Vol. 77 Tuesday, 

No. 64 April 3, 2012 

Part III 

Environmental Protection Agency 

40 CFR Part 50 

Secondary National Ambient Air Quality Standards for Oxides of Nitrogen 

and Sulfur; Final Rule 


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20218 Federal Register / Vol. 77, No. 64 / Tuesday, April 3, 2012 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 50 

[EPA–HQ–OAR–2007–1145; FRL–9654–4] 

RIN 2060–AO72 

Secondary National Ambient Air 

Quality Standards for Oxides of 

Nitrogen and Sulfur 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final rule. 

SUMMARY: This final rule is being issued 

as required by a consent decree 

governing the schedule for completion 

of this review of the air quality criteria 

and the secondary national ambient air 

quality standards (NAAQS) for oxides of 

nitrogen and oxides of sulfur. Based on 

its review, the EPA is retaining the 

current nitrogen dioxide (NO2) and 

sulfur dioxide (SO2) secondary 

standards to address the direct effects 

on vegetation of exposure to gaseous 

oxides of nitrogen and sulfur and, for 

reasons described in detail in this final 

preamble, is not adding new standards 

at this time to address effects associated 

with the deposition of oxides of 

nitrogen and sulfur on sensitive aquatic 

and terrestrial ecosystems. In addition, 

in this rule the EPA describes a field 

pilot program being developed to 

enhance our understanding of the 

degree of protectiveness that would 

likely be afforded by a multi-pollutant 

standard to address deposition-related 

acidification of sensitive aquatic 

ecosystems. 

DATES: This final rule is effective on 

June 4, 2012. 

ADDRESSES: The EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2007–1145. All 

documents in the docket are listed in 

the www.regulations.gov index. 

Although listed in the index, some 

information is not publicly available, 

e.g., confidential business information 

(CBI) or other information whose 

disclosure is restricted by statute. 

Certain other material, such as 

copyrighted material, will be publicly 

available only in hard copy. Publicly 

available docket materials are available 

either electronically in 

www.regulations.gov or in hard copy at 

the Air and Radiation Docket and 

Information Center, EPA/DC, EPA West, 

Room 3334, 1301 Constitution Ave. 

NW., Washington, DC. The Public 

Reading Room is open from 8:30 a.m. to 

4:30 p.m., Monday through Friday, 

excluding legal holidays. The telephone 

number for the Public Reading Room is 

(202) 566–1744 and the telephone 

number for the Air and Radiation 

Docket and Information Center is (202) 

566–1742. 

FOR FURTHER INFORMATION CONTACT: Mrs. 

Ginger Tennant, Office of Air Quality 

Planning and Standards (OAQPS), U.S. 

Environmental Protection Agency, Mail 

Code C504–06, Research Triangle Park, 

NC 27711; telephone: 919–541–4072; 

fax: 919–541–0237; email: 

tennant.ginger@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Table of Contents 

The following topics are discussed in 

this preamble: 

I. Background 

A. Legislative Requirements 

B. History of Reviews of NAAQS for 

Nitrogen Oxides and Sulfur Oxides 

1. NAAQS for Oxides of Nitrogen 

2. NAAQS for Oxides of Sulfur 

C. History of Related Assessments and 

Agency Actions 

D. History of the Current Review 

E. Scope of the Current Review 

1. Scope Presented in the Proposal 

2. Comments on the Scope of the Review 

II. Rationale for Final Decisions on the 

Adequacy of the Current Secondary 

Standards 

A. Introduction 

1. Overview of Effects 

a. Effects Associated With Gas-Phase 

Oxides of Nitrogen and Sulfur 

b. Effects Associated With Deposition of 

Oxides of Nitrogen and Sulfur 

2. Overview of Risk and Exposure 

Assessment 

a. Approach to REA Analyses 

b. Key Findings 

c. Other Welfare Effects 

3. Overview of Adversity of Effects to 

Public Welfare 

a. Ecosystem Services 

b. Effects on Ecosystem Services 

c. Summary 

B. Adequacy of the Current Standards 

1. Adequacy Considerations 

a. Adequacy of the Current Standards for 

Direct Effects 

b. Appropriateness and Adequacy of the 

Current Standards for Deposition-Related 

Effects 

c. Summary of Adequacy Considerations 

2. CASAC Views 

3. Administrator’s Proposed Conclusions 

C. Comments on Adequacy of the Current 

Standards 

1. Adequacy of the Current Standards To 

Address Direct Effects 

2. Adequacy of the Current Secondary 

Standards To Address DepositionRelated Effects 

D. Final Decisions on the Adequacy of the 

Current Standards 

III. Rationale for Final Decisions on 

Alternative Secondary Standards 

A. Overview of AAI Approach 

1. Ambient Air Indicators 

a. Oxides of Sulfur 

b. Oxides of Nitrogen 

2. Form 

a. Ecological Indicator 

b. Linking ANC to Deposition 

c. Linking Deposition to Ambient Air 

Indicators 

d. Aquatic Acidification Index 

e. Spatial Aggregation 

f. Summary of the AAI Form 

3. Averaging Time 

4. Level 

5. Characterization of Uncertainties 

B. CASAC Views 

C. Proposed Conclusions on Alternative 

Secondary Standards 

D. Comments on Alternative Secondary 

Standards 

1. Comments Related to an AAI-Based 

Standard 

a. Comments on Consideration of an AAIBased Standard 

b. Comments on Specific Aspects of an 

AAI-Based Approach 

2. Comments on 1-Hour NO2 and SO2

Secondary Standards 

E. Final Decisions on Alternative 

Secondary Standards for Oxides of 

Nitrogen and Sulfur 

IV. Field Pilot Program and Ambient 

Monitoring 

A. Overview of Proposed Field Pilot 

Program 

1. Complementary Measurements 

2. Complementary Areas of Research 

3. Implementation Challenges 

4. Monitoring Plan Development and 

Stakeholder Participation 

B. Summary of Proposed Evaluation of 

Monitoring Methods 

C. Comments on Field Pilot Program and 

Monitoring Methods Evaluation 

1. Goals, Objectives and Scope of Field 

Pilot Program 

2. Network Design and Role of CASTNET 

3. Complementary Measurements and 

Instrumentation 

4. Collaboration 

V. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

B. Paperwork Reduction Act 

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

and Safety Risks 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution, or Use 

I. National Technology Transfer and 

Advancement Act 

J. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

K. Congressional Review Act 

References 


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1The legislative history of Section 109 of the 

CAA indicates that a primary standard is to be set 

at ‘‘the maximum permissible ambient air level 

* * * which will protect the health of any 

[sensitive] group of the population,’’ and that for 

this purpose ‘‘reference should be made to a 

representative sample of persons comprising the 

sensitive group rather than to a single person in 

such a group’’ S. Rep. No. 91–1196, 91st Cong., 2d 

Sess. 10 (1970). 

I. Background 

A. Legislative Requirements 

Two sections of the Clean Air Act 

(CAA) govern the establishment and 

revision of the NAAQS. Section 108 (42 

U.S.C. Section 7408) directs the 

Administrator to identify and list 

certain air pollutants and then to issue 

air quality criteria for those pollutants. 

The Administrator is to list those air 

pollutants that in her ‘‘judgment, cause 

or contribute to air pollution which may 

reasonably be anticipated to endanger 

public health or welfare;’’ ‘‘the presence 

of which in the ambient air results from 

numerous or diverse mobile or 

stationary sources;’’ and ‘‘for which 

* * * [the Administrator] plans to issue 

air quality criteria * * *’’ Air quality 

criteria are intended to ‘‘accurately 

reflect the latest scientific knowledge 

useful in indicating the kind and extent 

of all identifiable effects on public 

health or welfare which may be 

expected from the presence of [a] 

pollutant in the ambient air * * *’’ 42 

U.S.C. Section 7408(b). Section 109 (42 

U.S.C. 7409) directs the Administrator 

to propose and promulgate ‘‘primary’’ 

and ‘‘secondary’’ NAAQS for pollutants 

for which air quality criteria are issued. 

Section 109(b)(1) defines a primary 

standard as one ‘‘the attainment and 

maintenance of which in the judgment 

of the Administrator, based on such 

criteria and allowing an adequate 

margin of safety, are requisite to protect 

the public health.’’ 1 A secondary 

standard, as defined in Section 

109(b)(2), must ‘‘specify a level of air 

quality the attainment and maintenance 

of which, in the judgment of the 

Administrator, based on such criteria, is 

requisite to protect the public welfare 

from any known or anticipated adverse 

effects associated with the presence of 

[the] pollutant in the ambient air.’’ 

Welfare effects as defined in Section 

302(h) (42 U.S.C. Section 7602(h)) 

include, but are not limited to, ‘‘effects 

on soils, water, crops, vegetation, manmade materials, animals, wildlife, 

weather, visibility and climate, damage 

to and deterioration of property, and 

hazards to transportation, as well as 

effects on economic values and on 

personal comfort and well-being.’’ 

In setting standards that are 

‘‘requisite’’ to protect public health and 

welfare, as provided in Section 109(b), 

the EPA’s task is to establish standards 

that are neither more nor less stringent 

than necessary for these purposes. In so 

doing, the EPA may not consider the 

costs of implementing the standards. 

See generally, Whitman v. American 

Trucking Associations, 531 U.S. 457, 

465–472, 475–76 (2001). Likewise, 

‘‘[a]ttainability and technological 

feasibility are not relevant 

considerations in the promulgation of 

national ambient air quality standards’’ 

(American Petroleum Institute v. Costle, 

665 F. 2d at 1185). Section 109(d)(1) 

requires that ‘‘not later than December 

31, 1980, and at 5-year intervals 

thereafter, the Administrator shall 

complete a thorough review of the 

criteria published under Section 108 

and the national ambient air quality 

standards * * * and shall make such 

revisions in such criteria and standards 

and promulgate such new standards as 

may be appropriate * * *.’’ Section 

109(d)(2) requires that an independent 

scientific review committee ‘‘shall 

complete a review of the criteria * * * 

and the national primary and secondary 

ambient air quality standards * * * and 

shall recommend to the Administrator 

any new * * * standards and revisions 

of existing criteria and standards as may 

be appropriate * * *.’’ Since the early 

1980’s, this independent review 

function has been performed by the 

Clean Air Scientific Advisory 

Committee (CASAC). 

B. History of Reviews of NAAQS for 

Nitrogen Oxides and Sulfur Oxides 

1. NAAQS for Oxides of Nitrogen 

After reviewing the relevant science 

on the public health and welfare effects 

associated with oxides of nitrogen, the 

EPA promulgated identical primary and 

secondary NAAQS for NO2 in April 

1971. These standards were set at a level 

of 0.053 parts per million (ppm) as an 

annual average (36 FR 8186). In 1982, 

the EPA published Air Quality Criteria 

Document for Oxides of Nitrogen (U.S. 

EPA, 1982), which updated the 

scientific criteria upon which the initial 

standards were based. In February 1984, 

the EPA proposed to retain the 

standards set in 1971 (49 FR 6866). 

After taking into account public 

comments, the EPA published the final 

decision to retain these standards in 

June 1985 (50 FR 25532). 

The EPA began the most recent 

previous review of the oxides of 

nitrogen secondary standards in 1987. 

In November 1991, the EPA released an 

updated draft air quality criteria 

document (AQCD) for CASAC and 

public review and comment (56 FR 

59285), which provided a 

comprehensive assessment of the 

available scientific and technical 

information on health and welfare 

effects associated with NO2 and other 

oxides of nitrogen. The CASAC 

reviewed the draft document at a 

meeting held on July 1, 1993, and 

concluded in a closure letter to the 

Administrator that the document 

‘‘provides a scientifically balanced and 

defensible summary of current 

knowledge of the effects of this 

pollutant and provides an adequate 

basis for the EPA to make a decision as 

to the appropriate NAAQS for NO2’’ 

(Wolff, 1993). The AQCD for Oxides of 

Nitrogen was then finalized (U.S. EPA, 

1995a). The EPA also prepared a Staff 

Paper that summarized and integrated 

the key studies and scientific evidence 

contained in the revised AQCD for 

oxides of nitrogen and identified the 

critical elements to be considered in the 

review of the NO2 NAAQS. The CASAC 

reviewed two drafts of the Staff Paper 

and concluded in a closure letter to the 

Administrator that the document 

provided a ‘‘scientifically adequate basis 

for regulatory decisions on nitrogen 

dioxide’’ (Wolff, 1995). 

In October 1995, the Administrator 

announced her proposed decision not to 

revise either the primary or secondary 

NAAQS for NO2 (60 FR 52874; October 

11, 1995). A year later, the 

Administrator made a final 

determination not to revise the NAAQS 

for NO2 after careful evaluation of the 

comments received on the proposal (61 

FR 52852; October 8, 1996). While the 

primary NO2 standard was revised in 

January 2010, by supplementing the 

existing annual standard with the 

establishment of a new 1-hour standard, 

set at a level of 100 parts per billion 

(ppb) (75 FR 6474), the secondary 

NAAQS for NO2 remains 0.053 ppm 

(100 micrograms per cubic meter [mg/ 

m3] of air), annual arithmetic average, 

calculated as the arithmetic mean of the 

1-hour NO2 concentrations. 

2. NAAQS for Oxides of Sulfur 

The EPA promulgated primary and 

secondary NAAQS for SO2 in April 

1971 (36 FR 8186). The secondary 

standards included a standard set at 

0.02 ppm, annual arithmetic mean, and 

a 3-hour average standard set at 0.5 

ppm, not to be exceeded more than once 

per year. These secondary standards 

were established solely on the basis of 

evidence of adverse effects on 

vegetation. In 1973, revisions made to 

Chapter 5 (‘‘Effects of Sulfur Oxide in 

the Atmosphere on Vegetation’’) of the 

AQCD for Sulfur Oxides (U.S. EPA, 

1973) indicated that it could not 


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properly be concluded that the 

vegetation injury reported resulted from 

the average SO2 exposure over the 

growing season, rather than from shortterm peak concentrations. Therefore, the 

EPA proposed (38 FR 11355) and then 

finalized (38 FR 25678) a revocation of 

the annual mean secondary standard. At 

that time, the EPA was aware that thencurrent concentrations of oxides of 

sulfur in the ambient air had other 

public welfare effects, including effects 

on materials, visibility, soils, and water. 

However, the available data were 

considered insufficient to establish a 

quantitative relationship between 

specific ambient concentrations of 

oxides of sulfur and such public welfare 

effects (38 FR 25679). 

In 1979, the EPA announced that it 

was revising the AQCD for oxides of 

sulfur concurrently with that for 

particulate matter (PM) and would 

produce a combined PM and oxides of 

sulfur criteria document. Following its 

review of a draft revised criteria 

document in August 1980, CASAC 

concluded that acid deposition was a 

topic of extreme scientific complexity 

because of the difficulty in establishing 

firm quantitative relationships among 

(1) Emissions of relevant pollutants 

(e.g., SO2 and oxides of nitrogen), (2) 

formation of acidic wet and dry 

deposition products, and (3) effects on 

terrestrial and aquatic ecosystems. The 

CASAC also noted that acid deposition 

involves, at a minimum, several 

different criteria pollutants: oxides of 

sulfur, oxides of nitrogen, and the fine 

particulate fraction of suspended 

particles. The CASAC felt that any 

document on this subject should 

address both wet and dry deposition, 

since dry deposition was believed to 

account for a substantial portion of the 

total acid deposition problem. 

For these reasons, CASAC 

recommended that a separate, 

comprehensive document on acid 

deposition be prepared prior to any 

consideration of using the NAAQS as a 

regulatory mechanism for the control of 

acid deposition. The CASAC also 

suggested that a discussion of acid 

deposition be included in the AQCDs 

for oxides of nitrogen and PM and 

oxides of sulfur. Following CASAC 

closure on the AQCD for oxides of 

sulfur in December 1981, the EPA 

published a Staff Paper in November 

1982, although the paper did not 

directly assess the issue of acid 

deposition. Instead, the EPA 

subsequently prepared the following 

documents to address acid deposition: 

The Acidic Deposition Phenomenon 

and Its Effects: Critical Assessment 

Review Papers, Volumes I and II (U.S. 

EPA, 1984a, b) and The Acidic 

Deposition Phenomenon and Its Effects: 

Critical Assessment Document (U.S. 

EPA, 1985) (53 FR 14935–14936). These 

documents, though they were not 

considered criteria documents and did 

not undergo CASAC review, represented 

the most comprehensive summary of 

scientific information relevant to acid 

deposition completed by the EPA at that 

point. 

In April 1988 (53 FR 14926), the EPA 

proposed not to revise the existing 

primary and secondary standards for 

SO2. This proposed decision with regard 

to the secondary SO2 NAAQS was due 

to the Administrator’s conclusions that: 

(1) Based upon the then-current 

scientific understanding of the acid 

deposition problem, it would be 

premature and unwise to prescribe any 

regulatory control program at that time; 

and (2) when the fundamental scientific 

uncertainties had been decreased 

through ongoing research efforts, the 

EPA would draft and support an 

appropriate set of control measures. 

Although the EPA revised the primary 

SO2 standard in June 2010 by 

establishing a new 1-hour standard at a 

level of 75 ppb and revoking the 

existing 24-hour and annual standards 

(75 FR 35520), no further decision on 

the secondary SO2 standard has been 

published. 

C. History of Related Assessments and 

Agency Actions 

In 1980, the Congress created the 

National Acid Precipitation Assessment 

Program (NAPAP) in response to 

growing concern about acidic 

deposition. The NAPAP was given a 

broad 10-year mandate to examine the 

causes and effects of acidic deposition 

and to explore alternative control 

options to alleviate acidic deposition 

and its effects. During the course of the 

program, the NAPAP issued a series of 

publicly available interim reports prior 

to the completion of a final report in 

1990 (NAPAP, 1990). 

In spite of the complexities and 

significant remaining uncertainties 

associated with the acid deposition 

problem, it soon became clear that a 

program to address acid deposition was 

needed. The CAA Amendments of 1990 

included numerous separate provisions 

related to the acid deposition problem. 

The primary and most important of the 

provisions, the amendments to Title IV 

of the Act, established the Acid Rain 

Program to reduce emissions of SO2 by 

10 million tons and emissions of 

nitrogen oxides by 2 million tons from 

1980 emission levels in order to achieve 

reductions over broad geographic 

regions. In this provision, Congress 

included a statement of findings that led 

them to take action, concluding that (1) 

The presence of acid compounds and 

their precursors in the atmosphere and 

in deposition from the atmosphere 

represents a threat to natural resources, 

ecosystems, materials, visibility, and 

public health; (2) the problem of acid 

deposition is of national and 

international significance; and (3) 

current and future generations of 

Americans will be adversely affected by 

delaying measures to remedy the 

problem. 

Second, Congress authorized the 

continuation of the NAPAP in order to 

assure that the research and monitoring 

efforts already undertaken would 

continue to be coordinated and would 

provide the basis for an impartial 

assessment of the effectiveness of the 

Title IV program. 

Third, Congress considered that 

further action might be necessary in the 

long-term to address any problems 

remaining after implementation of the 

Title IV program and, reserving 

judgment on the form that action could 

take, included Section 404 of the 1990 

Amendments (CAA Amendments of 

1990, Pub. L. 101–549, Section 404) 

requiring the EPA to conduct a study on 

the feasibility and effectiveness of an 

acid deposition standard or standards to 

protect ‘‘sensitive and critically 

sensitive aquatic and terrestrial 

resources.’’ At the conclusion of the 

study, the EPA was to submit a report 

to Congress. Five years later, the EPA 

submitted its report, entitled Acid 

Deposition Standard Feasibility Study: 

Report to Congress (U.S. EPA, 1995b) in 

fulfillment of this requirement. That 

report concluded that establishing acid 

deposition standards for sulfur and 

nitrogen deposition may at some point 

in the future be technically feasible, 

although appropriate deposition loads 

for these acidifying chemicals could not 

be defined with reasonable certainty at 

that time. 

Fourth, the 1990 Amendments also 

added new language to sections of the 

CAA pertaining to the scope and 

application of the secondary NAAQS 

designed to protect the public welfare. 

Specifically, the definition of ‘‘effects on 

welfare’’ in Section 302(h) was 

expanded to state that the welfare 

effects include effects ‘‘* * * whether 

caused by transformation, conversion, 

or combination with other air 

pollutants.’’ 

In 1999, seven Northeastern states 

cited this amended language in Section 

302(h) in a petition asking the EPA to 

use its authority under the NAAQS 

program to promulgate secondary 

NAAQS for the criteria pollutants 


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2Center for Biological Diversity, et al. v. Johnson, 

No. 05–1814 (D.D.C.). 

associated with the formation of acid 

rain. The petition stated that this 

language ‘‘clearly references the 

transformation of pollutants resulting in 

the inevitable formation of sulfate and 

nitrate aerosols and/or their ultimate 

environmental impacts as wet and dry 

deposition, clearly signaling 

Congressional intent that the welfare 

damage occasioned by sulfur and 

nitrogen oxides be addressed through 

the secondary standard provisions of 

Section 109 of the Act.’’ The petition 

further stated that ‘‘recent federal 

studies, including the NAPAP Biennial 

Report to Congress: An Integrated 

Assessment, document the continued 

and increasing damage being inflicted 

by acid deposition to the lakes and 

forests of New York, New England and 

other parts of our nation, demonstrating 

that the Title IV program had proven 

insufficient.’’ The petition also listed 

other adverse welfare effects associated 

with the transformation of these criteria 

pollutants, including impaired 

visibility, eutrophication of coastal 

estuaries, global warming, and 

tropospheric ozone and stratospheric 

ozone depletion. 

In a related matter, the Office of the 

Secretary of the U.S. Department of 

Interior (DOI) requested in 2000, that 

the EPA initiate a rulemaking 

proceeding to enhance the air quality in 

national parks and wilderness areas in 

order to protect resources and values 

that are being adversely affected by air 

pollution. Included among the effects of 

concern identified in the request were 

the acidification of streams, surface 

waters, and/or soils; eutrophication of 

coastal waters; visibility impairment; 

and foliar injury from ozone. 

In a Federal Register notice in 2001 

(65 FR 48699), the EPA announced 

receipt of these requests and asked for 

comment on the issues raised in them. 

The EPA stated that it would consider 

any relevant comments and information 

submitted, along with the information 

provided by the petitioners and DOI, 

before making any decision concerning 

a response to these requests for 

rulemaking. 

The 2005 NAPAP report states that 

‘‘* * * scientific studies indicate that 

the emission reductions achieved by 

Title IV are not sufficient to allow 

recovery of acid-sensitive ecosystems. 

Estimates from the literature of the 

scope of additional emission reductions 

that are necessary in order to protect 

acid-sensitive ecosystems range from 

approximately 40–80 percent beyond 

full implementation of Title IV * * *.’’ 

The results of the modeling presented in 

this Report to Congress indicate that 

broader recovery is not predicted 

without additional emission reductions 

(NAPAP, 2005). 

Given the state of the science as 

described in the Integrated Science 

Assessment (ISA), Risk and Exposure 

Assessment (REA), and in other recent 

reports, such as the NAPAP reports 

noted above, the EPA has decided, in 

the context of evaluating the adequacy 

of the current NO2 and SO2 secondary 

standards in this review, to revisit the 

question of the appropriateness of 

setting secondary NAAQS to address 

remaining known or anticipated adverse 

public welfare effects resulting from the 

acidic and nutrient deposition of these 

criteria pollutants. 

D. History of the Current Review 

The EPA initiated this current review 

in December 2005, with a call for 

information (70 FR 73236) for the 

development of a revised ISA. An 

Integrated Review Plan (IRP) was 

developed to provide the framework 

and schedule as well as the scope of the 

review and to identify policy-relevant 

questions to be addressed in the 

components of the review. The IRP was 

released in 2007 (U.S. EPA, 2007) for 

CASAC and public review. The EPA 

held a workshop in July 2007 on the ISA 

to obtain broad input from the relevant 

scientific communities. This workshop 

helped to inform the preparation of the 

first draft ISA, which was released for 

CASAC and public review in December 

2007; a CASAC meeting was held on 

April 2–3, 2008, to review the first draft 

ISA. A second draft ISA was released for 

CASAC and public review in August 

2008, and was discussed at a CASAC 

meeting held on October 1–2, 2008. The 

final ISA (U.S. EPA, 2008) was released 

in December 2008. 

Based on the science presented in the 

ISA, the EPA developed the REA to 

further assess the national impact of the 

effects documented in the ISA. The 

Draft Scope and Methods Plan for Risk/ 

Exposure Assessment: Secondary 

NAAQS Review for Oxides of Nitrogen 

and Oxides of Sulfur outlining the scope 

and design of the future REA was 

prepared for CASAC consultation and 

public review in March 2008. A first 

draft REA was presented to CASAC and 

the public for review in August 2008, 

and a second draft was presented for 

review in June 2009. The final REA 

(U.S. EPA, 2009) was released in 

September 2009. A first draft Policy 

Assessment (PA) was released in March 

2010, and reviewed by CASAC on April 

1–2, 2010. In a June 22, 2010, letter to 

the Administrator, CASAC provided 

advice and recommendations to the 

Agency concerning the first draft PA 

(Russell and Samet, 2010a). A second 

draft PA was released to CASAC and the 

public in September 2010, and reviewed 

by CASAC on October 6–7, 2010. The 

CASAC provided advice and 

recommendations to the Agency 

regarding the second draft PA in a 

December 9, 2010 letter (Russell and 

Samet 2010b). The CASAC and public 

comments on the second draft PA were 

considered by the EPA staff in 

developing a final PA (U.S. EPA, 2011). 

CASAC requested an additional meeting 

to provide additional advice to the 

Administrator based on the final PA on 

February 15–16, 2011. On January 14, 

2011 the EPA released a version of the 

final PA prior to final document 

production, to provide sufficient time 

for CASAC review of the document in 

advance of this meeting. The final PA, 

incorporating final reference checks and 

document formatting, was released in 

February 2011. In a May 17, 2011, letter 

(Russell and Samet, 2011a), CASAC 

offered additional advice and 

recommendations to the Administrator 

with regard to the review of the 

secondary NAAQS for oxides of 

nitrogen and oxides of sulfur. 

In 2005, the Center for Biological 

Diversity and four other plaintiffs filed 

a complaint alleging that the EPA had 

failed to complete the current review 

within the period provided by statute.2

The schedule for completion of this 

review is governed by a consent decree 

resolving that lawsuit and the 

subsequent extension agreed to by the 

parties. The schedule presented in the 

original consent decree that governs this 

review, entered by the court on 

November 19, 2007, was revised on 

October 22, 2009 to allow for a 17- 

month extension of the schedule. The 

current decree provides that the EPA 

sign for publication notices of proposed 

and final rulemaking concerning its 

review of the oxides of nitrogen and 

oxides of sulfur NAAQS no later than 

July 12, 2011 and March 20, 2012, 

respectively. 

This action presents the 

Administrator’s final decisions on the 

review of the current secondary oxides 

of nitrogen and oxides of sulfur 

standards. Throughout this preamble a 

number of conclusions, findings, and 

determinations by the Administrator are 

noted. 

E. Scope of the Current Review 

1. Scope Presented in the Proposal 

In conducting this periodic review of 

the secondary NAAQS for oxides of 

nitrogen and oxides of sulfur, as 

discussed in the IRP and REA, the EPA 


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decided to assess the scientific 

information, associated risks, and 

standards relevant to protecting the 

public welfare from adverse effects 

associated jointly with oxides of 

nitrogen and sulfur. Although the EPA 

has historically adopted separate 

secondary standards for oxides of 

nitrogen and oxides of sulfur, the EPA 

is conducting a joint review of these 

standards because oxides of nitrogen 

and sulfur, and their associated 

transformation products are linked from 

an atmospheric chemistry perspective, 

as well as from an environmental effects 

perspective. The National Research 

Council (NRC) has recommended that 

the EPA consider multiple pollutants, as 

appropriate, in forming the scientific 

basis for the NAAQS (NRC, 2004). As 

discussed in the ISA and REA, there is 

a strong basis for considering these 

pollutants together, building upon the 

EPA’s past recognition of the 

interactions of these pollutants and on 

the growing body of scientific 

information that is now available related 

to these interactions and associated 

ecological effects. 

In defining the scope of this review, 

it must be considered that the EPA has 

set secondary standards for two other 

criteria pollutants related to oxides of 

nitrogen and sulfur: ozone (O3) and PM. 

Oxides of nitrogen are precursors to the 

formation of ozone in the atmosphere, 

and under certain conditions, can 

combine with atmospheric ammonia to 

form ammonium nitrate, a component of 

fine PM. Oxides of sulfur are precursors 

to the formation of particulate sulfate, 

which is a significant component of fine 

PM in many parts of the United States. 

There are a number of welfare effects 

directly associated with ozone and fine 

PM, including ozone-related damage to 

vegetation and PM-related visibility 

impairment. Protection against those 

effects is provided by the ozone and fine 

PM secondary standards. This review 

focuses on evaluation of the protection 

provided by secondary standards for 

oxides of nitrogen and sulfur for two 

general types of effects: (1) direct effects 

on vegetation associated with exposure 

to gaseous oxides of nitrogen and sulfur 

in the ambient air, which are the effects 

that the current NO2 and SO2 secondary 

standards protect against; and (2) effects 

associated with the deposition of oxides 

of nitrogen and sulfur to sensitive 

aquatic and terrestrial ecosystems, 

including deposition in the form of 

particulate nitrate and particulate 

sulfate. 

