure requires the inspector
to evaluate an intrinsic optical property of the plumes (their opacity) by
observing their visibility, which is dependent not only on their opacity,
but also on their illumination and background viewing conditions.
Clearly, the trained-observer method of evaluating opacity has
limitations; e.g., at night when a plume cannot be seen, the method is
o
not generally applicable. Halow and Zeek have analyzed the visibility
of white plumes in daylight and have shown that observer evaluations depend
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upon the background and environmental lighting conditions of the plumes. A
white plume viewed against a white overcast sky is often invisible because
the amount of light it scatters toward the viewer equals the amount of
light that it attenuates from the background. The visibility of a white
plume viewed on a clear day is particularly sensitive to the viewing direc-
tion relative to the sun. The plume appears much brighter when the sun is
illuminating it from behind. Tests with trained observers have shown that
the evaluation of such plumes should be limited to times when they may be
seen with the sun illuminating them from the front. There are many plume
background and illuminating conditions confronting the smoke inspector in
the field that may impose limitations on the visual evaluation of the
plumes; consequently, it is important that the inspectors be trained
under viewing conditions that generally prevail in the field, and that
the inspectors know of any limitations that may result because of
deviations from these conditions.
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MEASUREMENT OF SMOKE OPACITY BY TRANSMISSOMETRY
The measurement of the light trausmittnixco or opacity of aerosols
by transmissometry requires the standardization of two important optical
characteristics of the transmissometer to obtain similar performance
between instruments. The need for standardizing the wavelength and
collimation characteristics of the transmissometer can best be illustrated
by examining the light-scattering characteristics of fine particles.
LIGHT TRANSMITTANCE OF AEROSOLS
A part of a parallel beam of incident light upon an aerosol such as
a smoke plume will always be removed from the beam by scattering and also
by absorption if the smoke is composed of absorbing particles. A measure
of the amount of light that passes through the smoke relative to the
amount of incident light is the transmittance of the smoke. The trans-
mitted light has not interacted with the particulates and will emerge
from the smoke still parallel to the incident beam.
The transmittance of light through an aerosol of monodisperse
9
spherical particles is defined by Bouguer's law:
T = - e - (7)
o
where: T = transmittance of the aerosol
F = light flux incident on the aerosol
F = light flux transmitted through aerosol
L = light path length through the aerosol
n = number concentration of the particles
a = projected area of one of the particles
Q = particle extinction coefficient
10
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The particle extinction coefficient (Q) is defined as the total flux
scattered and absorbed by a particle divided by the flux geometrically
incident on the particle. It is, in general, a function of the particle
size and shape, the refractive index, and the wavelength of the incident
light. The product naQ=o is sometimes called the turbidity coefficient
or the attenuation coefficient of the aerosol and has the dimensions (length )
WAVELENGTH CONSIDERATION
The effect of wavelength on the transmittance of smoke depends
primarily on the size of the particulates in the smoke and their associated
extinction coefficients. The particle size-extinction relationship can
usually be divided into three different light-scattering regions that
describe the scatter by particles much smaller than, larger than, and
comparable to the illuminating wavelength.
Particles that are much smaller than the illuminating wavelength
(d<0.05 micron in white light) are in the dipole or Rayleigh scattering
region. Particles in this region contribute little to the opacity of
smoke emissions since their extinction coefficients seldom exceed 10
-4
The extinction coefficients in this region are proportional to A if the
particles are transparent, and proportional to K~ if the particles are
absorbing.
Particles that are large compared to the illuminating wavelength
(d>2 micron in white light) are in the geometrical scattering region.
