OPTICAL STUDIES
OF AUTOMOTIVE AND NATURAL HAZES:
SCATTERING FROM SINGLE PARTICLES
(FINAL REPORT)
Authors
David T. Phillips, Ph.D.
Philip J. Wyatt, Ph.D
Supported by
Environmental Protection Agency
Air Pollution Control Office
and
The Coordinating Research Council, Inc.
CRC-APRAC Project No. CAPA 6-68
APCO Contract CPA-70-171
Science Spectrum, Inc.
Santa Barbara, California
February 1971
The findings of this report are not to be construed
as an official position of the Environmental Protection
Agency and/or of the Coordinating Research Council,
Inc., unless so designated by other authorized
documents.
-------
OPTICAL STUDIES
OF AUTOMOTIVE AND NATURAL HAZES:
SCATTERING FROM SINGLE PARTICLES
(FINAL REPORT)
Authors
David T. Phillips, Ph.D.
Philip J. Wyatt, Ph.D
Supported by
Environmental Protection Agency
Air Pollution Control Office
and
The Coordinating Research Council, Inc.
CRC-APRAC Project No. CAPA 6-68
APCO Contract CPA-70-171
Science Spectrum, Inc.
Santa Barbara, California
February 1971
The findings of this report are not to be construed
as an official position of the Environmental Protection
Agency and/or of the Coordinating Research Council,
Inc., unless so designated by other authorized
documents.
-------
ACKNOWLEDGMENTS
The authors thank Peter B. Schoefer and Dr. Chelcie
B. Liu for their many contributions to this project, and
Herman H. Brooks, who contributed essential electronic
innovation.
ii
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ABSTRACT
The use of single particle light scattering measure-
ments to determine the origin of atmospheric hazes has
been explored by measurement of laboratory aerosols,
field samples, and computer analysis of the light
scattering data.
Analytical methods are developed for the determination
of refractive index of such particles.
Analysis of scattering curves for larger laboratory
aerosol particles shows measurable differences in refractive
index between a photochemical pine tree aerosol and a
photochemical petroleum aerosol. For particles of diameter
less than 500 nanometers only the measurement of absolute
scattering intensity at two angles is required. Distinctive
non-spherical and absorbing particles were observed both in
automotive exhaust and atmospheric samples. Recommendations
are made for further development and testing of the single
particle optical scattering method for particulate air
pollution analysis.
iii
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No.
CONTENTS
Title
Page
3
4
5
ACKNOWLEDGMENTS ii
ABSTRACT iii
FIGURE CAPTIONS V
INTRODUCTION 1
Objective 1
Results 1
Recommendations 2
EXPERIMENTAL PROGRAM 3
Laboratory Aerosol Production 3
Differential II Instrument 3
Measurements in Laboratory Samples 5
Field Studies 6
ANALYTICAL PROGRAM 17
CONCLUSIONS AND DISCUSSION 36
REFERENCES 37
iv
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FIGURES
Number Title Page
1 Schematic representation of the light 4
scattering measurement. Scattered
light intensity is recorded as a
function of scattering angle by the
Differential II single particle
photometer.
2 Initial pine + UV particle differential 7
scattering intensity. Vertical and
horizontal polarization, incident
wavelength 514.5 nm.
3 Differential scattered intensity of a g
pine + UV particle after 90 minutes.
Vertical polarization, 514.5 nm
wavelength.
4 Differential scattered intensity of a 9
gasoline + NO + UV particle. Vertical
and horizontal polarization 514.5 nm
wavelength.
5 Automobile exhaust particle differential 10
scattered intensity, vertical polarization
514.5 nm wavelength. Note the signal
variation caused by rotation of this
irregular particle.
6 Haze particle differential scattered 12
intensity. Burbank (Los Angeles county).
Vertical polarization 514.5 nm wavelength.
7 Haze particle differential scattered 13
intensity. Goleta (Santa Barbara county).
Vertical polarization, 514.5 nm wavelength.
8 Irregular atmospheric particle differential 14
intensity, Santa Barbara. Vertical polar-
ization, 514.5 nm wavelength. Note the
similarity to the automobile exhaust
particle shown in Figure 5.
