REGIONAL WORKSHOPS ON
AIR QUALITY MODELING:
A SIWIARY REPORT
APRIL 1981
(Revised 1982)
Source Receptor Analysis Branch
Monitoring and Data .Analysis Division
Office of Air Quality Planning and Standards
U.S. Environmental Protection Agency
Research Triangle Park, Nor™ Carolina
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This report has been reviewed by the Office of Air Quality Planning
and Standards, EPA, and approved for publication. Mention of trade
names or commercial products is not intended to constitute endorsement
or recommendation for use.
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p
TABLE OF CONTENTS
Page
1.0 INTRODUCTION 1
2.0 DATABASES 3
2.1 Discussion 3
2.2 Recommendations 3
2.2.1 Acquisition of Data Bases 3
2.2.2 Background Concentrations 4
2.2.3 Source Data 4
2.2.4 Meteorological Data 4
3.0 FLAT TERRAIN MODELS 6
3.1 Discussion 6
3.2 Recoimiendations 6
3.2.1 Screening Techniques 6
3.2.2 Refined Analytical Techniques 6
3.2.3 Model Options 8
4.0 COMPLEX TERRAIN MODELS 9
4.1 Discussion 9
4.2 Recommendations 9
4.2.1 Screening Techniques 9
4.2.2 Refined Analytical Techniques 10
5.0 MOBILE SOURCE MODELS 11
6.0 GENERAL MODELING ISSUES 12
6.1 Discussion 12
6.2 Recommendations 12
6.2.1 Design Concentrations 12
6.2.2 Critical Receptor Sites 12
6.2.3 Long-Range Transport 13
6.2.4 Pollutant Half-Life 14
6.2.5 Urban/Rural Classification 14
6.2.6 General Model Evaluation 15
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7.0 USE OF NONGUIDELINE MODELS 16
7.1 Discussion 16
7.2 Definition of Guideline vs Nonguideline
Models . 16
7.3 Recomnendations 17
8.0 USE OF MEASURED DATA IN LIEU OF MODEL ESTIMATES 18
8.1 Discussion 18
8.2 Reconmendations 18
9.0 REGIONAL MODELING PROCEDURES 21
9.1 Discussion 21
9.2 Recommendations 21
Appendix A. Acquisition of Site Specific Meteorological Data . . A-l
Appendix B. Air Quality Analysis Checklist B-l
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1.0 INTRODUCTION
The requirements placed on air quality control agencies by the
Clean A1r Act have dramatically Increased the need for Improved air
quality modeling. The resulting increase in the use of models has also
led to a substantial increase in the number and complexity of situations
in which models are employed. The modeling guideline (Guideline on Air
Quality Models, EPA-450/2-78-027, April 1978) addresses many of the
problems in this relatively new and growing field, but much is left to
the discretion of the reviewing agency since many complex problems are
best solved on a case-by-case basis. However, because of the variety of
technically correct solutions to any complex problem, different approaches
with differing results have led to inconsistency in model applications
from Region tc Region. In an effort to improve consistency in the use
of modeling techniques, three in-house workshops have been held since
1978. These workshops provide a forum for the Regional Office and Head-
quarters groups to discuss common problem areas and arrive at generally
acceptable solutions.
Many recommendations were made in the course of the workshops.
These have been reviewed by OAQPS and some have necessarily been modi-
fied and supplemented to ensure consistency with other modeling policies.
This report clarifies preferred data bases and procedures for the appli-
cation of specific models and modeling techniques in situations where
the guideline permits a case-by-case analysis.
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Recommendations contained in this report should be followed by
the EPA Regional Offices until such time as the 1978 guideline is
formally revised. Issues concerning the use of models not specified
in this summary report or in the 1978 guideline, should be directed
to the OAQPS Model Clearinghouse for review. The current procedures
for submitting issues are provided in the Clearinghouse Operating Pla
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2.0 DATA BASES
2.1 Discussion
Estimated concentrations can vary widely depending on the
source, meteorological, and air quality data used in preparing the
estimates. Thus the need for consistency in the use of data and in the
selection of data bases is apparent. Also, an accurate and reliable air
quality data base is needed to evaluate the performance of a model.
Inconsistencies occur because adequate data frequently are not
available for model input. Requirements for pre-application monitoring
under PSD have alleviated some of the inconsistencies in data collection
and use. However, additional guidance is still needed in the collection
and interpretation of meteorological data.
Also, appropriate source data to reflect short-term variations
in emissions are often unavailable. The relationship of source emission
data to worst-case conditions can be another area of inconsistency.
This section identifies a few of the more frequent problem
areas and provides recommendations to ensure consistency in the select-
ion and use of data.
2.2 Recommendations
2.2.1 Acquisition of Data Bases
Guidance provided in the "Ambient Monitoring Guidelines
for Prevention of Significant Deterioration (PSD)," EPA-450/4-80-012 ,
November 1980 should be used for the establishment of a special monitor-
ing network for air quality analyses, including both air quality and
meteorological monitoring techniques. Additional information is avail-
able in 40CFR Part 58 and in the quality assurance and site selection
EPA guidance documents published on a pollutant-by-pollutant basis. The
EPA Regional Office should review the network design prior to operation.
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2.2.2 Background Concentrations
Techniques discussed in the Guideline on Air Quality
Models should be used in establishing background concentrations.
2.2.3 Source Data
The load or operating condition of a plant that causes
the highest ground-level concentrations should be determined through a
screening analysis and this load should be used to establish emission
limitations. As a-minimum all sources should be modeled using 100
percent design capacity; however, when modeling large sources, e.g., 500
MW power plants or equivalent, 50 and 75 percent capacity should also be
modeled.
Hourly sequential emissions determined for existing
sources from continuous in-stack monitoring should be used in model
evaluation where possible. Hourly emissions are critical where short-
term concentrations are of concern in such evaluations.
2.2.4 Meteorological Data
Five years of representative meteorological data should
be used when estimating concentrations with an air quality model.
Consecutive years from the most recently available five-year period are
preferred. The meteorological data may be data collected either on-site
or at the nearest National Weather Service (NWS) station. If the source
is large, e.g., emissions equivalent to a 500 MW power plant, the use of
five years of NWS meteorological data or at least one year of on-site
data is required.
Five years of on-site data are often not available.
When considering shorter periods of meteorological data, care must be
taken to ensure that the data used contain the appropriate worst-case
conditions. On-site data should also be subjected to quality assurance
procedures that will ensure that the data is at least as accurate and in
as much detail as NWS data.
Hourly average wind directions reported to the nearest
degree should be used where on-site data are used. The CRSTER randomi-
zation sequence (i.e., the established sequential random number set
designated for use with the meteorological preprocessor to CRSTER)
should be used with NWS wind data.
The surface wind reference height used in the model
should be defined to agree with the actual height of the surface wind
sensor. When wind is monitored at heights closer to plume height, the
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wind direction should be used to define the plume transport and the
speed should be utilized to develop the appropriate vertical wind speed
profile.
Guidance provided in Appendix A should be followed in
the design of site-specific, on-site meteorological data collection
programs.
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3.0 FLAT TERRAIN MODELS
3.1 Discussion
Flat terrain, as used here, is considered to be an area where
terrain features are all lower in elevation than the top of the stack of
the source in question. Most Gaussian models perform adequately in such
situations.
A number of models have been made available by EPA and others
for those applications where receptors are located at elevations less
than the top of the stack. However, inconsistencies have resulted from
the use of these models. Such inconsistencies occur in part because
models may be developed at different times for specific applications and
the various algorithms are improved, changed or added to accommodate a
specific problem or to reflect recent research. This section provides
recommendations to resolve these inconsistencies without limiting the
range of applicability of flat terrain models.
3.2 Recommendations
3.2.1 Screening Techniques
Screening techniques and options as provided in "Guide-
lines for Air Quality Maintenance Planning and Analysis Volume 10 (R):
Procedures for Evaluating Air Quality Impact of New Stationary Sources"
should be used.
Where possible, screening procedures should be site and
problem specific. Consideration should be given to: (1) terrain; (2)
urban or rural dispersion coefficients; and (3) worst-case conditions
when representative meteorological data or applicable detailed modeling
techniques are not available. If screening is the sole basis for the
analysis, adequate justification and documentation should be required
for the use of averaging time factors.
3.2.2 Refined Analytical Techniques
The following table lists the preferred models for the
indicated applications. These models should be used in Regional Office
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applications of models. For use of models and in applications that do
not appear in this table or in the 1978 guideline, Regional Offices
should follow Section 7 of this report.
Table 1. Preferred Model Use
SHORT TERM
MODEL
Single Source
Rural
CRSTER
Urban
RAM
Multiple Source
Rural
MPTER
Urban
RAM
Industrial Complexes
Rural/Urban
ISC
LONG TERM
Single Source
Rural
CRSTER
Urban
CDMQC i
Multiple Source
Rural
MPTER
Urban
CDMQC (
Industrial Complexes
Rural/Urban
ISC
For all
model applications
in a rural area,
the CRSTER
techniques for wind speed profile, plume rise and terrain adjustment
should be used unless other techniques can be shown on a case-by-case
basis to provide more appropriate and accurate estimates.
Dispersion coefficients appropriate to either urban or
rural settings should be used in accordance with Section 6.2.5. Sector
averaging should be accepted only for seasonal or annual estimates where
estimates are based on statistically summarized meteorological data.
*The choice of RAM or CDMQC in urban applications is a function of the
number of sources and the size of the area to be modeled, e.g., if only
three or four sources in an urban area are to be modeled, RAM should be
used.
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3.2.3 Model Options
The options that are found in the ISC, CRSTER, MPTER,
and PTPLU models have greatly increased the technical options available.
To ensure consistency in the use of these options, Regional Office users
should follow the guidance below:
a. Stack Tip Downwash (CRSTER, MPTER, ISC, PTPLU*)
This option should not be used unless demonstrated
to be applicable on a case-by-case basis. Although there is evidence
that this phenomenon can occur, there are no data to support wide use of
the option.
b. Plume Rise (CRSTER, MPTER, ISC, PTPLU)
In all cases, except for close-in receptors during
stable conditions in complex terrain or when the downwash (building
wake) algorithm of ISC is employed, the final plume rise option should
be used. The restriction on the use of gradual plume rise is based upon
the lack of specific data needed to quantify the dispersion during plume
rise. In complex terrain where plume impaction is the identified problem,
the use of transitional plume rise, during stable conditions, may be
required to ensure that impaction on close-in terrain is considered.