In this region, the extinction coefficient will have the value of 2 for
absorbing and irregularly shaped particles, but will oscillate around the
value of 2 for spherical transparent particles (Figure 2). For monodis-
perse spherical transparent particles in air, the oscillations in the
11
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OAVES FOR TRMSMKCNT MXTICUS
>,B< UONOOtSFCKC smCRtS, « -1.55. 1.5
C: IRREGULAR . RANDOM OmCNTMION , in • 1.9
Q4VC9 FOR »tJOB8WO UONOOHPIRSI WHERU, m • |.S»
0: TOTAL eiTINCIKM C.F: JUTTCRINO »
ABSORPTION COMPONENTS
PARTICLE-SIZE PARAMETER Q =7Td/\
0.5 1.0 1.5 2.0 2.5
AREA-MEAN PARTICLE DIAMETER (microns)
FOR WAVELENGTH OF 0.52 micron
Figure 2. Particle extinction coefficients for various aerosols
calculated from the Mie theory (except curve C).
extinction coefficient can result in extinction coefficient values in
this region being as high as 1.5 times the value of 2, to which it con-
verges at large particle sizes. If the particles are absorbing or
irregular in shape (randomly oriented), the limiting extinction coef-
ficient value of 2 can be obtained at particle sizes as small as 0.3
micron with little or no oscillation. In practice, the particulates in
smoke emissions are polydisperse and the oscillatory behavior of the
individual particle extinction coefficients is smoothed. Further
smoothing results when the particles vary in composition and the measure-
ments are made with polychromatic (white) light; consequently, smoke
emissions with mean particle sizes above 2 microns will generally have a
mean particle extinction coefficient of 2 and a transmittance that is not
a function of wavelength.
The intermediate size region (0.05
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region value of 10 to the plateau of 2 for absorbing and irregularly
shaped particles, and sometimes to a maximum value of 3 or 4 if the particles
are spherical, transparent, and monodisperse.
As indicated above, the extinction coefficient of highly absorbing and
irregularly shaped particles will reach the value of 2 between 0.3 and 0.7
micron. The maximum extinction coefficient value for transparent, spherical
particles is obtained between 0.67 and 1 micron in visible light before
oscillating around the value of 2.- Again, the oscillations are smoothed
toward the value of 2 for smoke emissions that are polydisperse and of
varying composition. Smoke emissions in this size region generally have
an extinction coefficient proportional to X~n(0
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COLLIMATION CONSIDERATIONS
To measure the amount of light transmitted by an aerosol, the trans-
missometer must be designed to exclude the light scattered by the aerosol
from the measurement. The poorly collimated transmissometer detects an
excessive amount of scattered light along with the transmitted light and
gives an erroneously high transmittance (low opacity) measurement of the
aerosol. Collimation of the in-stack transmissometer is obtained by
restricting the light projection and detection angles of the instrument
(Figure 5).
VIEW LIMITING APERATURE
DETECTOR
COMPACT
FILAMENT LAMP1
ANGLE OF VIEW -7 / PROJECTION ANGLE
i- -_-j' ^
COLLIMATING LENS
LENS CLEANING AIR
Figure 5. Schematic of transmissometer with collimating optics.
The error in the transmittance measurement due to the use of trans-
missometers with different light detection angles has been analyzed
theoretically by Ensor and Pilat and shown to be a function of detection
angle and aerosol particle size. They show that, in general, the error
associated with a given detector viewing angle increases with increased
particle mean diameter and, at a given particle mean diameter, decreases
14
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with increasing particle size geometric standard deviation (increasing
polydispersity of the particle size). These results were used to calculate
the apparent opacity of a 10-micron aerosol when measured with detection
angles of 0.2°, 2°, and 20° (Figure 6), and the apparent opacity of 2-, 5-,
10-, and 20-micron-sized aerosols when measured with a detection angle of
20° (Figure 7). Figure 6 shows that, for a given aerosol size, increased
error in the measurement of opacity results with increased detector angles
of view; Figure 7 shows that, for a given angle of view, increased error
in the measurement of opacity results with increased aerosol size.
