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Responses to Significant Comments on the 2011 Proposed 

Rule on the Secondary National Ambient Air Quality 

Standards for Oxides of Nitrogen and Oxides of Sulfur

(August 1, 2011; 76 FR 46084)

Docket Number EPA EPA-HQ-OAR-2007-1145

U.S. Environmental Protection Agency

March 20, 2012
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Table of Contents 

I. INTRODUCTION.......................................................................................................1

II. RESPONSES TO SIGNIFICANT COMMENTS ON PROPOSED 

RULEMAKING FOR OXIDES OF NITROGEN AND OXIDES OF SULFUR 

SECONDARY NAAQS ..........................................................................................2

A. Comments on Scope of the Review ...................................................................2

1. Authority of EPA to Address Deposition-related Effects.............................2

2. Other Comments Related to the Scope of the Review..................................5

B. Comments on the Adequacy of the Current Secondary Standards ....................6

1. Adequacy of the Current Standards to Address Direct Effects.....................6

2. Adequacy of the Current Standards to Address Deposition-related

Effects.........................................................................................................6

C. Comments on Setting an AAI-based Standard.................................................10

D. Comments on Specific Aspects of the AAI Approach ....................................19

E. Comments on Technical Elements of the AAI Approach ................................23

F. Comments on 1-Hour NO2 and SO2 Secondary Standards ..............................25

III. RESPONSES TO SIGNIFICANT COMMENTS RELATED TO THE FIELD 

PILOT PROGRAM AND MONITORING METHODS EVALUATION............29

A. Goals, Objectives, and Scope...........................................................................20

B. Monitoring Network and Site Selection...........................................................32

C. Complementary Measurements and Instrumentation.......................................34

D. Collaboration and Stakeholder Participation ...................................................35

IV. LEGAL, ADMINISTRATIVE, AND PROCEDURAL ISSUES AND 

MISPLACED COMMENTS .................................................................................38

A. Legal, Administrative, and Procedural Issues..................................................38

B. Misplaced Comments.......................................................................................39

V. REFERENCES..........................................................................................................40
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Responses to Significant Comments on the 2011 Proposed Rule on the 

Secondary National Ambient Air Quality Standards for Oxides of Nitrogen 

and Sulfur

I. INTRODUCTION 

This document, together with the preamble to the final rule on the review of the 

secondary national ambient air quality standards (NAAQS) for oxides of nitrogen and oxides of 

sulfur, presents the responses of the Environmental Protection Agency (EPA) to the significant 

public comments received on the 2011 proposed rule for secondary NAAQS for oxides of 

nitrogen and sulfur (76 FR 46094). All significant issues raised in the public comments have 

been addressed.

This response-to-comments document does not generally cross-reference each response 

to the commenter(s) who raised the particular issue involved, although commenters are identified 

in some cases where they provided particularly detailed comments that were used to frame the 

overall response on an issue. Comments were received about thirty commenters, including 

several industry groups, a few individual companies, several environmental groups, one federal 

agency, one organization of state and local air agencies, five individual states, a local 

government agency, and two tribes.

The comments addressed in this document include comments that are addressed in the 

preamble to the final rule as well as other comments that were not discussed in the preamble. As 

such, this document incorporates and augments the discussion of public comments, and the 

EPA’s responses to those comments, that appear in the preamble to the final rule. Although 

portions of the preamble to the final rule are paraphrased in this document where useful to add 

clarity to responses, the preamble itself remains the definitive statement of the rationale for the 

decisions presented in the final rule. 

Accordingly, this Response to Comments document, together with the preamble to the 

final rule on the review of the secondary NAAQS for oxides of nitrogen and sulfur and the 

information contained in the Integrated Science Assessment (ISA) (U.S. EPA, 2008), the Risk 

and Exposure Assessment (REA) (U.S. EPA, 2009), the Policy Assessment (PA) (U.S. EPA, 

2011), and the preamble to the 2011 proposed rule should be considered collectively as the 

EPA’s response to all of the significant comments submitted on the 2011 proposed rule.

Comments related to the proposed rule on the secondary NAAQS for oxides of nitrogen 

and sulfur, as discussed in sections I through III of the proposed rule, are addressed below in 

section II. Comments on the field pilot program and monitoring methods development, as 

discussed in section IV of the proposed rule, are addressed below in section III. Comments 

related to legal, administrative, and procedural issues, as well as misplaced comments related to 

issues that are not germane to the setting of the NAAQS, are addressed below in section IV.
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II. RESPONSES TO SIGNIFICANT COMMENTS ON PROPOSED RULE ON THE 

SECONDARY NAAQS FOR OXIDES OF NITROGEN AND SULFUR 

General comments related to the scope of the current review of the secondary standards 

for oxides of nitrogen and sulfur are addressed in section I.E.2 of the preamble to the final rule 

and in section II.A in this document. Comments related to the adequacy of the current secondary

standards for oxides of nitrogen and sulfur are addressed in section II.C.2 of the final rule and in 

section II.B of this document. Comments related to the multi-pollutant standard based on an 

aquatic acidification index (AAI) discussed in the proposal are addressed in section III.D.1 of the 

final rule and in sections II.C-E of this document. Comments related to setting new secondary

standards identical to the existing primary 1-hour nitrogen dioxide (NO2) and sulfur dioxide 

(SO2) standards are addressed in section III.D.2 of the final rule and in section II.F below.

A. Comments on Scope of the Review 

Comments related to the scope of the review are addressed in this section. These 

comments primarily questioned the EPA’s legal authority under Section 109 of the Clean Air Act 

(CAA) to set NAAQS that address deposition-related effects, focusing in particular on effects 

resulting from acidifying deposition to ecosystems (section II.A.1). Other comments related to 

the scope of the review raised the following issues (section II.A.2): (1) the authority of the EPA 

to consider pollutants jointly and develop joint standards and (2) issues related to the role of the 

secondary NAAQS in not only reducing pollution but in improving existing ecological 

conditions.

1. Authority of the EPA to Address Deposition-related Effects 

Comments: While environmental organizations and some other commenters urged the EPA to 

establish a NAAQS that would protect against the impacts on sensitive ecosystems associated 

with the acidifying deposition of nitrogen and sulfur, several industry commenters argued that 

the enactment of Title IV of the CAA in 1990 displaced the EPA’s authority to address 

acidification through the setting of NAAQS. These commenters contend that the existence of a 

specific regulatory program to address the acidification effects of oxides of nitrogen and sulfur

supplants the EPA’s general authority under the Clean Air Act. According to industry comments, 

this is demonstrated by the legislative history and a close reading of § 404, which required the 

EPA to report to Congress on the feasibility of developing an acid deposition standard and the 

actions that would be required to integrate such a program into the CAA. The required study

described in §404, commenters argue, demonstrates that Congress had concluded that the EPA

lacked the authority under section 109 of the CAA to establish a secondary NAAQS to address 

acid deposition. Commenters also claimed that until this review, the EPA had similarly

interpreted the CAA to limit its authority to address acidification through the NAAQS process.

Response: Although the EPA is not in this rulemaking adopting a secondary standard designed 

to protect the public welfare from the effects associated with the acidifying deposition of 

nitrogen and sulfur, the EPA does not agree that the enactment of Title IV displaced the EPA’s 

authority under §109 of the CAA to adopt such a NAAQS. We note that the purpose of Title IV 

“is to reduce the adverse effects of acid deposition,” CAA § 401(b), while section 109 directs the 

Administrator to go beyond this to set a standard that is “requisite to protect public welfare from 
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any known or anticipated adverse effects,” CAA § 109(b) (2). These provisions are not 

accordingly in conflict, but represent the often typical interlinked approach of Congress to 

address the frequently complex problems of air pollution.

In adding Title IV to the CAA, Congress created a new program to reduce the emissions 

of SO2 and NOx from electric generating units, the most significant sources of acidifying 

pollution in 1990. Nothing in the text or the legislative history of Title IV of the Act indicates 

that in creating additional authority Congress intended to foreclose the EPA’s authority to 

address acid deposition through the NAAQS process. Indeed, to the extent that Congress 

addressed the impact of Title IV on other provisions of the CAA, it made clear that Title IV had 

no impact on the compliance obligations of covered sources under other CAA provisions. CAA 

§ 413. The legislative history of the Title IV program makes clear that Congress was acting in 

the face of the EPA’s apparent inability to effectively address the problem of acidification 

through the NAAQS and other CAA programs, given the significant uncertainties associated 

with possible regulatory regimes to address acidification. See, e.g., S. Rep. No.101-228, at 289-

291(1989). Congress did not conclude that the EPA lacked the regulatory authority to address 

acidification but rather concluded that “a major acid deposition control program [was] warranted 

...because of the evidence of damage that had already occurred as well the likelihood of further 

damage in the absence of Congressional action.” H.R. Rep. No 101-490, at 360 (1990). The 

Senate Report made it clear that while the EPA envisioned section 109 as providing authority to 

adopt a secondary NAAQS to address the effects of acid deposition, the EPA remained 

concerned about the effectiveness of this and other regulatory approaches. S.Rep. No. 101-228, 

at 290-291. Congress addressed these issues by adding the new authorities found in Title IV, but 

made no mention of supplanting the EPA’s authority under section 109 to address acidification 

effects.

There is no discussion in the legislative history of Title IV of curtailing the EPA’s 

authority under the NAAQS program. As such, the requirement in § 404 of the 1990 CAA 

Amendments that the EPA send to Congress “a report on the feasibility and effectiveness of an 

acid deposition standard or standards” does not demonstrate that Congress concluded that an 

amendment to the CAA would be necessary to give the EPA the authority to issue regulations 

addressing acidification. See CAA § 401. The significance of the report required by § 404 can 

be understood in the overall context of the history of Congress’ and the EPA’s attempts to 

understand and to address the causes and effects of acid deposition and the EPA’s proposed 

conclusion in 1988 that the scientific uncertainties associated with acid deposition were too great 

to allow the Agency to establish a secondary NAAQS at that time to address those effects. In the 

proposed rule, the EPA noted that it was clear at the time of the 1990 CAA Amendments that a 

program to address acid deposition was needed and that the primary and most important of these 

provisions is Title IV of the Act, establishing the Acid Rain Program. The Report required under 

section 404 of the Amendments reflects this concern, and requires an evaluation of an acid 

deposition standard and a comparison of its effectiveness to various regulatory authorities under 

the Act, including the authority for a secondary NAAQS under section 109. CAA § 404 (note) 

(Pub.L. 101-549, §404 (6)). This indicates the existence of an ongoing authority under section 

109.

In assessing the import of § 404, the EPA has noted in the past and in the proposed rule at 

46087, that “Congress reserved judgment as to whether further action might be necessary or 
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appropriate in the longer term” to address any problems remaining after implementation of the 

Title IV program, and “if so, what form it should take.” See 58 Fed Reg. 21351, 21356 (April 21, 

1993). Such reservation of judgment by Congress concerned whether Congress should adopt 

additional statutory provisions to address the effects of acid deposition, as it did in 1990. It does 

not indicate a view that the EPA lacked authority under section 109 to establish a secondary

NAAQS to address acid deposition, but a recognition that the uncertainties associated with such 

a standard may be too significant to allow the Administrator to reach a reasoned conclusion to 

adopt such a standard, as in the 1988 proposal. The EPA’s decision in that rulemaking reflects 

the view that there is ongoing authority to address acid deposition under section 109 of the Act, 

and did not indicate that Title IV impliedly amended the CAA and removed all such regulatory 

authority outside of Title IV. As the EPA noted in its 1993 rulemaking addressing the question of 

whether to revise the secondary NAAQS for SO2, the EPA decided not to revise the standard at 

that time because of scientific uncertainty. The EPA noted the consistency of this decision with 

Congress’ actions in the 1990 amendments, but nowhere indicated that Congress’ actions meant 

the EPA no longer had the authority to adopt a secondary NAAQS to address acid deposition.

Instead the EPA stated it would not adopt a secondary NAAQS at that time and based on further 

studies and research would determine in the future what further action to take under section 109.

The EPA stated that:

In reaching a decision that revisions to the secondary standard for oxides of sulfur to 

address acidic deposition and related SO2 welfare effects are not appropriate at this time, 

the EPA has taken into account the significant reductions in SO2 emissions, ambient SO2 

concentrations, and ultimately the deposition of sulfur that will result from 

implementation of the title IV program. ... The EPA recognizes that the Congress left 

open the question whether further action to address acidic deposition might be necessary

in the longer term. The EPA is concerned, however, as was Congress, that a number of 

important scientific and implementation issues must be addressed through further 

research and study before a more informed decision can be made on whether such action 

is needed; if needed, what form it should take; and whether and how a given approach 

could be effectively integrated with the existing title IV program. Moreover, as discussed 

previously, setting either a secondary NAAQS or an acidic deposition standard would 

involve significant difficulties, especially as compared with the total-loading approach 

adopted in title IV. In the EPA's judgment, the prudent course of action is to await the 

results of the studies and research programs that are currently under way ... and then to 

determine whether additional control measures should be adopted or recommended to 

Congress. ... For these reasons, the EPA concludes that, under section 109(d)(1) of the 

Act, revision of the secondary standard for SOX to provide increased protection against 

acidic deposition and related welfare effects is not appropriate at this time. As provided 

for under the Act, the EPA will continue to assess the scientific information on acidic 

deposition and related SO2 welfare effects as it emerges from the ongoing research and 

monitoring programs, and [EPA]will update the air quality criteria for SOX accordingly. 

These revised criteria should provide more informed bases for reaching a decision on 

whether additional measures are needed to augment the title IV program. 58 FR 21351, 

21357-8 (April 21, 1993) (emphasis added).

Nearly twenty years later, we have made substantial progress toward the development 

of a NAAQS to protect against acidification, however we find ourselves in a position 
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where the Administrator has concluded again that the uncertainties associated with 

setting a NAAQS to address acidification are too substantial to allow her to set a standard 

that in her judgment would be requisite to protect the public welfare from such effects.

The EPA does not agree that the Administrator lacks the authority to set such a standard 

were it possible to do so with sufficient confidence that it would be requisite to protect 

public welfare.

2. Other Comments Related to the Scope of the Review

(1) Comment: Several commenters argued that the Clean Air Act requires that secondary

standards must specify a single air pollutant concentration that applies to each individual criteria 

pollutant. One commenter argued that the language in section 109(b) requiring the EPA to set a 

standard to protect the public welfare from the adverse effects associated with the presence of 

“such air pollutant” in the ambient air means that the EPA cannot set a NAAQS for more than 

one criteria pollutant. Another commenter stated that the Clean Air likely would not allow the 

EPA to set a standard that allows for tradeoffs between two pollutants but must specify a single 

air pollution concentration that applies to each individual criteria air pollutant. 

