# Requirements for Preparation, Adoption, and Submittal of State Implementation Plans (Guideline on Air Quality Models)

> Briefs, arguments, decisions, and more.

URL: https://www.frixlaw.com/law-library/documents/fr%3A00-4235

## Record

- **Collection:** Federal Register
- **Document type:** Proposed Rule
- **Published:** April 21, 2000
- **Citation:** 65 FR 21506

## Text

ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 51
[AH-FRL-6536-3]
RIN 2060-AF01
Requirements for Preparation, Adoption, and Submittal of State Implementation Plans (Guideline on Air Quality Models)

AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Proposed rule.

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 propose to make several additions and changes to the Guideline. We recommend two new dispersion models, AERMOD and CALPUFF, for adoption in appendix A of the
Guideline.
AERMOD would replace the Industrial Source Complex (ISC3) model in many assessments that now use it; AERMOD also would apply to complex terrain. CALPUFF would become a recommended technique for assessing long-range transport of pollutants and their impacts on Federal Class I areas. We revise two existing models: ISC3, by incorporating a new downwash algorithm (PRIME) and renaming the model ISC-PRIME, and the Emissions Dispersion Modeling System (EDMS), by incorporating improved emissions and dispersion modules. We make various editorial changes to update and reorganize information, and remove obsolete models (CDM, RAM and UAM).

DATES:

The period for comment on these proposed changes to the
Guideline
closes on July 20, 2000. We plan to hold a public hearing on the proposed changes in Summer 2000. The specific date and time will be announced in a separate document published in the
Federal Register
.

ADDRESSES:

We have established an official record for this rulemaking under docket number A-99-05. You may submit comments pertinent to this proposal to docket no. A-99-05 at the following address: Air Docket (6102), Room M-1500, Waterside Mall, U.S. Environmental Protection Agency, 401 M Street, S.W., Washington, DC. 20460. This docket is available for public inspection and copying between 8 a.m. and 5:30 p.m., Monday through Friday, at the address above. Please furnish duplicate comments to Tom Coulter, Air Quality Modeling Group (MD-14), U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. You may send electronic versions of comments pertinent to this proposal to: A-AND-R-DOCKET@epamail.epa.gov. Alternatively, comments are acceptable in WordPerfect 6.1 (or higher), preferably zipped (
e.g.,
PKware) as an attachment to the e-mail message. You must include the docket identification (A-99-05) with all electronic submittals. You may file electronic comments on this proposal online at many Federal Depository Libraries.

The hearing will be the main agenda for the 7th Conference on Air Quality Modeling, and the location will be announced in a separate document published in the
Federal Register
.

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) 541-5561 or C. Thomas Coulter, telephone (919) 541-0832.

SUPPLEMENTARY INFORMATION:

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.

Air Quality Modeling Conference

We held the Sixth Conference on Air Quality Modeling (6th conference) in Washington, DC on August 9-10, 1995. As required by Section 320 of the Clean Air Act, these conferences take place approximately every three years to standardize modeling procedures. The sixth conference featured presentations in several key modeling areas. One presentation, by the Interagency Workgroup on Air Quality Modeling (IWAQM
1

), covered long range transport modeling. Another presentation, by the American Meteorological Society (AMS)/EPA Regulatory Model Improvement Committee (AERMIC), covered developing an enhanced Gaussian dispersion model with boundary layer parameterization: AERMOD
2

. Also at the 6th conference, the Electric Power Research Institute (EPRI) presented recent research efforts to better define and characterize dispersion around buildings (downwash effects). These efforts were part of a program called the Plume RIse Model Enhancements (PRIME), and PRIME is proposed for integration within ISC3 (ISC-PRIME).

1
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.

2
AMS/EPA Regulatory MODel

The presentations were followed by a critical review/discussion of the CALPUFF and AERMOD modeling systems, 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. For the new and revised models described, we asked the public to address the following questions:

• What is the scientific merit of the models presented?

• What is their accuracy?

• What should be the regulatory use of individual models for specific applications?

• What implementation issues are apparent and what additional guidance is needed?

• What are the resource requirements of modeling systems presented?

• What additional information or analyses are needed?

We placed a transcript of the 6th conference proceedings and a copy of all written comments in Docket AQM-95-01. Answers to the above questions are reflected in the comments, which we reviewed and summarized (II-G-01). To the extent possible, we believe we have addressed the main concerns in the refinements proposed today, which focus on the two new modeling systems, as well as the enhancement of ISC3 with EPRI's PRIME downwash model (ISC-PRIME).

AERMOD

AERMOD is a state-of-the-practice Gaussian plume dispersion model whose formulation is based on planetary boundary layer principles. At the 6th conference, AERMIC members presented interim developmental and

evaluation results of AERMOD. AERMOD provides better characterization of plume dispersion than does the ISC3. Comprehensive comments were submitted on the AERMOD code and formulation document and on the AERMET draft User's Guide (AERMET is the meteorological preprocessor for AERMOD). The comments on the AERMET User's Guide were detailed and generally editorial in nature. Comments on AERMOD identified inconsistencies in the AERMOD code as well as among variables and recommended specific default values.

Commenters expressed concern that data bases historically used by EPA lack the variables required by AERMET and AERMOD. The deficiencies were thought to obstruct or weaken AERMOD's evaluation. We disagree that the data bases used for the AERMOD evaluations (Kincaid, Lovett, Martins Creek, Tracy, etc.) were not of the type used historically by EPA and furthermore believe that they contain the critical variables needed by AERMOD. One comment described a perceived “persistence of modeling procedures [by EPA] rather than an evolution to other techniques.” This tendency, the commenter believes, has been influenced by testing candidate techniques with the deficient data bases mentioned earlier. According to the commenter, this leaves the new candidate technique no way to show its possible superiority over existing techniques. The commenter argued for a change in this pattern. We disagree with this criticism in that we believe AERMOD has been adequately tested and represents, through its formulations, a technical advancement over its predecessors.

CALPUFF

CALPUFF is a Lagrangian dispersion model that simulates pollutant releases as a continuous series of
puffs.
IWAQM carefully studied the potential regulatory application of CALPUFF in its Phase 1 report.
3

At the 6th conference, IWAQM recommended that EPA consider CALPUFF as a preferred technique for long-range air pollution transport assessments (for example, for federal Class I areas). In its Phase 2 report,
4

IWAQM has, to the extent possible, attempted to resolve the concern and criticism over applying the CALPUFF modeling system.

3
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.

4
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.

On the whole, comments appeared to support IWAQM's efforts to simplify and clarify the modeling methods for addressing long-range transport and dispersion. The comments endorsed IWAQM's recommendation to employ one model for all sources and distances. The comments also endorsed IWAQM's recommendation of an approach whereby a group of stakeholders is established that, through consensus, defines the modeling methods, inventories, data bases, and significance criteria to be applied in assessing impacts for a given Class I area. This activity would precede an actual regulatory assessment.

Comments suggested that the Level 1 screen described in IWAQM's Phase I interim recommendations was not working well and needed improvement. IWAQM has attempted to do this by developing a screening procedure that uses CALPUFF with ISC-type meteorological input data, and has shown the results to be conservative for the case(s) tested (see footnote 4).
5

However, the screening approach may not give conservative concentration estimates in all cases (see below).

5
Environmental Protection Agency, 1998. Analyses of the CALMET/CALPUFF Modeling System in a Screening Mode. EPA Publication No. EPA-454/R-98-010. Office of Air Quality Planning & Standards, Research Triangle Park, NC.

Comments suggested that more comparisons with tracer studies were needed for transport distances of 50-200km. IWAQM sponsored four such evaluations.