The ISA focuses on the ecological 

effects associated with deposition of 

ambient oxides of nitrogen and sulfur to 

natural sensitive ecosystems, as 

distinguished from commercially 

managed forests and agricultural lands. 

This focus reflects the fact that the 

majority of the scientific evidence 

regarding acidification and nutrient 

enrichment is based on studies in 

unmanaged ecosystems. Non-managed 

terrestrial ecosystems tend to have a 

higher fraction of nitrogen deposition 

resulting from atmospheric nitrogen 

(U.S. EPA, 2008, section 3.3.2.5). In 

addition, the ISA notes that agricultural 

and commercial forest lands are 

routinely fertilized with amounts of 

nitrogen that exceed air pollutant inputs 

even in the most polluted areas (U.S. 

EPA, 2008, section 3.3.9). This review 

recognizes that the effects of nitrogen 

deposition in managed areas are viewed 

differently from a public welfare 

perspective than are the effects of 

nitrogen deposition in natural, 

unmanaged ecosystems, largely due to 

the more homogeneous, controlled 

nature of species composition and 

development in managed ecosystems 

and the potential for benefits of 

increased productivity in those 

ecosystems. 

In focusing on natural sensitive 

ecosystems, the PA primarily considers 

the effects of ambient oxides of nitrogen 

and sulfur via deposition on multiple 

ecological receptors. The ISA highlights 

effects including those associated with 

acidification and nitrogen nutrient 

enrichment. With a focus on these 

deposition-related effects the EPA’s 

objective is to develop a framework for 

oxides of nitrogen and sulfur standards 

that incorporates ecologically relevant 

factors and that recognizes the 

interactions between the two pollutants 

as they deposit to sensitive ecosystems. 

The overarching policy objective is to 

develop a secondary standard(s) based 

on the ecological criteria described in 

the ISA and the results of the 

assessments in the REA, and consistent 

with the requirement of the CAA to set 

secondary standards that are requisite to 

protect the public welfare from any 

known or anticipated adverse effects 

associated with the presence of these air 

pollutants in the ambient air. Consistent 

with the CAA, this policy objective 

includes consideration of ‘‘variable 

factors * * * which of themselves or in 

combination with other factors may 

alter the effects on public welfare’’ of 

the criteria air pollutants included in 

this review. 

In addition, we have chosen to focus 

on the effects of ambient oxides of 

nitrogen and sulfur on ecological 

impacts on sensitive aquatic ecosystems 

associated with acidifying deposition of 

nitrogen and sulfur, which is a 

transformation product of ambient 

oxides of nitrogen and sulfur. Based on 

the information in the ISA, the 

assessments presented in the REA, and 

advice from CASAC on earlier drafts of 

this PA (Russell and Samet, 2010a, 

2010b), and as discussed in detail in the 

PA, we have the greatest confidence in 

the causal linkages between oxides of 

nitrogen and sulfur and aquatic 

acidification effects relative to other 

deposition-related effects, including 

terrestrial acidification and aquatic and 

terrestrial nutrient enrichment. 

2. Comments on the Scope of the 

Review 

Comments received regarding the 

scope of the review were primarily 

those that questioned the EPA’s legal 

authority under Section 109 of the CAA 

to set NAAQS that address depositionrelated effects, focusing in particular on 

effects resulting from acidifying 

deposition to ecosystems. 

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 

CAA. According to industry comments, 

this is demonstrated by a close reading 

of Section 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 Section 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. 

Although the EPA is not 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 Section109 of the CAA to set such 

a NAAQS. We note that the purpose of 

Title IV ‘‘is to reduce the adverse effects 

of acid deposition,’’ CAA Section 

401(b), while Section 109 directs the 

Administrator to go beyond this to set 

a standard that is ‘‘requisite to protect 

public welfare from any known or 


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anticipated adverse effects,’’ CAA 

Section 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. 

Nothing in the text or the legislative 

history of Title IV of the Act indicates 

a clear intention by Congress to 

foreclose the EPA’s authority to address 

acid deposition through the NAAQS 

process. The requirement in Section 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 indicate that Congress had 

concluded that an amendment to the 

CAA would be necessary to give the 

EPA the authority to issue regulations 

addressing acidification. The 

significance of the report required by 

Section 404 cannot be understood 

clearly in isolation, but should be 

considered 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 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. In the 

proposed rule, we 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. In 

assessing the import of Section 404 in 

this overall context, the EPA has noted 

in the past and in section I.C above that 

‘‘Congress reserved judgment as to 

whether further action might be 

necessary or 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’’ (58 FR 21351, 21356 

(April 21, 1993)). Such reservation of 

judgment does not indicate that 

Congress viewed the EPA as lacking 

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 as to the appropriate 

standard. 

Having carefully considered the 

public comments, the EPA finds that the 

conclusions reached in the proposed 

rule with regard to the scope of the 

current review continue to be valid. The 

EPA concludes that the Agency has the 

authority under Section 109 of the CAA 

to consider deposition-related to 

ambient air concentrations of oxides of 

nitrogen and sulfur and the resulting 

effects on ecosystems and that the focus 

of the current review of the NAAQS for 

oxides of nitrogen and sulfur on aquatic 

acidification is appropriate. This issue 

is discussed in more detail in the EPA’s 

Response to Comments document. 

II. Rationale for Final Decisions on the 

Adequacy of the Current Secondary 

Standards 

This section presents the rationale for 

the Administrator’s final conclusions 

with regard to the adequacy of 

protection and ecological relevance of 

the current secondary standards for 

oxides of nitrogen and sulfur. As 

discussed more fully below, this 

rationale considered the latest scientific 

information on ecological effects 

associated with the presence of oxides 

of nitrogen and oxides of sulfur in the 

ambient air. This rationale also takes 

into account: (1) Staff assessments of the 

most policy-relevant information in the 

ISA and staff analyses of air quality, 

exposure, and ecological risks, 

presented more fully in the REA and in 

the PA, upon which staff conclusions on 

revisions to the secondary oxides of 

nitrogen and oxides of sulfur standards 

are based; (2) CASAC advice and 

recommendations, as reflected in 

discussions of drafts of the ISA, REA, 

and PA at public meetings, in separate 

written comments, and in CASAC’s 

letters to the Administrator; and (3) 

public comments received during the 

development of these documents, either 

in connection with CASAC meetings or 

separately as well as comments received 

on the proposal notice. 

In developing this rationale, the EPA 

has drawn upon an integrative synthesis 

of the entire body of evidence, 

published through early 2008, on 

ecological effects associated with the 

deposition of oxides of nitrogen and 

oxides of sulfur in the ambient air (U.S. 

EPA, 2008). As discussed below, this 

body of evidence addresses a broad 

range of ecological endpoints associated 

with ambient levels of oxides of 

nitrogen and oxides of sulfur. In 

considering this evidence, the EPA 

focuses on those ecological endpoints, 

such as aquatic acidification, for which 

the ISA judges associations with oxides 

of nitrogen and oxides of sulfur to be 

causal, likely causal, or for which the 

evidence is suggestive that oxides of 

nitrogen and/or sulfur contribute to the 

reported effects. The categories of 

causality determinations have been 

developed in the ISA (U.S. EPA, 2008) 

and are discussed in section 1.6 of the 

ISA. 

Decisions on retaining or revising the 

current secondary standards for oxides 

of nitrogen and sulfur are largely public 

welfare policy judgments based on the 

Administrator’s informed assessment of 

what constitutes requisite protection 

against adverse effects to public welfare. 

A public welfare policy decision should 

draw upon scientific information and 

analyses about welfare effects, exposure 

and risks, as well as judgments about 

the appropriate response to the range of 

uncertainties that are inherent in the 

scientific evidence and analyses. The 

ultimate determination as to what level 

of damage to ecosystems and the 

services provided by those ecosystems 

is adverse to public welfare is not 

wholly a scientific question, although it 

is informed by scientific studies linking 

ecosystem damage to losses in 

ecosystem services, and information on 

the value of those losses of ecosystem 

services. In reaching such decisions, the 

Administrator seeks to establish 

standards that are neither more nor less 

stringent than necessary for this 

purpose. 

Drawing from information in sections 

II.A–C of the proposal, section II.A 

below provides overviews of the public 

welfare effects considered in this 

review, the risk and exposure 

assessments, and the adversity of effects 

on public welfare. Section II.B presents 

conclusions in the ISA, REA, and PA on 

the adequacy of the current secondary 

standards for oxides of nitrogen and 

oxides of sulfur. Consideration is given 

to the adequacy of protection afforded 

by the current standards for both direct 

and deposition-related effects, as well as 

to the appropriateness of the 

fundamental structure and the basic 

elements of the current standards for 

providing protection from depositionrelated effects. The views of CASAC and 

a summary of the Administrator’s 

proposed conclusions are also included. 

Section II. C presents a discussion of the 

comments received on the proposal 

with regard to the adequacy of the 

current standards. Section II. D presents 

the Administrator’s final decisions with 

regard to the adequacy of the current 

standards for both direct and 

deposition-related effects on public 

welfare. 

A. Introduction 

A discussion of the effects associated 

with oxides of nitrogen and sulfur in the 

ambient air is presented below in 

section II.A.1. The discussion is 

organized around the types of effects 

being considered, including direct 

effects of gaseous oxides of nitrogen and 

sulfur, deposition-related effects related 

to acidification and nutrient 


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enrichment, and other effects such as 

materials damage, climate-related effects 

and mercury methylation. 

Section II.A.2 presents a summary 

and discussion of the risk and exposure 

assessment performed for each of the 

four major effects categories. The REA 

uses case studies representing the broad 

geographic variability of the impacts 

from oxides of nitrogen and sulfur to 

conclude that there are ongoing adverse 

effects in many ecosystems from 

deposition of oxides of nitrogen and 

sulfur and that under current emissions 

scenarios these effects are likely to 

continue. 

Section II.A.3 presents a discussion of 

adversity linking ecological effects to 

measures that can be used to 

characterize the extent to which such 

effects are reasonably considered to be 

adverse to public welfare. This involves 

consideration of how to characterize 

adversity from a public welfare 

perspective. In so doing, consideration 

is given to the concept of ecosystem 

services, the evidence of effects on 

ecosystem services, and how ecosystem 

services can be linked to ecological 

indicators. 

1. Overview of Effects 

This section discusses the known or 

anticipated ecological effects associated 

with oxides of nitrogen and sulfur, 

including the direct effects of gas-phase 

exposure to oxides of nitrogen and 

sulfur (section II.A.1.a) and effects 

associated with deposition-related 

exposure (section II.A.1.b). These 

sections also address questions about 

the nature and magnitude of ecosystem 

responses to reactive nitrogen and sulfur 

deposition, including responses related 

to acidification, nutrient depletion, and 

the mobilization of toxic metals in 

sensitive aquatic and terrestrial 

ecosystems. The uncertainties and 

limitations associated with the evidence 

of such effects are also discussed 

throughout this section. 

a. Effects Associated With Gas-Phase 

Oxides of Nitrogen and Sulfur 

Ecological effects on vegetation as 

discussed in earlier reviews as well as 

the ISA can be attributed to gas-phase 

oxides of nitrogen and sulfur. Acute and 

chronic exposures to gaseous pollutants 

such as SO2, NO2, nitric oxide (NO), 

nitric acid (HNO3) and peroxyacetyl 

nitrite (PAN) are associated with 

negative impacts to vegetation. The 

current secondary NAAQS were set to 

protect against direct damage to 

vegetation by exposure to gas-phase 

oxides of nitrogen and sulfur, such as 

foliar injury, decreased photosynthesis, 

and decreased growth. The following 

summary is a concise overview of the 

known or anticipated effects to 

vegetation caused by gas phase nitrogen 

and sulfur. Most phototoxic effects 

associated with gas phase oxides of 

nitrogen and sulfur occur at levels well 

above ambient concentrations observed 

in the United States (U.S. EPA, 2008, 

section 3.4.2.4). 

The 2008 ISA found that gas phase 

nitrogen and sulfur are associated with 

direct phytotoxic effects (U.S. EPA, 

2008, section 4.4). The evidence is 

sufficient to infer a causal relationship 

between exposure to SO2 and injury to 

vegetation (U.S. EPA, 2008, section 4.4.1 

and 3.4.2.1). Acute foliar injury to 

vegetation from SO2 may occur at levels 

above the current secondary standard 

(3-h average of 0.50 ppm). Effects on 

growth, reduced photosynthesis and 

decreased yield of vegetation are also 

associated with increased SO2 exposure 

concentration and time of exposure. 

The evidence is sufficient to infer a 

causal relationship between exposure to 

NO, NO2 and PAN and injury to 

vegetation (U.S. EPA, 2008, section 4.4.2 

and 3.4.2.2). At sufficient 

concentrations, NO, NO2 and PAN can 

decrease photosynthesis and induce 

visible foliar injury to plants. Evidence 

is also sufficient to infer a causal 

relationship between exposure to HNO3

and changes to vegetation (U.S. EPA, 

2008, section 4.4.3 and 3.4.2.3). 

Phytotoxic effects of this pollutant 

include damage to the leaf cuticle in 

vascular plants and disappearance of 

some sensitive lichen species. 

Vegetation in ecosystems near sources 

of gaseous oxides of nitrogen and sulfur 

or where SO2, NO, NO2, PAN and HNO3

are most concentrated are more likely to 

be impacted by these pollutants. Uptake 

of these pollutants in a plant canopy is 

a complex process involving adsorption 

to surfaces (leaves, stems and soil) and 

absorption into leaves (U.S. EPA, 2008, 

section 3.4.2). The functional 

relationship between ambient 

concentrations of gas phase oxides of 

nitrogen and sulfur and specific plant 

response are impacted by internal 

factors such as rate of stomatal 

conductance and plant detoxification 

mechanisms, and external factors 

including plant water status, light, 

temperature, humidity, and pollutant 

exposure regime (U.S. EPA, 2008, 

section 3.4.2). 

Entry of gases into a leaf is dependent 

upon physical and chemical processes 

of gas phase as well as to stomatal 

aperture. The aperture of the stomata is 

controlled largely by the prevailing 

environmental conditions, such as water 

availability, humidity, temperature, and 

light intensity. When the stomata are 

closed, resistance to gas uptake is high 

and the plant has a very low degree of 

susceptibility to injury. Mosses and 

lichens do not have a protective cuticle 

barrier to gaseous pollutants or stomata 

and are generally more sensitive to 

gaseous sulfur and nitrogen than 

vascular plants (U.S. EPA, 2008, section 

3.4.2). 

The appearance of foliar injury can 

vary significantly across species and 

growth conditions affecting stomatal 

conductance in vascular plants (U.S. 

EPA, 2009, section 6.4.1). For example, 

damage to lichens from SO2 exposure 

includes decreased photosynthesis and 

respiration, damage to the algal 

component of the lichen, leakage of 

electrolytes, inhibition of nitrogen 

fixation, decreased potassium (K+) 

absorption, and structural changes. 

The phytotoxic effects of gas phase 

oxides of nitrogen and sulfur are 

dependent on the exposure 

concentration and duration and species 

sensitivity to these pollutants. Effects to 

vegetation associated with oxides of 

nitrogen and sulfur are therefore 

variable across the United States and 

tend to be higher near sources of 

photochemical smog. For example, SO2

is considered to be the primary factor 

contributing to the death of lichens in 

many urban and industrial areas. 

The ISA states there is very limited 

new research on phytotoxic effects of 

NO, NO2, PAN and HNO3 at 

concentrations currently observed in the 

United States with the exception of 

some lichen species (U.S. EPA, 2008, 

section 4.4). Past and current HNO3

concentrations may be contributing to 

the decline in lichen species in the Los 

Angeles basin. Most phytotoxic effects 

associated with gas phase oxides of 

nitrogen and sulfur occur at levels well 

above ambient concentrations observed 

in the United States (U.S. EPA, 2008, 

section 3.4.2.4). 

b. Effects Associated With Deposition of 

Oxides of Nitrogen and Sulfur 

Ecological effects associated with the 

deposition of oxides of nitrogen and 

oxides of sulfur can be divided into 

endpoints related to the type of 

ecosystem affected and the type of 

effect. As more fully discussed in 

section II.A of the proposal and chapter 

3 of the PA, this section provides a brief 

summary of effects on ecosystems 

related to acidification, nutrient 

enrichment, and metal toxicity. 

i. Acidification Effects on Aquatic and 

Terrestrial Ecosystems 

Sulfur oxides and nitrogen oxides in 

the atmosphere undergo a complex mix 

of reactions in gaseous, liquid, and solid 


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phases to form various acidic 

compounds. These acidic compounds 

are removed from the atmosphere 

through deposition: either wet (e.g., 

rain, snow), fog or cloud, or dry (e.g., 

gases, particles). Deposition of these 

acidic compounds to aquatic and 

terrestrial ecosystems can lead to effects 

on ecosystem structure and function. 

Following deposition, these compounds 

can, in some instances, unless retained 

by soil or biota, leach out of the soils in 

the form of sulfate (SO4

2¥) and nitrate 

(NO3

¥), leading to the acidification of 

surface waters. The effects on 

ecosystems depend on the magnitude 

and rate of deposition, as well as a host 

of biogeochemical processes occurring 

in the soils and water bodies (U.S. EPA, 

2009, section 2.1). The chemical forms 

of nitrogen that may contribute to 

acidifying deposition include both 

oxidized and reduced chemical species, 

including reduced forms of nitrogen 

(NHX). 

The ISA concluded that deposition of 

oxides of nitrogen and sulfur and NHX

leads to the varying degrees of 

acidification of ecosystems (U.S. EPA, 

2008). In the process of acidification, 

biogeochemical components of 

terrestrial and freshwater aquatic 

ecosystems are altered in a way that 

leads to effects on biological organisms. 

Deposition to terrestrial ecosystems 

often moves through the soil and 

eventually leaches into adjacent water 

bodies. Principal factors governing the 

sensitivity of terrestrial and aquatic 

ecosystems to acidification from sulfur 

and nitrogen deposition include 

geology, plant uptake of nitrogen, soil 

depth, and elevation. Geologic 

formations having low base cation 

supply generally underlie the 

watersheds of acid-sensitive lakes and 

streams. Other factors that contribute to 

the sensitivity of soils and surface 

waters to acidifying deposition include 

topography, soil chemistry, land use, 

and hydrologic flowpath. Chronic as 

well as episodic acidification tends to 

occur primarily at relatively high 

elevations in areas that have base-poor 

bedrock, high relief, and shallow soils. 

With regard to aquatic acidification, 

the ISA concluded that the scientific 

evidence is sufficient to infer a causal 

relationship between acidifying 

deposition and effects on 

biogeochemistry and biota in aquatic 

ecosystems (U.S. EPA, 2008, section 

4.2.2). The strongest evidence comes 

from studies of surface water chemistry 

in which acidic deposition is observed 

to alter sulfate and nitrate 

concentrations in surface waters, the 

sum of base cations, acid neutralizing 

capacity (ANC), dissolved inorganic 

aluminum (Al) and pH (U.S. EPA, 2008, 

section 3.2.3.2). The ANC is a key 

indicator of acidification with relevance 

to both terrestrial and aquatic 

ecosystems. The ANC is useful because 

it integrates the overall acid-base status 

of a lake or stream and reflects how 

aquatic ecosystems respond to acidic 

deposition over time. There is also a 

relationship between ANC and the 

surface water constituents that directly 

contribute to or ameliorate acidityrelated stress, in particular, 

concentrations of hydrogen ion (as pH), 

calcium (Ca2∂) and Al. Moreover, low 

pH surface waters leach aluminum from 

soils, which is quite lethal to fish and 

other aquatic organisms. In aquatic 

systems, there is a direct relationship 

between ANC and fish and phytozooplankton diversity and abundance. 

Acidification in terrestrial ecosystems 

has been shown to cause decreased 

growth and increased susceptibility to 

disease and injury in sensitive tree 

species, including red spruce and sugar 

maple. 

Based on analyses of surface water 

data from freshwater ecosystem surveys 

and monitoring, the most sensitive lakes 

and streams are contained in New 

England, the Adirondack Mountains, 

the Appalachian Mountains (northern 

Appalachian Plateau and Ridge/Blue 

Ridge region), the mountainous West, 

and the Upper Midwest. ANC is the 

most widely used indicator of acid 

sensitivity and has been found in 

various studies to be the best single 

indicator of the biological response and 

health of aquatic communities in acid 

sensitive systems. Annual or multi-year 

average ANC is a good overall indicator 

of sensitivity, capturing the ability of an 

ecosystem to withstand chronic 

acidification as well as episodic events 

such as spring melting that can lower 

ANC over shorter time spans. Biota are 

generally not harmed when annual 

average ANC levels are >100 

microequivalents per liter (meq/L). At 

annual average ANC levels between 100 

and 50 meq/L, the fitness of sensitive 

species (e.g., brook trout, zooplankton) 

begins to decline. When annual average 

ANC is <50 meq/L, negative effects on 

aquatic biota are observed, including 

large reductions in diversity of fish 

species, and declines in health of fish 

populations, affecting reproductive 

ability and fitness. Annual average ANC 

levels below 0 meq/L are generally 

associated with complete loss of fish 

species and other biota that are sensitive 

to acidification. An example of the 

relationship between ANC level and 

aquatic effects based on lakes in the 

Adirondacks is illustrated in the 

following figure: 


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Recent studies indicate that 

acidification of lakes and streams can 

result in significant loss in economic 

value, which is one indicator of 

adversity associated with loss of 

ecosystem services. A 2006 study of 

New York residents found that they are 

willing to pay between $300 and $800 

million annually for the equivalent of 

improving lakes in the Adirondacks 

region to an ANC level of 50 meq/L. 

Several states have set goals for 

improving the acid status of lakes and 

streams, generally targeting ANC in the 

range of 50 to 60 meq/L, and have 

engaged in costly activities to decrease 

acidification. 

With regard to terrestrial ecosystems, 

the evidence is sufficient to infer a 

causal relationship between acidifying 

deposition and changes in 

biogeochemistry (U.S. EPA, 2008, 

section 4.2.1.1). The strongest evidence 

comes from studies of forested 

ecosystems, with supportive 

information on other plant taxa, 

including shrubs and lichens (U.S. EPA, 

2008, section 3.2.2.1.). Three useful 

indicators of chemical changes and 

acidification effects on terrestrial 

ecosystems, showing consistency among 

multiple studies are: soil base 

saturation, Al concentrations in soil 

water, and soil carbon to nitrogen (C:N) 

ratio (U.S. EPA, 2008, section 3.2.2.2). 

Forests of the Adirondack Mountains 

of New York, Green Mountains of 

Vermont, White Mountains of New 

Hampshire, the Allegheny Plateau of 

Pennsylvania, and high-elevation forest 

ecosystems in the southern 

Appalachians and mountainous regions 

in the West are the regions most 

sensitive to acidifying deposition. The 

health of at least a portion of the sugar 

maple and red spruce growing in the 

United States may have been 

compromised by acidifying total 

nitrogen and sulfur deposition in recent 

years. Soil acidification caused by 

acidic deposition has been shown to 

cause decreased growth and increased 

susceptibility to disease and injury in 

sensitive tree species. Red spruce 

dieback or decline has been observed 

across high elevation areas in the 

Adirondack, Green and White 

mountains. The frequency of freezing 

injury to red spruce needles has 

increased over the past 40 years, a 

period that coincided with increased 

emissions of sulfur and nitrogen oxides 

and increased acidifying deposition. 

Acidifying deposition can contribute to 

dieback in sugar maple through 

depletion of cations from soil with low 

levels of available calcium. Grasslands 

are likely less sensitive to acidification 

than forests due to grassland soils being 

generally rich in base cations. 

A commonly used indicator of 

terrestrial acidification is the base 

cation-to-aluminum ratio, Bc/Al. Many 

locations in sensitive areas of the United 

States have Bc/Al levels below 

benchmark levels we have classified as 

providing low to intermediate levels of 

protection to tree health. At a Bc/Al 

ratio of 1.2 (intermediate level of 

protection), red spruce growth can be 

reduced by 20 percent. At a Bc/Al ratio 

of 0.6 (low level of protection), sugar 

maple growth can be reduced by 20 

percent. While not defining whether a 

20 percent reduction in growth can be 

considered significant, existing 

economic studies suggest that avoiding 

significant declines in the health of 

spruce and sugar maple forests may be 

worth billions of dollars to residents of 

the Eastern United States. 

ii. Nutrient Enrichment Effects in 

Terrestrial and Aquatic Ecosystems 

The ISA found that deposition of 

nitrogen, including oxides of nitrogen 

and NHX, leads to the nitrogen 

enrichment of terrestrial, freshwater and 

estuarine ecosystems (U.S. EPA 2008). 

In the process of nitrogen enrichment, 

biogeochemical components of 

terrestrial and freshwater aquatic 

ecosystems are altered in a way that 

leads to effects on biological organisms. 

Nitrogen deposition is a major source of 

anthropogenic nitrogen. For many 

terrestrial and freshwater ecosystems 

other sources of nitrogen including 

fertilizer and waste treatment are greater 

than deposition. Nitrogen deposition 

often contributes to nitrogen-enrichment 

effects in estuaries, but does not drive 

the effects since other sources of 

nitrogen greatly exceed nitrogen 

deposition. Both oxides of nitrogen and 

NHX contribute to nitrogen deposition. 

For the most part, nitrogen effects on 

ecosystems do not depend on whether 

the nitrogen is in oxidized or reduced 

form. Thus, this summary focuses on 

the effects of nitrogen deposition in 

total. 

The numerous ecosystem types that 

occur across the United States have a 

broad range of sensitivity to nitrogen 

deposition. Organisms in their natural 


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environment are commonly adapted to 

a specific regime of nutrient availability. 

Change in the availability of one 

important nutrient, such as nitrogen, 

may result in imbalances in ecosystems, 

with effects on ecosystem processes, 

structure and function. In certain 

nitrogen-limited ecosystems, including 

many ecosystems managed for 

commercial production, nitrogen 

deposition can result in beneficial 

increases in productivity. Nutrient 

enrichment effects from deposition of 

oxides of nitrogen are difficult to 

disentangle from overall effects of 

nitrogen enrichment. This is caused by 

two factors: the inputs of reduced 

nitrogen from deposition and, in 

estuarine ecosystems, a large fraction of 

nitrogen inputs from non-atmospheric 

sources. 

The numerous ecosystem types that 

occur across the United States have a 

broad range of sensitivity to nitrogen 

deposition (U.S. EPA, 2008, Table 4–4). 

Increased deposition to nitrogen-limited 

ecosystems can lead to production 

increases that may be either beneficial 

or adverse depending on the system and 

management goals. Organisms in their 

natural environment are commonly 

adapted to a specific regime of nutrient 

availability. Change in the availability of 

one important nutrient, such as 

nitrogen, may result in an imbalance in 

ecological stoichiometry, with effects on 

ecosystem processes, structure and 

function. 

With regard to terrestrial ecosystems, 

the ISA concluded that the evidence is 

sufficient to infer a causal relationship 

between nitrogen deposition and the 

alteration of biogeochemical cycling in 

terrestrial ecosystems (U.S. EPA, 2008, 

section 4.3.1.1 and 3.3.2.1). Due to the 

complexity of interactions between the 

nitrogen and carbon cycling, the effects 

of nitrogen on carbon budgets 

(quantified input and output of carbon 

to the ecosystem) are variable. Regional 

trends in net ecosystem productivity 

(NEP) of forests (not managed for 

silviculture) have been estimated 

through models based on gradient 

studies and meta-analysis. Atmospheric 

nitrogen deposition has been shown to 

cause increased litter accumulation and 

carbon storage in above-ground woody 

biomass. In the West, this has lead to 

increased susceptibility to more severe 

fires. Less is known regarding the effects 

of nitrogen deposition on carbon 

budgets of non-forest ecosystems. The 

ISA also concludes that the evidence is 

sufficient to infer a causal relationship 

between nitrogen deposition on the 

alteration of species richness, species 

composition and biodiversity in 

terrestrial ecosystems (U.S. EPA, 2008, 

section 4.3.1.2). 

Little is known about the full extent 

and distribution of the terrestrial 

ecosystems in the United States that are 

most sensitive to impacts caused by 

nutrient enrichment from atmospheric 

nitrogen deposition. Effects are most 

likely to occur where areas of relatively 

high atmospheric N deposition intersect 

with nitrogen-limited plant 

communities. The alpine ecosystems of 

the Colorado Front Range, chaparral 

watersheds of the Sierra Nevada, lichen 

and vascular plant communities in the 

San Bernardino Mountains and the 

Pacific Northwest, and the southern 

California coastal sage scrub (CSS) 

community are among the most 

sensitive terrestrial ecosystems. There is 

growing evidence (U.S. EPA, 2008, 

section 4.3.1.2) that existing grassland 

ecosystems in the western United States 

are being altered by elevated levels of N 

inputs, including inputs from 

atmospheric deposition. 