When building downwash is considered a problem, transitional plume rise
calculations are used only to determine whether the plume will be affected
by building wake. If so, dispersion is handled by appropriate modified
dispersion parameters.
c. Rural/Urban Options (ISC)
The selection of the rural or urban option should
be based upon the determinations as outlined in Section 6.2.5 for deter-
mining whether an area is urban or rural.
d. Momentum Plume Rise (ISC, CRSTER, MPTER, PTPLU)
This is optional in the CRSTER and MPTER models and
an integral part of the ISC and PTPLU models. It should be used in the
CRSTER and MPTER models.
e. Deposition (ISC)
The deposition algorithm in ISC may be used whenever
deposition is considered to be a factor in the analysis.
PTPLU is found in UNAMAP (Version 4a) and is a replacement for PTMAX.
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(Revised August 1982)
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4.0 COMPLEX TERRAIN MODELING
4.1 Discussion
Although the need for a refined complex terrain dispersion model
has been acknowledged for several years, such a model has not yet been
developed. The lack of extensive data bases and basic knowledge concerning
the behavior of atmospheric variables in the vicinity of complex terrain
presents a considerable obstacle to the solution of the problem and the
development of a refined model.
A first step toward the solution of this problem has been taken.
The Environmental Sciences Research Laboratory initiated the multi-year
Complex Terrain Model Development project in 1979. The first field study
was conducted at Cinder Cone Butte and the first milestone report describing
this effort was issued in April 1982 (EPA-600/3-82-036). This report
includes an initial evaluation of three screening techniques. This
portion of the study represents the initial acquisition of basic data
needed to define the meteorological variables and plume behavior in
complex terrain. Until the behavior of plumes in various complex terrain
situations can be documented and new mathematical constructs developed,
the existing dispersion algorithms adapted to complex terrain must be
used.
For the purpose of this report, complex terrain is defined as
any terrain exceeding the height of the stack being modeled.
4.2 Recommendations
4.2.1 Screening Techniques
/
Two screening techniques are currently preferred:
Valley and Complex I. It is suggested that a two-tiered screening
approach be followed for complex terrain analyses.
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4.2.1.1 Initial Screening Technique
The initial screen is the use of the Valley
Model. For buoyant sources, the BID option in Valley may be used. The
following worst-case assumptions should be used in Valley to determine
24-hour averages: (1) P-G stability "F"; (2) wind speed of 2.5 m/s; (3)
6 hours of occurrence.
Multiple sources should be treated individual!
in the Valley Model and the concentrations for specific wind directions
summed. Only one wind direction should be used for 24-hour averages
(see User's Manual, pages 2-15) even if individual runs are made for
each source.
The receptor grid found in the Valley Model
User's Guide may not be sufficient for all analyses if only one geograph
ical scale factor is used. The Valley Model is very sensitive to ground
level elevation at the receptor, and the use of the standard polar grid
could miss the worst-case receptor. If this situation occurs, the user
should choose an additional set of receptors at appropriate downwind
distances whose elevations are equal to plume height minus 10 meters.
4.2.1.2 Second-Level Screening Technique
If a violation of any NAAQS or the controlling
increment is indicated, a second-level screening technique may be used.
An on-site data base of at least 1 full year of meteorological data,
collected in accordance with the recommendations in Appendix A, is
preferred for use with the second-level screening technique. All meteor
ological data used in the analysis must be reviewed for both spatial and
temporal representativeness.
At this time Complex I, available in UNAMAP
Version 4a, is the preferred second-level screening technique. Complex
I is the result of modifying the MPTER Model to incorporate the Valley
Model algorithm. As such it is a multi-source screening technique that
accepts hourly meteorological input. It differs from the Valley Model
only in its use of hourly data and a plume height correction. In Comple
I hourly wind direction input is specified to the nearest whole degree
(and the plume vectored accordingly), but the 22-1/2° Valley crosswind
sector averaging has been retained. During nonstable conditions Complex
I permits the plume to be transported horizontally relative to sea
level. As terrain elevation increases with distance, the plume height
above ground is allowed to decrease to no less than half its height
above plant grade. Beyond the point at which the height is halved, the
plume centerline parallels the terrain.
4.2.1.3 Restrictions
For screening analyses, the use of a sector
greater than 22-1/2° should not be allowed and full ground reflection
should always be used.
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(Revised August 1982)
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4.2.1.4 Options
For buoyant sources the option for buoyancy-
induced dispersion should be exercised.
Complex I requires the specification of an
approach distance and a 10-meter approach distance should be selected.
The Complex I program has five separate
complex terrain plume specification procedures in the designated IOPT
ODtion 25. Only the standard procedure labeled "1" should be selected,
(i.e., IOPT (25) = 1).
The option in the Valley model to use the 2.6
stable plume rise factor should be selected.
4.2.2 Refined Analytical Techniques
k'hen the results of the screening analysis demonstrate
a possible violation of NAAQS or the controlling PSD increments, a more
refined analysis should be conducted. Since there are no refined techniques
currently reconmended for complex terrain applications, a nonguideline
model may be applied in accordance with Section 7. In the absence of an
appropriate refined model, screening results may be acceptable.
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(Revised August 1982)
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5.0 MOBILE SOURCE MODELS
Regional meteorologists have not been involved in significant
consistency problems with carbon monoxide or ozone models. Some guid-
ance is found in the 1978 Guideline on Air Quality Models, and additional
guidance with respect to the use of models and data bases for SIP
revisions is contained in the Federal Register Volume 46, No. 14, p.
7182, entitled, "State Implementation Plans: Approval of 1982 Ozone and
Carbon Monoxide Plan Revisions for Areas Needing an Attainment Date
Extension."
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6.0 GENERAL MODELING ISSUES
6.1 Discussion
This section contains recommendations concerning a number of
different issues. The problem areas addressed are not specific to any
one program or modeling area but need resolution in nearly all modeling
si tuations.
6.2 Recommendations
6.2.1 Design Concentrations
If 5 years of NWS data are used in an analysis or if 1
year of on-site meteorological data are used, then the highest, second-
highest short-term concentration estimate should be used to determine
the impact of the source. If less than 5 years of NWS data or less than
1 year of on-site data are used, then the highest concentration.estimate
should be used as an approximation to the second-highest short-term
concentration.
Block averaging times should continue to be used for
modeling purposes.
6.2.2 Critical Receptor Sites
Receptor sites should be utilized in sufficient detail
to allow estimates of the highest concentrations and the probability of
a violation of a NAAQS or a PSD increment. • The procedures listed below
should be followed to locate receptor sites when a large source, such as
a 500 MW power plant, is being modeled.
a. Apply PTPLU to identify the distance to the highest
estimated concentration for each combination of atmospheric stability
class and wind speed. PTPLU should be run using 0.10, 0.15, 0.20, 0.25,
0.30, and 0.30 as wind profile exponents for the six stability Classes A
through F respectively. The receptor elevation in PTPLU should be set
to the highest terrain elevation above stack base and below stack top
found within a 1-kilometer radius of the stack. Identify the distance
to the highest concentration listed in the PTPLU output for each stability
class. Select the smallest of these distances obtained from PTPLU as
the first receptor distance.
b. Select eight more distances by multiplying the
first receptor distance by each of the following constants: 1.3, 1.7,
2.3, 3.0, 3.9, 5.2, 6.8, and 9.0. This geometric progression allows the
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(Revised August 1982)
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user to most closely approximate the location of maximum concentration.
There is no need for a receptor spacing closer than 0.1 kilometer. A
tenth receptor distance may be used to locate receptors in a potentially
high concentration area beyond the ninth receptor distance.
c. Check the PTPLU output to be sure that high con-
centrations with P-G D stability are not expected beyond the last
receptor distance. If high concentrations are expected beyond the last
receptor distance, then add additional rings to include those cases.
d. If the elevation of individual receptors is signi-
ficant, those elevations should be specified as the greatest terrain
elevation along the appropriate 10 degree arc for the receptor distance
of concern; the height should not be limited to the center of the 10
degree sector. In some instances it may be desirable to locate re-
ceptors at the plant boundary. Additional rings may be needed for this.
For models capable of using a rectangular grid, includ-
ing multi-source models, a one-kilometer square receptor grid extended
outward in all directions from the source to a distance of 10 kilometers,
or about 400 receptor sites, should be used. The grid should be ex-
tended farther if maximum 1-hour concentrations are estimated to occur
beyond 10 kilometers. For urban models, this grid should cover the
entire area being modeled. In addition, to identify concentrations that
might be missed by the spacing of the rectangular grid, individual
isolated receptor sites should be located downwind from the major source(s)
for prevailing wind directions during conditions of maximum concentration.
For each direction, four downwind distances associated with maximum one-
hour concentrations for Pasquil1-Gifford stability Classes A, B, C, and
D as determined by PTPLU should be selected. Receptor sites should also
be located at sites where monitored air quality data are available and
sites where plume interactions from multiple sources are likely to be
greatest. If the height of individual receptors is significant, those
should be specified as the actual terrain height at the receptor loca-
tion.
For sources smaller than those equivalent to a 500 MW
power plant, receptors should be located following the above procedures,
but in the actual model runs it may not be necessary to include all
receptors for all directions and all distances. The selection of recep-
tor sites is left to the discretion of the Regional Office, but should
be based on wind roses for the area and the results of calculations
using PTPLU or other comparable screening procedures.
6.2.3 Long-Range Transport
Long-range transport should be considered where impact
on Federal Class I areas is possible. The application of simple Gaussian
models for downwind transport distances greater than SO km should be
evaluated on a case-by-case basis. Models that are more appropriate
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at these transport distances should be evaluated as alternatives to the
simple Gaussian models. Models for long-range transport included in the
references to the 1978 Guideline on Air Quality Models can be used as
screening techniques. More complex and thoroughly documented models
such as MESOPUFF, MESOPLUME or MESOGRID may be considered on a case-by-
case basis for use as refined models within their established limitations.
6.2.4 Pol 1utant Half-1ife
Pollutant half-life should not be used in screening
analyses.