Commenters also sought clearer guidance on the limits of such modeling assessments, such as cases with intervening terrain between the sources and receptors of interest. IWAQM has attempted to make the modeling community(see footnote 4) aware that conducting a long-range transport assessment requires competent individuals, expert judgement, and strong interaction and coordination with the applicable reviewing authorities.

Comments suggested that comparisons were needed to assess whether CALPUFF can provide results similar to ISC3 and CTDMPLUS for steady-state meteorological conditions. We supported this work and examined CALPUFF for equivalency to ISC3,
6

both in a steady-state mode as well as non-steady-state (that is, when meteorological conditions varied hourly). For steady state conditions, CALPUFF mimicked ISC3 to a substantial degree. In non-steady state conditions, occurrences of calms and recirculations resulted in higher source impacts with CALPUFF than for ISC3 for most comparisons made.

6
Environmental Protection Agency, 1998. A Comparison of CALPUFF with ISC3. EPA Publication No. EPA-454/R-98-020. Office of Air Quality Planning & Standards, Reserch Triangle Park, NC.

ISC-PRIME

The development of PRIME by EPRI featured four key components: a field effort, laboratory modeling of fluids, developing model codes, and independently evaluating models.
7

The field measurements were made at a combustion turbine site in New Jersey in February and March 1994. Wind tunnel experiments have been done at EPA's Fluid Modeling Facility and at a facility at Monash University in Australia. PRIME is modular, it explicitly takes into account stack location and all three building dimensions, and attempts to model the shape of the ellipsoid cavity and the flow of the streamline descents over the top of the cavity. Plume rise calculations are enhanced to treat plumes that are not neutrally buoyant and have no vertical velocity. Unfortunately, at the time of the 6th modeling conference, evaluation work was incomplete and the PRIME code was unavailable for beta testing.

7
Schulman, L.L., D.G. Strimaitis, and J.S. Scire, 1999. Development and Evaluation of the PRIME Plume Rise and Building Downwash Model. 34pp. + 10 figures (submitted to Journal of the Air & Waste Management Association) (A-99-05, II-A-13).

Comments received at the 6th modeling conference commended EPRI's development of PRIME as “a significant improvement over the existing ISC algorithm” and one that could “provide accurate estimates for idealized building geometries.” Based on comments, potential problems were anticipated for proper treatment of the myriad combinations of building geometry, wind approach angle, upwind roughnesses, stabilities, etc. Commenters questioned whether all these effects could be parameterized into a robust algorithm to accurately treat downwash at actual sites. Another strong concern was the extent to which the algorithm would work under stable stratification, which is difficult to simulate in a wind tunnel. One commenter even suggested the application of a simpler approach,
i.e.
, the original work by Huber and Snyder who employed a “building downwash amplification factor”, as careful parameterization of this factor might

lead to acceptable accuracy with other benefits. The commenter also suggested that an integral plume rise model had been shown to yield good agreement with field and wind tunnel observations for treating plume trajectories. In terms of PRIME's evaluation, the commenter suggested using, as a basis for comparison, a version of ISC3 that excluded the Schulman-Scire downwash algorithm.

Since the 6th modeling conference, EPRI released a beta test version of PRIME, which was installed within ISC3 (hence, ISC-PRIME). Beta testing of ISC-PRIME shows significantly improved performance in comparison to ISC3.
8

To the extent possible, EPRI has attempted to address the comments on the PRIME algorithm and its documentation. A consequence analysis for using ISC-PRIME (versus ISC3) has also been prepared.
9

8
Paine, R.J. and F. Lew, 1997. Results of the Independent Evaluation of ISCST3 and ISC-PRIME. Prepared for the Electric Power Research Institute, Palo Alto, CA. ENSR Document Number 2460-026-440. (NTIS No. PB 98-156524)

9
Paine, R.J. and F. Lew, 1997. Consequence Analysis for ISC-PRIME. Prepared for the Electric Power Research Institute, Palo Alto, CA. ENSR Document Number 2460-026-450. (NTIS No. PB 98-156516)

Proposed Action

AERMOD

We propose revising section 4 of the
Guideline
to replace ISC3 by AERMOD as a state-of-the-practice technique for many air quality impact assessments. Applications for which AERMOD is suited are stated in subsequent sections of the
Guideline
and include assessment of plume impacts from traditional stationary sources in simple, intermediate, and complex terrain. In fact, since differentiation of simple versus complex terrain is unnecessary with AERMOD, we merged pertinent guidance in section 5 (Model Use in Complex Terrain) with that in section 4. You will find developmental, evaluation and peer scientific review references for AERMOD cited as appropriate. A model formulation document,
10

as well as a key evaluation reference for the AERMOD modeling system,
11

have been placed in the docket. We added a summary description of AERMOD to appendix A
12

of the
Guideline
, where you are directed to note additional evaluation references and a series of user's manuals. The essential codes, preprocessors, and test cases have been uploaded to our website (www.epa.gov/scram001; see 7th Conference).

10
Cimorelli, A.J., S.G. Perry, A. Venkatram, J.C. Weil, R.J. Paine, R.B. Wilson, R.F. Lee and W.D. Peters, 1998. AERMOD: Description of Model Formulation. (12/15/98 Draft Document) Prepared for Environmental Protection Agency, Research Triangle Park, NC. 113pp. (Docket No. A-99-05; II-A-1)

11
Paine, R.J., R.F. Lee, R.W. Brode, R.B. Wilson, A.J. Cimorelli, S.G., Perry, J.C. Weil, A. Venkatram and W.D. Peters, 1998: Model Evaluation Results for AERMOD (12/17/98 Draft). Prepared for Environmental Protection Agency, Research Triangle Park, NC. (Docket No. A-99-05, II-A-5)

12
Appendix A of appendix W is a repository for preferred, refined air quality models recommended for regulatory applications.

We invite your comment on whether we have reasonably addressed technical concerns and are on sound footing to recommend AERMOD for its intended applications. AERMOD lacks a general (all-terrain) screening tool, so we invite your comment on the practicality of using SCREEN3 as an interim tool for AERMOD and ISC-PRIME screening in simple terrain.

CALPUFF

In its Phase 2 recommendations, IWAQM recommended the CALPUFF modeling system 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 endorse its recommendation and are proposing CALPUFF for addition to appendix A of the
Guideline.
We have imposed conforming revisions to section 6 to recommend CALPUFF for regulatory applications involving long-range transport and have suggested a possible screening approach. We also propose CALPUFF for use for all downwind distances for those applications involving complex wind regimes, with case-by-case justification. Studies that support the above recommendations are summarized in IWAQM's Phase II Report (
op. cit.
).

The essential codes, utilities, preprocessors and test cases have been uploaded to the developers' Internet website (www.src.com/calpuff/calpuff1.htm). The documentation for CALMET and CALPUFF have been properly cited in the
Guideline
and are available from the aforementioned website. A peer review has also been cited and has been placed in the docket.

We solicit your comments on our proposal to recommend CALPUFF for its intended applications.

ISC-PRIME

We have proposed the use of ISC-PRIME
13

in section 4 of the
Guideline,
where we emphasize that if you are interested in treating aerodynamic downwash or dry deposition, ISC-PRIME is the recommended model. We have proposed editorial revisions in sections 5-7 of the
Guideline
to make it clear when use of ISC-PRIME is appropriate instead of AERMOD.

13
Schulman, L.L., D.G. Strimaitis, and J.S. Scire, 1997. Addendum to ISC3 User's Guide,
The PRIME Plume Rise and Building Downwash Model.
Prepared for the Electric Power Research Institute, Palo Alto, CA., Earth Tech Document A287. A-99-05, II-A-12)

The formulation and evaluation of the PRIME algorithm are described in open literature (
op. cit.
) The essential codes, utilities, and test cases have been uploaded to our website (www.epa.gov/scram001; see 7th Conference). We invite your comment on whether we are on sound footing to recommend use of ISC-PRIME as proposed.

