EPA-600/3-77-072
July 1977
Ecological Research Series
LIBBA^Y ^ PRO^0
S. B*YL-..,..: . ,L ...^EC.'ION AGEiKf?
I, IL 1 9
EP 600/3
77-072
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U S Environmental
Protection Agency, have been grouped into nine series These nine broad cate-
gories were established to facilitate further development and application of en-
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The nine series are
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describes research on the effects of pollution on humans, plant and animal spe-
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ences Investigations include formation, transport and pathway studies to deter-
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aquatic terrestrial, and atmospheric environments
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161
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EPA-600/3-77-072
July 1977
AEROSOL CHARACTERISTICS AND VISIBILITY
by
Alan P. Waggoner
Robert J. Charlson
University of Washington
Seattle, Washington 98195
Grant No. R800665
Project Officer
William E. Wilson
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. 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 commerical products constitute endorsement or
recommendation for use.
ii
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PREFACE
Atmospheric turbidity and visibility restrictions are perhaps the most
obvious manifestations of air pollution. Historically, from the days of the
caveman's primitive fire through the Industrial Revolution and onto the mod-
ern technological era, it has been clearly apparent that the emission of
smoke, dust and fumes into the air decreases its clarity, and destroys scenic
vistas. Until recently, however, there has not existed an adequate scientific
understanding of the relationship between airborne particulate matter and
reduced visibility, or the mechanisms by which aerosols interact with light
rays to produce atmospheric turbidity.
This lack of knowledge has been a severe impediment to early efforts by
air pollution control authorities to accurately quantify the extent of
visibility reduction caused by particulate pollutants. The latter was evi-
denced by the initial attempts to set visibility standards in California,
based simply on concentration of total suspended matter.
In the early 19&0's, W. Stoeber and I at the California Institute of
Technology concluded that such problems of visibility were amenable to
scientific investigation and solution. Accordingly, we proposed to undertake
a comprehensive study of the influence of aerosol characteristics on visi-
bility. The Public Health Service, recognizing the potential value of such
research, awarded the investigators a 3-year grant.
After the first year, the research project was transferred to the
University of Washington, Seattle, where I, joined in time by Masaki, Pueschel
and Charlson, continued the work. Ultimately Charlson and his associates,
Ahlquist and Waggoner, successfully expanded and deepened the scope of the
research.
The history of this research attests to the principle that a sound scien-
tific idea emerging at the proper time, adequately encouraged and supported,
111
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can not only add to the fund of knowledge, but also contribute greatly to the
technical solution of important social problems.
August T. Rossano
February 12, 1975
IV
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ABSTRACT
This report summarizes progress in measuring the optical properties of
aerosols and in relating aerosol characteristics to visibility reduction
made in the author's laboratory during the period 1965-1971. An instrument,
the integrating nephelometer, which measures the scattering component of
extinction, b , was developed and used in several field studies. Measured
sp
b and observer visibility have been shown to be highly correlated and to
sp
follow the Koschmieder relation. Measured b is highly correlated (0.95
sp
in Los Angeles) with suspended particle volume in the 0.1 to 1.0 ym size
range. A useful correlation (0.56 to 0.92 at various sites) has been found
between b and particle mass as collected on a filter. Techniques have
been developed to measure b as a function of relative humidity for ambient
sp
and model aerosols. Water, absorbed by hygroscopic aerosols, as H-SO,,
and/or deliquescent aerosols, as (NH,)_SO,, make a substantial contribution
to visibility reduction. Techniques were also developed to measure the
absorption component of extinction, b , ; to measure the forward/backward
3.U
scattering ratio; and to determine b as a function of wavelength.
sp
This report was submitted in fulfillment of Grant No. R800665 by the
University of Washington under the sponsorship of the U.S. Environmental
Protection Agency. This report covers the period April 1, 1971, to December
31, 1974, and work was completed as of December 31, 1974.
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CONTENTS
Preface iii
Abstract v
Acknowledgment viii
1. Introduction 1
2. Atmospheric Optics and Visibility 3
3. Particle Optics 5
4. Techniques for Measurement of Relevant
Optical Properties 19
5. Atmospheric Measurements and Data 25
6. Conclusions 28
7. Epilogue 30
References 31
Appendix 33
vii
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ACKNOWLEDGMENTS
We wish to acknowledge Prof. A.T. Rossano, who had the foresight in
1962 to begin this project. We certainly thank personnel of USPHS/D HEW,
NAPCA and EPA for having provided funds for this twelve-year effort. We also
thank the NSF under Grant GA 27662 which, although administratively unrelated
to this visibility project, provided useful resources and opportunities for
the data we report. Indeed, it is not possible to draw a hard line between
the science supported by these two agencies, even though there were separate
goals and purposes. Finally, we want to thank Prof. K.T. Whitby and his
colleagues, Dr. George Hidy, the California State Air Resources Board,
Dr. Rudolf Husar, Dr. John Winchester, and other colleagues who shared data
with us.
viii
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SECTION 1
INTRODUCTION
Air pollution, or more specifically the suspended particulate matter or
aerosol, has dramatic effects on the optical properties of air. Visibility is
often degraded from tens or hundreds of kilometers down to a few kilometers.
In highly polluted areas such as Los Angeles, visual ranges as small as one
kilometer occur.
The results reported here are from research started in 1962 with USPHS
grant AP336. At that time, the consensus of experts was that the problem of
visibility was too complex for generalization:
"It seems apparent...that any relation which is found between
visibility and particulate concentration...would be limited
to the specific location and time period when the sampling
was done. Like most atmospheric phenomena, these are very
complex measurements in spite of their apparent simplicity."
E. Robinson in Stern's 1962 edition of Air Pollution.
As reported in our publications and by others, the instrumental approach
we developed has made visibility degradation one of the best understood and
most easily quantified effects of air pollution.
A new instrument, the integrating nephelometer, was developed for our
visibility investigations. This instrument has provided an objective
measure of the optical effect of urban aerosol, and the measured scattering
coefficient has been shown to be highly correlated with both visual range
and mass concentration of particles, particularly those between 0.1 and 1.0 ym
in diameter. A 1973 report of the State of California Air Resources Board
recommends the integrating nephelometer as an instrument for routine air
quality monitoring (Samuels, et^ al^. , 1973).