For a refined analysis, if the need for half-life for
SCL can be demonstrated, site-specific data should be used to define a
rate of conversion of SO^. Otherwise only those refined models with
built-in conversion provisions should be used where conversion appears
to be an obvious problem.
For nitrogen oxides, complete conversion from NO to
nitrogen dioxide (N0?) should be used in screening analyses. In refined
analyses, case-by-case half-life conversion rates should be determined
on the basis of scientific technical studies appropriate to the site in
question. The methods suggested by Cole and Summerhays* should be
considered.
An infinite half-life should be used for estimates of
total suspended particulate concentrations when simple Gaussian models
with exponential decay terms are employed. Deposition and removal
should be directly considered in the model if it is a significant
factor.
6.2.5 Urban/Rural Classification
The selection of either rural or urban dispersion
coefficients in a specific application should follow the procedure below
using land use or population density.
Land Use Procedure: (1) Classify the land use within
the total area, A0, circumscribed by a 3 km radius circle about the
source using the meteorological land use typing scheme proposed by
Auer**; (2) If land use types II, 12, CI, R2, and R3 account for 50
percent or more of A0, use urban dispersion coefficients; otherwise, use
appropriate rural dispersion coefficients.
* Cole, H. 5., and J. E. Summerhays, A Review of Techniques for
Estimating Short Term N0? Concentrations, JAPCA, Vol. 29, No. S,
pp. 812-817, 1979. c
**Auer, A. H., Correlation of Land Use and Cover with Meteorological
Anomalies, JAM, Vol. 17, pp 636-643, 1978.
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Population Density Procedures: (1) Compute the
average population density, p, per square kilometer with Ac as defined
above; (2) If p, is greater than 750 people/km , use urban dispersion
coefficients; otherwise, use appropriate rural dispersion coefficients.
The land use procedure is considered the most defin-
itive. Population density should be used with caution, especially in a
highly industrialized area where the population density may be low but
the area is sufficiently built up so that the land use criteria would be
satisfied. Impacts from sources beyond the three (3) kilometers should
be included in the background.
For analyses of urban complexes, the entire area should
be modeled as an urban region if most of the sources are located in
areas classified as urban.
6.2.6 General Model Evaluation
A model evaluation study should assess how closely the
mathematical assumptions inherent to the model describe the physics
and/or chemistry of the atmosphere. The process of model evaluation
should consider all of the following: (1) assumptions inherent to
design/algorithms of model; (2) purpose/objective of model; (3) purpose/
objectives of monitors(s); (4) applicability of monitored data for
comparison; (5) comparison of model estimates with' monitor observations
for upper end of frequency distribution, statistical analyses, and
analyses of weakness in individual algorithms; and (6) analyses of
critical meteorological and source conditions and their effect on indi-
vidual algorithms. An analysis should also be made of the sensitivity
of the algorithms to the meteorological input data.
For short-term model evaluation, all input data should
be based on measured hourly averages. This includes mass emission
rates, stack dynamic operating parameters and meteorological input. The
spatial applicability of measured air quality data and tracer studies
should be consistent with the scale of the model comparisons.
Calibration for short-term air quality concentrations
is not recommended. Determination of the need for calibration and
calibration procedures are contained in the 1978 guideline.
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7.0 NONGUIDELINE MODELS
7.1 Discussion
Only a limited number of models or modeling techniques have
undergone a sufficient evaluation to be considered "guideline" models,
or to be recomnended procedures for modeling certain aspects of plume
behavior. There remain a large number of circumstances when no recom-
mended technique is available and no guideline model is totally appli-
cable. There are also circumstances when a model other than a recomnended
model may appear suitable. In those cases the Regional Office must
decide on the acceptable procedures and approve or disapprove specific
nonguideline modeling approaches for use in each specific situation.
7.2 Definition of Guideline vs. Nonguideline Models
Guideline models are those models specifically recommended
for (general) use in the 1978 Guideline on Air Quality Models. All
other models require review and evaluation on a case-by-case basis.
Changes made to a guideline model that do not affect the
concentration estimates do not change the guideline status of that
model. Examples of such changes are those required to run the program
on a different computer or those that affect only the format of the
model results. When such changes are made, the Regional Offices may
require a test case example to demonstrate that the concentrations
are not affected.
Use of a guideline model with other than recommended options
changes the status of the model to nonguideline. Similarly, if a guide-
line model has been revised or changed such that it produces concentra-
tions different from the original model for the same input data, the
status of the model 1s changed to nonguideline.
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7.3 Model Clearinghouse
A Model Clearinghouse has been established within OAOPS for
the purpose of assisting Regional Offices in the technical and con-
sistency reviews of nonguideline techniques or models. Regional Offices
may directly request assistance from the Clearinghouse once they have
completed an initial evaluation and prepared recommendations. The
Clearinghouse will also review all formal Federal Register proposals and
final rule-making packages to ensure modeling and data base consistency
and to evaluate the adequacy of nonguideline procedures. The Clearing-
house will also maintain a log of precedent-setting policy and technical
decisions and determine the transferability of models and data bases to
other situations. Details concerning the Clearinghouse and its operation
are found in the OAQPS document, "Model Clearinghouse: Operational
Plan," February 1981. Analyses using guideline techniques will not be
reviewed by the Clearinghouse.
7.4 Recommendations
The determination of the acceptability of a nonguideline model
is a Regional Office responsibility. Proposed models should be evaluated
from both a theoretical and a performance perspective. Proper support
and documentation for the use of a nonguideline model will normally
include air quality and meteorological data that have been collected
using appropriate techniques and procedures as outlined in the "Ambient
Monitoring Guideline for Prevention of Significant Deterioration (PSD)"
EPA 450/4-80-012, November 1980. Data bases for other than the specific
site in question may be acceptable if it can be shown that the data
available represent similar topography, climatology, and source configu-
rations. Any data base used must include appropriate periods of worst-
case conditions.
Procedures and techniques for determining the acceptability of a
nonguideline model on a case-by-case basis are contained in a document
entitled, "Interim Procedures for Evaluating Air Quality Models," August 1981.
In June 1982 an example application of these procedures, prepared by TRC
Environmental Consultants, Inc., was distributed to the Regional Offices.
Procedures outlined in these documents should be followed, as appropriate,
when evaluating nonguideline techniques.
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(Revised September 1982)
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8.0 USE OF MEASURED AIR QUALITY DATA IN LIEU OF MODEL ESTIMATES
8.1 Discussion
Dispersion model estimates, especially with air monitoring
support, are the preferred basis for air quality demonstration deci-
sions. Nevertheless, there may arise instances where the performance
of recomnended dispersion modeling techniques may be demonstrated by
observed air quality data to be less than acceptable. Occasionally
there may be no recommended modeling procedure. In these instances,
air pollutant emission limitations may be established on the basis of
observed air quality data.
8.2 Recommendations:
Modeling is the preferred method for determining emission
limitations for both new and existing sources. Where a wel1-accepted,
wel1-verified model is available, model results alone are sufficient.
Monitoring will normally not be accepted as the sole basis for emis-
sion limitation determination in flat terrain areas. In some instan-
ces where the modeling technique available is only a screening tech-
nique, the addition of air quality data to the analysis may lend
credence to model results.
In some instances a model that is applicable to the situa-
tion may not be available. Measured data may have to be used. Exampl
of such situations are: (1) complex terrain locations, (2) aerodynami
downwash situations, (3) land/water interface areas, and (4) urban
locations with a large fraction of particulate emissions from non-
traditional sources. However, only in the case of an existing source
would monitoring data alone be an acceptable basis for emission limits
In addition, there are other requirements for the acceptance of an
analysis based only on monitoring:
a. A monitoring network exists for the pollutants and
averaging times of concern;
b. It can be demonstrated that the monitors in the network
are located as close as possible to all points of maximum concentra-
tion;
c. The monitoring network and the data reduction and stor-
age procedures meet all EPA monitoring and quality assurance require-
ments;
18
-------
d. The data set and the analysis identify conclusively each
Individual source impact if more than one source or emission point is
involved;
e. At least one full year of valid ambient data is avail-
able and a demonstration that the year was not sufficiently atypical
to influence the resulting emission limits;
f. A demonstration that EPA recommended models are not
applicable through the comparison of the monitored data with model
results.
Sources should obtain approval from the Regional Office for
the monitoring network prior to the start of monitoring to ensure that
the situation requires the use of monitoring and to obtain approval of
the monitoring network design and procedures.
The following are examples for some coircnon situations where
monitored data might be considered. It should be noted, however, that
since the adequacy of a network 1s a function of the source configura-
tion as well as the topography and the meteorology of the site, a large
number of designs may need to be considered and no set pattern is appli-
cable to any one of the problem areas.
a. For aerodynamic downwash, consider one or two background
monitors plus two to four downwind monitors. The number of downwind
monitors should be determined by a consideration of the frequency of
the downwash events, the expected magnitude of the impact, and the
areal extent of the impact.
b. For shoreline conditions consider one to two background
monitors and three to eight downwind monitors. The number of downwind
monitors should be determined by considering site characteristics, the
magnitude and the areal extent of the predicted impact. It may be
necessary to complement the stationary monitoring network with mobile
sampling and plume tracking techniques.
c. For complex terrain, the air quality monitors should
assess the maximum impacts for each averaging period for which an air
quality violation is expected to occur. Approximately three to eight
monitors should be considered necessary to monitor for each such
averaging time. The exact number depends on the magnitude and extent
of expected violations. At least two monitors for each contiguous
area where violations are expected to occur is necessary except where
these areas are large. In this case, more than two monitors could be
required. As a guide, a 22-1/2° sector should define the maximum size
19
-------
of a large contiguous area. Based upon meteorological judgment,
additional monitors may be required to evaluate the source impact
depending on the complexity of the terrain.
d. For urban situations where the concern is particulates
and the sources of violations appear to be fugitive and/or reentrained
dust, extensive monitoring and receptor models may be needed to accurately
assess the problem.
20
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9.0 REVIEW OF PSD PERMIT APPLICATIONS
9.1 Discussion
Certain procedures with respect to the review and analysis of
PSD permits also should be standardized to ensure consistency. A few
of these are discussed below.