More is known about the sensitivity of 

particular plant communities. Based 

largely on results obtained in more 

extensive studies conducted in Europe, 

it is expected that the more sensitive 

terrestrial ecosystems include hardwood 

forests, alpine meadows, arid and semiarid lands, and grassland ecosystems 

(U.S. EPA, 2008, section 3.3.5). The REA 

used published research results (U.S. 

EPA, 2009, section 5.3.1 and U.S. EPA, 

2008, Table 4.4) to identify meaningful 

ecological benchmarks associated with 

different levels of atmospheric nitrogen 

deposition. These are illustrated in 

Figure 3–4 of the PA. The sensitive 

areas and ecological indicators 

identified by the ISA were analyzed 

further in the REA to create a national 

map that illustrates effects observed 

from ambient and experimental 

atmospheric nitrogen deposition loads 

in relation to Community Multi-scale 

Air Quality (CMAQ) 2002 modeling 

results and National Atmospheric 

Deposition Program (NADP) monitoring 

data. This map, reproduced in Figure 3– 

5 of the PA, depicts the sites where 

empirical effects of terrestrial nutrient 

enrichment have been observed and site 

proximity to elevated atmospheric 

nitrogen deposition. 

With regard to freshwater ecosystems, 

the ISA concluded that the evidence is 

sufficient to infer a causal relationship 

between nitrogen deposition and the 

alteration of biogeochemical cycling in 

freshwater aquatic ecosystems (U.S. 

EPA, 2008, section 3.3.2.3). Nitrogen 

deposition is the main source of 

nitrogen enrichment to headwater 

streams, lower order streams and high 

elevation lakes. The ISA also concludes 

that the evidence is sufficient to infer a 

causal relationship between nitrogen 

deposition and the alteration of species 

richness, species composition and 

biodiversity in freshwater aquatic 

ecosystems (U.S. EPA, 2008, section 

3.3.5.3). 

There are many examples of fresh 

waters that are nitrogen-limited or 

nitrogen and phosphorous (P) co-limited 

(U.S. EPA, 2008, section 3.3.3.2). Less is 

known about the extent and distribution 

of the terrestrial ecosystems in the 

United States that are most sensitive to 

the effects of nutrient enrichment from 

atmospheric nitrogen deposition 

compared to acidification. Grasslands in 

the western United States are typically 

nitrogen-limited ecosystems dominated 

by a diverse mix of perennial forbs and 

grass species. A meta-analysis discussed 

in the ISA (U.S. EPA, 2008, section 

3.3.3), indicated that nitrogen 

fertilization increased aboveground 

growth in all non-forest ecosystems 

except for deserts. Because the 

productivity of estuarine and near shore 

marine ecosystems is generally limited 

by the availability of nitrogen, they are 

also susceptible to the eutrophication 

effect of nitrogen deposition (U.S. EPA, 

2008, section 4.3.4.1). 

The magnitude of ecosystem response 

to nutrient enrichment may be thought 

of on two time scales, current 

conditions and how ecosystems have 

been altered since the onset of 

anthropogenic nitrogen deposition. As 

noted previously, studies found that 

nitrogen-limitation occurs as frequently 

as phosphorous-limitation in freshwater 

ecosystems (U.S. EPA, 2008, section 

3.3.3.2). Recently, a comprehensive 

study of available data from the 

northern hemisphere surveys of lakes 

along gradients of nitrogen deposition 

show increased inorganic nitrogen 

concentration and productivity to be 

correlated with atmospheric nitrogen 

deposition. The results are unequivocal 

evidence of nitrogen limitation in lakes 

with low ambient inputs of nitrogen, 

and increased nitrogen concentrations 

in lakes receiving nitrogen solely from 

atmospheric nitrogen deposition. It has 

been suggested that most lakes in the 

northern hemisphere may have 

originally been nitrogen-limited, and 

that atmospheric nitrogen deposition 

has changed the balance of nitrogen and 

phosphorous in lakes. 

Eutrophication effects from nitrogen 

deposition are most likely to be 

manifested in undisturbed, low nutrient 

surface waters such as those found in 

the higher elevation areas of the western 

United States. The most severe 

eutrophication from nitrogen deposition 

effects is expected downwind of major 


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urban and agricultural centers. High 

concentrations of lake or streamwater 

NO3

¥, indicative of ecosystem 

saturation, have been found at a variety 

of locations throughout the United 

States, including the San Bernardino 

and San Gabriel Mountains within the 

Los Angeles Air Basin, the Front Range 

of Colorado, the Allegheny mountains of 

West Virginia, the Catskill Mountains of 

New York, the Adirondack Mountains 

of New York, and the Great Smoky 

Mountains in Tennessee (U.S. EPA, 

2008, section 3.3.8). 

With regard to estuaries, the ISA 

concludes that the evidence is sufficient 

to infer a causal relationship between 

nitrogen deposition and the 

biogeochemical cycling of nitrogen and 

carbon in estuaries (U.S. EPA, 2008, 

section 4.3.4.1 and 3.3.2.3). In general, 

estuaries tend to be nitrogen-limited, 

and many currently receive high levels 

of nitrogen input from human activities 

(U.S. EPA, 2009, section 5.1.1). It is 

unknown if atmospheric deposition 

alone is sufficient to cause 

eutrophication; however, the 

contribution of atmospheric nitrogen 

deposition to total nitrogen load is 

calculated for some estuaries and can be 

>40 percent (U.S. EPA, 2009, section 

5.1.1). The evidence is also sufficient to 

infer a causal relationship between 

nitrogen deposition and the alteration of 

species richness, species composition 

and biodiversity in estuarine ecosystems 

(U.S. EPA, 2008, section 4.3.4.2 and 

3.3.5.4). Atmospheric and nonatmospheric sources of nitrogen 

contribute to increased phytoplankton 

and algal productivity, leading to 

eutrophication. Shifts in community 

composition, reduced hypolimnetic 

dissolved oxygen (DO), decreases in 

biodiversity, and mortality of 

submerged aquatic vegetation are 

associated with increased N deposition 

in estuarine systems. 

In contrast to terrestrial and 

freshwater systems, atmospheric 

nitrogen load to estuaries contributes to 

the total load but does not necessarily 

drive the effects since other combined 

sources of nitrogen often greatly exceed 

nitrogen deposition. In estuaries, 

nitrogen-loading from multiple 

anthropogenic and non-anthropogenic 

pathways leads to water quality 

deterioration, resulting in numerous 

effects including hypoxic zones, species 

mortality, changes in community 

composition and harmful algal blooms 

that are indicative of eutrophication. 

A recent national assessment of 

eutrophic conditions in estuaries found 

that 65 percent of the assessed systems 

had moderate to high overall eutrophic 

conditions. Most eutrophic estuaries 

occurred in the mid-Atlantic region and 

the estuaries with the lowest degree of 

eutrophication were in the North 

Atlantic. Other regions had mixtures of 

low, moderate, and high degrees of 

eutrophication (U.S. EPA, 2008, section 

4.3.4.3). The mid-Atlantic region is the 

most heavily impacted area in terms of 

moderate or high loss of submerged 

aquatic vegetation due to eutrophication 

(U.S. EPA, 2008, section 4.3.4.2). 

Submerged aquatic vegetation is 

important to the quality of estuarine 

ecosystem habitats because it provides 

habitat for a variety of aquatic 

organisms, absorbs excess nutrients, and 

traps sediments (U.S. EPA, 2008, section 

4.3.4.2). It is partly because many 

estuaries and near-coastal marine waters 

are degraded by nutrient enrichment 

that they are highly sensitive to 

potential negative impacts from nitrogen 

addition from atmospheric deposition. 

iii. Effects on Metal Toxicity 

As discussed in the ISA (U.S. EPA, 

2008, section 3.4.1 and 4.5), mercury is 

a highly neurotoxic contaminant that 

enters the food web as a methylated 

compound, methylmercury (MeHg). 

Mercury is principally methylated by 

sulfur-reducing bacteria and can be 

taken up by microorganisms, 

zooplankton and macroinvertebrates. 

The contaminant is concentrated in 

higher trophic levels, including fish 

eaten by humans. Experimental 

evidence has established that only 

inconsequential amounts of MeHg can 

be produced in the absence of sulfate. 

Once MeHg is present, other variables 

influence how much accumulates in 

fish, but elevated mercury levels in fish 

can only occur where substantial 

amounts of MeHg are present. Current 

evidence indicates that in watersheds 

where mercury is present, increased 

oxides of sulfur deposition very likely 

results in additional production of 

MeHg which leads to greater 

accumulation of MeHg concentrations 

in fish. With respect to sulfur deposition 

and mercury methylation, the final ISA 

determined that ‘‘[t]he evidence is 

sufficient to infer a causal relationship 

between sulfur deposition and increased 

mercury methylation in wetlands and 

aquatic environments.’’ 

The production of meaningful 

amounts of MeHg requires the presence 

of SO4

2¥ and mercury, and where 

mercury is present, increased 

availability of SO4

2¥ results in 

increased production of MeHg. There is 

increasing evidence on the relationship 

between sulfur deposition and increased 

methylation of mercury in aquatic 

environments; this effect occurs only 

where other factors are present at levels 

within a range to allow methylation. 

The production of MeHg requires the 

presence of SO4

2¥ and mercury, but the 

amount of MeHg produced varies with 

oxygen content, temperature, pH, and 

supply of labile organic carbon (U.S. 

EPA, 2008, section 3.4). In watersheds 

where changes in sulfate deposition did 

not produce an effect, one or several of 

those interacting factors were not in the 

range required for meaningful 

methylation to occur (U.S. EPA, 2008, 

section 3.4). Watersheds with 

conditions known to be conducive to 

mercury methylation can be found in 

the northeastern United States and 

southeastern Canada. 

While the ISA concluded that the 

evidence was sufficient to infer a causal 

relationship between sulfur deposition 

and increased MeHg production in 

wetlands and aquatic ecosystems, the 

REA concluded that there was 

insufficient evidence to quantify the 

relationship between sulfur deposition 

and MeHg production. Therefore, only a 

qualitative assessment was included in 

chapter 6 of the REA. As a result, the PA 

could not reach a conclusion as to the 

adequacy of the existing SO2 standards 

in protecting against welfare effects 

associated with increased mercury 

methylation. 

2. Overview of Risk and Exposure 

Assessment 

The risk and exposure assessment 

conducted for the current review was 

developed to describe potential risk 

from current and future deposition of 

oxides of nitrogen and sulfur to 

sensitive ecosystems. The case study 

analyses in the REA show that there is 

confidence that known or anticipated 

adverse ecological effects are occurring 

under current ambient loadings of 

nitrogen and sulfur in sensitive 

ecosystems across the United States. An 

overview of the analytic approaches 

used in the REA, a summary of the key 

findings from the air quality analyses 

and acidification and nutrient 

enrichment case studies, and general 

conclusions regarding other welfare 

effects are presented below. 

a. Approach to REA Analyses 

The REA evaluates the relationships 

between atmospheric concentrations, 

deposition, biologically relevant 

exposures, targeted ecosystem effects, 

and ecosystem services. To evaluate the 

nature and magnitude of adverse effects 

associated with deposition, the REA 

also examines various ways to quantify 

the relationships between air quality 

indicators, deposition of biologically 

available forms of nitrogen and sulfur, 

ecologically relevant indicators relating 


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to deposition, exposure and effects on 

sensitive receptors, and related effects 

resulting in changes in ecosystem 

structure and services. The intent is to 

determine the exposure metrics that 

incorporate the temporal considerations 

(i.e., biologically relevant timescales), 

pathways, and ecologically relevant 

indicators necessary to determine the 

effects on these ecosystems. To the 

extent feasible, the REA evaluates the 

overall load to the system for nitrogen 

and sulfur, as well as the variability in 

ecosystem responses to these pollutants. 

It also evaluates the contributions of 

atmospherically deposited nitrogen and 

sulfur individually relative to the 

combined atmospheric loadings of both 

elements together. Since oxidized 

nitrogen is the listed criteria pollutant 

(currently measured by the ambient air 

quality indicator NO2) for the 

atmospheric contribution to total 

nitrogen, the REA examines the 

contribution of nitrogen oxides to total 

reactive nitrogen in the atmosphere, 

relative to the contributions of reduced 

forms of nitrogen (e.g., ammonia, 

ammonium), to ultimately assess how a 

meaningful secondary NAAQS might be 

structured. 

The REA focuses on ecosystem 

welfare effects that result from the 

deposition of total reactive nitrogen and 

sulfur. Because ecosystems are diverse 

in biota, climate, geochemistry, and 

hydrology, response to pollutant 

exposures can vary greatly between 

ecosystems. In addition, these diverse 

ecosystems are not distributed evenly 

across the United States. To target 

nitrogen and sulfur acidification and 

nitrogen and sulfur enrichment, the 

REA addresses four main targeted 

ecosystem effects on terrestrial and 

aquatic systems identified by the ISA 

(U.S. EPA, 2008): Aquatic acidification 

due to nitrogen and sulfur; terrestrial 

acidification due to nitrogen and sulfur; 

aquatic nutrient enrichment, including 

eutrophication; and terrestrial nutrient 

enrichment. In addition to these four 

targeted ecosystem effects, the REA also 

qualitatively addresses the influence of 

sulfur oxides deposition on MeHg 

production; nitrous oxide (N2O) effects 

on climate; nitrogen effects on primary 

productivity and biogenic greenhouse 

gas (GHG) fluxes; and phytotoxic effects 

on plants. 

Because the targeted ecosystem effects 

outlined above are not evenly 

distributed across the United States, the 

REA identified case studies for each 

targeted effects based on ecosystems 

identified as sensitive to nitrogen and/ 

or sulfur deposition effects. Eight case 

study areas and two supplemental study 

areas (Rocky Mountain National Park 

and Little Rock Lake, Wisconsin) are 

summarized in the REA based on 

ecosystem characteristics, indicators, 

and ecosystem service information. Case 

studies selected for aquatic acidification 

effects were the Adirondack Mountains 

and Shenandoah National Park. Kane 

Experimental Forest in Pennsylvania 

and Hubbard Brook Experimental Forest 

in New Hampshire were selected as case 

studies for terrestrial acidification. 

Aquatic nutrient enrichment case study 

locations were selected in the Potomac 

River Basin upstream of Chesapeake Bay 

and the Neuse River Basin upstream of 

the Pamlico Sound in North Carolina. 

The CSS communities in southern 

California and the mixed conifer forest 

(MCF) communities in the San 

Bernardino and Sierra Nevada 

Mountains of California were selected as 

case studies for terrestrial nutrient 

enrichment. Two supplemental areas 

were also chosen, one in Rocky 

Mountain National Park for terrestrial 

nutrient enrichment and one in Little 

Rock Lake, Wisconsin for aquatic 

nutrient enrichment. 

For aquatic and terrestrial 

acidification effects, a similar 

conceptual approach was used (critical 

loads) to evaluate the impacts of 

multiple pollutants on an ecological 

endpoint, whereas the approaches used 

for aquatic and terrestrial nutrient 

enrichment were fundamentally 

distinct. Although the ecological 

indicators for aquatic and terrestrial 

acidification (i.e., ANC and BC/Al) are 

very different, both ecological indicators 

are well-correlated with effects such as 

reduced biodiversity and growth. While 

aquatic acidification is clearly the 

targeted effect area with the highest 

level of confidence, the relationship 

between atmospheric deposition and an 

ecological indicator is also quite strong 

for terrestrial acidification. The main 

drawback with the understanding of 

terrestrial acidification is that the data 

are based on laboratory responses rather 

than field measurements. Other 

stressors that are present in the field but 

that are not present in the laboratory 

may confound this relationship. 

For nutrient enrichment effects, the 

REA utilized different types of 

indicators for aquatic and terrestrial 

effects to assess both the likelihood of 

adverse effects to ecosystems and the 

relationship between adverse effects and 

atmospheric sources of oxides of 

nitrogen. The ecological indicator 

chosen for aquatic nutrient enrichment, 

the Assessment of Estuarine Trophic 

Status Eutrophication Index (ASSETS 

EI), seems to be inadequate to relate 

atmospheric deposition to the targeted 

ecological effect, likely due to the many 

other confounding factors. Further, 

there is far less confidence associated 

with the understanding of aquatic 

nutrient enrichment because of the large 

contributions from non-atmospheric 

sources of nitrogen and the influence of 

both oxidized and reduced forms of 

nitrogen, particularly in large 

watersheds and coastal areas. However, 

a strong relationship exists between 

atmospheric deposition of nitrogen and 

ecological effects in high alpine lakes in 

the Rocky Mountains because 

atmospheric deposition is the only 

source of nitrogen to these systems. 

There is also a strong weight-ofevidence regarding the relationships 

between ecological effects attributable to 

terrestrial nitrogen nutrient enrichment; 

however, ozone and climate change may 

be confounding factors. In addition, the 

response for other species or species in 

other regions of the United States has 

not been quantified. 

b. Key Findings 

In summary, based on case study 

analyses, the REA concludes that known 

or anticipated adverse ecological effects 

are occurring under current conditions 

and further concludes that these adverse 

effects continue into the future. Key 

findings from the air quality analyses, 

acidification and nutrient enrichment 

case studies, as well as general 

conclusions from evaluating additional 

welfare effects, are summarized below. 

i. Air Quality Analyses 

The air quality analyses in the REA 

encompass the current emissions 

sources of nitrogen and sulfur, as well 

as atmospheric concentrations, 

estimates of deposition of total nitrogen, 

policy-relevant background, and nonatmospheric loadings of nitrogen and 

sulfur to ecosystems, both nationwide 

and in the case study areas. Spatial 

fields of deposition were created using 

wet deposition measurements from the 

NADP National Trends Network and dry 

deposition predictions from the 2002 

CMAQ model simulation. Some key 

conclusions from this analysis are: 

(1) Total reactive nitrogen deposition 

and sulfur deposition are much greater 

in the East compared to most areas of 

the West. 

(2) These regional differences in 

deposition correspond to the regional 

differences in oxides of nitrogen and 

SO2 concentrations and emissions, 

which are also higher in the East. 

Oxides of nitrogen emissions are much 

greater and generally more widespread 

than ammonia (NH3) emissions 

nationwide; high NH3 emissions tend to 

be more local (e.g., eastern North 

Carolina) or sub-regional (e.g., the upper 


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Midwest and Plains states). The relative 

amounts of oxidized versus reduced 

nitrogen deposition are consistent with 

the relative amounts of oxides of 

nitrogen and NH3 emissions. Oxidized 

nitrogen deposition exceeds reduced 

nitrogen deposition in most of the case 

study areas; the major exception being 

the Neuse River/Neuse River Estuary 

Case Study Area. 

(3) Reduced nitrogen deposition 

exceeds oxidized nitrogen deposition in 

the vicinity of local sources of NH3. 

(4) There can be relatively large 

spatial variations in both total reactive 

nitrogen deposition and sulfur 

deposition within a case study area; this 

occurs particularly in those areas that 

contain or are near a high emissions 

source of oxides of nitrogen, NH3 and/ 

or SO2. 

(5) The seasonal patterns in 

deposition differ between the case study 

areas. For the case study areas in the 

East, the season with the greatest 

amounts of total reactive nitrogen 

deposition correspond to the season 

with the greatest amounts of sulfur 

deposition. Deposition peaks in spring 

in the Adirondack, Hubbard Brook 

Experimental Forest, and Kane 

Experimental Forest case study areas, 

and it peaks in summer in the Potomac 

River/Potomac Estuary, Shenandoah, 

and Neuse River/Neuse River Estuary 

case study areas. For the case study 

areas in the West, there is less 

consistency in the seasons with greatest 

total reactive nitrogen and sulfur 

deposition in a given area. In general, 

both nitrogen and/or sulfur deposition 

peaks in spring or summer. The 

exception to this is the Sierra Nevada 

Range portion of the MCF Case Study 

Area, in which sulfur deposition is 

greatest in winter. 

ii. Aquatic Acidification Case Studies 

The role of aquatic acidification in 

two eastern United States areas— 

northeastern New York’s Adirondack 

area and the Shenandoah area in 

Virginia—was analyzed in the REA to 

assess surface water trends in SO4

2¥

and NO3

¥concentrations and ANC 

levels and to affirm the understanding 

that reductions in deposition could 

influence the risk of acidification. 

Monitoring data from the EPAadministered Temporally Integrated 

Monitoring of Ecosystems/Long-Term 

Monitoring (TIME/LTM) programs and 

the Environmental Monitoring and 

Assessment Program (EMAP) were 

assessed for the years 1990 to 2006, and 

past, present and future water quality 

levels were estimated using both steadystate and dynamic biogeochemical 

models. 

Although wet deposition rates for SO2

and oxides of nitrogen in the 

Adirondack Case Study Area have 

reduced since the mid-1990s, current 

concentrations are still well above preacidification (1860) conditions. For a 

discussion of the uncertainties of preacidification, see U.S. EPA, 2011, 

Appendix F. The Model of Acidification 

of Groundwater in Catchments (MAGIC) 

modeling predicts NO3

¥ and SO4

2¥ are 

17- and 5-fold higher today, 

respectively. The estimated average 

ANC for 44 lakes in the Adirondack 

Case Study Area is 62.1 meq/L (±15.7 

meq/L); 78 percent of all monitored lakes 

in the Adirondack Case Study Area have 

a current risk of Elevated, Severe, or 

Acute. Of the 78 percent, 31 percent 

experience episodic acidification, and 

18 percent are chronically acidic today. 

(1) Based on the steady-state critical 

load model for the year 2002, 18 

percent, 28 percent, 44 percent, and 58 

percent of 169 modeled lakes received 

combined total sulfur and nitrogen 

deposition that exceeded critical loads 

corresponding to ANC limits of 0, 20, 

50, and 100 meq/L respectively. 

(2) Based on a deposition scenario 

that maintains current emission levels 

to 2020 and 2050, the simulation 

forecast indicates no improvement in 

water quality in the Adirondack Case 

Study Area. The percentage of lakes 

within the Elevated to Acute Concern 

classes remains the same in 2020 and 

2050. 

(3) Since the mid-1990s, streams in 

the Shenandoah Case Study Area have 

shown slight declines in NO3 and SO4

2¥

concentrations in surface waters. The 

ANC levels increased from about 50 

meq/L in the early 1990s to >75 meq/L 

until 2002, when ANC levels declined 

back to 1991–1992 levels. Current 

concentrations are still above preacidification (1860) conditions. The 

MAGIC modeling predicts surface water 

concentrations of NO3 and SO4

2¥ are 

10- and 32-fold higher today, 

respectively. The estimated average 

ANC for 60 streams in the Shenandoah 

Case Study Area is 57.9 meq/L (±4.5 meq/ 

L). Fifty-five percent of all monitored 

streams in the Shenandoah Case Study 

Area have a current risk of Elevated, 

Severe, or Acute. Of the 55 percent, 18 

percent experience episodic 

acidification, and 18 percent are 

chronically acidic today. 

(4) Based on the steady-state critical 

load model for the year 2002, 52 

percent, 72 percent, 85 percent and 93 

percent of 60 modeled streams received 

combined total sulfur and nitrogen 

deposition that exceeded critical loads 

corresponding to ANC limits of 0, 20, 

50, and 100 meq/L respectively. 

(5) Based on a deposition scenario 

that maintains current emission levels 

to 2020 and 2050, the simulation 

forecast indicates that a large number of 

streams would still have Elevated to 

Acute problems with acidity. 

iii. Terrestrial Acidification Case 

Studies 

The role of terrestrial acidification 

was examined in the REA using a 

critical load analysis for sugar maple 

and red spruce forests in the eastern 

United States by using the BC/Al ratio 

in acidified forest soils as an indicator 

to assess the impact of nitrogen and 

sulfur deposition on tree health. These 

are the two most commonly studied 

species in North America for impacts of 

acidification. At a BC/Al ratio of 1.2, red 

spruce growth can be reduced by 20 

percent. Sugar maple growth can be 

reduced by 20 percent at a BC/Al ratio 

of 0.6. Key findings of the case study are 

summarized below. 

(1) Case study results suggest that the 

health of at least a portion of the sugar 

maple and red spruce growing in the 

United States may have been 

compromised with acidifying total 

nitrogen and sulfur deposition in 2002. 

The 2002 CMAQ/NADP total nitrogen 

and sulfur deposition levels exceeded 

three selected critical loads in 3 percent 

to 75 percent of all sugar maple plots 

across 24 states. The three critical loads 

ranged from 6,008 to 107 eq/ha/yr for 

the BC/Al ratios of 0.6, 1.2, and 10.0 

(increasing levels of tree protection). 

The 2002 CMAQ/NADP total nitrogen 

and sulfur deposition levels exceeded 

three selected critical loads in 3 percent 

to 36 percent of all red spruce plots 

across eight states. The three critical 

loads ranged from 4,278 to 180 eq/ha/ 

yr for the BC/Al ratios of 0.6, 1.2, and 

10.0 (increasing levels of tree 

protection). 

(2) The SMB model assumptions 

made for base cation weathering (Bcw) 

and forest soil ANC input parameters 

are the main sources of uncertainty 

since these parameters are rarely 

measured and require researchers to use 

default values. 

(3) The pattern of case study results 

suggests that nitrogen and sulfur 

acidifying deposition in the sugar maple 

and red spruce forest areas studied were 

similar in magnitude to the critical 

loads for those areas and both 

ecosystems are likely to be sensitive to 

any future changes in the levels of 

deposition. 


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iv. Aquatic Nutrient Enrichment Case 

Studies 

The role of nitrogen deposition in two 

main stem rivers feeding their 

respective estuaries was analyzed in the 

REA to determine if decreases in 

deposition could influence the risk of 

eutrophication as predicted using the 

ASSETS EI scoring system in tandem 

with SPARROW (SPAtially Referenced 

Regression on Watershed Attributes) 

modeling. This modeling approach 

provides a transferrable, intermediatelevel analysis of the linkages between 

atmospheric deposition and receiving 

waters, while providing results on 

which conclusions could be drawn. A 

summary of findings follows: 

(1) The 2002 CMAQ/NADP results 

showed that an estimated 40,770,000 

kilograms (kg) of total nitrogen was 

deposited in the Potomac River 

watershed. The SPARROW modeling 

predicted that 7,380,000 kg N/yr of the 

deposited nitrogen reached the estuary 

(20 percent of the total load to the 

estuary). The overall ASSETS EI for the 

Potomac River and Potomac Estuary was 

Bad (based on all sources of N). 

(2) To improve the Potomac River and 

Potomac Estuary ASSETS EI score from 

Bad to Poor, a decrease of at least 78 

percent in the 2002 total nitrogen 

atmospheric deposition load to the 

watershed would be required. 

(3) The 2002 CMAQ/NADP results 

showed that an estimated 18,340,000 kg 

of total nitrogen was deposited in the 

Neuse River watershed. The SPARROW 

modeling predicted that 1,150,000 kg N/ 

yr of the deposited nitrogen reached the 

estuary (26 percent of the total load to 

the estuary). The overall ASSETS EI for 

the Neuse River/Neuse River Estuary 

was Bad. 

(4) It was found that the Neuse River/ 

Neuse River Estuary ASSETS EI score 

could not be improved from Bad to Poor 

with decreases only in the 2002 

atmospheric deposition load to the 

watershed. Additional reductions would 

be required from other nitrogen sources 

within the watershed. 

The small effect of decreasing 

atmospheric deposition in the Neuse 

River watershed is because the other 

nitrogen sources within the watershed 

are more influential than atmospheric 

deposition in affecting the total nitrogen 

loadings to the Neuse River Estuary, as 

estimated with the SPARROW model. A 

water body’s response to nutrient 

loading depends on the magnitude (e.g., 

agricultural sources have a higher 

influence in the Neuse than in the 

Potomac), spatial distribution, and other 

characteristics of the sources within the 

watershed; therefore a reduction in 

nitrogen deposition does not always 

produce a linear response in reduced 

load to the estuary, as demonstrated by 

these two case studies. 

v. Terrestrial Nutrient Enrichment Case 

Studies 

California CSS and MCF communities 

were the focus of the Terrestrial 

Nutrient Enrichment Case Studies of the 

REA. Geographic information systems 

analysis supported a qualitative review 

of past field research to identify 

ecological benchmarks associated with 

CSS and mycorrhizal communities, as 

well as MCF’s nutrient-sensitive 

acidophyte lichen communities, fineroot biomass in Ponderosa pine and 

leached nitrate in receiving waters. 

These benchmarks, ranging from 3.1 to 

17 kg N/ha/yr, were compared to 2002 

CMAQ/NADP data to discern any 

associations between atmospheric 

deposition and changing communities. 

Evidence supports the finding that 

nitrogen alters CSS and MCF. Key 

findings include the following: 

(1) The 2002 CMAQ/NADP nitrogen 

deposition data show that the 3.3 kg N/ 

ha/yr benchmark has been exceeded in 

more than 93 percent of CSS areas 

(654,048 ha). This suggests that such 

deposition is a driving force in the 

degradation of CSS communities. One 

potentially confounding factor is the 

role of fire. Although CSS decline has 

been observed in the absence of fire, the 

contributions of deposition and fire to 

the CSS decline require further research. 