The following sections summarize current knowledge of aerosol properties
necessary to describe integral effects of the aerosol-atmosphere system as
they relate to the problem of visibility. Included is research by others as
-------
well as that supported at the University of Washington. A list of publica-
tions supported by this grant is given in Appendix A.
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SECTION 2
ATMOSPHERIC OPTICS AND VISIBILITY
It is convenient to define several parameters commonly used to describe
atmospheric optics.
The extinction coefficient b of a real atmosphere defines the change
in intensity of light traversing a pathlength Ax by the Beer-Lambert law:
AI .
—=— = -b Ax
I ext
b is the sum of two terms:
ext
b = b . (gases) + b (Particles)
ext ext & ext
b (gases) = b H- b
ext v& Rg ag
where b Ax is the fraction of incident light scattered into all directions
Kg
by gas molecules in Ax.
b Ax is the fraction of incident light absorbed by gas molecules in
ag
Ax.
Our interest is in b (particles), which can be broken down as
GXt
follows:
b (particles) = b + b
ext r ap sp
where b Ax is the fraction of incident light absorbed by particles in Ax.
ap
b Ax is the fraction of incident light scattered into all directions
sp
by particles in Ax.
The observer visibility, or visual range, is that distance at which a
black object can be just discerned against the horizon. Koschmieder (1924)
showed that a turbid media, such as urban air, reduces the contrast (ratio
of brightness of an object to the horizon brightness, minus one) of distant
-------
objects as given by
-b x
C = c e (Middleton, 1968),
where C and C are the contrast relative to the horizon of an object at zero
distance and at distance x. A black object has a C of -1. Experiments have
determined that typical observers can detect objects on the horizon with a
visual contrast of 0.02 to 0.05. Assuming horizontal homogeneity of aerosol
properties and illumination and a 0.02 detectable contrast, the visible range
is
3 9
L = ^-
v b
ext
For a contrast of 0.05,
T - 3.0
Lv ~ b ^
ext
Usually the assumption is made that b = b
J r ext sp
b can be calculated from known or assumed aerosol particle size distribu-
sp
tion, concentration and refractive index, as discussed in Section 3.
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SECTION 3
PARTICLE OPTICS
The atmospheric aerosol is composed of particles that range in size
from smaller than 0.01 ym to larger than 10 ym diameter. The particles are
of various chemical compositions and each particle can be a mixture of sub-
stances or a single substance. The integral optical effect of the aerosol
particles is dependent on all of these parameters.
The integral properties of an aerosol can be expressed in a number of
ways: b , b , condensation nucleii count, mass of particles per volume of
sp ap
air, etc. Conversion from one integral aerosol property to another is
generally impossible without knowledge of the particle size distribution.
Earlier work by this laboratory (Charlson, 1969; Charlson, et^ al^, a,b 1974)
has shown that aerosol optical parameters depend predominantly on (1) size
distribution, (2) molecular composition, and (3) relative humidity.
PARTICLE SIZE
The optical properties of an individual particle depend on its effective
area, its refractive index, and, to an extent poorly understood, its shape.
Aerosol particle size distribution may be graphed in a number of ways:
(1) log (dN/dlnD) vs InD, (2) dN/dlnD vs InD, (3) dS/dlnD vs InD, and (4)
dV/dlnD vs InD, as shown in Figures 1 and 2 for urban Los Angeles data
taken during a period in 1969 by K.T. Whitby (1972).
When plotted in this way, the volume distribution is usually bi-modal
with one maximum between 0.2 and 1.0 urn and a second maximum between 3 and
20 ym in diameter, as shown in Figure 3. Using Mie solutions for spherical
particles, the optical scattering extinction coefficient (b ) per log size
sp
interval can be calculated and is shown in Figure 3 using the measured
Pomona aerosol size distribution. A similar plot of volume distribution and
b from Garland (1973) is shown in Figure 4 for high relative humidity
sp
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SMOG SIZE DISTRIBUTION
10? -
106
105
V)
o
oc
u
5
x
OQ
S
103
102
10
0.1
0.01
• GRAND AVG.L.A.-1969
.001 .01 0.1 1.0
PARTICLE DIAMETER, Dp,
10
100
Figure 1. Figure 1 and 2 show different ways of plotting the same
particle size distribution data taken during 1969 in Los Angeles.
The size distribution was measured using a combination of electrostatic
mobility and single particle optical counter techniques (Whitby,
et aj^. , 1972). Particle optical properties depend on particle surface
or volume. Hence this figure shows that the optical properties of
this sample are dominated by particles in the range 0.1 ym < D < 1 jjm.
-------
a
Q
CO
a.
O
o
o
I I I II ill I I I I Mill I 1 I Illlll
1.0 —
.01
.1 i
PARTICLE DIAMETER, MICRONS
10
Figure 2
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POMONA 21:40
10-5-72
MEASURED
<
a.
0.01
0.1
1.0
10
CJ
<
>
o
V)
o
£
o
CALCULATED
0.01
0.1 1.0
PARTICLE DIAMETER, y
10
Figure 3.
Top: Aerosol particle size distribution measured at Pomona during 1972 State
of California Air Resources Board ACHEX program (Hidy, et al., 1975).
Bottom: Calculated optical scattering by particles, bsp, for the measured size
distribution. The particles are assumed to be spheres of refractive index
1.5.
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07:14 h
10:23h
AV
Alogr
1.0
"L
10.0
1.0 10.0
radius, jUm
AV
Alogr
Alogr
IL
1.0
10.0
1.0 10.0
radius, urn
Figure 4. Light scattering coefficients in fog calculated from measured volume
distributions of Garland et at. (1973), showing the dominant contributions of
submicrometer particles to bSp.
-------
polluted British fog. It seems usual that sub-micrometer aerosol particles
dominate the aerosol scattering extinction In the visible spectrum although
there clearly are cases in fogs, rain, snow, clouds and dust storms in which
large particles influence or dominate visible extinction. A striking example
of the relationship of measured particle scattering to measured particle
volume in the 0.1 to 1.0 pm decade of particle size is shown in Figure 5.
