Treatment of PM2.5 and Ozone in 

PSD Compliance Demonstrations

11th Modeling Conference

Research Triangle Park, NC

August 12, 2015

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Updates for Secondary Pollutants

• EPA granted Sierra Club petition (Jan 4, 2012) with commitment to 

update the Guideline on Air Quality Models (Appendix W) to address 

O3 and secondary PM2.5 impacts

• Current final version provides little information about assessing single 

source secondary impacts (O3 and secondary PM2.5)

• Proposed revision includes an entire chapter (5) on secondarily formed 

pollutants (O3 and secondary PM2.5) and a chapter (6) focused on 

visibility, deposition (AQRVs)

• Intent for the proposed Appendix W revision is to develop appropriate 

detail that will be relevant over the long term

• Increasing technical detail and reflections of the current practice of 

model application in guidance documents which are more dynamic

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Process of updating Appendix W for single 

source secondary pollutant impacts

• Interagency Workgroup on Air Quality Modeling (IWAQM) has been a process 

historically used for collaborative work to update regulatory air quality modeling 

approaches

• IWAQM (phase 3) initiated in July 2013 to provide a mechanism for updating 

Appendix W and related guidance documents in partnership with the EPA 

Regional Offices and other federal agencies

• Goal is to understand and identify credible modeling techniques for single 

source secondary impacts for O3 and secondary PM2.5

• IWAQM3 consists of 2 working groups

– near-field impacts

– long-range transport (most similar to past IWAQM phases)

• Technical reports and guidance were developed to support the proposed 

revisions to the Guideline on Air Quality Models

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Appendix W Chapter 5: Outline

• 5.0 Models for Ozone and Secondarily Formed Particulate Matter

• 5.1 Discussion

– Describe ground level ozone and secondary PM2.5

• 5.2 Recommendations

– What types of modeling systems would be appropriate for either single or multi-source assessments 

for secondary pollutants? 

• 5.3 Recommended Models and Approaches for Ozone

– Background relevant to both permit and non-permit assessments

• 5.3.1 Models for NAAQS Attainment Demonstrations and Multi-Source Air Quality Assessments

– Projected future year O3 NAAQS attainment

• 5.3.2 Models for Single-Source Air Quality Assessments

– Current year O3 SIL or NAAQS comparison for permit related assessments

• 5.4 Recommended Models and Approaches for Secondarily Formed PM2.5

– Background relevant to both permit and non-permit assessments

• 5.4.1 Models for NAAQS Attainment Demonstrations and Multi-Source Air Quality Assessments

– Projected future year PM2.5 NAAQS attainment

• 5.4.2 Models for Single-Source Air Quality Assessments

– Current year PM2.5 SIL or NAAQS comparison for permit related assessments

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Appendix W Chapter 5: Summary

• Totally new chapter in Appendix W

• Clear distinction between nonattainment planning for NAAQS (multisource) vs. permit (single source) modeling requirements

• Emphasizes the importance to develop modeling protocols and consult 

with the reviewing authority

• Includes a screening approach with no preferred model

• Outlines a multi-tiered approach for single source permit assessments

• Emphasis on use of chemical transport models (e.g. photochemical & 

Lagrangian models) or techniques that reflect state of science 

atmospheric chemistry for the less-anticipated situations where a 

refined assessment is necessary

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Broader Considerations for O3 and 

secondary PM2.5 Permit Modeling – Model Emissions Rates for Precursors (MERPs) will be included as part of future 

rulemaking/guidance

– 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 secondarily-formed PM2.5 or ozone

• A source emitting above the SER but below the MERP would still need to follow administrative PSD 

requirements but may not need to do further technical demonstration to show impacts are below the 

appropriate SIL

– Our present understanding of the atmospheric science of ozone and secondary PM2.5 

formation indicates that MERP values will likely be higher than the SERs and more 

appropriate for evaluating the impacts of these criteria pollutants as precursors to PM2.5 

and ozone formation

– Separate memorandum have been submitted to the docket illustrating how permit 

assessments will work when MERPs are in place

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Guidance on the use of models for assessing the impacts of 

emissions from single sources on the secondarily formed 

pollutants ozone and PM2.5

• It is expected project source post-construction emissions will initially be 

compared with the level of the MERP (once available); if emissions are above 

the MERP then a first and possibly second tier assessment may be necessary

• For first tier assessments, it is generally expected that applicants would use 

existing empirical relationships between precursors and secondary impacts 

based on modeling systems appropriate for this purpose as detailed in this 

guidance

• It is also possible screening approaches based on full science chemical 

transport modeling systems (e.g. reduced form models) could provide 

information to satisfy the first tier in some situations

• The use of pre-existing credible technical information or a screening model for 

the purposes of estimating single source secondary impacts will be considered 

on a case-by-case basis and should be done in consultation with the appropriate 

permitting authority

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Guidance on the use of models for assessing the impacts of 

