MODEL CLEARINGHOUSE: FY-89 SUMMARY REPORT OCTOBER 1989 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-89 SUMMARY REPORT PAGE 1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . 3 2.0 FY-89 CLEARINGHOUSE ACTIVITIES . . . . . . . . . . . . . 4 2.1 Background and Responsibilities. . . . . . . . . . . 4 2.2 Summary of Regional Office Requests and Clearinghouse Responses. . . . . . . . . . . . . . . 6 2.3 Recurring Coordination . . . . . . . . . . . . . . . 7 2.4 Model Clearinghouse Information Storage and Retrieval System . . . . . . . . . . . . . . . . . .11 3.0 CLEARINGHOUSE RESPONSES TO RECURRING OR GENERIC ISSUES .13 3.1 Emission Rates for PSD Analyses. . . . . . . . . . .13 3.2 Modeling Background Sources. . . . . . . . . . . . .16 3.3 Mixed Terrain Modeling . . . . . . . . . . . . . . .20 3.4 PM10 SIP Modeling Guidance . . . . . . . . . . . . .22 Appendix A FY-89 MODEL CLEARINGHOUSE MEMORANDA . . . . . .A-1 Appendix B OUTLINE FOR PM10 SIP MODELING PROTOCOL. . . . .B-1 Appendix C SUMMARY OF THE EPA MODEL CLEARINGHOUSE. . . . .C-1 1.0 INTRODUCTION This report summarizes major activities of the Model Clearinghouse from August 15, 1988, to August 18, 1989. The report is divided into two parts. First, the FY-89 activities of the Clearinghouse are summarized. The second section summarizes Clearinghouse responses to several major categorical modeling problems; 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, 1988 to September 30, 1989. Appendix B is an outline for PM10 SIP modeling protocol, developed by Region X. Appendix C provides a brief summary of the EPA Model Clearinghouse responsibility structure and operational procedures. 2.0 FY-89 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. ----------------------- *Model Clearinghouse: Operational Plan, Revised, Source Receptor Analysis Branch, Office of Air Quality Planning and Standards, May 1988. 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-89 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. Completing the input of historical Clearinghouse records into a computerized data base. 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-89 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 27 regulatory modeling problems referred to the Clearinghouse which required a written response and another 78 referrals in which the issues were resolved orally. In addition, there were another 16 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-89 RECOMMENDATIONS REGION WRITTEN ORAL DISCUSSION/INFORMATION* TOTAL I 2 7 1 10 II 1 6 1 8 III 3 10 2 15 IV 4 15 1 20 V 1 12 5 18 VI 9 8 1 18 VII 1 3 0 4 VIII 4 12 2 18 IX 0 2 1 3 X 2 3 2 7 -- -- -- --- 27 78 16 121 -------------------- *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. 2.3 Recurring Coordination During FY-89 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 1988, a Clearinghouse report; this report served to inform Clearinghouse users about the issues and responses which occurred in FY-88. The report included a capsule listing of individual responses by the Clearinghouse to Regional Offices, and a summary of issues by pollutant, source type, etc. TABLE 2. CHARACTERIZATION OF 121 CLEARINGHOUSE REFERRALS DURING FY-89. 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 56 TSP 6 CO 15 Pb 5 NOx 8 O3 5 PM10 14 Unspecified 9 Unspecified Non-criteria 3 Generic 17 Source Types Power Plant 27 Power Boiler 3 Smelter 8 Steel Facility 3 Paper Mill 8 Refinery 2 Cogeneration Unit 5 Incineration 3 Mine 1 Chemical Plant 4 Roadway 9 Car Assembly Plant 1 Open Burning 1 Gas Compressor 1 Cement Plant 1 Generator (Gas or Oil) 1 Unspecified Stationary Source 9 Mixed/Multiple Sources 17 Generic to Any Stationary Source 11 Generic to Any Source 9 Urban or Rural Model Involved Urban Only 18 Rural Only 64 Both Urban and Rural 11 Not Relevant 28 Types of Response Written 27 Oral 78 Not Relevant 16 Applicable Regulations SIP 19 SIP Revision 22 SIP Revision - GEP 9 PSD 38 EIS 5 NSR 2 Redesignation 4 State Regulation 2 Study 1 Compliance Order 2 Superfund 1 Generic 13 Generic - SIP 3 Generic - PSD 4 Generic - GEP 1 Terrain Setting High Terrain (above stack height) 8 Low Terrain (below stack height) 48 Both High and Low Terrain 23 Essentially Flat Terrain 18 Not Relevant 24 Guideline or Nonguideline Model Involved Guideline 61 Non-guideline 19 Guideline and Non-guideline 18 Not Relevant 23 Data Bases Used for Model Input On-Site 29 Off-Site 39 Both off-site and on-site 8 Not Relevant 45 TABLE 3. SUBJECT AREAS INVOLVED IN FY-89 CLEARINGHOUSE REFERRALS SUBJECT AREA NUMBER OF OCCURRENCES* Adjustment of Model Estimates 4 Ambient Air 8 Ambient Monitoring 11 Area Wide Analysis 3 Block/Running Average 1 Calibration 3 Calm Winds 4 Deposition 1 Downwash 17 Emissions Characterization 4 Emission Rates for Model Input 19 Equivalency 5 Fluid Modeling 1 Grandfathering