9 Theoretical differential scattered intensity 15
for homogeneous spherical particles of
refractive indices 1.33, 1.42, 1.5, 1.59
at 514.5 nm wavelength. Particle diameter
100 nm, vertical polarization.
v
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Number Title page
10 Theoretical differential scattered 16
intensity for homogeneous spherical
particles of refractive indices 1.33,
1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 100 nm, horizontal
polarization.
11 Theoretical differential scattered 18
intensity for homogeneous spherical
particles of refractive indices 1.33,
1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 300 nm, vertical
polarization.
12 Theoretical differential scattered 19
intensity for homogeneous spherical
particles of refractive indices 1.33,
1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 300 nm, horizontal
polarization.
13 Theoretical differential scattered 20
intensity for homogeneous spherical
particles of refractive indices 1.33,
1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 500 nm, vertical
polarization.
14 Theoretical differential scattered 21
intensity for homogeneous spherical
particles of refractive indices 1.33,
1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 500 nm, horizontal
polarization.
15 Theoretical differential scattered 22
intensity for homogeneous spherical
particles of refractive indices 1.33,
1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 1,000 nm, vertical
polarization.
16 Theoretical differential scattered 23
intensity for homogeneous spherical
particles of refractive indices 1.33,
1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 1,000 nm, horizontal
polarization.
VI
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Number Title Page
17 Particle radius and refractive index 24
as a function of the scattered intensity
at 20° and the ratio of the scattered
intensity at 40° to that at 20°. Precise
absolute measurement of scattered intensity
at 20° and 40° allows the determination of
radius and index over a limited size range.
18 Theoretical differential scattered 25
intensity for the initial pine aerosol
particle (Fig. 2)/ 514.5 nm wavelength,
vertical polarization, n = 1.49 nm;
r = 540 nm, 550 nm, 560 nm. Crosses show
experimental data.
19 Theoretical differential scattered 27
intensity for the initial pine aerosol
particle (Pig. 2), 514.5 nm wavelength,
vertical polarization, r = 550 nm;
n= 1.49, 1.59, 1.50. Crosses show
experimental data.
20 Theoretical differential scattered 29
intensity for the initial pine aerosol
particle (Fig. 2), 514.5 nm wavelength,
horizontal polarization. n= 1.49;
r = 540 nm, 550 nm, 560 nm. Crosses
show experimental data.
21 Theoretical differential scattered 30
intensity for the initial pine aerosol
particle (Fig. 2), 514.5 nm wavelength,
horizontal polarization, r = 550 nm;
n « 1.43, 1.59, 1.50. Crosses show
experimental data.
22 Theoretical differential scattered 31
intensity for the 90-minute pine aerosol
particle (Fig. 3), 514.5 nm wavelength,
vertical polarization. n =* 1.49;
r - 495 nm, 505 nm, 515 nm. Crosses
show experimental data.
23 Theoretical differential scattered 32
intensity for the gasoline + NO aerosol
particle (Fig. 4), 514.5 nm wavelength,
verticle polarization. n = 1.54;
r = 400 nm, 410 nm, 420 nm. Crosses show
experimental data.
VI1
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Number Title Page
24 Theoretical differential scattered 33
intensity for the gasoline + NO aerosol
particle (Fig. 4), 514.5 nm wavelength,
vertical polarization, r = 410 nm;
n = 1.52, 1.54, 1.56. Crosses show
experimental data.
25 Theoretical differential scattered 34
intensity for the gasoline + NO
aerosol particle (Fig. 4), 514.5 nm
wavelength, ^vertical ^polarization:
n = 1.54; r = 400 nm, 410 nm,
420 nm. Crosses show experimental
data.
26 Theoretical differential scattered 35
intensity for the gasoline + NO
aerosol particle (Fig. 4), 514.5 nm
wavelength, horizontal polarization.
r = 410 nm; n = 1.48, 1.49, 1.50.
Crosses show experimental data.
viii
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1. INTRODUCTION
This project is a part of the continuing joint haze re-
search effort of the Air Pollution Control Office and the
Coordinating Research Council, Inc. A recent review of work
in this area was given by 0. A. Germogenova, et al, in Atmos-
pheric Saae: A Review, Bolt, Beranek and Newman, Report No.
1821, March 1970, a Final Report for Contracts CAPA-6-68 (1-68)
and CPA 22-69-29.
