United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-81-078/079 July 1983
&EPA
Project Summary
Evaluation of Short-Term
N02 Plume Models for Point
Sources: Volumes 1 and 2
M. A. Yocke, D. A. Stewart, J. Johnson, and R. J. Frost
Four mathematical models for
atmospheric pollution transport and
reactivity were tested on MISTT data
base for their ability to predict patterns
of ambient NO? concentrations in the
vicinity of large point sources. The
four models tested were: (1) the
Reactive Plume Model (RPM-II), (2)
the Total Conversion Method (TCM),
(3) the Oxidant Limiting Method
(OLM), and (4) the Conservation of
NO* and Oxidant Method (CNOM). Of
these methods, RPM-II was by far the
most sophisticated, and yielded more
accurate predictions than the other
three models.
This Project Summary was devel-
oped by EPA's Environmental Sciences
Research Laboratory. Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report order in-
formation at back).
Introduction
The burning of fossil fuels by automo-
biles, power plants, and refineries
produces appreciable amounts of oxides
of nitrogen (NOX), which are then
released into the atmosphere. These
emissions consist primarily of relatively
harmless nitric oxide (NO); however,
under suitable conditions, such as in the
presence of reactive hydrocarbons and
sunlight, NO can be oxidized to form
nitrogen dioxide (NOa).
Earlier epidemiological studies indi-
cated that long-term exposure to high
ambient NOa concentrations was asso-
ciated with a higher incidence of respira
tory disease. In addition, there has been
some recent concern about the effects
of short-term peak NOa concentrations
on the frequency and severity of acute
respiratory disease. The results of some
studies show that the magnitude of the
short-term peak NOa concentration had
a significantly greater influence than
either the duration or frequency of
exposure. Because of the importance of
these results, the U.S. Environmental
Protection Agency (EPA) is giving
serious consideration to promulgating
short-term NOa air quality regulations.
Part of this consideration is the review
of techniques that can be used to predict
short-term (about 1 h) average, ground-
level NOa concentrations. Specific
attention is being given to the ability to
predict NOa impacts due to NOX emis-
sions from large point sources (e.g.,
power plants and other large combustion
processes).
Under the sponsorship of EPA, Sys-
tems Applications, Inc. (SAI) has selected
several models that can be used to
predict ground-level NOa concentration
impacts from point sources and has
applied the selected models to MISTT
data base (a large aerometric and
meteorological data base collected near
St. Louis) for the purpose of evaluating
their performance. The MISTT data
were selected by EPA because the
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MISTT program involved a large urban
and point source monitoring effort that
included a significant number of aircraft
plume measurements.
Description of Models
The models evaluated in this study
were: (1) the Reactive Plume Model
(RPM-II); (2) the Total Conversion
Method (TCM); (3) the Oxidant Limiting
Method (OLM); and (4)the Conservation
of NOX and Oxidant Method (CNOM).
The last three are very simple methodo-
logies that have been suggested for use
with standard Gaussian point source
dispersion models. RPM-II, on the other
hand, is much more sophisticated,
allowing treatment of time-varying
winds, dispersion rates, reaction rates,
and can accommodate the use of very
large photochemical kinetic mechanisms
These models were chosen to provide a
range of complexity in NO2 predictions,
from the very simplest approach (TCM)
to one of the more advanced reactive
plume models (RPM-II).
In the Conservation of NOx and
oxidant method, the equilibrium rela-
tionship among NO, N02 and ozone (Oa)
is used in conjunction with the assump-
tions of oxidant a nd NOx conservation to
simplify the chemical kinetics. CNOM
suffers the limitation that beyond a few
kilometers from the source its simplified
treatment of chemistry is invalid in
hydrocarbon-rich environments. In
addition, any deficiencies in the disper-
sion parameter computations—the
Gaussian dispersion relationship, for
example, has many known limitations—
will be reflected in predicted N02
concentrations.
An even simpler model is the oxidant
limiting method (OLM). The OLM
assumes that N02 concentrations in the
plume are equal to the background Oa
plus some small fraction of the emitted
NO concentration. The dependence on
the background Os is an acknowledg-
ment that one NO2 molecule is formed
for every reaction of Oa and NO
molecules and that N02 thus may be
limited by the background Oa in an NO-
rich plume. However, it does not
explicitly account for diffusion, entrain-
ment, or the photostationary relationship.
