Revision to the Guideline on Air Quality Models: Adoption of a Preferred Long Range Transport Model and Other Revisions

[Federal Register: April 15, 2003 (Volume 68, Number 72)]
[Rules and Regulations]
[Page 18439-18482]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr15ap03-27]

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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 51
[AH-FRL-7478-3]
RIN 2060-AF01

Revision to the Guideline on Air Quality Models: Adoption of a
Preferred Long Range Transport Model and Other Revisions

AGENCY: Environmental Protection Agency (EPA).
ACTION: Final rule.

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SUMMARY: EPA's Guideline on Air Quality Models (``Guideline'')
addresses the regulatory application of air quality models for
assessing criteria pollutants under the Clean Air Act. In today's
action we promulgate several additions and changes to the Guideline. We
adopt a new dispersion model, CALPUFF, in appendix A of the Guideline.
CALPUFF becomes the preferred technique for assessing long range
transport of pollutants and their impacts on Federal Class I areas.
Action on AERMOD and the Emissions and Dispersion Modeling System
(EDMS) is deferred. We make various editorial changes to update and
reorganize information, and remove obsolete models.

DATES: This rule is effective May 15, 2003. Beginning April 15, 2003
the new model (i.e., CALPUFF) should be used for its intended purposes,
in accordance with today's document. The period before required
implementation of a new model allows user's sufficient time to prepare
meteorological data bases and to become familiar with model operation.
The new model may be used sooner, if desired.

ADDRESSES: All documents relevant to this rule have been placed in
Docket No. A-99-05 at the following address: EPA Docket Center, (EPA/
DC) EPA West (MC 6102T), 1301 Constitution Ave., NW., Washington, DC.
The EPA Docket Center Public Reading Room (B102) is open from 8:30 a.m.
to 4:30 p.m., Monday through Friday, excluding legal holidays. The
telephone number for the Air Docket is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT: Joseph A. Tikvart, Leader, Air Quality
Modeling Group (MD-14), Office of Air Quality Planning and Standards,
U.S. Environmental Protection Agency, Research Triangle Park, NC 27711;
telephone (919) 541-5562 (Tikvart.Joe@epa.gov).

SUPPLEMENTARY INFORMATION:

I. General Information

A. How Can I Get Copies of Related Information?

EPA established an official public docket for this action under
Docket ID No. A-99-05. The official public docket is the collection of
materials that is available for public viewing at the Air Docket in the
EPA Docket Center, (EPA/DC) EPA West (MC 6102T), 1301 Constitution
Ave., NW., Washington, DC. The EPA Docket Center Public Reading Room
(B102) is open from 8:30 a.m. to 4:30 p.m., Monday through Friday,
excluding legal holidays. The telephone number for the Reading Room is
(202) 566-1744, and the telephone number for the Air Docket is (202)
566-1742.
Our Air Quality Modeling Group maintains an Internet Web site
(Support Center for Regulatory Air Models--SCRAM) at: http://www.epa.gov/
scram001. You may find codes and documentation for models
referenced in today's action on the SCRAM Web site. We have also
uploaded various support documents (e.g., evaluation reports).

II. Background

The Guideline is used by EPA, States, and industry to prepare and
review new source permits and State Implementation Plan revisions. The
Guideline is intended to ensure consistent air quality analyses for
activities regulated at 40 CFR 51.112, 51.117, 51.150, 51.160, 51.166,
and 52.21. We originally published the Guideline in April 1978 and it
was incorporated by reference in the regulations for the Prevention of
Significant Deterioration (PSD) of Air Quality in June 1978. We revised
the Guideline in 1986, and updated it with supplement A in 1987,
supplement B in July 1993, and supplement C in August 1995. We
published the Guideline as appendix W to 40 CFR part 51 when we issued
supplement B. We republished the Guideline in August 1996 (61 FR 41838)
to adopt the CFR system for labeling paragraphs. On April 21, 2000 we
published proposed revisions in the Federal Register (65 FR 21506),
which is the basis for today's promulgation.
Today's notice promulgates those components of the proposal that
were clearly supported by public comments and that were otherwise not
controversial, notably:
? Adoption of CALPUFF in appendix A, as proposed, for
assessing long range transport of pollutants and their impacts on
Federal Class I areas;
? Removal of the Climatological Dispersion Model (CDM), RAM
and the Urban Airshed Model (UAM) from appendix A, as proposed;
? Simplification of complex terrain screening techniques in
section 5;
? Revision of section 9 to reflect our October 1997
settlement with the Utility Air Regulatory Group regarding
specification of emissions from background sources, as proposed;
? Updating information in appendix W and reorganizing its
structure; and
? Transfer of appendix B and appendix C to our Web site, as
proposed.
The proposal also included (1) adopting AERMOD \1\ to replace the
Industrial Source Complex (ISC3) model in many assessments that now use
it, (2) revising ISC3 by incorporating a new downwash algorithm (PRIME)
and renaming the model ISC-PRIME, and (3) updating the Emissions
Dispersion Modeling System (EDMS) by incorporating improved emissions
and dispersion modules. Regarding AERMOD, nearly every commenter urged
EPA to integrate aerodynamic downwash into AERMOD (i.e., not to require
two models for some analyses). The only cautions were associated with
the need for documentation, evaluation and review of the downwash
enhancement to AERMOD. As a result of AERMIC's (the American
Meteorological Society (AMS)/ EPA Regulatory Model Improvement
Committee) efforts to revise AERMOD, incorporating the PRIME algorithm
and making a few other incidental modifications and to respond to the
public's cautions, we believe that AERMOD, as modified for downwash,
merits another public examination of performance results. Also, since
the April 2000 proposal, the Federal Aviation Administration decided to
configure EDMS3.1 to incorporate the AERMOD dispersion model, and
results of its performance with AERMOD only recently became available.
Consequently, AERMOD and EDMS4.0, as well as other conforming changes
for the Guideline, will be reconsidered in a Supplemental Notice of
Proposed Rulemaking (SNPR) in the near future. Note that since AERMOD
is not included in today's promulgation, the proposed merger of the
Guideline's sections 4 and 5 will be deferred to AERMOD's adoption in
the future.
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\1\ AMS/EPA Regulatory MODel.
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III. Public Hearing on the Proposal

We held the 7th Conference on Air Quality Modeling (7th conference)
in Washington, DC on June 28-29, 2000. As required by section 320 of
the Clean Air Act, these conferences take place

[[Page 18441]]

approximately every three years to standardize modeling procedures.
This conference served as the forum for receiving public comments on
the Guideline revisions proposed in April 2000. The 7th conference
featured presentations in several key modeling areas that support the
revisions promulgated today. A presentation by the Interagency
Workgroup on Air Quality Modeling (IWAQM \2\) covered long range
transport modeling for point sources. This presentation was followed by
a critical review/discussion of the CALPUFF modeling system and
available performance evaluations, facilitated jointly by the Air &
Waste Management Association's AB-3 Committee and the American
Meteorological Society's Committee of Meteorological Aspects of Air
Pollution.
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\2\ IWAQM was formed in 1991 to provide a focus for development
of technically sound air quality models for regulatory assessments
of long range transport of pollutant source impacts on federal Class
I areas. IWAQM is an interagency collaboration that includes efforts
by EPA, U.S. Forest Service, National Park Service, and Fish and
Wildlife Service.
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We asked the public to address the following questions:
? Has the scientific merit of the models presented been
established?
? Are the models' accuracy sufficiently documented?
? Are the proposed regulatory uses of individual models for
specific applications appropriate and reasonable?
? Do significant implementation issues remain or is
additional guidance needed?
? Are there serious resource constraints imposed by modeling
systems presented?
? What additional analyses or information are needed?
We placed a transcript of the 7th conference proceedings and a copy
of all written comments, which embody answers to the above questions,
in Docket No. AQM-95-01.

IV. Discussion of Public Comments and Issues

All comments submitted to Docket No. A-99-05 are filed in Category
IV-D. We summarized these comments, developed detailed responses, and
drew conclusions on appropriate actions for today's action in the
summary of public comments and EPA responses.\3\ In this document, we
considered and discussed all significant comments. Whenever the
comments revealed any new information or suggested any alternative
solutions, we considered such in our final action.
The remainder of this preamble section provides an overview of the
primary issues encountered by the Agency during the public comment
period and summarizes our response-to-comments.\3\ This overview also
serves to explain the changes to the Guideline in today's action, and
the main technical and policy concerns addressed by the Agency.
Guidance and editorial changes associated with the resolution of these
issues are adopted in the appropriate sections of the Guideline. While
modeling by its nature involves approximation based on scientific
methodology, and entails utilization of advanced technology as it
evolves, we believe these changes respond to recent advances in the
area so that the Guideline continues to reflect the best and most
proven of the publicly available models and analytical techniques, as
well as to reflect reasonable policy choices.
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\3\ Summary of Public Comments and EPA Responses 7th Conference
on Air Quality Modeling, Washington, D.C., June 2000 (Air Docket A-
99-05, Item V-C-1). This document may also be examined from EPA's
SCRAM Web site (http://www.epa.gov/scram001). Note that comments/
responses re: AERMOD & EDMS are deferred to a companion document to
be released when the SNPR is published.
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CALPUFF

CALPUFF is a Lagrangian dispersion model that simulates pollutant
releases as a continuous series of puffs. Preceding our proposal to
adopt CALPUFF in the Guideline, IWAQM carefully studied the potential
regulatory application of CALPUFF in its Phase 1 report \4\ and in its
Phase 2 report.\5\
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\4\ Environmental Protection Agency, 1993. Interagency Workgroup
on Air Quality Modeling (IWAQM) Phase I report: Interim
Recommendation for Modeling Long range Transport and Impacts on
Regional Visibility; EPA Publication No. EPA-454/R-93-015.
\5\ Environmental Protection Agency, 1998. Interagency Workgroup
on Air Quality Modeling (IWAQM) Phase 2 Summary Report and
Recommendations for Modeling Long-Range Transport Impacts. EPA
Publication No. EPA-454/R-98-019.
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In our April 2000 Federal Register notice, we proposed adoption of
the CALPUFF modeling system, developed by Earth Tech, Inc., for refined
use in modeling long range transport and dispersion to characterize
reasonably attributable impacts from one or a few sources for PSD Class
I impacts. We also proposed use of CALPUFF for those applications
involving complex wind regimes, with case-by-case justification. We
sought comments on the use of CALPUFF for these applications, as well
as on related uses of meteorological information, e.g., on use of
prognostic mesoscale meteorological models and the length of record for
meteorological data.
Scientific merits and accuracy. In public comments there was a
general consensus that the technical basis of the CALPUFF modeling
system has merit and provides substantial capabilities to not only
address long range transport, but to address transport and dispersion
effects in some complex wind situations.
Commenters generally agreed that the CALPUFF modeling system has
adequate accuracy for use in the 50-200km range, with some studies
showing that acceptable results can be achieved at least out to 200 to
300km. Since the 7th Modeling Conference, enhancements were made to
CALPUFF that allow puffs to be split both horizontally (to address wind
direction shear) and vertically (to address spatial variation in
meteorological conditions). These enhancements likely will extend the
system's ability to treat transport and dispersion beyond 300km.
With respect to accuracy for complex wind situations, we believe
that the commenters agreed with our proposal to promote use of CALPUFF
for complex winds with prior approval by the reviewing authority.
CALPUFF has been demonstrated to perform as well as, or better than,
other short-range plume dispersion models for a few cases involving
complex winds, several with wind fields that are dominated by terrain
effects. Some suggested a need for more testing of CALPUFF, prior to
accepting its results in all cases involving complex wind situations.
We intend to post on our Web site citations to investigations for any
cases involving complex winds as they become available, and to build a
knowledge base from which determinations can be made on the use of
CALPUFF for various complex wind situations. This will support
consideration of new field study comparisons as they become available.
For the reasons stated above, it is apparent that CALPUFF contains the
scientific basis for more appropriately addressing long range transport
and dispersion effects in complex wind situations than do standard
plume models.
We conclude that, although the scientific advancements will
continue to emerge, CALPUFF in its current configuration is suitable
for regulatory use for long range transport, and on a case-by-case
basis for complex wind situations. We will require approval to be
obtained prior to accepting CALPUFF for complex wind situations, as
this will ensure that a protocol is agreed to between the parties
involved, and that all are willing to accept the results as binding. As
experience is gained in using CALPUFF for complex wind

