### 5900 Federal Register / Vol. 56, No. 30 / Wednesday, February 13,' i991 

# I Proposed Rules

ENVIRONMENTAL PROTECTION
AGENCY

40 CFR Parts 51 and 52

[AH-FRL-3820-7, Docket No.A-88-041

RIN 2060-AC43

Requirements forPreparation, Adoption,and Submittal of Implementation Plans

AGENCY: U.S. Environmental Protection Agency (EPA). ACTION: Notice ofproposed rulemaking.

SUMMARY: The "Guideline on Air Quality Models (Revised)", EPA-450/2- 78-027R (1986), and supplement A (1987), hereinafter referred to as the "Guideline", list air quality models for estimating ambient air concentrations due to sources ofair pollutants. The Guideline is presently incorporated by reference into the prevention of significant deterioration (PSD) regulations under the Clean AirAct at 40 CFR 51.166 and 52.21. EPA here
proposes to augment that Guideline with several new modeling techniques. The
purpose is to provide models for situations where specific procedures

were not previously available and to improve several previously adopted techniques. In addition, revisions are proposed that make the guidance and model requirements consistent with regulatory programs that have been formalized or changed since the last major Guideline revision in 1986 (51 FR 32176). EPA is proposing toestablish these additions and changes as supplement B to the Guideline. EPA is also proposing to amend40 CFR 51.166 and 52.21 to incorporate supplement B, and to amend 40 CFR 51.46, 51.63, 51.112, 51.117, 51.150, and 51.160 to codify the Guideline as the basis by which air quality models are to be used for demonstrations associated with SIP (State Implementation Plan) revisions, AQMA (Air Quality Maintenance Area) analyses, regional classifications for episode planning, and new source review Iin general (notjust PSD). The proposal, inpart, is based on public
comments received at the Fourth Conference on Air Quality Modeling, the
purpose ofwhich was to advise the public on these new techniques and to facilitate further technical review. Adoption of the new modeling techniques should significantly improve the technical basis for impact
assessment ofair pollution sources. DATES: A public hearing on the proposed

INew source review includes that ofmodified

sources. 

### changes will be held March 19, 1991 (Tuesday), from 9 a.m. to 5p.m. As needed to allow forpresentation ofall verbal comments, thehearing may be extended to noon ofthe nextday. The period forcomment onthese proposed changes closes May 6, 1991.
ADDRESSES: Comments:Written 

### comments should be submitted (in duplicate ifpossible) to: AirDocket (LE131), room M-1500, Waterside Mall, attention: Docket A-88-04, U.S. Environmental Protection Agency, 401 M Street SW., Washington, DC 20460 (also

see "Public Hearing").

Copies ofsupplement B (draft)to the "Guideline on Air Quality Models (Revised)" may be obtained bywriting

orcalling Joseph A.Tikvart, Source ReceptorAnalysis Branch, MD-14, U.S. Environmental ProtectionAgency, ResearchTriangle Park, NC 27711, phone (919) 541-5561. Supplement B (draft)is also available to registered

users of the Support Center for Regulatory Air Models Bulletin Board System by downloading the appropriate file. To register oraccess this electronic bulletinboard, users with a personal computer should dial (919) 541-5742.



### PublicHearing: GSA Auditorium, GSA National Capitol Region Building, 

### 7th and DStreets SW., Washington, DC.

Docket:Copies ofreports referenced herein, public statements and comments made in relation to the Fourth Conference on Air Quality Modeling, and public comments made on this Notice ofproposed rulemaking are maintained at Docket A-88-04. The docket is available for public inspection and copying between 8 a.m. and 4 p.m., Monday through Friday, at the address above. Areasonable fee maybe charged for copying.
FOR FURTHER INFORMATION CONTACT. Joseph A. Tikvart, Chief, Source Receptor Analysis Branch, Office of Air 

### Quality Planning and Standards, U.S. Environmental Protection Agency; Research Triangle Park, NC 27711; telephone (919) 541-5561 or (FTS) 629- 

### 5561.
SUPPLEMENTARY INFORMATION: 

### Background

-The "Guideline on Air Quality Models" ("Guideline") was originally published in April 1978. It was incorporated by reference in the regulations for the Prevention of Significant Deterioration of Air Quality in June 1978 (43 FR 26380). The purpose ofthe Guideline is to promote consistency in the use ofmodeling within the air management process. Such consistency is fostered by sections 110(a)(2), 165(e), 172 (a)and (b), 173, 301(a)(1) and 320 ofthe 1977 Clean Air Act Amendments, 242 U.S.C. 7410(a)(2), 7475(e), 7502 (a) and (b), 7503, 7601(a)(1) and 7620, respectively. The Guideline provides model users with a common basis forestimating pollution concentrations, assessing control strategies and specifying emission limits.

.InDecember 1984, EPA proposed revising the Guideline to incorporate knowledge concerning modeling analyses that had been developed since the original guidance was issued. Final revisions to the Guideline were promulgated in September 1986 (51 FR 32176) and, as a result of public comment, changes concerning four additional modeling techniques were simultaneously proposed. They were: (1) addition ofa specific version ofthe Rough Terrain Diffusion Model (RTDM), (2) modification ofthe downwash algorithm in the Industrial Source Complex (ISC) model, (3) addition to the Offshore and Coastal Dispersion (OCD) model toEPA's listof preferred models, and (4) addition of the AVACTA II model as an alternative model in the Guideline. In January 1988, EPA promulgated the four techniques as Supplement A to the Guideline (53 FR 392). The "Guideline on Air Quality Models (Revised)" (1986) and supplement A (1987) have been incorporated by reference into the PSD regulations at 40 CFR 51.166 and 52.21.