A specific impetus for the focus of the present project on the
difference between petroleum and vegetative based haze is the work
by R. A. Rasmussen of Washington State University, Pullman,
Washington, on the contributions of trees to air pollution. An
exhaustive discussion of light scattering and its applications may
be found in the book by M. Kerker, The Scattering of Light and Other
Electromagnetic Radiation, Academic Press, New York 1969.
Objective.
The objective of this study was to determine the feasibility
of using measurements of the angular variation of the intensity
of light scattered from single particles in the problem of iden-
tifying haze or aerosol particles originating from auto exhaust
emissions. Particles to be examined included reaction chamber
products produced by the following reactions:
1) Auto exhaust + NO, + UV radiation
2) Pine tree limbs + N02 + UV radiation
Scattering measurements were to be attemped on single "smog"
particles during a period of haze in Los Angeles.
Results.
Measurements of the angular variation of light scattered from
single particles produced in a reaction chamber by the reactions,
1) Gasoline + N02 + UV radiation
2) Pine cone + N02 + UV radiation,
show that these aerosols can be distinguished by single particle
light scattering measurements. The refractive index of a petroleum
aerosol particle was found to be 1.54 * 0.02 while the pine aerosol
particle index was 1.49 * 0.01. No attempt was made to accurately
reproduce atmospheric conditions in those laboratory experiments.
-------
Field measurements of single particle light scattering
were carried out in Los Angeles and Santa Barbara. During the
period available for field measurements pollutant levels were
not' high enough to cause eye irritation, though visibility was
reduced to 4-12 miles by photochemical haze. Haze particles
measured ranged from 200 nm (nanometers) to 500 nm in diameter.
A method by which the refractive index of particles of
diameter 100 to 500 nm can be determined using measurements of
the absolute intensity of the scattered light in addition to its
angular variation has been developed. The theoretical scattering
computations required for the method have been carried out.
Nonspherical particles, identified by flicker in the
scattered light, were found both in fresh samples of auto
exhaust and field samples of atmospheric haze.
Re commendations.
The feasibility of single particle optical determinations
of refractive index has been demonstrated for specific laboratory
aerosols. Field measurements show the method can be applied to
atmospheric haze. Single particle optical measurements promise
to be useful for study and control of particulate atmospheric
contamination. Further testing and development of the optical
method is required.
Precise measurement of absolute scattering intensity to-
gether with the angular dependence of scattering should be em-
ployed to determine the refractive index of small particles.
Depolarization, flicker, and dissymetry measurements should be
investigated in connection with nonspherical particles. Further
analytical studies are needed for these particles. Analysis of
scattering from absorbing metallic or combustion products should
be carried out. Optical field studies should be made routinely
in Los Angeles as well as measurements in contrasting rural and
urban areas. Joint chemical and optical studies, both in the
laboratory and of the Los Angeles atmosphere are needed to aid
the initial interpretation of optical data.
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2, EXPERIMENTAL PROGRAM
Laboratory Aerosol Production.
Four different laboratory aerosols have been produced in a
ten-gallon glass chamber. In each case, a 40-watt quartz envelope-
mercury discharge lamp was used as a source of ozone and ultra-
violet radiation. NO was generated for some experiments by
dropping a few strands of copper wire into a 40% solution of nitric
acid. The concentration of the reactants in these experiments were
higher than those normally found in the atmosphere.
1. A handful of lemon leaves, crushed and placed in
the chamber, quickly react with the ozone to
produce a fine aerosol of spherical particles.
2. A large resin-laden pine cone placed in the chamber
reacts with the ozone to produce a continuing
supply of spherical aerosol particles.
3. Samples of auto exhaust, captured in plastic bags,
cooled to remove excess moisture, and introduced
into the chamber contained particles of irregular
shape, which settled out in 24 hours. The low
concentration of hydrocarbons in the exhaust sample
prevented the growth of large particles in the
small reaction vessel.
4. Raw gasoline in an open dish together with NO
and ozone produced a heavy aerosol of spherical
particles as well as a tarry deposit on the walls
of the reaction chamber.
Differential II Instrument.
