United States       Office of Air Quality       EPA-450/2-78-028a
             Environmental Protection  Planning and Standards      OAQPS No. 1.2-097
             Agency         Research Triangle Park NC 27711   May 1978 « .
             _
v>EPA       OAQPS  Guideline
             Series

             Workbook for
             Comparison of Air
             Quality Models

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                              EPA-450/2-78-028a
                              OAQPS No. 1.2-097
Workbook for Comparison
                 of
     Air  Quality  Models
     U.S. ENVIRONMENTAL PROTECTION AGENCY
        Office of Air and Waste Management
      Office of Air Quality Planning and Standards
        Monitoring and Data Analysis Division
     Research Triangle Park, North Carolina 27711
                             *>- *
               May 1978

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                                      OAQPS GUIDELINE SERIES
The guideline series of reports is being issued by the Office of Air Quality Planning and Standards (OAQPS) to
provide  information to state and local air pollution control agencies; for example, to provide guidance on the
acquisition and processing of air quality data and on the planning and analysis requisite for the maintenance of air
quality.  Reports published in this series will be available - as supplies permit - from the Library Services Office
(MD-35), U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711, or, for a nominal
fee, from the National Technical Information Service, 5285 Port Royal Road, Springfield, Virginia 22161.
                                  Publication No. EPA-450/2-78-028a
                                    (OAQPS Guideline No. 1.2-097)
                                                 11

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                               ACKNOWLEDGEMENT

        This workbook was prepared by Argonne National Laboratory for the
Environmental Protection Agency under Interagency Agreement No.  EPA-IAG-D6-
0013.  The primary authors were Albert E.  Smith, Kenneth L. Brubaker,
Richard R. Cirillo, and Donald M. Rote of the Energy and Environmental Systems
Division, ANL.  Their contributions are gratefully acknowledged.  Extensive
technical and editorial review was provided by staff of the Source Receptor
Analysis Branch, Monitoring and Data Analysis Division.
                                      iii

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TABLE OF CONTENTS

1.
2.



3.






4.





5.












INTRODUCTION 	
PROCEDURE 	
2.1 GENERAL 	
2.2 OVERVIEW 	
2 . 3 STEP -BY-STEP INSTRUCTIONS 	
APPLICATION CLASSIFICATION 	
3.1 INTRODUCTION 	
3.2 INSTRUCTIONS 	
3. 3 CLASSIFICATION OF POLLUTANT CHARACTERISTICS 	
3.4 CLASSIFICATION OF AVERAGING TIMES 	
3.5 CLASSIFICATION OF SOURCE CHARACTERISTICS 	
3. 6 CLASSIFICATION OF TRANSPORT CHARACTERISTICS 	
PRELIMINARY ANALYSIS 	
4.1 INTRODUCTION 	
4.2 CHECK COMPATIBILITY OF STUDY MODEL WITH APPLICATION 	
4 . 3 CLASSIFICATION OF STUDY MODEL 	
4.4 IDENTIFICATION OF REFERENCE MODEL 	
4.5 REVIEW AND MODIFICATION OF IMPORTANCE RATINGS 	
TREATMENT OF APPLICATION ELEMENTS 	
5.1 INTRODUCTION 	
5.2 TREATMENT OF SOURCE-RECEPTOR RELATIONSHIP 	
5.3 TREATMENT OF EMISSION RATE 	
5.4 TREATMENT OF COMPOSITION OF EMISSIONS 	
5.5 TREATMENT OF PLUME BEHAVIOR 	
5.6 TREATMENT OF HORIZONTAL AND VERTICAL WIND FIELDS 	
5.7 TREATMENT OF HORIZONTAL AND VERTICAL DISPERSION 	
5.8 TREATMENT OF CHEMISTRY AND REACTION MECHANISM 	
5.9 TREATMENT OF PHYSICAL REMOVAL PROCESSES 	
5.10 TREATMENT OF BACKGROUND, BOUNDARY AND INITIAL CONDITIONS...
5. 11 TREATMENT OF TEMPORAL CORRELATIONS 	 , 	
Page
1
3
3
4
4
15
15
15
17
21
21
26
29
29
29
30
32
35
51
51
51
59
60
64
65
68
77
78
80
85
        IV

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                         TABLE OF CONTENTS (Cont'd)
                                                                      Pae
6.  COMPARATIVE EVALUATION ..........................................    93
    6.1  INTRODUCTION ...............................................    93
    6.2  TECHNICAL COMPARISON .......................................    93
         6.2.1  Comparing Treatments of Individual Elements
         6.2.2  Overall Technical Comparison
                6.2.2.1  Comparison .Based on HIGH-Rated
                         Elements	
                6.2.2.2  Comparison Based on MEDIUM- and LOW-Rated
                         Elements	,	    98
                6.2.2.3  Comparison with a CRITICAL Element	    "
7.  ROLLBACK/STATISTICAL MODELS	   101
    7.1  GENERAL	   101
    7.2  ADVANTAGES AND DISADVANTAGES	   102
    7. 3  COMPARISON OF ROLLBACK/STATISTICAL MODELS	   103

APPENDIX A,  TECHNICAL SUPPORT MATERIAL, t,,	   A-1
APPENDIX B.  BACKGROUND MATERIAL ON REFERENCE MODELS	   %_i
APPENDIX C.  APPLICATIONS TO SPECIFIC MODELS ,«....,,..,	   C-l
APPENDIX D.  APPLICATION CLASSIFICATION AND MODEL EVALUATION FORMS..   D_!

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                               LIST OF TABLES

Number                                                                  Page

 2.1   Workbook Section and Form for Each Step in Comparison	      4

 3 .1   Classification of Criteria Pollutants ,	    22

 3.2   Classification of NAAQS Averaging Times	     23

 3.3   Classification of Common Source Categories	     25

 4.1   Suggested Reference  Models for Indexed Applications	,     33

 4.2   Importance Ratings for Source-Receptor Relationship	     39

 4.3   Importance Ratings for Emission Rate	     40

 4.4   Importance Ratings for Chemical Composition of Emissions	     41

 4 .5   Importance Ratings for Plume Behavior	     42

 4.6   Importance Ratings for Horizontal Wind Field	     43

 4.7   Importance Ratings for Vertical Wind Field	     44

 4.8   Importance Ratings for Horizontal Dispersion	     45

 4.9   Importance Ratings for Vertical Dispersion	     46

 4.10  Importance Ratings for Chemistry and Reactor Mechanism	     47

 4.11  Importance Ratings for Physical Removal Processes	     48

 4.12  Importance Ratings for Background, Boundary and
       Initial Conditions	     49

 4.13  Importance Ratings for Temporal Correlations	     50

 5.1   Treatment of Source-Receptor Relationship	     52

 5.2   Treatment of Emission Rate	     61

 5.3   Treatment of Composition of Emissions	     63

 5.4   Treatment of Plume Behavior	     66

 5.5   Treatment of Horizontal Wind Field	     69

 5.6   Treatment of Vertical Wind Field	     70

 5.7   General Treatment of Dispersion	     71

 5.8   Treatment of Atmospheric Stability	     73
                                     VI

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                           LIST OF TABLES (Cont'd)
Number                                                                  Page
 5.9   Treatment of Surface Roughness	    73
 5.10  Possible Bases for Estimating Dispersion Parameter Values	    74
 5.11  Spatial and/or Temporal Dependence of Eddy Diffusivities	    76
 5.12  Treatment of Chemistry and Reaction Mechanism	    79
 5.13  Treatment of Physical Removal Processes	    81
 5.14  Treatment of Background, Boundary and Initial Conditions	    86
 5.15  Treatment of Temporal Correlations	    92
 6.1   Effect of Numbers of Elements on Ratings	    99

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                               LIST OF FIGURES

Number                                                                  Page
 2.1   Procedure for the Comparison of Air Quality
       Simulation Models	     5

 2.2   Evaluation Form - Part A	     8

 2.3   Application Elements as Major Factors Affecting
       Pollutant Concentrations	     9

 2 .4   Evaluation Form - Part B	    10

 2 .5   Evaluation Form - Part C	    12

 2 .6   Evaluat ion Form - Part D	    14

 3.1    Scheme for Classifying Model Applications	    16

 3.2   Sample Completed Application Classification Form	    18

 3.3   Decision Tree for Classifying Pollutant Characteristics	    19

 3.4   Decision Tree for Classifying Averaging Times	    22

 3.5   Decision Tree for Classifying Source Characteristics	    23

 3.6   Decision Tree for Classifying Transport Characteristics	    26
                                   viii

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                              1  INTRODUCTION

        A wide variety of air quality dispersion models are used by regulatory
agencies, consulting firms and industry to estimate the air quality impacts of
pollutant sources.  It is often desired to compare two or more models to
determine which is the most acceptable for a given application.  It is particu-
larly useful to be able to compare a given model with those which, by virtue
of their extensive use, are familiar to most of the modeling community.
        This workbook describes a technique for the qualitative comparison of
modeling approaches on technical grounds.  The methodology is based upon an
applications approach.  The results of the model comparison depend upon the
application for which the model is to be used as well as upon the model char-
acteristics.  In each application of the technique, the model of interest
(the "study" model) is compared with a "reference" model.  Any models may be
specified as study or reference models, as long as they are compatible with
the application of interest.
        The approach taken in this workbook is restricted to models that
mathematically simulate the physical phenomena which determine atmospheric
pollutant concentrations.  Simulation models may require locally measured
air quality data in order to fix the initial and boundary conditions or to
determine appropriate background pollutant levels.  Models excluded from
that category are any that make use of locally measured air quality data to
optimize or determine adjustable parameters unrelated to the physical pro-
cesses being simulated.  Thus, for example, calibration or averaging time
conversion procedures are not considered as part of a simulation model but
rather as statistical procedures which are applied to the calculations of a
simulation model.  Rollback models are also excluded from the category of
simulation models.  General considerations involved in the comparison of
rollback and statistical or empirical models are discussed briefly in Sec-
tion 7.
        Although the amount and detail of the technical material, particularly
that in Appendix A, may appear rather formidable to the user, it has been
included for reference purposes and as an aid in dealing with unfamiliar
treatments or situations.  The user should refer to the examples in Appendix  c

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                                      2
If examples of the application of the methodology are examined, it should
become clear that use of this workbook is not difficult.
        It is important to understand that the methodology described in this
workbook does not enable the user to evaluate the results obtained using a
particular air quality model.  That would require an evaluation of the
quality and suitability of the input data as well as of the model.  The
methodology enables the user to determine only if the model used is as tech-
nically adequate as another model for the application of interest.
        The workbook contains seven main sections and four appendices.
Section 2 contains an overall description of the methodology and general
instructions for its implementation.  Sections 3-6 contain specific instruc-
tions and guidelines for carrying out various steps in the procedure.  Sec-
tion 3 deals with the classification of the user's application.  Section 4 is
concerned with the general suitability and compatibility of a model with the
given application.  It also considers the importance of various aspects of
atmospheric dispersion in that application.  Selected reference models for
the various applications are also suggested in Section 4.  Section 5 provides
guidelines for identifying the treatments of various physical phenomena by a
given model.  Section 6 contains instructions and guidelines for making the
comparative evaluation.  Finally, Section 7 contains a discussion of some of
the considerations involved in comparing rollback/statistical models.
        The appendices contain general reference material useful in imple-
menting the methodology.  Appendix A contains technical discussions of the
physical phenomena that determine atmospheric pollutant concentrations.  It
also contains a discussion of the importance of each phenomenon in different
types of applications.  Appendix B contains information on the treatments of
these phenomena by selected reference models, including the working equations.
Appendix C provides several examples of the use of the methodology in various
common applications.  Finally, copies of each of the various forms used in
the methodology are provided for the user's convenience in Appendix D.

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                                2  PROCEDURE

2.1  GENERAL
       This section provides a general overview of the methodology,  outlines
the steps involved, and explains the types of information and decisions re-
quired at each step.  The user is required to exercise judgment and  to make
qualitative decisions at each step, since model comparison is not, and cannot
be made routine.  Information and guidelines are provided for each step in
the procedure.  Prior to the initial use of the methodology,  the user should
read and understand the guidelines in this section.  The experienced user
can proceed directly to the comparison with occasional consultation of the
reference materials.
       The procedure is application-specific; that is, the results depend
upon the specific situation to be modeled.  Initially, the user identifies
both the application of interest and an associated "reference model."  This
reference model serves as a standard of comparison against which the user
gages the "study model" being evaluated.  The way in which the study model
treats twelve aspects of atmospheric dispersion called "application elements,"
or simply "elements" is determined.  These application elements represent
physical and chemical phenomena that govern atmospheric pollutant concen-
trations and include such aspects as horizontal and vertical dispersion,
emission rate, and chemical reactions.  The importance of each element to the
application is defined in terms of an "importance rating."  Tables giving the
importance ratings for each element are provided, although the user
may modify them under some circumstances.  The heart of the procedure
involves an element-by-element comparison of the way in which each element
is treated by the two models.  These individual comparisons, together with
the importance ratings for each element in the given application, form the
basis upon which the final comparative evaluation of the two models is made.
       It is especially important that the user understand the physical
phenomena involved, because the comparison of two models with respect to the
way that they treat these phenomena is basic to the procedure.  Sufficient
information is provided in the text to permit these comparisons to be made
and the availability of expert advice in the event of difficulties is
assumed.

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 2.2  OVERVIEW
       Comparison of a study model to a reference model is carried out in
 nine steps.  Table 2.1 lists the section and tables to be used for each step
 in the comparison.  Figure 2.1 illustrates schematically the steps in the
 comparison and their relation to each other.  Greater detail, specific guide-
 lines, and tables are given in Sections 3-6 and Appendices A and B.  Forms
 for organizing and documenting a comparison are provided in Appendix D,
      Table 2.1.   Workbook Section and Form for Each Step in Comparison
                  Step
Number
Action
Workbook
Sections
  Form in
Appendix D
  1    Classify application

       Record study model information
  2    Document study model equations

  3    Check study model compatibility
                                o
  4    Classify study model type
  5    Identify reference model
  6    Review importance ratings
  7    Determine treatments of elements
  8    Compare treatments on element-by-
       element basis
  9    Synthesize individual comparisons
       into overall comparison
                           3       Application Classi-
                                   fication Form
                          2.3      Evaluation Form A
                          2.3      Reverse side of
                                   Evaluation Form A
                          4.2      Evaluation Form A
                          4.3      Evaluation Form A
                          4.4      Evaluation Form A
                          4.5      Evaluation Form B
                           5       Evaluation Form C

                         6.2.1     Evaluation Form C

                         6.2.2     Evaluation Form D
 If the study model has been classified as
 user should proceed directly to Section 7
                    a rollback/statistical model, the
                    wherein such models are discussed.
2.3  STEP-BY-STEP INSTRUCTIONS
       Step 1 - Classify Application; Record Basic Information
       The user first classifies the application of interest.  Specifically,
the user considers each of the following four aspects of the application:
          Pollutant characteristics,
          Averaging time,
       •  Source characteristics, and
       •  Transport characteristics.

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   2. DOCUMENT STUDY
      MODEL EQUATIONS
3. CHECK COMPATIBILITY
     OF STUDY MODEL
    WITH APPLICATION
   4. CLASSIFY STUDY
         MODEL
                               1.  CLASSIFY APPLICATION
                                     5.  IDENTIFY
                                   REFERENCE MODEL
7. DETERMINE TREATMENTS
      OF ELEMENTS
    BY BOTH MODELS
                              8.  COMPARE  TREATMENTS OF
                                 EACH ELEMENT  BASED ON
                               RELATIVE LEVEL  OF DETAIL
                            6.  REVIEW AND MODIFY
                              IMPORTANCE RATINGS
                                 IF NECESSARY
                           9.  COMBINE  TREATMENT COMPARISONS
                                 WITH  IMPORTANCE  RATINGS
                                       RESULT:
                               COMPARATIVE  EVALUATION
Note: Numbers in the boxes refer to  the  steps  in  the comparison procedure
      as given in Table 2.1.
   Figure 2.1.   Procedure  for  the Comparison  of  Air Quality Simulation Models

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The classification is based on an application tree approach as described in
Section 3.  An Application Classification Form (see Figure 3.2 or Appendix D)
is provided for carrying out this classification.  Next, the four-digit "appli-
cation index" is determined.  This index identifies the application throughout
the remainder of the comparison.   The application must be classified with
respect to the physical situation being modeled.   Some basic background infor-
mation about the study model is then recorded.