9.2 Recommendations
In those Regions where the Regional Office has the responsibility
for permitting new sources, the Regional Office should provide permit
applicants with a uniform PSD/NSR guidance package, including screening
and modeling requirements. The attached Air Quality Analyses Checklist
(Appendix B) is recommended as a standardized set of data and a standard
basic degree of analysis to be required of PSD and SIP revision applicants.
This checklist suggests a level of detail, including the necessary grid
resolution, required to assess both PSD increments and the NAAQS.
Special cases may require additional guidance.
A pre-application meeting between source owner and Regional
Office staff should be the norm and the Regional Meteorologist should be
represented.
PSD air quality analyses should be based on information
considered valid for the start-up date for the new or modified source.
The Regional Office should allow permit applicants to use
"Procedures for Evaluating Air Quality Impact of New Stationary Sources"
(EPA-45Q/4-77-QQ1) for screening purposes. Air quality concentration
estimates obtained using procedures in that Guideline on screening
techniques or using the refined analytical techniques incorporating
Pasquil1-Gifford or McElroy-Pooler sigmas, are equivalent to one-hour
values. Time-scaling of such estimates from any period shorter than one
hour is generally not acceptable. Time-scaling of one hour estimates to
longer period averages is not acceptable when the purpose is to obtain
the highest or highest, second-highest concentration estimates and a
refined analytical technique is appropriate for making one-hour estimates.
Regional Offices should require permit applicants to incorporate
the pollutant contributions of all sources into their analysis. This
should include emissions associated with area growth within the area of
the new or modified source's impact. PSD air quality assessments should
consider the amount of the allowable air quality increment that has
already been granted to any other new sources. The most recent source
applicant should be allowed the prerogative to re-model the existing or
permitted sources in addition to the one currently under consideration.
21
-------
This would permit the use of newly acquired data or improved modeling
techniques if such have become available since the last source was
permitted. When remodeling, the worst case conditions used in the
previous modeling analysis must be one set of conditions modeled in the
new analysis. All sources must be modeled for each set of meteorological
conditions selected and for all receptor sites used in the previous
applications as well as new sites specific to the new source.
22
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APPENDIX A
ACQUISITION OF SITE SPECIFIC METEOROLOGICAL DATA
Models recommended in the 1978 Guideline on Air Quality Models
require as input the following parameters:
0 transport wind speed and direction;
0 ambient air temperature;
0 Pasquill-Gifford stability category.
Wind Measurements
In addition to 10 m surface wind measurements, the transport wind
speed and direction should be measured at an elevation as close as
possible to the effective stack height. To approximate this, if a
source has a stack (or stacks) below 100 m, select the stack top height
as a wind measurement height. For sources with stacks extending above
100 m, a 100 m tower is suggested unless the stack top is significantly
above 100 m (200 m or more). For cases with stacks 200 m or above, the
Regional Meteorologist should determine the appropriate measurement
height on a case-by-case basis. Remote sensing may be a feasible
alternative.
For routine tower measurements and surface measurement the wind
speed should be measured using an anemometer and the wind direction
measured using a horizontal vane. The specifications for wind measuring
instruments contained in the "Ambient Monitoring Guidelines for Pre-
vention of Significant Deterioration (PSD)," EPA 450/4-80-012, November
1980 should be followed. Wind direction should be reported to the
nearest degree.
A-l
-------
Temperature
The temperature should be measured at or near standard instrument
shelter height. Ambient temperature can be reliably measured using good
quality linear thermistors or platinum resistance devices.
Stability Category
The Pasquil1-Gifford (P-G) stability categories, as originally
defined, incorporate subjectively determined insolation assessments
based on hourly cloud cover observations. In lieu of such observations
it is recommended that the P-G stability category be estimated using
Table 'A-1. Use of this table requires the direct measurement of the
elevation angle of the vertical wind direction. Measurements of ele-
vation angle are difficult to make without a substantial commitment in
maintenance, hence it is recommended that a be determined using the
-------
If a is computed using the output of the anemometer by other than
$
direct application of the formula for a variance, the method should be
demonstrated to be equivalent to direct computation.
Both the vertical wind speed fluctuations and the horizontal wind
speed should be measured at the same level. Moreover, these measure-
ments should be made at a height of 10 m for valid use in estimating the
RG stability category. Trees or land use might preclude measurements as
low as 10m and in such cases the measurements will have to be made at
heights above 10 m
If on-site measurements of either a, or a are not available,
9 u
stability categories may be determined using the horizontal wind direc-
~
tion fluctuation, a., as outlined by Mitchell and Timbre . This method
uses the NRC Safety Guide 1.23 categories of a0 listed in Table A
as an initial estimate of the P-G stability category. This relationship
is considered adequate for daytime use. During the nighttime (one hour
prior to sunset to one hour after sunrise) the adjustments given in
Table A-III should be applied. As with a . a should be adjusted for
q) 0
surface roughness by multiplying the measured a , by the average surface
roughness length within 1 to 3 km of the source.
If, due to maintenance or instrument failure, a and a. values are
u) 6
missing, the P-G stability categories can be estimated from the lower
level wind speed (if tower measurements are used) with estimates of sky
*Mitchell, A. Edger Jr., and K. 0. Timbre, Atmospheric Stability Class
from Horizontal Wind Fluctuation, 72nd Annual Meeting APCA, Cincinnati,
Ohio, June 24, 1979.
A-3
-------
cover and cloud heights from some suitable NWS site (or obtained on-
site) using the CRSTER preprocessor. However, the o categories and the
r
modified oc categories are anticipated to be better correlated to the
to
actual dispersion, especially in complex terrain, than employing either
estimates or measurements of insolation to estimate the P-G stability
category.
\
A-4
-------
Table A-I
P-fi Stability Category Versus
Vertical Hind Direction Fluctuation, a.
q
P-H Stability *Standard Deviation of
Category Vertical Wind Direction,
A > 12°
B 10° - 12°
C 7.8° - 10°
D 5° - 7.8°
E 2.4° - 5°
F < 2.4°
From: Smith, T. B. and S. M. Howard, "Methodology for Treating
Diffusivity," in MRI 72 FR-1030 6 September 1972.
0
*
Where it is anticipated that there may be increased dispersion because
of surface roughness, a factor of (z /15 cm) ' , where z is the average
surface roughness in centimeters witRin a radius of 1-3 Km of the source,
may be applied to the table values. It should be noted that this factor,
while theoretically sound, has not been subjected to rigorous testing
and may not improve the estimates in all circumstances. A table of zQ
values that may be used as a guide to estimating surface roughness is
given in: A. Smedman-Hogstrom and U. Hogstrom (1978). "A Practical
Method for Determining Wind Freguency Distributions for the Lowest 200 n
from Routine Meteorological Data" JAM, Vol. 17, p. 942.
A-5
(Revised August 1982)
-------
Table A-II
PG Stability Categories Versus
Horizontal Wind Direction Fluctuations, a.
y
PG Stability Range of Standard
Category Deviation, Degrees*
A
>
ro
ro
LTI
B
22.5
>
a q
>
17.5
C
17.5
>
n
"9
>
12.5
D
12.5
>
ae
s
7.5
C
u
7.5
>
ae
>
3.8
F
3.8
>
a„
>
Adapted from Nuclear Regulatory Commission (NRC) Regulatory Guide 1.23, 1972.
Where it. is anticipated that there may be^increased dispersion because of
surface roughness, a factor of (z /15 cm) ' , where z is the average surface
roughness in centimeters within a°radius of 1-3 km of the source, may be
applied to the table values. It should be noted that this factor, while
theoretically sound, has not been subjected to rigorous testing and may
not improve the estimates in all circumstances. A table of z values
that may be used as a guide to estimating surface roughness is given in:
A. Smedman-Hogstrom and U. Hogstrom (1978), "A.Practical Method for Deter-
mining Wind Frequency Distributions for the Lowest 200 m from Routine
Meteorological Data" JAM, Vol. 17, p. 942.
A-6
(Revised August 1982)
-------
If the a
stabi 1 i ty
class is
Table A-III
Night Time P-G Stability Categories Based on a
And if the 10m wind speed, u, is
D
E
F
m/s
u<2.9
2.9
-------
APPENDIX B
AIR QUALITY ANALYSIS CHECKLIST*
1. Source location map(s) showing location with respect to:
o Urban areas**
o PSD Class I areas within 100 km
o Nonattainment areas**
o Topographic features (terrain, lakes, river valleys, etc.)**
o Other major existing sources**
o Other major sources subject to PSD requirements
o NWS meteorological observations (surface and upper air)
o On-site/local meteorological observations (surface and upper
air)
o State/local/on-site air quality monitoring locations**
o Plant layout on a topographic map covering a 1-km radius of
the source with information sufficient to determine GEP
stack heights
2. Information on urban/rural characteristics:
o Land use within 3 km of source classified according to
Auer, A. H. (1978): Correlation of land use and cover with
meteoroloqical anomalies, J. of Applied Meteoroloqy, Vol. 17
p. 636-643.
o Population
- total
- density
o Based on current guidance determination of whether the area
should be addressed using urban or rural modeling methodology
* The "Guidelines for Air Quality Maintenance and Analyses," Volume 10
(Revised), EPA-450/4-77-001, October 1977 (OAQPS No. 1.2-029R) should
be used a screening tool to determine whether modeling analyses are
required. Screening procedures should be refined by the user to be
site/problem specific.
**Within 50 km or distance to which source has a significant impact,
whichever is less.
B-l
-------
3. Emission inventory and operating/design parameters for major
sources within region of significant impact of proposed site (same as
required for applicant:
o Actual and allowable annual emission rates (g/s) and opera-
ting rates*
o Maximum design load short-term emission rate (g/s)*
o Associated emissions/stack characteristics as a function of
load for maximum, average, and nominal operating conditions
if stack height is less than GEP or located in complex
terrain. Screening analyses as footnoted on B1 or detailed
analyses, if necessary, must be employed to determine the
constraining load condition (e. g., 50%, 75%, 100% load)
to be relied upon in the short-term modeling analysis.
- location (UTM's)
- height of stack (m) and grade level above MSL
- stack exit diameter (m)
- exit velocity (m/s)
- exit temperature (°K)
o Area source emissions (rates, size of area, height of area
source)*
o Location and dimensions of buildings (plant layout drawing)
- to determine GEP stack height
- to determine potential building downwash considerations
for stack heights less than GEP
o Associated parameters
- boiler size (megawatts, pounds/hr. steam, fuel consump-
tion, etc.)