Response: Section 109 of the CAA states than a secondary NAAQS must “protect the public 

welfare from known or anticipated adverse effects associated with the presence of such air 

pollutant in the ambient air.” The term “associated with...such air pollutant” describes the 

category of adverse affects to be associated with the air pollutant at issue, not the standard. The 

EPA does not agree with the commenter that this language limits our ability to set a standard that 

allows for tradeoffs between these two pollutants. For further discussion see section II. D (4). 

(2) Comment: One commenter stated that public welfare does not include remediating previous 

damage to ecosystems but only preventing further damage. This commenter stated that the 

development of such a standard as discussed in this review, one which looked at improving 

current conditions as well as preventing future degradation, was beyond the requirements for in 

the Clean Air Act for protection of public welfare.

Response: In setting primary and secondary NAAQS, the EPA takes into account the effect of 

a pollutant in the ambient air on public health and public welfare, respectively, by assessing the 

effects of criteria pollutants given the current state of public health and public welfare. In other 

words, in determining the level of air quality requisite to protect public health or welfare, the 

EPA takes into account existing conditions. The ambient levels of oxides of nitrogen and sulfur 

have a relationship to what the future degree of aquatic acidification will be, given current 

conditions, the EPA looks at what future conditions will be like given current conditions and 

varying ambient levels of oxides of nitrogen and sulfur. This future harm is what the EPA

addresses, looking at whether current conditions continue on in to the future or change. The EPA

does not believe that taking the approach suggested by commenter would allow the EPA to set 

standards that would provide requisite protection of public health or welfare, as they would call 

for the EPA to ignore current conditions, which are a relevant factor in determining what impact 

ambient levels of oxides of nitrogen and sulfur will have on aquatic acidification in the future.
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B. Comments on the Adequacy of the Current Secondary Standards

This section discusses the comments received from the public regarding the adequacy of 

the current secondary standards with regard to both direct and deposition-related effects.

1. Adequacy of the Current Standards to Address Direct Effects

The current secondary NO2 and SO2 secondary standards were set in 1971 to protect 

against direct effects of gaseous oxides of nitrogen and sulfur. For oxides of nitrogen, the current 

secondary NO2 standard is an annual standard set to protect against adverse effects on vegetation 

from direct exposure to ambient oxides of nitrogen. For oxides of sulfur, the current secondary

standard is a 3-hour standard intended to provide protection for plants from the direct foliar 

damage associated with atmospheric concentrations of SO2. As discussed in section II.B.1 of the 

preamble, the Administrator proposed to conclude that the current secondary standards are 

adequate to protect against direct phytotoxic effects on vegetation, and proposed to retain the 

current standards for that purpose.

Many commenters supported the EPA’s proposed decision to retain the current secondary

standards for various reasons related to their comments on alternative standards (as discussed in 

section III.D of the final rule and sections II.C and II. D below), a few commenters (Alliance of 

Automobile Manufacturers (AAM), Pennsylvania Dept. of Environmental Protection) 

specifically expressed the view that the current standards provide requisite protection from the 

direct effects on vegetation from exposures to gaseous oxides of nitrogen and sulfur, and no 

commenters opposed retention of the current secondary standards.

2. Adequacy of the Current Standards to Address Deposition-related Effects

As discussed in section II.B.2 of the final rule, with regard to deposition-related effects, 

the Administrator proposed to conclude that the elements of the current secondary standards are 

not ecologically relevant, and thus not appropriate to provide protection of ecosystems, and that 

they do not provide adequate protection from such acidification and nutrient enrichment effects 

in both aquatic and terrestrial ecosystems. Having reached these proposed conclusions, she 

determined that it was appropriate to consider alternative standards that are ecologically relevant.

(1) Comments: One group of commenters that addressed the adequacy of the current standards 

with regard to deposition-related effects included environmental organizations (Earthjustice, on 

behalf of the Appalachian Mountain Club, National Parks Conservation Association, Sierra Club, 

and Clean Air Council; the Center for Biological Diversity; the Nature Conservancy; Adirondack 

Council; Chesapeake Bay Foundation), the U.S. Department of the Interior, NESCAUM, New 

York Dept. of Environmental Conservation, and two tribes. These commenters generally

expressed the view that the current secondary standards do not provide adequate protection from 

deposition-related effects. More specifically, some of these commenters stated that there was 

overwhelming evidence of adversity to sensitive aquatic ecosystems from acidifying deposition. 

These commenters cited a broad range of scientific evidence that aquatic acidification was 

ongoing under current conditions allowed by the current secondary standards, and that this 

acidification represented an adverse effect on public welfare. Several commenters noted that the 
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Clean Air Scientific Advisory Committee (CASAC) had agreed that deposition-related effects 

were ongoing and harmful and that current standards were not adequate to prevent these effects.

Among these commenters, some also expressed the view that current standards were not 

adequate to protect against terrestrial acidification or nutrient enrichment. The Department of the 

Interior as well as Earthjustice noted that the current standards were not sufficient for these 

additional endpoints and cited ongoing harm under current conditions. Two tribes and the Center 

for Biological Diversity expressed the view that there was sufficient information to judge that the 

current standards were not adequate to protect against the adverse welfare effect of mercury

methylation, contrary to the EPA’s proposed conclusion that the available evidence was not 

sufficient to reach such a judgment. For example, The Forest County Potawatomi Community

provided several citations regarding the relationships between aquatic acidification and mercury

methylation and stated that there was sufficient evidence to find that the current standards were 

not adequate.

Response: With regard to the adequacy of the current secondary standards for NO2 and SO2, the 

EPA concurs with commenters’ assertions that the current standards do not provide adequate 

protection for ecosystems that are sensitive to aquatic acidification and that effects to these 

ecosystems are ongoing from ambient deposition of oxides of nitrogen and oxides of sulfur. The 

EPA also agrees that there is sufficient evidence to conclude that ambient deposition under the 

current secondary standards is causing or contributing to terrestrial acidification as well as

nutrient enrichment in sensitive ecosystems. A complete discussion of considerations with regard 

to adequacy can be found in section II.B of the final rule. In short, the ISA has established that 

the major effects of concern for this review of the oxides of nitrogen and sulfur standards are 

associated with deposition of nitrogen and sulfur caused by atmospheric concentrations of oxides 

of nitrogen and sulfur. The current standards are not directed toward depositional effects, and 

none of the elements of the current NAAQS – indicator, form, averaging time, and level – are 

suited for addressing the effects of nitrogen and sulfur deposition. Additionally, although the 

proportion of total nitrogen loadings associated with atmospheric deposition of nitrogen varies 

across locations, the ISA indicates that atmospheric nitrogen deposition is the main source of 

new anthropogenic nitrogen to most headwater streams, high elevation lakes, and low-order 

streams. Atmospheric nitrogen deposition contributes to the total nitrogen load in terrestrial, 

wetland, freshwater and estuarine ecosystems that receive nitrogen through multiple pathways.

There are expansive data to indicate that the levels of deposition under the current 

standards are not sufficient to prevent adverse effects in ecosystems. With regard to aquatic 

acidification, recent data indicate that in the Adirondacks and Shenandoah areas, rates of 

acidifying deposition of oxides of nitrogen and sulfur are still well above pre-acidification (1860) 

conditions. Forty-four percent of Adirondack lakes and 85 percent of Shenandoah streams 

evaluated exceed the critical load for an acid neutralizing capacity (ANC) of 50 microequivalents 

per liter (μeq/L), and have suffered loss of sensitive fish species. With regard to terrestrial 

acidification, the REA evaluated a small number of sensitive areas as case studies and showed 

the potential for reduced growth. When the methodology was extended to a 27-state region, 

similar results were found to indicate the potential for growth effects in sensitive forests. 

Nitrogen deposition can alter species composition and cause eutrophication in freshwater 

systems. In the Rocky Mountains, for example, current deposition levels, which are within the 

range associated with ambient nitrogen oxide levels meeting the current standard, are known to 
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cause changes in species composition in diatom communities indicating impaired water quality. 

With regard to terrestrial nutrient enrichment, most terrestrial ecosystems in the United States are 

nitrogen-limited, and therefore they are sensitive to perturbation caused by nitrogen additions. 

Under recent conditions, nearly all of the known sensitive mixed conifer forest ecosystems 

receive total nitrogen deposition levels above the ecological benchmark for changes in lichen 

species. In addition, in Coastal Sage Scrub ecosystems in California, nitrogen deposition exceeds 

the benchmark above which nitrogen is no longer a limiting nutrient, leading to potential 

alterations in ecosystem composition. Therefore, the EPA concludes that the current standards 

are not adequate for these effects.

The EPA, however, while agreeing that there is a causal effect between deposition of 

sulfur and mercury methylation disagrees that there is sufficient evidence to make the 

quantitative associations that would be necessary to determine that the current standards were not 

adequate to protect against mercury methylation. The ISA concluded that evidence is sufficient 

to infer a casual relationship between sulfur deposition and increased mercury methylation in 

wetlands and aquatic environments. Since the rate of mercury methylation varies according to 

several spatial and biogeochemical factors whose influence has not been fully quantified, the 

correlation between sulfur deposition and methylmercury could not be quantified for the purpose 

of interpolating the association across waterbodies or regions. Therefore, since we are unable to 

quantify the relationship between atmospherically deposited oxides of sulfur and mercury

methylation we cannot we cannot assess adequacy of protection. This subject is discussed more 

fully in section 6.2 of the REA (EPA 2009). 

(2) Comments: Another group of commenters, (e.g. Utility Air Regulatory Group (UARG), 

Electric Power Research Institute (EPRI), American Petroleum Institute (API), AAM, and 

American Road and Transportation Builders Association (ARTBA)) generally took the position 

that the currently available information was not sufficient to make informed judgments about the 

adequacy of the current standards to address aquatic acidification effects. These commenters 

generally based this view on the complex nature of the interactions between pollutants and 

ecosystems and uncertainties in the models and analyses considered in this review. Several 

commenters asserted that there was not sufficient data available to determine the relationship 

between acidifying deposition of oxides of nitrogen and sulfur and adverse effects on aquatic 

ecosystems, such that there was not sufficient information to allow for the assessment of the 

adequacy of the current standards to provide appropriate protection from this effect. For 

example, AAM noted the uncertainties in models relating to dry deposition and questioned the 

linkages between ambient concentrations of oxides of nitrogen and sulfur and the amount of 

nitrogen and sulfur deposition. In addition to commenting on data limitations, UARG also 

expressed the view that the ecosystem services analyses included in the proposal were 

insufficient to make judgments about adversity to aquatic ecosystems resulting from acidifying 

deposition and that there is a lack of evidence demonstrating that quantifiable changes in public 

welfare would result from reductions in acidifying deposition. Many commenters within this 

group did not directly comment on the adequacy of the current standards to protect against 

aquatic acidification or other deposition-related effects, but instead expressed the view that the 

EPA did not have the authority to consider deposition-related effects in general or aquatic 

acidification in particular through the NAAQS. This comment and the EPA’s response are 

discussed above in section I.E of the final rule. 
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Response: With regard to the adequacy of the current standards to protect against aquatic 

acidification, the EPA disagrees with commenters’ assertion that there is insufficient data to 

make linkages between deposition from the atmosphere and aquatic acidification effects. To the 

contrary, the EPA is confident that there is sufficient robust science to conclude that aquatic 

acidification is ongoing in sensitive ecosystems, that ambient deposition of oxides of nitrogen 

and oxides of sulfur are causative in many ecosystems nationwide and that the current standards 

are neither appropriate in form nor adequate in level to protect against such effects. The ISA 

concluded that there was a causal relationship between deposition of oxides of nitrogen and 

sulfur and reduced forms of nitrogen (NHx) and acidification of ecosystems. In addition, the ISA 

found that effects of acidifying deposition on ecosystems have been well studied over the past 

several decades, that vulnerable areas have been identified for the United States and that the 

wealth of available data has led to the development of robust ecological models used for 

predicting soil and surface water acidification. With regard to the scope of effects, the REA also 

concluded that the available data are robust and considered high quality. There is high 

confidence about the use of these data and their value for extrapolating to larger spatial areas.

The EPA Temporally Integrated Monitoring of Ecosystems and Long-Term Monitoring 

(TIME/LTM) network represents a source of long-term, representative sampling. Data on sulfate 

concentrations, nitrate concentrations and ANC from 1990 to 2006 used for this analysis as well 

as the EPA Environmental Monitoring and Assessment Program (EMAP) and Regional 

Environmental Monitoring and Assessment Program (REMAP) surveys, provide considerable 

data on surface water trends.

The EPA also disagrees with commenters’ assessment of limitations in wet and dry 

deposition modeling. Further discussion of characterizing deposition with models can be found 

in section IV.C of the final rule. Additionally, while the EPA recognizes that there are limitations 

associated with modeled deposition values, the linkages between model estimates of deposition 

and areas exhibiting aquatic acidification effects are consistent and persuasive in considering 

adequacy of the current standard. Section 2.3 of the PA and sections 2.8 and 2.10 of the ISA 

provide additional detailed discussions of deposition modeling and spatial resolution for 

deposition. CASAC concurred with the EPA’s conclusion on this matter and encouraged the 

EPA to move forward in developing a new form of a standard which would address aquatic 

acidification. Thus, while the EPA is fully mindful of the limitations and uncertainties associated 

with the data and models, the EPA concludes that the available evidence provides strong 

scientific support for the view that harm from aquatic acidification is ongoing and attributable in 

large part to atmospheric deposition of reactive nitrogen and sulfur.

With regard to the commenters’ reliance on ecosystem services analyses included in the 

proposal to make judgments about adversity and public welfare, the EPA disagrees that 

comprehensive ecosystems services analyses are necessary to determine adversity. Ecosystem 

services analyses are used in this review to inform the decisions made with regard to adequacy

and as such are used in conjunction with other considerations in the discussion of adversity to 

public welfare. Section 4 of the PA further refines this discussion of adversity to public welfare. 

Additionally, the paradigm of adversity to public welfare as deriving from disruptions in 

ecosystem structure and function has been used broadly by the EPA to categorize effects of 

pollutants from the cellular to the ecosystem level. An evaluation of adversity to public welfare 

might consider the likelihood, type, magnitude, and spatial scale of the effect as well as the 

potential for recovery and any uncertainties relating to these considerations. Within this context, 
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ecosystems services analyses are one of many tools used in this review to help inform the 

Administrator’s decision on adversity. The EPA concludes that the analyses performed as part of 

this review are sufficient to support the decisions made by the Administrator with regard to the 

adequacy of the current standards.