50
40
30
ui 20
cc
V)
10
PARTICLE REFRACTIVE INDEX = 1.5
LIGHT WAVELENGTH=0.55 n
GEOMETRIC STANDARD DEVIATION
PARTICLE SIZE DISTRIBUTION =
PARTICLE MASS MEAN DIAMETER = 10 fi'
LIGHT SOURCE PROJECTION ANGLE = 0
^
)NOF A?
fl || W
10 20 30 40
TRUE OPACITY, percent
50
40
£ 30
u
o
a
ui 20
tu
S
10
50
PARTICLE REFRACTIVE INDEX = 1.5
LIGHT WAVELENGTH = 0.55 \i
GEOMETRIC STANDARD DEVIATION OF
PARTICLE SIZE DISTRIBUTION = 3
dgw= PARTICLE MASS MEAN DIAMETER
LIGHT SOURCE PROJECTION ANGLE = 0
LIGHT DETECTION ANGLE = 20
10 '20 30 40
TRUE OPACITY, percent
Figure 6. Opacity of an aerosol as measured Figure 7. Opacity of aerosols containing
with transmissometers with different light particles of different sizes as measured with a
detection angles as a function of the true transmissometer with a 20 degree light detection
opacity of the aerosol. angle as a function of the true opacity of the
aerosol.
An experimental study of the effect of the collimating angles of the
detector and light source of a transmissometer on the measurement of the
opacity of emissions from a coal-fired steam generator was conducted by
Peterson and Tomaides. They show that the size of the collimating angles
15
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of the detector and light source produces similar errors, and both must be
restricted to minimize the error in the measured transmittance or opacity.
Detailed design and performance specifications for transmissometer opacity
12
measurement systems have been reported.
16
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MEASUREMENT OF MASS CONCENTRATION BY TRANSMISSOMETRY
The value of the transmissometer for monitoring the mass concentration
of particulate emissions from pollution sources has not been resolved.
Several investigators have reported the observation of good empirical cor-
relations between the mass concentration and light transmittance of specific
sources, whereas others have pointed out that the usefulness of such relation-
ships is too dependent on invariable particulate characteristics of the
sources. To understand the effect of various aerosol characteristics on the
relationship, it is useful to express Bouguer's transmittance law in terms
of the mass concentration of the aerosol.
For a polydisperse aerosol, Bouguer's law, shown in equation (7), may
be written as:
T = exp ^ Z n.d2Q. (8)
* -4 i ixi *• '
and the mass concentration (C ) of the aerosol may be written as:
where: T = transmittance of the aerosol
L = light path length through the aerosol
d. = diameter of the particles within the size increment i
n. = number of particles of size d.
Q. = particle extinction coefficient of the particles of size d.
p = density of the particles in the aerosol
Dividing equations (8) and (9) and solving for C gives:
2En.d.3
SEn.d. 0.
i i xi
17
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By defining a mean extinction coefficient, Q = En.d.^./En.d, 2, and a mean
particle diameter, d = En.d.3/En.d. 2, for the aerosol, equation (10) may
be written:
r 2dp 1 , .
Cm = —— In = (11)
3QL
d is often referred to as the volume-to-surface mean diameter of the
aerosol (sometimes called Sauter diameter).
Pilot and linsor express equation (10) as:
Cm = E2- ln T (12)
and define K as the specific particulate volume (cm3 particles/m3 air)
divided by the aerosol attenuation coefficient (m ). They show theoretical
values of K as a function of particle size for several aerosols with differ-
ent characteristics. These values of K were used in equation (12) to
calculate the effect of particle size and composition on the relationship
between the opacity and mass concentration of aerosols (Figure 8).
Figure 8 shows that particle size is the primary characteristic of the
aerosol affecting the opacity-mass concentration relationship when the
particle size of the aerosol is larger than 3 or 4 microns. However, at
particle sizes below 3 or 4 microns, the refractive index of the particu-
lates also has a pronounced effect on the relationship. This results in
the white particles in Figure 8 showing a maximum opacity per unit mass
concentration at about 0.6 micron particle size, whereas the black particles
show the maximum at about 0.15 micron.