        Step 2 - Document Study Model Equations
        The element-by-element comparisons are facilitated by listing the  (•
 major equations used by the study model on the back of the Evaluation Form -
 Part A.  The user must examine the study model documentation carefully to
 determine the equations which are actually used.  In some cases, when the
 study model documentation is inadequate or inconsistent, it may be necessary
 to examine the computer code itself to make the determination.  The equations
 used by selected reference models are documented in Appendix B.
        Step 3 - Check Model Compatibility
        In this step the general  compatibility of the study model with the
 application of interest is checked.   Brief guidelines are given in Section 4.2
 for determining whether
        •   Treatments of elements essential to the application of
           interest are incorporated in the study model and
        •   Output from the study  model meets the user's requirements.
 The documentation for the study  model should  contain the information to make
 the compatibility determination.
        Step 4 - Classify Study Model Type
        In this step, the study model is classified according to the general
 modeling  approach adopted.  This model classification is useful in identifying
 the way in which the model treats several application elements.   Specific
 guidelines can be found in Section 4.3.
        If the model is classified as a rollback/statistical model, or if it is
 found to  involve both simulation and statistical estimation procedures, the
 methodology described in this workbook does not  apply.  A general discussion
 of some of the considerations involved in the comparison of such models may be

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found in Section 7.  If the study model incorporates a calibration procedure
which empirically adjust the estimates made by a simulation model, the user
should consider only the simulation model and simply ignore the presence of the
calibration procedure when applying the methodology.
        Step 5 - Identify Reference Model
        Table 4.1,  Section 4.4,  identifies suggested reference models for some
 of the indexed applications.   This step only involves  looking up  the appro-
 priate reference model for the application index determined in Step 1.
        At this point,  the Evaluation Form - Part A should be completely
 filled out.  This form is shown in Figure 2.2.
        Step 6 - Review and Modify Importance Ratings
        This is the first step in which the user must consider the twelve
 application elements.   Each element specifies a particular physical aspect
 of the overall processes by which the emission of pollutants into the at-
 mosphere affects the air quality at some point.  Figure 2.3 depicts the re-
 lationship between these application elements and the estimation of atmos-
 pheric pollutant levels.  In order to compare the study model to the re-
 ference model, the user needs to know
           The importance of each element to the application of
           interest and
           How the treatment used by the study model compares to
           that used by the reference model.
 An estimate of the importance of each element to each application can be
 found from Tables 4.2-4.13 in Section 4.5.  Each element has been rated as
 being of HIGH, MEDIUM, or LOW importance to each application.  The user
 should review these importance ratings.  In light of the specific appli-
 cation, a particular  rating may either be  changed to another of the ratings
 given above or be designated as CRITICAL or IRRELEVANT.  Critical elements
 are those  to which it is desired  to give extraordinary weight when  the  two
 models are compared.  Irrelevant  elements  describe processes that are in-  •
 operative  in  the application of interest*  Generally, with the exception of
 irrelevant elements,  the user should expect to  make very few modifications
 to the given  ratings  and, at most, one  CRITICAL designation.
        The Evaluation Form - Part  B should now  be completed by indicating
 the importance  rating for each application element and any  changes  (see
 Figure 2.4).

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                                       8
                               EVALUATION FORM

                      Part A;   Abstract and References

Study Model;

References:
Abstract:
Classification:

Application Index:                            Reference Model:

Application Description;
Model Applicability:        Applicable
Not Applicable
Note:  The reverse side of this form for documenting the equations
       is not shown.
                   Figure 2.2.   Evaluation Form - Part A

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                                       10


                           EVALUATION FORM

                      Part B;  Importance Ratings


                      Application Index:


       Application                                       Importance Rating

         Element                                       Initial   Modified3



Source-Receptor Relationship

Emission Rate

Composition of Emissions

Plume Behavior

Horizontal Wind Field

Vertical Wind Speed

Horizontal Dispersion

Vertical Dispersion

Chemistry and Reaction Mechanism

Physical Removal Processes

Background, Boundary, Initial Conditions

Temporal Correlations
      the exception of the designation of IRRELEVANT elements, it is expected
 that at most one CRITICAL designation and possibly one other modification
 may be made.
                    Figure  2.4.  Evaluation Form  - Part B

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                                     11

        Step 7 - Determine Treatments of Application Elements
        In this step the treatments of the application elements by both the
study model and the reference model are determined.   The treatments for each
element should be described on the Evaluation Form - Part C.   The user
should consult that part of Section 5 that corresponds to each element.
Section 5 gives guidelines, questions, and tables to aid the  user in de-
termining the study model's treatment of each element.  If additional guid-
ance is needed, Appendix A provides detailed discussions of various common
treatments of the application elements.
        The treatment of each element by selected reference models can be
found in Tables B.2-B.13 in Appendix B.
        It is strongly recommended that the sections of Appendix A appropriate
to each element be read.  The general discussion in each section provides
information on the physical processes which are involved and  hence builds a
foundation for making the comparisons required in Step 8.
        Step 8 - Compare the Two Treatments of Each Element
        Once the treatment of a particular element has been determined, the
next step is to determine whether the study model's treatment of that element
is
        •  BETTER than,
        •  COMPARABLE to, or
        •  WORSE than
that of the reference model.  Detailed guidance on making this comparison is
given in Section 6.2.1.  The required comparison is qualitative.  Occasionally,
the user may have some difficulty in deciding whether, for instance, the
study model's treatment is better than or comparable to the reference model's.
In such cases, it is suggested that the user enter the best estimate and
note the other rating in parentheses for later consideration.  In general,
the user is urged to make a unique comparison whenever possible.
        The Evaluation Form - Part C should be completed at this point (see
Figure 2.5).  Note that the form in Figure 2.5 contains room for describing
the treatments of two elements only.  Additional copies of this form up to a
maximum total of six will be required in order to handle all relevant elements.

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                                     13

        Step 9 - Synthesize the Final Technical Comparison
        The information required for the final technical comparison can be
organized on the Evaluation Form - Part D (see Figure 2.6).   As  discussed in
Section 6.2.2, the user first summarizes the results of the element-by-element
comparisons.  An initial evaluation is made by considering the relative treat-
ments of the most important elements, and this initial evaluation is then
modified or not according to the relative treatments of the elements of
lesser importance.  Elements of low importance are considered only in am-
biguous cases. The final evaluation obtained in this way represents the
final result of applying the methodology and at this point the comparison
is complete.

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                                    15
                       3  APPLICATION CLASSIFICATION

3.1  INTRODUCTION
        The first step in the methodology is to suitably define the application
as discussed in Sections 3.2-3.6.   This workbook cannot treat all possible
factors in detail.  Thus, the user needs to consider the situation of interest
and to exercise judgment at all stages of the evaluation.  It is necessary,
however, to classify the application in as much detail as practicable.
        Four general aspects of air pollution simulation have been identified
as being suitable for this purpose.  In this section, the user classifies the
application with respect to these four aspects.  In the process, a four digit
number is generated.  This number serves to identify the class of applica-
tions to be considered in the comparison.  This number, the application index,
is used throughout the workbook for various purposes.  The user should not
consider in any way the details of the operation of either the reference or
the study model in classifying the application.  The classification should
reflect only the characteristics of the problem at hand without regard to
possible simulation techniques.
        Note that meteorological concepts are not used in the classification
scheme.  Relevant aspects of meteorology comprise several of the application
elements discussed in Section 5.

3.2  INSTRUCTIONS
        The following procedure enables the user to classify the application
of interest and to construct the application index.  Figure 3.1 shows the
classification process schematically.  Decision trees and the accompanying
guidelines are used to classify the application in the following four
categories:
        •  Pollutant characteristics
        •  Averaging time,
        •  Source characteristics, and
        •  Transport characteristics.
The trees require definite decisions at each branch point.  In cases of un-
certainty, the user may follow multiple branches and refer to the application

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                      16
( BEG
\
\


X METHOD OF
1M A PRODUCTION

/
PRIMARY ]/
1 K
/ v
POLLUTANT /
CHARACTERISTICS \
\ L
\ SECONDARY If
V\_
Y
LONG-TERM
AVERAGING S
TIME X SHORT-TERM
NUMBER

LIMITED /
SOURCE / \
REMOVAL
PROCESSES
NONE
CHEMICAL
PHYSICAL
CHEMICAL 8 PHYSICAL
NONE
CHEMICAL
PHYSICAL
CHEMICAL 8 PHYSICAL


GEOMETERY
POINT
AREA
LINE
CHARACTERISTICS \
\ MULTIPLE/COMBINATION
GEOGRAPHIC
FEATURES

COMPLEX /"
/ X.
TRANSPORT /
CHARACTERISTICS^
\ SIMPLE /~
X
APPLICATION
INDEX
TRANSPORT
DISTANCE
SHORT-RANGE
LONG-RANGE
SHORT-RANGE
LONG-RANGE

1
2
3
4
5
6
7
8
1
2
1
2
3
4
1
2
• 3
4
Figure 3.1.  Scheme for Classifying Model Applications

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                                    17
element descriptions in Appendix A.  The information in these detailed des-
criptions may facilitate the identification of the most pertinent classifica-
tion.  Each branch of each decision tree ends in a one-digit number that
classifies the given application within the category being considered.  After
classifying the application in all four categories, the index numbers are
combined into the four-digit composite application index.
       The steps in classifying the application are listed below:
       1.  Classify the application within each of the four categories
           shown In Figure 3.1.
       2.  Determine the one-digit numbers corresponding to the
           specific classification chosen within each of the four
           categories.
       3.  Form the composite application index from these four
           one-digit numbers.
       In classifying the application, the user should consult the guidelines
for each descriptive category contained in Sections 3.3-3.6.
       As an example, consider an application that involves estimating the
short-term concentrations of total suspended particulate (a primary pollutant)
due to a power plant (single point source) when (1) the interest is in the
area close to the plant; (2) particulate removal by fallout, deposition, etc.
can be ignored; and (3) the local geographic features are simple.  The appli-
cation index would be 1213, corresponding to the index numbers of 1, 2, 1 and
3 for the categories "pollutant characteristics," "averaging time," "source
characteristics," and "transport characteristics" respectively.  Figure 3.2
shows a completed sample Application Classification Form for this specific
example.

3.3  CLASSIFICATION OF POLLUTANT CHARACTERISTICS
       The decision tree for classifying pollutant characteristics is shown
in Figure 3.3.  The tree indicates processes by which pollutants are pro-
duced and/or removed from the atmosphere.  The first step is to classify the
pollutant as either primary or secondary.  Primary pollutants are defined as
those emitted directly into the atmosphere and secondary pollutants are de-
fined as those produced in the atmosphere by chemical reactions.  For example,
if the application involves estimating the additional CO concentration caused

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                                    19
                              METHOD  OF
                              PRODUCTION
              POLLUTANT
 REMOVAL        INDEX
PROCESSES      NUMBER
                                PRIMARY
         POLLUTANT
         CHARACTERISTICS
                               SECONDARY
                                               NONE
                                               CHEMICAL
                                               PHYSICAL
                                               CHEMICAL 8
                                               PHYSICAL
                  3
                  4
1
CHEMICAL
f
^ PHYSICAL
i CHEMICAL a
— — 3
,,„ 	 7
— a
    Figure 3.3.   Decision Tree for Classifying  Pollutant Characteristics

by a new highway link,  CO is classified as  a primary  pollutant because  it
comes directly from the vehicular sources using the link.  The most  common
example of a secondary pollutant is ozone,  which is produced  by  photo-
chemically initiated reactions involving reactive hydrocarbons and nitrogen
oxides.
       When the application involves a secondary pollutant,  it eventually
becomes necessary to determine the chemical identity  of the  primary  pre-
cursors of that pollutant.  Knowledge of the nature of the precursors and
their sources is required by the nature of  the  modeling process; namely, to
relate emissions from sources to atmospheric concentrations.  Primary pre-
cursors are defined here as those substances that are emitted directly  into
the atmosphere, undergo chemical reaction,  and  whose  ultimate reaction  pro-
ducts include the pollutant of interest.  Determination of the precursors
often requires expert advice but is not required at  this point in the
classification.  Knowledge of sources of the precursors is necessary when
the source characteristics are classified.

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                                     20
       A particular pollutant can be both emitted directly into the atmosphere
and produced by atmospheric chemical reactions; hence it can be both primary
and secondary.  For example, particulate sulfates are emitted directly from
catalytic converters on automobiles and result as well from atmospheric re-
actions involving S02.  In applications involving the total concentration of
such a pollutant, it may be possible to choose a single classification.  For
example, if primary sources are known to be dominant, the pollutant should
be classified primary.  On the other hand, if secondary generation dominates,
the secondary classification should be used.  In situations where neither
prevails, both branches should be explored.  The model used must be able to
handle both primary and secondary production.  In such cases, the comparison
will involve going through the evaluation process twice:  identifying two
applications, two reference models, two sets of importance ratings and making
two independent evaluations.  These evaluations can then be weighted by the
relative importance of primary and secondary production and combined to
arrive at the desired overall comparison.
       In applications involving a specific source, the classification should
be made based upon whether the pollutant emitted from that source is the
pollutant of interest or a precursor.
       After the method of production has been classified, the processes by
which the pollutant is removed from the atmosphere must be classified.  This
second classification should be made by considering removal processes that
are important over the range of time and distance involved in the applica-
tion.   In some situations, removal of the pollutant may be negligible and
the branch labeled "None" is chosen.  The determining factor is the rela-
tionship between the pollutant's removal rate under the conditions being
studied and its residence time within the study area.  Estimation of the
appropriate removal rates may require expert advice.  If roughly more than
one-quarter to one-third of the pollutant could be removed within the study
area,  removal processes should be accounted for within a simulation model.
       If removal is important,  a decision is required as to the type of re-
moval  processes involved.  Chemical removal processes are those in which the
pollutant reacts chemically in the atmosphere as,  for example, when S02
reacts to form sulfates and ceases to exist as the chemical species of inter-
est.   Physical removal processes produce a change in the amount of pollutant

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                                     21

in the air without causing an immediate change in the chemical species of the
material removed.  They include such processes as gravitational settling,
impaction, precipitation scavenging, and dry deposition.  In cases where both
chemical and physical removal processes are significant, the branch labeled
"Chemical and Physical" should be chosen, even if the user is interested pri-
marily in only one type.  Of course, if one type of removal process is clearly
predominant,  that branch should be chosen.
       Other terminology is frequently employed in discussing pollutant
characteristics.  Reactive pollutants are those that react chemically in
the atmosphere.  They thus belong on one of the four branches labelled
"chemical" or "chemical and physical" removal.  Pollutants that are not
reactive are called stable even if physical processes remove them from
the atmosphere.  Conservative pollutants are those for which no removal
process is significant enough to be considered.
       Common characteristics of the criteria pollutants are given in
Table 3.1.  The  indicated classifications are illustrative only and are not
intended as an exhaustive list of all possibilities.  In other circumstances,
the classification might be different from those shown in the table.