Codification ofthe Guideline in 40 CFR parts 51 and 52 has heretofore been for purposes of compliance with PSD regulations. However, as stated in the Introduction, the Guideline describes ".airquality modeling techniques that should be applied to State Implementation Plan (SIP)revisions for existing sources and to new source review 

### * * *", and thishas always been EPA policy. Use ofair quality models for such purposes is required at 51.112 (Demonstration ofAdequacy) and at 51.117 (Additional Provisions for Lead). Use of air quality models is also required: ForAir Quality Maintenance Area (AQMA) analyses at 51.46 (Projection ofAir Quality Concentrations); for classification of regions for episode plans at 51.150 (Prevention ofAir Pollution Emergency Episodes); and for new source review at

2OnNovember 15, 1990, theClean Air Act Amendments of 1990 ("Amendments"), Public Law 101-549. were enacted. As amended, the Clean Air Act provides continued authority forthe revisions
to the Guideline and the Code of Federal Regulations contained in this proposal. That authority Includes but is not limited to, the following provisions, some ofwhich may have been modified by theAmendments: Clean Air Act sections 110(a)(2). 165(e). 172 (a)and (c), 173, 301(a), and 320.

### Federal Register / Vol. 56, No. 30 / Wednesday, February 13, 1991 / Proposed Rules

51.160 (Review ofNew Sources and Modifications). EPA believes that its proposed incorporation by reference of the Guideline into these provisions is consistent with the authority granted by sections 110(a)(2), 165(e), 172 (a) and (b), 173, 301(a)(1) and 320 ofthe 1977 Clean AirAct Amendments (see footnote 2), 42 U.S.C. 7410(a)(2), 7475(e), 7502 (a) and (b), 7503, 7601(a)(1) and 7620, respectively.

To support theprocess ofdeveloping and revising the Guideline during the period 1977-1988, theFirst, Second and Third Conferences on Air Quality Modeling were held as required by section 320 ofthe Clean AirAct to help standardize modelingprocedures. These modeling conferences provided EPA with comments on the Guideline and associated revisions, thereby facilitating introduction ofimproved modeling techniques into the regulatory process.

In October 1988. the Fourth Conference on Air Quality Modeling

was held. Its purpose was to advise the public on new modeling techniques and
to solicit comments to guide EPA's consideration of any rulemaking needed
to further revise the Guideline. The new models provide techniques forsituations where specific procedures had not previously been available, and also improve several previously adopted techniques. These new techniques and the guidance discussed at the modeling conference are:

Complex Terrain Dispersion Model, Ozone precursor point source modeling,

Mobile source modeling at signalized intersections,

Emissions and dispersion modeling system for airports,

Long-range transport models. Models for estimating visibility impact of specific sources.

Model for shoreline dispersion. Valley stagnation, On-site meteorological program guidance.

Method forevaluating models, General screening techniques, Regional scale models, Modeling techniques forair pathway analyses. In general. EPA solicited comments on (1) whether the techniques are sound, (2) whether they should be adopted for regulatory application, and (3) whether limitations on use, including data base requirements, should be specified.

All comments presented at the fourth modeling conference and/or submitted
to Docket A-&-04 are encompassed in Docket Items II-D, II-E,and II-G. Also,

a verbatim transcript ofthe conference proceedings is available as Docket Item 

### I-G-3a andb.EPAhas summarized these comments, developed responses, and drawn conclusions on appropriate actions for this Notice ofProposed Rulemaking in the document "Summary ofPublic Comments andEPA Responses

onthe Fourth Conference on Air Quality Modeling-October 1988" (DocketItem II-0-1).

The following are hereby proposed as models or analysis procedures recommended forinclusion in the Guideline: ComplexTerrain Dispersion Model (refined model, accompanied by CTSCREEN and modifications in theuse of other screening models); CAL3QHC (carbonmonoxide estimates at signalized intersections); modeling for air pathway analysis oftoxic/hazardous pollutants (consistentmultimedia use of ISC and other models);EDMS (modeling system forairports); on-site meteorological program guidance (expansion ofcurrent guidance); general screening techniques andVISCREEN (updates and computerization ofcurrent screening techniques); method for evaluating models (enhancement of currentguidance). For themost part, these models and analysis procedures

are designed foroperation bnor to be compatible with a personal computer.

The following are being proposed for addition to Appendix Bofthe Guideline with an indication that they are available for application in theunique circumstances to which theyapply on a case-by-case basis: Shoreline Dispersion Model (sea/lakebreeze fumigation) and WYNDvalley (valley stagnation). Two models fallingin a similar category that

are already in appendix B are: MESOPUFF-Il (long-range transport) andPLUVUE-Il (visibility).

Besides adding new techniques, EPA is alsoproposing to clarify and update portions ofthe Guideline to make it consistent with current regulatory programs thathave been established throughother Agency activities.

Specified proposed actions on models, analysis procedures, and Guidelines modifications are discussed in the following section. Codes,user's guides, and other documents or guidance pertinent to specific modeling techniques are identified as individual Docket Items. These models and other changes willbe adopted in the Guidelines on Air Quality Models (Revised)through supplement B(Draft) (DocketItemrn-B-); seethe ADAESS Section of thisnotice for 

### general availability. Ofthe techniques considered atthe fourth modeling conference, onlytwo are notbeing proposed forfurther action atthis time; they are point source modeling for ozone precursors andregional scale models. Proposed Action

A. Complex Terrain Models
1.Refined Model

The ComplexTerrain Dispersion Model (CTDM) (Docket Item I-I-ib)is the result of amajor development and evaluation effort. It should fill a significant voidin the Guideline by providing a refined technique for determining pollutant concentrations due to plume impingement on the windward side ofterrain features during stable and neutral atmospheric conditions. Public comments generally support EPA's view that the model is technically sound,represents the best theoretical approach, and should be adopted for regulatory application as a refined model.