Pilot and Ensor 3 also show that the width of the size distribution
will usually affect the opacity-mass concentration relation. In addition,
irregular transparent particles smaller than 2 microns generally attenuate
18
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10
20
30
+*
§ 40
09
a
UJ
5°
2 60
70
80
\
\
\
\
LOG NORMAL DISTRIBUTION
STANDARD DEVIATION OQ = 4
PARTICLE DENSITY = 2 gram/cm?
WAVELENGTH = 0.55 /u
REFRACTIVE INDEX
WHITE = 1.5
BLACK = 1.96-0.66i
I N
0.1
0.2 0.3
MASS CONCENTRATION, e/m3
0.10
0.20
0.30
0.40
o
o
0.50
0.4
0.5
Figure 8. Opacity of smoke plumes containing particles of different sizes and refractive
indexes as a function of their mass concentration.
14
less light than spherical particles of the same projected area con-
sequently, the observed mass concentration indicated in Figure 8 for small
transparent particles will be two to three times too low for irregular
particles, but not appreciably different for absorbing particles.
The effect of particle size on the opacity-mass concentration relation-
ship has been studied experimentally in the laboratory by Uthe and Lapple.
They collected fly ash from a bituminous coal-fired power plant and
classified it into a series of size fractions. The various size fractions
were then pneumatically injected into an aerosol chamber at controlled
concentrations where the opacity was measured with a transmissometer.
19
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Figure 9 is the opacity-mass concentration relationship observed by Uthe
and Lapple for aerosols with four different mean sizes. Each point shown
in the figure generally represents an average of two to six runs. The
mass concentrations were calculated from aerosol generation rates.
§ 40 —
E
ro
u
4
0.1
0.2
0.3 0.4 0.5 0.6
MASS CONCENTRATION, g/m3
0.7
0.8
Figure 9. Opacity - mass concentration relationship of laboratory generated coal-
fired power plant emissions with different particle sizes.
These results were as expected and show the particle size dependence
indicated from theoretical considerations. Comparison of Figure 9 with
the theoretical calculations of Figure 8 for spherical particles with a
refractive index of 1.5 and a specific gravity of 2 indicates that the
coal-fired power plant fly ash attenuated around 50 percent as much light
20
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for similar particle sizes and mass concentrations. Considering the dif-
ferences in aerosol characteristics, better agreement was not to be
expected. The ash used for the aerosol generation was reported to have
size distributions of o =1.5 and contained a good number of particulates
o
with absorption characteristics; additionally, the specific gravity of 2
used for the theoretical calculations may be somewhat low for fly ash.
It is clear that for a useful relationship to exist between the
opacity and mass concentration of the particulate emissions from a pol-
lution source, the characteristics of the particulates (size, shape, and
composition) must be sufficiently constant, and for a conventional trans-
missometer to be useful as a monitor of the mass concentration, the
particulate characteristics must remain constant over a useful period of
time. Some experimental data are available that show good empirical
opacity-mass concentration calibrations can be obtained for transmissometers
on some sources (Figures 10, 11, and 12). Although these data indicate the
particle characteristics were sufficiently constant during the time of
calibration, no data on the long-terra usefulness of the calibrations seem
to be available. Considering the simplicity of the transmissometer, the
collection of such data seems to be highly desirable in order to evaluate
its potential as a mass monitor for specific sources. It is likely that
particulate emission control equipment will also control the characteristic
of the aerosol that most affects the relationship (particle size) and
improve the potential of the transmissometer as a mass monitor.