of Guidance 21 Interpretation of Modeling Guidance 13 Interpretation of Regulatory Guidance 17 Length of Record - Meteorological Data 1 Long-Range Transport 2 Meteorological Monitoring 15 Mixed Terrain Modeling 12 Modeled Background 10 Monitored Background 12 Monitoring in Lieu of Modeling 6 Performance Evaluations 17 Plume Rise 1 PSD Increment Calculation 3 Reactive Pollutant Modeling/Chemical Transformations 8 Receptor Modeling 4 Receptor Networks 4 Representativeness of Meteorological Data 19 Screening Procedures 8 Shoreline/Offshore Modeling 2 Significant Air Quality Impacts 8 Stability Classes 4 Supplementary Control Systems 1 Technical Credibility of Nonguideline Techniques 8 Terrain Correction Factors 3 Time Scaling 2 Urban/Rural 5 Stagnation 4 Visibility 3 ---------------------- *More than one subject was involved in the resolution of most referrals. During FY-89 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 covering the period October 1, 1988 to September 30, 1988, 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-89 such cases arose. In these cases the proposed Clearinghouse response was either provided to the Regions for comment or was discussed in some detail at the 1988 Regional/State Modelers Workshop before the response was finalized. Summaries of these issues are provided in Sections 3.1, 3.2, 3.3, and 3.4 of this report. 2.4 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 700 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 similar cases 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-89 the Clearinghouse completed the entry of the approximately 700 historical referrals into MCHISRS. Plans are underway for FY- 90 to provide access to this information by the Regional Offices. 3.0 CLEARINGHOUSE RESPONSES TO RECURRING OR GENERIC ISSUES During FY-89 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 Emission Rates for PSD Analyses During the past several years, a number of informal discussions have taken place between the Regions and various offices at OAQPS regarding what appropriate emission rates to use when performing the NAAQS modeling analysis for a PSD application. During FY-89, two Regions formally asked this question of the Model Clearinghouse. Of specific concern was the question of whether the required analysis for new major sources and major modifications is to be based on actual or allowable emissions from existing background sources. The FY-89 Clearinghouse position on the issue was that allowable emissions should generally be used (March 16, 1989 memorandum, J. Calcagni/W. Laxton to Regions III and VI). However, as explained below, certain allowances may be made, primarily with respect to the evaluation of impacts on the long-term NAAQS, to consider an existing source's actual annual operations. This position best resolves the inconsistencies between previous written guidance for PSD and the guidance applicable to NAAQS attainment demonstrations for State implementation plans (SIP's). The PSD regulations at 40 CFR 51.166(k) stipulate that "allowable emission increases from the proposed source or modification, in conjunction with all other applicable emissions increases...would not cause or contribute to air pollution in violation of [any National Ambient Air Quality Standard (NAAQS)]." (Emphasis added). While this provision clearly requires the use of allowable emissions for the new or modified source, it offers no similarly explicit requirement regarding emissions to be used for existing source contributions. Nationally, States and EPA Regional Offices have utilized several interpretations which have led to a consistency problem in implementing the requirement for a NAAQS demonstration under 40 CFR 51.166(k). Some States presently accept the use of actual source emissions for existing background point sources, and reference EPA guidance to support their position. Regions, on the other hand, encourage the use of emissions estimates more closely reflecting legally allowable emissions. Available EPA guidance for PSD, which dates back to 1980, supports the use of actual emissions to project the air quality impacts caused by existing point sources. Specifically, the Prevention of Significant Deterioration Workshop Manual (EPA- 450/2-80-081, October 1980) states that "actual emissions should be used... to reflect the impact that would be detected by ambient air monitors" for the PSD NAAQS analysis. However, because many sources typically emit at rates well below their legally allowable emission rate on an annual basis, the use of actual emissions to demonstrate NAAQS attainment could substantially underestimate the potential air quality impacts resulting from existing sources. The EPA's policy for demonstrating stationary point source compliance with the NAAQS for SIP purposes clearly requires the use of