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 February 4, 2011 

 

MEMORANDUM 

SUBJECT: Transmittal of Policy Assessment for the Review of the Secondary National 

Ambient Air Quality Standards for Oxides of Nitrogen and Sulfur – Final 

Document 

FROM: Lydia N. Wegman, Director /s/ 

Health and Environmental Impacts Division 

Office of Air Quality Planning and Standards 

TO: Holly Stallworth 

Designated Federal Officer 

Clean Air Scientific Advisory Committee 

EPA Science Advisory Board Staff Office 

 Attached is the final document, Policy Assessment for the Review of the Secondary 

National Ambient Air Quality Standards for Oxides of Nitrogen and Sulfur (Policy 

Assessment), prepared by the Environmental Protection Agency’s (EPA) Office of Air Quality 

Planning and Standards (OAQPS) staff as part of EPA’s ongoing review of the secondary national 

ambient air quality standards (NAAQS) for oxides of nitrogen and oxides of sulfur. An earlier 

version of this document – the January 14, 2011 version – was made available to the Clean Air 

Scientific Advisory Committee (CASAC) Oxides of Nitrogen and Sulfur Secondary NAAQS 

Review Panel (the Panel) prior to final document production to provide sufficient time for the Panel 

to consider a final Policy Assessment in advance of the Panel meeting scheduled for February 15-

16, 2011. The CASAC requested the upcoming meeting (Russell and Samet, 2010) so as to develop 

CASAC recommendations on key elements of the NAAQS for oxides of nitrogen and sulfur based 

on a final, more complete Policy Assessment than the second draft Policy Assessment that the Panel 

reviewed in October 2010. 

 Since I conveyed the earlier version of the Policy Assessment to the Panel (Wegman 

memorandum, January 18, 2011), we have completed document production, including final 

reference checks and document formatting. This final Policy Assessment, like the January 14, 2011 

version, presents staff analyses and conclusions relevant to EPA’s review of the secondary (welfarebased) standards for oxides of nitrogen and sulfur. It reflects consideration of, and is responsive to, 

comments from the Panel (Russell and Samet, 2010), as well as public comments, on the second 

draft Policy Assessment. Attachment 1 presents a broader discussion of changes made to the 

second draft Policy Assessment than was included with my earlier memo. The attachment also 

notes the following minor changes that have been made relative to the January 14, 2011 version: 

modified Figure 2-32; expanded discussion of the range of effects related to nutrient enrichment in 

section 6.3.2; added ecoregion-level water quality data to Appendix C; and expanded discussion 

of future research and data collection (Appendix F, section F.5).
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This final Policy Assessment is being made available to the Panel in the form of the attached 

electronic files, one for the main body of the document and the other for the appendices, which we 

request that you forward to members of the Panel. This document is also available on the same EPA 

website as the earlier version: http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_pa.html. 

Printed copies of this document will be sent to members of the Panel via Federal Express. 

As I expressed previously, we appreciate the effort that the Panel has made in reviewing draft 

documents in conjunction with our review of the secondary NAAQS for oxides of nitrogen and 

sulfur, and welcome further comment and advice from the Panel on the key elements of the NAAQS. 

We look forward to the Panel’s discussion at its upcoming meeting in February. Should you have 

any questions regarding this document, please contact Dr. Rich Scheffe (919-541-4650; email 

scheffe.rich@epa.gov). 

cc: Vanessa Vu, SAB/OA 

 Rosalina Rodriguez, OAQPS/OAR 

 Karen Martin, OAQPS/ OAR 

 Rich Scheffe, OAQPS/OAR 

 Ginger Tennant, OAQPS/OAR 

 Randy Waite, OAQPS/OAR 

 John Vandenberg, ORD/NCEA-RTP 

 Debra Walsh, ORD/NCEA-RTP 

 Doug Johns, ORD/NCEA-RTP 

 Tara Greaver, ORD/NCEA-RTP 
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Attachment 1 

Staff Responses to CASAC Comments on the second draft 

Policy Assessment for the Review of the Secondary National Ambient 

Air Quality Standards for Oxides of Nitrogen and Sulfur

 Changes made in the final Policy Assessment in response to the CASAC Panel’s comments 

on the second draft Policy Assessment are summarized below by chapter. We also note major 

organizational changes made in the document to eliminate redundancy and improve the logical flow 

of information presented. Further, we note a few minor changes made to the January 14, 2011 

version of this document beyond formatting, referencing, and editorial changes. 