One concern hadbeen the ability to estimate concentrations during unstable conditions. Public comments discussed several ways ofcoping withthis matter, but in themeantime, EPA has elected to deal directly with the problem by augmenting CTDM with a module for unstable conditions based on current scientific theory. The new model is knownas CTDMPLUS and has undergone sensitivity analysis and evaluation (DocketItems 11--30,11-1-- 31). The user's guide documenting the scientific assumptions, Input and operating requirements, model options anddefaults, andthe code is available

as EPA-600/8-89/041 (NTISNo.PB89- 181424).



### Other concerns related to limitations ofthe model orits data base requirements. Based on those comments, EPA is limiting application of CTDMPLUS to the windward side of terrain features and makes no attempt at this time torecommend techniques for lee side effects orother complex terrain phenomena.However, CTDMPLUS is applicable to all terrain receptors above stackheight; thus,for applications of this model the "intermediate-terrain" problem that hasbothered many users of complex terrainmodels is eliminated. (Otherregulatory models in the Guideline are currently recommended forreceptors at or below stackheight).

Withregard to data base requirements, commenters would like these tobe set forth as specifically as possible. However, due to the intense datarequirements ofthe model (Docket Items Il4-lo-d) and limited experience with itsuse for regulatory applications, these specifications mustremain somewhat general in the Guideline. Meteorological inputs to CTDMPLUS shouldbe based on multi-level wind speed anddirection data obtained upto 5901

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representative plume height; SODAR data may be appropriate. Direct turbulence measurements and vertical potential temperature gradient should be obtained for at least three measurement levels with the topmost level as close to plume height as practical: generally, data up to 15Dm above stack base on an instrumented tower shouldbe adequate. Elements ofthe on-site meteorological
measurement program, and development ofa receptor network, shouldbe developed on a case-by-case basis with the concurrence of the EPA Regional Office.

2. Screening Techniques

EPA has also received varied
comment on the advisability of establishing a screening technique derived from CTDM; such a model would have a better scientific basis than currently available screening techniques. Some commenters recognize the desirability of such a technique, while others express concerns about the actual need and difficulty ofestablishing

a standard set ofmeteorological conditions for the screening technique. EPA has elected a screening technique entitled CTSCREEN, EPA-600/8-90/ XXX (Docket Item I-J-7). This technique is essentially CTDMPLUS with a predetermined range of meteorological conditions that are thought to bracket the highest concentrations formost source-terrain configurations. The preparation of this technique isjustified by the improved scientific basis derived from the CTDMPLUS code. It willbe allowed as

an alternate screening technique.

3. Sensitivity Analysis

Adoption of a new model can affect the design concentrations on which emission limits are based, thus, sensitivity analyses for CTDMPLUS as a refined model were conducted (Docket Item II-I-28a-l). These analyses compared design concentrations from CTDMPLUS to those from available complex terrain screening techniques for

a variety ofsource-terrain configurations. The expectation was that the screening techniques would provide generally higher design concentrations than CTDMPLUS; by intent,the screening techniques should be more conservative than this refined technique which has also been established as the most accurate ofall complex terrain techniques considered (Docket Item 1-1-2 and 11-1-3). This expectation was satisfied in most cases

so that CTDMPLUS provides less stringent and more appropriate emission limitations than previously possible with the conservative screening techniques in use.The degree of conservatism, though, among the screening models was not always consistent within the present hierarchical structure ofsuch techniques. The addition ofCTSCREEN further complicates the situation and, in combination with findings ofthe sensitivity analyses, places in doubt the merits of such a hierarchical structure. Thus, the structure will be dropped so that any acceptable screening technique
may be used consistent with the needs,

resources, and available data ofthe

user.

In a few cases, the screening techniques (RTDM more so than other models) have the potential to provide less stringent design concentrations than does CTDMPLUS and may not be acting effectively as conservative screening techniques. However, for several

reasons the limited findings cited here

are not thought to be a significant enough matter to justify any further changes in the use of these models. For instance: (1) Recent evaluations with the
current version ofRTDM-default show the model to provide conservative estimates compared to measured data; (2) the experience with CTDMPLUS in relation to screening models is insufficient to draw conclusions with
any confidence; (3) the extent to which RTDM might underestimate concentrations is not unequivocally delineated in the various analyses; (4) estimates lower than those of CTDMPLUS occasionally occur with other screening models, although rarely; and (5) since RTDM is the most refined ofthe screening models, it is expected to provide the most realistic of the conservative estimates and occasional

crossover to lower estimates than those derived from CTDMPLUS might not indicate significant inaccuracy. Nevertheless, screening models should generally function as conservative estimators of ambient concentrations, and EPA seeks public comment on limited exceptions like those noted here. As additional information becomes available and experience with CTDMPLUS is gained over the next few
years, it may be necessary to further consider this matter.
4. Conclusion

