United States Office of Air Quality EPA-450/4-83-018
Environmental Protection Planning and Standards June 1983
Agency Research Triangle Park NC 27711
__
&EFA RECEPTOR
MODEL
TECHNICAL
SERIES
VOLUME IV
Summary of Particle
Identification
Techniques
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EPA-4SO/4-83-018
RECEPTOR MODEL TECHNICAL
SERIES
VOLUME IV
Summary of Particle Identification Techniques
Prepared By
George E. Weant
J. Calvin Thames
Contract No. 68-02-3509
Work Assignment No. 24
EPA Project Officer: Thompson G. Pace
Engineering-Science
501 Willard Street
Durham, NC 27701 y.S. Environmental Protection Agency
Region V, Library
230 South Dearborn Street
Chir-cFQ, nijnois 60304
Prepared For
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Monitoring And Data Analysis Division
Research Triangle Park, NC 27711
June 1983
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DISCLAIMER
This report has been reviewed by the Office of Air Quality Planning
and Standards, U. S. Environmental Protection Agency, and approved for
publication as received from Engineering-Science, Inc. Approval does not
signify that the contents necessarily reflect the views and policies of
the U. S. Environmental Protection Agency, nor does mention of trade names
or commercial products constitute endorsement or recommendations for use.
U.S. Environmental Protection Agency
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TABLE OF CONTENTS
1.0 INTRODUCTION 1
2.0 PARTICLE IDENTIFICATION METHODS 2
2.1 Sampl i ng Met hods 2
2.1.1 Samplers 2
2.1.2 Filter Media 3
2.2 Analytical Methods 3
2.2.1 Optical Microscopy 3
2.2.2 Scanning Electron Microscopy 7
2.2.3 X-Ray Diffraction 9
3.0 TYPES AND PROPERTIES OF PARTICLES 11
3.1 Particle Type Terminology 11
3.2 Particle Types and Properties 14
3.2.1 Minerals 14
3.2.2 Combustion Products 14
3.2.3 Other Particles 20
3.3 Secondary Particles 20
4.0 EVALUATING AND APPLYING RESULTS 23
4.1 Sampl i ng Devices 23
4.1.1 Sampler Location 23
4.1.2 Sampler Biases 24
4.1.3 Sampling Time 26
4.2 Filter Material 26
4.2.1 Fiber Filters 27
4.2.2 Membrane Filters 27
4.2.3 Artifact Formation 29
4.2.4 Negative Artifacts 31
4.3 Preparation of Samples 32
4.3.1 Nature of Filters ...32
4.3.2 Handling of Filters and Removal of
Particles From Filters 32
4.3.3 Mounting of Samples 34
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TABLE OF CONTENTS (CONTINUED)
4.4 Particle Size 35
4.4.2 Microscopic Size Measurement Techniques 35
4.4.2 Direct Measurement of Volume and Determination
of Mass 37
4.4.3 Aerodynamic Size 38
4.4.4 Conversion of Particle Sizing Data 39
4.4.5 Errors, Biases, and Limitations 39
4.4.6 Particle Size Distributions 40
4.4.7 Creation of Size Distributions 43
4.5 Source Fingerprints 44
4.6 Quality Assurance 48
5.0 SELECTION OF METHODS 50
5.1 Methods 50
5.2 Cost of Analyses 53
6.0 SELECTION OF A LABORATORY 55
7.0 REFERENCES 57
APPENDIX 59
IV
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LIST OF TABLES
Table Page
3.1 Common Minerals, Composition, and Possible Origins 15
4.1 Characterization of Volume (or Mass) Particle Size
Distribution Modes 41
4.2 Semi-Automatic and Automatic Particle Counting and
Measuring System 45
5.1 Selection of Monitors and Analysis Techniques Based on
Sampling Conditions 51
5.2 Selection of Filters and Analytical Techniques 52
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LIST OF FIGURES
2-1 Generalized Particle Identification System for Transparent
Mineral Particles 5
4-1 Relationships Between Particle Size Measuring Techniques.......36
4-2 Idealized Graphical Representation of a Bimodal Distribution...42
4-3 Combustion Particles and Associated X-Ray Spectra 47
VI
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RECEPTOR MODEL TECHNICAL SERIES
Vol. 1 - Overview of Receptor Model Application to Participate Source
Apportionment, EPA-450/4-81-016a, NTIS PB82-139429, Code
A05, $11.50
Vol. 2 - Chemical Mass Balance, EPA-450/4-81-016b, NTIS PB82-187345, Code
A07, $14.50
Vol. 3 - User's Manual for Chemical Mass Balance Model, EPA-450/4-83-014
Vol. 4 - Summary of Particle Identification Techniques, EPA-450/4-83-018
VI1
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1.0 INTRODUCTION
The purpose of this document is to provide a generalized discussion
of the methods used in particle identification and characterization for
source apportionment studies. This document can be used by agency per-
sonnel as a guide in commissioning source apportionment studies and
interpreting the results from these studies. It is not meant to be a
"how-to" manual of microscopic methods.
In the analysis of a heterogeneous mixture of particles collected on
ambient filters, chemical analysis will give only the composition in terms
of chemical elements or radicals. In source apportionment studies, a
more detailed analysis, in which the individual particles are identified
and characterized, is desirable to provide an overall picture of the
origins of these particles. In many cases, particle identification
provides the information necessary to evaluate the impacts of various
sources on the receptors.
To accomplish the characterization and identification of individual
particles, certain physical methods are used either independently or in
combination with chemical analyses. This document describes these physi-
cal methods and, where appropriate, the chemical methods associated with
them.
This document provides the basic information on the techniques, des-
cribes the types and properties of particles, discusses the selection of
methods, provides the basic information to evaluate and apply the results
of studies, and discusses the parameters to be used to select a laboratory
to perform the analyses.
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2.0 PARTICLE IDENTIFICATION METHODS
2.1 SAMPLING METHODS
2.1.1
Many methods have been used for the collection of ambient particles
for analysis. These include settling, impinging, and filtration. The
filtration method is the only procedure discussed here since these collec-
tion devices are usually already an integral part of an agency's ambient
monitoring program.
The most common ambient particulate sampler is the high volume samp-
ler (hi-vol). The hi-vol is a simple device which draws ambient air at
a rate of 1.1 to 1.7 m^/min. (40-60 ft^/min) through an 20-cm by 25-cm
(8-in. by 10-in.) filter. The sampler is designed so that all particles
smaller than about 100pm are collected, however, collection efficiency
is low for the larger particles. 1
If only particles of a given size range are of interest, accessories
are available for this purpose. The standard inlet to the hi-vol can be
replaced with a cascade impactor which separates the particles according
to aerodynamic diameters into several size ranges. If inhalable particu-
lates are of concern, a size-selective inlet can be used in place of
the standard inlet. This allows the collection only of those particles
less than 10 ym in size.
An alternate sampling device for inhalable particulates is the di-
chotomous sampler. This sampler, also called a virtual impactor, frac-
tionates and collects particle into two size ranges; coarse and fine. The
coarse particles are considered to be 10 to 2.5 ym (aerodynamic) while
the fine particles are less than 2.5 ym. Particles larger than 10 ym
are not collected.
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2.1.2 Filter Media
Several types of filter media are available for use with all of the
air samplers. Each filter type has its own advantages and disadvantages
depending on their intended purpose. The two major filter types are fi-
ber and membrane. The glass-fiber filters are those normally used in
ambient monitoring. These filters are not usually suitable for chemical
analysis because of high background levels of impurities. Cellulose-fiber
filters share the advantages of the glass fiber filters; low pressure
drop and cost. They are also free of many of the impurities found in
the glass fiber and are more suitable for chemical analysis. However,
cellulose is a very hygroscopic material. Fiber filters are not particu-
larily suitable for use in microscopy because particles tend to be buried
in the fibers and cannot be easily seen.
Membrane filters are available in several different materials includ-
ing mixed cellulose esters, polyvinyl chloride (PVC), polytetrafluoroethy-
lene (PTFE), and silver. These filters provide fairly untextured surfaces
with low particle penetration. This type of surface allows optical micro-
scopy and SEM to be conducted easily. However, the filters do have high
pressure drops, so that longer sampling times at lower flow rates are
required to obtain a representative sample.
The polycarbonate filters have a nearly smooth surface. The parti-
cles collected on these filters can be easily examined through microscopy
techniques and removed for elemental analysis. However, like the membrane
filters, the polycarbonate filters have high pressure drops.
A more detailed discussion of filters is presented in Section 4.2.
2.2 ANALYTICAL METHODS
2.2.1 Optical Microscopy
Optical microscopy is the oldest particle identification method in
use today. Despite its age, it is still a rapid, accurate technique.
The information obtained by this method includes the particle structure,
transparency, color, and optical properties. These characteristics can
be used to identify particles.
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Depending on the particle under study, reflected and/or trans-
mitted light is used. Reflected light is used on opaque particles. Sur-
face features, shape, color, and luster can be examined with reflected
light, and in some instances, these parameters are sufficient to adequate-
ly identify the particle. Reflected light can also be used for the
direct examination of particles on membrane and polycarbonate filters.
Light transmitted from below the microscope stage, passes through the
particle and into the microscope tube. During the light's travel, it is
subjected to various distortions and alterations by the components of
the microscope and by the particle itself. One of the most important
alterations to light is its polarization, i.e., the restriction of
vibration to one direction. The use of polarized light is a valuable
tool in particle identification.
The intent of this report is to describe the various techniques that
are used to identify particles and not to present theory of operation.
A generalized particle identification system for transparent particles
with brief explanations of terms and techniques is presented. The system
would follow that shown in Figure 2.1. The system is presented for miner-
al particles because of their predominance in collected particles and be-
cause most other particles would be readily identified as non-mineral. An
exception to this occurs with non-spherical , non-carbonaneous combustion
fly ash particles which are commonly misidentified as being minerals.
The identification system is meant to be used with a series of tables
that associate the properties or parameters with lists of minerals having
these properties. Reference 6 has such tables as do other texts on opti-
cal mineralogy. In many cases, a particle can be identified at a glance.
In others, only a few of the grain's properties must be examined. In
others, the entire system must be followed. Many of the terms present-
ed here are defined in Section 3.1.
Color Color is often a key to mineral identification. In addition
to color, a grain should be checked for pleochroism using polarized light.
Pleochroism is the change in color exhibited by anisotropic minerals when
the microscope stage is rotated. Isotropic minerals do not show pleochro-
ism.