The correlation coefficient of b , measured with a nephelometer, and
0.1 to 1.0 Mm particle volume, measured using electroststic mobility and
single particle optical counting techniques, was 0.95 at various locations in
the Los Angeles basin. The correlation of b with aerosol mass as collected
sp
on a filter is generally poorer, although still useful, as shown in Table 1.
MOLECULAR COMPOSITION
The particle interaction with water, biological effects and complex
refractive index depend on the molecular composition. Therefore, it is
important that the composition of various aerosol systems be classified,
particularly insofar as this determines the imaginary part of the refractive
index and hygroscopicity. Unfortunately, this is an area in which very little
work has been done so far. Rasmussen and Went (1965) suggested that organic
materials (terpenes) are a major source of atmospheric particles, but did
not quantify their work adequately for application to optics. Junge (1954)
has shown that the reaction products of S0_ with water and ammonia play an
important part in urban and rural aerosols, although he did not attempt to
relate quantitatively the composition with optical effects. We have pre-
limir.ary data suggesting that continental aerosol optics is often dominated
by H SO, and the products of its neutralization with NH (Charlson et al. ,
1974a; Charlson et al., 1974b).
There are two features of particulate chemistry which simplify the
situation considerably in some locations. First, relatively pure (i.e., mole
fraction >50%) molecular species (e.g. (NH.KSO, , H SO, or seasalt) dominate
optical scattering in some atmospheric aerosols and second, certain compounds
are found almost exclusively in the submicrometer fraction (Patterson and
Wagman, 1974, Dzubay and Stevens, 1973), as shown in Table 2 and in Figure 6.
The molecular nature of individual particles is a function of the source
and removal mechanisms for these particles. The most important observable
10
-------
200
m
£
«• 100
0
VOLUME,O.M.O/umVS.bsp
TWO HOUR AVERAGES FROM
WESTCOVENIA, RUBIDOUX
POMONA, DOMINGUEZ HILLS
CORRELATION COEFFICIENT = 0.948
10
15
20
Figure 5. Plot of measured aerosol particle volume including only those of 0.1 to 1.0ju m diameter versus
measured bsp. Measurements were part of State of California Air Resources Board ACHEX program (Hidy,
et al., 1975). Data was supplied by Dr. Clark of North American Rockwell.
11
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1L/M
TO PUMP
Figure 6a. Schematic view of a dichotomous sampler which contains
a virtual impactor. The flow rate at the inlet is 50 liters per minute,
and the flow rates at the outlets are 49 and 1 liters per minute (Loo
andJaKlevic, 1974).
O
S, Zn, Br, Pb.
SMALLER THAN 2 fim.
1.8-mg DEPOSIT
Al, Si. Ca. Ti, Fe.
LARGER THAN 2 fj
1.3-mg DEPOSIT
Figure 6b. Photograph of the two filters used in the dichotomous
sampler for the 23 hour period beginning 10:1530 August, 1973,
at Wash. Univ. in St. Louis, Mo. The sampled air volume was 68 m3.
This reproduction shows, as does the original photograph, that most
of the optical absorption is due to the small particle mode (Dzubay,
1973).
12
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effect of composition on particle optics is the relationship of b and
sp
relative humidity.
RELATIVE HUMIDITY
The humidity effects in aerosol optics fall into three categories:
RH _<^ 100%: particles between and above water clouds
(including high RH hazes);
RH > 100%: unactivated particles in water clouds and fog;
RH > 100%: activated cloud droplets.
Our efforts have been limited to the first two and are discussed in the
following paragraphs.
RH < 100%
Since a large fraction of submicrometer particles are hygroscopic or
deliquescent (Winkler, 1973; Junge, 1954; Hanel, 1971; Covert, 1974), the
size distribution of an atmospheric aerosol, and hence its optical or climat-
ological properties, depend largely on relative humidities, even at RH < 50%.
Figures 7, 8 and 9 show the total light scattering coefficient, b ,as
sp
a function of relative humidity for several different aerosol types as found
in the real atmosphere. These curves are representative of those taken over
a wide variety of locations and have certain highly reproducible features.
First of all, it wil] be noted that light scattering always increases with
humidity,, although for relatively hygrophobic systems the increase may be
very slight up to extremely high RH (for example, Figure 9a). While for
most aerosols, such as H SO, droplets, the curve increases monotonically,
definite inflection points due to deliquescent salts (see Figures 7 and 8a)
are seen frequently, indicating the dominance by rather pure inorganic
subtances such as (NH,)?SO,.
The evolution of a distribution of droplets under conditions of changing,
subsaturation RH modifies the optical interactions between radiation and
particles, thus changing the temperature of the environment of the particles
and hence in turn the relative humidity. This complex chain of events
cannot be satisfactorily modelled until the parameters which go into the
models (dependence of particle growth on chemistry, optical properties of
saturated and supersaturated droplets, etc.) and the basic physical principles
13
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i i i i i i i
J I I
50
RELATIVE HUMIDITY. PER CENT
100
Figure 7a. Averaged humidograms, Pt Reyes Lighthouse, California, 1630
24 Aug. 72 to 0600 25 Aug. 72.
0 50 100
RELATIVE HUMIDITY, PER CENT
Figure 7b. Humidograms, laboratory aerosol, NaCI and Sea Salt.
14
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i I I I I I I
0 50
RELATIVE HUMIDITY. PER CENT
Figure 8a. Humidograms at Tyson, Missouri, a.
b. 1223 23 Sep. 73.
I I I I I I I
100
2330 24 Sep. 73.
ACIDSULFATE ,' / AC/D
SULFATE
I I I I I I I I I I 1
I I I
SO
100
RELATIVE HUMIDITY, PER CENT
Figure 8b. Humidograms, lab acid sulfate aerosol with addition
of 0.1 ppm NH .
15
-------
1 r—f
i r T r i i i i i
i i _ i
i i i i i
0 50
RELATIVE HUMIDITY, PER CENT
Figure 9a. Averaged humidograms, Pasadena, California.
1032 21 Sep. 72. b. 1300 to 1700 22 Sep. 72.