EPA is proposing to identify CTDMPLUS in chapter 5 of the Guideline as a recommended refined model for complex terrain with a minimum set ofrequirements for data used in the model.More detailed elements ofthe meteorological
measurement program and the selection ofreceptor sites are to be developed on

a case-by-case basis with concurrence ofthe EPA Regional Office. The model 

### will be included in appendix A ofthe Guideline. CTSCREEN willbe added as
an acceptable technique in chapter 5. The hierarchical structure ofacceptable screening techniques is proposed to be dropped, allowing discretion ofthe user and the EPA Regional Office or reviewing authority as to which technique is most useful for a given application.
B.MobileSourceModelingat Signalized Intersections
Public comment indicates concern that current guidance on modeling carbon monoxide concentrations for intersections (Worksheet 2 ofthe "Guideline forAir Quality Maintenance Planning and Analysis", Volume 9, EPA450/4-75-001) is difficult to use, 

### outdated, and cannot adequately model over-capacity situations. In response, EPA developed a program to review traffic and emissions components of intersection models and has consulted withthe Federal Highway Administration (FHWA) on such analyses. A plan for conducting these analyses and developing an improved intersection model was presented at the fourth modeling conference. Public comments supported the need for an improved modeling technique and more specifically the program that EPA proposed.

As a result ofthis program, EPAhas completed development ofthe CAL3QHC intersection model which has improved traffic and emissions components; it incorporates CALINE3 which is the recommended Guideline dispersion model for roadway sources. Documentation for the code and user's guide is available as Docket Item II-J--8. CAL3QHC and a related screening technique appear to be suitable to replace Worksheet 2 for refined analyses and theHot Spot Guidelines for screening analyses in estimating CO concentrations. Since this is thefirst public view ofthe complete model, no restrictions on use were identified in the public comments other than restrictions to roadways at intersections. EPA has alsoprepared "Guideline for Modeling CarbonMonoxide from Roadway Intersections" (DocketItem 11-J-9) and solicits comments regarding it.

Performance evaluation ofCAL3QHC (DocketItem 11-1-32) indicates that this is among the most accurate ofseveral models tested for signalized intersections. A sensitivity analysis (DocketItem 11-1-29) was also performed to determine the impact of CAL3QHC on design concentrations relative to the other available techniques. Contrary to expectations, it I 

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was found that CAL3QHC produced higher estimates and the need formore stringent controls than did Worksheet 2 and some other techniques for the intersection scenarios considered. This indicates that EPA'sexisting guidance

on modeling forroadway intersections up untilthis time may tend to lead to underestimates ofhighest ambient concentrations.

Conclusion:EPA isproposing to include CAL3QHC and a related screening technique as the recommended models for roadway intersections in Section6.2 of the Guideline. They would replace Worksheet 2 forrefined analyses and the Hot Spot Guidelines for screening analyses. CALINE3remains the recommended dispersion model in appendix A forroadways.
C.Emissionsand Dispersion Modeling SystemforAirports

The Federal Aviation Administration and theU.S. AirForce have jointly developed theEmissions andDispersion Modeling System (EDMS} to provide an integrated assessment ofpollution from multiple sources (e.g., aircraft,motor vehicles, and power plants) at airports and airbases. These agencies have requested that EPAadopt this modeling system to fill the void in the Guideline which does notcontain any models or recommendations oriented specifically to these operations. While theemissions components ofEDMS are a unique new application, the dispersion components

are consistent with models already recommended in the Guideline and contained in appendix A. A user's guide and code for theEDMS model are contained in DocketItem 11-1-8.

Thepublic comments on thismodel

were mixed and appeared to confuse the identified model with an earlier version. Nevertheless, given the apparent need for such a model and the fact that dispersion components are consistent with techniques alreadyrecommended in the Guideline, EPA believes that EDMS should be considered for inclusion in appendix A ofthe Guideline. However, itis appropriate to restrict theregulatory application of EDMS to thoseimpact assessments which contain orhave as a majorfocus changes in aircraft operations which willresult in a significant change in emission and pollutant concentration patterns.For example, ifa combustion unit at an airport requires a PSD permit byitself, independent ofaircraft operations ormobile source peripheral to,but offairport property, then a model recommended in the Guideline for these specific applications should be used in such an analysis. Conclusion.EPA is proposing to 

### include EDMS as the recommended model for,butlimited to, airport operations as identified in section 7.2.8 ofthe Guideline. The model will be included in appendix A ofthe Guideline.
D.Modeling forAirPathway Analyses
Atthe fourth modeling conference, EPA sought comment ontechniques for developing air pathway analyses of toxic and hazardous waste pollutants. Modelingfor these pollutants is done

across anumber ofthe EPA offices with varying levels ofdetail in the analysis performed. While the public comments did notgive as much attention to this matter as had been expected, there was general support for uniform modeling guidance and standardized models both through direct availability ofmodels and through improvedguidance in specific

areas. The comments have already been satisfied in part, since ISC is the model that serves as thebasis formostair pathway analyses ofcontinuous pollutant emissions.In addition, the "Workbook ofScreening Techniques for Assessing Impacts ofToxicAir Pollutants", EPA-450/4-88-009 (NTIS No. PB 89-134340) provides the basis for 

### obtainingpreliminary estimates ofshortterm concentrations due to a wide range oftoxic/hazardous pollutant release scenarios. Also, DEGADIS 2.1 is available as a refinedmodel for treating releases ofheavier-than-air gases over timeperiods oflimited duration,EPA450/4-89-019 (NTIS No. PB 90-213893).