C. Comments on Setting an AAI-based Standard

General comments that either supported or opposed the proposed decision not to set an 

AAI-based standard in this review are addressed in this section. Comments related specifically to 

the nature and degree of the uncertainties surrounding the setting of an AAI-based standard are 

also addressed in this section. Two groups of commenters offered sharply divergent views on 

whether it is appropriate for the EPA to set or even consider an AAI-based standard to protect 

against the effects in aquatic ecosystems from acidifying deposition associated with ambient 

concentrations of oxides of nitrogen and sulfur. These groups provided strongly contrasting 

views on the strength and limitations in the underlying scientific information upon which such a 

standard could be based, as well as on the legal authority and requirements in the CAA for the 

EPA to set such a standard. These comments are discussed in section III.D of the final rule, and 

build in part on the overarching issue raised by some commenters as to the EPA’s authority 

under the CAA to include deposition-related effects within the scope of a NAAQS review, which 

is discussed in section I.E of the final rule. Some commenters also expressed views about 

specific aspects of an AAI-based approach, as discussed in section III.D of the final rule and are 

included in section II.D of this document. Comments on more technical elements of the AAI 

approach are included in section II.E of this document. 

Comments: The first group of commenters, including several industry groups (e.g., EPRI, 

UARG, and API), individual companies (e.g., East Kentucky Power Cooperative), and two states 

(TX, SD), strongly supported the EPA’s proposed decision not to set an AAI-based standard in 

this review. These commenters generally focused on the limitations and uncertainties in the 

scientific evidence used by the EPA as a basis for its consideration of an AAI-based standard, 

expressing the view that these limitations and uncertainties were so great as to preclude setting 

such a standard at this time. Several industry commenters felt the uncertainties were of sufficient 

magnitude as to invalidate the AAI approach for use in the NAAQS, while others agreed with the 

EPA’s finding that further information and analysis is needed, and further noted that this work 

should be completed before the EPA could propose a new multi-pollutant standard. More 

fundamentally, some commenters in this group expressed the view that any consideration of such 

a standard is inconsistent with various provisions of the CAA and thus unlawful.

With regard to their views on the underlying scientific information, many of these 

commenters focused on what they asserted were areas of substantial uncertainty in the AAI 

approach including uncertainties in the individual F factors of the AAI, air deposition modeling, 

critical loads modeling, and available water quality and watershed data. Several commenters felt 

a more rigorous uncertainty and variability analysis of the AAI, beyond the analyses that the 

EPA presented in the PA, would be needed if the EPA were to consider such a standard in the 

future.

Some commenters expressed concerns with specific aspects of the AAI, such as the 

adequacy of the Omernik ecoregion approach as a method of waterbody aggregation for critical 
11

load calculations and whether ANC was an appropriate ecological indicator. The commenters 

asserted that the EPA needed to explore different methods for calculating critical loads, collect 

essential data, and employ mechanistic water chemistry models. The commenters also felt that 

the EPA was arbitrary in choosing its criteria for sensitive ecoregions and percent waterbodies, 

and that there was a bias in the field data toward sensitive areas. Several commenters felt a more 

comprehensive research program was needed to improve characterization of the biogeochemical 

and deposition processes incorporated into the AAI.

Some industry groups commented on uncertainties in the Community Multi-scale Air 

Quality (CMAQ) modeling, including high levels of uncertainty surrounding measurement and 

modeling of chemically reduced forms of nitrogen (NHx). Other commenters were also critical 

of the reliance of the AAI on modeling, and expressed the view that CMAQ would require 

intensive deposition-focused evaluation.

A second group of commenters, including several environmental groups (e.g., Center for 

Biological Diversity, Earthjustice, and Adirondack Council), the U.S. Department of Interior and 

the National Park Service, the New York Department of Environmental Conservation, and two 

tribes (Fond du Lac Band and Potawatomi) strongly disagreed with the EPA’s proposed decision 

not to set an AAI-based standard in this review. These commenters generally focused on the 

strengths of the evidence of deposition-related effects, the extent to which analyses presented in 

the PA addressed uncertainties and limitations in the evidence, and on information regarding the 

adversity of such effects as a basis for their views that such a standard was warranted at this 

time. Many of these commenters pointed to CASAC’s review of the underlying scientific 

evidence and its support for moving forward with an AAI-based standard at this time as support 

for their views.

In general, the environmental group commenters expressed the view that the current 

standards are clearly not adequate and that a combined oxides of nitrogen and sulfurstandard that 

links ambient air quality to an ecosystem indicator is appropriate, founded in science, and 

necessary for protection of public welfare. The commenters stated the current standards are 

neither sufficiently protective nor appropriate to address deposition-related effects. They also 

noted that the EPA has worked for decades to solve the acid deposition problem and that in their 

view the AAI represents an elegant solution to that problem. 

With regard to their views on the underlying scientific information, these commenters 

generally agreed with the EPA’s proposed conclusions that there are well-established water 

quality and biological indicators of aquatic deposition and well-established models that address 

air deposition, water quality impacts, and effects on biota. Many of these commenters expressed 

the view that the uncertainties and limitations in the scientific evidence were adequately 

addressed in the PA, which was reviewed by CASAC. Many of these commenters pointed to 

CASAC’s support for adopting an AAI-based standard in this review while concurrently 

conducting additional field monitoring and longer-term research that might reduce uncertainties 

in future reviews of secondary NAAQS for oxides of nitrogen and sulfur.

With regard to their views on the uncertainties and limitations in the scientific evidence, 

these commenters generally disagreed that there was sufficient uncertainty to preclude setting an 

AAI-based standard and pointed to several analyses and conclusions presented in the PA to 
12

support their position. For example, Earthjustice and the Center for Biological Diversity 

specifically cited the PA in presenting their views including the following points:

 that the PA concluded that “the confidence level in the information and processes 

associated with the linkages from ecological effects to atmospheric conditions 

through deposition and ecosystem modeling that is very high” (PA at 7-67);

 that the AAI performed well in the cumulative analysis of uncertainty cited in the PA, 

and specifically that mean results were “very close” to the observed values in the two 

regions analyzed (PA at 7-68), and that there was “no apparent directional bias in the 

[un]certainty regarding the biological, chemical and physical processes incorporated 

in the AAI” (PA at 7-69);

 that the PA lays out alternative levels and percentages of an AAI that would be 

appropriate to consider in setting a new AAI-based standard, while recognizing that 

selection from among alternative standards will necessarily reflect consideration of 

uncertainties, noting that nowhere does the PA suggest that this cannot be reasonably

accomplished, and instead proposes specific ranges for levels of the standard and 

discusses in detail criteria to weigh in setting the standard within that range (PA at 7-

78 to -80); and

 that there is an apparent disconnect between the assessment of uncertainty in the PA 

and the claim in the proposed rule that there is “no reasoned way” to choose a specific 

AAI-based standard.

Some governmental agency commenters were strongly supportive of an AAI-based 

standard and clearly felt such a standard should be adopted now. They also noted that the current 

ambient concentrations of oxides of nitrogen and sulfur are causing adverse ecological impacts 

and they believe that ongoing damage due to acidic deposition and the risks to ecosystems far 

outweigh the risk of setting an AAI-based standard while some uncertainties remain. They assert 

that deposition of oxides of nitrogen and sulfur is causing adversity to public welfare and that the 

scientific uncertainties do not preclude setting an AAI-based standard, and point to CASAC as 

generally supporting this view. The commenters believe that the EPA has ample evidence to 

support a new ecologically based standard and that the AAI is reasonable and scientifically 

defensible. NY specifically recommended an AAI of 50 with some flexibility built into the F 

factors.

Some of these agency and environmental group commenters also referenced CASAC’s 

support for specific elements of the AAI-based standard developed in the PA, including 1) the 

use of ANC as an appropriate ecological indicator for such a standard, 2) the use of total reactive 

oxidized nitrogen (NOy) and SOx as well-justified indicators of atmospheric concentrations of 

oxides of nitrogen and sulfur, 3) the use of Omernik Level III ecoregions, 4) the division of 

ecoregions into sensitive and non-sensitive categories, 5) the use of a 3 to 5 year averaging time, 

and 6) the appropriateness of an AAI level between 20 to 75 μeq/L.

With regard to their views on the requirements of the CAA, several environmental group 

commenters stated that given the large body of evidence supporting significant ongoing harm to 

the public welfare and the EPA’s finding the current standards are neither sufficiently protective 

nor appropriate to address deposition-related effects, the EPA’s reliance on uncertainty as 
13

grounds for failing to propose protective standards is irrational, arbitrary, and legally flawed. 

They believe that the EPA cannot lawfully reject a new AAI-based standard while continuing to 

rely solely on a form of the standard that is inadequate and allows serious harms to the public 

welfare to continue. When confronted with scientific uncertainties and incomplete data, they feel 

the EPA must act in a precautionary manner that errs toward stronger protections. Further, they

believe that the EPA’s reliance on scientific uncertainty as a basis for its inaction is 

unsupportable in light of CASAC’s advice and the EPA staff’s conclusions in the ISA, REA and 

PA.

In addition to the two broad groups of commenters discussed above, other commenters 

offered more general views on an AAI-based standard. For example, some state commenters 

(NC and PA) expressed support for the concept of developing a multi-pollutant, AAI-based 

standard, but felt that it would be important to gather additional information before proposing 

any such standard.

Response: The EPA has carefully considered these comments on whether or not an AAI-based 

secondary standard for oxides of nitrogen and sulfur is appropriate at this time. The EPA agrees 

with the second group of commenters and CASAC’s advice (outlined in section III.B of the final 

rule) that there is a strong scientific basis for development of the structure of such a standard, 

specifically with regard to a standard that would provide protection from deposition-related 

aquatic acidification in sensitive ecosystems across the country. As discussed in section II.A of 

the final rule and supported by several commenters, the available scientific evidence is sufficient 

to infer a causal relationship between acidifying deposition of nitrogen and sulfur and potential 

adverse effects to aquatic ecosystems, and that the deposition of oxides of nitrogen and sulfur 

both cause such acidification under current conditions that are allowed by the current secondary 

standards (U.S. EPA, 2008, chapter 3). The EPA agrees with commenters that there are wellestablished water quality and biological indicators of aquatic acidification as well as wellestablished models that address deposition, water quality, and effects on ecosystem biota, and 

that ecosystem sensitivity to acidification varies across the country (U.S. EPA, 2011, chapter 7).

The EPA also agrees with the second group of commenters and CASAC that ANC would 

be an appropriate ecological indicator, reflecting the acidifying effects of deposition of nitrogen 

and sulfur (U.S. EPA, 2011, chapter 7.2 and Russell and Samet, 2011a). Further, the EPA agrees 

that the structure of an AAI-based standard is well-grounded in science and would address the 

combined effects of deposition from oxides of nitrogen and sulfur by characterizing the linkages 

between ambient concentrations, deposition, and aquatic acidification, and that the structure of 

the standard takes into account relevant variations in these linkages across the country (section 

III.B. of the final rule and U.S. EPA, 2011, chapter 7).

The EPA disagrees with the first group of commenters that the use of Omernik 

ecoregions would be inadequate. A full explanation of the EPA’s rationale for selecting the 

Omernik ecoregion scheme for spatial aggregation is found in section 7.2.5 of the PA. Omernik 

ecoregions include consideration of geology, physiology, vegetation, climate, soils, land use, 

wildlife, and hydrology. These factors also relate well to sensitivity to acidification. The EPA 

also evaluated the National Ecological Observatory Network (NEON) and Bailey’s ecoregions 

developed for the US Forest Service and concluded that the Omernik ecoregion classification 

would be the most appropriate for an AAI-based standard. It offers several levels of spatial 
14

delineation, has undergone extensive scientific peer review, and has explicitly been applied to 

delineating acid sensitive areas of the US.

Nonetheless, the EPA agrees with the first group of commenters that there are important 

remaining scientific uncertainties within the derivation of the AAI, with the data used to specify

the factors within the AAI equation, and with the models themselves. These uncertainties are 

more fully discussed in Appendix F and G of the PA and in section III.A.5 of the final rule. 

These uncertainties have been reviewed by CASAC, and the EPA recognizes that further 

research would help to reduce the uncertainties. In general, the EPA also recognizes that the AAI 

would depend on atmospheric and ecological modeling, with inherent uncertainties, to specify

the terms of an AAI equation that incorporate the linkages between ambient concentrations, 

deposition, and aquatic acidification. 

The EPA agrees with the first group of commenters that there are several important 

limitations in the available data upon which elements of the AAI are based (U.S. EPA, 2011, 

Chapter 7). For example, existing monitors for NOy are generally not located in areas that are 

representative of sensitive aquatic ecosystems, and there is relatively sparse water quality data 

coverage in sensitive mountainous western areas. Further, even in areas where relevant data are 

available, small sample sizes impede efforts to characterize the representativeness of the 

available data for some ecoregions, which was noted by CASAC as being of particular concern 

(Russell and Samet, 2011a). Also, measurements of reduced forms of nitrogen are available from 

only a small number of monitoring sites, and emission inventories for reduced forms of nitrogen 

used in atmospheric modeling are subject to considerable uncertainty.

The EPA agrees with the first group of commenters that uncertainties related to the use of 

ecological and atmospheric models are difficult to evaluate due to a lack of relevant 

observational data. For example, relatively large uncertainties are introduced by a lack of data 

with regard to pre-industrial environmental conditions and other parameters that are necessary

inputs to critical load models that are the basis for factor F1 in the AAI equation. Also, 

observational data are not generally available to evaluate the modeled relationships between 

nitrogen and sulfur in the ambient air and associated deposition, which are the basis for the other 

factors (i.e., F2, F3, and F4) in the AAI equation.

The EPA agrees that these data limitations and model uncertainties create a number of 

inherent uncertainties and complexities in the quantification of the F factors of the AAI and the 

representativeness of the F factors at an ecoregion scale (U.S. EPA, 2011, Appendix F). These 

uncertainties and complexities currently lead to large uncertainty in characterizing the degree of 

protectiveness that would be afforded by an AAI-based standard with quantified F factors 

derived as discussed in the final rule, within the ranges of levels and forms identified in section 

III.A of the final rule.

The EPA disagrees with the first set of commenters that the selection of sensitive 

ecoregions and percentile waterbodies would be arbitrary. The EPA fully discussed its rationale 

and selection of sensitive ecoregions and the range of percentiles used in section 7.2.5 of the PA. 

The EPA relied on available alkalinity and ANC data to draw distinctions between sensitive and 

non-sensitive ecoregions. The EPA used its judgment in selecting the range of percentiles for 
15

sensitive and non-sensitive ecoregions, attempting to be neither over-protective nor underprotective of the set of waterbodies in each ecoregion. 

