UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK, NC 27711 

JUN 3 0 2015 

MEMORANDUM 

SUBJECT: 

FROM: 

TO: 

Proposed Approach for Demonstrating Ozone PSD Compliance 

Air 

Tyler 

Quality 

J. Fox, 

Modeling 

Group Leade Group~~ 

, C4JIJ-ef I ( 

Office of Air Quality Planning and Standards 

Proposed Regulatory Docket No. EP A-HQ-OAR-2015-0310 

Introduction 

OFFICE OF 

AIR QUALITY PLANNING 

AND STANDARDS 

To specifically assist the public in commenting on the "Revision to the Guideline on Air 

Quality Models: Enhancements to the AERMOD Dispersion Modeling System and Incorporation 

of Approaches to Address Ozone and Fine Particulate Matter" proposed rule within the overall 

context of the New Source Review (NSR) program, including Prevention of Significant 

Deterioration (PSD), the Environmental Protection Agency (EPA) has added two memoranda to 

the docket of this proposed rule. 1 • 

2 These memoranda provide details on how a future approach 

to PSD compliance demonstrations will work for secondary fine particulate matter (PM2.s) and 

our expectations for how such an approach might work for ozone based on future, separate 

actions to establish a Significant Impact Level (SIL) and Model Emissions Rates for Precursors 

(MERPs) for PM2.s and ozone. This memorandum will focus on ozone and the related precursors 

of volatile organic compounds (VOC) and nitrogen oxides (NOx). 

The complex chemistry of ozone formation is well-documented and has historically 

presented significant challenges with the identification and establishment of particular models 

for assessing the impacts of individual stationary sources on the formation of this air pollutant.3

• 

4 

1 U.S. EPA, 2015. "Proposed Approach for Demonstrating PM2.5 PSD Compliance", Memorandum to Docket 

No. EPA-HQ-OAR-2015-0310 by Tyler J Fox, U.S. EPA/OAQPS, Research Triangle Park, NC. June 30, 2015 . 

2 U.S. EPA, 2015. "Proposed Approach for Demonstrating Ozone PSD Compliance", Memorandum to Docket 

No. EPA-HQ-OAR-2015-0310 by Tyler J Fox, U.S. EPNOAQPS, Research Triangle Park, NC. June 30, 2015. 

3 NARSTO, 2000. An Assessment of Tropospheric Ozone Pollution: A North American Perspective. The 

NARSTO Synthesis Team. NARSTO Management Office (Envair), Pasco, Washington. 

4 Seinfeld and Pandis. 1998. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. J. 

Seinfeld and S. Pandis. Wiley Interscience. New York, New York. ISBN 0 47 117815 2. 

Recycled/Recyclable •Printed with Vegetable Oil Based Inks on 100% Recycled Paper (40% Postconsumer) 
2

Because of these considerations, the EPA's judgment in the past has been that it was not 

technically sound to assign with particularity specific models that must be used to assess the 

impacts of a single source on ozone concentrations. Instead, the EPA has chosen to satisfy the 

requirements of Section 165(e) (3) (D) of the Clean Air Act (CAA) by determining particular 

models or other analytical techniques that should be used on a case-by-case basis.5 As stated in 

Section 5.2.1.c of the current Guideline on Air Quality Models (published as Appendix W to 40 

CFR Part 51, hereafter Guideline), the “[c]hoice of methods used to assess the [ozone] impact of 

an individual source depends on the nature of the source and its emissions.” Under this approach, 

the appropriate methods for assessing ozone impacts are determined in consultation with the 

appropriate EPA Regional Office and/or permit reviewing authority on a case-by-case basis. A 

modeling protocol should be developed by the permit applicant and approved by the appropriate 

permitting authority to ensure that the analysis conducted will conform to the recommendations, 

requirements, and principles of Section 10.2.1 of the current Guideline.

In the January 2012, Sierra Club petition grant,

6 the EPA committed to engage in 

rulemaking to evaluate whether updates to the Guideline are warranted and, as appropriate, 

incorporate new analytical techniques or models for ozone and secondarily formed PM2.5. The

EPA’s proposed revisions to the Guideline satisfies the EPA’s commitment in the petition grant. 

As a part of this commitment and in compliance with CAA section 320, the EPA conducted the 

Tenth Modeling Conference in March 2012, where there were presentations of ongoing research 

of single-source plume chemistry and photochemical grid modeling techniques, as well as 

several public forums, and the EPA subsequently received written comments pertaining to such 

modeling. The EPA initiated Phase 3 of the Interagency Workgroup on Air Quality Modeling 

(IWAQM) process in June 2013 to inform this process to update the Guideline to address 

chemically reactive pollutants for near-field and long-range transport applications. Comments 

received from stakeholders at the Tenth Modeling Conference supported this collaborative effort 

to provide additional guidance for modeling single-source impacts of secondarily formed 

pollutants in the near-field and for long-range transport. Stakeholder comments also supported 

the idea of this collaborative effort occurring in parallel with stakeholders’ efforts to further 

model development and evaluation. The EPA’s recommended revisions to the Guideline are 

largely based on detailed review and assessment of this input.