3.4  CLASSIFICATION OF AVERAGING TIMES
       The decision tree for classifying averaging times is shown in Figure
3.4.  The time of interest is that over which the estimated concentrations
are to be averaged.  For a particular pollutant,  averaging times from several
minutes to a year may be of interest.  Applications involving concentrations
averaged for 24 hours or less should be considered as distinct from those in
which seasonal or annual averages are estimated.   The former are classified
as short-term.  Longer averaging times like a month, season, or year are
classified as long-term. Table  3.2 classifies  the  averaging times  specified
in the NAAQS.

3.5  CLASSIFICATION OF SOURCE CHARACTERISTICS
       In order to classify source characteristics,  the number and  the geometry
of the sources involved  in  the application must be  determined.   The decision
tree for  classifying  source characteristics is  shown in Figure 3.5.   The

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                                    22
            Table  3.1   Classification of Criteria Pollutants'
Pollutant
Conditions
Method of
Production
 Removal
Processes
                                                           Pollutant
                                                          Index Number
TSP
S02
Particles smaller
than about 30pm.
Particles larger
than about 30ym.
Residence time,
under 5-8 hrs.
Primary     None               1

Primary     Physical           3
            (gravitational
            settling)

CO
NO 2
Oxidants
Non-methane
Hydrocarbons
Residence time
over 5-8 hrs.
Most conditions.
Most sources emit
mainly NO which
reacts to form N02 •
Primary sources
are generally
negligible.
-
Primary Chemical and
Physical
Primary None
Secondary Chemical
Secondary Chemical
Primary Chemical
4
1
6
6
2
a
 These characteristics are for short-range urban  situations without pre-
 cipitation and could change under other circumstances.
 "Residence time" is approximated by the time taken  to traverse the
 region of interest at a characteristic wind  speed.
                                    LONG-TERM
                                                     AVERAGING
                                                    TIME INDEX
                                                      NUMBER
              AVERAGING
                 TIME
                                    SHORT-TERM
      Figure 3.4.  Decision Tree for Classifying  Averaging Times

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                                     23

          Table 3.2  Classification of NAAQS Averaging Times
Pollutant
TSPa
so2a
CO
Oxidants
Hydrocarbons
N02
Averaging Time
Annual
2 4 -hour
Annual
2 4 -hour
3 -hour
8 -hour
1-hour
1-hour
3 -hour
Annual
Classification
Long-term
Short-term
Long-term
Short-term
Short-term
Short-term
Short-term
Short-term
Short-term
Long-term
Averaging
Time
Index No.
1
2
1
2
2
2
2
2
2
1
o
 These averaging  times are also specified for the Prevention  of Significant

 Deterioration  (PSD)  increments.
                                                            SOURCE
                                                        CHARACTERISTICS
                            NUMBER        GEOMETRY        INDEX NUMBER
                                           POINT
                            LIMITED    f    AREA	 2


                                            LINE	 ,
     SOURCE
     CHARACTERISTICS
                        \
                            MULTIPLE/COMBINATION
     Figure  3.5.  Decision Tree for Classifying Source Characteristics

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                                     24
proper branch of the tree is chosen by considering both the number and the
geometry together.  The user should choose the branch appropriate to the
physical characteristics of the sources as inventoried for use in the model.
       Whenever the dimensions of a stationary source are small compared to
the distance at which concentrations are to be estimated, the source can be
treated as a geometric point.  Generally, sources are treated as points if
they emit a substantial amount of any criteria pollutant, e.g. 100 tons per
year.  Some control agencies treat sources with as little as 1 ton per year
of emissions as points.
       Sources that emit small amounts of pollutant are usually aggregated
and treated as uniform area sources.  Their large number and highly variable
emission rates preclude treating them as individual sources.  In addition,
some sources, such as open quarries or windblown fields, are "true" area
sources due to their geometry.  They are not aggregates of many point or
line sources.
       The line source designation is usually reserved for special problems
involving aircraft or microscale analyses of vehicular impacts.  As is the
case with multiple point sources, closely packed line sources such as streets
in downtown areas are frequently treated as uniform area sources.  If indivi-
dual line source data exists, the more appropriate line classification should
be made; if the data base aggregates the lines to areas, the area classifica-
tion should be made.
       A small number of sources (generally less than 10-20) having the same
geometry should be classified "limited" in number.  Thus, if the impact of a
power plant with five stacks were being estimated, the "limited" branch should
be chosen.  The same choice should also be made if five spatially separated
sources are involved.  However, in the latter case, the user should be aware
that some models for treating a limited number of sources assume that these
sources are coincident in space or equivalently that the separations between

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                                      25
 sources  are small compared  to  the  transport  distances.  The  choice between  the
 "limited" or the "multiple" branch should be based  on the number of  sources to
 be modeled, not  the number  of  actual  sources.  Thus, if several hundred  small
 incinerators have been  inventoried as six area sources, the  sources  would be
 characterized as limited  in number (six  area sources), not multiple.
       If either multiple sources  or  several source geometries are to be
 modeled,  the "Multiple/Combination" branch is chosen.  If a  limited  number  of
 sources  of a single geometry are to be modeled, the appropriate branch along
 the "Limited" path is chosen.
       The source characteristics  of  some common source categories are given
 in Table  3.3.  In particular applications, these general guidelines  may be
 modified.   For example, if  the impact of a small number of industrial process
 sources  is being investigated, the source characteristics should be  classified
 as a limited number of  point sources  and not as part of an aggregated area
 source.
             Table 3.3.  Classification of Common Source Categories
Source Category
Industrial
Process
Fuel Combustion
(Internal and
External)
Transpor tat ion
Electricity
Generation
Incineration
Urban Area
Number
Limited
Multiple
Limited
Multiple
Limited
Multiple
Limited
Multiple
Limited
Multiple
Multiple
Source
Size3
Large
Small
Large
Small



Various
Geometric
Description
Point
Area
Point
Area
Line
Line or Area
Point
Point
Point
Area
Combination
Source Characteristics
Index Number
1
4
1
4
3
4
1
4
1
4
4
a
 Source size is measured relative to the point source size specified in the
 emission inventory.   For  example,  major (large)  sources  are  generally  consid-
 ered  to  be those  that  emit  more  than  100  tons  per  year.

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                                      26
3.6  CLASSIFICATION OF TRANSPORT CHARACTERISTICS
       The decision tree for  classifying transport characteristics is shown in
Figure 3.6.  This tree classifies  the application with respect to geographic
features and the transport  distance  from sources to receptors.  One important
general factor affecting transport has been omitted from the tree:  meteorology.
Meteorology has been omitted  so that the application can be defined by rela-
tively fixed or invariant characteristics of the application rather than by the
variable factors which affect transport and dispersion.
       The application is defined  by specific pollutant characteristics,
specific averaging times, well-defined source characteristics, well-defined
geography, and a specific region.  However, a wide range of meteorological
conditions frequently must  be analyzed.  The relevant meteorological con-
siderations and transport under certain specific adverse meteorological
conditions are discussed in Appendices A.2 and A.4.
                                                          TRANSPORT
                            GEOGRAPHIC     TRANSPORT   CHARACTERISTICS
                             FEATURES       DISTANCE     INDEX NUMBER
                                           SHORT-RANGE
                              COMPLEX
                                           LONG-RANGE
        TRANSPORT
        CHARACTERISTICS
                                           SHORT-RANGE
                              SIMPLE
                                           LONG-RANGE
    Figure  3.6.  Decision Tree for Classifying Transport Characteristics

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                                      27
       Geographic features should be classified as complex if their presence
requires the consideration of point-to-point variations in the speed or
direction of transport winds.  Applications involving areas with rough
terrain, areas containing or close to a large body of water, or areas in
valleys or street canyons should generally be classified as having "complex"
geographic features.   Areas with level or gently rolling terrain should
generally be classified as having "simple" geographic features.  Determining
the appropriate classification may require the advice of an expert air
pollution meteorologist.  The determination may also depend upon other
features of the specific application.  For example, variations in terrain that
are significant when short-range transport is being considered might be in-
significant when long-range transport is of interest.  In coastal areas or
near the Great Lakes, the potential effects of sea or lake breezes may
require the complex classification.  The precise distance inland over which
the complex classification must be maintained cannot be specified without
knowing the details of the application.  If in doubt, the user should consult
the technical material in Appendices A.3 and A.4 or obtain expert advice on
the particular situation.
       A different set of applications arises when the effects of sources are
estimated over long rather than short distances.  As a general rule-of-thumb,
distances between sources and receptors that exceed 60-100 km (40-60 mi)
should be classified  long-range.  The transport distance is also related to
the residence time considered when the pollutant characteristics were
classified.  Care should be taken to see that the two choices are made on a
consistent basis.

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                                       29

                           4  PRELIMINARY  ANALYSIS

 4.1  INTRODUCTION
        In this  section,  guidelines and instructions  are provided  to  enable
 the user to carry out  steps 3-6  as given  in  Table 2.1  and  shown in Figure 2.1.
 These steps involve  a  check to see that the  study model is compatible with
 the application in certain essential  respects,  the classification of the
 study model,  the identification  of the reference  model to  be  used in the
 comparison, and the  review and possible modification of the importance ratings.
 These steps are considered in Sections 4.2-4.5, respectively.   Sufficient
 guidelines  for  carrying  out Step 2, the documentation  of the  study model
 equations,  may  be found  in Section 2.3.

 4.2  CHECK  COMPATIBILITY OF STUDY  MODEL WITH APPLICATION
        It is  desirable,  before proceeding  further, to  determine that the study
 model meets two specific requirements  of  the application of interest and is
 therefore compatible with it.  Generally,  reference  models can  be used as
 standards for comparison even when they are  not strictly compatible  with the
 application of  interest.
        The  first  requirement  is  that the  study model should contain  treatments
 of  all  elements  in which the  user  has  a specific  interest  or which are required
 by  the  nature of  the application.   For  example, if the  user has determined that
 some  specific physical removal process  is  important  for  the application of
 interest, and has as a result chosen the appropriate branch in  the Application
 Tree,  the study model  should  incorporate a treatment, however simplified,  of
 that  process.   If the pollutant  of  interest  is subject  to  chemical removal
 processes,  and/or if it  is  a  secondary pollutant,  the  study model should  incor-
 porate  a  treatment of the  effects  of the relevant  chemical  reactions.   This
 may be  extended to processes not covered in  this workbook.  If  the user is
 interested  in the effects of any process whatever, the  study model must  incor-
 porate  a  treatment of that  process.
        The  second requirement is that the study model should provide  the user
with  the desired results.  No attempt is made to list the various possibi-
 lities here.  It is assumed that the user knows what estimates are desired
and can determine whether or not the study model provides  them.  For  example,

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                                     30

the application might involve calculating the frequency distribution of
twenty-four hour averages at some location.   In this case,  a model which
gives only the maximum twenty-four hour average is not compatible with the
user's application, because the desired information is not  available as an
output from the model.

4.3  CLASSIFICATION OF STUDY MODEL
       The possible treatments of several application elements depend upon the
general type of model being considered.  It is, therefore,  useful to classify
the study model with regard to certain general characteristics.  Guidelines
are provided in this section to enable the user to carry out this classifica-
tion; further discussion may be found in Appendix A.4.2.
       The study model should first be categorized as either a simulation
model or as a rollback/statistical model.  The distinctions between these two
broad categories are based upon
       •  The extent to which the model in question attempts to
          simulate the relevant physical and chemical processes
          which significantly affect atmospheric pollution levels
          and
       •  The degree to which locally measured air quality data are
          required for model usage or for the adjustment of model
          parameters.
A simulation model should attempt to describe mathematically the effects of
all relevant physical phenomena expected to have a significant effect on air
quality in the application of interest.  It should not absolutely require the
existence and use of locally measured air quality data except possibly to fix
initial and boundary conditions.  For example, a model which incorporates a
model calibration procedure involving a statistical adjustment of concentration
estimates should still be classified as a simulation model.
       Rollback/statistical models are also formulated mathematically and may
require many of the same input variables, such as wind speed or mixing height,
as do simulation models.  They do not, however, attempt to describe the physi-
cal processes involved in the transport and dispersion of pollutants from
source to receptor in order to estimate pollutant concentrations.  Instead,
the relationship between concentrations and the model  input variables  is deter-
mined empirically.  This is usually done by assuming  some simple  functional

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                                      31
 relationship  containing  one  or more  adjustable  parameters and  then determining
 the values  of these  parameters that  produce  the best agreement with air
 quality  data.

       If the study  model  is classified as a rollback/statistical model, or
 if  statistical conversion  of averaging times is used by the model, the
 methodology described  in this workbook does  not apply.  General guidelines

 regarding the comparison of  rollback/statistical models may be found  in

 Section  7 but the user should consult with an expert to determine the proper
 way to perform the evaluation in  such cases.

       If the study  model  is a simulation model, it may be categorized
 according to  the following two additional features:

       1.   General modeling  approach adopted

               Numerical, involving the solution by numerical pro-
               cedures of equations based upon the conservation of
               mass (K-theory), or

            •   Semiempirical, involving the assumption of a particular
               functional form for the pollutant distribution; and

       2.   Treatment of  the  time dependence  of  pollutant concentrations

               Steady-state,  involving no time dependence; only a
               single constant set of conditions is used,

            •   Dynamic, involving the estimation of pollutant con-
               centrations  as functions of time  or as functions of
               position along a dynamic trajectory; evolution of the
               system in  time is described in a  causal manner,

            •   Sequential,  in which a sequence of conditions is con-
               sidered; a separate independent calculation is done for
               each, or

            •   Climatological, in which a number of different conditions
               are considered, each weighted by  its frequency of
               occurrence; a  separate calculation is done for each.

The user should be able to categorize the study model according to the general

modeling approach from the definitions given and the study model documentation.
If further discussion is needed,  Appendix A.4.2 may be consulted.   It should

be pointed out that the first categorization relates to the treatment of

dispersion by  the study model and that different classifications may be needed
for the treatments of horizontal  and vertical dispersion.

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                                     32
       The treatment of the time dependence of pollutant concentrations must
be determined in order to carry out the second classification,  and the user
should be able to make this determination from the definitions  provided
together with the study model documentation.  Again,  further discussion is
given in Appendix A.4.2.
       Two points need to be made here regarding the  classification of time
dependence.  The first relates to the difference between a dynamic model and
a sequential model.  Dynamic models often divide the  total simulated time
interval into a series of time steps and treat the pollutant distribution at
the end of one step as the initial distribution for the next, thereby handling
the time dependence in a causal manner.  Sequential models also consider a
series of time periods, but ignore the causal relation between pollutant dis-
tributions at each time step and do independent calculations for each.
       The second point is that for sequential and climatological models,
the individual calculations made for each of the conditions considered may
be carried out using either dynamic or steady-state methods; a steady-state
model is almost always used.  This classification should also be indicated.
Thus combinations like "climatological (steady-state)" or "sequential
(steady-state)" arise.
       Table B.I in Appendix B gives the classification for selected reference
models suggested for use in this workbook.

4.4  IDENTIFICATION OF REFERENCE MODEL
       Table 4.1 suggests reference models  that may be associated with some of
the indexed applications.  Each suggested  reference model is briefly described
in Appendix B.  In the table, many applications do not have an associated
reference model.  In these cases, the user  is encouraged to compare his model
with some other applicable simulation model.  If no such model is available,
there is no reference model and no comparison can be made.  Since not all
aspects of an application have been classified by the foregoing procedure,
footnotes have been provided where additional information is required in deter-
mining the appropriate reference model.

-------An error occurred while trying to OCR this image.