21
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30
25
* 20
I «
CO
to
n
i
0.10
0.09
0.08
0.07
0.06
0.05 £
STANDARD CONDITIONS
DRY GAS
0.02
0.12
0.14
0.04 0.06 0.08 0.10
MASS CONCENTRATION, g/m3
Figure 10. Opacity-mass concentration relationship for particulate emissions
from a kraft recovery furnace.16
UJ
O
o
0.04 z
o
0.03 ^
2
ui
0.02 £
0.01
0
22
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45
40
~ 30
o 20
10
0.30
STANDARD CONDITIONS
DRY GAS
0.1 0.2 0.3
MASS CONCENTRATION, g/m3
0.4
0.20
0.15 it
o
o
z
o
<
0.1 Oz
0.05
0.5
Figure 11. Opacity-mass concentration relationship for paniculate
emissions from a cement plant kiln.17
§
2
o
60
50
40
30
20
10
STANDARD CONDITIONS
DRY GAS
0.20
0.18.
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.1 0.2 0.3 0.4 0.5 0.6
MASS CONCENTRATION. g/m3
0.7
0.8
Figure 12. Opacity-mass concentration relationship for particulate
emissions from a bituminous coal-fired boilerJo
23
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REMOTE MEASUREMENT OF PLUME OPACITY
There are three instrumental methods that have been described for
remote measurement of plume opacity. Two of the methods use photometry of
the sun or of contrasting targets through the plumes for the measurements.
These methods are considered methods of opportunity since they are applicable
only under certain conditions. The third method is more general and uses a
laser radar (lidar) technique. With this method, the plume opacity is
determined by shooting a pulse of light through the plume and measuring the
ratio of the light backscattered from the pulse by the atmosphere before and
after the plume.
SUN PHOTOMETRY
Measurement of plume opacity by sun photometry is restricted to times
when the sun is unobstructed by clouds and may be viewed through a discrete
cross section of the plume at the stack exit. Conditions for the measure-
ment are usually best on clear days when the sun is relatively low in the
sky, the sun can be viewed through the plume at the stack exit, and the
direction of flow of the dispersing plume in the atmosphere is away from
the sun. A sun photometer specifically designed for plume opacity measure-
ments is available from the Shell Development Company. The measurement can
19
also be made with a standard sun photometer, e.g., the Volz sun photometer.
However, it may be necessary to modify the spectral response of the standard
sun photometer to obtain good sensitivity to visible light. A Volz sun
photometer modified for plume opacity measurements has been described.
24
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CONTRASTING TARGET TELEPHOTOMETRY
Measurement of plume opacity by contrasting target telephotometry is
restricted to times when contrasting targets can be viewed through the plume
at the stack exit and to times when the ambient illumination of the plume is
stable during the measurement. The contrasting targets may be distant hills,
tall buildings, or towers and the sky adjacent to them. For the plume
illumination to be sufficiently stable during the measurement normally
requires that the sky be clear or uniformly overcast. The plume opacity is
determined by using a narrow-angle-view (less than 1/2 degree) telephotometer
to measure the ratio of the luminance difference between targets when viewed
through and beside the plume. Narrow-angle telephotometers suitable for the
measurement are commercially available.
It is also feasible to determine the opacity of a plume by photo-
graphing the contrasting targets through them, e.g., from a helicopter. The
ratio of the luminance differences between the targets is then measured from
the film images with a laboratory densitometer. Plume opacity measurements
by telephotometry and photography of contrasting targets have been described
• j * -i 6,20
in detail.
LIDAR
The technique of determining plume opacity by lidar was first proposed
to the Edison Electric Institute and the U. S. Public Health Service by the
late M.G.H. Ligda of Stanford Research Institute (SRI) during the middle
1960's. EEI and PHS pursued the development of the technique as part of a
cooperative research study in the measurement of the opacity of plumes from
stacks of steam electric power plants. They contracted SRI to conduct a
laboratory and field study of the method using existing SRI lidar equipment
25
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designed for atmospheric studies. The results indicated the method was
feasible and defined many of the design parameters needed for a lidar
21
system for plume opacity measurements. The General Electric Company
was then contracted to develop a mobile lidar system specifically designed
20 22
for remote measurement of plume opacity. ' The performance of the lidar
is presently being evaluated by the Environmental Protection Agency in con-
junction with studies to develop and evaluate opacity measurement methods
for various sources.
