United States
Environmental Protection
Agency
Office of Environmental Sciences Research
Research and Laboratory
Development Research Triangle Park, North Carolina 27711
EPA-l>00, 7-77-020
March 1977
POWER PLANT STACK PLUMES
IN COMPLEX TERRAIN: An
Appraisal of Current
Research
Interagency
Energy-Environment
Research and Development
Program Report
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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 seven series.
These seven broad categories were established to facilitate further
development and application of environmental technology. Elimination
of traditional grouping was consciously planned to foster technology
transfer and a maximum interface in related fields. The seven series
are:
1. Environmental Health Effects Research
2, Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from
the effort funded under the 17-agency Federal Energy/Environment
Research and Development Program. These studies relate to EPA's
mission to protect the public health and welfare from adverse effects
of pollutants associated with energy systems. The goal of the Program
is to assure the rapid development of domestic energy supplies in an
environmentally—compatible manner by providing the necessary
environmental data and control technology. Investigations include
analyses of the transport of energy-related pollutants and their health
and ecological effects; assessments of, and development of, control
technologies for energy systems; and integrated assessments of a wide
range of energy-related environmental issues.
This document is available to the public through the National Technical
Information Service, Springfield, Virginia 22161.
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EPA-600/7-77-020
March 1977
POWER PLANT STACK PLUMES IN COMPLEX TERRAIN
An Appraisal of Current Research
by
Robert C. Koch, W. Gale Biggs, Paul H. Hwang,
Irving Leichter, Kenneth E. Pickering,
Eric R. Sawdey, and John L. Swift
GEOMET, Incorporated
15 Firstfield Road
Gaithersburg, Maryland 20760
Phase I - Contract Number 68-02-2260
Project Officer
George C. Holzworth
Meteorology and Assessment 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
publication. Mention of trade names or commercial products does not con-
stitute endorsement or recommendation for use.
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ABSTRACT
This report reviews the literature of scientific studies of the behavior
of stack plumes from fossil-fueled electric power plants in complex (hilly or
mountainous) terrain. Non-conservative chemical transformation and depletion,
and conservative transport and diffusion of pollutants, are considered. Studies
of S02 oxidation rates in power plant plumes are described and the primary
mechanisms for conversion to sulfate are detailed. Homogeneous gas phase
reaction of S02 with OH is of importance, with conversion rates reported from
1 to 13 percent per hour. Aqueous phase SC>2 oxidation has higher conversion
rates, in presence of NH3 and with iron, lead or manganese as catalysts, but
is less well-studied. Early depletion of background ozone by stack plumes is
documented; evidence indicates that ozone above background levels reappears in
plumes at great distances from the stack. Little information on field studies
of plume chemistry of NOX, CO, or hydrocarbons was found. Scavenging of S02
from plumes by precipitation is shown to increase with distance from the source
and has been modeled. Surface contact may be an important plume depletion pro-
cess but quantitative models have not been validated.
Current theories of airflow, turbulence and diffusion phenomena in
complex terratn are described, and are exemplified through the review of sixteen
field observation programs of the physical behavior of plumes from continuous
elevated sources in complex terrain. The review discusses program objectives,
data, sampling methodologies, model-to-measurement comparisons associated with
the field programs and program conclusions. The types of models available to
simulate plume behavior numerically are discussed in the contexts of these
programs. In addition, results are reported of an Independent GEOMET analysis
of
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A view of the plume from the Four Corners Power Plant in New Mexico, crowing ihe Hogback and going through two
Ice-wave oscillations. The plant U situated to the east (right) of this photograph.
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CONTENTS
Abstract 11i
Photograph of the Four Corners Power Plant Plume v
Figures ix
Tables , xiii
Part 1. Scope of Effort and Results 1
Section I. Introduction .... 1
Background 1
Objectives 3
Approach . 4
Section II. Summary and Conclusions 6
Plume Chemistry 7
Plume Depletion 9
Plume Transport and Diffusion 10
Part 2. Nonconservative Aspects of Plume Behavior
Section III. Principles and Laboratory Research 13
Direct Photo-Oxidation T£
Indirect Photo-Oxidation ";'3
Air Oxidation in Liquid Droplets 2Z
Metallic-Catalyzed Oxidation in Liquid Droplets f 6
Other Aqueous S02 Oxidation Mechanisms ol
Heterogeneous Studies 32
Related Chemical Processes 36
Precipitation Scavenging 39
Section IV. Models of Plume Chemistry and Depletion-Deposition
Processes 42
Section V. Field Studies of Plumes 55
Colbert Power Plant , 55
Crystal River Power Plant. ']'•.
Morgantown Power Plant .\?
Keystone ana Nort.-port Plant , . .5:
Four Corners Plant 5~
MISTT Program 66
S02 Deposition Studies FJ
Precipitation Scavenging Studies 75
(Continued;
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Section VI
CONTENTS (Concluded)
Evaluation 7b
Part 3. Transport and Diffusion of Plumes Under Influence
of Complex Terrain. 84
Section VII. Principles and Observations 85
Airflow in Complex Terrain . 85
Turbulence and Diffusion Enhancement ...... 91
Section VIII. Modeling of Terrain Influence on Plume Behavior . . 101
"NOAA" Gaussian Model 106
U.S. EPA Single Source (CRSTER) and "VALLEY"
Models .... ...... 109
ERT PSDM Model 110
Aerovironment Statistical Turbulence Model ... Ill
INTERA Model 114
TVA Model 116
Section IX. Field Studies 117
Dugway, Utah 117
Vandenberg Air Force Base 119
Dickerson Power Plant. 121
Naughton Plant 122
Four Corners Plant ...... 125
Navajo Plant 129
Huntington Canyon 137
Garfield Smelter 146
Kaiparowits 152
LAPPES Program ........ 155
Kingston Power Plant 167
Summary of Field Studies 173
Section X. Recapitulation and Commentary . 181
Enhanced Turbulence "81
Terrain Imp-; Demerit "82
Plume Dimensions 133
Plume Center-line Concentrations 184
State-of-tne-Art for Modeling Transport and
Diffusion of Continuous Elevated Plumes in
Complex Terrain. . 187
Appendices
A. Bibliography on Nonconservative Plume Behavior 188
B. Bibliography on Transport and Diffusion of Plumes
under the Influence of Complex Terrain . ... 206
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FIGURES
Number Page
1 Possible atmospheric paths for S02 upon emission
from a power plant stack 15
2 Effect of hydrocarbon structure on nitrogen dioxide
formation rate 38
3 A falling raindrop and the associated transport and
washout mechanisms acting upon it in the atmosphere. . 41
4 Schematic of physical interactions modeled by SMICK. . . 48
5a Profiles of crossplume ozone concentration at various
elevations downwind of the Morgantown Power Plant. . . 61
5b Profiles of sulfur dioxide and ozone crossplume con-
centrations at 600 m altitude at various downwind
distances from the Morgantown Power Plant 62
6a Conceptual illustration of the variation of both
homogeneous and heterogeneous S02 conversion
mechanisms as a function of time of year for a
specific stack location. .... 8C
6b Conceptual illustration of the variation of both
homogeneous and heterogeneous S02 conversion
mechanisms as a function of time of day for a
specific stack location 80
6c Conceptual illustration of the variation of both
homogeneous and heterogeneous S02 conversion
mechanisms as a function of distance from the stack. . 3".
7a A schematic cross section of the lower turbulence
zone (LTZ) based on six case studies, showing the
potential temperature field 89
7b Strong vertical shears and large lor.c'ltudinal speed
changes, especially in the vicinity or the uodrafts
and the rotor are associated with the wind speed
maximum in the stable layer at the top of tne i_TZ . . b^
(Continued)
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FIGURES (Continued)
Number Page
7c Distribution of moderate or greater intensities
of turbulence found within the LTZ in the wave
cases ............ 89
7d The larger dimensions of the LTZ and the greater
extent of the area of severe turbulence distinguish
the streamline and turbulence distribution of
hydraulic jump types from wave types 89
8 Schematic illustration of mountain top influences upon
the gradient level flow component and the downward
transporting of gradient flow momentum . 95
9 Schematic illustration of circulations triggered by
slope density flows and air drainage from a side
feeder canyon . 95
lOa Schematic view of the type of terrain capable of <
affecting the wake turbulence ..... 96
lOb Schematic illustration of turbulent wake effects
caused by obstacles protruding into the primary
flow pattern. 97
11 Coordinate system showing Gaussian distributions in
the horizontal and vertical 102
12 Configuration of elevated line source releases with
respect to wind direction and orientation of mountain
system near Dugway, Utah 118
13 Vertical plume dimensions (az) inferred from cross
width (a ) and surface center!ine concentration
assuming^SO percer.t deposition at -;he surface ... ;2G
14a Helicopter and grour.c samples for D stability ... . "i39
14b Helicopter samples only for D stability elevated
release tests ',39
(Continuea'
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FIGURES (Concluded)
Number Page
15a Helicopter and ground samples for F stability
release at ground-level conditions of down-
canyon flow 140
15b Helicopter samples only for F stability release
at ground-level in conditions of down-canyon flow . . 140
16 Terrain cross-sections at three sites of varying
degrees of roughness 143
17a Results of Garfield aerial sampling test with
stability class "B" conditions 148
17b Results of Garfield aerial sampling test with
stability class "C" conditions 148
18 Profiles of elevation toward 14 directions from
Keystone Power Plant 162
19 Elevation profile from Kingston plant to each
receptor 171
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TABLES
Number Page
1 Mechanisms that convert sulfur dioxide to sulfates. . . 16
2 Studies of the photo-oxidation of sulfur dioxide in
sunlight 18
3 Nine initiating reactions for homogeneous gas phase
oxidation of sulfur dioxide and their lifetimes for
conversion to H2S04 in the troposphere 19
4 Summary of teflon bag experimental conditions for
S02/NO/N02/l-Butene system 23
5 Nomenclature and values used for plume S02 oxidation
calculations 28
6 Summary of eight experiments involving S02 reactions
in the presence of various particulates 34
7 Summary of typical secondary atmospheric sulfate types. 35
8 Key reactions proposed for the $62 conversion to
sulfate via OH radicals . 44
9 S02 oxidation studies - Colbert Steam Plant Plume ... 56
10 Normalized and averaged data from 11 plume sampling
runs made at the Keystone Power Plant 64
11 Meteorological stability categories ... 103
12 Model and measurement comparisons for peak hourly
average S02 concentrations (ppm) observed at
elevated terrain features near Navajo Generating
Station 134
13 Average ratio and range of calculated to observed
relative concentration values x' cal/x' obs for
three different terrain configurations 144
(Continued)
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TABLES (Concluded)
Number Page
14 Garfield data model-to-measurement comparisons of
ground-level concentrations 150
15 Comparative model results of the ratios of the mean
calculated to mean observed values of each of four
tracer tests near Kaiparowits 154
16 Dispersion parameters a and a derived from plume
cross-section measurements in the LAPPES program. . . 158
17 Summary of observed and standard a value, observed
minus standard, and equivalent Pasquill stability
class for combined Keystone and Conemaugh data. ... 161
18 a values as obtained by Nilsson (1975) as well as
•^correlation coefficients and slopes of the regression
curves 169
19 Comparison of computed and observed mean ground-level
S02 concentrations 172
20 Summary of field studies of transport and diffusion
in complex terrain from elevated sources 174
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PART 1
SCOPE OF EFFORT AND RESULTS
Section I
INTRODUCTION
At the end of October, 1975, the Meteorology and Assessment
Division of the U.S. Environmental Protection Agency, at Research Triangle
Park, North Carolina, awarded GEOMET, Incorporated a contract to under-
take a study - including literature research, field observations, and
analytic modeling - into the behavior of stack plumes from large fossil -
fueled power plants in complex terrain. The first product of that study,
an appraisal of the extent, utility and limitations of existing knowledge
in this field, is the subject of this report.
This report is the culmination of Phase I of GEOMET's work
under EPA Contract Number 68-02-2260. A Phase II report, describing
results of a long-term program for field monitoring of sulfur oxides
and other plume components begun by GEOMET in the vicinity of the 700 MW
coal-fired Clinch River Power Plant at Carbo, Virginia in April, 1976,
will be issued in late 1977.
BACKGROUND
Fossil-fueled electric power plants are major sources of air
pollution. Particulate matter, sulfur dioxide, nitrogen oxides, and
trace metals are - in the absence of effective control equipment -
dispersed into the air from power plant stacks. All can be hazardous
to health and welfare in the plant environs. Chemical reactions of
pollutants in power plant plumes may result in formation of sulfates,
nitrates, and photochemical oxidants which are advected over great dis-
tances, resulting in health hazards in areas remote from the power plant
source of the pollution. Permissible maximum ambient concentrations of
sulfur dioxide (S02), nitrogen oxides (NOX), total suspended particu-
late (TSP), and oxidants (Ox) have been established in law and are
enforced under State Air Quality Control Implementation Plans. National
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standards for ambient sulfate and nitrate concentrations do not yet
exist, but may be expected in the future.
Particulate matter emissions can be effectively controlled
at large power plants through electrostatic precipitation. Sulfur
dioxide emission control procedures are technically feasible but are
expensive; they have not been widely adopted. The electric power industry
has typically relied upon the dispersive power of the atmosphere to reduce
ambient ground level S02 concentrations, through discharge of emissions
from tall stacks. Effective means for the control of NOX emissions are
still in development, not yet applicable to large-scale commercial pro-
duction of electricity.
Growing reliance upon coal as a fuel for power generation -
switching from imported oil and scarce gas - has increased the national
concern about the SOg and sulfate air pollution problem. Much of the
coal available in the United States has a relatively high sulfur content
(more than one percent), but this fuel is our most sure source of future
energy for many years to come.
Coal and oil-fired power plants were originally built near
load centers, typically in or near urban areas. With the development
of improved high voltage, long distance power transmission systems it
has become economical to build very large (1000 MW and more) coal-fired
power plants in coal mining areas, transmitting power hundreds of miles
to load centers. Power plants in such remote areas are often located
in complex mountainous terrain, in whicri ground elevations at ..-, o.^
higher than the top of plant stacks occur,within the area traversed cy
the pollutant plumes from the plant. This condition may also occur -In
hilly, settled country, particularly where smaller or older power plants
have stacks less than about 150 m in height.
For the design and siting of new power plants, and for the
control of pollutants from existing ones, it is necessary to have reliable
techniques for calculating the ambient air quality impact of stack
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emissions from large power plants - particularly for S02, NOX and their
transformation products. The problem has been studied in depth for many
years. A body of modeling technology exists, suitable for calculating the
physical behavior of a stack plume based upon input knowledge of pollutant
emissions, stack configuration, and meteorology - but the models perform
well only under limited conditions of terrain uniformity; and they do
not perform well for non-conservative, reactive pollutants.
Existing models generally assume a uniform terrain roughness.
Some models take differential terrain elevations into account, with broad
simplifications and limited success. Models available to planners are
typically weak (or totally lacking) in ability to handle plume depletion
due to chemical transformation and non-conservative behavior.
A need exists to develop improved techniques for calculating
the behavior of pollutants in the plumes of large power plants in com-
plex terrain. Realistic goals for new studies can only be established
through an examination of the extent, capability, and limitations of
the existing body of knowledge. That examination is the subject of
this report.
OBJECTIVES
The study which produced this report had the following
objectives:
• Provision of an up-to-date description of existing
knowledge of (1) concentrations of S02 and sulfate
(and pollutants interacting with these), resulting
from the operations of large fossil-fueled power
plants in complex terrain, (2) relationships between
meteorological data, terrain characteristics, and
plume behavior, and (3) the pollutant transfor-
mation and depletion.
• Appraisal of the adequacy of typical meteorology,
air quality and emissions data - of kinds routinely
available to power plant plume analysts in non-research
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situations - for use in calculating ground level
concentrations of S02 and sulfates, due to power
plant emissions, in areas of complex terrain.
Implicit in these objectives are the concepts of identifying
deficiencies in current knowledge and recommending new research which
could improve our ability to calculate the air pollution impact of power
plant plumes in complex terrain.
Precise definitions of "large power plant" or of "complex
terrain" are difficult to outline. In general, a large power plant may
be thought of as one in which more than 700 MW of capacity are installed
at one location. For plume study purposes, the complexity of terrain is
a function of its influence upon plume behavior. As a working definition,
complex terrain is terrain in which stack pollutants appear at ground
surfaces in concentrations significantly different from those which would
be expected to occur if the terrain were essentially level.
APPROACH
The study was essentially performed in a six-month period,
from November, 1975 through April, 1976. The method was primarily
that of a literature search, supplemented by discussions with researchers
active in related areas. Research literature was sought out, reviewed,
summarized, and appraised in the two major categories of (1) chemical
transformations in power plant plumes and (2) the concentrations of
pollutants in plumes moving under meteorological influences over complex
terrain. In these categories information was sought on field observa-
tions of relevant data and also upon analytic efforts to develop, improve,
and validate models for calculating pollutant behavior. GEOMET made
some independent modeling analyses of S02 and meteorological data taken
by others in previous field studies. In these modeling analyses GEGMET
sought preliminary insights into relationships between terrain complexity
and variations in the horizontal and vertical standard deviations (ay ar.d
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az) of pollutant distribution models conventionally used to describe the
dispersion of a plume. The results have been used to suggest further
directions for study.
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Section II
SUMMARY AND CONCLUSIONS
Over 200 studies, directly or indirectly relating to the subject
of plume behavior from elevated point sources in complex terrain, were
reviewed for the preparation of this report. The titles and authors of
these studies are listed in an appended Bibliography. Aspects of plume
behavior have been broadly categorized into two areas of information,
namely, (1) non-conservative aspects relating to chemical transformation
and depletion processes and (2) transport and diffusion aspects.*
Information presented regarding plume chemistry includes a
discussion of the principles and results of investigative laboratory
work for each of four classes of sulfate formation processes, a review
of the approaches which have been used in plume chemistry modeling, and
descriptions of major field observation programs. Other information pre-
sented regarding plume depletion processes includes a discussion of the
principles of precipitation scavenging and chemical processes other than
sulfate formation, modeling approaches for treating wet and dry deposition,
and a description of major field observation programs involving precipi-
tation scavenging measurements. An evaluation of the current state-of-
the-art knowledge of the non-conservative aspects of plume behavior
follows the discussions of principles, modeling, and field programs.
With regard to transport and diffusion aspects of plume
behavior, the principles and observations of airflow and turbulence
and diffusion enhancement in complex terrain are reviewed, followed by
a discussion of the modeling of terrain influence on plume behavior and
by detailed descriptions of 16 major field studies. An evaluation is
made of the current state-of-the-art knowledge of the transport and
diffusion of plumes under the influence of complex terrain.
* In this review we have used transport, diffusion and dispersion with the following meanings: Transport -
Movement of plume components by winds, at scales large enough to be treated by the equation of motion-
Diffusion Exchange of plume components and other atmospheric elements between regions in space, in
apparently random motions, at a scale too small to be treated by the equations of motion; Dispersion - The
combined result of the transport and diffusion processes.
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Based on the review of previous studies of plume behavior and
on a new analysis of the behavior of plumes from three power plants
located in the Appalachian mountain regions of Pennsylvania and in
Tennessee, we have drawn conclusions regarding the present state-of-the-
art and needs for further research to treat plume behavior from a coal-
fired power plant in complex terrain. Conclusions reached regarding the
current state-of-the-art of non-conservative plume processes are presented
first. The findings concern available measurements, modeling, sulfate
conversion mechanisms, and plume depletion-deposition processes. The
conclusions reached with regard to plume transport and diffusion then
follow.
PLUME CHEMISTRY
A wide range of S02 oxidation rates (varying from zero to over
50 percent per hour) have been deduced from field study measurements.
The rates reflect variations in meteorology (especially relative humidity),
time of year, location, source emissions, and background atmospheric con-
taminants; most of these variables cannot be controlled in field experi-
ments. A field program design that can serve as a guide in planning future
studies is needed. Normally plume measurements show a depletion of ozone
below the normal background concentration. However, levels greater than
background have been evident in several studies of plant plumes at distances
in excess of 50 km from the plume source. Current and future field pro-
grams are being designed to determine if ozone is formed in plumes under
certain conditions. Very limited field observations have been made of chem-
ical processes (other than the S02 to sulfate conversion) which involve ozone,
NO, N02, CO, hydrocarbons, water, and radical chemistry.
Although several plume chemistry models have been developed,
there is a definite lack of data to test these models. Additional research
is needed in the areas of plume physics, plume photochemistry, and plume
measurement methodology. Improved information on the dominant S02 con-
version mechanisms and their reaction rates are required for realistic
depiction of plume chemical processes.
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Different S02 oxidation mechanisms may be important in power
plant plumes under different environmental conditions. The significance
of different conditions can only be hypothesized at this time. The four
major processes of sulfate formation that have been identified from
theoretical considerations to be potentially active in plant plumes
include homogeneous indirect photo-oxidation, homogeneous air oxidation
in liquid droplets, quasi-homogeneous catalyzed oxidation in liquid drop-
lets and heteorogeneous catalyzed oxidation on dry surfaces.
The gas phase reaction of S02 with OH is the most notable
homogeneous sulfate formation process. An S02 sulfate conversion rate
of 2 percent hr"1 is a representative average rate for this reaction.
Further quantification of the importance of the homogeneous OH-S02
mechanism compared to heterogeneous processes can be determined with
more complete information on OH atmospheric profiles. S02 conversion
rates for homogeneous gas phase reactions vary from 1 to 13 percent hr~'.
Sulfate conversion rates of 0.1 to 2.0 percent per minute have
been reported in studies of aqueous phase S02 oxidation. The process
rate is highly variable due to its sensitivity to several parameters
including temperature, relative humidity, and pH. The important role of
NH3 in promoting this conversion by maintaining a high pH has been docu-
mented. Available information for measuring NH3 in the short time frames
and low concentrations typical of airborne plume measurements is not
adequate for field studies; similar problems exist with regard to field
monitoring of the roles of trace metal catalysts.
Various transition metal ions (e.g., MnT+, Cu^, Fe++, and
Co++) have a catalytic effect on the conversion of S02 to sulfate Dy a
wide variety of heterogeneous reactions in solution conditions. Studies
point to S02 conversion rates from 0 to 6 percent hr"1, with the most
efficient catalysts being leac and iron. Research has shown ory ^art<-
culate vanadium to be relatively inactive as an effective catalyst -.n
the heterogeneous oxidation of S02-
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PLUME DEPLETION
The few field studies of precipitation scavenging of reactive
pollutants from power plant plumes have measured the washout process by
examining ground rainwater samples. The most comprehensive and recent
study at the Centralia power plant in Washington showed that scavenging
rates of S02 increase with distance from the plant, and that sulfate
concentrations in rainwater due to the plume presence are not significant
less than 10 km from the stack. The Scavenging Model Incorporating Chemical
Kinetics (SMICK) has been developed and has the capability to predict preci-
pitation scavenging of reactive pollutants from power plant plumes.
The complexities involved in measuring the numerous parameters
necessary to represent the rainout mechanism as it applies to power plant
plume depletion have resulted in a large information gap. To realistically
model the rainout mechanism requires data on specific cloud parameters
such as depth, temperature and water distributions, electrical activity,
as well as information about the sulfate aerosol, such as size distribu-
tion, concentration, wettability and activity as a nucleus.
While surface contact is probably an important plume depletion
process for S02, methods of estimating this effect are not sufficiently
well developed to make accurate quantitative estimates of a depletion
rate. The rate of pollutant removal by vegetation and soil has been
shown in laboratory studies to be affected by wind velocity, canopy
height, light intensity, soil pH, and moisture content. Based on the
laboratory findings, as much as 49 percent of the S02 emitted from a
large coal-fired plant would be depleted under some conditions within
—32 km due to the average uptake rate of a 40 cm alfalfa canopy
during daylight.
Numerical models of the dry deposition process, still rather
primitive, have relied mainly on wind tunnel data to predict deposition
velocities. They require atmospheric data for validation purposes. The
source depletion approach, in which source strength is reduced as a
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function of downwind distance has been commonly used in Gaussian plume
equations to represent the plume depletion processes. A steady-state one
dimensional mass transfer model is also available.
PLUME TRANSPORT AND DIFFUSION
Sixteen field observation programs directly related to the
diffusion of a plume from a continuous elevated point in complex terrain
were reviewed. Most of the field programs have been very limited in
scope, either in terms of the intensity of observations or the duration
of observations; typically a program consists of one week of plume mea-
surements. The majority of the field programs have focused on observing
the worst case situations which have usually been judged to be associated
with very stable conditions.
The principal characteristics of flow over complex terrain
which create enhanced turbulence are (1) thermally driven drainage and
up-slope flows and (2) waves in the lee of a ridge. Channeling and
physical diversion of flow may also increase the turbulence.
The most abundant data available to confirm turbulence enhance-
ment due to complex terrain are from bivanes and tetroons. Measurements
of plume contaminants confirm the relevance of these measurements to p'iurne
behavior. Wind fluctuation data are especially important for estimating
plume dispersion rates in rough terrain.
There is some evidence that horizontal spreading (i.e., ay),
is amplified to a greater extent than vertical spreading in complex
terrain, particularly under stable conditions. The resultant increase
of ay has been attributed primarily to plume separation processes ana
plume meandering which occur when the higher terrain elevations are con-
fronted by the plume. Overall, the observed plume dimensions ir. complex
terrain (compared to flat terrain for similar stability conditions) nave
a larger cross-section and the plume is generally more dilute in the
core by a factor of two or more.
Although several models are available for estimating ground-
level concentrations from an elevated point-source in complex terrain,
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validation analyses suggest that each is subject to systematic errors which
may occur because the effects of terrain are not adequately represented. In
general, the models are capable of estimating ground concentrations, measured
at distances greater than 10 km from the source, within a factor of two.
Larger errors occur for distances closer to the-source.
Practical methods of characterizing the effects of terrain on the
path and the diffusion of an elevated plume need to be developed. There are
many difficulties in providing a working model of mountain wind fields in com-
plex terrain even though adequate theories for local wind in simple ridge and
valley situations have been developed. The topography amplifies all the prob-
lems typically associated with initializing and grid meshing in mesoscale
dynamics modeling. Analytic methods for generating and quantifying terrain
characteristics with sufficient reality and economy for modeling are needed.
Results from one series of tracer studies suggest that the turbulent
downward transport of the plume during transitional fumigation periods between
stable morning and neutral/unstable afternoon conditions, may cause higher
ground-level concentrations than the passing of the plume close to or impinging
on elevated features under stable conditions. More information (for example,
extensive temperature profile data atop ridges near point sources when potential
fumigation conditions exist) must be available from field measurement programs
to define the behavior mechanisms associated with the highest ground concentra-
tions in complex terrain situations.
Meteorological data used in previous studies, obtained from local
radiosondes, pilot balloons, constant-volume balloon flights, aircraft, and
surface networks, may not have been an adequate representation of the variation
of meteorological conditions throughout the areas of complex terrain which
affected behavior of the plumes under study. Readings obtained do not appear
to have fully reflected orographically induced eddies, directional wind shear,
and thermal convection processes which are vital to the plume transport pro-
cess. Even with batteries of specially placed sensors, mobile ground vans,
and helicopter observations„ (such as have characterized most of-the 16 f-':.d
studies of plume advection and dispersion in complex terrain examined in
this report) it has not so far been possible to assemble and process field
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data in ways which define the circumstances under which plume impingement
(or near impingement) occurs in complex terrain. If this has been the
case for data gathered in specially-equipped research programs, an inescap-
able conclusion is that the kinds of meteorological, air quality, and pol-
lution emissions data now routinely available to air quality analysts for
power plant plume studies are inadequate - in the present state-of-the-art
plume modeling - for the acceptably accurate calculation of ground-level
concentrations due to a power plant plume in complex terrain.
There is a need for the development of improved modeling tech-
niques which will permit the calculation of validatable plume concentration
estimates (and plume impingements if these occur) in complex terrain. These
modeling techniques should be based upon data which are routinely measured
and are routinely available, at an economically affordable cost, for the
vicinities of power plant sites.
-12-
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PART 2
NON-CONSERVATIVE ASPECTS OF PLUME BEHAVIOR
The dispersion and the fate of effluents from power plant
stacks are a function of conservative and non-conservative aspects of
plume behavior. If the effluent plume were to consist entirely of
non-reactive gases it would be transported by wind and diluted by tur-
bulence, but the total mass of the original gas would remain unchanged
in the dispersion process. The plume behavior would be conservative.
The ideal gas described by conventional Gdussian plume dispersion models
has a conservative behavior. In reality such behavior can be approxi-
mated for short periods of time by plumes of fine particulates and of
relatively non-reactive stack effluents such as sulfur dioxide, but the
approximation is inadequate for periods of more than a few hours (or
less for more reactive effluents) because of chemical and physical
transformations which may change both the composition and the total
mass of material remaining in the plume. These chemical and physical
transformations are the non-conservative aspects of plume behavior.
Considerations of non-conservative plume behavior must neces-
sarily modify all efforts to provide a realistic, quantitative description
of the overall behavior and the ambient air pollution impact of stack
plumes from large fossil-fueled power plants - whether in complex or in
simple terrain.
A discussion of existing research into non-conservative plume
behavior is presented in the next four sections (III, IV, V, and VI) of
this report. Conservative plume behavior is discussed subsequently in
Part 3.
Section III
PRINCIPLES AND LABORATORY RESEARCH
The combustion of fossil fuels, whether coal, petroleum
products or natural gas, is an oxidation process in which carbon and
-13-
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hydrocarbons plus air are converted principally to carbon dioxide and
water vapor which are exhausted from the stack as effluents. A residue
of solid ash may be removed in the power plant (through the use of
precipitators and similar cleaning devices) or the ash may be largely
discharged from the stack as particulate matter. Most fossil fuels con-
tain some sulfur, which is largely oxidized to sulfur dioxide and leaves
the plant in the stack plume unless in-plant sulfur removal processes
are used. Trace metals in the fuels appear in the stack effluents. A
by-product of high-temperature fuel combustion is the oxidation of some
of the molecular nitrogen in the combustion air to nitric oxide and
nitrogen dioxide; these gases also appear in the stack effluent.
Sulfur dioxide is the stack effluent of major interest,
because of its known adverse effects upon human health and vegetation,
and because of its propensity to oxidize to sulfates which may cause
greater damage. All identified pollutants in stack gases are, however,
of interest for plume behavior studies.
The main combustion processes of fossil fuels in power plants
are accompanied by numerous intermediate side reactions, resulting from
the variety of trace chemical constituents, temperatures, and oxygen
concentrations encountered within the combustion system. Upon emission,
the S02 component of the plume is not in chemical equilibrium with its
rapidly changing surroundings and enters into further reactions in the
plume. Knowledge of the chemistry of sulfur oxides in the atmosphere
is still incomplete but, strong evidence indicates that the ultimate
fate of sulfur oxides in the atmosphere is oxidation to some form of
sulfate, whether by a homogeneous path with reactants in the same phase
(the gas phase, liquid phase, or solid phase), or a multi-phased heter-
ogeneous path. The plume-atmosphere interactions which are known to
exert an influence on ultimate S02 fate are depicted in Figure 1 from
Hales et al. (1971a).
It is the intent of this Section III to provide a review of
the mechanisms that convert sulfur dioxide to sulfates, the detailed
chemistry and dynamic processes associated with these changes, and the
-14-
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Figure 1. Possible Atmospheric Paths for SO2 Upon Emission From a
Power Plant Stack (after Hales et al. 1971).
-15-
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principles of the active scavenging and deposition processes which may
ultimately deplete the gaseous and aerosol products from the atmosphere.
Five important sulfate formation mechanisms have been identified
(see Table 1), most of which involve reactive agents such as photochemical
smog, ammonia, catalytic metals and fine particulates. Relative humidity,
temperature and the concentrations of the reactants control the oxidation
rates associated with these mechanisms.
TABLE 1. MECHANISMS THAT CONVERT SULFUR DIOXIDE TO SULFATES (from U.S. EPA 1974)
Mechanism
Overall Reaction
Factors on Which Sulfate
Formation Primarily Depends
1. Direct Photo-oxidation
2. Indirect Photo-oxidation
3. Air Oxidation in Liquid
droplets
4. Catalyzed oxidation in
liquid droplets
5. Catalyzed oxidation on
dry surfaces
SO,
SO,
'
SO
Light, Oxygen.
Water
;, Water, NO.
H2S04
Organic oxidants, OH
2 4
Liquid water
— a -
NH4 + H SO,
o 2 3
+ SOT
44
Oxygen, liquid water^
T ' , *
Heavy metal ions
SO
Carbon, water
SO
so
2 4
Sulfur dioxide concentration,
sunlight intensity
Sulfur dioxide concentration,
organic oxidant concentration,
OH, NOX
Ammonia concentration
Concentration of heavy metal
(Fe, Mn) ions
Carbon particle concentration
(Surface area)
DIRECT PHOTO-OXIDATION
Exposure of pure sulfur dioxide and sulfur dioxide-oxygen
mixtures to certain ultraviolet radiation wavelength regions can result
in slow oxidation and the formation of sulfur trioxide. If water is
present in the environment, the $03 is rapidly hydrated, condensation
of the resulting H2S04 acid takes place and subsequent acid reactions
with particulate matter are common. Laboratory chamber investigations
of the reaction kinetics of homogeneous photo-oxidation are subject to
uncontrolled variables and unknown experimental parameters, such as
intensity and spectral distribution of radiation, humidity, temperature,
-16-
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gaseous impurities, aerosol impurities, and reactive container surfaces.
For example, Cox and Penkett (1970) found the conversion rate for the
decay of gaseous SC>2 in natural sunlight was as high as 100 percent per
hour and attributed the results to possible wall effects. The interaction
of S02 with trace impurities in the air was suspected to be the cause of
increased condensation nuclei in the dark when S02 was activated in the
same experiment.
.Examination of the absorption spectrum for S02 shows two bands
O
above 2900 A (the second significantly stronger than the first) involved
in the transition of S02 to an excited state. Early studies sought to
examine the quantum yields (<|>) for the photo-oxidation of S02 in the first
allowed region and the outcomes of these efforts resulted in a variety of
values for a variety of experimental conditions and reactant concentrations.
The results with the S02 pressures used were generally extrapolated to ppm
concentrations usually found in the atmosphere. Of the experiments designed
to investigate the photo-oxidation of S02 at low concentrations in the air
using natural sunlight (see Table 2), conversion rates indicated by the
first order rate constants obtained are seen to vary from the low values
of 0.05% hr'1 - 0.65% hr'1 (Hall 1953; Gerhard and Johnstone 1955; Urone
et al. 1968; and Cox and Penkett 1970) to that of 24% hr'1 (Renzetti and
Doyle 1960). The low values are believed the more realistic for low con-
centrations of S02> since most of these workers used very pure gases,
resulting in minimal interference from trace impurities. Sethi et al.
(1969), Cadle and Allen (1970) and McQuigg and Allen (1970) have presented
results which tend to eliminate homogeneous direct photo-oxidation as an
important mechanism for oxidation of S02 in the troposphere. Their low
quantum yield results of ~ 5 x 10-3 and the more recent lower value of
1 x 10~9 reported by Friend et al. (1973) indicate that the mechanism
is too slow to account for transformations observed in periods of a few
hours. Direct photo-oxidation is possibly important only for extremely
long-range transport of low levels of pollution such as on a global scale.
-17-
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TABLE 2. STUDIES OF THE PHOTO-OXIDATION OF SULFUR DIOXIDE IN SUNLIGHT
Researchers
Laboratory Technique Utilized
Reported
Quantum
Yield
Conversion
Rate Indicated (for Sunlight)
By First Order Rate Constant
co
i
Hall (1953)
Measured the photo-oxidation rate to SO3 (56-230 nm)
with O2 (50-200 nm) in sunlight.
10
-2
Gerhard and
Johnstone
(1955)
Measured the amount of sulfuric acid aerosol found in
the photolysis of SO^ at low concentrations in moist
air using both a mercury sunlamp (295-305 nm) and
natural sunlight.
