UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK, NC 27711 

OCT 2 3 2015 

OFFICE OF 

AIR QUALITY PLANNING 

AND STANDARDS 

MEMORANDUM 

SUBJECT: 

FROM: 

TO: 

Appendix W Webinars for State and Tribal Associations 

George M. Bridgers, Model Clearinghouse Director 

Air Quality Modeling Group, C439-01 

Office of Air Quality Planning and Standards 

Chi ~ 

Proposed Regulatory Docket No. EPA-HQ-OAR-2015-0310 

The Office of Air Quality Planning and Standards (OAQPS) participated in a series of 

conference calls with the National Association of Clean Air Agencies (NACAA), the 

Association of Air Pollution Control Agencies (AAPCA), and the National Tribal Air 

Association (NTAA) per each of these association's respective requests for a summary briefing 

on the proposed changes to the Guideline on Air Quality Models. The NACAA conference call 

was on August 6, 2015; the AAPCA conference call was on August 25, 2015; and the NTAA 

conference call was on October 20, 2015. For each of these engagements, the same presentation 

was provided to the participants and included summary slides from the material that the OAQPS 

presented at the 11th Conference on Air Quality Modeling. That presentation is included as an 

attachment to this docket submission. On each of the conference calls, participants asked general 

clarifying questions about the proposed rulemaking package, which were answered by OAQPS 

staff. 

Attachments: Proposal to Revise to the Guideline on Air Quality Models Overview and Status 

Briefing, August 6, 2015 

Internet Address (URL) • http://www.epa.gov 

Recycled/Recyclable •Printed with Vegetable Oil Based Inks on Recycled Paper (Minimum 25% Postconsumer) 
Proposal to Revise to the

Guideline on Air Quality Models

(Appendix W to 40 CFR Part 51)

Overview and Status Briefing

August 6, 2015

(The material for this briefing is publically available through previous EPA 

presentations, the Preamble and Fact Sheet for the proposed rulemaking, 

and the 11th Conference on Air Quality Modeling informational website.)
Challenges to Our Current Air Quality Models

• States and sources report difficulty demonstrating 

compliance with 1-hour NO2 and SO2 NAAQS.

– Accuracy of models receiving greater scrutiny so need to address 

model inputs and science of our existing regulatory models.

– Past practices for NAAQS compliance demonstrations under 

NSR/PSD that may be “overly conservative” in some cases.

• Sierra Club Petition Grant – Ozone and PM2.5.

– In January 2012, the EPA granted a petition submitted by the Sierra 

Club.

– In the petition grant, the EPA committed to engage in rulemaking to 

evaluate updates to Appendix W to 40 CFR 51, and, as 

appropriate, incorporate new analytical techniques or models for 

ozone and secondary PM2.5 for new and modified sources.

2
Background

• EPA’s Guideline on Air Quality Models (Guideline) is 

published as Appendix W to 40 CFR Part 51.

• EPA developed the Guideline to help EPA, States, and 

industry prepare and review new source permits and State 

or Tribal Implementation Plan revisions.

• The Guideline is important because it specifies models for 

regulatory application and provides guidance for their use.

• The Guideline provides a common basis for estimating the 

air quality concentrations of criteria pollutants used in 

assessing control strategies and developing emissions 

limits.

3
Background

• Originally published in 1978, the Guideline was last 

updated in 2003 and 2005 with the promulgation of 

CALPUFF as the preferred long-range transport 

dispersion model and AERMOD as the preferred nearfield dispersion model, respectively.

• Per section 320 of CAA, EPA holds triennial conferences 

on air quality modeling to engage with public on 

appropriate modeling practices and, as appropriate, 

inform revisions to the Guideline.

• The last modeling conference was the 10th Conference on 

Air Quality Modeling held in March 2012.

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Proposed Rulemaking and

11th Conference on Air Quality Modeling

• On July 14, 2015, the EPA proposed to update to the 

Guideline on Air Quality Models.

– Published in the Federal Register on July 29, 2015.

(80 FR 45340)

– Docket ID No. EPA-HQ-OAR-2015-0310

– 90-day public comment window ending on October, 27, 2015.

• 11th Conference on Air Quality Modeling

– August 12 and 13, 2015 at the EPA RTP, NC Campus.