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                                     34
                                 Table 4.1.  (Contd.)
Index Reference
Number Model
5211 SAI, DIFKIN (f)
5212
5213 SAI, DIFKIN (f)
5214
5221 SAI, DIFKIN (f)
5222
5223 SAI, DIFKIN (f)
5224
5231
5232
5233
5234
5241 SAI, DIFKIN (f)
5242
5243 SAI, DIFKIN (f)
5244
6111
6112
6113
6114
6121
6122
6123
6124
6131
6132
6133
6134
6141
6142
6143
6144
6211 SAI, DIFKIN (f)
6212
6213 STRAM, SAI, DIFKIN (f,g)
6214 STRAM
6221 SAI, DIFKIN (f)
6222
Index Reference
Number Model
6223 SAI, DIFKIN (f)
6224
6231
6232
6233
6234
6241 SAI, DIFKIN (f)
6242
6243 SAI, DIFKIN (f)
6244
7111
7112
7113
7114
7121
7122
7123
7124
7131
7132
7133
7134
7141
7142
7143
7144
7211
7212
7213 STRAM
7214 STRAM
7221
7222
7223
7224
7231
7232
7233

Index
Number
7234
7241
7242
7243
7244
8111
8112
8113
8114
8121
8122
8123
8124
8131
8132
8133
8134
8141
8142
8143
8144
8211
8212
8213
8214
8221
8222
8223
8224
8231
8232
8233
8234
8241
8242
8243
8244

Reference
Model























STRAM
STRAM













Note:  References to users' guides for each suggested reference model can be
       found in Appendix B.
«a
 For applications for which no reference is listed, the user should compare
 his model with another applicable simulation model.

 Valley should be used when the receptor height exceeds the stack height
 (plume impaction case) .
Q
 CRSTER should be used only when receptor is below stack height.

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                                     35

                            Table 4.1  (Contd.)
 HIWAY is used for analysis of single highway link, APRAC for urban highway
 systems.
g
 Choose RAM for a combination of source types, APRAC for multiple line sources
 (highway systems).
 SAI is a regional grid model; DIFKIN is a trajectory model.  Which to choose
 as a reference model depends upon aspects of the user's application not
 classified in the tree.  SAI treats only photochemical smog.  DIFKIN is also
 designed to treat photochemical smog, but provision is made for user-speci-
 fication of an arbitrary chemical mechanism involving arbitrary, user-defined
 chemical species.  Area sources may require other than emission data for pre-
 processor or preprocessing by user in both models.
gSTRAM allows treatments of other than photochemical reactions.  If the interest
 is in photochemistry choose SAI or DIFKIN.
 CRSTER assumes all sources are located at the same point.  For a single source
 with multiple stacks, where this is a reasonable approximation, choose CRSTER
 rather than COM as the reference model.
 CRSTER assumes all sources are located at the same point.  For a single source
 with multiple stacks, where this is a reasonable approximation, choose CRSTER
 rather than RAM as the reference model.
4.5  REVIEW AND MODIFICATION OF IMPORTANCE RATINGS
       As indicated in Section 2.1, the importance rating of an element is a
measure of the importance of that element as a factor in determining atmos-
pheric pollutant concentrations in the application of interest.  The importance
ratings are used as weighting factors in combining the individual element-by-
element comparisons of the study and reference models into a final comparative
evaluation.
       Tables 4.2-4.13 at the end of this section give the importance rating
of each element in each of the indexed applications.  Brief discussions of
these ratings may be found in Appendix A.9.  At this point in the methodology,
the user should review the importance ratings corresponding to the application
index previously constructed and determine the need for modification of these
ratings in the specific situation of interest.

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                                     36

       Three types of situations arise that may justify modifying the impor-
tance ratings given in the tables at the end of this section:
          When the particular circumstances being modeled,  or  the
          particular interests of the user indicate that an element
          is either more or less important than the tabulated  entry,
          When a particular element is of such overwhelming interest
          or importance that its treatment is critical to the  appli-
          cation, and
       •  When a particular element has no bearing on the application
          and hence is irrelevant.
       In the first situation, the decision to change a rating depends very
much on the particular situation involved and the particular interests of the
user.  The user should review these ratings considering those  aspects of the
specific application not considered in the Application Tree.  If necessary,
the user should modify these ratings.  Such changes should  be  made only after
thoughtful reflection and consultation with an expert.  For example,  the user
may be interested in estimating the amount of S02 removed by deposition in a
region where deposition would usually be small and hence rated as of  LOW
importance.  Given this particular interest, the importance rating for
physical removal might be changed from LOW to MEDIUM or HIGH with the con-
currence of an expert.  The choice between MEDIUM and HIGH must be left to the
user's discretion and depends upon how much weight it is felt  that deposition
deserves relative to the other application elements.
       It may also be desirable to give one element exceptional weight in the
technical evaluation.  In this case, the element is designated CRITICAL to the
overall technical evaluation.  The comparison of the way that  element is
treated is weighted even more heavily than that of an element  of HIGH impor-
tance.  The CRITICAL designation should be used sparingly and  then only when
the user has a very strong interest in an effect associated specifically with
that element.  As an example, if it is desired to pick one of  several alter-
native sites for a new source so as to give the "best" resulting air pollution
estimates, source-receptor relationship might be treated as a CRITICAL element.
In this example, the user needs to determine the differences in pollutant
concentrations as the horizontal location of the source changes, perhaps by
only relatively small amounts.  Only models that handle horizontal location in
a detailed manner would be acceptable.  However, horizontal location is only

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                                     37
one aspect of the application element "source-receptor relationship."  In
such situations, the user must identify clearly which aspect is critical in
making the comparison between two models.
       Lastly, some elements may be IRRELEVANT in the application of interest
and should be so designated.  These elements are not considered at all in the
technical comparison.  Examples of irrelevant elements include chemistry and
reaction mechanism for primary conservative pollutants and physical removal
in situations for which dry deposition and precipitation scavenging are unim-
portant.  Although it may appear a rather simple matter to decide whether a
given element is irrelevant, the user must be cautioned against the indis-
criminate designation of irrelevant elements.  Except in clear-cut instances
such as the examples given above, most elements will have at least some LOW
importance to the application and may need to be considered in cases where
ambiguity exists.  Irrelevant elements, however, are never considered in the
comparison under any circumstances.
       It bears repeating that changes in the importance ratings and especially
the designation of an application element as CRITICAL should be undertaken only
with expert advice and a firm conviction that the specific situation to be
simulated clearly dictates such changes.  Otherwise, the uniformity of evalua-
tion that this workbook can provide is nullified.  At most, no more than one
or two changes in the tabulated importance ratings or a single critical desig-
nation, if any, should be made.

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                          38
         TABLES 4.2-4.13 - IMPORTANCE RATINGS
These ratings are based on the relative importance of
each element to each class of applications as defined
by the branches of the Application Tree.

For an interpretation of the Application Index as it
applies to each table, see Fig. 3.1.

Brief discussions of these ratings are contained in
Append ix A.9.

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                                      51

                     5  TREATMENT OF APPLICATION ELEMENTS

 5.1   INTRODUCTION
        This  section  gives  specific guidelines for determining, for each
 application  element, the treatment used by a given model.  In making these
 determinations, reference  to the dispersion equation and the other working
 equations documented on Evaluation Form-Part A is useful.  The user must be
 careful in determining the treatments used by the study model.  Model docu-
 mentation is often inadequate and occasionally inconsistent, and in some
 cases  it may be necessary  to examine  the computer code itself to determine
 what  the real treatment of some element is.  The list of input variables
 should  be examined and checked for consistency with the working equations
 and general  formulation to insure that sufficient information is input and
 that no seemingly irrelevant data are required.  If these general guidelines
 are followed, the effort involved in determining the treatments of the appli-
 cation  elements will be minimized.

 5.2  TREATMENT OF SOURCE-RECEPTOR RELATIONSHIP
        There are six factors to be considered:
          Horizontal location of sources,
        •  Release heights,
        •  Downwind and crosswind distances,
        •  Orientation of area and line sources,
          Horizontal location of receptors, and
        •  Height of receptors.
Guidance to aid the user in determining the treatment of these factors is given
below.*  This guidance and Table 5.1 listing the various treatments assume a
multiple source application.  In the context of source location,  receptor loca-
tion, release height, and receptor height,  the user should not compare treat-
ments solely on their respective levels of  detail.   Consideration should also
be given to whether the level of detail employed is required for  the applica-
tion of interest.   For example,  if a single source-receptor pair  is of interest,
*Refer to Appendix A.1.2 for detailed discussion of the treatment of the
 source-receptor relationship.

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                                     52
            Table 5.1.   Treatment of Source-Receptor Relationship'
Source Type

  Point
   a. Horizontal Source Location

                  Method of Treatment

1.  Sources located at specific, arbitrary points.

2.  Sources aggregated onto many subareas.

3.  Sources aggregated onto only a few subareas.

4.  Sources aggregated on a basis other than location.
    No information on location.

5.  Not treated explicitly; all sources treated alike
    regardless of location.
  Area
1.  Sources located at arbitrary locations; not
    located as blocks on a grid network.

2.  Sources located as blocks on grid network.  User
    can change scale of grid.

3.  Sources located as blocks on a grid network.  User
    cannot change scale of grid.

4.  Sources aggregated on a basis other than location.
    No information on location.

5.  Not treated explicitly.
  Line
                     3,

                     4,

                     5,
    Line located at any desired position; line treated
    as volume source with width and height as well as
    length.

    Lines treated as one-dimensional (i.e., no width
    or height) with arbitrary location.

    Sources aggregated onto many subareas.

    Sources aggregated onto only a few subareas.

    Sources aggregated on a basis other than location.
    No information on location.
                     6.   Not treated explicitly.

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                                     53
                            Table 5.1.   (Contd.)
                              b.  Release Height
Source Type

  Point
  Area
                  Method  of  Treatment

 1.   Accounts  for both elevation of  stack base  and
     physical  stack height.

 2.   Assumes flat terrain; no  elevation  corrections.
     Release at  any physical stack height.

 3.   Several representative  release  heights  can be
     specified for each grid cell or category when
     sources have been aggregated.

 4.   Model  assumes all releases  take place at same
     user-defined height.

 5.   Model  assumes all releases  take place at same
     height which user cannot  change.

 6.   Not  treated  explicitly.


 1.   Accounts  for both average elevation  of  area and
     allows several  arbitrary  release heights.

 2.   Assumes flat terrain; no  elevation corrections.
     Several arbitrary release heights for each area.

 3.   Assumes flat  terrain.   Only  one release height
     for  each  area.

 4.   Only one  release  height may  be  specified for all
     areas.

 5.   Model  assumes all  releases take place at same
     height which user  cannot change.

 6.   Not treated  explicitly.
 Line
1.  Release height and elevation may both be specified,

2.  Flat terrain assumed, arbitrary release height for
    each source.

3.  Several representative release heights can be spec-
    ified for each grid square when sources have been
    aggregated.

4.  Assumes all releases at same height which user can-
    not change.

5.  Not treated explicitly.

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                                     54
                            Table  5.1.   (Contd.)
                       c.  Downwind/Crosswind  Distances
Source Type

  Point
  Area
                 Method of Treatment

1.  Precise downwind and crosswind  distances  calculated
    for each source-receptor pair.

2.  Single representative or average  value used  for
    aggregate of several point sources.

3.  Not treated explicitly.

1.  Calculated for various points within each area
    source.

2.  Single representative or average  value used  for
    each area source.

3.  Not treated explicitly.
  Line
1.  Single value calculated for each segment along line.

2.  Single representative or average value used for en-
    tire line.

3.  Single representative or average value used for
    aggregate of several line sources.

4.  Not treated explicitly.
                            d. Source Orientation
Source Type
  Point

  Area
                 Method of Treatment
1.  Not applicable.

1.  Areas can assume any orientation; sides not restric-
    ted to lie along specific directions.

2.  Sides of areas restricted to be along specific grid
    directions.

3.  Not treated explicitly.
  Line
1.  Line can assume any orientation with respect to
    receptor and may be inclined.

2.  Line assumed to be horizontal; orientation arbitrary,

3.  Applies to only a restricted range of orientations.

4.  Not treated explicitly.

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                                  55


                         Table 5.1.  (Contd.)
                         e. Receptor Location
                          Method of Treatment

1.  Receptor located at specific, arbitrary location.

2.  Receptor located at specific, arbitrary location within some spe-
    cific area, commonly an area source emission grid.

3.  Receptors located at points on a separate user-defined grid.

4.  Receptors located only at points defined within the model.

5.  Receptor locations defined only as being within certain boundaries
    smaller than the entire region of interest, e.g., within a given
    grid cell.

6.  Receptor location not treated explicitly.  Concentration estimate
    independent of receptor location within region of interest.
                          f. Receptor Height

                          Method of Treatment
I.  Receptor located at specific, arbitrary height above ground.

2.  Receptor located at specific, arbitrary height above ground sub-
    ject to upper limit constraint, e.g., effective stack height.

3.  Receptor located at one of several discrete user-defined heights.

4.  Receptor located at one of several model-defined heights.

5.  Receptor heights defined only as being within certain ranges.

6.  Receptors constrained to be all at the same height.

7.  Receptors constrained to be all at gound level.
8.  Not treated explicitly.

 Within each source type, treatments are listed in order of decreasing
 level of detail.   Various combinations of the treatments for each as-
 pect of the source-receptor may be used; the user can arrive at an
 overall comparison by determining how the models compare in their
 treatment of each aspect.

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                                     56

a model need not be able to locate many sources and receptors as long as a
single pair can be treated in its true relationship.
       Horizontal Location
       The treatment of the horizontal location of point sources can be found
by determining:
          Whether each source can be located arbitrarily at its
          true location.
          If not, the level of aggregation imposed by the model
          as distinct from that imposed by the emission inventory.

          The basis for this aggregation.
The key consideration is the level or degree of aggregation required.  Little
or none implies a relatively detailed treatment in contrast to treatments that
require a lot.  Some treatments aggregate on the basis of a parameter such as
source type and hence convey little information on source location.
       For area sources, the considerations are similar:
          Whether each source can be located arbritarily or must be
          blocks in a fixed  grid network.
          Whether the user can change the scale of the grid to suit
          the detail available in the inventory.
        •  Whether the model  imposes aggregation in addition to any
          imposed by the inventory in developing  the area sources.
        •  The basis for the model's aggregation.
The major difference between the point and area sources comes from the  two-
dimensional nature of the area sources.  They must be located as blocks rather
than as points; otherwise the progression from the most to the least detailed
treatments  is the same for both types.
        For  line  sources, the situation is similar  except that the line  may be
treated as  an elongated volume source having length, width, and height.   For
the location of a line the user should determine:
          Whether the  line can have width and/or  height or is truly
          considered a one-dimensional source.
          If the receptor height  is arbitrary  or  if  only one height
           (usually ground  level)  is assumed.

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                                     57
        •  Whether the line can be uniquely located, usually by the
          location of its endpoints.
        •  If not, the degree of aggregation imposed by the model.
          The basis for that aggregation.
       Release Height
       As used here, release height means the height above ground level at
which emissions are physically released into the atmosphere and does not in-
clude any considerations of plume rise or other types of plume behavior, which
are dealt with in Section 5.5.  The user should be aware, however, that some
effective plume rise may frequently be included in the user-specified release
height supplied as input to models that do not treat plume rise explicitly.
This approach is discussed in Section 5.5; the comparison of treatments of
release height should proceed as if the release heights were the heights at
which emissions actually enter the atmosphere.  There are three considerations
in determining the level of detail with which release height is treated for
point, area, and line sources:
          Whether the model treats elevation differences due to
          terrain.
        •  Whether the model allows the physical stack height (or
          height of release above ground level for area and line
          sources) to vary between different sources or source
          categories.
        •  Whether release heights can be specified arbitrarily by
          the user or whether specific values or sets of values are
          imposed by the model.
       Receptor Location and Receptor Height
       Since receptors are generally taken to be points, the user should be
able to describe the treatment of receptor location and height by making the
same considerations as outlined above.  Specifically, the user should deter-
mine:
       •  Whether the receptors can be located arbitrarily or are
          limited to specific points within the given area.
          If the receptor height is arbitrary or if only one height
          (usually ground level) is assumed.