We intend to consider AERMOD, ISC-PRIME, and CALPUFF as our recommended techniques for their intended applications (as specified in the
Guideline
) starting one year after we issue the final rule, and that the models be used in their regulatory default modes. The models
may
be used in the interim (
i.e.
, as soon as we issue the final rule). We invite your comment on the reasonableness of the timing of this implementation schedule.

We are aware that, where downwash is of concern, some potential users of AERMOD and ISC-PRIME might find joint application of the two models burdensome. We invite comment on this matter and seek input on alternative approaches that ensure that the latest science is used (as included in both AERMOD and PRIME) for regulatory modeling applications. One alternative considered by AERMIC is the direct inclusion of the PRIME algorithm in AERMOD. This effort, including testing, performance evaluation for the PRIME data bases, and peer scientific review, could take up to 12 months.

Proposed Editorial Changes

Editorial changes are described by affected sections. For a more detailed showing of before/after effects, you are referred to a redline/strikeout version (WordPerfect format) of appendix W that has been posted on our website (www.epa.gov/scram001; see 7th Conference).

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.

Section 3

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.

Section 4

As mentioned earlier, we revised section 4 to present AERMOD, ISC-PRIME, and CALPUFF as regulatory modeling techniques for particular applications. 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.
14

14
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

As mentioned above, we merged pertinent guidance in section 5 (Modeling in Complex Terrain) with that in section 4. With the anticipated widespread use of AERMOD for all terrain types, there is no longer any utility in the previous differentiation between simple and complex terrain for model selection. To further 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), and RTDM remain available for use in applicable cases where established/accepted procedures are used. Consultation with the appropriate Regional Office is still advised for application of these screening models.

Section 6

We revised section 6 (renumbered to section 5) 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). Footnotes have been inserted to provide caveats pertaining to the recent Court decision to remand or vacate parts of these new standards. You will note that we inserted respective subsections for particulate matter and lead from section 7, so that section 5 now primarily contains modeling guidance for the criteria pollutants regulated in Part 51 (SO
2
analyses are covered in section 4).

• 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.
15

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. We believe that it will frequently be necessary to consider the regional scale for such demonstrations and that, since the last revision to appendix W, newer models have become available. We invite comment on the need to integrate ozone and fine particle impacts (
i.e.,
the “one atmosphere” approach). Are modeling tools and air program policies sufficiently developed to provide guidance on an integrated approach at this time? We also invite comments on whether specific validated tools have been sufficiently developed to calculate impacts of individual point sources of ozone and PM-2.5 precursor pollutants. Are there any models that can be recommended for source-specific ozone and PM-2.5 assessments?

15
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 website, 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 NO
2
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
16

in section 5. 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 umbrella for possible use to develop and evaluate national policy and assist State and local control agencies. For long range transport analyses, we present and 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. Finally, in the subsection on air pathway analyses, we identify the availability of AERMOD and removed specific reference to DEGADIS (other heavy gas models are also available on a case-by-case basis).

16
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, D.C.

Section 8

We revised section 8 (renumbered to section 7) to better reflect our current regulatory practice for the general modeling considerations addressed.

• In subsection 7.2.4, we introduce the atmospheric stability characterization for AERMOD.

• In subsection 7.2.5, we describe the plume rise approaches used by AERMOD and ISC-PRIME.

• We revised subsection 7.2.6 to refer back to subsection 5.2.3 for details on chemical transformation of NO
X
.

• We merged subsection 7.2.8 (Urban/Rural Classification) with subsection 7.2.3 (Dispersion Coefficients).

• We merged discussions in subsections 7.2.9 (Fumigation) and 7.2.10 (Stagnation) into one new subsection (Complex Winds), and 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

We renumbered section 9 as section 8 and made the following changes:

• We revised subsection 8.2.3 (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). This petition,
Appalachian Power Company et al.
v.
EPA
(D.C. Circuit), No. 93-1631, was filed on November 3, 1993. The plaintiffs challenged the modeling assumptions required for existing point sources and new (or modified) existing point source compliance demonstrations as set forth in tables 9-1 and 9-2 of the
Guideline.
In accordance with the settlement, we are clarifying the definition of “nearby sources.” The “maximum allowable emission limit,” specified in Tables 8-1 and 8-1 (formerly 9-1 and 9-2), is tied in certain circumstances
17

to the emission rate representative of a nearby source's maximum physical capacity to emit. We are also clarifying 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 reviewing authority that this is, in fact, the case. We added footnotes to tables 8-1 and 8-2 to refer back to applicable paragraphs of subsection 8.2.3 that provide the necessary clarification.

17
See section 8.2.3. of the
Guideline.

• We enhanced section 8.3 (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
18, 19, 20

or MM5
21

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 8.3.1.2) for long-range transport and complex wind situations.

18
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-scal data.
Monthly Weather Review,
118:1250-1277.

19
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.

20
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 website @ WWW.NNDC.NOAA.GOV/.

21
www.mmm.ucar.edu/mm5/mm5-home.html

• We revised subsection 8.3.2 (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 invite you to comment on the usefulness of ASOS meteorological data for air quality modeling. More specifically, we invite comment on whether the policy of modeling with the most recent 5 years of NWS meteorological data (section 8.3.1.2) should include ASOS data. We also invite comment on whether the period of record must be the most recent 5 years—regardless of whether it contains ASOS data. Similarly, should the policy to model with the most recent full year of meteorological data (
i.e.,
section 10.2.3.4) include ASOS data?

• We revised subsection 8.3.3.1 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 AERMOD and CTDMPLUS were moved to their respective 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 8.3.3.2 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 8.3.4 (Treatment of Calms) to increase accuracy and to include information pertaining to AERMOD.

Section 10

We revised section 10 (renumbered section 9) to include AERMOD, ISC-PRIME, and CALPUFF.

Section 11

We propose minor revisions for section 11 (renumbered section 10) to reflect the new ambient air quality standards for fine particles and ozone. Because EPA has retreated from its emissions trading (“bubble”) policy for SO
2
, we have deleted subsection 11.2.3.4.

Section 12 & 13

We redesignated section 13 (Bibliography) as section 11 and retained section 12 (References). We revised them by adding some references, deleting obsolete/superseded ones, and resequencing. You will note that peer scientific reviews for AERMOD, CALPUFF and ISC-PRIME have 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 intend to revise the glossary and place it on EPA's Internet SCRAM website.

We invite your comment on any of the changes proposed above (Proposed editorial changes) for appendix W text, including the merging of sections 4 and 5.

Appendix A

We updated the introduction to appendix A (section A.0). As mentioned before, we added AERMOD and CALPUFF to appendix A, and modified the ISC3 description (now, ISC-PRIME) to include the EPRI downwash

algorithm. We propose removing 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.

In the mid-1980s, the Federal Aviation Administration (FAA) developed the Emissions and Dispersion Modeling System (EDMS) to assess the air quality of proposed airport development projects by. In response to the growing needs of the air quality analysis community and changes in regulations (
e.g.,
conformity requirements from the Clean Air Act Amendment of 1990), FAA updated EDMS to version 3.1. Accordingly, we included a revised summary description for EDMS in appendix A. The emissions module of EDMS 3.1 includes input and methodology enhancements. The dispersion module of EDMS 3.1 also has improved and has been refined to incorporate code from two EPA dispersion models: PAL2 and CALINE3. The dispersion module also has been revised to allow the user greater flexibility in specifying inputs such as dispersion settings and coefficients, hourly operational profiles for aircraft queues, and meteorological data. EDMS 3.1 features provide greater resolution in defining emissions and dispersion concentrations, and have the potential to increase or decrease the results, depending on the individual scenario. EDMS has never been subjected to performance evaluation, and no studies of its performance have been cited. We invite comment on whether this compromises its viability as a recommended/preferred model for assessing airport impacts on air quality. We also invite suggestions as to how this deficiency can be addressed.