Habit (Shape and Form) Many minerals consistently assume a parti-
cular shape or form.Such a tendency is called habit.
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Transparent Mineral
Does Not
Exhibit
Birefringence
I s o t ro p i c
Index of Refraction
Color
Shape or Form
Cleavage
Surface Topography
Internal Structure
Does
Exhibit
Anisotropic
U n i a x i a 1
Biaxial
Figure 2-1.
Generalized Particle Identification System
for Transparent Mineral Particles
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Cleavage Cleavage, parting, and fracture are sometimes character-
istic of minerals.
Index of Refraction The index of refraction for mineral particles
can be determined with the microscope by two methods; central illumination
(Becke line) and oblique illumination. Both methods employ the use of
liquids with precise indices of refraction. The index of the particle
is compared to the liquid until equal indices are found, and the particle
index is then known. The central illumination method employs a refraction
phenomenon known as the Becke Line to measure differences in refractive in-
dices. The Becke Line is a blur or irregular white line just inside or
outside the mineral grain. As the microscope tube is raised, the line
moves toward the medium of higher refractive index. A series of these
tests are run with immersion oils of various refractive indices until the
equal index is found.
The oblique illumination method uses a card which is inserted below
the stage, and half of the light is cut off. This procedure darkens one
half of the field and illuminates the other half with oblique rays. The
oblique rays cause a shadow on one side of the mineral grain. If the sha-
dow appears next to the dark side of the stage the index of the particle
is less than the index of the immersion oil. When the indices are equal,
the particle will be blue on one side and red on the other when white
light is used for illumination.
Isotropic vs. Anisotrophic Minerals which crystalize in the iso-
metric system and mineraloids are isotropic (having the same properties
in all directions). In most cases, these minerals can be identified by
their physical properties. Minerals crystallizing in other crystal
systems are anisotropic (having properties that vary with direction).
Birefringence (Double Refraction) Anisotropic substances split
polarized light into two vectors which have different refractive indices,
which travel through the crystal along different paths of different
lengths, and which vibrate in planes perpendicular to each other. The
two rays pass through the analyzer (a polarizer or nicol prism) where
they are aligned to vibrate in the same plane. The rays are in a posi-
tion to interfere and interference colors are produced. The color pro-
duced depends on the amount of retardation (one ray is retarded due to
the longer distance of travel through the grain).
The difference in the refractive indices of the two vector components
is the birefringence. The maximum birefringence, or the greatest differ-
ence between the two indices of refraction, is approximately constant for
a given mineral. Experienced microscopists can use the birefringence, as
shown by interference colors, of a particle to assist in its identifica-
tion.
Optical Character Anisotropic minerals are either uniaxial (having
but one direction in which the light passing through the crystal is not
doubly refracted) or biaxial (having two optic axes or lines of no double
refraction). Interference figures are used to determine the optical
character of the mineral.
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Calcite and aragonite are chemically identical substances; both are
CaC03. By using microscopy, these two substances may be distinguishable
by noting that aragonite lacks the rhombohedral cleavage of calcite and
is biaxial.
Dispersion staining is another particle identification technique that
is used in optical microscopy. The sample is immersed in a liquid medium
of known refractive index. The dispersion colors created by the differ-
ence between the refractive index dispersion of the sample and the immer-
sion medium make it possible to identify particles. The procedure has
been used to identify toxic dusts, minerals, and fibers. It is valid for
use with both isotropic and anisotropic substances.
2.2.2 Scanning Electron Microscopy
Scanning electron microscopy (SEM) can provide information on parti-
cle size, shape, texture, and topography. Particles are examined on a
one-at-a-time basis, or automated systems can be used. Elemental composi-
tion can also be determined when the appropriate analyzer is used with
the microscope. In many respects SEM produces results similar to those
obtained with a light microscope. It has a magnification range of 20 to
100,000 X with a depth of field which is 300 times that of the light
microscope.- It also has a much greater resolution ability. In the
normal examination of small particles, the SEM uses magnifications of
100 to 5000 X. This provides for a resolution of 20 nm as compared to
200 nm for the light microscope.
The principle of operation of the SEM is electron optical imaging of
the sample when the sample is scanned by a high energy beam of electrons.
The imaging is made possible by the resulting low-energy secondary elec-
trons and backscattered electrons. The secondary electrons emitted from
the surface of the sample provide an image which appears three-demension-
al. During the scan, the sample can be rotated and tilted to almost
any position so that all sides of the sample can be examined. This
allows for the examination of irregularly shaped particles.
The usual source of the electron beam is a tungsten filament in the
electron gun. Better resolution is possible by using newer filament
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materials which have higher current densities and produce a smaller
scanning spot. The usual diameter of the spot is about lOOft.2 Before
the electron beam reaches the sample, it must pass through a series of
magnetic lenses. The final lens contains scanning coils which deflect
the beam to impact the sample. The secondary electrons emitted from the
sample are captured and amplified by an electron collector for display
on a CRT. The magnification is controlled by the area of the sample
scanned. The smaller the scan area, the greater the degree of magnifica-
tion.
The sample to be scanned can be coated with a conductive material
after being secured to an appropriate mount. The coating of the sample
is critical for many materials since an uncoated sample can become charged
and begin to deflect the electron beam. If charging occurs, very little
information can be obtained from the sample's surface. The coating is
applied by placing the mounted sample in a vacuum evaporator. To ensure
that the coating thickness is uniform in all depressions and protrusions
on the sample's surface, the sample is rotated and tilted while being
coated with carbon or a metal.
The preferred conductive coatings are alloys such as gold-palladium
and platinum-palladium because they provide a more uniform coat. Nonalloy
metals used are aluminum and gold. The usual thickness of the coating is
5 to . 10 nm. Carbon is also used as a coating material particularly
where elemental composition is to be determined by X-ray analysis.
In order to provide sufficient information for accurate identifica-
tion and characterization of particles, other data in addition to surface
morphology are necessary. The information required is elemental composi-
tion and may be obtained by adding an energy dispersive X-ray analyzer
(EDXRA) or a wavelength dispersive X-ray analyzer (WDXRA) to the SEM.
The EDXRA can provide elemental composition for the elements above mag-
nesium (1=12), while the WDXRA is good for elements above lithium (Z=3).
The EDXRA's are photodiodes sensitive to the X-ray photons emitted
from the sample's surface when exposed to the electron beam. These
solid-state detectors produce electrical pulses for each photon received.
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These pulses are recorded and can be displayed to provide a characteriza-
tion of the X-ray wavelengths emitted by the sample. The EDXRA is a
sensitive, rapid technique for determining elemental composition but is
not without its limitations. The most serious problem is its inability
to detect hydrogen, lithium, beryllium, boron, carbon, and nitrogen. Oxy-
gen can only be detected with the addition of very expensive detectors.
It also has fairly poor resolution for elements with adjoining X-ray
lines. These problems can be overcome by applying the WDXRA to these
portions of the analysis.
The WDXRA is a crystal diffraction spectrometer. By this we mean,
the X-ray wavelengths from the sample are selected by each crystal in
the spectrometer by diffraction. By changing the diffraction angle, all
of the wavelengths can be scanned and recorded usually by a proportional
counter. Although this method provides useful information, the scanning
of all of the elements is time consuming. Also, the intensity of the
X-rays measured is much less than the EDXRA. These shortcomings are over-
come by the expanded range of elements (Z^3) that can be analyzed.
2.2.3 X-Ray Diffraction
X-ray diffraction (XRD) is a nondestructive analytical procedure to
identify and characterize individual particles or whole samples of crys-
talline substances. Once a compound has been identified, the results
may be catalogued for future reference, an advantage as compared to other
particle identification methods. Another somewhat unique ability of XRD
is the identification of substances with identical chemical compositions
in the solid phase. Many substances such as calcite and aragonite and
quartz and cristobalite exist in chemically identical forms but are dis-
tinguishable by XRD.
XRD systems are composed of three basic components: an X-ray source,
the sample, and a detection system. Depending on the application, the de-
tector may use some type of photographic film or an electronic counter.
The basic principle behind XRD is the determination of diffraction pat-
terns of the X-rays passing through the crystal structure of the sample.
This pattern is used as a measure of the interplanar spacings of the
atoms in the crystal which can be used to identify the sample.
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The diffraction patterns can be determined from single crystals or a
powder, although fine powders are preferred. The drawback to the XRD pro-
cedure is the sample size. Good diffraction patterns are difficult to
obtain with small samples. A sample size decrease is accompanied by a
decrease in the number of crystals in the sample. This in turn leads
to a spotty X-ray pattern. To some extent, this problem can be overcome
by finely grinding the sample before analysis and increasing the detector
exposure time. However, this increase in exposure time reduces the con-
trast between the diffraction pattern and the background, and the diffrac-
tion lines are difficult to see and measure. The decrease in contrast
is caused by air-scatter of the X-rays and scatter of the X-rays caused
by the sample support system. The problems can be partially overcome by
removing air from the X-ray path, reducing the X-ray path length, and min-
imizing interference from the sample support by appropriate selection of
support material and size. Many materials have been used to mount the
sample including nylon, gelatin, and nitrocellulose. However, glass fi-
bers are preferred because they are inert to solvents and allow very small
particles to be easily seen.
There are many materials for which XRD is ideally suited. These in-
clude fertilizers, minerals, pigments, abrasives, corrosion products, and
all other crystalline materials. The noncrystalline materials do not pro-
duce X-ray diffraction patterns and are, therefore, not suitable for anal-
ysis by this method. These include combustion glass products, biological
materials, other glasses, organics, and polymers. Those materials which
have large atomic numbers are much easier to analyze than light element
samples because the heavier materials produce darker X-ray diffraction
lines. The particle size of the sample affects the ability of XRD to
provide an accurate analysis. Depending on the apparatus used, 5- vm
particles can be routinely analyzed, while 1-ym particles are considered
the limitation of the technique.
10
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Chain like - An extended number of particles in a line.
Cleavage - The splitting, or tendency to split, along planes deter-
mined by the crystal structure. Cleavage is always parallel to a
possible crystal face, that is, to a rational lattice plane of the
crystal.
Columnar - Refers to the growth of crystals in column shapes.
Crystal - The regular polyhedral form, bounded by plane surfaces,
which is the outward expression of a periodic or regularily repeating
internal arrangement of atoms.
Dendritic - A branched pattern, like a tree or shrub.
Dull - Lacking in luster because of surface texture.
Ellipsoid - An elongated sphere.