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                                     58

With these considerations and the entries in Table 5.1,  the user should be
able to describe and compare treatments of receptor location and receptor
height.
       Downwind/Crosswind Distances
       Some models contain parameters whose values depend explicitly on down-
wind or crosswind distances between source-receptor pairs.   In such cases,
these treatments must be described and compared.  In other  cases, it should
simply be noted on the technical evaluation form that this  aspect of the
source-receptor relationship is "not applicable."  The key  consideration for
all source types is whether the treatment causes a loss in  the precision with
which the downwind and crosswind distances can be specified.  Aspects for the
user to consider are:
       •  Whether precise downwind/crosswind distances are  calcu-
          lated for each point source-receptor pair or for  various
          points within area sources or along line sources.
          Whether, for area and line sources, single representative
          or average distances are used.
It should be noted that when the model aggregates sources,  the values of
downwind/crosswind distance are representative of the grid  blocks used in
the aggregation and not of the sources themselves.
       Orientation
       Orientation does not apply to point sources.  For area sources, the
user needs to consider:
       •  Whether the sides of the area sources can be arbitrarily
          oriented, or
          Whether they are restricted to lie along specific directions
          specified by a grid.
       For line sources, the situation is slightly more complex, because a line
presents a significantly different appearance depending upon the angle from
which it is viewed.  The considerations are:
          Whether the line can be inclined or must be horizontal.
          Whether the line can be arbitrarily oriented with respect
          to the wind direction; some models can treat only a re-
          stricted range of orientations.

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                                      59
        When these considerations have been made, the user should have
 little trouble in locating the study model's treatment of each aspect of
 the source-receptor relationship in Table 5.1 and in briefly describing that
 treatment on the Evaluation Form-Part C.   In applications involving several
 source types, an overall evaluation must  be reached based upon the comparative
 treatment of each source type individually.  Each treatment must be weighted
 by the expected importance of that source type in the application of interest.
        Table 5.1 gives the treatment of the source-receptor relationship by
 models in general.   Table B.2 gives the treatment of the source-receptor
 relationship by selected  reference  models.

 5.3  TREATMENT OF EMISSION RATE
        The degree of spatial and temporal resolution must both be assessed
 in determining the  treatment of  emission  rate.*   Regardless of the aspect
 or type of source,  the user is basically  interested in determining:
        •  Whether an emission rate  and  emission  pattern unique to
           each source can be specified, or
        •  Whether average emission  rates  and  general patterns  must
           be  used.
        For the spatial aspect of  point  sources,  the user  need  only determine
 whether each  source  can have an  arbitrary emission  rate  or  whether all  sources
 must  have  identical  rates.
        For  area  sources,  there are  several  methods  for  treating  the  emission
 rate,  depending  on how the  concentration  estimates  are made.   The  user must
 determine:
        •  Whether variation  in the  emission rate  is  allowed within
          a single source or  whether each area is assumed to emit
          uniformly.
        •  If the areas must be uniform, whether the  emission rates
          are arbitrary or whether  they must be the  same or selected
          from a specific set of values.
        •  Whether concentration estimates  are obtained by numerical
          integration over the area or by  replacing  the area by a
          small number of effective point  sources.
*Refer to Appendix A.1.3 for detailed discussion of the treatment of emission
 rate.

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                                    60
       •   Whether  contributions  from all  individual area  sources are estimated,
       •   Whether  a  single  estimate of  the  total area  source contribution  is
          made.
       Similarly,  for  line  sources the  considerations  are:
       •   Whether  the  emission rate can vary along  the line or
          whether  the  line  is  assumed to  be a uniform  source.
       •   If the line  must  be  uniform,  whether the  emission rate
          is arbitrary.
       •   Whether  concentration  estimates are obtained by inte-
          gration  or by  replacing the  line by a small  number  of
          effective  point sources.
       For the temporal  aspect,  the considerations  are the same for all types
of sources:
       •   Whether the  emissions  can vary  with time, or
       •   Whether constant emission rates are assumed.
          If the emissions can vary with  time, whether the model
          uses
             - An actual time sequence of emission rates,
             - A sampled set of emission rates, or
             - A set of rates partially arranged in
               sequence or correlated with other variables.
       With these considerations, the user should be able to determine a
model's treatment of emission rate and to describe it on the evaluation Form-
Part C.
       Table 5.2 gives the general treatment  of emission rate.  Table B.3
gives  the treatment of emission rate by the reference models.

5.4  TREATMENT OF COMPOSITION OF EMISSIONS
       The user must deal with  two aspects of  the composition  of emissions.
For all types of  emissions, the chemical composition must be treated except
when dealing with primary, stable pollutants.  In addition, for particulate
matter, the size  distribution may need to  be  considered.*
 *Refer  to  Appendix  A.1.4  for  detailed  discussion  of  the  treatment  of compo-
  sition of emissions.

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                                               61
                       Table 5.2.   Treatment of Emission Rate
Variation
Source Type
                                                 Method of Treatment
Spatial
Temporal
  Point     1.    Allows arbitrary emission rate  for  each  source.
            2,    All sources assumed to  have identical  emission  rates.

  Area      1.    Allows variation over each area source.
                  Total contribution estimated by summing  all  individual contributions.
                  Individual contributions estimated  by  integration.
            2.    Assumes uniform area sources with arbitrary  emission  rates.
                  Total contribution estimated by summing  all  individual contributions.
                  Individual contributions estimated  by  integration.
            3.    Assumes uniform area sources with arbitrary  emission  rates.
                  Total contribution estimated by integration  without estimating  indi-
                  vidual source contributions.
            4.    Assumes uniform area sources with arbitrary  emission  rates.
                  Total contribution estimated by summing  all  individual contributions.
                  Individual contributions estimated  by  replacing each  area  source  by a
                  small number of effective point sources.
            5.    Assumes uniform area sources, all with the same emission rate.
                  Total contribution estimated by replacing entire area source  distri-
                  bution by a small number of effective  point  sources.

  Line      1.    Allows variation along  the line.
                  Integrates to obtain concentration.

            2.    Assumes arbitrary emission rate is  uniform along the  line.
                  Integrates to obtain concentration.
            3.    Allows variation along  the line.
                  Replaces line by a small number of  effective point sources with
                  emission rates dependent upon position along line.

            4.    Assumes uniform lines with arbitrary emission rates.
                  Replaces line by small  number of effective point sources.
            5.    Assumes all lines have  the same uniform  emission rate.
                  Replaces line by small  number of effective point sources.

  All       1.    Treats an actual time sequence  of emission rates averaged  over  a
                  short interval (typically one hour).
            2.a.  Treats a sampled set of the possible emission rates appropriate
                  to a short interval.
              b.  Treats time sequence of short-term  emission  rates derived  by  model
                  from input emission rates appropriate  to longer term.
            3.    Treats a time sequence  of emission  rates averaged over a long
                  interval (e.g., one day).
            4.    Uses a set of emission  rates which  are at most  partially arranged
                  in sequence.
            5.    Treats only constant emission rates.
Within each source type,  the treatments  are  listed  in order of decreasing level of detail.
 In addition to the level  of detail,  the  suitability of  the technique of treating the variations
 must be assessed as discussed in Appendix  A.1.3 whenever  the  treatment allows temporal variations
 to be specified.

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                                     62

       If treatment of chemical composition of emissions is required by the

application, the user should consider:

       •  Whether all relevant compounds are treated individually
          or whether certain compounds are lumped into classes with
          each class treated by a representative or hypothetical
          compound.  The specific compounds or classes treated
          should be listed or described on the Evaluation Form-Part C.

          If lumping is used, the number of classes considered and
          whether these classes are appropriate to the application
          of interest.

       •  Whether the model determines emissions of certain compounds
          as fixed fractions of input emissions regardless of source
          type.

          Whether further assumptions are made regarding the compo-
          sition of user-input emissions.

       •  Whether the model treats only one of many compounds known to
          interact.

In making these determinations, expert advice may be required.

       When fallout, deposition, or precipitation scavenging (rainout or wash-

out) are involved, the size distribution of particulate matter is important.

The treatment used may be found by determining:

       •  Whether a size distribution is used and if so, whether it
          is continuous or discrete.

              - If continuous, whether the functional form can
                be specified or whether parameters are input
                for a fixed distribution.

              - If discrete, whether there are many narrow size
                ranges or a few broad ranges.

          If a size distribution is not used, whether some given
          fraction of the emissions is affected by the size-dependent
          mechanism.

          If so, whether the fraction is arbitrary.

These considerations should enable the user to determine the treatment  of com-

position of emissions.

       Table 5.3 gives the general treatments of the composition of emissions.

Table B.4 gives the treatments used by the selected reference models.

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                                     64


5.5  TREATMENT OF PLUME BEHAVIOR

       By considering the following,  the user should be able to determine the
study model's treatment of plume behavior:*

          Whether a plume rise formula is used by the model.

       •  If so,  whether the formula  has been extensively vali-
          dated for the specific application of interest or for
          similar applications.

       •  Whether the formula is one  of the common ones:  Briggs'
          2/3, Holland, CONCAWE, CONCAWE simplified, or the ASME
          formula.

       •  If the  formula has not been validated and is not one of
          the four common formulae:

              - To what power of the  inverse wind speed
               the plume rise is proportional.

              - Whether there is a buoyancy term and the
               power to which it is  raised.

              - Whether there is a momentum term.

              - Whether the formula accounts for differences
                in atmospheric stability.

              - Whether plume rise is a function of downwind
               distance.

       •  If no plume rise formula used, whether the product of wind
          speed and plume rise is assumed constant.

       •  If so,  whether the constant can be changed from source to
          source.

          Upon what other parameters, like stability, the constant can
          depend.

       •  Whether downwash and/or fumigation are treated.

In comparing treatments of plume rise, the main consideration should be vali-

dation in either  the application of interest or similar applications.  A

properly validated formula should always be considered better than an unvali-
dated formula.
*Refer to Appendix A.2.2 for detailed discussion of the treatment of plume
 behavior.

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                                     65

       As pointed out in Section 5.2, some effective plume rise may be included

in the values of the stack or release height specified by the user as input to
a model.  If this approach is adopted,  the treatment of plume rise must be

determined from the algorithm or formulae by which the effective plume rise is
estimated prior to its inclusion as part of the release height.  Otherwise,

the treatment implemented in the computer program should be determined.

       When the "tilted plume" approximation has been used to treat deposi-

tion from particulate plumes, this treatment should be included under dry

deposition (Table 5.13).

       Table 5.4 gives the general treatments of plume behavior, and Table B.5

gives the treatments used by the selected reference models.


5.6  TREATMENT OF HORIZONTAL AND VERTICAL WIND FIELDS

       The treatment of the horizontal wind field by the study model may be

determined from the following considerations:*

          Whether the horizontal components of the wind velocity
          may depend on horizontal location.

        •  If not, whether both wind speed and direction are treated
          explicitly or whether one or both are not treated explicitly.

          If the horizontal components depend on position, whether
          the dependence is arbitrary or assumed a priori.

          If arbitrary, whether the components are specified at
          discrete points or as continuous functions of position.

        •  Whether both the horizontal components may depend on
          height above ground.

          If so, whether they are specified at discrete heights or
          as continuous functions of height.

        •  If the wind direction is constant with height, whether
          the wind speed can depend upon height.

        •  If so, whether the dependence is arbitrary.

        •  If arbitrary, whether the wind speed is specified at dis-
          crete intervals or given as a continuous function of height.
*Refer to Appendix A.3.2 for a discussion of the possible treatments of the
 horizontal and vertical wind fields.

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                                        66
                Table  5.4.   Treatment  of Plume  Behavior3
                             Method of Treatment
 1.  Plume rise formulation validated for the specific application of interest.

 2.  Plume rise formulation validated for applications similar to the one of
    interest.

 3.  Fully detailed treatment with plume rise and dispersion treated simultan-
    eously.  Limited validation.13

 4.  Two-step treatments.  Plume rise is calculated and then dispersion is cal-
    culated from a virtual source at an effective stack height.

    a.  Analytical formulas, depending upon meteorology and stack parameters,
        used to estimate plume rise:

        •  Plume rise treated by one of the common formulas:  Brigg's 2/3,
           Holland, CONCAWE, CONCAWE simplified, or ASME.

        •  Plume rise treated by unvalidated formula satisfying  the general
           guidelines and comparing favorably with common formulas.0  (See
           Appendix A.2 and Table A.I for these guidelines.)

        •  Plume rise treated by unvalidated formula satisfying  some of the
           general guidelines or comparing unfavorably with common formulas.0
           (See Appendix A.2 and Table A.I for these guidelines.)

    b.  Product of plume rise and wind speed assumed constant.   User can choose
        value of constant for each source and use different constants for a
        small number of meteorological or source parameters, e.g.,  stability.

    c.  Product of plume rise and wind speed assumed constant.   User can choose
        value of constant for each source.   Constant is  independent of  other
        source and meteorological parameters.

    d.  As in 4 (c),  but one value of the constant  used  for all  sources.

    e.  Single value  of plume rise used for all sources.   (Could be included
        in release height.)

5.  Not treated explicitly.

 Treatments  are listed in order  of decreasing  level of detail.

 Not used  in most  models at this time;  used only in special applications.

 Special weight should be given  to formulas treating plume  rise  as  a  function
 of downwind distance.   This  consideration  is more  important for low-level
 than  for  elevated releases.
b
c
 Notes:  1.   In addition  to  comparing the treatments of plume rise, the user
            should  consider whether the models treat downwash or fumigation.
            Treatment  of either or both by a model tends to make that model's
            treatment  of plume behavior better than the treatment by a model
            that  ignores these effects.

        2.   Where the  "tilted plume" approximation has been used for particulate
            plumes,  the  user should include this under the treatment of dry
            deposition.   (See Table 5.12.)

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                                     67


       •  Whether a specified  dependence with height  of  either  or
          both wind speed and  direction is  assumed  a  priori.

       •  If either or both wind speed  and  direction  are constant
          with height, whether different values  are used for
          different source heights.

       •  Whether the horizontal components depend  on time.

          If so,  whether they  are specified continuously or at  a
          sequence of elapsed  times.

       •  If specified at a sequence of times, whether they are
          assumed constant within each  interval  or  interpolated
          between times.

       •  Whether the treatment is climatological.

       These considerations should guide the user to  an identification of the
treatment of the horizontal wind field.  In addition, the following specific

information should be provided on the Evaluation Form-Part C:

          The model classification as determined in Section 4.3.

          A description of the nature of any constraints on the
          allowable values of  the wind  speed and direction, e.g.,
          the number of sectors and wind speed classes used in  a
          climatological model.

          A description of the nature of any parameters, either
          user-specified or built into  the  model, that govern the
          dependence of wind speed and  direction on position,  height
          above ground, or time, e.g.,  power law dependence of  wind
          speed on height with exponents dependent  on atmospheric
          stability.

       •  A description of any dependence of the horizontal wind
          components on position, height, or time that explicitly
          or implicitly is assumed a priori by the  study model.

       •  The source of nature of any non-standard  meteorological
          data required in the model determination of the wind
          field.

       •  Any additional information relating to the treatment of
          the horizontal wind  field not covered  in this discussion.

       The treatment of the vertical component of the wind field may be deter-

mined in a similar manner from the following considerations:

          Whether the vertical wind field is treated  explicitly;
          whether an implicit  treatment is  used; or whether  the
          vertical wind field  is not considered  in the study  model
          formulation.