Appendix B: To Be Moved to Website (www.epa.gov/scram001)

Appendix B of the
Guideline
has been a repository for over 20 alternate models to be used with case-by-case justification. These models have not necessarily been the subject of any performance evaluation, and their inclusion in appendix B does not mean the Agency sanctions their use. They are listed for convenience, and have been used in few regulatory applications. Production and maintenance of the appendix B information currently in CFR text presents a real burden to EPA. Accordingly, we propose to move the appendix B repository of alternate model summary descriptions to our Internet SCRAM website (www.epa.gov/scram001). Placement of this material on the website offers many advantages. In this format, we will be able to maintain the list and model descriptions more easily and inexpensively. We could, for example, routinely make revisions on a nominally annual basis, whereas the current system imposes a nominally 3-year cycle for such revisions. Model developers could list their own website address for users to obtain more information. We invite your comments on the proposed movement of the list of alternative model descriptions to our website.

Several model developers have submitted new dispersion models for inclusion in this website repository of alternate models:

• Second-Order Closure Integrated Puff Model (SCIPUFF);

• Open Burn/Open Detonation Dispersion Model (OBODM);

• Atmospheric Dispersion Modeling System (ADMS); and

• Comprehensive Air Quality Model with extensions (CAMx).

As described below, codes for these models, as well as applicable documentation, have been uploaded to our Internet SCRAM website for your review. We have included summary descriptions in docket no. A-99-05 for your review and comment. Finally, we propose deleting a model currently listed in appendix B, MESOPUFF II, which CALPUFF replaces.

Appendix C

We also propose removing appendix C (Example Air Quality Analysis Checklist) from the CFR. We believe this checklist is outdated, in need of revision, and would be more practical to maintain if posted on EPA's Internet SCRAM website (as is our intention for appendix B).

Availability of Related Information

Our Air Quality Modeling Group maintains an Internet website (Support Center for Regulatory Air Models—SCRAM) at: www.epa.gov/scram001. You may find codes and documentation for models proposed for adoption in today's action on the SCRAM website. In addition, we have uploaded various support documents (
e.g.,
evaluation reports) that are now available for review.

Administrative Requirements

A. Executive Order 12866

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, 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 proposed 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.

For purposes of assessing the impacts of today's rule on small entities, small entity is 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.

We do not anticipate that today's proposal will 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 proposal, the impacts of using updated modeling techniques would be minimal, if not non-existent. The action proposed today incorporates comments received at the 6th Conference on Air Quality Modeling in August 1995 in Washington, D.C. The proposal features several new modeling systems and serves to increase efficiency and accuracy. These systems employ procedural concepts that are very similar to those currently used, changing only mathematical formulations and specific data elements. Any impact on small entities would mainly be ascribed to the proposed use of AERMOD, which will replace ISC3. Computer run times for AERMOD may be longer than those for ISC3, owing to AERMOD's increased sophistication so that more time may be involved in preparing input data using AERMOD's preprocessors (AERMET and AERMAP) relative to an ISC3 run. However, this is more than compensated by AERMOD's capability to treat simple and complex terrain problems in one model, which actually affords a timesaving advantage. Moreover, we designed AERMOD's output formats to mimic those of ISC3, thus easing interpretation of results. Therefore, we do not believe that AERMOD's use poses a significant or unreasonable burden on any small entities. The proposed action imposes no new regulatory burdens and, as such, there will be no additional impact on small entities regarding reporting, recordkeeping, compliance requirements.

After considering the economic impacts of today's proposed rule on small entities, I certify that this action will not have a significant economic impact on a substantial number of small entities.

D. 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 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.”

Under Section 6 of Executive Order 13132, EPA may not issue a regulation that has federalism implications, that imposes substantial direct compliance costs, and that is not required by statute, unless the Federal government provides the funds necessary to pay the direct compliance costs incurred by State and local governments, or EPA consults with State and local officials early in the process of developing the proposed regulation. EPA also may not issue a regulation that has federalism implications and that preempts State law, unless the Agency consults with State and local officials early in the process of developing the proposed regulation.

This proposed 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. The proposal 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, the requirements of section 6 of the Executive Order do not apply to this rule.

E. Executive Order 13084: Consultation and Coordination With Indian Tribal Governments

Under Executive Order 13084, EPA may not issue a regulation that is not required by statute, that significantly or uniquely affects the communities of Indian tribal governments, and that imposes substantial direct compliance costs on those communities, unless the Federal government provides the funds necessary to pay the direct compliance costs incurred by the tribal governments, or EPA consults with those governments. If EPA complies by consulting, Executive Order 13084 requires EPA to provide to the Office of Management and Budget, in a separately identified section of the preamble to the rule, a description of the extent of EPA's prior consultation with representatives of affected tribal governments, a summary of the nature of their concerns, and a statement supporting the need to issue the regulation. In addition, Executive Order 13084 requires EPA to develop an effective process permitting elected officials and other representatives of Indian tribal governments “to provide meaningful and timely input in the development of regulatory policies on matters that significantly or uniquely affect their communities.”

Today's proposed rule does not significantly or uniquely affect the communities of Indian tribal governments. As stated above with respect to Executive Order 12875, the proposal does not impose any additional requirements for the regulated community, including Indian Tribal Governments. Accordingly, the requirements of section 3(b) of Executive Order 13084 do not apply to this rule.

F. Executive Order 13045: Protection of Children From Environmental Health Risks 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 proposed 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 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.

G. Unfunded Mandates Reform Act

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 contains no Federal mandates (under the regulatory provisions of Title II of the UMRA) for State, local, or tribal governments or the private sector.

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: February 8, 2000.
Carol M. Browner,
Administrator.

Part 51, chapter I, title 40 of the Code of Federal Regulations is proposed to be 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:

42 U.S.C. 7410, 7414, 7421, 7470-7479, 7491, 7492, 7601, and 7602.

2. Appendix W to Part 51 is 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 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 Models for Ozone, Particulate Matter, Carbon Monoxide, Nitrogen Dioxide, and Lead

5.1 Discussion

5.2 Recommendations

5.2.1 Models for Ozone

5.2.2 Models for Particulate Matter

5.2.2.1 PM-2.5

5.2.2.2 PM-10

5.2.3 Models for Carbon Monoxide

5.2.4 Models for Nitrogen Dioxide (Annual Average)

5.2.5 Models for Lead

6.0 Other Model Requirements

6.1 Discussion

6.2 Recommendations

6.2.1 Visibility

6.2.2 Good Engineering Practice Stack Height

6.2.3 Long Range Transport (i.e., beyond 50km)

6.2.4 Modeling Guidance for Other Governmental Programs

7.0 General Modeling Considerations

7.1 Discussion

7.2 Recommendations

7.2.1 Design Concentrations

7.2.2 Critical Receptor Sites

7.2.3 Dispersion Coefficients

7.2.4 Stability Categories

7.2.5  Plume Rise

7.2.6 Chemical Transformation

7.2.7 Gravitational Settling and Deposition

7.2.8 Complex Winds

7.2.9 Calibration of Models

8.0 Model Input Data

8.1 Source Data

8.1.1 Discussion

8.1.2 Recommendations

8.2 Background Concentrations

8.2.1 Discussion

8.2.2 Recommendations (Isolated Single Source)

8.2.3 Recommendations (Multi-Source Areas)

8.3 Meteorological Input Data

8.3.1 Length of Record of Meteorological Data

8.3.2 National Weather Service Data

8.3.3 Site-Specific Data

8.3.4 Treatment of Calms

9.0 Accuracy and Uncertainty of Models

9.1 Discussion

9.1.1 Overview of Model Uncertainty

9.1.2 Studies of Model Accuracy

9.1.3 Use of Uncertainty in Decision-Making

9.1.4 Evaluation of Models

9.2 Recommendations

10.0 Regulatory Application of Models

10.1 Discussion

10.2 Recommendations

10.2.1 Analysis Requirements

10.2.2 Use of Measured Data in Lieu of Model Estimates

10.2.3 Emission Limits

11.0 Bibliography

12.0 References

Appendix A to Appendix W of Part 51—Summaries of Preferred Air Quality Models

List of Tables

Table No.
and
Title

4-1a Neutral/Stable Meteorological Matrix for CTSCREEN

4-1b Unstable/Convective Meteorological Matrix for CTSCREEN

8-1 Model Emission Input Data for Point Sources

8-2 Point Source Model Input Data (Emissions) for PSD NAAQS Compliance Demonstrations