The CSS is fragmented into many small 

parcels, and the 2002 CMAQ/NADP 12- 

km grid data are not fine enough to fully 

validate the relationship between CSS 

distribution, nitrogen deposition, and 

fire. 

(2) The 2002 CMAQ/NADP nitrogen 

deposition data exceeds the 3.1 kg N/ha/ 

yr benchmark in more than 38percent 

(1,099,133 ha) of MCF areas, and nitrate 

leaching has been observed in surface 

waters. Ozone effects confound nitrogen 

effects on MCF acidophyte lichen, and 

the interrelationship between fire and 

nitrogen cycling requires additional 

research. 

c. Other Welfare Effects 

Ecological effects have also been 

documented across the United States 

where elevated nitrogen deposition has 

been observed, including the eastern 

slope of the Rocky Mountains where 

shifts in dominant algal species in 

alpine lakes have occurred where wet 

nitrogen deposition was only about 1.5 

kg N/ha/yr. High alpine terrestrial 

communities have a low capacity to 

sequester nitrogen deposition, and 

monitored deposition exceeding 3 to 4 

kg N/ha/yr could lead to communitylevel changes in plant species, lichens 

and mycorrhizae. 

Additional welfare effects are 

documented, but examined less 

extensively, in the REA. These effects 

include qualitative discussions related 

to visibility and materials damage, such 

as corrosion, erosion, and soiling of 

paint and buildings which are being 

addressed in the PM NAAQS review 

currently underway. A discussion of the 

causal relationship between sulfur 

deposition (as sulfate, SO4

2¥) and 

increased mercury methylation in 

wetlands and aquatic environments is 

also included in the REA. On this 

subject the REA concludes that 

decreases in SO4

2¥ deposition will 

likely result in decreases in MeHg 

concentration; however, spatial and 

biogeochemical variations nationally 

hinder establishing large scale doseresponse relationships. 

Several additional issues concerning 

oxides of nitrogen were addressed in the 

REA. Consideration was also given to 

N2O, a potent GHG. The REA concluded 

that it is most appropriate to analyze the 

role of N2O in the context of all of the 

GHGs rather than as part of the REA for 

this review. The REA considered 

nitrogen deposition and its correlation 

with the rate of photosynthesis and net 

primary productivity. Nitrogen addition 

ranging from 15.4 to 300 kg N/ha/yr is 

documented as increasing wetland N2O 

production by an average of 207 percent 

across all ecosystems. Nitrogen addition 

ranging from 30 to 240 kg N/ha/yr 

increased methane (CH4) emissions by 

115 percent, averaged across all 

ecosystems, and methane uptake was 

reduced by 38 percent averaged across 

all ecosystems when nitrogen addition 

ranged from 10 to 560 kg N/ha/yr, but 

reductions were only significant for 

coniferous and deciduous forests. The 

heterogeneity of ecosystems across the 

United States, however, introduces 

variations into dose-response 

relationships. 

The phytotoxic effects of oxides of 

nitrogen and sulfur on vegetation were 

also briefly discussed in the REA which 

concluded that since a unique 

secondary NAAQS exists for SO2, and 

concentrations of nitric oxide (NO), NO2

and PAN are rarely high enough to have 

phytotoxic effects on vegetation, further 

assessment was not warranted at this 

time. 

3. Overview of Adversity of Effects to 

Public Welfare 

Characterizing a known or anticipated 

adverse effect to public welfare is an 

important component of developing any 

secondary NAAQS. According to the 


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CAA, welfare effects include: ‘‘effects on 

soils, water, crops, vegetation, manmade 

materials, animals, wildlife, weather, 

visibility, and climate, damage to and 

deterioration of property, and hazards to 

transportation, as well as effect on 

economic values and on personal 

comfort and well-being, whether caused 

by transformation, conversion, or 

combination with other air pollutants’’ 

(CAA, Section 302(h)). While the text 

above lists a number of welfare effects, 

these effects do not define public 

welfare in and of themselves. 

Although there is no specific 

definition of adversity to public welfare, 

the paradigm of linking adversity to 

public welfare to 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. 

Similar concepts were used in past 

reviews of secondary NAAQS for ozone 

and PM (relating to visibility), as well as 

in initial reviews of effects from lead 

deposition. Because oxides of nitrogen 

and sulfur are deposited from ambient 

sources into ecosystems where they 

affect changes to organisms, populations 

and ecosystems, the concept of 

adversity to public welfare as a result of 

alterations in structure and function of 

ecosystems is an appropriate 

consideration for this review. 

Based on information provided in the 

PA, the following section discusses how 

ecological effects from deposition of 

oxides of nitrogen and sulfur relate to 

adversity to public welfare. In the PA, 

public welfare was discussed in terms of 

loss of ecosystem services (defined 

below), which in some cases can be 

monetized. Each of the four main effect 

areas (aquatic and terrestrial 

acidification and aquatic and terrestrial 

nutrient over-enrichment) are discussed 

including current ecological effects and 

associated ecosystem services. 

a. Ecosystem Services 

The PA defines ecosystem services as 

the benefits individuals and 

organizations obtain from ecosystems. 

Ecosystem services can be classified as 

provisioning (food and water), 

regulating (control of climate and 

disease), cultural (recreational, 

existence, spiritual, educational), and 

supporting (nutrient cycling). 

Conceptually, changes in ecosystem 

services may be used to aid in 

characterizing a known or anticipated 

adverse effect to public welfare. In the 

REA and PA ecosystem services are 

discussed as a method of assessing the 

magnitude and significance to the 

public of resources affected by ambient 

concentrations of oxides of nitrogen and 

sulfur and deposition in sensitive 

ecosystems. 

The EPA has in previous NAAQS 

reviews defined ecological goods and 

services for the purposes of a Regulatory 

Impact Analysis (RIA) as the ‘‘outputs of 

ecological functions or processes that 

directly or indirectly contribute to social 

welfare or have the potential to do so in 

the future. Some outputs may be bought 

and sold, but most are not marketed.’’ It 

is especially important to acknowledge 

that it is difficult to measure and/or 

monetize the goods and services 

supplied by ecosystems. It can be 

informative in characterizing adversity 

to public welfare to attempt to place an 

economic valuation on the set of goods 

and services that have been identified 

with respect to a change in policy 

however it must be noted that this 

valuation will be incomplete and 

illustrative only. 

Knowledge about the relationships 

linking ambient concentrations and 

ecosystem services is considered in the 

PA as one method by which to inform 

a policy judgment on a known or 

anticipated adverse public welfare 

effect. For example, a change in an 

ecosystem structure and process, such 

as foliar injury, would be classified as 

an ecological effect, with the associated 

changes in ecosystem services, such as 

primary productivity, food availability, 

forest products, and aesthetics (e.g., 

scenic viewing), classified as public 

welfare effects. Additionally, changes in 

biodiversity would be classified as an 

ecological effect, and the associated 

changes in ecosystem services— 

productivity, existence (nonuse) value, 

recreational viewing and aesthetics— 

would also be classified as public 

welfare effects. 

As described in chapters 4 and 5 of 

the REA, case study analyses were 

performed that link deposition in 

sensitive ecosystems to changes in a 

given ecological indicator (e.g., for 

aquatic acidification, to changes in 

ANC) and then to changes in 

ecosystems. Appendix 8 of the REA 

links the changes in ecosystems to the 

services they provide (e.g., fish species 

richness and its influence on 

recreational fishing). To the extent 

possible for each targeted effect area, the 

REA linked ambient concentrations of 

nitrogen and sulfur (i.e., ambient air 

quality indicators) to deposition in 

sensitive ecosystems (i.e., exposure 

pathways), and then to system response 

as measured by a given ecological 

indicator (e.g., lake and stream 

acidification as measured by ANC). The 

ecological effect (e.g., changes in fish 

species richness) was then, where 

possible, associated with changes in 

ecosystem services and the 

corresponding public welfare effects 

(e.g., recreational fishing). 

b. Effects on Ecosystem Services 

The process used to link ecological 

indicators to ecosystem services is 

discussed extensively in appendix 8 of 

the REA. In brief, for each case study 

area assessed, the ecological indicators 

are linked to an ecological response that 

is subsequently linked to associated 

services to the extent possible. For 

example, in the case study for aquatic 

acidification the chosen ecological 

indicator is ANC which can be linked to 

the ecosystem service of recreational 

fishing. Although recreational fishing 

losses are the only service effects that 

can be independently quantified or 

monetized at this time, there are 

numerous other ecosystem services that 

may be related to the ecological effects 

of acidification. 

While aquatic acidification is the 

focus of this proposed standard, the 

other effect areas were also analyzed in 

the REA and these ecosystems are being 

harmed by nitrogen and sulfur 

deposition and will obtain some 

measure of protection with any decrease 

in that deposition regardless of the 

reason for the decrease. The following 

summarizes the current levels of 

specific ecosystem services for aquatic 

and terrestrial acidification and aquatic 

and terrestrial nutrient over-enrichment 

and attempts to quantify and when 

possible monetize the harm to public 

welfare, as represented by ecosystem 

services, due to nitrogen and sulfur 

deposition. 

i. Aquatic Acidification 

Acidification of aquatic ecosystems 

primarily affects the ecosystem services 

that are derived from the fish and other 

aquatic life found in surface waters. In 

the northeastern United States, the 

surface waters affected by acidification 

are not a major source of commercially 

raised or caught fish; however, they are 

a source of food for some recreational 

and subsistence fishers and for other 

consumers. Although data and models 

are available for examining the effects 

on recreational fishing, relatively little 

data are available for measuring the 

effects on subsistence and other 

consumers. Inland waters also provide 

aesthetic and educational services along 

with non-use services, such as existence 

value (protection and preservation with 


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no expectation of direct use). In general, 

inland surface waters such as lakes, 

rivers, and streams also provide a 

number of regulating services, playing a 

role in hydrological regimes and climate 

regulation. There is little evidence that 

acidification of freshwaters in the 

northeastern United States has 

significantly degraded these specific 

services; however, freshwater 

ecosystems also provide biological 

control services by providing 

environments that sustain delicate 

aquatic food chains. The toxic effects of 

acidification on fish and other aquatic 

life impair these services by disrupting 

the trophic structure of surface waters. 

Although it is difficult to quantify these 

services and how they are affected by 

acidification, it is worth noting that 

some of these services may be captured 

through measures of provisioning and 

cultural services. For example, these 

biological control services may serve as 

‘‘intermediate’’ inputs that support the 

production of ‘‘final’’ recreational 

fishing and other cultural services. 

As summarized in Chapter 4 of the 

PA, recent studies indicate that 

acidification of lakes and streams can 

result in significant loss in economic 

value. Embedded in these numbers is a 

degree of harm to recreational fishing 

services due to acidification that has 

occurred over time. These harms have 

not been quantified on a regional scale; 

however, a case study was conducted in 

the Adirondacks area (U.S. EPA, 2011, 

section 4.4.2). 

In the Adirondacks case study, 

estimates of changes in recreational 

fishing services were determined, as 

well as changes more broadly in 

‘‘cultural’’ ecosystem services 

(including recreational, aesthetic, and 

nonuse services). First, the MAGIC 

model (U.S. EPA, 2009, Appendix 8 and 

section 2.2) was applied to 44 lakes to 

predict what ANC levels would be 

under both ‘‘business as usual’’ 

conditions (i.e., allowing for some 

decline in deposition due to existing 

regulations) and pre-emission (i.e., 

background) conditions. Second, to 

estimate the recreational fishing impacts 

of aquatic acidification in these lakes, 

an existing model of recreational fishing 

demand and site choice was applied. 

This model predicts how recreational 

fishing patterns in the Adirondacks 

would differ and how much higher the 

average annual value of recreational 

fishing services would be for New York 

residents if lake ANC levels 

corresponded to background (rather 

than business as usual) conditions. To 

estimate impacts on a broader category 

of cultural (and some provisioning) 

ecosystem services, results from the 

Banzhaf et al (2006) valuation survey of 

New York residents were adapted and 

applied to this context. The focus of the 

survey was on impacts on aquatic 

resources. Pretesting of the survey 

indicated that respondents nonetheless 

tended to assume that benefits would 

occur in the condition of birds and 

forests as well as in recreational fishing. 

The REA estimated 44 percent of the 

Adirondack lakes currently fall below 

an ANC of 50 meq/L. Several states have 

set goals for improving the acid status 

of lakes and streams, generally targeting 

ANC in the range of 50 to 60 meq/L, and 

have engaged in costly activities to 

decrease acidification. 

These results imply significant value 

to the public in addition to those 

derived from recreational fishing 

services. Note that the results are only 

applicable to improvements in the 

Adirondacks valued by residents of New 

York. If similar benefits exist in other 

acid-impacted areas, benefits for the 

nation as a whole could be substantial. 

The analysis provides results on only a 

subset of the impacts of acidification on 

ecosystem services and suggests that the 

overall impact on these services could 

be substantial. 

ii. Terrestrial Acidification 

Chapters 4.4.3 and 4.4.4 of the PA 

review several economic studies of areas 

sensitive to terrestrial acidification. 

Forests in the northeastern United 

States provide several important and 

valuable provisioning ecosystem 

services, which are reflected in the 

production and sales of tree products. 

Sugar maples are a particularly 

important commercial hardwood tree 

species in the United States, producing 

timber and maple syrup that provide 

hundreds of millions of dollars in 

economic value annually. Red spruce is 

also used in a variety of wood products 

and provides up to $100 million in 

economic value annually. Although the 

data do not exist to directly link 

acidification damages to economic 

values of lost recreational ecosystem 

services in forests, these resources are 

valuable to the public. The EPA is not 

able to quantify at this time the specific 

effects on these values of acid 

deposition, or of any specific reductions 

in deposition, relative to the effects of 

many other factors that may affect them. 

iii. Nutrient Enrichment 

Chapters 4.4.5 and 4.4.6 of the PA 

summarize economic studies of east 

coast estuaries affected by nutrient overenrichment or eutrophication. Estuaries 

in the eastern United States are 

important for fish and shellfish 

production. The estuaries are capable of 

supporting large stocks of resident 

commercial species, and they serve as 

the breeding grounds and interim 

habitat for several migratory species. To 

provide an indication of the magnitude 

of provisioning services associated with 

coastal fisheries, from 2005 to 2007, the 

average value of total catch was $1.5 

billion per year in 15 East Coast states. 

Estuaries also provide an important and 

substantial variety of cultural ecosystem 

services, including water-based 

recreational and aesthetic services. For 

example, data indicate that 4.8 percent 

of the population in coastal states from 

North Carolina to Massachusetts 

participated in saltwater fishing, with a 

total of 26 million saltwater fishing days 

in 2006. Recreational participation 

estimates for 1999–2000 showed almost 

6 million individuals participated in 

motor boating in coastal states from 

North Carolina to Massachusetts. The 

EPA is not able to quantify at this time 

the specific effects on these values of 

nitrogen deposition, or of any specific 

reductions in deposition, relative to the 

effects of many other factors that may 

affect them. 

Terrestrial ecosystems can also suffer 

from nutrient over-enrichment. Each 

ecosystem is different in its composition 

of species and nutrient requirements. 

Changes to individual ecosystems from 

changes in nitrogen deposition can be 

hard to assess economically. Relative 

recreational values are often determined 

by public use information. Chapter 4.4.7 

of the PA reviewed studies related to 

park use in California. Data from 

California State Parks indicate that in 

2002, 68.7 percent of surveyed 

individuals participated in trail hiking 

for an average of 24.1 days per year. The 

EPA is not able to quantify at this time 

the specific effects on these values of 

nitrogen deposition, or of any specific 

reductions in deposition, relative to the 

effects of many other factors that may 

affect them. 

The PA also identified fire regulation 

as a service that could be affected by 

nutrient over-enrichment of the CSS and 

MCF ecosystems by encouraging growth 

of more flammable grasses, increasing 

fuel loads, and altering the fire cycle. 

Over the 5-year period from 2004 to 

2008, Southern California experienced, 

on average, over 4,000 fires per year, 

burning, on average, over 400,000 acres 

per year. It is not possible at this time 

to quantify the contribution of nitrogen 

deposition, among many other factors, 

to increased fire risk. 

c. Summary 

Adversity to public welfare can be 

understood by looking at how 

deposition of oxides of nitrogen and 


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20234 Federal Register / Vol. 77, No. 64 / Tuesday, April 3, 2012 / Rules and Regulations 

3The current primary NO2 standard has recently 

been changed to the 3-year average of the 98th 

percentile of the annual distribution of the 1 hour 

daily maximum of the concentration of NO2. The 

current secondary standard remains as it was set in 

1971. 

sulfur affect the ecological functions of 

an ecosystem (see II.A.), and then 

understanding the ecosystem services 

that are degraded. The monetized value 

of the ecosystem services provided by 

ecosystems that are sensitive to 

deposition of oxides of nitrogen and 

sulfur are in the billions of dollars each 

year, though it is not possible to 

quantify or monetize at this time the 

effects on these values of nitrogen and 

sulfur deposition or of any changes in 

deposition that may result from new 

secondary standards. Many lakes and 

streams are known to be degraded by 

acidic deposition which affects 

recreational fishing and tourism. Forest 

growth is likely suffering from acidic 

deposition in sensitive areas affecting 

red spruce and sugar maple timber 

production, sugar maple syrup 

production, hiking, aesthetic enjoyment 

and tourism. Nitrogen deposition 

contributes significantly to 

eutrophication in many estuaries 

affecting fish production, swimming, 

boating, aesthetic enjoyment and 

tourism. Ecosystem services are likely 

affected by nutrient enrichment in many 

natural and scenic terrestrial areas, 

affecting biodiversity, including habitat 

for rare and endangered species, fire 

control, hiking, aesthetic enjoyment and 

tourism. 

B. Adequacy of the Current Standards 

An important issue to be addressed in 

this review of the secondary standards 

for oxides of nitrogen and sulfur is 

whether, in view of the scientific 

evidence reflected in the ISA, additional 

information on exposure and risk 

discussed in the REA, and conclusions 

drawn from the PA, the current 

standards provide adequate protection 

of public welfare. In this review, 

consideration is given to the adequacy 

of the current standards with regard to 

both the direct effects of exposure to 

gaseous oxides of nitrogen and sulfur on 

vegetation and on potentially adverse 

deposition-related effects on sensitive 

aquatic and terrestrial ecosystems. This 

section is drawn from section II.D of the 

proposal. The following discussion 

summarizes the considerations related 

to the adequacy of the standards as 

discussed in the PA (section II.B.1), 

CASAC’s views on adequacy (section 

II.B.2), and the Administrator’s 

proposed conclusions on the adequacy 

of the current standards. 

1. Adequacy Considerations 

This discussion is based on the 

information presented in the PA and 

includes considerations related to the 

adequacy of the current NO2 and SO2

secondary standards with regard to 

direct effects (section II.B.1.a), as well as 

considerations related to both the 

appropriateness and the adequacy of 

protection of the current standards with 

regard to deposition-related effects 

(section II.B.1.b). 

a. Adequacy of the Current Standards 

for Direct Effects 

For oxides of nitrogen, the current 

secondary standard was set identical to 

the primary standard,3 i.e., an annual 

standard set for NO2 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. In considering 

the adequacy of these standards, it is 

appropriate to consider whether they 

are adequate to protect against the direct 

effects on vegetation resulting from 

exposure to ambient oxides of nitrogen 

and sulfur, which was the basis for 

initially setting the standards in 1971. 

The ISA concludes that there was 

sufficient evidence to infer a causal 

relationship between exposure to SO2, 

NO, NO2 and PAN and injury to 

vegetation. Additional research on acute 

foliar injury has been limited and there 

is no evidence to suggest foliar injury 

below the levels of the current 

secondary standards. Based on 

information in the ISA, the PA 

concludes that there is sufficient 

evidence to suggest that the levels of the 

current standards are likely adequate to 

protect against phytotoxic effects caused 

by direct gas-phase exposure. 

b. Appropriateness and Adequacy of the 

Current Standards for Depositionrelated Effects 

This section addresses two concepts 

necessary to evaluate the current 

standards in the context of depositionrelated effects. First, appropriateness of 

the current standards is considered with 

regard to indicator, form, level and 

averaging time. This discussion 

includes particular emphasis on the 

indicators and forms of the current 

standards and the degree to which they 

are ecologically relevant with regard to 

deposition-related effects that vary 

spatially and temporally. Second, this 

section considers the current standards 

in terms of adequacy of protection. 

i. Appropriateness 

The ISA has established that the 

major effects of concern for this review 

are associated with deposition of 

nitrogen and sulfur caused by 

atmospheric concentrations of oxides of 

nitrogen and sulfur. As discussed 

below, 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. 

Four issues arise that call into 

question the ecological relevance of the 

structure of the current secondary 

standards for oxides of nitrogen and 

sulfur. 

(1) The current SO2 secondary 

standard (0.5 ppm SO2 over a 3-hour 

average) does not utilize an averaging 

time that relates to an exposure period 

that is relevant for ecosystem impacts. 

The majority of deposition-related 

impacts are associated with depositional 

loads that occur over periods of months 

to years. This differs significantly from 

exposures associated with hourly 

concentrations of SO2 as measured by 

the current secondary standard. By 

addressing short-term concentrations, 

the current SO2 secondary standard, 

while protective against direct foliar 

effects from gaseous oxides of sulfur, 

does not take into account the findings 

of effects in the ISA, which notes the 

relationship between annual deposition 

of sulfur and acidification effects which 

are likely to be more severe and 

widespread than phytotoxic effects 

under current ambient conditions, and 

include effects from long-term and 

short-term deposition. Acidification is a 

process that occurs over time because 

the ability of an aquatic system to 

counteract acidic inputs is reduced as 

natural buffers are used more rapidly 

than they can be replaced through 

geologic weathering. The relevant 

period of exposure for ecosystems is, 

therefore, not the exposures captured in 

the short averaging time of the current 

SO2 secondary standard. The current 

secondary standard for oxides of 

nitrogen is an annual standard (0.053 

ppm averaged over 1 year) and as such 

the averaging time of the standard is 

more ecologically relevant. 

(2) Current standards do not utilize 

appropriate atmospheric indicators. 

Nitrogen dioxide and SO2 are used as 

the species of oxides of nitrogen and 

sulfur that are measured to determine 

compliance with the standards, but they 

do not capture all relevant chemical 

species of oxides of nitrogen and sulfur 

that contribute to deposition-related 


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effects. The ISA provides evidence that 

deposition-related effects are associated 

with total nitrogen and total sulfur 

deposition, and thus all chemical 

species of oxidized nitrogen and 

oxidized sulfur that are deposited will 

contribute to effects on ecosystems. 

Thus, by using atmospheric NO2 and 

SO2 concentrations as indicators, the 

current standards address only a 

fraction of total atmospheric oxides of 

nitrogen and sulfur, and do not take into 

account the effects from deposition of 

total atmospheric oxides of nitrogen and 

sulfur. This suggests that more 

comprehensive atmospheric indicators 

should be considered in designing 

ecologically relevant standards. 

(3) Current standards reflect separate 

assessments of the two individual 

pollutants, NO2 and SO2, rather than 

assessing the joint impacts of deposition 

of nitrogen and sulfur to ecosystems. 

Recognizing the role that each pollutant 

plays in jointly affecting ecosystem 

indicators, functions, and services is 

vital to developing a meaningful 

standard. The clearest example of this 

interaction is in assessment of the 

impacts of acidifying deposition on 

aquatic ecosystems. Acidification in an 

aquatic ecosystem depends on the total 

acidifying potential of the nitrogen and 

sulfur deposition resulting from oxides 

of nitrogen and sulfur as well as the 

inputs from other sources of nitrogen 

and sulfur such as reduced nitrogen and 

non-atmospheric sources. It is the joint 

impact of the two pollutants that 

determines the ultimate effect on 

organisms within the ecosystem, and 

critical ecosystem functions such as 

habitat provision and biodiversity. 

Standards that are set independently are 

less able to account for the contribution 

of the other pollutant. This suggests that 

interactions between oxides of nitrogen 

and oxides of sulfur should be a critical 

element of the conceptual framework for 

ecologically relevant standards. There 

are also important interactions between 

oxides of nitrogen and sulfur and 

reduced forms of nitrogen, which also 

contribute to acidification and nutrient 

enrichment. It is important that the 

structure of the standards address the 

role of reduced nitrogen in determining 

the ecological effects resulting from 

deposition of atmospheric oxides of 

nitrogen and sulfur. Consideration will 

also have to be given to total loadings 

as ecosystems respond to all sources of 

nitrogen and sulfur. 

(4) Current standards do not take into 

account variability in ecosystem 

sensitivity. Ecosystems are not 

uniformly distributed either spatially or 

temporally in their sensitivity to oxides 

of nitrogen and sulfur. Therefore, failure 

to account for the major determinants of 

variability, including geological and soil 

characteristics related to the sensitivity 

to acidification or nutrient enrichment, 

as well as atmospheric and landscape 

characteristics that govern rates of 

deposition, may lead to standards that 

do not provide requisite levels of 

protection across ecosystems. The 

current structures of the standards do 

not address the complexities in the 

responses of ecosystems to deposition of 

oxides of nitrogen and sulfur. 

Ecosystems contain complex groupings 

of organisms that respond in various 

ways to the alterations of soil and water 

that result from deposition of nitrogen 

and sulfur compounds. Different 

ecosystems therefore respond 

depending on a multitude of factors that 

control how deposition is integrated 

into the system. For example, the same 

levels of deposition falling on limestone 

dominated soils have a very different 

effect from those falling on shallow 

glaciated soils underlain with granite. 

One system may over time display no 

obvious detriment while the other may 

experience a catastrophic loss in fish 

communities. This degree of sensitivity 

is a function of many atmospheric 

factors that control rates of deposition as 

well as ecological factors that control 

how an ecosystem responds to that 

deposition. The current standards do 

not take into account spatial and 

seasonal variations, not only in 

depositional loadings, but also in 

sensitivity of ecosystems exposed to 

those loadings. Based on the discussion 

summarized above, the PA concludes 

that the current secondary standards for 

oxides of nitrogen and oxides of sulfur 

are not ecologically relevant in terms of 

averaging time, form, level or indicator. 

ii. Adequacy of Protection 

As described in the PA, ambient 

conditions in 2005 indicate that the 

current SO2 and NO2 secondary 

standards were not exceeded at that 

time (U.S. EPA, 2011, Figures 6–1 and 

6–2) in locations where negative 

ecological effects have been observed. In 

many locations, SO2 and NO2

concentrations are substantially below 

the levels of the secondary standards. 

This pattern suggests that levels of 

deposition and any negative effects on 

ecosystems due to deposition of oxides 

of nitrogen and sulfur under recent 

conditions are occurring even though 

areas meet or are below current 

standards. In addition, based on 

conclusions in the REA, these levels 

will not decline in the future to levels 

below which it is reasonable to 

anticipate effects. 

In determining the adequacy of the 

current secondary standards for oxides 

of nitrogen and sulfur the PA 

considered the extent to which ambient 

deposition contributes to loadings in 

ecosystems. Since the last review of the 

secondary standard for oxides of 

nitrogen, a great deal of information on 

the contribution of atmospheric 

deposition associated with ambient 

oxides of nitrogen has become available. 

The REA presents a thorough 

assessment of the contribution of 

oxidized nitrogen relative to total 

nitrogen deposition throughout the 

United States, and the relative 

contributions of ambient oxidized and 

reduced forms of nitrogen. The REA 

concludes that based on that analysis, 

ambient oxides of nitrogen are a 

significant component of atmospheric 

nitrogen deposition, even in areas with 

relatively high rates of reduced nitrogen 

deposition. In addition, atmospheric 

deposition of oxidized nitrogen 

contributes significantly to total 

nitrogen loadings in nitrogen sensitive 

ecosystems. 

The ISA summarizes the available 

studies of relative nitrogen contribution 

and finds that in much of the United 

States, oxides of nitrogen contribute 

from 50 to 75 percent of total 

atmospheric deposition relative to total 

reactive nitrogen, which includes 

oxidized and reduced nitrogen species 

(U.S. EPA, 2008, section 2.8.4). 

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. In several 

large estuarine systems, including the 

Chesapeake Bay, atmospheric 

deposition accounts for between 10 and 

40 percent of total nitrogen loadings 

(U.S. EPA, 2008). 

Atmospheric concentrations of oxides 

of sulfur account for nearly all sulfur 

deposition in the U.S. For the period 

2004–2006, mean sulfur deposition in 

the United States was greatest east of the 

Mississippi River with the highest 

deposition amount, 21.3 kg S/ha-yr, in 

the Ohio River Valley where most 

recording stations reported 3-year 

averages >10 kg S/ha-yr. Numerous 

other stations in the East reported S 

deposition >5 kg S/ha-yr. Total sulfur 

deposition in the United States west of 


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the 100th meridian was relatively low, 

with all recording stations reporting <2 

kg S/ha-yr and many reporting <1 kg S/ 

ha-yr. Sulfur was primarily deposited in 

the form of wet SO4

2¥ followed in 

decreasing order by a smaller proportion 

of dry SO2 and a much smaller 

proportion of deposition as dry SO4

2¥.