Equant (equant grains) - Crystals or grains that have the same or
nearly the same diameter in every direction.
Fibrous - A flexible particle at least 20x longer than its diameter.
Floe (flocculate) - A fine aggregation of grains or particles.
Foliated - Made up of thin leaves.
Fracture - The manner of breaking and appearance of a mineral when
broken, which is distinctive for certain minerals, as conchoidal
fracture.
Habit - The characteristic shape of a crystal as determined by the
crystal faces developed and their shapes and relative proportions.
Inclusion - Solid, liquid or gas sealed within a particle.
Laminar - Composed of layers.
Luster - The character of light reflected by mineral surfaces. See
adamantine, metallic, resinous, silky, pearly, and vitreous.
Metallic - The luster of metals.
Oolitic - A spherical to ellipsoidal body that has a concentric or
radial structure or both.
Opaque - An object that does not transmit light.
Pearly - Having the luster of pearls.
12
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PI |_e rosjjhe^re - The encapsulated solid paticles inside of cenospheres.
Prismatic - A form of three or more similar faces parallel to a single
axis.
Rad_i_ate_d - Applied to crystal aggregates that radiate from a center.
Rjgjjjef - The appearance or visibility of outline and surface of a mineral
or other particle and caused by the difference between the index of re-
fraction of the mineral and the mounting medium. The greater the differ-
ence, the stronger is the relief.
- Resembling resin in appearance.
Rigsette - A symmetrical growth form, resembling a rose. Common in gypsum,
barite, and pyrite.
Rounded - Not sharp; rounded edges are common to weathered materials.
^oundne^sjs - The ratio of the average radius of the corners and edges to
the radius of the maximum inscribed circle. One of the two shape measures
of a particle; the other being sphericity. Roundness is a measure of
the sharpness of the corners and edges of a grain.
Shaj'd - A curved, spiculelike fragment.
Shinny - Having a surface luster.
Silky - Having the luster of silk.
Sphjej^ijcity - The ratio of the normal diameter to the maximum intercept
tru^gTTtne particle. The second of the two measures of grain shape; the
other being roundness. The measure of the form of the grain independent
of the sharpness of the grain edges.
_S_tr i [atjgjrs - A series of parallel grooves on the surface of a grain.
_ - A crystal flattened paralled to any face.
Jr. an_s_1 [ucervt - Transmitting light but not capable of being seen through.
TrarTsmT TttTn g light diffusely.
- May be seen through. Transmitting light without diffusing
^
or scattering its rays.
- Having a wavy surface.
_\/ฃSjjcul_a_r - Containing vesicles; small, circular, enclosed spaces.
yi_tre_ou_s_ - Having the luster of broken glass, quartz, or calcite.
13
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3.2 PARTICLE TYPES AND PROPERTIES
Most particle-identification, source apportionment studies divide the
collected particles into four classes; minerals, combustion products, bio-
logical materials, and miscellaneous. The procedures used in this report
to describe each of these classes vary due to the nature of the particle.
For the minerals, possible origins of the particles are presented. For
combustion products, the properties of the particles are discussed because
these properties can give clues to the origin of the particles. Biologi-
cal materials are not discussed because in the majority of instances,
these particles are related to natural sources. Miscellaneous particles
are handled similarily to the minerals.
3.2.1 Minerals
Minerals are commonly collected in airborne particulate samples. In
a study of 300 filter analyses, minerals comprised an average of 65 per-
cent (mass) of the particles on the filters.5 Approximately 1,700 mineral
species have been recognized, many of which are very rare. Some of the
more common minerals which may be found on ambient filters are discussed
in Table 3-1 along with their possible origins.
3.2.2 Combustion Products
Combustion products are important components of ambient particulate
loading, averaging 25 percent of the filter catch.5 Most ambient particu-
late studies list the following types of combustion product categories:
Combustion Products
Soot:
Oil
Coal
Very fine, unidentified
Glassy fly ash
Incinerator fly ash
Burned paper
Burned wood
Oil, coal, and wood combustion products can take many forms depend-
ing on the chemical composition of the particles, the combustion zone
temperature, and the particle residence time in the combustion zone. At
14
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least two generalized classification schemes for coal fly ash have been
reported.8ป9 The first, based on both optical and SEM studies, indicates
five classes of fly ash; large irregular particles, solid spherical parti-
cles, particles, hollow spherical particles, hollow spherical particles
containing a number of small solid particles encapsulated inside them,
and agglomerates of many small spherical particles.^
The second classification scheme was based on optical microscopy and
had eleven classes of particles.9 The shapes of the particles were re-
lated to the exposure to combustion (i.e., residence time or temperature).
As exposure increased, particles, including silicates, iron oxides and
coal, tended to become more rounded until the spherical shapes were ob-
tained.
The one exception to the exposure/roundness hypothesis appears to be
particles described as angular, lacy, and opaque. These particles appear
in the coarser fractions and are thought to be incompletely combusted car-
bonaceous material from internal boiler surfaces.
In many cases, the spherical particles may have particles adhering to
their surfaces. These particles are crystals and may be acicular, elon-
gate blades, or cuboid.9 These crystals are probably formed from the re-
action of sulfuric acid with the metals in the flyash. Very little infor-
mation has been reported on these crystals, and they could be a method of
fingerprinting sources if they are examined.
Wood and oil droplets undergo morphological changes due to exposure
to combustion. Sawdust becomes brown, then charred black, then rounded
with its edges white or gray, and then completely white or gray-white
ash. The oil droplets begin to darken as volatiles are oxidized, then
form a brown crusty surface; then become a rough, black, cenosphere-like
material (petroleum coke); then a cloudy, vitreous particle; and then
very small, clear to yellow particles.
The category of incinerator flyash is not easily defined. In general,
one particle type can not be used to fingerprint all incinerators because
an incinerator's emissions are dependent on the type and amount of the
material to be incinerated.
19
-------
Two approaches to classifying particles as incinerator flyash were
obtained.3.4 in the first, a two-step procedure is employed. In the
qualitative (first) step, particles are classified by morphology, chemis-
try, and other criteria. In the quantitative (second) step, optical
criteria are developed for each of the classes. Interferences are taken
into account. Assembledges of particles based on the optical criteria
are then related to source types. The optical criteria must be strictly
followed for this procedure to work.
In the second approach, characterizations of source samples from in-
cinerators are compared to the ambient samples. Assembledges of inciner-
ator emission particle types are located in the ambient sample and "sub-
tracted" out.
3.2.3 Other Particles
Other particles are commonly classified as biological material and
miscellaneous. Biological material usually includes pollen, spores,
paper, starch, and miscellaneous plant tissue. These are not usually
indicative of sources and, therefore, are not discussed.
Miscellaneous particles, materials such as iron or steel, textile
fibers and rubber particles are usually indicative of sources. Most
rubber particles probably originated from the wear of tires. However,
some may come from various tire making operations. Tire particles may
be quite large and in some cases have been mistaken for combustion pro-
ducts.
3.3 SECONDARY PARTICLES
Although not specifically addressed in source apportionment studies,
secondary particles are potentially important contributors to the ambient
particle loading. The presence of secondary particles and their nuclei
causes the multimodal particle distributions discussed in Section 4.4.6.
In addition, secondary particles have been characterized in studies of
coal fly-ash particles.8.9
Secondary particles are formed in the atmosphere as a result of
chemical reactions. Important secondary particles include sulfates,
nitrates, ammonia compounds, and organics. Important atmospheric gases
which may react to form secondary particles include ammonia, nitric
20
-------
oxide, nitrogen dioxide, nitric acid, sulfur dioxide, hydrogen sulfide,
sulfuric acid, hydrogen chloride, and organics.
The formation of homogeneous secondary particles has at least two
distinct phases; the gas phase and the particle phase. Often, the re-
action mechanisms are highly complex and involve many precursors. In
the gas phase, two or more gases react to form an intermediate vapor
which nucleates small particles ( 10~3 y m). These small particles
then grow by a number of mechanisms, including:1^
1) diffusion and condensation of the intermediate vapor on the par-
ticle surface,
2) adsorption on the particle surface by some or all of the reactant
gases,
3) adsorption of the reactant gases into the particle,
4) scavenging of nucleating vapor molecular by the particles, and
5) coagulation of the particles.
The particles grow (accumulate) into the size range of 0.2 to 2 ym.
Heterogeneous secondary particles occur as condensation of molecules
of a supersaturated vapor onto foreign particles or ions due to gas-phase
or catalytic reactions on the particle surface. They can also form on
the surfaces of other particles in the exhausts of industrial processes.
Studies of fly ash from coal combustion have shown the presence of acicu-
lar, elongate blades, and cuboid particles adhering to the fly-ash sur-
faces.9 The particles were identified as CaS04 and resulted from the re-
action of sulfuric acid with the metal oxides in the fly ash. Ammonium
sulfates can also be formed through the reaction of sulfuric acid with
ambient ammonia in the power plant stacks and on hi-vol filters.9'11
The examination of ambient filters which contain secondary particles
may lead to several problems, the biggest of which is the possible mis-
identification or misclassification of the particles. A secondary parti-
cle can be mistaken for another particle and placed in an inappropriate
category. Other problems include:
1) Artifacts may form on the front or inside the filter and then
migrate to the backs of filters. The backs of the filters should
be examined.
2) Organic particles (those with high vapor pressures) are
volatilized by the SEM and are lost to the microscopist.
21
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3) Many sulfates form on other particles. An EDXA analysis
would show sulfur and the entire particle would be considered
to be a sulfate when only the surface would be.
To summarize, the formation of secondary particles follows several
routes. Secondary particles can be important contributors to the ambient
particulate loading. However, they may not be "seen" by the microscopist,
or if seen, they may be mistaken for other particles or placed in incorrect
categories.
22
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4.0 EVALUATING AND APPLYING RESULTS
The primary purpose of this section is to present enough general in-
formation to allow the user to evaluate and/or apply the results of source
apportionment studies. Secondary purposes of this section are (1) to pro-
vide enough information to explain some of the concepts involved in per-
forming these studies, and (2) to present biases and impacts on the re-
sults due to both inherent limitations of the techniques and improper use
of the techniques. With these purposes in mind, the approaches to pre-
senting the information vary in relation to the nature of the topic.
Topics discussed include sampling devices, filters, preparation of
samples, particle size, fingerprinting, and quality assurance.
4.1 SAMPLING DEVICES
The collection of particles for analysis can be made with the hi-vol
which is the standard ambient particle sampler used by control agencies.