Conclusion: EPA proposes to amend sections ofthe Guideline appropriate to air toxics to discuss several items. For instance, the ISC model willbe identified in section 7.2.7 and in appendix A as the recommended air quality model for analyses oftoxics/ hazardous pollutants, especially those that involve continuousreleases. Screening techniques contained in the "Workbook ofScreening Techniques for Assessing Impacts ofToxic Air Pollutants" will be identified as available for airtoxics analyses. References in section 12 will be expanded to identify guidance developed by other EPA programs which, in general, heavilyrely on the ISC model, and to provide more information ontoxic chemical properties. In addition, DEGADIS 2.1 will be added to appendix Bas a refined model for densegases that maybe used.

as appropriate, on a case-by-case basis.
E. On-SiteMeteorological Program Guidance

The use ofon-site meteorological data to support air quality impact analyses has grown steadily over recent years. EPAhas published a document titled "On-site Meteorological Program Guidance forRegulatory Modeling Applications", EPA-450/4-87-013 (DocketItem 11-1-17]. The purpose of this document is to supplement the limited guidance on this subject currently in chapter 9 ofthe Guideline. It also consolidates into a single document specific guidance on thecollection and

use ofon-site meteorological data for air quality modeling analyses. This document hadundergone extensive external peer review by State agencies, meteorological instrument manufacturers, and other Federal agencies prior to thefourth modeling conference.

Also, in response to the needfor consistency in theprocessing ofon-site meteorological data, EPAhas developed

a Meteorological Processor for Regulatory Models (MPRM), EPA-600/ 3-88/043 (Docket Item 11-1-18). This computer algorithm is designed to process on-site meteorological data following theprocedures recommended in the on-site meteorological program guidance document, and produces output necessary torun the EPA regulatory dispersion models. Itis designed to provide considerable flexibility tousers interms ofinput data formats, and maybe easily expanded to accommodate the data requirements of future models.

Public comments generally supported the basis for and adoption ofthe on-site meteorological guidance document and MPRM inthe Guideline. In addition, a variety ofminor technical points were made about both.EPA has assessed these technical pointsindividually and finds It unnecessary or inappropriate (as documented in the response to comments document) to act onmost of them for thisproposal. However,EPA agrees with comments suggesting a revised stability classification scheme. EPA developed a scheme and conducted

a sensitivity analysis (DocketItem 11-j12). The scheme is included in an addendum (DocketItem 11-1-14) to the guidance document, the Guideline and
MPRM. I



### Conclusion:EPA proposes to reference the document "On-Site Meteorological Program Guidance for Regulatory Modeling Applications" in section 9.3.3 asthe primary source of supplementary guidance onthe collection and use ofon-site meteorological data.The hierarchy of stability classification schemes in that document willbe changed to the preferred scheme that is based on solar radiation and temperature difference measurements, but the useof other 5903

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techniques prior to a year following promulgation willbe exempt from this provision. As part ofincorporation of the on-site guidance document, MPRM willbe identified as the standard processor ofon-site meteorological data input forregulatory model applications.

F. General Screening Techniques
The Guideline references Volume 10(R), EPA-450/4-77-001, as the basis of recommended screening techniques for stationary point sources. This document has been revised to incorporate additions to the technical approach and changes so that the techniques may easily be executed on a personal microcomputer. The revised document is "Screening Procedures for Estimating the Air Quality Impact ofStationary Sources", EPA-450/4-88-010 (Docket Item 11-1-20), including an addendum (DocketItem lI-I-20a). The Guideline also references a visibility workbook, EPA-450/4-80-031. A revision ofthe document, "Workbook for Plume Visual Impact Screening and Analysis", EPA450/4-88-015 (Docket Item 11-1-13), has been prepared to reflect current technical information and experience. This revised document contains procedures and a visibility model that

can be used for screening purposes to estimate visibility impairment from specific sources. Both of these revised documents were discussed at the fourth modeling conference.

Comments on the two sets of screening techniques were mixed, although EPA believes that these new personal computer based techniques are
an improvement over the existing handcalculation techniques. EPAhas addressed a variety ofcriticisms and suggested changes in the response to comments document, where it isnoted that many ofthese comments are either inappropriate or misinterpret the intent and basis ofthe guidance. Therefore, since there has been time forthe public
to gain additional experience with these techniques, their attributes will be more apparent and their basis relative toprior screening techniques better understood. These computerized techniques are not intended to be inherently different or
more conservative, but are instead
meant to better organize and ease the computational burden.

Conclusion:EPA proposes to identify "Screening Procedures forEstimating the Air Quality Impact of Stationary Sources" with the SCREEN model as the recommended screening technique in section 4.2 of the Guideline. It also proposes to identify "Workbook for Plume Visual Impact Screening and Analysis" with the VISCREEN model as the recommended screening technique 

### for visibility assessments in section 7.2.4.

G.Method forEvaluating Models

The Guideline refers to "Interim Procedures for Evaluating Air Quality Models", EPA-450/4-84-023 as the basis for determining which of two or more models is most appropriate for a given application. However, EPA has recently refined these methods and advanced the statistical methodology for determining which model is better suited to a particular situation. With this newer methodology, it is feasible to combine results from different averaging periods and different data bases into a probabilistic framework. This methodology is described in an EPA report titled "Protocol forDetermining the BestPerforming Model" (Docket Item 11-1-19) with an appendix that provides examples of a protocol comparing the performance oftwo rural air quality models.The method is now also documented in a user's guide (Docket Item 11--33).

Thepublic comments were cautious in their support of this improved methodology. Concerns primarily addressed flexibility of the techniques and limitations inexperience with their

use. Nevertheless, EPAbelieves that these techniques may be exercised with great flexibility: The example presented is not intended to be the only set of statistical comparisons. When used with this built-in flexibility, these techniques should enhance the ability to assess the relative merits of the available models for a given application.