Application of the pulsed lidar system described above is primarily
^
for opacity research studies and for further development of the method. In
its present form the instrument would have little application for surveil-
lance work by a control agency because of cost, operating power, and com-
plexity. For the development of a smaller, low-powered, and less expensive
instrument for remote measurement of plume opacity, a lidar technique using
a frequency modulated, continuous wave (CW) laser is being investigated.
The initial investigation of the CW lidar indicates that the method is
23
feasible, and further development is planned.
26
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SUMMARY AND CONCLUSIONS
Visible emission standards have been used for regulating the parti-
culate emissions of stationary sources for more than 75 years. During
most of this period, the regulations applied only to emissions that
appeared gray or black, and the primary method of measuring the blackness
of the emissions was the Ringelmann Chart comparators. However, during
the past 20 years, visible emissions regulations have been extended to
any emission affecting visibility regardless of color. This general
application of the visible emissions regulations has been accomplished by
defining the emission standards in terms of the opacity of the plumes
emitted by the sources. Plume opacity is an intrinsic optical property
of the emissions that is measured instrumentally by light transmission
methods. To permit the older Ringelmann number standards to also be
measured by light transmission methods, a Ringelmann number 1, 2, 3, 4,-
opacity 20, 40, 60, 80 equivalency is normally accepted.
Opacity of plumes is also measured by trained smoke observers who
attend a smoke-reading course where they are taught to assign opacity
values to plumes based on their appearance. The inspectors become certified
smoke readers when they learn to assign opacity values to a range of black
and white training plumes with a specified accuracy relative to a calibrated
in-stack transmission meter. It should be noted that the trained observer
is not actually evaluating the visibility of plumes but is evaluating the
opacity of plumes by observing their visibility. The actual visibility of
plumes is generally too dependent upon ambient illumination and background
viewing conditions to be well regulated by opacity standards. The plume
27
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visual effect best regulated by the opacity standards is the obscuration
of visibility.
The transmissometer used to measure the opacity of particulates must
have two essential design features. The spectral response of the instru-
ment should be limited to visible light and, moreover, should be mostly
sensitive to green light. The instrument should also be designed with
light collimating optics to exclude light scattered by the particulates
from the measurement. A properly designed and installed in-stack transmis-
someter should be capable of monitoring the opacity of the plume emitted
from the stack provided water in condensed form is not present and the
characteristics of the aerosols in the effluent are maintained between the
in-stack measurement point and the plume.
In considering the relationship between opacity and mass concentration
of a particulate emission, it should be recognized that the opacity of the
emission is a stack-size dependent measurement, whereas its mass concentra-
tion is not. In addition, the relationship between the opacity or light
transmittance and mass concentration of particulate emissions is complicated
due to its dependence upon the characteristics of the particulates, e.g.,
size, shape, composition, etc. Nevertheless, as the opacity of the particu-
late emissions approaches zero, their mass concentrations also approach zero;
consequently, limiting the opacity of the emissions has the effect of
limiting their mass concentration.
Although only a limited opacity-mass concentration relationship can be
expected between sources of different sizes and particulate characteristics,
or for sources with fluctuating particulate characteristics, a good opacity-
28
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mass concentration relationship can be expected for sources with particulate
characteristics that are stable. Development of such relationshipj is
usually done by empirical calibration of a transmissometer on the source with
an accepted manual gravimetric sampling method. Good empirical opacity-mass
concentration calibrations have been obtained for transmissometers on a
number of stationary sources; however, the precision of the calibrations for
long-term mass monitoring of the sources by transmissometry has not been
determined nor has the precision of the calibration within a source category
been determined. Clearly, the usefulness of the calibration depends on the
stability of the particulate characteristics of the source and the similarity
of the particulate characteristics between sources within a source category.
