MODEL CLEARINGHOUSE: FY-90 SUMMARY REPORT OCTOBER 1990 Source Receptor Analysis Branch Technical Support Division Office of Air Quality Planning and Standards U.S. Environmental Protection Agency Research Triangle Park, North Carolina 27711 TABLE OF CONTENTS MODEL CLEARINGHOUSE: FY-90 SUMMARY REPORT 1.0 INTRODUCTION 2.0 FY-90 CLEARINGHOUSE ACTIVITIES . . . . . . . . . . . . . . . 2.1 Background and Responsibilities . . . . . . . . . . . . 2.2 Summary of Regional Office Requests and Clearinghouse Responses . . . . . . . . . . . . . . . . . . . . . . . 2.3 Recurring Coordination . . . . . . . . . . . . . . . . 2.4 Support Center for Regulatory Air Models Bulletin Board System (SCRAM BBS) . . . . . . . . . . . . . . . 2.5 Model Clearinghouse Information Storage and Retrieval System . . . . . . . . . . . . . . . . . . . . . . . . 3.0 CLEARINGHOUSE RESPONSES TO RECURRING OR GENERIC ISSUES . . . 3.1 Guidance for Justifying PM-10 SIP's Based on Receptor Modeling . . . . . . . . . . . . . . . . . . . 3.2 Modeling of Area Sources with ISC . . . . . . . . . . . Appendix A FY-90 MODEL CLEARINGHOUSE MEMORANDA . . . . . . . . . Appendix B PM-10 SIP DEMONSTRATIONS FOR SMALL ISOLATED AREAS WITH SPATIALLY UNIFORM EMISSIONS . . . . . . . Appendix C SUMMARY OF THE EPA MODEL CLEARINGHOUSE . . . . . . . 1.0 INTRODUCTION This report summarizes major activities of the Model Clearinghouse from August 18, 1989, to August 17, 1990. The report is divided into two parts. First, the FY-90 activities of the Clearinghouse are summarized. The second section summarizes Clearinghouse responses to major categorical modeling problems occurring during FY-90; within each of these there were several issues/questions of a similar nature. Appendix A provides a list of Model Clearinghouse memoranda covering the period October 1, 1989 to September 30, 1990. Appendix B contains a guidance memorandum on PM-10 SIP modeling developed in conjunction with the Model Clearinghouse. Appendix C provides a brief summary of the EPA Model Clearinghouse responsibility, structure and operational procedures. 2.0 FY-90 CLEARINGHOUSE ACTIVITIES 2.1 Background and Responsibilities The Model Clearinghouse was established on November 20, 1980. The Regional Offices were notified of its general purposes, procedures and limitations on that date. A detailed operational plan describing the functions, structure, procedures, and schedule for implementation was issued on February 25, 1981. Initially the operation of the Model Clearinghouse was limited to the review of proposed deviations from modeling guidance for the regulatory modeling of iron/steel facilities, smelters, and power plants. As such, the reviews were usually limited to SO2, TSP, and Pb. On a gradual basis the limitation to these facilities was relaxed to the point where the Clearinghouse, by the mid-1980's, was accepting referrals for all point sources of stable pollutants. Beginning in 1986, the Clearinghouse began accepting referrals for mobile source problems involving CO. During FY-88, the Model Clearinghouse was expanded to cover O3 and NO2. A revised Model Clearinghouse operational plan was issued to reflect that the Model Clearinghouse purview includes all criteria pollutants. The Model Clearinghouse Operational Plan states that the primary purposes of the Model Clearinghouse are to provide: 1. A mechanism whereby the proposed acceptance by a Regional Office of a nonguideline model or alternative technique can be reviewed for national consistency before final approval by the Regional Administrator. Such reviews promote the use of equivalent acceptance criteria by all Regions. 2. A mechanism whereby the in-depth technical evaluation and/or performance evaluation of a proposed technique can be reviewed by those EPA personnel who are most familiar with the types of techniques to be employed. 