100
a. 0232 to
J 2
T T i
I I i I I I I I I I I I I I ! i I I
Figure 9b.
SO
RELATIVE HUMIDITY, PER CENT
Humidogram, (NH )2SO laboratory aerosol,
100
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of the component processes are understood.
A system has been designed and operated by this laboratory that (over a
period of about 120 seconds) sweeps the relative humidity of air containing
aerosol particles from 30% to 95%. Changes in particle diameter are detected
as changes in the scattering coefficient of the aerosol particles (Covert,
1974; Charlson et_ al_., a,b 1974).
In the midcontinent region 30 km southwest of St. Louis, this system
detected H2S04/(NH4)HS04/(NH4)2S04 as dominate materials in the 0.1 to 1 ym
decade of aerosol size. Injection of sub ppm concentrations of NH converted
the b (RH) response characteristic of H0SO. to that of (NH.^SO.. The
sp / 4 42 4
(NH,)~ SO, is detected by comparing the value of relative humidity at the
deliquescence point for the unknown sample with that of laboratory generated
(NH4)2S04 aerosol. 98% of the time either H SO or (NH4) S04 was the dominant
substance in terms of optical effect (Charlson et al., 1974a). Figure 10
shows non-urban turbidity and SO, and suggests a possible relationship
between the two parameters similar to that found during our measurements near
St. Louis.
RII > 100%, Deactivated Particles
When RH > 100%, and in the presence of suitable cloud condensation
nuclei, some of the droplets grow to much larger sizes, forming fog and water
clouds. The study of the processes leading up to the formation of the large
drops is a cornerstone of cloud physics. In addition to the activated par-
ticles, there are unactivated particles which often outnumber the cloud or
fog drops by orders of magnitude (Twomey, 1972) , and which may still influence
or even dominate some optical properties of clouds. Both light scattering
and absorption by these unactivated particles may be important.
17
-------
Figure 10, Non-urban turbid,-ty, decad,
I., 1969).
Figure 10b. Non-urban SO^, Mgrn/M3 (NASN
dgta)
-------
SECTION 4
TECHNIQUES FOR MEASUREMENT OF RELEVANT OPTICAL PROPERTIES
In the past several years our efforts havevbeen focused on design and
testing of methods to measure aerosol optical properties that directly
determine aerosol radiative interactions. Methods for measurement of these
relevant integral aerosol optical properties—namely, b , b, , b (RH) , and
sp osp ^P
b —are described in the following sections.
ap
b
sp
Consider a layer of thickness dx illuminated by a parallel beam of
wavelength A and intensity I . For perpendicular incidence, the intensity
o ,A
of light scattered into solid angle dQ is
dIX (O)dfl = I .B.(0)dx.
A visibility meter using the operator's eye as a detector was devised by
Buettell and Brewer (1949) that geometrically performs the integration of
3, (9) over solid angle to measure b , ,.,. , ,n ,r./-0\ AI -, j
Ax 6 sp,A (Middleton, 1968). Ahlquist and
Charlson (1967) increased the original instrument sensitivity by using a
photomultiplier tube to detect scattered light from a xenon flash lamp.
Ahlquist e_t^ ai^. (1974, patent application) improved the sensitivity,
stability and dynamic range by substituting an incandescent lamp for the
xenon flash lamp and detecting the scattered light using digital photon
counting techniques. This instrument, called an integrating nephelometer , is
shown in Figure 11. Modern versions of Beuttell and Brewer's device have
sufficient sensitivity to be calibratable in an absolute sense with b , the
scattering coefficient of particle-free gases such as He, CO „, CC1 F_ .
19
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TUNGSTEN FILAMENT
LIGHT SOURCE
CLEAN AIR
PURGE
I /
NARROWBAND
OPTICAL FILTER
AEROSOL
OUTLET
JtLA
GLASS
COLLIMATING DISKS
AEROSOL
INLET
CLEAN AIR
PURGE
AEROSOL
OUTLET
TUNGSTEN FILAMENT
LIGHT SOURCE
SCATTERING VOLUME
Figure 11. Diagram of nephelometer with enlarged view of the partial
shutter. Without the shutter, the instrument integrates the particle
scattering coefficient over ~7° to 170° to measure bsp. With the
shutter in place, the instrument integrates over ~9Qo to 17(K» to
measure t>bsp-
20
-------
The geometric errors of the instrument have been studied by Middleton
(1968), Ensor and Waggoner (1970), Heintzenberg and Quenzel (1973), and Rabin-
off and Herman (1973) and are estimated to be 10% or less for the aerosol
particle size distributions normally found in the atmosphere.
The modern instrument is alternately filled with ambient and particle-
free air and the difference in scattered light intensity is proportional to
the scattering extinction coefficient due to aerosol particles, b . The
-7-1 SP
measured values of b in the atmosphere range from 10 m at Mauna Loa
SP
Observatory to 3 x lO"-^"1 in polluted Los Angeles (0.005 to 150 times the
Rayleigh scattering coefficient at 530nm).
The integrating nephelometer has become an accepted instrument for
measurement of aerosol scattering extinction. A series of patents, based on
the designs of the authors of this report and covering various aspects of
the nephelometer, have been issued to the University of Washington. Several
hundred instruments have been produced and are in regular use for both
research and monitoring. High sensitivity, multiwavelength instruments have
been purchased by Institute fur Meteorologie, Mainz, Germany, Air Force
Cambridge Research Lab and the National Oceanographic and Atmospheric
Administration.
The draft version of Volume I of the ACHEX final report from Rockwell
International to the Air Resources Board, State of California, recommends the
integrating nephelometer for both long term monitoring and short term sur-
veillance of aerosol properties.
bsp
An optically thin aerosol layer over a dark surface increases the albedo
by scattering incident radiation backwards into space. The albedo per unit
thickness of an aerosol layer illuminated by a zenith sun can be determined
by integrating the aerosol volume scattering function over the backward
hemisphere of scattering angle. A partial shutter, shown in Figure 11, can
change the angle of integration of the nephelometer so that the scattered
light intensity is proportional to the backward hemisphere scattering
ACHEX Aerosol Characterization Experiment of the California Air Resources
Board Prime contractor is Rockwell International Science Center (Hidy
et aU, 1975).