The Science Spectrum Differential II single particle
scattering photometer was used to make the measurements
described in this report. A schematic representation of a
light scattering photometer is shown in Fig. 1. Samples
of the aerosol were removed from the reaction chamber with
a 50 cc syringe, and injected into the scattering cell in a
slowly moving air stream. They can be viewed by eye as they
cross a laser beam. Particles selected for study are held
indefinitely in the cell by electrostatic fields. When the
particle is first selected, the electric fields are varied
under manual control to hold the particle near the center of
the laser beam. Within a few seconds the particle chosen is
pulled to the center of the cell by the radial component of the
field, while other particles are swept away, because their
charge-to-mass ratio and initial position are different than
the particle selected for study.
-------
DETECTOR
LASER
6 SCATTERING ANGLE
SCATTERER
Figure 1 -
Schematic representation of the light scattering
measurement. Scattered light intensity is recorded
as a function of scattering angle by the Differential
II single particle photometer.
-------
The electric charge of the laboratory aerosols was found
to vary. To hold particles with only one or two electrons,
electrode potentials were increased to 1000 volts. Aerosols
formed with zero charge were charged with a 20 kv corona
discharge electron gun.
When the chosen particle is alone in the beam, control
of the fields is transferred to an automatic servo system which
photoelectrically monitors the particle position. While the
particle is held steady in the laser beam, a 1P21 photomultiplier
detector with appropriate masks and filters is rotated around
the cell. An X-Y recorder plots the scattered light intensity
as a function of angle.
The light source is a Science Spectrum/TRW special pulsed
argon-ion laser tunable to the following wavelengths:
Wavelength Average
(nm) _ Relative Intensity Power (mW)
514.5 1.00 .5
496.5 .34 .17
488.0 .6 .3
476.5 .36 .18
457.9 .08 .04
The beam is single transverse mode, plane polarized, with
1 milliradian divergence.
Other characteristics of the Differential II are:
Angular Resolution (scattered light): 2.0°
Angular Incremental Advance; 2.5 minutes
Scanning Speeds: 180°/min. , 90°/min. , 45°/min. , 22.5°/ndn. ,
Slewing Speed; 720°/min.
Angular Range: 8° to 172°
Angular Accuracy (.readout) * 40 minutes
Measurements of Laboratory Samples.
Plots of light scattering intensity versus scattering angle
for single particles of reaction chamber aerosols are shown on
the following pages. In all cases the light was plane polarized
-------
and of wavelength 514.5 nm. The laser was mounted to provide
vertically polarized light (perpendicular to the scattering
plane) and a half wave plate was used to obtain horizontal
polarization (parallel to the plane of scattering), without
much loss of light intensity. The curves presented are the
basis for the laboratory test of the feasibility of the use
of single particle light scattering measurements to determine
the origin of haze particles. The sensitivity of the instru-
ment has been adjusted to place the first peak near the top
of the scale.
Figure 2 shows the vertically and horizontally polarized
light scattering patterns from a pine-ozone haze particle when
first captured. Note that the vertical curve has six peaks.
Figure 3 shows the vertical scattering from the same particle
90 minutes later. Evaporation has reduced the particle size
and only five peaks remain. This vegetative particle is to
be distinguished from the petroleum based particle whose
vertical and horizontal polarization scattering curves are
shown in Figure 4. The analysis of this data is discussed in
Section 3.
Fresh auto exhaust samples, taken from an idling 50,000
mile Volkswagen sedan burning leaded regular gas was found to
contain a large number of particles exhibiting a distinct
forward peak in scattered light intensity. A typical measure-
ment, shown in Figure 5, illustrates the steep forward peak
and flicker characteristic of these particles. It is reasonable
to suppose that these samples are mixtures of lead and solid
combustion products like carbon, though chemical studies and
theoretical analysis of the scattering from such objects have
not been performed. Such particles are distinctive and may
provide an indication of fossil fuel air contamination.
Field Studies.
For the field measurements a mobile laboratory was con-
structed. The Differential II photometer and its recorder
were mounted on a table in the rear of an I. H. Scout vehicle.
A portable power pack utilizing two lead-acid storage batteries
and a 275 watt inverter was used for power. Though the voltage
was reasonably stable, the supply was not regulated and absolute
intensity calibration was not attempted. The operator was
seated in the rear seat in a convenient position to operate
the instrument. Extreme temperature variations caused some
difficulty with optical alignment but not enough to prevent
the use of the instrument. Hundreds of particles were visible
in each sample. Haze particles with sufficient charge to hold
were relatively infrequent, but enough were easily found for
the measurements.