10
-3
Renzetti and
Doyle (1960)
Measurements of the decrease in the gas phase SO2 con-
centration after SO2 air mixtures had been exposed to a
mercury u. v. lamp for a known time.
3 x 10
-1
Urone et al.
(1968)
Used technique similar to Renzetti and Doyle (1960);
compared their 310-420 nm u. v. source with natural
sunlight and suggest that lower estimates of photo -
oxidation rates in sunlight are more realistic.
0.05% hr
-1
0. 1 to 0. Z% hr
-1
24% hr
-1
0. 1% hr'1
Cox and Penkett
(1970)
Low concentrations of gaseous SO2 and a sulfur-containing
aerosol were monitored both in the dark and sunlight using
radioactive SO2 and the rate of the photochemical
reaction determined from SO2 decay and aerosol yield.
10
-2
Friend et aL
(1973)
A system was designed for the purpose of studying
aerosol formation when trace gases in air are
irradiated with ultraviolet light.
1.0 x 10
-9
0. 65% hr
-1
<0.01% hr
-1
-------
INDIRECT PHOTO-OXIDATION
Chemical reactions involving S02 in mixtures of air containing
oxides of nitrogen only, or with oxides of nitrogen and hydrocarbons have
a wide range of potential reaction paths. Researchers are seeking to
define these paths and the S02 oxidation kinetics associated with each of
these reaction systems.
Davis (1975b) identified nine possible initiating reactions
(see Table 3) of homogeneous gas-phase oxidation processes which might be
involved in the conversion of S02 to H2S04 aerosol in the troposhere. The
lifetimes for conversion range from 10 days down to 3 days for the dif-
ferent reactant species.
TABLE 3. NINE INITIATING REACTIONS FOR HOMOGENEOUS GAS PHASE OXIDATION
OF SULFUR DIOXIDE AND THEIR LIFETIMES FOR CONVERSION TO H2SO4 IN THE TROPOSPHERE
(after Davis 1975b)
Number
la
Ib
2
3
4
5
6
7
8
9
Reaction
SO2 + hl>(2400-3400A) ->SO|
SO|+02->(S04)
SO2 + O( P) + M->SOs+M
S02+02(1A)+(S04)
SO2 +O3-»-SO2 +O2
SO2 + NOs -*• 803 + NO2
SO2 +N2O5 -»-SO3 + N2©4
SO2 + HO2 ->SOs + OH
S02 + OH + M -V HSOs + M
SO2 + CH3O2 -vSOs + CHsO
Estimated Concentration of
2nd Species (Molec/cm^)
- —
~ 1 x 104
~ 106
1 x 1012
~ 1 x 107
., 6 x 106
~ 5 x 108
.. 5 x 106
.. 108
Troposphere
Lifetime (Days)
4x 107
~ 6 x 104
~108
~105
-1011
~109
-23
~3
-100
-19-
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In the absence of high organic radical concentrations and catalytic
surface sites the prime conversion mechanisms in the atmosphere, as
reported by Davis (1975b) and other researchers (Friend et al. 1973,
Crutzen 1974), appear to be the S02 reaction with either OH or H02
(Reactions 7 and 8 in Table 3). Free radicals are the neutral fragments
of molecules. In unpolluted tropospheric air, the most important radicals
appear to be OH and H02 because they are essentially inert toward the main
atmospheric constituents (N2, 02, Ar, C02, and H20), but react readily
with many trace gases. In particular, the reaction of OH with N02 has
been suggested as a significant source of atmospheric nitric acid and
the reaction of OH with methane has been suggested as a significant
(although disputed) source of atmospheric carbon monoxide. The source
of the OH radical is the reaction of water vapor with metastable oxygen
(0('D)), a species produced by photolysis of ozone. For this reason,
the presence of ozone may be indicative of the coexistence of the OH
radical (Davis 1976, personal communication). Rate constants must be
determined for numerous reactions involving the OH radical to determine
the importance of these reactions as efficient conversion mechanisms in
the atmosphere. The concentrations of S02 and other transient chemical
species must be jointly measured to effectively study reactions; a laser-
induced fluorescence system has been developed (Lubkin 1975) so that
hydroxyl radical concentrations can be analyzed in a more quantitative
manner than other currently available techniques permit. Despite the
low rates of Reaction 2 at atmospheric concentration levels, this reaction.
is apparently the most important S02 oxidation mechanism in experimental
tests of irradiated S02-NOx-air systems. N02 added to an S02-Air
mixture will undergo photolysis and liberate oxygen molecules which
encourage Reaction 2. In a study of the N020s-S02-03 reaction system,
Daubendiek and Calvert (1975) suggest the homogeneous gas-phase reaction
systems N03 + S02 -»• N02 + $03 and N205 + S02 -> ^04 + SOs (Reactions 5
and 6) are not important removal paths based on their recent
of the rate constants of these reactions.
-20-
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The addition of hydrocarbons to a system of S02 and NOX and
subsequent irradiation results in observed S02 oxidation rates that are
much greater than in the S02~NOX system. The direct reaction of SC^
and hydrocarbons has been the subject of study for the past quarter of a
century and unfortunately only a few of these studies are available which
report a base of experimental data spanning a wide range of initial reac-
tant concentrations to deduce the chemical kinetics of a particular photo-
chemical system. The smog-chamber work of Wilson and Levy (1970 and 1972)
and Smith and Urone (1974) are two of the very few studies available which
report at least partial concentration-time data.
Smith and Urone (1974) conducted a laboratory experiment utilizing
pyrex flasks in which they irradiated 2 ppm [S02] and 0, 0.85, 1.7, 3.4,
5.1, and 10.2 ppm [N02] respectively in the presence of H20. The S02
oxidation rate reached a maximum of 3.3 ppm min-1 for an initial N02 con-
centration of 0.85 ppm. An initial N02 concentration of 10.2 ppm and
similar experiment conditions resulted in an observed S02 oxidation rate
of 1.8 ppm nrin~l. The initial photochemical rate was 1.7 x 10~4 ppm/min
in air with no N02 present. The rate was found to increase when the
S02/N02 ratio was 1 or 2, but a decrease was evident when the ratio was
0.6 or less. In experimental conditions duplicating those just described,
propane was added to the system in initial concentrations ranging from 3
to 12 ppm. Both sets of data exhibited an inhibition in the S02 oxidation
rate with increasing N02- Hydroperoxyl radicals in the N0x-Hydrocarbon-
S02 system may have been responsible for the inhibition effect on the S02
oxidation.
Wilson and Levy (1970 and 1972) studied the smog-chamber irra-
diation of a number of S02-N0x-Hydrocarbon mixtures for hydrocarbon
species such as 1-butene, 1-heptene, 2,2,4-trimethylpentane, and toluene.
Quantitative values did not come out of these studies because of the high
degree of scatter in the experimental data. The rate of S02 disappearance
was found to correlate with hydrocarbon activity. Relative humidity was
found to have a dramatic effect on the overall reaction rate in certain
high humidity runs.
-21-
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Groblicki and Nebel (1971), Cox and Penkett (1972), and McNeils
(1974) have investigated the oxidation of S02 to H2S04 aerosol when S02
was added to olefin-03 systems. A (biradical) short-lived intermediate
product of the ozone olefin reaction was postulated to react with the S02.
The consequence of this reaction in plume chemistry can only be speculated
upon at this time.
Roberts and Friedlander (1976) irradiated a 96m3 teflon bag in
natural sunlight to study unfiltered ambient air containing added amounts
of S02, 1-heptene and NOX. Other hydrocarbons such as cyclohexene, 2-methyl
2-butene, 2,3-dimethyl-2-butene, and 1,7-octadiene were also studied in
less detail. The experiments were designed to study the formation of
sulfur containing aerosols under photochemical smog environment conditions.
A sulfur balance was carried out on the system by simultaneous measurements
of the conversion of S02 and the formation of particulate sulfur. The
results for all of the experiments were qualitatively similar: S02 decay
was found to be slight until the 03 concentration became larger than 5 pphm,
at which point there was a sharp downward trend in the S02 concentration.
Two major behavior regimes were observed in these experiments:
1. As NO is converted to N02, the concentrations of S02, 03,
1-heptene and bscat (decrease in visibility) are constant
2. Aerosol formation results from the 03 induced decay of
both S02 and 1-heptene.
Depending on the initial S02 level (and even with surprisingly
low relative humidities), the reaction rates varied from 0 to 99 percent
per hour, based on psuedo first order depletion of S02- Table 4 summarizes
the teflon bag experimental conditions.
-22-
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TABLE 4. SUMMARY OF TEFLON BAG EXPERIMENTAL CONDITIONS
FOR S02/NO/N02/1-BUTENE SYSTEM
(fiom Roberts and Friedlander 1976)
Hydrocarbon
[Hc]o(ppm)
[NO]0 (ppm)
[N02]0(ppm)
[S02]o(ppb)
Abscat (10" m~ )
k (% hr-1)
£03] max (pphm)
%RH
T (° C)
Run Number
1
1-Heptene
0.89
0.20
0.20
0
2.4
0
53
45
31
2
1-Heptene
1.85
0.19
0.10
39
9.5
25-82
41
16
35
3
1-Heptene
1.66
0.10
0.10
80
14.5
21-99
33
14
38
4
1-Heptene
1.47
0.20
0.11
86
14.0
27-72
26
16
36
In the Roberts and Friedlander study a kinetic scheme was pro-
posed for aerosol formation based on a mechanism similar to that of Cox
and Penkett (1972). In the gas phase:
03 + Olefin -»- reactive intermediate
reactive intermediate -> condensable organic compounds
reactive intermediate + S02 -»• SOa + condensable organic compounds
$03 + H20 •> H2S04 + sulfates
What has been determined from all, the studies of S02 oxidation
rates in NOx-hydrocarbon-S02 systems is that known reactions (involving
S02 for which rate constants have been determined) are in themselves insuf-
ficient to account for the oxidation rates in these chemical systems.
AIR OXIDATION IN LIQUID DROPLETS
Important oxidation reactions in the aqueous phase are at least
quasi-homogeneous by nature, since all of the participating reactants are
in solution. Aerosol particles, which constitute the nucleation sites for
-23-
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droplet formation, are considered as participants in these reactions
because molecules of S02 and other gases quickly diffuse into the interior
of a water droplet where they may directly encounter the nucleating particle
and allow the oxidation process to proceed throughout the droplet. This
process consists of the hydration and subsequent dissociation of the dis-
solved gases and oxidation of sulfite or bisulfite ions.
Van den Heuvel and Mason (1963), Scott and Hobbs (1967), McKay
(1971), and Miller and DePena (1972) investigated the aqueous phase mech-
anism of S02 oxidation catalyzed by NH3. Van den Heuvel and Mason (1963)
examined the following reaction paths:
2S02 + 02 -*• 2S03
S03 + H20 •* H2S04 ->2H+ + S04=
NH3 + H20 -> NH40H £ NH4+ + OH"
The rate of formation of (NH4)2S04 was measured by supporting
approximately 100 drops of H20 on a quartz fiber grid in a 5 liter glass
flask exposed to air containing gaseous S02 and NH3. An oxidation con-
version rate of 2.5% min'1 was obtained by extrapolation of data to con-
ditions in an industrial atmosphere containing 100 ygm~3 S02 and 10 pgrrf3
NH3. The amount of fonmed sulfate when the sulfur dioxide alone was in
air was at least two orders of magnitude smaller than when the sulfur
dioxide was mixed with ammonia.
Scott and Hobbs (1967) considered a set of nine chemical equi-
librium equations for the S02-NH3-liquid H20 system, including caroon
dioxide equilibria. The rate of production of S04~ was assumed to be
limited by the oxidation of the sulfite ion described by the first, order
rate equation
d[S04=]
-24-
-------
Analysis of the data of Van den Heuvel and Mason (1963) resulted in a
choice of K = 0.1 min"^ as the rate constant. A plot of the theoretical
results obtained from the calculations made with the above assumptions
did not result in a limiting value for sulfate formation or a direct
proportionality between sulfate and the initial S02 partial pressure.
The theoretical curves indicated that for the concentration of sulfate
in waterdrops as a function of time, the presence of ammonia will increase
the amount of sulfate produced by the reaction after it has run for some
time. An adequate explanation of the S02-NH3-liquid HoO system can
be based on the theory of this research. The rate of sulfur dioxide
oxidation derived in this study was about 2.5 percent hr-1.
McKay (1971) examined the kinetics of sulfate oxidation by
taking the rate law of Fuller and Crist (1941) and substituting ionization
constants from the Scott and Hobbs (1967) study, suggesting that the
reaction is roughly on an order of magnitude faster than Scott and Hobbs
had assumed. Evident from the research was the enhancement of the reaction
rates with a lowering of the temperatures; the increased solubility of
ammonia and sulfur dioxide at lower temperatures are cited as the cause by
the authors. At the time of the McKay (1971) research, the large negative
temperature correlation in his work had not been noted by other workers in
the field, although this feature has now been noted by various other investi-
gators. An oxidation rate of approximately 13% hr~l is suggested from this
work.
Miller and De Pena (1972) measured the rate of sulfate ion for-
mation in distilled water drops (raindrop size) for a range of SC>2 partial
pressures, following the experimental methodology of Van den Heuvel and
Mason (1963), and utilizing the basic model of Scott and Hobbs (1972).
The region of experimental values for the first order reaction (d[S04=]/dt =
K [S03=] showed K =3 x 10"3 sec'1, in good agreement with the operative
value in the study of Scott and Hobbs. Oxidation rates of only 0.1% hr'1
were calculated from the Miller and De Pena data.
-25-
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METALLIC-CATALYZED OXIDATION IN LIQUID DROPLETS
Sulfur dioxide may be oxidized to H2S04 in aqueous solution accord-
ing to the following process:
2S02 + 2H20 + 02 catalySt->2H2S04
which requires water, dissolved oxygen and the presence of a catalyst.
Ammonia has already been previously mentioned as one such catalyst; metal
ions such as Fe3+, Mn2+, and Cu2+ constitute the other major grouping of
catalyst-oxidation candidates in the atmosphere (Johnstone and Coughanowr
1958). These metal ions are contained in iron and manganese salts normally
found as suspended particulate matter in the ambient air from burned coal
fly ash. The particulates behave as sites of nucleation which at higher
humidities may either dissolve in the droplets or remain as solid con-
densation nuclei centers. The role of iron as an efficient catalyst
in the oxidation of S02 in the atmosphere has been studied by Junge and
Ryan (1958), Johnstone and Coughanowr (1958), Foster (1969), Brimblecombe
and Spedding (1974) and Freiberg (1974 and 1975).
Junge and Ryan (1958) examined the iron-catalyzed reaction of S02
oxidation in acid solutions, without investigation of the kinetics. Air
containing S02 was bubbled through dilute catalyst solutions and a linear
relationship was found between the final extent of $04" formation and the
S02 partial pressure of the reaction. The pH of the catalyst solutions
was cited as the governing factor of the S02 oxidation process; S02 solu-
bility decreases as acidity is increased due to sulfate formation, anc
eventually a situation is attained where sulfate formation ceases oecause
of liquid phase S02 depletion. This bubbling technique has been criticized
by Cheng et al. (1971) as an unrealistic representation of an aerosol gas
system due to the influences of mixing effects and mass transfer mechanisms
on the kinetics of the reaction.
Johnstone and Coughanowr (1958) approached the absorption and
reaction aspects of their catalytic oxidation studies by mathematically
-26-
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considering the reaction system as one of gas diffusion into a liquid drop
with a concurrent zero order reaction in the liquid phase. The value of
the zero order rate constant was acquired from 'previous experiments invol-
ving homogeneous S02 liquid phase oxidation. In an experiment which
exposed a single drop of manganese sulfate solution to an environment
consisting of air and trace amounts of S02, it was estimated that mangan-
ese sulfate crystals of 1 micron at 1 ppm S02 yielded a rate of oxidation
in fog droplets on the order of 1 percent min"1. Cheng et al . (1971) had
doubts about the validity of the assumption that the reaction rate was
controlled by liquid phase gas diffusion.
Foster (1969) derived theoretical growth rates of H2S04 droplets
to test the effectiveness of MnS04 crystallites as a catalyst for S02 oxi-
dation in humid, polluted surroundings. Review and comparison with pre-
vious theoretical works on the subjects of manganese and iron solution
catalytic oxidation of S02 led to the following rate expressions, the
symbols of which are explained in Table 5, along with the values (when
provided) utilized for the calculations:
Rate of S02 oxidation = 22.4 KiCiV loor - -1
by Mn+2 catalyst GD x IUU/0 min
* on -j 4-- DWKu (1 - fo) K.n-f.
Rate of SO-2 oxidation _ o? /, H _ '_ i i i inno/ min
by Fe+2 catalyst ' 22'4 - - M1 x 100/0 min
When these values are substituted into the equations, S02 plume oxidation
rates of 0.09% min~^ for Mn and rates of 0.15 - 1.5% min~l for Fe are the
results. This work implies that the major catalytic role for aqueous phase
S02 oxidation in plumes is played by oxides of iron.
-27-
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TABLE 5. NOMENCLATURE AND VALUES USED FOR PLUME SO2 OXIDATION
CALCULATIONS (from Foster 1969)
General
Manganese
(Mn-O.)
Iron
(Fe203)
W
G
D
f
o
S
M.
i
n.
fj1
Ki
Ci
Mi
ni
fi
KH
Nomenclature
Effluent dust burden, g/ liter
Effluent SO2 content, ppm
Effluent dilution factor
Fraction of total sulfur oxidized
Droplet sulfate concentrations, mo I/ liter
H20
Oxide molecular weight, g/mol
Number of catalytic ions per molecule
Fraction by weight of dust soluble
Rate constant
Catalytic concentration within the droplets
Oxide molecular weight, g/mol
Number of catalytic ions per molecule
Fraction by weight of dust soluble
Solubility constant
Value
_3
2x 10
2-5 x 103
ID'3
ID'1
1
2
2- 29 x 10
3
2 x 10-4
*
*
l-60x 102
2
io-2
i
* Values not provided.
Matteson et al. (1969) studied the mechanism of S02 oxidation
catalyzed by manganese sulfate aerosol. The kinetic theory proposed for
the oxidation process was based on a four-step chemical reaction involving
the formation of intermediate complexes.
Mn2+ + S02 t Mn-S022+
2Mn-S022+ + Oj
Mn-S032+
HS04-
H
H2Q 2 Mn2+
2 H2S04.
02] + 2Mn-S03
2+
The rate expression which was determined was
d(S°2)
where the experimental rate constant = 2.4 x 105 mole-"! s'1
-28-
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Negligible S04~ concentrations were formed for relative humidities below
95 percent in the experiments apparently as a result of inadequate hydration
of the metal salt. Rates on the order of 0.5% day"^ '-were recorded. The
experimental data obtained were basically incorrect (Cheng et al. 1971)
due to errors in the recorded reaction times.
Selected metal salts which had been reported to act as aerosol
catalysts (MnSCty, MnCl2 and CuSCty) were tested for their effectiveness in
promoting atmospheric oxidation of S02 to H2S04 in laboratory work by
Cheng et al. (1971). The experiment was carried out using an aerosol
stabilizing technique, whereby teflon beads with deposited aerosol particles
were packed into a flow reactor, and exposed to influent S02 concentrations
ranging from 3 to 18 ppm in humid air. Higher S02 oxidation rates were
found to be positively correlated with higher relative humidities; in
addition, the chemical reaction in the liquid phase in conjunction with
absorption of S02 by aqueous catalyst drops controlled the overall rate
of reaction. Using NaCl as a base for comparison, aerosols of MnS04,
MnCl2> and CuS04 were found to be 12.3, 3.5, and 2.4 times more effective
than NaCl in assisting the oxidation of S02 on a milligram-to-milligram
basis. To approximate the rate of absorption and oxidation of S02 by fog
droplets in a natural fog, laboratory results were extrapolated to condi-
tions typical of an urban industrial atmospheric environment. The following
assumptions were made for this estimation:
1. 15 ym was the average diameter of fog droplets
2. S02 concentration in the atmosphere was 0.1 ppm.
3. Fog droplet concentration of catalyst is equivalent
to 500 ppm MnS04; one-half of the fog droplets con-
tain the catalyst capable of oxidizing S02 to H2S04
4. 0.2 g of water per cubic meter of air defined the
fog concentrations.
The atmospheric oxidation rate under these conditions was found to be
2 percent hr"1.
-29-
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Brimblecombe and Spedding (1974) noted that many of the experi-
mental data used in the interpretation of the oxidation of S02 within the
atmosphere in previous studies were characterized by concentrations several
orders of magnitude higher than those expected in the aqueous aerosol, and
were therefore unrealistic. These researchers attempted to establish the
rate of oxidation of low concentrations of S02 (about 10~5 molar) in aqueous
solutions containing traces of Fe3+ (about 10-6 molar) as these concentra-
tions approach those which might be likely in the atmospheric aerosol. A
radiochemical method utilizing 35$02 monitored the oxidation process in
the experimental procedure over a range of added Fe3+ catalyst concentrations
up to 1.6 x 10~8 molar. A radical mechanism was favored due to the high
reaction rates noted in dilute solution; hydroxylated Fe3+ was postulated
as the initiator in the production of $03" radicals:
SOg" + FeOOH + 3H+ + Fe2+ + 2H20 + SOs"
To explain the termination of the radical chain, Brimblecombe
and Spedding (1974) state that the Fe2+ is oxidized back to the +3 oxidation
state by reacting as follows:
SOg + Fe2+ -*• SO2.- + Fe3+
Small amounts of Fe3+ in the atmosphere would be capable of oxidizing a
large quantity of sulfur dioxide by such a step which would allow regen-
eration of Fe3+: 2Fe3+ + H20 + H2S03 -*• SO^" + 4H+ + 2Fe2+. Assuming an
S0£ concentration of 28 yg/m3 and a Fe3+ concentration of lO'6 molar,
Brimblecombe and Spedding arrive at a S02 conversion rate in fog of ~ 3.2
percent/day.
Freiberg (1974, 1975) examined the mechanism of iron-catalyzed
oxidation of S02 in oxygenated acid solutions by beginning with the rate
expression:
d[S04=] KCK/ [H2S03]2 [Fe3+]
-30-
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where
Kc = rate constant
k^ = first dissociation constant of sulfurous acid
Since the pH increases as relative humidity rises, the reaction rates were
found to be especially dependent on high relative humidity; an increase
from 80 percent to 90 percent RH resulted in an eightfold increase of i>on-
catalyzed oxidation of S02» whereas a relative humidity increase from
80 percent to 95 percent resulted in a sixty-four-fold increase in the oxi-
dation reaction rate. Freiberg also examined the dependence of the oxidation
rate of S02 on temperature. The relationship involves numerous physical-
chemical processes which influence the reaction in a way different from the
expected reaction rate increase with temperature alone; the end result is
a significant decrease in the yield of the reaction as the temperature
increases. An increment of 5°C results in a net decrease in reaction rate
of iron catalyzed oxidation of S02 by about an order of magnitude.
OTHER AQUEOUS S02 OXIDATION MECHANISMS
When water droplets are present in a mixture of S02, 03 and air,
both S02 and 03 are observed to vanish swiftly, indicating that the reaction
S02 + 03 -*• $03 + 02 proceeds more efficiently in the liquid phase than it
does in the gas phase.
The mechanism of aqueous phase S02 oxidation by ozone has been
investigated by Espenson and Taube (1965), Penkett (1972), and Penkett and
Garland (1974). In the study by Espenson and Taube (1965), two elements
of this mechanism were examined; namely the acid solution reaction of S02 +
03 + H20 -*• H2S04 + H+ + 02 and the basic solution reaction of SOs" + 03 ->
S04= + 02. Unfortunately, quantitative information did not result from the
work.
Penkett (1972) investigated the reaction path of HS03~ + 03 ->
HS04~ + 02 (S02 in the form of bisulfite ions) with the rate of oxidation
provided by d[0s]/dt = K[0s] [HSOs'J. In the kinetic work, the rate
-31-
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constant K = 3.32 +_0.13 x TO5 NHs-1, and a 0.05 ppm ozone concentration
resulted in an extrapolated S02 oxidation rate of - 0.21% min"1 at 9.6°C;
this compares to the slower rate of 2.5% hr-1 predicted by Scott and Hobbs
(1967) for S02 oxidation catalyzed by NHs.
Penkett and Garland (1974) studied the same reaction path as
Penkett (1972) and, for a pH range from 4 to 7 at 10°C with 0.1 ppm 862
and 0.05 ppm 63 in fog water, determined the reaction rate coefficient of
the expression -d[S03=]/dt = K[S03=] to be K = 4.18 x 10~4 + 1.77 [H+]1/2 sec'1.
HETEROGENEOUS STUDIES
Corn and Cheng (1972) examined the heterogeneous catalysis of sul-
fur dioxide in the laboratory with respect to interactions with insoluble
particulate matter. Micron and submicron particles of CaC03, \/205, Fe-Oo
and flyash from a coal-burning power plant were utilized along with suspended
particulate matter from an urban environment, Mn02 and activated carbon.
Through the use of an aerosol stabilizing technique, the aerosol particles
were deposited on teflon beads which were packed into a flow reactor in order
to obtain kinetic data. Mn02, Fe203 and the suspended particulate matter
from Pittsburgh air were found to sorb S02, with physical adsorption as the
primary process. Steady-state rates of conversion of 0.013 and 0.021 yg of
S02 min~l mg~^ at S02 concentrations of 8.0 and 14.4 ppm, respectively, were
found for the activated charcoal. Under the conditions of the tests, CaC03,
V20s and flyash were discovered to be essentially inert to S02 at room temper-
ature. Twenty mg of \/205 at 95 percent relative humidity removed only ',.2 per-
cent of the total S02 after 50 minutes of exposure, which is surprising,
considering that at a high temperature of 500°C or beyond, vanadium pentox'.ae
is known to participate in the oxidation of S02 to 503 according to the
reaction:
V2°5 + S02 •*• V2°4 + S03
Low et al. (1971) studied the gas-solid reaction of S02 with CaO
by means of infrared spectroscopic techniques as a segment of research designed
to investigate the removal of S02 from power-plant stack gases by way of
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catalytic oxidation of powdered limestone introduced into the stack stream.
A similar study was conducted by Goodsel et al (1972) for sorption of S02
on MgO in research to find the removal rates of S02 from plant stack gases
by catalytic oxidation with introduced dolomite. Both studies showed the
formation of a surface sulfite as the main irreversible interaction product
when the CaO and MgO were degassed. Sulfates were formed when the surface
sulfite was exposed to heat in an oxygen environment.
A combination of colorimetric and radio-tracer techniques were
employed by Urone et al. (1968) in studies of reactions of parts per
million concentrations of sulfur dioxide, with laboratory environments con-
taining hydrocarbons, nitrogen dioxide, moisture, particulates and ultra-
violet radiation. Powdered oxides of aluminum, calcium, chromium, iron,
lead and vanadium were found to react with S02 within minutes without the
presence of sunlight or ultraviolet radiation, whereas gaseous mixtures
of S02 and clean air kept in the dark for days did not react. A reaction
rate on the order of 0.1 percent hr~^ was found for the photochemical
reaction of sulfur dioxide in clean air with ultraviolet radiation equiv-
alent to noonday sunlight. Inert solids such as NaCl and calcium carbonate
resulted in negligible heterogeneous reaction rates with $02; however the
reaction was significant for heterogeneous reactions involving other
particulate matter such as iron and lead in some instances, as shown in
Table 6.
-33-
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TABLE 6. SUMMARY OF EIGHT EXPERIMENTS INVOLVING SO2
REACTIONS IN THE PRESENCE OF VARIOUS PARTICULATES
(from Urone et aL, 1968)
Original
SO2 Concentration
14 (ppm)
14
8
17
18
10
14
8
2nd
Reactant
CaC03
CrO3
V205
PbO
Pb02
(CaO + Ag203)
Ag203
Fe304
2nd Reactant
Concentration
30.3 (mg)
11.0
17.2
11.0
12.0
(16.6, 19.8)
33.1
14.0
UV
Exposure
0 (min)
0
180
0
0
(0, 0)
0
0
Total Time
in F lask
1100 (min)
1030
810
15
9
30
1145
4
SO2 Reaction
Rate
0.2% hr"1
0.5% hr"1
0. 7% hr" 1
1.7% min"1
6.0% min"1
3.0% min"1
.04% min-1
17% of SO2 reacted
Smith et al. (1969) developed a method for studying adsorption
at solid-gas interfaces in aerocolloidal systems by generating submicron
sized aerosols (0.01 to 0.1 ym) using an exploding-wire technique and
radio-labeled-gases to achieve adequate sensitivity in sorption measure-
ments. Preliminary measurements of 35S-labeled S02 with Fes04, Al20a,
and platinum aerosols at ambient conditions, showed that low S02 concen-
trations resulted in preferential chemisorption followed by multilayered
physical adsorption at higher concentrations. Although significant adsorp-
tion was evident on the Fe304, Al20s and platinum surfaces, the oxiGotion
rates were not presented in the Smith et al. (1969) research. Overall, it
is seen that dry heterogeneously catalyzed S02 oxidation systems are rela-
tively ineffective when compared to aqueous phase oxidation systems.
The sulfate forms that result as products of S02 oxidation in the
atmosphere are tabulated in Table 7 along with brief descriptions of their
chemical nature (Wilson et al. 1976). The sulfate types are secondary in
nature; primary surface types (e.g., those emitted directly by industrial
or by natural sources) are not listed. The probable size class by mass
particle diameter for each of these sulfate forms is between 0.1 and 1.0 ym.
Whitby and Liv (1974) examined the size distribution of the general atmosphere
-34-
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aerosol, and found the number concentration to be dominated by particles in
the 0.002 to 0.02 ym range and the major part of the aerosol surface area in
the 0.05 to 0.5 ym size range; a bimodal distribution characterized particle
volume and mass. The "accumulation mode," defined as the size range between
0.1 and 1.0 ym, is estimated to be the size fit for 80 percent of atmospheric
sulfate. Secondary accumulation-mode aerosol can have a very long life time
in the atmosphere, traveling great distances, and having the opportunity to
participate in other chemical processes before removal by fallout or rainout.
TABLE 7. A SUMMARY OF TYPICAL SECONDARY ATMOSPHERIC SULFATE TYPES
(from Wilson et al. 1976)
Formula
H2SO4
NH4HS04
(NH4)3H(S04)2
(NH4)2S04
Names
Sulfuric acid (oil of vitriol)
Acid ammonium sulfate
Triammonium acid disul-
fate (Letovicite)
Ammonium sulfate (Mascag-
nite)
Sources
Atmospheric oxidation of
SO2; direct from manufac-
turing.
Oxidation of SO2 with NH3
addition.
Oxidation of SO2 plus NHj.
Oxidation of SO2 plus NHj.
Notable Chemical
Properties
Strong acid, very hygro-
scopic (drying agent at
low RH).
Strong acid, hygroscopic.
Acidic, deliquescent at
~ 65 percent RH.
Weak acid, water soluble
deliquescent at 80 percent
RH.
Charlson et al. (1974) investigated the hygroscopic properties
of the sulfate aerosol in the St. Louis region and found that the
can absorb water and increase in size at all humidities, as can
while (Nh^^SO* can do so at relative humidities above about 65 percent.
SuTfuric acid is known to give rise to heteromolecular nucleation of drop-
lets in the presence of water vapor, and will condense on preexisting
nuclei (Stauffer et al. 1973 and Cox 1973). Unfortunately, detailed esti-
ma-tes of. growth parameters for aqueous solution droplets of sulfuric acid
are not available at this time. Research on the growth of solution drop-
lets of sulfuric acid as well as other important solutes as a function of
-35-
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relative humidity will be useful in the analysis of aerosol growth curves
to identify the chemical composition and understanding of the solute-
solvent interaction in the process of growth of solution droplets. Nair
and Vohra (1975) indicate that the relative increases in size and mass
depend on the quantity of solute per particle. They show that sulfuric
acid nuclei in the Aitken size range (< 0.1 urn) can grow to cloud nuclei
as the relative humidity increases. Nuclei can grow continuously at rela-
tive humidities below 100 percent due to the instability produced by the
chain process of sequential condensation of water and solute molecules onto
the nuclei. Katz and Mirabel (1974) examined how certain kinds of suspended
liquid aerosols can be generated in the atmosphere by the binary homogeneous
nucleation of water vapor and a polluting reactant such as sulfuric or nitric
acid; humidities as low as 10 percent in the presence of ^$04 were found to
result in large nucleation rates.
RELATED CHEMICAL PROCESSES
Field programs designed to investigate the plume chemistry of
processes other than sulfate formation are rare in the literature. An
excellent example of one such study is the Davis et al. (1974) investigation
of nitrogen oxides and S02 chemistry in the Morgantown power plant plume, in
which the power plant plume was postulated to have had a significant effect
in producing increased ambient 03 concentrations far downwind of the plant.
The Morgantown power plant plume investigation is described in Section V.
Whereas sulfur dioxide plume chemistry primarily involves the
transformation of S02 to sulfate (S04=), nitrogen oxide chemistry involves
the transformation of NO to N02, with the subsequent formation of ozone,
other oxidants, and nitrates. The literature is filled with photochemical
reactions involving 03, NO, N02, hydrocarbons and free radicals. These
data have been a product of laboratory work concerned with the polluted
urban environment, or more recently have been data gathered in studies of
rural ozone and in attempts to determine the downflux of stratospheric ozone.
-36-
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The photolysis of nitrogen dioxide is the only reaction known to
generate important numbers of oxygen atoms necessary for the production of
ozone:
N02 + hv + NO + 0 (1)
Once the oxygen atoms are present, they react with oxygen molecules, involv-
ing a nitrogen molecule or another oxygen molecule as the third body to form
ozone according to:
02 + 0 + M -> 03 + M (2)
Reaction (1), the photolysis of N02, is a very rapid reaction, but simul-
taneously, the reaction of ozone with NO is a rapid 03 removal mechanism:
NO + 03 •*> N02 + 02 (3)
and additionally, ozone is also removed by a relatively slow reaction with
N02:
N02 + 03 •* NOs + 02 (4)
Chemical reactions involving 'organic molecules or radicals which drive
or maintain the concentration ratio of N02 to NO at high levels are
responsible for high steady-state (i.e., 03 production rate = 03 destruc
tion rate) levels of ozone. In a polluted urban environment, depending
on the nature of the hydrocarbons present (i.e., paraffin, olefin or
aromatic), a branching reaction sequence occurs which allows photolysis
of N02 to create additional N02 at the expense of NO. Figure 2 from
Hanst and Bufalini (1971) illustrates how various hydrocarbons will oxi-
dize NO at different rates in smog-chamber experiments. It has been
theorized that for power plant plumes, a mechanism not affiliated with
hydrocarbons could oxidize NO + N02 to elevate the [N02l/[NO] value,
-37-
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thus increasing ozone concentrations. Two such mechanisms have been
suggested; the first, a sulfur mechanism and the second, a chlorine
mechanism (Tesche et al. 1976); however recent studies of plumes in areas
with low background hydrocarbon concentrations have detected no net ozone
production.
03
On
a
4«>
rt
Pi
a
o
O
a
a
10
Internal Olefins
1, 3 Butadiene
Multialkylbenzenes
Terminal Olefins
LI
Monoalkylbenzenes
P araf ins
D
Benzene
Figure 2. Effect of Hydrocarbon Structure on Nitrogen Dioxide Formation Rate (From Hanst and Bulfalini 1971).
-38-
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PRECIPITATION SCAVENGING
Two basic processes act to scavenge an aerosol from the atmosphere.
The first process is the result of the aerosol serving as a cloud nucleus or
undergoing capture by cloud water or ice particles, and is termed "rainout."
The removal of the subcloud aerosol by raindrops as they fall is the second
process, "washout."