Organizational changes include: 

• A new technical chapter (chapter 2) has been added, covering emissions, air quality 

monitoring networks and modeling, deposition, water chemistry and ecosystem modeling, 

and trends of each component (emissions, air, water). 

• A new integrated policy chapter (chapter 7) has added that addresses all the elements of a 

new multi-pollutant, multi-media standard for aquatic acidification (indicators, form, 

averaging time and level), implications of alternative levels and forms, uncertainties 

associated with individual and combined components of such a standard, and staff 

conclusions regarding these elements. This chapter integrates and replaces chapters 5 and 9 

of the second draft Policy Assessment. 

• An expanded uncertainty analysis is now presented in Appendices F and G, building on and 

replacing the analyses that were presented in chapter 7 of the second draft Policy 

Assessment. 

• Additional Appendices have been added to provide detailed information on spatial 

aggregation (Appendix C), the implications of alternative standards (Appendix D), and 

additional analysis of alternative indicators of oxides of nitrogen (Appendix E). 

Executive Summary and Chapter 1 – Introduction 

 These sections have been shortened substantially and focused on the key policy-relevant 

questions and issues, consistent with the framework used to organize the information presented 

throughout this final document. 

Chapter 2 -- Characterizing Emissions, Air Quality, Deposition, and Water Quality

 This new chapter was developed to consolidate much of the underlying technical 

information that is relied on to develop the policy discussion in chapter 7. This chapter includes 

many of the technical discussions, maps and tables that were in chapters 2, 4 and 8 of the second 

draft Policy Assessment. Figure 2-32 (Figure 3-2.12 in the second draft document) has been 

modified by adding a panel showing relative changes in concentrations and deposition and revising 

the figure caption and legends accordingly. New material, some of which was extracted from the 

Risk and Exposure Assessment and the Integrated Science Assessment, was added to address 

water chemistry and trends of emissions, air concentrations, deposition and water quality. Much 
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of the content in chapter 2 is presented in the context of how this information relates to specific 

aspects of developing the standard in chapter 7. 

 A generalized trends analysis that combines time series of emissions, air quality, 

deposition, and water quality is provided in section 2.5 of chapter 2. While this trends analysis is 

not an explicit reconstruction of historical values of the aquatic acidification index (AAI), it does 

demonstrate that the directional responses across all of these components are consistent with the 

structure of the form of the standard presented in this final Policy Assessment. We would expect 

a retrospective analysis to exhibit a pattern of response similar to the prospective analysis that 

has been included in chapter 7, as described below. We explored ways of reconstructing 

historical AAI values, although the lack of both observed and modeled data bases precluded such 

a study. 

Chapter 3 – Known of Anticipated Ecological Effects

 No major changes were made to this chapter other than to move some of the material on 

ecosystem modeling to the new chapter 2 and to add material on biological effects that had been in 

chapter 5 of the second draft document. Both of these changes were made to consolidate biological 

effects into one chapter and reduce redundancy. Some editing was done to improve the logical 

presentation of the material, remove redundancy, and add citations as appropriate. 

Chapter 4 – Considerations of Adversity to Public Welfare

 In response to comments on this chapter, we added an explanation of non-linear values in 

the aquatic acidification section and included more emphasis on nonuse (existence), preservation 

and bequest values. For terrestrial acidification values, we included actual survey values for 

existence and bequest values as well as a discussion explaining why the model results for 

provisioning services were smaller than the values for recreational and other uses. Also, we 

added more discussion of the beneficial effects of nitrogen, including potential effects on carbon 

sequestration, as well as more explanation of climate regulating services by ecosystems. 

Chapter 5 – Co-Protection for Other Effects Potentially Afforded by an Aquatic Acidification 

Standard

 Revisions to this chapter mostly included editorial corrections and clarifications. 

Chapter 6 – Addressing the Adequacy of the Current Standards

 In response to comments on this chapter, section 6.3.2 has been revised to be more 

inclusive of the range of effects nutrient enrichment can have on ecosystems. We note that 

changes to this section were inadvertently left out of the January 14, 2011 version of the 

document, but have been included in this final Policy Assessment.

Chapter 7 -- Consideration of Alternative Standards for Aquatic Acidification

 Chapter 7 serves as the policy chapter, effectively replacing chapters 5 and 9 of the second 

draft Policy Assessment. This new policy chapter provides the rationale underlying staff’s 

conclusions on each element of the standard, building from a more simplified conceptual model to a 

full expression of the standard. Calculations are provided to demonstrate how the standard would be 
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applied and to provide estimates of areas likely not to meet alternative standards, defined in terms of 

alternative levels and alternative percentiles that are an integral part of the specification of the form 

of the standard. 

