United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-83-085 Dec. 1983
SERA Project Summary
The Chemistry, Physics, and
Optical Properties of Plumes
and Background Air in the
Southwest United States
D.L Blumenthal, LW. Richards, E.S. Macias,
R.W. Bergstrom, and P.S. Bhardwaja
VISTTA (Visibility Impairment due to
Sulfur Transport and Transformation in
the Atmosphere) is a cooperative
program involving numerous govern-
ment agencies, private companies, and
universities and is jointly sponsored by
the U.S. Environmental Protection
Agency and Salt River Project. During
the summer and winter of 1979, plume
measurement programs were conducted
near the Navajo Generating Station
(NGS), at Page, Arizona.
During VISTTA, ground and airborne
measurements of aerosol size distribution,
chemistry and optical properties, as
well as trace gas concentrations, were
made in the plume and in background
air. Extensive regional and plume
telephotometer measurements, airborne
measurements along telephotometer
sight paths, background meteorological
measurements, and source aerosol and
chemistry measurements were also
made. Various types of visibility measure-
ments were compared with one another
and with calculations of light extinction
made using aerosol and NO2 data. The
measured plume optical effects were
compared to those predicted using the
EPA-Systems Applications, Inc. plume
visibility model (PLUVUE).
The data base collected during VISTTA
can be used for plume visibility model
evaluation. Specific data from 27 sight
paths through the NGS plume on June
28, July 13, December 4, 7, and 15
have been tabulated and are available
for model evaluation. These data
include:
• Emission characteristics of the
NGS.
• Meteorological data and sun-observ-
er-plume geometry.
• Background trace gas, aerosol, and
visibility data.
• Telephotometer measurements of
the plume made from four sites.
• Aircraft measurements made along
the telephotometer sight path,
which include plume width, pollu-
tant concentrations, and particle
light scattering (bsp).
The results of the study to date
indicate that:
• For the NGS plume, under most
lighting and viewing conditions,
NOz dominates the blue light
extinction and brown coloration
due to the plume.
• For distances up to 100 km or
more for power plants like NGS,
secondary aerosol formation can
be ignored in visibility models
under the dry conditions studied.
• Widespread areas of elevated
aerosol concentrations were docu-
mented in the Southwest due to
long range transport from the
Southern California area and wild-
fires. Other causes of regional
haze are known to exist but were
not documented in this study.
• Evaluation of the chemistry, aero-
sol growth, and optics'components
of the PLUVUE plume visibility
model showed predictions to be in
reasonable agreement with the
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measurements. More uncertainty
was encountered with the diffu-
sion component. A set of nine
reactions among NO, NO2, Oa, Oz,
SO2, OH, H2O, and O('D) was
found to adequately simulate the
plume chemistry for the clean dry
background conditions at NGS.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory. Research Triangle
Park, NC. to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title fsee Project Report ordering in-
formation at back).
Introduction
In 1977, the U.S. Congress amended
the Clean Air Act to set as a national goal
the "prevention of any future, and the
remedying of any existing, impairment of
visibility in mandatory class I Federal
areas which impairment results from
man-made pollution" (Section 169A). In
order to accomplish this goal, an understand-
ing of the effects of current and proposed
emissions on visibility is necessary. In
the southwestern U.S., visibility impair-
ment is believed to be caused in large part
by sulfur-emitting sources such as urban
areas, smelters, and power plants. The
percentage of the total sulfur emissions
due to power plants is expected to
increase in future years as more coal-fired
plants are built and as sulfur emissions
from smelters are further reduced by new
controls. To develop the technical informa-
tion required to understand the contribution
of coal-fired power plants to visibility
impairment in western pristine areas, the
U.S. Environmental Protection Agency
(EPA) initiated the VISTTA project (Visibility
Impairment due to Sulfur Transport and
Transformation in the Atmosphere).
The VISTTA project is a multiyear
cooperative effort involving several
government agencies, private companies,
and universities, with the overall coordin-
ation of the program the responsibility of
Meteorology Research, Inc.(MRI).*
The general goals of the VISTTA
project are to:
• Determine the relationship between
emissions of coal-fired power
plants and their downwind optical
effects.
"In October 1982 this responsibility was
transferred, along with the project scientists
involved, to Sonoma Technology Inc., Santa
Rosa, California
• Document the current natural and
anthropogenic contributors to vis-
ibility impairment in the south-
western U.S. and the relative
contribution of the various sources,
so that the contribution of power
plants to the total can be determined.
• Provide data for evaluation of
plume visibility models.
During the summers of 1977and 1978,
preliminary VISTTA experiments were
performed using an instrumented aircraft
to measure chemical and physical proper-
ties of selected point source plumes as
well as regional haze in the southwestern
U.S.
In 1979, two major field studies were
conducted at and around the Navajo
Generating Station (NGS) at Page,
Arizona—one from June 26 to July 14
and the second from December 1 to 16.
The specific objectives of the 1979
studies were to:
• Measure the chemical, optical, and
physical properties of the NGS
plume.