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1
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Figure 5 - Automobile exhaust particle differential scattered
intensity, vertical polarization 514.5 nm wavelength,
Note the signal variation caused by rotation of this
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AUTO EXHAUST PARTICLE
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SCATTERING ANGLE
10
-------
Field studies were made on January 25 through 31 in Santa
Barbara, as well as 15 miles to the west in Goleta and 100 miles
to the east in Los Angeles during light haze. No severe smog
conditions occurred in Los Angeles during the period that the
mobile laboratory was in operation. Measurements were made in
Burbank and Universal City areas of Los Angeles on January 27
when visibility was limited to 6-12 miles by photochemical haze.
No eye irritation was present. Typical scattering curves are
shown in Figures 6 through 10. The size of the spherical haze
particles observed was typically 500 nm or smaller/ as deter-
mined by comparison with theoretical curves shown in Section 3.
Figure 10 shows an atmospheric particle with fluctuations
similar to those observed in automotive exhaust samples. The
accurate optical classification of such irregular particles
remains to be done, though precise measurement of the fluctua-
tion amplitude is technically straightforward.
11
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Figure 9 - Theoretical differential scattered.intensity for
homogeneous spherical particles of refractive indices
1.33,. 1.42, 1.5, 1.59 at 514.5 nm wavelength. Particle
diameter 100 nm, vertical polarization.
VERTICAL POLARIZATION
100 nm DIAMETER
60° 80° 100°
SCATTERING ANGLE
15
-------
Figure 10 - Theoretical differential scattered intensity for
homogeneous spherical particles of refractive indices
1.33, 1.42, 1.5, 1.59 at 514.5 run wavelength.
Particle diameter 100 nm, horizontal polarization.
HORIZONTAL POLARIZATION
100 nm DIAMETER
80' 100°
SCATTERING ANGlf
16
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3, ANALYTICAL PROGRAM: DEDUCTION
OF SIZE AND REFRACTIVE INDEX
The objective of the analysis of the scattering data is
to identify the source of the measured particle. Many of the
particles seen appear to be spherical as evidenced by steady
scattering, as would be expected from droplets of water or
tarry liquids, in light scattering, spherical particles are
characterized by their radius and refractive index. Though
the radius of the particles may be of meteorological interest,
the refractive index is a direct consequence of the chemical
composition of the particle, and thus offers an important
guide for identification. In this study the primary analytical
problem has been the determination of refractive index from
light scattering data.
Though most of the particles scattered light steadily,
indicating spherical symmetry, some particles were seen to
flicker violently. These may be particles with irregular
shape such as combustion products composed of metals, carbon, or
other absorbing materials. Such materials are common air
pollutants. No attempt was made to analyze the scattering
of nonspherical or absorbing particles, though the presence
of such particles in fresh auto exhaust samples suggests
they may provide important information in atmospheric
studies.
Layered structures formed by the condensation of one
material on a nucleus of a different material are also
important in the formation of haze. No attempt was made to
test the measured curves for the presence of such particles
though this should be possible if the nucleus is 50 nm or
larger in diameter.1
Extensive digital computer calculations have been used
to quantitatively interpret the measured single spherical
particle scattering curves. Theoretical scattering curves for
vertically and horizontally polarized 514.5 nm wavelength light
incident on homogeneous, transparent spheres of diameter
100 nm, 300 nm, 500 nm and 1000 nm with refractive index 1.33
(water), 1.42 (protein), 1.5 (glass), 1.59 (latex), are
given in Figs. 11 through 18.
Note that the numbers of peaks in the vertical scattering
curve is given approximately by 2Dn/X, which provides a
simple way to size particles within about 0.1 micron. Note
also that the curves for particles with higher refractive
index tend to have variations of larger relative amplitude in
the back angles.
17
-------
Figure 11 - Theoretical differential scattered intensity for
homogeneous spherical particles of refractive indices
1.33, 1.42f 1.5, 1.59 at 514.5 run wavelength.
Particle diameter 300 nm, vertical polarization.