The remainder of this memo provides a summary of the current approach and the 

proposed approach, as anticipated under the proposed revisions to the Guideline and other future 

 5 We note that this technical judgment has no effect on the obligation of sources subject to PSD to conduct a 

source impact analysis and demonstrate that a proposed source or modification will not cause or contribute to a 

violation of any NAAQS. 40 CFR 51.166(k); 52.21(k). That is, the inclusion of a process rather than a specific 

preferred model in Appendix W does not relieve the source of the requirement to make this demonstration, which 

necessarily involves an analysis.

6 U.S. EPA, 2012. Gina McCarthy Letter to Robert Ukeiley dated January 4, 2012, Washington, D.C. 20460. 

http://www.epa.gov/ttn/scram/10thmodconf/review_material/Sierra_Club_Petition_OAR-11-002-1093.pdf.
3

EPA rulemakings and guidance, for demonstrating compliance with the ozone National Ambient 

Air Quality Standards (NAAQS).

Summary of Current Approach 

As described above, the current approach to assess the ozone impacts of an individual 

source is performed on a case-by-case basis in consultation with the appropriate EPA Regional 

Office and/or permit reviewing authority. Section 5.2.1.c of the current Guideline states, the 

“[c]hoice of methods used to assess the [ozone] impact of an individual source depends on the 

nature of the source and its emissions.” There is currently not a preferred or recommended 

analytical technique or modeling system to perform ozone compliance demonstration 

assessments for individual sources. Some assessments have been completely qualitative in 

nature; while others have utilized chemical transport (e.g., photochemical grid) models as part of 

the assessment. Through the consultation process, a modeling protocol should be developed by 

the permit applicant and approved by the appropriate permitting authority to ensure that the 

analysis conducted will conform to the recommendations, requirements, and principles of

Section 10.2.1 of the current Guideline.

Summary of Proposed Approach under Future EPA Rulemakings

For the proposed revisions to the Guideline, the EPA has determined that advances in 

photochemical modeling science indicate it is now reasonable to provide more specific, 

generally-applicable 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. 

For assessing ozone impacts from single sources, the degree of complexity required to assess 

potential impacts varies depending on the nature of the source, its emissions, and the background 

environment. In order to provide the user community flexibility in estimating single-source 

ozone impacts and given the emphasis on the use of chemical transport models for these 

purposes, the EPA is proposing a two-tiered demonstration approach for addressing singlesource impacts on ozone, i.e., 

• First tier involves use of technically credible relationships between precursor 

emissions and a source’s impacts that may be published in the peer-reviewed 

literature; developed from modeling that was previously conducted for an area by a 

source, a governmental agency, or some other entity and that is deemed sufficient; or 

generated by a peer-reviewed reduced form model. 

• Second tier involves application of more sophisticated case-specific chemical 

transport models (e.g., photochemical grid models) to be determined in consultation 

with the EPA Regional Office and conducted consistent with new EPA single-source 
4

modeling guidance.7

To fully implement these proposed changes to the Guideline related to addressing ozone

impacts, the EPA intends to pursue a separate rulemaking to establish a technical basis and new 

values for ozone Significant Impact Levels (SILs) and to introduce a new demonstration tool for 

ozone precursors referred to as Model Emissions Rates for Precursors (MERP). Figure 1 

illustrates how the demonstration tools would be implemented in the future proposed approach. 

A MERP would neither replace the existing Significant Emissions Rates (SERs) for these 

pollutants nor serve as the basis for the applicability of PSD requirements to sources with 

emissions above the SER. However, a MERP would represent a level of emissions of precursors 

that is not expected to contribute significantly to concentrations of ozone. Our present 

understanding of the atmospheric science of ozone formation indicates that MERP values for 

VOC and NOx will likely be higher than their SERs as criteria pollutants and more appropriate 

for evaluating their impacts as precursor pollutants to ozone formation. As part of the separate 

rulemaking, the EPA intends to demonstrate that a source with precursor emissions of VOC or 

NOx below the MERP level will have ambient impacts that will be less than the SIL and, 

thereby, provide a sufficient demonstration that the source will not cause or contribute to a 

violation of the ozone NAAQS. 