3. A communication outlet for EPA's experience with the use of nonguideline models, data bases or other deviations from current guidance. The Clearinghouse maintains a high level of expertise on the applicability of various models and classes of models and allowed deviations from accepted procedures in specified circumstances. This information is communicated on a periodic basis to EPA personnel involved in regulatory model applications. The Clearinghouse is formally located in the Source Receptor Analysis Branch (SRAB) of OAQPS. However, the Air Quality Management Division (AQMD) also participates in Clearinghouse matters involving SIP attainment strategies and other regulatory functions. The primary responsibility for managing the Clearinghouse and ensuring that all of its functions are carried out is performed by a person full-time within SRAB. The responsibility for responding to requests for review of modeling issues is assigned, on a pollutant/program basis to three SRAB individuals. In addition, AQMD supports the Clearinghouse with staff who are also knowledgeable in modeling policy. These individuals are responsible for screening SIP submittals and related documents, referring modeling issues to SRAB through the Clearinghouse, and documenting the final (and any significant interim) decision on disposition of the issues. The FY-90 activities for the Model Clearinghouse included: 1. Responding to Regional Office requests for review of nonguideline models proposed for use. 2. Reviewing draft and formally submitted Federal Register actions. 3. Documenting Clearinghouse decisions and discussions. 4. Summarizing Clearinghouse activities at various meetings. 5. Issuing, internally, a summary report for activities of FY-89. 6. Inputting FY-90 Clearinghouse records into a computerized data base. 7. Providing for Regional Office direct access to the computerized data base by modem. 8. Disseminating Clearinghouse memoranda and reports to the public through a bulletin board system. 2.2 Summary of Regional Office Requests and Clearinghouse Responses The number and type of contacts from the Regional Offices to the Clearinghouse during FY-90 are summarized in Table 1. The data in the table under "Recommendations" include those issues where a coordinated clarification of policy was involved or an investigation had to be undertaken by the Clearinghouse. In addition, there were numerous other inquiries to the Clearinghouse regarding procedures, technical considerations, and policies which were readily resolved or referred elsewhere. Note from the table that there were a total of 23 regulatory modeling problems referred to the Clearinghouse which required a written response and another 67 referrals in which the issues were resolved orally. In addition, there were another 13 referrals where discussions took place but a Clearinghouse recommendation was not requested. Note also that requests for assistance, either written or by telephone, came from all the Regional Offices. This suggests that there is an awareness and a desire for Clearinghouse support throughout the Agency. TABLE 1. TYPES OF CLEARINGHOUSE RESPONSES DURING FY-90 RECOMMENDATIONS REGION WRITTEN ORAL DISCUSSION/INFORMATION * TOTAL I 0 5 1 6 II 1 5 0 6 III 3 5 4 12 IV 3 9 0 12 V 1 6 0 7 VI 4 8 4 16 VII 1 5 0 6 VIII 8 18 3 29 IX 2 2 0 4 X 0 4 1 5 Totals 23 67 13 103 -------------------- * Did not include a specific Clearinghouse recommendation. Table 2 contains a summary, by category, of the nature of the referrals and requests for assistance. The table is useful for identifying commonly recurring problems and problem areas. Table 3 tabulates the regulatory/modeling subject area involved in resolution of issues for each referral. More than one subject area was involved in the resolution of issues for most referrals. Also, it should be noted that in Table 3 referrals for Interpretation of Modeling Guidance and Interpretation of Regulatory Guidance usually involved broad issues where the applicability of the guidance in general was either called into question or that the issue involved a subject not included in the remainder of the subject area list. TABLE 2. CHARACTERIZATION OF 103 CLEARINGHOUSE REFERRALS DURING FY-90. ONLY REFERRALS CONTAINING SIGNIFICANT MODELING ISSUES ARE INCLUDED. SOME REFERRALS INVOLVED MORE THAN ONE POLLUTANT, SOURCE TYPE, OR REGULATION. Pollutants Involved Number of Referrals SO2 42 TSP 7 CO 12 Pb 5 NOx 7 O3 7 As 1 PM-10 21 Methylene Chloride 1 Unspecified 5 Unspecified Non-criteria 5 Generic 6 Source Types Power Plant 7 Power Boiler 2 Smelter 7 Steel Facility 3 Paper Mill 2 Refinery 5 Cogeneration Unit 9 Incineration 5 Mine 7 Chemical Plant 4 Roadway 8 Aircraft 1 Lime Kiln 1 Lumber/Wood Products Plant 1 Unspecified Stationary Source 9 Mixed/Multiple Sources 20 Generic to Any Stationary Source 11 Generic to Any Source 2 Urban or Rural Model Involved Urban Only 24 Rural Only 60 Both Urban and Rural 5 Not Relevant 14 Types of Response Written 23 Oral 67 Not Relevant 13 Applicable Regulations SIP 19 SIP Revision 18 SIP Revision - GEP 1 PSD 33 EIS 4 NSR 11 Bubble 