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.

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              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.

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                       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.

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     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.

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                                        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.

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     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

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 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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Figure 5 - Automobile  exhaust particle differential  scattered
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     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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 Ln  (~h (~h t~h
 •   O  H- Hi
       O  (D
    (~t* 3  (^
    3"^   CD
    CD     3

    £)  |—J t-f.
    C^  »t^ &>
    rt •   M
    O  m
    3     H-
    033
    CT 3  ri-
    I-1-    CD
    I-1    3
    CD     01
          H-
          rt

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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).
                                37

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