- boiler parameters (% excess air, boiler type, type of
firing, etc.)
- operating conditions (pollutant content in fuel, hours of
operation, capacity factor, % load for winter, summer,
etc.)
- pollutant control equipment parameters (design efficiency,
operation record, e.g., can it be bypassed?, etc.)
o Anticipated growth changes
*Particulate emissions should be specified as a function of particulate
diameter and density ranges.
B-2
-------
4. Air quality monitoring data:
o Summary of existing observations for latest five years
(including any additional quality assured measured data
which can be obtained from any state or local agency or
company)*
o Comparison with standards
o Discussion of background due to uninventoried sources and
contributions from outside the inventoried area and descrip-
tion of the method used for determination of background
(should be consistent with the Guideline on Air Quality
Models)
5. Meteorological data:
o Five consecutive years of the most recent representative
sequential hourly National Weather Service (NWS) data, or
one or more years of hourly sequential on-site data
o Discussion of meteorological conditions observed (as applied
or modified for the site-specific area, i.e., identify
possible variations due to difference between the monitoring
site and the specific site of the source)
o Discussion of topographic/land use influences
Air quality modeling analyses:
o Model each individual year for which data are available with
a recommended model or model demonstrated to be acceptable
on a case-by-case basis
- urban dispersion coefficients for urban areas
- rural dispersion coefficients for rural areas
o Evaluate.downwash if stack height is less than GEP
o Define worst case meteorology
o Determine background and document method
- long-term
- short-term
o Provide topographic map(s) of receptor network with respect
to location of all sources
*See ** on page B1 of checklist.
B-3
-------
o Follow current guidance on selection of receptor sites for
refined analyses
o Include receptor terrain heights (if applicable) used in
analyses
o Compare model estimates with measurements considering the
upper ends of the frequency distribution
o Determine extent of significant impact—provide maps
o Define areas of maximum and highest, second-highest impacts
due to applicant source (refer to format suggested in Air
Quality Summary Tables)
- long-term
- short-term
7. Comparison with acceptable air quality levels:
o NAAQS
o PSD increments
o Emission offset impacts if nonattainment
8. Documentation and guidelines for modeling methodology:
o Follow guidance documents
- Guideline on Air Quality Models, EPA-450/2-78-027,
April 1978
- Workbook for Comparison of Air Quality Models, EPA-450/2-
78-Q28a>b, May 1978
- Guide!ines for AQMA, Vol. 10(R), EPA-450/4-77-001, October
1977
- Technical Support Document for Determination of Good
Engineering Practice Stack Height (Draft), EPA, July 1978
- Ambient Air Monitoring Guidelines for PSD, EPA-450/2-78-
019, May 1978
- Requirements for the Preparation, Adoption and Submittal of
Implementation Plans; Approval and Promulgation of Implemen-
tation Plans, Federal Register, Volume 43, No. 118, pp 52676-
52748, August 1980.
B-4
-------
AIR QUALITY SUMMARY
For New Source Alone
** *~
Pollutant *
Highest Highest Highest Hi ghest Annual
2nd High 2nd High
Concentration Due to
Modeled Source (yg/m3)
Background Concentration
(yg/m3)
Total Concentration (yg/m3)
Receptor Distance (Km)
(or 'JTM Easting)
Receptor Direction (°)
(or UTM Northing)
Receptor Elevation (m)
Wind Speed (m/s)
Wind Direction (°)
Mixing Depth (m)
Temperature (°K)
Stability
Day/Month/Year
of Occurrence
~Use separate sheet for each pollutant (S0~, TSP, CO, NO , HC, Pb,
Hg, Asbestos, etc.)
**List all appropriate averaging period (1-hr, 3-hr, 8-hr, 24-hr,
30-day, 90-day, etc.) for which an air quality standard exists
Surface Air Data From Surface Station Elevation (m)
Anemometer Height Above Local Ground Level (m) '
Upper Air Data From
Period of Record Analyzed '
Model Used
Recoirmended Model
B-5
-------
AIR QUALITY SUMMARY
For All New Sources
** **
Pollutant * ~ ————
Highest Highest Highest Highest Annual
2nd High 2nd High
Concentration Due to
Modeled Source (ug/m3)
Background Concentration
(yg/m3)
Total Concentration (yg/m3)
Receptor Distance (Km)
(or UTM Easting)
Receptor Direction (°)
(or UTM Northing)
Receptor Elevation (m)
Wind Speed (m/s)
Wind Direction (°)
Mixing Depth (m)
Temperature (°K)
Stability
Day/Month/Year
of Occurrence
*Use separate sheet for each pollutant (S02, TSP, CO, NO , HC, Pb,
Hg, Asbestos, etc.)
**List all appropriate averaging period (1-hr, 3-hr, 8-hr, 24-hr,
30-day, 90-day, etc.) for which an air quality standard exists
Surface Air Data From Surface Station Elevation (m)
Anemometer Height Above Local Ground Level (m)
Upper Air Data From
Period of Record Analyzed
¦ Model Used' ___
Recomnended Model
B-6
-------
AIR QUALITY SUMMARY
For All Sources
Pollutant * __ZIZZ^ZZZ zm
Highest Highest Highest Highest Annual
2nd High 2nd High
Concentration Due to
Modeled Source (yg/m3)
Background Concentration
(yg/m3)
Total Concentration (yg/m3)
Receptor Distance (Km)
(or UTM Easting)
Receptor Direction (°)
(or UTM Northing)
Receptor Elevation (m)
Wind Speed (m/s)
Wind Direction (°)
Mixing Depth (m)
Temperature (°K)
Stability
Day/Month/Year
of Occurrence
*Use separate sheet for each pollutant (S0?, TSP, CO, NO , HC, Pb,
Hg, Asbestos, etc.)
**List all appropriate averaging period (1-hr, 3-hr, 8-hr, 24-hr,
30-day, 90-day, etc.) for which an air quality standard exists
Surface Air Data From Surface Station Elevation (m)
Anemometer Height Above Local Ground Level (m)
Upper Air Data From
Period of Record Analyzed '
Model Used
Recornnended Model
B-7
-------
STACK PARAMETERS FOR ANNUAL MODELING
Stack Serving Emission Rate Stack Exit Stack Exit Stack Exit Physical Stack GEP Stack Base
No. for each Diameter Velocity Temperature Height (m) Stack Elevation
Pollutant (g/s) [m] (m/s) (°K) Height (m) (m)
CO
cv> ! tp
: <Ł
-------
STACK PARAMETERS FOR SHORT TERM MODELING*
Stack Serving Emission Rate Stack Exit Stack Exit Stack Exit Physical Stack 6EP Stack Base
No. for each Diameter Velocity Temperature Height (m) Stack Elevation
Pollutant (g/s) [m] (m/s) (°K) Height (m) (m)
. \
*Separate tables for 50%, 75%, 100% of full load operating condition (and any other operating conditions as
determined by screening or detailed modeling analyses to represent constraining operating conditions) should
be provided.
-------
ADDENDUM
to
REGIONAL WORKSHOPS ON AIR QUALITY MODELING: A SUMMARY REPORT,
EPA-450/4-82-015, (PB 83-150573), April 1981
Replace the cover and pages 4, 5, 6, 8, 10, 10a, 15, A-1, A-2, A-3, A-4,
A-5, A-6, A-7, vri th the attached revised pages. Add new pages 5a, 15a,
A-8, A-9, C-l, C-2, C-3, C-4, C-5 and C-6.
October 1983
-------
EPA-450/4-82-015
REGIONAL WORKSHOPS OM
AIR QUALITY MODELING:
A SUMMARY REPORT
APRIL 1981
Source Receptor Analysis Branch
Monitoring and Data Analysis Division
Office of Air Quality Planning and Standards
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina
6-tt
-------
2.2.2 Background Concentrations
Techniques discussed in the Guideline on Air Quality
Models should be used to establishing background concentrations.
2.2.3 Source Data _
The load or operating condition of a plant that
causes the highest ground-level concentrations should be determined
through a screening analysis and this load should be used to establish
emission limitations. As a minimum all sources should be modeled using
100 percent design capacity; however, when modeling large sources,
e.g., 500 MW power plants or equivalent, 50 and 75 percent capacity
should also be modeled.
Hourly sequential emissions determined for existing
sources from continuous in-stack monitoring should be used in model
evaluation where possible. Hourly emissions are critical where short-
term concentrations are of concern in such evaluations.
2.2.4 Meteorological Data
2.2.4.1 Period of Record
Five years of representative meteorological
data should be used when estimating concentrations with an air quality
model. Consecutive years from the most recently available five-year
period are preferred. The meteorological data may be data collected
either on-site or at the nearest National Weather Service (NWS) station.
If the source is large, e.g., a 500 MW power plant, the use of five
years of NWS meteorological data or at least one year of on-site data
is required. As many years of on-site data as are available should be
used.
Five years of on-site data are often not
available. When considering shorter periods of meteorological data,
care must be taken to ensure that the data used contain the appropriate
worst-case conditions. On-site data should also be subjected to quality
assurance procedures that will ensure that the data are at least as
accurate and in as much detail as NWS data.
2.2.4.2 Wind Direction
Hourly average wind directions reported to
the nearest degree should be used where on-site data are used. The
CRSTER randomizaton sequence (i.e., the established sequential random
number set designated for use with the meteorological preprocessor to
CRSTER) should be used with NWS wind data.
4
(Revised October 1983)
-------
2.2.4.3 Reference Height
The surface wind reference height used in
the model should be defined to agree with the actual height of the
surface wind sensor. When wind is monitored at heights closer to
plume height, the wind direction should be used to define the plume
transport and the speed should be utilized to develop the appropriate
vertical wind speed profile.
2.2.4.4 Collection of Field Data
Guidance provided in Appendix A should be
followed in the design of site-specific, on-site meteorological data
collection programs.
2.2.4.5 Use of National Weather Service (NWS) Calms
This section should be applied to situations
where: (1) the available data are hourly NWS records or comparable
military or FAA data, (2) the model of choice i s Gaussian.
Wind speeds less than 2 knots are herein
defined as calm. Although the estimated concentrations based on using
the "calm" data as model input may be significant, Gaussian models
cannot realistically handle these near zero wind conditions. There-
fore, the hourly concentrations calculated using the calm data should
not be considered valid concentrations; the wind and concentration data
for these hours should be considered missing.