emissions which are more closely tied to allowable emissions. The model emission input data and requirements for such SIP demonstrations are contained in Table 9-1 of the Guideline for Air Quality Models (Revised) (Guideline). For "nearby background sources" an adjustment to the allowable emission rate* may be made only for determinations of compliance with the annual and quarterly NAAQS, and only with respect to the annual operating factor. For "other background sources" an adjustment to both the operating level and the operating factor, as explained in Table 9-1, could be made for determinations of compliance with the long-term and short-term NAAQS. ------------------ *Emission rates for model input consist of three components: (1) the emission limit, e.g., #/mBTU; (2) the operating level, e.g., mmBtu/hour; and (3) the operating factor, e.g., hours/day, hours/year. The referenced model emission input data requirements for existing point sources are contained in the Guideline which has undergone rulemaking and is incorporated by reference in EPA's PSD regulations under Parts 51 and 52. Although a footnote in Table 9-1 indicates that the model input data requirements may not apply to PSD NAAQS analyses, such requirements should now be applied to PSD rather than using actual emissions as indicated in the 1980 PSD guidance. Thus, compliance demonstrations for PSD and for stationary source control strategies under SIP's will be accomplished in a consistent manner. In order to apply Table 9-1 in the Guideline to PSD NAAQS analyses, certain clarifications need to be provided. First, the proposed major new source or major modification must be modeled at its maximum allowable emission rate. Second, the existing facility to which a major modification has been proposed, but whose actual emissions (not including emissions from the proposed modification) will remain unchanged, may be considered as the "stationary point source subject to SIP emission limit(s)..." to determine the model emission input requirements. Portions of the existing facility where the emission rate is expected to increase as a result of the proposed modification should be modeled at the allowable emission rate. Finally, background point sources (1) having already received their construction permit but not yet in operation, or (2) with less than 2 years of operational history, should also be modeled at their allowable emission rate. Of course, an analysis which demonstrates no contravention of the standards, based entirely on maximum allowable emissions rates (including full operation for the entire year) for all modeled point sources is acceptable. If a violation of any NAAQS is revealed by this type of analysis, then the adjustments described above may be made in cases where it can be shown to the satisfaction of the permit granting agency that historical operating levels and/or operating factors will be representative of future conditions. This use of Table 9-1 of the Guideline for accomplishing the required PSD NAAQS analysis supersedes the various procedural interpretations formerly applied. However, since different procedures have been in use, a grace period until October 1, 1989 for implementing the required procedure was provided. Current plans are to include this table in the NSR Review Manual, currently under development. 3.2 Modeling Background Sources During FY-89 the Model Clearinghouse was successful in educing an OAQPS policy statement regarding issues on modeling background sources that have been outstanding for several years. Specifically, the two major issues are: (1) Approval of a proposed SIP emission limit for a source under consideration when there are modeled violations of the National Ambient Air Quality Standards (NAAQS) due to nearby background sources in the surrounding area. Regulatory modelers have found that there are numerous cases, especially in the eastern United States, where the background sources are by themselves causing a calculated violation of the NAAQS. This problem arises because of gradual changes in SIP demonstration policies and modeling policies since the early-1970's. Also, technical changes to regulatory dispersion models, e.g., addition of a downwash algorithm, changes the estimated impacts of individual sources, often resulting in a modeled violation of a NAAQS or PSD increment. (2) The resource burden associated with assembling the data necessary for modeling the background sources. The resource burden sub-issue arises because the information needed to model the background sources is frequently not contained in the State's existing emission inventory. Many of the background sources are old and small, but pose a threat to the NAAQS in their immediate vicinity because of building downwash or impacts on nearby hillsides. In order to model these sources