1 Redesignation 1 State Regulation 5 Study 3 Superfund 3 FIP 2 Generic 2 Generic - PSD 4 Not Relevant 2 Terrain Setting High Terrain (above stack height) 8 Low Terrain (below stack height) 55 Both High and Low Terrain 14 Essentially Flat Terrain 12 Not Relevant 14 Guideline or Nonguideline Model Involved Guideline 38 Non-guideline 23 Guideline and Non-guideline 20 Not Relevant 22 Data Bases Used for Model Input On-Site 25 Off-Site 33 Both Off-site and On-site 3 Not Relevant 42 TABLE 3. SUBJECT AREAS INVOLVED IN FY-90 CLEARINGHOUSE REFERRALS SUBJECT AREA NUMBER OF OCCURRENCES * Adjustment of Model Estimates 3 Ambient Air 9 Ambient Monitoring 16 Area Wide Analysis 2 Calm Winds 2 Clearinghouse Operating Procedures 4 Deposition 2 Downwash 15 Emissions Characterization 11 Emission Rates for Model Input 14 Equivalency 3 Fluid Modeling 4 Grandfathering of Guidance 5 Interpretation of Modeling Guidance 7 Interpretation of Regulatory Guidance 14 Length of Record - Meteorological Data 4 Load Level 1 Long-Range Transport 2 Meteorological Monitoring 10 Mixed Terrain Modeling 5 Model Development Projects 3 Modeled Background 7 Monitored Background 4 Monitoring in Lieu of Modeling 12 Open Dust Fugitives 5 Performance Evaluations 12 Plume Rise 1 PSD Increment Calculation 2 Reactive Pollutant Modeling/Chemical Transformations 11 Receptor Modeling 11 Receptor Networks 4 Representativeness of Meteorological Data 3 Screening Procedures 5 Shoreline/Offshore Modeling 1 Significant Air Quality Impacts 10 Statistical Source-Receptor Analysis Techniques 1 Technical Credibility of Nonguideline Techniques 10 Urban/Rural 2 Wind Speed Profiles 2 Worst Case Meteorology 3 Stagnation 6 Visibility 2 ------------------------- * More than one subject was involved in the resolution of most referrals. 2.3 Recurring Coordination During FY-90 the Clearinghouse conducted or participated in a number of activities that can be categorized as coordination and information exchanges with the Regional Offices. One of the first activities was to prepare and distribute to the Regional Offices in October 1989, a Clearinghouse report; this report served to inform Clearinghouse users about the issues and responses which occurred in FY-89. The report included summaries of several generic issues which occurred during the fiscal year. During FY-90 the Clearinghouse continued its policy of sending copies of its written responses (along with incoming requests) to all the Regional Offices. In this way, the Regional Offices are made aware, in a timely fashion, of decisions that may affect their modeling activities. Also, in order for the Regions to be assured that their records are complete, the Clearinghouse attaches to each response, an updated list of all Clearinghouse memoranda issued during the fiscal year. The list is provided as Appendix A. The Model Clearinghouse has a policy whereby an advance opinion of the Regions is sought on particularly sensitive issues with national implications. During FY-90 one such case arose; it is summarized in Section 3.1. For this case the proposed Clearinghouse response was discussed in some detail at the 1990 Regional/State Modelers Workshop before the response was finalized. 2.4 Support Center for Regulatory Air Models Bulletin Board System (SCRAM BBS) During FY-90 EPA began disseminating Agency memoranda and reports generated by the Clearinghouse to the public through the SCRAM BBS. Three types of information are included: 1) Selected historical memoranda on generic/recurring issues that have been generated by the Clearinghouse in the FY-81 to FY-90 timeframe; 2) The most recent, i.e. FY-89 and FY-90, Clearinghouse memoranda; and 3) The FY-89 Model Clearinghouse annual report. Information on the SCRAM BBS is not intended to be a complete set of information relative to any subject. Rather, it serves as a means to communicate to regulatory model users significant decisions involving the interpretation of current modeling guidance and experience with the use of nonguideline models, data bases and other deviations from the current guidance. The SCRAM BBS cautions users to consult the Guideline on Air Quality Models and contact the appropriate State or Regional Office before using the information for individual/regulatory applications. As new Clearinghouse information becomes available it is placed on the bulletin board in one of the three categories mentioned above. However, in regard to the current year Model Clearinghouse memoranda, a one to two month delay occurs before the information is placed on the bulletin board. 