8-3 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, including prevention of significant deterioration (PSD).
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 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 3.0. 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. Chapter 2 gives an overview of models and their appropriate use. Chapter 3 provides specific guidance on the use of “preferred” air quality models and on the selection of alternative techniques. Chapters 4 through 6 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. Chapter 7 discusses issues common to many modeling analyses, including acceptable model components. Chapter 8 makes recommendations for data inputs to models including source, meteorological and background air quality data. Chapter 9 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 website at
www.epa.gov/scram001.
A list of alternate models that can be used with case-by-case justification (Section 3.2), a glossary of terms, and an example air quality analysis checklist are also posted on this website. This is a site with which modelers should become familiar.

3.0 Recommended Air Quality Models

a. This section recommends refined modeling techniques that are preferred 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
5
and periodic Regional/State/Local Modelers workshops.

b. 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 website (Section 2.3), and the preparation of a written protocol help to keep misunderstandings at a minimum.

c. 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.

d. The 1980 solicitation of new or different models from the technical community
6
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

16
that include statistical measures of model performance in comparison with measured air quality data as suggested by the American Meteorological Society
17
and, where possible, peer scientific reviews.
18

19

20

21

22

23

24

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 Section 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
25

26
are available to assist in developing a consistent approach when justifying the use of other than the preferred modeling techniques recommended in the Guideline. Reference
27
contains advanced statistical techniques for determining which model performs better than other competing models. In many cases, this protocol should be considered preferentially to the material in Chapter 3 of reference 25. 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 below. 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 website (Section 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 3.2.2b, 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 identified below are satisfied.

d. For condition (2) in paragraph 3.2.2 b, the procedures and techniques for determining the acceptability of a model for an individual case based on superior performance are contained in references 25-27 and 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 3.2.2b, 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 Traditional Stationary Source Models

4.1 Discussion

a. Guidance in this section applies to modeling analyses for which the predominant meteorological conditions that control the design concentration are steady state and for which the transport distances are nominally 50km or less. 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. 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. Complex terrain is defined as terrain exceeding the height of the stack being modeled.

c. In the early 1980s, model evaluation exercises were conducted to determine the “best, most appropriate point source model” for use in simple terrain.
8 18
No one model was found to be clearly superior, and, based on past use, public familiarity, and

availability, ISC (predecessor to ISC3
28
) 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).

d. Encouraged by the development of pragmatic methods for better characterization of plume dispersion
29 30 31 32
, the AMS/EPA Regulatory Model Improvement Committee (AERMIC) developed AERMOD
33
. AERMOD employs state-of-practice parameterizations for characterizing the meteorological influences and dispersion. The model utilizes a probability density function (pdf) and the superposition of several Gaussian plumes to characterize the distinctly non-Gaussian nature of the vertical pollutant distribution for elevated plumes during convective conditions; otherwise the distribution is Gaussian. Also, nighttime urban boundary layers (and plumes within them) have the turbulence enhanced by AERMOD to simulate the influence of the urban heat island. AERMOD has been evaluated using a variety of data sets and has been found to perform better than ISC3 for many applications, and as well or better than CTDMPLUS for several complex terrain data sets (Section A.1; subsection n). Currently, AERMOD does not contain algorithms for dry deposition.

e. A new building downwash algorithm was developed and tested within the ISC3 construct, ISC-PRIME,
24
which is in Appendix A. ISC-PRIME has been evaluated using a variety of data sets and has been found to perform better than ISC3 (Section A.7; subsection n). ISC-PRIME retains the dry deposition inherent in ISC3.

4.2 Recommendations

4.2.1 Screening Techniques

4.2.1.1 Simple Terrain

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,
34
and a computerized version of the recommended screening technique, SCREEN, is available.
35

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 Section 7.2.3. 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 reviewing authority on the choice of the screening model for each analysis, and on the input data as well as the ultimate use of the results.

4.2.1.2 Complex Terrain

a. CTSCREEN
36
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.
36
The terrain data must be digitized in the same manner as for CTDMPLUS and a terrain processor is available.
37
A discussion of the model's performance characteristics is provided in a technical paper.
38
CTSCREEN is designed to execute a fixed matrix of meteorological values for wind speed (u), standard deviation of horizontal and vertical wind speeds (σ
v
, σ
w
), vertical potential temperature gradient (dθ/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 4-1 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 Regional Office 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
37
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
28 35 39
may be acceptable for complex terrain cases where established procedures are used. The user is encouraged to confer with the Regional Office if any unresolvable problems are encountered,
e.g.,
applicability, meteorological data, receptor siting, or terrain contour processing issues.

4.2.2 Refined Analytical Techniques

a. A brief description of each preferred model for refined applications is found in Appendix A. Also listed in that appendix are availability, the model input requirements, the standard options that should be selected when running the program, and output options.

b. For a wide range of regulatory applications in all types of terrain, the recommended model is AERMOD. This recommendation is based on extensive developmental and performance evaluation (Section A.1; subsection n). Differentiation of simple versus complex terrain is unnecessary with AERMOD. In complex terrain, AERMOD employs the well-known dividing-streamline concept in a simplified simulation of the effects of plume-terrain interactions.

c. If dry deposition or aerodynamic building downwash is important for the modeling analysis,
e.g.,
paragraphs 5.2.2.2(e), 5.2.5(b), 6.2.2(b), and 7.2.7(b), the recommended model is ISC-PRIME. Line sources can be simulated with ISC-PRIME if point or volume sources are appropriately combined. If buoyant plume rise from line sources is important for the modeling analysis, the recommended model is BLP. For other special modeling applications, CALINE3 (or CAL3QHCR on a case-by-case basis), OCD, and EDMS are recommended as described in Sections 5 and 6.

d. If the modeling application involves a well defined hill or ridge and a detailed dispersion analysis of the spatial pattern of plume impacts is of interest, CTDMPLUS, listed in Appendix A, is available. CDTMPLUS provides greater resolution of concentrations about the contour of the hill feature than does AERMOD through a different plume-terrain interaction algorithm.

Table 4-1a.—Neutral/Stable Meteorological Matrix for CTSCREEN

Variable:
Specific values

U‸(m/s)
1.0
2.0
3.0
4.0
5.0

σ
v
(m/s)

0.3
0.75

σ
w
(m/s)

0.08
0.15
0.30
0.75

Δθ/Δz (K/m)
0.01
0.02
0.035

WD
(Wind direction is optimized internally for each meteorological combination.)

Exceptions:

(1) If U ≤ 2 m/s and σ
v
≤ 0.3 m/s, then include σ
w
= 0.04 m/s.

(2) If σ
w
= 0.75 m/s and U ≥ 3.0 m/s, then Δθ/Δz is limited to ≤ 0.01 K/m.