Fundamentals of the influence of rain on the precipitation scaven-
ging of gases have been reviewed by Postma (1970) and Hales (1972). The
possibility that a gas plume could become redistributed in the atmosphere
due to the action of "reversible" washout has been discussed by Hales. The
gas concentration in each raindrop, as it traverses a plume, approaches a
concentration saturated with respect to the concentration of the plume (Slinn
1974). The drop can emerge from beneath the plume with a concentration super-
saturated with respect to ambient conditions, if no irreversible chemical con-
version of the gas occurs within the drop, and desorption of the gas is
possible. An increased dosage to low-level receptors is the effective result
of this lowering of the plume center!ine elevation. A complete mathematical
description of the rate at which this sorption-desorption phenomenon might
lower a gas plume is a formidable undertaking unless a number of assumptions
are imposed. Slinn (1974) assumed it was reasonable to use a convective
diffusion equation (assuming the stack exit is high enough and the atmosphere
stable enough), ignoring longitudinal diffusion compared with transport,
neglecting wind shear, and treating the diffusion coefficients as constants
in his attempt to predict this rate of plume redistribution. The "washdown"
speed of the plume is a function of the rainfall rate and Henry's law con-
stant (dimensionless). An example cited shows that an S02 plume exposed
to a rainfall rate of 10~4 cm s"1, with a Henry's law constant of 3.3 x 103,
results in a velocity of 3.33 x 10~ cm s~ . Another example demonstrated
4
that a plume would have fallen 30 m after a time of 10 s^
Bolin et al. (1974) described the efficiency of the rainout
mechanism as dependent on (a) the efficiency of the microphysical processes
in the clouds and (b) the frequency of precipitation. The consumption of
-39-
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aerosol particles by the cloud drop varies greatly with particle radius; and
the absorption of gases to cloud elements depends on the chemical composition
of the cloud drops. More work is required to understand these processes
quantitatively but it is interesting to note that there is evidence of a
rather rapid transformation of sulfur dioxide into sulfuric acid in clouds
as long as the pH of the cloud drop water is significantly higher than 4
(Brosset 1973). It is clear, however, that the most important factor in
determining the efficiency of the rainout mechanism for removal of pollutants
from the atmosphere is the frequency of rains (Rodhe and Grande!1 1973). If
rainout were the only sink mechanism, the average residence time for a pollu-
tant in the atmosphere estimated in the Rodhe and Grande!! Swedish study
would be about 40 hours in the winter and 90 hours in the summer based on
seasonal precipitation data collected during a one-year period. Low intensity
precipitation periods were appreciably longer in winter than in summer. Rodhe
and Grande!! also derived the distribution function for the probability of
rainout of a pollutant released at an arbitrary instant.
Hales et a!. (1971) describe the complex processes of liquid
mixing in a falling drop and their relation to mass-transfer, obscured
further by film effects at the interface and by the possibility of
chemical reaction within the liquid (see Figure 3). Hales (1972) explains
that in order for pollutant gas molecules to be captured, they must first
migrate from the atmosphere to the surface of the liquid. They then may
pass through the liquid-vapor interface and, finally, migrate into the
liquid interior where they may react chemically or exist simply as unreacted
dissolved gas molecules. The existence of dissolved gas molecules results
in a concentration of gas in the liquid which, owing to the finite solubil-
ity of the gas, will be characterized by a vapor pressure. Thus, there wi'i".
be a tendency for gas molecules to desorb from the liquid and, retracing the
mechanisms by which they were captured, return to the atmosphere. The gen-
eral problem of modeling microphysical systems in the atmosphere is discusses
in Section IV as it relates to the pertinent microphysical phenomena of drop-
gas interactions.
-40-
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STREAMLINE
DROPLET SURFACE
CIRCULATION PATTERNS
,+r
CHEMICAL
REACTION
LIQUID-PHASE
TRANSPORT
INTERFACIAL
TRANSPORT
GAS-PHASE
TRANSPORT
Figure 3. A Falling Raindrop and the Associated Transport and Washout Mechanisms Acting Upon it
in the Atmosphere.
-------
Section IV
MODELS OF PLUME CHEMISTRY AND DEPLETION-DEPOSITION PROCESSES
The ideal chemistry model must be able to identify the principal
chemical reactions that occur in a plume and to explain the rates of change
of concentrations of the various components. Such a model generally com-
prises a set of differential equations, each of which expresses the conser-
vation of mass of a species. In an atmosphere containing numerous chemical
species, simulation of the chemical reactions involved could^be extremely
difficult due to uncertainty of what reactions take place, doubt about rate
constants, and nonlinearities in reaction rates. Add this to the mathema-
tical complexities of the nonlinear transport equations defining physical
behavior of plumes under the influence of complex terrain and one can
see why no one all-inclusive chemical reaction-depletion atmospheric
dispersion model is an operational reality at this time. The SO^-sulfate
mechanism, the rates of S02 transformation, the extent of reaction and
the final state are not well known at this time for operational modeling
of the S02 chemical reactions of a given power plant plume, although
the basic chemical kinetic features of a reactive plume from a large
point source are now receiving attention. The research trend up until
now has been to focus on individual conversion mechanisms for S02 to
sulfate.
Takahashi et al. (1975) proposed a purely theoretical kinetic
model of sulfuric acid aerosol formation from the photo-oxidation of
sulfur dioxide vapor based on thermodynamic principles. The three stage
mechanism consisted of 1) the photo-oxidation of sulfur dioxide to
sulfur trioxide, and through the combination with water molecules, the
formation of sulfuric acid vapor; 2) the combination of several numbers of
water molecules and sulfuric acid resulting in the nucleation of sulfuric
acid vapor to a critical sized cluster or embryo; and 3) the condensation
of sulfuric acid and water molecules, as well as through the coagulation
with other particles to allow growth of the nucleated embryo to a larger
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aerosol particle. The riucleation rate for a system of water and sulfuric
acid vapor was calculated at various relative humidities according to the
expression:
I = C exp (-AG/RT)
where
C is a factor containing the probability of capture of
a molecule of vapor by a critical embryo
AG is the free energy to form an embryo
R is the gas constant
T is the absolute temperature
The nucleation rate was found to increase rapidly with an increase of
sulfuric acid vapor pressure, and was relative humidity dependent.
Equations were formulated for the number concentration of sulfuric
acid vapor and particles, the time change of the number of sulfuric acid
molecules in a particle, the deposition rate of sulfuric acid vapor to
already formed aerosol particles, and the equilibrium particle radius.
Generally the sulfuric acid vapor molecule concentration was found to
increase rapidly at the initiation of irradiation and then slowly decrease
after reaching a maximum value, determined by the photo-oxidation rate.
Indications were that nucleation was predominant in the early stage of
the process, with vapor condensation becoming significant in the later
stage. In the early stages when nucleation was predominant, the vapor
concentration was not always high enough for effective condensation to
occur, leading to growth in particle size. However, in some cases of
low relative humidity, the nucleation rate was very small so that most of
the sulfuric acid vapor condensed to make the particles grow larger.
Thus, the relative humidity and the acid vapor oxidation rate characterized
the rate of growth of particles. Although its time change is very slow,
the sulfuric acid concentration in particles increased, as the condensation
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of vapor became effective. Unpublished experimental data by Cox (1973)
and Takahashi et al. (1975) indicated that particle number concentration
at a relative humidity as low as 10 percent or less was much higher than
that predicted by this kinetic model; differences were partially attributed
to uncertainity in the calculated values of the nucleation rate. At such
a low relative humidity, natural ionizing radiation may play a significant
role in nucleation of sulfuric acid embryos (Vohra et al. 1972). Problems
mentioned by Takahashi et al. (1975) concerning aerosol modeling include
the proper assumption for the particle size distribution change with time,
and the effect of wall deposition and air flow pattern in future model
applications to chamber studies.
Davis (1976) proposes the possible key reactions of the S02 con-
version to sulfate via OH radicals (see Table 8) to explain the ozone bulge
noted in the study of the Morgantown Power Plant (see Section V).
TABLE 8. KEY REACTIONS PROPOSED FOR THE SO2 CONVERSION TO SULFATE VIA OH RADICALS *
(a) S02 +
(b) HS03
(c) HS03
(d) HSO5
(e) HSO5
(f) HS04
(g) HS04
(h) HSO4
(i) HS06
(j) HS06
OH + M
+ O2 + M
+ HO2
+ NO
+ H02
+ H02
+ HS04
+ 02 + M
+ NO
+ SO
-*• HS03 +
+ HSOS +
•*• H2S03 -
-> HS04 +
M
M
t-02
NO2
•*• H2S05** + 02
-+ H2S04 -
-> H2S208
-»• HSOg +
-> HS05 +
-»• HSOr +
*-o2
**
1
M
N02
SO,
5
* It should be noted that the detailed kinetics of only reaction (a.) have been well-studied in the
laboratory. Chamber studies, however, indicate that some combination of processes (b)-(j) are
quite fast compared to (a).
** Both H2SO$ and H2S2Og are known to hydrolyze in the presence of water to form H2SO4.
-44-
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The HSOg, proposed to be produced when 02 is added to HS04, oxidizes two
NO molecules to reform HS04 which then repeats the process. This NO oxida-
tion is postulated to explain the ozone bulge (Davis et al. 1974), but has
been disputed in the literature.
Systems Applications, Inc. (Liu 1975) has developed a chemical
kinetic model to study the basic features of a reactive plume from a large
point source. The Reactive Plume Model (RPM) handles the transport and
diffusion processes in a simple fashion, while the chemical kinetics are
given more detailed attention. The considerations which were taken into
account to arrive at such a model format included the exceedingly complex
photochemistry prevalent in plumes where NOX/HC ratios are high and the
difficulty of solving nonlinear transport equations as compared to the
ordinary controlling differential equations which describe the chemical
kinetics. The following is the derived modeling equation for a plume
along a wind trajectory:
where the rate of production of species i through chemical reactions is
represented by the first term oh the right-hand side, and the rate of
dilution of pollutant species i within the plume along the trajectory
path (x) due to horizontal spreading (w) and the variation in mixing
depth (h), respectively are represented by the second and third terms
on the right-hand side. A simple mass balance for the pollutant species
characterizes the model equation.
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A modified version of the Hecht-Seinfeld-Dodge kinetic reaction
mechanism (1974) for photochemical smog formation, employing five reaction
steps involving S02, describes the rate of change of species i due to
chemical reactions. These S02 reactions include:
H02 + S02 •> OH- + S03 where k = 1.3 ppm'1 min'1
S02 + N03 + NOo + SO, where k = 10.0 ppm'1 min'1
I
RO^ + S02 # RO- + S03 where k = 1.5 ppm'1 min'1
RCO^ + S02 + (02) -> R0£ + S03 + C02 where k = 1.5 ppm"1 min'1
OH- + S02 & [HS03J 52 H02 + S03 where k = 900 ppm'1 min'1
Uncertainties about atmospheric S02 chemistry dictate that these
reaction steps remain provisional until the appropriateness of the
S02 mechanism is better asserted by results of planned field studies.
The reactive plume model is found to be especially sensitive
to variations in four of the input variables, specifically 1) plume
geometry as a function of downwind distance; 2) pollutant emission
rates; 3) ultraviolet radiation flux and; 4) the initial concentrations
of NO, N02, S02, 03, and hydrocarbons.
Huang et al. (1974) followed the methodology of Hazbun et al.
(1971) in the Tennessee Valley Authority's derivation of a kinetic model
designed to provide a detailed account of the rates of change in concen-
trations of the various components resulting from their chemical inter*
actions and the effect of meteorological conditions upon these reactions.
The Hazbun approach was to construct profiles by measuring the concentra-
tions of the various species under differing meteorological conditions at
a number of downwind distances. On the basis of then available literature,
a kinetic model was devised for probable chemical reactions and various
-46-
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rate constants determined from experimental data were selected to formu-
late a set of rate equations. The set of rate equations was integrated
to give the theoretical profiles of the various components. The chemical
reactions possible (from a literature search at that time) were upwards
of 60; however, only 29 reactions were considered important from con-
siderations of the magnitudes of the rate constants and reactant concen-
trations. Unfortunately, exclusion of the OH + S02 reaction and numerous
updated values of the rate constants have basically antiquated this model.
Model validation studies have not as yet been conducted in the
atmosphere since data in sufficient detail to validate model predictions
in plumes are simply nonexistent.
A numerical model has been developed for predicting precipita-
tion scavenging of reactive pollutants from power plant plumes (Dana et
al. 1976). The Scavenging Model Incorporating Chemical Kinetics (SMICK)
can calculate collection, liquid-phase chemical reaction, and (possible)
desorption of multiple plume-bound pollutants as they interact with fall-
ing raindrops to be ultimately deposited on the ground. A schematic of
these physical interactions is shown in Figure 4. SMICK also possesses
the capability to perform calculations for any specific aqueous-phase
kinetics mechanism expressed in a proper subroutine form. Initial appli-
cations of the Scott and Hobbs (1967) aqueous-phase oxidation mechanism
for SOo (which depends strongly on the presence of dissolved ammonia)
showed that in comparisons of these predicted results with field measure-
ments, taken at the Centralia power plant (see Section V), the mechanism
is not adequate to account for the sulfate formation rates observed. To
calculate washout concentrations, the following equation is utilized:
-47-
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I
^
00
Volatile
Material
in Raindrop
c
o
•H
iJ
O
Nonvolatile
Material in
Raindrop
Figure 4. Schematic of Physical Interactions Modeled by SMICK (from Dana et al. 1976).
-------
where
c = the liquid phase concentration (moles/cm^)
z = distance above ground at stack base (cm)
Ky = overall mass-transfer coefficient (mole/cm^-s)
Vz = the terminal fall velocity of a raindrop of
radius a (cm/s)
a = radius of a raindrop (cm)
Y = mole fraction of pollutant a in local gas phase
H1 = solubility parameter (cm3/mole).
Provided values of Ky, Vz, Y, and H1, the above equation is numerically
solved for a number of selected drop sizes; the results are then distri-
buted according to the raindrop spectrum to obtain average concentrations.
In addition to performing this function, the computer code allows for a
number of sophistications including nonlinear solubility behavior, lofting
plumes and nonvertical rainfall.
A steady-state, one-dimensional mass transfer model describing
the dry deposition process has been developed by Sehmel et al. (1973) and
has been employed by Sehmel and Hodgson (1974) to predict particle deposi-
tion velocities to simple surfaces. The minimum experimental data required
to successfully employ the model are friction velocity, roughness height,
particle size distributions, and atmospheric stability. Conceptually,
the model is a three-box simulation of particle transport utilizing the
continuity equation. The three elements consist of:
Box 1 - The atmospheric turbulent layer in which the
transfer processes are best described by micro-
meteorological eddy diffusivity.
-49-
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Box 2 - A layer just above the vegetative canopy or
surface elements in which the transfer processes
are modified by the presence or structure of the
canopy or surface.
Box 3 - A layer (occupied by the canopy or surface elements)
in which the final transfer process is best expressed
by surface mass transfer coefficients, where the
interaction between the surface material and the
pollutant is important.
The basic model assumptions, as stated by Sehmel and Hodgson (1974) are:
1. Particles diffuse at a constant flux from a uniform
concentration of particles.
2. A relationship for particle eddy diffusivity can be
determined.
3. The effect of gravity can be described by the terminal
settling velocity.
4. Particle agglomeration does not occur.
5. The particles are completely retained by the surface.
With the basic model assumptions, the deposition flux to a surface is
described by
N = -U+D) - VtC
-50-
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where
N = deposition flux to environmental surface (particles cnr2s-"l)
e = particle eddy diffusivity (cm^s"')
D = Brownian diffusivity (cm^s-l)
C = particle concentration (particles cm~3)
z = height (cm)
Vt = absolute value of terminal settling velocity (cm s~l)
This equation is then integrated to provide integral diffusional
resistances, which may be related to deposition velocity. Predicted depo-
sition velocities were graphically presented for a 1 meter reference
concentration height, for stable and unstable atmospheres, with the
parameters for each figure being either friction velocity (range of 10
to 200 cm s'1) or roughness height (10~3 to 10 cm). In general, depo-
sition velocities increase with an increase in either friction velocity or
roughness height parameters. For all combinations of these two parameters
the minimum deposition velocities were found to occur in the particle
diameter range of 0.04 to 0.4um. Deposition velocities were determined
to be nearly independent of particle diameter only in the 0.1 to I.Oym
size range (note; this is the'accumulation mode" where 80 percent of
atmospheric sulfate is found (E.P.A. 1974)).
Horst (1974) developed a surface depletion model which improves on
the usual Gaussian plume model "source depletion" representation of loss of
airborne contaminant due to deposition. The Gaussian source depletion
approach appropriately reduces the source strength as a function of down-
wind distance. In the Horst approach, the air concentration at any location
-51-
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can be calculated as the sum of the nondepositing diffusion from a
primary source at coordinates (0,0,h) plus the diffusion from all of
the upwind surface sources which account for deposition. The derived
equation for the source depletion model considering the crosswind-
integrated air concentration is:
fX
7 (x,z) = IT (x,z,h) (QQ - JQ Vd x U, zd) d e)
where
x (x,z) is the crosswind integrated air concentration
Q0 is the undepleted source strength at x = o
Vd is the deposition velocity (constant of proportionality)
{ represents deposition coordinates at downwind point (x,y,z)
Zd is the reference height
D" is a diffusion function.
The Horst model and its predecessor were compared for a variety of
conditions to judge the efficiency of the source depletion model with respect
to the surface depletion model. Calculations were made for a variety of source
heights (2 m, 10 m, and 100 m) and for three Pasquill stability categories,
(A, D, and F). It was determined that the differences between the two models
increase with increasing stability. The source depletion model was charac-
terized by an overprediction of total deposition and a consequent under-
prediction of the remaining airborne material. In one case of moderately
strong deposition and stable conditions, the source depletion model for
parameters at a downwind distance of 10 km was found to be in error by
factors of 3 and 4. At greater downwind distances and in cases of stronger
deposition, these factors can increase substantially. The source depletion
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model was found to be entirely sufficient for low deposition conditions
but to increase in error for high deposition case. For example, for
Vd/u~ = 10~3 the source depletion model is in error by 10 to 20 percent
for three release heights and three stabilities; for V^/u" = 10~2 and
unstable thermal stability the greatest error is approximately 35 per-
cent.
Droppo (1974) described the types of input necessary for
developmental modeling of dry deposition processes on high and low
vegetation canopies. Four regimes characterize the model of overall
deposition over a canopy, these include 1) air layer above the canopy;
2) air-vegetation layer within the canopy; 3) the surface layers over
the individual plant elements and 4) inside the plant elements where
internal processes limit the rate of assimilation of certain materials
into the plant. With the assumption that atmospheric and canopy
characteristics can be expressed as height and time functions, the
functional relationship for a given site is:
r(z,t) = r^z.t) + r2(z,t) + r3(z,t)
where
r is the total resistance (to flow of material)
r-| is the resistance in the first regime
TI is the resistance in the second regime
r3 is the resistance in the third regime
There is no defined r^ since these processes will directly affect the
third regime.
The model input variables consist of a series of plant character-
istics and processes (functions of height and time). These are: Physical
Characteristics, such as canopy height, density of foliage, canopy
roughness and leaf area index; Physiological Characteristics, such as
-53-
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stomata openings and surface characteristics and finally Concurrent
Fluxes such as momentum, latent heat, and sensible heat. No quantitative
data were provided in the Droppo (1974) paper, but a series of qualitative
results were given. The deposition process is expected to differ
depending on physical characteristics of the canopy; greater amounts of
deposition will occur in the lower regions of the low canopies than
in higher less dense canopies. The upper portions of higher vegetation
canopies are the locations for significant deposition of material. Results
of diffusion, wind profiles, energy budget and physiological research
relating to vegetation canopies will be incorporated into this model
as it is developed.
-54-
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Section V
FIELD STUDIES OF PLUMES
Data have been gathered on plume transformation chemistry, and
theories have been developed and tested, in field studies of specific plumes.
The methods and results of a number of these studies are discussed below.
COLBERT POWER PLANT
In pioneering work of the use of the helicopter as a plume track-
ing tool, a limited field study program was undertaken at the TVA's coal
burning Colbert Power Plant to measure the rate of atmospheric oxidation of
S02 during different weather conditions (Gartrell et al. 1963). At that
time the plant, located approximately 20 km southwest of Wilson Dam, Alabama,
had four 200 mw units, each with a 97 m (300 feet) stack. About 70 percent
of the flyash was removed by mechanical flyash collectors. The reported
results of earlier laboratory studies were utilized in the planning of this
project to provide the investigators with a basis of comparison and indica-
tions of what to expect from the field study.
Sampling from a helicopter was designed to directly determine
S02 and 803 components, utilizing a filter paper separation technique.
(The instrumentation at that stage of the technology was not adequate for
making S03 or H2SO, measurements; unfortunately reliable measurements of
ambient HUSO^ levels are still not a reality even today.) Auxiliary in-
strumentation included the altimeter, a spring-wound clock, and in later
tests, wet and dry temperature probes. A voice recorder was utilized to
store data from these instruments for subsequent analysis. Measurements
were made during early morning inversion conditions for easy plume detec-
tion and presence of maximum S02 plume concentrations. Close to the plant
(1 to 2 miles), longitudinal flights were made along the center!ine of the
relatively narrow plume to shorten the total sampling time necessary for
each sample. Further away from the plant, where the plume was wider, the
-55-
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helicopter conducted replicate flights across the center!ine of the plume
in progressive sections. A data summary of the flights (Table 9), taken
from the Gartrell et al. (1963) study, depicts the range of S02 oxidation
rates encountered for a variety of different time, distance, and relative
humidity conditions. The oxidation rates are seen to range from 0 to
3.7 percent for measurements characterized by low humidity, to rates of
21 and 32 percent for humidities close to 75 percent. An S02 oxida-
tion rate of 55.5 percent for a time of 108 minutes was observed in one
sample run. A common starting time for calculating oxidation rates was
not a condition of the sampling methodology.
TABLE 9. SO, OXIDATION STUDIES COLBERT STEAM PLANT PLUME (from Gartrell et al. 1963)
Dace Sample
1960 No.
8/2 1
2
3
£_
9/2 1
2
10/14 1
2
10/26 1
2
10/2S 1
2
5/3 I
2
3
8/19 1
2
10/11 1
2
Travel from Point of Emission
Time
(min)
Distance
(miles)
Relative
Humidity
in Plume
(Percent)
"Low Rates"
5
5
5
15
30
78
12
60
6
84
12
84
13
13
13
108
23
12
96
.25-1
.25-1
.25-1
1-1.5
2-3
8
.5-1.5
5-6
.25-1.25
8-9
.5-1.5
8-9
62
54
45
48
68
L_ 70
"Hi?h Rates"
1.1
1.1
1.1
8-10
.75-2
.5-1.5
8
74
73
SO 2
Oxidation
(Percent)
0
0
1.20
0
3.70
2.20
2.15
3.23
1.50
2.70
1.10
4.10
13.80
10.00
19.20
55.50
3.00
21.60
32.00
-56-
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Gartrell et al. (1963) state that the limited data obtained in
their study do not provide a basis for accurate estimates of the post-
emission absolute rate of S02 oxidation. The investigators postulate
that the dominating factor controlling the oxidation rates was moisture
within the plume or ambient strata; especially at relative humidity
values greater than 70 percent. Newman et al. (1975) contend that upon
closer inspection of the Gartrell et al. (1963) data, a dependence on
humidity is not that clear, and attribute the observed high S02 oxidation
rates to very high particulate loading in the plume.
The TVA's Colbert plant was selected once again for the collection
of data on the characteristics of a coal-fired power plant plume, including
the chemical constituents and their concentrations with respect to downwind
distance (Huang et al. 1974). A kinetic model of plume chemical interac-
tions was developed from data gathered, and has been described in Section IV.
At the time of the field program late in 1972, the plant was characterized
by a 980 mw capacity, with three stacks, two at 93 m (300 feet) and one at
152 m (500 feet).
A Sikorsky model S-58 helicopter was equipped with instruments
designed to measure sulfur dioxide, total sulfur, nitrogen dioxide, nitric
oxide, ozone, nitrous acid, toluene, particles, humidity and temperature
for the project duration. During the program, the helicopter would leave
the TVA airport in Muscle Shoals, Alabama, at sunrise and fly to the
vicinity of the Colbert plant. During the flight to the steam plant the
background (ambient) concentrations of the pollutants were measured. Upon
arrival at the plant a vertical temperature profile was made to -an altitude
of 1200 m (4000 feet). Once the temperature profile was completed, plume
measurements were initiated across the plume centerline at predetermined
distances downwind. Samples were taken and continuous gas analysis was
started when the helicopter entered the plume. A minimum of 60 seconds
in the plume was allowed for plume sampling; circling within the plume
was required for samples in the narrow portions of the plume. To allow
-57-
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the sampling probes to function properly, an airspeed of near 35 knots
was maintained during a sampling run. The sulfur analyzer was continuously
operated and utilized as an indication of plume boundary location. Temp-
erature and dew point were measured simultaneously with the pollutants, and
before entering the plume. The plume traverses were continued until the
morning inversion was completely broken and the plume was too dispersed to
measure. A second vertical temperature profile was made at the end of the
sampling periods. The traverses of the plume by the helicopter were made
at downwind distances of 0.8, 1.6, 3.2, 6.4, and 9.5 kilometers. The plume
was traversed at distances greater than 16.1 km (10 miles) when the atmosphere
was extremely stable. Measurements of ground level meteorological para-
meters, specifically dry and wet-bulb temperatures, solar radiation, and wind
speed and direction were made concurrent with the airborne studies. Winds
aloft were determined by releasing and tracking pibals.
The gathered experimental data were utilized to construct concen-
tration profiles. Theoretical concentration profiles were constructed by
the integration of 35 rate equations for the kinetic model chemical reac-
tions assumed to be the most likely to occur. These theoretical profiles
were altered by varying the rate constants until the resulting profiles
agree as closely as desired with the observed profiles, and these were
taken to be a likely description of the plume chemistry. However, it appears
that the 35 reactions used by Huang et al. form a weak representation of
reality. The reactions of radical chemistry were poorly represented; rate
constants could be estimated with an accuracy of no more than an order of
magnitude.
Due to the suspected inaccuracy of the'experimental results, the
overall rate of oxidation of S02 could not be determined from either (1) the
change in concentration of SC>2 by separating the effects of dispersion and
oxidation or (2) a comparison of concentrations of S02 and sulfates. S02
oxidation rates appeared to be slow in any event. The authors expressed
the desire that the results in their report should not be accepted as quanti-
tative due to the limitations involved.
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CRYSTAL RIVER POWER PLANT
A series of investigations at the Florida Power Corporation's
coal burning Crystal River Power Plant, utilizing aircraft, had a goal
of testing a method for evaluating S02 decay rates in the plume
(Stephens and McCaldin 1971). The 375 MW capacity plant has a single
152 m (500 ft.) discharge stack and is situated on the flat Gulf Coast
plain 129 km (80 miles) north of Tampa, Florida.
To differentiate between a decrease in S02 concentration due to
atmospheric dispersion processes and a decrease in concentration due to
oxidation or any other chemical or physical decay factors, a technique
believed by Stephens and McCaldin (1971) to be "conservative" in nature
was utilized. The technique consisted of a comparative ratio of sulfur
dioxide concentration to a conservative tracer concentration (consisting
of submicron-sized particulate matter emitted from the stack); supposedly
the ratio of particulates to S02 would approximate a constant as the plume
aged and diffused, if the SOp were conservative. The ratio would increase
as the plume ages if the S02 demonstrates a measurable decay rate. Such
a procedure has been challenged by Friend et al. (1972), who explained that
particulates cannot be used as a conservative tracer since the sulfate
particles formed as a result of the oxidation of sulfur dioxide can sig-
nificantly contribute to the total mass.
It was determined that the time required for the decay of one-
half of the sulfur dioxide indicated a first order rate equation for the
reaction. The range of SO- loss rates were described to vary from a
negligible value at low relative humidity (30-40 percent), to a half-life
of approximately 140 minutes at medium relative humidity (40-50 percent),
and 70 minutes at high relative humidity (78-80 percent).
MORGANTOWN POWER PLANT
Field studies at the Morgantown Power Plant of the Potomac
Electric Power Company were conducted by Davis et al. (1974) to determine
-59-
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the extent to which NOY and S02 chemistry occurs as a function of distance
/\
from the emission source. The plant, which burned an oil-coal fuel mix-
ture (75 percent residual #6 oil and 25 percent coal) is located 64 km
(40 miles) south of Washington, D.C., and has a 1250 MW capacity. The
collection of field data, which took place from October 1973 to August
1974, was accomplished through use of a single engine Cessna 205 airplane
early in the program and a Cessna 172 later in the program. Extensive
field data were collected from instrumentation aboard the Cessna 205
aircraft which included a Monitor Labs 8410 chemiluminescence ozone
analyzer, a Monitor Labs 8440 chemiluminescence NO-NO--NO analyzer and
£• J\
a Meloy SA-160R flame photometric SOp analyzer. Instruments designed to
monitor CO and hydrocarbons were also employed. Horizontal cross-
sectional traverses were made of the plume at altitudes ranging from 213
to 914 meters at various downwind distances up to 56 km. A major obser-
vation in the field program was the depletion of ambient ozone within the
plume near the stack at distances of 5 to 8 km, and an increase in ozone
at 50 km downwind with a decrease in plume S02. The plume depletion
features close to the plant are illustrated in Figures 5a and 5b from the
Davis et al. (1974) report. An increase of roughly 30 percent over ambient
ozone levels was noted at 56 km, but the profiles at this downwind distance
were not available. The details of the atmospheric chemistry theory pro-
posed by Davis et al. (1974) to explain the net generation of ozone in
plumes have been described earlier in Section IV and will not be repeated
here.
KEYSTONE AND NORTHPORT PLANT
The extent of oxidation of sulfur dioxide to sulfate was examined
in a study of the Pennsylvania Electric Company's Keystone coal-fired power
plant by Newman et al. (1975a), and the Long Island Lighting Company's
Northport, New York oil-fired power plant by Newman et al. (1975b). The
Keystone Plant generates a total of 1800 MW using two boilers-whose units
are equipped with electrostatic precipitators rated at 99.5 percent efficiency,
-60-
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CO
o
Q_
Q.
60-
40-
20-
0-
60-
40-
20
0
60-
40-
20-
0.
1300 EST 23 NOV., 1975
1.6 KM OUT
1900 FT ALT
4.8 KM OUT
1900 FT ALT
11.2 KM OUT
1700 FT ALT
16 KM OUT
1700 FT ALT
202
Kilometers (Approximate)
Figure 5a. Profiles of Crossplume Ozone Concentration at Various Elevations Downwind
of the Morgantown Power Plant (from Davis et al. 1974a).
-61-
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2 ppm
so2 o3
1 ppm 50ppb-|
22 JLINR, 1974
14SO-1500 EST
S09
1.6 KM
TOO-,
50-
0-
24 KM
SO
2 ,_-''
TOOn
50-
0
SO,
32 KM
'
-1 KM-
Figure 5b. Profiles of Sulfur Dioxide and Ozone Crossplume Concentrations at 600 m Altitude
at Various Downwind Distances from the Morgantown Power Plant (from Davis et al. 1974).
-62-
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each unit served by a 244 m stack. At Northport, the 760 mw plant
operates with two boilers, each with a 183 m stack. Mechanical cyclone
type precipitators on these units have a rated efficiency of 85 percent.
In both plant studies, a single-engine Cessna 182 was utilized
to obtain plume samples for isotopic ratio and concentration measurements
of the 32S:34S naturally occurring in the fuel sulfur. The aircraft was
instrumented with a hi-vol for sampling of sulfate (on fiberglass filters)
and S02 (on alkaline impregnated filters), an electroconductivity S02
analyzer for locating the plume, and auxiliary equipment for measuring
temperature, relative humidity, altitude, airspeed and aircraft location.
Background measurements of S02 and sulfates were obtained upwind of the
plume at plume altitude and repeated crosswind traverses at a minimum of
five distances (when feasible) were made in the plume during data gathering,
Sulfate sampling under the plume was conducted at two distances downwind.
Concentrations of the conservative tracer SFs, compared to S02
as a function of distance, were used to assess the S02 losses by oxidation
and deposition. Hi-vol samplings of sulfate particulates were so small
that a reliable analysis could not be conducted to fit the data obtained
to the oxidation process postulated for both coal and oil-fired plumes;
measurements of S02 converted to sulfate were therefore based upon the
assumption that reductions of S02 concentrations greater than those found
for SFs should be attributed to conversion to 504.
The fraction of S02 converted at measurement points in the plume
downwind from the stack was also calculated from the expression:
(1 - f) = 6S°2 (fue1) ~ 6S°2
1000 a
where
f = the fraction of S02 converted
6S02 = the "del value" which is a measure of deviations
of the ratio 32$/34s at the point of measurement
from a standard isotopic sulfur ratio for
32$/34s of 22.210
-63-
-------
a = k-1, where k is the equilibrium constant between
S02 and S03 for the reaction
34so2 + 32so3 5 32so2 + 34so3.
Eleven standard plume sampling runs were made at the Keystone
power plant. The S02 concentrations were measured in the flue gas at
the stack and at six downwind distances. The data obtained on these runs
were combined, normalized and averaged with the results summarized in
Table 10.
TABLE 10. NORMALIZED AND AVERAGED DATA FROM 11 PLUME SAMPLING RUNS MADE AT THE
KEYSTONE POWER PLANT (from Newman et al. 1975a)
Downwind Distance from Plant (km)
6 * (del value)
Percent SO conversion to SO based on
6 calculations
Percent SO- conversion to SO based on
' 4
measured concentrations
0.0
2.8
0.0
0.0
0.8
2.5
1.3
3.0
1.6
2.1
3.6
4.1
3.2
1.9
4.4
3.6
4.8
2.2
3.0
1.8
16.1
2.2
3.0
0.3
48.3
2.1
3.5
0.0
* The del values are normalized to the average flue gas value of 2.8.
The percents of S02 conversion shown in this table refer to con-
version in the atmosphere after leaving the stack, and to a common base of
0.0 percent at the stack. Reductions in conversion with increasing distance
were attributed to removal of 864 from the atmosphere after its formation.
The following observations were obtained from these data:
1. The indicated decrease with distance in percent
SO;? converted based on concentration supports
evidence for the apparent dropping out of sulfate
from the plume.
2. The percent 862 conversion to sulfate based on isotopic
ratio measurement tends to attain a steady value of 3
to 4 percent, which is independent of time or distance;
however, the authors point out that super-imposed on
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these percentages, which are depicted as a function of
distance, are wind velocities that ranged over a factor
of four. Part of the variability may be associated with
a large range in the isotopic ratio of the sulfur in the
coal at the plant.
The low S02 conversion rates of less than 5 percent are attri-
buted to a rate limiting process. This is supported by the low particulate
content of the Keystone plant plume, on the order of 105 yg/m3.
A calculated S02 plume oxidation half-life of 10 hours was
derived from a humidity-independent second order kinetic expression,
and from a resulting rate constant of 1 pprrT1 hr"1. The oil-fired plume
had no reported particulate loadings. Vanadium particulates, originating
from fuel oil were postulated (by Newman et al. 1975b) to be the catalyst
in a mechanism whereby the S02 is in equilibrium with water on these par-
ticulates and catalytic oxidation subsequently occurs. Newman et al.
(1975a) did not postulate a similar mechanism involving Fe2C>3 in the
coal-fired particulate matter. The noted highly variable S02 conversion
in both oil- and coal-fired plumes and the lack of a definite correlation
of the percent S02 conversion with relative humidity are major research
outcomes of these studies.
FOUR CORNERS PLANT
Further evidence for low S02 oxidation rates in coal-fired
power plant plumes was provided by the University of Utah Research
Institute (1974) in a study carried out at the Four Corners power plant
in New Mexico. The program objectives were to measure the degree of
S02 conversion and to evaluate its impact on concentrations of atmospheric
sulfate and visibility. Parameters measured to determine the conversions
of S02 to particulate sulfate included particulate ammonium sulfate;
upwind and downwind difference in concentrations compared to ambient
S02, and the speed of wind. One test resulted in a 0.37 percent hr-1
conversion of $62 to particulate sulfate at about 21 km downwind, and
relative humidities of 30-51 percent. Another test resulted in a 0.76 per-
cent hr~l conversion 21 km downwind with relative humidities of 26-40 per-
cent. Conversions of 0.45 percent hr~^ at about 60 km downwind were noted
-65-
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in this same test. Both tests were characterized by S02 concentrations
ranging from 0.01-0.06 ppm. This study possibly provides a more realistic
measure of plume S02 oxidation rates due to the fact that the S02 rates
of oxidation provided are the result of direct, positive sulfate analysis
and not deduced from the comparison of S02 loss with plume tracer mater-
ial.
