Overview of Proposed 

Revisions to Appendix W

Tyler Fox

OAQPS

11th Modeling Conference

Research Triangle Park, NC

August 12, 2015
Regulatory Revisions to Appendix W: Schedule

• Proposed Rulemaking, “Revision to the Guideline on Air 

Quality Models”, July 29, 2015

– Proposes updates to current EPA-preferred models to address 

input and science issues

– Incorporates new analytical techniques to address ozone and 

secondary PM2.5

– Updates for conducting individual source and cumulative 

impact analysis for NAAQS pollutants

• EPA will accept public comments on the proposal for 90 

days through October 27, 2015

– Public comment #1 = “Lot of work. Good job folks at OAQPS.”

• Final Rulemaking, “Revision to the Guideline on Air 

Quality Models”, Spring 2016 2
Setting the Foundation: Sections 1-3

• Section 1: Introduction

– Purpose and Applicability of the Guideline

• Section 2: Overview of Model Use

– Suitability of Models (Several factors)

– Sophistication of Air Quality Models (Screening/Refined)

• Section 3: Preferred & Alternative Air Quality 

Models

– Preferred models: Conditions for Appendix A models

– Alternative models: Conditions and their approval 

– Role of Model Clearinghouse

3
Identifying Modeling Approaches: Sections 4-6

• Section 4: Models for “inert pollutants’

– AERSCREEN as screening model

– AERMOD Modeling System as preferred model

– Multi-tiered approach for NO2

• Section 5: Models for O3 & Secondary PM2.5

– No preferred model or technique

– Recommends two-tiered approach with detailed guidance

• Section 6: Modeling for AQRVs & Other Govt

Programs

– Role of FLMs, FLAG & AQRVs (visibility & deposition)

– BOEM & OCS air quality modeling

– FAA and use of AEDT for air quality assessments

4
Proposed Updates to AERMOD 

Modeling System: Preamble

• EPA updated regulatory version from v14134 

to v15181 (June 30, 2015) to address several 

bug fixes.

• In the NPRM, EPA has proposed to 

incorporate specific updates to the regulatory 

version that are the subject of public review 

and comment and then would be codified as 

part of the final rule action, as appropriate.

– These options have thus remained “beta” in 

v15181 to allow for public testing & evaluation

5
Long-Range Transport Assessments: 

Section 4

• EPA proposing that Appendix W no longer contain 

language requiring use of CALPUFF or other Lagrangian 

puff model for LRT assessments.

• For NAAQS demonstrations: Near-field modeling is 

sufficient to address whether a source will cause of 

contribute to a NAAQS violation so EPA does not 

consider a LRT assessment beyond 50 km necessary for 

inert pollutants.

• EPA recognizes that LRT assessments may be 

necessary in limited situations for PSD increment and 

proposes a screening approach where CALPUFF or 

other appropriate model/tool may be used. 6
How to Inform & Apply Models: Sections 7-9

• Section 7: General Modeling Considerations

– Recommendations for dispersion modeling 

• Section 8: Model Input Data

– Modeling Domain & Source Data

– Background Concentrations

– Meteorological Input Data

• Section 9: Regulatory Application of Models

– Modeling Protocol 

– Multi-stage approach to demonstrating compliance

– Use of Measured Data in lieu of Model Estimates 7