Remote instrumental measurement of plume opacity is complicated by the
ambient light scattered by the plume in the direction of the viewer. This
scattered light generally prevents opacity measurement by simple plume-to-
background contrast measurements by direct telephotometry or by photometry
of plume photographs although such methods may at times give reasonable
estimates of plume opacity if the plume consists of light-absorbing particles
such that the amount of light scattered by the plume toward the viewer is
small when compared to the brightness of the plume background.
Two relatively simple methods exist for remote instrumental measurement
of opacity, but both are methods of opportunity. They are through-the-plume
photometry of the sun and telephotometry of contrasting targets. The sun
photometer method is most applicable in those areas with a large number of
clear days. It is also limited to those sources whose plumes can be viewed with
29
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the sun behind them. The telephotometry-of-contrasting-target method is
limited to those sources whose plumes can be viewed with contrasting scenes
behind them, e.g., a hill or building and the adjacent sky. The method is
difficult to apply if the environmental or background lighting conditions
of the plumes or the opacity of the plumes is fluctuating.
A more general laser radar (lidar) method for remote measurement of
plume opacity is being developed, but at present the instrumentation is
complicated and expensive. Development of simpler instrumentation for the
method is being pursued.
30
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REFERENCES
1. Ringelmann, M. Method of Estimating Smoke Produced by Industrial
Installations. Rev. Techniques. 268-271, June 1898.
2. Coons, J. D., H. A. James, H. C. Johnson, and M. S. Walker. Develop-
ment, Calibration, and Use of a Plume Evaluation Training Unit.
Journal of Air Pollution Control Association lj^: 199-203, May 1965.
3. Rose, A. H., J. S. Nader, and P. A. Drinker. Development of an
Improved Smoke Inspection Guide. Journal of Air Pollution Control
Association 8^:112-116, August 1958.
4. Rose, A. H., and J. S. Nader. Field Evaluation of an Improved Smoke
Inspection Guide. Journal of Air Pollution Control Association 8_:117-
119, August 1958.
5. Conner, W. D., C. F. Smith, and J. S. Nader. Development of a Smoke
Guide for the Evaluation of White Plumes. Journal of Air Pollution
Control Association l&i748-758, November 1968.
6. Conner, W. D., and J. R. Hodkinson. Optical Properties and Visual
Effects of Smoke-Stack Plumes. U. S. Public Health Service Publica-
tion No. 999-AP-30. NTIS Publication Number PB 174-705. Springfield,
Virginia, 1967. 89pp.
7. Standards of Performance for New Stationary Sources. Federal Register
36 (247):24876-24895, December 23, 1971.
8. Halow, J. S., and S. J. Zeek. Predicting Ringelmann Number and Optical
Characteristics of Plumes. Journal of Air Pollution Control Association
23^:676-684, August 1973.
9. Hodkinson, J. R. The Optical Measurement of Aerosols. In: Aerosol
Science, Davies, C. N. (ed.). New York, Academic Press, 1966. p.288.
10. Ensor, D. S., and M. J. Pilat. The Effect of Particle Size Distribution
on Light Transmittance Measurement. American Industrial Hygiene Associa-
tion Journal 32_: 287-292, May 1971.
11. Peterson, C. M., and M. Tomaides. In-Stack Transmittance Techniques for
Measuring Opacities of Particulate Emissions from Stationary Sources.
Prepared for EPA by Environmental Research Corporation under Contract
Number 68-02-0309. NTIS Publication Number PB 212-741. Springfield,
Virginia, April 1972. 87pp.
12. Nader, J. S., F. Jaye, and W. Conner. Performance Specifications for
Stationary Source Monitoring Systems for Gases and Visible Emissions.
Environmental Protection Agency. Publication Number EPA-650/2-74-013.
Research Triangle Park, N. C. January 1974. 74p.