[[Page 18442]]

situations, acceptance will become clear and those cases that are
problematic will be better identified. As suggested by comments, we
have removed reference to WYNDvalley from the Guideline.
Implementation issues/additional guidance. Some comments suggested
that the CALMET (meteorological preprocessor for CALPUFF) and CALPUFF
options should be defined for a variety of specific situations. We
believe that more experience is needed before specific guidance can be
offered for the variety of applications envisioned that might use the
CALPUFF modeling system. We placed emphasis on (1) amplifying the
available guidance information, (2) expanding the data formats for
meteorological input data, and (3) making the code more robust to
various choices in compilers. When sufficient experience has been
attained, and it has become obvious what settings should be employed
for best results for certain situations, we will promulgate expanded
guidance after allowing opportunity for public review and comment. In
the meantime, we will release interim guidance as it becomes available
to assist users in tailoring CALPUFF for application. We have created a
series of frequently asked questions (FAQ) with answers which the
public can access via Earth Tech's Internet Web site: (
http://www.src.com/calpuff/calpuff1.htm). This interim FAQ
list will be extended as resources permit.
For long range transport and complex winds applications, we
proposed that if only National Weather Service (NWS) or comparable
standard meteorological observations are employed, then five
consecutive years of data should be used. We further proposed that less
than five years of data were acceptable if appropriate NWS data are
merged with available mesoscale meteorological fields. These proposals
were generally supported by public comments,\3\ but the commenters did
provide a variety of opinions about how many years of data should be
minimally acceptable, ranging from 1 to 5 years. As we explained in our
response-to-comments, we sought to strike a balance between the need
for a sufficiently robust meteorological record to ensure results of
reliable integrity, while maintaining administrative and computational
burdens at a practical level. In consultation with the Regional
Offices, we therefore have agreed to allow use of less than five, but
at least three, years of assimilated mesoscale meteorological data.
More than 3 years may lead to the objectionable computations burdens
noted here, whereas less than 3 provides insufficient variation in
meteorological conditions to capture the range of possible
concentrations. We have also clarified that when merging NWS data with
mesoscale meteorological fields, the NWS data should be shown to be
relevant and appropriate.
For long range transport, we proposed use of a CALPUFF screening
approach on a case-by-case basis that was first outlined in the IWAQM
Phase 2 report (op. cit.) and was generally supported by commenters.
The full scope of public comments is presented and addressed in our
response-to-comments document.\3\ We agree with the comments suggesting
use of terrain heights for each receptor ring to be representative of
the Class I areas of interest. Furthermore, to ensure an appropriate
degree of flexibility, we will allow the permitting agency to decide
whether it will accept the CALPUFF screening results as proposed, and
in that decision process will defer to the appropriate reviewing
authority to decide on the details of how the CALPUFF screen is to be
implemented.
Resource constraints. The full scope of public comments is
presented and addressed in our response-to-comments document.\3\ There
was a general sense from commenters that a skilled person having
experience with CALMET can perform the required processing steps. Still
some commenters encouraged us to find and promote a simplification to
the CALMET meteorological processing steps. We did not support the
suggestion to use screening level (ISC-like) meteorological data until
such time as packaged data sets are made available. This would negate
the benefits of using the system to simulate trajectories over large
downwind distances, thereby undermining the purpose for which CALPUFF
is intended. Although the processing steps are numerous and complex,
they can be managed by competent staff.
Long range transport and complex wind situations are not trivial
modeling problems. All commenters were aware that to address these
situations requires more information (e.g., terrain heights, land use
mosaic, time and space variations in meteorological conditions) than is
typical when using standard plume models. Processing the input data is
a necessary but demanding task. The complexity of these situations
requires a selection of options to provide the flexibility to tailor
the model to specific situations. The CALPUFF system is currently
configured to support a specific applied approach for long range
transport, while at the same time, it has the flexibility for case-by-
case applications involving complex winds.
Additional analyses. Some commenters questioned whether CALPUFF has
undergone sufficient testing to secure its accuracy for assessing
impacts on air quality related values (AQRVs). We believe the available
testing for assessing AQRVs addresses many of these concerns. In
addition, it should be recognized that the FLMs are responsible for
defining the relevant AQRV's of interest and the procedures to employ
to assess whether there is an adverse impact. When CALPUFF is used for
a visibility impact assessment, this would likely be for a Class I AQRV
assessment, and the reviewing authorities are the FLMs responsible for
the management and protection of the resources for the particular Class
I areas involved. The Federal Land Managers' Air Quality Related Values
Work Group (FLAG) was formed in 1997 to provide a more consistent
approach for FLMs to evaluate air pollution effects on their resources.
In IWAQM's Phase 2 report, we indicated that EPA would use the
procedures specified by the FLMs as a consequence of their
deliberations (e.g., in their FLAG report: http://www2.nature.nps.gov/air/
Permits/flag/htm/index.html). To assist permit applicants, the FLMs
have provided procedures in the December 2000 (Phase I) FLAG report for
performing such analyses as may be required. Included in these
instructions, they have identified significance thresholds for
potential adverse impacts, and methodologies for computing a visibility
impact. The commenters are in fact addressing the FLAG procedures which
are not the subject of today's action. To the extent that they were
addressed in the response to comments developed by the FLMs in the FLAG
Phase I report, we refer commenters to that document.
Criticism was also directed at CALPUFF's treatment of chemical
transformations, which affect AQRVs. Specific concern was expressed
about the sulfate and aqueous phase chemistry algorithms. As chronicled
on the FLAG Web site (above), these procedures and criteria have been
published and received review and comment. However, today's rule
addresses the suitability of CALPUFF for PSD increment consumption and
for complex wind situations (with case-by-case approval), not AQRV
analyses.

Other Modeling Systems

Our proposal to remove UAM-IV from appendix A as a recommended
model for ozone and to remove reference to ROM and RADM for

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regional scale applications was supported by some commenters who
understood that these models were no longer state-of-the-science. Those
who objected to removal of UAM-IV were concerned that the Models-3/CMAQ
(Community Multi-scale Air Quality) model, as a replacement for UAM-IV,
was not sufficiently tested. In fact, Models-3/CMAQ is identified as
only one option among currently available models that are appropriate
in simulating the highly complex ozone/PM-2.5 formation and transport
processes. It is the responsibility of the appropriate control
agency(ies) with jurisdiction for the model application to exercise
discretion in the choice of models. Alternately, criteria for using
models not in appendix A are clearly delineated in revised wording that
we proposed for subsection 3.2.2 of appendix W. These options should
more than mitigate concerns expressed by the commenters.
We generally agree that Models-3/CMAQ and REMSAD will continue to
benefit from further evaluation and testing for use in urban/regional
scale assessments of ozone and PM-2.5, and are not the only models
available for these applications. The same is true of all similar
regional scale models. However, CMAQ and REMSAD have been successfully
subjected to peer scientific reviews and are currently undergoing
performance evaluations that will extend over several years as data
bases become more extensive and complete for both ozone and PM-2.5.
While comment was solicited on the need to integrate ozone and fine
particle impacts (i.e., the ``one atmosphere'' approach) for regional
scale assessments, we did not receive substantial comment. Comments on
integrating analyses were supportive and comments on source-specific
analyses indicated that more work was needed in this area. It is clear
that further developmental efforts on estimating the impact of
individual sources is necessary before specific modeling requirements
are identified for such applications.
Comments \3\ were generally supportive of our proposal to remove
appendix B (Summaries of Alternative Air Quality Models) from appendix
W and maintaining it as a PDF file on our SCRAM Internet Web site. As
we stated in the preamble to the notice of proposed rulemaking for this
action, appendix B of the Guideline was created solely for the
convenience of those seeking information about alternatives to the
models adopted in appendix A. The models described in appendix B may or
may not have not been the subject of performance evaluations and their
inclusion in appendix B does not confer special status or EPA sanction
on their use. Conversely, the fact that a model has not been listed in
appendix B carries no implication that its performance or acceptability
for use is any poorer than appendix B listed models. Whether or not a
model is listed, potential users will be subject to the same
requirements, i.e., to demonstrate that the model performs acceptably
for its intended regulatory application. Because production and
maintenance of appendix B information in the Code of Federal
Regulations presents a substantial administrative burden for EPA and is
not updated frequently enough to provide current information to
potential users, we are moving the appendix B repository of alternative
model summary descriptions to our Internet SCRAM Web site. This action
offers the advantages of easier and less expensive maintenance, as well
as more frequent updating, and is thus more likely to contain a
comprehensive description of alternative models which have been brought
to our attention. Similarly, the air quality checklist (formerly
appendix C of the Guideline) will be available on the Web site as a PDF
file.
The appendix B listing will therefore now appear as a list of
Alternative Models (PDF file) on our Web site. We have clarified in its
Introduction and Availability section that new models added to the list
were/are not necessarily the subject of review upon their addition. On
the other hand, it should be noted that the models identified in our
proposal (i.e., ADMS, SCIPUFF, OBODM, and CAMx) were included in the
review process for today's action concerning the list of alternative
models. At the request of the developer, we will remove MESOPUFF from
appendix B since its function is replaced by CALPUFF.
Comments on the dispersion model ADMS argued that proprietary
limitations on the availability of ADMS should not preclude it from
having equal status with other Appendix A models and that it should be
recommended in appendix A. However, as specified by Guideline paragraph
3.1.1(c)(vi), air quality models used in U.S. regulatory programs must
be in the public domain at reasonable cost. This is because the source
code needs to be open for public access and scrutiny to enable
meaningful opportunity for public comment on new source permits, PSD
increment consumption and SIPs. These criteria have been in place in
U.S. regulatory programs since the inception of the Guideline and are
needed to meet EPA's obligations under the CAA and the Administrative
Procedure Act. Until the joint issues of availability (source code) and
cost are addressed by the authors of ADMS, it is most appropriately
listed as an alternative model for use on a case-by-case basis. Even if
the model is justified on a case-by-case basis, users are responsible
for making the model available for public review and comment for
specific applications.
A similar comment regarding the puff model SCIPUFF did not consider
that the model has not gone through the same extensive testing and
regulatory evaluation as has CALPUFF, nor has it been as widely used as
CALPUFF for regulatory applications. As has been done by CALPUFF's
developers, a commitment to support public availability of SCIPUFF
would have to be made by its supporter before it could be considered
for adoption in appendix A.
Developers of neither ADMS nor SCIPUFF have addressed conflicts
associated with multiple models for the same application in such a way
as to assist EPA in resolving this issue. Moreover, we believe that
neither ADMS nor SCIPUFF technically fill a particular technical need
that is different from that occupied by the suite of refined dispersion
models that EPA has promulgated for regulatory purposes after public
review and comment.
Based on public comments and the rationale provided in our notice
of proposed rulemaking, our decision to reference the ozone limiting
method (OLM) and CAL3QHC for use in specific circumstances is
justified.