If more precise information is desired on different particle size ranges,
several samplers are available from which to choose. Two of these devices
were designed as modifications to the basic hi-vol sampling system. The
cascade impactor is located under the roof of the hi-vol and can provide
particle sizing on several size ranges. The size selective inlet replaces
the hi-vol roof altogether and essentially allows only inhalable particles
of 10 ym and less to enter. The virtual impactor or dichotomous sampler is
an entirely independent sampling system which collects only inhalable par-
ticles (<10 ym) and separates the sample into fine (< 2.5 ym) and coarse
(> 2.5pm) fractions.
4.1.1 Sampler Location
All of the particulate samplers previously described are governed by
the same set of siting criteria. These criteria ensure that the air flow
around the sampler is unobstructed and that the sampler is not unduly
23
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influenced by localized sources of particles. These siting criteria
are summarized as follows:^
1. The sampler inlet must be located between 2 and 15 m above the
ground.
2. The sampler inlet must be located more than 2 m from any sup-
porting structure.
3. The sampler must be located more than 20 m from any trees.
4. The distance from the sampler to any obstacle must be at least
twice the height of the obstacle protrusion above the sampler.
5. There must be an unobstructed airflow for 270 degrees around
the sampler.
6. The sampler must not be located close to any incinerator or
furnace flues.
7. Depending on the height of the sampler inlet, there are cer-
tain minimum spacing requirements from roads (see Figure 1,
Appendix E of 40 CFR 58).
4.1.2 Sampler Biases
Each of the sampling devices has its own particular biases associated
with the collection of particulates. These biases should be considered
when designing a study or analyzing the results from a study.
One of the biases associated with all ambient samplers is the collec-
tion of particles that are not truly representative of the distribution
of particles in the atmosphere. To be representative, the sampler must
collect particles in a manner that does not physically or chemically al-
ter them or bias the distribution. Most samplers are not able to do this
because particles may be crushed or altered through reactions, and the
particles are rarely, if ever, uniformly dispersed in the air. In addi-
tion, particles are not uniformly collected. In most cases, a biased
population (e.g., respirable dust, mineral dust) is collected.
The hi-vol has different collection efficiencies depending on the
size of the particle. For particles less than about 5 ym, the hi-vol
sampler has nearly a 100-percent efficiency.^ However, the collection
efficiency decreases significantly as the particle size increases up to
50 or 60 m where the efficiency approaches zero. Around 30 ym is the
24
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generality accepted value for the 50-percent cutpoint. The collection
efficiency of particles above 5 ym is significantly affected by wind
speed, sampling flowrate, and the orientation of the sampler to the wind
direction. The particle collection is least affected by orientation when
the hi-vol's roof ridge is at a 45-degree angle to the wind direction.13
This variability in the collection of the larger particle sizes along
with the non-uniform deposition of particles in the filter makes the
accurate characterization of hi-vol samples difficult.
The hi-vol also collects particles when it is not operating. In areas
of high wind speeds, large particles can settle on the filters while the
hi-vol is in its standby mode.
The high-volume cascade impactor shares all of the biases associated
with the hi-vol, since it is basically an internal modification to that
device. Additionally the cascade impactor has a problem inherent in its
design as a particle sizing instrument. Because the particles are aero-
dynamically sized by impacting on a solid surface, particle bounce and
reentrainment can occur. This trait is particularly pronounced at the
higher mass loadings. Another problem with the impaction against solid
surfaces is that single large particles can be broken into many smaller
particles and produce a positive bias towards the smaller size range.
Also, as with the hi-vol, the particle deposits are not uniform which
hinders accurate sample analysis.
The other particle sizing modification available for the hi-vol is
the size-selective inlet. The inlet is designed to correct the wind
speed and directional orientation problems of the standard hi-vol shelter.
The size selective inlet design also minimizes the particle breaking,
bounce, and reentrainment problems of the cascade impactor. But, because
particle impaction against a solid surface does occur, some reentrainment
should be expected. The problem of nonuniform particle deposits on the
filter also exists, as with the other hi-vol methods.
The virtual impactor or dichotomous sampler's design eliminates the
particle breaking, bounce, and reentrainment problems since particle
impaction on a solid is not a part of the sizing technique. The possibi-
lity of some fine-particle carryover into the coarse airstreams of the
impactor does exist, but the mass data can be adjusted to compensate for
25
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this. The sampler is not without some problems. Some of the inlets
have been shown to be sensitive to wind speeds such that the outpoint
can vary. There are also fairly high internal wall losses because of
large particle deposition on the internal surfaces. The sampler also
has some sensitivity to ambient relative humidity. At very high humidity
levels, the sampling flow rate is reduced sufficiently to affect the sam-
pling effectiveness.
4.1.3 Sampling Time
The various methods of analysis have certain requirements on the
amount of material on the filter necessary for accurate characterizations.
Since the particle sampling devices are designed for optimum collection
efficiencies at particular flow rates, the sampling time is the parameter
which can be varied to provide some control over filter loading. In de-
signing a sampling system, consideration should always be given to the
type of analysis to be performed on the sample. Generally, microscopy
requires the smallest sample be collected, preferably only a single lay-
er of particles. The X-ray diffraction technique requires large samples
for accurate analyses. The other methods of analysis' requirements are
usually between these two, although the smaller samples will most often
be sufficient.
Multiple samplers, each with a different sampling time, can be used
to optimize the filter loadings. Also, samplers equipped with wind-
direction activated motors can be used either to selectively sample
ambient air or to measure the impact from a particular source.
4.2 FILTER MATERIAL
The filter material is extremely important to the proper collection
and analysis of an ambient sample. The filter material is even more
critical when the chemical composition of the sample is to be determined.
If the selection of the filter material can be made in developing a
study, care should be given to select a filter which is easily handled
in the field and is compatible with the analytical techniques that will
be used. If the analysis is conducted on filters already in use in a
sampling network, some knowledge of the effects of the filter material on
the analysis will be useful in interpreting the results. The following
26
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discussion provides information on the chracteristics of the various
types of filter media.
4.2.1 Fiber Filters
The glass-fiber filter is the most common in use in air pollution
monitoring. This filter has many advantages when used to determine mass
by gravimetric analysis. These include low pressure drop, high capacity,
low cost, and ease of weighing. They also have the advantage of with-
standing fairly high temperatures and of being nonhygroscopic. They
typically exhibit a sampling efficiency of 99.7 percent for particles of
0.3-ym diameter. Although not directly applicable to source-receptor
studies, the filters are not soluble in organic solvents so that organic
material can be extracted by appropriate solvents for subsequent analysis.
The disadvantages of the glass fiber filter for particle identifica-
tion outweigh its advantages for most applications. The fiber structure
allows particles to penetrate deep into the filters. The fibers may hide
particles from viewing by the microscopy techniques. This deep penetra-
tion also causes problems for the X-ray techniques by attenuating the
characteristic X-rays from many of the light elements. The highest
grade glass fiber filters have high background levels because of trace
element contamination. The filters themselves are responsible for scat-
tering large amounts of the X-rays. The high elemental background levels
create an additional problem of artifact formation. This artifact forma-
tion has the potential to significantly distort the analytical results.
A discussion of artifact formation is presented in Section 4.2.3. These
problems make the glass fiber filters generally unsuitable for micro-
scopic or elemental analysis.
Fiber filters are also available in cellulose. These filters can be
obtained in grades which are very pure and have a low ash content. The
disadvantages of these filters are that they have low collection efficien-
cies, they are not uniform, they exhibit high hygroscopic properties,
and their fibers are birefringent.
4.2.2 Membrane Filters
From a microscopy standpoint, membrane filters are the best of the
available filters. These filters are made from a variety of polymeric
27
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materials including polyvinylchloride, mixtures of cellulose, cellulose
triacetate, acrylonitrile, fluorinated polymers, polytetrafluoroethylene,
and polycarbonate. The membrane filters can be classified into two main
categories. These are the types in which the pores are formed during
the creation of the filter sheets (e.g., Milliporeฎ) and the types which
have the pores formed after the sheets are made (e.g., Milliporeฎ).
In general,the membrane filters are much preferred to the fiber fil-
ters. They have low uniform levels of trace element contamination and
produce low X-ray scattering. The membrane filters are soluble in many
organic solvents when particle extraction procedures are required. An
advantage for analysis by microscopy is that the filter becomes transpar-
ent with the addition of a drop of liquid of the same refractive index
as the filter.^ The disadvantages of membrane filters include relatively
high pressure drops, low capacity for filter loading, and a brittleness
which makes them difficult to handle in the field.
Each of the membrane filter types has those advantages and disadvan-
tages to different degrees. The Millipore-type filter has a low trace
element background, a higher capacity for particulate loading, and a
greater ease of handling. The filter allows for easy removal of particles
for analysis. However the sponge-like structure of the Millipore makes
the filter unsuitable for direct examination on the filter by SEM. The
filters are also very sensitive to ambient moisture and can produce
significantly different weights if analyzed gravimetrically.*5
The Nucleporeฎ filter is the ideal filter for SEM analysis since it
has a flat, featureless surface which allows direct examination on the
filter surface and allows particles to be easily removed. It is also
suitable for X-ray analysis, since it provides a low trace element back-
ground. The filter does have some disadvantages which could preclude
its use for certain sampling situations. The flatness of the filter
allows particles to easily become detached during handling. The filter
structure also has a very low particle loading capacity which necessitates
the use of a series of filters to collect sufficient mass for gravimetric
analysis. The fragility of the Nucleporeฎ filter makes it difficult to
handle in the field.1? Since the unique pores of the Nucleporeฎ filter
are produced as circular holes in the filter's surface, a cluster of
28
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holes can deteriorate to form a single large hole. This can lead to a
change in filter efficiency, sample loss, and nonuniform particle de-
posits.16
4.2.3 Artifact Formation
During and after the ambient sampling, extraneous particles (arti-
facts) may be formed on filters. Artifacts are formed from the reactions
of gases and particles with the filter itself and/or other collected
particles. The extent of artifact formation depends on the type of
filter used, the presence or absence of reactant components, the relative
humidity, and other factors.