In general, the first set of commenters tends to treat all aspects of the AAI as subject to a 

high to very high degree of uncertainty. The EPA disagrees with this view, and instead views 

some parts of the AAI as based on more certain scientific information than others. For example, 

the EPA believes there is a solid scientific basis for the general framework of the AAI and for the 

relationship between ANC and effects on aquatic life. There is a strong basis for selection of 

ANC as an ecological indicator, for selection of NOy and SOx as ambient air indicators, for 

selection of the annual and 3- to5-year averaging time frame, and for selection of the range of 

ANC and percentile of water bodies for consideration. Likewise, the EPA believes there is a 

solid scientific basis for selection of Omernik ecoregions as the geographic basis for 

development of the AAI F factors. The EPA believes that for many areas there is a strong basis 

for determining whether an ecoregion is acid sensitive or not acid sensitive, while recognizing 

there is some uncertainty in some areas as to which category the area should fall in. The EPA’s 

decision not to adopt an AAI-based standard at this time is not driven by uncertainty in these 

elements of the AAI, but instead in the elements needed to derive the quantified F factors for 

ecoregions across the country and our ability to evaluate the representativeness of those F factors 

for an entire ecoregion. The greatest uncertainties concern the F1 and F2 factors, which relate to 

development of a single critical load to represent a specified percentile of all of the waterbodies 

in an ecoregion and development of the value for deposition of reduced nitrogen. In addition, 

there are also important uncertainties related to development of the F3 and F4 factors, which 

concern the quantified relationship between ambient levels of NOy and SOx and deposition rates 

of nitrogen and sulfur. The bases for these uncertainties are discussed in more detail in sections 

III.A.5 and section III.E of the final rule. Thus, while the EPA agrees in part with the first group

of commenters, in general they paint with too broad a brush. The EPA’s decision is based instead 

on taking into account the areas where there is less scientific uncertainty as well as the areas 

where there remain significant scientific uncertainties. 

In general, the second set of commenters does not contest the scientific evidence as 

discussed by the EPA or the scientific conclusions the EPA draws. They do not contest the 

existence of scientific uncertainty or the causes of it, and do not present scientific or technical 

arguments to contest the nature or magnitude of the uncertainty. Instead, they disagree with the 

conclusions or judgments to draw from the uncertainty. In the view of these commenters, the 

degree of uncertainty is low enough to warrant setting an AAI standard at this time. They

disagree with the Administrator's policy judgment that the nature and magnitude of uncertainty

warrants not setting an AAI standard at this time. Their primary disagreement is with this 

judgment, not with the EPA’s underlying views on the science and its uncertainties. As discussed 

in the proposal and below, however, the Administrator's reasoned judgment is that it is not 

appropriate to establish an AAI-based secondary standard at this time. The uncertainties 

discussed above prevent an understanding of the degree of protectiveness that would be afforded 

to various ecoregions across the country by a new standard defined in terms of a specific 

nationwide target ANC level and a specific percentile of water bodies for acid-sensitive 

ecoregions. Therefore, the Administrator is unable to identify an appropriate standard. 

The EPA recognizes that the AAI equation, with factors quantified in the ranges 

discussed in section III.A of the final rule and described more fully in chapter 7 of the PA, 
16

generally performs well in identifying areas of the country that are sensitive to such acidifying 

deposition and indicates, as expected, that lower ambient levels of oxides of nitrogen and sulfur 

would lead to higher calculated AAI values (PA, chapter 7). However, the various uncertainties 

discussed above are critical for determining with any degree of confidence the actual degree of 

protection that would be afforded such areas by any specific target ANC level and percentile of 

water bodies that would be chosen in setting a new AAI-based standard with quantified F factors, 

and thus for determining an appropriate AAI-based standard that meets the requirements of 

Section 109 of the CAA. The EPA recognizes that these limitations and uncertainties result in a 

considerable degree of uncertainty as to how well the quantified elements of the AAI standard 

would predict the actual relationship between varying ambient concentrations of oxides of 

nitrogen and sulfur and steady-state ANC levels across the distribution of water bodies within the 

various ecoregions in the United States. Because of this, there is considerable uncertainty as to 

the actual degree of protectiveness that such a standard would provide, especially for acidsensitive ecoregions.

With regard to the specific comments raised by Earthjustice and the Center for Biological 

Diversity noted above, that the uncertainties are not so great as to preclude setting an AAI-based 

standard, the EPA disagrees. As an initial matter, the EPA agrees that staff did reach the 

conclusions in the PA noted in the first three points raised by Earthjustice, however EPA does 

not agree with the implications of those conclusions drawn by the commenter. First, while staff 

did conclude that the “confidence level in the information and processes associated with the 

linkages from ecological effects to atmospheric conditions through deposition and ecosystem 

modeling is very high,” (PA, p. 7-67), that conclusion refers to the structure of the AAI, not to 

the quantification of the F factors that ultimately determine the degree of protectiveness afforded 

by any specific combination of level and form. While the scientific bases for the linkages 

between these factors of the AAI as presented in the PA are strongly supported, it is the 

quantification of these factors and the use of ecological and atmospheric modeling at an 

ecoregion scale that is necessary to specify the terms of the AAI equation and to determine the 

degree of protectiveness that any specific AAI-based standard would afford. Further, the 

appropriateness of these factors depends upon an understanding of the representativeness of the 

available distributions of critical load values at an ecoregion scale.

Second, while staff concluded that the AAI generally did perform well in the cumulative 

uncertainty analysis,

1 we note that analysis was limited to two ecoregions, only one of which was 

an acid-sensitive region. Further, the PA observed that these results indicate “that there is no 

systematic bias in the results despite what can be relatively high levels of uncertainty in the input 

parameters” (PA at 7-68). It is important to recognize that bias and uncertainty are not the same, 

and that a result indicative of minimal directional bias based on a very limited analysis does not 

negate conclusions with regard to the degree of uncertainty present in the various elements of the 

AAI. For example, the PA found that uncertainties in various elements of the AAI can be as 

high as 100% as was found for the transference ratios used in the AAI equation, as discussed in 

Appendix G of the PA. Extension of such results, which is inherent in drawing upon results from 

 

1 The cited PA text in this comment includes the phrase: “that mean results were ‘very close’ to 

the observed values” (PA at 7-68). For clarification, we note that the term “observed values” 

does not refer to measured values, but rather to the entire distribution of calculated values using 

the AAI equation, to which the mean values were compared. 
17

only two areas as a basis for setting a national standard, likely results in potentially large and 

unquantifiable uncertainties.

Third, in considering the last two points raised by Earthjustice and the Center for 

Biological Diversity, we first observe that there is a substantive difference in the nature of the 

questions addressed in the PA and the nature of the policy judgment that needs to be made by the 

Administrator in determining if the available information is sufficient to provide an adequate 

basis for setting an AAI-based standard. The questions addressed in the PA focus on determining 

a range of alternative standards that would be appropriate for the Administrator to consider, and 

characterizing the uncertainties associated with such a standard. This is clearly not the same as 

reaching a judgment as to whether the uncertainties, aggregated across all the various elements 

of the AAI, allow for sufficient confidence in determining a specific AAI-based standard that 

would provide requisite protection. The EPA agrees that the PA lays out staff views on 

alternative levels and forms (in terms of percentiles of water bodies) of an AAI that would be 

appropriate to consider in setting a new AAI-based standard. While this is a reasonable 

conclusion to be drawn in the PA, it does not address the broader question of the overall 

uncertainty associated with the degree of protection afforded by specifying a level and percentile 

form.

As noted above, there is a considerable degree of uncertainty as to how well the 

quantified elements of the AAI standard would predict the actual relationship between varying 

ambient concentrations of oxides of nitrogen and sulfur and steady-state ANC levels across the 

distribution of water bodies at an ecoregion scale. In particular, defining percentile values for 

acid-sensitive and non-acid sensitive ecoregions, incorporated within factor F1 of the AAI 

equation, relies upon an understanding of the extent to which the available critical load 

distributions are representative of the set of water bodies within an ecoregion. Both EPA and

CASAC have reflected concern with the degree of uncertainty that exists in our understanding of

the representativeness of the available critical load distributions at this time. The CASAC 

expressly noted in its advice to the Administrator that “[a]s EPA moves forward in the regulatory

process, we recommend some attention be given to our residual concern that the available data 

may reflect the more sensitive water bodies and thus, the selection of percentiles of water bodies 

to be protected could be conservatively biased” (Russell and Samet, 2011a). In EPA’s view, a 

better understanding of this issue is central to characterizing the degree of protectiveness that an 

AAI-based standard would afford, and the extent of our understanding at this time is not 

sufficient to allow for a reasoned characterization of the degree of protectiveness that any

specific AAI-based standard would afford. In light of these considerations, the EPA does not 

agree that there is a disconnect between the assessment of uncertainty in the PA and the policy

judgment made by the Administrator that the nature and degree of uncertainties that are uniquely 

present in this rulemaking are such as to preclude reaching a reasoned decision that a new AAIbased standard would be requisite to protect public welfare from deposition-related effects from 

aquatic acidification.

With regard to comments that the EPA cannot lawfully reject a new AAI-based standard, 

the EPA disagrees with the second group of commenters that the Administrator is required to set 

an AAI-based standard at this time. Although the Administrator has concluded that the current 

secondary standards are neither appropriate nor adequate to protect against potentially adverse 

deposition-related effects associated with ambient concentrations of oxides of nitrogen and 
18

sulfur, such a conclusion does not require the EPA to adopt a new NAAQS where the 

Administrator cannot reasonably judge that it would meet the criteria for a secondary NAAQS. 

The Administrator judges that the current limitations in relevant data and the 

uncertainties associated with specifying the elements of a new AAI-based NAAQS defined in 

terms of modeled factors are of such nature and degree as to prevent her from reaching a 

reasoned decision as to what level and form (in terms of a selected percentile) of such a standard 

would provide any particular intended degree of protection of public welfare that the 

Administrator determined satisfied the requirements to set an appropriate standard under Section 

109 of the CAA. As a result, the Administrator has determined that she cannot establish an AAIbased standard that is requisite to protect public welfare. The Administrator has made a similar 

judgment in deciding not to adopt new secondary NAAQS in the form of 1-hour standards 

identical to the primary NO2 and SO2 standards, as discussed below. No other NAAQS revisions 

to address the effects of acid deposition associated with oxides of nitrogen and sulfur in the 

ambient air have been suggested or considered by the EPA, CASAC, or commenters in this 

review.2 As such, all possible revisions to the secondary NAAQS to address the effects of acid 

deposition would involve adoption of new secondary standards that are judged by the 

Administrator to have such a high degree of uncertainty that she cannot make a reasoned 

decision that a new standard would satisfy the criteria of Section 109(b) of the CAA.

Commenters have pointed to the requirement in Section 109(b)(2) of the CAA that any

secondary NAAQS “must specify a level of air quality the attainment and maintenance of which 

...is requisite to protect the public welfare from any know or anticipated adverse effects...” in 

support of the argument that the EPA must adopt a new standard that provides requisite 

protection, having concluded that the current secondary standards are not sufficient to protect 

against adverse effects. In considering this comment, the EPA has taken into account the 

statutory language, as well as the bases for the EPA’s conclusion that the current standards for 

oxides of nitrogen and sulfur are neither appropriate nor adequate to provide protection against 

potentially adverse deposition-related effects and the data and model uncertainties that limit our 

efforts to characterize the degree of protectiveness that would be afforded by either an AAIbased standard or a 1-hour standard. We have concluded that Section 109 of the CAA does not 

require the EPA to adopt a new secondary standard where, as here, in the reasoned judgment of 

the Administrator, the uncertainties associated with such a standard would prevent her from 

determining whether or not such a NAAQS is requisite to protect public welfare. Section 109(b) 

of the CAA does not require the EPA to set a new standard under circumstances where the 

Administrator cannot reasonably judge that it would meet the criteria for a secondary NAAQS. 

The EPA recognizes and agrees with the comment from one environmental group that the 

EPA is not “foreclosed from setting a standard unless it can identify ...a ‘perfect’ standard level 

that is free from any noteworthy uncertainty.” However, that is not the situation in this 

rulemaking. The Agency has concluded that it would not be appropriate to promulgate a standard 

to address the public welfare effects of acidifying deposition where the remaining scientific 

 

2

 No one has suggested that EPA should revise the current 3-hour or annual secondary

standards to address the effects of acidifying deposition associated with oxides of nitrogen and 

sulfur in the ambient air. All revisions under consideration have involved adopting new 

secondary NAAQS.
19

uncertainties are of such import that they limit the EPA’s ability to make a reasoned decision on 

the degree of protectiveness that would be afforded by such a standard. The EPA recognizes that 

the result of this decision is that the current secondary standards continue in place and continue 

to be neither appropriate nor adequate to protect against potentially adverse deposition-related 

effects associated with ambient concentrations of oxides of nitrogen and sulfur. However, in the 

Administrator's view the proper response under the current circumstances is to continue to 

develop the scientific and technical basis for a future revision to the standards, and not to adopt 

at this time a new secondary standard that she cannot reasonably judge would comply with 

Section 109 of the CAA.

Further, the EPA agrees with both groups of commenters and CASAC that collecting 

further field data would be beneficial. A field pilot program is discussed in detail in section IV of 

the final rule. However, the EPA disagrees with the first group of commenters’ assertions that 

these uncertainties should invalidate or preclude the further development of an AAI-based 

standard.

D. Comments on Specific Aspects of the AAI Approach

This section discusses comments on the following four specific aspects of an AAI-based 

approach to setting a secondary standard for oxides of nitrogen and sulfur: (1) the inclusion of 

chemically reduced nitrogen (NHx), in addition to oxides of nitrogen, in the AAI equation; (2) 

whether such a standard would be appropriately construed as a national standard versus a 

regional standard; (3) whether such a standard would be appropriately construed as an ambient 

air quality standard versus a water quality standard; and (4) whether the EPA has authority under 

the CAA to set a multi-pollutant NAAQS.

(1) Comments: As described in section III.A of the final rule, the AAI equation contains a 

separate factor that accounts for the acidifying potential of NHx, in addition to the factor that 

accounts for the acidifying potential of oxides of nitrogen. Several industry commenters 

addressed this issue explicitly, with some expressing the view that NHx should be treated the 

same as NOx in the AAI, while others felt it should not be included at all in the AAI. Several 

commenters expressed the view that accounting for NHx in the AAI equation represents a de 

facto regulation of ammonia, which they assert is unlawful since reduced nitrogen is not a listed 

air pollutant under Section 108 of the CAA.

Other commenters, including environmental groups and governmental agency

commenters, did not explicitly comment on the inclusion of NHx in the AAI equation; however 

several commenters made note of CASAC’s advice on this issue. CASAC advised that it is 

necessary to include a factor for NHx in the AAI equation, even though it is not a listed pollutant, 

since aquatic ecosystems respond to inputs of NHx to create acidity just like they do with inputs 

of NOx and SOx. 

Response: The EPA has included NHx deposition explicitly as part of factor F2 in the AAI 

expression to account for the acidifying potential afforded by ammonia gas and ammonium ion. 

Inclusion of NHx deposition, in addition to deposition of oxides of nitrogen, is necessary to 

account for potential effects of all reactive nitrogen species which, in turn, allows for 

determining the contributions of oxides of N and S to aquatic acidification. This approach is 
20

consistent with the requirement in the CAA that where the state of the science provides a basis 

for considering such effects, the review of the air quality criteria for a pollutant should 

encompass the ways in which other air pollutants may interact with the criteria pollutant to 

produce adverse effects. See CAA Section 108(a)(2). In effect, the inclusion of NHx deposition 

can be viewed as a necessary component consistent with our scientific understanding that links 

deposition of all nitrogen species to ecological effects.