2.5 Model Clearinghouse Information Storage and Retrieval System A primary purpose of the Model Clearinghouse is to provide a mechanism whereby the proposed acceptance by a Regional Office of a nonguideline model or alternative technique can be reviewed for national consistency before final approval by the Regional Administrator. To accomplish this purpose the Clearinghouse needs to be cognizant of the historical usage of nonguideline models and techniques in each application. To maintain this awareness the Clearinghouse maintains "paper" files (case histories) of referrals from the Regional Offices. These files serve to document the usage of nonguideline models and alternative techniques. Periodically, the information in the files is summarized and communicated to the Regional Offices such that they, too, can be aware of precedents when considering any new proposals from States or industry to apply nonguideline models or alternative techniques. Since FY-81 the Model Clearinghouse has accumulated approximately 800 records of referrals from the Regional Offices involving the interpretation of modeling guidance and the use of nonguideline models/techniques. This voluminous amount of information began to present problems in locating precedents and cases of a similar nature in order to promote the desired consistency when a new referral is received. In recognition of this growing problem, in FY-87 the Clearinghouse installed a PC software system whereby certain key information on each record can be stored in a data bank. The software, in turn, gives the user the capability to electronically search the data base to locate records of like characteristics that can be referred to in order to consider the consistency aspects of new referrals. This software is known as the Model Clearinghouse Information Storage and Retrieval System (MCHISRS). During FY-90 the Clearinghouse added approximately 100 entries to the approximately 700 FY-81 to FY-89 referrals into MCHISRS. Also during FY-90 a mechanism was developed under contract whereby the Regional Offices can now access by modem the MCHISRS in a search and retrieve mode for any national precedents that might affect their position on a current specific modeling issue. 3.0 CLEARINGHOUSE RESPONSES TO RECURRING OR GENERIC ISSUES During FY-90 the Clearinghouse responded to a number of queries/referrals which had common themes or common consistency/technical issues. The more significant issues, and the Clearinghouse responses to them, are described below. 3.1 Guidance for Justifying PM-10 SIP's based on Receptor Modeling In relation to their efforts to develop PM-10 SIP's in small isolated areas, some Regional Offices have expressed the need for flexibility in the SIP demonstration requirements where receptor modeling (RM), coupled with proportional (rollback) modeling, is considered to be adequate to identify source contributions and demonstrate attainment. In response to this need, during FY-90 the Model Clearinghouse participated in the development of the rationale and justification necessary to exercise this flexibility. This guidance was provided to the Regional Offices through a July 5, 1990 memorandum to the Air Branch Chiefs. A copy of this memorandum is included as Appendix B to this report. The guidance essentially says that there may be certain limited circumstances where RM alone may be adequate to demonstrate attainment of the PM-10 NAAQS. However, before using such a technique, a State would need to obtain approval from the Regional Office. The Regional Office would need to justify the use of RM against criteria provided in the guidance. The guidance recommends that the Regional Offices consult with the Model Clearinghouse for advice when making their case-by-case determinations. The guidance provides three basic criteria for approval of RM alone. The first deals with the need for a justification that the monitoring network is spatially adequate to measure the highest concentration attributable to the predominant sources. The second deals with the need for a justification that the monitoring data are temporally adequate to establish the maximum concentration. The third criterion is that the area contains only a few, relatively well characterized source categories. For details of these criteria see Appendix B. The guidance cautions the Regions that the prerogative to use RM demonstrations should be exercised judiciously. For those situations where RM is approved, the Regions are urged to arrange for contingencies that would allow the use of dispersion modeling at a later date should the control strategy be found to be inadequate. 