Sulfates and nitrates are common artifacts. The formation of arti-
facts can lead to serious sampling errors. For example, sulfur dioxide
reactions on glass fiber filters can cause a 30-percent error in total
particulate mass.*? Similarily, on glass fiber filters, nitrate artifact
formation averaged 10yg/m3 compared to 8.5yg/m3 true particulate nitrate
in one study.18
Three types of reactions were reported to occur which could lead to
the formation of sulfate artifacts. These were reactions between 1) sul-
furic acid aerosol and the filter itself, 2) gaseous ammonia and collected
acidic sulfate, and 3) acidic aerosol and other collected particles.28
Sulfuric acid and sulfur dioxide react with the components of glass
fiber filters and many membrane filters. To avoid filter reactions with
sulfuric acid in the sampled gas stream, polycarbonate (Nucleporeฎ) and
PTFE (Mitexฎ) membrane filters-should be used.19
Gaseous ammonia had little effect on dry acidic sulfates in one ex-
perimental situation.19 However, with increased relative humidity, acidic
sulfate particles were solvated. Various products, such as ammonium
sulfate and ammonium sulfamate can be formed.20
The reaction of sulfate particles with other particles has been ob-
served.19 Experiments with sulfuric acid aerosol and halide (Cl ,1) salts
showed considerable loss of the volatile halogen acids and the formation
of stable sulphate salts.19
In these same experiments, a difference was seen in the reactions on
Nucleporeฎ and Mitexฎ filters. When using KC1 as the halide salt, no
29
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reaction occurred on dry Mitexฎ filters when exposed to sulfuric acid
treatment, while a significant decline in the Cl/K ratio was observed on
Nucleporeฎ filters.19 With relative humidity at 76 percent, the Mitexฎ
filter showed a 96-percent reduction in the Cl/K ratio when exposed to
the sulfuric acid aerosol.
This same article proposed that a possible preliminary condition for
particle/particle interactions was the inhomogeneous distribution of the
particles on the filters. This type of distribution can result from both
the improper design of the inlet to the filter holder which can lead to
the local enrichment of large particles and from the agglomeration of
particles around the pores of membrane filters.19
The oxidation of sulfur dioxide on carbon particles was investigated
in two studies. 21,22 ^ ]ow temperatures, the following observations
were recorded:
1) in the presence of oxygen, SOg is oxidized on carbon to form 863
or H2S04,
2) S03 and HgSC^ poison the surface active sites of the filters,
because they do not desorb after their formation,
3) nitrogen dioxide is chemisorbed or decomposed to NO, and
4) at ambient temperatures, the amount of acid formed on carbon
surfaces is increased with the addition of NO to the
In addition to carbon, Fe203, MnOg and'particulate matter from Pitts-
burg have been shown to sorb S02.23
Active sites of certain filter types can fix gaseous nitrogen com-
pounds, especially nitric acid, and cause the formation of artifact par-
ticulate nitrates. The chief artifact producing reaction appears to be
the gas-filter interaction. 2* The discussion of nitrate artifact forma-
tion is separated into filter types.
Glass-fiber Glass-fiber filters display high nitrate artifact for-
mation. In one laboratory study, clean filters were exposed to nitrogen
dioxide and nitric acid mists. ^ The nitrogen dioxide caused comparative-
ly little artifact formation on all filters. On glass fiber filters,
the addition of ozone or ammonia caused small increases in artifact
formation. With nitric acid, artifact formation was one to two orders
of magnitude higher.
30
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Another laboratory study examined the impact on artifact formation
from the exposure of clean filters to N02, HN03, NH3, PAN, and N20.24
Glassfiber filters showed significant artifact formation when exposed
to nitric acid and nitrogen dioxide at high humidity.
Quartz-Fiber -- Quartz-fiber filters show some nitrate artifact for-
mation in field tests and upon laboratory exposure to nitric acid.23,25
as much nitrate as quartz-fiber filters, although the collection effici-
encies of the two filters are essentially equivalent.25 in the lab
tests, the quartz-fiber filter was judged to be superior to the glass-
fiber, although some nitrate artifacts were formed from exposure to
nitric acid.^4
Spectrograde Nitric acid attacks the coatings on Spectrograde fil-
ters and forms a high level of artifact nitrate.25
Nylon High levels of nitrate artifacts were formed upon exposure to
nitric acid and nitrogen dioxide at high humidijties.24
Cellulose-Acetate Nitric acid caused the formation of substantial
artifacts on cellulose-acetate filters.24 However, other nitrogen gases
had no effect.
Polycarbonate -- No significant nitrate artifacts were produced by
exposing clean polycarbonate filters to nitrogen gases.24
Tef1 onฎ Nitrogen artifact formation is negligible on Teflonฎ fil-
ters when exposed to nitrogen gases.24
Teflonฎ filters have shown a condition termed "negative artifact for-
mation." In one study as much as 90 percent of the collected nitrates
were lost.26 The study equated the loss to temperature on a directly
proportional basis. The explanation of this phenomenon was that the
nitrates were lost due to evaporation caused by the disequilibrium between
the solid and gaseous nitrate phases.
4.2.4 Negative Artifacts
In addition to the evaporation of nitrates described above, negative
artifacts can occur as a result of chemical reactions on the filter sur-
face. One study described the reaction of sulfuric acid produced by the
photochemical oxidation of sulfur dioxide with nitrates.27 Nitric acid
volatilizes, the sulfate ions react with the available cations to form
salts, and the reduced nitrogen species are retained in the aerosol. A
31
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conclusion expressed by this study was that in eastern cities where S02
is prevalent, nitrate levels would be depressed.27
4.3 PREPARATION OF SAMPLES
During the preparation of samples for microscopic analyses, many of
the techniques used can lead to sample loss and/or changes in the samples
which lead to biased results. These biased results can occur due to the
nature of some filters, in the handling of the filters, in the removal
of particles from the filters, in the preparation of the samples for
analysis, and in the analysis of the samples.
4.3.1 Nature of Filters
Two conditions of filters are difficult to handle by microscopic
examination. The first is the layered filter. In this instance, the
filter has become covered with multiple layers of particles due to
overloading or "blinding" by flat particles. This condition poses
special problems because only the top layer can be examined. If this
condition occurs, it should be noted in the examination, and the analyst
should remember that the top layer may not be representative of the
other layers.
The second condition is embedded particles. In this instance, parti-
cles have become embedded between fibers of the glass fiber filter or in
the pores of other filters. Generally, these particles are difficult to
examine or remove. This condition should also be noted in the report.
Both of these conditions can lead to biased results in that all the
particles are not examined. In the case of layered filters, the top
layer may or may not be representative of the other layers. In the case
of embedded particles, those that are embedded are probably small, and
an examination of the filter without the inclusion of these particles
interjects a size-selective bias to the results.
4.3.2 Handling of Filters and Removal of Particles From Filters
Filters used to collect particles must be properly handled to avoid
the loss of particles or the contamination of the filters. The proper
handling techniques are discussed in the Quality Assurance Section.
32
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A particle-laden filter received at the laboratory should be examined
as is. If the filter is coated with multiple layers of particles or if
further analysis of individual particles is necessary, particles can be
removed from the filter or vice versa. The removal of particles can
induce errors in the analysis due to loss of particles, changes in the
physical structure of the particle by crushing or disaggregation, and
biasing the particle examination due to size-selective removal.
To remove particles, from a filter, various techniques have been em-
ployed. Particles as small as 100 u mean be picked by tweezers, and those
as small as 3 pm can be picked by a tungsten needle, sometimes dipped in
an adhesive. For particles in the range of 2-4 vim, a thin film of col-
lodion is dried around the particle, and then a block which encloses the
particle is cut out and mounted.2
These particle-picking techniques are very size selective. In addi-
tion to being time consuming, the probability exists for particle loss
and changes in the physical state of the particle through crushing or
disaggregation. The high degree of skill necessary for the picking
operations also limits the applicability of the technique.
Another method of particle removal from a filter is ultrasonic clean-
ing of the filter in a fluid. This technique is commonly used for remov-
ing particles from membrane filters where alcohol or toluene is the fluid.
Problems with this technique may include:
1. disaggregation of agglomerated particles,
2. uncertain removal efficiency for particles,
3. size-biased removal, and
4. solubilization of some particles in the solvent.
The alternative to particle removal is filter removal. The two
methods of filter removal, low-temperature ashing and solvent dissolving,
also have drawbacks. The ashing can result in the contamination of the
sample with nonashless filter material, the loss of sample, or the
chemical alternation of the sample.
The dissolving of the filter is an appropriate method for membrane
filters. A suitable solvent, such as acetone for cellulose ester filters
and hydrofuran for PVC filters, can be used for particle/filter separa-
tion. This technique uses centri.fugation and several solvent exchanges
33
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for dissolving. The main drawback to this method is the possibility of
particle loss or solubilization.
4.3.3 Mounting of Samples
The mounting of particles for analysis involves different techniques
depending on which analysis method is being used. The mounting technique
and media should be specified along with the analyses results.
For optical microscopy, glass slides and cover glasses are used as
mounts. The particles, either on or off the filters, are placed in a
mounting liquid (medium).
To obtain the relief (sharpness of outline due to contrast) that is
necessary to adequately see the particles, the mounting medium should
have a refractive .index that is much different from the refractive index
of the particle. Most particles have a refractive index in the range of
1.50 to 1.55.2 yne Aroclorsฎ, with refractive indices of 1.64 to 1.67,
are suitable viscous mounting media for most mineral applications. Aro-
clorsฎ are restricted chemicals.
For electron microscopy, the sample must be mounted on a suitable
substrate and mounting media. The substrate should be chosen to elimi-
nate spurious X-ray readings when it is hit by backscattered electrons
and X-rays. Beryllium, carbon, graphite, lithium fluoride, and cellulose
sheets are suitable substrates. Glass microscope slides are not used
because they may add sodium, calcium, silicon, and potassium to the
X-ray spectra.I6
The proper choice of mounting media is necessary to avoid sample
loss due to charging of the particles and their subsequent ejection into
the interior of the SEM. In addition, if the particle is submerged in
the media, it may not be "seen."
For X-ray analysis, better results are obtained from uncoated parti-
cles. However, charging may occur, and a light carbon coating may be
necessary.
For X-ray diffraction (XRD), a particle is selected and mounted on
the end of a fiber. Supposedly, particles as small as 1 m can be ana-
lyzed using this method. The problems associated with the handling of
34
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these small particles have been discussed. The XRD technique can also be
used for examining a thick layer of particles "in place" on filters.
4.4 PARTICLE SIZE
The measurement of particle size is important to the characteriza-
tion of ambient particulates. Particle size measurements are conducted
in two ways; the physical measurement of size using the microscope, and
the measurement of aerodynamic size using sampling equipment and particle
counting equipment. The various sizes obtained from these methods are
used to statistically characterize the particle distribution.