Meteorological Data Issues

In our proposal we solicited comment on terminology and meaning of
``site-specific'' data and on use of surface meteorological data
derived from the NWS's Automated Surface Observing System (ASOS). More
specifically, we invited comment on whether the policy of modeling with
the most recent 5 years of NWS meteorological data should include ASOS
data and whether the period of record must be the most recent 5 years,
regardless of whether it contains ASOS data.
No one provided negative comments on the use of the term ``site-
specific'' or associated definitions as used in the proposed revisions.
Thus, for the reasons discussed in the proposal, we will retain this
terminology.
The majority of commenters who addressed the topic of ASOS data
felt that the ASOS data were inferior for use with Gaussian models,
though not all commenters agreed. With respect to the

[[Page 18444]]

use of the most recent 5 years of meteorological data, there was some
concern about the reliability of ASOS data. We revised guidance to
specifically address this concern by allowing flexibility in the choice
of ASOS or observer-based observations depending on which provided the
most representative meteorological information.
Final Action
Today's action amends appendix W of 40 CFR part 51 as detailed
below:

CALPUFF

The public comments provided constructive suggestions but did not
suggest altering promulgation of the CALPUFF modeling system. We will
therefore promulgate use of the CALPUFF modeling system as follows:
(A) Long Range Transport
CALPUFF will be adopted as a refined model for use in sulfur
dioxide and particulate matter ambient air quality standards and PSD
increment impact analyses involving (1) transport greater than 50km
from one or several closely spaced sources, and (2) analyses involving
a mixture of both long range and short-range source-receptor
relationships in a large modeling domain (e.g., several industrialized
areas located along a river or valley). The screening approach outlined
in the IWAQM Phase 2 report is available for use on a case-by-case
basis that generally provides concentrations that are higher than those
obtained using refined characterizations of the meteorological
conditions.
Given the judgement and refinement involved, conducting a long
range transport modeling assessment will require significant
consultation with the appropriate reviewing authority, and for Class I
analyses the appropriate FLM. To facilitate use of complex air quality
and meteorological modeling systems, a written protocol may be
considered for developing consensus in the methods and procedures to be
followed.
(B) Complex Winds
(1) On a case-by-case basis, the CALPUFF modeling system may be
applied for air quality estimates involving complex meteorological
conditions, where the assumptions of steady-state straight-line
transport both in time and space are inappropriate.
(2) In such situations, where the otherwise preferred dispersion
model is found to be less appropriate, use of the CALPUFF modeling
system will be in accordance with the procedures and requirements
outlined in paragraph 3.2.2(e) of the Guideline.
The public comments provided constructive suggestions, but did not
suggest altering the meteorological data requirements for refined
modeling assessments using the CALPUFF modeling system. Therefore, we
will promulgate use of the CALPUFF modeling system with the following
meteorological data requirements. For long range transport and for
complex winds situations, there are two possibilities:
(A) If only NWS or comparable standard meteorological observations
are employed, then five years of meteorological data should be used.
(B) If mesoscale meteorological fields are employed with
appropriate NWS observations, then less than five years but at least
three years of meteorological data may be used. Following the
suggestions provided in public comments, we revised the Guideline to
emphasize that appropriate NWS observations should be used in
conjunction with mesoscale meteorological data.
In response to the suggestions provided in public comments, we: (1)
Created a series of frequently asked questions to provide additional
technical information to users, which will be made publicly available
via Earth Tech's Internet Web site, (2) expanded the meteorological and
precipitation data formats that can be processed, (3) have tested and
made changes as necessary that allow the modeling software to be
compiled by several Fortran compilers, thus making the code more robust
to various choices in compilers, and (4) will maintain and make
publicly available via our Web site, a list of technical papers and
reports that describe testing and evaluation of the CALPUFF modeling
system in a variety of situations and thus provide a basis for wider
use of the CALPUFF modeling system.
For appropriate applications, CALPUFF may be used during the one-
year period following the promulgation of today's notice. After one
year following promulgation of today's notice, CALPUFF should be used
for appropriate applications.

Other Modeling Systems

We have removed UAM-IV from appendix A for urban ozone applications
and removed reference to ROM and RADM for regional scale applications
to reflect the current state-of-science. Similarly, we have identified
Models-3/CMAQ and REMSAD as example modeling systems that have been
evaluated and peer reviewed for regional scale applications, and make
clear that this does not preclude the use of other models.
We have removed appendix B and appendix C from appendix W and
placed equivalent counterparts on our SCRAM Internet Web site. Former
appendix B will simply become a list of alternative model summaries,
and should be readily updated as new models in the proper format are
submitted and not on a restrictive schedule. Given the current status
of ADMS and SCIPUFF, as well as OBODM, CAMx and UAMV (an update to UAM-
IV), all have now been included in the web-based Alternative Models
list.
As proposed, we have referenced OLM and CAL3QHC for use in specific
circumstances, and removed RAM and CDM from appendix A.

Meteorological Data Issues

The terminology for ``site-specific'' has been implemented as
proposed since there was a lack of negative comment. The prevailing
concept is, as commenters recognized, representativeness, and this is
now emphasized in our guidance.
Due to limitations of ASOS data for use with standard dispersion
models, paragraph 8.3.1.2(a) of appendix W has been revised to indicate
that where the latest 5 years of data includes ASOS data (now the
typical situation) discretion should be used. Where judgment indicates
ASOS data are inadequate for cloud cover observations, the most recent
5 years of NWS data that are observer-based may be considered for use.
In response to public comment, we have updated our meteorological
data processors (i.e., MPRM and CALMET) to allow processing of
meteorological data formats from the National Climatic Data Center
necessary to operate associated air quality models; no further updates
to MPRM are necessary at this time. The meteorological monitoring
guidance \6\ has been updated.
------------------------------------------------------------------------

\6\ Environmental Protection Agency, 2000. Meteorological
Monitoring Guidance for Regulatory Modeling Applications. EPA
Publication No. EPA-454/R-99-005. U.S. Environmental Protection
Agency, Research Triangle Park, NC. (www.epa.gov/scram001).
------------------------------------------------------------------------

Final Editorial Changes to Appendix W
Preface
You will note some minor revisions to reflect current EPA practice.
Section 2
In a streamlining effort, we removed section 2.2 and added a new
section 2.3 to address model availability.

[[Page 18445]]

Section 3
As proposed, we revised section 3 to more accurately reflect
current EPA practice, e.g., functions of the Model Clearinghouse and
enhanced criteria for the use of alternative models. Requirements for
alternative models when preferred models are less appropriate for
specific applications have been clarified. These requirements include
scientific peer review and the establishment of an acceptable protocol
prior to the model's use.
Section 4
We revised section 4.2.2 to reflect the widespread use of short-
term models for all averaging periods. Hence, we no longer reference
long-term models (e.g., ISCLT) in the Guideline.\7\
------------------------------------------------------------------------

\7\ Note that because appendix W is designed to guide
assessments for criteria pollutants, the proposed discontinuation of
ISCLT for purposes herein does not preclude its use for other
pollutant assessments, as applicable. For example, the ASPEN model
(Assessment System for Population Exposure Nationwide) uses the
capabilities of ISCLT to estimate ambient concentrations of toxic
pollutants nationwide by census tract. Such applications require the
abbreviated computing possible with ISCLT.
------------------------------------------------------------------------

Section 5
To simplify, the list of acceptable, yet equivalent, screening
techniques for complex terrain was removed. CTSCREEN and guidance for
its use are retained; CTSCREEN remains acceptable for all terrain above
stack top. The screening techniques whose descriptions we removed,
i.e., Valley (as implemented in SCREEN3), COMPLEX I (as implemented in
ISC3), SHORTZ/LONGZ, and RTDM remain available for use in applicable
cases where established/accepted procedures are used. Consultation with
the appropriate reviewing authority is still advised for application of
these screening models.
Section 6
As proposed, we revised section 6 to reflect the new PM-2.5 and
ozone ambient air quality standards that were issued on July 18, 1997
(62 FR 38652 & 62 FR 38856). You will note that we inserted respective
subsections for particulate matter and lead from section 8, so that
section 6 now primarily contains modeling guidance for the criteria
pollutants regulated in Part 51 (SO2 analyses are covered in section
4). We also updated information on receptor models.
? We enhanced the subsection on particulate matter as much as
possible to reflect the Agency's current thinking on approaches for
fine particulates (PM-2.5). You will note that we removed the
references to the Climatological Dispersion Model (CDM 2.0) as well as
to RAM from this section, and also deleted CDM and RAM from appendix A
(see below).
? We enhanced the subsection on ozone to better reflect
modeling approaches we currently envision, and added a reference for
current guidance on ozone attainment demonstrations.\8\ You will note
that we removed the reference to the Urban Airshed Model (UAM-IV) from
this section, and deleted UAM from appendix A. UAM-IV is no longer the
recommended photochemical model for attainment demonstrations for
ozone.
------------------------------------------------------------------------

\8\ Environmental Protection Agency, 1998. Use of Models and
Other Analyses in Attainment Demonstrations for the 8-hr Ozone NAAQS
(Draft). Office of Air Quality Planning & Standards, Research
Triangle Park, NC. (Docket No. A-99-05, II-A-14) (Also available on
SCRAM Web site, http://www.epa.gov/scram001, as draft8hr.pdf)
------------------------------------------------------------------------

? We updated the subsection on carbon monoxide by removing
reference to RAM. While UAM-IV is deleted from appendix A, reference to
areawide analyses is retained. For refined intersection modeling,
CAL3QHCR is specifically mentioned for use on a case-by-case basis.
? In the subsection on NO₂ models, we added a
third tier for the screening approach that allows the use of the ozone
limiting method on a case-by-case basis. You may recall that this
approach was removed with the Guideline update promulgated on August 9,
1995 (60 FR 40465).
? In the subsection on lead, we deleted references to 40 CFR
51.83, 51.84, and 51.85, conforming to previous EPA action (51 FR
40661).
Section 7
For regional scale modeling, we removed reference to the Regional
Oxidant Model (ROM) and the Regional Acid Deposition Model (RADM) from
section 7 because they are outdated and replaced by a reference to
Models-3 \9\ in section 6. We enhanced the subsection on visibility to
reflect the provisions of the Clean Air Act, including those for
reasonable attribution of visibility impairment and regional haze, as
well as the new NAAQS for PM-2.5. For assessment of reasonably
attributable haze impairment due to one or a small group of sources,
CALPUFF is available for use on a case-by-case basis. We identify
REMSAD and new approaches under the Models-3/CMAQ umbrella for possible
use to develop and evaluate national policy and assist State and local
control agencies. For long range transport analyses, we recommend the
CALPUFF modeling system. To facilitate use of a complex air quality and
meteorological modeling system like CALPUFF, we stipulate that a
written protocol may be considered for developing consensus in the
methods and procedures to be followed.
------------------------------------------------------------------------

\9\ Environmental Protection Agency, 1998. EPA Third-Generation
Air Quality Modeling System. Models-3, Volume 9b: User Manual. EPA
Publication No. EPA-600/R-98/069(b). Office of Research and
Development, Washington, DC.
------------------------------------------------------------------------

Section 8
As proposed, we revised section 8 to better reflect our current
regulatory practice for the general modeling considerations addressed.
? We revised subsection 8.2.6 to refer to subsection 6.2.3
for details on chemical transformation of NO_X.
? We merged subsection 8.2.8 (Urban/Rural Classification)
with subsection 8.2.3 (Dispersion Coefficients), and removed reference
to WYNDvalley.
? We merged discussions in subsections 8.2.9 (Fumigation) and
8.2.10 (Stagnation) into one new subsection (8.2.8--Complex Winds), and
specifically identify the availability of CALPUFF for certain
situations on a case-by-case basis.
? We removed the distinction between short-term and long-term
models because when assessing the impacts from criteria air pollutants,
long-term estimates are now practicable using hour-by-hour
meteorological data.
Section 9
As proposed,
? We revised subsection 9.2.3 (recommendations for estimating
background concentrations from nearby sources) to reflect a settlement
reached on October 16, 1997 in a petition brought by the Utility Air
Regulatory Group (UARG). In accordance with the settlement, we are
clarifying the definition of ``nearby sources.'' The ``maximum
allowable emission limit,'' specified in Tables 9-1 and 9-2, is tied in
certain circumstances \10\ to the emission rate representative of a
nearby source's maximum physical capacity to emit. We also clarify that
nearby sources should be modeled only when they operate at the same
time as the primary source(s) being modeled. Where a nearby source does
not, by its nature, operate at the same time as the primary source
being modeled, the burden is on the primary source to demonstrate to
the satisfaction of the appropriate reviewing authority that this is,
in fact, the case. We added footnotes to Tables 9-1 and 9-2 to refer
back to applicable paragraphs of subsection 9.2.3 that provide the
necessary clarification.
---------------------------------------------------------------------------

\10\ See section 8.2.3 of the Guideline.