(3) If U ≥ 4 m/s, then σ
w
≥ 0.15 m/s.

(4) σ
w
≤σ
V

Table 4-1B.—Unstable/Convective Meteorological Matrix for CTSCREEN

Variable:
Specific values

U (m/s)
1.0
2.0
3.0
4.0
5.0

U
*
(m/s)

0.1
0.3
0.5

L (m)
−10
−50
−90

Δθ/Δz (K/m)
0.030

(potential temperature gradient above Z
i
)

Z
i
(m)

0.5h
1.0h
1.5h
h= terrain height)

5.0 Models for Ozone, Particulate Matter, Carbon Monoxide, Nitrogen Dioxide, and Lead

5.1 Discussion

a. This section identifies modeling approaches or models appropriate for addressing ozone (O
3
),
a
carbon monoxide (CO), nitrogen dioxide (NO
2
), particulates (PM-2.5 and PM-10),
b,

c
and lead. These pollutants are often associated with emissions from numerous sources. Generally, mobile sources contribute significantly to emissions of these pollutants or their precursors. For cases where it is of interest to estimate concentrations of CO or NO
2
near a single or small group of stationary sources, refer to Section 4. (Modeling approaches for SO
2
are discussed in Section 4.)

b. Several of the pollutants mentioned in the preceding paragraph are closely related to each other in that they share common sources of emissions and/or are subject to chemical transformations of similar precursors.
40

41
For example, strategies designed to reduce ozone could have an effect on the secondary component of PM-2.5 and vice versa. Thus, it makes sense to use models which take into account the chemical coupling between O
3
and PM-2.5, when feasible. This should promote consistency among methods used to evaluate strategies for reducing different pollutants as well as consistency among the strategies themselves. Regulatory requirements for the different pollutants are likely to be due at different times. Thus, the following paragraphs identify appropriate modeling approaches for pollutants individually.

c. The NAAQS for ozone was revised on July 18, 1997 and is now based on an 8-hour averaging period (62 FR 38856). Models for ozone are needed primarily to guide choice of strategies to correct an observed ozone problem in an area not attaining the NAAQS for ozone. Use of photochemical grid models is the recommended means for identifying strategies needed to correct high ozone concentrations in such areas. Such models need to consider emissions of volatile organic compounds (VOC), nitrogen oxides (NO
X
) and carbon monoxide (CO), as well as means for generating meteorological data governing transport and dispersion of ozone and its precursors. Other approaches, such as Lagrangian or observational models may be used to guide choice of appropriate strategies to consider with a photochemical grid model. These other approaches may be sufficient to address ozone in an area where observed concentrations are near the NAAQS or only

slightly above it. Such a decision needs to be made on a case-by-case basis in concert with the appropriate Regional Office.

d. A control agency with jurisdiction over one or more areas with significant ozone problems should review available ambient air quality data to assess whether the problem is likely to be significantly impacted by regional transport.
42
Choice of a modeling approach depends on the outcome of this review. In cases where transport is considered significant, use of a nested regional model may be the preferred approach. If the observed problem is believed to be primarily of local origin, use of a model, with a single horizontal grid resolution and geographical coverage that is less than that of a regional model, may suffice.

e. The fine particulate matter NAAQS, promulgated on July 18, 1997 (62 FR 38652), includes particles with an aerodynamic diameter nominally less than or equal to 2.5 micrometers (PM-2.5). Models for PM-2.5 are needed to assess adequacy of a proposed strategy for meeting annual and/or 24-hour NAAQS for PM-2.5. PM-2.5 is a mixture consisting of several diverse components. Because chemical/physical properties and origins of each component differ, it may be appropriate to use either a single model capable of addressing several of the important components or to model primary and secondary components using different models. Effects of a control strategy on PM-2.5 is estimated from the sum of the effects on the components composing PM-2.5. Model users may refer to guidance
43
for further details concerning appropriate modeling approaches.

f. A control agency with jurisdiction over one or more areas with PM-2.5 problems should review available ambient air quality data to assess which components of PM-2.5 are likely to be major contributors to the problem. If it is determined that regional transport of secondary particulates, such as sulfates or nitrates, is likely to contribute significantly to the problem, use of a regional model may be the preferred approach. Otherwise, coverage may be limited to a domain that is urban scale or less. Special care should be taken to select appropriate geographical coverage for a modeling application.
43

g. The NAAQS for PM-10 was promulgated in July 1987 (40 CFR 50.6). A SIP development guide
44
is available to assist in PM-10 analyses and control strategy development. EPA promulgated regulations for PSD increments measured as PM-10 in a document published on June 3, 1993 (§ 51.166(c)). As an aid to assessing the impact on ambient air quality of particulate matter generated from prescribed burning activities, a reference
45
is available.

h. Models for assessing the impact of CO emissions are needed for a number of different purposes. Examples include evaluating effects of point sources, congested intersections and highways, as well as the cumulative effect of numerous sources of CO in an urban area.

i. Models for assessing the impact of sources on ambient NO
2
concentrations are primarily needed to meet new source review requirements, such as addressing the effect of a proposed source on PSD increments for annual concentrations of NO
2
. Impact of an individual source on ambient NO
2
depends, in part, on the chemical environment into which the source's plume is to be emitted. There are several approaches for estimating effects of an individual source on ambient NO
2
. One approach is through use of a plume-in-grid algorithm imbedded within a photochemical grid model. However, because of the rigor and complexity involved, and because this approach may not be capable of defining sub-grid concentration gradients, the plume-in-grid approach may be impractical for estimating effects on an annual PSD increment. A second approach is to develop site-specific conversion factors based on measurements. If it is not possible to develop site-specific conversion factors and use of the plume-in-grid algorithm is also not feasible, other screening procedures may be considered.

j. In January 1999 (40 CFR Part 58, Appendix D), EPA gave notice that concern about ambient lead impacts was being shifted away from roadways and toward a focus on stationary point sources. EPA has also issued guidance on siting ambient monitors in the vicinity of such sources.
46
For lead, the SIP should contain an air quality analysis to determine the maximum quarterly lead concentration resulting from major lead point sources, such as smelters, gasoline additive plants, etc. General guidance for lead SIP development is also available.
47

5.2 Recommendations

5.2.1 Models for Ozone

a.
Choice of Models for Multi-source Applications.
Simulation of ozone formation and transport is a highly complex and resource intensive exercise. Control agencies with jurisdiction over areas with ozone problems are encouraged to use photochemical grid models, such as the Models-3/Community Multi-scale Air Quality (CMAQ) modeling system,
48
to evaluate the relationship between precursor species and ozone. Judgement on the suitability of a model for a given application should consider factors that include use of the model in an attainment test, development of emissions and meteorological inputs to the model and choice of episodes to model.
42
Similar models for the 8-hour NAAQS and for the 1-hour NAAQS are appropriate.

b.
Choice of Models to Complement Photochemical Grid Models.
As previously noted, observational models, Lagrangian models, or the Empirical Kinetics Modeling Approach (EKMA)
49, 50
may be used to help guide choice of strategies to simulate with a photochemical grid model and to corroborate results obtained with a grid model. EPA has issued guidance
42
in selecting appropriate techniques.

c.
Estimating the Impact of Individual Sources.
Choice of methods used to assess the impact of an individual source depends on the nature of the source and its emissions. Thus, model users should consult with the appropriate Regional Office to determine the most suitable approach on a case-by-case basis (Section 3.2.2).