31
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13. Pilat, M. J., and D. S. Ensor. Plume Opacity and Particulate Mass
Concentration. Atmospheric Environment. 4_: 163-173, April 1970.
14. Hodkinson, J. R. The Physical Basis of Dust Measurement by Light
Scattering. In: Aerosols: Physical Chemistry and Application.
Spurney, K. (ed.). Publishing House of the Czechoslovak Academy of
Science, 1965. p.184.
15. Uthe, E. E., and C. E. Lapple. Study of Laser Backscatter by Particu-
lates in Stack Emissions. Prepared for EPA by Stanford Research
Institute under Contract Number CPA 70-173. NTIS Publication Number
PB 212-530. Springfield, Virginia. January 1972. 58p.
16. National Council for Air and Stream Improvement Test Report. Pre-
sented at West Coast Regional NCASI Meeting, Portland, Oregon, 1972.
17. Buhne, K.W., and L. Duwel. Recording Dust Emission Measurements in
the Cement Industry with the RM4 Smoke Density Meter made by Messrs
Sick. Staub ^2(8):19-26, August 1972.
18. Schneider, W. A. Opacity Monitoring of Stack Emissions: A Design
Tool with Promising Results. The 1974 Electric Utility - Generation
Planbook. New York, Mc-Graw-Hill, 1974. p.73.
19. Volz. F. A Photometer for Measuring Solar Radiation. Arch. Met.,
Geophys. and Blokl. BIO;100-131. 1959.
20. Cook, C. S., G. W. Bethke, and W. D. Conner. Remote Measurement of
Smoke Plume Transmittance Using Lidar. Applied Optics 11:1742-1748,
August 1972.
21. Evens, W. E. Development of Lidar Stack Effluent Opacity Measuring
System. Prepared for Edison Electric Institute by Stanford Research
Institute under SRI Project Number 6529. NTIS Publication Number
PB 233-135/AS. Springfield, Virginia, July 1967.
22. Bethke, G. W. Development of Range Squared and Off-Gating Modifications
for a Lidar System. Prepared for EPA by General Electric Company under
Contract 68-02-0570. Publication Number EPA-650/2-73-040. Research
Triangle Park, N. C., December 1973. 47pp.
23. Ferguson, R. A. Feasibility of a CW Lidar Technique for Measurement of
Plume Opacity. Prepared for EPA by Stanford Research Institute under
Contract Number 68-02-0543. Publication Number EPA-650/2-73-037.
Research Triangle Park, N. C., November 1973. 90p.
32
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
IiPA-650/2-74-128
2.
3. RECIPIENT'S ACCESSION*NO.
4. TITLE AND SUBTITLE
Measurement of the Opacity and Mass Concentration
of Particulate Emissions by Transmissometry
6. REPORT DATE
November 1974
6. PERFORMING ORGANIZATION CODE
7. AUTHORIS)
William D. Conner
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Chemistry and Physics Laboratory
Environmental Protection Agency
National Environmental Research Center
Research Triangle Park. N.C. 27711
10. PROGRAM ELEMENT NO.
10 ROAP No. 26AAM
11.C
ACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
IB. SUPPLEMENTARY NOTES
16. ABSTRACT
This report is a review of particulate opacity measurement techniques.
The relationship between the opacity of plumes and their visual effects,
and the relationship between the opacity of plumes and their particulate
mass concentration are discussed. The report also discusses optical
design characteristics of transmissometers that are necessary for
measurement of the opacity of particulate emissions. Various methods
for remote measurement of plume opacity are reviewed including the
visible emission observer.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lOENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
opacity
visible emissions
transmissometry
particulate emissions
lidar
photometry
18. DISTRIBUTION STATEMENT
Release unlimited
19. SECURITY CLASS (This Report)
None
21. NO. OF PAGES
39
20. SECURITY CLASS (Thispage)
None
22. PRICE
EPA Form 2220-1 (9-73)
33
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