21
-------
extinction coefficient b due to aerosol particles, b, normally is in the
range 0.1 to 0.2 times the aerosol scattering extinction coefficient b
b
ap
The two aerosol parameters needed in simple radiative climatic models
are the particle backward hemisphere scattering coefficient, b , and the
particle absorption extinction coefficient, b . There are a number of ways
ap
of measuring b , and none is entirely satisfactory.
ap
Long path extinction cannot be used because b is 10 m to 10 m or
ap
smaller. Various techniques based on inverting angular scattering information
have been used by Eiden (1966) and Grams et^ aK (1974), etc., but these
methods require precise knowledge of the aerosol size distribution, and
contain errors of unknown size and magnitude, since the scattering by
irregular particles is calculated using Mie formulae for spheres. The ab-
sorption coefficient of collected aerosol samples can be estimated with low
precision from measurement of the transmission of KBr pellets containing
dispersed aerosol (Volz, 1972). Lindberg and Laude (1974) measured aerosol
absorption by measuring the decrease of diffuse reflectance of a white powder
when a small amount of aerosol is dispersed in it.
All of the above methods, in our opinion, are poorly suited for measure-
ments in background locations. Measurement of the angular dependence of
the aerosol volume scattering function is difficult when molecular scattering
dominates. The methods of Volz and Lindberg require collecting an aerosol
sample over several days, scraping the sample off the collecting surface, and
dispersing the sample in another media. Any treatment of the sample that
alters the aerosol size distribution will alter the optical absorption
coefficient (Waggoner et^ al^., 1973; Bergstrom, 1973). A different technique
for measurement of b has been developed in our laboratory that we believe
ap
is superior to those described above.
Atmospheric aerosol is collected by passing ambient air through a
Nuclepore filter. The filter consists of a 10 ym thick film of polycarbonate
plastic with 0.4 urn holes etched through it. The holes are etched along
damage tracks from highly ionizing particles and are round and perpendicular
to the surface of the film. Individual particles with a mean separation of
22
-------
several diameters are collected on the surface of the filter. The filter
and the particles are placed in an optical system that illuminates the
particles and the filter with a parallel beam of, in this case, green light,
and collects both direct transmitted and forward scattered light. The
extinction or change in transmission between a clean filter and the filter
plus aerosol is assumed to be the same as absorption by the same aerosol
dispersed in a long column of air. Knowing the volume of air passed through
the filter during collection of the aerosol, one can calculate the optical
absorption coefficient due to particles, b
This method has been checked for accuracy using laboratory aerosols of
known (including zero) absorption coefficient and is described by Lin et _al.
(1973). The disadvantages of the method center on errors introduced by
sample alteration that may take place during collection, but the sample
alteration is probably much less than in the techniques of Volz and Lindberg.
2
The sample collection is simple and only requires 10 to 20 yg/cm of aerosol
on the filter. (Data is presented in Figure 12.)
23
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i i i r
i i \ r
ST. LOUIS UNIVERSITY
9/28/73-10/4/73
cc
DC
3
U
u
o
u
Si
a
Ul
cc
WASHINGTON UNIVERSITY
8/22/73-8/30/73
TYSON, MO.
9/5/73-9/26/73
I I
I I
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Figure 12. Histograms showing the absorption fraction of
extinction at three sites in industrial-urban residential-urban
and rural Missouri. bap (530 nm) was measured via the method
of Lin (1973). bsp (530 nm) was measured with a University
of Washington nephelometer.
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SECTION 5
ATMOSPHERIC MEASUREMENTS AND DATA
b AND VISIBILITY
sp
As discussed in Section 2, Koschmieder related b to the distance at
ext
which a black object is just visible when viewed against the horizon sky.
The distance of visibility is given by
V = —• (Middleton, 1968),
ext
assuming aerosol homogeniety, uniform Illumination and a 0.02 detectable
contrast. Commonly it is assumed that b = b , i.e. b , =0. Measure-
ext scat abs
mentis of b and observer visibility show good agreement with the formula
SC 3.L
above.
Horvath and Noll (1969) conducted a study in Seattle between total light
scattering, b , measured with an integrating nephelometer, and prevailing
SC3.L
visibility observed by two separate people. Their results were in good agree-
ment with the theoretical expression of Koschmieder for RH < 65% RH.
Apparently the location of the nephelometer in a heated room caused a reduced
RH in the light scattering measurements. In the cases where RH < 65%, the
correlation between b and the prevailing visibility was 0.89 and 0.91,
S C cL t
respectively, with a coefficient in the Koschmieder expression of 3.5 + 0.36
and 3.2 + 0.25, respectively. This can be compared with the theoretical value
of 3.9, indicating a slightly lower prevailing visibility than meteorological
range. Since no ideal black targets were used (only trees, buildings, etc.),
these would have caused just such a deviation.
Samuels e_t al. (1973) conducted the most extensive tests to date of the
relationship of prevailing visibility to light scattering and various mass
concentration measures as discussed earlier.
25
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They conclude that b as measured with the integrating nephelometer
SCelL
is a good predictor of prevailing visibility and that the regression analysis
is in agreement with Koschmieder's theory. These workers noted that there
was a smaller observed prevailing visibility than that predicted from theory
and b measurement, which they suggested was due to non-ideal black
SC3.C
visibility targets.
MEASUREMENTS OF SCATTERING PARAMETERS
Under support from the Environmental Protection Agency, National Science
Foundation and the California Air Resources Board, we have measured various
aerosol scattering parameters in urban and rural locations in California,
Colorado, Missouri and Washington. In all locations the incoming air was
heated 5 to 20 C above ambient to lower relative humidity of the sample.
The measured parameters were:
b - Scattering extinction coefficient of particles at 530 nm. (Rayleigh
-^ at 530 nm = 0.15 x 10~4 nT1)
a - Wavelength dependence of b parameterized
sp
b = KA'a
sp
Two values of a were computed from Red-Green b and Blue-Green
b . Red is 640 nm. Blue is 430 nm. Green iIP530 nm.
sp
Scat, ratio - Ratio of half sphere back scatter to b from particles at 530nm.