MISTT PROGRAM
Extensive studies involving three-dimensional mapping of large
plumes from multiple urban sources were accomplished in the St. Louis area
as part of the Regional Air Pollution Study (RAPS) and the Midwest Inter-
state Sulfur Transport and Transformation Study (MISTT) during the summers
of 1973-1975 and during February of 1975 (Wilson et al. 1976). These
studies challenged the adequacy of the usual technique of calculating the
fractional conversion of S02 to sulfate from S02 concentration and sulfate
data obtained by aircraft flying through the plume on a variety of flight
paths. Based on observational data 'of typic'al cross-plume profiles of
Aitken nuclei concentration, light scattering aerosol, S02 and ozone, it
is believed that the S02 conversion rate ifc different in various parts of
the plume, the higher rates favoring the plume edges rather than center,
although this difference is not evident in all cases. The conclusion
reached by Wilson et al. (1976) is that measurements of S02 and sulfate mass
flow rates in the plume are the best indicators of conversion rate.
Aircraft mapping of the plumes of the 2400 MW Labadie plant,
emanating from three stacks 213.5 m (700 ft) tall located west-southwest
of St. Louis was conducted out to plume distances approaching 100 km. The
basic concept was the measurement of the gaseous and particulate sulfur
flow rates at increasing distances from the source to derive pollutant
budgets and ultimately to allow estimates of the rate at which trans-
formations and removal processes act on the individual pollutants. The
coordinated efforts of two instrumented aircraft, an instrumented van
and three mobile pilot balloon units were utilized in the plume mapping
program.
Plume aerosol volume flows were calculated from aircraft size
distribution measurements with an electrical aerosol analyzer and an
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optical particle counter made during horizontal passes and from upwind
spirals through the plume. The results of plume aerosol flow rates and
calculated sulfur conversion obtained for one test day showed little
sulfate formation in the first 30 km, and a conversion of some 2% hr
in the region 30-50 km from the source. An important observation of the
MISTT studies is low SOT formation until NO is converted to NO,, and then
subsequent higher S07 formation: this stresses the necessity of long
distance measurements from the stack source, since at closer distances
(i.e., 16-32 km) only the stack sulfate, derived from $03 from in the com-
bustion process, may be present.
S02 DEPOSITION STUDIES
Pacific Northwest Laboratories (Nickola and Clark 1974) designed
a technique for field measurements of deposition of a particulate plume by
tracking with an inert gas plume. The inert gas, Krypton-85, was simulta-
neously released with fluorescent zinc sulfide particulate tracer from a
single site 26 m in elevation; the air concentrations of both plumes were
examined downwind of the release point. The mass median diameter of the
zinc sulfide was 5 ym; removal processes such as impaction and gravitational
settling became important if the particle diameter was greater than this
value (Dabberdt and Smith 1975). The deposition process can be credited
with the expected deficit of mass in the particulate plume as both chem-
ically stable plumes travel downwind; an estimation of the amount of
zinc sulfide deposited was conducted by a graphical study of the differ-
ences between the two vertical profiles of concentration near the ground.
The field sampling grid was comprised of more than 600 filters at 1.5 m
height placed about the tracer release tower in a concentric fashion, on
sectors of six arcs at distances of approximately 0.2, 0.4, 0.8, 1.2, and
3.2 km* from the point of release. The results showed that under moderately
* Arcs at 0.8, 1.6 and 3.2 km were actually concentric about a point 100 m due south of the release
point. Thus, the distance from release point to arc varies with azimuth.
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stable conditions about 7 percent of the particulate plume was deposited
during passage to a distance of 842 m. Zero to 1 percent of the particu-
late tracer was deposited within 812 m from the release point under more
stable atmospheric conditions. Approximately 1 percent of the zinc sulfide
tracer is estimated to have touched down within 200 m of the source during
a case characterized as flat with sagebrush and steppe grasses dominating
the region. _ ____ __ ______
Garland~~(T974)~has measured the rate of S0? dry deposition both
35
in the field and in the laboratory using S02, a radioactive tracer as
one technique and the concentration gradient method as the other. Several
of the field efforts, utilizing the radioactive tracer method to measure
the deposition velocity to grass, consisted of releasing a small amount
of S02 containing 50 millicuries 35S over a 30-minute_period at a
height of 1 m, 50 m upwind of an arranged sampling area near the center
of a large field. S0? concentrations were determined by air bubbler
samplers mounted above marked plots. Exposure samples of the grass were
35
removed from several of these plots and the uptake of S was resolved by
transferring the total extracted sulfate to a liquid scintillation counter.
The results were interpreted as a deposition velocity defined by:
y _ radioactive particle counts in grass per unit ground area
y concentration in air X time of exposure
The gradient method, when used to determine the flux of S02,
assumes that momentum, heat, and S02 fluxes are constant with height over
the range of measurement. An area of uniform surface extending for at
least 100 times the measurement height upwind is required. The diffus-
ivity of momentum or the diffusivity of heat are used as approximations
of the flux of S02; the diffusivity for momentum was used in the Garland
(1974) study. These diffusivities are defined as:
Km = l or Ku =
4> H
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where
Km = diffusivity of momentum (m^ s~l)
KH = diffusivity of heat (cal m'1 s-1)
k = von Karman's constant, an empirical constant of value 0.4
u* = friction velocity defined in terms of shear stress
on the surface due to the wind (nrl)
z = height above surface (m)
d = reflects the effect of tall crops in elevating the height
of the momentum sink above the surface (m)
n
n
the dimension! ess shear of momentum
the dimensionless shear of heat.
The results of three sets of field measurements of the dry
deposition of S02 over grass vary depending on the time of year and
characteristics of the canopy. For short grass in March and June, the
mean Vg was 0.55 cm s"U medium grass (radioactive method) in June-November
gave a mean Vg of 1.19 cm s~^ ; using the gradient method from November -
January over medium grass gave a mean Vg of 0.77 cm s~T. Over long grass,
using the radioactive method, a mean result of 0.85 cm s~1 was found; this
compares to a Vg of 0.61 cm s~^ for 14 measurements reported by Shepherd
(1974), and to the deposition velocity of 0.4 cm s over a wheat field
found by Fowler and Unsworth (1974).
Most of the research concerned with the absorption of atmo-
spheric water soluble gases by vegetetion has been conducted using
environmental chambers constructed to simulate different field environ-
ment conditions. In the research of Hill and Chamberlain (1974), two
environmental chambers were designed that accomplished this simulation
by controlling wind velocity, ($2 concentration, temperature, relative
humidity, light intensity, and pollutant concentration. The standard
alfalfa canopies used in the chamber were found to result in S02 and N02
concentration profiles that indicated that these substances were
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efficiently removed from the atmosphere by the surface and subsurface
vegetation of the upper portion of the plant canopy. The rate of
pollutant removal by vegetation was shown to be affected by wind velocity,
canopy height, and light intensity, with the removal rate of pollutant
found to increase linearly with the increase of pollutant concentration
until those pollutant levels when serious physiological affects take over
the plant. Due to the lack of actual field data, the authors describe a
hypothetical situation in which they attempt to evaluate the role of vege-
tation as a sink for air pollutants in any specific area. A large coal-
fired power plant was assumed to produce average ambient S02 concentrations
of about 6 ppb within 32 km of it. Vegetation cover in this area, with
an average S02 uptake rate equivalent to that of a 40 cm alfalfa canopy
(17 yl min"1 nr2 pphnr1) during daylight hours, would give an uptake rate
of 1.24 x 105 kg/day in the 32.5 x 108 m2 within 32 km of the plant. This
would be 49 percent of the S02 emitted from the plant; the percentage removed
would be greater if the area were a forest, or less if the vegetation were
scarce. The authors bring up an important point, relating the conversion of
S02 to 864 in the atmosphere to vegetation pollutant sinks. The low conver-
sion rate of 862 to S0| particles, in rural areas where photochemical smog
reactions are less important, will result in high percentages of S02 removed
by vegetation before it is oxidized to sulfate, if vegetation acts as an
S02 sink.
PRECIPITATION SCAVENGING STUDIES
To assess the extent of depletion from a diffusing plume, an
experimental investigation was conducted at the Keystone Power Plant
(located in a polluted area) as part of the LAPPES program to measure the
sulfur dioxide washout by natural precipitation (Hales et al. 1971).
Precipitation samplers were strategically placed on a surface sampling
line which extended across the overhead plume. The compound of primary
interest was S02, although sulfate, nitrite, and nitrate levels were also
determined from selected samples. Samples were delivered to a mobile
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laboratory for analysis as quickly as possible upon their collection.
The mass of water-soluble particulate recovered from each precipitation
collector indicated the degree of pollutant deposition from the plume through
precipitation scavenging and dry deposition. The sulfate and nitrate results
exhibited a large amount of scatter which according to the authors was suspected
to have arisen due to artifacts within the samples. The sulfur dioxide distri-
butions observed in the experiments were not sufficiently well-defined for the
calculation of washout coefficients. Observed values of S02 washout flux were
stated to be less than theoretical estimates by factors of 50 to 100. Notable
was the absence of a positive correlation of washout sulfur dioxide concentration
with the location of the Keystone plume, attributed to the plume shallowness
at short distances from the source which permits the least time for sulfur
dioxide sorption and the greatest time for its desorption. In several cases,
a negative correlation of washout beneath the plume was discovered. The
results indicated that the S02 scavenging process was more complicated than
the investigators had suspected. The conclusions reached, based on the data
obtained, were as follows:
1. The actual removal rate of S02 from the plume by the washout
mechanism from the stack source at 244 m (800 ft) above ground level was
up to two orders of magnitude less than predicted by preexisting theory.
Washout fluxes in rainfall and snowfall were clustered about a value of
10 ymoles (m2 hr)~l for precipitation rate normalized to 1 mm hr-1.
2. A factor of two difference was noted from the relationship
between the observed washout and precipitation rate and the relationship
derived from preexisting theory. More efficient washout by large drops
than is predicted by preexisting theory is attributed to the larger
exponent of precipitation rate in the observed relationship.
3. Increases in washout concentrations of SOo were apparent
with distance from the stack. Lateral diffusion of the plume, according
to preexisting theory, is responsible for decreases with distance of con-
centrations proportional to the vertically integrated mass of sulfur
dioxide.
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4. Washout of background sulfur dioxide concentrations
appeared to interfere strongly with plume-washout observations. Remote
sources about 20 km to the southeast of the Keystone plant contributed
heavily to the S02 washout observed.
5. There were indications that sulfur dioxide washout was
inhibited in some way by the Keystone plant or that the S02 was converted
to sulfate. A depression of S02 concentration observed in precipitation
collected beneath the plume under certain circumstances when the washout
of background sulfur dioxide was relatively high supported this.
Three experimental studies of the washout of power plant
effluents were conducted at the State of Washington's Centralia coal-
fired plant (Dana et al. 1975, 1976). The Centralia stacks have a
height of 143 m above the ground. Heavily forested hills, rising to
within 50 km of the stack tops comprise the rugged terrain within a few
kilometers in the normal downwind direction, however no problems were
encountered in site selections for precipitation sampling. In contrast
to the washout studies performed near the heavily polluted Keystone plant,
the effects of the Centralia effluent plume were isolated by rain con-
centration measurements. The effect of the plant on local levels of S02»
SO^, and H was determined from laboratory analysis of the precipitation
samples. The rain sampling positions were concentrated in accordance
with the predominant wind direction during rainfall at Centralia. Ninety
fixed collection sites were arranged to reflect the prevalent area under
the plume at distances from 0.4 to 11 km downwind in the latest experi-
ment study. A scavenging "plume" of SOp on the ground sampling array
was clearly indicated. The measured scavenging rates for SCL as a
function of downwind distance show rates of 3.0 and 27.0 (gram-moles/cm
9 Q
s) x 10 at 0.4-0.5 km downwind, 100 and 42 (gram/mo!es/cm-s) x 10
g
at 4.5 km, 54 (gram-moles/cm-s) x 10 at 7.0 km and 115 ar.d 129 (gram-
n
moles/cm-s) x 10 at 10-11.5 km downwind. Sulfate molar concentrations
were, in general, not large compared with those of S02, so a deposition
plume for sulfate was difficult to define, except for some discernible
plume-related sulfate deposition at downwind distances greater than about
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7 km. No correlations were found between locations of the power plant
plume and the measured concentrations in rainwater of ammonium, nitrite,
nitrate, or phosphate ions.
Along with the involved field experiments at the Centralia
plant was a research program directed at the development of a numerical
model for predicting precipitation scavenging of reactive pollutants
from power-plant plumes. The model is described in Section IV. Compari-
sons of the calculated results were conducted using the model with experi-
mental data employing the liquid phase ammonia (Scott and Hobbs 1967)
mechanism. Indications were that the mechanism is insufficient to explain
observed sulfate formation rates in the tested plume and raindrop situations.
The primary conclusions drawn from the Centralia field program
were: (1) At downwind distances less than about 10 km, sulfate concen-
trations in rainwater due to the presence of the plume do not appear
significant. (2) Ammonium, nitrate, and soluble (ortho) phosphate ion
concentrations in the samples obtained were close to normal background
levels. (3) SCL and hydrogen ion were the only measured species which
showed plume related deposition patterns at all distances from 0.4 to 11 km
where samples were collected.
In a different investigation seasonally averaged sulfate concentration
in rainwater samples collected near the TVA Colbert plant were analyzed by con-
verting the data to isopleths of sulfate washout at 17 sampling sites located
in an area 1.6 to 25.7 km (1-16 miles) from the plant (Hutcheson and Hall 1974).
The washout process was defined as the mass of sulfate per unit area per inch
of rain. The washout pattern was found to be skewed in the general direction
of maximum precipitation rates. Unlike the Keystone power plant, the Colbert
plant is situated in an area of low background atmospheric sulfur concentration
and has stacks only 91.5 m (300 ft) high. Little S02 washout compared to the
sulfate aerosol washout was observed at Keystone; at Colbert, the major features
of the washout pattern derived from the data cannot be accounted for by sulfate
aerosol scavenging but can be accounted for if most of the washout is attributed
to S02 scavenging. Kale's (1971) theoretical argument is supported by this
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study, which indicates that the background acidity of rain (with large
raindrops being less acid than small drops and therefore absorbing greater
quantities of SCL), the height of the pollutant source and the rainfall
rate (high precipitation rates are associated with larger mean raindrop
size) all strongly influence the relative importance of sulfate aerosol
and S$ scavenging in the very complex sulfur washout process.
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Section VI
EVALUATION
Studies of S02 oxidation rates in power plant plumes show a
diversity of results, with reported values ranging from under 1 percent
per hour to as much as 30 percent per hour. The Gartrell et al. (1963)
study of the coal-fired Colbert power plant plume in Alabama presented
S02 oxidation rates of about 1-3 percent per hour for low humidity cases,
while higher rates on the order of 30 percent per hour were found for
higher relative humidities, with one case yielding an S02 oxidation rate
of 55 percent during a period of 108 minutes. The Stephens and McCaldin
(1971) field study of the coal-fired Crystal River power plant plume in
Florida resulted in data which showed the S02 half-life to be 144 minutes
at relative humidities ranging from 40-55 percent, and 70 minutes for rela-
tive humidities of 78-80 percent. Although there is controversy concern-
ing the assumption made by Stephens and McCaldin that particulate matter
in the submicron range can be used as a conservative tracer, their results
bring out the concept of the importance of relative humidity in the plume
and ambient air as a major factor in the atmospheric oxidation of S02.
Studies of the coal-fired Keystone plant in Pennsylvania (Newman et al.
1975a) and the oil-fired Northport plant in New York (Newman et al. 1975b)
yielded S02 oxidation rates in the coal-fired plumes that were surprisingly
lower than those of the oil-fired plant. The oxidation rates ranged from
0 to 5 percent in the Keystone plumes and from 0 to 26 percent in the
Northport plumes; the low particulate content of the coal-fired plume is
believed to be responsible for the correspondingly low S02 oxidation rates.
Definite correlations of oxidation with relative humidities were not
evident for the experimental humidity range of 40-95 percent. At the Four
Corners power plant in New Mexico (University of Utah Research Institute
1974) a recent trend of observing lower S02 oxidation rates in studies of
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coal-fired plant plumes was evident. Reported values for conversions of
S02 to particulate sulfate ranged from 0.37 percent per hour at a dis-
tance of 21 km downwind (at relative humidities of 36-51 percent in one
test), to 0.76 percent per hour at 21 km downwind, and 0.45 percent per
hour at 60 km downwind, with relative humidities of 26-40 percent in
another test. The dryness of the region is believed to be responsible
for the low conversion rates. In the MISTT program, mapping of the Labadie
plant plume near St. Louis resulted in pollutant budgets inferred from
estimates of rates of transformation and removal in the plume which showed
almost no sulfate formation within the first 30 km of the plant, and a
conversion of some 2% hr" at downwind distances of 30-50 km.
A study of the Morgantown plant plume in Maryland by Davis et
al. (1974) is controversial because of the evidence of an "ozone bulge" in
the plume profile data 50 km downwind of the plant. A series of reaction
steps was postulated to help interpret the bulge and there is the possi-
bility that under certain conditions, the initiating S0? + OH mechanism
in this reaction sequence may be the dominant S02 conversion mechanism to
ultimately result in ^$04 and other aerosol products under certain con-
ditions. This conveniently leads into an evaluation of what the literature
has shown to be the relevant S02 conversion mechanism in power plant plumes.
Five primary mechanisms for SOp oxidation in the atmosphere have
been postulated. These include the homogeneous processes of direct photo-
oxidation, indirect photo-oxidation, aqueous phase oxidation, the hetero-
geneous process involving catalyzed oxidation in liquids and dry hetero-
geneous reaction. Homogeneous photo-oxidation of S02 has generally been
agreed to be an insignificant cause of sulfate formation in plumes, with
the slowest rates of S02 conversion typically occurring in pure air and
sunshine. The reported rates range from about 0.1 percent per hour in
laboratory work (Gerhard and Johnstone 1958, Urone et al. 1968, Cox and
Penkett 1970) to actual atmospheric observations such as a 2 percent per
hour value described by Katz (1950) and one value of 12 percent per minute
found by Shirai et al. (1962).
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Indirect homogeneous photo-oxidation of S02 may be a significant
reaction sequence in plume chemistry. The dominant gas phase removal mech-
anism for SCL may be the oxidation of S02 by OH and H02 as shown by Davis
et al. (1974), Castleman et al. (1974), and Calvert and McQuigg (1975).
A better understanding of the impact of this mechanism on S02 plume con-
version rates will be forthcoming as more reliable measurements of OH
concentrations in plumes are available. One current estimate of 2 percent
per hour for S02 conversion through OH and H20 oxidation has been put
forth by Castleman et al. (1975). It is difficult to calculate the homo-
geneous rate of oxidation of S02 in photochemical smog systems due to the
diversity of test results and complexities introduced by the use of dif-
ferent S02-NOx-hydrocarbon and $02-03-0!efin mixtures and conditions in
varying experimental laboratory procedures.
Studies of homogeneous aqueous phase reactions yield S02
conversion rates ranging from the order of 0.1 percent hr~l (Miller and
dePena 1972), through 2.5 percent hr'1 (Scott and Hobbs 1967), to 13 per-
cent hr~l (McKay 1971) for the ammonia-S02-water system. Dana et al.
(1976), in their field work regarding precipitation scavenging of reactive
fossil-fuel plumes, place strong emphasis on the need for experimental
measurements of ammonia solubility in water at low concentrations prior
to any further speculation regarding the role of ammonia in S02 oxidation
processes.
The effect of metal catalysts in the aqueous phase oxidation of
S02 has been shown to result in a range of from 0.1 to 2.0 percent per
minute, with possible higher rates. Foster (1969) estimated the rate of
oxidation by a Mn catalyst to be 0.09 percent min"1 and 0.15-1.5 percent
min~^ for the rate of oxidation for Fe, showing the importance of iron as
a catalyst. Cheng et al. (1971) extrapolated their laboratory results to
the atmosphere and arrived at an S02 oxidation rate of about 2 percent
hr-1 for a MnS04 catalyst concentration believed to be typical in a power
plant plume. The oxidation of S02 for the Cheng et al. study is seen
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to be four times larger than Foster's level. The high variability in
oxidation rates is attributed to the sensitivity of this mechanism to
parameters such as pH, relative humidity, temperature and catalyst con-
centration. The most effective catalysis occurs in solution under pre-
cipitation conditions; Dana et al. (1976) have pointed to faster S02
chemical conversion rates in plumes, with a chemistry believed to be more
complex than for fair weather. The mechanisms that could be active under
precipitating conditions include (1) an accelerated gas-phase reaction
process, (2) a liquid phase reaction process in precipitation, (3) a liquid
phase process in cloud droplets and subsequent efficient scavenging of cloud
drops by precipitation, or (4) combinations of the above.
Heterogeneous solid-gas phase oxidation studies in the laboratory
may not extrapolate quantitatively to the atmosphere. In power plant plumes,
it is the small particles, less than 1 ym in diameter, that are potential
catalysts for chemical reactions because of their large specific surface
area and probably high trace metal content. A realistic estimation of the
surface area available for catalytic aqueous phase oxidation or catalytic
oxidation on dry surfaces requires data on particle surface composition,
particle surface structure, and particle concentration in the plume; such
data has not been gathered in past studies. Of interest is the role of
dry particulate vanadium in the heterogeneous oxidation of S02; but the
role of vanadium as an effective catalyst is disputable as Corn and Cheng
(1972) and Urone et al. (1968) have shown it to be relatively inactive.
Researchers have presented theories which favor either homo-
geneous or heterogeneous mechanisms as being dominant in S02 oxidation in
plumes, but a review of all studies suggests that each mechanism probably
is of major importance but under quite different conditions. For example,
the homogeneous gas phase process involving OH radicals most definitely
has a strong positive temperature dependence due to the requirement of
high H20, high 03, and high UV flux for the generation of maximum OH con-
centrations (the very small negative temperature dependence of the OH + S02
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reaction would be insignificant compared with the strong positive tem-
perature dependence for OH production). In the case of heterogeneous
processes, things are much more complex; here both positive and negative
temperature dependence terms are likely to be involved. Other significant
negative effects result from absorption and adsorption of gaseous species
(e.g., 03 or $02) on or into a liquid or solid surface. In general, it
appears that in the more complex heterogeneous systems, counterbalancing
temperature terms render these processes rather insensitive to temperature
or that this reaction mode has a net negative temperature dependence
(McKay 1971). If so, this could mean that under cool weather or winter-
time conditions (also possibly summertime night conditions) heterogeneous
reactions are the major S02 conversion mode (Davis, personal communication,
1976).
One line of thought may be that at a given power plant location,
there may be no one predominant SO- conversion mechanism (homogeneous or
heterogeneous) acting on an annual basis, as each mechanism can operate
efficiently only when a specific (and as yet undefined) set of environ-
mental parameters are present. For example, in the summer months with
characteristically higher temperatures and relative humidities prevalent,
the production of OH free radicals may be accelerated, thus favoring enhan-
ced homogeneous processes of S02 plume oxidation in the plume. On the
other hand, it may be that heterogeneous processes are less effective at
higher temperatures. The entire situation may be reversed in the winter
months; the entire concept is qualitatively illustrated in Figure 6a as a
cyclic phenomenon with annual periodicity. On a diurnal basis, the high
UV flux during daylight favors the homogeneous mechanisms, while hetero-
geneous reactions dominate the nighttime hours (see Figure 6b). At a
given time, examination of plume S02 conversion mechanisms as a function
of distance may show heterogeneous processes dominating close to the stack
but dropping off with downwind distance. The ready availability of metal
catalysts from the plume allows the heterogeneous mechanisms to operate
almost immediately, whereas the homogeneous processes (specifically the
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.2
c
rf
_c
o
p
5
D
Homogeneous
Mechanisms
Heterogeneous
Mechanisms
Winter
Summer
Season
s
o
Q
Figure 6a. Conceptual Illustration of the Variation of Both Homogeneous and Heterogeneous
SO2 Conversion Mechanisms as a Function of Time of Year for a Specific
Stack Location.
Homogeneous Mechanisms
eterogeneous
Mechanisms
Early Morning
Mid-day
Early Evening
Late Night
Diurnal Cycle
Figure 6b. Conceptual Illustration of the Variation of Both Homogeneous and Heterogeneous
SO Conversion Mechanisms as a Function of Time of Day for a Specific
Stack Location.
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S02 + OH reaction) take some distance downwind (10's of kilometers) to
become important, possibly closely correlated with the rate of ozone pro-
duction, and reaching a maximum at some point where formation is limited
by dilution of S02 (see Figure 6c).
Homogeneous Mechanisms
fl
o
1
-------
Chemical kinetic models to study the basic features of a reactive
plume from a large point source are currently not available, but certainly
will be forthcoming as quantitative information becomes available on the
subjects of plume SCL-sulfate conversion mechanisms, rates of S02 trans-
formation and the extent and final state of reaction. Systems Applications,
Inc. (Liu 1975) has developed one such model which employs a provisional
set of SOp reaction steps until field studies can validate the appropriate-
ness of the SO- mechanism. The recent recognition of the importance of
S02 conversion to ^04 and other sulfates via OH radicals by Davis (1976)
and others is a key accomplishment in the state-of-the-art.
Dry deposition in the atmosphere is a function of the type of
pollutant material, the deposition substrate character, and the meteoro-
logical conditions, such as wind speed and direction, surface roughness
length, friction velocity and atmospheric stability. Recent investigations
(Owers and Powell 1974, Shepherd 1974, Garland et al. 1974) establish the
2 1
velocity of deposition of SC^ to be in the order of 10 m s . Submicron
atmospheric sulfate particles may have deposition velocities about an order
of magnitude smaller than the corresponding values established for S02
(Chamberlain 1966).
Numerical modeling of the dry deposition process is still in the
early stages, and field measurements, rather than laboratory-simulated con-
ditions, are needed for validation purposes. Sehmel and Hodgson (1974)
developed a steady-state, one-dimensional mass transfer model and with wind
tunnel data, determined that for their conditions of experimentation depo-
sition velocities were nearly independent of particle diameter only in the
0.1 to 1.0 ym size range. This corresponds to the "accumulation mode"
where 80 percent of atmospheric sulfate is found. The minimum deposition
velocities were found to occur in the particle diameter range of 0.04 to
0.4 ym.
Laboratory studies of the absorption of atmospheric water-soluble
gases by vegetation show the rate of pollutant removal to be affected by
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wind velocity, canopy height, and light intensity, with the removal rate
linearly increasing with increase of pollutant concentration to a limit.
The effects of the washout mechanism on power plant plume deple-
tion are documented mostly through the research of the Battelle group in
northwest Washington. The washout process depends greatly on rain charac-
teristics such as the raindrop size distribution and the aerosol collection
efficiency of the raindrops. The S02 rainfall deposition pattern in the
region of the Centralia coal-fired plant based on rain collector samplers
located on the ground along the cross-plume line clearly indicated a
scavenged "plume" of S02 on the sampling array (Dana et al. 1976). The
field experiments concluded that sulfate concentrations in rainwater due
to the plume's presence do not appear to be significant at downwind dis-
tances less than 10 km; ammonium nitrate and soluble (ortho) phosphate ion
concentrations were at or near normal background levels; and, the only
species measured which showed plume-related deposition patterns at all
distances where samples were collected (0.4-11 km) were S02 and hydrogen
ion.
The Battelle group has also developed one of the few numerical
models for predicting precipitation scavenging of reactive pollutants from
power plant plumes (Dana et al. 1975, 1976). The model can predict ground
level washout fluxes and average concentrations from precipitation as a
function of location beneath a plume. Initial applications and comparisons
of the model predictions to field data indicated that the Scott and Hobbs
(1967) aqueous-phase dissolved ammonia oxidation mechanism for S02 is not
adequate to account for observed rates of sulfate formation.
The rainout process is known to be a function of the nature of
the aerosol, its size distribution, concentration, wettability, activity
as a nuclei; cloud parameters such as the nature of the cloud itself, its
depth, temperature and water distributions, electrical activity and other
parameters. Research into this mechanism, as it applies to power plant
plumes has been virtually non-existent due to the complexities involved
with data gathering.
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PART 3
TRANSPORT AND DIFFUSION OF PLUMES UNDER INFLUENCE OF
COMPLEX TERRAIN
In contrast to the previous section, which discussed various
aspects of atmospheric plume chemical transformations, this section deals
with the physical and conservative behavior of plume components, i.e., with
conditions in which chemical transformations do not occur (or are ignored)
and in which the total mass of pollutant in the plume is treated as remain-
ing unchanged over time. Neither of these situations is truly representa-
tive of the behavior of actual S02 and NOX plumes from power plants, although
both are often assumed to be sufficiently true to provide acceptable results
in short-term modeling calculations of pollutant concentrations from power
plant stacks.
Our ability to address directly questions of pollutant disper-
sion* in complex terrain is a function of our understanding of the major
physical laws that govern airflow, turbulence and diffusion mechanisms
under differing meteorological and topographical conditions. It is,
therefore, appropriate to begin this evaluation of the state-of-the-art
of plume transport and diffusion in complicated terrain with a review of
the current theories and observations based upon research concerning air-
flow, turbulence and diffusion phenomena in such topographical surround-
ings. These subjects are discussed in Section VII, "Principles and
Observations." Section VIII will describe the types of models available
to numerically simulate these phenomena. The next section (Section IX)
will be a review of field work with regard to complex terrain situations;
studies concerned with the transport and diffusion aspects of actual stack
plumes or tracer material will be discussed in detail, with emphasis on the
program objectives, the methodologies of data sampling utilized, model-to-
measurement comparisons associated with these field studies, and project results
and conclusions. Included within the field studies is our own analysis of c
* Dispersion in this review is defined as the combined result of transport and diffusion processes.
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and az relationships in the Gaussian plume model over complex terrain as
derived from LAPPES and TVA power plant plume concentration data. An over-
all matrix summary of these field studies concludes Section IX. Finally,
Section X is an appraisal of current capabilities for calculating airflow
and diffusion based upon analysis of the field studies and modeling com-
parisons.
Section VII
PRINCIPLES AND OBSERVATIONS
AIRFLOW IN COMPLEX TERRAIN
For the purposes of this document, consideration of airflow in
complex terrain will include only studies of phenomena related to orographic
features, e.g., problems of up-slope and down-slope flow. Other flows of
a local nature which are not immediately relatable to differences in terrain
elevation (e.g., lake and sea breezes) will not be discussed in this report.
With the exceptions of mountain lee waves - only briefly touched upon in this
document - and flow channeling described subsequently in discussions of
field studies, the conditions described are totally influenced by diurnal
effects. Daytime solar heating and nighttime radiational cooling generate
the driving force for the mountain and valley flows. Whether or not these
flows establish themselves is largely dependent upon the gradient conditions
existing in the synoptic flows. Gradient conditions can either assist or
inhibit the development of these local flows.
Several investigators have determined that a gradient wind speed
of approximately 6 m sec" is necessary for the breakup of the nocturnal
drainage flow. Kangos et al. (1969) have summarized much of the research
that has been undertaken concerning the extent of mountain and valley winds.
Maximum velocities have been found at altitudes from 1/4 to 1/2 the ridge
height for upslope and downslope winds. Smith (1965) found that the depth
of the drainage flow for a ridge at Twentynine Palms, California, was at
least 80 m. In the same study a dome of cold air was found to develop in
the valley to the west of the ridge as a result of drainage.
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Egami et al. (1974) conducted a study of plume dispersion near
the Mohave Generating Plant which is located in a river valley with ridges
extending to 1000-1300 meters above the valley floor. Pibal measurements
of wind speed and direction were taken and the summary of results concen-
trated heavily on the mountain and valley wind patterns. The up-valley
winds were of greater depth (4000 m) than the down-valley flow (500-1000 m).
The greater extent of the up-valley flow was attributed to daytime trans-
port of momentum from higher levels due to unstable conditions.
Two wind systems have been noted by Tyson and Preston-Whyte (1972)
in Natal: (1) mountain and valley winds occurring in the deep valleys and
(2) mountain-plain and plain-mountain winds occurring above ridge level and
above the influence of surface topography. At the time of maximum solar
radiation the entire up-valley flow was as much as 1000-1250 m deep, with
the maximum wind speed of about 10 m sec~l at the 250-300 m level. The
mountain-plain drainage wind was most fully developed at 2 hours before sun-
rise and became as deep as 1000 m.
The thermal circulations in a small canyon located in the Wasatch
Range of northern Utah were studied by Thompson (1967). The major events
of a typical night of radiational cooling were as follows: (1) formation
of a thin film of cold air on the floor of the canyon and a short distance
up the canyon slopes; (2) rapid wind shift from up-canyon to down-canyon
wind; (3) rapidly increasing wind speed accompanying the continued cooling
followed by steady wind the remainder of the night; (4) cold core persisted
in the canyon as morning heating progressed and (5) rapid wind shift fror
down-canyon to up-canyon in the morning. The major difference be^een rr,-,i
and conditions in other larger canyons is that the inversion and nocturnal
flow regime probably begin earlier and develop more rapidly in this .artic-
ular small canyon.
Kangos et al. (1969) summarized work relating to air flows over
mountain peaks and ridges and the resulting production of lee waves. The
following summarization will confine itself only to those cases where the
wind blows perpendicular to a ridge. In an attempt to classify motions
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over a ridge, wave characteristics such as length and amplitude were found
to be important and on the basis of these, three categories were defined,
namely, laminar flow, small-wave flow, and large-wave flow.
Small-wave flow is characterized by a wavelength less than or
equal to the width of the ridge and by moderate winds which increase with
height through a considerable depth. Stabilities are not as great as for
large waves, where the wavelength exceeds the ridge width. When stream-
lines become packed as air passes over the ridge, large waves are produced.
Rotor flow or hydraulic jump flow is usually present with large waves.
Jump flow is characterized by a marked temperature inversion at some level
above ridge height. The waves form a single prominent peak in the vicinity
of the ridge. Large wind speeds result in the subsequent trough, and then
the flow "jumps" to a higher level downstream with no further wave activity.
Such a situation is depicted in Figure 7d (Lester and Fingerhut 1974).
From energy conservation considerations the turbulence in this region must
therefore be great.
In addition to wind speed, an important consideration in defining
air flow over ridges would be the stability of the atmosphere. Those
slopes facing the sun would undergo heating and hence generate upflow winds.
Those slopes on the backside or receiving no sunshine could have forces act-
ing to generate down-slope flows. The forcing equations for these up- or
down-slope conditions would then need to be superimposed upon the general
airflow (or synoptic flow) existing at the time.
Thus a leeward-facing slope that was receiving direct sunshine
could tend to promote and enforce the rotary flow that might be generated
in the lee of a mountain. Conversely, during the nighttime, or with a
shaded flow the forces generating a rotary circulation would tend to be
inhibited. Studies at NCAR have shown that the extremely damaging winds
that occur in the Boulder area are a combination of down-slope and synoptic
flows reinforcing each other.
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In test releases of fluorescent particles (FP) upwind of a 300 m
_1 _]
ridge by Smith (1965), airflows of 5 to 7 m sec and 1 to 5 m sec pro-
duced waves. According to theory, wind speeds this light should be charac-
terized by flow that is displaced upward by the ridge and then returns to
its previous height behind the ridge without the oscillatory pattern of
well-developed lee waves. Highest concentrations of a plume should occur
well above the lee slope. In the case of this particular ridge at Twenty-
nine Palms, California, the lee slope was still quite gentle and only
slight downward motions were required to bring the FP plume to the surface.
The change from drainage flow to wave flow occurred abruptly as stability
increased during the evening.
