September 15, 2010

MEMORANDUM

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

Ambient Air Quality Standards for NOx and SOx: Second External Review Draft

FROM: Lydia N. Wegman, Director /s/

Health and Environmental Impacts Division (C504-02)

Office of Air Quality Planning and Standards

United States Environmental Protection Agency

TO: Angela Nugent

Designated Federal Officer

Clean Air Scientific Advisory Committee

EPA Science Advisory Board Staff Office

Attached is the draft document, Policy Assessment for the Review of the Secondary National 

Ambient Air Quality Standards for NOx and SOx: Second External Review Draft (September

2010), 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

(welfare-based) national ambient air quality standards (NAAQS) for oxides of nitrogen (NOx) 

and sulfur (SOx). This document will be the focus of a review by the Clean Air Scientific 

Advisory Committee (CASAC) Oxides of Nitrogen (NOx) and Sulfur Oxides (SOx) Secondary

Review Panel (the CASAC NOx SOx Secondary Panel) at a public meeting to be held in 

Durham NC on October 6-7, 2010. I am requesting that you forward these draft documents to 

the CASAC NOx SOx Secondary Panel to prepare for the October meeting. 

As part of the review of the current secondary (welfare-based) NAAQS for NOx and SOx, 

EPA’s OAQPS staff has prepared a second draft Policy Assessment. This draft Policy

Assessment evaluates the policy implications of the key scientific information contained in the 

document Integrated Science Assessment for Oxides of Nitrogen and Sulfur-Ecological Criteria, 

prepared by EPA’s National Center for Environmental Assessment (NCEA) and the results from 

the analyses contained in the Risk and Exposure Assessment for Review of the Secondary

National Ambient Air Quality Standards for Oxides of Nitrogen and Oxides of Sulfur. These 

documents, along with EPA’s Integrated Review Plan, can be found at 

http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html.

The second draft Policy Assessment reflects consideration of comments from the Panel, as well 

as public comments, on the first draft Policy Assessment, which was reviewed by the Panel on 

April 1-2 and May 3, 2010. The main comments from the Panel on the first draft Policy
Assessment were provided to us in a June 22, 2010 letter (Russell and Samet, 2010)1

. These 

comments, and the changes made in the second draft Policy Assessment in response to them, are 

summarized in Attachment 1. Note that the structure of the Policy Assessment has changed 

somewhat from the first draft. While there are still 9 chapters, Chapters 5 and 6 in the first draft 

have been combined into Chapter 5 in the current draft; Chapter 7 in the first draft is now 

Chapter 6 in the current draft, Chapter 7 in the current draft is a new chapter on uncertainty, 

variability and sensitivity analyses, and Chapter 8 in the current draft is a new chapter on 

monitoring issues.

The current draft contains several placeholders indicating missing tables or discussions. We 

expect to provide the Panel with these tables and discussions in a separate submission two weeks 

prior to the October 6-7 Panel meeting. These will include a table in Chapter 9 summarizing the 

options for elements of the standards, a table in Chapter 7 summarizing key uncertainties, and a 

discussion in Chapter 5 of the implications of a range of choices of the target percent of lakes 

and streams to protect for alternative target ANC levels. This discussion will also cover the 

various spatial aggregation choices and resulting implications on the target percentages. We 

recognize that there are some formatting issues in several chapters, and these will be addressed in 

the final PA. 

The CASAC and public comments on the draft Policy Assessment will be taken into 

consideration in making revisions to the draft document. A final Policy Assessment will be 

released in December of 2010. Following completion of the final Policy Assessment, EPA will 

conduct a rulemaking with regard to its review of the secondary NOx and SOx NAAQS. 

Consistent with the terms of a consent decree, the EPA Administrator will sign a notice of 

proposed rulemaking by July 12, 2011 and a final rulemaking notice by March 20, 2012. Draft 

documents are being made available to the CASAC NOx SOx Secondary Panel in the form of

attached electronic files. The documents are also available from the EPA website at

http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html. Printed copies of these

documents will be sent to CASAC NOx SOx Secondary Panel members via UPS. A set of 

charge questions related to the draft Policy Assessment will be submitted separately.

We look forward to discussing the second draft Policy Assessment with the CASAC NOx SOx 

Secondary Panel at our upcoming meeting. Should you have any questions regarding the first 

draft NOx SOx Secondary NAAQS Policy Assessment, please contact me (919-541-5505; email 

wegman.lydia@epa.gov) or Dr. Bryan Hubbell (919-541-0621; email hubbell.bryan@epa.gov).