The EPA’s Guidance for PM2.5 Permit Modeling8 provides for a three-tiered approach to 

address secondary PM2.5 that includes:

1) a qualitative assessment; 

2) a hybrid qualitative/quantitative assessment utilizing existing technical work; and 

3) a full quantitative modeling exercise.

Aspects of this three-tiered approach provides a parallel for consideration of conducting permit 

modeling for ozone. As such, the EPA expects that MERPs as a demonstration tool will absolve 

the need for a first tier of a qualitative assessment since sources that would potentially provide a 

qualitative assessment of ozone impacts are expected to have precursor emissions levels below 

the MERP. The second and third tiers of assessment would then be consistent with the EPA’s 

proposed two-tiered demonstration approach for ozone reflected in Section 5.3.2 of the proposed 

revisions to the Guideline.

While the development of MERPs for ozone precursors is expected to address a number 

of PSD permitting situations, the EPA believes that most of the remaining situations in which a 

source must demonstrate compliance under the proposed Guideline will be addressed sufficiently 

 7 U.S. EPA, 2015. Guidance on the use of models for assessing the impacts from single sources on secondarily 

formed pollutants ozone and PM2.5. Publication No. EPA 454/P-15-001. Office of Air Quality Planning & 

Standards, Research Triangle Park, North Carolina 27711.

8 U.S. EPA, 2014. Guidance for PM2.5 Modeling. May 20, 2014, Publication No. EPA-454/B-14-001. Office of 

Air Quality Planning & Standards, Research Triangle Park, NC. 

http://www.epa.gov/ttn/scram/guidance/guide/Guidance_for_PM25_Permit_Modeling.pdf.
5

under the proposed first tier where existing technical information could be used in combination 

with other supportive information and analysis for the purposes of estimating ozone impacts 

from a particular source. The existing technical information should provide a credible and 

representative estimate of the ozone impacts from the project source. In these situations, a more 

refined approach for estimating ozone impacts from project sources may not be necessary. The 

EPA has been compiling and reviewing screening approaches that are based on technically 

credible tools (e.g., photochemical grid models) that relate source precursor emissions to ozone

impacts. In review of existing approaches detailed in peer reviewed journal articles and non-peer 

reviewed forms (e.g., technical reports, conference presentations), it is not clear that a single 

approach has been clearly proposed to and evaluated by the modeling community for estimating 

screening level ozone impacts from single sources. Other screening level alternatives to chemical 

transport model application may include the use of existing credible photochemical model 

impacts for sources deemed to be similar in terms of emission rates, release parameters, and 

background environment. The EPA will continue to engage with the modeling community to 

identify credible alternative approaches for estimating single-source ozone impacts which 

provide flexibility and are less resource intensive for permit demonstration purposes.
6

Figure 1. Overview of NAAQS Compliance Demonstration for New or Modifying Sources 

Under NSR/PSD Programs: Under Future EPA Rulemakings & Guidance

New or Modified

Source

Nonattainment

Area?

Attainment or Unclassified 

Area

Source Emissions

Greater Than or 

Equal to SER(s)?

Analysis of Ambient Air Quality 

Impacts Not Required for the 

Particular Pollutant

Source Impact 

Above SIL? Satisfies NAAQS AQ Analysis

Projected

NAAQS 

Violations?

Satisfies NAAQS AQ Analysis

Source Impact Greater 

Than or Equal to SIL 

at Projected 

Violations?

Compliance Demonstration is Not 

Adequate.

Y

Y

Y

N Y

N

Source Impact Analysis

N

N

Cumulative Impact Analysis

Nonattainment NSR 

Requirements Apply

Nonattainment Area

N

Y

Satisfies NAAQS AQ Analysis

Source Emissions 

Above MERP(s)?

Y

Y

Satisfies NAAQS AQ Analysis

N
7

Summary

To summarize, the EPA is providing this memorandum to the docket of the “Revision to 

the Guideline on Air Quality Models: Enhancements to the AERMOD Dispersion Modeling 

System and Incorporation of Approaches to Address Ozone and Fine Particulate Matter”

proposed rule to specifically assist the public in providing comment. This memoranda provides

details within the overall context of the NSR program, including PSD, on how a future approach 

to PSD compliance demonstrations will work for ozone based on a future action to establish a 

SIL and MERPs for ozone and its precursors. If you have additional questions regarding this 

memorandum or the proposed rulemaking, please feel free to contact me, (919) 541-5562 or 

fox.tyler@epa.gov, or contact George Bridgers, (919) 541-5563 or bridgers.george@epa.gov.

cc: Air Program Managers

Regional Office Modeling Contacts

Richard Wayland, C304-02

Anna Wood, C504-01

Raj Rao, C504-01

Brian Doster, OGC

Melina Williams, OGC

George Bridgers, C439-01