United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/023 May 1985
&EPA Project Summary
Haze Over Eastern
North America:
Part 1. Haze Properties
Rudolf B. Husar
The fundamental properties of at-
mospheric aerosols pertinent to at-
mospheric optics are the size, shape,
and refractive index for the entire
aerosol population at a given point.
Once these properties are established,
their interaction with visible radiation
can be calculated, and the relevant
optical properties can be measured.
The most important properties are the
concentration and size distribution of
particles within the accumulation
mode size range of 0.1 to 1.0 /tm in
diameter. The chemical composition
of the aerosol population weakly in-
fluences optical properties by chang-
ing the refractive index, but it is vital
for the identification of the origin of
the light-scattering aerosol. This Sum-
mary reviews the properties of aero-
sols relevant to their optical behavior.
Regularities in these properties sug-
gest that relatively simple parameteri-
zations can be developed that will
relate optical properties to aerosol
mass. These relationships can then be
used with aerosol models to predict
visibilty and with optical meas-
urements to infer aerosol mass.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory, Research Tri-
angle Park, North Carolina to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back}.
Introduction
This summary is a review of the optical
characteristics of atmospheric aerosols,
the physical and chemical properties and
behavior that influence optical properties,
and the relationships between optical
properties and chemical and physical
properties. We search for regularities in
aerosol properties that will assist us in
parameterizing the optical behavior of the
atmospheric aerosol. Such parameteriza-
tions are needed to transform aerosol
mass calculated in regional or urban
models into visual range and other
visibility-related air quality parameters.
Parameterizations are also needed to
estimate aerosol mass loading from op-
tical measurements such as those ob-
tained from teleradiometers or from im-
ages taken by satellites.
Results
Aerosol Properties
Size distribution. The size of at-
mospheric aerosols ranges over five
orders of magnitude. The smallest ob-
served particles are on the order of 10° A
in diameter, and they could well be called
molecular clusters. At the other extreme,
large dust particles of 100 /im or more
may be suspended in the air for extended
periods of time. Within the five decades
of size range, there is a region in which
the particle diameter is comparable to the
wavelength of visible radiation (0.4 to 0.7
/im). Particles of this size interact strongly
with visible light.
Recent intensive studies on the
physical-chemical properties and behavior
of man-made aerosols have revealed some
important regularities. In a study of the
characteristics of the Los Angeles smog
aerosol, Whitby et al. (1) discovered and
substantiated by a variety of data that
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most of the atmospheric aerosol volume
and mass is distributed bimodally; the
lower mass mode is in the size range 0.1
to 1 jim, while the upper mode was over
5 nm. The saddle point between the two
volume or mass modes is generally be-
tween 0.8 and 3 ^m. Their finding of
bimodal aerosol mass distribution is
significant in that the two modes were
subsequently identified as having distinct-
ly different physical characteristics (pani-
cle shape, volatility) and chemical com-
position; ordinarily the two modes are
produced by different sources; and finally
the lower (fine) and upper (coarse) mass
modes are associated with distinctly dif-
ferent effects.
Atmospheric optics is influenced almost
exclusively by the accumulation mode
aerosols in th 0.1- to 1-^m size range.
Particles in this size range are the most
efficient scatterers per unit mass; also,
fine particles tend to accumulate in that
size range, which prompted Whitby to call
it the accumulation mode. The volume
concentration in the accumulation mode
ranges from O.Vm'/cm3 for marine
background aerosols (which corresponds
to concentration of 0.1/tg/m3 at unit den-
sity) to about 100 jtnWcm3 in regional
hazes. In spite of almost three orders of
magnitude volume concentration variabili-
ty, the mean size and the fitted log-
arithmic standard deviation of the accum-
ulation mode aerosol have been found to
be remarkably constant; the volume mean
size normally ranges from 0.18 to 0.3 ^m,
while the standard deviation is between
1.84 and 2.11.
Chemical composition. The knowledge
of the chemical composition of light-
scattering aerosols is essential to the
understanding of the cause of visibility im-
pairment. It is not so much that the
chemical composition of the aerosol
changes the optical properties; rather, the
chemical composition serves as a tracer of
the probable origin of the light-scattering
aerosol. In fact, for atmospheric haze in
general, the chemical composition is the
most important clue currently available
regarding its probable orgin.