------------------------------------------------------------------------

[[Page 18446]]

? We enhanced section 9.3 (Meteorological Input Data) to
develop concepts of meteorological data representativeness, minimum
meteorological data requirements, and the use of prognostic mesoscale
meteorological models in certain situations. These models (e.g., the
Penn State/NCAR MM4 11,12,13 or MM5 \14\ model) assimilate
meteorological data from several surface and upper air stations in or
near a domain and generate a 3-dimensional field of wind, temperature
and relative humidity profiles. We revised recommendations for length
of record for meteorological data (subsection 9.3.1.2) for long range
transport and complex wind situations. In paragraph 9.3.1.2(d) we
specifically allow the use of at least three years (need not be
consecutive) of assimilated mesoscale meteorological data.
------------------------------------------------------------------------

\11\ Stauffer, D.R. and Seaman, N.L., 1990. Use of four-
dimensional data assimilation in a limited-area mesoscale model.
Part I: Experiments with synoptic-scale data. Monthly Weather
Review, 118: 1250-1277.
\12\ Stauffer, D.R., Seaman, N.L., and Binkowski, F.S., 1991.
Use of four-dimensional data assimilation in a limited-area
mesoscale model. Part II: Effect of data assimilation within the
planetary boundary layer. Monthly Weather Review, 119: 734-754.
\13\ Hourly Modeled Sounding Data. MM4--1990 Meteorological
Data, 12-volume CD-ROM. Jointly produced by NOAA's National Climatic
Data Center and Atmospheric Sciences Modeling Division. August 1995.
Can be ordered from NOAA National Data Center's Internet Web site @
www.nndc.noaa.gov/.
\14\ http://www.mmm.ucar.edu/mm5/mm5-home.html

? We revised subsection 9.3.2 (National Weather Service Data)
to inform users that National Weather Service (NWS) surface and upper
air meteorological data are available on CD-ROM from the National
Climatic Data Center. Recent years of such surface data are derived
from the NWS's Automated Surface Observing System (ASOS). We revised
subsection 9.3.1.2 to address the possible occurrence of ASOS data
within 5-year sets of meteorological data.
? We revised subsection 9.3.3.1 to clarify that, while site-
specific measurements are frequently made ``on-property'' (i.e., on the
source's premises), acquisition of adequately representative site-
specific data does not preclude collecting data from a location off
property. Conversely, collection of meteorological data on property
does not of itself guarantee adequate representativeness. The
subsection was also enhanced by improving the discussion of collection
of temperature difference measurements; a paragraph was developed that
focuses on measurement of aloft winds for simulation of plume rise,
dispersion and transport (some details for CTDMPLUS were moved to its
appendix A descriptions); a paragraph was added to address collection
and use of direct turbulence measurements; and the paragraph that
discusses meteorological data preprocessor has been enhanced.
? We revised subsection 9.3.3.2 by removing reference to the
STAR processing routine because ISCLT and CDM 2.0 (for which STAR
formatted data were developed) have been removed.
? We revised subsection 9.3.4 (Treatment of Calms) to
increase accuracy.
Section 10
We updated section 10 to reflect current thinking and state-of-the-
practice regarding model accuracy and uncertainty.
Section 11
As proposed, we made minor revisions to section 11 to reflect the
new ambient air quality standards for fine particles and ozone. Because
EPA has revised its emissions trading program for SO₂, we
have deleted subsection 11.2.3.4.
Section 12 & 13
We redesignated section 13 (Bibliography) as section 12
(References) and vice-versa. We revised them by adding some references,
deleting obsolete/superseded ones, and resequencing. You will note that
a peer scientific review for CALPUFF has been included.
Section 14
In a streamlining effort, we removed section 14 (Glossary). Given
current familiarity with modeling terminology, we no longer consider
that maintenance of such a glossary is as necessary as it once may have
been. For these and other reasons relating to Office of Federal
Register policy (see discussion of appendix B below), we have revised
the glossary and placed it on our Internet Web site.

Appendix A

We updated the introduction to appendix A (section A.0). As
mentioned before, we added CALPUFF to appendix A. We removed the
Climatological Dispersion Model (CDM 2.0), the Gaussian-Plume Multiple
Source Air Quality Algorithm (RAM), and the Urban Airshed Model (UAM)
from appendix A. These models have been superseded and are no longer
considered preferred techniques.

Appendix B

We have moved the appendix B repository of alternate model summary
descriptions to our Internet SCRAM Web site (http://www.epa.gov/scram001).
Placement of this material on the Web site offers many
advantages. In this format, we will be able to maintain the list and
model descriptions more easily and inexpensively.
Several model developers have submitted new dispersion models for
inclusion in this Web site repository of alternate models:
? Second-Order Closure Integrated Puff Model (SCIPUFF);
? Open Burn/Open Detonation Dispersion Model (OBODM);
? Atmospheric Dispersion Modeling System (ADMS);
? Comprehensive Air Quality Model with extensions (CAMx); and
? Urban Airshed Model--V (UAMV).
As described below, codes (executables) for these models, as well
as applicable documentation, have been uploaded to our Internet SCRAM
Web site. Finally, we deleted a model currently listed in appendix B,
MESOPUFF II, which CALPUFF replaces.

Appendix C

As proposed, we also moved appendix C (Example Air Quality Analysis
Checklist) from the CFR to our Internet SCRAM Web site. We believe this
checklist is outdated, in need of revision, and would be more practical
to maintain if posted on EPA's Internet SCRAM Web site.

Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

Under Executive Order 12866 (58 FR 51735 (October 4, 1993)), the
Agency must determine whether the regulatory action is ``significant''
and therefore subject to review by the Office of Management and Budget
(OMB) and the requirements of the Executive Order. The Order defines
``significant regulatory action'' as one that is likely to result in a
rule that may:
(1) Have an annual effect on the economy of $100 million or more or
adversely affect in a material way the economy, a sector of the
economy, productivity, competition, jobs, the environment, public
health or safety, or State, local, or tribal governments or
communities;
(2) Create a serious inconsistency or otherwise interfere with an
action taken or planned by another agency;
(3) Materially alter the budgetary impact of entitlements, grants,
user fees,

[[Page 18447]]

or loan programs of the rights and obligations of recipients thereof;
or
(4) Raise novel legal or policy issues arising out of legal
mandates, the President's priorities, or the principles set forth in
the Order.
This rule is not a ``significant regulatory action'' under the
terms of Executive Order 12866 and is therefore not subject to OMB
review.

B. Paperwork Reduction Act

This final rule does not contain any information collection
requirements subject to review by OMB under the Paperwork Reduction
Act, 44 U.S.C. 3501 et seq.

C. Regulatory Flexibility Act (RFA), as amended by the Small Business
Regulatory Enforcement Fairness Act of 1996 (SBREFA), 5 U.S.C. 601 et
seq.

The RFA generally requires an agency to prepare a regulatory
flexibility analysis of any rule subject to notice and comment
rulemaking requirements under the Administrative Procedure Act or any
other statute unless the agency certifies that the rule will not have a
significant economic impact on a substantial number of small entities.
Small entities include small businesses, small organizations, and small
governmental jurisdictions.
EPA has determined that it is not necessary to prepare a regulatory
flexibility analysis in connection with this final rule. EPA has also
determined that this rule will not have a significant economic impact
on a substantial number of small entities. For purposes of assessing
the impact of today's rule on small entities, small entities are
defined as: (1) A small business that meets the RFA default definitions
for small business (based on Small Business Administration size
standards), as described in 13 CFR 121.201; (2) a small governmental
jurisdiction that is a government of a city, county, town, school
district or special district with a population of less than 50,000; and
(3) a small organization that is any not-for-profit enterprise which is
independently owned and operated and is not dominant in its field.
After considering the economic impacts of today's final rule on
small entities, EPA has concluded that this action will not have a
significant economic impact on a substantial number of small entities.
This final rule will not impose any requirements on small entities.
Today's rule will not have any impacts on small entities because
existing and new sources of air emissions that model air quality for
State Implementation Plans and the prevention of significant
deterioration are typically not small entities. The modeling techniques
described today are primarily used by state air control agencies and by
industry.
To the extent that any small entities would ever have to model air
quality using the modeling techniques described in today's rule, the
impacts of using updated modeling techniques would be minimal, if not
non-existent. The action promulgated today incorporates comments
received at the 7th Conference on Air Quality Modeling in June 2000 in
Washington, DC. The rule features a new modeling system for calculating
PSD increment consumption--CALPUFF--and serves to increase efficiency
and accuracy. This system employs procedural concepts that are very
similar to those currently used, changing only mathematical
formulations and specific data elements. No impacts on small entities
in the use of CALPUFF are anticipated. We do not believe that CALPUFF's
use poses a significant or unreasonable burden on any small entities.
This final action imposes no new regulatory burdens and, as such, there
will be no additional impact on small entities regarding reporting,
recordkeeping, compliance requirements.

D. Unfunded Mandates Reform Act of 1995

Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public
Law 104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and tribal
governments and the private sector. Under section 202 of the UMRA, EPA
generally must prepare a written statement, including a cost-benefit
analysis, for proposed and final rules with ``Federal mandates'' that
may result in expenditures to State, local, and tribal governments, in
the aggregate, or to the private sector, of $100 million or more in any
one year. Before promulgating an EPA rule for which a written statement
is needed, section 205 of the UMRA generally requires EPA to identify
and consider a reasonable number of regulatory alternatives and adopt
the least costly, most cost-effective or least burdensome alternative
that achieves the objectives of the rule. The provisions of section 205
do not apply when they are inconsistent with applicable law. Moreover,
section 205 allows EPA to adopt an alternative other than the least
costly, most cost-effective or least burdensome alternative if the
Administrator publishes with the final rule an explanation why that
alternative was not adopted. Before EPA establishes any regulatory
requirements that may significantly or uniquely affect small
governments, including tribal governments, it must have developed under
section 203 of the UMRA a small government agency plan.
The plan must provide for notifying potentially affected small
governments, enabling officials of affected small governments to have
meaningful and timely input in the development of EPA regulatory
proposals with significant Federal intergovernmental mandates, and
informing, educating, and advising small governments on compliance with
the regulatory requirements.
Today's rule recommends a new modeling system for calculating PSD
increment consumption--CALPUFF--that increases efficiency and accuracy.
CALPUFF has been used for these purposes on a case-by-case basis (per
Guideline subsection 3.2.2) for several years, as has its predecessor--
MESOPUFF II. While Guideline subsection 3.2.2 still allows for
alternative models to be used, EPA is now sufficiently confident in
CALPUFF's technical formulation and performance to adopt it in appendix
A of the Guideline. Since the two modeling systems are comparable in
scope and purpose, use of CALPUFF itself does not involve any increase
in costs. The optional use of prognostic meteorological data (e.g.,
MM5) input files, however, may result in a small incremental cost
increase. To the extent that the use of more refined models with
comprehensive input data bases reduces the potential for over-or
underprediction of air quality impacts, air quality management programs
become more economically efficient. Moreover, modeling costs (which
include those for input data acquisition) are typically among the
implementation costs that are considered as part of the programs (i.e.,
PSD) that establish and periodically revise requirements for
compliance. Any incremental modeling costs attributable to today's rule
do not approach the $100 million threshold prescribed by UMRA. EPA has
determined that this rule contains no regulatory requirements that
might significantly or uniquely affect small governments. This rule
therefore contains no Federal mandates (under the regulatory provisions
of Title II of the UMRA) for State, local, or tribal governments or the
private sector.

