Appendix W Refresher

Clarification Memoranda 

Roger W. Brode

U.S. EPA/OAQPS

Air Quality Modeling Group

9th Conference on Air Quality Modeling

October 9, 2008

Research Triangle Park, NC
Outline

• Role of Clarification Memoranda• Clarification Memo Process• Appendix W References to “Clarific

and “Consistency”

• Issued Clarification Memos• Pending/Potential Issues for Clarific

Memos
Role of Clarification Mem

• Clarification memos address issues 

may arise with broad implications, i.focused on specific permit modeling applications, which should be addrethrough Model Clearinghouse proce

• Serve as reminders/clarifications in 

response to new issues that may arconcerns that Appendix W not being adequately followed

• Intended to foster consistency in the 

application of Appendix W guidance
Clarification Memo Proce

• Issues may arise through ongoing OAQPS 

assessments or through regular interaction 

Regional Office modeling contacts

• OAQPS internal review of memos through 

Quality Assessment Division (AQAD) Direcas needed, through Air Quality Policy Divi(AQPD) and Office of General Council (OG

• Memos also reviewed by EPA Regional Of

modeling contacts

• Memos issued to EPA Regional Offices thr

modeling contacts or ADD’s as appropriatedistribution based on scope or focus of me
Clarification Memo Proce

• Finalized memos distributed to comm

through SCRAM website, identified u“Recent Additions”

• Clarification memos archived on sepa

SCRAM webpage, with link from “PerModeling Guidance” page


Appendix W on Clarificatio

• Appendix W includes the following referenc

the need for consistency and clarification:

– “Industry and control agencies have long expre

need for consistency in the application of air q

models for regulatory purposes.” [Preface, para

– “Historically, three primary activities have provi

input to revisions of the Guideline. The first is a 

annual EPA workshops conducted for the purpensuring consistency and providing clarific

the application of models.” [Preface, paragraph 
Appendix W on Clarificatio

• Appendix W references (cont.):

– “From time to time situations arise requiring clarification 

intent of the guidance on a specific topic. Periodic worheld with the headquarters, Regional Office, State, and lmodeling representatives to ensure consistency in mo

guidance and to promote the use of more accurate air qmodels and data bases. The workshops serve to provide explanations of Guideline requirements to the Regional workshop reports are issued with this clarifying informa

[Paragraph 1.0(f)]

– “The model that most accurately estimates concentration

area of interest is always sought. However, it is clear froexpressed by the States and EPA Regional Offices, by mindustries and trade associations, and also by the delibeCongress, that consistency in the selection and applimodels and data bases should also be sought, even in 

case analyses. Consistency ensures that air quality co

agencies and the general public have a common basis festimating pollutant concentrations, assessing control stspecifying emission limits.” [Paragraph 1.0(f)]
Issued Clarification Mem

• Regulatory status of proprietary vers

of AERMOD – December 12, 2007

– Motivated in response to frequent ques

regarding “parallelized” versions of AE

• Regulatory status of CALPUFF for n

field applications – August 14, 2008

– Motivated by concerns that Appendix W 

guidance not being followed, and technissues and concerns
CALPUFF Near-field Clarifica

• EPA-preferred model for near-field regula

applications is AERMOD

• CALPUFF may be considered on a case-

case basis as an alternative model for neapplications involving complex winds

– Subject to approval by reviewing authority– Subject to requirements in paragraph 3.2.2(e

Appendix W, when there is no preferred mod

• Staff memorandum regarding technical is

related to CALPUFF near-field applicationposted on SCRAM on Sept. 26, 2008
Pending/Potential Issues fo

Clarification Memos

• Use of ASOS vs. observer-based Na

Weather Service (NWS) data and treof missing NWS data in AERMOD 

• Implementation of EPA formula for G

Engineering Practice (GEP) stack he

AERMOD (with PRIME downwash)
Pending Clarification Memo 

NWS Met Data Issues in AERM

• AERMOD requirements for data completeness diff

ISCST3, which required 100% completeness underegulatory default option

• AERMOD sensitivity to ASOS vs. observer-based 

been assessed; generally less of an issue with AEthan ISCST3 (AERMOD ASOS study documentatiprovided)

• Missing NWS data more extensive with advent of A

METAR, with data gaps biased toward low wind spraising concerns regarding representativeness for applications

• Potential use of 1-minute ASOS data to suppleme

NCDC archived data with hourly averaged winds; significantly reduce number of calm and missing h
Pending Clarification Memo 

GEP Formula Height in AERM

• AERMOD currently turns off building dow

effects if stack height is greater than or eq

EPA formula for GEP stack height, Hgep

:

– Hgep = H

b + 1.5L, where

H

b = building height above stack base

L = lesser of building height and projected width

• AERMOD implementation is consistent w

previous versions of AERMOD and ISC

• Significant discontinuities in AERMOD im

have been noted for stacks that straddle formula height; orders of magnitude in so
Pending Clarification Memo 

GEP Formula Height in AERM

• Comments at 7th Modeling Conference in 

recommended that EPA consider changinPRIME to eliminate discontinuity at GEP 

• EPA response was that current implemen

a requirement imposed by GEP Stack HeRegulations in Section 123 of CAA

• Magnitude of discontinuities found in som

has prompted reexamination of this posit

• Current assessment is that AERMOD sho

modified to remove this criterion for turnindownwash influences
Pending Clarification Memo 

GEP Formula Height in AERM

• GEP Stack Height regulations define 

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 

does not apply below 65 meters

• Discontinuities are primarily a conce

shorter stacks, usually with squat bu
Pending Clarification Memo 

GEP Formula Height in AERM

• Pre-PRIME downwash algorithms de

vertical extent of wake influence genconsistent with EPA formula height, resulting in little, if any, discontinuity

• Vertical extent of wake influence in P

formulation can extend well above thformula height

• Wind tunnel studies clearly support w

influences above EPA formula heighsome stack/building geometries
Questions?