Attachments

cc: Vanessa Vu, SAB, OA 

John Vandenberg, ORD/NCEA-RTP 

Mary Ross, ORD/NCEA-RTP

Ron Evans, OAQPS/HEID

Dale Evarts, OAQPS/HEID

 

1 Russell, A. and Samet, J. (2010). Letter from Clean Air Scientific Advisory Committee to the Honorable Lisa P. 

Jackson, Administrator, US EPA. Review of the Policy Assessment for the Review of the Secondary National 

Ambient Air Quality Standards for NOx and SOx: First Draft (March 2010). June 22, 2010.
Richard Haeuber, OAP/CAMD

Karen Martin, OAQPS/HEID

Christine Davis, OAQPS/HEID

Tara Greaver, ORD/NCEA-RTP

Bryan Hubbell, OAQPS/HEID

Meredith Lassiter, OAQPS/HEID

Jason Lynch, OAP/CAMD

Norm Possiel, OAQPS/AQAD

Adam Reff, OAQPS/AQAD

Rich Scheffe, OAQPS/AQAD

Travis Smith, OAQPS/HEID

Ginger Tennant, OAQPS/HEID

Randy Waite, OAQPS/HEID

Nealson Watkins, OAQPS/AQAD
Attachment 1

CASAC comments on first draft PM Policy Assessment and responses to those comments

The CASAC Panel’s comments and recommendations on the first draft of the NOx and 

SOx Secondary NAAQS Policy Assessment, as well as changes made in the second draft in 

response to those comments, are summarized below. Overarching comments and 

recommendations are summarized below, followed by comments and recommendations on 

chapters 2 through 9.

Overarching comments and recommendations

The Panel made several overarching comments and recommendations, including:

 The second draft should include in a separate chapter a more comprehensive sensitivity

and uncertainty analyses of the proposed approach. This chapter should provide more 

reasoning to support EPA’s assessment of the relative values of the uncertainties. The 

sensitivity analyses should include the sensitivity of the AAPI to component terms. The 

chapter should provide a more thorough evaluation of the ability of CMAQ to simulate 

the ambient concentrations and wet and dry deposition fluxes of the SOx, NOx and NHx 

species used in determining the AAPI.

 The second draft should include a succinct discussion of other potential impacts of NOx 

deposition to N-limited ecosystems, including potential production increases that may be 

either beneficial or adverse depending on the system and management goals.

 The second draft should ensure the use of a consistent set of units in its presentation of 

data, results and equations.

In response to these comments, we have made extensive edits throughout the second draft Policy

Assessment to ensure consistent units are used, and have included a table in Chapter 1 which 

clearly identifies units and conversions used in the document (Table 1-1, Section 1.1). We have 

added a new chapter (Chapter 7) which provides a more extensive discussion of uncertainties and 

variability in the science, data, and models which inform our considerations of the standards. 

This chapter also provides assessments of the sensitivity of models and the AAPI to inputs. We 

have added additional discussions of the broader set of potential impacts of NOx deposition, 

while continuing to focus the overall review on the effects of NOx and SOx deposition on 

sensitive aquatic ecosystems, and protection against adverse effects of NOx additions (ES-4, 

Section 1.6, Section 4.5.2, Section 2.2.2, and Section 3.4).

Chapter 2 (Known or Anticipated Ecological Effects)

The panel was generally supportive of the review of effects provided in the chapter. The panel 

asked primarily for clarifications of the text and additional references. The panel asked for a 

summary of the assumptions and limitations of the steady-state models used in the ecological 

effects studies. The panel requested that we clarify that ecological effects due to deposition of N 

and S occur in a relatively small set of sensitive ecosystems, rather than throughout all 
ecosystems. The panel also asked for a more thorough discussion of uncertainties, with 

justifications for rankings of uncertainties, and sensitivity analyses for the different models used 

for aquatic acidification.

Changes made in chapter 2 in the second draft Policy Assessment were primarily focused on

streamlining the chapter as well as on improving and clarifying the approach for characterizing 

uncertainty. In addition, we included a new table (Table 2-2) that summarizes the assumptions 

and limitations of the ecological effects models.

Chapter 3 (Considerations of Adversity to Public Welfare)

The panel asked for a more focused discussion on how and why the current effects of SOx/NOy 

deposition on sensitive ecosystems matter in terms of services and functions of ecosystems that 

are important to people, emphasizing that effects occur only in a limited set of sensitive areas.