E. Executive Order 13132: Federalism

Executive Order 13132, entitled ``Federalism `` (64 FR 43255,
August 10, 1999), requires EPA to develop an accountable process to
ensure ``meaningful and timely input by State and local officials in
the development of regulatory policies that have federalism

[[Page 18448]]

implications.'' ``Policies that have federalism implications `` is
defined in the Executive Order to include regulations that have
``substantial direct effects on the States, on the relationship between
the national government and the States, or on the distribution of power
and responsibilities among the various levels of government.''
This final rule does not have federalism implications. It will not
have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132. This rule does not create a
mandate on State, local or tribal governments. The rule does not impose
any enforceable duties on these entities (see D. Unfunded Mandates
Reform Act of 1995, above). The rule would add better, more accurate
techniques for air dispersion modeling analyses and does not impose any
additional requirements for any of the affected parties covered under
Executive Order 13132. Thus, Executive Order 13132 does not apply to
this rule.

F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments

Executive Order 13175, entitled ``Consultation and Coordination
with Indian Tribal Governments'' (59 FR 22951, November 9, 2000),
requires EPA to develop an accountable process to ensure ``meaningful
and timely input by tribal officials in the development of regulatory
policies that have tribal implications.'' This final rule does not have
tribal implications, as specified in Executive Order 13175. As stated
above (see D. Unfunded Mandates Reform Act of 1995, above), the rule
does not impose any new requirements for calculating PSD increment
consumption, and does not impose any additional requirements for the
regulated community, including Indian Tribal Governments. Thus,
Executive Order 13175 does not apply to this rule.
Today's final rule does not significantly or uniquely affect the
communities of Indian tribal governments. Accordingly, the requirements
of section 3(b) of Executive Order 13175 do not apply to this rule.

G. Executive Order 13045: Protection of Children From Environmental
Health and Safety Risks

Executive Order 13045 applies to any rule that EPA determines (1)
to be ``economically significant '' as defined under Executive Order
12866, and (2) the environmental health or safety risk addressed by the
rule has a disproportionate effect on children. If the regulatory
action meets both the criteria, the Agency must evaluate the
environmental health or safety effects of the planned rule on children;
and explain why the planned regulation is preferable to other
potentially effective and reasonably feasible alternatives considered
by the Agency.
This final rule is not subject to Executive Order 13045, entitled
``Protection of Children from Environmental Health Risks and Safety
Risks '' (62 FR 19885, April 23, 1997) because it does not impose an
economically significant regulatory action as defined by Executive
Order 12866 and the action does not involve decisions on environmental
health or safety risks that may disproportionately affect children.

H. Executive Order 13211: Actions that Significantly Affect Energy
Supply, Distribution, or Use

This rule is not subject to Executive Order 13211, ``Actions
Concerning Regulations That Significantly Affect Energy Supply,
Distribution, or Use'' (66 FR 28355 (May 22, 2001)) because it is not a
significant regulatory action under Executive Order 12866.

I. National Technology Transfer and Advancement Act of 1995

Section 12(d) of the National Technology Transfer and Advancement
Act of 1995 (``NTTAA''), Public Law 104-113, section 12(d) (15 U.S.C.
272 note) directs EPA to use voluntary consensus standards in its
regulatory activities unless to do so would be inconsistent with
applicable law or otherwise impractical. Voluntary consensus standards
are technical standards (e.g., materials specifications, test methods,
sampling procedures, and business practices) that are developed or
adopted by voluntary consensus standards bodies. The NTTAA directs EPA
to provide Congress, through OMB, explanations when the Agency decides
not to use available and applicable voluntary consensus standards.
This action does not involve technical standards. Therefore, EPA
did not consider the use of any voluntary consensus standards.

J. Congressional Review Act of 1998

The Congressional Review Act, 5 U.S.C. 801 et seq., as added by the
Small Business Regulatory Enforcement Fairness Act of 1996, generally
provides that before a rule may take effect, the agency promulgating
the rule must submit a rule report, which includes a copy of the rule,
to each House of the Congress and to the Comptroller General of the
United States. EPA will submit a report containing this rule and other
required information to the U.S. Senate, the U.S. House of
Representatives, and the Comptroller General of the United States prior
to publication of the rule in the Federal Register. A Major rule cannot
take effect until 60 days after it is published in the Federal
Register. This action is not a ``major rule'' as defined by 5 U.S.C.
804(2), and will be effective 30 days from the publication date of this
notice.

List of Subjects in 40 CFR Part 51

Environmental protection, Administrative practice and procedure,
Air pollution control, Carbon monoxide, Intergovernmental relations,
Nitrogen oxides, Ozone, Particulate matter, Reporting and recordkeeping
requirements, Sulfur oxides.

Dated: April 2, 2003.
Christine Todd Whitman,
Administrator.

? Part 51, chapter I, title 40 of the Code of Federal Regulations is
amended as follows:

PART 51--REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF
IMPLEMENTATION PLANS

? 1. The authority citation for part 51 continues to read as follows:

Authority: 23 U.S.C. 100; 42 U.S.C. 7401-7671q.

? 2. Appendix W to Part 51 revised to read as follows:

Appendix W to Part 51--Guideline on Air Quality Models

Preface

a. Industry and control agencies have long expressed a need for
consistency in the application of air quality models for regulatory
purposes. In the 1977 Clean Air Act, Congress mandated such
consistency and encouraged the standardization of model
applications. The Guideline on Air Quality Models (hereafter,
Guideline) was first published in April 1978 to satisfy these
requirements by specifying models and providing guidance for their
use. The Guideline provides a common basis for estimating the air
quality concentrations of criteria pollutants used in assessing
control strategies and developing emission limits.
b. The continuing development of new air quality models in
response to regulatory requirements and the expanded requirements
for models to cover even more complex problems have emphasized the
need for periodic review and update of guidance on these techniques.
Three primary on-going activities provide direct input to revisions
of the Guideline. The first is a series of annual

[[Page 18449]]

EPA workshops conducted for the purpose of ensuring consistency and
providing clarification in the application of models. The second
activity is the solicitation and review of new models from the
technical and user community. In the March 27, 1980 Federal
Register, a procedure was outlined for the submittal to EPA of
privately developed models. After extensive evaluation and
scientific review, these models, as well as those made available by
EPA, are considered for recognition in the Guideline. The third
activity is the extensive on-going research efforts by EPA and
others in air quality and meteorological modeling.
c. Based primarily on these three activities, new sections and
topics are included as needed. EPA does not make changes to the
guidance on a predetermined schedule, but rather on an as needed
basis. EPA believes that revisions of the Guideline should be timely
and responsive to user needs and should involve public participation
to the greatest possible extent. All future changes to the guidance
will be proposed and finalized in the Federal Register. Information
on the current status of modeling guidance can always be obtained
from EPA's Regional Offices.

Table of Contents

List of Tables

1.0 Introduction
2.0 Overview of Model Use
2.1 Suitability of Models
2.2 Levels of Sophistication of Models
2.3 Availability of Models
3.0 Recommended Air Quality Models
3.1 Preferred Modeling Techniques
3.1.1 Discussion
3.1.2 Recommendations
3.2 Use of Alternative Models
3.2.1 Discussion
3.2.2 Recommendations
3.3 Availability of Supplementary Modeling Guidance
4.0 Traditional Stationary-Source Models
4.1 Discussion
4.2 Recommendations
4.2.1 Screening Techniques
4.2.1.1 Simple Terrain
4.2.1.2 Complex Terrain
4.2.2 Refined Analytical Techniques
5.0 Model Use in Complex Terrain
5.1 Discussion
5.2 Recommendations
5.2.1 Screening Techniques
5.2.2 Refined Analytical Techniques
6.0 Models for Ozone, Particulate Matter, Carbon Monoxide, Nitrogen
Dioxide, and Lead
6.1 Discussion
6.2 Recommendations
6.2.1 Models for Ozone
6.2.1 Models for Particulate Matter
6.2.2.1 PM-2.5
6.2.2.2 PM-10
6.2.3 Models for Carbon Monoxide
6.2.4 Models for Nitrogen Dioxide (Annual Average)
6.2.5 Models for Lead
7.0 Other Model Requirements
7.1 Discussion
7.2 Recommendations
7.2.1 Visibility
7.2.2 Good Engineering Practice Stack Height
7.2.3 Long Range Transport (i.e., beyond 50km)
7.2.4 Modeling Guidance for Other Governmental Programs
8.0 General Modeling Considerations
8.1 Discussion
8.2 Recommendations
8.2.1 Design Concentrations
8.2.2 Critical Receptor Sites
8.2.3 Dispersion Coefficients
8.2.4 Stability Categories
8.2.5 Plume Rise
8.2.6 Chemical Transformation
8.2.7 Gravitational Settling and Deposition
8.2.8 Complex Winds
8.2.9 Calibration of Models
9.0 Model Input Data
9.1 Source Data
9.1.1 Discussion
9.1.2 Recommendations
9.2 Background Concentrations
9.2.1 Discussion
9.2.2 Recommendations (Isolated Single Source)
9.2.3 Recommendations (Multi-Source Areas)
9.3 Meteorological Input Data
9.3.1 Length of Record of Meteorological Data
9.3.2 National Weather Service Data
9.3.3 Site Specific Data
9.3.4 Treatment of Calms
10.0 Accuracy and Uncertainty of Models
10.1 Discussion
10.1.1 Overview of Model Uncertainty
10.1.2 Studies of Model Accuracy
10.1.3 Use of Uncertainty in Decision-Making
10.1.4 Evaluation of Models
10.2 Recommendations
11.0 Regulatory Application of Models
11.1 Discussion
11.2 Recommendations
11.2.1 Analysis Requirements
11.2.2 Use of Measured Data in Lieu of Model Estimates
11.2.3 Emission Limits
12.0 Bibliography
13.0 References

Appendix A to Appendix W of 40 CFR Part 51--Summaries of Preferred Air
Quality Models

List of Tables
------------------------------------------------------------------------
Table No. Title
------------------------------------------------------------------------
5-1............................... Neutral/Stable Meteorological Matrix
for CTSCREEN.
5-1............................... Unstable/Convective Meteorological
Matrix for CTSCREEN.
9-1............................... Model Emission Input Data for Point
Sources.
9-2............................... Point Source Model Input Data
(Emissions) for PSD NAAQS
Compliance Demonstrations.
9-3............................... Averaging Times for Site Specific
Wind and Turbulence Measurements.
------------------------------------------------------------------------