Lester and Fingerhut (1974) investigated the lower turbulent
zone (LTZ), a highly turbulent region of near-neutral stability located
immediately to the lee of a range of mountains, between the ground and
the stable layer in which wave motion occurs. Aircraft flights measured
potential temperature, horizontal velocity, and longitudinal gust velocity.
Cross-sections of wind speed and direction and potential temperature were
analyzed for a case of large amplitude lee waves and strong turbulence
over the plains just east of the Colorado Rockies. The effect of the
passage of a weak cold front was noted. The lee wavelength increased from
20 to 32 km, amplitude increased from 0.5 to 0.75 km, and there was an
increase in overall turbulence intensity. It was not possible to isolate
spatial changes in LTZ structure resulting from differences in upstream
terrain since synoptic changes masked these effects. The LTZ extended
40 km downstream of the first lee wave trough. Maximum turbulence was
associated with the updraft area just upstream of the rotor (see Figures 7a
through 7c) and minimum turbulence was found in and near the troughs. How-
ever, in another case a stronger and more rapidly moving cold front had
the opposite effect (i.e., wavelength, amplitude, depth of LTZ, and turbu-
lent levels were all decreased). In the hydraulic jump cases (see Figure 7d),
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0 1O 20 30 40 50 60 7O 80
1
E
Isentropes ^ Streamlines
Figure 7a. A Schematic Cross Section of the Lower Turbulence Zone^LTZ) Based
On Six Case Studies, Showing the Potential Temperature Field. Evident Are
The Typically Weak Potential Temperature Gradient, and the Relatively
Strong Stability at the Top; The Low Level Potential Temperature Minima
Are Interpreted as Evidence of Air Moving Upward From the Surface Layers
In the Rotor Circulation,
Figure 7c. Distribution of Moderate or Greater Intensities of Turbulence Found
1 Within the LTZ In the Wave Cases.
CO
10
20
30
80
HI Wind Speed Minimum
Max. Wind Speed Gradients
Figure 7b. Strong Vertical Shears and Large Longitudinal Speed Changes,
Especially In the Vicinity of the Updrafts and the Rotor are Associated with
The Wind Speed Maximum In the Stable Layer at the Top of the LTZ.
10 20 30 40
Distance (km) from Ridge
Figure 7d. The Larger Dimensions of the LTZ and the Greater Extent of the Area
Of Severe Turbulence Distinguish the Streamline and Turbulence
Distribution of Hydraulic Jump Types from Wave Types.
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severe turbulence was found over larger dimensions of the LTZ. In this
study, the rotor circulations were deduced from examination of the streamline
patterns implied by aircraft Doppler wind measurements and cross-sectional
potential temperature distributions.
Channeling of airflow by mountain and valley systems was studied
by Nappo (1975) as a portion of ETTEX (Eastern Tennessee Trajectory Experi-
ment). Wind soundings were made simultaneously at five stations over a
mesoscale network to determine the topographic effects on airflow caused
by the Great Smoky Mountains, Cumberland Plateau, and the Tennessee Valley.
It was anticipated that channeling by the Tennessee Valley would be most
evident with stable night conditions. However, pronounced up-valley turn-
ing of the wind in the lowest 700 m was noted for an unstable case, and
the channeling effect increased with lower altitudes. In a stable case
slight up-valley channeling was noted even at approximately 1200 m above
the valley floor; between 600 and 700 m a sharp change in wind direction
was observed. This was the transition from the free atmosphere to the
surface layer which was being influenced by the strong terrain effects.
The vertical wind profiles, highly localized, showed a linear shear layer
near the ground and smooth profile above, characteristic of stable con-
ditions. Cross-valley and down-valley drainage winds were found in this
lower layer. From limited analyses of the data, it was concluded that
during stable conditions, large scale topographic features always affect
the mesoscale flow, with the effects extending to elevations greater than
2000 m above the ground and to distances beyond 50 km.
Potential flow theory, involving use of either the stream func-
tion or velocity potential, has been extremely useful in assessing theoret-
ical pollutant transport over mountainous terrain. The major limitations
to this approach are the neglect of viscous effects and the inability to
include stratification effects. Results of this type of modeling have
shown that for two-dimensional flow and in the absence of diffusive mixing,
ground-level concentrations resulting from an elevated point source are
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the same over vertically differentiated terrain as over flat terrain. The
plume is closer to the surface near the top of ridges but there is also
a decrease in the plume's vertical dimension (packing of streamlines) and
an increase in velocity over the ridge. Streamline deformation is smaller
for flow over a "hemispherical hill" than over a ridge aligned normal to
the flow. Thus, there is potentially greater impact of elevated plumes
on the two-dimensional hill.
For stratified flow most theoretical work has centered on solu-
tions of the "mountain wave" equation in two dimensions. For neutral
stability or for a narrow ridge, streamlines are symmetrical about the
crest and the perturbations decrease with height, Egan (1975) has esti-
mated that the effect on the ground-level concentrations at the upwind side
of a ridge under such conditions would be similar to those produced by
potential flow over a ridge. Streamline crests tilt upwind for stably
stratified flow or for flow over broad ridges. The closest approach of
a plume and the greatest velocity occur on the leeward slope. A major
conclusion reached from this type of theoretical study is that streamlines
approach the surface more closely under stable conditions than under neu-
tral conditions.
TURBULENCE AND DIFFUSION ENHANCEMENT
Since turbulence is partly caused by the roughness of the sur-
face over which air flows and since turbulence is the principal mechanism
which results in the dispersion of the pollutants from a power plant stack,
the dimensions of the plume, e.g., a and cz, are frequently used to charac-
terize the effect of terrain on the plume. Measurements of the degree of
turbulence have normally been taken by means of wind observations, using
bivanes to obtain both horizontal and vertical wind components or by use
of tetroons to follow a particular parcel of air.
Kao et al. (1974) utilized a network of 15 continuous wind record-
ing stations in the Salt Lake Valley to investigate typical wind patterns
and the topographical effect on diffusion in valleys. The general flow
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patterns exhibited the expected diurnal variation with drainage winds pre-
vailing in the early morning and evening and up-valley winds prevailing
in the afternoon. Analysis of the data found that the distributions of
the mean and turbulent motions in the valley were generally non-homogeneous
and non-stationary. Maxima of both the mean and turbulent kinetic energy
occurred in the late morning and again in the early to mid-afternoon.
Alternately, trajectories of marked air parcels released successively at
10 minute intervals were computed also by Kao et al. (1974). Air trajec-
tories of the center of mass of all marked particles released in a 3-hour
period and the ensemble averages of the mean squares of the north-south
and east-west components of the distance between particle trajectories for
every 3-hour time period were computed for analysis of mean transport and
turbulent diffusion, respectively. The mean spread of the parcels was
generally greater in the afternoon than in the morning, and the east-west
component of the spread (along the narrow dimension of the valley) was
greater than for the north-south component. Spread was greater during the
transitional evening hours than during early morning stable conditions.
Since cooling and the subsequent development of temperature inver-
sions in valleys causes drainage of air down the slopes, inversions can be
said to enhance turbulence in this situation. Leahey and Halitsky (1973)
investigated turbulence at very low wind speeds at the Consolidated Edison
Nuclear Plant at Indian Point, New York, located in the Hudson Valley.
Wind fluctuation data were obtained from a bivane, and a turbulence clima-
tology for winds <_ 2 m s~* was derived. The bivane observations were
taken on a 30.5 m tower which was approximately 500 m from the river. The
Hudson valley at this location is about 5 km wide and mountains from 150
to 350 m high enclose the valley. Diffusion was related to the turbulent wind
fluctuations by the method of Hay and Pasquill (1959). Estimates were made
for a distance of 1100 m from a ground-level source for all wind directions
and thermal stratifications. Investigation of the data showed no occurrence
of diffusion rates lower than those associated with the neutral (D) sta-
bility category.* Turbulence levels in the down-valley and down-slope
* For definition, see Table 11.
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flow increased as the intensity of the inversion increased, probably due
to the role of inversions in producing katabatic winds.
Leahey and Rowe (1974) completed a bivane study of airflow over
a river bank near a coal-fired generating station in Alberta. Vertical
distance from valley floor to rim was 52 m and the valley width was 1.5 km.
The bivane was on a 30 m tower located on the valley rim. Data including
four tetroon trials, each with four tetroon flights, were collected in
situations ranging from very stable to near neutral. Dispersion coeffi-
cients were calculated using the mean tetroon trajectory for each trial
and computing the root-mean-square of the lateral and vertical distances
between individual trajectories at several downwind positions. The median
values of the horizontal diffusion coefficient were twice as large as the
expected flat terrain coefficient (Turner 1969) out to 3 km downwind, and
about equal beyond this distance. The median values of the vertical dif-
fusion coefficient were about twice those of Turner; the increased vertical
diffusion was attributed to terrain influences.
In another study in Alberta, over Copithorn Ridge, S02 measure-
ments were taken by helicopter by Leahey (1974). Copithorn Ridge is 99 m
high and is located 1.2 km downwind of a stack which had an emission
height of 69 m. A comparison of a and a during neutral conditions with
the expected dimensions using a D category* indicated that the experimental
results were respectively 1.7 and 1.2 times larger than would be anticipated
over flat terrain, i.e., greater diffusion than expected took place.
In the study of diffusion in a deep, steep-walled canyon in Utah
(Start et al. 1973), three physical mechanisms were postulated to enhance
mechanical turbulence. The first effect is the downward transfer of momen-
tum into the canyons from the considerable turbulence generated around
rough mountain tops. This effect is illustrated in Figure 8 where the
downward transport of momentum is related to the cross-canyon component
* For definition, see Table 11.
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flow. During conditions of strong temperature inversions, and with down-
ward momentum transport from the mountain tops minimal, downslope density
flows from side or feeder canyons may drain out into the main canyon and
may have enough momentum to reach the opposite main canyon slope. They
may then rise in a form of overshoot. Helical-like circulations may result
when these feeder canyon air drainage flows act in conjunction with down-
slope density flows along the main canyon wall (Figure 9). The third tur-
bulence-enhancing mechanism is wake turbulence. It is brought about
by flow over and around protruding cliffs, and across small, steep-
walled draws and indentations (see Figures Ida and lOb). The wake
region downwind of these topographical features is characterized by a
region, called a cavity, of reduced wind speed and enhanced turbulence.
Wake turbulence is the smallest in scale of the three turbulence mech-
anisms.
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GRADIENT FLOW
Figure 8. Schematic Illustration of Mountain Top Influences Upon the Gradient Level Flow Component and the
Downward Transporting of Gradient Flow Momentum.
(from Start et al. 1973)
Figure 9. Schematic Illustration of Circulations Triggered by Slope Density Flows and
Air Drainage from a Side Feeder Canyon.
(from Start et al. 1973)
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Figure lOa. Schematic View of the Type of Terrain Capable of Affecting the Wake Turbulence.
This same terrain is depicted in Figure lOb with arrows added to show the type of secondary air
flow caused by such protrusions into the primary flow of the canyon.
(from Start et al. 1973)
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TWO DIMENSIONAL CANYON FLOW
Figure lOb. Schematic Illustration of Turbulent Wake Effects Caused by Obstacles Protruding Into
the Primary Flow Pattern.
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A portion of the ETTEX project (Eastern Tennessee Trajectory
Experiment) involved a study of the relative diffusion of tetroon pairs
during convective conditions (Hanna 1975). Radar was used to track
tetroons within the afternoon mixing layer between 500 and 1500 m above
the surface. Instantaneous readings of tetroon range, elevation angle,
and azimuth angle were taken by the radar every two minutes. Thirteen
relative dispersion experiments were conducted near mid-day for cases of
sunny or partly cloudy skies and light wind conditions. Two tetroons
were released in each run; az values based on the tetroon positions ranged
from 60 to 430 m at distances varying between 4 and 26 km downwind. Con-
vective eddies caused considerable vertical motion from near the surface
to 2000 m. The turbulence of the wind speed (aw) did not correlate with
the wind speed, which is typical of turbulence production due to convec-
tion. Wind directions generally had a SW or NE component, which are
directions corresponding to the alignment of the ridges and valleys of
eastern Tennessee.
Based on similarity theory, the predicted separation was three
times greater than the observed. Part of this discrepancy may have been
caus.ed by the inability of tetroons to respond W rapid fluctuations of
air motion. Statistical turbulence theory is valid if both tetroons
remain within the same atmospheric layer; however, the large vertical
separations detected between individual balloons may render statistical
theory void for this application. Other'types of experiments such as
the use of smoke puffs might give better results. No discussion of the
specific effects of topography on the results of the tetroon runs was
contained in the report, since the findings did not indicate an increase
in turbulence over the predicted values for flat terrain.
Analysis by MacCready et al. (1974) of turbulence characteristics
of LO-LOCAT data (from the Lo-Low Altitude Clear Air Turbulence Project)
which was conducted over a variety of terrain in the southwest U.S.
including high and low mountains showed that the turbulence spectra
obtained from the study fit the von Karman spectrum formula rather well,
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especially the -5/3 law at the small end of the spectrum. For very stable
cases in complex terrain the minimum lateral turbulence observed was inde-
_1
pendent of wind speed up to about 5 m sec and then increased with wind
speed. Values of the standard deviation of vertical wind speed (a ) were
generally 60-70 percent of the standard deviation of crosswind speed (a )
under stable and very stable conditions. Standard deviations of horizontal
wind direction (aQ) were greater than "standard" a. values for the stability
D t)
classes as presented in AEC Safety Guide 23, at low wind speeds for all
stability classes. The values for stable and very stable cases were always
much greater than would have been inferred by the standard method of relat-
ing turbulence to temperature stability classifications* which is applicable
for essentially flat terrain.
MacCready et al. (1974) also devised a method for relating a
roughness terrain factor (atr)» obtained from U.S. Geological Survey
topographic maps, to estimates of horizontal turbulence (a ). A regres-
sion analysis was employed to determine a best fit curve relating turbu-
lence intensity to wind speed, height above the ground, and a. . Results
showed that turbulence values were influenced strongly by a. and only
very weakly by the former two variables. However, there was no effort
made to stratify the data according to location in complex terrain (i.e.,
in the lee of a mountain, valley, ridge top, etc.) or by the type of
meteorological conditions. It is also questionable whether the particular
terrain-turbulence relation derived in MacCready's study is applicable in
other areas of the U.S. in addition to the southwest.
Most studies have indicated that complex terrain contributes to
alterations in air flow and to increased amounts of turbulent diffusion
compared to those for flat terrain. Airborne concentration measurements
appear to be the best way to observe these effects; tetroon studies have
limitations in their usefulness in determining the degree of turbulence.
Drainage flow and lee waves seem to be the flow characteristics most
responsible for increased turbulent effects.
* See Table 11 for category definition.
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Another method of air flow investigation has been laboratory
simulation, but this has proved difficult for estimating dispersion and
especially over irregular terrain. This has been mainly due to the lack
of an ability to achieve similarity of Reynolds Number, Richardson
Number, Rayleigh Number, and other nondimensional parameters (Snyder 1972).
Perhaps the largest difficulty to overcome in achieving similarity in these
numbers relates to the dual nature of turbulence. For those classes of
studies for which mechanical turbulence is the dominant influence, labora-
tory simulation has proven to be extremely beneficial and can be scaled
appropriately for similarity between the simulation and the real world.
For those classes of problems in which thermally induced turbulence is an
important factor (which includes almost all classes of atmospheric disper-
sions) there does not appear to be a satisfactory means of scaling the
problem for laboratory simulation. Until a relationship can be obtained
for scaling thermally induced turbulence, the use of laboratory simulation
for dispersion studies will be a limited application.
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Section VIII
MODELING OF TERRAIN INFLUENCE ON PLUME BEHAVIOR
A large number of mathematical models, most but not all being
adaptations of a Gaussian distribution concept of plume dispersion com-
bined with a plume rise formula, have been used in concert with terrain
data to calculate plume dispersion under influences of elevated and com-
plex terrain. We begin this section with a brief description of classical
Gaussian atmospheric dispersion estimates over flat terrain, and we then
discuss models which have been developed to date for calculating plume
dispersion from elevated point sources in complex terrain, with emphasis
on their assumptions and modifications to the standard Gaussian equations.
Gaussian empirical formulas are derived from data obtained over
flat terrain, utilizing the various atmospheric stability categories.*
Turner's (1969) workbook adequately delineates the classical use of the
empirical formulas for flat terrain situations, as recommended by
Pasquill (1961) and advanced by Gifford (1961). The typical Gaussian
coordinate system, containing both horizontal and vertical planes is shown
in Figure 11. Equation 1 describes the concentration of gas or aerosols
at x, y, z from a continuous source with an effective emission height.
The basic assumption includes a Gaussian distribution in both horizontal
and vertical planes of plume spread with oy and az respectively representing
horizontal and vertical standard deviations of plume concentration
distribution.
**Note: exp - a/b = e ~a' where e is the base of natural
logarithms and is approximately equal to 2.7183.
* See Table 11 for class definitions.
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(0, 0, 0)
(*, -y,
(*, -y, 0)
Figure 11. Coordinate System Showing Gaussian Distributions in the Horizontal and Vertical.
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where
x = concentration of effluent (g m )
Q = emission rate of effluent (g s~^)
u~ = mean wind speed affecting the plume (m s~^)
a = standard deviation of plume concentration in the
horizontal (m)
az = standard deviation of plume concentration in the
vertical (m)
H = h + Ah = effective plume height (m)
hs = physical stack height (m)
Ah = plume rise (m)
Several popular, similar, diffusion categorization schemes
have evolved, stating relationships between the standard deviation of
pollutant concentration as a function of distance and measured meteorological
parameters. One of the first diffusion categorization schemes, utilized
insolation, time of day, wind speed and cloud cover to define stability
(see Table 11). '
TABLE 11. METEOROLOGICAL STABILITY CATEGORIES (after Turner 1970)
Surface
wind
Nighttime Conditions
Thin
overcast
speed Daytime insolation or >4/8 <3/8
(m s~l) Strong Moderate Slight cloudiness cloudiness
<2 A A-B
2-3 A-B B
3-5 B B-C
5-6 C C-D
>6 C D
A. Extremely unstable conditions.
B. Moderately unstable conditions.
C. Slightly unstable conditions.
D. Neutral conditions.*
E. Slightly stable conditions.
F. Moderately stable conditions.
B
C E F
CD E
D D D
D D D
* The neutral condition D, should
be assumed for overcast condi -
tions during day or night.
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The stability classification scheme as modified by Turner (1970)
and defined in Table 11 has been computerized and is readily available for
use on surface data. This program is available through the NOAA National
Climatic Center in Asheville and is entitled STAR. The STAR program uses
as input data the hourly or three-hourly data as collected at National
Weather Service or FAA observing stations and the longitude and latitude of
the observing station.
Another stability categorization scheme developed by TVA (1970)
uses dispersion parameters for six stability classes each representing one
of the following average potential temperature changes with height:
Neutral 0.00°C/100m
Slightly Stable 0.27°C/100m
Stable 0.64°C/100m
Isothermal 1.00°C/100m
Moderate Inversion 1.36°C/100m
Strong Inversion 1.73°C/100m
This categorization only considers the stable cases and is not intended for
use during unstable conditions.
The NRC (formerly the AEC) uses a stability classification based
upon both a temperature difference as exists close to the surface and a
wind direction standard deviation (oe) value as follows:
Stability Class AT (°C/100m) oe_
A <-1.9 25°
B -1.9 to -1.7 20°
C -1.7 to -1.5 15'
D -1.5 to -0.5 10C
E -0.5 to +1.5 5°
F +1.5 to +4.0 2.5°
G >+4.0 1.7°
;o
•\o
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The present standard for estimating pollutant dispersion in the
atmosphere is the Gaussian plume equation. The use of this equation implies
that the dimensions of the plume can be reasonably well defined as func-
tions of travel distance, and in terms of a a and a as defined in
Turner's Workbook (1970) or other curves, e.g., the TVA curves (TVA 1970),
obtained in previous studies. We describe subsequently, in Section IX,
GEOMET's efforts to obtain a preliminary insight into the impacts of ter-
rain in degrading the performance of the conventional Gaussian plume dis-
persion model. Since ground-level concentrations are often incorrectly
estimated by the model, it is of interest to know how much the use of
measured estimates of a and a improve the model agreement with measured
concentrations. If significant improvement occurs, it is of interest, as
a second step, to investigate the extent to which variations of measured
o and a can be correlated with terrain features. For this analysis we
needed airborne monitoring data describing plume concentrations from large
power plants in complex terrain. We selected LAPPES and Kingston Plant
data because previous analyses had not addressed this data from the view-
point taken here, which is to examine whether terrain influenced dispersion
within the plume. A description of our analyses, addressed toward this
end, is in Section IX. Before presenting the GEOMET analysis we present
a review of model modifications by other investigators and review the
results by others of applying these models to available field data.
The results of several concentration measurement programs per-
formed in complex terrain areas of both Eastern and Western United States,
have led to the development of modifications to the flat-terrain diffusion
model to account for the effects of terrain. Several modifications, each
with differing assumptions of terrain modified wind flow and enhanced
diffusion are delineated in what follows.
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"NOAA" GAUSSIAN MODEL
Diffusion calculations were made in the Southwest Energy Study
(Van der Hoven et al. 1972) for five planned or existing power plants
located in areas of generally complex terrain (Mohave, Navajo, Four
Corners, San Juan, and Huntington Canyon) with a Gaussian plume model
having the following terrain-related assumptions:
• For neutral and unstable conditions, the plume center-
line elevation above surrounding terrain was always
equal to the initial effective plume height. Thus, the
following standard Gaussian equation for ground-level
concentrations beneath the plume centerline was used:
exp-J-%-^ (2)
where:
x = short-term concentration, (applicable for
periods of 10 minutes to about one hour)
For stability classes* E and F, the plume was assumed to
remain at a constant elevation above mean sea level and
to impact upon surrounding terrain of equal or greater
elevations. In this case, the above equation is modified
so that the numerator of the term in brackets is H-hj,
where hy = terrain height above stack base (m).
When the terrain height exceeds H, then H-hy is
set to zero.
For fumigation conditions, centerline concentrations
were estimated by
X:
Zf /2TZT (CT +2J) zf 0)
where
zf = H - hT.
* See Table 11 for class definitions.
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Maximum concentrations resulting from vertical fumigation
through a layer from the terrain to the plume center were
assumed to apply as follows: a critical distance, x, at
which maximum concentrations caused by_fumigation should
be considered, was determined by X.T = u t 4 where
JH3
fp°p 9Z \, vMh-h-i
r \l AU U_M I i I. 1 Ml1
and
30
^- = change of potential temperature with
az height (°C m-1)
Cp = specific heat of air at constant pressure
(ergs g-1 °C-1)
tm = time required for the inversion to rise from
the top of the stack to the plume center (s)
o
p = ambient air density (g nf )
R = net rate of sensible heating of an air column
by solar radiation (ergs s-' cm-2)
and the rest of the symbols are as defined for Equation (1)
The following equation was applied for limited mixing
conditions when the pollutant is uniformly mixed in the
vertical and the receptor is below the plume centerline:
where
is the mixing depth of constant height above the
terrain in meters.
* The form of equations 3 and 4 differs somewhat from the equations presented in D. B. Turner's Workbook
for Atmospheric Dispersion Estimates for fumigation conditions. Equations 3 and 4 assumed fumigation
conditions between ground and plume centerline, while the Workbook assumes the mixing layer to be
between the ground and the plume top (where the top is defined as 2<7Z above plume centerline). As a
result, centerline concentrations under fumigation conditions as estimated in the Southwest Energy Study
are greater, by a. factor of 1 + 2
-------
Long-term concentrations (monthly, seasonal or annual
time periods) are represented by two equations:
u 0xa
z
(hs + Ah-hj)
21
2a
2
z
(6)
where
f = relative frequency with which winds blow in
a given wind sector
e'= angular width of sector (radians),
and
n-p
(7)
for limited mixing conditions.
Equations (5) and (6) were used (Martin and Tikvart 1968) with
average mixing heights, frequency distributions of wind speed and direction
by stability class (generated by the "STAR" program), average terrain
height profiles in each of eight sectors from the plants and assumed
average plant loads of about 80 to 85 percent to calculate annual concen-
trations at various locations near each of the five plants.
Output of this model for short-term concentration calculations
under stable conditions, with the associated terrain impaction assumption,
has been compared with observed concentrations (both tracers and $02)
more often, and for more different elevated sources in complex terrain (at
least five), than has been the case with any other model. However, the
comparisons in all cases have been limited to results for only a few days
(< 10) at each site; further, little or no validation of the fumigation
or limited mixing models has been attempted.
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The consensus is that the NOAA model representation is an over-
simplification of the complicated wind field-terrain interaction, and of the
important terrain parameters such as the slope of the terrain and the
width of raised terrain barriers in the plume path.
U.S. EPA SINGLE SOURCE (CRSTER) AND "VALLEY*1 MODELS
The Office of Air Quality Planning and Standards (OAQPS) of the
U.S. Environmental Protection Agency has used two variations of the
conventional Gaussian model for the evaluation of air quality in complex
terrain (Mears 1975). The CRSTER model uses equations very similar to
those presented above for the standard and limited mixing conditions in
the NOAA model. The terrain adjustment procedure is the same except that
it is applied for all stability conditions.* The stability class, wind
speed and direction, and mixing depth can be determined through subprograms
from standard U.S. Weather Service surface and upper air (radiosonde) data;
these processed data can then be input to the model with effective plume
height data to obtain hourly concentration values for selected receptor
points which are then arithmetically averaged to obtain average values
which correspond to the time scales of the various air quality standards
(i.e., 3-hour, 24-hour, annual average). Although this model has been
extensively used to evaluate the air quality impact of a number of power
plants in complex terrain, only limited comparisons with measured concen-
trations have been published.
The second of OAQPS's complex terrain models is called "VALLEY"
and is used for a worst case 24-hour average prediction of plume impact
for cases where the plume centerline is below the elevation of receptors
located on nearby terrain. It also utilizes a Gaussian formula very
similar to that of the NOAA model for stable conditions (i.e., with the
same basic terrain adjustment formula) with the following differences:
t The height of the plume centerline above the ground is
decreased by the elevation of the ground above the base
of the stack.
* See Table 11 for stability definitions.
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• The effluent is uniformly distributed in the
horizontal over a 22.5° sector for estimates of
crosswind dispersion.
« The plume center!ine is never less than 10 meters
from the ground.
® The wind direction is within a given 22.5° sector
for 6 hours out of 24.
« Constant 2.5 m/s winds and E stability class are
assumed throughout the 24-hour period.
ERT PSDM MODEL
Environmental Research and Technology, Inc. (Egan 1975) has
developed a modified Gaussian model for application to point sources in
complex terrain, for neutral, unstable and stable conditions with assump-
tions as follows:
9 For receptors located on terrain above the
effective plume height, the model assumes the
plume to remain at a distance generally taken
to be one-half of the effective plume height
over flat terrain away from the surface, for
neutral and unstable conditions.
• For stable conditions the plume is assumed to
pass around the side of a hill rather than
over it.
There is no evidence of validation of this model in the literature.
Egan (1975) has made further modifications to the Gaussian model
based on results of laboratory analyses of flow oast a cylinder and sphere
for neutral conditions. The modified ground-level centerline plume equation,
including reflection at the surface, is
2Q
x = *—i exp
(8)
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where
D , DZ = Coefficients used to modify the ay and az values due
^ to the effects of complex terrain, (i.e., Dy and Dz would
equal one for flat terrain)
C = (3^/9H)/(9i}i/3z), the ratio of the vertical gradient of the
stream function at the effective stack height to the
gradient at the plume centerline over the surface (dimen-
sionless). The ratio reduces to one for flat terrain.
n = (z - z )/H, where z (m) is the local height of the plume,
z (m) is the local height of the terrain, and H(m)
is the effective height of the stack. This ratio reduces
to one for flat terrain.
This model constitutes a departure from the conventional
Gaussian formula in that it (1) accounts for the alteration of flow
fields by complex terrain (through introduction of the parameter "c"),
and (2) allows a conceptual separation of the effects of complex terrain
on the flow field and turbulent diffusivities. Unfortunately, these
changes require estimates (either from a flow-model or from measurement
studies) of the relative height above the surface of the plume centerline
(a function of terrain), and the streamline spacing in the vicinity of
the plume in order to be applied effectively. This requires a more extensive
meteorological data base (at least with respect to spatial resolution) than
is normally available for estimating diffusion in complex terrain with the
standard Gaussian models.
AEROVIRONMENT .STATISTICAL TURBULENCE MODEL
MacCready et al. (1974) have developed an interesting model
in which they have used classical turbulence-diffusion theory together
with turbulence spectra determined from the Air Force - Boeing LO-LOCAT
program conducted in areas of mountainous terrain of the southwest to
develop a modified Gaussian model for use in complex terrain.
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The relationship between turbulence and diffusion is developed
starting with Pasquill's (1974) theoretical equation describing expanding
crosswind dispersion of a puff in an isotropic homogeneous environment, given by
2
°y2 = v'2t2/ F»
'/'
sin Trnt/3
Trnt/g
0 S-
where
dn (9)
a is the crosswind dispersion coefficient (m)
V is the turbulent velocity in the direction crosswind to the
mean wind direction with the averaging referring to an
infinite ensemble average (m s~l)
t is the time of travel (s)
3 is the Lagrangian* correlation coefficient (dimensionless), and
Fe(n) dn is the Eulerian** power spectral density in the frequency width dn.
The averaging refers to an infinite ensemble average, and material is
released at time t = 0. MacCready et al. (1974) states that Taylor's
hypothesis which converts temporal to spatial frequency, together with
empirical estimates of e and the von Karman turbulence spectra (which
the LO-LOCAT data were shown to represent well) were then used to arrive
at the formulas
ay2 = 0.44 avU (10)
az2 = 0.44 awLt (11)
where
L is the Eulerian longitudinal length scale (m)
a and a are Eulerian lateral and vertical components of turbulence,
w
respectively (m s"').
* The concentration statistics are described in terms of the statistical properties
of the displacements of groups of particles released in a. fluid.
** Statistics are formulated in terms of the statistical properties of the velocities measured
at fixed points in the fluid.
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Substitution of (10) .and (11) into the basic Gaussian formula gives
X/Q =
2.761
for centerline concentrations, for stack height = 0 and continuous emissions,
The above equation was deemed to be applicable for one hour
concentration averages, and does not take into account ground reflection
or vertical shear. Regarding the latter, the authors suggest use of a
a rather than a given by
eff
a =0.07 u + 1.2a
where
a is the measured lateral turbulence (m s~ ).
The validity of this assumption has not been tested; the authors
suggest confirmation from additional turbulence data when they become
available. The principal drawbacks of this approach are:
• Turbulence statistics ay and aw and the length
scale L must either be measured at the site for
which concentration estimates are desired (at
a point of sufficient height to be at least
midway between the ground and effective stack
height) or estimated from other data.
• It has not been successfully validated.
The authors have suggested a method whereby av can be estimated
from a terrain roughness factor O^R, which is found by spectrum analysis
of terrain height values in the vicinity of the source in question. Once
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otR is known, the relationship between av and otR derived from LO-LOCAT
data can be used to derive av. The following objective method is sug-
gested for determining atR:
(1) On a 1:250,000 USGS map plot four symmetrical lines of
45 nautical mile length in a tic-tac-toe pattern around
the site of interest, with the middle of each line coming
to 10 miles from the center point,
(2) Take height readings at 0.5 mile intervals for each line,
(3) For each 7.5 mile segment on a line calculate the root-
mean-square about the mean height (call it 07.5). and
(4) Find the average of all the a7 5 for all segments on all
lines, and multiply by 2.3 to give atR.
This method was derived by taking root-mean-square values for segments of
various lengths for the legs of various LO-LOCAT flights over rough
terrain, and seeing what segment length appeared to work best and what
multiplication factor gave the best fit.
Supporting the validity of this objective method is the fact
that the data on which it is based (LO-LOCAT) are extensive, consisting
of 8,800 turbulent spectra determined for complex terrain of varying
degrees (i.e., high mountains, low mountains) and in different geographic
areas (California, Colorado, New York). The spectra were also determined
for all seasons and times of day. The chief drawbacks are that (1) the
spectra are representative of only two altitudes, 76 m and 229 m above
ground; (2) the relationship between ay and atR show a large degree of
scatter, and (3) aw must be inferred from av based on the correlation
between the two.
INTERA MODEL
INTERA Environmental Consultants (Hoffnagle et al. 1975) have
recently developed a combined potential flow-turbulent diffusion model
for use over complex terrain. The model consists of the modified Navier-Stokes
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equations with some basic assumptions. These assumptions are that pressure
changes do not affect density; density changes have a negligible effect on
viscous terms of the equations; turbulent effects are expressed in terms of
eddy viscosity (vy); and density can be presented as a function of position.
Turbulence is expressed in terms of the pressure equation as:
V2p =
£
% dt
8 / dw\
az fpdt}
^T v^vu + ^7 v^vv + 37 VMVW
where the first bracketed term represents the inertial sources and the second
bracketed term represents the viscous sources (a complex tensor function of
the flow field).
As far as the boundary conditions are concerned, the' z direction
is bounded by the ground surface with variable elevation, and by the top
of the turbulent boundary layer. It is assumed that
u = f (y,z) (known)
v = w = 0
at x = 0
The y boundaries and the top of the turbulent boundary layer are assumed
to be frictionless, and u = v = w = o at the ground. A vertical pressure
gradient is specified at the outflow face. Viscosity is calculated from
the measurement of shear stress at the top of the turbulent boundary layer.
For the detailed equations, the reader is referred to "Evaluation of Selected
Air Pollution Dispersion Models Applicable to Complex Terrain" (INTERA
1975).
The INTERA model computes a terrain induced wind field which is
then used in the diffusion equation solution, thus avoiding the parallel
or horizontal flow assumptions made in the NOAA and EPA Gaussian models.
This model has been rather extensively compared with observed concentrations
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of either S02 or tracer resulting from elevated point source releases at
several different sites in complex terrain, where power plants or smelters
currently exist (i.e., Navajo Generating Station, Garfield Smelter), or have
been proposed (Kaiparowits Power Plant). The results of these comparisons
are described in Section IX, for the respective study sites. The limitations
in the use of the INTERA model are that the model, as it currently exists,
prevents the formation of recirculating flow in the lee of an obstacle (i.e.,
flows as depicted in Figure 7d) and that the elimination of consideration of
the energy balance equation prevents calculation of natural convective flows
such as mountain/valley flows and lake and sea breezes.
TVA MODEL
The TVA model (Montgomery et al. 1973) is actually a set of
Gaussian calculational techniques very similar to the NOAA model, with
each technique being appropriate to a different meteorological condition.
It is not specifically designed for application in complex terrain situ-
ations. The model differs from the other standard Gaussian models (EPA
"VALLEY," NOAA) in that the diffusion coefficients used (a and o ) are
not the same as the standard coefficients (Turner 1969), and were
essentially derived from observations of power plant plumes.
The diffusion coefficients obtained were primarily derived for
the stable atmospheric conditions of limited mixing and inversion breakup.
It is interesting to note that one recent study (Dames and Moore 1976)
compared the NOAA and TVA coefficients using Denver, Colorado data. The
NOAA model identified inversion breakup as a potential problem in meeting
Colorado standards and showed limited mixing as being no problem. Using
the TVA approach, just the opposite was found, i.e., limited mixing was
the problem and inversion breakup was not.
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Section IX
FIELD STUDIES
Field studies concerned with the physical behavior (i.e., transport
and/or diffusion) of actual stack plumes or tracer material in complex ter-
rain will be discussed in this section. Many of the 12 study locations dis-
cussed deal with tracer releases and not with actual effluents. The text
will first describe some of the early fluorescent particle (FP) diffusion
trials in complex terrain (McMullen and Perkins 1963, Hinds 1970), and
discuss the turbulence measurements near the Dickerson Power Plant in
Maryland (Weil 1974). Programs at the Naughton Plant (Spangler et al.
1973), the Four Corners Plant (Niemann 1973, Smith and Anderson 1974),
the Navajo Plant (Hovind et al. 1973b, NAWC 1974, Rockwell, MRI and SAI
1975), the Huntington Canyon (Hovind et al. 1973a, c, 1974, Start et al.