To examine these techniques and to present the problems associated
with these measurements, this section will briefly explain the techniques
used, the relationships between the techniques, the biases and errors
associated with the techniques, and particle size distributions.
4.4.1 Microscopic Size Measurement Techniques
Particle sizes are commmonly reported as either linear dimensions,
such as diameter, or as areas. Statistical linear measures and measures
based on the projected area of the grain are the most common sizing tech-
niques.
Four types of statistical linear measurements are currently used
and are summarized below:
1. Martin's Statistical Diameter -- The Martin's diameter is the
length of the line in a fixed direction which divides the pro-
jected grain area into two equal halves.
2. Feret's Statistical Diameter The maximum projected length of
a grain calculated in a fixed direction.
3. Maximum Horizontal Intercept The maximum length of a line
in a fixed direction limited by the contour of the grain.
4. Apparent Long Axis The maximum length of a grain.
The nominal diameter, the diameter of a circle of the same area as
the grain, is the other method used for particle sizing. Graticules are
used for measuring the areas. Equilization of the two areas is accom-
plished by fitting the circle to the grain.
Figure 4-1 shows the relationships between these size measuring tech-
niques. The mean values obtained by Martin's diameter and the nominal dia-
35
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Nominal Diameter
(diameter of circle)
Horizontal Intercept
Traverse
Direction
of
Microscope
Figure 4-1. Relationships Between Particle Size Measuring Techniques
36
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meter have been shown to agree closely with exact area measurements of
"normally" shaped mineral particles.2^ For elongated particles, Martin's
diameter is smaller and Feret's diameter is greater than the nominal di-
ameter.
Since particle shape is very important to accurate particle size mea-
surements, a description of the particle shape should be given when stat-
ing particle sizes. In addition, the method employed for measurement
should also be stated.
4.4.2 Direct Measurement of Volume and Determination of Mass
The direct measurement of volume of particles is easily accomplished
when the particles are perfect spheres. In the case of spheres, the vol-
ume is defined by the diameter of the sphere. As particle shape departs
from that of a sphere, the volume of the particle changes in relation to
the volume of a sphere of the same diameter.
As an example of this problem, the following discussion is presented.
Assume that a cube shaped particle is oriented such that the
length of one of its sides is equal to the diameter measured by
Martin's method. The diameter is 1 y m. The volume of the cube-
shaped particle is:
V = a3 = 1 ym3.
The volume of a sphere with a diameter of 1 m is:
V = n d3 = 0.5236 urn3.
B-
Thus, the volume of the particle as measured by Martin's diameter
and assumed to be a sphere is 52.4 percent of the true volume.
If the same cube-shaped particle (l^mon side) is oriented so that
the diagonal across one face is measured as the Martin's diameter,
its diameter is:
d2 = a2 + b2 =2 wm2
d = \/2 um.
Its volume is still 1 ym3, but the volume of a sphere of diameter
\/2 y m i s :
V = n(V2)3 = 1.481 ym3.
Thus, the volume of the particle as measured by Martin's diameter
and assumed to be a sphere is 148 percent of the true value.
37
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As can be seen, the determination of volume for non-spherical particles
can lead to large errors.
Another method that can be used to estimate the volume of particles
is the measurement of vertical dimensions. This technique is applicable
only to large grains that can be handled individually.
The mass of the particles can be calculated by simply multiplying
the volume by the density of the particle. However, mass is not easily
obtained due to the problems of measuring the volume of the particles.
In addition, the density is often not known or is not correctly assumed.
4.4.3 Aerodynamic Size
In addition to the two-dimensional size measurements of particle pro-
jections, a particle can be defined by its aerodynamic size. Aerodynamic
size is usually measured by the sampling device. If not measured, some
sampling devices aerodynamically fractionate the particles into size
cuts, e.g., the dicotomous sampler fractionates particles into two sizes,
greater than 2.5 ym and less than 2.5ym. In some instances, particles
are removed from filters, resuspended in a fluid, and settled to calculate
their settling velocities.
The most commonly employed measures of aerodynamic diameter are the
Stokes1 diameter, the classical aerodynamic diameter, and the aerodynamic
impaction diameter (also known as the Lovelace or aerodynamic resistance
diameters).
The Stokes1 diameter is defined by:
Vs = gpC Ds2, Re <0.5,
where: Vs = terminal settling velocity of a particle in free fall,
m/sec,
g = gravitational constant (9.80665 m/sec^),
p = particle density, kg/m3,
Ds = Stokes1 diameter, m,
n = fluid viscosity, kg/m-sec,
C = slip correction factor (Cunningham correction factor) for
spherical particle of diameter Ds, dimensionless, and
Re = Reynold's number, dimensionless.
The classical aerodynamic diameter (D/\e) is defined as:
gc
33
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This diameter differs from the Stokes' diameter by assuming that the den-
sity of the sphere is unity and by calculating the slip correction factor
for the classical aerodynamic diameter.
The aerodynamic impaction diameter is used for impactors and inertial
classifiers and is defined as:
DAi=
4.4.4 Conversion of Particle Sizing Data
Sets of particle sizing data should specify the particle sizing tech-
nique used to generate the data. In order to compare data generated by
one method to data generated by another, formulas have been developed
for the data conversion.
To convert particle physical diameter obtained by microscopy to aero-
dynamic diameter, the most commonly used method is:^
Where: da = particle aerodynamic diameter, u m,
d = particle physical diameter, \i m
P = particle density
C = Cunningham Correction Factor = 1 + 0.000621 T/d
T = temperature, ฐK, and
Ca = Cunningham correction for da.
In practice C - Ca, therefore,
da = dVfT.
To compare the various aerodynamic diameters to each other, Refer-
ence 29 should be consulted.
4.4.5 Errors, Biases, and Limitations
The measurement of particle size with microscopic techniques can
easily introduce errors, and the precision is limited by various parame-
ters. To begin with, the "true" size of particles is difficult to deter-
mine in terms of the three-dimensional form of the particle. Therefore,
particle size is based on either linear or area measurements of the
two-dimensional projection of the particle, and the third dimension is
ignored.
39
-------
This projection leads to two problems. The first is the stability
of the resting position of the particle. In general, particles mounted
in low viscosity media will probably settle into their position of maximum
stability. However, this may not be the case in viscous media.28
The second problem results from the orientation of the particle with
respect to the direction of measurement. The diameter of the particle,
using Martin's, Feret's, or the maximum horizontal intercept methods, is
measured in a fixed direction,,usually the traverse of the microscope
stage. The solution to this problem is not simple. Since the microscope
traverse can easily be changed by rotating the stage, many microscopists
set the traverse to either the long or short axis of most of the grains.
No matter what method is used, some measure or description of randomness
or orientation of the particles should be given.
Other limiting parameters include:
1) Nominal diameter, measured by comparison to circles, is based on
an eye-fit of the circle to the particle. This procedure re-
quires a high degree of skill.
2) The practical limitation of the optical microscope for the sizing
of particles is approximately 0.8 pm. Even with particles of
the 2 or 3ym size, errors may be high unless the microscopist
is highly trained.2
3) Few microscope objectives have precisely marked nominal magnifi-
cations.2 Thus, errors in size measurements are compounded
by factors introduced by improper magnifications.
4) Operator fatigue has a great effect on size observations.
4.4.6 Particle Size Distributions
Suspensions of particles in the ambient atmosphere never consist of
particles of homogeneous sizes. Thus, a distribution of particle sizes
(i.e., the relative number of particles in each size cut) is important
to study these particles. Distributions can be made of particle counts,
particle mass, and particle volume. Distributions are usually described
by a mean and a measure of the spread of the sizes, usually the standard
deviation, in each size class.
Particle size distributions may be multimodal (with mode defined as
an integral of the distribution between the minima on each side of a
40
-------
maximum). In urban aerosols, there are usually one number mode, two
surface modes, and two or three volume or mass modes.
The volume (or mass) distribution normally is composed of two modes;
coarse and accumulation (fine). In some areas that are influenced by a
source of "fresh" nuclei, a third mode, known as the Aitken mode, may
occur. In these instances, the fine mode is the sum of the accumulation
and Aitken modes. Characteristics of each mode are shown in Table 4.1.
In general, the coarse mode is composed of primary particles that have
resulted from wind entrainment of soil or dust or from other mechanical
actions. The fine mode consists of secondary particles that have formed
as the result of particle growth from the gas phase through condensation,
combustion, or atmospheric transformation.
TABLE 4.1. CHARACTERIZATION OF VOLUME (OR MASS) PARTICLE SIZE
DISTRIBUTION MODES
Mode
Size Range
of Particles
Type of Particle
Coarse
%2-3 Mm to ^ lOOy m
Primary; mineral dusts, industrial
particles, sea salts, etc.
Fine
Accumulation ^0.02 pm to
Aitken
^0.005 to 0.05 urn
Secondary
2-
NO-:
NH/
and organics) & primary combustion
An idealized graphical representation of a bimodal distribution show-
ing the coarse and fine modes is illustrated in Figure 4-2. Several
features of the distribution should be explained.
1. The particle diameters at the minimum points can vary (see
Table 4-1),
2. The minimum points are not zero because the size ranges overlap,
i.e., some primary particles are < 1pm in size and some second-
ary particles may grow to >3y m, and
3. The magnitude of the mode integrals can vary substantially with-
out apparent correlation to influencing factors. The primary
influences are combustion sources, secondary sources, natural
sources, windspeed, and atmospheric turbulence.
41
-------
CO
CO
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1.0
PARTICLE DIAMETER,
10.0
Figure 4-2. Idealized Graphical Representation of a Bimodal Distribution
42
-------
The existance of multimodal particle size distributions has an impact
on the collection and analysis of ambient particles. The dichotomous
sampler cuts the particles at 2.5 y m. The 2.5 ym cut-size is in the
minimum range between the coarse and fine modes. Therefore, the dichoto-
mous sampler appears to be well adapted to sampling particles in two
modes of urban atmospheres.
The minimum between the coarse and fine modes roughly corresponds to
the lower operating range of the optical microscope. To examine the
fine mode, SEM should be used.
4.4.7 Creation of Size Distributions
A size class can be any size fractinated component, such as the two
cut sizes of the dichotomous samples. In some cases, the size classes
can be chosen based on microscopic examination of an unfractionated sample
by using ocular micrometer-scale divisions when measuring particle size.