The chemical composition of the am-
bient aerosol may be used to trace its
origin. Characteristic tracer elements such
as vanadium (which comes primarily from
fuel oil) and lead (which comes primarily
from auto exhaust) can be used to tell
how much fuel oil and auto exhaust, re-
spectively, contribute to the ambient
aerosol. This method is called chemical
element balance and requires knowledge
of source compositions. A second tech-
nique starts with the chemical composi-
tion and uses statistical techniques to in-
fer source compositions. The composition
of each identified source may be deter-
mined.
The first comprehensive study of the
size-chemical composition of the haze
aerosol was conducted in the Los Angeles
air basin by the California Air Resources
Board as part of the Aerosol Characteriza-
tion Experiment (ACHEX). As part of this
study of the nature and origins of vis-
ibility-reducing aerosols in Los Angeles, a
chemical mass balance was constructed
for the measured aerosol at seven loca-
tions in the basin. The key contributing
species for the total aerosol mass concen-
tration were nitrates, sulfates, organics,
and other unidentified substances. Based
on statistical analysis of bscat, and
chemical composition data, they con-
cluded that sulfates are the most efficient
scatterers among the measured chemical
species.
For several years an extensive air pollu-
tion monitoring program was conducted
in St. Louis as part of project RAPS
(Regional Air Pollution Study). Size-
segregated aerosol samples were auto-
maticaly collected by the dichotomous
samplers and analyzed for the elemental
composition and mass concentration of
fine and coarse particles. The results from
the ten-station monitoring network were
analyzed to determine the distribution of
the aerosol species within the St. Louis
metropolitan area. Sulfur compounds
were again the dominant species of the
fine aerosol fraction, contributing about
60% of the mass. For stations within the
central city, motor vehicle contribution
was estimated to be about 10% of the
fine particle mass. For the peripheral sta-
tions, 25 kms removed from the city
center, the sulfate concentrations were
comparable to those within the city, but
motor vehicles accounted for only a few
percent of the fine particle mass. It was
concluded that the automobile contribu-
tions were of local origin, while the
sulfate is distributed regionally, and the
addition of the sulfate by the St. Louis
metropolitan area is only 10-20% over the
regional background.
In a mass balance analysis, it was
possible to account for practically all the
coarse particle mass by contributions of
crustal shale and limestone. The fine par-
ticle mass concentration for a Smoky
Mountain National Park site at Elkmont,
Tennessee, was about 25/ig/m3, com-
parable to the values in the St. Louis
region outside the city. Mere again about
60% of the fine particle mass was con-
tributed by sulfur compounds. The 40%
of unknown compounds may include car-
bonaceous compounds, nitrates, and
water. The coarse particle mass concen-
tration at the Smoky Mountain site was
only 6 /*g/m3, substantially below that in
the St. Louis region.
More recent studies have shown that
carbon particles, which contribute to ex-
tinction by both absorption and scatter-
ing, are important in urban areas. How-
ever, carbon particles are less important in
rural areas unless the area is impacted by
a forest fire.
Summary - Aerosol Properties. The at-
mospheric aerosol mass or volume dis-
tribution over the continental United
States is generally bimodal, the fine parti-
cle volume mean diameter is almost in-
variant in size at about 0.3 /tm. Most ac-
cumulation mode particles are spherical
droplets. The chemical composition of the
light-scattering aerosols provides a
valuable clue regarding their probable
sources. Sulfur compounds evidently con-
tribute about half of the fine particle mass
over most of the continental United
States. They generally occur in the most
effective light-scattering size range, 0.4 to
0.7 /tm, and they contribute 50% or more
of the light-scattering. From the above it
is apparent that as far as optical effects
are concerned, sulfur compounds con-
stitute the most significant chemical com-
ponent of the fine particle mass over the
eastern United States.
Relationship Between Aerosol
Number, Mass Concentration,
and Optical Properties
Ever since John Aitken developed a
condensation nuclei counter, a quest has
been in progress to establish a generally
applicable quantitative relationship be-
tween aerosol concentrations and size and
aerosol effects on atmospheric optics.