correctly it is often necessary to obtain building dimensions and other data. In a few cases it is necessary to collect on-site meteorological data in order to ascertain impacts on nearby hillsides. On the SIP approval issue the May 3, 1989 OAQPS policy memorandum restates the existing policy developed by the Model Clearinghouse and discusses limited exceptions to the policy. The general policy may be summarized as follows: 1. Background concentrations are an essential part of the total air quality concentration to be considered in determining source impacts. Nearby sources which are expected to cause a significant concentration gradient in the vicinity of the source under consideration should be explicitly modeled (as "background" sources). 2. Under Section 110 of the Clean Air Act, each SIP must provide for attainment and maintenance of the NAAQS. Where background sources are found to cause or contribute to a violation, a SIP revision for the source under consideration generally should not be approved until each violation in the modeled Region is prevented or eliminated through the SIP rules. This policy avoids approval of a SIP revision which does not provide for attainment throughout the modeled area. The policy memorandum also recognizes that Section 110 of the Clean Air Act allows for approval of portions of SIPs. Therefore, exceptions to the general policy may be warranted in certain circumstances. Before any exception will be considered, it must be clearly shown that the SIP would be improved as a result of the partial approval. As a minimum, the following factors should be considered in determining exceptions to the general policy: 1. Approval would not interfere with expeditious attainment (i.e., emissions from the source under consideration do not cause or contribute to the modeled violation). 2. There would be an environmental benefit (i.e., the SIP revision would result in an actual emissions decrease and ambient air quality improvement). 3. Enforcement of the SIP would be improved (e.g., without approval there would be no federally enforceable measure for the source under consideration or ambiguities in the previous limit serve to frustrate enforcement efforts). Where it is found that an exception should be made based on the above factors, it is expected that the proposed approval notice will specifically identify the background source violations and clearly state that the State retains an obligation to take action expeditiously to correct the background violations. The final approval notice for the source under consideration should not be promulgated before the State acknowledges the background violations and submits an acceptable schedule for corrective action. The schedule would then be included in the final notice as the State's response to EPA's identification of violations. A SIP call pursuant to Section 110(a)(2)(H) should be issued where a State fails to acknowledge its obligation and submit a schedule for resolution of violations during the comment period. Regarding the resources issue, the policy memorandum noted that the Guideline states that the nearby (background) source inventory should be determined in consultation with the local air pollution control agency. Specifically, the Guideline states that "The number of (background) sources is expected to be small except in unusual situations." In this and in other areas, the Guideline necessarily provides flexibility and requires judgment to be exercised by the reviewing agency. The resource burden may be mitigated somewhat by application of this judgment. Given the flexibility that is provided by existing guidance and the tendency for more explicit policy to reduce this flexibility, no further guidance was judged necessary. The Regional Offices generally have been able to work with their States to collect sufficient data to support the necessary modeling. Consequently, there was little support for the suggestion to revise the current policy to more explicitly limit the number of sources that should be modeled for downwash. 3.3 Mixed Terrain Modeling The Guideline indicates that where complex terrain receptors are located between stack height and plume height, estimates should be made at such receptors with both a complex terrain model and a simple terrain model (with terrain cut off at stack height). The higher of the two estimates would be used for regulatory purposes. The Guideline does not elaborate on how the estimates are to be made/compared. A number of practical complications were the subject of several referrals to the Clearinghouse. The major issue is whether the comparisons between complex terrain model estimates and simple terrain model estimates should be made on an hourly basis or on an annual basis (higher of the high-second-highs from each model). A complication with the latter