1973, 1975), the Garfield Smelter (Start et al. 1974) and Kaiparowits
(NAWC 1974) are presented next, followed by descriptions and GEOMET
analyses of LAPPES program data (Schiermeier 1970, 1972) and data from
the TVA Kingston Plant.
DUGWAY, UTAH
Some of the earliest FP diffusion trials conducted in complex
terrain involved four instantaneous elevated line source releases over
mountainous terrain near Dugway, Utah (McMullen and Perkins 1963). The
site chosen for the trials provided successively, in a west to east
direction, a large flat area at an elevation of approximately 1300 m MSL,
a ridge rising to 2000 m MSL, followed by a broad valley. Elevated line
source releases were carried out from 305 to 670 m. above the terrain.
The objective of the trials was to determine the degree to which an FP
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tracer cloud disseminated above a ground inversion enters into the drain-
age wind system characteristic of nighttime flow in a canyon-like complex
terrain environment. The orientations of all of the elevated cross-wind
line releases were approximately perpendicular to the main north-south
ridge of the mountain system (see Figure 12). Three of the four trials
were characterized by moderate to strong low level inversions, while high
winds prevented well-established drainage conditions during the other trial
The wind direction was predominantly south-southeast during each of the
trials, and thus lent similarity to the roughness fetches in each case.
Line Source Plume
Ridge
Axis
Wind
Figure 12, Configuration of Elevated Line Source Releases with
Respect to Wind Direction and Orientation of Mountain
System Near Dugway, Utah.
Larger amounts of tracer particles were recovered by Rotorod
samplers at ground level over the rough terrain than over the flat terrain,
thereby indicating enhanced rates of mixing in the irregular terrain. This
was further substantiated by observing that over rough terrain FP was recov-
ered much closer to the source. McMullen and Perkins suggest that two sep-
arate diffusion regimes contributed to the total tracer recoveries: direct
diffusion from the main cloud as it passed overhead, and diffusion within
the drainage flow. The drainage transport would lag in time the detection
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of the initial pjume. A sampler would first detect the passage of the main
plume as it went overhead; then, at some later time, the flows that were coming
out of the canyons which had accumulated some FP material would start to
show up on the sampler as their drainage flows would pass over the sampler.
The percentage of FP material recovery was determined by first knowing
the mean efficiency of the Rotorod samplers for the FP material used;
then calculating the effective flow rate and dividing this flow rate into
the reported recovery, thus giving the corresponding dosage. Direct dif-
fusion was estimated to account for 85 percent of the recovery for non-
canyon samples, however, drainage transport was estimated to account for
as much as 50 percent of the recovery at samples within the canyon. The
plume was definitely ill-defined, as there was no systematic variation in
the total recovery with sampler location in the canyon.
McMullen and Perkins found that terrain-induced mechanical
turbulence was sufficient to give significant FP recoveries at the surface
in all trials both in the lateral canyons and over the open sampling area,
despite low-level stability. They also found that vertical mixing is
much more rapid over complex terrain than for an equal distance of travel
over flat terrain. The authors suggest that the second conclusion would
probably not be true for FP particles released with the flow perpendicular
to the ridges instead of parallel to them.
VANDENBERG AIR FORCE BASE
Hinds (1970) reported on a series of 113 diffusion tests over
the mountainous terrain in the southern portion of Vandenberg Air Force
Base in Southern California. The releases were from ground-level point
sources and consisted of 15-30 minute releases of zinc sulfide. Elevations
in the area varied from 30-500 m AMSL. A comparison of diffusion over
mountainous terrain and flat terrain in the same maritime climate showed
that daytime unstable conditions minimized the importance of terrain,
whereas nighttime stable conditions especially in winter led to apparently
significant interactions between terrain-induced drainage flow and
synoptic-scale onshore gradient flow in the complex terrain. Figure 13,
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from the Hinds study, shows this comparison. The solid line represents
moderately unstable conditions (Stability B) as suggested by Pasquill (1962)
Night tests
Doy tests
* * I ••
1000
Distance, m
10,000
Figure 13, Vertical Plume Dimensions (oz) Inferred from Cross
Width (
-------
The variables chosen were travel time, standard deviation of wind direction
(a0), temperature lapse rate, and wind speed. Downwind travel time was
found to be a relatively good predictor, but ae was the poorest indicator
of dispersion. The lack of dependence on OQ was attributed to the non-
homogeneity of the flow field in complex terrain. Impingement of the
plume was probably great on the ridges, as the canyon exposures were
typically only half the concentrations found on the ridge tops.
DICKERSON POWER PLANT
Increased turbulence due to rough terrain was clearly demon-
strated at the Dickerson Power Plant along the Potomac River in Maryland.
The study for the Maryland Power Plant Siting Program (Weil 1974) involved
comparison of stability classes from Dulles Airport, which is located on
flat terrain, and from the Dickerson site in moderately rough terrain,
30 km distant. The terrain in the vicinity of the Dickerson plant is
characterized by generally rolling terrain but with mountains located
8 km west of the plant (Catoctin Mountains) with elevations above the
plant stack top and which could equal or exceed the effective plume
height on occasion. The stack base is situated approximately 90 m above
MSL. The topography rises to approximately 65 m above the stack top at
10-15 km to the east of the plant. One hundred twenty-eight crosswind
traverses were made with a mobile van during which the S02 plume from the
Dickerson Plant was encountered. Sampling from the van was supplemented
by pibal releases during the sampling period, by temperature and wind
profiles measured two or three times a day with radiosondes and instru-
mented aircraft, and by wind and lateral turbulence measurements at two
levels (10 and 100 meters) on an instrumented tower near the plant.
Stability classes based on Dulles data were computed every
3 hours for June 1973, and for each such stability class the standard
deviation of the fluctuation of horizontal wind direction (ae) was com-
puted from lower and upper (10 m and 100 m) tower winds at Dickerson for
three 10-minute periods centered around the Dulles reading. Wind direction
observations were taken every 10 seconds at the Dickerson tower. A
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classification system was developed (Slade 1960) with a ae value of 25°
assumed to be equivalent to the A stability class and with oe values
ranging down to 2.5° for F stability. This system was then applied to
both the 10 m and 100 m levels. For the 10 m level the median stability
class using Dickerson data for each 30-minute period were tabulated against
the stability class using Dulles data. Low-level turbulence was greater at
Dickerson than at Dulles, as the medians for Dickerson were more often
less stable than the respective stability classes for Dulles. However,
the Dickerson 100 m level tended to be more stable than Dulles. This was
to be expected since Dulles represents low-level turbulence over flat
land. The Dickerson topography caused extra low-level turbulence, but
this mechanical turbulence was dissipated by the time it reached 100 m.
Using the standard Gaussian formula without terrain adjustment
(i.e., assuming the plume remains at a height equal to the Initial effective
stack height over complex terrain), and with the assumption that stability
classes determined from weather observations at nearby Dulles Airport are
applicable in the area of the plant, calculated concentrations stated by
the authors to under-estimate 128 measured five minute ground level S02
center!ine concentrations by as much as two orders of magnitude. The
model was then rerun using the assumptions that (1) the plume remained at
a constant height above MSL regardless of location (this was applied for
all stability classes, as for the EPA single-source CRSTER model), and
(2) the presence of rougher terrain effectively shifted the stability class
by one class toward more unstable conditions. The modification resulted
in an improved relationship between calculated and measured values, the
former being within a factor of two of the latter most of the time.
NAUGHTON PLANT
North American Weather Consultants (Spangler et al. 1973) con-
ducted a six-day plume tracking experiment in the vicinity of the Naughton
Plant (owned by Utah Power and Light) near Kemmerer, Wyoming, under "a
variety" of meteorological conditions. Their initial draft summary report
pertained mainly to results obtained during one day (March 25, 1973), on
which an extremely stable air mass dominated the area, and the wind was
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blowing towards the three surface S02 monitors. The study had three
objectives: (1) to measure the initial plume dilution during the momentum-
dominated phase of plume rise, (2) to measure the magnitude of the plume
centerline concentration downwind, and (3) to relate plume measurements
to ground concentration measurements from three existing monitoring
stations. Along with S02 measurements, aerial temperature profiles were
taken; one just upwind of the Naughton Plant and the other 4,8 km down-
wind. Surface wind observations were made at the three SC^ monitoring sites,
and two pibals were launched at mid-morning on March 25, The temperature
profiles showed the very strong inversion in the lowest 470 m, and the winds
at the plume level were 4,8 m/s and 4,6 m/s from the west for the two
pibals launched. This led the researchers to classify the morning as a
class F stability case.
In trying to discover the amount of initial plume dilution after
the momentum-dominated phase of plume rise, a helicopter penetration into
the plume, in an upwind direction (toward the stacks) done at various
angles and altitudes, was used. The results indicate that the calculated
concentration directly above the stack centerline was higher than the
measured values. Out of 8 dilution factors calculated from measurements
on various days, 5 were in the 45-50 range, with the full range being
23-60. No pattern of dilution factor versus stability condition was
immediately obvious. On the morning of March 25 (the extremely stable case),
the initial stack concentration was calculated to be 211 ppm S02- Initial
dilution after the momentum-dominated phase of plume rise was measured at
4.90 ppm at the plume centerline. In this case, the reduction factor was 43.
The second objective of the study was to determine the plume
centerline concentration downwind. The centerline of the plume was
located and measured with the helicopter by flying -downwind and "wagging"
(side to side) and "porpoising" (up and down) to stay on the centerline
of the plume. The centerline S02 concentration results obtained during
the March 25 case are presented below. At 4.8 km and 24.1 km downwind of
the plant, cross-wind traverses were flown at incremental altitudes. At
4.8 km the range of the maximum concentration at the different levels
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ranged from 0.08 ppm at 2300 m MSL to 1.06 ppm at 2450 m MSL to 0.07 ppm
at 2600 m MSL. At 24.1 km the range was from 0.11 ppm at 2350 m MSL to
0.59 ppm at 2450 m MSL to 0.07 ppm at 2500 m MSL.* No traverses were made
below 2350 m MSL or above 2500 m MSL 24.1 km downwind of the plant. A
"porpoising" maneuver between 2400 m MSL and 2550 m MSL 14.5 km downwind
from the plant gave a range of S02 concentrations of 0.05 ppm at 2400 m MSL
to 0.78 ppm at 2450 m MSL to 0.06 ppm at 2550 m MSL. These measured
results were compared with the SOp concentrations predicted by the
Gaussian distributions used in the NOAA model (Turner 1969).
The comparisons were for one day during which the area was under
the influence of a high pressure system, with accompanying very stable
lapse rate conditions. Further, the comparison was made only for measured
values obtained in the plume at distances out to about 20 km east of the
plant, where terrain features are much less rugged than the mountainous
areas to the west of the plant. The results of the comparison were simi-
lar to those obtained under similar stability conditions at the Four
Corners and Navajo Plants, specifically, greater model overestimation of
concentrations occurred at distances close to the plant than at greater
distances. The NOAA model S0£ centerline concentrations were greater
than 7 times those observed at distances within 2 km of the Naughton plant,
but were within a factor of 2 at distances beyond 7 km. As for the compari-
sons made at the Four Corners and Navajo plants, the failure of the model
to adequately take into account the initial dilution (a factor probably not
related to surrounding terrain) was also the cause of the model-to-measure-
ment discrepancies close to the plant. _
The third objective of the study was to relate plume measure-
ments to ground concentration measurements from three existing air quality
monitoring stations located at about 2100 m MSL. Data from the ground con-
centration measurements at the three existing air quality monitoring stations
were only reported for the very stable case on March 25, 1973. The plume
was observed to clear all terrain by a minimum of 120 m. No significant
The plant elevation, not given in the report, is about 2100 m MSL.
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concentrations were recorded in the morning, but after destabilization of
the lower atmosphere up to the height of the plume at 1045 MST (marked by a
sudden decrease in elevated centerline concentrations), concentrations
slightly above the background concentrations were recorded. The maximum
concentrations before 1045 MST were 0.015 ppm at 0900 and 1000 MST. The
maximum afterwards was 0.046 ppm at 1800 MST. While the S02 concentrations
measured after 1045 MST were higher than the background levels, they were
still considered to be low.
All three surface SOo monitors were east or northeast of the
plant; no surface samplers were located in the more mountainous terrain
to the west of the plant where plume impingement on elevated terrain might
be more likely to occur. It is not known whether any airborne sampling
was conducted in the latter area; if it was, results were not discussed
in the initial summary report. Owing to the limited nature of the surface
sampling network, as well as the short time period characterizing the
study as a whole, any conclusions which could be made from the data would
be of limited significance with regard to the effect of complex terrain
on the behavior of the Naughton plume.
FOUR CORNERS PLANT
Plume tracking studies covering somewhat similar time periods
(less than one week) were conducted 1n the vicinity of the New Mexico Four
Corners Power Plant by North American Weather Consultants (Niemann 1973)
and Meteorology Research, Inc. (Smith and Anderson 1974). Both of the
studies Involved airborne sampling of pollutants (S02 1n the first case
and both S02 and NOX in the latter case).
The objective of the MRI study was to test certain factors in
the NOAA Model. These factors were: (1) The possible effects of buoyancy
and exit velocity on the initial plume dilution, and (2) The subsequent
rate of plume growth downwind. The objectives of the NAWC study were to
measure: (1) the initial dilution factor in the initial momentum-dominated
phase, (2) the plume centerline concentration, (3) plume geometry, (4) the
crosswind concentration distribution downwind, and (5) the dissipation due
to turbulence over flat and complex terrain.
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Three days were selected as test cases by MRI: February 17, 18,
and 19, 1974.* Terrain downwind of the Four Corners Plant for the Febru-
ary 19 case was not exceedingly rough, but the important features included
a drop of 122 m in terrain within the first 1.6 km of the plant site, with
several subsequent rises at various buttes within 16.1 km. Turbulence
dissipation measurements were taken for each case by a "Universal Indicated
Turbulence System" aboard the aircraft. Greater dilution rates were observed
for February 17 and 18 than for February 19.
Considerable variation existed from one day to the next, depend-
ing on the particular roughness fetch, without any change in stability
category as given by the temperature lapse rate. On February 17, concen-
trations remained lower than predicted throughout the entire observed
plume travel (29 km), indicating enhanced dilution. Smith and Anderson
considered turbulence measurements necessary in accurately estimating
plume diffusion, particularly when the layer containing the plume is not
connected to the surface layers by turbulent mechanisms (i.e., the plume
was above the low-level drainage flow). They concluded that significant
differences in plume dimension and dilution could occur without any major
change in the stability category as a result of turbulence variations at
the plume level.
—" Comparisons of normalized plume center!ine concentrations of S02
(i.e., Xu/Q values) obtained aloft from airborne plume tracking observations
under stable (E and F) conditions were made to those calculated using the
NOAA model. For E stability, the model calculated values were greater than
the observed values at a distance of 1 km downwind by a factor of about 5
on February 17, 1974, and by a factor of 15 on February 15, 1974 and
February 19, 1974. For the F category, model calculated values for 1 km
downwind were greater than observed by a factor of about 7 on February 17,
1974, and a factor of about 30 on February 15, 1974 and February 19, 1974.
At distances approaching 12 km downwind the measured values approached the
model predictions to a very close degree; within a factor of 2 for the
E category and within a factor of 3 for the F category in all cases.
* Data were collected on a total of five days - February 15, 17, 18, 19, and 20, 1974.
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Niemann (1973) has compared instantaneous S02 plume center!ine
concentrations observed aloft for the Four Corners Plant under neutral and
stable conditions to corresponding values calculated from the conventional
Gaussian formula for flat terrain and from the stable version of the NOAA
model, respectively. He found that both models overestimated the measured
concentrations close to the plant by a factor of 10 (i.e., under both
neutral and stable conditions), while at distances greater than 15 km the
conventional Gaussian calculations generally agreed with observed values
while the NOAA model overpredicted center!ine concentrations by a factor
of two under stable conditions.
These results agree at least qualitatively with the comparisons
made by Smith and Anderson (1974), in that the Gaussian models show greater
overestimation near the plant, while being within a factor of two at
greater distances. In both works (by Niemann, and by Smith and Anderson)
the authors attribute the initial overestimation to enhanced initial
dilution from plume momentum and buoyancy. In one of the three stable
cases examined by Smith and Anderson, and in the two cases examined by
Niemann, the observed centerline concentrations at larger downwind dis-
tances remained less than the model values, by factors ranging from 2 to 5.
Niemann suggested that this was a result of increased horizontal plume
meander and oscillation, and vertical undulation over the complex terrain
in the vicinity of the Four Corners Plant. Smith and Anderson have
suggested that observed differences in turbulence dissipation rates
accounted for the variations which they observed in downwind plume
behavior under similar stability conditions (i.e., stable) as defined
by the vertical temperature gradient.
In the February 19, 1974 case, near the stack (0.8 km), both
the width and height of the plume exceeded the model values for both
stability categories (i.e., a by almost a factor of 3 for F stability).
This effect was probably caused by rapid initial dilution of the plume
by buoyancy and turbulent mechanisms which may or may not have been
terrain induced. At 8.85 km downwind the observed lateral spread was
still much greater than the model values (about a factor of 2), but the
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observed az became less than either the model E or F values. The observed
a dimension was found to lie between the model E and F values at 29 km;
the observed a remained much less than the model value for either cate-
gory (i.e., a factor of 3 less for E stability). Therefore, rapid growth
of the plume occurred initially followed by much slower growth than pre-
dicted by the model for either E or F stability.
Assuming that ay and az are power functions of downwind distance
from the source, the following exponent powers were found to be applicable
at downwind distances of 8.8 to 29 km compared to the powers assumed in
the NOAA model for February 19, 1974:
For the relation:
a(x) = xp
x = downwind distance
Observed
NOAA model E
Stability
Model F Stability
p = exponent
ay
0.35
0.88
0.88
az
0.32
0.44
0.38
These data indicate that the downwind lateral spread was much
less than predicted by standard techniques and the vertical spread (oz)
was somewhat less than suggested by the observed temperature lapse rate.
In retrospect, had the additional time and financial resources been
available, certain supplemental data would have been extremely helpful
for further insights into the behavior of the plume. The NAWC study was
carried out with limited meteorological observations, with one vertical
temperature sounding being taken near the plant before each plume tracking
flight and no upper level winds in the vicinity of the plant. Each of the
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MRI flights consisted of more frequent temperature soundings at various
distances downwind, and were supplemented by upper-level winds. With
regard to the NAWC study, an additional downwind vertical temperature
measurement would have been useful in at least one case, when the plume
seemed to be fumigating to the surface after the lone early morning
sounding had indicated a low level inversion. In neither study were
sufficient wind data obtained to accurately determine flow characteristics
over the entire area being sampled (e.g., the frequent upper level wind
soundings taken in the MRI study were at one location, about 5 km north
of the plant).
One of the two major objectives of the program of study conducted
by Smith and Anderson (1974) at the Four Corners Power Plant was to investigate
the dynamics of plume impingement on obstacles in the plume path. However,
these investigators soon learned that only limited knowledge of the plume's
impact on the surface could be gained without a ground-level sampling system.
An important factor that affects the ground-level concentration is whether the
plume remains in the drainage flow or is able to penetrate it into the synoptic
flow above. Smith and Anderson concluded that for drainage flows of less than
1.5 m s~^ the plume would penetrate and rise above the drainage flow but
absence of a ground sampling system prevented verification of this conclusion.
The analysis of data was a bit more detailed in the MRI study; both observed
horizontal and vertical plume dimensions (standard deviations) and normalized
concentration data were compared to values predicted by standard Gaussian model
values. However, the comparisons were only for a very limited range of meteor-
ological conditions (stable lapse rate and light drainage winds). Although the
NAWC study compared only observed and calculated center!ine concentrations, the
comparisons were for a greater (although not appreciably wide) range of meteor-
ological conditions (neutral stability with mode&ate winds and a stable lapse
rate with light winds). Neither study encompassed more than a few days.
NAVAJO PLANT
The Navajo Plant, the largest electric generating plant in the State
of Arizona, has been the subject of the greatest number of studies of plants
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operating in complex terrain. The research performed to date include daytime
and nighttime tracer studies by North American Weather Consultants (NAWC 1974,
Hovind et al. 1973b), extensive ground-level and aircraft monitoring of S02
(Rockwell International, MRI and SAI, 1975) and model-to-model comparisons
of Navajo data (INTERA 1975).
The Navajo station is located at 1326 m MSL in a large natural
basin with the Colorado River cutting across the center. Flat mesas rise
up to 1850-2450 m MSL within 40 km of the plant, and Navajo Mountain
(3166 m MSL) is located 48 km to the east. The two most significant terrain
elements for plume impaction are Vermilion Cliffs 22.5 km to the west and
Leche-e Rock 8 km to the southeast. The Vermilion Cliffs are part of a
larger elevated landform (Paria Plateau) which rises somewhat abruptly
in the vicinity of the Cliffs, while Leche-e Rock is a relatively narrow
landform with a flat top; thus, these features represents two distinct
types of elevated terrain which could conceivably alter the plume direction
in different ways (i.e., the plume could flow around Leche-e Rock much more
easily than around the Paria Plateau). The two locations also represent
areas whose elevations are close to that of the predicted plume centerline
under stable conditions.
The daytime tracer study (NAWC 1974) was designed for the
purpose of evaluating the quantitative impact of fluorescent particles
(FP), smoke, and equivalent SO^ impact upon surrounding terrain as well
as the documentation of possible unfavorable meteorological-topographic
effects on plume rise, transport, and diffusion during conditions other
than those postulated as 'worst case1 (i.e.. stable, light-wind) conditions
in the NOAA (Van der Hoven 1972) Southwest Energy Study. The nighttime
tracer study (Hovind et al. 1973b) had as its objective the simulation of
plume transport and diffusion under postulated worst case (plume impingement
conditions.
In both studies, FP tracer was released from the 236 m stack
and from aircraft simulating an effective stack height of 210 to 250 meters
above the stack. The period of the nighttime tracer program was characterized
by nocturnal inversions extending above stack height, but not as high as
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the simulated effective stack height. Continuous recording of winds at
stack top and numerous pibal and temperature soundings at various locations
near the plant site were taken during the releases. The nighttime tracer
study was supplemented by six continuously recording wind stations, most
of which were located on elevated terrain, 8 to 40 km from the plant, which
equaled or exceeded the effective stack height under stable conditions.
Both the daytime and nighttime tracer studies were part of a larger diffusion
climatology program conducted by NAWC at the Navajo site from May to September
1973. The meteorological data included pibal and temperature soundings,
and surface and stack top wind measurements. The extensive meteorological
data were used to construct time cross-sections of the wind flow fields for
periods including the tracer releases. No attempt was made to quantify the
tracer behavior in terms of model parameters which could use this data as
input (i.e., plume geometry as a function of downwind distance, or various
types of terrain). However, a Rotorod network strategically distributed
around the Navajo Plant allowed for a quantitative evaluation of the impact
of FP tracer upon the most prominent elevated terrain features at critical
elevations between stack top and effective stack height.
The FP releases did not support the theory of direct center!ine
impingement on elevated terrain. In the night case of June 5-6, 1973,
the maximum impact from a stack top release was on elevated terrain to
the south and southwest of the plant within 24 km, but there was no
distinct maximum impact zone. High FP counts were distributed over a large
sector of elevations. On June 6-7, 1973, the FP plume was initially trans-
ported toward elevated terrain, but the plume never reached the surface.
The authors have hypothesized that the plume may have become entrained in
a downdraft associated with strong wind shear induced by the rising terrain,
since the maximum impact was at an elevation lower than the release point.
This phenomenon is evidence that strong vertical transport can occur near
elevated terrain even in stable conditions. FP tracer releases showed
greatest impact on the sampling network under light wind looping conditions
on summer days. No quantitative information on the degree of turbulence
enhancement by elevated terrain was given, however, the effect was expected
to be less than for nighttime stable conditions.
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The possibility exists, of courses that the plume centerline in
the FP experiments may have reached the surface, but not directly over the
network. Conditions in wintertime may be more stable and similar experi-
ments may show even further enhancement of plume meandering. In the day-
time study, plume impingement was investigated under limited mixing
conditions and during inversion "break-up" conditions. Visual smoke
plumes showed downwash within 1.6 km of the plant with strong winds.
Smoke releases upwind of elevated terrain showed no direct plume center-
line impingement as predicted by the NOAA model, as the plume was trans-
ported either over or around obstacles. The greatest impact on elevated
terrain occurred with turbulent downward transport of the plume during
transitional fumigation periods between stable morning conditions and
neutral/unstable afternoon conditions. Plume meandering occurred fre-
quently at Navajo under stable, light wind conditions due to interaction
between the gradient flow and the diurnal thermal circulations induced
by terrain.
Oil smoke was also released just upwind of Leche-e Rock and
Ka1parow1ts Plateau, two prominent terrain features in the vicinity of
the Navajo Plant, There was no evidence of direct centerline impinge-
ment on Leche-e Rock. However, later in the morning increased mixing
associated with a transitional fumigation period allowed transport of
the smoke into the rills of Leche-e Rock. At Kaiparowits Plateau, down-
slope drainage flow caught some of the smoke, but the remainder rose
305 m over the plateau and there was very little diffusion to the surface.
Hovind et al. (1973b) have made comparisons of NOAA model calculations
with equivalent $03 ground concentrations determined from measurement of FP
tracer concentrations resulting from releases from the Navajo stack (or by air-
craft at the estimated effective plume height prior to the beginning of the
plant's operation). The comparisons were for nighttime tracer releases under
stable conditions, and for maximum hourly concentrations resulting from releases
of 1 to 3 hours duration. For both the aerial and stack top releases, the NOAA
model values ranged from a factor of about 20 to a factor close to 300 times
greater than the measured equivalent $63 concentrations. The possibility
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of less than TOO percent dispersing efficiency was discussed in the NAWC
report describing these comparisons; however, the point was made that even
if an extreme value of 50 percent reduction in dispensing efficiency is
assumed, the maximum S02 measured would still be nearly two orders of
magnitude below the calculations. Their comparison between equivalent
maximum hourly S02 concentration values and values predicted with the
NOAA Impaction Model in 7 tracer releases show the model over-predicting
by factors of 20 to close to 300.
INTERA Consultants (1975) have made comparisons between calculated
S02 values, using the NOAA, EPA "VALLEY" and INTERA models, and maximum
measured concentrations collected in the vicinity of the Navajo Plant.
The S02 data representing Vermilion Cliffs were taken from five monitoring
sites located in a line atop the Cliffs which are approximately crosswind
when the wind is blowing from the plant toward the Paria Plateau. Hourly
average S02 data from these five sites for the three days on which the
highest S02 hourly averages occurred atop the Cliffs were compared with
models. The highest hourly average concentration atop Leche-e Rock was
also compared to model calculations. Aerial plume centerline measurements
for each of these four days were additionally compared to values calculated
by the standard Gaussian model (as presented in the EPA workbook, Turner
1970) and the INTERA model values.
The model input parameters for wind speed and direction were
specified from pibal data near the plant site (Page Airport) and surface
wind data (combined to estimate likely speed and direction at time of
high concentration). Temperature soundings made mostly near the plant
provided the basis for stability estimates. Comparisons are made for
the range of stabilities and windspeeds thought to be appropriate for the
travel time to Vermilion Cliffs. Limited mixing was assumed when appro-
priate, as in the Leche-e Rock case. The wind direction which provided
the "best fit" of values calculated by the INTERA model to the observed
data was used for all three models. The effective plume height was
obtained from aerial S02 measurements on each day. The comparisons for
the three Vermilion Cliffs cases and the Leche-e Rock case are shown in
Table 12.
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TABLE 12. MODEL AND MEASUREMENT COMPARISONS FOR PEAK HOURLY AVERAGE
SO2 CONCENTRATIONS (PPM) OBSERVED AT ELEVATED TERRAIN FEATURES NEAR
NAVAJO GENERATING STATION
Site -Date
Vermillion Cliffs
(10/16/74)
Vermillion Cliffs
(11/24/74)
Vermillion Cliffs
(12/2/74)
Leche-e Rock
(12/10/74)
Stability
(Windspeed,
m/sec)
F(l.S)
F(2.1)
E(l.S)
E(2.1)
F(1.5)
F(2.1)
E(1.5)
E(2.1)
F(1.5)
F(2.1)
E(1.5)
E(2.1)
Limited
mixing
(1.5)
Model Calculations (ppm)
NOAA
1.25
0.89
0.46
0.33
1.02
0.73
0.38
0.27
0.80
0.57
0.31
0.22
0.39
EPA "VALLEY"
0.20
0. 14
0.11
0.08
0.17
0.12
0.08
0.06
0.13
0.09 '
0.07
0.05
0.21
INTERA
0.11
0.08
0.078
0.058
0.087
0.057
0.066
0.048
0.16
Maximum
Measured Peak
Hourly SO2
(ppm)
0.102
0.102
0.102
0.102
0.070
0.070
0.070
0.070
0.069
0.069
0.069
0.069
0.071
Note: The EPA "VALLEY" model was not designed for the calculation of hourly
averages, hence the results shown here for "VALLEY" are of limited relevance.
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It is seen that for the input values specified, the NOAA model
overpredicts by a factor of 3-15, while the VALLEY and INTERA models are
generally within a factor of three. For the Vermilion Cliffs cases, the
VALLEY model performs the best if E stability is assumed, while the
INTERA model performs the best if F stability is assumed.
The comparisons with aerial measurements were made in each case
for three or four downwind distances, generally between 3 and 22 km from
the plant. The model calculations were expected to be somewhat lower than
the measured values since the former corresponded to a one-hour average
at a particular point and the latter to an "instantaneous" value. However,
this was not the case for the aerial data collected on the high concentration
days at Vermilion Cliffs. At closer distances to the plant (4-8 km), the
INTERA model overpredicted the plume centerline concentration on all three
days by a factor of 2 to 3 for the meteorological conditions under which
it performs best with respect to ground level concentrations (F stability,
1.5 m/s); while approximately equaling the centerline values for E
stability and 1.5 m/s. The INTERA model calculations for either condition
at 5 km were generally within 20 percent of observed values but low by a
factor of 2 to 4 at 22 km. The Gaussian calculations for either E or F
stability (without sector averaging as employed by the EPA model] were
somewhat higher than the INTERA model predictions (by up to a factor of
1.5) at close distances to the plant (about 8 km), while the calculations
were of about equal value at greater distances.
Extensive ground-level and aircraft monitoring of S02 was under-
taken near the Navajo Generating Station and the results were presented in
a report assembled by Rockwell International, MRI, and SAI (1975). Since
the purpose of the project was to determine if additional SO,, removal
would be necessary at the plant, field measurements were conducted during
the season of the greatest occurrence of stable conditions (fall and
winter), as determined from climatological records.
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The principal measurements obtained from the field program
included three-hour average stack emissions, ambient S02 concentrations,
surface winds from a wide range of terrain elevations (including those
potentially affected by plume impingement), pibal soundings at least four
times daily (increased to 12 times daily during intensive study), and
radiosonde and aircraft temperature soundings (once and twice daily,
respectively). Particular to this study among all of those reviewed
were the following:
• Classification by terrain location and meteoro-
logical conditions of the highest measured ground-
level concentrations.
® Comparison of S02 peak (five minute) to mean
(3 hour) ratios for elevated terrain near the
Navajo Plant (high enough to be impacted by the
plume) to equivalent ratios obtained by Montgomery
et al. (1972) at TVA's Paradise Plant located in
relatively flat terrain.
• Spatial interpolation of three-hourly SO? con-
centrations for samples located on elevated
terrain (at or near plume impingement height) to
estimate daily three-hour SC^ maxima.
• Utilization of results from several previous field
programs* conducted at the same site in the design
of the field sampling program.
Vermilion Cliffs was the area most significantly affected, with
direct impingement being the dominant mechanism. The study looked at three
peak cases, and compared these high concentration days to 23 other non-zero
low concentration days. It was concluded that the peak concentrations of
S02, determined with respect to Q, resulted from: (1) the plume centerline
* Two of these studies were the daytime and nighttime tracer studies conducted by Hovind et al. which
were previously discussed; the other two were background measurements obtained by Dames and Moore
(1973), and another six-day field study conducted by Smith and Anderson (1975) which was not
available at the time of this report but is summarized in the Rockwell report.
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being near the top of the cliffs; (2) a steady plume direction over at
least a 3-hour period; and (3) light winds (1 to 4 m/s) at plume level.
The major factor separating cases with slightly lower S02 values from
the peak cases was variable wind direction. Cases with the greatest S02
impingement on the cliffs usually were characterized by the plume being
forced down in front of the cliff.
The prime requirement for significant S02 concentrations on
Leche-e Rock was steady wind direction toward this area for several hours.
This condition was met only twice during the program. Plume hetght and
wind speed were then critical for determining the actual concentrations.
Monitoring sites were located on top of each element of elevated terrain
to insure interception of the plume if impingement occurred.
HUNTINGTON CANYON
Several independent tracer studies have been carried out in
Huntington Canyon, Utah, a very steep-walled canyon with generally moun-
tainous terrain.
A study by the Air Resources Laboratory of NOAA (Start et al.
1973) was conducted in March and April of 1973, with the purposes of
(1) determining the nature and extent of plume impaction on elevated ter-
rain and (2) comparing dilution within rough terrain to that expected
over flat, open terrain. The instrumentation network was of appreciable
density, consisting of (1) 40 SFs tracer gas surface samplers located at
various points on the canyon walls and floor (sampling averaging time of
about one hour), primarily up-canyon from the stack release point, and
within 10 km of the latter; (2) nine surface wind speed and direction
recording stations, six of which were up-canyon from the stack release point;
(3) aspirated resistance thermometers at three levels on the 183 meter stack
of a power plant which was under construction at the site; and (4) a Bell
Helicopter which collected one minute plume samples. The number of SFs
tracer releases was limited to 11, with 6 of the releases being from the
elevated stack; The latter were made exclusively during daytime lapse
conditions. One elevated release was made during inversion conditions,
that being from a point of elevated terrain which jutted into the canyon.
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Comparisons were made between observed center!ine values and corresponding
values calculated by the standard Gaussian plume model and stratified by
stability categories.* Calculated versus observed plume center con-
centrations were found to range from minimal differences during moderate
to strong temperature lapse, to canyon dilutions of five times that of
"standard" flat terrain curves, to values about fifteen times more dilute
than standard curves for strong inversions. The phenomenon of enhanced
mechanical turbulence is believed to be responsible for the greater
effluent dilutions with increasing stability in this study.
The data summarizing the results of the elevated release tests
for D stability* are presented in Figures 14a and 14b from Start et al.
(1973). Aerial centerline samples are represented by 0, ground-level
samples are depicted by X; Figure 14a includes both helicopter and ground
samples, while 14b represents helicopter samples only. Figures 15a and
15b are comparable to Figures 14a and 14b but are for ground-level releases
for conditions of stable down-canyon flow. The solid curves represent
the expected flat terrain concentrations.
* See Table 11 for category definitions.
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io
-2
io-3
10-4
10-5
o-
3
1Q
-6
io
-7
10-9
102
103
Distance (m)
10°
Figure 14a. Helicopter and Ground Samples for D stability.
0 Aerial samples
X - Canyon wall and floor samples
10'
,-2
10'
,-3
io-4
O
"3- 10-s
X
10
,-6
10-8
10'
,-9
102
IO3 IO4
Distance (m)
10°
Figure 14b. Helicopter Samples only for D Stability Elevated Release Tests
0 - Aerial samples
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10
-2
X 10"
io-
io-
-7
IO
Distance (m)
Figure 15a. Helicopter and Ground Samples for F Stability Release at
Ground-Level in Conditions of Down-Canyon Flow
0 - Aerial Samples
X Canyon Wall and Floor Samples
10
-2
10
-3
10
-4
O
10
-5
10
-6
10
10
-7
"oN
10
-9
IO2 IO3 IO4 IO5
Distance (m)
Figure 15b. Helicopter Samples only for F Stability Release at Ground-Level
in Conditions of Down-Canyon Flow
0 - Aerial Samples
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North American Weather Consultants (Hovind et al. 1973a) con-
ducted a series of three separate tracer experiments, each of one week's
duration, in order to provide a quantitative description of the air
quality impact from a 430 MW power plant site in Huntington Canyon, and
also to evaluate the applicability of the NOAA and Standard Gaussian
plume centerline models.