– Conference to focus on the proposed revisions to the Guideline.

– Serves as public hearing for NPRM as part of public comment period. 

Transcripts and presentation will be posted to Docket and on the 11th

Conference on Air Quality Modeling informational website.

• Final rulemaking anticipated in mid-2016.

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Appendix W: A Piece of the NSR 

Program Puzzle

6

6

Bringing it together . . .

PM2.5 SILs Rule

Appendix W

Ambient Air & Related 

Policy Issues

O3 SILs Rule
Proposed Actions: AERMOD

• The EPA is proposing enhancements to the scientific 

formulation of the preferred near-field dispersion model, 

AERMOD, to address technical concerns expressed by the 

stakeholder community and improve model performance in its 

regulatory applications.

– 1. A proposed option incorporated in AERMET to adjust the surface friction 

velocity (u*) to address issues with AERMOD model overprediction under 

stable, low wind speed conditions. (ADJ_U*)

– 2. A proposed low wind option in AERMOD to address issues with model 

overprediction under low wind speed conditions. (LOWWIND3)

– 3. Modifications to AERMOD formulation to address issues with 

overprediction for applications involving relatively tall stacks located near 

relatively small urban areas.

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Proposed Actions: AERMOD (Cont.)

• The EPA is proposing enhancements to the scientific 

formulation of the preferred near-field dispersion model, 

AERMOD, to address technical concerns expressed by the 

stakeholder community and improve model performance in its 

regulatory applications.

– 4. Proposed regulatory default options in AERMOD to address plume rise 

for horizontal and capped stacks based on the July 9, 1993, Model 

Clearinghouse memorandum, with adjustments to account for the PRIME 

algorithm for sources subject to building downwash.

– 5. A proposed buoyant line source option, based on the BLP model, has 

been incorporated in AERMOD.

– 6. Proposed updates to the NO2 Tier 2 and Tier 3 screening techniques 

coded within AERMOD, including the replacement of the Ambient Ratio 

Method (ARM) Tier 2 option with a revised ARM2 option and the 

replacement of the Plume Volume Molar Ratio Method (PVMRM) Tier 3 

option with a revised PVMRM2 option. 8
AERMET/AERMOD (v15181) Release

• At the same time as the NPRM, the regulatory version of the 

AERMOD Modeling System was revised to version 15181.

– Numerous bug fixes and limited enhancements were made to 

version 14134 as described in Model Change Bulletin #11.

– Included new “beta” options for LOWWIND3 and PVMRM2.

– Incorporated a buoyant line source option, based on BLP.

• Applicants with permits in process can still use AERMOD 

version 14134 but should review the MCB with the appropriate 

reviewing authority to determine if any of the changes would 

adversely impact concentration estimates and/or compliance 

status. Rerunning final results with version 15181 is preferred.

• The proposed status of any “beta” or “non-default” option in the 

NPRM does not infer that they can be used immediately without 

appropriate justification and approval by the Regional Office with 

concurrence from the Model Clearinghouse. 9
Proposed Actions: AERSCREEN & 

CALINE3

• The EPA is proposing AERSCREEN as the recommended 

screening model for simple and complex terrain for single 

sources and options for multi-source screening with AERMOD.

– AERSCREEN now includes inversion break-up and coastal 

fumigation which were previously parts of SCREEN3.

• The EPA is proposing to replace the model known as CALINE3 

for mobile source applications including fine particle pollution 

(PM2.5, PM10), and carbon monoxide (CO) hot-spot analyses 

based on evidence of a more scientifically sound basis for the 

use of AERMOD, improved model performance over CALINE3, 

and its ability to use more recent and representative 

meteorological input data. 10
Proposed Actions: Single-Source 

Impacts on Ozone and Secondary PM2.5

• For this proposed revision to the Guideline, the EPA has 

determined that advances in photochemical modeling science 

indicate it is now reasonable to provide more specific, generallyapplicable guidance that identifies particular models or analytical 

techniques that may be used under specific circumstances for 

assessing the impacts of an individual source on ozone and 

secondary PM2.5.

• The EPA believes photochemical grid models are generally most 

appropriate for addressing ozone and secondary PM2.5 because 

they provide a spatially and temporally dynamic realistic chemical 

and physical environment for plume growth and chemical 

transformation.