option is that plume rise is variable and a given receptor may be a simple terrain receptor relative to one stack, a "comparison" receptor relative to another stack, and even a complex terrain receptor (above plume height) relative to a third stack. The FY-87 Clearinghouse position relative to these issues was as follows. The intent of the guidance was to make the comparison on an annual basis, i.e., use the higher of the high- second-highs. This entails the identification of receptors that are "usually" located between stack and plume height for stable conditions. For multiple stacks separate estimates would have to be made and post processor software would have to be written on a case-by-case basis to sum up the contributions from the various stacks. However, in some situations such as cases where performance evaluations are involved, it makes more sense to perform the comparisons on an hourly basis. In such cases, this option was left open to the Region. During FY-88 it was pointed out to the Clearinghouse that its position with respect to modeling multiple sources is technically questionable since the critical impacts from different stacks would likely occur on different days. It was agreed that it would not make sense to sum the critical concentrations for each stack on an annual basis. The Clearinghouse agreed to modify its position with respect to multiple stacks such that comparison of complex terrain/simple terrain estimates should be made on an hour- by-hour/receptor-by-receptor basis. During FY-89 two additional complicating issues were pointed out to the Clearinghouse. First, while it is clear that the hour-by-hour technique must be used for the multi-source case, there has been no such understanding regarding the single source case. The Clearinghouse was asked to establish a position that a single technique should apply to both situations since there would appear to be a clear inequity to require a more conservative approach of a source simply because there are other sources to be modeled. A second issue involves the situation in which an applicant intends to use the Valley screen for the complex terrain model and sequential modeling for the simple terrain model. In this situation, it is not possible to perform the hour-by-hour approach since the Valley model is incapable of making predictions for the various hourly meteorological conditions. The recommendation made to the Clearinghouse was to allow for a case-by-case analysis designed to determine how likely it would be that the controlling concentrations would occur at the intermediate terrain receptors. If it could be demonstrated that this would be unlikely, then the applicant could ignore those receptors. On the other hand, if it was determined to be likely, then the applicant would be required to either collect on-site data, for use with either Complex I or RTDM, or develop a case specific screen. The Clearinghouse essentially agreed with the recommendations in both cases (June 6, 1989 memorandum, J. Tikvart to Region III). Thus, the statements in the Guideline regarding modeling in intermediate terrain should be implemented as follows: 1. When on-site meteorological data are available, receptors that are located in intermediate terrain, i.e., between stack height and plume height, should be modeled with both a simple terrain model (with terrain "cut-off" at stack height) and a complex terrain model and the highest of the two estimates chosen on an hour- by-hour basis. Estimates for averaging times longer than 1 hour would be determined in a standard fashion and may contain a mixture of simple terrain and complex terrain model estimates. This procedure would be used for both single and multiple stack situations. 2. When on-site meteorological data are not available and only the Valley screen is available for the complex terrain estimates, case-by-case judgments can be made on whether the controlling (design) concentration would be associated with the simple terrain model estimates or the Valley model estimates. In those cases where judgmental considerations do not lead to a probable conclusion in that regard, it may be necessary to require the source to collect 1 year of on-site meteorological data so that the procedure in Paragraph 1 can be used. It should be noted that the eventual availability of CTDMPLUS will likely ameliorate the intermediate terrain problem. CTDMPLUS should be applicable to all receptors above stack height; it will do away with the need for using two different models and comparing the estimates. There will still be some ambiguity for multi-stack situations; an attempt will be made to develop straightforward guidance for such situations. 