Another simplification, called total or
partial conversion (TCM or PCM),
assumes that all or some fraction of the
emitted NOX is ultimately converted to
N02.
The TCM and OLM suffer the same
deficiencies as CNOM; moreover, these
methods are further limited by their
own, more restrictive, assumptions.
However, in many cases they are useful
screening procedures. TCM and OLM
can be usedquicklyand inexpensively to
obtain an estimate of the magnitude of
the potential NOa impacts.
RPM-II considers an array of well
mixed reactors or cells across the
plume, each of which moves downwind
and disperses along the mean-wind
trajectory. The model can accommodate
temporally and spatially varying wind
speeds, reaction rates, ambient concen-
tration levels, and dispersion or entrain-
ment rates. As currently configured,
RPM-II uses a state-of-the-art 68-step
carbon bond photochemical reaction
mechanism developed by Whitten and
Hogo*; other mechanisms can be easily
substituted for the carbon bond mech-
anism if desired. The most severe
limitation of an involved reactive plume
model like RPM-II is the requirement for
valid ambient concentration estimates
of reactants along the plume trajectory.
These must by measured, predicted, or
hypothesized. Ambient hydrocarbons
are extremely difficult to characterize
even by measurement, yet predictions
of plume NO2 are very sensitive to
hydrocarbons. Like most other dispersion
models, RPM-II is also limited by the
user's ability to properly specify wind
speeds and dispersion rates, especially
in complex terrain applications. But
despite such limitations, reactive plume
models like RPM-II are clearly more
realistic in their approach to NO2
estimation than are CNOM, OLM, or
TCM.
Data Base
The data required for the application
of the models selected are:
• Atmospheric stability, dispersion
coefficient, or horizontal plume
spread as a function of downwind
distance and time.
• Point source emissions rates for
NO, NO2, sulfur dioxide (SO2),
sulfates, and hydrocarbons.
• Source data:
- Plume heights.
- Source parameters, flow rates,
stack temperatures.
- Source locations.
• Ambient concentrations for all
species as functions of location and
time.
'Whitten, G Z and H Hogo, Mathematical
Modeling of Simulated Photochemical Smog. EPA-
600/3-77-011, U.S. Environmental Protection
Agency, Research Triangle Park, IMC, 1977, 307 pp.
• Sunlight intensity as a function o
location and time.
• Inversion heights or vertical plume
spreads as functions of location anc
time.
• Wind field.
These data will also satisfy all require-
ments of both RPM-II and the three
simple NO2 estimation procedures.
At the start of this study, the useful-
ness of the MISTT data base for applying
and evaluating N02 prediction techni-
ques was uncertain, because the data
base was designed to characterize
plume sulfur and aerosol transport and
transformations, not to support reactive
plume modeling. However, a significant
amount of plume data was collected
during the MISTT study and much of il
for a large point source (Labadie power
plant plume). This study focused on the
Labadie power plant plume data collected
during the MISTT program. Most of the
required data were available as part of
the MISTT data base or from other data
sources. However, after careful exami-
nation of the MISTT data, the airborne
hydrocarbon concentration measure-
ments were found invalid. Plume
dispersion parameters were usually
difficult to estimate with much certainty
from the airborne concentration profiles.
To compensate for uncertainties in
the dispersion data, the input dispersion
rates were adjusted so that the following
equation was approximately satisfied:
A(S) = t(E,/U(S) - (C,aA0)]/(C,p(S) - C,a)
where
A(S) = plume cross-section area at
downwind distance S
E, = emissions rate of species i
(NOx or SO2)
U(S) = wind speed at distance S
Cia = measured ambient concen-
tration of species i
Ao = exit area of stacks (121.2 m2)
C,P(S)= measured average plume
concentration of species i at
distance S
Adjustments were made to the total
ambient hydrocarbon levels so that the
plume Oa concentrations predicted by
RPM-II approximately matched the
measured values. The reactivity of the
hydrocarbon mass (i.e., the species
distribution of hydrocarbons) was
determined from hydrocarbon analyses
carried out for the RAPS program (a
ground-level monitoring program con-
ducted in the same area during the
MISTT program.)