Conclusion:EPA proposes to include the "Protocol for Determining the Best Performing Model" as an adjunct to the "Interim Procedures" when the use ofan alternative model is beingjustified for a site specific application. This analysis procedure will be referenced in sections 3.2 and 10.1 ofthe Guideline.

H.AlternateModels inAppendixB

At the Fourth Conference on Air Quality Modeling, EPA identified four additional models that have unique applications for which there is currently

no recommended model. Those models (and their applications) are: Shoreline Dispersion Model (SDM) (sea/lake breeze fumigation), WYNDvalley (valley stagnation), MESOPUFF-1I (long-range transport), and PLUVUE-1I (visibility). Actually no one long range transport model was specifically referenced, but from the limited statistical data available MESOPUFF-I appears to be the most accurate ofthe models identified in EPA's performance evaluation. MESOPUFF-II is also the model for which EPA prepared an example protocol.

The public comments on these four models were mixed, but no fatal flaws

were identified. However, there was general comment directed toward these models with which EPA basically
agrees: that is, the performance evaluation ofthese models may be of limited utility due to sparse data bases. As a result, information on the performance ofthese models is either inadequate tojustify recommending generic use orthe evaluation has identified limitations which need to be further investigated before a recommendation may be issued. Information on the user's guides, codes and accuracy of these models beyond that information already in the Guideline follows:

SDM

User's Guide to SDM-A Shoreline Dispersion Model, EPA-450/4--88-017 (Docket Item 11-1-15).

Analysis and Evaluation ofStatistical Coastal Fumigation Models, EPA-450/4- 87-002 (Docket Item 11-1-14). WYNDvalley

A User's Guide to WYNDvalley 3.11 (Docket Item II-I-16a-c).

MESOPUFF II

Evaluation ofShort-Term, Long Range Transport Models, Volumes I and 2, EPA-450/4-86--016a and 016b (Docket Item lI-I-9a and 9b).

Review ofShort-Term Long Range Transport Models (Docket Item 11-1-10).

A Modeling Protocol for Applying MESOPUFF-I1 to Long Range Transport Problems (Docket lI-I-11).

Also see the Guideline.

PLUVUE II

See the Guideline. Even with the limitations identified above and in the public comments, EPA finds that these models fill a unique void in available modeling applications and that they may be at least as accurate, under appropriate conditions, as other available techniques.

Conclusion:EPA is proposing to add SDM and WYNDvalley to appendix B of the Guideline. In addition, these models along with MESOPUFF-II and PLUVUEII will beidentified as models available for unique applications on a case-bycase basis in the following respective sections ofthe Guideline, sections 8.2.9, 8.2.10, 7.2.6, and 7.2.4.

L SupplementaryChanges

Besides the addition ofnew models, EPA is also proposing revisions to make

### Federal Register / Vol. 56, No. 30 / Wednesday, February 13, 1991 / Proposed Rules

the Guideline consistent with regulatory programs that have been formalized or changed since thelast major Guideline revision in 1986 (51 FR 32176).

In addition to minor wording changes, the following procedural changes/ additions tothe Guideline are worthy of note:

1.A recommendation for consultation with the Regional Office on complex issues concerning VOC/NO. point

source remodeling (section6.2.1);

2. Clarification ofmodeling requirements applicable to prevention of significant deterioration for NOa (section 6.2.3);

3. Changing references to particulate matter, i.e., PM-10 replacing TSP (sections7.2.2 and 11.2.3);

4. Clarification on the use ofemissions data for NAAQS analyses associated with prevention ofsignificant deterioration (PSD) compliance demonstrations (Table9-2), in particular,estimating emissions in a
mannerwhich more closely reflects the allowable ratherthan theactual emissions forbackground sourcesS;

5.Deletion oftheERTAQ and MPSDM models from Appendix B at the request ofthe model developer since these models will no longerbe supported and they have not beenwidely used;

6. Updates to theOffshore and Coastal Dispersion (OCD) model to reflect recent improvements prepared by the Minerals Management Service which has funded development ofthis model (appendix A),

7. Updates to the Urban Airshed Model and to EKMA (OZIPP)to include thelatest chemical mechanism (CB-4) and toreflect improved and updated guidance (section 6.2.1 and appendix A).

The CB-4 mechanism is being substitutedbecause it Isa clearly superior technique. It hasbeen the subject ofscientific and evaluation studies (DocketItem 11--27) and the scientific community hasgenerally found thischemical mechanism tobe technically sound andto accurately represent detailed smog chamber findings (DocketItem 11-1-26). Also, results from a study ofSt.Louis, where comparison between estimates and observations forCB-4 vs. CB-2 is possible, indicate more accurate ozone estimates and less underprediction with CB-4 (DocketItem 114-34).

EPA has also reviewed the definition ofmodel calibration in section 8.2.11 of

. Thi crifcation does notapp ywhen calculatingincrement consumption under the PSI) program,since the regulatlon applicabletothat analysis require, eapy optate.actual (vs. allowable) mInkme. Sea 4 CFR 52.mb}(1J3Xi/ 45 FR 52717-2M Agu*4 7.29M 

### the Guideline to determine if unacceptable practices are clearly and precisely identified and prohibited. This

was donein response to a report by the General Accounting Office that addressed, in part,the consistent use of air quality modelsin regulatory decisions (Docket Item II-F-1). Upon review, EPA has found the Guideline to be clear and appropriately definitive; it states in part that "model calibration is unacceptable". Past experience has indicated that this guidance is unambiguous; further detail applied to prohibiting calibration ofthese complex mathematical tools could confuse and
create the appearance of "loopholes". No furtheraction on calibration or changes to section 8.2.11 are here proposed.