* sp
The sites were:
Richmond - Northeast corner of San Francisco Bay in vicinity of petro-
chemical plants.
Point Reyes - Coast Guard station on cliff 150 meters above the sea
surface, 50 km NW of San Francisco.
Fresno - Central Valley of California, urban agricultural site.
Hunter Liggett - Rural California site 20 km inland from ocean. Local
elevation 400 m. Local vegetation consisted of dry
grass and sparce trees.
Cal. Tec. - Site on campus in Pasadena in Los Angeles basin.
Pomona - Site at county fairgrounds in inland area of Los Angeles basin.
Washington Univ. - Campus site located in residential area of St. Louis,
MO.
Tyson - Rural area 25 km WSW of St. Louis.
26
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St. Louis Univ. - Campus site in industrial St. Louis
Henderson - Site 10 km NE of Denver.
Trout Farm - Site 8 km N of Denver.
Table 3 lists the measured values at each site. For each measurement
parameter, the range of that parameter containing 63% of the data is specified*
-4 -1
For b , the units are 10 M and the range low to high contains 63% of data.
b MEASUREMENTS
ap
Using the technique described in Section 4 (Lin et^ jjl^ , 1973), we
measured b at three locations near St. Louis during fall of 1973. The sites
ap
are discussed in the previous section. The measurements are presented in
Figure 12 as the ratio of b to b , where b is the sum of b and b
ap ext ext ap sp
The results are as expected in that the rural area has much less absorption
than the industrial area. The magnitude of absorption is very high in the
industrial location: b is nearly equal to b . In terms of reducing solar
ap ^ sp
energy at the surface, at Tyson backscatter and absorption have equal effect.
At St. Louis University absorption dominates.
27
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SECTION 6
CONCLUSIONS
In 1962, experts in the field considered the relationship of visibility
to measureable aerosol parameters to be intractable (see the Robinson quota-
tion on page 1).
As described in Section 5, we now have a good understanding of the
relationship of visibility to aerosol parameters, as well as instruments to
measure those aerosol parameters. In the aerosol field the contributions of
K.T. Whitby, B.Y.H. Liu and co-workers at the University of Minnesota cannot
be over-emphasized.
We would summarize the result of the past 12 years of research on aero-
sol properties by ourselves and others as:
1. The integrating nephelometer is a useful instrument to measure the
scattering component of extinction, b
sp
2. Measured b and observer visibility are highly correlated and
sp
follow the Koschmieder relation.
3. Measured b , using a commercial integrating nephelometer, has been
sp
shown to have a very high correlation coefficient (e.g. 0.95 in measurements
at several Los Angeles basin sites) with measured suspended particle volume
concentration in the 0.1 to 1.0 ym decade of particle diameter.
4. A useful correlation exists between b and particle mass as collec-
sp
ted on filters. Measured correlation coefficients at various sites range
from 0.56 to 0.92.
5. As Whitby and others have shown, a plot of particle volume concentra-
tion per log radius interval usually has two log normal modes, Our optical
results are consistent with this model.
6, Whitby's coarse particle mode, centered on 6 to 20 pm diameter, is
the product of mechanical operations, grinding fracture, etc., has the chemi-
cal properties of its local sources, usually has short atmospheric lifetime
28
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and transport and usually has little or no optical effect, at least in all of
our measurements.
7. The fine particle mode, centered on 0.3 to 0.6 ym diameter, is the
product of high or low temperature condensation, coagulation and gas to par-
ticle conversion of natural or anthropogenic source materials. This mode is
I —
dominated by NH,, SO,, Pb, Br and organic matter, has long atmospheric life-
time and transport, and dominates light scattering.
8. Visibility reduction is predominately due to the fine particle mode.
9. Our measurements have shown that sulfates, sometimes as H?SO,/
NH.HSO, and sometimes as (NH ) SO,, dominate the small particle mode in rural
Missouri.
10. In terms of aerosol optical scattering properties (i.e. b , etc.),
sp
the differences between rural and urban sites seems to be small, with the
exception of Los Angeles and clean coastal sites such as Point Reyes.
29
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SECTION 7
EPILOGUE
As pointed out in the second paragraph of this report, when Prof. A.T.
Rossano began this project, the probability of success seemed slim. The
granting agencies (USPHS/DHEW) had to undertake support with an element of
faith in the process of basic research. In retrospect, we feel this was
warranted. The project successfully explained those aerosol characteristics
which control visibility and developed an instrument, the integrating nephe-
lometer, which is widely and successfully used. The entire project has been
conducted as basic research as opposed to directed research.
In this day of increased control of research, of demands for relevance
and application to natural needs, we are pleased to note that basic, undirec-
ted research still works. We feel it is safe for granting agencies to support
some research with an element of faith that the results will be useful.
30
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REFERENCES
1. Ahlquist, N.C. and R.J. Charlson, Journal of the Air Pollution Control
Association 17, 467-469 (1967).
2. Ahlquist, N. et_ ail. , Patent filed (1974).
3. Bergstrom, R.W., Beitr. z. Phys. Atm. 46_, 223 (1973).
4. Beutell, R.G. and A.W. Brewer, J. Sci. Instruments 26, 357 (1949).
5. Charlson, R.J. £t al., Atm. Env. 2, 455 (1968)
6. Charlson, R.J., Env. Sci. and Tec., ^, 913 (1969)
7. Charlson, R.J., et al., (a) Science 184, 156 (1974).
8. Charlson, R.J., et^ al. , (b) Atm. Env. 8, 12 (to be published) (1974).
9. Charlson, R.J., et. a±., (c) Tellus 26, 3 (1974).
10. Covert, D.S., Ph.D. Thesis, University of Washington (1974).
11. Dzubay, T.G. and R.K. Stevens, Report presented at 2nd Joint Session of
Environmental Pollutants, Washington, B.C. (1973).