The EPA recognizes that the NAAQS is established to address the pollutants oxides of 

nitrogen and oxides of sulfur. Consequently, the ambient concentrations of oxides of sulfur (as 

SOx) and nitrogen (as NOy) are accounted for separately from the deposition of NHx in the AAI 

equation, thus defining the standard specifically in terms of the acidifying potential of levels of 

oxides of nitrogen and sulfur in the ambient air. More specifically, compliance with an AAIbased standard would be based on using federal reference or equivalent monitoring methods to 

measure ambient concentrations of NOy and SOx to determine an area’s attainment status. 

Conversely, there would be no requirement to measure concentrations of NHx to determine 

compliance with an AAI-based standard. Rather, ecoregion-specific values of NHx deposition 

would be determined by modeling and would be specified by the EPA in conjunction with 

setting such a standard, and would not be a variable in the AAI equation as would SOx and NOy.

The contribution of reduced forms of nitrogen to total nitrogen deposition would represent an 

ecosystem-specific environmental factor that plays a necessary background role in characterizing 

the relationship between the measured, variable levels of the ambient air indicators of oxides of 

nitrogen and sulfur (NOy and SOx) and the associated degree of aquatic acidification. Section 

108 requires the air quality criteria to evaluate to the extent practicable the variable factors such 

as atmospheric conditions that affect the impact of the ambient air pollutant (in this case oxides 

of nitrogen and sulfur) on the public welfare. In this review, such variable factors include the 

deposition of reduced nitrogen in an ecoregion, as well as all of the other elements reflected in 

the factors F1 to F4, and the designation of an area as acid-sensitive or not acid-sensitive. Section 

109 calls for the EPA to base the NAAQS on the air quality criteria, and accounting for the role 

of reduced nitrogen deposition in the AAI reflects this.

In considering this aspect of an AAI-based standard, the EPA took into account that in 

applying the AAI equation, all factors, including NHx deposition, would be updated as 

appropriate as part of the periodic reviews of the NAAQS, called for at five-year intervals by the 

CAA, to account for changing environmental conditions and new data. In determining an 

ecoregion’s status with regard to meeting a particular AAI-based standard, NHx deposition 

reflected in the F2 factor would be treated just as all of the other environmental terms – e.g. 

critical loads and transference ratios -- which influence factors F1, F3 and F4. To the extent that 

changes in NHx deposition occur from one review to the next, the ecoregion-specific F2 factors 

would be updated to reflect such changes. To the extent that NHx deposition decreased from one 

review to the next, an AAI-based standard updated during a periodic review to reflect this change 

would allow for potentially higher levels of NOy and SOx that would meet a specific AAI-based 

standard; conversely, increased levels of NHx deposition would allow for potentially lower levels 

of NOy and SOx. Meeting a specific AAI-based standard would only require that the combined 

levels of NOy and SOx be such that a calculated AAI value meet or exceed the AAI value set as 

the level of the standard. Consequently, while the contribution of NHx deposition would be 

accounted for, NHx emissions would not be regulated through the implementation of an AAIbased standard. NHx deposition would be treated as an ecologically relevant background value 
21

that could be updated over time to reflect changes in circumstances, but accounting for such 

changes would not be required for purposes of determining compliance with an AAI-based 

standard. Thus, the incorporation of NHx in the AAI equation would not result in de facto

regulation of NHx emissions.

(2) Comments: Some commenters raised the issue of whether an AAI-based standard would be a 

national standard, as required by Section 109 of the CAA, or whether it is in essence a regional 

standard. One group of commenters (the Center for Biological Diversity and the National Park 

Service) generally expressed the view that an AAI-based standard would be a national standard, 

whereas another group, including industry commenters, asserted that an AAI-based standard 

would be a regional standard and thus not consistent with the requirements of the CAA.

The first group of commenters supported the use of a national ANC indicator, 

recognizing that an AAI approach would account for regional differences in sensitivity and 

relevant environmental factors while providing a nationally consistent degree of protection 

across sensitive ecoregions. For example, the National Park Service stated that the AAI approach 

provides a uniform level of protection to sensitive ecosystems while appropriately taking into 

account the variability in deposition, meteorology, and other relevant environmental factors 

across ecoregions.

The second group of commenters noted that application of the AAI equation in different 

areas of the country produced different allowable concentrations of NOy and SOx, asserting as a 

result that an AAI-based standard would be a regional standard. These commenters asserted that 

the EPA lacks authority under the CAA to set such a regional NAAQS. For example, UARG 

states that the AAI is applied differently in different regions of the country (e.g., sensitive vs. 

non-sensitive ecoregions). The Alliance of Automobile Manufacturers commented that both the 

EPA and Congress historically have decided that secondary national air quality standards are not 

an appropriate approach to address regionally variable welfare effects.

Response: The EPA believes that a secondary NAAQS based on the AAI approach could be a 

national standard, consistent with the CAA. An AAI-based standard would apply all across the 

country. It would be defined in part by a single level of the AAI – that is, every part of the 

country would be expected to meet or exceed a specified AAI level. The scientific basis for 

setting a national AAI level is rooted in the similarity between AAI and acid neutralizing 

capacity (ANC), which is a widely accepted ecological health indicator for aquatic acidification. 

The rationale underlying the use of ANC is that the ecosystem health reflected by an ANC value 

in one part of the country is generally similar to that in another location, irrespective of regional 

differences in biogeochemistry and atmospheric conditions. The EPA recognizes that allowable 

concentrations of the ambient air pollutant indicators for oxides of nitrogen and sulfur in the AAI 

equation can vary from one location to another and result in the same calculated AAI. The 

difference between an AAI-based standard and the existing primary standards is that the level of 

the standard is defined directly in terms of the measured ambient air pollutant indicator. That is, 

the health-based indicator and the measured ambient air indicator are based on the same 

chemical entity. In an AAI-based standard, the level of the standard, reflecting a nationally 

consistent degree of protection, would be defined in terms of an ecological indicator, ANC, and 

compliance would be determined based on concentrations of the ambient air indicators, NOy and 

SOx. From an ecosystem health perspective, it is most relevant to use the ecological indicator, 
22

ANC, to establish a single level that, in the context of an AAI, leads to a similar degree of 

protection across the country. The allowable levels of NOy and SOx could vary across the 

country, while the specified AAI level and the corresponding degree of protection, would not. 

This would facilitate ensuring that such a NAAQS would provide sufficient protection, but not 

more than was necessary. It should be noted that in the 2006 particulate matter (PM) NAAQS 

decision the EPA set a NAAQS that envisions variation in allowable ambient levels of certain 

kinds of PM. The EPA set a PM10 standard with a single numerical level, which then allowed 

varying levels of coarse PM, a subset of PM10. The PM10 standard was designed to allow lower 

levels of coarse PM in urban areas and higher levels of coarse PM in non-urban, rural areas. The 

EPA’s goal was to target protection at urban areas, where the evidence showed coarse particles 

presented a greater risk to public health. The single numerical standard for PM10 allowed 

variable levels of coarse PM, with higher allowable levels where there was less evidence of risk 

and lower allowable levels where the evidence of risk was greater. This approach was upheld in 

American Farm Bur. Fed. v. EPA, 559 F.3d 512, 533- 536 (D.C. Cir. 2009).

In conjunction with consideration of an AAI-based standard, the EPA has recognized that 

the nation includes some relatively acid-sensitive and some relatively non-acid sensitive 

ecoregions. This delineation allows for an appropriate application of the AAI equation that 

increases its relevancy from a national perspective as it avoids creating more than requisite 

protection in areas that are not acid sensitive. The AAI equation and the selected level of such a 

standard would be applicable everywhere; however, factors in the AAI equation are 

appropriately dependent on the sensitive and non-sensitive ecoregion classification. Therefore, 

the delineation of sensitive and non-sensitive regions allows for a nationally consistent 

application of the AAI equation as it targets protection on those areas most likely to benefit from 

reductions in acidifying deposition of oxides of nitrogen and sulfur, and avoids more than 

requisite protection in areas that would not benefit from such reductions.

(3) Comments: Some commenters expressed the view that an AAI-based standard would 

essentially be a water quality standard, since it would use ANC, a water quality property, as the 

ecological indicator. For example, UARG expressed this view by noting that an AAI standard 

would be defined in terms of a single water quality level with multiple allowable air quality 

concentrations of oxides of nitrogen and sulfur.

Response: The EPA notes that the AAI relates aquatic acidification to ambient air 

concentrations of oxides of nitrogen and sulfur. An AAI-based standard would be set at a level 

such that ambient air concentrations would not cause harmful acidification effects to water 

quality resources, which is within the scope of welfare effects that secondary NAAQS are to 

address (i.e., welfare effects include, but are not limited to, “effects on soils, water, ...”). 

Accordingly, while an AAI-based standard would address effects on water quality, it would do 

so by defining the allowable ambient air concentrations of oxides of nitrogen and sulfur that 

would provide appropriate protection against such effects. Compliance with such a standard 

would be determined by measuring ambient air concentrations of NOy and SOx, not by

measuring the water quality property of ANC. The actual water quality of any body of water 

would not be used to determine compliance with the air quality standard, and no body of water 

would be considered in “non-compliance” with an AAI air quality standard. Thus, an AAI-based 

standard is appropriately construed as an air quality standard, not a water quality standard.
23

(4) Comments: Some commenters questioned whether the EPA has the authority to establish a 

NAAQS that jointly addresses ambient concentrations of oxides of nitrogen and oxides of sulfur. 

Pointing to language in Section 109(b)(2) that a NAAQS must address “adverse effects 

associated with the presence of such air pollutant in the ambient air,” these commenters took the 

position that the EPA may not allow for tradeoffs between two pollutants in setting a NAAQS. 

See Section109 (b)(2)(emphasis added). These commenters suggest the NAAQS must be set for 

“such air pollutant” only.

Response: The EPA disagrees that the phrase “such air pollutant” in Section109 (b) (2) would 

prohibit the Agency from setting a multi-pollutant NAAQS in the form of an AAI. When the 

Administrator sets a NAAQS, the standard must be “requisite to protect the public welfare from 

any known or anticipated adverse effects associated with the presence of such air pollutant.” 

CAA Section109 (b)(2). The term “associated with...such air pollutant” describes the category

of adverse affects to be associated with the air pollutant at issue, not the standard.

Oxides of nitrogen and sulfur, pollutants for which the EPA has issued air quality criteria, 

both cause acidification of aquatic ecosystems, effects that could be considered adverse to public 

welfare. As such, aquatic acidification is a “known or anticipated adverse effect associated with 

the presence of [oxides of nitrogen] in the ambient air.” This known or anticipated adverse effect 

is also associated with the presence of oxides of sulfur in the ambient air. Given the scientific 

links between ambient air concentrations of oxides of nitrogen and sulfur, the related deposition 

of nitrogen and sulfur, and the associated ecological responses, the EPA appropriately considered 

a multi-pollutant NAAQS in the form of an AAI to protect against the effects of acidifying 

deposition to aquatic ecosystems that took into account these linkages. Rather than limiting the 

EPA’s authority, the language cited by the commenters goes to the breadth of the EPA’s 

obligation and authority to set standards to protect against “any known or anticipated adverse 

effects.” In addition, the NAAQS are to be based on the air quality criteria, which under Section 

108 (a)(2) are required to consider the kind of multi-pollutant linkage evident in this review. The 

EPA does not read the language of Section 109(b) as prohibiting the Administrator from setting a 

multi-pollutant NAAQS such as the AAI where such an approach would be judged as the 

appropriate way to satisfy Section 109(b)’s requirements for each of the pollutants involved.

E. Comments on Technical Elements of the AAI Approach

This section summarizes comments received on specific technical elements of the AAI 

approach that are not fully addressed above. Since the EPA is not promulgating an AAI-based 

standard in this review, these technical comments are noted for their value to the next review of 

the secondary NAAQS for oxides of nitrogen and sulfur. Accordingly, the EPA recognizes the 

comments and will consider them in the context of the next review progresses.

Comments: Some commenters (e.g., EPRI, UARG, ASARCO, AAM, Tri-State Generation and 

Transmission Association) provided in-depth technical comments and analysis of the AAI 

approach. In some cases, the commenters disagreed with the models and indicators that the EPA 

had considered. In other cases, the commenters provided additional information that in their view 

would help inform an expanded uncertainty analysis. Other commenters (e.g., Center for 

Biological Diversity, Earthjustice, Adirondack Council and the National Park Service) expressed 

support for the models and indicators that EPA had considered, noting that the EPA used well-
24

established water quality and biological indicators and well-established models that address air 

deposition, water quality impacts, and effects on biota.

With regard to the EPA’s consideration of ANC as an ecological indicator for effects 

related to aquatic acidification, some commenters expressed concerns that ANC was not the best 

indicator of NOx and SOx depositional effects on aquatic biota. These commenters expressed the 

view that in certain areas of the country, ANC does not act as a good predictor of aquatic health 

as measured by fish species diversity. Some such commenters provided references to studies not 

reviewed in the ISA and discussed the interactions of ANC with the presence of dissolved 

organic carbon and inorganic monomeric aluminum and its contribution to pH. These 

commenters noted that inorganic monomeric aluminum and pH are more directly causally 

associated with the toxic mechanisms of impaired species richness and fish mortality; hence, in 

their view, are better ecological indicators than ANC.

On the other hand, other commenters supported EPA’s consideration of ANC as an 

appropriate ecological indicator, referencing CASAC’s support of ANC as an appropriate 

ecological indicator for an AAI-based standard and support for focusing on target ANC levels as 

the basis for defining the degree of protection that an AAI-based standard would afford.

With regard to EPA’s consideration of the Omernik ecoregion scheme, some commenters 

expressed reservations over the choice of the Omernik ecoregion scheme and felt that the EPA 

had not fully evaluated the relationship to sensitive waterbodies. They felt a more thorough 

analysis was warranted including distribution of sensitive lakes and underlying watershed 

characteristics. They felt data representativeness had not been adequately analyzed and that the 

acid sensitivity classification was flawed. They submitted in-depth evaluations and suggestions 

for additional data. They also felt that the current approach did not adequately account for 

naturally acidic areas and anthropogenic non-emission sources of acidity. In addition, they felt 

that the EPA needs to evaluate areas of the country that are more representative, including nonsensitive regions. They recommended that the EPA should develop a national database, including 

key water quality parameters, aquatic biota at several trophic levels, soil chemistry variables, and 

watershed characteristics.

Other commenters expressed support for the use of the Omernik ecoregion scheme by 

way of referencing CASAC’s review and support of the EPA rationale for selecting the Omernik 

Level III ecoregions over other regionalization schemes discussed in the PA (U.S. EPA, 2011). 