VERTICAL POLARIZATION
300 nm DIAMETER
JIM; BSifi s&s
60° 80e 100°
SCATTERING ANGLE
180°
18
-------
Figure 12 - Theoretical differential scattered intensity for
homogeneous spherical particles of refractive indices
1.33, 1.42, 1.5, 1.59 at 514.5. nm wavelength.
Particle diameter 300 nm, horizontal polarization.
HORIZONTAL POLARIZATION
300 nm DIAMETER
o°
20°
40*
60° 80' 100*
SCATTERING ANGLE
120'
140°
160°
19
-------
Figure 13 - Theoretical differential scattered intensity for
homogeneous spherical particles of refractive indices
1.33, 1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 500 nm, vertical polarization.
it iiiitiii
VERTICAL POLARIZATION
500 nm DIAMETER
60° 80° 100C 120° 140c 160'
SCATTERING ANGLE
180°
20
-------
Figure 14 - Theoretical differential scattered intensity for
homogeneous spherical particles of refractive indices
1.33, 1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 500 nm, horizontal polarization.
HORIZONTAL POLARIZATION
500 nm DIAMETER
0°
20'
40°
60° 80° 100° 120°
SCATTERING ANGLE
21
-------
Figure 15 - Theoretical differential scattered intensity for
homogeneous spherical particles of refractive indices
1.33, 1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 1,000 nm, vertical polarization.
VERTICAL POLARIZATION
1000 nm DIAMETER
60C 80° 100°
SCATTERING ANGLE
22
-------
Figure 16 - Theoretical differential scattered intensity for
homogeneous spherical particles of refractive indices
1.33, 1.42, 1.5, 1.59 at 514.5 nm wavelength.
Particle diameter 1,000 nm, horizontal polarization.
HORIZONTAL POLARIZATION
1000 nm DIAMETER
80° 100°
SCATTERING ANGLE
23
-------
Figure 17 - Particle radius and refractive index as a function of the
scattered intensity at 20° and the ratio of the scattered
intensity at 40° to that at 20°. Precise absolute measure-
ment of scattered intensity at 20° and 40° allows the
determination of radius and index over a limited size range,
0.5
0.6 0.7
RELATIVE INTENSITY. I(4W/I(20*>
0.8
0.9
24
-------
Figure 18 - Theoretical differential scattered intensity for the
initial pine aerosol particle (Fig. 2), 514.5 nm wavelength,
vertical polarization. n = 1.49 nm; r = 540 nm, 550 nm,
560 nm. Crosses show experimental data.
INITIAL PINE + \\v PARTICLE
THEORETICAL SCATTERING
H VERTICAL POLARIZATION
VARIOUS RADII, N - 1.49
80' 100-
SC AFTER ING ANGLE
25
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The theoretical scattering curves show that particles
smaller than 500 nm diameter exhibit no secondary scattering
peaks. As the particle diameter becomes greater than 500 nm
secondary scattering peaks in the differential scattering
intensity become quite pronounced. For small particles the
relative differential scattering intensity is (approximately) a
universal function of the parameter nD, wh^re n is the refractive
index and D is the diameter. For larger particles the effects
of radius and index on the shape of the scattering curve can
be distinguished. For particles of diameter smaller than 500 nm,
a relative scattering intensity measurement is not adequate to
determine the refractive index. However, the absolute value
of scattering intensity depends on higher powers of refractive
index. Thus the shape of the scattering curve and its absolute
intensity together provide a unique determination of radius
and index for small dielectric spheres. The ratio of the in-
tensity of vertically polarized scattered light at 40°, 1(40°),
to the_intensity at 20°, 1(20°), has been found to provide a
convenient measure of the shape of the scattering curve. The
relation of 1(20°) and I(40°)/I (20°) to radius and index is
shown, in Figure 19 by curves for various values of refractive
index with particle radius as a running parameter. Using this
figure, a particle radius and an index of refraction can be
determined for a measured pair 1(20°) and I(40°)/I(20°).
This method of analysis required highly accurate measure-
ments and absolute intensity calibration of the instrument.
To be useful in categorizing hydrocarbon aerosols/ the index
must be determined to approximately one per cent. This requires
measurement of the ratio of the intensities at two angles to
approximately 0.2% accuracy. The requirement for the absolute
intensity measurement is somewhat less, about 1%. Absolute cali-
bration can be carried out using Dow latex spheres as a standard.