As discussed throughout the REA 

(U.S. EPA, 2009 and section II.B above), 

there are several key areas of risk that 

are associated with ambient 

concentrations of oxides of nitrogen and 

sulfur. As noted earlier, in previous 

reviews of the secondary standards for 

oxides of nitrogen and sulfur, the 

standards were designed to protect 

against direct exposure of plants to 

ambient concentrations of the 

pollutants. A significant shift in 

understanding of the effects of oxides of 

nitrogen and sulfur has occurred since 

the last reviews, reflecting the large 

amount of research that has been 

conducted on the effects of deposition 

of nitrogen and sulfur to ecosystems. 

The most significant current risks of 

adverse effects to public welfare are 

those related to deposition of oxides of 

nitrogen and sulfur to both terrestrial 

and aquatic ecosystems. These risks fall 

into two categories, acidification and 

nutrient enrichment, which were 

emphasized in the REA as most relevant 

to evaluating the adequacy of the 

existing standards in protecting public 

welfare from adverse ecological effects. 

(a) Aquatic Acidification 

The focus of the REA case studies was 

to determine whether deposition of 

sulfur and oxidized nitrogen in 

locations where ambient oxides of 

nitrogen and sulfur were at or below the 

current standards resulted in 

acidification and related effects, 

including episodic acidification and 

mercury methylation. Based on the case 

studies conducted for lakes in the 

Adirondacks and streams in 

Shenandoah National Park (case studies 

are discussed more fully in section II.B 

and U.S. EPA, 2009), there is significant 

risk to acid sensitive aquatic ecosystems 

at atmospheric concentrations of oxides 

of nitrogen and sulfur at or below the 

current standards. The REA also 

strongly supports a relationship 

between atmospheric deposition of 

oxides of nitrogen and sulfur and loss of 

ANC in sensitive ecosystems and 

indicates that ANC is an excellent 

indicator of aquatic acidification. The 

REA also concludes that at levels of 

deposition associated with oxides of 

nitrogen and sulfur concentrations at or 

below the current standards, ANC levels 

are expected to be below benchmark 

values that are associated with 

significant losses in fish species 

richness. 

Significant portions of the United 

States are acid sensitive, and current 

deposition levels exceed those that 

would allow recovery of the most acid 

sensitive lakes in the Adirondacks (U.S. 

EPA, 2008, Executive Summary). In 

addition, because of past loadings, areas 

of the Shenandoah are sensitive to 

current deposition levels (U.S. EPA, 

2008, Executive Summary). Parts of the 

West are naturally less sensitive to 

acidification and subjected to lower 

deposition (particularly oxides of sulfur) 

levels relative to the eastern United 

States, and as such, less focus in the ISA 

is placed on the adequacy of the existing 

standards in these areas, with the 

exception of the mountainous areas of 

the West, which experience episodic 

acidification due to deposition. 

In describing the effects of 

acidification in the two case study areas 

the REA uses the approach of describing 

benchmarks in terms of ANC values. 

Many locations in sensitive areas of the 

United States have ANC levels below 

benchmark levels for ANC classified as 

severe, elevated, or moderate concern 

(U.S. EPA, 2011, Figure 2–1). The 

average current ANC levels across 44 

lakes in the Adirondack case study area 

is 62.1 meq/L (moderate concern). 

However, 44 percent of lakes had 

deposition levels exceeding the critical 

load for an ANC of 50 meq/L (elevated), 

and 28 percent of lakes had deposition 

levels exceeding the (higher) critical 

load for an ANC of 20 meq/L (severe) 

(U.S. EPA, 2009, section 4.2.4.2). This 

information indicates that almost half of 

the 44 lakes in the Adirondacks case 

study area are at an elevated concern 

level, and almost a third are at a severe 

concern level. These levels are 

associated with greatly diminished fish 

species diversity, and losses in the 

health and reproductive capacity of 

remaining populations. Based on 

assessments of the relationship between 

number of fish species and ANC level in 

both the Adirondacks and Shenandoah 

areas, the number of fish species is 

decreased by over half at an ANC level 

of 20 meq/L relative to an ANC level at 

100 meq/L (U.S. EPA, 2009, Figure 4.2– 

1). When extrapolated to the full 

population of lakes in the Adirondacks 

area using weights based on the EMAP 

probability survey (U.S. EPA, 2009, 

section 4.2.6.1), 36 percent of lakes 

exceeded the critical load for an ANC of 

50 meq/L and 13 percent of lakes 

exceeded the critical load for an ANC of 

20 meq/L. 

Many streams in the Shenandoah case 

study area also have levels of deposition 

that are associated with ANC levels 

classified as severe, elevated, or 

moderate concern. The average ANC 

under recent conditions for the 60 

streams evaluated in the Shenandoah 

case study area is 57.9 meq/L, indicating 

moderate concern. However, 85 percent 

of these streams had recent deposition 

exceeding the critical load for an ANC 

of 50 meq/L, and 72 percent exceeded 

the critical load for an ANC of 20 meq/ 

L. As with the Adirondacks area, this 

information suggests that ANC levels 

may decline in the future and 

significant numbers of sensitive streams 

in the Shenandoah area are at risk of 

adverse impacts on fish populations if 

recent conditions persist. Many other 

streams in the Shenandoah area are also 

likely to experience conditions of 

elevated to severe concern based on the 

prevalence in the area of bedrock 

geology associated with increased 

sensitivity to acidification suggesting 

that effects due to stream acidification 

could be widespread in the Shenandoah 

area (U.S. EPA, 2009, section 4.2.6.2). 

In addition to these chronic 

acidification effects, the ISA notes that 

‘‘consideration of episodic acidification 

greatly increases the extent and degree 

of estimated effects for acidifying 

deposition on surface waters’’ (U.S. 

EPA, 2008, section 3.2.1.6). Some 

studies show that the number of lakes 

that could be classified as acid-impacted 

based on episodic acidification is 2 to 3 

times the number of lakes classified as 

acid-impacted based on chronic ANC. 

These episodic acidification events can 

have long-term effects on fish 

populations (U.S. EPA, 2008, section 

3.2.1.6). Under recent conditions, 

episodic acidification has been observed 

in locations in the eastern United States 

and in the mountainous western United 

States (U.S. EPA, 2008, section 3.2.1.6). 

The ISA, REA and PA all conclude 

that the current standards are not 

adequate to protect against the adverse 

impacts of aquatic acidification on 

sensitive ecosystems. A recent survey, 

as reported in the ISA, found sensitive 

streams in many locations in the United 

States, including the Appalachian 

Mountains, the Coastal Plain, and the 

Mountainous West (U.S. EPA, 2008, 

section 4.2.2.3). In these sensitive areas, 

between 1 and 6 percent of stream 

kilometers are chronically acidified. The 

REA further concludes that both the 

Adirondack and Shenandoah case study 

areas are currently receiving deposition 

from ambient oxides of nitrogen and 

sulfur in excess of their ability to 

neutralize such inputs. In addition, 

based on the current emission scenarios, 

forecast modeling out to the year 2020 

as well as 2050 indicates a large number 

of streams in these areas will still be 


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adversely impacted (section II.B). Based 

on these considerations, the PA 

concludes that the current secondary 

NAAQS for oxides of nitrogen and 

sulfur do not provide adequate 

protection of sensitive ecosystems with 

regard to aquatic acidification. 

(b) Terrestrial Acidification 

Based on the terrestrial acidification 

case studies, Kane Experimental Forest 

in Pennsylvania and Hubbard Brook 

Experimental Forest described in 

section II.B of sugar maple and red 

spruce habitat, the REA concludes that 

there is significant risk to sensitive 

terrestrial ecosystems from acidification 

at atmospheric concentrations of NO2

and SO2 at or below the current 

standards. The ecological indicator 

selected for terrestrial acidification is 

the BC/Al, which has been linked to tree 

health and growth. The results of the 

REA strongly support a relationship 

between atmospheric deposition of 

oxides of nitrogen and sulfur and BC/Al, 

and that BC/Al is a good indicator of 

terrestrial acidification. At levels of 

deposition associated with oxides of 

nitrogen and sulfur concentrations at or 

below the current standards, BC/Al 

levels are expected to be below 

benchmark values that are associated 

with significant effects on tree health 

and growth. Such degradation of 

terrestrial ecosystems could affect 

ecosystem services such as habitat 

provisioning, endangered species, goods 

production (timber, syrup, etc.) among 

others. 

Many locations in sensitive areas of 

the United States have BC/Al levels 

below benchmark levels classified as 

providing low to intermediate levels of 

protection to tree health. At a BC/Al 

ratio of 1.2 (intermediate level of 

protection), red spruce growth can be 

reduced by 20 percent. At a BC/Al ratio 

of 0.6 (low level of protection), sugar 

maple growth can be decreased by 20 

percent. The REA did not evaluate 

broad sensitive regions. However, in the 

sugar maple case study area (Kane 

Experimental Forest), recent deposition 

levels are associated with a BC/Al ratio 

below 1.2, indicating between 

intermediate and low level of 

protection, which would indicate the 

potential for a greater than 20 percent 

reduction in growth. In the red spruce 

case study area (Hubbard Brook 

Experimental Forest), recent deposition 

levels are associated with a BC/Al ratio 

slightly above 1.2, indicating slightly 

better than an intermediate level of 

protection (U.S. EPA, 2009, section 

4.3.5.1). 

Over the full range of sugar maple, 12 

percent of evaluated forest plots 

exceeded the critical loads for a BC/Al 

ratio of 1.2, and 3 percent exceeded the 

critical load for a BC/Al ratio of 0.6. 

However, there was large variability 

across states. In New Jersey, 67 percent 

of plots exceeded the critical load for a 

BC/Al ratio of 1.2, while in several 

states on the outskirts of the range for 

sugar maple (e.g. Arkansas, Illinois) no 

plots exceeded the critical load for a BC/ 

Al ratio of 1.2. For red spruce, overall 

5 percent of plots exceeded the critical 

load for a BC/Al ratio of 1.2, and 3 

percent exceeded the critical load for a 

BC/Al ratio of 0.6. In the major red 

spruce producing states (Maine, New 

Hampshire, and Vermont), critical loads 

for a BC/Al ratio of 1.2 were exceeded 

in 0.5, 38, and 6 percent of plots, 

respectively. 

The ISA, REA and PA all conclude 

that the current standards are not 

adequate to protect against the adverse 

impacts of terrestrial acidification on 

sensitive ecosystems. As stated in the 

REA and PA, the main drawback, with 

the understanding of terrestrial 

acidification lies in the sparseness of 

available data by which we can predict 

critical loads and that the data are based 

on laboratory responses rather than field 

measurements. Other stressors that are 

present in the field but that are not 

present in the laboratory may confound 

this relationship. The REA does 

however, conclude that the case study 

results, when extended to a 27 state 

region, show that nitrogen and sulfur 

acidifying deposition in the sugar maple 

and red spruce forest areas caused the 

calculated Bc/Al ratio to fall below 1.2 

(the intermediate level of protection) in 

12 percent of the sugar maple plots and 

5 percent of the red spruce plots; 

however, results from individual states 

ranged from 0 to 67 percent of the plots 

for sugar maple and 0 to 100 percent of 

the plots for red spruce. 

(c) Terrestrial Nutrient Enrichment 

Nutrient enrichment effects are due to 

nitrogen loadings from both 

atmospheric and non-atmospheric 

sources. Evaluation of nutrient 

enrichment effects requires an 

understanding that nutrient inputs are 

essential to ecosystem health and that 

specific long-term levels of nutrients in 

a system affect the types of species that 

occur over long periods of time. Shortterm additions of nutrients can affect 

species competition, and even small 

additions of nitrogen in areas that are 

traditionally nutrient poor can have 

significant impacts on productivity as 

well as species composition. Most 

ecosystems in the United States are 

nitrogen-limited, so regional decreases 

in emissions and deposition of airborne 

nitrogen compounds could lead to some 

decrease in growth of the vegetation that 

surrounds the targeted aquatic system 

but as discussed below evidence for this 

is mixed. Whether these changes in 

plant growth are seen as beneficial or 

adverse will depend on the nature of the 

ecosystem being assessed. 

Information on the effects of changes 

in nitrogen deposition on forestlands 

and other terrestrial ecosystems is very 

limited. The multiplicity of factors 

affecting forests, including other 

potential stressors such as ozone, and 

limiting factors such as moisture and 

other nutrients, confound assessments 

of marginal changes in any one stressor 

or nutrient in forest ecosystems. The 

ISA notes that only a fraction of the 

deposited nitrogen is taken up by the 

forests, most of the nitrogen is retained 

in the soils (U.S. EPA, 2008, section 

3.3.2.1). In addition, the ISA indicates 

that forest management practices can 

significantly affect the nitrogen cycling 

within a forest ecosystem, and as such, 

the response of managed forests to 

nitrogen deposition will be variable 

depending on the forest management 

practices employed in a given forest 

ecosystem (U.S. EPA, 2008, Annex C 

C.6.3). Increases in the availability of 

nitrogen in nitrogen-limited forests via 

atmospheric deposition could increase 

forest production over large nonmanaged areas, but the evidence is 

mixed, with some studies showing 

increased production and other showing 

little effect on wood production (U.S. 

EPA, 2008, section 3.3.9). Because 

leaching of nitrate can promote cation 

losses, which in some cases create 

nutrient imbalances, slower growth and 

lessened disease and freezing tolerances 

for forest trees, the net effect of 

increased N on forests in the United 

States is uncertain (U.S. EPA, 2008, 

section 3.3.9). 

The scientific literature has many 

examples of the deleterious effects 

caused by excessive nitrogen loadings to 

terrestrial systems. Several studies have 

set benchmark values for levels of N 

deposition at which scientifically 

adverse effects are known to occur. 

Large areas of the country appear to be 

experiencing deposition above these 

benchmarks. The ISA indicates studies 

that have found that at 3.1 kg N/ha/yr, 

the community of lichens begins to 

change from acidophytic to tolerant 

species; at 5.2 kg N/ha/yr, the typical 

dominance by acidophytic species no 

longer occurs; and at 10.2 kg N/ha/yr, 

acidophytic lichens are totally lost from 

the community. Additional studies in 

the Colorado Front Range of the Rocky 

Mountain National Park support these 

findings. These three values (3.1, 5.2, 


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and 10.2 kg/ha/yr) are one set of 

ecologically meaningful benchmarks for 

the mixed conifer forest (MCF) of the 

pacific coast regions. Nearly all of the 

known sensitive communities receive 

total nitrogen deposition levels above 

the 3.1 N kg/ha/yr ecological benchmark 

according to the 12 km, 2002 CMAQ/ 

NADP data, with the exception of the 

easternmost Sierra Nevadas. The MCFs 

in the southern portion of the Sierra 

Nevada forests and nearly all MCF 

communities in the San Bernardino 

forests receive total nitrogen deposition 

levels above the 5.2 N kg/ha/yr 

ecological benchmark. 

Coastal Sage Scrub communities are 

also known to be sensitive to 

community shifts caused by excess 

nitrogen loadings. Studies have 

investigated the amount of nitrogen 

utilized by healthy and degraded CSS 

systems. In healthy stands, the authors 

estimated that 3.3 kg N/ha/yr was used 

for CSS plant growth. It is assumed that 

3.3 kg N/ha/yr is near the point where 

nitrogen is no longer limiting in the CSS 

community and above which level 

community changes occur, including 

dominance by invasive species and loss 

of coastal sage scrub. Therefore, this 

amount can be considered an ecological 

benchmark for the CSS community. The 

majority of the known CSS range is 

currently receiving deposition in excess 

of this benchmark. Thus, the REA 

concludes that recent conditions where 

oxides of nitrogen ambient 

concentrations are at or below the 

current oxides of nitrogen secondary 

standards are not adequate to protect 

against anticipated adverse impacts 

from N nutrient enrichment in sensitive 

ecosystems. 

(d) Aquatic Nutrient Enrichment 

The REA aquatic nutrient enrichment 

case studies focused on coastal estuaries 

and revealed that while current ambient 

loadings of atmospheric oxides of 

nitrogen are contributing to the overall 

depositional loading of coastal estuaries, 

other non-atmospheric sources are 

contributing in far greater amounts in 

total, although atmospheric 

contributions are as large as some other 

individual source types. The ability of 

current data and models to characterize 

the incremental adverse impacts of 

nitrogen deposition is limited, both by 

the available ecological indicators, and 

by the inability to attribute specific 

effects to atmospheric sources of 

nitrogen. The REA case studies used 

ASSETS EI as the ecological indicator 

for aquatic nutrient enrichment. This 

index is a six level index characterizing 

overall eutrophication risk in a water 

body. This indictor is not sensitive to 

changes in nitrogen deposition within a 

single level of the index. In addition, 

this type of indicator does not reflect the 

impact of nitrogen deposition in 

conjunction with other sources of 

nitrogen. 

Based on the above considerations, 

the REA concludes that the ASSETS EI 

is not an appropriate ecological 

indicator for estuarine aquatic 

eutrophication and that additional 

analysis is required to develop an 

appropriate indicator for determining 

the appropriate levels of protection from 

N nutrient enrichment effects in 

estuaries related to deposition of oxides 

of nitrogen. As a result, the EPA is 

unable to make a determination as to the 

adequacy of the existing secondary 

oxides of nitrogen standard in 

protecting public welfare from nitrogen 

nutrient enrichment effects in estuarine 

aquatic ecosystems. 

Additionally, nitrogen deposition can 

alter species composition and cause 

eutrophication in freshwater systems. In 

the Rocky Mountains, for example, 

deposition loads of 1.5 to 2 kg/ha/yr 

which are well within current ambient 

levels are known to cause changes in 

species composition in diatom 

communities indicating impaired water 

quality (U.S. EPA, 2008, section 3.3.5.3). 

This suggests that the existing 

secondary standard for oxides of 

nitrogen does not protect such 

ecosystems and their resulting services 

from impairment. 

(e) Other Effects 

An important consideration in 

looking at the effects of deposition of 

oxides of sulfur in aquatic ecosystems is 

the potential for production of MeHg, a 

neurotoxic contaminant. The 

production of meaningful amounts of 

MeHg requires the presence of SO4

2¥

and mercury, and where mercury is 

present, increased availability of SO4

2¥

results in increased production of 

MeHg. There is increasing evidence on 

the relationship between sulfur 

deposition and increased methylation of 

mercury in aquatic environments; this 

effect occurs only where other factors 

are present at levels within a range to 

allow methylation. The production of 

MeHg requires the presence of SO4

2¥

and mercury, but the amount of MeHg 

produced varies with oxygen content, 

temperature, pH and supply of labile 

organic carbon (U.S. EPA, 2008, section 

3.4). In watersheds where changes in 

sulfate deposition did not produce an 

effect, one or several of those interacting 

factors were not in the range required 

for meaningful methylation to occur 

(U.S. EPA, 2008, section 3.4). 

Watersheds with conditions known to 

be conducive to mercury methylation 

can be found in the northeastern United 

States and southeastern Canada (U.S. 

EPA, 2009, section 6). 

With respect to sulfur deposition and 

mercury methylation, the final ISA 

determined that ‘‘[t]he evidence is 

sufficient to infer a causal relationship 

between sulfur deposition and increased 

mercury methylation in wetlands and 

aquatic environments.’’ However, the 

EPA did not conduct a quantitative 

assessment of the risks associated with 

increased mercury methylation under 

current conditions. As such, the EPA is 

unable to make a determination as to the 

adequacy of the existing SO2 secondary 

standards in protecting against welfare 

effects associated with increased 

mercury methylation. 

c. Summary of Adequacy 

Considerations 

In summary, the PA concludes that 

currently available scientific evidence 

and assessments clearly call into 

question the adequacy of the current 

standards with regard to depositionrelated effects on sensitive aquatic and 

terrestrial ecosystems, including 

acidification and nutrient enrichment. 

Further, the PA recognizes that the 

elements of the current standards— 

indicator, averaging time, level and 

form—are not ecologically relevant, and 

are thus not appropriate for standards 

designed to provide such protection. 

Thus, the PA concludes that 

consideration should be given to 

establishing a new ecologically relevant 

multi-pollutant, multimedia standard to 

provide appropriate protection from 

deposition-related ecological effects of 

oxides of nitrogen and sulfur on 

sensitive ecosystems, with a focus on 

protecting against adverse effects 

associated with acidifying deposition in 

sensitive aquatic ecosystems. 

2. CASAC Views 

In a letter to the Administrator 

(Russell and Samet 2011a), the CASAC 

Oxides of Nitrogen and Oxides of Sulfur 

Panel, with full endorsement of the 

chartered CASAC, unanimously 

concluded that: 

‘‘EPA staff has demonstrated through the 

Integrated Science Assessment (ISA), Risk 

and Exposure Characterization (REA) and the 

draft PA that ambient NOX and SOX can 

have, and are having, adverse environmental 

impacts. The Panel views that the current 

NOX and SOX secondary standards should be 

retained to protect against direct adverse 

impacts to vegetation from exposure to gas 

phase exposures of these two families of air 

pollutants. Further, the ISA, REA and draft 

PA demonstrate that adverse impacts to 

aquatic ecosystems are also occurring due to 

deposition of NOX and SOX. Those impacts 


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include acidification and undesirable levels 

of nutrient enrichment in some aquatic 

ecosystems. The levels of the current NOX

and SOX secondary NAAQS are not 

sufficient, nor the forms of those standards 

appropriate, to protect against adverse 

depositional effects; thus a revised NAAQS is 

warranted.’’ 

In addition, with regard to the joint 

consideration of both oxides of nitrogen 

and oxides of sulfur as well as the 

consideration of deposition-related 

effects, CASAC concluded that the PA 

had developed a credible methodology 

for considering such effects. The Panel 

stated that ‘‘the Policy Assessment 

develops a framework for a multipollutant, multimedia standard that is 

ecologically relevant and reflects the 

combined impacts of these two 

pollutants as they deposit to sensitive 

aquatic ecosystems.’’ 

3. Administrator’s Proposed 

Conclusions 

Based on the above considerations 

and taking into account CASAC advice, 

in the proposed rule the Administrator 

considered the adequacy of the current 

NO2 and SO2 secondary standards with 

regard to both direct effects on 

vegetation, as well as on depositionrelated effects on sensitive ecosystems. 

With regard to direct phytotoxic effects 

on vegetation, the Administrator 

concluded that the current secondary 

standards are adequately protective, and 

thus proposed to retain the current NO2

and SO2 secondary standards for that 

purpose. 

With regard to deposition-related 

effects, the Administrator first 

considered the appropriateness of the 

structure of the current standards to 

address ecological effects of concern. 

Based on the evidence as well as 

considering the advice given by CASAC, 

the Administrator concluded that the 

elements of the current standards are 

not ecologically relevant and thus are 

not appropriate to provide protection of 

ecosystems. In considering the adequacy 

of protection with regard to depositionrelated effects, the Administrator 

considered the full nature of ecological 

effects related to the deposition of 

ambient oxides of nitrogen and sulfur 

into sensitive ecosystems across the 

country. Based on the evidence and 

information evaluated in the ISA, REA, 

and PA, and taking into account CASAC 

advice, the Administrator concluded 

that current levels of oxides of nitrogen 

and sulfur are sufficient to cause 

acidification of both aquatic and 

terrestrial ecosystems, nutrient 

enrichment of terrestrial ecosystems and 

contribute to nutrient enrichment effects 

in estuaries that could be considered 

adverse, and that the current secondary 

standards do not provide adequate 

protection from such effects. 

Having reached these conclusions, the 

Administrator determined that it was 

appropriate to consider alternative 

standards that are ecologically relevant. 

These considerations, as discussed 

below in section III, supported the 

conclusion that the current secondary 

standards are neither appropriate nor 

adequate to protect against depositionrelated effects. 

C. Comments on Adequacy of the 

Current Standards 

The above sections outline the effects 

evidence and assessments (section II.A) 

used by the Administrator to inform her 

proposed judgments about the adequacy 

of the current secondary NO2 and SO2

standards with regard to both direct 

effects associated with gas-phase oxides 

of nitrogen and sulfur (section II.B.1) as 

well effects associated with deposition 

of oxides of nitrogen and sulfur to 

sensitive aquatic and terrestrial 

ecosystems (section II.B.2). 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. Comments related to the EPA’s 

authority to address deposition-related 

effects through the NAAQS are 

discussed above in section I.E. 

Comments related to the EPA’s 

proposed conclusions regarding 

alternative secondary standards are 

discussed below in section III.D. 

1. Adequacy of Current Secondary 

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 above in section II.B.1, 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 below in section III.D), 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 Current Secondary 

Standards to Address DepositionRelated Effects 

As discussed above in section II.B.2, 

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. 

One group of commenters that 

addressed the adequacy of the current 

standards with regard to depositionrelated 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 depositionrelated 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 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 


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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. 

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 

above. 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 ANC of 50 meq/ 

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 

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 assess adequacy 

of protection. This subject is discussed 

more fully in section 6.2 of the REA 

(U.S. EPA, 2009). 

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 


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4The annual secondary standard for oxides of 

nitrogen is being specified in units of ppb to 

conform to the current version of the annual 

primary standard, as specified in the final rule for 

the most recent review of the NO2 primary NAAQS 

(75 FR 6531; February 9, 2010). 

the EPA’s response are discussed above 

in section I.E. 

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 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 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 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. 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, 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. 

D. Final Decisions on the Adequacy of 

the Current Standards 

Based on the considerations discussed 

above, including CASAC advice and 

public comments, the Administrator 

believes that the conclusions reached in 

the proposed rule with regard to the 

adequacy of the current secondary 

standards for oxides of nitrogen and 

sulfur for direct and deposition-related 

effects continue to be valid. The 

Administrator recognizes that the 

purpose of the secondary standard is to 

protect against ‘‘adverse’’ effects 

resulting from exposure to oxides of 

nitrogen and sulfur, discussed above in 

section II.A. The Administrator also 

recognizes the need for conclusions as 

to the adequacy of the current standards 

for both direct and deposition-related 

effects as well as conclusions as to the 

appropriateness and ecological 

relevance of the current standards. 

In considering what constitutes an 

ecological effect that is also adverse to 

the public welfare, the Administrator 

took into account the ISA conclusions 

regarding the nature and strength of the 

effects evidence, the risk and exposure 

assessment results, the degree to which 

the associated uncertainties should be 

considered in interpreting the results, 

the conclusions presented in the PA, 

and the views of CASAC and members 

of the public. On these bases, the 

Administrator concludes that the 

current secondary standards are 

adequate to protect against direct 

phytotoxic effects on vegetation. Thus, 

the Administrator has decided to retain 

the current secondary standards for 

oxides of nitrogen at 53 ppb,4 annual 

average concentration, measured in the 

ambient air as NO2, and the current 

secondary standard for oxides of sulfur 

at 0.5 ppm, 3-hour average 

concentration, measured in the ambient 

air as SO2. 

With regard to deposition-related 

effects, the Administrator first 

considered the appropriateness of the 

structure of the current secondary 

standards to address ecological effects of 

concern. Based on the evidence as well 

as considering the advice given by 

CASAC and public comments on this 

matter, the Administrator concludes 

that the elements of the current 

standards are not ecologically relevant 

and thus are not appropriate to provide 

protection of ecosystems. On the subject 

of adequacy of protection with regard to 

deposition-related effects, the 

Administrator considered the full nature 

of ecological effects related to the 

deposition of ambient oxides of nitrogen 

and sulfur into sensitive ecosystems 

across the country. Her conclusions are 

based on the evidence presented in the 

ISA with regard to acidification and 

nutrient enrichment effects, the findings 

of the REA with regard to scope and 

severity of the current and likely future 

effects of deposition, the synthesis of 

both the scientific evidence and risk and 

exposure results in the PA as to the 

adequacy of the current standards, and 

the advice of CASAC and public 

comments. After such consideration, the 

Administrator concludes that current 

levels of oxides of nitrogen and sulfur 

are sufficient to cause acidification of 


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both aquatic and terrestrial ecosystems, 

nutrient enrichment of terrestrial 

ecosystems and contribute to nutrient 

enrichment effects in estuaries that 

could be considered adverse, and the 

current secondary standards do not 

provide adequate protection from such 

effects. 

Having reached these conclusions, the 

Administrator determined that it was 

appropriate to consider alternative 

standards that are ecologically relevant, 

as discussed below in section III. These 

considerations further support her 

conclusion that the current secondary 

standards for oxides of nitrogen and 

sulfur are neither appropriate nor 

adequate to protect against depositionrelated effects. 

III. Rationale for Final Decisions on 

Alternative Secondary Standards 

This section presents the rationale for 

the Administrator’s final decisions 

regarding alternative secondary 

standards for oxides of nitrogen and 

sulfur to address deposition-related 

effects. Section III.A provides an 

overview of the aquatic acidification 

index (AAI) approach presented in the 

PA to address such effects related to 

aquatic acidification. Advice from 

CASAC on such a new approach is 

presented in section III.B. The 

Administrator’s proposed conclusions 

on an AAI-based standard are presented 

in section III.C. Comments on an AAIbased standard are discussed in section 

III.D as well as in the Response to 

Comments document. The 

Administrator’s final decisions 

regarding alternative secondary 

standards are presented in section III.E. 