While several commenters called for more data to be generated in relatively unstudied 

areas, one commenter wanted the EPA to further study the Appalachian Mountain region to 

explain why it is a heavily impacted outlier, showing the largest relative exceedances of 

estimated critical loads by current deposition of nitrogen and sulfur (see chapter 7, U.S. EPA, 

2011). This commenter urged the EPA to further explore whether the AAI is accurately assessing 

and identifying areas of concern based on atmospheric and water conditions, including the 

influence of fertilizer deposition. Another commenter recommended that the EPA select the 

Chesapeake Bay watershed for further study, noting that additional data would help with the 

nitrogen total maximum daily load (TMDL) that is being implemented.
25

With regard to critical load models, some commenters questioned the EPA’s choice of 

acidification models for the critical load calculations used to develop the AAI equation. They

expressed concern with the use of steady-state assumptions and simple aquatic models, and 

expressed the view that the Steady-State Water Chemistry model considered by EPA did not 

adequately capture the complex processes involved in acidification of waterbodies. They

expressed concerns with how base cation weathering was determined and how base cation 

deposition is handled within the AAI. Some commenters suggested the EPA engage in further 

development and testing of mechanistic models that include key physical and chemical 

processes, e.g., ion uptake, ion exchange, and weathering rates. They provided detailed 

discussion of alternative models and approaches. Some commenters also expressed concerns 

about how the AAI treats nitrogen uptake by terrestrial plants, expressing the view that it 

provides insufficient consideration of soil sulfur retention.

Other commenters supported the EPA’s choice of critical load models and recommended 

extending the approach to other ecosystem effects in addition to aquatic acidification, such as 

terrestrial acidification, terrestrial nutrient enrichment, and aquatic eutrophication. They

recommended that the EPA fully consider research and policy decisions that have occurred in 

parts of Europe in the area of terrestrial nutrient enrichment from nitrogen deposition.

With regard to the transference ratios used in the AAI, some commenters expressed 

concerns that there is no unique link between ground-level NOx and SOx concentrations and the 

deposition that may lead to effects. In their view, there is a lack of deposition data and the 

models are inadequate for deposition predictions. They expressed concerns with the transference 

ratio as a method to link ambient air concentrations to deposition. One commenter submitted 

analyses of the variability in the transference ratios using various air deposition models and 

argued that the transference ratios were unstable and thus created great uncertainty within the 

AAI. Another commenter felt that emissions of NOx and SOx, instead of ambient concentrations, 

were better predictors of aquatic acidification effects; therefore an air quality standard was 

inappropriate. These commenters included detailed analyses and rationales for their positions.

Other commenters supported the EPA’s overall approach to an AAI-based standard, 

including the use of transference ratios in the AAI equation for an ecoregion. These commenters 

noted CASAC’s review of the PA and support for such an approach.

Response: The EPA has carefully considered these technical comments and analyses and 

believes that the issues raised in the comments are appropriately considered in the next review. 

The information presented in these comment will help inform EPA’s consideration of the 

scientific aspects of developing an ecologically relevant, multi-pollutant standard in the next 

review.

F. Comments on 1-Hour NO2 and SO2 Secondary Standards

Comments: Comments received on the proposal related to setting new 1-hour NO2 and SO2

secondary standards are addressed in this section. Most generally, there was broad and strong 

opposition to the EPA’s proposed decision to set 1-hour NO2 and SO2 secondary standards 

identical to the 1-hour NO2 and SO2 primary standards. For example, strong opposition to this 

proposed decision was expressed by a diverse set of commenters, including some environmental 
26

groups (e.g., Environmental Justice, the Adirondack Council) and industry groups (e.g., UARG, 

AAM, ASARCO, API, Portland Cement Association, Tri-State Generation and Transmission 

Association, Louisiana Chemical Association, East Kentucky Power Cooperative, FMMI, Rio 

Tinto), the U.S. Department of the Interior, and some states (e.g., NY, PA, TX). These 

commenters offered various arguments in support of their views that the proposed decision is 

unlawful, arbitrary, and not supported by the record of this rulemaking, as outlined below. One 

commenter (NC) supported setting secondary standards identical to the 1-hour NO2 and SO2

primary standards, while also supporting the EPA’s decision to take additional time to develop a 

multi-pollutant AAI-based secondary standard. Another commenter (SD) simply supported 

setting secondary standards that are no more stringent than the primary standards.

In proposing the 1-hour secondary standards, the EPA recognized that such standards 

would not be ecologically relevant, but concluded that they would nonetheless “directionally 

provide some degree of additional protection” by reducing deposition to sensitive ecosystems. 

The EPA also noted that this was consistent with the view that the current secondary standards 

are neither sufficiently protective nor appropriate in form, but that it is not appropriate to propose 

to set a new, ecologically relevant multi-pollutant secondary standard at this time.

In arguing that the proposed decision to set 1-hour NO2 and SO2 secondary standards 

identical to the 1-hour NO2 and SO2 primary standards is unlawful, commenters asserted that the 

EPA’s rationale is not consistent with the requirements of Section 109 of the CAA. Commenters 

argue that this rationale is not consistent with the CAA requirement that the EPA set secondary

NAAQS that are “requisite to protect public welfare;” that is, a standard that is neither more nor 

less stringent than necessary for this purpose. More specifically, these commenters argue that a 

standard that is based solely on “directionally” improving the environment, without any evidence 

or judgment that it would provide “requisite” protection, is not consistent with the requirements 

of the CAA and is thus unlawful. Some commenters also note that the CAA requires that the 

EPA revise previously adopted NAAQS as “appropriate” to provide such protection. These 

commenters assert that since the EPA’s proposal concludes that the 1-hour NO2 and SO2

standards are not ecologically relevant to address deposition-related effects on sensitive 

ecosystems, adding such standards cannot be considered to be an appropriate revision to the 

NAAQS for the purpose of addressing adverse ecological effects.

Commenters also raised a number of issues in support of their views that the proposed 

decision is arbitrary and unsupported by the available information in the record of this 

rulemaking. Some commenters noted that there is no evidence or analysis in the record that 

addresses the degree of protection that would likely be afforded by 1-hour NO2 and SO2

standards, and, further, that the EPA does not claim otherwise. In the absence of such 

information, commenters argue that the EPA cannot make a reasoned judgment as to what levels 

of such 1-hour NO2 and SO2 standards would be requisite to protect public welfare; in particular, 

some commenters emphasized that the EPA cannot demonstrate that such standards would not be 

more stringent than necessary to protect against adverse deposition-related effects to sensitive 

ecosystems. Thus, in the commenters’ view, any such 1-hour standards would be arbitrary.

One commenter also expressed the view that the EPA’s proposed decision to set new 1-

hour NO2 and SO2 secondary standards is inconsistent with the reasoning the EPA used as a 

basis for proposing not to set a new ecologically relevant AAI-based secondary standard at this 
27

time. As summarized above, the EPA based its proposed decision not to set an AAI-based 

standard, which is expressly designed to address important differences in ecosystem sensitivities, 

in part on uncertainties and limitations in relevant information that were of such nature and 

degree as to prevent the Administrator from reaching a reasoned decision at this time as to what 

level and form of such a standard would provide a particular degree of protection. This 

commenter asserts that the proposed decision to set new 1-hour NO2 and SO2 secondary

standards completely ignores such uncertainties inherent in 1-hour standards, which are not even 

structured to account for differences in ecosystem sensitivities. 

Some commenters asserted not only that the EPA has failed to provide any information 

on the degree of protection that would likely be afforded by the proposed 1-hour NO2 and SO2

standards, but that such an analysis cannot be done since there is no rational connection between 

any of the elements of the proposed 1-hour secondary standards – including the averaging time 

and level – and the ecological effects the proposed standards are intended to address. In 

particular, commenters noted that the EPA has not presented any rational basis for concluding 

that standards designed to reduce human health risks associated with short-term peak 

concentrations of NO2 and SO2 have any connection whatsoever to addressing long-term 

deposition of oxides of nitrogen and sulfur and associated impacts on sensitive ecosystems.

Further, commenters argued that there is no evidence in the record that demonstrates the 

proposed 1-hour secondary standards would provide any environmental benefit. For example, 

commenters noted that such standards do not take into account ecosystem sensitivity; they may

not result in reductions to long-term deposition that is the relevant time frame for depositionrelated effects on sensitive ecosystems; and they would not provide any benefit beyond that 

which might accrue from the identical primary standards that are already in effect. Some 

commenters have also noted that many other environmental regulations are already in place that 

will provide reductions in ambient oxides of nitrogen and sulfur, and that the EPA has not 

demonstrated that any additional reductions are needed to provide requisite protection.

Response: The EPA agrees that the Agency has not presented evidence or analysis in the record 

that addresses the degree of protection that would likely be afforded by secondary standards set 

identical to the current 1-hour NO2 and SO2 primary standards. The EPA further agrees that such 

an analysis cannot be done since there is no demonstrable linkage between peak 1-hour average 

concentrations of NO2 and SO2 in the ambient air and the impact of deposition-related 

acidification associated with oxides of nitrogen and sulfur on sensitive aquatic ecosystems that 

the proposed standards were intended to address. As a result, the EPA agrees that there is no 

factual basis to make a reasoned judgment as to what levels of 1-hour NO2 and SO2 standards 

would provide a desired degree of protection of the public welfare, such that the EPA cannot 

demonstrate or judge that the proposed standards would not be more or less stringent than 

necessary to provide the desired degree of protection against potentially adverse depositionrelated effects to sensitive ecosystems.

As to whether the proposed standards would provide any environmental benefit, it is the 

EPA’s view that it is reasonable to conclude that any standard that would lead to reductions in 

NO2 and SO2 emissions would likely result in some environmental benefit for some acidsensitive areas. Nonetheless, the EPA recognizes that any such environmental benefit that would 

result from reductions in NO2 and SO2 emissions sufficient to attain the 1-hour standards cannot 
28

be specifically quantified or linked to reductions in aquatic acidification in specific ecoregions. 

In addition, unlike an AAI-based standard, the 1-hour standards would tend to provide more 

protection than is warranted in areas that are not acid-sensitive.

Further, the EPA recognizes that any such benefits would accrue from the 1-hour NO2

and SO2 primary standards that are in effect, regardless of whether identical secondary standards 

are adopted. The EPA does not agree, however, that the Agency needs to consider future 

reductions that may accrue from other environmental regulations in the context of reaching a 

judgment as to what NAAQS is requisite to protect public welfare.

The EPA notes that the strongly held view of the commenters with respect to the 

proposed 1-hour standards is that the EPA should reject and not adopt a standard where there is 

not an adequate scientific or technical basis for judging the degree of protection which such a 

standard would provide. The EPA agrees with that general point. According to commenters, the 

1-hour standards should be rejected because they do not have such a basis, and, as discussed 

below, the EPA agrees. This is consistent with the reasoning that the EPA has applied to 

consideration of an AAI-based standard, as discussed above in response to comments related to 

an AAI-based standard. As noted above, the limitations and uncertainties in the scientific and 

technical basis for developing a specific AAI-based standard result in a great degree of 

uncertainty as to how well the quantified elements of the AAI would predict the actual 

relationship between varying ambient concentrations of oxides of nitrogen and sulfur and steadystate ANC levels across the distribution of water bodies within the various ecoregions in the 

United States. Because of this, there is great uncertainty as to the actual degree of protectiveness 

that such a standard would provide, especially for acid-sensitive ecoregions. The Administrator 

judges that the uncertainties are of such a nature and magnitude that there is no reasoned way to 

choose a specific AAI-based standard, in terms of a specific nationwide target ANC level or 

percentile of water bodies that would appropriately account for the uncertainties, since neither 

the direction nor the magnitude of change from the target level and percentile that would 

otherwise be chosen can reasonably be ascertained at this time.

3

The EPA has also considered, in light of the public comments, whether it is necessary or 

appropriate under Section 109 of the CAA to make any revision to the current secondary

standards for oxides of nitrogen and sulfur, having concluded that the current standards are 

neither adequate nor appropriate. As discussed in section III.D.1.a of the final rule, with regard to 

comments on the EPA’s proposed decision not to set a new multi-pollutant AAI-based standard 

at this time, some commenters argued that the EPA cannot lawfully use uncertainty as a basis to 

decline to set an ecologically relevant standard, having concluded that the current secondary

standards are neither adequately protective nor appropriate to provide protection to ecosystems. 

In response, the EPA disagrees, stating that data limitations and uncertainties in key elements of 

a standard, which are of such nature and degree as to prevent the Administrator from reaching a 

reasoned decision as to what specific standard would be appropriate to provide requisite 

 

3

Thus, as discussed above, EPA’s disagreement with commenters concerning adoption of an 

AAI-based standard at this time appears to stem from differing views on whether or not there is 

an adequate scientific or technical basis for judging the degree of protection which an AAI-based 

standard would afford. There does not appear to be a disagreement with the view that EPA 

should not adopt a standard absent such a scientific or technical basis.
29

protection, are an appropriate basis for deciding not to set such a standard, even one that is of an 

ecologically relevant form. The EPA concludes that it is appropriate to apply the same reasoning 

in reaching a decision as to whether to set new 1-hour NO2 and SO2 secondary standards. In this 

case, the uncertainties are arguably greater than with an AAI-based standard, since as noted 

above there is no demonstrable linkage between the elements of such standards and impacts on 

sensitive ecosystems that the standards would be intended to address.

III. RESPONSES TO SIGNIFICANT COMMENTS RELATED TO THE FIELD 

PILOT PROGRAM AND MONITORING METHODS EVALUATION

Public comments on EPA’s proposed plans for a field pilot program and related 

evaluation of monitoring methods generally fell into the following four topic areas: goals, 

objectives, and scope; monitoring network and site selection; complementary measurements and 

instrumentation; and collaboration and stakeholder participation. An overview of these 

comments and EPA’s responses are discussed in section of IV.D of the final rule and in sections 

III.A-D below.

In addition, many commenters generally requested that EPA provide clarification of its 

plans regarding the field pilot program. As outlined below as part of section III.D, EPA is 

developing a draft white paper that will reflect input from EPA’s ongoing consultation with its 

partners on the National Association of Clean Air Agencies (NACAA) monitor steering 

committee as well as EPA’s consideration of the public comments made on the proposal. The 

draft white paper in intended to provide further clarity regarding the purpose, scope, program 

elements, and process for participation in the development and implementation of a field pilot 

program. The EPA plans to make this draft white paper available for additional public comment 

later this year. After taking into consideration further input from the public, the EPA plans to 

prepare a final white paper that will serve as a work plan for the field pilot program.

A. Goals, Objectives, and Scope

Comments: There was a mix of comments regarding the need for and the overall purpose and 

scope of the field pilot program. In general commenters that supported the AAI approach ((e.g., 

U.S. Department of Interior (DOI)/National Park Service (NPS), Nature Conservancy, 

Adirondack Council, NESCAUM, NY, PA, NC) also supported the concept of deploying a field 

pilot program as well the proposed goals and objectives, while offering specific comments on the 

scope of the proposed monitoring effort. Other commenters supporting the AAI approach, 

including Earthjustice and the Center for Biological Diversity expressed the view that a field 

pilot program was not needed to support adoption of such a standard in this review. A variety of 

commenters expressed the view that a field pilot program in 3 to 5 ecoregions was too limited to 

adequately capture differences in concentrations and deposition patterns across the nation.