This Notice alsoproposes to amend 40 CFRpart 51 to give regulatory status to long-standing EPA policy regarding the

use ofair quality models forpurposes of control strategy development, AQMA analyses, classification ofregions for episode plans, andnew source review, thereby clarifying and codifying thatthe Guideline is applicable for those purposes. In addition to explicit reference toregulatory modeling in section 166 (aswell as §52.21), the introduction tothe Guideline makes clear that its "airquality modeling techniques * * *should be applied to State Implementation Plan (SIP) revisions for existing sources and to

new source reviews * * ". Therefore, 

### because use ofairquality models is required inJ§51.46 (Projection ofAir Quality Concentrations) forprojections associatedwith AQMA analyses, 51.112 (Demonstration ofAdequacy) forSIP revisions, 51.117 (Additional Provisions forLead) for analysis oflead concentrations, 51.150 (Prevention ofAir Pollution Emergency Episodes) for classification ofregions forepisode planning, and 51.160 (Review ofNew Sources andModifications) fornew

source review, this amendment to 40 CFR part51 will serve to close a gap not addressed in previousrulemakings. Note that with respect tothe proposed revision of40CFR 51.160,it is intended that theGuideline applyto theentire subpart, includingbut notlimited tonew and modified source review under §

### §51.165 (a)and (b) and 51.166(1).

J.PublicCommentsan OtherTopics

At theFourth Conference onAir Quality Modeling, EPA solicited comments on (1) the technical validity of

a screening technique forpoint sources ofozone precursors and (2) the usefulness ofregional scale models to regulatory programs. Public comments

onthe screening techniquefound them inappropriate, too conservative, and requiring further research and testing. Based on these comments, EPA does not intend to propose these screening techniques for inclusion in the Guideline
at this time.

With regard to regional scale models,
commenters supported the need for and

use ofthese models. However. it was felt that they are too computationally complex and costly in terms ofcomputer and human resources to include in the Guideline-at this time. EPA agrees that further testing and experience with these models is needed forregulatory application, and norecommendations on their use are being made at this time.

In addition, a variety ofpublic
comments were provided on subjects for which EPA did not seek input; most prominent were thevariable emissions for SO2 and the Industrial Source Complex (ISC model. Commenters urgeduse of the expected exceedances method for estimating SO2 concentrations and an expanded use of the multi-point rollback method (46 FR 58101, November 30, 1981). However, EPAdoes not intend to modifygeneral provisions ofthe Guideline to include these methods. With the current deterministic form of the SO2 NAAQS, the expected exceedances method would provide no assurance that the NAAQS would be attained and maintained. Also, EPAdoes not generally endorse multi-point rollback and has limited its use to a few cases for

a single source type.Also, comments on ISC identified operatingproblems and

concerns about limitations ofthe model. EPA has corrected the problems and is actively working to keep the model
current. There Is an ancillary ongoing
program to upgrade the model to make it easier to use and maintain.

Other comments considered: (1)Issues which havebeen addressed elsewhere in EPA policy memoranda outside the
scope ofthe Guideline; (2) issues that

were previously resolved in prior Guideline revisions subjected to public comment; (3) narrow technical issues which are premature to address; and (4) issues upon which EPA has already acted, e.g.. implementation of an electronic bulletin board. For obvious

reasons, EPA plans no further action on these matters, as discussed in the
response to comments document.
E.O. 12291

Under Executive Order 12291, EPA
must decide whether a rule is "major" and therefore subject to the requirement ofa Regulatory Impact Analysis. The Administrator finds thisproposed rule
not major because it will not have an 5905

### Federal Register / Vol: 56, No. -30 / Wednesday, February 13.,- 1991 ,/ 

### Proposed Rules

annual effect on the economy of$100 million or more; itwill not result in a major increase in costs orprices; and there will be no significant adverse effects on competition, employment, investment, productivity, innovation or

on the abilityofU.S.-based enterprises to compete with foreign-based enterprises in domestic or export markets. This regulation will result in no significant environmental or energy impacts. Thus,no Regulatory Impact Analysis was conducted. Regulatory Flexibility Act

Pursuant to the provisions of5U.S.C. 605(b), Ihereby certify that the attached proposed rule willnothave a significant impact on a substantial number ofsmall entities.This rule merely updates existing technical requirements for air quality modeling analyses required by other Clean Air Actprograms (e.g., prevention ofsignificant deterioration,

new source review, SIP revisions) and imposes no new regulatory burdens.
Economic Impact Assessment

The requirement forperforming an economic impact assessment in section 317 ofthe Act, 42U.S.C. 7617, does not apply to this proposed action since the revisions included do not constitute a substantial change in theregulatory burden imposed by theregulation. Paperwork Reduction Act

This proposed rule does notcontain any information collection requirements subject to review by the Office of Management and Budget (OMB) under thePaperwork Reduction Actof 1980, U.S.C. 3501 etseq.EPA has submitted this regulation to the OMB for review under Executive Order 12291 and their written comments onthe revisions and any EPA responses have been placed in the docket forthis proceeding.
List ofSubjects
40 CFRPart 51
Administrative practice and procedure, Airpollution control, Carbon monoxide, Hydrocarbons, Intergovernmental relations, Lead, Nitrogen dioxide, Ozone, Particulate matter,Reporting and recordkeeping requirements, Sulfur oxides.