• Characterize the regional background
aerosol and chemical environment
near the power plant.
• Estimate the contributions of various
sources to the regional haze using
chemical element balance and
extinction budget techniques.
• Analyze background and plume
data to determine the effects of
various plume components on
plume optical effects and to evaluate
EPA's PLUVUE visibility model.
This report summarizes the measure-
ments and results to date of the June-
July and December 1979 plume studies.
Procedures
During the 1979 field projects, ground
and airborne measurements of aerosol
size distribution, chemical and optical
properties, as well as gaseous reactant
concentrations, were made in both the
NGS plume and background atmosphere.
In-stack size distribution and aerosol
chemistry measurements were made to
characterize the emissions. Extensive
regional and plume telephotometer
measurements, airborne measurements
along telephotometer sight paths, and
meteorological data were collected.
The following types of specific experi-
ments were performed:
• Plume chemistry flights. Sampling
flights were made during which
plume samples were collected for
later chemical analysis. These
flights consisted of a series of orbits
(approximately one hour each) at
fixed locations in the plume and in
background air with one set of grab
samples (filters, impactors, etc.)
collected during each orbit. Plume
samples were usually collected at
two or three distances from the
plant, ranging up to 100 km down-
wind. Samples were analyzed for
SOi, NOs , HNG-3, NH3, NHj, C, and
trace elements.
• Telephotometer sight path measure-
ments. Sampling flights were made
along numerous telephotometer
sight paths both through the plume
and through background air from
the telephotometer to the target.
Aerosol concentrations and size
distribution as well as gas concentra-
tions were measured along the
path.
• Regional flights (summer only).
Sampling flights were made by two
aircraft to determine the composition
and spatial variation of the regional
haze. Aerosol size and chemistry as
well as gaseous pollutants were
measured. Regional flights were
also made by one aircraft in the
region upwind of Page on several
days when the other aircraft was
measuring plume chemistry.
• Ground based measurements of
gases, aerosol chemistry and size
distribution, and atmospheric optics.
These measurements were made
continually throughout the field
programs to document the temporal
and spatial variations in the back-
ground conditions, and to occasion-
ally document plume characteristics
at ground level.
• Regional light extinction and scatter-
ing measurements. Telephotometer
and Integrating Nephelometer data
were obtained continuously from
several sites throughout the sampling
region. These instruments provided
supporting measurements of the
temporal and spatial variations in
visibility and provided an opportunity
for intermethod comparisons.
• Method comparisons. Laboratory
and field comparisons were performed
between all the telephotometers
used during the program, including
commercial and research instruments
to compare the results from telepho-
tometers and Integrating Nephe-
lometers. Comparisons were made
between extinction from telephoto-
meter data and calculations of
extinction made by integrating i
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sight path aerosol data. Compari-
sons of several techniques for
measuring sulfur aerosol and size
distributions were made.
Throughout the program, all flights
were supported by upper-air wind sound-
ings every hour and temperature sound-
ings every four hours in the Page vicinity.
In addition, National Weather Service
(NWS) surface and upper-air data were
obtained for most of the Southwest for
the period of the program. Two extra
radiosonde measurements per day (total
of four per day) were made at each of five
NWS stations in the Southwest during
the periods of the summer regional
flights. Source measurements of aerosol
size distribution and chemistry were
made on several days. Emission rates,
sulfur content, and other source opera-
tional parameters of NGS were made
available by SRP for the duration of the
program.
Results and Conclusions
As the report for which this summary is
prepared is itself a summary of an
extensive measurement and analysis
program, a discussion of the experimental
results is beyond the scope of this
summary. In general, sufficient data were
obtained to determine the quantity and
variability of the aerosol and gas emissions,
the size distribution of the aerosol
emissions and plume aerosol, the formation
rates in the plume of sulfates, nitrates,
nitric acid, and fine particle aerosol, and
the background aerosol composition for a
few different synoptic situations. The
optical properties of the plume were
determined and compared to model
predictions for 15 different case studies.
The conclusions drawn from the data are
presented below.
• For the NGS plume, under most
lighting and viewing conditions
within 50 km of the plant, NOz
dominates the blue light extinction
and brown coloration due to the
plume. At distances less than 10
km, primary particles greaterthan 1
/urn in diameter can dominate the
light extinction at all visible wave-
lengths.The primary particle size
distribution in the plume near the
source has a small maximum under
0.1 /urn diameter and a dominant
maximum near 3.5 /urn diameter.
For modeling purposes, a mass
median diameter of 3.5 /urn should
be used for NGS emissions.
• Sulfate formation rates in the NGS
plume in both summer and winter
under dry conditions were between
0 and 1% per hour. The NO2 to
HNOs conversion rate was substan-
tially more rapid than the SO2 to
sulfate conversion rate, but the
nitric acid remained in the gaseous
state. The sulfate formed was
largely ammonium sulfate. No
particulate nitrates were measured
in the plume. Atdistances upto 100
km in the plume, particle formation
and growth were only detected in
the size range below 0.1 jum and
resulted in insignificant optical
effects compared to those due to
the background aerosol. Some
aerosol probably was formed above
0.1 /urn, but this formation was
obscured by variations in the
background aerosol concentrations.