Commenters that did not support the adoption or future development of an AAI-based 

secondary NAAQS (e.g., EPRI, UARG, AAM, NCBA, Aluminum Association, and TX) 

expressed the view that a field pilot program was therefore not needed. However, these 

commenters nonetheless expressed the view that if EPA intended to consider such a standard in 

future reviews, the field pilot program would need to expand in coverage and incorporate a much 
30

more comprehensive research program to address data gaps and uncertainties inherent in such an 

approach. These commenters suggested that the field pilot program should be more responsive to 

the issues raised by the members of the CASAC review panel. One commenter (API) expressed 

the view that even if EPA intended to consider such a standard in the future, a field study was not 

appropriate at this time on the basis that the AAI-based approach was still only very preliminary

in nature.

These commenters not supporting the AAI and the field pilot program as proposed 

contended that the proposed program fails to address key scientific uncertainties and data needs 

with regard to a methodology based on the AAI, and cannot meaningfully reduce the 

uncertainties that would be associated with such a standard. Some of these commenters offered 

specific recommendations for areas of research, noted below, that in their view would be 

necessary to support any further consideration of such a standard. For example, these 

commenters contended that it was necessary to conduct research in the following areas before 

further consideration of an AAI-based standard: (1) the effect of other sources, including 

wastewater pollution from permitted or unpermitted sources and fertilization of farm lands, on 

aquatic acidification; (2) relationships between measured air quality and deposition rates and 

related model performance evaluations; (3) improved methods for measuring dry deposition; and 

(4) characterization of NHx concentrations that are representative of specific ecoregions for all 

ecoregions based on a model performance evaluation. More specific suggestions regarding longterm research activities include:

 Exploration of several alternate methods to measure concentrations of SOx, NOy and NHx

in the atmosphere, including methods with high-time resolution and methods that target 

specific components of these three families of chemical compounds; and target methods 

that can differentiate between organic and inorganic constituents of gases and particles. 

 Development of instruments to measure actual dry deposition fluxes of these species 

instead of relying on a combination of ambient concentrations and modeled deposition 

velocities. An emphasis should be given to methods that would be amenable to operation 

in routine networks; these methods should be robust, relatively low-cost, and provide 

continuous data on the flux of SOx, NOy and NHx. Consideration should be given to the 

use of Leaf Areas Indices (LAI) in combination with flux measurements to better develop 

algorithms for estimating dry deposition fluxes that accurately represent the role of the 

forest canopy.

 Collection of actual deposition data in addition to concentrations in order to test the 

suitability of the transference ratio concept, given the role of atmospheric models in 

defining the AAI. In doing so, data should be collected at different locations of any 

regionalization schemes being considered in order to evaluate the spatial variability of 

these ratios across different geospatial classification methods. These data should then be 

compared to model simulations as part of a rigorous model evaluation study, which 

should include more than one atmospheric model and various model configurations. 

 Continued development of atmospheric models such that they include missing emissions 

sources (such as emissions or organic reduced-nitrogen gases) and the accurate temporal 

and spatial distribution of emissions. In addition, atmospheric models should simulate the 

bi-directional flux of NOy and NHx species, which would require better land-use and 

characterization data for use within the modeling systems. Atmospheric models should 

also accurately represent the gas-particle partitioning of chemical compounds, in 
31

particular the formation of ammonium nitrate under different meteorological and air 

quality conditions.

Additional views were expressed by various commenters in regard to implementation, 

site selection and data availability. Many commenters from State agencies and industry agreed 

with the EPA that implementation challenges should be addressed during the course of the field 

pilot program. For example, commenters expressed the view that guidance should emerge for 

monitoring network design accounting for the influence of variability of air concentration and 

deposition patterns within specific ecoregions. Some commenters also noted that much of the 

underlying information for the AAI was based on the Adirondacks and Shenandoah regions 

which are relatively rich data sources and the field pilot program should consider under-sampled 

areas in other parts of country such as the mountainous West. Also, some commenters requested 

that relatively non-acid sensitive areas be included in the field pilot program in the interest of 

broader national applicability or, as one state agency suggested, the availability of a rich data 

base in the Chesapeake Bay region. Some commenters also expressed the view that results from 

the field pilot program would not be available for the next periodic review of the secondary

standards for oxides of nitrogen and sulfur.

Response: Having considered these comments contending that the scope of the field pilot 

program is too limited spatially and not sufficiently comprehensive, EPA maintains that the 

purpose and scope of the pilot studies program as presented in the proposal remain appropriate, 

with further clarification provided in IV.D of the final rule. As summarized in section IV.A of 

the final rule, the primary goal of the field pilot program is to collect and analyze data so as to 

enhance the Agency’s understanding of the degree of protectiveness that would likely be 

afforded by an AAI-based standard. EPA also intends that data generated by this program would 

support development of an appropriate monitoring network for such a standard. This field pilot 

program is not intended to be a research program, but rather to be a more targeted data collection 

and analysis effort, which will be done in conjunction with ongoing research efforts that are 

better suited to address some of the issues raised by commenters on the breadth of the field pilot 

program.

EPA largely agrees that the scope of the field pilot program is not adequate to address 

many of the issues raised by the commenters regarding either the ability to adequately capture air 

quality and deposition patterns in all ecoregions or fully addressing scientific uncertainties 

related to numerous investigations into measurement development methods and biogeochemical 

and atmospheric deposition processes. However, as noted earlier, a field pilot program by

definition is limited in scope and intended to guide future broader applications. Toward that end, 

the field pilot program is intended to provide an intermediate link between initial conceptual 

design and potential future development and adoption of a standard, where the breadth and depth 

of spatial coverage would explicitly be addressed through monitoring network rules and 

implementation guidance. 

The relevant ongoing programs addressing underlying atmospheric deposition 

uncertainties and development of critical load models include EPA’s atmospheric deposition 

research program and the multi-agency National Critical Load Data Base (NCLDB) program, 

respectively. In addition, the NAAQS review process of iterative science review and assessment 
32

provides a framework for evaluating newly available information that may address current data 

gaps and scientific uncertainties. These research programs are appropriate venues for addressing 

comments, including relevant CASAC recommendations, regarding desired improvements in the 

science underlying an AAI-based standard. In light of these ongoing research programs, it is not 

appropriate to duplicate these efforts through an expanded scope of the field pilot program. 

Rather, the most efficient approach is to increase the coordination between the field pilot 

program and these existing efforts, an important theme of the field pilot program white paper 

(section IV.D of the final rule). For example, EPA will explore co-locating planned dry

deposition studies at field pilot program sites that would result in mutually beneficial data 

enhancements that support both pilot program and research program objectives.

With regard to views regarding the importance of water quality monitoring, the EPA 

agrees with comments recommending increased coordination with water quality sampling and 

critical load modeling programs. In addition to working closely with the NCLDB, EPA will 

factor in availability of water quality monitoring data in selecting field pilot program sites. The 

field pilot program has the potential to spur increased water quality monitoring in under-sampled 

areas which would improve confidence in generating ecoregion representative critical loads, as 

well as enhancing longer-term assessment of progress.

In addressing the last group of comments concerning implementation, site selection and 

data availability, EPA offers the following views. The field pilot program does provide an 

opportunity to assist in answering a number of implementation challenges, including the design 

of a future network that could support an AAI-based secondary standard. Toward that end, EPA 

will work closely with its state and local agency partners in utilizing the field pilot program as a 

test case for implementation-based issues. In optimizing the design of a field pilot program, 

emphasis will be placed on relatively acid-sensitive areas given that those are areas an AAIbased standard would be intended to protect. Nevertheless, EPA will consider ecoregions that 

may offer advantages in having multiple deposition-based effects beyond aquatic acidification 

that potentially could support future reviews that consider multiple ecological effects. In 

addition, nearly all ecoregions have a mix of acid-sensitive and non-acid sensitive water bodies 

which will allow for assessing some of the AAI applicability to different aquatic systems. EPA 

also notes that the field pilot program will provide data and analyses that will help inform 

consideration of an AAI-based standard in the next review. For example, data and analyses 

generated as part of the field pilot program will be incorporated into EPA’s characterization of 

environmental factors and evaluations of alternative approaches to specifying the terms of an 

AAI that would be included in the exposure/risk assessment and policy assessment prepared as 

part of the next review.

B. Monitoring Network and Site Selection

Comments: Most commenters expressed the view that the Clean Air Status and Trends Network 

(CASTNET) was an appropriate program to support the field pilot program and a potential AAIbased standard. While government agencies generally supported the use of CASTNET, some 

State organizations suggested that the NCore monitoring network may be more efficient given 

that the costs of adding CASTNET filter packs (CFPs) to NCore locations is less than that of 

adding NOy instruments, which exist at NCore locations, to CASTNET locations. Support also 
33

was expressed by New York State and NESCAUM for the use of rural NCore monitoring 

stations, where appropriate, in combination with CASTNET sites. Some states requested that 

access to the sampling methods and laboratory analyses used in the program and all data results 

be made through a national contract for States and local agencies, a concern related to 

CASTNET operations being managed by EPA. Environmental groups also supported the use of 

CASTNET and encouraged EPA to adopt the multiple stakeholder process of the NCLDB 

program and to align CASTNET sites with the Temporally Integrated Monitoring of Ecosystems 

and Long-Term Monitoring (TIME/LTM) water sampling programs. These water sampling 

programs should also be extended to other under-sampled areas of the country that are acid 

sensitive. Some industry commenters (e.g., API) raised concerns regarding the CFPs as they

have measurement artifacts associated with both mass loss and gain.

Some state agencies commented that states should not be required to fund or implement 

the pilot monitoring studies, and funding should arise from sources other than State and 

Territorial Air Grant (STAG) funds. In related comments, the NPS and environmental groups 

encouraged EPA to make this effort a priority for funding. 

Some commenters, including some industry groups, were less supportive in their 

comments, with some expressing the view that there are very significant data gaps that a field 

monitoring program should be designed to fill. Some of these commenters noted that the field 

program described in the proposal is limited to measuring ambient concentrations with very

coarse technologies. In the view of these commenters, complementary measurements beyond the 

atmospheric indicators that had been considered by EPA should be included, along with 

collocation of multiple measurements at all site locations. Some of these commenters supported 

the selection of monitoring sites that include locations with extreme temperature and relative 

humidity conditions. One commenter suggested that a minimum of three ecoregions in the 

eastern United States should be included in the field pilot program.

Response: The EPA has considered all available monitoring networks in the interest of locating 

the most suitable sites for a pilot study and to effectively leverage resources. The CASTNET 

monitoring program offers substantially more available platforms in acid-sensitive ecoregions 

relative to rural NCore sites and CASTNET sites already include the CFP method for 

measurements of key atmospheric species. Consequently, the financial burden on states, tribes 

and local air monitoring agencies would be less using this existing infrastructure instead of 

expanding measurements at or relocating rural NCore sites. The CASTNET siting design 

originally was intended to discern contributions of acidifying deposition of NOx and SOx to 

sensitive ecosystems, which is especially relevant for the AAI applications. NCore was designed 

as a more generalized network to collect measurements in a variety of geographical areas, with 

no specific focus on acid-sensitive ecosystems. Moreover, CASTNET has established a track 

record over the last two decades of providing quality measurements, whereas NCore is a 

relatively new network that has been fully deployed for less than two years and therefore not 

been subjected to review and analysis commensurate with the CASTNET program. Nevertheless, 

as some states suggested, this pilot program should afford an opportunity to explore the use of 

existing rural NCore sites in acid-sensitive ecoregions. EPA welcomes the inclusion of rural 

NCore sites into the pilot study in cases where there are clear advantages of using such sites, and 

especially where such sites provide additional information likely resulting in more conclusive 
34

data findings. The development of site selection criteria and site selection will be conducted in 

partnership with other federal, state and local agencies. Although CASTNET is managed by 

EPA, the agency has aggressively supported the user community management approach adopted 

in the National Atmospheric Deposition Program (NADP) and views the field pilot program as 

an opportunity to expand ownership of CASTNET analysis and data products, which currently

can be accessed by the public.

While the field pilot program resources are focused on atmospheric measurements, as 

noted above EPA will try to leverage existing water quality monitoring programs such as 

TIME/LTM in selecting field pilot program site locations. EPA would rely heavily on the 

NCLDB critical load work for generating AAI values at monitoring locations as part of the field 

pilot program. In regard to issues raised by commenters regarding artifacts in the CFP, which 

would be the basis for SOx data in the field pilot program, EPA notes that these methods have 

been extensively deployed and evaluated and have exhibited generally excellent performance. As 

part of the CASAC review on measurement methods, CASAC pointed out that the CFPs are 

preferred methods for measuring SOx in rural, low concentration environments due to the 

sensitivity of the CFP method.

C. Complementary Measurements and Instrumentation

Comments: In general, commenters across government agencies, environmental groups and 

industry supported the use of complementary measurements that would be deployed in addition 

to the CFP and NOy instruments used to measure the indicators, NOy and SOx. Comments 

regarding these measurements were provided in different contexts. For example, industry views 

reflected a position that complementary measurements were necessary to address information 

gaps, whereas state agencies and environmental groups expressed more general support in the 

interest of adding additional useful data, but not as a required component of the field pilot 

program. 

Commenters expressed support for including trace gas continuous SO2 and speciated 

PM2.5 measurements in the field pilot program to provide test data for determining the suitability

of continuous SO2 measurements as a Federal Equivalent Method (FEM) for secondary standards 

and to characterize the relationship between CFP-based particulate sulfate and the national 

network of speciation samplers used throughout the state and local air quality networks. Industry

commenters suggested that dry deposition flux measurements be conducted at the field pilot 

program sites, while also indicating that having sites in only 3 to 5 ecoregions would be 

inadequate. Industry commenters also suggested deploying multiple co-located methods 

measuring the same species as a quality assurance step and advocated measuring individual NOy

species. Several commenters suggested adding NADP wet deposition samplers.

There were several comments from industry and government organizations supporting 

the development of a Federal Reference Method (FRM) for NOy and CFP-based SO2 and sulfate 

measurements. Greater attention was addressed to NOy measurements as the technology has only 

recently been used in routine monitoring applications. Some commenters supported EPA’s 

approach of using EPA’s research office to conduct instrument evaluation as a related but 
35

separate program from the field pilot program. Some commenters also recommended testing 

NOy at locations with extreme temperature and relative humidity regimes. 