A. Overview of AAI Approach 

Having reached the conclusion in the 

proposal that the current NO2 and SO2

secondary standards are not adequate to 

provide appropriate protection against 

potentially adverse deposition-related 

effects associated with oxides of 

nitrogen and sulfur, the Administrator 

then considered what new multipollutant standard might be appropriate, 

at this time, to address such effects on 

public welfare. The Administrator 

recognizes that the inherently complex 

and variable linkages between ambient 

concentrations of nitrogen and sulfur 

oxides, the related deposited forms of 

nitrogen and sulfur, and the ecological 

responses that are associated with 

public welfare effects call for 

consideration of a standard with an 

ecologically relevant design that reflects 

these linkages. The Administrator also 

recognizes that characterization of such 

complex and variable linkages in this 

review requires consideration of 

information and analyses that have 

important limitations and uncertainties. 

Despite its complexity, an 

ecologically relevant multi-pollutant 

standard to address deposition-related 

effects would still appropriately be 

defined in terms of the same basic 

elements that are used to define any 

NAAQS—indicator, form, averaging 

time, and level. The form would 

incorporate additional structural 

elements that reflect relevant multipollutant and multimedia attributes. 

These structural elements include the 

use of an ecological indicator, tied to the 

ecological effect we are focused on, and 

other elements that account for 

ecologically relevant factors other than 

ambient air concentrations. All of these 

elements would be needed to enable a 

linkage from ambient air indicators to 

the relevant ecological effect to define 

an ecologically relevant standard. As a 

result, such a standard would 

necessarily be more complex than the 

NAAQS that have been set historically 

to address effects associated with 

ambient concentrations of a single 

pollutant. 

More specifically, the Administrator 

considered an ecologically relevant 

multi-pollutant standard to address 

effects associated with acidifying 

deposition-related to ambient 

concentrations of oxides of nitrogen and 

sulfur in sensitive aquatic ecosystems. 

This focus is consistent with the 

information presented in the ISA, REA, 

and PA, which highlighted the greater 

quantity and quality of the available 

evidence and assessments associated 

with aquatic acidification relative to the 

information and assessments available 

for other deposition-related effects, 

including terrestrial acidification and 

aquatic and terrestrial nutrient 

enrichment. Based on its review of these 

documents, CASAC agreed that aquatic 

acidification should be the focus for 

developing a new multi-pollutant 

standard in this review. In reaching 

conclusions about an air quality 

standard designed to address 

deposition-related aquatic acidification 

effects, the Administrator also 

recognizes that such a standard may 

also provide some degree of protection 

against other deposition-related effects. 

As discussed in chapter 7 of the PA, 

the development of a new multipollutant ambient air quality standard to 

address deposition-related aquatic 

acidification effects recognizes that it is 

appropriate to consider a nationally 

applicable standard for protection 

against adverse effects of aquatic 

acidification on public welfare. At the 

same time, the PA recognizes the 

complex and heterogeneous interactions 

between ambient air concentrations of 

nitrogen and sulfur oxides, the related 

deposition of nitrogen and sulfur, and 

associated ecological responses. The 

development of such a standard also 

needs to take into account the 

limitations and uncertainties in the 

available information and analyses upon 

which characterization of such 

interactions are based. The approach 

used in the PA also recognizes that 

while such a standard would be 

national in scope and coverage, the 

effects to public welfare from aquatic 

acidification will not occur to the same 

extent in all locations in the United 

States, given the inherent variability of 

the responses of aquatic systems to the 

effects of acidifying deposition. This 

contrasts with the relatively more 

homogeneous relationships between 

ambient air concentrations of air 

pollutants and the associated inhalation 

exposures and related public health 

responses that are typically considered 

in setting primary NAAQS. 

As discussed above in section II–A, 

many locations in the United States are 

naturally protected against acid 

deposition due to underlying geological 

conditions. Likewise, some locations in 

the United States, including lands 

managed for commercial agriculture and 

forestry, are not likely to be negatively 

impacted by current levels of nitrogen 

and sulfur deposition. As a result, while 

a new ecologically relevant secondary 

standard would apply everywhere, it 

would be structured to account for 

differences in the sensitivity of 

ecosystems across the country. This 

would allow for appropriate protection 

of sensitive aquatic ecosystems, which 

are relatively pristine and wild and 

generally in rural areas, and the services 

provided by such sensitive ecosystems, 

without requiring more protection than 

is needed elsewhere. 

As discussed below, the multipollutant standard developed in the PA 

would employ (1) Total reactive 

oxidized nitrogen (NOy) and oxides of 

sulfur (SOX) as the atmospheric ambient 

air indicators; (2) a form that takes into 

account variable factors, such as 

atmospheric and ecosystem conditions 

that modify the amounts of deposited 

nitrogen and sulfur; the distinction 

between oxidized and reduced forms of 

nitrogen; effects of deposited nitrogen 

and sulfur on aquatic ecosystems in 

terms of the ecological indicator ANC; 

and the representativeness of water 

bodies within a defined spatial area; (3) 

a multi-year averaging time, and (4) a 

standard level defined in terms of a 

single, national target ANC value that, 

in the context of the above form, 

identifies the various levels of 


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concentrations of NOy and SOX in the 

ambient air that would meet the 

standard. The form of such a standard 

has been defined by an index, AAI, 

which reflects the relationship between 

ambient concentrations of NOy and SOX

and aquatic acidification effects that 

result from nitrogen and sulfur 

deposition-related to these ambient 

concentrations. 

In summarizing the considerations 

associated with such an air quality 

standard to address deposition-related 

aquatic acidification effects, as 

discussed more fully in sections III.A– 

F of the proposal and in the PA, the 

following sections focus on each 

element of the standard, including 

ambient air indicators (section III.A.1), 

form (section III.A.2), averaging time 

(section III.A.3), and level (section 

III.A.4). Considerations related to 

important uncertainties inherent in such 

an approach are discussed in section 

III.A.5. 

1. Ambient Air Indicators 

The PA concludes that ambient air 

indicators other than NO2 and SO2

should be considered as the appropriate 

indicators of oxides of nitrogen and 

sulfur in the ambient air for protection 

against the acidification effects 

associated with deposition of the 

associated nitrogen and sulfur. This 

conclusion is based on the recognition 

that all forms of nitrogen and sulfur in 

the ambient air contribute to deposition 

and resulting acidification, and as such, 

NO2 and SO2 are incomplete ambient air 

indicators. In principle, the indicators 

should represent the species that are 

associated with oxides of nitrogen and 

sulfur in the ambient air and can 

contribute acidifying deposition. This 

includes both the species of oxides of 

nitrogen and sulfur that are directly 

emitted as well as species transformed 

in the atmosphere from oxides of 

nitrogen and sulfur that retain the 

nitrogen and sulfur atoms from directly 

emitted oxides of nitrogen and sulfur. 

All of these compounds are associated 

with oxides of nitrogen and sulfur in the 

ambient air and can contribute to 

acidifying deposition. 

The PA focuses in particular on the 

various compounds with nitrogen or 

sulfur atoms that are associated with 

oxides of nitrogen and sulfur, because 

the acidifying potential is specific to 

nitrogen and sulfur, and not other atoms 

(e.g., H, C, O) whether derived from the 

original source of oxides of nitrogen and 

sulfur emissions or from atmospheric 

transformations. For example, the 

acidifying potential of each molecule of 

NO2, NO, HNO3 or PAN is identical, as 

is the potential for each molecule of SO2

or ion of particulate sulfate (p-SO4). 

Each atom of sulfur affords twice the 

acidifying potential of each atom of 

nitrogen. 

a. Oxides of Sulfur 

As discussed in the PA (U.S. EPA, 

2011, section 7.1.1), oxides of sulfur 

include the gases sulfur monoxide (SO), 

SO2, sulfur trioxide (SO3), disulfur 

monoxide (S2O), and particulate-phase 

sulfur compounds (referred to as SO4) 

that result from gas-phase sulfur oxides 

interacting with particles. However, the 

sum of SO2 and SO4 does represent 

virtually the entire ambient air mass of 

sulfur that contributes to acidification. 

In addition to accounting for virtually 

all the potential for acidification from 

oxidized sulfur in the ambient air, there 

are reliable methods to monitor the 

concentrations of SO2 and particulate 

SO4. The PA concludes that the sum of 

SO2 and SO4, referred to as SOX, are 

appropriate ambient air indicators of 

oxides of sulfur because they represent 

virtually all of the acidification 

potential of ambient air oxides of sulfur 

and there are reliable methods suitable 

for measuring SO2 and SO4. 

b. Oxides of Nitrogen 

As discussed in the PA (U.S. EPA, 

2011, section 7.1.2), NOy, as defined in 

chapter 2 of the PA, incorporates 

basically all of the oxidized nitrogen 

species that have acidifying potential 

and as such, NOy should be considered 

as an appropriate indicator for oxides of 

nitrogen. Total reactive oxidized 

nitrogen is an aggregate measure of NO 

and NO2 and all of the reactive oxidized 

products of NO and NO2. That is, NOy

is a group of nitrogen compounds in 

which all of the compounds are either 

an oxide of nitrogen or compounds in 

which the nitrogen atoms came from 

oxides of nitrogen. Total reactive 

oxidized nitrogen is especially relevant 

as an ambient indicator for acidification 

in that it both relates to the oxides of 

nitrogen in the ambient air and also 

represents the acidification potential of 

all oxidized nitrogen species in the 

ambient air, whether an oxide of 

nitrogen or derived from oxides of 

nitrogen. The merits of other individual 

NOy species, particularly total nitrate, 

are discussed in section 2 of the PA. 

2. Form 

Based on the evidence of the aquatic 

acidification effects caused by the 

deposition of NOy and SOX, the PA 

(U.S. EPA, 2011, section 7.2) presents 

the development of a new form that is 

ecologically relevant for addressing 

such effects. The conceptual design for 

the form of such a standard includes 

three main components: an ecological 

indicator, deposition metrics that relate 

to the ecological indicator, and a 

function that relates ambient air 

indicators to deposition metrics. 

Collectively, these three components 

link the ecological indicator to ambient 

air indicators, as illustrated below in Fig 

III–1. 


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5This section discusses the linkages between 

deposition of nitrogen and sulfur and ANC. Section 

III.A.2.c then discusses the linkages between atmospheric concentrations of NOy and SOX and 

deposition of nitrogen and sulfur. 

The simplified flow diagram in Figure 

III–1 compresses the various 

atmospheric, biological, and 

geochemical processes associated with 

acidifying deposition to aquatic 

ecosystems into a simplified conceptual 

picture. The ecological indicator (left 

box) is related to atmospheric 

deposition through biogeochemical 

ecosystem models (middle box), which 

associate a target deposition load to a 

target ecological indicator. Once a target 

deposition is established, associated 

allowable air concentrations are 

determined (right box) through the 

relationships between ambient air 

concentration and deposition that are 

embodied in air quality models such as 

CMAQ. The PA describes the 

development and rationale for each of 

these components, as well the 

integration of these components into the 

full expression of the form of the 

standard using the concept of a national 

AAI that represents a target ANC level 

as a function of ambient air 

concentrations. 

The AAI was designed to be an 

ecologically relevant form of the 

standard that determines the levels of 

NOy and SOX in the ambient air that 

would achieve a target ANC limit for the 

United States. The intent of the AAI is 

to weight atmospheric concentrations of 

oxides of nitrogen and sulfur by their 

propensity to contribute to acidification 

through deposition, given the 

fundamental acidifying potential of each 

pollutant, and to take into account the 

ecological factors that govern acid 

sensitivity in different ecosystems. The 

index also accounts for the contribution 

of reduced nitrogen to acidification. 

Thus, the AAI encompasses those 

attributes of specific relevance to 

protecting ecosystems from the 

acidifying potential of ambient air 

concentrations of NOy and SOX. 

a. Ecological Indicator 

This section summarizes the rationale 

in the PA for selecting ANC as the 

appropriate ecological indicator for 

consideration. Recognizing that ANC is 

not itself the causative or toxic agent for 

adverse aquatic acidification effects, the 

rationale for using ANC as the relevant 

ecological indicator is based on the 

following: 

(1) The ANC is directly associated 

with the causative agents, pH and 

dissolved Al, both through empirical 

evidence and mechanistic relationships; 

(2) Empirical evidence shows very 

clear and strong relationships between 

adverse effects and ANC; 

(3) The ANC is a more reliable 

indicator from a modeling perspective, 

allowing use of a body of studies and 

technical analyses related to ANC and 

acidification to inform the development 

of the standard; and 

(4) The ANC embodies the concept of 

acidification as posed by the basic 

principles of acid base chemistry and 

the measurement method used to 

estimate ANC and, therefore, serves as 

a direct index to protect against 

acidification. 

Because ANC clearly links both to 

biological effects of aquatic acidification 

as well as to acidifying inputs of NOy

and SOX deposition, the PA concludes 

that ANC is an appropriate ecological 

indicator for relating adverse aquatic 

ecosystem effects to acidifying 

atmospheric deposition of SOX and 

NOy, and is preferred to other potential 

indicators. In reaching this conclusion, 

the PA notes that in its review of the 

first draft PA, CASAC concluded that 

‘‘information on levels of ANC 

protective to fish and other aquatic biota 

has been well developed and presents 

probably the lowest level of uncertainty 

in the entire methodology’’ (Russell and 

Samet, 2010a). In its more recent review 

of the second draft PA, CASAC agreed 

‘‘that acid neutralizing capacity is an 

appropriate ecological measure for 

reflecting the effects of aquatic 

acidification’’ (Russell and Samet, 

2010b; p. 4). 

b. Linking ANC to Deposition 

There is evidence to support a 

quantified relationship between 

deposition of nitrogen and sulfur and 

ANC. This relationship was analyzed in 

the REA for two case study areas, the 

Adirondack and Shenandoah 

Mountains, based on time-series 

modeling and observed trends. In the 

REA analysis, long-term trends in 

surface water nitrate, sulfate and ANC 

were modeled using MAGIC for the two 

case study areas. These data were used 

to compare recent surface water 

conditions in 2006 with preindustrial 

conditions (i.e., preacidification 1860). 

The results showed a marked increase 

in the number of lakes affected by 

acidifying deposition, characterized as a 

decrease in ANC levels, since the onset 

of anthropogenic nitrogen and sulfur 

deposition, as discussed in chapter 2 of 

the PA. 

In the REA, the quantified 

relationship between deposition and 

ANC was investigated using ecosystem 

acidification models, also referred to as 

acid balance models or critical loads 

models (U.S. EPA, 2011, section 2 and 

U.S. EPA, 2009, section 4 and Appendix 

4). These models quantify the 

relationship between deposition of 

nitrogen and sulfur and the resulting 

ANC in surface waters based on an 

ecosystem’s inherent generation of ANC 

and ability to neutralize nitrogen 

deposition through biological and 

physical processes. A critical load is 

defined as the amount of acidifying 

atmospheric deposition of nitrogen and 

sulfur beyond which a target ANC is not 

reached. Relatively high critical load 

values imply that an ecosystem can 

accommodate greater deposition levels 

than lower critical loads for a specific 

target ANC level. Ecosystem models that 

calculate critical loads form the basis for 

linking deposition to ANC. 

As discussed in chapter 2 of the PA, 

both dynamic and steady-state models 

calculate ANC as a function of 

ecosystem attributes and atmospheric 

nitrogen and sulfur deposition, and can 

be used to calculate critical loads. 

Steady-state models are time invariant 

and reflect the long-term consequences 

associated with an ecosystem reaching 

equilibrium under a constant level of 

atmospheric deposition. Dynamic 

models are time variant and take into 

account the time dependencies inherent 

in ecosystem hydrology, soil and 

biological processes. Dynamic models 

like MAGIC can provide the time-series 

response of ANC to deposition whereas 

steady-state models provide a single 

ANC relationship to any fixed 

deposition level. Dynamic models 

naturally are more complex than steadystate models as they attempt to capture 

as much of the fundamental 

biogeochemical processes as practicable, 

whereas steady-state models depend on 

far greater parameterization and 

generalization of processes that is 

afforded, to some degree, by not having 

to account for temporal variability. 

In the PA, a steady-state model is 

used to define the relevant critical load, 

which is the amount of atmospheric 

deposition of nitrogen (N) and sulfur (S) 

beyond which a target ANC is not 

achieved and sustained.5 It is expressed 

as: 


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6Because Neco is only relevant to nitrogen 

deposition, in rare cases where Neco is greater than 

the total nitrogen deposition, the critical load 

would be defined only in terms of acidifying 

deposition of sulfur and the Neco term in equation 

III–1 would be set to zero. 

Where: 

CLANClim(N + S) is the critical load of 

deposition, with units of equivalent 

charge/(area-time); 

[BC]0* is the natural contribution of base 

cations from weathering, soil processes 

and preindustrial deposition, with units 

of equivalent charge/volume; 

[ANClim] is the target ANC value, with units 

of equivalent charge/volume; 

Q is the catchment level runoff rate governed 

by water mass balance and dominated by 

precipitation, with units of distance/ 

time; and 

Neco is the amount of nitrogen deposition 

that is effectively neutralized by a variety 

of biological (e.g., nutrient uptake) and 

physical processes, with units of 

equivalent charge/(area-time). 

Equation III–1 is a modified 

expression that adopts the basic 

formulation of the steady-state models 

that are described in chapter 2 of the 

PA. More detailed discussion of the 

rationale, assumptions and derivation of 

equation III–1, as well as all of the 

equations in this section, are included 

in Appendix B of the PA. The equation 

simply reflects the amount of deposition 

of nitrogen and sulfur from the 

atmosphere, CLANClim(N + S), that is 

associated with a sustainable long-term 

ANC target, [ANClim], given the capacity 

of the natural system to generate ANC, 

[BC]0*, and the capacity of the natural 

system to neutralize nitrogen 

deposition, Neco. This expression of 

critical load is valid when nitrogen 

deposition is greater than Neco.6 The 

runoff rate, Q, allows for balancing mass 

in the two environmental mediums— 

atmosphere and catchment. This critical 

load expression can be focused on a 

single water system or more broadly. To 

extend applicability of the critical load 

expression (equation III–1) from the 

catchment level to broader spatial areas, 

the terms Qr and CLr, are used, which 

are the runoff rate and critical load, 

respectively, of the region over which 

all the atmospheric terms in the 

equation are defined. 

As presented above, the terms S and 

N in the CLANClim (N + S) term broadly 

represent all species of sulfur or 

nitrogen that can contribute to 

acidifying deposition. This follows 

conventions used in the scientific 

literature that addresses critical loads, 

and it reflects all possible acidifying 

contributions from any sulfur or 

nitrogen species. For all practical 

purposes, S reflects SOX as described 

above, the sum of sulfur dioxide gas and 

particulate sulfate. However, N in 

equation III–1 includes both oxidized 

forms, consistent with the ambient 

indicator, NOy, in addition to the 

reduced nitrogen species, ammonia and 

ammonium ion, referred to as NHX. The 

NHX is included in the critical load 

formulation because it contributes to 

potentially acidifying nitrogen 

deposition. Consequently, from a mass 

balance or modeling perspective, the 

form of the standard needs to account 

for NHX, as described below. 

c. Linking Deposition to Ambient Air 

Indicators 

The last major component of the form 

illustrated in Figure III–1 addresses the 

linkage between deposition of nitrogen 

and sulfur and concentrations of the 

ambient air indicators, NOy and SOX. To 

link ambient air concentrations with 

deposition, the PA defines a 

transference ratio, T, as the ratio of total 

wet and dry deposition to ambient 

concentration, consistent with the area 

and time period over which the 

standard is defined. To express 

deposition of NOy and SOX in terms of 

NOy and SOX ambient concentrations, 

two transference ratios were defined, 

where TSOx equals the ratio of the 

combined dry and wet deposition of 

SOX to the ambient air concentration of 

SOX, and TNOy equals the ratio of the 

combined dry and wet deposition of 

NOy to the ambient air concentration of 

NOy. 

As described in chapter 7 of the PA, 

reduced forms of nitrogen (NHX) are 

included in total nitrogen in the critical 

load equation, III–1. Reduced forms of 

nitrogen are treated separately, as are 

NOy and SOX, and the transference 

ratios are applied. This results in the 

following critical load expression that is 

defined explicitly in terms of the 

indicators NOy and SOX: 

This is the same equation as III–1, with 

the deposition associated with the 

critical load translated to deposition 

from ambient air concentrations via 

transference ratios. In addition, 

deposition of reduced nitrogen, 

oxidized nitrogen and oxidized sulfur 

are treated separately. 

Transference ratios are a modeled 

construct, and therefore cannot be 

compared directly to measurable 

quantities. Section III.B.3 of the 

proposal discusses approaches to 

quantifying these ratios that consider 

blending observational data and models. 

The PA more fully discusses the 

rationale underlying transference ratios, 

as well as analyses illustrating the 

relative stability and variability of these 

ratios. 

d. Aquatic Acidification Index 

Having established the transference 

ratios that translate atmospheric 

concentrations to deposition of nitrogen 

and sulfur and the various expressions 

that link atmospheric deposition of 

nitrogen and sulfur to ANC, the PA 

derived the following expression of 

these linkages, which separates reduced 

forms of nitrogen, NHx, from oxidized 

forms: 


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7Because NHX is characterized directly as 

deposition, not as an ambient concentration in this 

equation, no transference ratio is needed for this 

term. 

8We note that an 85th area within Omernik’s 

Ecoregion Level III is currently being developed for 

California. 

Equation III–3 is the basic expression 

of the form of a standard that translates 

the conceptual framework into an 

explicit expression that defines ANC as 

a function of the ambient air indicators, 

NOy and SOX, reduced nitrogen 

deposition,7 and the critical load 

necessary to achieve a target ANC level. 

This equation calculates an expected 

ANC value based on ambient 

concentrations of NOy and SOX. The 

calculated ANC will differ from the 

target ANC (ANClim) depending on how 

much the nitrogen and sulfur deposition 

associated with NOy, SOX, and NHX

differs from the critical load associated 

with just achieving the target ANC. 

Based on equation III–3, the PA 

defines an AAI that is more simply 

stated using terms that highlight the 

ambient air indicators: 

where the AAI represents the long-term (or 

steady-state) ANC level associated with 

ambient air concentrations of NOy and SOX. 

The factors F1 through F4 convey three 

attributes: a relative measure of the 

ecosystem’s ability to neutralize acids (F1), 

the acidifying potential of reduced nitrogen 

deposition (F2), and the deposition-toconcentration translators for NOy (F3) and 

SOX (F4). 

Specifically: 

F1 = ANClim + CLr/Qr ; 

F2 = NHX/Qr = NHX deposition divided by 

Qr; 

F3 = TNOy/Qr ; TNOy is the transference ratio 

that converts ambient air concentrations 

of NOy to deposition of NOy; and 

F4 = TSOx/Qr ; TSOx is the transference ratio 

that converts ambient air concentrations 

of SOX to deposition of SOX. 

All of these factors include 

representative Qr to maintain unit (and 

mass) consistency between the AAI and 

the terms on the right side of equation 

III–4. 

The F1 factor is the target ANC level 

plus the amount of deposition (critical 

load) the ecosystem can receive and still 

achieve the target level. It incorporates 

an ecosystem’s ability to generate acid 

neutralizing capacity through base 

cation supply ([BC]*0) and to neutralize 

acidifying nitrogen deposition through 

Neco, both of which are incorporated in 

the CL term. As noted above, because 

Neco can only neutralize nitrogen 

deposition (oxidized or reduced) there 

may be rare cases where Neco exceeds 

the combination of reduced and 

oxidized nitrogen deposition. 

Consequently, to ensure that the AAI 

equation is applicable in all cases that 

may occur, equation III–4 is conditional 

on total nitrogen deposition, {NHX + 

F3[NOy]}, being greater than Neco. In 

rare cases where Neco is greater than 

{NHX + F3[NOy]}, F2, F3, and Neco 

would be set equal to 0 in the AAI 

equation. The consequence of setting F2 

and F3 to zero is simply to constrain the 

AAI calculation just to SOX, as nitrogen 

would have no bearing on acidifying 

contributions in this case. 

The PA concludes that equation III–4 

(U.S. EPA, 2011, equation 7–12), which 

defines the AAI, is ecologically relevant 

and appropriate for use as the form of 

a national standard designed to provide 

protection for aquatic ecosystems from 

the effects of acidifying deposition 

associated with concentrations of oxides 

of nitrogen and sulfur in the ambient 

air. This AAI equation does not, 

however, in itself, define the spatial 

areas over which the terms of the 

equation would apply. To specify values 

for factors F1 through F4, it is necessary 

to define spatial areas over which these 

factors are determined. Thus, it is 

necessary to identify an approach for 

spatially aggregating water bodies into 

ecologically meaningful regions across 

the United States, as discussed below. 

e. Spatial Aggregation 

As discussed in the PA, one of the 

unique aspects of this form is the need 

to consider the spatial areas over which 

values for the F factors in the AAI 

equation are quantified. Ecosystems 

across the United States exhibit a wide 

range of geological, hydrological and 

vegetation characteristics that influence 

greatly the ecosystem parameters, Q, 

BC0* and Neco that are incorporated in 

the AAI. Variations in ecosystem 

attributes naturally lead to wide 

variability in the sensitivities of water 

bodies in the United States to 

acidification, as well as in the 

responsiveness of water bodies to 

changes in acidifying deposition. 

Consequently, variations in ecosystem 

sensitivity, and the uncertainties 

inherent in characterizing these 

variations, must be taken into account 

in developing a national standard. In 

developing a secondary NAAQS to 

protect public welfare, the focus of the 

PA is on protecting sensitive 

populations of water bodies, not on each 

individual water body, which is 

consistent with the Agency’s approach 

to protecting public health through 

primary NAAQS that focus on 

susceptible populations, not on each 

individual. 

The approach used for defining 

ecologically relevant regions across the 

United States, along with approaches to 

characterizing each region as acid 

sensitive or relatively non-acid sensitive 

is discussed in detail in the PA (U.S. 

EPA, 2011, section 7.2.5). This 

characterization facilitates a more 

detailed analysis and focus on those 

regions that are relatively more acid 

sensitive, as well as avoiding overprotection in relatively non-acid 

sensitive regions that would receive 

limited benefit from reductions in the 

deposition of oxides of nitrogen and 

sulfur with respect to aquatic 

acidification effects. 

Based on considering available 

classification schemes for spatial 

aggregation, the PA concludes that 

Omernik’s ecoregion classification (as 

described at http://www.epa.gov/wed/ 

pages/ecoregions) is the most 

appropriate method to consider for the 

purposes of this review. The PA 

concludes that ecoregion level III 

(Figure IV–1) resolution, with 84 

defined ecoregions in the contiguous 

United States,8 is the most appropriate 

level to consider for this purpose. The 

PA notes that the use of ecoregions is an 

appropriate spatial aggregation scheme 

for an AAI-based standard focused on 

deposition-related aquatic acidification 

effects, while many of the same 

ecoregion attributes may be applicable 

in subsequent NAAQS reviews that may 

address other deposition-related aquatic 

and terrestrial ecological effects. 

Because atmospheric deposition is 

modified by ecosystem attributes, the 

types of vegetation, soils, bedrock 

geology, and topographic features that 

are the basis of this ecoregion 

classification approach also will likely 

be key attributes for other depositionrelated effects (e.g., terrestrial 

acidification, nutrient enrichment) that 

link atmospheric concentrations to an 

aquatic or terrestrial ecological 

indicator. 

The PA used Omernik’s original 

alkalinity data (U.S. EPA, 2011, section 


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9The distribution of critical loads was based on 

CL values calculated with Neco at the lake level. 

Consideration could also be given to using a 

distribution of CLs without Neco and adding the 

ecoregion average Neco value to the nth percentile 

critical load. This would avoid cases where the 

lake-level Neco value potentially could be greater 

than total nitrogen deposition. The CL at the lake 

level represents the CL for the lake to achieve the 

specified national target ANC value. 10The PA judged the data to be sufficient for this 

purpose if data are available from more than 10 

water bodies in an ecoregion. 

2) and more recent ANC data to 

delineate two broad groupings of 

ecoregions: acid-sensitive and relatively 

non-acid sensitive ecoregions. This 

delineation was made to facilitate 

greater focus on those ecoregions with 

water bodies that generally have greater 

acid sensitivity and to avoid overprotection in regions with generally less 

sensitive water bodies. The approach 

used to delineate acid-sensitive and 

relatively non-acid sensitive regions 

included an initial numerical-based 

sorting scheme using ANC data, which 

categorized ecoregions with relatively 

high ANC values as being relatively 

non-acid sensitive. This initial 

delineation resulted in 29 of the 84 

Omernik ecoregions being categorized 

as acid sensitive. Subsequently, land 

use data based on the 2006 National 

Land Cover Data base (NLCD, http:// 

www.epa.gov/mrlc/nlcd-2006.html) 

were also considered to determine to 

what extent an ecoregion is of a 

relatively pristine and rural nature by 

quantifying the degree to which active 

management practices related to 

development and agriculture occur in 

each ecoregion, resulting in 22 relatively 

acid-sensitive ecoregions (Table III–1). 