3.4 PM10 SIP Modeling Guidance During 1988, Region X developed the Appendix B "Outline for PM10 SIP Modeling Protocol" to assist the State and local agencies in their Region in preparing modeling protocols as part of the PM10 SIP development process. In developing the outline, Region X followed the principles contained in the Guideline. That is, one should first consider the use of an Appendix A Guideline model for the situation. Then, if there are concerns that the Appendix A model may not be applicable to the meteorology, topography, or source characteristics of the problem, one should consider the use of an appropriate nonguideline technique. If a nonguideline model is to be selected, Section 3.2.2 indicates that if the alternative model is theoretically sound, and the data bases to run the model are available, then its applicability needs to be demonstrated through some kind of performance evaluation. If the Appendix A model is clearly not applicable, then the "3a" criteria of Section 3.2.2 may be used, whereby it is shown that the nonguideline model is not biased toward underestimation. If it is not clear whether the Appendix A model is appropriate, then the "3b" criteria applies, and a rigorous application of the Interim Procedures for Evaluating Air Quality Models (Revised) is necessary. When Region X applied that general guidance to the Group I areas in the Region, a consistent theme arose. Most of the PM10 problems occurred during stagnation conditions, suggesting that no Appendix A model may be applicable. Thus, the use of a nonguideline model became necessary. Region X generally required that agencies perform modeling with both a stagnation model, WYNDvalley, and a Guideline model, such as RAM or ISC. Application of WYNDvalley as a nonguideline model really fits the "3a" criteria of the Guideline. However, because very little data exist to demonstrate that the model does not underpredict for each application, some comparison is necessary of the nonguideline model estimates with both measured air quality concentrations, and, as a point of reference, with estimates from a Guideline model. As guidance for selecting the appropriate Guideline model, RAM has generally been recommended where area sources in the urban area are more important, relative to point sources. On the other hand, ISC is recommended where industrial point sources are relatively important, and the potential exists for downwash. In these nonguideline model situations, a limited performance evaluation using the "Protocol for Determining the Best Performing Model" is applied to select the model to use for determining the design concentration and for evaluating control strategies. One deviation from the protocol is that the predictions and observations are paired in time and space. This is because: 1. as a practical matter, WYNDvalley is applied on an episode day basis, and 2. evaluating individual source contributions to exceedance concentrations at a particular location during a particular time is the most important aspect of this type of modeling exercise. The performance evaluations are limited in the sense that usually only a few monitoring stations are available to provide data for comparison with model predictions, and because of significant uncertainties in the input data bases of emissions and meteorology. Region X was somewhat concerned that there may be a lack of consistency in how nonguideline models are selected, evaluated, and applied to PM10 stagnation situations. The ability to effect a credible modeling program for stagnation in the Regions depends to some extent on what other Regions/States are doing for such situations. Thus, the Region asked the Clearinghouse to review the outline, and if it is determined to be appropriate, share it with other Regions to promote consistency. The Model Clearinghouse agreed that the outline is appropriate for modeling PM10 areas where valley stagnation or other unique meteorological phenomena may govern the design concentration (March 23, 1989 memorandum, J. Tikvart to Region X). The requirement for a limited performance evaluation based on the "Protocol for Determining the Best Performing Model" is an acceptable way of implementing the principles in the outline. The Clearinghouse then distributed the outline to all the Regions urging them to follow the principles contained in the outline, to promote national consistency. APPENDIX A FY 89 MODEL CLEARINGHOUSE MEMORANDA Date Region Subject 10/11/88 VI Use of ISC UNAMAP 6, Change 7 11/07/88 VI Compilation of Most Recent, Available 5-Year Meteorological Data By Texas 11/08/88 V State of Indiana Meteorological Preprocessor Program 11/09/88 VI Information Regarding Refinery Tank Farms and Their Rural/Urban Designation 11/09/88 VI Request for Use of ISC 6.2 11/21/88 VI Request for Use of ISCST and ISCLT Version 6.2 in Twin Oak Steam Electric Station PSD Application 11/28/88 VI Request for Use of ISCST and ISCLT Version 6.2 in