emissions from single sources on the secondarily formed 

pollutants ozone and PM2.5

• For second tier assessments (Sections 5.3.2.c and 5.4.2.c) when 

necessary, guidance is provided on the air quality models, inputs, run 

time options, receptor placement, and application approach for the 

purposes of estimating the impacts on ozone and secondarily formed 

PM2.5 from single project sources

• Within the second tier described in Appendix W, applicants are 

provided flexibility in terms of the complexity of model application for 

comparison to both the SIL and NAAQS

• These sub-tiers of the Appendix W second tier allow for simpler 

approaches to be compared conservatively to the SIL and NAAQS and 

more sophisticated approaches could be applied to provide a more 

representative impact for a source

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Guidance on the use of models for assessing the impacts of 

emissions from single sources on the secondarily formed 

pollutants ozone and PM2.5

• For second tier assessments, chemical transport models are recommended 

for estimating single source O3 and secondary PM2.5 impacts

• Chemical transport models include Lagrangian puff models and Eulerian grid 

(e.g. photochemical transport) models

• Lagrangian puff models need as input a realistic chemical environment

• Photochemical transport models typically estimate a realistic chemical 

environment

• Even though single source emissions are injected into a grid volume, 

comparisons with in-plume measurements indicate these types of models can 

capture downwind secondary pollutant impacts when applied appropriately for 

this purpose

• Further testing is needed for both types of chemical transport modeling systems 

(Lagrangian and Eulerian) to best understand the configurations appropriate for 

permit related assessments

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IWAQM3: Near-Field Impacts Workgroup 

• Updates to preamble & Chapter 5: Ozone and Secondary PM2.5

• New guidance document: Guidance on the use of models for assessing 

the impacts of emissions from single sources on the secondarily formed 

pollutants ozone and PM2.5 (available in docket)

• IWAQM Phase 3 Summary Report: Near-Field Single Source 

Secondary Impacts (available in docket)

– What do we know about the relationships between single source 

precursors and secondary impacts?

– Overview of published emissions and secondary impacts from 

single sources to provide context for expected impacts

– Recommended models, approaches, screening tools, etc.

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Estimating Source Contribution with Chemical 

Transport Models

• Lagrangian puff models output single source impacts

• Single source impacts estimated by photochemical grid models can be done by 

comparing a 1) model simulation with all sources and the project source at preconstruction levels and 2) model simulation with all sources and the project source at 

post-construction levels

• Alternatively, post-construction emissions could be tracked with photochemical grid 

model source apportionment or source sensitivity model extensions

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Baseline + Source 

modification

Baseline Contribution of Source 

modification
Appendix W Chapter 6: Outline

• 6.0 Modeling for Air Quality Related Values and Other Governmental Programs

• 6.1 Discussion

– Brief description of scope of this section and the need to consult the appropriate guidance and reviewing Agencies

• 6.2 Air Quality Related Values

– Describes what AQRVs entail, the regulatory authority, the reviewing authority, and FLAG guidance

• 6.2.1 Visibility

– Describes the nature of visibility impairment and appropriate guidance

• 6.2.1.1 Models for Estimating Near-Field Visibility Impairment

• Describes the types of modeling systems appropriate for this purpose and directs to appropriate guidance

• 6.2.1.2 Models for Estimating Visibility Impairment for Long-Range Transport

– Describes the types of modeling systems appropriate for this purpose and directs to appropriate guidance

• 6.2.2 Models for Estimating Deposition Impacts

• Describes the types of modeling systems appropriate for this purpose and directs to appropriate guidance

• 6.3 Modeling Guidance for Other Governmental Programs

– Directs to guidance for outer continental shelf where BOEM is the authority and airports (FAA)

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Appendix W Chapter 6: Summary

• Previous version of Appendix W chapter 6 contained a variety of incongruous information

• New chapter 6 focuses only on air quality related values (visibility and deposition) with small 

notes for other governmental programs

• Section describes visibility and deposition and the types of models that are appropriate for 

estimating these endpoints

• Emphasis on use of chemical transport models (e.g. photochemical models) or techniques 

that reflect state of science atmospheric chemistry

• Specific guidance for model selection and model application for visibility and deposition at 

Class I areas available from Federal Land Managers

– Federal Land Managers' Air Quality Related Values Work Group (FLAG) Phase I Report - Revised 2010

• Specific guidance for models and model application available from FAA for airports and 

BOEM for offshore sources within their jurisdiction

• Screening approach for primary pollutant impacts on PSD/NAAQS increment at Class I 

areas described in Chapter 4 of the proposed Appendix W revision

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IWAQM3: Long Range Transport Workgroup 

• Updates to preamble & Chapter 6: Modeling for Air Quality 

Related Values and Other Governmental Programs

• IWAQM Phase 3 Summary Report: Long Range Transport and 

Air Quality Related Values (in docket)

– Recommended models, approaches, screening tools, etc. for 

long range transport assessments of secondary pollutants 

and deposition

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