Tracer Program III, the most extensive of the three tracer
experiments, consisted of FP tracer and smoke releases from the estimated
effective stack height as well as helicopter smoke releases near elevated
terrain. The FP tracer concentrations were subsequently converted to
equivalent S02 concentrations. Six surface stations monitored temperature
and winds at various locations within the canyon, while pibal and aircraft
temperature soundings were made up to three times a day. Included in the
sampling network was a special sampling grid on one of the canyon walls
aligned in a principally crosswind direction.
Aircraft measurements of the turbulence dissipation parameter
(e) were made with a Universal Indicated Turbulence System. This param-
eter, which is actually a measure of the turbulence dissipation rate in the
inertial subrange of eddy sizes, indicates the rate of energy flow from
larger to smaller eddies, and has been related to a turbulence magnitude
scale. Values of e were recorded that indicated moderate to heavy turbu-
lence even when stabilities were classified as stable or extreme:ly stable.
Greater values of e were found over Huntington Canyon than over Castle
Valley, a nearby terrain feature characterized by less roughness.
In the plume tracer studies by Hovind et al. (1973a), the plume
tended to follow a meandering path during up-canyon transport along a
mean direction outlined by the elevated boundaries of the main and side
canyon. Persistent plume transport in the stable air mass across Hunting-
ton Canyon toward the major topographic features was never observed,
although the plume would at times move briefly toward that direction only
to be deflected into an up-canyon direction while approaching the canyon
walls. Therefore, the NOAA model assumption of plume transport across
canyon onto elevated terrain was in this case unjustified.
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During a week of observations in January 1973, all except one
of the days were characterized by stable conditions, and the remaining
day was neutral. Model-to-measurement comparisons by Hovind et al. (1973c)
indicated that the NOAA model overestimated centerline concentrations by
10 times at 2.5 km downwind, but was closer to unity beyond 10 km from the
plant site under stable, light wind conditions. No information was given
regarding ay and az, except that the plume influence on the sampling grid was
rather uniform for stable conditions. This could either mean that a was
much greater than the lateral extent of the sampling grid (400 m) or that
the plume was non-Gaussian. There is also serious question as to whether the
aircraft release was equivalent to a point source, thus making comparisons
to model calculations unjustified.
In another analysis effort by NAWC, Hovind et al. (1974) described
studies of FP and oil fog dispersion that were undertaken at three sites
with different roughness characteristics in the southwestern U.S. to deter-
mine the effect of terrain roughness upon plume dispersion from an elevated
source. The three sites were characterized as follows: (1) a canyon with
gradually rising canyon floors and steep walls; (2) a plant site located
at relatively high elevation compared with surrounding terrain, with the
general area being characterized by considerable ground roughness; and
(3) relatively smooth terrain but with some low bluffs and cliffs in the
direction of rising terrain. The authors do not disclose the names of the
three sites in the report, although the first site seems representative
of Huntington Canyon. Figure 16, from the report, provides a depiction
of cross-section profiles at the three sites.
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'^-" PLAN
I.'OJ -
9 CO -
SITE A
PLANT SITE B
PLANT SITE C
25 20 15 10 5 0 5 10 15 20 25
HORIZONTAL P'STAHCE [KILCUETERSI
Figure 16. Terrain Cross-Sections at Three Sites of Varying Degrees
of Roughness (from Hovind et aL 1974)
Aircraft FP tracer releases upwind of selected elevated terrain
areas were employed at the first site. Six such releases were studied in
January 1973, under stagnant atmospheric conditions with temperature
inversions existing immediately above the release altitude. Oil fog was
used as a tracer at the latter two sites. Data representing four cases
characterized by neutral stability with moderate winds were summarized for
the second site, while nine cases having a stable atmosphere were investi-
gated for the site representing the smoothest terrain.
-143-
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Hovind's et al. (1974) comparison study of three sites with
different roughness characteristics in the southwest U.S. provided ratios
of calculated centerline concentrations to observed values using a stan-
dard Gaussian model with dispersion coefficients as defined in Turner
(1971). The average ratios for each of the three sites were 10.2 for the
canyon site, 6.0 for the rugged open terrain site, and 1.2 for the rela-
tively flat, open terrain site (see Table 13). The authors concluded that
average departures of observed values from model calculations were signif-
icantly high for the two sites with rougher terrain, but minimal for the
open, level terrain. The departures were attributed to increased turbu-
lence in the rough terrain.
TABLE 13. AVERAGE RATIO AND RANGE OF CALCULATED
TO OBSERVED RELATIVE CONCENTRATION VALUES X' cal/X'obs
FOR THREE DIFFERENT TERRAIN CONFIGURATIONS (from Hovind
et al. 1974)
Confined Canyon
(Site A)
Rugged open
terrain (Site B)
Relatively flat,
open terrain
(Site C)
y ' /Y'
cal/X obs
io.2(137;p4)
6.0 (5;53)
1-2 £J>
Pasquill
Stability
F
D
E
No . of
Cases
6
ft
9
For the canyon site the authors used a and a values correspond-
ing to stability classes* C and D as inputs for the model, while observed
meteorological conditions indicated an F stability condition. Since
tracer releases were made below the inversion layer, the plumes, at
least initially, were not in an atmosphere characterized by F stability.
Three mechanisms were hypothesized for the enhanced mechanical
turbulence: (1) gradient flow momentum transport into the canyon;
(2) drainage winds from feeder canyons which, when combined with additional
slope drainage have enough momentum to be carried up the other side of
the canyon; (3) turbulence effects created by the plume winding about
* See Table 11 for stability class definitions.
144-
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protrusions or bluffs into the canyon center. However, there was no
analysis of wind observations and no turbulence measurements to substan-
tiate these flow hypotheses.
The objectives of the study, to observe impaction and dilution
within the canyon were met. It was concluded that transient impactions
on the order of a few minutes may occur at any point during lapse condi-
tions. The results are somewhat limited in significance because the topog-
raphy of the release points varied greatly for the differing meteorological
conditions and because sampling was only conducted for flow to the steep
canyon terrain (the latter fact was acknowledged by the authors), i.e., no
sampling was conducted to the northwest for flows which carried the plume
toward the main valley to the east, away from the mountains.
Looking back upon the series of Huntington Canyon tracer programs
conducted by NAWC, certain supplemental information gathering would have
proven helpful with respect to evaluating the tracer impact under stable or
very stable light wind conditions (e.g., those consistent with hypothesized
worst case conditions). An extension of the study sampling period could
have allowed for investigation of such phenomena as fumigation (observed
in the NOAA study) or the possibility of lower level coupling with the
gradient flow and subsequent cross-canyon flow. The aircraft FP tracer
and turbulence measurements could also have been related in a systematic
way to plume behavior (e.g., attempt to assess plume geometry from com-
bined surface and aircraft FP data, and to relate plume centerline con-
centrations to turbulence measurements). An additional point to be made
is that both the NOAA and NAWC Huntington Canyon programs involved tracer
releases less than or equal to one hour in duration. Had these release
time lengths been of longer duration, concentrations for longer averaging
times might have been calculated, or peak to mean ratios (assuredly appli-
cable for complex terrain) might have been utilized.
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GARFIELD SMELTER
An investigation by NOAA ARL (Start et al. 1974) involved a
short canyon near Garfield, Utah, in a mountain range along the south
shore of the Great Salt Lake. The diurnal cycling of the lake-valley
winds made this site more complex than Huntington Canyon. The tracer
diffusion tests in this mountainous terrain involved the 122 meter Garfield
Smelter stack and had the following objectives: (1) to measure ground-
level and elevated centerline concentrations to quantify the plume dilu-
tion, if any, attributable to rough terrain; (2) to examine the degree of
impaction against the mountain slopes; and (3) to establish a modest data
base as an aid to mathematical analysis of diffusion in complex terrain.
Seven SFg tracer releases were made in as many days from the
elevated stack. Forty samplers were positioned on the mountain slopes
and canyon floor; in addition, 10 continuous wind speed and direction
recorders were placed within the testing area, and pibals were taken both
before and during the release period. Radiosonde ascents were also made
while each test was in progress. The seven elevated releases were all
made under similar estimated stability conditions* (B or C); two ground-
level nighttime tracer releases (no visible tracer was released with the
SF5) were made under inversion conditions, but their discussion was limited
in the report because of difficulties in locating a defined plume centerline.
Model-to-measurement comparisons of centerline concentrations
and lateral plume dimensions were made for all of the elevated releases;
for three of the releases, the ay values from the data were not derived
from samplers positioned in a crosswind direction but rather from esti-
mated plume widths or second moment** calculations. Similarly, az values
were derived from transformations of the Gaussian equation or with cross-
wind integrated concentrations; no actual measurements of vertical con-
centration gradients were made in the study. Plume measurements were not
taken during early morning inversion conditions and so the effect of the
* See Table 11 for stability definition.
** Second moment of the lateral distribution of tracer mass about the center of gravity of the mass
distribution.
-146-
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drainage wind flowing out toward the lake could not be documented. Start
et al. state in their report that "the plume impacted significantly" against
the steep terrain on the west side of the canyon in one test case, while
in other tests the plume conformed more or less to the terrain undulations,
without significant impingement.
Based on visual observations during the tests, the authors sug-
gested several mechanisms which are believed to contribute to the enhanced
lateral spreading and lower center!ine concentrations. These are sum-
marized as follows:
e A plume approaching steeply rising terrain can
experience lateral deflection in an attempt to
flow around the blocking obstacle. This is
particularly true during plume looping conditions
when descending segments neared the ground.
9 Mechanical turbulence, and vertical shear,
believed to arise over and around the peaks
and ridges of mountainous terrain results in
an enhanced turbulent state (one visual
observation of the actual plumes emitted
from two of the smelter stacks revealed that
the plume from the lower stack was being
carried up-canyon while the plume from the
higher stack was moving in a direction 90°
from this, across a ridge in response to the
near gradient flow aloft).
The results of Garfield aerial center!ine measurements for sta-
bility classes B and C are exhibited in Figure !7a, b from Start et al. (1974)
A least-squares, first-order curve fitting of the data points is repre-
sented by the dashed line in both graphs. Plume measurements conducted
over level terrain are denoted by the 0 symbols in Figure 17b. The solid
lines are the expected curves over flat terrain.
-147-
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x 10
10
10 10
Distance (Meters)
10
Figure 17a. Results of Garfield Aerial Sampling
Test with Stability Class "B" Conditions
10
10
10
-9
10 10 10
Distance (Meters)
10
Figure 17b. Results of Garfield Aerial Sampling
Test with Stability Class "C" Conditions
The researchers who conducted the Garfield SF6 study (Start et
al. 1974) made comparisons of observed normalized (xu/Q) plume centerline
concentrations with calculations from standard curves presented in the
EPA Workbook (Turner 1970). Observed and calculated lateral plume spread-
ing were also compared. Xu/Q values of the seven elevated releases of
SFg conducted in lapse conditions were two to four times less than pre-
dicted by the standard curves. However, data from four of the tests were
not analyzed because of complicated plume geometry. In the three tests
that did have a reasonably Gaussian shaped plume, a exceeded the expected
value by a factor of two. The enhanced lateral spreading may be due to
avoidance of terrain by the plume. There was a tendency for the plumes
to be deflected laterally in an attempt to flow out and around blocking
obstacles. Values of oz however, were in general less than those for
flat terrain. The aerial plume centerline comparisons were essentially
the same as those described in the next paragraph, made by INTERA for the
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NOAA model (since the latter is based on the same curves), and thus gave
the same results. Lateral spreading was examined in a quantitative manner
in the Start et al . (1974) study by (1) selecting slices across the iso-
pleth patterns which were normal to the observed direction of plume travel;
(2) extracting analyzed concentration values along each slice to obtain
reasonable approximations of the concentration distributions; and (3) cal-
culating a values by both the plume "width" method and the second moment
J
method (both methods resulted in a values which were approximately the
same). A comparison of these observed values with standard a values for
the appropriate stability classes (B or C) showed the average ratio of
observed a to Pasquill-Gifford a to be 1.79.
The data from these three NOAA Garfield tests were also compared
with the NOAA, EPA "VALLEY," and INTERA model calculations (Intera
Consultants 1975).
The input data were determined as follows:
• The plume rise was obtained by extrapolating the plume
centerline as determined from helicopter observations,
back to the source.
• The INTERA model calculations were performed using wind
profiles (1) from pibal data at the site and (2) from a
standard power law for unstable conditions.
• The stability class was determined from the objective
criteria suggested by Turner* (1970).
The models were compared by INTERA Consultants (1975) by using
two different statistical measures. One method, involving the ratio of
the calculated mean value to the observed mean value ground-level concen-
tration, tends to give a greater weight to the larger measured and calcu-
lated values. This ratio (Rl) can be mathematically stated as:
Rl =
(OBS
* See Table 11 for criteria used.
-149-
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where
XCALC
XOBS
= calculated concentration
= observed concentration.
The second method gives an average of the calculated to observed ratio
at every specific sample point. This logarithmic mean ratio (R2) is
mathematically described as:
R2 = exp
1n (XCALC/XOBS)
N
N = number of points.
The log mean ratio gives approximately equal weight to large
and small measured and calculated values. Model-to-measurement compart
sons for ground-level concentrations are summarized in Table 14.
TABLE 14. GARFIELD DATA MODEL-TO-MEASUREMENT COMPARISONS
OF GROUND-LEVEL CONCENTRATIONS
Test
2
3
7
Estimated Pasquill Stability
C
C
B
Model
INTERA - Pibal
INTERA Standard
NOAA
EPA VALLEY
INTERA - Pibal
INTERA Standard
NOAA
EPA VALLEY
INTERA - Pibal
INTERA - Standard
NOAA
EPA VALLEY
Rl
1.66
0.92
0.26
0.36
0.57
0.45
0.35
0.34
2.50
1.50
1.50
2.67
R2
1.13
0.82
0.20
0.36
0.40
0.44
0.26
0.36
1.56
1.25
0.78
2.23
-150-
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Analysis of these three tests, using R1, show the INTERA model
ground concentration results to be within a factor of 2.5 and the results
of the NOAA model to be within a factor of 3.9. For Test 7, the NOAA node!
performs as well as the INTERA model when the standard profile was input to
the latter (Rl = 1.50), and better than the INTERA model with a pibal wind
profile.
Additional comparisons were made with observed data in the
three tests for predicted plume centerline concentrations as a function
of downwind distance, and for how well the observed concentration pat-
terns were reproduced spatially.
The helicopter samples, corresponding to an averaging time of
the order of one minute, were converted to one hour values by the EPA
Workbook technique (Turner 1970) to correspond to the model calculations,
which represent one hour averages. The results of the comparisons for
the aerial centerline data are 35 follows:
• The INTERA model calculations using standard
wind profile data were lower than observed
values by factors from 1.3 to 4.0 in two of
the cases and were within a factor of 1.5 in
the last test.
• The INTERA model values calculated using
pibal wind data were within a factor of
1.5 in two of the three tests, but were
higher than plume centerline values by a
factor of 2 to 4 at distances beyond 1 km
in the last test.
• The NOAA model calculations were greater
than observed centerline values by a factor
of 2 to 5 at distances within 4 km of the
plant in two of the three tests and for all
distances in the last test; calculated values
approached observed values at distances
greater than 4 km in Tests 2 and 3.
The results of the model comparisons for accuracy of reproduction of the
observed concentration pattern showed the INTERA model predicting the
-151-
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location of the maximum concentrations accurately in two of the three
tests, while the NOAA model was accurate in this regard in only one of
the tests. The NOAA model predicted the location of the maximum concen-
tration in a valley in two of the three cases.
Comparison of the heights of the maximum in-plumes concentrations
indicate that the NOAA model assumption of parallel to terrain plume center-
line flow in complex terrain under unstable conditions is invalid for the
unstable conditions characterizing the days evaluated. For these days,
the heights of the maximum in-plume concentrations over the ridges was
in general half those over the valleys. The INTERA model usually predicted
the heights with good accuracy, although it did not show quite as much con-
formance to the terrain features as actually observed.
KAIPAROWITS
A recent FP and oil fog tracer study has been conducted by
North American Weather Consultants (1974) in the vicinity of the proposed
Kaiparowits Generating Station which was to be located in the rugged canyonland
of southern Utah. Some of the details of this study were not available at
the time of this report. However, it is known that the study was fairly
limited in duration, consisting of 10 FP tracer releases in May of 1974,
7 at one location in the area and 3 at another. Each of the releases was
supplemented by measurements of the vertical temperature profile, vertical
wind profile, and turbulence measurements. FP recoveries from four ele-
vated tracer releases made by aircraft, two of which were at one proposed
site (Fourmile Bench), and two of which were at the Nipple Bench site,
were chosen for evaluation. One of the Fourmile Bench tests was character-
ized by early morning stable conditions, while the other three tests were
characterized by unstable conditons. The principal interest in the study
lies in the subsequent use of the data in model-to-measurement comparisons
by Hoffnagle et al. (1975).
Initial model calculations were made using the stability cate-
gory observed at the time of each test, and pibal wind profiles constructed
for each test. Once the initial model calculations had been compared to
-152-
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the measured data, the models were adjusted individually to provide the
best possible simulation within the limits of model flexibility.
Hoffnagle et al. (1975) have tested the EPA "VALLEY," NOAA, TVA
coning model and INTERA models against fluorescent particle tracer data
obtained by North American Weather Consultants at two proposed sites for
the Kaiparowits Generating Station. It is important to note that the TVA
model (which, like the NOAA model, is actually a set of Gaussian calcula-
tional techniques, with each technique being appropriate to a different
meteorological condition) was not specifically designed for application
in complex terrain situations; however, the comparison here is of interest
because the model differs from the other standard Gaussian models (EPA
"VALLEY," NOAA) in that the diffusion coefficients used (a and a ) are
not the same as the standard coefficients as described in Turner (1969),
and were essentially derived from observations of power plant plumes. A
different set of equations is used for computing maximum S02 concentra-
tions from power plants under coning (neutral stability), inversion breakup,
and limited mixing conditions; the INTERA comparisons were based on TVA
"coning" model calculations in each case.
Initial model calculations were made using the stability cate-
gory, observed at the time of each test, and pibal wind profiles made
during each test (for determination of wind speed in the case of the
Gaussian models, and wind profile in the case of the INTERA model). Once
the initial model calculations had been compared to the measured data,
the models were adjusted individually to provide the best possible simu-
lation within the limits of model flexibility.
Table 15 shows the results of the comparison of initial model
calculations with the FP data.
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TABLE 15. COMPARATIVE MODEL RESULTS OF THE RATIOS OF THE MEAN CALCULATED TO
MEAN OBSERVED VALUES OF EACH OF FOUR TRACER TESTS NEAR KAIPAROWITS (after Hoffnagle et 3.1. 1975)
Model
NOAA
TVA
VALLEY
INTERA
Tracer Test HI
No. of Receptors
Evaluated
20
20
8
20
Ratio
0.7
O.S
1.5
1.7
Tracer Test 12
No. of Receptors
Evaluated
12
12
3
12
Ratio
1.3
0.4
2.3
1.7
Tracer Test #3
No. of Receptors
Evaluated
18
18
7
18
Ratio
0.8
1.5
1.1
1.9
Tracer Test #4
No. of Receptors
Evaluated
20
20
6
20
Ratio
1.2
0.3
0.8
0.9
• Of the 3 to 20 receptors evaluated in each case, the NOAA
model had the best mean ratio of calculated to observed
concentrations in three of the four cases, and its mean
ratio was within 30 percent of unity in all four cases.
• None of the remaining three models had consistently better
mean ratios than the other two. The TVA coning model
generally underestimated (three of the four cases); the
EPA "VALLEY" model had mean ratios from 0.8 to 2.3; and
the INTERA model had mean ratios from 0.9 to 1.9.
• None of the four models had consistently good correlation
coefficients, and thus did not accurately calculate plume
behavior as a whole. The INTERA model had the highest
single case correlation of 0.8. The INTERA, TVA and
NOAA models had correlations exceeding 0.5 in only one
case, while the "VALLEY" model always gave correlations
below 0.5. (But it should be noted that the "VALLEY"
model was never intended for use in short-term
predictions.)
The "plume" recoveries in at least one of the cases (the release
under assumed stable conditions) suggest that inversion breakup may have
been occurring; the fact that no meteorological data were available to
confirm this (as had been the case in the comparisons for the Navajo
Generating Station) points out the need for better documentation. It also
suggests that application of TVA's "inversion breakup" Gaussian equation
might have been more appropriate in this case. The results of the other
-154-
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cases imply that the terrain in the Kaiparowits area is characterized by
more enhanced dispersion for the given situation than would be expected
from conventional Pasquill-Gifford measures of stability.
LAPPES PROGRAM
Description of the Site
The Large Power Plant Effluent Study (LAPPES) was initiated
during 1967, and conducted during a subsequent five-year period, to evalu-
ate the extent and effects of air pollution resulting from three coal
burning generating stations with tall stacks (Conemaugh, Keystone, and
Homer City) in rolling to mountainous terrain 1n Western Pennsylvania.
The four-volume report on the LAPPES program by Schiermeier et al. (1970-
1972) dealt mainly with data presentation and was not concerned for the most
part with any data analysis, other than to qualitatively describe a few
obvious cases of terrain influence on dispersion from the Conemaugh Station
(located in the roughest terrain of any of the plants) as evidenced from
airborne monitoring near that plant during April and October of 1971,
The terrain in the vicinity of the Keystone, Homer City and Conemaugh
Plants is composed of rolling hills rising 100 - 200 meters above
valley floors and ridges rising 200 - 450 meters above valley floors.
The major features are Chestnut Ridge, oriented NE-SW between the Homer
City and Conemaugh stations, and Laurel Ridge which is considerably
higher and located to the SE of Conemaugh.
The Keystone Station is situated in a shallow rural valley with two
244 m stacks the base of which are at 305 m MSL. Except for this valley, the
surrounding terrain within 5 km is hilly, with the highest peak reaching no
more than mid-stack height, i.e., about 120 m above the plant. The Keystone
Power Station is about 25 km to the northwest of the closest ridge (Chest-
nut Ridge). So the influence of topography was not evident from ground-
level measurements near Keystone Plant.
The Homer City Station, with twin stacks at a height of 244 m, is
located on a plateau, with the surrounding terrain falling off as much as
100 m from the stack base height of 366 m MSL. The highest peaks within 5 km
-155-
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are those of Chestnut Ridge to the SE, which range slightly higher than
midstack height, i.e., about 170 m above the plant. East of the ridge
is another plateau which received higher S02 concentrations at ground
level than were found at a similar distance in any other direction. This
phenomenon may have been a lee effect of Chestnut Ridge.
The Conemaugh Station is the most susceptible to terrain influ-
ences. Separating this plant from Johnstown to the SE is Laurel Ridge,
which has peaks reaching 180 meters above the stack top of 305 meters
within 10 km of the plant. These hills along with the slightly lower
Chestnut Ridge to the west may cause channeling of the plume. Schiermeier
(1972) observed that under neutral conditions flow from the SE quadrant
(passing over Laurel Ridge before reaching the plant) brought the plume
to the surface very quickly. Ground-level concentrations then diminished
with further distance, but then increased again on the lea side of Chestnut
Ridge. Downwash on the lee side of Laurel Ridge, confirmed by subsidence
of pilot balloons in the vicinity of the plant, was responsible for
bringing the plume to the surface near the plant. Surface heating on
sunny days precluded downwash. However, under northwest flow the plume
rose over Laurel Ridge and mixed in a deep layer in the lee of the ridge.
Only low concentrations were measured at ground level. No lee waves were
detected downwind of Laurel Ridge. This lack of downwash supports the
idea that roughness fetch plays a large role with regard to plume transport
and dispersion characteristics. It is known from previous studies (Egan
1975) that flow separation in complex terrain may either take the form of
large, reverse, stationary eddies, or "shed" vortices. It is conceivable
that the former was being observed near the Conemaugh Plant with southeast
winds (the downwash conditions), while upwind roughness characteristics
favored the second type of separation flow with northwest winds, since down-
wash was then absent.
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Terrain Roughness Parameters
In the course of preparing this present report GEOMET undertook
special analyses of data from the LAPPES and other studies to assess the
impact of terrain-induced turbulence on the diffusion parameters a and
j
az in a Gaussian dispersion model. In order to quantitatively measure
the terrain variation, ground elevations beneath several plume center!ine
paths were plotted from USGS topographic maps. The standard deviations of
the elevations along the plume center!ine paths out to various distances
were computed. A space interval of 200 m was used for plotting elevations.
Calculated Standard Deviation of Elevations (SDE's) are tabulated for com-
parison (Table 17) with the differences between plume parameters measured
in the plume and conventional values for the same plume estimated by the
methods of Turner's Workbook (1970).
Figure 18 shows the terrain cross-sections of 14 plume paths,
plotted in various compass directions from the Keystone plant out to 16 km
distance, together with the corresponding SDE values. The abscissa shows
downwind distance in kilometers; the ordinate shows elevation above sea
level in meters. (Note that the vertical to horizontal scale ratio is
0.06). The figure shows that larger SDE numbers correspond to more com-
plex terrain. For example, the highest calculated SDE value, 58.2 m, is
noted for the direction toward the east; the smallest calculated SDE value,
24.3 m, occurs to the south-southwest. The elevation profile to the south-
southwest contains smaller variations than that of the profile toward the
east. Figure 18 is included as an example of the technique for characterizing
terrain cross-sections. Similar calculations were made for the other SDE
values shown in Tables 16 and 17.
Plume Cross-Sections and Meteorological Data
Standard deviations (a and a ) of the observed S02 concentrations
in LAPPES plume cross-sections were statistically computed by GEOMET through
integration of the vertical concentrations and crosswind concentrations
-157-
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TABLE 16. DISPERSION PARAMETERS a AND
-------
TABLE 16. (Continued)
Plant
Keystone
Wind Sector
4
5
6
7
8
10
11
13
14
15
Distance
From Plant
(km)
16
4
10
16
4
10
16
4
4
10
10
16
4
10
4
16
4
SDE
(m)
37.9
25.2
33.3
58.2
32.4
40.0
36.5
27.1
19.8
24.3
31.5
30.6
34.6
32.3
42.0
42.5
33.1
Field
Test
1
1
2
3
4
1
2
3
1
2
1
1
1
1
1
2
1
1
1
1
1
1
2
1
1
2
a
(m)
417.8
473.9
291.5
365.9
386.6
687.5
527.0
561.9
783.1
757.5
276.3
441.5
1807. 3
565.3
881.7
220.0
1699.9
378.0
558.8
220.9
546.1
261.4
672.5
553.5
406.4
634.4
CTZ
(m)
50.6
41.3
103.6
64.7
80.4
60.2
57.4
108.4
52.2
34.9
53.6
58.7
78.2
63.3
29.4
49.2
66.2
48.5
55.9
109.4
47.4
88.7
84.4
63.7
121.3
108.6
(Continued)
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TABLE 16. (Concluded)
Plant
Keystone
Conemaugh
Wind Sector
15
16
1
2
3
4
5
6
7
15
16
Distance
From Plant
(km)
10
16
10
10
4
4
10
4
10
10
4
4
10
16
10
SDE
(m)
31.0
30.1
33.7
66.6
43.1
40.8
57.9
25.4
92.3
175.3
105.0
117.6
166.8
65.0
66.8
Field
Test
1
1
1
1
1
1
2
3
1
1
2
3
4
1
1
1
1
1
1
1
ay
(m)
880.4
841.1
336.2
781.6
322.5
304.8
321.4
280.7
1109.1
357.4
515.9
386.5
314.4
657.1
1169.4
465.1
587.8
1142.3
1461.2
945.1
OT.
(m)
81.4
78.6
39.6
46.8
45.2
38.3
42.8
62.6
79.8
48.9
73.2
56.6
74.0
39.4
90.5
93.9
91.3
74.1
106.2
79.3
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TABLE 17. SUMMARY OF OBSERVED AND STANDARD a VALUE, OBSERVED MINUS STANDARD, AND EQUIVALENT PASQUILL
STABILITY CLASS FOR COMBINED KEYSTONE AND CONEMAUGH DATA
Travel
Distance
(km)-
4
10
16
Number
of
Cases
4
14
6
5
2
7
12
4
3
1
2
10
7
3
SDE
(M)
16.8- 19.8
25.2- 27.2
30.5- 34.7
40.8- 43.1
105.0-117.6
23.1- 24.3
31.0- 33.7
37.3- 40.0
58.2- 66.8
92.3
166.8-175.3
28.5- 30.7
36.5- 42.5
58.2- 65.0
ay (m)
Observed
430.3
401.8
512.8
371.7
526.4
1016.5
776.5
578.5
945.3
675.1
1155.8
1110.8
882.3
985.7
WADE
Estimate*
239.3
239.3
239.3
239.3
239.3
543.6
543.6
543.6
543.6
543.6
543.6
825.1
825.1
825.1
Observed
Minus
Estimate
191.0
162.5
273.5
132.4
287.1
472.9
232.9
34.'9
401.7
131.5
612.2
285.7
57.2
160.6
KY**
2.6
2.8
2.1
3.0
2.0
2.4
3.2
3.8
2.6
3.5
2.1
3.3
3.9
3.5
°z(m)
Observed
53.0
63.1
90.4
86.6
92.6
66.8
57.6
57.6
68.6
39.4
82.3
62.4
79.2
43.6
WADE
Estimate*
77.5
77.5
77.5
77.5
77.5
134.9
134.9
134.9
134.9
134.9
134.9
176.0
176.0
176.0
Observed
Minus
Estimate
-24.5
-14.4
12.9
9.1
15.1
^-68.1
-77.3
-77.3
-66.3
-95.5
-52.6
-113.6
-97.8
-132.4
KZ**
4.9
4.4
3.8
3.9
3.8
5.3
5.6
5.6
5.2
6.0
4.9
5.8
5.3
6.0
cr>
* Estimate from Workbook for Atmospheric Dispersion Estimates (Turner 1970).
** KY and KZ are the approximate Pasquill Stability Class (i.e., A = 1, B = 2, etc. ) required to represent the observed value.
-------
455
394
333
CTi
ro
i
W
394
333
272
455
394
333
272
455
394 _
333 _
272 —
•f—H-
I
10
I
n
i
12
I
13
I
14
r
15
16
N
SDE = 33.5 (m)
Wind Sector 1
NNE
SDE = 28.3 (m)
Wind Sector 2
NE
SDE = 37.1 (m)
Wind Sector 3
ENE
SDE= 37.7 (m)
Wind Sector 4
Down Wind Distance (km)
Figure 18 , Profile* of Elevation Toward 14 Directions from Keysto".: Power Plant. Standard deviation of
elevation rcai.1 by an increment of 200 m horizontally arc also indicated. (Continued)
-------
CO
I
455
394
333
272
455 _
394
333
394
333
272
I I I 1 I I I I I I I I I I I
H 1 j 1 1 1 1 1 1 1 1 1 1 1 h
1 1—H 1 1 1 1 1 K-
H 1 1 1 H
I 1 1 1 1 1—H 1 1 1
I I I I I I I I I I
67 8 9 10 11 12 13 14 15
Down Wind Distance (km)
16
SDE = 57.8 (m)
Wind Sector 5
ESE
SDE = 32. 2 (m)
Wind Sector 6
SE
SDE = 36. 2 (m)
Wind Sector 7
SSE
SDE = 27.0(m)
Wind Sector 8
Figure ^ • Profiles of Elevation Toward 14 Directions from Keystone Power Plant. Standard deviation of
elevation read by an increment of 200 in horizontally are also indicated. (Continued)
-------
I
cr>
i
272
6789
Down Wind Distance (km)
16
SSW
SDE = 24.2 (m)
Wind Sector 9
SW
SDE = 30.4 (m)
Wind Sector 10
W
SDE = 31.6 (m)
Wind Sector 11
WNW
SDE =42.4 (m)
Wind Sector 12
Figure 18 . Profiles of Elevation Toward 14 Directions from Keystone Power Plant. Standard deviation of
elevation read by increment of 200 m horizontally are also indicated. (Continued)
-------
cr>
en
455
394
333
§ 272
I I 1 I
rt
I
W
455 —
394
333
272
6 78 9 10 11
Down Wind Distance (km) •
I
12
13 14
\
15
16
NW
SDE = 29.9 (m)
Wind Sector 13
NNW
SDE = 33.5 (m)
Wind Sector 14
Figure 18 . Profiles of Elevation Toward 14 Directions from Keystone Power Plant. Standard deviation of
elevation read by an increment of 200 m horizontally are also indicated. (Concluded)
-------
respectively. Because the statistical method used is valid for Gaussian
distribution curves only, 436 plume cross-sections were examined manually
and 28 percent (122), which appeared to be near the Gaussian bell shape,
were selected for further study. Wind speed and solar radiation, measured
at Jimmy Stewart Airport in Indiana, Pennsylvania in the LAPPES program,
were used to determine the stability classes* for each measurement period.
Skewness was calculated for horizontally integrated concentra-
tions measured at the Conemaugh Power Plant. Although the plume cross-
sections used were selected to be nearly Gaussian in shape, vertical
concentrations (horizontally integrated) were generally skewed toward the
upper half of the plume. Seventy-two percent of the cross-sections were
skewed toward the upper half of the plume, 17 percent were skewed toward
the lower half of the plume, and 11 percent had no skewness. It is our
speculation that these results indicate more rapid mixing in the lower
portions of the plume than in the upper portions, perhaps due to a terrain-
induced turbulence effect or due to thermally induced turbulence due to
heating of the ground surface. A stronger possibility may be that the
skewness was caused by a shearing-off of plume remnants as the main part
of the plume rose to its buoyant level or into an inversion aloft.
Dispersion parameters, a and a in a Gaussian dispersion model
are considered to be a function of downwind distance and atmospheric sta-
bility. Conventionally, atmospheric stability is treated as a function of
wind speed and solar radiation or lapse rate. This primarily represents
the effects of thermally-induced turbulence; but the terrain-induced
turbulence, which is very important in a complex terrain area, is not
accommodated by the Gaussian dispersion model. If terrain-induced turbu-
lence has no effect on plume dispersion, then we would expect that the
differences between observed ay and az and the values calculated by con-
ventional methods (i.e.. Turner's Workbook) will on the average be zero.
If significant differences are observed, the deviation should correlate
with terrain characteristics such as the SDE values.
* See Table 11 for category definitions.
-166-
-------
Plume measurements were sorted by their center!ine direction
into 16 compass directions. Observed values of a and a were averaged
for each stability class, plume travel direction and distance. All cases
selected were for neutral conditions (class D). Homer City data were
omitted because of the small number of observations. Table 16 shows
observed a and a values and SDE values for each plume-centerline direc-
J
tion and distance from the plant. In order to see the influence of SDE
on a and a , we grouped the data by ranges of SDE values and downwind
distances (4, 10 and 16 km) and computed the mean ay and az for each
group in Table 17. Also tabulated are ay and az values which have been
measured, values estimated from Turner's Workbook (1970), and their dif-
ferences. The measured ay and az values were used to determine equivalent
stability classes, KY and KZ, based upon the observed horizontal and
vertical concentration distributions, respectively. These values are
also tabulated in Table 17. From this table one can observe that the
impact of terrain on the horizontal dispersion parameter (ay) is more
significant than on the vertical dispersion parameter (az). If one
examines the average of KY values for a fixed travel distance, an increase
in the average KY value with distance is noted. At 4 km distance, the
average KY is 2.5, while out to 16 km distance, the average KY is 3.6;
this is closer to the actual stability class of 4. This indicates that
the terrain influences on ay are more significant at closer distances
from the plant; the terrain influence on az is more important at further
distances from the plant. With increasing distance the influence of
terrain on the plume becomes smaller. The correlation between SDE and KZ
is not recognizable. It should be recognized that these calculated SDE's
depend to some extent on downwind distance and AX (the distance increment)
as well as the nature of the terrain; instead of a specific direction,
consideration of a sector may be warranted.