3.2 Modeling of Area Sources with ISC It is sometimes necessary to model area source impacts with ISCST or ISCLT. The most common situations where this is necessary are cases where area sources are part of an industrial complex or where road dust and/or materials handling sources associated with a mining operation are present. (For modeling gridded emissions inventories in urban areas, RAM or CDM is recommended.) In recent years a number of modelers have expressed concerns that the ISC estimates for area sources are "suspect," either in general or especially so when receptors are placed in or in close proximity to area sources. The advice given to these modelers has been to heed the information in the ISC User's Manual regarding the size and shape of area sources and to judiciously limit the size of area sources to minimize problems. Receptors should not be placed within the boundaries of area sources. During FY-90 the perceived problems of spurious and/or biased estimates for ISC area sources was investigated by the Clearinghouse and others in the SRAB. These investigations included an EPA contract where several different area source algorithms were examined. The contractor report concluded that there are some shortcomings in each of them. The report emphasizes the need to subdivide area sources in ISCST, if concentration estimates are required in the near field. In a second EPA FY-90 effort, Region VI asked the Clearinghouse to look into some findings by the State of Texas that ISCLT yields spurious spikes in concentration estimates when used for modeling landfills. The Clearinghouse examined the Texas results and made additional ISC model runs. The Clearinghouse found that while the spurious estimates found by the State were real, they could be eliminated by reducing the area source size such that its dimension is small compared to the distance to the nearest receptor. Based on these investigations the Clearinghouse concluded that the limitations of current area source algorithms in general necessitate case-by- case professional judgment when determining the appropriate sizes of area sources to be modeled. This makes the results obtained from different modelers not necessarily replicable. While this is not an ideal situation from a regulatory point of view, it is a practical necessity when dealing with area sources and is an outcome that SRAB has been aware of for a long time. Similarly, the occurrence of spurious spikes can be mitigated if the sizes of any area sources are made small, i.e. subdivision of an area to a number of smaller areas, compared to the distance to the nearest receptor. This latter mitigating measure may introduce some problems in practicality in cases where receptors need to be placed very close to the edge of an area source. Prudent judgment and perhaps some "microscale" remodeling may be useful in such cases. Recognizing that this conclusion may not be entirely satisfactory in the long term, consideration is being given to incorporating a different area source algorithm in the ISC model. APPENDIX A FY 90 MODEL CLEARINGHOUSE MEMORANDA Date Region Subject 10/17/89 VI Ambient Air 11/7/89 II Interpretation of On-site Meteorological Data Requirements and the Use of RTDM for a PSD Source 11/28/89 VIII Utah PM-10 Secondary Sulfate and Nitrate Calculations 01/02/90 IV Effect of Changing Stack Heights on Prevention of Significant Deterioration (PSD) Modeling and Monitoring 01/10/90 VIII Utah PM-10, Secondary Sulfate and Projections 01/10/90 VIII Review of The Utah County PM-10 Draft SIP 01/11/90 VI Alternative Emission Reduction (Bubble) SIP Revision Authorizing Operation of a New Sulfur Recovery Plant at the Conoco Inc. Ponca City Refinery 01/16/90 VI Recent Texas Air Control Board (TACB) Evaluation of the ISC Area Source Algorithm 01/16/90 V Refined Metals Lead Modeling Analysis 02/22/90 III Approval of Equivalence Demonstration Plan Integrated Intermediate Terrain Model 03/01/90 VIII East Helena Lead SIP 03/23/90 III Mon Valley SO2 Study Allegheny County, PA 05/10/90 VIII Four Billings Montana Modeling Proposals 05/14/90 VIII Comments on the Overview of Geneva Steel's PM10 Control Plan 05/24/90 VI Review of El Paso/Juarez Modeling