The use of the microscope to determine size distributions of parti-
cles is difficult mainly because of the number of particles that must be
counted. Sufficient particles in each size range must be counted to get
significant data. The exact number of particles to be measured and
counted depends on the number of size ranges and the diversity of shapes.
Fairly homogeneously sized particles of nearly equal "shapes may require
only 100 or so particles to be counted, while highly irregularily shaped
particles in many sizes may require thousands.^
A study discussed in Reference 28 reported that for a size range rep-
resenting 20 percent by weight of the total, 400 particles within that
size range should be sufficient for a 1-percent accuracy. The study
also reported that to get a 0.1-percent accuracy, 40,000 particles should
be counted.
The large number of particles to be counted is somewhat offset by the
needed accuracy in the particle measurements. For particle distribution
counts, the measurements of particle sizes do not have to be as accurate
as the size measurements on single particles, because, when many particles
are measured, small errors in one direction are offset and compensated
for by small errors in another direction.
43
-------
Another problem with microscopically measured distributions is a rep-
resentative sample; the sample selected for analysis must be representa-
tive of the entire sample. To counter this problem several portions of
the whole should be selected for analysis.
As shown by the above discussion, microscopic methods have limited
applications when employed for the creation of particle-size distribu-
tions, especially when non-fractionated samples are being examined. For
particle sizing applications, the microscope, especially the optical,
should be used for the examination of relatively small numbers of parti-
cles, as a verification of aerodynamic cut-size data from samplers, and
for the verification of automatic counting data.
To handle large numbers of particles, semi-automatic and automatic
counting and measuring systems have been developed. Some of these de-
vices are relatively simple, while others are complex and computer aided.
A list of some commercially available instruments along with their general
mode of operation is shown in Table 4-2.
4.5 SOURCE FINGERPRINTS
Particles and their characteristics can be used to trace back to
their sources with varying degrees of success. These characteristics
include type of particle, particle size, particle shape and freshness of
cut, and particle composition. Figure 4-3 shows combustion particles
and their associated X-ray spectra. The degree of succe.ss in source ap-
portionment of particles varies on how well the particles can be charac-
terized, on how much effort and cost are expended, and on how many sources
with similar particle emissions exist in the area of impact.
In source apportionment studies, a single particle is rarely used to
characterize a source. Instead, a particle assembledge is characterized
by relative distributions of particle types or chemical characteristics
in size ranges. These assembledges can then be compared to collected
particles to reveal a relative contribution of each source.
All of these variables are related. With a variety of similar souce
types, a more detailed analysis is needed, the cost increases, and cer-
tainty of results decreases. A special study can be conducted to evaluate
44
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Figure 4-3. Combustion Particles and Associated X-Ray Spectra
(Magnification: 1000 x ; Spectra show the presence of Al, SI,
S, K, Fe, and Cu)30
47
-------
the impact of single similar sources on ambient particle loadings. The
study is designed to take advantage of special meterological conditions
that segregate a particular source's emissions. These special conditions
can be predicted by using dispersion modeling.
To produce source fingerprints, two major analytical techniques are
used. These techniques are normally used in combination. The advantages
and disadvantages of microscopy and chemical analysis as used separately
and as used in combination can be stated as follows.
Methods Advantage Disadvantage
Microscopy Highly accurate for Large errors possible
identifying source in estimating relative
types source contribution
Chemical/Ele- Highly accurate for Large errors possible
mental chemical/elemental in estimating relative
Analysis composition source contribution
Combination of Highly accurate for Large errors possible
Microscopy & source types and in estimating relative
Chemical/Ele- chemical/elemental source contribution
mental composition. Agreement
Analysis between the two methods
is strong indication that
source-type contributiion
assignment is valid.
4.6 QUALITY ASSURANCE
All aspects of source apportionment studies should be covered by
quality assurance (QA) procedures. The QA procedures for ambient sampling
and filter handling are discussed in Reference 12 and are not repeated
here.
The QA procedures used in the particle identification and character-
ization aspects of the studies appear to be fairly well standardized. A
good QA program should include the following procedures, at a minimum:
Optical Microscopy^
1. Graticule and reticule calibration,
2. Calibration of microscope objectives, and
3. Replicate analyses (usually at least 10 percent).
48
-------
SEM4
1. Calibration of SEM magnification,
2. Replicate analyses, and
3. Calibration of X-ray spectrometer peaks,
49
-------
5.0 SELECTION OF METHODS
The selection of the appropriate sampling and analytical methods de-
pends on a number of factors including the purpose of the study, the
characteristics of the particles collected, the size of the sample col-
lected, and the available budget. Each portion of the study, i.e.,
sampling, sample preparation, and analysis, must be evaluated in light
of the restraining factors.
5.1 METHODS
The purpose of source apportionment studies is to designate, locate,
and determine the contribution of various air pollution sources on the
ambient particulate loading. The types of equipment and analysis techni-
ques used have an impact on the results of the study and on the quality
of the data collected (see Section 4.0). The information presented in
Table 5-1 and 5-2 can be used to assist in the selection of sampling and
analytical methods.
In general, the selection of the proper monitors, filters, and analy-
tical techniques is dependent on three major influences; the type of
sampling to be performed, the type of sources impacting the study area,
and the budget constraints. In most situations, all three of these
influences can be active at the same time.
On Table 5-1, the type of monitor and the analysis techniques are
compared to the influences described above. The impact of each influence
can be seen on the selections. A low budget program limits the selection
of samplers and analysis techniques. In effect, a low budget may require
the analysis of filters from an existing TSP monitoring program. Thus,
the sampling sites and the types of monitors and analyses may be severely
limited.
The type of source has its greatest impact on the type of analysis.
The size range of industrial process emissions allows the use of all
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three analysis techniques. However, fugitive dust should require only
optical microscopy, unless one type of particle may require X-ray analysis
for identification. The SEM could also be used for this.
In areas which are impacted by a source of fresh nuclei, such as in
the vicinity of power plants, SEM is the appropriate technique because of
the size range involved. In areas of high sulfate impacts, SEM is pre-
ferred but optical microscopy may be used.
The type of sampling influences the type of monitor being used. For
both the IP and TSP sampling, the type of monitor is specified, and gravi-
metric analyses are performed.
For Table 5-2, filters and analysis techniques are compared to types
of monitors. For the hi-vol, both optical and SEM are specified. The
difficulties with using the techniques on fiber filters have been discuss-
ed before. When using this table, reference should be made to the sec-
tions of the report that deal with the filter media and the use of certain
filters in certain types of atmospheres. For example, when sampling in
high sulfate areas, PTFE filters should be used.
5.2 COST OF ANALYSES
Arriving at typical costs for the various analysis procedures is
difficult because of the many different variables associated with particle
identification techniques. Of prime importance to the cost level is the
final use of the data received. While the costs of confirming data gath-
ered by another technique should be relatively low, the costs for planning
purposes should be higher, and those for litigation would be much higher.
The use of the data dictates the precision involved and the movement from
a qualitative to a semi-quantitative effort. Thus, the costs increase.
The costs to perform the two major analysis techniques are reported
to be approximately $400 per filter for optical microscopy-^ and $200 to
$300 per filter for scanning electron microscopy.^ Both costs include
sample preparation, sample analysis, particle sizing, and quality assur-
ance. The SEM cost includes X-ray analysis. These costs could be typical
for those associated with a planning study.
Often, both techniques are used in one study. In these cases, the
costs should approach the total of $600 to $700 per filter. Somewhat
53
-------
higher costs may be incurred when individual particles are "picked" from
the filters for SEM analysis.
54
-------
6.0 SELECTION OF A LABORATORY
The most critical decision to be made by an organization seeking as-
sistance with particle analysis is the selection of a suitable laboratory
to conduct the work. Certain parameters enter into this selection and
include experience of personnel, availability of equipment, quality
assurance procedures employed, backlog of work, past performance and
cost.
The most important area for consideration of a laboratory to perform
the work is the experience of the microscopists. Information on the back-
ground and current work areas of the microscopists at the lab should be
requested. This information should include their experience with each
of the specific analytical techniques.
Discussions elsewhere in this report identify the appropriate analy-
sis methods depending on the desired results. The availability of these
particular methods can be used as an initial screening criteria when eval-
uating several labs.
The next step in the evaluation should be much more detailed. The
labs under consideration should be requested to supply a written copy of
their quality assurance procedures. These can be compared to the minimum
QA requirements presented in this report. Since the integrity of the
sample is so important in the accuracy of the final results, the quality
assurance and chain of custody procedures are one of the most important
evaluation criteria.
In order for the information provided by the analysis to be useful,
it must be available as soon after the sample is collected as possible.
To ensure the expenditious handling of samples, the work backlog of the
microscopists in the lab should be requested. This will provide some
estimate of when the results of the analysis will be available.
55
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There are two methods which can be used to judge the past performance
of each lab under consideration. First, a list of former clients of each
lab should be requested and several of those listed should be contacted.
These clients can provide some insight as to whether the lab produced
the expected results in a timely manner and in a form that was easily
usable. Second, a request can also be made for the lab to supply a
sample report of a study of a similar nature. The agency can then decide
if the report is sufficiently detailed or requires further elaboration.
Depending on the agency's capabilities, a report showing only the analysis
results may be sufficient. Other agencies may want explanations and
conclusions drawn from the results included in the report from the
lab.
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7.0 REFERENCES
1. "Reference Method for the Determination of Suspended Participates
in the Atmosphere (High Volume Method)," Appendix B of "National
Primary and Secondary Ambient Air Quality Standards," Fed. Reg.,
Vol. 36, 7.0 November 25, 1971, pg. 22384.
2. McCrone, W. C. & J. G. Delly, The Particle Atlas, Vol. 1 Principles
and Techniques, 2nd Edition, Ann Arbor Science Publishers Inc., Ann
Arbor, Mich., 1973.
3. Crutcher, Russ, Boeing Corp., personal communication, May 25, 1982.
4. Janocko, P., Energy Technology Consultants, personal communication,
May 25, 1982.
5. Bradway, R. M., and F. A. Record, "National Assessment of the Urban
Particulate Problem, Vol. II, Particulate Characterization," EPA-
450/3-76-025, July 1976.
6. Kerr, P. F., Optical Minerology, McGraw-Hill Book Company, Inc., New
York, 1959.
7. Pough, F. H., A Field Guide to Rocks and Minerals, 3rd Edition,
Houghton Mifflin Company, Boston, 1960.