Gas-to-particle conversion was
undetectable at night and while
plume SOa concentrations were
greater than about 100 ppb.
• The greatest causes of visibility
impairment in the region during the
1979 summer study were widespread
areas of elevated aerosol concentra-
tions at different times due to:
1. Long-range transport from the
Southern California area 800
km away, and
2. Wildfires.
• Although shown to be important by
other studies, significant regional
visibility impairment due to smelters
or soil-derived elements (wind-
blown dust) was not measured
during the short 1979 summer
VISTTA study.
• During this study, visibility impair-
ment was generally caused by
nonsoil aerosol. On these days,
sulfates and organic carbon compounds
dominated the fine particle aerosol
composition. Fine particle chemical
species contributed to light extinction
approximately in proportion to their
contribution to aerosol mass.
• Visibility reduction due to regional
haze can equal or exceed the
reduction caused by a plume from a
nearby well-controlled power plant.
• In the NGS plume, N02 from
primary emissions and the termolec-
ular reaction can account for 3% to
6% of the measured NOX near the
source. For a given total amount of
NO, in any plume cross section, the
N02 concentrations can be adequate-
ly modeled by assuming the photo-
stationary-state relationship of NO,
Os, and NO2. For dilute southwest-
ern plumes «100 ppb S02at NGS),
the reaction of N02 with OH repre-
sents a significant NO2 loss mecha-
nism, which should be accounted
for in models.
• Evaluation of the chemistry, aero-
sol growth, and optics components
of the EPA visibility model (PLUVUE)
showed predictions to be in reason-
able agreement with the measure-
ments. More uncertainty was encoun-
tered with the dispersion component.
The nine reactions used in PLUVUE
adequately model the plume chemis-
try measured at NGS. For visibility
modeling at power plants like NGS,
for distances up to 100 km, secondary
aerosol formation can be ignored
under clean dry conditions.
• Comparisons between ground-
based contrast telephotometer and
Integrating Nephelometer data
obtained during the program showed
good agreement on days of regional
homogeneity. The choice of targets
was found to be important; the best
correlations (r > 0.9) were for
targets to the north. Inherent
contrast changes in the targets
with sun angle can substantially
vary the correlation.
• Good agreement was found between
the extinction coefficients determined
by an Integrating Nephelometer on
a sight path flight and the telepho-
tometer viewing along the same
sight path.
• Comparison and calibration of 12
multiwavelength contrast telephoto-
meters showed an inherent error of
about 1% or less in the contrast
(ratio) mode and about 10% in the
absolute radiance mode. Operator
differences can increase the variation
in contrast mode to 2% with a
resulting variation in the calculated
extinction coefficient of up to 15%.
• The S02 emission rate per BTU
from NGS typically was found to
vary less than 15%. Emission rates
of NOx and particulate matter per
BTU, on the other hand, varied by
about a factor of 2, complicating the
prediction of visual impact in the
definable plume.
• The chemical composition of the
southwestern aerosol is substantial-
ly different for aerosol above and
below about 1 #m diameter. For
optimum separation of the coarse
and fine aerosol, size-segregating
samplers used in the southwestern
U.S. should have a cut point near 1
/urn diameter.
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The following findings suggest that
NOx and its oxidation products will
be removed from the atmosphere
more rapidly than S02 and its
oxidation products, with the result
that NOx emissions have an impact
over a smaller region than SOa
emissions: (1) NOX is oxidized to
nitric acid (gas) several times more
rapidly than SQz is oxidized to
sulfate (particles); (2) gases are
removed from the atmosphere
more rapidly than particles; and (3)
the air masses transported from
Southern California contained
easily measurable sulfate concen-
trations, but the nitrate concentra-
tions were near the detection limit.
The June-July and December 1979
VISTTA field programs provided an
extensive data base for the evaluation
of plume visibility models and of
their individual components (e.g.,
dispersion, chemistry, and optics
components). It is important to
note, however, that the performance
of a plume visibility model can vary
according to the conditions consid-
ered. The VISTTA case studies
presented in this report were
obtained for a specific set of
conditions (i.e., a power plant
plume with particulate emission
control in a relatively clean and dry
atmospheric environment at down-
wind distances of less than 35 km).
Further evaluation of plume visibility
models could require the develop-
ment of other data bases.
D. L BlumenthalandL W. Richards are presently with Sonoma Technology, Inc.,
Santa Rosa, CA;E. S. Maciasis with Washington University, St. Louis, MO 63130;
R. W. Bergstrom is with Systems Applications. Inc., San Rafael, CA 94903; and P.
S. Bhardwa/a is with the Salt River Project, Phoenix, AZ 85001.
William E. Wilson, Jr., and William D. Conner are the EPA Project Officers (see
below).
The complete report, entitled "The Chemistry, Physics, and Optical Properties of
Plumes and Background Air in the Southwest United States," (Order No. PB
83-261 768; Cost: $11.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officers can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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