5.2.2 Models for Particulate Matter

5.2.2.1 PM-2.5

a.
Choice of Models for Multi-source Applications.
Simulation of phenomena resulting in high ambient PM-2.5 can be a multi-faceted and complex problem resulting from PM-2.5's existence as an aerosol mixture. Treating secondary components of PM-2.5, such as sulfates and nitrates, can be a highly complex and resource-intensive exercise. Control agencies with jurisdiction over areas with secondary PM-2.5 problems are encouraged to use models which integrate chemical and physical processes important in the formation, decay and transport of these species (
e.g.
, Models-3/CMAQ
48
or REMSAD
51
). Primary components can be simulated using less resource-intensive techniques. Suitability of a modeling approach or mix of modeling approaches for a given application requires technical judgement
43
, as well as professional experience in choice of models, use of the model(s) in an attainment test, development of emissions and meteorological inputs to the model and selection of days to model.

b.
Choice of Analysis Techniques to Complement Air Quality Simulation Models.
Observational models may be used to corroborate predictions obtained with one or more air quality simulation models. They may also be potentially useful in helping to define specific source categories contributing to major components of PM-2.5.
43

c.
Estimating the Impact of Individual Sources.
Choice of methods used to assess the impact of an individual source depends on the nature of the source and its emissions. Thus, model users should consult with the appropriate Regional Office to determine the most suitable approach on a case-by-case basis (Section 3.2.2).

5.2.2.2 PM-10

a. Screening techniques like those identified in Section 4 are applicable to PM-10. Conservative assumptions which do not allow removal or transformation are suggested for screening. Thus, it is recommended that subjectively determined values for “half-life” or pollutant decay not be used as a surrogate for particle removal. Proportional models (rollback/forward) may not be applied for screening analysis, unless such techniques are used in conjunction with receptor modeling.
44

b. Refined models such as those discussed in Section 4 are recommended for PM-10. However, where possible, particle size, gas-to-particle formation, and their effect on ambient concentrations may be considered. For point sources of small particles and for source-specific analyses of complicated sources, use the appropriate recommended steady-state plume dispersion model (Section 4.2.2). For guidance on determination of design concentrations, see paragraph 7.2.1.1(e).

c. Receptor models
52

53

54
have proven useful for helping validate emission inventories and for corroborating source-specific impacts estimated by dispersion models. In regulatory applications, dispersion models have been used in conjunction with receptor models to attribute source (or source category)

contributions. Guidance is available for PM-10 sampling and analysis applicable to receptor modeling.
55

d. Under certain conditions, recommended dispersion models may not be reliable. In such circumstances, the modeling approach should be approved by the appropriate Regional Office on a case-by-case basis. Analyses involving model calculations for stagnation conditions should also be justified on a case-by-case basis (Section 7.2.8).

e. Fugitive dust usually refers to dust put into the atmosphere by the wind blowing over plowed fields, dirt roads or desert or sandy areas with little or no vegetation. Reentrained dust is that which is put into the air by reason of vehicles driving over dirt roads (or dirty roads) and dusty areas. Such sources can be characterized as line, area or volume sources. Emission rates may be based on site-specific data or values from the general literature. Fugitive emissions include the emissions resulting from the industrial process that are not captured and vented through a stack but may be released from various locations within the complex. Where such fugitive emissions can be properly specified, use the recommended steady-state dispersion model (Section 4.2.2) that handles gravitational settling and dry deposition. In some unique cases a model developed specifically for the situation may be needed. Due to the difficult nature of characterizing and modeling fugitive dust and fugitive emissions, it is recommended that the proposed procedure be cleared by the appropriate Regional Office for each specific situation before the modeling exercise is begun.

5.2.3 Models for Carbon Monoxide

a. Guidance is available for analyzing CO impacts at roadway intersections.
56
The recommended screening model for such analyses is CAL3QHC.
57

58
This model combines CALINE3 (listed in Appendix A) with a traffic model to calculate delays and queues that occur at signalized intersections. The screening approach is described in reference 56; a refined approach may be considered on a case-by-case basis with CAL3QHCR.
59
The latest version of the MOBILE (mobile source emission factor) model should be used for emissions input to intersection models.

b. For analyses of highways characterized by uninterrupted traffic flows, CALINE3 is recommended, with emissions input from the latest version of the MOBILE model.

c. For urban area wide analyses of CO, an Eulerian grid model should be used. Information on SIP development and requirements for using such models can be found in several references.
56

60

61

62

d. Where point sources of CO are of concern, they should be treated using the screening and refined techniques described in Section 4 of the Guideline.

5.2.4 Models for Nitrogen Dioxide (Annual Average)

a. A tiered screening approach is recommended to obtain annual average estimates of NO
2
from point sources for New Source Review analysis, including PSD, and for SIP planning purposes. This multi-tiered approach is conceptually shown in Figure 5-1 and described in paragraphs 5.2.4 b through d:

b. For Tier 1 (the initial screen), use an appropriate model from Appendix A to estimate the maximum annual average concentration and assume a total conversion of NO to NO
2
. If the concentration exceeds the NAAQS and/or PSD increments for NO
2
, proceed to the 2nd level screen.

c. For Tier 2 (2nd level) screening analysis, multiply the Tier 1 estimate(s) by an empirically derived NO
2
/ NO
x
value of 0.75 (annual national default).
63
The reviewing agency may establish an alternative default NO
2
/ NO
x
ratio based on ambient annual average NO
2
and annual average NO
x
data representative of area wide quasi-equilibrium conditions Alternative default NO
2
/NO
x
ratios should be based on data satisfying quality assurance procedures that ensure data accuracy for both NO
2
and NO
x
within the typical range of measured values. In areas with relatively low NO
x
concentrations, the quality assurance procedures used to determine compliance with the NO
2
national ambient air quality standard may not be adequate. In addition, default NO
2
/NO
x
ratios, including the 0.75 national default value, can underestimate long range NO2 impacts and should be used with caution in long range transport scenarios.

d. For Tier 3 (3rd level) analysis, a detailed screening method may be selected on a case-by-case basis. For point source modeling, other refined screening methods, such as the ozone limiting method
64
, may also be considered. Also, a site-specific NO
2
/NO
x
ratio may be used as a detailed screening method if it meets the same restrictions as described for alternative default NO
2
/NO
x
ratios. Ambient NO
2x
monitors used to develop a site-specific ratio should be sited to obtain the NO
2
and NO
x
concentrations under quasi-equilibrium conditions. Data obtained from monitors sited at the maximum NO
x
impact site, as may be required in a PSD pre-construction monitoring program, likely reflect transitional NO
x
conditions. Therefore, NO
x
data from maximum impact sites may not be suitable for determining a site-specific NO
2
/NO
x
ratio that is applicable for the entire modeling analysis. A site-specific ratio derived from maximum impact data can only be used to estimate NO
2
impacts at receptors located within the same distance of the source as the source-to-monitor distance.

e. In urban areas (Section 7.2.3), a proportional model may be used as a preliminary assessment to evaluate control strategies to meet the NAAQS for multiple minor sources, i.e., minor point, area and mobile sources of NO
x
; concentrations resulting from major point sources should be estimated separately as discussed above, then added to the impact of the minor sources. An acceptable screening technique for urban complexes is to assume that all NO
x
is emitted in the form of NO
2
and to use a model from Appendix A for nonreactive pollutants to estimate NO
2
concentrations. A more accurate estimate can be obtained by: (1) calculating the annual average concentrations of NO
x
with an urban model, and (2) converting these estimates to NO
2
concentrations using an empirically derived annual NO
2
/ NO
x
ratio. A value of 0.75 is recommended for this ratio. However, a spatially averaged alternative default annual NO
2
/ NO
x
ratio may be determined from an existing air quality monitoring network and used in lieu of the 0.75 value if it is determined to be representative of prevailing ratios in the urban area by the reviewing agency. To ensure use of appropriate locally derived annual average NO
2
/ NO
x
ratios, monitoring data under consideration should be limited to those collected at monitors meeting siting criteria defined in 40 CFR Part 58, Appendix D as representative of “neighborhood”, “urban”, or “regional” scales. Furthermore, the highest annual spatially averaged NO
2
/ NO
x
ratio from the most recent 3 years of complete data should be used to foster conservatism in estimated impacts.