• The EPA is proposing a two-tiered demonstration approach for 

addressing single-source impacts on ozone and secondary PM2.5

that will be used in concert with future demonstration tools (e.g., 

MERPs).

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Proposed Actions: CALPUFF & Met Data

• For long-range air quality assessments, the EPA is proposing to 

remove CALPUFF as a preferred model and recommending its 

use as a screening technique along with other Lagrangian 

models for addressing PSD increment beyond 50 km from a 

new or modifying source. (This proposed change does not affect 

the EPA’s recommendation in the 2005 BART Guidelines to use 

CALPUFF in the BART determination process.)

• To provide more flexibility and improve the meteorological inputs 

used for regulatory modeling, the EPA is proposing to allow the 

use of prognostic meteorological data in AERMOD where there 

is no representative National Weather Service (NWS) station, 

and it is prohibitive or not feasible to collect adequately 

representative site-specific data.

– Mesoscale Model Interface Program (MMIF) 12
Proposed Actions: Model Clearinghouse

• The EPA is proposing to codify the long-standing process of the 

Regional Offices consulting and coordinating with the Model 

Clearinghouse on all approvals of alternative models or 

techniques.

• While the Regional Administrators are the delegated authority to 

issue such approvals under section 3.2 of the Guideline, all 

alternative model approvals will only be issued after consultation 

with the EPA’s Model Clearinghouse and formal documentation 

through a concurrence memorandum which demonstrates that 

the requirements within section 3.2 for use of an alternative 

model have been met.

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Proposed Actions: Revisions to the 

Appendix W Reg Text

• The EPA is proposing to make modifications to section 8 of the 

Guideline regarding model inputs and background 

concentrations to provide much needed clarity associated with 

input and database selection for use in PSD and SIP modeling.

– More definitive definition of the appropriate modeling domain and how to 

best characterize the various contributions to air quality concentrations 

within that domain.

– Revised requirements on how to characterize emissions from nearby 

sources to be explicitly modeled for purposes of a cumulative impact 

assessment under PSD and new language regarding how to characterize 

direct and precursor emissions from modeled sources for SIP attainment 

demonstrations for ozone, PM2.5, and regional haze.

– Revised recommendations on how to determine background concentrations 

in constructing the design concentration, or total air quality concentration, 

as part of a cumulative impact analysis for NAAQS and PSD increments. 14
Proposed Actions: Revisions to the 

Appendix W Reg Text (Cont.)

• The EPA proposes to revise the discussion portion of section 9 

to more clearly summarize the general concepts presented in 

earlier sections of the Guideline and to set the stage for the 

appropriate regulatory application of models and/or, in rare 

circumstances, air quality monitoring data.

– Strongly encourage adherence to the recommendations in section 

9.2.1 of the proposed Guideline regarding development of a 

modeling protocol.

• The EPA is proposing to make additional editorial changes to 

update and reorganize information throughout the Guideline. 

These revisions are not intended to meaningfully change the 

substance of the Guideline, but rather to make the Guideline 

easier to use. 15
Reminder...

• The EPA proposed to update to the Guideline on Air Quality 

Models:

– Published in the Federal Register on July 29, 2015.

(80 FR 45340)

– Docket ID No. EPA-HQ-OAR-2015-0310

– 90-day public comment window ending on October, 27, 2015.

• 11th Conference on Air Quality Modeling

– August 12 and 13, 2015 at the EPA RTP, NC Campus.

– Conference to focus on the proposed revisions to the Guideline.

– Serves as public hearing for NPRM as part of public comment period. 

Transcripts and presentation will be posted to Docket and on the 11th

Conference on Air Quality Modeling informational website. 16
For More Information

• 11th Conference on Air Quality Modeling informational website:

http://www.epa.gov/ttn/scram/11thmodconf.htm

• Proposed Rulemaking Docket

(ID No. EPA-HQ-OAQ-2015-0310):

http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OAR-2015-

0310-0001

• Conference Registration (Not required but recommended):

http://www.cleanairinfo.com/regionalstatelocalmodelingworkshop/

• Additional Questions About the Conference or Proposed 

Rulemaking:

Bridgers.George@epa.gov

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