12. Eiden, R. , Applied Optics 5, 4_, 569 (1966).
13. Ensor, D. and A.P. Waggoner, Atmos. Env. 4, 48 (1970).
14. Flowers, E.G. et al., J. Appl. Met. 8_, 6_, 955 (1969).
15. Garland, J.A. et_ a^. , Atmos. Env. 7_, I (1973).
16. Grams, G. W. et^ al., J. Appl. Met. 13, 459 (1974).
17. Hanel, G. , Beitr. z. Phys. Atm. 44-, 137 (1971).
18. Heintzenberg, J. and H. Quenzel, Atmos. Env. _7, 509 (1973).
19. Hidy, George H. et^ al., "Summary of California ACHEX", Air Pollution
Control Association Journal 25, 11, 1107-1114 (Nov. 1975).
20. Horvath, H. and K.E. Noll, Atmos. Env. _3> 543 (1969).
31
-------
21. Junge, C., J. Meteorol. 11_, 323 (1954).
22. Koschmieder, H., Beitr. Phys. Freien Atm 12, 33-53 & 171-181 (1924).
23. Lin, C. I. et al. , Applied Optics 12_ 1356 (1973) .
24. Lindberg, J.D. and L.S. Laude, Applied Optics 13, _8, 1923 (1974).
25. Loo, B.W. and J.M. Jaklevic, LBL-2468 UC-4 Chemistry TID-4500-R61 (1974).
26. Middleton, W.E., Vision Through the Atmosphere, University of Toronto
Press, Toronto, Canada (1968).
27. Patterson, R.K. and J. Wagman, Presented before Am. Chem. Soc., Los
Angeles, (1974).
28. Rabinoff, R. and B. Herman, J. Appl. Met. 12_, 184 (1973).
29. Rasmussen, R.A. and F.W. Went, PNAS 53, l_, 215 (1965).
30. Samuels, H.J. et_ aJU , "Visibility, Light Scattering and Mass Concen-
tration of Particulate Matter", Report of California Air Resources
Board, (1973).
31. Simmons, W.A. and W. Young, "Correlation of the Integrating Nephelo-
meter to High Volume Air Sampler", Mass. Dept. of Pub. Health (1970)
32. Twomey, S., J. Atmos. Sci. 29_, 456 (1972).
33. Volz, F.E., JGR 77, £, 1017 (1972).
34. Waggoner, A.P. et^ al_. , Applied Optics 12, 896 (1973).
35. Whitby, K.T., in Aerosols and Atmospheric Chemistry, ed. by G.M. Hidy,
Acad. Press, New York (1972).
36. Kinkier, P., Aerosol Sci. 4, 373 (1973).
32
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APPENDIX A
PUBLICATIONS RESULTING FROM GRANT R 800665
R.J. Charlson and M.J. Pilat, "Theoretical and Optical Studies of Humidity
Effects of the Size Distributions of a Hygroscopic Aerosol." Recherche
Atmospherique, Vol. II. No. 2-3, p. 165 (1967).
N.C. Ahlquist, and R.J. Charlson, "A New Instrument for Monitoring the
Visual Quality of Air," Journal of the Air Pollution Control Association,
17, 467-469, (1967).
R.J. Charlson, H. Horvath, and R.F. Pueschel, "The Direct Measurement of
Atmospheric Light Scattering Coefficient for Studies of Visibility and
Pollution," Atmospheric Environment, 1_, 469-478, (1967).
W.E. Buchan and R.J. Charlson, "Urban Haze: The Extent of Automotive
Contribution," Science,, 12, January, 1968.
N.C. Ahlquist and R.J. Charlson, "Measurement of the Vertical and Horizontal
Profile of Aerosol Concentration in Urban Air with the Integrating Nephe-
lometer." Presented at the ABCA-PNWIS meeting, November, 1967, Salem,
Oregon. Environmental Science & Technology, 2^ 363-366, (1968). (Featured
Cover Article.)
R.J. Charlson, "Atmospheric Visibility Related to Aerosol Mass Concentration:
A Review." Presented at the national meeting of the American Chemical
Society, Atlantic City, September, 1968 in a Symposium on colloids in air
and water pollution. Environmental Science and Technology, 3, 913-918
(1969).
Ahlquist, N.C. and R.J. Charlson, "Measurement of the Wavelength Dependence
of Atmospheric Extinction due to Scatter." Presented at the PNWIS-APCA
meeting, 21-22 November, 1968, Vancouver, B.C., Atmospheric Environment, _3_,
551-564 (1969).
R.J. Charlson, N.C. Ahlquist and H. Selvidge, "The Use of the Integrating
Nephelometer for Monitoring Particulate Pollution." Presented at the 10th
conference on Methods in Air Pollution and Industrial Hygience Studies,
San Francisco, 19-21, February, 1969.
R.J. Charlson, N.C. Ahlquist and H. Selvidge, "The Use of the Integrating
Nephelometer for Monitoring Particulate Pollution." Presented at the 10th
conference on Methods in Air Pollution and Industrial Hygience Studies,
San Francisco, 19-21, February, 1969.
33
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U.S.D.H.E.W., "Air Quality Criterion for Particulate Matter," February, 1969,
(Major author of Chapter 3).
R.J. Charlson, "Progress in Atmospheric Aerosol Research at the University
of Washington," The Trend in Engineering, College of Engineering, University
of Washington, May, 1969.
H, Horvath and R.J. Charlson, "The Direct Optical Measurement of Atmospheric
Air Pollution," American Industrial Hygiene Association Journal, 30, 500-509
(1969) .
W.M. Porch, R.J. Charlson and L.F. Radke, Atmospheric Aerosol: Does a
Background Level Exist? Science, 170. 315-317 (1970).
Charlson, R.J., "Multiwavelength Nephelometer Measurements in Los Angeles
Smog Aerosol I, II and III," Abstract and Preface, J. Colloid and Interface
Sci. 39, 240-241, 1972. Also presented at Kendall Award Sympsium of
American Chemical Society Annual Meeting, Los Angeles, Calif., May, 1971.