Response: The EPA appreciates the support expressed by commenters regarding the use of 

complementary measurements. While EPA agrees with views expressing the importance of 

additional measurements, complementary measurements will not have the same funding priority 

as indictor measurements for NOy and SOx. Nevertheless, it is reasonable to expect that all field 

pilot program sites will also include NADP precipitation samplers and NADP passive ammonia 

samplers, both of which are located in roughly half of all CASTNET sites. EPA agrees that the 

formal NOy FRM development should be decoupled from the pilot studies, while recognizing 

that separate NOy measurements are an important component of the pilot study. Although NOy

measurement technology is relatively mature, the effort to develop FRM certification will 

promote more confidence in the data due to standardized operational and quality assurance 

protocols.

D. Collaboration and Stakeholder Participation

Comments: Most commenters agreed with EPA’s intention to broaden review and participation 

in the field pilot program, given that the AAI approach cuts across multiple organizations and 

technical disciplines. Both industry and state governments suggested that some level of initial 

and ongoing external peer review is needed for evaluating design of the field pilot program and 

subsequent data analyses, with one state suggesting using NACAA’s Monitoring Steering 

Committee. Some state commenters also reasoned that an agency’s participation in the pilot 

program should be optional, because some states cannot support additional monitoring even if it 

were to be fully funded. The NPS in particular indicated a desire to participate with EPA in the 

field pilot program. Clearly, many of the comments described above suggesting added emphasis 

on water quality monitoring and research collectively emphasize strengthening the collaborative 

aspects of this field pilot program.

Response: EPA is encouraged by commenters’ interest in the field pilot program. While EPA’s 

Office of Air and Radiation (OAR) will assume primary leadership of this program, EPA OAR 

will take several actions to promote collaboration across internal EPA research programs and 

other government agencies as described in section IV.D of the final rule. Paralleling this effort, 

EPA will solicit comment on a draft white paper to enable ongoing review and input from the 

public. Taking into consideration comments received on the draft white paper, EPA will prepare 

a final white paper that will serve as a program management and communication document. Key

topics areas that will be addressed in the draft white paper are described briefly below.

(1) Background, purpose and products. An introductory section will include a statement of 

program objectives. Since the field pilot program emphasizes atmospheric measurements in 3 

to 5 ecoregions, the role and leveraging opportunities of the field pilot program in relation to 

ongoing water quality modeling, critical load work and atmospheric deposition research will 

be introduced here and expanded upon in later sections. A brief description of intended data 

and analysis products from this effort – highlighting the types of AAI calculations and 

variability analyses of NOy and SOx concentrations and deposition rates -- will be introduced 

here and expanded upon in more depth in the data analysis section. The introductory section 
36

will also describe the process for broader public engagement and review. The EPA intends to 

work closely with two existing venues – the NADP critical load and total deposition science 

committees and the NACAA monitoring subcommittee. Background technical information 

will include an overview of the AAI equation.

(2) Site selection. Factors to be considered in selecting ecoregions for inclusion in the field pilot 

program will be discussed and initial thoughts on recommended locations will be presented. 

Site location considerations include available infrastructure resources (e.g., air monitoring 

site platforms, nearby water quality monitoring, forest/ecosystem experimental studies), 

location in acid-sensitive ecoregions, geographic/ecosystem diversity (e.g., east and west 

locations) and partnership opportunities. To assist in site selection, a series of resource-based 

maps will be available illustrating infrastructure assets and partnership opportunities. A 

paring down of the 22 acid-sensitive ecoregions to less than 10 will be accompanied by

discussion of the merits of each area, and initial thoughts on groupings of 3, 4 and 5 

ecoregions will be provided. This site selection discussion is particularly important as it is 

initiates broader discussion to enlist interest from potential partners.

(3) Linking atmospheric measurements to the AAI. A suite of atmospheric measurements to be 

made at the field pilot program sites will be discussed, with a focus on measurements of 

indicators for oxides of nitrogen and sulfur based on the CFPs and commercial NOy

instruments. Complementary measurements such as ammonia gas, precipitation chemistry

and other monitoring methods under consideration (e.g., speciated NOy samplers and 

continuous SO2) will also be discussed, and information on the potential benefits and costs 

associated with gathering such data will be presented to help inform the program planning 

effort. 

(4) Linking water quality data and critical load calculations to the AAI. The biogeochemical 

components which support the determination of representative critical loads are an important 

complement to atmospheric measurements as both biogeochemical and atmospheric 

processes work together in the calculation of area-specific AAI values. In parallel with the 

atmospheric measurements, a revised set of critical load estimates will be developed under 

the umbrella of the NCLDB, which is addressed by the NADP critical loads science 

committee. In addition to revising critical load estimates, this effort will help inform ongoing 

critical load model evaluation and development efforts. Topic areas to be considered for 

inclusion in the program include evaluating the F-factor approach, implications of sulfate 

retention on steady-state critical load modeling, the role of dynamic models as evaluation 

tools for generating national critical load data and assessment of Neco characterization 

approaches. These critical load model evaluation efforts will focus on field pilot program 

sites contingent on the ability to leverage water quality and soil sampling efforts underway, 

or new efforts that may be spurred by the pilot program. As part of this effort, attention will 

be given to characterizing the critical load representativeness at the ecoregion level using 

existing water quality surveys, which is an important aspect of the AAI equation. 

(5) AAI proof-of-concept. Based on newly assembled atmospheric measurements and critical 

load data, as discussed above, AAI calculations will be performed. As there will be air 

quality measurements in selected ecoregions for the indicators, SOx and NOy, observation-
37

based AAI values can be calculated for those ecoregions in a manner that would be 

illustrative of the implementation of a potential future AAI-based standard. Further, based on 

related national-level efforts to expand the critical load data base and to update CMAQ air 

quality model simulations, a national set of AAI values will be calculated using approaches 

similar to those presented in the PA. In addition, the availability of both site-specific 

measurements and national-scale modeling will allow for assessments that can support 

monitoring network design. 

(6) Coordination with atmospheric deposition research. The field pilot program presents an 

opportunity to leverage planned EPA dry deposition flux studies and a window of time to 

more fully explore atmospheric deposition characterization approaches. These research 

efforts are not part of the field pilot program, but the pilot program provides an opportunity

for mutual leveraging between EPA air management and research efforts. The field pilot 

program offers an extended suite of atmospheric measurements in locations useful for 

diagnosing model (air quality and deposition focused) behavior. Leveraging refers to locating 

planned deposition flux studies at field pilot program locations. In addition, the time window 

afforded by the pilot program allows for continued efforts at improving deposition 

characterizations in general. Examples of improved characterization approaches include the 

use of the Parameter-elevation Regressions on Independent Slopes Model (PRISM) gridded 

precipitation data to improve wet deposition characterization and the merging of modeled 

and observed data in generating the best available deposition estimates. As part of these 

research elements, further evaluation of transference ratios can be addressed and the pilot 

program will include at least one mountainous west ecoregion enabling insights into 

deposition and transference ratio variability in western regions.

(7) Role of the field pilot program and implementation. As discussed in section IV.A above, the 

field pilot program provides an opportunity to further explore the issues that would be 

associated with implementing an AAI-based secondary standard. The field pilot program 

provides a reasonable time period for EPA and states to become familiar with the AAI 

approach and develop practical approaches related to implementation issues. For example, 

such issues are associated with the use of ecoregion boundaries as a planning basis, a focus 

on rural environments, and the myriad reporting and analysis requirements that are inherent 

in implementing any such standard. The draft white paper will outline a process for 

addressing such issues that will involve creating appropriate forums for bringing together 

EPA and state government experts in technical and air management disciplines.

(8) Data Analysis Products. The atmospheric and biogeochemical measurements to be made as 

part of the field pilot program, together with updated model simulations that become 

available during the course of the pilot program, will be used to support analyses that can 

help inform further development and evaluation of the AAI. Such analyses will be identified 

and described in terms of how they could advance our understanding and characterization of 

the degree of protectiveness that would likely be afforded by AAI-based standards that could 

be considered in the next review of the NAAQS for oxides of nitrogen and sulfur.

The field pilot program affords an excellent opportunity to coordinate air quality

monitoring and related critical load and water quality assessment activities (modeling and 

measurements). As part of the planning effort for this program, EPA will engage other federal 
38

agencies (U.S. Geological Survey, NPS, U.S. Forest Service) and state and local agencies 

primarily through existing NADP and NACAA committee structures.

IV. RESPONSES TO LEGAL, ADMINISTRATIVE AND PROCEDURAL ISSUES 

AND MISPLACED COMMENTS

This section is intended to address those comments received regarding legal, 

administrative, and procedural issues as well as issues raised in public comments that are not 

considered when setting a NAAQS. Many legal comments are presented in previous sections of 

this document and throughout the rule. Comments included here address issues related to 

interactions with CASAC, the EPA’s authority to define a new indicator of ambient air quality, 

and responsibility of the EPA under tribal trust agreements. Additionally, a number of comments 

were submitted related to implementation issues and regulatory impact analyses. As the EPA is 

not permitted to consider implementation-related issues or the cost associated with meeting a 

standard when developing NAAQS, these comments are considered misplaced as they are not 

relevant to determining the appropriate secondary NAAQS for oxides of nitrogen and sulfur. We 

note that some implementation issues are generally addressed for informational purposes only in 

section I.C of the preamble to the final rule.

A. Legal, Administrative and Procedural Issues

(1) Comment: One commenter (NESCAUM) expressed concern that the proposal was not 

consistent with CASAC’s recommendation to apply an ecologically relevant level and form in 

this review. 

Response: In considering CASAC’s advice and recommendations, the EPA pays close attention 

to the content of the advice and weighs it carefully, whether it is advice on the science or advice 

on policy matters related to new or revised NAAQS. The EPA in fact has done this in this 

review, as seen in the many and varied revisions made to the draft documents CASAC has 

reviewed and commented on, such as the ISA, the Risk and Exposure Assessment and the Policy

Assessment. It can also be seen in the careful attention paid by the Administrator to CASAC’s 

advice in both the proposed rule and in the final rule. The Administrator has carefully explained 

where she has accepted CASAC’s advice and where she has not, and explained in detail her 

reasoning. We acknowledge that in previous comments on the Policy Assessment, CASAC 

expressed support for setting ecologically relevant standards but also identified areas of concern 

with regard to uncertainty in modeling and data. The EPA is not, however, required to follow 

CASAC’s recommendations where the Administrator explains the reasons for such differences 

where she differs in important respects from CASAC’s advice (CAA section 307(d) (3), (d) (6)).

(2) Comment: Some commenters asserted that the consideration of NOy as the indicator for 

oxides of nitrogen in the AAI is contrary to the EPA’s statutory authority. One such commenter 

asserted that NOy was a broader indicator than had been defined in listing oxides of nitrogen as a 

criteria pollutant and as such the EPA was prevented from modifying the scope of the criteria 

pollutant addressed in this review.
39

Response: While this issue will be addressed more fully in future reviews, the EPA disagrees 

that NOy, as considered in this review, is beyond what is permissible in conjunction with the 

listed criteria pollutant for oxides of nitrogen. In the primary standards, NO2 is used as the 

indicator of oxides of nitrogen because it is relevant to the exposures of concern to public health. 

Similarly, in this review NOy is considered as the indicator of oxides of nitrogen in that it is 

considered to be the most relevant indicator of exposures of concern to aquatic ecosystems with 

regard to acidifying deposition. 

(2) Comment: Two tribes, the Forest County Potawatomi Community and the Fond du Lac 

Reservation, commented that failure of the EPA to implement standards and determine adequacy

with regard to mercury methylation and deposition of oxides of sulfur represents a violation of 

tribal trust responsibilities. The commenters explained that methylation of mercury from sulfur 

deposition continues to be a significant adverse effect on tribal lands and that the EPA has the 

responsibility to prevent such effects. 

Response: The adequacy of the current standards to address mercury methylation is discussed in 

section II.B.2. above and in greater detail in the final rule section II.B. With regard to tribal trust, 

the EPA agrees with the commenters that the definition of “tribal implications” can also include 

rule impacts to tribal resources, whether occurring on reservations or on lands managed by 

federal agencies under the federal Trust Responsibility. These resources include tribes’ lands, 

waters, and natural, religious and cultural resources. NAAQS are intended to provide additional 

protection, as warranted, to adversely impacted systems and as such are expected to also provide 

additional protection to such resources that fall on tribal lands or lands held in trust by federal 

agencies. On August 3, 2011, the EPA sent letters to all Tribal leaders offering to consult with 

the tribes on the proposed rule. On October 6, 2011 the EPA held a call consultation call with the

Forest County Potawatomi Community and 5 other tribes participated for informational 

purposes.

With respect to EO 13175, the EPA has determined however, that this rule does not have 

tribal implications. The rule concerns the establishment of a secondary Oxides of Nitrogen and 

Oxides of Sulfur NAAQS. The Tribal Authority Rule gives Tribes the opportunity to develop 

and implement CAA programs such as the PM NAAQS, but it leaves to the discretion of the 

Tribe whether to develop these programs and which programs, or appropriate elements of a 

program, they will adopt. Thus, the rule does not have substantial direct effects on Tribes, which 

means it does not have tribal implications and is not subject to EO 13175. Nevertheless, the 

Agency has, as explained in section x, worked to obtain timely and meaningful input from Indian 

Tribes and their leaders.

B. Misplaced Comments

Comment: Several commenters stated that additional implementation language should have 

been included in the proposal both with regard how the EPA would implement an AAI-based 

standard in the future and with regard to the proposed decision to set the secondary NAAQS for 

oxides of nitrogen and sulfur equal to the primary NAAQS for NO2 and SO2. Some commenters 

also noted that further regulatory analyses such as regulatory impact analyses and regulatory

flexibility analyses should have been conducted. 
40

Response: Comments concerning implementation of a NAAQS are legally irrelevant in 

determining which standards are requisite to protect public welfare. (API v. Costle, 665 F. 2d at 

1185-86). The Clean Air Act also specifically prohibits the EPA from considering costs

associated with implementation or setting of NAAQS. The EPA therefore did not consider these

comments in its decision making process on the NAAQS standard.

V. REFERENCES

Russell, A and J. M. Samet, 2011a. Review of the Policy Assessment for the Review of the 

Secondary National Ambient Air Quality Standard for NOx and SOx: FINAL. EPACASAC-11-005.

U.S. EPA 2008. Integrated Science Assessment (ISA) for Oxides of Nitrogen and Sulfur 

Ecological Criteria (Final Report). U.S. Environmental Protection Agency, Washington, 

D.C., EPA/600/R-08/082F, 2008.

U.S. EPA 2009. Risk and Exposure Assessment for Review of the Secondary National Ambient 

Air Quality Standards for Oxides of Nitrogen and Oxides of Sulfur-Main Content - Final 

Report. U.S. Environmental Protection Agency, Washington, D.C., EPA-452/R-09-008a.

U.S. EPA 2011. Policy Assessment for the Review of the Secondary National Ambient Air 

Quality Standards for Oxides of Nitrogen and Oxides of Sulfur. U.S. Environmental 

Protection Agency, Washington, D.C., EPA-452/R-11-005a.