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                                      68

        •  If treated explicitly,  whether the vertical component
           depends arbitrarily upon horizontal position and/or
           height above ground.
        •  If arbitrarily dependent on position and/or height,
           whether the vertical component is a continuous function
           of these variables or whether it is specified at dis-
           crete points.
        •  Whether there is any assumed a priori dependence of the
           vertical component on position or height  above ground.
           Whether the vertical component depends on time.
           If so, whether it is specified continuously or at a
           sequence of elapsed times.
        •  If specified at a sequence  of times,  whether it  is assumed
           constant within each interval or whether  its value is
           interpolated.
        Although these questions will  enable the user  to identify  the  study
 model's treatment of vertical wind field,  additional  information  should be
 provided.   This additional information is  the same  as required above  for  the
 horizontal wind field.  If the study  model uses an  implicit treatment of  the
 vertical wind field, the treatment should  be described in  sufficient  detail
 to  make clear the assumptions involved.
        Tables 5.5 and 5.6, respectively,  give the treatments of the horizontal
 wind  field and the vertical wind  field by  models in general.   Tables  B.6  and
 B.7 give the treatments  used by selected reference  models.

 5.7  TREATMENT OF HORIZONTAL AND  VERTICAL  DISPERSION
        The treatment of  both horizontal  and vertical  dispersion by the study
 model may  represent different modeling approaches.*  For example, horizontal
 dispersion may be described with  a semiempirical method and vertical  dispersion
 may be  described with a  numerical method.   On the other hand,  the two treat-
 ments may  be closely related.   In either case,  separate descriptions  of the
 treatments of these two  elements  are  required on the  Evaluation Form-Part  C.
 General treatments of dispersion  corresponding  to the different types of models
 identified in the classification  procedure outlined in Section 4.3 are given
 in  Table 5.7.   Certain information is  required  to adequately specify  the  par-
 ticular treatment used by the study model.   Regardless of  the model type,  the
*Refer to Appendix A.4.2 for a detailed discussion of the treatment  of hori-
 zontal and vertical dispersion.

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                                      72
 following information should  be  provided:

        •   The model  classification  as  determined  in  Section  4.3.

        •   The treatment  or  parameterization  of  atmospheric
           stability  used in estimating the values  of eddy
           diffusivities  or  standard deviations  (see  Table 5.8).
           The name  (or names)  of  any parameter  or  turbulence
           classification scheme used to  specify the  stability
           of  the  atmosphere or the  level of  turbulence  should be
           noted.  If a classification  scheme is used, the number
           of  classes used should  be noted.   Limits or other
           constraints to allowable  parameter values  should be
           indicated.

        •   The treatment  or  parameterization  of  surface  roughness
           effects used in estimating the values of eddy diffusi-
           vities  or  standard deviations  (see Table 5.9).  Infor-
           mation  similar to that  required for the  treatment  of
           atmospheric stability should be provided.

        •   The origin or  basis  for choosing the  specific model
           parameter  values  (see Table  5.10).  If the specific
           parameters  are widely used or  have been  published  in
           the scientific literature, it  is sufficient to identify
           them (for  example, the Pasquill-Gifford  dispersion
           coefficients for  the Gaussian  plume model) and provide
           an  appropriate reference;  if they  are not  widely used
           and  not publicly  available,  the basis for  using them
           should  be  noted.

        •   The  averaging  time for meteorological variables.

This information may  be  omitted in  certain cases in  which dispersion is des-
cribed  implicitly, rather than explicitly.    This is  the case in the narrow
plume or  sector averaging approximations for horizontal dispersion or in the
uniform mixing approximation for vertical dispersion, for example.

       Additional information is also required,  depending on the treatment
classification.

       SemiempiricaiL.  The  functional form assumed for the pollutant distri-
bution should be identified if commonly used  (e.g., Gaussian plume, narrow

plume approximation,  sector averaging,  uniform mixing),  or described briefly

if not widely used,  including some  indication of its origin.   Dependences of

model parameters on position,  height, time,  meteorological,  or other variables

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                                 73
           Table 5.8.  Treatment of Atmospheric Stability3
1.  Atmospheric stability characterized by the numerical value of a
    stability parameter (see for example Table A. 3) that may take
    on any value within the range for which the model is designed.

2.  Atmospheric stability divided into discrete classes within which
    no variation is recognized.

         a.  Many classes, including time of day or other para-
             meters.

         b.  Many classes, not including other factors, or
             Few classes, but including additional factors.

         c.  Few classes, no additional factors included.

3.  Atmospheric stability not explicitly treated.

 Treatments given in order of decreasing level of detail.
            Table 5.9.  Treatment of Surface Roughness
                                                      a
1.  Surface roughness characterized by the numerical value of a
    roughness parameter (such as the roughness length)  which can
    take on any value within some appropriate range.

2.  Surface roughness characterized in terms of discrete classes
    or types of surface.

         a.  Many classes.

         b.  Few classes.

3.  Surface roughness not  treated explicitly.
Q
 Treatments given in order  of decreasing level of detail.

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                                     75

should be indicated if not adequately described under the general information
above.
       Numerical.  The dependence of the eddy diffusivity on position, height,
time, meteorological, or other variables should be indicated, in addition to
the basis for the values or for the specific parameterization that is used by
the study model  (see Table 5.11) .  The general name or type of numerical
method used to solve the diffusion equation should be mentioned, although
comparison based on the details of the numerical techniques is beyond the
scope of this methodology.
       Steady-State.  The averaging time, or the period of time over which
a steady-state condition is assumed to hold, should be indicated.
       Sequential or Climatological.  No specific additional information is
required if the  study model has been classified as sequential or climatologi-
cal.
       Dynamic.  The time dependence should be described to a sufficient ex-
tent to indicate its general nature.  For example, a model that has been
classified dynamic because it involves the determination of a trajectory, but
in which the time dependence of the pollutant concentration is not explicit,
should be distinguished from a model in which it is.  In cases involving
explicit time dependence, the method of describing the temporal variation,
either continuously or in a sequence of time steps, should be indicated
along with the size of the time step used.
       Finally, any additional information relevant to the treatment of dis-
persion that may affect the evaluation or clarify the operation of the study
model should be given.  This information may include:
          Different modes of operation under different meteorolo-
          gical circumstances,
       •  Different treatments for different source geometries, and
       •  Any additional or different assumptions made in the for-
          mulation of the study model not covered in the discussion
          in this workbook.
       Table 5.7 gives the general treatments of dispersion.   Tables 5.8 and
5.9, respectively, give the treatments of atmospheric stability and surface
roughness used in estimating the values of the dispersion parameters (the

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                                     77


eddy diffusivities or standard deviations).  Table 5.10 gives possible bases

for comparing different choices of dispersion parameter values.  Table 5.11

gives possible treatments of the spatial and temporal dependence of eddy

diffusivities.  Treatments of horizontal and vertical dispersion by selected

reference models are given in Tables B.8 and B.9.


5.8  TREATMENT OF CHEMISTRY AND REACTION MECHANISM

       The treatment of chemistry and reaction mechanism used by the study

model may be determined from the following considerations (expert advice may

be required in some cases):

       •  Whether all relevant reactions and chemical species
          are handled.

          Whether similar compounds are treated together in one
          or more classes (lumping approximation) or whether only
          some of the relevant compounds are treated.

          Whether the equilibrium approximation is applied to the
          system of reactions.

          Whether all reactions are truly first-order (linear).

          Whether the appearance and disappearance of pollutants
          of interest are approximated by one or more first-order
          processes.

       In addition to these considerations, the user must also determine:

       •  Whether any adjustment is made for the effects of in-
          complete turbulent mixing.

       Finally, the following additional information should also be provided;

          A two-or three-word description of the general chemical
          system being  treated (e.g., photochemical smog system).

       •  The number of distinct chemical reactions considered.

       •  The number and an explicit list of the chemical species,
          real or "lumped," treated in the mechanism.

       •  If the lumping approximation is used, a list of those
          species treated in the mechanism that are representative
          compounds for the classes being used.
*Refer to Appendix A.5.2 for a discussion of the possible treatments of
 chemistry and reaction mechanism.

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                                     78
          If the steady-state approximation is used, a list of those
          species to which it is applied.

        •  A description, including the extent of any user input,
          of any spatial or temporal variation attributed to any
          chemical reaction rate constant.

          A description of any optional modes of operation, such
          as user-specification of an arbitrary reaction mechanism.

       Table 5.12 gives the general treatments of chemistry and reaction mech-
anism.  The treatments used by selected reference models are given in Table B..10.


5.9  TREATMENT OF PHYSICAL REMOVAL PROCESSES

       Two physical removal processes are considered in this workbook:  dry

deposition and precipitation scavenging.  The guidelines in this section

enable the user to identify the treatment of each used by the study model.*

       Dry Deposition

       The treatment of dry deposition may be determined from the following

considerations:

          Whether both the effect on the vertical concentration
          profile and the effect of pollutant removal are treated.

        •  If so, whether the deposition velocity is assumed constant
          and/or independent of position.

        •  Whether only the effect of pollutant removal is treated.
          (Effective source-strength treatment.)

          If so, how the effective source strength is determined.

             - Integration of mass removal rate proportional
               to ground level concentration, or

             - Assumed exponential decay.

          Whether the deposition velocity is a function of meteorolo-
          gical variables.

        •  Whether the tilted plume approximation is used to treat
          gravitational settling.

       Additional information relating to assumed deposition velocity values

or assumed dependences on meteorological or other parameters should be
*Refer to Appendix A.6.2 and A.6.3 for discussions of possible treatments of
 dry deposition and precipitation scavenging.

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                                 79
                                                                  a
        Table 5.12.   Treatment of Chemistry and Reaction Mechanism
1.  Complete mechanism used:

    a.  System of truly first-order reactions - exact treatment
        possible independent of dispersion.

    b.  More general system of reactions -

           Includes all known relevant reactions except those
           definitely known to be insignificant and

        •  Treats all chemical species explicitly.

2.  Simplified mechanism used, incorporating the steady-state
    approximation for highly reactive intermediates but not the
    lumping approximation for similar species.

3.  Simplified mechanism used, involving the lumping of similar
    chemical species into classes and/or treatment  of only some
    of the relevant compounds but not the steady-state approxi-
    mation.

4.  Simplified mechanism used, involving both steady-state and
    either the lumping approximation and/or the treatment of only
    some of the relevant compounds.

5.  Equilibrium approximation used, with or without the lumping
    approximation.

6.  Approximates the disappearance and/or appearance of a pollu-
    tant of interest and/or the appearance of its reaction pro-
    ducts as first-order reactions and uses exact treatment for
    the first-order (exponential) processes.

7.  Not treated explicitly.
o
 Treatments given in order of decreasing level of detail.

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                                      80

 included  in  the description on the Evaluation Form-Part C.
       Precipitation Scavenging
       The treatment of precipitation scavenging may be determined from the
 following considerations:
        •  Whether the study model allows the washout coefficient
          to be an arbitrary function of time.
        •  Whether the washout coefficient is obtained from a
          user-supplied time-dependent rainfall rate.
        •  Whether the washout coefficient is assumed to be con-
          stant over the period during which precipitation occurs.
        •  Whether each occurrence of rainfall is assumed to remove
          the same fraction of pollutant.
        •  Whether the user may specify the frequency of rainfall.
       As before, additional information relating to any model assumptions
 such as washout coefficient values, rainfall frequencies, etc. should be
 provided.  Precipitation scavenging should be assumed to occur only during
 precipitation.  If a study model always applies an exponential decay factor,
 that treatment should not^ be considered as a treatment of precipitation
 scavenging.
       Table 5.13 gives the general treatments of these two removal processes
 and Table B.ll gives the treatments of physical removal processes by selected
 reference models.

 5.10  TREATMENT OF BACKGROUND,  BOUNDARY AND INITIAL CONDITIONS
       There are five aspects of the element to be considered:
       •  Background,
       •  Upper boundary condition at the mixing height,
       •  Lower boundary condition at the earth's surface,
       •  Boundary condition at the vertical sides of the region
          of interest,  and
       •  Initial conditions in the region of interest.
The treatments of each  of these aspects depends upon the classification of the

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                                     82

study model as numerical/dynamic, numerical/steady-state, semiempirical/dyna-
mic, or semiempirical/steady-state.*  The treatment used by a climatological
model depends upon the treatment employed by the model used to do the basic
dispersion calculations.  Sequential steady-state treatments may account for
variations by using different values for each time interval involved and
hence provide more detail than treatments that treat only a single steady-
state.
       Background
       Numerical models treat upwind pollutant levels in terms of the
boundary condition at the upwind vertical side; hence background does not
need to be considered for them.
       For semiempirical/dynamic models the user must determine whether
spatial and temporal variations in background are treated:
          Whether the background is time-dependent or constant.
       •  Whether the background can change arbitrarily from re-
          ceptor to receptor, or whether it must be uniform across
          the region of interest.
       For semiempirical/steady-state models, only the second consideration
need be made, because there can be no time dependence.
       Upper Boundary Condition
       The upper boundary condition refers to the way in which dispersion
is treated at the mixing height.  The user should focus on the variations
in mixing height which can be accommodated by the model and whether the
top of the mixing layer is treated as an absolute or partial barrier to
pollutants.
       For numerical/dynamic models, the user should determine:
          Whether pollutants dispersing upward are allowed to
          be only partially reflected at the mixing height, or
          whether perfect reflection is specified.
       •  Whether pollutants can be entrained into the region of
          interest from above as the mixing height increases.
          Whether the mixing height can vary with location.
*Refer to Appendix A.7.2 for a detailed discussion of treatments of background,
 boundary and initial conditions.

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                                     83

       For a numerical/steady-state model only the first three considerations
apply.
       For semiempirical/dynamic models,  the user must determine:
       •  Whether the mixing height can be a function of time.
       •  Which method of treatment is used:
          - Perfect reflection simulated by multiple image
            sources and evaluation of the resulting infinite sum,  or
          - Perfect reflection with interpolation used between the region
            where the mixing height has no effect and the region of uni-
            form mixing.
       •  Whether a functional form has been chosen that implicity assumes
          the mixing height is large enough so as not to affect pollutant
          concentrations.
       •  Whether an implicit treatment has been used.
Implicit treatments simulate the effect of the mixing height by limiting the
appropriate parameters of the semiempirical dispersion function.  For example,
the vertical dispersion coefficient in a Gaussian plume formulation may be lim-
ited  to simulate the limited upward spread of a plume.
       Only the last three considerations apply to a semiempirical/steady-state
model.

       Lower Boundary Condition
       The lower boundary condition refers to the way in which dispersion is
treated at the earth's surface.
       For a numerical/dynamic model the following considerations should be
made:
          Whether partial reflection treated, or whether perfect reflection
          must be used.
       •  If partial reflection is treated, a numerical model will generally
          do so in terms of a dry deposition velocity.  In this case the user
          must determine:
          - Whether the dry deposition velocity can vary with location within
            the region of interest.
          - Whether the dry deposition velocity can vary with time.
       For numerical/steady-state models, the consideration of time dependence
is irrelevant.

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                                     84

       The considerations (and treatments) are the same for both semiempirical/
dynamic and semiempirical/steady-state models.  The user should consider:
       •  Whether partial reflection is treated, or whether only perfect
          reflection is treated.
          If only perfect reflection is treated:
          - Whether multiple image sources or a single image source is used.
          - Whether the infinite sum giving the concentration estimate is
            evaluated or whether interpolation is used between the region
            where the mixing height has no effect and the region of uni-
            form mixing.
Note also that some semiempirical models use only a single image source to
treat perfect reflection at the earth's surface.  In this case, no multiple
reflections can occur and the upper boundary is implicitly assumed to have
no effect on pollutant concentrations.  If partial reflection is treated by
a semiempirical model, the functional form used is normally a solution to
the diffusion equation appropriate to the partial reflection boundary con-
dition.