These adjustments to the input data
preclude a strict evaluation of model
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performances or verification of model
predictions. However, it is possible to
judge whether the models are capable
of acceptable performance when reason-
able input values are selected.
In addition, a "restricted" data base
was assembled for input to the models.
The purpose of using this data set was to
evaluate model performance for the
limited data normally available to a
prospective user. Model input values for
all parameters in the "restricted" data
set were identical to those in the
"unrestricted" set, except for wind
speeds, plume widths, and plume
depths. These exceptions were instead
derived from local National Weather
Service station data and standard
Pasquill-Gifford-type dispersion curves.
A compendium of data collected by
Meteorology Research, Inc. and the
University of Washington during the
MISTT program is presented in plotted
form in Volume 2 of the Project Report.
This includes maps showing aircraft
flight paths and graphs of the data
collected along the paths.
Results
Ten case-study days were selected
from the MISTT study data base, and 28
observations of average plume concen-
trations at various downwind distances
were derived from the data on those 10
days. The results of the model runs with
the "unrestricted" and "restricted" data
bases were studied with the aid of
several statistical and graphic tech-
niques. The RPM-II performed better than
the simple NO2 models in both the
"restricted" and "unrestricted" cases.
This result might be expected because
RPM-II is intended to account in greater
detail for more processes than are the
simpler models.
Inspection of scatter diagrams reveals
that RPM-II is capable of matching the
observations more closely than the
simple models across a wide range of
NO2 concentrations. The least-squares
best fit line has a slope of 0.887 and an
intercept of 1.6 ppb, compared with an
ideal slope of 1.0. The RPM-II clearly
performed within the expected accuracy
of the monitoring instruments (~ ± 15
percent of the true va lue). The computed
statistics are also good, but they do not
constitute validation of RPM-II perfor-
mance because of the input data
uncertainties. Rather, the following
conclusions about RPM-II are war-
ranted:
• The model is capable of acceptable
performance when reasonable
ambient hydrocarbon and plume
dispersion input values are selected.
• No apparent defect that affects
performance is revealed in the
model's formulation, although the
formulation does contain several
simplifying assumptions.
Because the simple models contain a
relatively simplistic treatment of chem-
istry, they may not be appropriate for
use in situations where hydrocarbon
oxidation significantly influences NOa
concentrations. It is therefore desirable
to define concisely the character of the
situations requiring more sophisticated
models. On the basis of the simulations
carried out in this study, however, it is
clear that several important parameters
interact to determine NOa production
rates. Thus, a concise definition of
conditions requiring a sophisticated
model is not possible on the basis of
these results. NOa production appears
generally to depend on the existence of
a sufficient amount of ambient ozone
and hydrocarbons, solar insolation, and
plume dispersion in combination. The
precise combinations depend on the
following variables:
• Amount of NOX present in the
plume and in the ambient air.
• Time of release from the stack.
• Ratios of NO to NOs in the plume
and in the ambient air.
• Wind speed and variability.
• Split of the reactive hydrocarbons.
Conclusions
The four models, RPM-II, TCM, OLM,
and CNOM performed reasonably well
by comparison with MISTT data both
"unrestricted" and "restricted" input
data sets. Of the four, RPM-II was the
superior performer using both data sets
over a wide range of concentration
levels. TCM, OLM, and CNOM tended to
underpredict but mostly at the high
concentration ranges. It must be stres-
sed that model performances in this
study cannot be construed as a verifica-
tion of model performances because of
input data deficiencies and the input
data adjustments that were necessary.
M. A. Yocke, D. A. Stewart, J. Johnson, and R. J. Frost are with Systems
Applications. Inc., Sam Rafael, CA 94903
K. L. Schere is the EPA Project Officer (see below).
The complete report consists of two volumes entitled "Evaluation of Short- Term
NOz Plume Models for Point Sources:"
"Volume I. Technical Discussion," (Order No. PB 82-234 329; Cost: $17.50,
subject to change).
"Volume II. Data," (Order No. PB 83-217 232; Cost: $4.50, microfiche only,
subject to change).
The above reports will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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Fees Paid
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Agency
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Penalty for Private Use $300
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