Measurements to the required accuracy are difficult but are
believed to be achievable. Since the accuracy of optical measure-
ments are ultimately limited by the number of photons detected,
an increase in laser power will be very helpful for the smaller
particles, since many natural haze particles are smaller than
500 nm diameter, and this approach shows promise for particle
identification in this size range, effort to achieve such
measurements should be continued. Furthermore, since intensity
measurements of only two angles are needed the analysis is
compatible with a rapid particle counting and sorting requirement.
For particles of diameter greater than about 500 nm, the
determination of radius and refractive index can be based
entirely on the vertical and horizontal scattering curves,
without absolute intensity calibration. This method has been
previously reported.2 visual comparison of the measured
scattering curves with an atlas of theoretical curves allows
the approximate determination of radius and refractive index.
26
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Figure 19 - Theoretical differential scattered intensity for the initial
pine aerosol particle (Fig. 2), 514.5 ira wavelength,
vertical polarization, r = 550 nm; n » 1.48, 1.49, 1.50.
Crosses show experimental data.
INITIAL PINE + hv PARTICLE
THEORETICAL SCATTERING
VERTICAL POLARIZATION
VARIOUS REFRACTIVE INDICES
60* 80* 100' 120' 140*
SCATTERING ANGLE
27
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Then a net of curves with closely spaced value of radius and
index are computed in the neighborhood of the measured particle.
Careful examination of the number and position of the peaks and
valleys, and particularly the relative height of the peaks and
valleys allows selection of the fit. Theoretical curves
bracketing the best fit for the representative Pine + UV particle
(Figures 2 and 3) and the representative Gasoline + NO + UV
particle (Figure 4) are shown in Figures 20 through 25. Con-
siderable theoretical and experimental attention has been given
to the uniqueness of size and index determinations by light
scattering by other workers.3'1*
Further development of analytical techniques will be re-
quired for an easily automated index measurement for large
particles. However, since these curves have characteristic
properties recognizable by eye, it seems probable that analytical
methods could be developed which would provide a rapid and
accurate determination of refractive index and radius for large
spherical particles. Fourier, or spherical harmonic analysis,
promise to offer a more systematic approach to this problem.
The measurement of absolute intensities required for smaller
particles may also be of use in this size range. More theoret-
ical work is needed in this area.
28
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Figure 20
Theoretical differential scattered intensity for the
initial pine aerosol particle (Fig. 2), 514.5 nm wavelength,
horizontal polarization, n = 1.49; r = 540 nm, 550 nra,
560 nm. Crosses show experimental data.
INITIAL PINE + hi/ PARTICLE «
-] THEORETICAL SCATTERING
^i|:::i:u:CT:-|;::1 HORIZONTAL POLARIZATION
VARIOUS RADII, N • 1.49
ft
60* 80' 100* 120*
SCATTERING ANGLE
29
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Figure 21
Theoretical differential scattered intensity for the
initial pine aerosol particle (Fig. 2), 514.5 nm wavelength,
horizontal polarization. r = 550 nm; n = 1.48, 1.49,
1.50. Crosses show experimental data.
INITIAL PINE + hi/ PARTICLE -t-
THEORETICAL SCATTERING
HORIZONTAL POLARIZATION 4-
VARIOUS REFRACTIVE INDICES
80° 100- 120'
SCATTERING ANGLE
30
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Figure 22
Theoretical differential scattered intensity for the
90-minute pine aerosol particle (Fig. 3), 514.5 nm
wavelength, vertical polarization, n = 1.49; r = 495 nm,
505 nm, 515 nm. Crosses show experimental data.
FINAL PINE + hi/ PARTICLE
THEORETICAL SCATTERING
VERTICAL POLARIZATION
VARIOUS RADII, N - 1.49
60' 80' 100*
SCATTERING ANGLE
31
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Figure 23
Theoretical differential scattered intensity for the
gasoline + NO aerosol particle (Fig. 4), 514.5 nm
wavelength, verticle polarization, n = 1.54; r = 400
410 nm, 420 nm. Crosses show experimental data.
nm,
GASOLINE + NOX + hi/ PARTICLE
THEORETICAL SCATTERING
VERTICAL POLARIZATION
VARIOUS RADII, N - 1.54
80' 100*
SCATTERING ANGLE
32
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Figure 24
Theoretical differential scattered intensity for the
gasoline + NO aerosol particle (Fig. 4), 514.5 nm
wavelength, vertical polarization, r = 410 nm; n = 1.52,
1.54, 1.56. Crosses show experimental data..