Ensor, D.S., R.J. Charlson, N.C. Ahlquist, K.T. Whitby, R.B. Husar and
B.Y.H. Liu, "Multiwavelength Nephelometer Measurements in Los Angeles Smog
Aerosol I: Comparison of Calculated and Measured Light Scattering,"
J. Colloid and Interface Sci., 39_, 242-251, 1972.
Thielke, J.F., R.J. Charlson, J.W. Winter, N.C. Ahlquist, K.T. Whitby,
R.B. Husar, and R.Y.H. Liu. "Multiwavelength Nephelometer Measurements in
Los Angeles Smog Aerosols II: Correlation with Size Distributions, Volume
Concentrations and Broad Band Light Scattering," J. Colloid and Interface
Sc±. , 39, 252-259, 1972.
Charlson, R.J., D.S. Covert, Y. Tokiwa and P.K. Mueller, "Multiwavelength
Nephelometer Measurements in Los Angeles Smog Aerosol III: Comparison to
Light Extinction by N02," J. Colloid and Interface Sci., 39_, 260-265, 1972.
Zeigler, C.S., R.J. Charlson and S.H. Forler, "Mt. Rainier: Now You See It,
Now You Don't," Weatherwise, 24_, 115-119, (1971).
Ensor, D.S., W.M. Porch, M.J. Pilat and R.J. Charlson, "Influence of
Atmospheric Aerosol on Albedo," J. Applied Meteorol., 10, (1971).
Covert, D.S., R.J. Charlson and N.C. Ahlquist, "A Study of the Relationship
of Chemical Composition and Humidity to Light Scattering by Aerosols,"
J. Applied Meteorol. , _11, 968-976 (1972).
Waggoner, A.P., N.C. Ahlquist and R.J. Charlson, "Measurement of the Aerosol
Total Scatter - Backscatter Ratio," Applied Optics, 11, 2886-2889 (1972).
Porch, W.M., D.S. Ensor, R.J. Charlson and J. Heintzenberg, "Blue Moon: Is
This a Property of Background Aerosol?" Applied Optics^ 12, 34-36 (1973) .
34
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Lin, C., M.B. Baker, and R.J. Charlson, "Absorption Coeeficient of Atmosphe-
ric Aerosol: A Method for Measurement," Applied Optics, 12, 1356-1363
(1973).
Charlson, R.J., W.M., Porch, A.P. Waggoner and N.C. Ahlquist, "Background
Aerosol Light Scattering Characteristics: Nephelometric Observations at
Mauna Loa Observatory Compared with Results at Other Remote Locations,"
Tellus, accepted October 1973 for press.
McJilton, C., R. Frank and R.J. Charlson, "The Role of Relative Humidity
in the Synergistic Effect of S0? -Aerosol Mixture on the Lung," Science
in press, 1973.
Frank, R., C.E. McJilton and R.J. Charlson, "Sulfur Oxides and Particles;
Effects on Pulmonary Physiology in Man and Animals," Presented at the
Conference on Health Effects of Air Pollutants, National Academy of Sciences
Washington D.C., October 3-5. 1973.
Larson, T.V., R.J. Charlson, E.J. Knudson, G.D. Christian, H.H. Harrison,
"The Influence of a S0» Point Source on the Ii.airi Chemistry of a Single
Storm in the Puget Sound Region," Water Air and Soil Pollution, 4_ (1975).
Vanderpol, A.H., F.D. Carsey, D.S. Covert, R.J. Charlson, A.P. Waggoneer,
"Aerosol Chemical Parameters and Air Mass Character in the St. Louis Region,"
Science, 190, 7 Nov. (1975)
Waggoner, A.P., A.H. Vanderpol, R.J. Charlson, T.V. Larsen, L. Granat,
C. Tragardh, "Sulfate as a Cause of Tropospheric Haze," accepted by Nature.
Porch, W.M., D.S. Ensor, R.J. Charlson, "Visibility of Distant Mountains as
a Measure of Background Aerosol Pollution," Applied Optics, 14, (1975).
Weiss, R.V., A.P. Waggoner, R.J. Charlson, N.C. Ahlquist, "Sulfate Aerosol:
Its Geographical Extent," Submitted to Science.
Bolin, R., R.J. Charlson, "On the Role of the Tropospheric Sulfer Cycle in
the Short-Wave Radiative Climate of the Earth," AMBIO, 5_, No. 2, (1976).
Covert, D.S., R.J. Charlson, R. Rasmussen, H. Harrison, "Atmospheric
Chemistry and Air Quality," Reviews of Geophysics and Space Physics, 13,
No. 3 (1975).
Scheutzle, D., D. Cronn, A.L. Crittenden, R.J. Charlson, "Molecular Com-
position of Secondary Aerosol and Its Possible Origin," Env. Sci. and Tech.,
9, No. 9, (1975)
35
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/3-77-072
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
AEROSOL CHARACTERISTICS AND VISIBILITY
5. REPORT DATE
July 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Alan P. Waggoner and Robert J. Charlson
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
University of Washington
Department of Civil Engineering
Seattle, Washington 98195
10. PROGRAM ELEMENT NO.
1AA603 AG-11 (FY77)
11. CONTRACT/GRANT NO.
R800665
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory - RTF, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Interim 4/71 - 12/74
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report summarizes progress in measuring the optical properties of
aerosols and in relating aerosol characteristics to visibility reduction made in
the author's laboratory during the period 1965-1971. An instrument, the integrating
nephelometer, which measures the scattering component of extinction, b , was
developed and used in several field studies. Measured b and observerpvisibility
have been shown to be highly correlated and to follow the Koschmieder relation.
Measured b is highly correlated (0.95 in Los Angeles) with suspended particle
volume in fRe 0.1 to 1.0 urn size range. A useful correlation (0.56 to 0.92 at
various sites) has been found between b and particle mass as collected on a
filter. Techniques have been developed to measure b as a function of relative
humidicy for ambient and model aerosols. Water, absorbed by hygroscopic aerosols,
as H?SO,, and/or deliquescent aerosols, as (NH.KSO,, make a substantial contri-
bution to visibility reduction. Techniques were also developed to measure the
absorption component of extinction, b , ; to measure the forward/backward scattering
ratio; and to determine b as a function of wavelength.
sp
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
*Air pollution
*Aerosols
*Particles
Visibility
*Light scattering
*Nephelometers
13&
07D
14B
20F
2 ON
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
44
20 SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
36
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