1.0 Introduction

a. The Guideline recommends air quality modeling techniques that
should be applied to State Implementation Plan (SIP) revisions for
existing sources and to new source reviews (NSR), including
prevention of significant deterioration (PSD). (See Ref. 1, 2, 3).
Applicable only to criteria air pollutants, it is intended for use
by EPA Regional Offices in judging the adequacy of modeling analyses
performed by EPA, State and local agencies and by industry. The
guidance is appropriate for use by other Federal agencies and by
State agencies with air quality and land management
responsibilities. The Guideline serves to identify, for all
interested parties, those techniques and data bases EPA considers
acceptable. The Guideline is not intended to be a compendium of
modeling techniques. Rather, it should serve as a common measure of
acceptable technical analysis when supported by sound scientific
judgement.
b. Due to limitations in the spatial and temporal coverage of
air quality measurements, monitoring data normally are not
sufficient as the sole basis for demonstrating the adequacy of
emission limits for existing sources. Also, the impacts of new
sources that do not yet exist can only be determined through
modeling. Thus, models, while uniquely filling one program need,
have become a primary analytical tool in most air quality
assessments. Air quality measurements can be used in a complementary
manner to dispersion models, with due regard for the strengths and
weaknesses of both analysis techniques. Measurements are
particularly useful in assessing the accuracy of model estimates.
The use of air quality measurements alone however could be
preferable, as detailed in a later section of this document, when
models are found to be unacceptable and monitoring data with
sufficient spatial and temporal coverage are available.
c. It would be advantageous to categorize the various regulatory
programs and to apply a designated model to each proposed source
needing analysis under a given program. However, the diversity of
the nation's topography and climate, and variations in source
configurations and operating characteristics dictate against a
strict modeling ``cookbook''. There is no one model capable of
properly addressing all conceivable situations even within a broad
category such as point sources. Meteorological phenomena associated
with threats to air quality standards are rarely amenable to a
single mathematical treatment; thus, case-by-case analysis and
judgement are frequently required. As modeling efforts become more
complex, it is increasingly important that they be directed by
highly competent individuals with a broad range of experience and
knowledge in air quality meteorology. Further, they should be
coordinated closely with specialists in emissions characteristics,
air monitoring and data processing. The judgement of experienced
meteorologists and analysts is essential.
d. The model that most accurately estimates concentrations in
the area of interest is always sought. However, it is clear from the
needs expressed by the States and EPA Regional Offices, by many
industries and trade associations, and also by the deliberations of
Congress, that consistency in

[[Page 18450]]

the selection and application of models and data bases should also
be sought, even in case-by-case analyses. Consistency ensures that
air quality control agencies and the general public have a common
basis for estimating pollutant concentrations, assessing control
strategies and specifying emission limits. Such consistency is not,
however, promoted at the expense of model and data base accuracy.
The Guideline provides a consistent basis for selection of the most
accurate models and data bases for use in air quality assessments.
e. Recommendations are made in the Guideline concerning air
quality models, data bases, requirements for concentration
estimates, the use of measured data in lieu of model estimates, and
model evaluation procedures. Models are identified for some specific
applications. The guidance provided here should be followed in air
quality analyses relative to State Implementation Plans and in
supporting analyses required by EPA, State and local agency air
programs. EPA may approve the use of another technique that can be
demonstrated to be more appropriate than those recommended in this
guide. This is discussed at greater length in Section 3. In all
cases, the model applied to a given situation should be the one that
provides the most accurate representation of atmospheric transport,
dispersion, and chemical transformations in the area of interest.
However, to ensure consistency, deviations from this guide should be
carefully documented and fully supported.
f. From time to time situations arise requiring clarification of
the intent of the guidance on a specific topic. Periodic workshops
are held with the headquarters, Regional Office, State, and local
agency modeling representatives to ensure consistency in modeling
guidance and to promote the use of more accurate air quality models
and data bases. The workshops serve to provide further explanations
of Guideline requirements to the Regional Offices and workshop
reports are issued with this clarifying information. In addition,
findings from on-going research programs, new model submittals, or
results from model evaluations and applications are continuously
evaluated. Based on this information changes in the guidance may be
indicated.
g. All changes to the Guideline must follow rulemaking
requirements since the Guideline is codified in Appendix W of Part
51. EPA will promulgate proposed and final rules in the Federal
Register to amend this Appendix. Ample opportunity for public
comment will be provided for each proposed change and public
hearings scheduled if requested.
h. A wide range of topics on modeling and data bases are
discussed in the Guideline. Section 2 gives an overview of models
and their appropriate use. Section 3 provides specific guidance on
the use of ``preferred'' air quality models and on the selection of
alternative techniques. Sections 4 through 7 provide recommendations
on modeling techniques for application to simple-terrain stationary
source problems, complex terrain problems, and mobile source
problems. Specific modeling requirements for selected regulatory
issues are also addressed. Section 8 discusses issues common to many
modeling analyses, including acceptable model components. Section 9
makes recommendations for data inputs to models including source,
meteorological and background air quality data. Section 10 covers
the uncertainty in model estimates and how that information can be
useful to the regulatory decision-maker. The last chapter summarizes
how estimates and measurements of air quality are used in assessing
source impact and in evaluating control strategies.
i. Appendix W to 40 CFR Part 51 itself contains an appendix:
Appendix A. Thus, when reference is made to ``Appendix A'' in this
document, it refers to Appendix A to Appendix W to 40 CFR Part 51.
Appendix A contains summaries of refined air quality models that are
``preferred'' for specific applications; both EPA models and models
developed by others are included.

2.0 Overview of Model Use

a. Before attempting to implement the guidance contained in this
document, the reader should be aware of certain general information
concerning air quality models and their use. Such information is
provided in this section.

2.1 Suitability of Models

a. The extent to which a specific air quality model is suitable
for the evaluation of source impact depends upon several factors.
These include: (1) The meteorological and topographic complexities
of the area; (2) the level of detail and accuracy needed for the
analysis; (3) the technical competence of those undertaking such
simulation modeling; (4) the resources available; and (5) the detail
and accuracy of the data base, i.e., emissions inventory,
meteorological data, and air quality data. Appropriate data should
be available before any attempt is made to apply a model. A model
that requires detailed, precise, input data should not be used when
such data are unavailable. However, assuming the data are adequate,
the greater the detail with which a model considers the spatial and
temporal variations in emissions and meteorological conditions, the
greater the ability to evaluate the source impact and to distinguish
the effects of various control strategies.
b. Air quality models have been applied with the most accuracy,
or the least degree of uncertainty, to simulations of long term
averages in areas with relatively simple topography. Areas subject
to major topographic influences experience meteorological
complexities that are extremely difficult to simulate. Although
models are available for such circumstances, they are frequently
site specific and resource intensive. In the absence of a model
capable of simulating such complexities, only a preliminary
approximation may be feasible until such time as better models and
data bases become available.
c. Models are highly specialized tools. Competent and
experienced personnel are an essential prerequisite to the
successful application of simulation models. The need for
specialists is critical when the more sophisticated models are used
or the area being investigated has complicated meteorological or
topographic features. A model applied improperly, or with
inappropriate data, can lead to serious misjudgements regarding the
source impact or the effectiveness of a control strategy.
d. The resource demands generated by use of air quality models
vary widely depending on the specific application. The resources
required depend on the nature of the model and its complexity, the
detail of the data base, the difficulty of the application, and the
amount and level of expertise required. The costs of manpower and
computational facilities may also be important factors in the
selection and use of a model for a specific analysis. However, it
should be recognized that under some sets of physical circumstances
and accuracy requirements, no present model may be appropriate.
Thus, consideration of these factors should lead to selection of an
appropriate model.

2.2 Levels of Sophistication of Models

a. There are two levels of sophistication of models. The first
level consists of relatively simple estimation techniques that
generally use preset, worst-case meteorological conditions to
provide conservative estimates of the air quality impact of a
specific source, or source category. These are called screening
techniques or screening models. The purpose of such techniques is to
eliminate the need of more detailed modeling for those sources that
clearly will not cause or contribute to ambient concentrations in
excess of either the National Ambient Air Quality Standards
(NAAQS)\4\ or the allowable prevention of significant deterioration
(PSD) concentration increments.2,3 If a screening
technique indicates that the concentration contributed by the source
exceeds the PSD increment or the increment remaining to just meet
the NAAQS, then the second level of more sophisticated models should
be applied.
b. The second level consists of those analytical techniques that
provide more detailed treatment of physical and chemical atmospheric
processes, require more detailed and precise input data, and provide
more specialized concentration estimates. As a result they provide a
more refined and, at least theoretically, a more accurate estimate
of source impact and the effectiveness of control strategies. These
are referred to as refined models.
c. The use of screening techniques followed, as appropriate, by
a more refined analysis is always desirable, however there are
situations where the screening techniques are practically and
technically the only viable option for estimating source impact. In
such cases, an attempt should be made to acquire or improve the
necessary data bases and to develop appropriate analytical
techniques.

2.3 Availability of Models

a. For most of the screening and refined models discussed in the
Guideline, codes, associated documentation and other useful
information are available for download from EPA's Support Center for
Regulatory Air Modeling (SCRAM) Internet Web site at
http://www.epa.gov/scram001. A list of

[[Page 18451]]

alternate models that can be used with case-by-case justification
(subsection 3.2) and an example air quality analysis checklist are
also posted on this Web site. This is a site with which modelers
should become familiar.

3.0 Recommended Air Quality Models

a. This section recommends the approach to be taken in
determining refined modeling techniques for use in regulatory air
quality programs. The status of models developed by EPA, as well as
those submitted to EPA for review and possible inclusion in this
guidance, is discussed. The section also addresses the selection of
models for individual cases and provides recommendations for
situations where the preferred models are not applicable. Two
additional sources of modeling guidance are the Model Clearinghouse
and periodic Regional/State/Local Modelers workshops.
b. In this guidance, when approval is required for a particular
modeling technique or analytical procedure, we often refer to the
``appropriate reviewing authority''. In some EPA regions, authority
for NSR and PSD permitting and related activities has been delegated
to State and even local agencies. In these cases, such agencies are
``representatives'' of the respective regions. Even in these
circumstances, the Regional Office retains the ultimate authority in
decisions and approvals. Therefore, as discussed above and depending
on the circumstances, the appropriate reviewing authority may be the
Regional Office, Federal Land Manager(s), State agency(ies), or
perhaps local agency(ies). In cases where review and approval comes
solely from the Regional Office (sometimes stated as ``Regional
Administrator''), this will be stipulated. If there is any question
as to the appropriate reviewing authority, you should contact the
Regional modeling contact (http://www.epa.gov/scram001/tt28.htm
#regionalmodelingcontacts) in the appropriate EPA Regional
Office, whose jurisdiction generally includes the physical location
of the source in question and its expected impacts.
c. In all regulatory analyses, especially if other than
preferred models are selected for use, early discussions among
Regional Office staff, State and local control agencies, industry
representatives, and where appropriate, the Federal Land Manager,
are invaluable and are encouraged. Agreement on the data base(s) to
be used, modeling techniques to be applied and the overall technical
approach, prior to the actual analyses, helps avoid
misunderstandings concerning the final results and may reduce the
later need for additional analyses. The use of an air quality
analysis checklist, such as is posted on EPA's Internet SCRAM Web
site (subsection 2.3), and the preparation of a written protocol
help to keep misunderstandings at a minimum.
d. It should not be construed that the preferred models
identified here are to be permanently used to the exclusion of all
others or that they are the only models available for relating
emissions to air quality. The model that most accurately estimates
concentrations in the area of interest is always sought. However,
designation of specific models is needed to promote consistency in
model selection and application.
e. The 1980 solicitation of new or different models from the
technical community and the program whereby these models were
evaluated, established a means by which new models are identified,
reviewed and made available in the Guideline. There is a pressing
need for the development of models for a wide range of regulatory
applications. Refined models that more realistically simulate the
physical and chemical process in the atmosphere and that more
reliably estimate pollutant concentrations are needed. Thus, the
solicitation of models is considered to be continuous.