The panel asked for more explanation of the policy implications of the monetary values of 

ecosystem services discussed in the chapter. The panel also asked for a table summarizing the 

monetary valuation associated with various ecosystem services estimates. The panel asked for 

more focus on the effects that are likely occurring due to NOx and SOx deposition and less on 

those that potentially could be affected. The panel also asked for greater acknowledgement of 

the potential beneficial effects of nitrogen inputs to nutrient limited ecosystems.

Change made in chapter 3 in the second draft Policy Assessment were focused on describing 

those ecosystem services that were likely impacted by current NOx and SOx deposition. In 

addition, we have expanded the discussion of insights that can be gained by evaluating public 

decisionmaking regarding protection of aquatic resources (Section 3.1.2.4.3), and through 

evaluation of programs in Europe that have established targets for ecosystem protection as part 

of critical loads policies (Section 3.1.2.4.4). We have modified Figures 3.4 and 3.5 to include 

deposition levels in a wider range of public lands, including Federal Class I Areas, as well as 

state and local parks and wilderness areas. We have added an additional assessment of the value 

of decreases in acidification in the Adirondacks based on the Banzhaf et al (2006) valuation 

survey of NY residents (Section 3.4.2). We have added a number of additional tables, including 

a summary of values of ecosystem services affected by current N deposition (Table 3-7 and 3-8). 

We have included a discussion of the potential beneficial effects of N deposition in nitrogenlimited areas in Chapter 4, and have added a clarification to this chapter acknowledging these 

effects and reiterating our focus on adverse effects (Section 3.4.5).

Chapter 4 (Addressing the Adequacy of the Current Standards)

The panel was generally supportive of the information provided demonstrating that current NOx 

and SOx secondary standards are not adequate to protect against effects of deposition of NOx 

and SOx to sensitive ecosystems. While the panel agreed that steady state models are 

appropriate to evaluate the adequacy of the standards, they note that approach could be enhanced 

by also applying dynamic acidification modeling techniques to “hindcast”changes in ANC. The 

panel recommended that the evaluation of adequacy should consider the extent to which and 

rates at which some less sensitive systems may continue to recover at current S + N deposition, 

while other more sensitive systems will not recover or may experience further deterioration. The 
panel also asked for additional plots showing the relationship of NOy to NOx, compared with 

NO2 to NOx. The panel also asked that a summary of the performance assessment of CMAQ be 

included.

The performance assessment of CMAQ has been incorporated in the new Chapter 7 addressing 

uncertainty, variability, and sensitivity. Much of the monitoring discussion has been moved to a 

separate Chapter 8 on monitoring issues, and that chapter also provides more discussion on the 

ambient concentration to deposition relationship of individual sulfur and nitrogen species. New 

plots of NOy to NOx and NO2 to NOx have been added to chapter 4. 

While the panel recognized that the current review was focused on sensitive, non-managed 

ecosystems, the panel requested more discussion of fertilization effects. In response to these 

comments an additional discussion has been added to Chapter 4 (Section 4.5.2) regarding 

nutrient enrichment effects in both managed and non-managed systems. In addition, language 

has been added in several other chapters highlighting the range of potential responses to nitrogen 

nutrient enrichment in different types of ecosystems, especially in N-limited ecosystems.

Chapters 5 (Conceptual Design of an Ecologically Relevant Multi-pollutant Standard) and 6 

(Options for Elements of a Standard to Protect Against Effects from Aquatic Acidification)

Note: We have combined Chapters 5 and 6 from the first draft Policy Assessment into a single 

Chapter 5 in the second draft Policy Assessment to reduce redundancy and improve the flow of 

the document. 

The panel was generally supportive of both the conceptual framework developed to address 

effects associated with deposition of NOx and SOx, and the Atmospheric Acidification 

Protection Index introduced as a new form of the standards. The panel agreed with selection of 

NOy and SOx as atmospheric indicators, and with the selection of ANC as the ecological 

indicator. The panel agreed that a multi-year averaging time from 3 to 5 years was appropriate. 

The panel also agreed that a multipollutant standard is appropriate to protect aquatic ecosystems 

from acid deposition. 

The panel asked that reduced nitrogen be treated as a variable component of the AAPI standard 

that can be updated. The panel asked that the underlying assumption of steady state behavior 

should be evaluated with a parallel effort using sensitivity analysis and dynamic modeling. The 

panel asked that the AAPI equation for the proposed standard should include the geographical 

extent and numerical range of its applicability. The panel recommended that the mass balance 

approach to determining Neco be used over the dynamic modeling approach. The panel 

recommended that in evaluating methods for aggregating catchment scale critical loads, 

consideration be given to approaches by which the water bodies within any specified area could 

be first stratified to include only those which were considered potentially susceptible to 

acidification, based on estimates of preindustrial ANC, or acid sensitivity class. The panel 

advised EPA to focus on an AAPI standard driven by aquatic effects concerns, given both the 

availability of data, and the likelihood that such a standard would decrease N and S inputs to 

ecosystems, which would provide protection against additional effects. The panel recommended 
that EPA conduct a sensitivity study to characterize uncertainty associated with different 

components of the conceptual framework and propagate the resulting uncertainty at every step to 

arrive at an ensemble of SOx and NOy response surfaces to meet a given standard. The panel 

asked for inclusion of a separate conceptual diagram which shows the important contributing 

factors for each term in the AAPI equation.