40 CFRPart 52


### Air pollution control,Lead, Nitrogen dioxide, Ozone, Sulfur oxides.

Authority-This Notice ofProposed Rulemaking isissued underthe authority granted by sections 110(a)(2), 165(e), 172 (a) & (b), 173,301(a)(1) and320 ofthe 1977 Clean AirAct Amendments, 42 U.S.C. 7410(a)(2), 7475(e). 7502(a) & (b). 7503, 7601(a)(1) and 7620, respectively (see Footnote 2under
"SUPPLEMENTARY INFORMATION").

Dated: February 5,1991. William K. Reilly, Administrator.

It is proposed to amend part 51, chapter I,title 40 ofthe Code ofFederal Regulations as follows:

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

1.The authority citation forpart 51 is revised toread as follows:

Authority: 42 U.S.C. 7410(a)(2), 7475(e), 7502 (a) and (b), 7503, 7601(a)(1) and 7620.

2. Section 51.46 is amended by revising paragraph (b) and removing paragraph (c) to read as follows: §51.46 AOMAanalysis: Projectionofair quality concentrations.

Such concentrations shall be projected using techniques consistent with the requirements in §51.112(a).

§51.63 [Amended]

3.In 

### §51.63, paragraph (a) is amended byremoving "51.46,".

4. In §51.112, paragraph (a) is amended by removing the second
sentence and adding paragraphs (a)(1) and (a)(2) to read as follows: 

### §51.112 Demonstration ofadequacy.



### (a) * * * (1)The adequacy ofa control strategy shall be demonstrated by means of applicable air quality models, data bases, and other requirements specified in the "Guideline on Air Quality Models (Revised)" (1986), supplement A (1987) and supplement B (1990), which are incorporated by reference. The Guideline and its Supplements (EPA Publication No. 450/2-78-027R) are for sale from the U.S. Department of Commerce, National Technical Information Service, 5825 Port Royal Road, Springfield, VA 22161. They are also available for inspection at the Office ofthe Federal Register, room 8301, 1100 L Street NW., Washington, DC. These materials are incorporated as they exist on the date ofapproval and a notice ofany change willbe published in the Federal Register.

(2) Where an air quality model specified in the "Guideline on Air Quality Models (Revised)" (1986), supplement A (1987) and supplement B (1990) is inappropriate, the model may be modified or another model substituted. Such a modification or substitution ofa model may be made on
a case-by-case basis or, where appropriate, on a generic basis for a specific state program. Written approval of the Administrator must be obtained for any modification or substitution. In addition, use ofa modified or substituted model mustbe subject to notice and opportunity forpublic
comment under procedures and set forth in § 51.102.

§51.117 [Amended]

5. In §51.117, paragraph (c)(1) is amended by adding the phrase ", consistent with requirements contained in § 51.112(a)" immediately after ".* * ifdesired". Paragraph (c)(2) is amended by adding the phrase ", consistent with requirements contained in §51.112(a)" immediately after " * *for demonstration ofattainment". Paragraph (c)(3) is amended by adding the phrase ", consistent with requirements contained in § 51.112(a)" immediately after " *..for the demonstration ofattainment".

§51.150 [Amended]

6. In § 51.150, paragraph (e) is amended by adding the phrase ", consistent with therequirements contained in §51.112(a)" immediately after " * * ofthis section" in the first sentence, and by removing the second
sentence.

7. In § 51.160, is amended by adding paragraphs (f)(1) and (f)(2) to read as follows:


### §51.160 Legallyenforceable procedures.

* * * * *

(1) All applications of air quality modeling involved in this subpart shall be based on the applicable models, data bases, and other requirements specified in the "Guideline on Air Quality Models (Revised)" (1986), supplement A (1987) and supplement B (1990), which are incorporated by reference. The Guideline and its Supplements (EPA Publication No. 450/2-78-027R) are for sale from the U.S. Department of Commerce, National Technical Information Service, 5825 PortRoyal Road, Springfield, VA, 22161. They are also available for inspection at the Office ofthe Federal Register, room 8301, 1100 L Street, NW., Washington, DC. These materials are incorporated as they exist on the date ofapproval and a notice ofany change will be published in the Federal Register.

(2) Where an air quality model specified in the "Guideline on Air Quality Models (Revised)" (1986). 

### ,,,506

### Federal Register / Vol. 56, No. 3 Wednesday, February 13, 1991 / Propdsed Rules

supplement A (1987) and supplement B (1990) is inappropriate, the model may be modified or another model substituted. Such a modification or substitution of amodel may be made on

a case-by-case basis or, where appropriate, on a generic basis for a specific state program. Written approval ofthe Administrator mustbe obtained for any modification or substitution. In addition, use ofa modified or substituted model mustbe subject to notice and opportunity for public
comment under procedures set forth in § 51.102. §51.166 [Amended] 8. In § 51.166, paragraph (1)(1) and (1)(2) are amended by removing "and supplement A (1987)" and by adding ", supplement A (1987) and supplement B (1990)".

It is proposed to amend part 52, chapter I of title 40 ofthe Code of Federal Regulations as follows:

PART 52-APPROVALAND PROMULGATION OF IMPLEMENTATION PLANS

1. The authority citation for part52 continues to read as follows: Authority: 42 U.S.C. 7401-7642. 

### §52.21 [Amended]

2. In § 52.21, paragraph (1)(1) and (1)(21

are amended by removing "and supplement A (1987)" and by adding ", supplement A (1987) and supplement B (1990)". [FR Doc. 91-3448 Filed 2-12-91; 8:45 am]

BILUNG CODE 6560-50-M 

### 5907