Plan 06/04/90 III Definition of Postapproval Monitoring 06/14/90 VII Doe Run, Herculaneum Lead SIP 06/18/90 IX Attainment Demonstration and Modeling Discussion for the South Coast FIP Notice of Proposed Rule- making 06/21/90 IV Offshore and Coastal Dispersion (OCD) Model 07/05/90 VIII PM-10 SIP Demonstrations for Small Isolated Areas with Spatially Uniform Emissions 07/27/90 VIII ASARCO, East Helena Overall Modeling Protocol 08/09/90 IX Phoenix CO Design Concentrations APPENDIX B PM-10 SIP DEMONSTRATIONS FOR SMALL ISOLATED AREAS WITH SPATIALLY UNIFORM EMISSIONS July 5, 1990 MEMORANDUM SUBJECT: PM-10 SIP Demonstrations for Small Isolated Areas With Spatially Uniform Emissions FROM: Robert D. Bauman, Chief /s/ SO2/Particulate Matter Programs Branch (MD-15) Joseph A. Tikvart, Chief /s/ Source Receptor Analysis Branch (MD-14) TO: Chief, Air Branch Regions I-X This memorandum is in response to recent conversations between the Particulate Matter Programs Section and Regions VIII and X. The Regions have repeatedly expressed the need for flexibility in control strategy demonstration requirements when confronted with air-sheds where receptor modeling, coupled with proportional (rollback) modeling is considered to be adequate to identify source contributions and demonstrate attainment. The purpose of this memorandum is to discuss the rationale and justification for exercising this flexibility. It is appropriate in certain situations to rely on a receptor model (RM) demonstration (i.e., use of receptor modeling, emission inventories, design value obtained by air quality monitoring, and proportional modeling) as the basis for a control strategy demonstration. This approach is an option provided for in sections 4 and 6 of the PM-10 SIP Development Guideline. While it is clear from the guideline that the use of dispersion models in combination with receptor models is the preferred approach, in certain limited situations, the use of an RM demonstration alone may be adequate to demonstrate attainment. The State must obtain approval to use the RM demonstration option prior to SIP submittal. The decision that an RM demonstration is adequate to demonstrate attainment is the responsibility of the Regional Office; however, the Region should consult the Model Clearinghouse for advice in making this determination. The Region must justify the determination and, in doing so, must consider all of the following: 1. The spatial representativeness of the monitoring network and the spatial uniformity of emissions. The PM-10 monitoring network must be representative of the maximum air quality impacts from the predominant (i.e., generally on the order of 90 percent) sources and source categories in the PM-10 emission inventory. Emissions from area source categories are often distributed nearly uniformly across the area. This implies that ambient patterns would not be characterized by strong concentration gradients, thus lessening the need for an extensive monitoring network. However, areas with point sources will generally find an RM demonstration difficult to justify because the concentration pattern would be characterized by local "hot spots." In such cases, a dispersion model, along with representative meteorological data are typically required. In a few areas, emissions of antiskid materials from a small number of road surfaces constitute the predominant PM-10 source category. These emissions should be uniformly distributed along these road surfaces. The monitoring network must be shown to be in accordance with EPA's monitoring guidance and spatially representative of the maximum air quality impact from this source category. 2. The temporal representativeness of the monitoring network. If the 24-hour NAAQS is controlling, the network must have samples collected at sufficiently frequent intervals to ensure that the impacts from the governing emission sources are adequately monitored. 