The panel noted a number of challenges in specifying the AAPI form. The panel highlighted a 

number of areas of uncertainty, including

 Uncertainty in pre-industrial levels of ANC

 Uncertainty in the use of steady state models

 Uncertainty in the atmospheric deposition transformation ratios

 Uncertainty in modeled estimates of reduced nitrogen deposition

In addition, the panel recommended that close attention be given to the issue of matching 

temporal and spatial scales in various components of the AAPI.

The panel recommended disaggregating the g(·) term in the AAPI equation, and include a term to 

account for naturally occurring organic acids. The panel recommends that consideration of 

chronically acidic or highly acid sensitive surface waters that would not be recoverable at a given 

ANC limit should be given.

The panel recommended that the CMAQ modeling that is used to develop the atmospheric 

deposition transformation ratios be evaluated for sensitivity to emissions and chemistry, and that 

characterization of the uncertainties in precipitation and wet deposition be included. The panel 

recommended several specific evaluations of CMAQ, including:

 Model performance for nitric oxide, nitrogen dioxide, sulfur dioxide, nitrate, 

ammonium and aerosol nitrate, ammonium, and sulfate for different networks for 

which the data are routinely available

 Model performance for wet deposition of sulfate, nitrate, and ammonium using 

the National Atmospheric Deposition Program (NADP) network

 A regional model evaluation using the continuous measurements of nitric oxide, 

nitrogen dioxide, nitric acid and NOY from the SEARCH network in the 

southeastern U.S.

The panel expressed support for the use of NOx/SOx tradeoff curves, but recommended that 

these curves include the impact of reduced nitrogen.

In response to the comments from the panel, Chapter 5 has been revised in the following ways:

 We have added a section comparing critical loads calculated using steady-state versus 

dynamic modeling has been added (5.3.2.3).

 The mass balance approach to determining Neco has been used (5.3.2).

 The tradeoff curves now show the impact of NHx, based on a range of NHx deposition 

values which are modeled to occur across the U.S. (5.3.9).

 Criteria have been identified to only consider critical loads from catchments that are 

susceptible to acidification from atmospheric deposition, while excluding those 
catchments in which acidification results from high organic acid content, acid mine 

drainage or naturally low base cation weathering (5.3.2.6).

 The focus of the conceptual design is on aquatic acidification (5.3.1) with co-protection 

to other effects presented in Chapter 6.

 A new conceptual diagram has been added that outlines each step to develop the tradeoff 

curves and the AAPI (5.3.1)

 The g(·) term from the first draft PA has been disaggregated so that each component is 

represented in the AAPI equation (5.3.4). As stated above, the critical load dataset is 

screened to exclude those catchments which would not be able to recover based on preindustrial ANC values (5.3.2.6).

Chapter 7 (Co-protection)

Note: The chapter on co-protection is now Chapter 6.

The panel supported evaluation of co-protection provided by an aquatic acidification based 

standard for terrestrial acidification endpoints and terrestrial and aquatic nutrient enrichment 

endpoints. The panel recommended that the chapter be expanded to better explain the potential 

interrelationships between aquatic and related terrestrial responses to acidification. The panel 

recommended that further consideration not be given to developing a single standard addressing 

both acidification and nutrient enrichment.

Chapter 9 (Conclusions)

The panel generally supported the preliminary staff conclusions regarding the adequacy of 

existing standards, and options for the indicators, averaging times, and form of additional 

standards to protect against the effects of NOx and SOx deposition to sensitive aquatic 

ecosystems. The panel reiterated the request for a separate uncertainty chapter. They also asked 

that discussions of future research needs be linked to the identification of critical uncertainties.

Chapter 9 has been expanded to include a discussion of staff conclusions regarding the options 

for elements of the standard. We have included discussions of how critical uncertainties might 

influence the AAPI, with references to the broader uncertainty discussions in Chapter 7. 

Research needs are discussed briefly in the new Chapter 7, but will need to be more fully 

developed in the final PA.