TABLE III–1—LIST OF 22 ACIDSENSITIVE AREAS

Ecoregion name Ecoregion 

number 

Ridge and Valley ........................ 8.4.1 

Northern Appalachian Plateau 

and Uplands ............................ 8.1.3 

Piedmont ..................................... 8.3.4 

Western Allegheny Plateau ........ 8.4.3 

Southwestern Appalachians ....... 8.4.9 

Boston Mountains ....................... 8.4.6 

Blue Ridge .................................. 8.4.4 

Ouachita Mountains .................... 8.4.8 

Central Appalachians ................. 8.4.2 

Northern Lakes and Forests ....... 5.2.1 

Maine/New Brunswick Plains 

and Hills .................................. 8.1.8 

North Central Appalachians ....... 5.3.3 

Northern Appalachian and Atlantic Maritime Highlands ............ 5.3.1 

Columbia Mountains/Northern 

Rockies ................................... 6.2.3 

Middle Rockies ........................... 6.2.10 

Wasatch and Uinta Mountains ... 6.2.13 

North Cascades .......................... 6.2.5 

Cascades .................................... 6.2.7 

Southern Rockies ....................... 6.2.14 

Sierra Nevada ............................. 6.2.12 

Idaho Batholith ............................ 6.2.15 

Canadian Rockies ...................... 6.2.4 

Consideration was also given to the 

use of naturally acidic conditions in 

defining relatively non-acid sensitive 

areas. For example, several of the 

ecoregions located in plains near the 

coast exhibit elevated dissolved organic 

carbon (DOC) levels, which is associated 

with naturally acidic conditions. The 

DOC in surface waters is derived from 

a variety of weak organic acid 

compounds generated from the natural 

availability and decomposition of 

organic matter from biota. 

Consequently, high DOC is associated 

with ‘‘natural’’ acidity, with the 

implication that a standard intended to 

protect against atmospheric 

contributions to acidity is not an area of 

focus. The evidence suggests that 

several of the more highly managed 

ecoregions in coastal or near coastal 

transition zones are associated with 

relatively high DOC values, typically 

exceeding on average 5 milligrams per 

liter, compared to other acid sensitive 

areas. Although there is sound logic to 

interpret naturally acidic areas as 

relatively non-acid sensitive, natural 

acidity indicators were not explicitly 

included in defining relatively non-acid 

sensitive areas as there does not exist a 

generally accepted quantifiable 

scientific definition of natural acidity. 

Approaches to explicitly define natural 

acidity likely will be pursued in future 

reviews of the standard. 

Having concluded that the Omernik 

level III ecoregions are an appropriate 

approach to spatial aggregation for the 

purpose of a standard to address 

deposition-related aquatic acidification 

effects, the PA uses those ecoregions to 

define each of the factors in the AAI 

equation. As discussed below, factors F1 

through F4 in equation III–4 are defined 

for each ecoregion by specifying 

ecoregion-specific values for each factor 

based on measured and modeled data. 

i. Factor F1 

As discussed above, factor F1 reflects 

a relative measure of an ecosystem’s 

ability to neutralize acidifying 

deposition, and is defined as: F1 = 

ANClim + CLr/Qr. The value of F1 for 

each ecoregion would be based on a 

calculated critical load used to represent 

the ecoregion (CLr) associated with a 

single national target ANC level 

(ANClim, discussed below in section 

III.D), as well as on a runoff rate (Qr) to 

represent the region. To specify 

ecoregion-specific values for the term 

Qr, the PA used the median value of the 

distribution of Q values that are 

available for water bodies within each 

ecoregion. To specify ecoregion-specific 

values for the term CLr in factor F1, a 

distribution 9 of calculated critical loads 

was created for the water bodies in each 

ecoregion for which sufficient water 

quality and hydrology data are 

available.10 The specified critical load 

was then defined to be a specific 

percentile of the distribution of critical 

loads in the ecoregion. Thus, for 

example, using the 90th percentile 

means that within an ecoregion, the goal 

would be for 90 percent of the water 

bodies to have higher calculated critical 

loads than the specified critical load. 

That is, if the specified critical load 

were to occur across the ecoregion, the 

goal would be for 90 percent of the 

water bodies to achieve the national 

ANC target or better. 

The specific percentile selected as 

part of the definition of F1 is an 

important parameter that directly 

impacts the critical load specified to 

represent each ecoregion, and therefore 

the degree of protectiveness of the 

standard. A higher percentile 

corresponds to a lower critical load and, 

therefore, to lower allowable ambient air 

concentrations of NOy and SOX and 

related deposition to achieve a target 

AAI level. In conjunction with the other 

terms in the AAI equation, alternative 

forms can be appropriately 

characterized in part by identifying a 

range of alternative percentiles. The 

choice of an appropriate range of 

percentiles to consider for acid-sensitive 

and relatively non-acid sensitive 

ecoregions, respectively, is discussed 

below. 

For relatively acid-sensitive 

ecoregions, the PA concludes it is 

appropriate to consider percentiles in 

the range of the 70th to the 90th 

percentile (of sensitivity). This 

conclusion is based on the judgment 

that it would not be appropriate to 

represent an ecoregion with the lowest 

or near lowest critical load, so as to 

avoid potential extreme outliers that can 

be seen to exist at the extreme end of the 

data distributions, which would not be 

representative of the population of acid 

sensitive water bodies within the 

ecoregion and could lead to an overly 

protective standard. At the same time, 

in considering ecoregions that are 

inherently acid sensitive, it is judged to 

be appropriate to limit the lower end of 

the range for consideration to the 70th 

percentile, a value well above the 

median of the distribution, so that a 

substantial majority of acid-sensitive 

water bodies are protected. Since the 

percentile value influences the relative 


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11Unlike other NAAQS, where the standard is 

met when the relevant value is at or below the level 

of the standard since a lower standard level is more 

protective, in this case a higher standard level is 

more protective. 

degree of protectiveness afforded by the 

AAI approach, the degree of confidence 

in characterizing the representativeness 

of sampled water bodies relative to all 

water bodies within an ecoregion is a 

critical issue, and it is important to 

continually improve this confidence. 

For relatively non-acid sensitive 

ecoregions, the PA concludes it is 

appropriate to consider the use of a 

range of percentiles that extends lower 

than the range identified above for acidsensitive ecoregions. Consideration of a 

lower percentile would avoid 

representing a relatively non-acid 

sensitive ecoregion by a critical load 

associated with relatively more acidsensitive water bodies. In particular, the 

PA concludes it is appropriate to focus 

on the median or 50th percentile of the 

distribution of critical loads so as to 

avoid over-protection in such 

ecoregions. 

ii. Factor F2, F3 and F4 

As discussed above, factor F2 is the 

amount of reduced nitrogen deposition 

within an ecoregion, including the 

deposition of both ammonia gas and 

ammonium ion, and is defined as: F2 = 

NHX/Qr. The PA calculated the 

representative runoff rate, Qr, using a 

similar approach as noted above for 

factor F1; i.e., the median value of the 

distribution of Q values that are 

available for water bodies within each 

ecoregion. In the PA, 2005 CMAQ 

model simulations over 12-km grids are 

used to calculate an average value of 

NHX for each ecoregion. The NHX term 

is based on annual average model 

outputs for each grid cell, which are 

spatially averaged across all the grid 

cells contained in each ecoregion to 

calculate a representative annual 

average value for each ecoregion. The 

PA concludes that this approach of 

using spatially averaged values is 

appropriate for modeling, largely due to 

the relatively rapid mixing of air masses 

that typically results in relatively 

homogeneous air quality patterns for 

regionally dispersed pollutants. In 

addition, there is greater confidence in 

using spatially averaged modeled 

atmospheric fields than in using 

modeled point-specific fields. 

This averaging approach is also used 

for the air concentration and deposition 

terms in factors F3 and F4, which are 

the ratios that relate ambient air 

concentrations of NOy and SOX to the 

associated deposition, and are defined 

as follows: F3 = TNOy/Qr and F4 = TSOx/ 

Qr. TNOy is the transference ratio that 

converts ambient air concentrations of 

NOy to deposition of NOy and TSOx is 

the transference ratio that converts 

ambient air concentrations of SOX to 

deposition of SOX. The transference 

ratios are based on the 2005 CMAQ 

simulations, using average values for 

each ecoregion, as noted above for factor 

F2. More specifically, the transference 

ratios are calculated as the annual 

deposition of NOy or SOX spatially 

averaged across the ecoregion and 

divided by the annual ambient air 

concentration of NOy or SOX, 

respectively, spatially averaged across 

the ecoregion. 

f. Summary of the AAI Form 

The PA developed an ecologically 

relevant form of an ambient air quality 

standard to address deposition-related 

aquatic acidification effects using an 

equation to calculate an AAI value in 

terms of the ambient air indicators of 

oxides of nitrogen and sulfur and the 

relevant ecological and atmospheric 

factors that modify the relationships 

between the ambient air indicators and 

ANC. Recognizing the spatial variability 

of such factors across the United States, 

the PA concludes it is appropriate to 

divide the country into ecologically 

relevant regions, characterized as acidsensitive or relatively non-acid 

sensitive, and specify the value of each 

of the factors in the AAI equation for 

each such region. 

Using the equation, a value of AAI 

can be calculated for any measured 

values of ambient NOy and SOX. For 

such a NAAQS, the Administrator 

would set a single, national value for the 

level of the AAI used to determine 

achievement of the NAAQS, as 

summarized below in section III.A.4. 

The ecoregion-specific values for factors 

F1 through F4 would be specified by the 

EPA based on the most recent data and 

CMAQ model simulations, and codified 

as part of such a standard. These factors 

would be reviewed and updated as 

appropriate in the context of each 

periodic review of the NAAQS. 

3. Averaging Time 

Reflecting a focus on long-term effects 

of acidifying deposition, the PA 

developed the AAI that links ambient 

air indicators to deposition-related 

ecological effects, in terms of several 

factors, F1 through F4. As discussed 

above, these factors are all calculated as 

annual average values, whether based 

on water quality and hydrology data or 

on CMAQ model simulations. In the 

context of a standard defined in terms 

of the AAI, the PA concludes that it is 

appropriate to consider the same annual 

averaging time for the ambient air 

indicators as is used for the factors in 

the AAI equation. As noted in chapter 

3 of the ISA, protection against episodic 

acidity events can be achieved by 

establishing a higher chronic ANC level. 

The PA also considered interannual 

variability in both ambient air quality 

and in precipitation, which is directly 

related to the deposition of oxides of 

nitrogen and sulfur from the ambient 

air. While ambient air concentrations 

show year-to-year variability, often the 

year-to-year variability in precipitation 

is considerably greater, given the highly 

stochastic nature of precipitation. The 

use of multiple years over which annual 

averages are determined would dampen 

the effects of interannual variability in 

both air quality and precipitation. 

Consequently, the PA concludes that an 

annual averaging time based on the 

average of each year over a consecutive 

3- to 5-year period is appropriate to 

consider for the ambient air indicators 

NOy and SOX. 

4. Level 

The PA concludes that the level of a 

standard for aquatic acidification based 

on the AAI would be defined in terms 

of a single, national value of the AAI. 

Such a standard would be met at a 

monitoring site when the multi-year 

average of the calculated annual values 

of the AAI was equal to or above the 

specified level of the standard.11 The 

annual values of the AAI would be 

calculated based on the AAI equation 

using the assigned ecoregion-specific 

values for factors F1 through F4 and 

monitored annual average NOy and SOX

concentrations. Since the AAI equation 

is based on chronic ANC as the 

ecological indicator, the level chosen for 

the standard would reflect a target 

chronic ANC value. The combination of 

the form of the standard, discussed 

above in section III.A.2, defined by the 

AAI equation and the assigned values of 

the F factors in the equation, other 

elements of the standard including the 

ambient air indicators (section III.A.1) 

and their averaging time (section 

III.A.3), and the level of the standard 

determines the allowable levels of NOy

and SOX in the ambient air within each 

ecoregion. All of the elements of the 

standard together determine the degree 

of protection from adverse aquatic 

acidification effects associated with 

oxides of nitrogen and sulfur in the 

ambient air. The level of the standard 

plays a central role in determining the 

degree of protection provided and is 

discussed below. 

Based on associations between pH 

levels and target ANC levels and 


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12Field measurements of NHX have been 

extremely limited, but have begun to be enhanced 

through the NADP’s passive ammonia network 

(AMoN). 

between ANC levels and aquatic 

ecosystem effects, as well as 

consideration of episodic acidity, 

ecosystem response time, precedent 

uses of target ANC levels, and public 

welfare benefits, the PA concludes that 

consideration should be given to a range 

of standard AAI levels from 20 to 75 

meq/L. The available evidence indicates 

that target ANC levels below 20 meq/L 

would be inadequate to protect against 

substantial ecological effects and 

potential catastrophic loss of ecosystem 

function in some sensitive aquatic 

ecosystems. While ecological effects 

occur at ANC levels below 50 meq/L in 

some sensitive ecosystems, the degree 

and nature of those effects are less 

significant than at levels below 20 meq/ 

L. Levels at and above 50 meq/L would 

be expected to provide additional 

protection, although uncertainties 

regarding the potential for additional 

protection from adverse ecological 

effects are much larger for target ANC 

levels above about 75 meq/L, as effects 

are generally appreciably less sensitive 

to changes in ANC at such higher levels. 

The PA recognizes that the level of 

the standard together with the other 

elements of the standard, including the 

ambient air indicators, averaging time, 

and form, determine the overall 

protectiveness of the standard. Thus, 

consideration of a standard level should 

reflect the strengths and limitations of 

the evidence and assessments as well as 

the inherent uncertainties in the 

development of each of the elements of 

the standard. The implications of 

considering alternative standards, 

defined in terms of alternative 

combinations of levels and percentile 

values that are a critical component of 

factor F1 in the form of the standard, are 

discussed in section III.E of the proposal 

and more fully in the PA. 

5. Characterization of Uncertainties 

The characterization of uncertainties 

is intended to address the relative 

confidence associated with the linked 

atmospheric-ecological effects system 

described above, and is described in 

detail in the PA (U.S. EPA, 2011, section 

7.6 and Appendices F and G) and 

summarized in section III.F of the 

proposal. A brief overview of 

uncertainties is presented here in the 

context of the major structural 

components underlying the standard, as 

well as with regard to areas of relatively 

high uncertainty. 

As discussed in the PA (U.S. EPA, 

2011, Table 7–3), there is relatively low 

uncertainty with regard to the 

conceptual formulation of the overall 

structure of the AAI-based standard that 

incorporates the major associations 

linking biological effects to air 

concentrations. Based on the strength of 

the evidence that links species richness 

and mortality to water quality, the 

associations are strongly causal and 

without any obvious confounding 

influence. The strong association 

between the ecosystem indicator (ANC) 

and the causative water chemistry 

species (dissolved aluminum and 

hydrogen ion) reinforces the confidence 

in the linkage between deposition of 

nitrogen and sulfur and effects. This 

strong association between ANC and 

effects is supported by a sound 

mechanistic foundation between 

deposition and ANC. The same 

mechanistic strength holds true for the 

relationship between ambient air levels 

of nitrogen and sulfur and deposition, 

which completes the linkage from 

ambient air indicators through 

deposition to ecological effects. 

There are much higher uncertainties, 

however, in considering and quantifying 

the specific elements within the 

structure of an AAI-based standard, 

including the deposition of SOX, NOy, 

and NHX as well as the critical loadrelated component, each of which can 

vary within and across ecoregions. 

Overall system uncertainty with an AAI 

approach relates not just to the 

uncertainty in each element, but also to 

the combined uncertainties that result 

from linking these elements together 

within the AAI-based structure and over 

the defined spatial scale (i.e., 

ecoregions). Some of these elements— 

including, for example, dry deposition, 

pre-industrial base cation production, 

and reduced nitrogen deposition—are 

estimated with less confidence than 

other elements (U.S. EPA, 2011, Table 

7.3). The uncertainties associated with 

all of these elements, and the 

combination of these elements through 

the AAI equation and over the ecoregion 

spatial scale, are summarized below. 

The lack of observed dry deposition 

data, which affects confidence in the 

AAI on an ecoregion scale, is 

constrained in part by the lack of 

efficient measurement technologies. 

Progress in reducing uncertainties in 

dry deposition will depend on 

improved atmospheric concentration 

data and direct deposition flux 

measurements of the relevant suite of 

NOy and SOX species. 

Pre-industrial base cation 

productivity by definition is not 

observable. Contemporary observations 

and inter-model comparisons are useful 

tools that help reduce the uncertainty in 

estimates of pre-industrial base cation 

productivity used in the AAI equation. 

In characterizing contemporary base 

cation flux using basic water quality 

measurements (i.e., major anion and 

cation species as defined in equation 

2.11 in the PA), it is reasonable to 

assume that a major component of 

contemporary base cation flux is 

associated with pre-industrial 

weathering rates. To the extent that 

multiple models converge on similar 

solutions within and across ecoregions, 

greater confidence in estimating preindustrial base cation production within 

the AAI and ecoregion frameworks 

would be achieved. 

While characterization of NHX

deposition has been evolving over the 

last decade, the high uncertainty in 

characterizing NHX deposition is due to 

both the lack of field measurements and 

the inherent complexity of 

characterizing NHX with respect to 

source emissions and dry deposition.12

Because ammonia emissions are 

generated through a combination of 

man-made and biological activities, and 

ammonia is semi-volatile, the ability to 

characterize spatial and temporal 

distributions of NHX concentrations and 

deposition patterns is limited. While 

direct measurement of NHX deposition 

is resource intensive because of the 

diffuse nature of sources (i.e., area-wide 

and non-point sources), there have been 

more frequent deposition flux studies, 

relative to other nitrogen species, that 

enable the estimation of both emissions 

and dry deposition. Also, while 

ammonia has a relatively high 

deposition velocity and traditionally 

was thought to deposit close to the 

emissions release areas, the semivolatile nature of ammonia results in reentrainment back into the lower 

boundary layer of the atmosphere 

resulting in a more dispersed 

concentration pattern exhibiting 

transport characteristics similar to 

longer lived atmospheric species. These 

inherent complexities in source 

characterization and ambient 

concentration patterns significantly 

increase the degree of uncertainty in 

NHX deposition in general, and in the 

AAI equation applied on an ecoregion 

scale in particular. However, the PA 

notes that progress is being made in 

measuring ammonia with cost efficient 

samplers and anticipates the gradual 

evolution of a spatially robust ammonia 

sampling network that would help 

support analyses to reduce underlying 

uncertainties in NHX deposition. 

In characterizing uncertainties with 

respect to available measurement data 

and the use of ecological and 


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atmospheric models, as summarized in 

sections III.F.2–3 of the proposal, the PA 

identified data gaps and model 

uncertainties in relative terms by 

comparing, for example, the relative 

richness of data between geographic 

areas or environmental media. As 

discussed in the proposal and more 

fully in the PA, from an uncertainty 

perspective, gaps in field measurement 

data increase uncertainties in modeled 

processes and in the specific application 

of such models. As noted above, 

processes that are embodied in an AAIbased standard are modeled using the 

CMAQ atmospheric model and steadystate ecological models. These models 

are characterized in the ISA as being 

well-established and have undergone 

extensive peer review. Nonetheless, the 

application of these models for purposes 

of specifying the factors in the AAI 

equation, on an ecoregion scale, is a 

new application that introduces 

uncertainties, especially in areas with 

limited observational data that can be 

used to evaluate this specific 

application. Understanding 

uncertainties in relevant modeled 

processes thus involves consideration of 

the uncertainties associated with 

applying each model as well as the 

combination of these uncertainties as 

the models are applied in combination 

within the AAI framework applied on 

an ecoregion scale. 

Our confidence in improving critical 

load estimates can be increased by 

expanding water quality data bases used 

as inputs and evaluation metrics for 

critical load models. With regard to 

water quality data, the PA notes that 

such data are typically limited relative 

to air quality data sets, and are also 

relatively sparse in the western United 

States. While there are several state and 

local agency water quality data bases, it 

is unclear the extent to which 

differences in sampling, chemical 

analysis and reporting protocols would 

impact the use of such data for the 

purpose of better understanding the 

degree of protectiveness that would be 

afforded by an AAI-based standard 

within sensitive ecoregions across the 

country. In addition, our understanding 

of water quality in Alaska and Hawaii 

and the acid sensitivity of their 

ecoregions is particularly limited. 

Expanding the water quality data bases 

would enable clearer delineation of 

ecoregion representative critical loads in 

terms of the nth percentile. This would 

provide more refined characterization of 

the degree of protection afforded by a 

given standard. Longer term, the 

availability of water quality trend data 

(annual to monthly sampled) would 

support accountability assessments that 

examine if an ecoregion’s response to air 

management efforts is as predicted by 

earlier model forecasting. The most 

obvious example is the long-term 

response of water quality ANC change 

to changes in calculated AAI, 

deposition, ambient NOY and SOX

concentrations, and emissions. In 

addition, water quality trends data 

provide a basis for evaluating and 

improving the parameterizations of 

processes in critical load models 

applied at the ecoregion scale related to 

nitrogen retention and base cation 

supply. A better understanding of soil 

processes, especially in the southern 

Appalachians, would enhance efforts to 

examine the variability within 

ecoregions of the soil-based adsorption 

and exchange processes which moderate 

the supply of major cations and anions 

to surface waters and strongly influence 

the response of surface water ANC to 

changes in deposition of nitrogen and 

sulfur. 

Steady-state biogeochemical 

ecosystem modeling is used to develop 

critical load estimates that are 

incorporated in the AAI equation 

through factor F1. Consequently, the PA 

notes that an estimate of the temporal 

response of surface water ANC to 

deposition and air concentration 

changes is not directly available. 

Lacking a predicted temporal response 

impairs the ability to conduct 

accountability assessments down to the 

effects level. Accountability assessments 

would examine the response of each 

step in the emissions source through air 

concentration—deposition—surface 

water quality—biota continuum. The 

steady-state assumption at the 

ecosystem level does not impair 

accountability assessments through the 

air concentration/deposition range of 

that continuum. However, in using 

steady-state ecosystem modeling, 

several assumptions are made relative to 

the long-term importance of processes 

related to soil adsorption of major ions 

and ecosystem nitrogen dynamics. 

Because these models often were 

developed and applied in glaciated 

areas with relatively thin and 

organically rich soils, their applicability 

is relatively more uncertain in areas 

such as those in the non-glaciated claybased soil regions of the central 

Appalachians. Consequently, it is 

desirable to develop the information 

bases to drive simple dynamic 

ecosystem models that incorporate more 

detailed treatment of subsurface 

processes, such as adsorption and 

exchange processes and sulfate 

absorption. 

B. CASAC Views 

The CASAC has advised the EPA 

concerning the ISA, the REA, and the 

PA. The CASAC supported the EPA’s 

interpretation of the science embodied 

in the ISA and the assessment 

approaches and conclusions 

incorporated in the REA. 

Most recently, CASAC considered the 

information in the final PA in providing 

its recommendations on the review of 

the new multi-pollutant standard 

developed in that document and 

discussed above (Russell and Samet, 

2011a). In so doing, CASAC expressed 

general support for the conceptual 

framework of the standard based on the 

underlying scientific information, as 

well as for the conclusions in the PA 

with regard to indicators, averaging 

time, form and level of the standard that 

are appropriate for consideration by the 

Agency in reaching decisions on the 

review of the secondary NAAQS for 

oxides of nitrogen and sulfur: 

‘‘The final Policy Assessment clearly sets 

out the basis for the recommended ranges for 

each of the four elements (indicator, 

averaging time, level and form) of a potential 

NAAQS that uses ambient air indicators to 

address the combined effects of oxides of 

nitrogen and oxides of sulfur on aquatic 

ecosystems, primarily streams and lakes. As 

requested in our previous letters, the Policy 

Assessment also describes the implications of 

choosing specific combinations of elements 

and provides numerous maps and tabular 

estimates of the spatial extent and degree of 

severity of NAAQS exceedances expected to 

result from possible combinations of the 

elements of the standard.’’ 

‘‘We believe this final PA is appropriate for 

use in determining a secondary standard to 

help protect aquatic ecosystems from 

acidifying deposition of oxides of sulfur and 

nitrogen. The EPA staff has done a 

commendable job developing the innovative 

Aquatic Acidification Index (AAI), which 

provides a framework for a national standard 

based on ambient concentrations that also 

takes into account regional differences in 

sensitivities of ecosystems across the country 

to effects of acidifying deposition.’’ 

(Russell and Samet, 2011a). 

With respect to indicators, CASAC 

supported the use of SOX and NOy as 

ambient air indicators (discussed above 

in section III.A) and ANC as the 

ecological indicator (discussed above in 

section III.B.1). With respect to 

averaging time (discussed above in 

section III.C), CASAC agreed with the 

conclusions in the PA that ‘‘an 

averaging time of three to five years for 

the AAI parameters is appropriate.’’ 

CASAC noted that ‘‘a longer averaging 

time would mask possible trends of 

AAI, while a shorter averaging time 

would make the AAI being more 

influenced by the conditions of the 


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particular years selected’’ (Russell and 

Samet, 2011a). 

With respect to the form of the 

standard (discussed above in section 

III.B), CASAC stated the following: 

‘‘EPA has developed the AAI, an 

innovative ‘‘form’’ of the NAAQS itself that 

incorporates the multi-pollutant, multimedia, environmentally modified, 

geographically variable nature of SOX/NOy

deposition-related aquatic acidification 

effects. With the caveats noted below, 

CASAC believes that this form of the NAAQS 

as described in the final Policy Assessment 

is consistent with and directly reflective of 

current scientific understanding of effects of 

acidifying deposition on aquatic 

ecosystems.’’ (Russell and Samet, 2011a) 

‘‘CASAC agrees that the spatial 

components of the form in the Policy 

Assessment are reasonable and that use of 

Omernik’s ecoregions (Level III) is 

appropriate for a secondary NAAQS intended 

to protect the aquatic environment from 

acidification * * *’’ 

(Russell and Samet, 2011a). 

The caveats noted by CASAC include 

a recognition of the importance of 

continuing to evaluate the performance 

of the CMAQ and ecological models to 

account for model uncertainties and to 

make the model-dependent factors in 

the AAI more transparent. In addition, 

CASAC noted that the role of DOC and 

its effects on ANC would benefit from 

further refinement and clarification 

(Russell and Samet, 2011a). While 

CASAC expressed the view that the 

‘‘division of ecoregions into ‘sensitive’ 

and ‘non-sensitive’ subsets, with a more 

protective percentile applied to the 

sensitive areas, also seems reasonable’’ 

(Russell and Samet, 2011a), CASAC also 

noted that there was the need for greater 

clarity in specifying how appropriate 

screening criteria would be applied in 

assigning ecoregions to these categories. 

Further, CASAC identified potential 

biases in critical load calculations and 

in the regional representativeness of 

available water chemistry data, leading 

to the observation that a given 

percentile of the distribution of 

estimated critical loads may be 

protective of a higher percentage of 

surface waters in some regions (Russell 

and Samet, 2011a). Such potential 

biases led CASAC to recommend that 

‘‘some attention be given to our residual 

concern that the available data may 

reflect the more sensitive water bodies 

and thus, the selection of the percentiles 

of waterbodies to be protected could be 

conservatively biased’’ (Russell and 

Samet, 2011a). 

With respect to level as well as the 

combination of level and form as they 

are presented as alternative standards 

(discussed above in sections III.D–E), 

CASAC agreed with the PA conclusions 

that consideration should be given to 

standard levels within the range of 20 

and 75 meq/L. CASAC also recognized 

that the level and the form of any AAIbased standard are so closely linked that 

these two elements should be 

considered together: 

‘‘When considered in isolation, it is 

difficult to evaluate the logic or implications 

of selecting from percentiles (70th to 90th) of 

the distribution of estimated critical loads for 

lakes in sensitive ecoregions to determine an 

acceptable amount of deposition for a given 

ecoregion. However, when these percentile 

ranges are combined with alternative levels 

within the staff-recommended ANC range of 

20 to 75 microequivalents per liter (meq/L), 

the results using the AAI point to the 

ecoregions across the country that would be 

expected to require additional protection 

from acidifying deposition. Reasonable 

choices were made in developing the form. 

The number of acid sensitive regions not 

likely to meet the standard will be affected 

both by choice of ANC level and the 

percentile of the distribution of critical loads 

for lakes to meet alternative ANC levels in 

each region. These combined 

recommendations provide the Administrator 

with a broad but reasonable range of 

minimally to substantially protective options 

for the standard.’’ 

(Russell and Samet, 2011a). 

CASAC also commented on the EPA’s 

uncertainty analysis, and provided 

advice on areas requiring further 

clarification in the proposed rule and 

future research. The CASAC found it 

‘‘difficult to judge the adequacy of the 

uncertainty analysis performed by the 

EPA because of lack of details on data 

inputs and the methodology used, and 

lack of clarity in presentation’’ (Russell 

and Samet, 2011a). In particular, 

CASAC identified the need for more 

thorough model evaluations of critical 

load and atmospheric modeling, 

recognizing the important role of 

models as they are incorporated in the 

form of the standard. In light of the 

innovative nature of the standard 

developed in the PA, CASAC identified 

‘‘a number of areas that should be the 

focus of further research’’ (Russell and 

Samet, 2011a). While CASAC 

recognized that the EPA staff was able 

to address some of the issues in the PA, 

they also noted areas ‘‘that would 

benefit from further study or 

consideration in potential revisions or 

modifications to the form of the 

standard.’’ Such research areas include 

‘‘sulfur retention and mobilization in 

the soils, aluminum availability, soil 

versus water acidification and 

ecosystem recovery times.’’ Further, 

CASAC encouraged future efforts to 

monitor individual ambient nitrogen 

species, which would help inform 

further CMAQ evaluations and the 

specification of model-derived elements 

in the AAI equation (Russell and Samet, 

2011a). 