8. Natusch, D. F. S., "Characterization of Fly Ash from Coal Combustion,"
Workshop Proceeding on Primary Sulfate Emissions from Combustion
Sources. U.S. EPA 600/9-78-Q20b. Vol. 2, 1978. pp. 149-163.
9. Fisher, G. L., (et al), "Physical and Morphological Studies of Size -
Classified Coal Fly Ash," ES&T, Vol. 12, No. 4, April 1978, pp. 447-
451.
10. Dahlin, R. S., Ja-an Su, and L. K. Peters, "Aerosol Formation in
Reacting Gases: Theory and Application to the Anhydrous NH3 - HC1
System," AIChE Journal, Vol. 27, No. 3, May 1981, pp. 404-418.
11. Draftz, R. G., "Types and Sources of Suspended Particles in Chicago,"
111. Inst. of Tech. Res. Inst., Report No. C 9914-C01, Chicago.
12. Quality Assurance Handbook for Air Pollution Measurement Systems,
Vol. II, Ambient Air Specific Methods, EPA-600/4-77-027a, May 1977.
13. McFarland, A. R., C. A. Ortiz, and C. E. Rodes, "Characteristics of
Aerosol Samplers Used in Ambient Air Monitoring," paper presented at
86th national meeting, AIChE, Houston, April 1-5, 1979.
14. Cadle, R. D., The Measurement of Airborne Particles, John Wiley &
Sons, New York, 1975.
15. Knapp, K. T., R. L. Bennett, R. J. Griffin, & R. C. Steward, "Collec-
tion Methods for the Determination of Stationary Source Particulate
Sulfur and Other Elements," in Workshop Proceedings on Primary Sulfate
Emissions from Combustion Sources, Vol. 1 - Meaurement Technology^
EPA-600/9-78-020a, August 1978, pp. 145-159.
57
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16. DeNee, P. B., "Collecting, Handling and Mounting Particles for SEM,"
in Scanning Electron Microscopy, Vol. 1, SEM Inc., AMF O'Hare, ILL,
1978, pp. 479-486.
17. Gelman, C. and J. C. Marshall, "High Purity Fibrous Air Sampling
Media," J. Amer. Ind. Hyg. Assoc., Vol. 36, July 1975, p. 36.
18. Appel, B. R., S. M. Wall, Y. Tokiwa, & M. Haik, "Interference Effects
in Sampling Particulate Nitrate In Ambient Air," Atmospheric Environ-
ment, Vol. 13, 1979, pp. 319-325.
19. Klockow, D., B. Jablonski, & R. Niebner, "Possible Artifacts in
Filter Sampling of Atmospheric Sulfphric Acid and Acidic Sulfates,"
Atmospheric Environment, Vol. 13, 1979, pp. 1665-1676.
20. Hartley, E. M., Jr. and M. J. Matteson, "Sulfur Dioxide Reactions
with Ammonia in Humid Air," I & EC Fundamentals, Vol. 14, Feb. 1975,
pp. 67-72.
21. Gofer, W. R., Ill, D. R. Schryer, and R. S. Rogowski, "The Enhanced
Oxidation of S02 by N02 on Carbon Particles," Atmospheric Environment,
Vol. 14, 1980, pp. 571-575.
22. Britton, L. G., and A. G. Clarke, "Heterogeneous Reactions of Sulphur
Dioxide and S02/N02 Mixtures with a Carbon Soot Aerosol," Atmospheric
Environment. Vol. 14, 1980, pp. 829-839.
23. Corn, M. & R. T. Cheng, "Interactions of Sulfur Dioxide with Insoluble
Suspended Particulate Matter," Presented at 65th Annual Meeting of
APCA, Miami Beach, June 18-22, 1972.
24. Spicer, C. W., & P. M. Schumacher, "Particulate Nitrate: Laboratory
and Field Studies of Major Sampling Interferences," Atmospheric
Environment, Vol. 13, 1979, pp. 543-552.
25. Spicer, C. W., "Photochemical Atmospheric Pollutants Derived from
Nitrogen Oxides," Atmospheric Environment, Vol. 11, 1977, pp. 1089-
1095.
26. Shaw, R. W., et al, "Measurements of Atmospheric Nitrate and Nitric
Acid: The Denuder Difference Experiment," Atmospheric Environment,
Vol. 16, No. 4, 1982, pp. 845-853.
27. Marker, A. B., L. W. Richards, & W. F. Clark, "The Effect of Atmos-
pheric S02 Photochemistry Upon Observed Nitrate Concentration in Aero-
sols," Atmospheric Environment, Vol. 11, 1977, pp. 87-91.
28. Humpheries, D. W., "Mensuration Methods in Optical Microscopy," in
Advances in Optical and Electron Microscopy, Vol. 3, Academic Press,
London, 1969, pp. 33-98.
29. Galeski, J. B., "Particle Size Definitions for Particulate Data Analy-
sis," EPA-600/7-77-129, November 1977.
30. Meant, G. E., "Airborne Particulate Production from Feldspar Process-
ing," M. S. Thesis, N. C. State Univ., 1973.
58
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APPENDIX
BIBLIOGRAPHY OF REPORTS
SOURCE APPORTIONMENT USING PHYSICAL METHODS
59
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MICROSCOPY
Casuccio, G. S., P. B. Janocko, R. J. Lee, and J. F. Kelly, "The Role of
Computer Controlled Scanning Electron Microscopy in Receptor Modeling,"
Paper presented at APCA 75th annual meeting, New Orleans, La., 1982.
Crutcher, E. R., "Light Microscopy As An Analytical Approach to Receptor
Modeling," Paper presented at APCA Specialty Conference Receptor Models
Applied to Contemporary Air Pollution Problems, Danvers, Mass., October
17-20, 1982.
Crutcher, E. R. and L. S. Nishimura, "A Standardized Approach to Quantita-
tive Light Microscopy," Proceedings of 4th International Symposium on
Contamination Control, Washington, D.C., 1978, p. 150.
Draftz, R. G., "Aerosol Source Characterization Study in Miami, Florida:
Microscopical Analysis," EPA-600/3-79-097, 1979.
Draftz, R. G. and K. Severin, "Microscopical Analysis of Aerosols Collected
in St. Louis, Missouri," EPA-600/3-870-027, 1980.
Graf, J., R. H. Snow, and R. G. Draftz, "Aerosol Sampling and Analysis --
Phoenix, Arizona," EPA-600/3-77-015, 1977.
Janocko, P. B. et. al., "The El Paso Airshed: Source Apportionment Using
Complimentary Analyses and Receptor Models," Paper presented at APCA
Specialty Conference Receptor Models Applied to Contemporary Air
Pollution Problems, Danvers, Mass., October 17-20, 1982.
Johnson, D. L., et. al., "A Chemical Element Comparison of Individual
Particle Analysis and Bulk Chemical Analysis," Scanning Elect. Microsc.,
Vol. I, 1981, p. 469.
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X-RAY DIFFRACTION
Davis, B. L., "Additional Suggestions for X-Ray Quantitative Analysis of
High-Volume Filter Samples," Atmos. Environ, Vol. 12, 1978, p. 2403.
Davis, B. L., "Standardless X-Ray Diffraction Quantitative Analysis,"
Atmos. Environ., Vol. 14, 1980, p. 217.
Davis, B. L., "The Use of X-Ray Diffraction Quantitative Analysis in Air
Quality Source Studies," In Electron Microscopy and X-Ray Applications to
Environmental and Occupational Health Analysis, Vol. 2, P. A. Russell,
ed., Ann Arbor Science Publishers, Ann Arbor, Mich., 1981.
Davis, B. L., "A Study of the Errors in X-Ray Quantitative Analysis
Procedures for Aerosols Collected on Filter Media," Atmos. Environ., Vol.
15, 1981, p. 291.
Davis, B. L., "Hybrid Model for Source Apportionment of the Houston
Aerosol for the Period September 10-18, 1980, "Institute of Atmospheric
Sciences Report No. 82-3, S. D. School of Mines and Technology, Rapid
City, S.D., 1982.
Davis, B. L., L. R. Johnson, and M. J. Flannagan, "Provenance Factor
Analysis of Fugitive Dust Produced in Rapid City, South Dakota," JAPCA,
Vol . 31, 1981, p. 241.
Davis, B. L., D. Maughn, and u. Carlson, "X-Ray Studies of Airborne
Particulate Matter Observed During Wintertime at Missoula, Missouri," In
Electron Microscopy and X-Ray Applications to Environmental and Occupational
Health Analysis, Ann Arbor Science Publisher, Ann Arbor, Mich. In Press.
61
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TECHNICAL REPORT DATA
(Please rc^'J Instructions on the rewrtt. Before completing)
1. RE.POPT NO
EPA-450/4-83-014
2
13. RECIPIENT'S ACCESSION NO.
4. TITLE ANDSUBTITLfc
Receptor Model Technical Series
Volume IV
Summary of Particle Identification Technique
j
5 REPORT DATE
June 1983
6. PERFORMING ORGANIZATION CODE
7 AUTHOR(S)
George E. Meant and
J. Calvin Thames
8. PERFORMING ORGANIZATION REPORT NO
9 PERFORMING ORGANIZATION NAME AND ADDRESS
Engineering-Science
501 Mi Hard Street
Durham, North Carolina 27701
10. PROGRAM ELEMENT NO.
11 CONTRACT/GRANT NO.
68-02-3509
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Monitoring and Data Analysis Division
Research Triangle Park, North Carolina 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
EPA Project Officer: Thompson G. Pace
16. ABSTRACT
In recent years there has been continuing interest in source apportionment by
studying the particles collected by the sampler. These receptor methods of source
apportionment are useful for control strategy development. The purpose of this
document is to provide a general discussion of the methods used for particle identi-
fication, as collected by ambient samplers.
This document provides the basic information on the techniques, describes the
types and properties of particles, discusses the selection of methods, provides the
basic information to evaluate and apply the results of studies, and discusses the
parameters to be used to select a laboratory to perform the analyses.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Receptor Model
Optical Microscopy
Scanning Electron Miscroscopy
Source Apportionment
Particulate Matter
b IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
18. DISTRIBUTION STATEMENT
I Release Unlimited
19 SECURITY CLASS (This Report/
\ Unclassified
j 20. SECURITY CLASS tThispagei
Unclassified
21. NO. OF PAGES
69
22. PRICE
EPA Form 2220-1 (Rev. 4-77! PREVIOUS,
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U.S. Environmental Protection Agency
Region V, Library
230 South Dearborn Street
Chicago, Illinois 60604
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