       Vertical Sides
       For a numerical/dynamic model, the following considerations should
enable the user to determine the model's treatment of the boundary condi-
tions at the vertical sides.  The considerations are almost the same for
numerical/steady-state models, except that time dependence need not be
considered.  The user should determine:
       •  Whether the flux into the region can vary from point to point.
       •  Whether the flux can differ at different elevations.
       •  Whether the flux can vary with time, or whether it must be con-
          stant .
The user should recall that numerical models treat background pollutant con-
centrations as a boundary condition at the vertical sides.
       Semiempirical models treat this boundary condition at the vertical
sides as background.  Hence, this condition need not be considered for
such models.

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                                     85


       Initial Conditions

       Initial conditions need only be considered for dynamic models.   For  both
numerical/dynamic and semiempirical/dynamic models,  the user should determine:

       •  Whether the initial concentrations can vary from point to point,  or
          whether they must be uniform throughout the region.

In making this determination, both the dependence on horizontal location and

elevation should be considered.

       After taking these considerations into account, the user should be able

to determine the study model's treatment of each relevant aspect of background,

boundary and initial conditions.

       Any additional information which would clarify the study model's treat-

ment of a particular element  should also be included with the description of
the treatment on the Evaluation Form-Part C.  Examples of such information

include:

        •  How background levels are determined at different  locations or times.
          For example, whether  they are user-specified at each time interval or
          are interpolated between one initial value and one  final value.

          How mixing height  is  determined and upon what parameters it depends.

        •  The parameters limited and  the limiting values or  the method used  in
          an  implicit treatment of an upper or lower boundary condition  should
          be noted.

        •  How the  flux through  the vertical sides of the region of interest  is
          determined for different locations and times.

          Any additional assumptions  made  in the study model's  treatment that
          are not  covered by this workbook.

        General  treatments of background, boundary and  initial conditions are

given  in  Table  5.14  and  the  treatments used by selected reference models are

given  in Table  B.12.


5.11   TREATMENT OF TEMPORAL  CORRELATIONS

        In dealing  with temporal correlations, the quantities of primary  interest

are  emissions,  meteorological variables, removal processes,  background,  and the

various boundary conditions.*
 *Detailed discussions of treatments of  temporal  correlations  can be found in
  Appendix A.8.2.

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                                        86
                Table 5.14.  Treatment of Background, Boundary
                             and Initial Conditions3
                               a.  Background
Type of
 Model

Numerical/
dynamic

Numerical/
steady-state

Semiempirical/
dynamic
Semiempirical/
steady-state0
Climatological
                   Method of Treatment

 (Treated as a boundary condition at vertical boundaries.)

 (Treated as a boundary condition  at vertical boundaries.)


1.  Time-dependent level added to calculated concentra-
    tions; value can change at each receptor.

2.  As in No. 1, but value is uniform throughout region of
    interest at all times.

3.  Single uniform, constant level added to calculated
    concentrations.
4.  No treated explicitly.

1.  Background level can vary with location within region.
2.  Single uniform level for entire region added to calcu-
    lated concentrations.
3.  Not treated explicitly.

Level of detail depends upon treatment employed by model
used to do basic dispersion calculations.
               b.  Upper Boundary Condition (at Mixing Height)
Type of
 Model

Numerical/
dynamic
                   Method of Treatment

    Treats both partial reflection and entrainment of pol-
    lutants from above mixing layer.^
    Mixing height depends upon location and time.
    Treats either partial reflection or entrainment,  but
    not both.

    Treats only perfect reflection.
    Mixing height depends upon location and time.
    Treats only perfect reflection.
    Mixing height is a constant and uniform over region of
    interest.

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                                      87
                             Table 5.14 (Contd.)
               b.  Upper Boundary Condition (at Mixing Height)
Type of
 Model

Numerical/
steady-state
Semiempirical/
dynamic
Semiempirical/
s teady—statec
Climatological
                   Method of Treatment

1.  Treats both partial reflection and entrainment of pol-
    lutants from above mixing layer."
    Mixing height depends upon location only.

2.  Treats either partial reflection or entrainment.

3.  Treats only perfect reflection.
    Mixing height depends upon location only.

4.  Treats only perfect reflection.
    Mixing height uniform over region of interest.

1.  Uses perfect reflection boundary condition.
    Mixing height is a function of time.
    Uses method of multiple images and evaluates infinite sum.

2.  Same as No. 1, but with constant mixing height.

3.  Uses perfect reflection with interpolation between
    region where mixing height has no effect and region of
    uniform mixing.

4.  Functional form implicitly assumes mixing height large
    enough to have no effect.

5.  Implicit treatment limits appropriate parameters, such
    as vertical dispersion coefficient.

1.  Uses perfect reflection with constant mixing height.
    Uses method of multiple images and evaluates infinite sum.

2.  Uses perfect reflection with interpolation between
    region where mixing height has no effect and region of
    uniform mixing.
3.  Functional form implicitly assumes mixing height large
    enough to have no effect.

4.  Implicit treatment limits appropriate parameters, such
    as vertical dispersion coefficient.

Level of detail depends upon treatment employed by model
used to do basic dispersion calculations.

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                                       88
                              Table 5.14  (Contd.)
               c.  Lower Boundary Condition (at Earth's Surface)
Type of
Model
Numerical/
dynamic
Method of Treatment3

1. Allows for partial reflection in terms of a dry depo-
sition velocity, which can depend on location and time
2. Treats partial reflection usine constant drv denosi-
Numerical/
steady-state
Semiempirical/
dynamic
Semiempirical/
steady-state
     tion velocity.

 3.   Treats perfect reflection only.

 1.   Allows for partial reflection in terms of a dry depo-
     sition velocity, which can depend on location.

 2.   Treats partial reflection using constant dry deposi-
     tion velocity.

 3.   Treats perfect reflection only.

 1.   Assumed form of vertical concentration profile based
     on solution to diffusion equation which includes
     boundary condition describing partial reflection.

 2.   Treats only perfect reflection by method of multiple
     images; evaluates infinite sum.

 3.   Treats only perfect reflection; uses single image
     source treatment and interpolates between region where
     mixing height has no effect and region of uniform mix-
     ing.

 4.   Treats only perfect reflection; uses single image
     source treatment with an implicit treatment of upper
     boundary condition (hence no multiple reflections).

 1.   Assumed form of vertical concentration profile based
     on solution to diffusion equation which includes
     boundary condition describing partial reflection.

 2.   Treats only perfect reflection by method of multipe
     images; evaluates infinite sum.

 3.   Treats only perfect reflection; uses single image
     source treatment and interpolates between region where
    mixing height has no effect and region of uniform mixing,

4.   Treats only perfect reflection; uses single image
     source treatment with an implicit treatment of upper
    boundary condition (hence no multiple reflections).
Climatological
Level of detail depends upon treatment employed by model
used to do basic dispersion calculations.

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                                       89
                             Table 5.14 (Contd.)
                             d.  Vertical Sides
Type of
 Model

Numerical/
dynamic
Numerical/
steady-state
Semiempirical/
dynamic

Semiempirical/
steady-state

Climatological
                   Method of Treatment

1.  Treats flux into region as function of location
    (including elevation) and time.

2.  a.  Treats flux into region as function of horizontal
        location (no elevation dependence) and time, or

    b.  Treats flux as a function of either location
        (including elevation) or time, but not both.

3.  Treats flux as a function of horizontal location only
    (no elevation or time dependence).

4.  Treats flux as a constant.

5.  Not treated explicitly; assumes horizontal uniformity.

1.  Treats flux into region as function of location
    (including elevation).

2.  Treats flux as a function of horizontal location only.

3.  Treats flux as a constant.

4.  Not treated explicitly; assumes horizontal uniformity.

(Treated as background.)


(Treated as background.)


Level of detail depends upon treatment employed by model
used to do basic dispersion calculations.

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                                      90
                             Table 5.14 (Contd.)
                               Initial Conditions
Type of
 Model

Numerical/
dynamic
Numerical/
steady-state

Semiempirical/
dynamic


Semiempirical/
steady-state

Climatological
                   Method of Treatment

1.  Treated as a function of position (including elevation),

2.  Specified as uniform, independent of position.

Not applicable.


1.  Treated as a function of position (including elevation),

2.  Treated as uniform, independent of position.

Not applicable.
Level of detail depends upon treatment employed by model
used to do basic dispersion calculations.
 Treatments are listed in order of decreasing level of detail within each
 type of model.

""Fumigation may also be treated by numerical models through an appropriate
 choice of the upper boundary condition during the time of the fumigation.

"Sequential steady-state treatments can account for variations in parameters
 like background and mixing height by assigning a different value to each of
 the time intervals involved.  Models providing this capability give more
 detailed treatments of this element than those which do not.

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                                    91
       If the study model is dynamic or sequential, the temporal correlations

between these various quantities are usually accounted for automatically; in

other cases, the degree or correlation between the various time-dependent

quantities must be assessed by the user.  For both sequential and non-sequential
treatments the user must determine:
                                                                                »
       •  The degree of temporal resolution available to describe the temporal
          variations.  The degree may be different for different parameters.

       •  Whether these quantities are correlated to a high degree.   In other
          words, whether a concentration estimate for a particular time is
       ,   made on the basis of values of the varying parameters that are appro-
          priate to that particular time.

These considerations should enable the user to determine the study model's
treatment of temporal correlations.

       In addition, when comparing treatments by two different models, the user
should consider:

       •  Whether the magnitude of the variations in the specific application
          of interest are sufficiently large to require detailed correlation.

       •  Whether the quantities correlated by the model are important to the
          specific application.

For example, if emission rates are truly constant, it is unnecessary to cor-

relate them with changes in meteorology.  Or, it may be more important to

correlate two highly critical quantities than to correlate four quantities
of lesser importance to the application of interest.

       When describing the treatment on Evaluation Form-Part C, the  user should
list:

       •   The quantities correlated,

       •   The degree of temporal resolution;  for example,  day-night  differences
          or hourly differences in emission rates,

       •   The method used  to accomplish the correlations;  for example, via a
          stability wind rose or by hourly values input by the user,

       •   Any scheme used  to correlate variables; for example,  adjusting input
          mixing height  by stability class,

       •   Any correlations particularly important in the application  of inter-
          est,  and

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                                    92


         Any correlations  that  are unimportant  because  certain variables  are
         constant in the particular application of  interest.

      General treatments of temporal correlations are given  in Table  5.15 and

treatments by selected reference models are given in Table B.13.

                                                             a
               Table 5.15.   Treatment of  Temporal Correlations


                            Method of Treatment
                                                   Q
1.  Sequential treatments  (correlations  automatic).

    a.  High degree of temporal resolution (usually one hour) of all  quanti-
        ties.
    b.  High degree of temporal resolution of time-dependent quantities most
        important to the application.
    c.  Either low degree of temporal resolution or failure  to  correlate  some
        important quantities.

2.  Non-sequential treatments with limited correlation.

    a.  High degree of temporal resolution and correlation of time-dependent
        quantities important to application.
    b.  Either low degree of temporal resolution or failure  to  correlate  some
        important quantities.

3.  Correlations not treated explicitly.
£1
 The quantities of interest here are those used to  determine emissions, mete-
 orology, removal processes, background,  and boundary conditions.
 Treatments are listed in order of decreasing level of  detail.
s*
 Found in dynamic and sequential models.

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                                     93

                          6  COMPARATIVE EVALUATION

6.1  INTRODUCTION
       This section provides detailed guidance on comparing two simulation
models.  The comparison is made after the importance rating of each element
has been reviewed and the treatment of each element has been determined.  The
comparative evaluation is technical in nature and consists of two steps
described in Sections 6.2.1 and 6.2.2, respectively.  In the first step, the
models are compared on an element-by-element basis; in the second, these indi-
vidual comparisons and the importance ratings are combined into the technical
evaluation.

6.2  TECHNICAL COMPARISON
       Before initially attempting to compare the treatments of an element by
two models, it is strongly recommended that the user be familiar with the
technical material presented in the appropriate section of Appendix A.  Exper-
ienced users may have a lesser need for such reference material.

6.2.1  Comparing Treatments of Individual Elements
       The study model's treatment of a particular element should be rated as
BETTER than, COMPARABLE to, or WORSE that the reference model's treatment.
This rating depends on the relative level of detail with which the two models
treat the element and the need for a detailed treatment.  If the study model's
treatment is significantly more detailed than that used by the reference model
and considerable detail is needed in the application,  the study model's treat-
ment is rated BETTER.  If both treatments are essentially the same,  the study
model is rated COMPARABLE.  Finally, if the study model incorporates sub-
stantially less of the detail needed in the application, it is rated WORSE.
Guidance on determining the relative level of detail is available from three
sources:
       •   The general tables in Section 5,
          The discussions in Appendix A,  and
       •   The degree of flexibility the user has in defining input to
          the model.

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                                     94

       The tables in Section 5 list various treatments of each element in
order of decreasing level of detail.  These tables are the basic tools to
be used in determining the relative level of detail used in a given treat-
ment.  Some elements have been further subdivided into several aspects,
each of which must be considered in making the comparison for those ele-
ments.
       When there is only a single aspect, the user should locate the treatment
used by the study model and the treatment used by the reference model on the
appropriate table in Section 5.  Tables B.2-B.13 in Appendix B give the treatments
used by selected reference models.    Since not all possible treatments are listed
in the tables, the user may need to infer at what level a given treatment would
be located if it were explicitly included in the table.   On the basis of the
relative level at which the two treatments occur, the user should decide upon a
comparative rating of the study model with respect to the reference model.
Judgment must be exercised at this stage; a treatment occurring at a slightly
higher level in the table than another is not necessarily a BETTER treatment.
COMPARABLE should be interpreted as meaning "near" or "approximately the same"
and should not be interpreted as meaning exact equality.  A significant or
substantial difference makes the comparative rating BETTER or WORSE.
       When the relative level of detail of two treatments is determined, the
relevance of that detail to the particular situation should also be considered.
The tables of treatments in Section 6 are general  in nature and may indicate a
difference in the level of detail that is irrelevant in the application of
interest.  For example, assume that the application involves level terrain,
that the study model accounts for physical stack height, and that  the  reference
model accounts for both the topographic elevation  of the source as well as the
physical stack height.  In this case, the study model would not be rated as
WORSE than the reference model, since  elevation corrections are not required in
the specific situation being modeled.
       The situation becomes more complicated for  those elements for which
several aspects are discussed.  The comparison  is  made for each aspect sepa-
rately and the results combined to give a comparative rating  for the  element
as a whole.  The user should be guided by two things:
        •  The likely importance of the various  aspects in the  particular
          situation to be  simulated, and
        •  The expected sensitivity of model estimates to changes in each
          aspect separately.

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                                     95

       The actual situation can be examined to determine which aspects might
require more detailed treatment.  These would then weigh more heavily in the
evaluation of the element as a whole.  Treatments of aspects to which the
concentration estimates are most sensitive would weigh more heavily than
treatments of aspects to which the estimates are least sensitive.   A quali-
tative judgement on these matters is being sought; quantitative information
is not required.  It may become necessary to consult an expert if  questions
regarding sensitivity arise.
       In difficult situations, the discussions in Appendix A may  help resolve
the problem.
       Specific guidance for all possible situations is clearly not feasible,
but the general principles discussed above should enable the user  to compare
the treatments in most situations.  If this guidance does not allow the user
to reach a decision, an expert should be consulted.
       One final point needs to be made regarding the comparative  rating of
treatments.  For elements like dispersion for which the model itself must be
classified, the user must consider the appropriateness of the type of model
to the application at hand.  Many factors could go into determing  whether a
particular type of model is appropriate and no specific guidance can be given.
The user must make a decision based on specific needs of the problem.  For
example, the tables for dispersion in Section 5 list numerical/dynamic treat-
ments above the semiempirical/steady-state treatments.  However, as discussed
in Appendix A, if certain conditions are satisfied, a semiempirical/steady-
state treatment may be just as good as a numerical/dynamic treatment.  In
fact, if a high degree of spatial resolution is required, the semiempirical
approach may be more applicable.  Thus, the user would be warned that a highly
detailed approach may be inapplicable given the user's specific needs.  This
example illustrates the need for the user to be familiar with the  material
in Appendix A.