The model output containing concentrations
calculated using the existing CRSTER procedure for handling calms should
be examined for critical concentrations. If a critical concentration has
been calculated using a calm, then recalculate the concentration consider-
ing the hours using the calms as missing. The new 3, 8 and 24-hour
average critical concentrations should be calculated by dividing the
total concentration for the period by the number of valid or non-missing
hours. If the total number of valid hours is less than 18 for 24-hour
averages, less than 6 for 8-hour averages or less than 3 for 3-hour
averages, the total concentration should be divided by 18 for the
24-hour average, 6 for the 8-hour average and 3 for the 3-hour average.
For annual averages, the sum of all valid hourly concentrations is divided
by the number of non-calm hours during the year.
Computer software ("CALMPRO") has been dev-
eloped by Region I and may be used to process either the meteorological
input data or the output from CRSTER, MPTER, ISC, RAM or Complex I according
to the procedures in this section.
5
(Revised October 1983)
tr IS
-------
These procedures do not apply to annual average
concentrations determined with a model using "STAR" data input. The treat-
ment of calms in such joint frequency distributions should not be changed.
2.2.4.6 Use of On-Site "Calm" or Light Wind Data
Measured on-site wind speeds of less than
1 m/s should be set equal to 1 m/s when used as input to Gaussian models.
Wind direction for these low wind speed hours may be determined on a
case-by-case basis from the available on-site records. If the wind
is indeterminate with respect to speed or direction, it should be treated
as missing data and short term averages may then be calculated as in
2.2.4.5 above.
2.2.4.7 Extended Periods of Calms
Stagnant conditions that include extended
periods of calms often produce high concentrations over wide areas for
relatively long averaging periods. The standard Gaussian models are
often not applicable to such situations. When stagnation conditions
are of concern, other modeling techniques may be considered on a
case-by-case basis.
5a
(Revised October 1983)
-------
3.0 FLAT TERRAIN POINT SOURCE MODELS
3 .1 Discussion
Flat terrain, as used here, is considered to be an area where
terrain features are all lower in elevation than the top of the stack of
the source in question.
A number of models have been made available by EPA and others
for those applications where receptors are located at elevations less than
the top of the stack. However, inconsistencies have resulted from the
use of these models. Such Inconsistencies occur in part because models
may be developed at different times for specific applications and
the various algorithms are improved, changed or added to accommodate a
specific problem or to reflect recent research. This section provides
recommendations to resolve these inconsistencies without limiting the
range of applicability of flat terrain models.
3.2 Recommendations
3.2.1 Screening Techniques
Screening techniques and options as provided in "Guide-
lines for Air Quality Maintenance Planning and Analysis Volume 10 (R):
Procedures for Evaluating Air Quality Impact of Mew Stationary Sources"
should be used.
Where possible screening procedures should be site and
problem specific. Consideration should be given to: (1) terrain; (2)
urban or rural dispersion coefficients; (3) building downwash potential
(see Sections 3.2.3f, 6.2.7 and Appendix C); and (4) worst-case conditions
when representative meteorological data or applicable detailed modeling
techniques are not available. If screening is the sole basis for the
analysis, adequate justification and documentation should be required
for the use of averaging time factors.
3.2.2 Refined Analytical Techniques
The following table lists the preferred models for the
indicated applications. These models should be used in Regional Office
6
(Revised October 1983)
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3.2.3 Model Options
The options that are found in ISC, CRSTER, MPTER,
and PTPLU* have greatly increased the technical options available. To
ensure consistency in the use of these options, Regional Office users
should follow the guidance below:
a. Stack Tip Downwash (CRSTER, MPTER, ISC, PTPLU)
This option should not be used unless demonstrated
to be applicable on a case-by-case basis. Although there is evidence
that this phenomenon can occur, there are no data to support wide use of
the option.
b. Plume Rise (CRSTER, MPTER, ISC, PTPLU)
In all cases, except when the downwash (building
wake) algorithm of ISC is employed, the final plume rise option should be
used. The restriction on the use of gradual plume rise is based upon
the lack of specific data needed to quantify the dispersion during plume
rise. When building downwash is considered a problem, transitional
plume rise calculations are used only to determine whether the plume
will be affected by building wake. If so, dispersion is then handled by
appropriate modified dispersion parameters.
c. Rural/Urban Options (ISC)
The selection of the rural or urban option should
be based upon the determinations as outlined in Section 6.2.5 for deter-
mining whether an area is urban or rural.
d. Momentum Plume Rise (ISC, CRSTER, MPTER, PTPLU)
This is optional in the CRSTER and MPTER models
and an integral part of the ISC and PTPLU models. It should be used in the
CRSTER and MPTER models.
e. Deposition (ISC)
The deposition algorithm in ISC should be used
whenever deposition is considered to be a factor in the analysis.
v f. Building Wake Effects (ISC)
The building wake effects option in ISC should be used
whenever the height of the stack to be modeled is less than Good Engineering
Practice (GEP) stack height. (See also Section 6.2.7)
*PTPLU is found in ONAMAP (Version 5)(PB 83-244 368) and is a replacement for
PTMAX.
8
(Revised October 1983)
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4.2.1.1 Initial Screening Technique
The initial screen is the use of the Valley
Model Screening Technique. The following worst-case assumptions should I
be used in the Valley Screening Technique to determine 24-hour averages: i
(1) P-G stability "F"; (2) wind speed of 2.5 m/s; (3) 6 hours of occurrence.
Multiple sources should be treated individually
in the Valley Model Screening Technique and the concentrations for I
specific wind directions summed. Only one wind direction should be used
for 24-hour averages (see User's Manual, pages 2-15) even if individual
runs are made for each source.
The receptor grid found in the Valley Model
User's Guide may not be sufficient for all analyses if only one geographical
scale factor is used. The Valley Model is very sensitive to ground-level
elevation at the receptor, and the use of the standard polar grid could
miss the vorst-case receptor. If this situation occurs, the user should
choose an additional set of receptors at appropriate downwind distances
whose elevations are equal to plume height minus 10 meters.
4.2.1.2 Second Level Screening Technique
If a violation of any NAAQS or the controlling
increment is indicated, a second-level screening technique may be used.
An on-site data base of at least 1 full year of meteorological data,
collected in accordance with the recommendations in Appendix A, is
preferred for use with the second-level screening technique. All meteor-
ological data used in the analysis must be reviewed for both spatial and
temporal representativeness.
At this time, Complex I, available in UNAMAP
Version 5, is the preferred second-level screening technique. Complex I
is the result of modifying the MPTER Model to incorporate the Valley
Model algorithm. As such it is a multi-source screening technique that
accepts hourly meteorological input. It differs from the Valley Model in
that it uses hourly sequential meteorological data. In Complex I, hourly
wind direction input is specified to the nearest whole degree (and the
plume vectored accordingly), but the 22 1/2° Valley Model crosswind sector
averaging has been retained.
4.2.1.3 Restrictions
For screening analyses, the use of a sector
greater than 22-1/2° should not be allowed and full ground reflection
should always be used.
10
(Revised October 1983)
6-fl
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4.2.1.4 Options
Complex I should be used with the following input:
Card 5:
(1) select terrain adjustment I OPT (1) = 1
(2) select buoyancy induced dispersion I0PT(4) = 1
(3) set IOPT(25) = 1
Card 6:
(4) use the following terrain adjustment values:
0.5, 0.5, 0.5, 0.5, 0.0, 0.0.
(5) set ZMIN = 10.
If gradual plume rise is used to estimate concen-
trations at nearby elevated receptors, each of the concentrations listed in
the model output table of high values should be carefully examined prior
to regulatory application. The gradual plume rise option in COMPLEX I is
not specific to stable conditions and the high concentrations could be
the result of the larger dispersion coefficients assigned to unstable or
neutral conditions and plume touchdown on other than elevated terrain.
Only those concentrations specifically recorded at receptors above plume
height should be used.
The option in the Valley Model to use the 2.6
stable plume rise factor should be selected and for buoyant sources the option
for buoyancy-induced dispersion should be exercised.
4.2.2 Refined Analytical Techniques
When the results of the screening analysis demonstrate
a possible violation of NAAQS or the controlling PSD increments, a more
refined analysis should be conducted. Since there are no refined techniques
currently recommended for complex terrain applications, a nonguideline
model may be applied in accordance with Section 7. In the absence of an
appropriate refined model, screening results may be acceptable.
10a
(Revised October 1983)
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Population Density Procedures: (1) Compute the
average population density, per square kilometer with A0 as defined
above; (2) If j), is greater than 7 50 people/km^, use urban dispersion
coefficients; otherwise, use appropriate rural dispersion coefficients.
The land use procedure is considered the most defin-
itive. Population density should be used with caution, especially in a
highly industrialized area where the population density may be low but
the area is sufficiently built up so that the land use criteria would be
satisfied. Impacts from sources beyond the three (3) kilometers should
be included in the background.
For analyses of urban complexes, the entire area should
be modeled as an urban region if most of the sources are located in
areas classified as urban.
6.2.6 General Model Evaluation
A model evaluation study should assess how closely the
mathematical assumptions inherent to the model describe the physics
and/or chemistry of the atmosphere. The process of model evaluation
should consider all of the following; (1) assumptions inherent to
design/algorithms of model; (2) purpose/objective of model; (3) purpose/
objectives of monitor(s); (4) applicability of monitored data for
comparison; (5) comparison rif model estimates with monitor observations
for upper end of frequency distribution, statistical analyses, and
analyses of weakness in individual algorithms; and (6) analyses of
critical meteorological and source conditions and their effect on indi-
vidual algorithms. An analysis should also be made of the sensitivity
of the algorithms to the meteorological input data.
For short-term model evaluation, all input data should
be based on measured hourly averages. This includes mass emission
rates, stack dynamic operating parameters and meteorological input. The
spatial applicability of measured air quality data and tracer studies
should be consistent with the scale of the model comparisons.
Calibration for short-term air quality concentrations
is not recommended. Determination of the need for calibration and
calibration procedures are contained in the 1978 guideline.