By the 1970's, it was recognized that
there are distinct advantages to monitor-
ing the aerosol mass concentration in at
least two size-segregated classes, the fine
particle mass and the coarse particle
mass. The separation size is normally set
at the saddle point of the bimodal mass
distribution (0.8 to 3 ^m). From physical
considerations, most of the light scatter-
ing by haze is contributed by fine par-
ticles. More recently, there has been
statistical confirmation of the strong rela-
tionship between the fine particle mass
and the light scattering coefficient
measured by the integrating nephelo-
meter. In various parts of the country,
simultaneous monitoring of bscat and FPM
yielded correlation coefficients exceeding t
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0.9 and ratios bscat to FPM of 3 to 4 m2/g,
as given in Table 1.
Table 1. Correlation of Fine Particle Mass
with Light Scattering Coefficient
Location rr?*/g r" N*
Riverside, CA
Los Angeles, CA
Oakland, CA
Sacramento, CA
Los Angeles, CA
Portland, OR
Mesa Verde, CO
Pugetlsl., WA
Seattle, WA
Seattle, WA
Denver, CO
Houston, TX
3.7
3.2
4.4
3.2
3.2
2.9
3.0
3.1
3.2
3.3
3.3
0.88
0.69
0.62
0.96
0.83
0.98
0.94
0.90
0.94
0.96
0.88
88
39
20
6
58
108
5
26
58
64
268
88
"rrf/g = ratio of bsca, to fine particle mass.
V = coefficient of determination, square of cor-
relation coefficient.
*/V = number of points.
The remarkable feature of these recent
data sets is the narrow range of the ratio
of bscat to fine particle mass for various
sampling locations, times, and mass con-
centrations. This ratio has units of square
meters of light-scattering cross-section
per unit mass of aerosol. It is thus a scat-
tering efficiency factor, and its absolute
magnitude is directly comparable to
calculations of light scattering using Mie
theory. The spatialtemporal invariance of
the scattering efficiency factor has two
possible explanations: either the size
distribution of fine particles that con-
tribute to light scattering is invariant or
the scattering per unit mass of FPM is in-
dependent of particle size. As discussed
in the full Project Report, it is a combina-
tion of both.
The above brief review, along with
numerous reports not discussed here,
reveals a century-long search for laws that
describe the regularities of aerosol optics,
aerosol size distributions, and their in-
terdependence. However, at this time,
generally applicable laws that consistently
describe the size distribution and optical
properties of atmospheric fine particles
are not at hand. The increasing need for
electro-optical information about the at-
mosphere along with the encouraging
hints gathered over the past century dic-
tates that the search be pursued.
Relationships Among the
Optical Parameters
Spectral extinction. Spectral extinction
data from the early 1900's show that the
blue/green extinction ratio and the
red/green extinction ratio converge
^systematically to unity as the extinction
coefficient increases from near Rayleigh
scattering to fog. Thus, the spectral ex-
tinction of aerosol (defined by Middleton
as air and particles) becomes increasingly
"white" with larger values of extinction.
An interesting abrupt transition from
spectral to white scattering at bext =
0.5km"1 may be used to define "mist",
i.e., the transition phase betwen haze and
fog.
Aerosol phase function. The regularities
of the aerosol phase function (variation in
scattering intensity with scttering angle)
have also been examined. Various work-
ers have observed a systematic shift of
light scattering toward forward angles
with increasing extinction coefficient
(decreasing visual range). An impressive
systematic investigation of this phe-
nomena was conducted by Barteneva (2)
at five locations in the Soviet Union, also
showing the elongation of the phase
function with decreasing visual range.
These regularities of phase function have
also been confirmed in a measurement
program conducted by the United States
Air Force in the United States and Europe
using a polar nephelometer.
Backscattering. One of the more con-
venient means of monitoring the light-
scattering by aerosols is detection of the
light scattered backwards to the light
source, whether the source is coherent or
incoherent. The back-scattering intensity
(R) has been simultaneously measured
with the visual range (V) and gives a
remarkably well-defined relationship for
several sites in the United States: V =
constant/R15. An equally consistent rela-
tionship between total extinction and
backscattering has been observed in the
Soviet Union. These two data sets are
consistent in that both the phase function
and backscattering data show that with
increasing extinction coefficient the
backscatterng phase function declines
systematically.
inversion of Optical Data Using
Interactive Graphics
Aerosol size distributions can be fitted
iteratively to measured phase function
data. An attractive data set for illustration
and other purposes is the set measured
and tabulated by Barteneva (2).