 Spatial aggregation methodology is discussed as part of the discussion of the form of the 

standard (section 7.2.5), with a focus on Omernik ecoregion Level III categorization, which is used 

for defining several of the water quality and atmospheric variables incorporated in the standard. 

Extensive background information on the Omernik level III ecoregions is provided in a new 

Appendix C. We note that water quality data have been added for each such ecoregion beyond the 

information presented in the January 14, 2011 version of Appendix C. 

We have conducted a prospective analysis of the behavior of the standard, as discussed in 

section 7.5 and more fully presented in Appendix D. This analysis includes calculated AAI 

values based on both 2005 current conditions and an emissions sensitivity run with significant 

reductions of oxides of nitrogen and sulfur used as proxies for potential future conditions. We 

interpret the results of the prospective calculations as demonstrating expected and interpretable 

behavior of the standard. 

 To facilitate the flow across the sections within this chapter addressing individual elements of 

the standard, many of the details regarding model assumptions and derivations are appended, and 

much of the effects discussion relevant to establishing a range of ANC levels for consideration that 

had been in the previous chapter 5 was moved to chapter 3. 

 Overall, this new integrated chapter addresses comments related to clarifying the description 

of spatial aggregation as well as clarifying how it all elements of the standard work together. 

Greater clarity describing the linkage of all elements of the standard is enabled by moving much of 

the underlying technical information to chapters 2 and 3. 

Appendix F – Evaluation of Variability, Sensitivity, and Uncertainty in the Acidification Index

 With regard to comments related to further evaluation of the stability of the transference 

ratios, we have done additional analyses, which are presented in Appendix F and discussed in 

section 7.6 of chapter 7. Previously, we reported on the spatial and temporal variability of 

transference ratios based on the spatial variability across 12 km grid cells in the Adirondacks and 

Shenandoah areas, as well as the sensitivity to major reductions in emissions of nitrogen and 

sulfur oxides. For this final Policy Assessment, we added three additional analyses of the 

transference ratios that are included in Appendix F. These include a comparison of CMAQ 12 

km and 36 km generated transference ratios, transference ratios generated by the Canadian 

AURAMS model, and construction of transference ratios based on combining observations of 

wet deposition, ambient air concentrations from CASTNET, and dry deposition using the 

CASTNET dry deposition model. We were not able to perform a chemical mechanism 

sensitivity analysis that held all modeled platform attributes the same other than the chemical 

mechanism. We currently plan to continue probing the behavior of transference ratios both 

through modeled sensitivity studies as well as through constructing observation-based ratios. 

 With regard to comments related to further CMAQ evaluation and improvement, we have 

conducted comparisons of CMAQ-predicted SO2 with observed SO2 at rural SEARCH sites; 

these results were added to Appendix F. The most relevant model improvement steps for CMAQ 

with regard to deposition also are listed in Appendix F. These include adoption of PRISM-
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adjusted precipitation, bi-directional ammonia flux, and addition of lightning-generated nitrogen 

oxides. Further, a planned dry deposition flux study for nitrogen and sulfur will provide a more 

rigorous observational basis for evaluating the deposition schemes in CMAQ. Collectively, 

these tasks, as well as related model improvement efforts in chemical mechanism development 

and emissions and meteorological processing, are all intended to enhance the underlying 

scientific foundation of CMAQ and reduce biases of model estimates relative to observations. 

 With regard to comments on future research areas, we have expanded the discussion in 

Appendix F (section F.5) to incorporate additional areas suggested by the Panel that should be 

considered for future research and data collection. We note that this expanded discussion was 

not included in the January 14, 2011 version of this document. 

Appendix G – Cumulative Uncertainty Analysis 

 A cumulative uncertainty analysis was developed in response to comments 

recommending a comprehensive quantitative analysis of uncertainties of all components of the 

standard. This analysis is included in Appendix G. Staff designed and performed a Monte 

Carlo-style analysis to assess the cumulative effects of uncertainty in the AAI equation. This 

analysis included bootstrap analyses to generate uncertainty distributions for the main 

components of the AAI (Neco, NHx, TNOy, TSOx, and CL, which includes BCo and Q). To assess 

the cumulative effect of the uncertainty, the results of the bootstrap analyses were randomly 

sampled to generate repeated AAI trade-off curves. The results of these analyses were illustrated 

graphically, including estimates of 95% confidence intervals.