6.2.7 GEP and Downwash Analysis
All SIP revisions and PSD permits for major sources
(potential to emit greater than 100 tons per year) should include a GEP
analysis for each stack. For area-wide SIPs, where a GEP analysis for
each source may prove impractical, those major sources with known or
suspected downwash problems, and all large sources (e.g. emissions equivalent
15
(Revised October 1983)
6-11
-------
to a 500 MW power plant), should have GEP analyses. All GEP analyses
should be performed in accordance with the "Guideline for Determination
of Good Engineering Practice Stack Height (Technical Support for the Stack
Height Regulations)EPA-450/4-80-023, July 1981. If the analysis
indicates that the stack is below the GEP height, a downwash analysis
should be performed. Detailed downwash screening procedures are provided
in Appendix C.
15a
(Revised October 1983)
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APPENDIX A
ACQUISITION OF SITE SPECIFIC METEOROLOGICAL DATA
Models recommended in the 1978 Guideline on Air Quality Models require
input the following parameter's:
° transport wind speed and direction;
° ambient air- temperature;
° Pasquil 1-Gifford stability category.
Transport Wind 5peed and Direction
For- stacks below 100 m, select the stack top height as the transport
wind speed and direction measurement height. For- sources with stacks
extending above 100 m, a 100 m tower- is suggested unless the stack top is
significantly above 100 m (200 m or- more). For- cases with stacks 200 m or
above, the Regional Meteorologist should determine the appropriate measure-
ment height on a case-by-case basis. Remote sensing may be a feasible
al ter native.
The wind speed should be measured using an anemometer- and the wind
direction measured using a vane with a vertical tail. The specifications
for wind measuring instruments contained in the "Ambient Monitoring Guidelin
for- Prevention of Significant Deterioration (PSD)," EPA 450/4-80-012,
November 1980 should be followed. Wind direction should be reported to the
nearest degree.
Ambient Air Temperature
Ambient temperature should be measured at or near standard instrument
shelter height. Temperature can be reliably measured using good quality
linear- thermistors or- platinum resistance devices.
A-l
(Revised October 1983)
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Pa squill-Gif ford Stability Category
The Pasquil1-Gifford (P-G) stability categories, as originally
defined, couple near-surface measurements of wind speed with subjectively
determined insolation assessments based on hourly cloud covet- and ceiling
observations. The wind speed measurements are made at or near 10 m. The
insolation rate is typically assessed using the cloud cover and ceiling height
criteria outlined by Turner (1964)*. Often the cloud cover data are not
available in site-specific data sets. In the absence of such observations,
it is recommended that the P-G stability category be estimated using
Table A-I. This table requires , the standard deviation of the vertical
wind direction fluctuations. If the surface roughness of the area surrounding
the source is different from the 15 cm roughness length upon which the table
is based, an adjustment may be made as indicated in footnote 2 of Table A-I.
is computed from direct measurements of the elevation angle of the
vertical wind directions.
If measurements of elevation angle are not available, oŁ may be
determined using the transform:
(radians) = ow/u
where: = the standard deviation of the vertical wind
direction fluctuations over- a one-hour- period;
*Turner, D. B., 1964. A Diffusion Model for- an Urban Area. Journal of
Applied Meteorology, 3(1):83-91.
A-2
(Revised October- 1983)
b-yz-
-------
cw = the standard deviation of the vertical wind speed
fluctuations over a one-hour- period;
u = the average horizontal wind speed for a one-hour period.
Since both cw and u are in meters per- second, ~Ł is in radians. To
use qr in Table A-I, of must be converted to degrees. It is recommended
that a vertically mounted propeller anemometer- be used to measure the
vertical wind speed fluctuations. The instrument should meet the specifica-
tions given in the Ambient Monitoring Guidelines referenced above.
Compute cw directly each hour- using at least 3600 values based on a
recommended readout interval of 1 second. If is computed using the
output of the anemometer- by other- than direct application of the formula
for- a variance, the method should be demonstrated to be equivalent to
direct computation. Both the vertical wind speed fluctuations and the
horizontal wind speed should be measured at the same level. Moreover-, these
measurements should be made at a height of 10 m for- use in estimating the
P-G stability category. Where trees or land use preclude measurements as
low as 10 m, measurements should be made at a height above the obstructions.
If on-site measurement of either oe or- cw are not available,
stability categories may be determined using the horizontal wind direction
fluctuation, o^, as outlined by John Irwin in Dispersion Estimate
Suggestion No. 8. He includes the Mitchell and Timbre* method that uses
*Mi tchell, A. Edgar-, Jr., and K. 0. Timbre, Atmospheric Stability Class
from Horizontal Wind Fluctuation, 72nd Annual Meeting APCA, Cincinnati,
Ohio, June 24, 1979
A-3
(Revised October 1983)
-------
the NRC Safety Guide 1.23 categories of o^ listed in Table A-II as an
initial estimate of the P-G stability category. This relationship is
considered adequate for daytime use. During the nighttime (one hour-
prior to sunset to one hour- after sunrise) the adjustments given in Table
A-II I should be applied to these categories. As with cr in Table A-I,
an hourly average may be adjusted for- surface roughness by multiplying
the table values of by a factor- based on the average surface roughness
length determined within 1 to 3 km of the source. The need for- such
adjustments should be determined on a case-by-case basis.
Wind direction meander- may, at times, lead to an erroneous deter-mination
of P-G stability category based on aTo minimize wind direction meander-
contributions, may be determined for each of four- 15-minute periods in
an hour-. To obtain the for- stability deter-minations in these situations,
take the square root of one-quarter of the sum of the squares of the four
15-minute c^'s. While this appr-oach is acceptable for- determining stability
category, a^'s calculated in this manner- are not likely to be suitable
for- input to models under development that are designed to accept on-site
hourly o's based on 60-minute periods.
There has not been widespread use of and to determine P-G
categories. As mentioned in the footnotes to Tables A-I and A-II, the
techniques outlined have not been extensively tested. The criteria listed
in Tables A-I, A-II, and A-111 are for and a^ values at 10m. For best
results, the aŁ and values should be for- heights near- the surface as
A-4
(Revised October- 1983)
fo-w
-------
close to 10 m as practicable. Obstacles and large r-oughness elements may
preclude measurements as low as 10 m. If circumstances preclude measurements
below 30 m, the Regional Meteorol ogi st should be consulted to determine
the appropriate measurements to be taken on a case-by-case basis. The
criteria listed in Tables A-1, A-11, and A-111 result from studies conducted
in relatively flat terrain in rather ideal circumstances. For' routine
applications where conditions are often less than ideal, it is recommended
that a temporary program be initiated at each site to spot-check the stability
class estimates. Irwin's method using oŁ or- oa should be compared with
P-G stability class estimates using on-site wind speed and subjective
assessments of the insolation based on ceiling height and cloud cover-
observations. The Regional Meteorologist should be consulted when using
the spot-check results to refine and adjust the preliminary criteria
outlined in Tables A-I, A-II, and A-111.
In summary, when on-site data sets are being used, Pasquil 1-Gifford
stability categories should be determined from one of the following
schemes listed in the order- of preference:
(1) Turner's 1964 method using on-site data which include cloud
cover, ceiling height and surface (~ 10m) winds.
(2) oŁ from on-site measurements and Table A-I.
(oŁ may be determined from elevation angle measurements or may be
estimated from measursnents of aw according to the transform: = aw/u
(see page A-2).)
A-5
(Revised October- 1983)
-------
(3)
(4)
cover' and
oa f''om on-site measurements and Table A-II and A-111.
Turner's 1964 method using on-site wind speed with cloud
ceiling height from a nearby NWS site.
A-6
(Revised October 1983)
-------
Table A-I
P-G Stability Category Versus Standard Deviation of
Vertical Wind Direction Fluctuation,
P-G Stability Standard Deviation of Vertical Wind
Category Direction Fluctuations (in degrees) 1 >2
A > 11.5°3
B 10° to 11.5°
C 7.8° to 10°
D 5° to 7.8°
E 2.4° to 5.0°
F < 2.4°
Adapted from: Irwin, John, 1980. Estimation of Pasquill Stability
Categor ies, Dispersion Estimate Suggestion No. 8, Environ-
mental Applications Branch, Environmental Protection Agency,
Research Triangle Park, NC.
Icare should be taken that the wind sensor- is responsive enough for- use in
measuring vertical wind direction fluctuations.
^A surface r-oughness factor- of (zQ/15 cm)^, where zQ is the average surface
roughness in centimeters within a radius of 1-3 km of the source, may be applie
to the tabl_e values. It should be noted that this factor-, while theoretically
sound, has not been subjected to rigorous testing and may not impr-ove the
estimates in all circumstances. A table of z0 values that may be used as a
guide to estimating surface roughness is given in: A. Smedman-Hogstrom and
U. Hogstrom (1978). "A Practical Method for- Determining Wind Frequency
Distributions for- the Lowest 200 m from Routine Meteorological Data." Journal
of Applied Meteorology, 17 (7 ) :942-953.
^These criteria were adapted fr-om those presented by Smith and Howard ( 1972).*
It would seem reasonable to restrict the possible categories to A through D
during daytime hours and to categories D through F during the nighttime hours.
During daytime hours for- wind speeds above 6 m/s, conditions ar e neutral.
During the night, conditions are neutral for- wind speeds equal to or- greater
than 5 m/s.
*Smith, T. B. and S. M. Howard, 1972. Methodology for- Treating Diffusivity.
Mil 72FR-1030. Meteorology Research, Inc, 464 W. Woodbury Road, Altadena, CA.
A-7
(Revised October- 1983)
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Table A-II
P-G Stability Categories Versus Standard Deviation of
Horizontal Wind Direction Fluctuations, oa
P-G Stability Standard Deviation of the Horizontal
Category Wind Direction F1 unctuations (in degrees)^ >2
>
22.53
17.5
to
22.5
12.5
to
17.5
7.5
to
12.5
3.8
to
7.5
<
3.8
Adapted from: Irwin, John, 1980. Estimation of Pasquill Stability
Categories, Dispersion Estimate Suggestion No. 8, Environmental
Applications Branch, Environmental Protection Agency, Research
Triangl e Park , NC.
ICare should be taken that the wind sensor is responsive enough for use in
measuring wind direction fluctuations.