3. The impact of only a few, relatively well characterized source categories. Receptor models can generally well characterize only a limited number of chemically distinguishable sources or source categories. The above criteria imply that the area should be relatively small, characterized by uniform areawide emissions of one or two source categories, and geographically isolated from other PM-10 source areas. Examples of circumstances where RM demonstrations may be justifiable are small air-sheds where the only significant emission sources are residential wood combustion and/or road antiskid materials. It must be noted that the prerogative to use RM demonstrations should be exercised judiciously. Even when a RM is employed, consideration should be given to initiation of basic meteorological measurements as a contingency to the control program being found inadequate and predictive dispersion modeling being necessary at a later time. The use of dispersion modeling and receptor modeling in combination remains the preferred approach when both models are applicable to a particular circumstance. cc: T. Pace D. Stonefield D. Wilson Regional Modeling Contact, Regions I-X PM-10 Contact, Regions I-X APPENDIX C SUMMARY OF THE EPA MODEL CLEARINGHOUSE EPA Model Clearinghouse Summary The Model Clearinghouse is the single EPA focal point for reviewing the use of modeling techniques for criteria pollutants in specific regulatory applications. The Clearinghouse also serves to compile and periodically report for Regional Office benefit Agency decisions concerning deviations from the requirements of the Guideline on Air Quality Models (Revised) (Guideline). Need for the Model Clearinghouse The Guideline states that when a recommended model or data base is not used, the Regional Administrator may approve the use of other techniques that are demonstrated to be more appropriate. However, there is also a need to provide for a mechanism that promotes fairness and consistency in modeling decisions among the various Regional Offices and the States. The Model Clearinghouse promotes this fairness and uniformity and also serves as a focal point for technical review of "nonguideline" techniques proposed for use/approval by a Regional Administrator. Functions of the Model Clearinghouse The major function of the Clearinghouse is to review specific proposed actions which involve interpretation of modeling guidance, deviations from strict interpretation of such guidance and the use of options in the guidance, e.g., Regional Office acceptance of nonguideline models and data bases. This is handled in two ways: (1) the Clearinghouse, on request from the Regional Office, will review the Region's position on proposed (specific case) use of a nonguideline model for technical soundness and national consistency, and (2) the Clearinghouse will screen Federal Register regulatory packages for adherence to modeling policy and make recommendations for resolution of any issues identified. A secondary function of the Model Clearinghouse is to communicate to regulatory model users in EPA significant decisions involving the interpretation of modeling guidance. This is accomplished through an annual "Clearinghouse Report" which itemizes the significant decisions that have been made and the circumstances involved. This report serves to improve consistency in future decisions and as a source of technical information for the Regional Offices. In addition to the annual report the Clearinghouse informs users on a contemporary basis of significant decisions through copies of written decisions and briefings at various meetings and workshops; the public has access to Agency memoranda and reports through an EPA electronic bulletin board system. Structure of the Clearinghouse The Clearinghouse is formally located in the Source Receptor Analysis Branch (SRAB) of OAQPS. However, the Air Quality Management Division (AQMD) also participates in Clearinghouse matters involving SIP attainment strategies and other regulatory functions. The primary responsibility for managing the Clearinghouse and ensuring that all of its functions are carried out is performed by a person full-time within SRAB. The responsibility for responding to requests for review of modeling issues is assigned, on a pollutant/program basis to three SRAB individuals. In addition, AQMD supports the Clearinghouse with staff who are also knowledgeable in modeling policy. These individuals are responsible for screening SIP submittals and related documents, referring modeling issues to SRAB through the Clearinghouse and documenting the final (and any significant interim) decision on disposition of the issues. Communication Chain The Model Clearinghouse functions within the organizational structure of EPA. As such the Clearinghouse serves the EPA Regional Offices. It coordinates with and communicates decisions to the Regional Offices. Any coordination with State and local agencies and individual sources on Clearinghouse activities is a function of the EPA Regional Offices.