**Appendix W Refresher: Clarification Memoranda** **Roger W. Brode U.S. EPA/OAQPS** **Air Quality Modeling Group 9** **th Conference on Air Quality Modeling** **October 9, 2008 Research Triangle Park, NC** **Outline • Role of Clarification Memoranda** **• Clarification Memo Process • Appendix W References to “Clarification”** **and “Consistency” • Issued Clarification Memos** **• Pending/Potential Issues for Clarification Memos** **Role of Clarification Memos • Clarification memos address issues that** **may arise with broad implications, i.e., not focused on specific permit modeling** **applications, which should be addressed through Model Clearinghouse process** **• Serve as reminders/clarifications in response to new issues that may arise or** **concerns that Appendix W not being adequately followed** **• Intended to foster consistency in the application of Appendix W guidance** **Clarification Memo Process • Issues may arise through ongoing OAQPS** **assessments or through regular interaction with EPA Regional Office modeling contacts** **• OAQPS internal review of memos through Air Quality Assessment Division (AQAD) Director and,** **as needed, through Air Quality Policy Division (AQPD) and Office of General Council (OGC)** **• Memos also reviewed by EPA Regional Office modeling contacts** **• Memos issued to EPA Regional Offices through modeling contacts or ADD’s as appropriate, with** **distribution based on scope or focus of memo Clarification Memo Process** **• Finalized memos distributed to community through SCRAM website, identified under** **“Recent Additions” • Clarification memos archived on separate** **SCRAM webpage, with link from “Permit Modeling Guidance” page** **Appendix W on Clarification • Appendix W includes the following references to** **the need for consistency and clarification: – “Industry and control agencies have long expressed** **a need for** consistency**in the application of air quality models for regulatory purposes.” [Preface, paragraph** **(a)] – “Historically, three primary activities have provi** **ded direct input to revisions of the** Guideline**. The first is a series of annual EPA workshops conducted for the purpose of** ensuring consistency**and**providing clarification**in the application of models.” [Preface, paragraph (b)]** **Appendix W on Clarification • Appendix W references (cont.):** **– “From time to time situations arise requiring**clarification of theintent of the guidance**on a specific topic. Periodic workshops are held with the headquarters, Regional Office, State,** **and local agency modeling representatives to** ensure consistency**in modeling guidance and to promote the use of more accurate ai** **r quality models and data bases. The workshops serve to provi** **de further explanations of Guideline requirements to the Regio** **nal Offices and workshop reports are issued with this** clarifying information**.” [Paragraph 1.0(f)]** **– “The model that most accurately estimates concentr ations in the** **area of interest is always sought. However, it is c lear from the needs** **expressed by the States and EPA Regional Offices, b y many** **industries and trade associations, and also by the deliberations of** **Congress, that**consistency in the selection and application ofmodels**and data bases should also be sought, even in case- by** **case analyses.**Consistency ensures**that air quality control agencies and the general public have a common basis** **for estimating pollutant concentrations, assessing cont** **rol strategies and specifying emission limits.” [Paragraph 1.0(f)]** **Issued Clarification Memos • Regulatory status of proprietary versions** **of AERMOD – December 12, 2007 – Motivated in response to frequent questions** **regarding “parallelized” versions of AERMOD • Regulatory status of CALPUFF for near** **field applications – August 14, 2008 – Motivated by concerns that Appendix W** **guidance not being followed, and technical issues and concerns** **CALPUFF Near-field Clarification • EPA-preferred model for near-field regulatory** **applications is AERMOD • CALPUFF may be considered on a case-by** **case basis as an alternative model for near-field applications involving complex winds** **– Subject to approval by reviewing authority – Subject to requirements in paragraph 3.2.2(e) of** **Appendix W, when there is no preferred model • Staff memorandum regarding technical issues** **related to CALPUFF near-field applications posted on SCRAM on Sept. 26, 2008** **Pending/Potential Issues for Clarification Memos** **• Use of ASOS vs. observer-based National Weather Service (NWS) data and treatment** **of missing NWS data in AERMOD • Implementation of EPA formula for Good** **Engineering Practice (GEP) stack height in AERMOD (with PRIME downwash)** **Pending Clarification Memo – NWS Met Data Issues in AERMOD** **• AERMOD requirements for data completeness differ f rom** **ISCST3, which required 100% completeness under regulatory default option** **• AERMOD sensitivity to ASOS vs. observer-based data has** **been assessed; generally less of an issue with AERM OD** **than ISCST3 (AERMOD ASOS study documentation to be provided)** **• Missing NWS data more extensive with advent of ASO S and** **METAR, with data gaps biased toward low wind speeds ,** **raising concerns regarding representativeness for s ome** **applications • Potential use of 1-minute ASOS data to supplement** **standard NCDC archived data with hourly averaged winds; coul** **d significantly reduce number of calm and missing hou** **rs Pending Clarification Memo –** **GEP Formula Height in AERMOD • AERMOD currently turns off building downwash** **effects if stack height is greater than or equal to EPA formula for GEP stack height, H** **gep :** **– H gep** **= H b** **+ 1.5L, where H** **b = building height above stack base** **L = lesser of building height and projected wid th** **• AERMOD implementation is consistent with all previous versions of AERMOD and ISC** **• Significant discontinuities in AERMOD impacts have been noted for stacks that straddle the GEP** **formula height; orders of magnitude in some cases Pending Clarification Memo –** **GEP Formula Height in AERMOD • Comments at 7** **th Modeling Conference in 2000** **recommended that EPA consider changing ISC PRIME to eliminate discontinuity at GEP height** **• EPA response was that current implementation is a requirement imposed by GEP Stack Height** **Regulations in Section 123 of CAA • Magnitude of discontinuities found in some cases** **has prompted reexamination of this position • Current assessment is that AERMOD should be** **modified to remove this criterion for turning off downwash influences** **Pending Clarification Memo – GEP Formula Height in AERMOD** **• GEP Stack Height regulations define GEP stack height as the greater** **of: – 65 meters (de minimis GEP height);** **– EPA formula height; or – height determined by field study or fluid** **modeling demonstration • Based on the definition, EPA formula height** **does not apply below 65 meters • Discontinuities are primarily a concern for** **shorter stacks, usually with squat buildings Pending Clarification Memo –** **GEP Formula Height in AERMOD • Pre-PRIME downwash algorithms defined** **vertical extent of wake influence generally consistent with EPA formula height,** **resulting in little, if any, discontinuity • Vertical extent of wake influence in PRIME** **formulation can extend well above the EPA formula height** **• Wind tunnel studies clearly support wake influences above EPA formula height for** **some stack/building geometries Questions?**