3.1 Preferred Modeling Techniques

3.1.1 Discussion

a. EPA has developed models suitable for regulatory application.
Other models have been submitted by private developers for possible
inclusion in the Guideline. These refined models have undergone
evaluation exercises 7,8,9,10,11,12,13,14,15 that include
statistical measures of model performance in comparison with
measured air quality data as suggested by the American
Meteorological Society \16\ and, where possible, peer scientific
reviews. \17,18,19,20,21\
b. When a single model is found to perform better than others,
it is recommended for application as a preferred model and listed in
Appendix A. If no one model is found to clearly perform better
through the evaluation exercise, then the preferred model listed in
Appendix A is selected on the basis of other factors such as past
use, public familiarity, cost or resource requirements, and
availability. No further evaluation of a preferred model is required
for a particular application if the EPA recommendations for
regulatory use specified for the model in the Guideline are
followed. Alternative models to those listed in Appendix A should
generally be compared with measured air quality data when they are
used for regulatory applications consistent with recommendations in
subsection 3.2.
c. The solicitation of new refined models which are based on
sounder scientific principles and which more reliably estimate
pollutant concentrations is considered by EPA to be continuous.
Models that are submitted in accordance with the established
provisions will be evaluated as submitted. These requirements are:
i. The model must be computerized and functioning in a common
computer code suitable for use on a variety of computer systems.
ii. The model must be documented in a user's guide which
identifies the mathematics of the model, data requirements and
program operating characteristics at a level of detail comparable to
that available for currently recommended models.
iii. The model must be accompanied by a complete test data set
including input parameters and output results. The test data must be
included in the user's guide as well as provided in computer-
readable form.
iv. The model must be useful to typical users, e.g., State air
pollution control agencies, for specific air quality control
problems. Such users should be able to operate the computer
program(s) from available documentation.
v. The model documentation must include a comparison with air
quality data (and/or tracer measurements) or with other well-
established analytical techniques.
vi. The developer must be willing to make the model available to
users at reasonable cost or make it available for public access
through the Internet or National Technical Information Service: the
model cannot be proprietary.
d. The evaluation process will include a determination of
technical merit, in accordance with the above six items including
the practicality of the model for use in ongoing regulatory
programs. Each model will also be subjected to a performance
evaluation for an appropriate data base and to a peer scientific
review. Models for wide use (not just an isolated case) that are
found to perform better will be proposed for inclusion as preferred
models in future Guideline revisions.

3.1.2 Recommendations

a. Appendix A identifies refined models that are preferred for
use in regulatory applications. If a model is required for a
particular application, the user should select a model from that
appendix. These models may be used without a formal demonstration of
applicability as long as they are used as indicated in each model
summary of Appendix A. Further recommendations for the application
of these models to specific source problems are found in subsequent
sections of the Guideline.
b. If changes are made to a preferred model without affecting
the concentration estimates, the preferred status of the model is
unchanged. Examples of modifications that do not affect
concentrations are those made to enable use of a different computer
or those that affect only the format or averaging time of the model
results. However, when any changes are made, the Regional
Administrator should require a test case example to demonstrate that
the concentration estimates are not affected.
c. A preferred model should be operated with the options listed
in Appendix A as ``Recommendations for Regulatory Use.'' If other
options are exercised, the model is no longer ``preferred.'' Any
other modification to a preferred model that would result in a
change in the concentration estimates likewise alters its status as
a preferred model. Use of the model must then be justified on a
case-by-case basis.

3.2 Use of Alternative Models

3.2.1 Discussion

a. Selection of the best techniques for each individual air
quality analysis is always encouraged, but the selection should be
done in a consistent manner. A simple listing of models in this
guide cannot alone achieve that consistency nor can it necessarily
provide the best model for all possible situations. EPA reports
22,23 are available to assist in developing a consistent
approach when justifying the use of other than the preferred
modeling techniques recommended

[[Page 18452]]

in the Guideline. An ASTM reference 24 provides a general
philosophy for developing and implementing advanced statistical
evaluations of atmospheric dispersion models, and provides an
example statistical technique to illustrate the application of this
philosophy. An EPA reference 25 provides a statistical
technique for evaluating model performance for predicting peak
concentration values, as might be observed at individual monitoring
locations. In many cases, this protocol should be considered
preferentially to the material in Chapter 3 of reference 22. The
procedures in these documents provide a general framework for
objective decision-making on the acceptability of an alternative
model for a given regulatory application. The documents contain
procedures for conducting both the technical evaluation of the model
and the field test or performance evaluation.
b. This section discusses the use of alternate modeling
techniques and defines three situations when alternative models may
be used.

3.2.2 Recommendations

a. Determination of acceptability of a model is a Regional
Office responsibility. Where the Regional Administrator finds that
an alternative model is more appropriate than a preferred model,
that model may be used subject to the recommendations of this
subsection. This finding will normally result from a determination
that (1) a preferred air quality model is not appropriate for the
particular application; or (2) a more appropriate model or
analytical procedure is available and applicable.
b. An alternative model should be evaluated from both a
theoretical and a performance perspective before it is selected for
use. There are three separate conditions under which such a model
may normally be approved for use: (1) If a demonstration can be made
that the model produces concentration estimates equivalent to the
estimates obtained using a preferred model; (2) if a statistical
performance evaluation has been conducted using measured air quality
data and the results of that evaluation indicate the alternative
model performs better for the given application than a comparable
model in Appendix A; or (3) if the preferred model is less
appropriate for the specific application, or there is no preferred
model. Any one of these three separate conditions may make use of an
alternative model acceptable. Some known alternative models that are
applicable for selected situations are listed on EPA's SCRAM
Internet Web site (subsection 2.3). However, inclusion there does
not confer any unique status relative to other alternative models
that are being or will be developed in the future.
c. Equivalency, condition (1) in paragraph (b) of this
subsection, is established by demonstrating that the maximum or
highest, second highest concentrations are within 2 percent of the
estimates obtained from the preferred model. The option to show
equivalency is intended as a simple demonstration of acceptability
for an alternative model that is so nearly identical (or contains
options that can make it identical) to a preferred model that it can
be treated for practical purposes as the preferred model. Two
percent was selected as the basis for equivalency since it is a
rough approximation of the fraction that PSD Class I increments are
of the NAAQS for SO\2\, i.e., the difference in concentrations that
is judged to be significant. However, notwithstanding this
demonstration, models that are not equivalent may be used when one
of the two other conditions described in paragraphs (d) and (e) of
this subsection are satisfied.
d. For condition (2) in paragraph (b) of this subsection, the
procedures and techniques for determining the acceptability of a
model for an individual case based on superior performance are
contained in references 22-25 should be followed, as appropriate.
Preparation and implementation of an evaluation protocol which is
acceptable to both control agencies and regulated industry is an
important element in such an evaluation.
e. Finally, for condition (3) in paragraph (b) of this
subsection, an alternative refined model may be used provided that:
i. The model has received a scientific peer review;
ii. The model can be demonstrated to be applicable to the
problem on a theoretical basis;
iii. The data bases which are necessary to perform the analysis
are available and adequate;
iv. Appropriate performance evaluations of the model have shown
that the model is not biased toward underestimates; and
v. A protocol on methods and procedures to be followed has been
established.

3.3 Availability of Supplementary Modeling Guidance

a. The Regional Administrator has the authority to select models
that are appropriate for use in a given situation. However, there is
a need for assistance and guidance in the selection process so that
fairness and consistency in modeling decisions is fostered among the
various Regional Offices and the States. To satisfy that need, EPA
established the Model Clearinghouse \5\ and also holds periodic
workshops with headquarters, Regional Office, State, and local
agency modeling representatives.
b. The Regional Office should always be consulted for
information and guidance concerning modeling methods and
interpretations of modeling guidance, and to ensure that the air
quality model user has available the latest most up-to-date policy
and procedures. As appropriate, the Regional Office may request
assistance from the Model Clearinghouse after an initial evaluation
and decision has been reached concerning the application of a model,
analytical technique or data base in a particular regulatory action.

4.0 Simple-Terrain Stationary Source Models

4.1 Discussion

a. Simple terrain, as used here, is considered to be an area
where terrain features are all lower in elevation than the top of
the stack of the source(s) in question. The models recommended in
this section are generally used in the air quality impact analysis
of stationary sources for most criteria pollutants. The averaging
time of the concentration estimates produced by these models ranges
from 1 hour to an annual average.
b. In the early 1980s, model evaluation exercises were conducted
to determine the ``best, most appropriate point source model'' for
use in simple terrain.8,17 No one model was found to be
clearly superior and, based on past use, public familiarity, and
availability, ISC (predecessor to ISC3 \26\) became the recommended
model for a wide range of regulatory applications. Other refined
models which also employed the basic Gaussian kernel, i.e., BLP,
CALINE3, OCD, and EDMS, were developed for specialized applications
(Appendix A). Performance evaluations were also made for these
models, which are identified in Appendix A.

4.2 Recommendations

4.2.1 Screening Techniques

a. Where a preliminary or conservative estimate is desired,
point source screening techniques are an acceptable approach to air
quality analyses. EPA has published guidance for screening
procedures,\27\ and a computerized version of the recommended
screening technique, SCREEN3, is available.\28\
b. All screening procedures should be adjusted to the site and
problem at hand. Close attention should be paid to whether the area
should be classified urban or rural in accordance with subsection
8.2.3. The climatology of the area should be studied to help define
the worst-case meteorological conditions. Agreement should be
reached between the model user and the appropriate reviewing
authority (paragraph 3.0(b)) on the choice of the screening model
for each analysis, and on the input data as well as the ultimate use
of the results.

4.2.2 Refined Analytical Techniques

a. A brief description of preferred models for refined
applications is found in Appendix A. Also listed in that appendix
are the model input requirements, the standard options that should
be selected when running the program, and output options.
b. When modeling for compliance with short term NAAQS and PSD
increments is of primary concern, a short term model may be used to
provide long term concentration estimates. The conversion from long
term to short term concentration averages by any transformation
technique is not acceptable in regulatory applications.
c. The state-of-the-science for modeling atmospheric deposition
is evolving and the best techniques are currently being assessed and
their results are being compared with observations. Consequently,
the approach taken for any purpose should be coordinated with the
appropriate reviewing authority (paragraph 3.0(b)).