f. To demonstrate compliance with NO
2
PSD increments in urban areas, emissions from major and minor sources should be included in the modeling analysis. Point and area source emissions should be modeled as discussed above. If mobile source emissions do not contribute to localized areas of high ambient NO
2
concentrations, they should be modeled as area sources. When modeled as area sources, mobile source emissions should be assumed uniform over the entire highway link and allocated to each area source grid square based on the portion of highway link within each grid square. If localized areas of high concentrations are likely, then mobile sources should be modeled as line sources using an appropriate steady-state plume dispersion model (e.g., CAL3QHCR; Section 5.2.3).

g. More refined techniques to handle special circumstances may be considered on a case-by-case basis and agreement with the reviewing authority should be obtained. Such techniques should consider individual quantities of NO and NO
2
emissions, atmospheric transport and dispersion, and atmospheric transformation of NO to NO
2
. Where they are available, site-specific data on the conversion of NO to NO
2
may be used. Photochemical dispersion models, if used for other pollutants in the area, may also be applied to the NO
x
problem.

5.2.5 Models for Lead

a. For major lead point sources, such as smelters, which contribute fugitive emissions and for which deposition is important, use the appropriate recommended steady-state plume dispersion model (Section 4.2.2). To model an entire major urban area or to model areas without significant sources of lead emissions, as a minimum a proportional (rollback) model may be used for air quality analysis. The rollback philosophy assumes that measured pollutant concentrations are proportional to emissions. However, urban or other dispersion models are encouraged in these circumstances where the use of such models is feasible.

b. In modeling the effect of traditional line sources (such as a specific roadway or highway) on lead air quality, dispersion models applied for other pollutants can be used. Dispersion models such as CALINE3 and CAL3QHCR have been used for modeling

carbon monoxide emissions from highways and intersections (Section 5.2.3). However, where deposition is of concern, ISC-PRIME may be used. Also, where there is a point source in the middle of a substantial road network, the lead concentrations that result from the road network should be treated as background (Section 8.2); the point source and any nearby major roadways should be modeled separately using the appropriate recommended steady-state plume dispersion model (Section 4.2.2).

6.0 Other Model Requirements

6.1 Discussion

a. This section covers those cases where specific techniques have been developed for special regulatory programs. Most of the programs have, or will have when fully developed, separate guidance documents that cover the program and a discussion of the tools that are needed. The following paragraphs reference those guidance documents, when they are available. No attempt has been made to provide a comprehensive discussion of each topic since the reference documents were designed to do that. This section will undergo periodic revision as new programs are added and new techniques are developed.

b. Other Federal agencies have also developed specific modeling approaches for their own regulatory or other requirements.
65
Although such regulatory requirements and manuals may have come about because of EPA rules or standards, the implementation of such regulations and the use of the modeling techniques is under the jurisdiction of the agency issuing the manual or directive.

c. The need to estimate impacts at distances greater than 50km (the nominal distance to which EPA considers most steady-state Gaussian plume models are applicable) is an important one especially when considering the effects from secondary pollutants. Unfortunately, models originally available to EPA had not undergone sufficient field evaluation to be recommended for general use. Data bases from field studies at mesoscale and long range transport distances were limited in detail. This limitation was a result of the expense to perform the field studies required to verify and improve mesoscale and long range transport models. Meteorological data adequate for generating three-dimensional wind fields were particularly sparse. Application of models to complicated terrain compounds the difficulty of making good assessments of long range transport impacts. EPA completed limited evaluation of several long range transport (LRT) models against two sets of field data and evaluated results.
13
Based on the results, EPA concluded that long range and mesoscale transport models were limited for regulatory use to a case-by-case basis. However a more recent series of comparisons has been completed for a new model, CALPUFF (Section A.4). Several of these field studies involved three-to-four hour releases of tracer gas sampled along arcs of receptors at distances greater than 50km downwind. In some cases, short-term concentration sampling was available, such that the transport of the tracer puff as it passed the arc could be monitored. Differences on the order of 10 to 20 degrees were found between the location of the simulated and observed center of mass of the tracer puff. Most of the simulated centerline concentration maxima along each arc were within a factor of two of those observed. It was concluded from these case studies that the CALPUFF dispersion model had performed in a reasonable manner, and had no apparent bias toward over or under prediction, so long as the transport distance was limited to less than 300km.
66

6.2 Recommendations

6.2.1 Visibility

a. Visibility in important natural areas (
e.g.
, Federal Class I areas) is protected under a number of provisions of the Clean Air Act, including Sections 169A and 169B (addressing impacts primarily from existing sources) and Section 165 (new source review). Visibility impairment is caused by light scattering and light absorption associated with particles and gases in the atmosphere. In most areas of the country, light scattering by PM-2.5 is the most significant component of visibility impairment. The key components of PM-2.5 contributing to visibility impairment include sulfates, nitrates, organic carbon, elemental carbon, and crustal material.

b. The visibility regulations as promulgated in December 1980 (40 CFR 51.300—51.307) require States to mitigate visibility impairment, in any of the 156 mandatory Federal Class I areas, that is found to be “reasonably attributable” to a single source or a small group of sources. In 1985, EPA promulgated Federal Implementation Plans (FIPs) for several States without approved visibility provisions in their SIPs. The IMPROVE (Interagency Monitoring for Protected Visual Environments) monitoring network, a cooperative effort between EPA, the States, and Federal land management agencies, was established to implement the monitoring requirements in these FIPs. Data has been collected by the IMPROVE network since 1988.

c. In 1999, EPA issued revisions to the 1980 regulations to address visibility impairment in the form of regional haze, which is caused by numerous, diverse sources (
e.g.
, stationary, mobile, and area sources) located across a broad region (40 CFR 51.308—51.309). The state of relevant scientific knowledge has expanded significantly since the Clean Air Act Amendments of 1977. A number of studies and reports
67

68
have concluded that long range transport (
e.g.
, up to hundreds of kilometers) of fine particulate matter plays a significant role in visibility impairment across the country. Section 169A of the Act requires states to develop SIPs containing long-term strategies for remedying existing and preventing future visibility impairment in 156 mandatory Class I federal areas. In order to develop long-term strategies to address regional haze, many States will need to conduct regional-scale modeling of fine particulate concentrations and associated visibility impairment (
e.g.
, light extinction and deciview metrics).

d. Guidance and a screening model, VISCREEN, are available.
69
VISCREEN can be used to calculate the potential impact of a plume of specified emissions for specific transport and dispersion conditions. If a more comprehensive analysis is required, any refined model should be selected in consultation with the EPA Regional Office and the appropriate Federal Land Manager who is responsible for determining whether there is an adverse effect by a plume on a Class I area. PLUVUE II, an alternative model listed on EPA's Internet SCRAM website (Section 2.3), may be applied on a case-by-case basis when refined plume visibility evaluations are needed.

e. CALPUFF (Section A.4) may be applied on a case-by-case basis when assessment is needed of reasonably attributable haze impairment due to one or a small group of sources. The procedures and analyses should be determined in consultation with the appropriate Regional Office, the appropriate regulatory permitting authority, and the appropriate Federal Land Manager (FLM).

f. Regional scale models are used by EPA to develop and evaluate national policy and assist State and local control agencies. Two such models which can be used to assess visibility impacts from source emissions are Models-3
48
and REMSAD
51
. Model users should consult with the appropriate Regional Office to determine the most suitable approach on a case-by-case basis (Section 3.2.2).

6.2.2 Good Engineering

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A00-4235. Public record. Not legal advice.