The phase function data for eight
classes of haze from light haze (class, #1)
to heavy haze (class, #2), have been fitted
to various size distributions using Center
for Air Pollution Information and Trends
Analysis's interactive graphics capabilities.
This numerical fitting process illustrates
that for light hazes the choice of the real
refractive index is not crucial. For heavy
haze (#8), on the other hand, a fit can
only be obtained with n = 1.4 or 1.33 but
not with 1.5 or 1.6, suggesting a major
contribution by water. The best fit size
distributions also show that for light haze
(#4 and #5) an appreciable quantity of
0.15-^m size particles is required in addi-
tion to Rayleigh scattering in order to ob-
tain the measured amount of backscatter-
ing. Heavy hazes (#7 and #8) can be fitted
with a narrower size distribution with the
volume mean diameter centered at the
wavelength of light.
Regularities
There are two regularities emerging
from the extensive aerosol-optical data
sets on fine particles: the systematic for-
ward elongation of the aerosol phase
function with increasing extinction coeffi-
cient (2) and the existence of the ac-
cumulation mode, i.e., condensation-
coagulation aerosol growth into a relative-
ly narrow size range (1). For some time it
has been felt that the fine particle mass
mode is rather invariant in volume mean
diameter. This, however, is inconsistent
with the inverted size distributions from
Barteneva's phase function data (2). In
order to fit this data, it was necessary to
increase the volume mean diameter from
0.3 to 0.6 nm, corresponding to light and
heavy haze, respectively. Thus, there ap-
pears to be an additional regularity - a
systematic increase in volume or mass
mean diameter as the mass concentration
increases.
Conclusions
The atmospheric aerosol exhibits
regularities in its physical, chemical, and
optical properties. These regularities
should permit relatively straight-forward
parameterization of optical properties
based on the total aerosol mass and the
relative humidity. Because of the impor-
tance of sulfate for scattering and carbon
for absorption, knowledge of the concen-
tration of these two species, along with
relative humidity, should provide a useful
first estimate of optical extinction from
which visual range may be inferred.
The idea of the self-preserving size
distribution must be replaced by that of
the accumulation mode in which at-
mospheric particulate matter piles up on
accumulation. The accumulation mode of
well-aged particulate matter does have a
relatively constant mass mean diameter.
However, as the total mass concentration
increases there appears to be a small in-
crease in the mass mean diameter and a
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corresponding shift in the phase function
toward more scattering in the forward
direction. Information on phase function
variations are important for calculating
aerosol mass from optical properties, for
determining changes in visual air quality
such as contrast ir objects at distances
less than the visual range, and for
estimating changes in haze color and in-
tensity as a function of viewing angle.
The regularities in these relationships ap-
pear to be adequate to permit parameter-
ization of phase function in terms of
aerosol mass loading.
Recommendations
It is recommended that the visibility
research program proceed to develop par-
ameters that relate optical extinction and
phase function to aerosol mass; to use
these parameters to calculate visibility
from predictive aerosol models; and to
estimate aerosol mass loading from
satellite imagery.
References
1. Whitby, K.T., Husar, R.B. and Liu,
B.Y.H. (1972) The aerosol size distribu-
tion of Los Angeles smog. J. Colloid
Interface Sci. 39, 177-204.
2. Barteneva, O.D. (1960) Scattering
functions of light in the atmospheric
boundary layer. Izv. Akad. Nauk
SSSR, Ser. Geofiz. 1852-1865.
RudolfB. Husar is with Center for Air Pollution Information and Trends Analysis,
Washington, University, St. Louis, MO 63130.
William E. Wilson is the EPA Project Officer (see below).
The complete report, entitled "Haze Over Eastern North America: Part 1. Haze
Properties" (Order No. PB85-181 857'/AS; Cost: $8.50, subject to change) will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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