6.2.2  Overall Technical Comparison
       This section describes the procedure for synthesizing the comparisons of
the individual elements into the TECHNICAL EVALUATION.

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                                     96

       When there have been no  designations  of  CRITICAL  elements,  the  user
should proceed as follows:
       1.  Examine the distribution of the treatments of the HIGH-rated ele-
           ments among the  categories BETTER, COMPARABLE, and WORSE.   Based
           on the type of distribution and the guidance  below, formulate a
           tentative evaluation.
       2.  Based on the distribution and number of the MEDIUM-rated elements,
           determine the need to  modify the tentative rating assigned  in step
           1.  Modify as required.
       3.  The modified rating obtained in Step 2 will normally be the TECH-
           NICAL EVALUATION.  In ambiguous situations, the LOW-rated elements
           may need to be considered.
       If the user is unable to complete the TECHNICAL EVALUATION given the
following guidelines, an expert should be consulted.
       It is also strongly  suggested that the user document the reasons for
the decisions made, particularly in ambiguous cases.

       6.2.2.1  Comparison Based  on HIGH-Rated Elements  (Step  1)
       In step 1, only the  high-rated elements are considered.  The user should
take the following points into consideration:
          The relative numbers of treatments in each category (the distribution),
          The specific application elements in each category,
       •  The relative importance of these elements in the application of
          interest,
          Factors unique to the application of interest, and
          Possible ambiguous ratings in parentheses.
Five types of distributions can arise.  Each is described below along with the
manner in which the other considerations affect the evaluation.  It is not
always necessary for the user to identify the particular distribution as long
as the application of the general ideas is understood.
      Case  1  - All  Treatments of High-Rated Elements  in  Same  Category  (BETTER,
              COMPARABLE, WORSE)
       In this case, the comparison is obvious.  For example, if there are three
highly important elements and the treatment of each by the study model is rated
better, BETTER is the unambiguous entry in the "Comparative Rating" column.

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                                     97

        Case  -  2 Equal Numbers of Treatments  in Each Category
        With  equal numbers of treatments in each category, the user cannot simply
 assume  that  one better treatment balances out one worse treatment.  Rather, the
 user  should  identify those elements whose treatments are being considered in the
 three categories.  The importance of these elements to the specific situation to
 be modeled should then be reassessed and any factors unique to that situation
 should  be considered.  This process should not be interpreted as a reassignment
 of the  importance ratings but rather as a means to "fine tune" the HIGH ratings
 in order to  resolve an otherwise ambiguous situation.  Although all these ele-
 ments are of generally high importance in the application, the HIGH category
 itself  covers  a rather broad range and some of these elements may be more
 important than others.  If such a determination can be made, a basis may exist
 for making a comparison other then COMPARABLE.
        Other information may be gained by looking at any ambiguous element-by-
 element comparisons.  For example, if one of the COMPARABLE ratings is ambigu-
 ous and is indicated as perhaps belonging in the BETTER category, the judgement
 would lean toward BETTER as the Comparative Rating.  This judgment is made only
 after the relative importances of the elements rated as better and worse have
 been  considered.

        Case 3  - A Large Number of Treatments in One Category and a Smaller
                Number in the Other Two
       The considerations here are similar to those described above.  The compar-
 ative rating is that category with the largest number of elements.  The confi-
 dence in this  rating increases as the relative number of treatments in this
 category increases.   For example, a rating of COMPARABLE based on three compar-
 able treatments and two worse ones is much more tenuous than a COMPARABLE rat-
 ing based on four comparable treatments and one worse one.  Some additional
 insight may also be gained by examining any ambiguous ratings.  Since these
 ratings are second guesses,  their major use should be in cases where they tend
 to remove ambiguity from the situation, that is,  if they support the initial
 comparison.   In any case,  no final assignment of  the Comparative Rating should
be made until the elements whose treatments are listed in each column are identi-
 fied and their relative importance reevaluated.   It may be possible, for instance,
 for a single WORSE treatment of a highly rated element to justify a Comparative
Rating of WORSE even if the  treatments of  several other high-rated elements are
COMPARABLE.

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                                     98

       Case 4 - Large Numbers of Treatments in COMPARABLE and
                One Other Category and a Small Number in the Third
       The same considerations apply here as enumerated in Case 3.  Here,
however, any change in rating from the category with the largest number of
treatments would generally be to that associated with the next larger number
of elements.

       Case 5 - A Large Number of Treatments in the BETTER and
                WORSE Categories and a Small Number in COMPARABLE
       This type of distribution should be considered anomalous and the user
should review the individual comparisons and importance ratings before pro-
ceeding.  If the anomaly cannot be resolved, an expert should be consulted for
aid in the comparison.
       6.2.2.2.  Comparison Based on MEDIUM- and LOW-RATED ELEMENTS
                 (S-teps 2 and 3)
       In step 2, the MEDIUM-rated elements are considered and the user must
decide whether or not to change the Comparative Evaluation reached at the end
of step 1.  Keeping in mind that the MEDIUM-rated elements are by definition
less important individually than the HIGH-rated elements, the user should
consider the following:
          The same considerations as involved in step 1, and
       •  The relative numbers of HIGH- and MEDIUM-rated elements.
       The consideration of the distribution of the MEDIUM-rated elements and
factors unique to the application follow lines parallel to those discussed in
Section 6.2.2.1.  If the distribution of high and medium elements are similar,
no change is made in the rating.  If the distributions are different, the
desirability of a change becomes stronger as the relative number of medium
elements increases.  When the numbers of high and medium elements are considered,
the user must think about which elements are involved and their relative impor-
tance in the specific situation of interest.  For example, consider the two
cases outlined in Table 6.1.  In case A, the user would be much more likely to
change the initial Comparative Rating from BETTER to COMPARABLE than in case B,
because of the relatively larger number of MEDIUM-rated elements in case A.

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                      Table 6.1.
   99
Effect of Numbers of
Elements on Ratings
Case
A
B
Importance
Rating
HIGH
MEDIUM
HIGH
MEDIUM
Better
1
0
3
0
Comparable
1
6
1
4
Worse
0
0
0
0
Comparative
Rating
BETTER
COMPARABLE
BETTER
BETTER

       In most cases, step 3 simply involves writing down the Comparative
Rating reached at the end of step 2 as the TECHNICAL EVALUATION.  The LOW-
rated elements are usually not considered.  In cases in which the user
considers that a contemplated change based on the MEDIUM-rated elements is
ambiguous, it may be necessary to look at the LOW-rated elements in an
attempt to resolve the ambiguity.  The rules for the comparison are essentially
the same as in step 2, except that in this case the user is examining the dis-
tribution, number of elements, and importance to the application to see if the
indicated direction of change is the same as that being contemplated.  If it is,
the ambiguity may be removed.  If not, the change should probably not be made.
If problems still remain, an expert should be consulted.
       At this point, the TECHNICAL EVALUATION is complete.
       6.2.2.3  Comparison with a CRITICAL Element
       If a CRITICAL element has been designated, the procedure is almost the
same, except that the initial rating is based upon consideration of that element
alone.  Possible subsequent modifications are then based first upon consideration
of the treatments of the HIGH-rated elements and then upon those of the MEDIUM-
rated elements.  However, if a critical element is treated worse, the user may
consider making this single comparison the basis of the entire TECHNICAL
EVALUATION.  For comparable and better treatments of the critical element, the
entire procedure should be applied.  Since there will generally be only a
single critical element and several HIGH-rated elements, the user must decide
how many high elements would be required to override an initial Comparative
Rating based on the critical element alone.  Again, the user must reach a
qualitative judgment based on the considerations discussed above and must not
base an entire comparison on a single critical element.

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                                     101

                       7  ROLLBACK/STATISTICAL MODELS

7.1  GENERAL
       The emphasis in this workbook is on the comparison of simulation
models.  By definition, these models simulate mathematically the effects of
physical and chemical processes that affect air quality.  Furthermore, the
relevant phenomena are considered and described in terms of fundamental
physical principles of general applicability.  As a result, a simulation model
has the property of being transferable from one location to another as long as
the limitations imposed by assumptions made in the basic formulation of the
model are not violated.  Air quality data taken within the region of interest
is not required except to fix boundary and/or initial conditions.
       Models which do not satisfy these criteria may be encountered by the
user.  This section provides general guidelines for their evaluation and
comparison.  Such models are termed rollback/statistical models throughout
this workbook and are characterized by one or both of the following proper-
ties:
       •  Only a very few of the factors relating to emissions,
          meteorological, transformation, and removal processes
          are explicitly considered in the formulation of the model.
       •  Locally measured air quality data is required in order to
          determine empirically the values of various coefficients
          or parameters in the model.
       Included in this classification is the simple rollback model and
various extensions of it.  The simple rollback model assumes a linear relation
between total pollutant emissions within some region and pollutant concentra-
tions within that region.  It does not explicitly consider the spatial distri-
bution of emissions, the location of the receptors of interest, nor any meteoro-
logical factors.
       Also included in the rollback/statistical category are those models that
require the use of actual atmospheric monitoring data, including both air
quality and meteorological variables, for the empirical adjustment of model
parameters.  The relationship between air quality and selected meteorological
parameters is determined empirically in these models, commonly through the
use of regression or other statistical techniques.

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                                      102
 7.2  ADVANTAGES AND DISADVANTAGES
       The major technical advantage enjoyed by statistical models
 is the close relationship between concentration estimates and values actually
 measured under more or less similar circumstances.  The effects of all those
 factors that determine atmospheric pollutant concentrations are implicitly
 accounted for in the air quality data used to develop and optimize the model.
 The same may be said about rollback, since an air quality measurement is used
 to estimate the ratio of emissions to atmospheric concentrations.  Due to the
 nature of the procedures used in developing a statistical model, information
 is often available regarding the statistical significance of the variables
 that are taken into account and the magnitude of the statistical error made
 by the model; no such information is available for rollback.  Other advan-
 tages of rollback/statistical models, such as their low development cost and
 low resource requirements, are primarily pragmatic in nature.
       The major disadvantage of rollback/statistical models also arises from
 their dependence on air quality data.  A statistical model is not, in general,
 applicable under conditions that are outside the range of conditions included
within the data used in its development and optimization.  The range of condi-
 tions commonly includes variations in meteorological variables, but for practi-
 cal reasons very little variation in the spatial distribution of emissions can
be investigated.  Consequently,  statistical models are not generally suited for
applications that involve the consideration of significant changes in the dis-
tribution of emissions and, as a result, are not transferable without re-
evaluation of their specific empirical parameters or coefficients.  Rollback
considers only one set of conditions, specifically that set of meteorological
conditions and emission rates which existed over that period of time in which
the air quality measurement used in the model was made.  Therefore, it is also
unsuited for the consideration of changes in the emission distribution.
       The applicability of rollback/statistical models is more limited than
that of a simulation model for other reasons as well.  For example, only
rather general concentration estimates may be obtained.  The dependence of
pollutant concentrations on position cannot normally be predicted, nor can
individual source contributions  at any given position of interest.

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                                    103

       If a model that falls into the rollback/statistical category is to be
evaluated, the user should consider the applicability of the particular model
to the application of interest, keeping in mind the generally limited appli-
cability of such models.  Statistical models are often formulated for a very
specific purpose.  If that purpose coincides with the requirements of the
user, and if the range of conditions of interest to the user are included
within those used to develop the model, the model is applicable and may give
better results than a simulation model.  Otherwise, the model may not be
appropriate.

7.3  COMPARISON OF ROLLBACK/STATISTICAL MODELS
       The only basis that may exist for the comparison of a given statistical
model with a given simulation model is the observed performance of each in
previous practical applications.  In general, the philosophies and goals of
these two approaches to estimating atmospheric pollutant levels are sufficiently
different as to preclude any systematic, objective, a priori technical comparison.
If, however, estimates of the errors made by the simulation model in applications
similar to the one of interest are available, they may be compared directly with
the expected error for the statistical model, thereby allowing a comparative
evaluation to be made.  Unfortunately, even this basis does not exist for roll-
back, since the emission-concentration relationship is an assumed one, rather
than one obtained using accepted statistical procedures.
       It may be possible in some cases to make a comparison of a rollback/
statistical model with a simulation model by determining the approximations that
must be made to reduce the working equations to a linear relation with constant
coefficients between the pollutant concentration and the total emission rate.
The degree of validity of the necessary approximations in the situation to be
modeled is a measure of the degree of comparability of the simulation model and
the rollback/statistical model.  If this approach is adopted, the Evaluation
Form provides a convenient format for recording the necessary information.
Furthermore, if the element-by-element comparisons are interpreted as a measure
of the  validity of the relevant approximations in going from the simulation
model treatment  to the  rollback or statistical model treatment, the information
can be summarized easily on Part D of  the  form.  The technical comparison can

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                                     104

in this case be made following the same guidelines as are used for the
comparison of two simulation models as described in the other sections of this
workbook.

        It may not always be possible to make a comparison in this manner.  A
statistical model may still be evaluated on its past performance.  However,
since rollback is:  (1) not really based upon consideration of the application
elements discussed in this workbook; (2) not obtained by statistical analysis
of appropriate air quality data;  and (3) as yet unverified, no basis exists
for objectively evaluating the performance of a rollback model.

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                                  TECHNICAL REPORT DATA
                           (Please read Imtructions on the reverse before completing)
 REPORT NO.
       EPA-450/2-78-028a
                                                          3. RECIPIENT'
OAQPS No.  1.2-097
 TITLE AND SUBTITLE

 Workbook for  the Comparison of Air  Quality Models
                            5. REPORT DATE
                                May 1 978
                                                          6. PERFORMING ORGANIZATION CODE
                                                                     ACCESSION-NO.
 AUTHOR(S)
                                                          8. PERFORM!
                                                            OAQPS No. 1.2-097
. PERFORMING ORGANIZATION NAME AND ADDRESS
 U.S. Environmental  Protection Agency
 Office of Air  and Waste Management
 Office of Air  Quality Planning  and Standards
 Research  Triangle Park, NC  27711
                                                           10. PROGRAM ELEMENT NO.
                            11. CONTRACT/GRANT NO.
2. SPONSORING AGENCY NAME AND ADDRESS
                                                           13. TYPE OF REPORT AND PERIOD COVERED
                                                            EPA/OAQPS Guideline
                                                           14. SPONSORING AGENCY CODE


                                                               200/04
5. SUPPLEMENTARY NOTES
6. ABSTRACT
      The  workbook describes a  technique for the qualitative comparison  of  modeling
 approaches  on technical grounds.   The methodology  is  based upon an applications
 approach.   The results of the  model  comparison depend upon the application for which
 the model  is  to be used as well  as upon the model  characteristics.   In  each
 application of the technique,  the  model of interest  is compared with a  reference
 model.  Any model may be specified as the reference model, as long as it  is compati-
 ble with  the  application of interest.
                                KEY WORDS AND DOCUMENT ANALYSIS
                  DESCRIPTORS
 Atmospheric Models
 Air  Pollution Abatement
 Mathematical Models
 Atmospheric Diffusion
                                              b.IDENTIFIERS/OPEN ENDED TERMS
                Implementation  Air
                  Pollution  Planning
                Gaussian Plume  Models
                Diffusion Modeling
                                                                         c. COSATI Field/Group
13B
13. DISTRIBUTION STATEMENT
  Release  Unlimited
                19. SECURITY CLASS (This Report}

                 None
                                                                         21. NO. OF PAGES
 no
                                              20. SECURITY CLASS (This page)

                                               None
                                                                         22. PRICE
EPA Form 2220-1 (9-73)

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