KINGSTON POWER PLANT
The TVA's Kingston Power Plant has been the focal point for
several intensive field programs. These programs exceed the magnitude of
the Navajo and LAPPES studies in scope of data collections. A computer
-167-
-------
analysis of Kingston Plant data similar to the analysis of LAPPES data was
performed by GEOMET to examine terrain-influenced dispersion within the plume;
this work is described below.
Description of the Site and Monitoring Program
The Kingston Steam Plant is situated in eastern Tennessee about
56 km west-southwest of Knoxville and 3.2 km northeast of Kingston. The
plant lies on a peninsula formed by the junction of the Emory and Clinch
Rivers. Flowing southward from the plant site, the Clinch River combines
with the Tennessee River some 4.8 km downstream. Within a radius of
1.6 to 4.8 km, the plant is almost totally encompassed by ridges 107 to
122 m higher than the plant grade. The terrain continues to be hilly
outside the immediate area, with ridges aligned northeast-southwest and
extending to elevations about 244 m higher than the plant grade.
Intensive field studies of plumes were conducted in the
vicinity of TVA's Kingston Power Plant for three two-to-three month peri-
ods in the winter and early spring of 1973 - 1975. Only preliminary
analyses of data had been made at the time of this report and these were
essentially limited to calculations of ay. The most impressive aspect
of this field monitoring program compared to the others previously described
was the number of different modes of S02 monitoring used, and more impor-
tantly, the coordination between these modes which resulted in use of each
to its optimal advantage.
For example, in the 1975 sampling period, a fixed-wing aircraft,
van, helicopter, and 11 fixed surface monitors were used on a daily basis,
in addition to plume photography. The van and helicopter were both equipped
with S02 monitors and served to locate the portable monitoring in areas of
high S02 concentrations. Airborne crosswind plume measurements were made
at fixed distances downwind. Thuss the analyses of plume geometry could
be made for this two-month period in late winter-early $pring, The three
data collection periods featured intense collection (from the standpoint
of temporal resolution) of upper air meteorological data, especially with
respect to temperature soundings, which were made on an hourly basis by
-168-
-------
aircraft. In the LAPPES program four to six helicopter temperature pro-
files plus two radiosondes were taken each day. However, it was evident
from the discussion of the Navajo results that spatial resolution in
temperature soundings may be more important than temporal resolutions in
complex terrain.
In a series of experimental studies at the TVA Kingston and
Johnsonville Steam Plants plume S02 concentration data taken from flights
of fixed-wing aircraft were used by Nilsson (1975) in a preliminary anal-
ysis, to calculate a values, employing three different methods. The first
method is a statistical approach in which the Gaussian curve is approximated
by a series of straight lines by taking several points on the graph. The
second method is the area under a curve approach, where a (area) is obtained
by integrating the Gaussian curve equation. The third method is a peak-based
approach whereby the curve peak is measured and the standard deviation is
obtained by dividing the calculated width between the points where the ordin-
ates of the graph are a certain fraction of the peak. Linear regression is
performed on the a values calculated by one method versus the a values
•J J
produced by another. For the Kingston Power Plant a (stat) is greater
•J
than a (area) in all cases, but when the S02 data taken near the Johnsonville
Plant are adjusted to account for instrument time lag (this data adjustment
procedure is not done for the Kingston graphs), the resulting regression
showed a slope much closer to unity. These results are summarized in Table 18
The study did not focus on plume-terrain effects at all in the program, but
was primarily concerned with the application of different statistical tests
on plume location and dimensions. An analysis of the Kingston data by TVA
is in progress.
TABLE 18. <*Y VALUES AS OBTAINED BY NILSSON (1975) AS WELL AS CORRELATION
COEFFICIENTS AND SLOPES OF THE REGRESSION CURVES
(Ty
Correlation
Coefficient
Slope
Kingston
Area vs Stat
396,612
.716
.464
Johnsonville
Area vs Stat
489,420
.879
.755
10 Percent vs Stat
490,415
.963
.816
10 Percent vs Area
415,414
.915
.914
-169-
-------
In the GEOMEl analysis of Kingston data, terrain characteristics
were determined using elevations read from USGS topographic maps for 200 m
increments between the source and the six S02 monitoring stations. Figure 19
shows elevation cross-sections from the Kingston Steam plant to six monitor-
ing stations, together with SDE values. Ground-level S02 measurement stations
were labeled from 1 through 6. Data were analyzed for the following time
periods.
Station No. 1 1/72 to 6/72, 1/74 to 12/74
Station No. 2 1/74 to 12/74
Station No. 3 1/74 to 12/74
Station No. 4 1/74 to 12/74
Station No. 5 1/74 to 12/74
Station No. 6 7/72 to 12/72, 1/74 to 12/74.
Wind speed, wind direction and temperature measured at about 46 m above
the ground, and temperature measured at 1.22 m above the ground, were
available in the Kingston data. Wind direction was coded in 16 compass
directions. Stack parameters were obtained from FPC Form 67. Gas tem-
perature and exit velocity were calculated from daily coal consumption
rates, BTU content of coal, and BTU rating of the unit. S02 emission
rates were provided by TVA in tons per day for each unit.
An analysis was made by GEOMET between measured concentrations
and those estimated using a standard Gaussian plume dispersion model for
point sources. The dispersion model is a multiple-source Gaussian plume
model, adopted from EPA's UNAMAP (Users' Network for Applied Modeling of
Air Pollution). Plume rise is calculated according to Brigg's plume rise
estimates. The program was modified to use dispersion parameters c and
y
az, based on the TVA curves (TVA 1970). Temperature gradient measurements
were used to determine the closest stability class for the observation
period; no unstable condition (less than -0.13°K/100m) was included in
this analysis.
-170-
-------
272
212
455
394
333
272 —
a 303
.2
4J
at
I
w
242
303
242
272
272
212
I 1 1 1 1 1 1
0 1
SDK = 7.
78
Down Wind Distance (km)
10 11
Figure 19. Elevation Profile from Kingston Plant to Each Receptor
-171-
-------
About 125 cases of 24-hour mean S02 concentrations were cal-
culated by the model using hourly meteorological and emission data for
each of six observation stations. The mean concentrations are summarized
together with SDE values in Table 19. Fifty percent of the model values
are in the same order of magnitude as the observed values; the other
50 percent are off by an order of magnitude or more. The ratios of
computed concentrations to the corresponding observed values are also
shown in Table 19. The principal conclusion to be drawn from these
comparisons is that wind direction resolved to 16 compass directions
is not sufficient to define the location of plume relative to the
monitoring stations. Between adjacent elevation cross-sections there
could be hills or valleys. This emphasizes the short-comings of com-
puting SDE along a small number of specific directions. In reality,
a plume travels in the downwind direction in a narrow-angle sector; and
terrain features in the entire sector influence the diffusion of the
plume. Therefore, it is logical to compute the SDE of a sector.
TABLE 19. COMPARISON OF COMPUTED AND OBSERVED MEAN GROUND-LEVEL SO2
CONCENTRATIONS*
SDE (m)
Mean Competed
Concentration (fig/m )
Mean Observed
Concentration (^jg/m )
Computed Minus
Observed (/^g/m )
Computed Divided
by Observed
Station
1
13.7
63
237
-174
0.27
2
67.4
0
12
-12
3
21.0
56
17
39
3.3
4
29.3
1
13
-12
0.08
5
4.9
176
20
156
8.8
6
7.0
22
63
-41
0.35
About 125 cases were used for each station.
-172-
-------
Correlations between SDE values and the values of xcalc/Xobs
are not evident. In addition to the definition of wind direction utilized,
and the deficiency in the computational method of the SDE, the reasons
for the poor correlation can be (1) the terrain induced aerodynamic effect
is not included, and (2) the small amount of data in the preliminary GEOMET
analyses suggest that a plume dispersion model for complex terrain requires
at least the following capabilities:
1. Inclusion of a means of categorizing terrain
numerically by sectors and by distance from the
source.
2. Inclusion of wind flows of varying pattern rather
than just a uniform flow.
SUMMARY OF FIELD STUDIES
Table 20 (Part A, Plume Sampling Characteristics and Part B,
Meteorological Data) is a compendium of the 16 field studies that were
reviewed in Section IX. The major objective of each of these research
efforts has been the compilation of data regarding the physical behavior
of either stack plumes or tracer material in complex terrain. The format
of this summary includes (in Table 20A) a statement of study title and
investigators, the nature of pollutant (tracer) and source, and a brief
project comment. The information matrix categorically depicts the nature
of the field observations for each study, describing the characteristics
of ground and airborne sampling efforts. The details concerning ground
sampling include the number of samplers, the distance range, the terrain
characteristics of samplers and the number of sampling days. The distance
range, number of sampling days and the sampled plume characteristics are
listed for airborne sampling. Meteorological observations are portrayed
in Table 20B with information concerning methodology of temperature measure-
ments, type and range of aerial turbulence measurements, frequencies of
stable, neutral, or unstable stabilities and type of wind data along with
range of wind speeds and wind direction characteristics.
-173-
-------
TABU iO. SUMMARY OF FIELD STUDIES OF TRANSPORT AND DIFFUSION IN COMPLEX TERRAIN FROM ELEVATED SOURCES
A. Plume and Sampling Characteristics
Study Number
1
2
3
Study Title and Investigators
"P articulate Diffusion Over
Irregular Terrain,
McMuLlen and Perkins 1963
"Diffusion Over Coastal
Mountains of Southern
California,
Hinds 1970
"Plume Dispersion
Studies and Modeling
for Fossil -Fueled Power
Plants,
Well 1974
Nature of Pollutant
(Tracer) and Source
Fluorescent
particles released
from straight line
airplane flight
Releases from ZnS
from ground-level
point sources
SO plume from
Plant
Comments
Some of the earliest FP
diffusion trials ever con-
ducted in complex ter-
rain
Diffusion comparisons
over mountains and
flat terrain show night-
time stable conditions
lead to interactions
between terrain-
induced drainage flow
and synoptic scale
flow in complex ter-
rain
Study clearly demon-
bulence due to rough
terrain
Ground Sampling
Number
of Samplers
=: 100
43
Mobile van
(1)
Distance
Range
11-21 km
= 8 km
Unknown
Terrain
Characteristics
at Samplers
450-600 rn
above valley
floor
30-500 m
above sea level
Unknown
Number
of Days
4
113 tests
Oct 1972
thru Apr
1973 (no.
of days
not known
precisely)
Airborne Sampling
Distance
Range
27 km
-
Number
of Days
4
-
Plume
Characteristics
Sampled
Downwind and
return to release
line
-
-P"
I
-------
TABLE 20. (CONTINUED)
A. Plume and Sampling Characteristics
In
V
|
T3
3
4
5
6
7
8
Study Title and Investigators
"An Initial Evaluation of
Sulfur Dioxide Measurements
Conducted at Naughton
Power Plant,"
Sp angler et al. 1973
"Airborne SO^ Measure-
ments at Four Corners
Power Plant, '
Niemann 1973
"Analysis of Plume Data -
Four Corners Power Plant, '
Smith and Anderson 1974
"Nighttime Tracer Studies
of Power Plant Plumes at
the Navajo Generating
Station, "
Hovind et al. 1973
"Daytime Plume Simu-
lation Tracer and Clima-
tology Program at the
Navajo Generating
Station, ••
North American Weather
Consultants 1974
Nature of Pollutant
(Tracer) and Source
SO from power
plant stack
SC"2 from power
plant stack, com-
parison with NCAA
model results
Visible (oil fog)
, smoke and fluor-
escent particle
tracers
Visible (oil fog)
smoke released
upwind of ele-
vated terrain.
. Fluorescent
particle tracer
to simulate SO2
released by air-
craft at stack
and effective
stack heights.
Comments
During stable flow the
plume remained aloft with
no ground-level impaction
upon the elevated terrain
downwind of the plant
Turbulence dissipation
measurements indicated
enhanced dilution over
complex terrain than
that expected over flat
land
Two cafe* where FP relesie* under
stable nighttime condition! did not
rapport the theory of direct center-
line Impingement OB elevated ter-
rain
Tha greatest lampllng network
bnpkcrj were noted under light
wind looping condition* on
nunmer day?
Ground Sampling
Number
of Samplers
3
None
85
75
Distance
Range
1-5 km
-
Up to
45 km
-
Up to
10km
from
plant or
release
point
Terrain
Characteristics
of Samplers
« 100-250 km
above stack
base
-
*=s up to 10OO nr
above stack
base
s= up to 600 m
above stack
base
Number
of Days
6
-
4
7
Airborne Sampling
Distance
Range
0-15 km
for
extremely
stable day,
not known
for other
five days
Up to
45 km
Up to
18 km
.
Number
of Days
6
4
4
.
Plume
Characteristics
Sampled
Crosswlnd
Ascending spiral,
crosswlnd, along
wind
Crosswind and
vertical
_
I
—I
01
(ContinueHl
-------
TABLE 20. (CONTINUED)
u
1
r
1
9
10
11
12
13
A. Plume and Sampling Characteristics
Study Title and Investigators
"Navajo Generating Station
SO Field Monitoring Pro-
gram,
Rockwell Inl., MRI, SAI
1975
"Summary Report on
Meteorological Power
Plant Plume Simulation
Tracer Studies in
Huntington Canyon, Utah,
Hovind et al. I973a
"The Influence of Rough
Mountainous Terrain Upon
Plume Dispersion from An
Elevated Source,
HovJnd et al. 1974
"Diffusion in Canyon with
Rough Mountainous Ter-
rain, '
Start et al. 1975
"Effluent Dilutions Over
Mountainous Terrain,
Start et al. 1974
Nature of Pollutant
(Tracer) and Source
SO2 from power
plant stack
FF and smoke
releases from
estimated
effective stack
height, by sky-
writer, in addi-
tion to helicopter
smoke releases
in area inacces-
sible by plane *
Fluorescent
particles at one
site and oil fog
dispersion at two
sites
SFg tracer from
stack, elevated
terrain, and can-
yon floor. Also
oil fog flow.
SF^ tracer released
from smelter stack
Comments
(Same aj Study
No. 8)
Extensive supplemental
observational informa-
tion including photo-
graphs of various types
of plume behavior -
much of subjective infor-
mation reported valuable
to understanding flow in
complex terrain - several
physical mechanisms for
the increase of lateral
dispersion observed
Ground Sampling
Number
of Samplers
26
86 rotorod
stations
Used at
FP site
only
Number not
provided
40
^40
Distance
Range
Up to
30 km
Up to
15 km
from
plant
site*
Up to
25 km
0-10 km
0-5 km
Terrain
Characteristics
of Samplers
= up to 600 m
above stack
base
Up to 850 m
above stack
base*
Unknown
Up to =350 m
above canyon
floor
Up to =850 m
above smelter
level
Number
of Days
145 days
total, 80
non-zero
observa-
tion days
7
6
11
7
Airborne Sampling
Distance
Range
Up to
28 km
Sampling
was made,
but details
not discus-
sed
Up to
25 km
10 km
Up to
6 km
Number
of Days
144
_
13
9
7
Plume
Characteristics
Sampled
Crosswlad, along
wind
_
Crosswind and
ceiiterline
Center line
Center! in e
cr>
i
(Continued)
* Refers to Tracer Program III, the most extensive of seven separate field studies (each of about one
week's duration), conducted by NAWC at the Huntington Canyon Plant Site.
-------
TABLE 20. (CONTINUED)
JD
X
1
14
15
16
A. Plume and Sampling Characteristics
Study Title and Investigators
"Elevated FP Tracer Releases
from Kaiparowits Power Plant
Site, "
North American Weather
Consultants 1974
"Large Power Plant Effluent
Study - Vol. 4, "
Schiermeler 1971
TVA Field Smelters in the
Vicinity of Kingston Power
Plant,
1973-1975
Nature of Pollutant
(Tracer) and Source
FP tracer released
from aircraft at
two sites supple-
mented by oil fog
releases
SOT from power
plant stack
SO™ from power
plant stack
Comments
Vicinity of proposed plant
site subject of tracer
studies and model simu-
lation-comparisons
Airborne sampling data
provide large data base
for future studies of ter-
rain influences
Ground Sampling
Number
of Samplers
Not stated
precisely,
>20
Up to 14
11 fixed
sites, plus
portable
bubblers
Distance
Range
_
Variable -
op to
27 km
Up to
10km
Terrain
Characteristics
of Samplers
_
Up to mid-
stack height
Up to 250 m
above the
plant grade
Number
of Days
Not stated
but total of
10 tracer
releases
made
27
Nov-Dec
1973, Mar-
Apr 1974,
Jan-Mar
1975
Airborne Sampling
Distance
Range
Unknown
Up to
16 kjn
Up to
21 km
Number
of Days
_
175
Unknown
Plume
Characteristics
Sampled
_
Crosswind at
several altitudes
Crosswind at
several altitudes
-------
TABU 20. (CONTINUED)
S tudy N um ber
1
2
3
4
B. Meteorological Data
Tpmpe nature
Aircraft Profiles
Sampling at ground
and tower
Hourly radiosondes
Sampling at surface
and 100 m
Aircraft Profiles
2 to 3 Radiosondes
Per Day
Aircraft Profiles
Turbulence
Types of
Measurements
_
_
Lateral (ff )
_
Range of Aerial
Measurements
_
_
_
_
Observed Stability Conditions
Frequency
of Stable
_
_
_
_
Frequency
of Neutral
_
-
_
^
Frequency
of Unstable
_
-
_
_
Other Stability
Information
_
Not known, precisely,
but mostly neutral and
unstable
Not precisely knowna but
majority of sampling
was probabhy conducted
under neutral or unstable
conditions (between H)am
and 4pm only)
Not exactly known, but
stated that plume track-
ing over 6-day period
covered a variety of
meteorological condi-
tions including 1 day
with extremely stable
air mass.
Wind
Types of Wind Data
Radiosonde, tower,
plbal, surface
Radiosonde,, tower,
surface
Radiosonde, tower,
pibal
Pibal, surface
RanRe of Wind Speeds
Light (3)
Strong (1)
Unknown
-
Not known
Wind Direction Characterise! cs
Predominantly SSE,
Unknown
-
Not known, but both easterly
and westerly flow sampled
00
-------
TABLE 20. (CONTINUED)
8
Study Num
5
6
7
8
9
10
n
12
13
14
B. Meteorological Data
Temperature
Aircraft Profiles
Aircraft Profiles
Aircraft ProflUs
Ground Sampling
Aircraft Profiles
Aircraft Profiles
1 Radiosonde Per
Day
Aircraft Profiles
Ground Sampling
A Ircraf t Profiles
Sampling at 3 levels
on 183 m stack
Ground Sampling
1 Radiosonde Per
Day
Some vertical temp-
erature profiles
taken, but number
and mediod unknown
Turbulence
Types of
Measurements
Dissipation Rate
Dissipation Rate
-
Vertical
-
Dissipation Rate
Dissipation Rate
-
-
Unknown
Range of Aerial
Measurements
Yes, sfc. to plume
top
Yes, sfc. to plume
top
-
Not precisely known
-
-
Yes to top of plume
-
-
Unknown
Observed Stability Conditions
Frequency
of Stable
2
4
4
-
-
7
15
5
-
Frequency
of Neutral
1-1/2
-
-
-
-
-
4
5
-
Frequency
of Unstable
-
-
-
-
-
-
0
1
7
Other Stability
Information
1/2 (Transitory inver-
sion breakup)
-
-
7
Not known precisely, but
majority of cases stable
or neutral
-
~
-
~
Not precisely known, but
testa during neutral,
stable, and slightly
unstable conditions
Wind
Types of Wind Data
Radiosonde, surface
Pibal
Pibal, stack top,
surface
Pibal, stack top
Radiosonde, plbal,
stack top, surface
Plbal, stack top,
surface
-
Tower, stack top,
surface
Radiosonde, pibal,
surface
Plbal
Range of Wind Speeds
Nov. cases moderate
Jan, cases calm to
very light
2 days light,
2 days moderate
Very light
Moderate to strong
Wide range
0 to 4 m/sec
Light to moderate
Light
Light to moderate
Unknown
Wind Direction Characteristics
Steady easterly variable
Generally easterly near surface
Great variability
Mostly southwest
Observations divided Into 3 cate-
gories - 2 distinct directions and
remainder
Mostly east-southeast, westerly
on one occasion
Unknown
Mostly southeast and north at
stack top
All between northwest and
east- northeast
Unknown
(Continued)
-------
TABLE 20. (CONCLUDED)
SJ
-0
e
X
~a
2
15
16
B. Meteorological Data
Temperature
Aircraft Profiles
Ground Sampling
2 Radiosondes Per
Day
Aircraft Profiles
Sampling at 1 , 2,
and 48 m levels
Turbulence
Types of
Measurements
Lence measure-
ments by NtRI as
part of LAPPES
LQ spring and
fall of 1968
Dissipation Rate
Range of Aerial
Measurements
Yes, up to 5000 m
Observed Stability Conditions
Frequency
of Stable
Frequency
of Neutral
Frequency
of Unstable
Odier Stability
Information
Measurements made during stable morning and inversion breakup
for each date
-
-
-
Unknown
W!nd
Types of Wind Data
Radiosonde, pibal,
surface
Tower, plbal,
surface
Range of Wind Speeds
Light to moderate
Unknown
Wind Direction Characteristics
Southwest and west dominant
Unknown
co
o
-------
Section X
RECAPITULATION AND COMMENTARY
The analysis of the scope of previous studies of plume (or
simulated plume) behavior in complex terrain, and its relationship to
meteorological, emission, and topographical factors has revealed that
a modest data base, of both theoretical and observational information has
been gathered relative to important phenomena associated with flow in com-
plex terrain. Financial restraints in conjunction with other practical
considerations have dictated that field measurement programs each focus
on a small number of specific objectives. The diversity of geographic
sites selected, the variety in the experimental methods utilized, and
the differences in the analysis of results present a formidable under-
taking if the goal is to consolidate the outcomes of each of these pro-
grams into a flexible, yet coherent, universal set of rules for plume
diffusion in complex terrain. However, certain common denominators have
been found to link the diversity of research projects in the literature,
thus allowing a number of qualitative generalizations regarding plume
diffusion and transport in complex terrain to be presented.
ENHANCED TURBULENCE
Plume studies have indicated that an expected increase in turbu-
lence exists in complex terrain over that found over relatively flat sur-
faces. This conclusion has been achieved through analyses of wind direction
fluctuations, comparison of ay and az measured values to the Pasquill-
Gifford values applicable to flat terrain, and by model-to-measurement
comparisons of effluent or tracer concentrations. As earlier references
have made clear, a major purpose of many field investigations has been
the collection of plume geometry data from which inferences can be made
concerning the levels of turbulence present near power plants located in
complex terrain. It appears from a study of this literature that in
general, aircraft measurements of plume cross-sections yield the best
information about plume geometry. When aircraft measurements are not
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available, comparisons of measured ground-level concentrations to model
calculations of ground-level concentrations in flat terrain may be used
to estimate turbulence enhancement due to the complex terrain.
TERRAIN IMPINGEMENT
One of the important questions to be approached is how stability
affects the proximity of the plume to the surrounding complex terrain.
For those studies in which the stability of the atmosphere was reported
in detail, the stable conditons most frequently characterized the cases
studied, principally because study objectives often were to examine the
validity of hypothesized NOAA model plume impingement on elevated terrain
under these conditions. The Navajo studies (Rockwell, MRI and SRI, 1975)
contained two cases where the few FP releases under stable nighttime con-
ditions did not support the theory of direct centerline impingement on
elevated terrain. Even though the tracer was initially transported towards
critical elevations (between stack top and effective stack height), no
distinct maximum impact zones were indicated, except for one case, and that
was for a location below the release elevation (however, the highest S02
ground concentrations occurred on the cliffs). Evidently, strong vertical
mixing was occurring near the elevated terrain even with the stable con-
ditions, probably as a result of strong wind shear induced by the rising
terrain. Direct impingement upon elevated terrain was also not evident in
the Huntington Canyon studies for stable conditions;* when the plume was
directed toward the canyon walls, the observations indicate that it was
deflected away from the wall before impingement.* During lapse conditions
at Huntington Canyon it was concluded that transient impactions on the
order of several minutes may indeed occur at any point, however the results are
limited in significance; one reason being because the topography of the release
points varied greatly for the differing meterorological conditions. Direct
impingement of a plume against steep canyon walls was observed by Start et al.
* The lone stable release from an elevated position on a natural prominence visually showed the plume
to impinge on the canyon wall, however, it moved up the slope, away from the samplers, so ao
quantitative data were obtained. Kinematically produced deformation effects may have been responsible.
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in one Stability B test case near the Garfield Smelter. In the Navajo
studies, oil smoke releases during transitional fumigation conditions
allowed transport of the smoke into the rills of one of the elevated
terrain features (Leche-e Rock) 8 km downwind; earlier stable conditions
had presented no evidence of direct centerline impingement. The turbulent
downward transport of the plume was expected to result in the greatest
impact on elevated terrain during transitional fumigation periods between
stable morning and neutral unstable afternoon conditions. The greatest
Navajo sampling network impacts were noted under light wind looping con-
ditions on summer days. Most of the field studies have been of limited
duration (generally less than two weeks, and in most cases of the order
of one week), and as a result have examined dispersion over a relatively
limited range of atmospheric stability conditions. However, plume behavior
in complex terrain during documented transitory conditions (especially
inversion breakup) has been investigated and discussed in detail (i.e.,
over periods greater than one week) in the six-month Navajo study by
Rockwell International, Meteorology Research, Inc., and Systems Applica-
tions, Inc. (1975). Many of the observed impingement events occurred
during unstable conditions and primarily at the initiation of unstable
conditions. Temperature soundings atop a relatively flat, elevated ter-
rain feature for the days on which the plume passed close to or inter-
sected that terrain (resulting in the highest observed ground concentrations)
were not conducted, but soundings near the plant suggested that fumigation
rather than stable conditions may have resulted in the high concentrations
observed.
PLUME DIMENSIONS
There is some evidence from the LAPPES program (Schiermeier 1970,
1972), GEOMET's analyses of the LAPPES data (1976), and the Four Corners
studies (Niemann 1973, Smith and Anderson 1974) that horizontal spreading
(i.e., a ) is amplified to a greater extent than vertical spreading in
complex terrain, particularly under stable conditions. The resultant
increase of a has been attributed primarily to plume separation processes
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-------
and plume meandering which occur when the higher terrain elevations are
confronted by the plume. Plume meandering was frequent in the Navajo region
under stable light conditions due to gradient flow interaction with the
nocturnal thermal circulations induced by terrain. Both Smith and Anderson
and Niemann concluded that significant differences in plume dimension and
dilution could occur without any major change in the stability category
as determined by turbulence variations at the plume level. This indicates
that perhaps a more concise way of characterizing dispersion other than
atmospheric stability may be needed in complex terrain. At Huntington
Canyon, moderate to heavy turbulence conditions have been documented with
stable conditions, with enhanced dilution as stabilities increased. Recent
acoustic sounding data shows extensive waves and breaking waves in stable
layers, thus indicating that stable layers are not quiescent. The cooling
and subsequent development of radiation inversions in irregular topography
cause drainage of air down the slopes, and this acceleration of air has to
set off compensating circulations. Thus, inversions and their possible
subsequent anti-mountain circulations can be said to enhance turbulence
in this complex terrain situation. Studies have also shown turbulence
levels in the down-valley and downslope flow to increase as a function of
the inversion intensity, probably due to the role of inversions in the
production of katabatic winds. Lee wave phenomena have also been pointed
out as responsible for increased turbulent effects, and mechanical tur-
bulence and vertical shear are postulated to arise over and around peaks
and ridges of mountainous terrain. As was evident in the Navajo program,
significant interactions between terrain-induced drainage flow and the
prevailing gradient flow aloft may occur near elevated features.
PLUME CENTERLINE CONCENTRATIONS
A comparison study of measured aerial plume centerline S0? con-
centrations to the concentrations predicted by the Gaussian plume model
on one test day characterized by stable conditions near the Naughton Plant
site showed results similar to those obtained near the Four Corners and
Navajo plants for stable conditions. Within 2 km of Naughton for the one
case situation described by Spangler et al. (1973) model S0? centerline
-184-
-------
concentrations were greater than seven times those observed; while beyond
7 km the calculations were within a factor of 2 of the measured values. At
Four Corners (Smith and Anderson 1974) 11 data points representing three
test days were overpredicted by factors of 5 to 30 at 1 km from the stack;
the measured/calculated values were within a factor of 2 beyond 12 km,
and results were similar at 18 km from the plant. Niemann's (1973) com-
parison of Four Corners measured instantaneous S02 plume centerline con-
centrations for six flights under neutral and very stable conditions in two
different seasons support the Spang!er et al. (1973) and the Smith and
Anderson (1974) results. For the neutral condition data, the conventional
plume model for centerline dispersion was used for comparison, and for the
stable data, the NOAA model predictions were used. Both models overesti-
mated the measured concentrations by a factor of 10 under both stability
conditions close to the plant. At distances greater than 15 km, the con-
ventional Gaussian calculations generally agreed with the observed values,
while the NOAA model over-predicted centerline concentrations by a factor
of 2 under stable conditions. Seven comparisons of equivalent maximum
hourly ground concentrations obtained from airborne and stacktop FP releases
and the calculations of the NOAA centerline prediction equation were made
at Navajo (Hovind et al. 1973 draft report). Under stated stable conditions,
model overestimations ranged from 20 to 300 times the actual observed S02
values. At Huntington Canyon, 11 test comparisons between observed plume
centerline concentrations and corresponding values calculated by the standard
Gaussian plume model showed minimal differences during moderate to strong
temperature lapse; canyon dilutions of 5 times to 15 times that of
"standard" curves were evident for strong inversions (Start et al. 1975).
In the Hovind et al. (1973c) study in the same region, the NOAA model
overestimated centerline concentrations by 10 times at 2.5 km downwind,
but approached actual measured values beyond 10 km during one week of
observations in which all except one of the days were characterized by
stable conditions; the remaining day was neutral. The Hovind et al. (1974)
study, compared the ratio of calculated standard Gaussian plume centerline
concentrations to observed values at three sites with different roughness
-185-
-------
characteristics. These ratios ranged from 10.2 for the Huntington Canyon
site under F stability for six cases, to 6.0 for four cases of D stability
in the rugged open canyon site, to 1.2 for nine cases of E stability in
flat open terrain. Near the Garfield Smelter, Start et al. (1974) found
that for this location, characterized by complex air circulations due to
the presence of the nearby Great Salt Lake, measured xu/Q values were 2
to 4 times less than predicted by standard curves in seven elevated FP
releases in lapse conditions. INTERA (1975) verifies these results in
three tests which found NOAA model concentrations to be greater than
observed centerline values by factors of 2 to 5 (for stability categories
of B and C) at distances within 4 km of the plant in two of the three tests,
and for all distances in the last test. Beyond 4 km, calculated values
approached observed values. Hoffnagle et al. (1975) compared the measured
data of four FP tests at two proposed plant sites in complex terrain to
NOAA model calculations where one test was characterized by stable condi-
tions, and the other three characterized by unstable conditions. The
mean ratio of calculated to observed values in all four cases (7 to 20
receptors evaluated in each case) was within 30 percent of unity.
Comparisons of the NOAA, EPA, and INTERA models systematically
show greater overestimation with respect to plume centerline values close
to the Navajo, Garfield, and Kaiparowits sites than at greater distances,
suggesting that neither the Gaussian nor INTERA models in these cases
adequately take into account the initial dilution due to emission-related
parameters (buoyancy and turbulence generated by stack exit velocity and
possible immediate terrain effects). The validity of the comparisons for
aerial data may be limited because of the different averaging times repre-
senting the model calculations (one hour, 24 hours for Valley) and measure-
ments (one minute). The effective plume height used in the model calculations
was determined from aerial data, rather than from plume rise formula; thus,
the true predictive capability of these differing formulae could not be
realistically assessed for the given meteorological and topographical con-
ditions.
-186-
-------
STATE-OF-THE-ART FOR MODELING TRANSPORT AND DIFFUSION OF CONTINUOUS
ELEVATED PLUMES IN COMPLEX TERRAIN
Comparisons of measured plume dimensions to the Pasquill-Gifford
diffusion parameters (a and a ) in complex terrain indicate that, for
both E and F stabilities, rapid growth of the plume occurs initially
followed by slower growth than predicted by the standard Gaussian plume
model. The model values in general underestimate initial lateral spread,
and overestimate vertical spread at greater distances.
Several studies have revealed that greater discrepancies between
modeled/observed concentrations occur under stable conditions than under
neutral conditions. The NOAA Huntington Canyon study, the Niemann 1973
study at Four Corners, and the NOAA model/measurement comparisons made by
INTERA (1975), for Navajo (under stable conditions), Kaiparowits (unstable
conditions) and Garfield (unstable conditions) support this conclusion.
Analysis of the available numerical models developed or
adapted for use in complex terrain from the literature reveal no evidence
of validation for the Environmental Research and Technology (Egan 1975)
modified Gaussian model or the Aerovironment (MacCready et al. 1974)
modified Gaussian model. Research has been done in the past on effects of
topography on airflow (with two outstanding examples being drainage-
counter-drainage winds and hydraulic "jump" flow) but nothing surfaced
during this review that revealed an attempt to develop and validate a
point source air quality model to incorporate these effects. Current
models are incapable of simulating plume behavior when the lower part of
the plume is caught in drainage flow and the upper part of the plume is
in the counter flow. There exists a need to determine how severely down-
wash conditions affect the performance of the available models. Presently
available models represent the influences of terrain by a receptor height
adjustment procedure, or in the case of the INTERA model, by means of a
wind speed profile. The detailed turbulence influences of terrain
characteristics and terrain obstructions are required for reliable esti-
mations of air quality impact; future emphasis must be placed on relating
terrain roughness to dispersion in complex terrain in a quantitative
manner.
-187-
-------
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Appendix B
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/7-77-020
2.
3. RECIPIENT'S ACCESSION1 NO.
4. TITLE AND SUBTITLE
POWER PLANT STACK PLUMES IN COMPLEX TERRAIN:
An Appraisal of Current Research
5. REPORT DATE
March 1977
6. PERFORMING ORGANIZATION CODE
AUTHOR(s)Robert C. Koch, W. Gale Biggs, Paul H. Hwang,
Irving Leichter, Kenneth E. Pickering, Eric R. Swadey,
and John I . Swift . _
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
10. PROGRAM ELEMENT NO.
GEOMET, Incorporated
15 Firstfield Road
Gaithersburg, MD 20760
1NE625
11. CONTRACT/GRANT NO.
68-02-2260
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory - RTP, NC
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
13. TYPE OF REPORT AND PERIOD COVERED
Interim 11/75 - 10/76
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report reviews the literature of scientific studies of the behavior of stack
plumes from fossil-fueled electric power plants in complex (hilly or mountainous)
terrain. Non-conservative chemical transformation and depletion, and conservative
transport and diffusion of pollutants are considered. Studies of S02 oxidation
rates in power plant plumes are described and the primary mechanisms for conversion
to sulfate are detailed. Scavenging of S02 from plumes by precipitation is reviewed
along with surface contact and deposition as important plume depletion processes.
Current theories of airflow, turbulence and diffusion phenomena in complex terrain
are described, and are exemplified through the review of sixteen field observation
programs of the physical behavior of plumes from continuous elevated sources in
complex terrain. The review discusses program objectives, data sampling methodolo-
gies, model-to-measurement comparisons associated with the field program conclusions,
The types of models available to simulate plume behavior numerically are discussed
in the contexts of these programs. In addition, results are reported of an indepen-
dent GEOMET analysis of ay and CTZ relationships in the Gaussian plume model over
complex terrain, using data from LAPPES and TVA field measurement programs.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
cos AT I Field/Group
*Air pollution *Mountains
*Reviews *Atmospheric diffusion
*Electric power plants *Transport properties
*Plumes *Chemical reactions
*Sulfur dioxide *Field tests
*Terrain
*Hills
13B
058
10B
21B
07B
08F
04A
07D
14B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
235
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
221
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