^A surface roughness factor- of (z0/15 cm)^, where zQ is the average surface
roughness in centimeters within a radius of 1-3 km of the source, may be
applied to the table values. It should be noted that this factor-, while
theoretical ly sound, has not been subjected to rigorous testing and may not
improved the estimates in all circumstances. A table of z0 values that may
be used as a guide to estimating surface roughness is given in: A Smedman-
Hogstr-om and U. Hogstrom (1978). "A Practical Method for- Determining Wind
Frequency Distributions for* the Lowest 200 m from Routine Meteorological
Data." Journal of Applied Meteorology, 17(7 ) :942-953.
•^These criteria are from NRC Safety Guide 1.23. It would seem reasonable
to restrict the possible categories to A through D during daytime hours
with a restriction that for- wind speeds above 6 m/s, conditions are
neutral. Likewise, during the nighttime hours, some restrictions, as in
Table A-111, are needed to preclude occurrences of categories A through C.
A-8
(Revised October- 1983)
6-J*
-------
Table A-111
Nighttime^ P-G Stability Categories Based on CA from Table A-II
If the aA
stabil i ty
class is
And if the 10 m wind speed, u, is
m71 mi /hr
u<2.9
2.9
-------
APPENDIX C
BUILDING DOWNWASH SCREENING PROCEDURES
When a GEP analysis indicates that a stack is less than the GEP height,
the following screening procedures should be applied to assess the potential
for air quality problems. The building downwash screening procedure is
divided into two major areas of concern. Within the cavity region (up to
3L downwind, where L = the lesser of the building height or projected width),
a series of simple hand calculations can be used. Within the wake region
(3L to 10L downwind), the ISC model can be used in a screening mode.
Details on both procedures are provided below.
Cavity Region
The cavity effects screening procedure consists of four sequential
steps.
Step 1. Compare the stack height to the cavity height. Calculate
the cavity height hc:
hc = H + 0.5 (L),
where: H = height of structure (m) and
L = lesser dimension (height or projected width) of structure (m).
If the stack height is greater than or equal to the cavity height,
then it may be assumed that maximum impacts will be dominated by the wake
effects, and no further cavity analysis is required. Proceed to perform
the wake effects analysis. If the stack height is less than the cavity
height, proceed to Step 2.
C-l
(October 1983)
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Step 2. Estimate the momentum plume rise for neutral atmospheric con-
ditions. First compute the momentum flux, Fm:
Fm " 'W v2d2/4
where Ta = ambient air temperature (°K) (assume 293°K),
Ts = stack exit temperature (°K),
v = stack exit velocity (m/s) and
d = stack inner diameter (m).
Next, compute the momentum plume rise hm:
1/3
3Fm(x)
hm =
b2 u2
where b = (1/3 + u/vs),
u = critical wind speed (m/s) (assume 7.5 m/s),
x = downwind distance (m) (assume 2 building heights downwind).
The plume height can be calculated by adding the momentum plume rise
to the stack height. If the plume height is greater than or equal to the
cavity height calculated in Step 1, then it may be assumed that maximum
impacts will be dominated by the wake effects and no further cavity
analysis is required. Proceed to the wake effects analysis. If the plume
height is less than the cavity height, proceed to Step 3.
C-2
(October 1983)
-------
Step 3. Estimate the downwind extent of the cavity. Compute the cavity
length (xr), measured from the lee side of the building:
for short buildings (Y/H<2):
xr « (A)(W)
1.0 + B(W/H)
for long buildings (Y/H _> 2):
xr = 1.75(W) ,
1.0 + 0.25{W/H)
where: H = building height (m)
Y = alongwind building dimension (m),
W = crosswind building dimension (m),
A = -2.0 + 3.7{Y/H)-l/3 and
B = -0.15 + 0-305(Y/H)"1/3.
Next, compare the cavity length to the closest distance to the plant
property line. Consider only plant property to which public access is
precluded. If the cavity does not exceed this distance, then it may be
assumed that cavity effects will not impact ambient air, and no further
cavity analysis is required. Proceed to the wake effects analysis. If
the cavity extends beyond plant property, proceed to Step 4.
Step 4. Estimate Impacts within the cavity. "Worst case" concentration
impacts (X) can be estimated by the following approximation:
X = Q
1.5{A)(u)
where: Q = emission rate (g/s),
A = cross-sectional area of building normal to wind (m^) and
u = wind speed (m/s).
C-3
(October 1983)
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For u, one should choose the lowest wind speed likely to result in entrap-
ment of most or all of the pollutant into the cavity. If no data are
available from which the minimum speed can be estimated, assume a worst
case wind speed of 3 m/s.
This concludes the cavity effects screening procedure. It is con-
sidered to be conservative. If this conservative estimate proves un-
acceptable, one may wish to consider a field study or fluid modeling
demonstration to show maintenance of the NAAQS or PSD increments within
the cavity. If such options are pursued, prior agreement on the study
plan and methodology should be reached with the Regional Office.
Make Region
Wake effects screening can be performed with ISC using a set of
representative "worst case" meteorological conditions. The procedure
consists of three steps.
Step 1. Determine the "worst case" building dimensions for input to
the model. To model "worst case" conditions, care should be taken to use
the same critical building dimensions (maximum projected width and/or
height) that gave the greatest stack height in the GEP analysis. The way
ISC is constructed, the user inputs a building length and width, instead
of the projected width used in the GEP analysis. The model calculates an
area based on this length and width and then determines the diameter of a
circle with equal area. This so called "effective diameter" (D) is used
in all other model calculations as the projected width of the building.
C-4
(October 1983)
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Thus ISC assumes:
(L ) (W) = U/4) D2.
To model the projected width determined in the GEP analysis, set D equal
to the projected width and solve the above equation assuming L = W. The
calculated value should then be used as inputs to ISC for L and W. For
example, if a building i s 60 m tall, 40 m long and 30 m wide, the greatest
GEP height is found by maximizing the projected width (using the 50 m
diagonal). In this case, set D = 50 and solve the above equation to find
L = W = 44 m. This dimension is then used as the input for L and W in ISC
Step 2. Calculate maximum hourly concentrations using ISC. The
following procedures should be followed:
A. Use the wake effects option with building dimensions determined
in Step 1, transitional plume rise (ISW(24)=2), and no stack tip downwash
(ISVJ (2 5 ) = 1).
B. With the source at the center of the grid, place receptors down-
wind along a single radial. Receptors should be spaced no more than 100 m
apart within 2000 m of the source. Additional receptors may be needed
on a case specific basis to ensure prediction of the maximum concentration
C. A set of representative "wrst case" meteorological conditions
should be used in conjunction with the model option that reads hourly
data in card image format (ISW(19)=2). The following combinations of
stability class and wind speed should be used in the model to insure use
of the "worst case" meteorological conditions:
C-5
(October 1983)
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Stabil i ty Class
Wind Speed (m/s)
A
, 3
B
, 3, 5
C
, 3, 5, 10
0
, 3, 5, 10, 20
E
, 3, 5
F (rural only)
, 3, 5.
A temperature of 293°K, a mixing height of 5000 m, and a wind direction
along the line of receptors should be used for each hour. If other
combinations of parameters (stability, wind speed, temperature, etc) are
known or suspected to cause problems, they should also be modeled.
Step 3. Obtain concentration estimates for the averaging times of
concern. The maximum 1-hour concentration is the highest of the concen-
trations estimated in Step 2. Maximum concentrations for longer averaging
times should be estimated using the procedures described in EPA's "Guide-
lines for Air Quality Maintenance Planning and Analysis, Volume 10 (Revised)
Procedures for Evaluating Air Quality Impact of New Stationary Sources."
EPA-4-50/4-77-001 , October 1977 (pp 4-20 thru 4-22).
C-6
(October 1983)
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TECHNICAL REPORT DATA
(Please read Insiructions on the reverse before completing)
1; ^c5CRT N'O. 1 2.
:EPA-450/4-82-015 |
3. RECIPIENT'S ACCES5IOf*NO.
-.TiTL; A\E SUBTITLE
Regional Workshops on Air Quality Modeling
5. REPORT DATE
April 1981
A.Summary Report
6. PERFORMING ORGAN'ZA7ION CODE
7. A.UTHORIS]
Monitoring and Data Analysis Division
Office of Air Quality Planning and Standards
8. PERFORMING OR
GANIZATION REPORT NO.
9: PERFORMING ORGANIZATION' NAME AND ADDRESS
10. PROGRAM ELEMENT NO.
U. S. Environmental Protection Agency
Research Triangle Park, N. C. 27711
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRE5S
13. TYPE OF REPORT AND PERIOD COVERED
1979 - 1982
Same as box 5»
1U. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
•EPA Project Officer: C. J. Hopper
IS. AcS'TRACT
The requirements placed on air quality control agencies by the Clean Air Act have
draraticslly increased the need for improved air quality modeling. The resulting
increase in the use of models has also led to a substantial increase in the number
and cocpleiity of situations in which models are employed. The modeling guideline
(Guideline on Air Quality Models, EPA-450/2—78-027, April 1978) addresses many of the
problems in this relatively new and growing field, but much is left to the discretion
of the reviewing agency since many complex problems are best solved on a case-by-case
basis. However, because of the variety of technically correct solutions to any
complex problem, different approaches with differing results have led to inconsist-
ency in model applications from Region to Region. In an effort to improve consist-
ency several workshops were held to provide"a forum for the Regional Office and
Headquarters groups to discuss common problem areas and arrive at generally accept-
able solutions. Many recommendateions were made in the course of the workshops.
These were reviewed by OAQPS and some have necessarily been modified and supple-
mented to ensure consistency with other modeling policies. This report clarifies
preferred data bases and procedures for the application of specific models and model-
ing techniques in situations where the guideline permits a case-by-case analysis.
17. KEY WORDS AND DC
CUMENT ANALYSIS
i. DESCRIPTORS
t, IDENTIFIERS/OPEN ENDED TERMS
c. cosati Field/Group
Air Pollution
Atmospheric Models
Atmospheric Diffusion
Meteorology
Air. Pollution Abatement
Implementation Air
Pollution Planning
Diffusion Modeling
Gaussian Plume Models
Clean Air Act
13B
•2. ~.S7 =.: sUTlON STATEMENT
unlimited release
19. SECURITY CLASS (This Report)
none
21. NO. OF PAGES
4^
20 SECURITY CLASS (This page J
none
22. PRICE
EPA Form 2220-1 (9-73)
9
I
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