Formosa Plastics PSD Application 01/30/89 VIII E. Helena Lead SIP 02/08/89 IV Yates Power Plant GEP SIP 02/10/89 VIII Denver PM10 SIP 02/27/89 IV Paradise Power Plant 02/28/89 III Martins Creek -- Regulations for Redesignation 03/20/89 VI Proposed Region VI Responses to Louisiana About Modeling Issues 03/20/89 III & VI Use of Allowable Emissions for National Ambient Air Quality Standards (NAAQS) Impact Analyses Under the Requirements for Prevention of Significant Deterioration (PSD) 03/23/89 X Model Clearinghouse Review of Outline for PM10 SIP Modeling Protocol 04/06/89 I "Connecticut Ambient Impact Analysis Guideline" 4/25/89 I MassPower PSD -- Urban vs Rural for Background Source 5/11/89 I - X Issues Associated Modeling Background Sources 6/8/89 III Policy Interpretation - Modeling for Intermediate Terrain 6/28/89 I - X Clarification of Stack-Structure Relationships 6/28/89 IV Response to Region VI Position on PSD Modeling Issue 7/31/89 VI Request for Support in Evaluating Meteorological Data bases 08/16/89 VIII Review of the Utah PM10 Draft SIP (your memo 8/1/89) 09/20/89 IV Stack Located Within 5L of Dam APPENDIX B OUTLINE FOR PM10 SIP MODELING PROTOCOL I. Analysis of High Concentration Days This analysis should be based on least 2 years of the most recent PM10 monitoring data from the areas of concern. The data should be from everyday monitoring. Justification should be provided that the monitoring location represents maximum concentration locations. The highest 15 concentration days, or all exceedance days, whichever is greater should be analyzed. Representative meteorological data associated with the high concentration days should be evaluated to assess worst-case conditions. For example, are average wind speeds low, what are prevalent wind directions, were there meteorological conditions typical throughout a regional area, etc. Important sources contributing to the air quality problem should be identified (area sources, point sources, downwash, terrain impact, etc.) The identification of important sources may be based on other prior modeling, emission inventories, or other knowledge of the area. The purpose of this analysis is to provide a basis to determine what dispersion models might be applicable to the problem. II. Determination of Applicable Models Considering the sources, topography, and meteorology associated with the air quality problems, appropriate modeling techniques should be identified. First consideration should be given to EPA modeling guidance ("Guideline on Air Quality Models," EPA-450/2- 78-027R, July 1986, including Supplement A, January 1988). Guideline techniques should be used as applicable and appropriate. Where Guideline models are not applicable, consideration should be given to the use of non-Guideline models. For example, in the case of a pulp mill with elevated terrain in the area, the use of two Guideline models, ISC and Complex I, is appropriate. In another example, there are no Guideline models which are applicable to stagnation problems. In this case, the use of the non-Guideline model WYNDvalley should be considered. In some cases, the data bases necessary to perform the modeling are not available. A data collection program will be required to develop the necessary data. For example, some models require at least 1 year of on-site meteorological data to perform an adequate analysis. III.Model Evaluation Procedure (for use of a non-Guideline model) Where a non-Guideline model is used, justification must be provided in a comparison with a Guideline model (see "Interim Procedures for Evaluating Air Quality Models, EPA-450/4-84-023, September 1984). This justification should include a comparison of the two models on a theoretical basis and on a performance basis. The model performance evaluation is the most important part. A protocol for the performance evaluation should be developed which establishes the basis for the selection of the best model to use in the SIP demonstration modeling. The protocol will identify the measured concentrations, the meteorological data, the model options, the statistics to be generated based on comparing predicted and observed values, the selection criteria, etc. IV. SIP Demonstration Modeling The selected model(s) is then used to model the high concentration days. Various control strategies can be included to evaluate effectiveness. If the model is a Guideline technique, it must be used according to recommendations in the Guideline. If it is a non-Guideline technique, the model must be used in a manner consistent with the methodology used in the performance evaluation which was the basis for its selection. Compliance with standards should be demonstrated in all areas at all times. APPENDIX C 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. 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.