5.0 Model Use in Complex Terrain

5.1 Discussion

a. For the purpose of the Guideline, complex terrain is defined
as terrain exceeding the height of the stack being

[[Page 18453]]

modeled. Complex terrain dispersion models are normally applied to
stationary sources of pollutants such as SO₂ and
particulates.
b. A major outcome from the EPA Complex Terrain Model
Development project has been the publication of a refined dispersion
model (CTDM) suitable for regulatory application to plume impaction
assessments in complex terrain.\29\ Although CTDM as originally
produced was only applicable to those hours characterized as neutral
or stable, a computer code for all stability conditions--CTDMPLUS--
together with a user's guide,\30\ and site specific meteorological
and terrain data processors \31,32\ is available. Moreover,
CTSCREEN,\33\ a version of CTDMPLUS that does not require site
specific meteorological data inputs, is also available as a
screening technique.
c. The methods discussed in this section should be considered in
two categories: (1) Screening techniques, and (2) the refined
dispersion model, CTDMPLUS, discussed in this subsection and listed
in Appendix A.
d. Continued improvements in ability to accurately model plume
dispersion in complex terrain situations can be expected, e.g., from
research on lee side effects due to terrain obstacles. New
approaches to improve the ability of models to realistically
simulate atmospheric physics, e.g., hybrid models which incorporate
an accurate wind field analysis, will ultimately provide more
appropriate tools for analyses. Such hybrid modeling techniques are
also acceptable for regulatory applications after the appropriate
demonstration and evaluation.\22\

5.2 Recommendations

a. Recommendations in this section apply primarily to those
situations where the impaction of plumes on terrain at elevations
equal to or greater than the plume centerline during stable
atmospheric conditions are determined to be the problem. If a
violation of any NAAQS or the controlling increment is indicated by
using any of the preferred screening techniques, then a refined
complex terrain model may be used. Phenomena such as fumigation,
wind direction shear, lee-side effects, building wake- or terrain-
induced downwash, deposition, chemical transformation, variable
plume trajectories, and long range transport are not addressed by
the recommendations in this section.
b. Where site specific data are used for either screening or
refined complex terrain models, a data base of at least 1 full-year
of meteorological data is preferred. If more data are available,
they should be used. Meteorological data used in the analysis should
be reviewed for both spatial and temporal representativeness.
c. Placement of receptors requires very careful attention when
modeling in complex terrain. Often the highest concentrations are
predicted to occur under very stable conditions, when the plume is
near, or impinges on, the terrain. The plume under such conditions
may be quite narrow in the vertical, so that even relatively small
changes in a receptor's location may substantially affect the
predicted concentration. Receptors within about a kilometer of the
source may be even more sensitive to location. Thus, a dense array
of receptors may be required in some cases. In order to avoid
excessively large computer runs due to such a large array of
receptors, it is often desirable to model the area twice. The first
model run would use a moderate number of receptors carefully located
over the area of interest. The second model run would use a more
dense array of receptors in areas showing potential for high
concentrations, as indicated by the results of the first model run.
d. When CTSCREEN or CTDMPLUS is used, digitized contour data
must be first processed by the CTDM Terrain Processor \32\ to
provide hill shape parameters in a format suitable for direct input
to CTDMPLUS. Then the user supplies receptors either through an
interactive program that is part of the model or directly, by using
a text editor; using both methods to select receptors will generally
be necessary to assure that the maximum concentrations are estimated
by either model. In cases where a terrain feature may ``appear to
the plume'' as smaller, multiple hills, it may be necessary to model
the terrain both as a single feature and as multiple hills to
determine design concentrations.
e. The user is encouraged to confer with the Regional Office if
any unresolvable problems are encountered with any screening or
refined analytical procedures, e.g., meteorological data, receptor
siting, or terrain contour processing issues.

5.2.1 Screening Techniques

a. CTSCREEN \33\ can be used to obtain conservative, yet
realistic, worst-case estimates for receptors located on terrain
above stack height. CTSCREEN accounts for the three-dimensional
nature of plume and terrain interaction and requires detailed
terrain data representative of the modeling domain. The model
description and user's instructions are contained in the user's
guide.\33\ The terrain data must be digitized in the same manner as
for CTDMPLUS and a terrain processor is available.\32\ A discussion
of the model's performance characteristics is provided in a
technical paper.\34\ CTSCREEN is designed to execute a fixed matrix
of meteorological values for wind speed (u), standard deviation of
horizontal and vertical wind speeds ([sigma]_v,
[sigma]_w), vertical potential temperature gradient
(d[thetas]/dz), friction velocity (u_*), Monin-Obukhov
length (L), mixing height (z_i) as a function of terrain
height, and wind directions for both neutral/stable conditions and
unstable convective conditions. Table 5-1 contains the matrix of
meteorological variables that is used for each CTSCREEN analysis.
There are 96 combinations, including exceptions, for each wind
direction for the neutral/stable case, and 108 combinations for the
unstable case. The specification of wind direction, however, is
handled internally, based on the source and terrain geometry.
Although CTSCREEN is designed to address a single source scenario,
there are a number of options that can be selected on a case-by-case
basis to address multi-source situations. However, the appropriate
reviewing authority (paragraph 3.0(b)) should be consulted, and
concurrence obtained, on the protocol for modeling multiple sources
with CTSCREEN to ensure that the worst case is identified and
assessed. The maximum concentration output from CTSCREEN represents
a worst-case 1-hour concentration. Time-scaling factors of 0.7 for
3-hour, 0.15 for 24-hour and 0.03 for annual concentration averages
are applied internally by CTSCREEN to the highest 1-hour
concentration calculated by the model.
b. Placement of receptors requires very careful attention when
modeling in complex terrain. Often the highest concentrations are
predicted to occur under very stable conditions, when the plume is
near, or impinges on, the terrain. The plume under such conditions
may be quite narrow in the vertical, so that even relatively small
changes in a receptor's location may substantially affect the
predicted concentration. Receptors within about a kilometer of the
source may be even more sensitive to location. Thus, a dense array
of receptors may be required in some cases. In order to avoid
excessively large computer runs due to such a large array of
receptors, it is often desirable to model the area twice. The first
model run would use a moderate number of receptors carefully located
over the area of interest. The second model run would use a more
dense array of receptors in areas showing potential for high
concentrations, as indicated by the results of the first model run.
c. As mentioned above, digitized contour data must be
preprocessed \32\ to provide hill shape parameters in suitable input
format. The user then supplies receptors either through an
interactive program that is part of the model or directly, by using
a text editor; using both methods to select receptors will generally
be necessary to assure that the maximum concentrations are estimated
by either model. In cases where a terrain feature may ``appear to
the plume'' as smaller, multiple hills, it may be necessary to model
the terrain both as a single feature and as multiple hills to
determine design concentrations.
d. Other screening techniques, e.g., Valley (as implemented in
SCREEN3 \28\), COMPLEX I (as implemented in ISC3 \26\), SHORTZ/LONGZ
\35\, and RTDM \36\ may be acceptable for complex terrain cases
where established procedures are used. The user is encouraged to
confer with the appropriate reviewing authority (paragraph 3.0(b))
if any unresolvable problems are encountered, e.g., applicability,
meteorological data, receptor siting, or terrain contour processing
issues.

5.2.2 Refined Analytical Techniques

a. When the results of the screening analysis demonstrate a
possible violation of NAAQS or the controlling PSD increments, a
more refined analysis may need to be conducted.
b. The Complex Terrain Dispersion Model PLus Algorithms for
Unstable Situations (CTDMPLUS) is a refined air quality model that
is preferred for use in all stability conditions for complex terrain
applications. CTDMPLUS is a sequential model that requires five
input files: (1) General program specifications; (2) a terrain data
file; (3) a receptor file; (4) a surface meteorological data file;
and (5) a user created meteorological profile data file. Two
optional input files consist of hourly emissions parameters and a
file containing upper air data from rawinsonde data files, e.g., a
National Climatic Data Center TD-6201 file, unless

[[Page 18454]]

there are no hours categorized as unstable in the record. The model
description and user instructions are contained in Volume 1 of the
User's Guide.\30\ Separate publications 32,31 describe
the terrain preprocessor system and the meteorological preprocessor
program. In Part I of a technical article \37\ is a discussion of
the model and its preprocessors; the model's performance
characteristics are discussed in Part II of the same article.\38\
The size of the CTDMPLUS executable file on a personal computer is
approximately 360K bytes. The model produces hourly average
concentrations of stable pollutants, i.e., chemical transformation
or decay of species and settling/deposition are not simulated. To
obtain concentration averages corresponding to the NAAQS, e.g., 3-
or 24-hour, or annual averages, the user must execute a
postprocessor program such as CHAVG. CTDMPLUS is applicable to all
receptors on terrain elevations above stack top. However, the model
contains no algorithms for simulating building downwash or the
mixing or recirculation found in cavity zones in the lee of a hill.
The path taken by a plume through an array of hills cannot be
simulated. CTDMPLUS does not explicitly simulate calm meteorological
periods, and for those situations the user should follow the
guidance in subsection 9.3.4. The user should follow the
recommendations in the User's Guide under General Program
Specifications for: (1) Selecting mixed layer heights, (2) setting
minimum scalar wind speed to 1 m/s, and (3) scaling wind direction
with height. Close coordination with the Regional Office is
essential to insure a consistent, technically sound application of
this model.
c. The performance of CTDMPLUS is greatly improved by the use of
meteorological data from several levels up to plume height. However,
due to the vast range of source-plume-hill geometries possible in
complex terrain, detailed requirements for meteorological monitoring
in support of refined analyses using CTDMPLUS should be determined
on a case-by-case basis. The following general guidance should be
considered in the development of a meteorological monitoring
protocol for regulatory applications of CTDMPLUS and reviewed in
detail by the Regional Office before initiating any monitoring. As
appropriate, EPA guidance (see reference 100) should be consulted
for specific guidance on siting requirements for meteorological
towers, selection and exposure of sensors, etc. As more experience
is gained with the model in a variety of circumstances, more
specific guidance may be developed.
d. Site specific meteorological data are critical to dispersion
modeling in complex terrain and, consequently, the meteorological
requirements are more demanding than for simple terrain. Generally,
three different meteorological files (referred to as surface,
profile, and rawin files) are needed to run CTDMPLUS in a regulatory
mode.
e. The surface file is created by the meteorological
preprocessor (METPRO) \31\ based on site specific measurements or
estimates of solar and/or net radiation, cloud cover and ceiling,
and the mixed layer height. These data are used in METPRO to
calculate the various surface layer scaling parameters (roughness
length, friction velocity, and Monin-Obukhov length) which are
needed to run the model. All of the user inputs required for the
surface file are based either on surface observations or on
measurements at or below 10m.
f. The profile data file is prepared by the user with site
specific measurements (from at least three levels) of wind speed,
wind direction, turbulence, and potential temperature. These
measurements should be obtained up to the representative plume
height(s) of interest (i.e., the plume height(s) under those
conditions important to the determination of the design
concentration). The representative plume height(s) of interest
should be determined using an appropriate complex terrain screening
procedure (e.g., CTSCREEN) and should be documented in the
monitoring/modeling protocol. The necessary meteorological
measurements should be obtained from an appropriately sited
meteorological tower augmented by SODAR if the representative plume
height(s) of interest exceed 100m. The meteorological tower need not
exceed the lesser of the representative plume height of interest
(the highest plume height if there is more than one plume height of
interest) or 100m.
g. Locating towers on nearby terrain to obtain stack height or
plume height measurements for use in profiles by CTDMPLUS should be
avoided unless it can clearly be demonstrated that such measurements
would be representative of conditions affecting the plume.
h. The rawin file is created by a second meteorological
preprocessor (READ62) \31\ based on NWS (National Weather Service)
upper air data. The rawin file is used in CTDMPLUS to calculate
vertical potential temperature gradients for use in estimating plume
penetration in unstable conditions. The representativeness of the
off-site NWS upper air data should be evaluated on a case-by-case
basis.
i. In the absence of an appropriate refined model, screening
results may need to be used to determine air quality impact and/or
emission limits.

Table 5-1a.--Neutral/Stable Meteorological Matrix for CTSCREEN
----------------------------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------
Variable Specific values
------------------------------------------------------
U (m/s).............................................. 1.0 2.0 3.0 4.0 5.0
[sigma]_v (m/s)....................................... 0.3 0.75 .......... .......... .........
[sigma]_w (m/s)....................................... 0.08 0.15 0.30 0.75 .........
[Delta][thetas]/[Delta]z (K/m)....................... 0.01 0.02 0.035 .......... .........
WD................................................... (Wind direction optimized internally for each
meteorological combination)
----------------------------------------------------------------------------------------------------------------
Exceptions:
(1) If U <= 2 m/s and [sigma]_v <= 0.3 m/s, then include [sigma]_w = 0.04 m/s.
(2) If [sigma]_w = 0.75 m/s and U >= 3.0 m/s, then [Delta][thetas]/[Delta]z is limited to <= 0.01 K/m.
(3) If U £= 4 m/s, then [sigma]w £= 0.15 m/s.
(4) [sigma]_w <= [sigma]_v

Table 5-1b.--Unstable/Convective Meteorological Matrix for CTSCREEN
----------------------------------------------------------------------------------------------------------------

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