GASOLINE + NOx + hi/ PARTICLE
THEORETICAL SCATTERING
VERTICAL POLARIZATION
VARIOUS REFRACTIVE INDICES
60' 80* 100-
SCATTERING ANGLE
33
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Figure 25 Theoretical differential scattered intensity for the
gasoline + NO aerosol particle (Fig. 4), 514.5 nm
wavelength, vertical polarization. n = 1.54; r = 400 nm,
410 nm, 420 nm. Crosses show experimental data.
GASOLINE + NOx + h^ PARTICLE
THEORETICAL SCATTERING
HORIZONTAL POLARIZATION
VARIOUS RADII, N - 1.54
W 100'
SCATTERING ANGLE
34
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Figure 26
Theoretical differential scattered intensity for the
gasoline + NO aerosol particle (Fig. 4), 514.5 nm
wavelength, horizontal polarization, r = 410 nm;
n = 1.48, 1.49, 1.50. Crosses show experimental data.
GASOLINE + NOX + hi/ PARTICLE
THEORETICAL SCATTERING
HORIZONTAL POLARIZATION
VARIOUS REFRACTIVE INDICES
60* 80* 10(r 120'
SCATTERING ANGLE
35
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CONCLUSIONS AND DISCUSSION
Reaction chamber aerosol single particle light scattering
measurements have been used to determine the refractive indices
of pine + UV and gasoline + N0x + UV. The values obtained are
1.49 ± .01 and 1.54 * .02, respectively. Thus these aerosols
can be distinguished by single particle optical scattering
measurements. Differential light scattering intensity measure-
ments of single atmospheric haze particles have been made
successfully in the Santa Barbara and Los Angeles areas.
It appears feasible to use such measurements to distinguish
the origin of certain classes of individual atmospheric haze
particles. More extensive field studies are required to
properly evaluate the usefulness of the method. Joint chemical
and light scattering studies would be particularly useful.
An analytical method to determine the refractive index of
particles in the 100 to 500 nm diameter range by using absolute
intensity of the scattering at two fixed angles has been
developed. By adjusting the angles of measurement the method
can be extended to smaller or larger particle diameters. This
promises to be an effective tool, well adapted for rapid
measurements of large numbers of particles, for air pollution
studies.
Nonspherical particles have been observed in automotive
exhaust samples. Such particles are expected to change the
polarization of the scattered light. This readily observable
effect offers a potentially powerful but untested tool for
the categorization of particles. Also, an unsymmetric particle
does not scatter light with equal intensity at equal scattering
angles on opposite sides of the incident beam as a spherical
particle does. Measurement of the correlation of the signals
in opposed detectors would provide additional information about
the symmetry of the scatterer. Flicker amplitude measurements
may also be useful.
The speed limitations of chart recorders and the manual
manipulation of single particles are not inherent in the light
scattering measurement. Once the utility of scattering measure-
ments has been proven, the use of special purpose computers
and high speed scattering detectors can make the routine measure-
ment of airborne particles a convenient and precise tool.
36
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5, REFERENCES
1. A. L. Aden and M. Kerker, J. Appl. Phys. 22^ 1242 (1951) .
A. Guttler, Ann. Phyaik, [6] 11 65 (1952).
W. F. Espensheid, E. Willis/ E. Matijevic, and M. Kerker,
J. Colloid & Int. Soi. 2jO 501 (1965).
2. D. T. Phillips, P. J. Wyatt, and R. M. Berkman, J. Colloid
Int. Soi. 3£ 159 (1970).
3. W. A. Farone and M. Kerker, J. Opt. Soo. Am. 56_ 476 (1966),
D. Cooke, and M. Kerker, J. Opt. Soo. Am. 59_ 43 (1969).
4. R. Mireles, J. Math, and Phys. 45_ 127 (1966).
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