C. Proposed Conclusions on Alternative 

Secondary Standards 

As discussed in section III.H of the 

proposal, the Administrator considered 

whether it is appropriate at this time to 

set a new multi-pollutant standard to 

address deposition-related effects 

associated with oxides of nitrogen and 

sulfur, with a structure that would 

better reflect the available science 

regarding acidifying deposition to 

sensitive aquatic ecosystems. In so 

doing, she recognized that such a 

standard, for purposes of Section 109(b) 

and (d) of the CAA, must in her 

judgment be requisite to protect public 

welfare, such that it would be neither 

more nor less stringent than necessary 

for that purpose. In particular, she 

focused on the AAI-based standard 

developed in the PA and reviewed by 

CASAC, as discussed above. Based on 

consideration of the scientific basis for 

such a standard and the conclusions 

reached in the ISA, the Administrator 

agreed with the conclusion in the PA, 

and supported by CASAC, that there is 

a strong scientific basis for development 

of a standard with the general structure 

presented in the PA. She recognized 

that while the standard is innovative 

and unique, the structure of the 

standard is well-grounded in the science 

underlying the relationships between 

ambient concentrations of oxides of 

nitrogen and sulfur and the aquatic 

acidification related to deposition of 

nitrogen and sulfur associated with such 

ambient concentrations. 

Nonetheless, the Administrator also 

recognized that such a standard would 

depend on atmospheric and ecological 

modeling, based on appropriate data, to 

specify the terms of an equation that 

incorporates the linkages between 

ambient concentrations, deposition, and 

aquatic acidification, for each separate 

ecoregion, and that there are a number 

of inherent uncertainties and 

complexities that are relevant to the 

question of whether it is appropriate 

under Section 109 of the CAA to set a 

specific AAI-based standard at this time. 

Based on her consideration of these 

important uncertainties and limitations, 

the Administrator recognized that in 

combination, 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 


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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. The Administrator 

recognized that the AAI equation, with 

factors quantified in the ranges 

discussed above and described more 

fully in the PA, 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. However, the 

uncertainties discussed here are critical 

for determining 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 AAIbased standard, and thus for 

determining an appropriate AAI-based 

standard that meets the requirements of 

Section 109. 

The Administrator noted that setting 

a NAAQS generally involves 

consideration of the degree of 

uncertainties in the science and other 

information, such as gaps in the relevant 

data and, in this case, limitations in the 

evaluation of the application of relevant 

ecological and atmospheric models at an 

ecoregion scale. She noted that the issue 

here is not a question of uncertainties 

about the scientific soundness of the 

structure of the AAI, but instead 

uncertainties in the quantification and 

representativeness of the elements of the 

AAI as they vary in ecoregions across 

the country. At present, these 

uncertainties 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 and thus 

prevent identification of an appropriate 

standard. 

The Administrator judged that the 

uncertainties are of such 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. Further, she 

noted that CASAC acknowledged that 

important uncertainties remain that 

would benefit from further study and 

data collection efforts, which might lead 

to potential revisions or modifications 

to the form of the standard developed in 

the PA, and that CASAC encouraged the 

Agency to engage in future monitoring 

and model evaluation efforts to help 

inform further development of the 

elements of an AAI-based standard. 

Based on these considerations the 

Administrator judged that the current 

limitations in relevant data and the 

uncertainties associated with specifying 

the elements of the AAI based on 

modeled factors are of such nature and 

degree as to prevent her from reaching 

a reasoned decision such that she is 

adequately confident 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. 

Based on the above considerations, 

the Administrator provisionally 

concluded that it is premature to set a 

new, multi-pollutant secondary 

standard for oxides of nitrogen and 

sulfur at this time, and as such she 

proposed not to set such a new 

secondary standard. Nonetheless, while 

the Administrator concluded that it is 

premature to set such a multi-pollutant 

standard at this time, she determined 

that the Agency should undertake a 

field pilot program to gather additional 

data (discussed below in section IV). 

She concluded that it is appropriate that 

such a program be undertaken before, 

rather than after, reaching a decision to 

set such a standard. 

In reaching her proposed decision not 

to set a new AAI-based standard at this 

time, the Administrator recognized that 

the new NO2 and SO2 primary 1-hour 

standards set in 2010, while not 

ecologically relevant for a secondary 

standard, will nonetheless result in 

reductions in oxides of nitrogen and 

sulfur that will directionally benefit the 

environment by reducing NOy and SOX

deposition to sensitive ecosystems. The 

Administrator proposed to revise the 

secondary standards by adding 

secondary standards identical to the 

NO2 and SO2 primary 1-hour standards 

set in 2010, including a 1-hour 

secondary NO2 standard set at a level of 

100 ppb and a 1-hour secondary SO2

standard set at a level of 75 ppb. The 

EPA noted that while this will not add 

secondary standards of an ecologically 

relevant form to address depositionrelated effects, it will provide additional 

protection for sensitive areas. The EPA 

further noted that this proposed 

decision is 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, for the 

reasons summarized above. 

The EPA solicited comment on all 

aspects of this proposed decision, as 

discussed in the following section. 

D. Comments on Alternative Secondary 

Standards 

In this section, comments received on 

the proposal related to an AAI-based 

standard are discussed in section III.D.1 

and comments related to the proposed 

decision to set 1-hour NO2 and SO2

secondary standards are discussed in 

section III.D.2. 

1. Comments Related to 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. 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 below in section III.D.1.a, 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 above in section I.E. 

Some commenters also expressed views 

about specific aspects of an AAI-based 

approach, as discussed below in section 

III.D.1.b. More technical comments on 

specific elements and factors of the AAI 

are discussed in the Response to 

Comments document. General 

comments based on implementationrelated factors that are not a permissible 

basis for considering an alternative 

standard are noted in the Response to 

Comments document. 

a. Comments on Consideration of an 

AAI-Based Standard 

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 


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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 AAIbased 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 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 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 depositionrelated 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 NOX/SOX

standard 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 depositionrelated 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 AAIbased 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. 

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 NOX and SOX

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 NOX and SOX deposition 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 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 nonsensitive categories, (5) the use of a 3 to 

5 year averaging time, and (6) the 

appropriateness of an AAI level between 

20 to 75 meq/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 

grounds for failing to propose protective 

standards is irrational, arbitrary, and 

legally flawed. They believe that the 

EPA cannot lawfully reject a new AAIbased 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, a few 

other commenters offered more general 


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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. One state organization 

(NESCAUM) expressed concern that the 

EPA was not following CASAC’s 

recommendation to propose an 

ecologically relevant level and form for 

this NAAQS. 

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) 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 depositionrelated aquatic acidification in sensitive 

ecosystems across the country. As 

discussed in section II.A 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 well-established 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 AAIbased 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. above 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 U.S. 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 delineation, has undergone 

extensive scientific peer review, and has 

explicitly been applied to delineating 

acid sensitive areas of the U.S. 

Nonetheless, the EPA agrees with the 

first group of commenters that there are 

important and significant 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 above. 

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 a high degree of 

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 recognizes that, in contrast, such 

model-related uncertainties are not 

relevant in the consideration of other 

NAAQS since those NAAQS are not 

defined in terms of factors based on 

such models. 

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 a 

high degree of uncertainty in 

characterizing the degree of 

protectiveness that would be afforded 

by an AAI-based standard with 

quantified F factors derived as 

discussed above, within the ranges of 

levels and forms identified in section 

III.A above. 

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 sensitive and nonsensitive 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 


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13No one has suggested that the 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. 

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- to 5-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 and significant 

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 above and are considered as well 

in section III.E below. 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 is of such 

significance that it 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 a reasoned 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 above 

and described more fully in chapter 7 of 

the PA, 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 high 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 a high degree 

of uncertainty as to the actual degree of 

protectiveness that such a standard 

would provide, especially for acidsensitive ecoregions. 

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 AAIbased 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 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 AAIbased NAAQS defined in terms of 

modeled factors are of such a significant 

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 

AAI-based 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.13 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 


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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 are of 

such significance that they 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. 

This is consistent with the decision 

by the Supreme Court in Massachusetts 

v. EPA, 549 U.S. 497 (2007), which 

concerned the EPA’s authority under 

Section 202(a) of the CAA. There the 

Supreme Court determined that 

scientific uncertainty that ‘‘is so 

profound that it precludes the EPA from 

making a reasoned judgment’’ 

concerning endangerment to public 

health and welfare from air pollution 

would justify the EPA not making a 

finding on endangerment. Id at 534. The 

Court noted that ‘‘[t]he statutory 

question is whether sufficient 

information exists to make’’ an 

endangerment finding. Id. In this 

review, the scientific uncertainty is of 

such a significant nature and degree that 

sufficient information does not exist for 

the EPA to make a reasoned judgment 

as to whether a new secondary standard 

addressing aquatic acidification would 

satisfy the criteria of Section 109(b). As 

such, adding a new AAI secondary 

standard at this time would not ‘‘be 

appropriate under [Section 109(b)].’’ 

CAA Section 109(d)(1). 

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 uncertainties are of 

such significance that they preclude the 

EPA from making a reasoned 

determination of the degree of 

protectiveness that would be afforded 

by such a standard. The EPA recognizes 

that as a result of not setting a new 

secondary standard 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 below. 

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. 

b. Comments on Specific Aspects of an 

AAI-Based 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 multipollutant NAAQS. 

(1) As described above in section 

III.A, 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. 

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 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 AAI-based 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 


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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 AAIbased 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 AAIbased 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 AAI-based 

standard. NHX deposition would be 

treated as an ecologically relevant 

background value 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) 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. 

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, 

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 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 


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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 nonsensitive 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) 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. 

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. 

(4) 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 Section 109(b)(2) 

(emphasis added). These commenters 

suggest the NAAQS must be set for 

‘‘such air pollutant’’ only. The EPA 

disagrees that the phrase ‘‘such air 

pollutant’’ in Section 109(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 Section 

109(b)(2). 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, acidifying 

deposition 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. 

2. Comments on 1-Hour NO2 and SO2

Secondary Standards 

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 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 


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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 time. As 

summarized above, the EPA based its 

proposed decision not to set an AAIbased 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 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 

deposition-related 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. 

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 reasonably be 

done in the absence of a 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 

deposition-related 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 acid-sensitive 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 be specifically 

quantified or linked to reductions in 

aquatic acidification in specific 

ecoregions. In addition, unlike an AAIbased 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 

benefits that would accrue as a result of 

actions taken to meet the 2010 1-hour 

NO2 and SO2 primary standards will 

occur regardless of whether we adopt 

identical secondary standards. Thus, 

there is no additional environmental 

benefit to be gained by making the 

standards identical. 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 


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14Thus, as discussed above, EPA’s disagreement 

with commenters concerning adoption of an AAIbased 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. 

15Section 109(d)(1) of the CAA requires that 

‘‘* * * the Administrator shall complete a 

thorough review * * * and shall make such 

revisions in such criteria and standards and 

promulgate such new standards as may be 

appropriate under * * * subsection 109(b) of this 

section.’’ 

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 

steady-state ANC levels across the 

distribution of water bodies within the 

various ecoregions in the United States. 

Because of this, there is a high degree 

of uncertainty as to the actual degree of 

protectiveness that such a standard 

would provide, especially for acidsensitive ecoregions. At this time, the 

Administrator judges that the 

uncertainties are of such a significant 

nature and degree that there is no 

reasoned way to choose a specific AAIbased 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.14

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 above in section III.D.1.a, 

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 significant nature and degree as to 

prevent the Administrator from reaching 

a reasoned decision as to what specific 

standard would be appropriate to 

provide requisite 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 

even 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. 

E. Final Decisions on Alternative 

Secondary Standards for Oxides of 

Nitrogen and Sulfur 

In considering the appropriateness of 

establishing a new multi-pollutant AAIbased standard to provide protection 

against potentially adverse depositionrelated effects associated with oxides of 

nitrogen and sulfur, or setting new 

secondary standards identical to the 

current 1-hour NO2 and SO2 primary 

standards, the Administrator took into 

account the information and 

conclusions in the ISA, REA, and PA, 

CASAC advice, and the views of public 

commenters. This consideration follows 

from her conclusion, discussed above in 

section II.D, that the existing NO2 and 

SO2 secondary standards are neither 

appropriate nor sufficiently protective 

for this purpose. 

As an initial matter, the Administrator 

has again considered whether it is 

appropriate at this time to set a new 

multi-pollutant standard to provide 

protection against potentially adverse 

deposition-related effects associated 

with oxides of nitrogen and sulfur, with 

a structure that would better reflect the 

available science regarding acidifying 

deposition. In considering this, she 

recognizes that such a standard, for 

purposes of Section 109(b) and (d) of the 

CAA,15 must in her judgment be 

requisite to protect public welfare, such 

that it would be neither more nor less 

stringent than necessary for that 

purpose. In particular, she has focused 

on the new AAI-based standard 

developed in the PA and reviewed by 

CASAC, as discussed above in section 

III.A. In so doing, the Administrator has 

again considered the extent to which 

there is a scientific basis for 

development of such a standard, 

specifically with regard to a standard 

that would provide protection from 

deposition-related aquatic acidification 

in sensitive aquatic ecosystems in areas 

across the country. 

The Administrator notes that the ISA 

concludes that the available scientific 

evidence is sufficient to infer a causal 

relationship between acidifying 

deposition of nitrogen and sulfur in 

aquatic ecosystems, and that the 

deposition of oxides of nitrogen and 

sulfur both cause such acidification 

under current conditions in the United 

States. Further, the ISA concludes that 

there are well-established water quality 

and biological indicators of aquatic 

acidification as well as well-established 

models that address deposition, water 

quality, and effects on ecosystem biota, 

and that ecosystem sensitivity to 

acidification varies across the country 

according to present and historic 

nitrogen and sulfur deposition as well 

as geologic, soil, vegetative, and 

hydrologic factors. In considering public 

comments on the relevant scientific 

evidence, the Administrator notes that 

some commenters agree with these 

conclusions in the ISA, whereas other 

commenters question the extent to 

which the scientific information 

provides evidence of well-established 

water quality and biological indicators 

of aquatic acidification and the extent to 

which relevant models appropriately 

account for important factors or have 

been adequately evaluated. The 

Administrator has carefully considered 

these comments and the Agency’s 

responses to these comments, as 

discussed above in section III.D. The 

Administrator also has considered the 

views of CASAC, including its general 

support for the conceptual framework of 

the AAI-based standard developed in 

the PA based on the assessments of the 

underlying scientific information in the 

ISA and REA. 

Based on these considerations, the 

Administrator again concludes that the 

general structure of an AAI-based 

standard addresses the combined effects 

of deposition from oxides of nitrogen 

and sulfur by characterizing the linkages 

between ambient concentrations, 

deposition, and aquatic acidification, 

and that it takes into account relevant 

variations in these linkages across the 

country. She recognizes that while such 

a standard clearly would be quite 

innovative and unique, the general 

structure of such a standard is 

nonetheless well-grounded in the 

science underlying the relationships 

between ambient concentrations of 

oxides of nitrogen and sulfur and the 

aquatic acidification related to 

deposition of nitrogen and sulfur 


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associated with such ambient 

concentrations. Based on these 

considerations, the Administrator 

continues to agree with the conclusion 

in the PA, and supported by CASAC, 

that there is a strong scientific basis for 

continued development of a standard 

with the general structure presented in 

the PA. Further, the Administrator 

recognizes that the AAI equation, with 

factors quantified in the ranges 

discussed above and described more 

fully in the PA, 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 directionally would 

lead to higher calculated AAI values. 

Nonetheless, while the Administrator 

recognizes the strong scientific 

foundation for the general structure of 

an AAI-based standard, she also 

recognizes that a specific AAI-based 

standard would depend to a great degree 

on atmospheric and ecological 

modeling, in combination with 

appropriate data, to specify the 

quantified terms of an equation that 

incorporates the linkages between 

ambient concentrations, deposition, and 

aquatic acidification. This equation, 

which defines an aquatic acidification 

index (AAI), has the effect of translating 

spatially variable ambient 

concentrations and ecological effects 

into a potential national standard. 

With respect to establishing the 

specific terms of this equation, there are 

a number of important and significant 

uncertainties and complexities that are 

critical to the question of whether it is 

appropriate under Section 109 of the 

CAA to set a specific AAI-based 

standard at this time, recognizing that 

such a standard must be one that in the 

judgment of the Administrator is 

requisite to protect public welfare 

without being either more or less 

stringent than necessary for this 

purpose. As discussed above in section 

III.A, these uncertainties and 

complexities generally relate not to the 

structure of the standard, but to the 

quantification of the various elements of 

the standard, i.e., the F factors, and their 

representativeness at an ecoregion scale. 

These uncertainties and complexities, 

which are unique to this NAAQS 

review, currently preclude the 

characterization of the degree of 

protectiveness that would be afforded 

by such a standard, within the ranges of 

levels and forms identified in the PA, 

and the representativeness of F factors 

in the AAI equation described above 

and in the PA. These uncertainties have 

been generally categorized as limitations 

in available field data as well as 

uncertainties that are related to reliance 

on the application of ecological and 

atmospheric modeling at the ecoregion 

scale to specify the various elements of 

the AAI. 

With regard to data limitations, the 

Administrator observes that there are 

several key limitations in the available 

data upon which elements of the AAI 

are based. For example, while ambient 

measurements of NOy are made as part 

of a national monitoring network, the 

monitors are not located in locations 

that have been determined to be 

representative of sensitive aquatic 

ecosystems or individual ecoregions. 

Further, while air and water quality data 

are generally available in areas in the 

eastern United States, there is relatively 

sparse coverage in mountainous western 

areas where a number of sensitive 

aquatic ecosystems are located. Even in 

areas where relevant data are available, 

small sample sizes in some areas 

impede efforts to characterize the 

representativeness of the available data 

at an ecoregion scale, which was noted 

by CASAC and some commenters as 

being of particular concern. 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. 

With regard to uncertainties related to 

the use of ecological and atmospheric 

modeling, the Administrator notes in 

particular that model results are 

difficult to evaluate due to a lack of 

relevant observational data. For 

example, large uncertainties are 

introduced by a lack of data to inform 

the 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. 

Taking into account the above 

considerations, the Administrator 

recognizes that characterization of the 

uncertainties in the AAI equation as a 

whole represents a unique challenge in 

this review primarily as a result of the 

complexity in the structure of an AAIbased standard. In this case, the very 

nature of some of the uncertainties is 

fundamentally different than 

uncertainties that have been relevant in 

other NAAQS reviews. She notes, for 

example, some of the uncertainties 

uniquely associated with the 

quantification of various elements of the 

AAI result from limitations in the extent 

to which ecological and atmospheric 

models, which have not been used to 

define other NAAQS, have been 

evaluated. Another important type of 

uncertainty relates to limitations in the 

extent to which the representativeness 

of various factors can be determined at 

an ecoregion scale, which has not been 

a consideration in other NAAQS. 

In combination, these limitations and 

uncertainties are of such a nature and 

degree as to result in a high 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, the EPA cannot 

reasonably characterize the actual 

degree of protectiveness that such a 

standard would provide, especially for 

acid-sensitive ecoregions. The 

uncertainties discussed here are critical 

for determining 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 AAIbased standard, and thus for 

determining an appropriate AAI-based 

standard that meets the requirements of 

Section 109 of the CAA. 

In considering these uncertainties in 

light of CASAC’s advice, the 

Administrator notes that CASAC 

acknowledged that important 

uncertainties remain that would benefit 

from further study and data collection 

efforts, which might lead to potential 

revisions or modifications to the form of 

the standard developed in the PA. She 

also notes that CASAC encouraged the 

Agency to engage in future monitoring 

and model evaluation efforts to help 

inform the specification of modelderived elements in the AAI equation. 

CASAC supported the view in the PA 

that there was a scientific basis for 

consideration of an AAI, and that is 

what the Administrator has done in that 

she has fully considered an AAI-based 

standard. However, CASAC did not 

indicate that there was such a degree of 

scientific support for quantifying the 

terms of the AAI equation and setting a 

specific AAI-based standard at this time 

that it would be inappropriate to 

consider not setting an AAI-based 

standard in this review in light of the 

uncertainties that CASAC itself 

recognized. 

Further, in considering these 

uncertainties in light of the public 

comments discussed above, the 

Administrator notes that these 


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uncertainties and limitations have been 

highlighted by a number of public 

commenters in support of their view 

that it would be inappropriate to 

establish an AAI-based standard at this 

time. Other commenters, however, 

noted that NAAQS decisions are always 

made in the face of uncertainties, and 

expressed the view that the 

uncertainties in this NAAQS review are 

not so great as to preclude establishing 

such a standard at this time. 

The Administrator agrees with the 

commenters that note that NAAQS 

decisions are always made in the face of 

uncertainties, since the latest available 

scientific information upon which 

NAAQS are to be based is often at the 

leading edge of research. Thus, the EPA 

Administrator must always consider 

uncertainties in scientific and other 

information in reaching decisions on 

whether to retain or revise an existing 

NAAQS or to adopt a new NAAQS. As 

a result, it is clear that the existence of 

scientific uncertainty does not preclude 

adoption of a new or revised NAAQS. 

The issue here, however, is not whether 

uncertainty exists, but whether it is of 

such a significant nature and magnitude 

that it warrants not adopting an AAIbased standard at this time. In that 

context, the Administrator recognizes 

that the AAI-based standard considered 

in this review is by far the most 

complex form of a NAAQS standard that 

the EPA has considered, to date, and 

that this is the first review in which the 

scientific and technical details of an 

AAI-based standard have been 

developed for consideration. This 

review has served to bring into focus for 

the first time the nature and degree of 

the uncertainties associated with 

quantifying the specific factors in the 

equation that defines the AAI. Thus, in 

this review, the Administrator must 

newly consider not only the scientific 

basis for the conceptual framework of 

such a standard, but also the extent to 

which the available data, models, and 

analyses provide a reasoned basis to 

choose a specific AAI-based standard 

consistent with the requirements of 

Section 109 of the CAA. 

The nature of the uncertainties 

present in this review, and the 

implications of those uncertainties for 

reaching a reasoned decision as to 

whether an AAI-based NAAQS could be 

set consistent with the requirements of 

section 109(b), are in sharp contrast to 

the nature of uncertainties present in 

other NAAQS reviews. In other NAAQS 

reviews, studies are generally available 

directly linking ambient air 

concentrations of the pollutant to 

evidence of effects on public health or 

welfare. For example, in reviewing a 

health-based primary NAAQS the EPA 

typically considers a wide range of 

clinical, epidemiologic, toxicologic, and 

other studies that evaluate the 

relationship between direct exposure to 

an ambient air pollutant and human 

health. The EPA also often considers 

laboratory or field studies or surveys 

that evaluate and characterize the 

relationship between ambient levels of 

an air pollutant and welfare effects, 

such as effects of the ambient air 

pollutant on the growth of plants or on 

injury to plants. These kinds of 

scientific studies have provided a 

reasoned basis in other reviews for the 

selection of an appropriate level and 

form of a standard, with the EPA taking 

into account the nature and degree of 

uncertainties, for example, in the 

relationships between varying ambient 

air concentrations and the impact on 

human health or the environment. 

Further, the uncertainties present in 

the evidence available for other NAAQS 

reviews have not been of such a 

significant nature that they have 

precluded a reasoned assessment of the 

degree of protectiveness that would 

likely be afforded by specific alternative 

standards under consideration. In this 

case, however, unlike in other NAAQS 

reviews, multi-pollutant and multimedia pathways of exposure must be 

considered, and characterized in terms 

of an equation with several factors, 

where the values of those factors vary 

from ecoregion to ecoregion. The 

quantification of these factors must be 

based on the use of ecological and 

atmospheric modeling at an ecoregion 

scale. Further, the appropriateness of 

these factors depends upon analyses 

that could be used to determine the 

representativeness of the data at an 

ecoregion level. These circumstances, 

which are unique to this review, result 

in such large uncertainties at this time 

that in the aggregate they preclude the 

development of a reasoned assessment 

of the degree of protectiveness that 

specific alternative AAI-based standards 

would provide. 

Based on the above considerations, 

the Administrator has determined that 

at this time it is not appropriate under 

Section 109 of the CAA to set a new 

multi-pollutant standard to address 

deposition-related effects of oxides of 

nitrogen and sulfur on aquatic 

acidification. As the Administrator 

noted in the proposal, setting a NAAQS 

properly involves consideration of the 

degree of uncertainties in the science 

and other information, such as gaps in 

the relevant data and, in this case, 

limitations in the evaluation of the 

application of relevant ecological and 

atmospheric models at an ecoregion 

scale. As noted above, the issue here is 

not a question of uncertainties about the 

scientific soundness of the structure of 

the AAI, but instead uncertainties in the 

quantification and representativeness of 

the elements of the AAI as they vary in 

ecoregions across the country. At 

present, in the Administrator’s 

judgment, the unique uncertainties 

present in this review are of such 

significance that they preclude a 

reasoned 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, together 

with an AAI defined in terms of 

ecoregion-specific F factors. The 

Administrator has considered whether 

these uncertainties could be 

appropriately accounted for by choosing 

either a more or less protective target 

ANC level and percentile of water 

bodies than would otherwise be chosen 

if the uncertainties did not prevent a 

reasoned judgment on the quantification 

of the AAI factors. However, in the 

Administrator’s judgment, the 

uncertainties are of such a significant 

nature and degree that there is no 

reasoned way to choose such 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. 

Based on the above considerations, 

the Administrator judges that the 

current limitations in relevant data and 

the uncertainties associated with 

specifying the elements of the AAI are 

of such nature and degree as to prevent 

her from reaching a reasoned judgment 

as to what level and form (in terms of 

a selected percentile) of an AAI-based 

standard would provide the degree of 

protection that the Administrator 

determined was requisite. While 

acknowledging that CASAC supported 

consideration of moving forward to 

establish the standard developed in the 

PA at this time, the Administrator also 

observes that CASAC supported 

conducting further field studies that 

would better inform the continued 

development or modification of such a 

standard. Given the current high degree 

of uncertainties and the large 

complexities inherent in quantifying the 

elements of such a standard, largely 

deriving from the nature of the standard 

under consideration for the first time in 

this review, and having fully considered 


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CASAC’s advice and public comments, 

the Administrator concludes that it 

would be premature and not appropriate 

to set a new, multi-pollutant AAI-based 

secondary standard for oxides of 

nitrogen and sulfur at this time. 

While the Administrator has 

concluded that it is not appropriate to 

set such a multi-pollutant standard at 

this time, she has determined that the 

Agency should undertake a field pilot 

program to gather additional data, and 

that it is appropriate that such a 

program be undertaken before, rather 

than after, reaching a decision to set 

such a standard. As described below in 

section IV, the purpose of the program 

is to collect and analyze data so as to 

enhance our understanding of the 

degree of protectiveness that would 

likely be afforded by a standard based 

on the AAI as developed in the PA. This 

will provide additional information to 

aid the Agency in considering an 

appropriate multi-pollutant standard in 

future reviews, specifically with respect 

to the acidifying effects of deposition of 

oxides of nitrogen and sulfur. Data 

generated by this field program will also 

support development of an appropriate 

monitoring network that would work in 

concert with such a standard to result in 

the intended degree of protection. The 

information generated during the field 

program can also be used to help state 

agencies and the EPA better understand 

how an AAI-based standard would work 

in terms of the implementation of such 

a standard. 

While not a basis for this decision, the 

Administrator also recognizes, as she 

did at the time of the proposal, that a 

new, innovative AAI-based standard 

would raise significant implementation 

issues that would need to be addressed 

consistent with the CAA requirements 

for implementation-related actions 

following the setting of a new NAAQS. 

It will take time to address these issues, 

during which the Agency will be 

conducting a field pilot program to 

gather relevant data and the 

environment will benefit from 

reductions in oxides of nitrogen and 

sulfur resulting from the new NO2 and 

SO2 primary standards, as noted above, 

as well as reductions expected to be 

achieved from the EPA’s Cross-State Air 

Pollution Rule and Mercury and Air 

Toxics standards. These 

implementation-related issues are 

discussed in more detail below in 

section IV.A.5. 

The Administrator has also 

reconsidered whether it is appropriate 

at this time to set new secondary 

standards identical to the current 1-hour 

NO2 and SO2 primary standards. In the 

proposal, the Administrator recognized 

that the new NO2 and SO2 primary 1- 

hour standards set in 2010 were not 

ecologically relevant for a secondary 

standard to address deposition-related 

effects associated with oxides of 

nitrogen and sulfur. Nonetheless, the 

Administrator proposed to set new 

secondary standards identical to the 1- 

hour NO2 and SO2 primary standards on 

the basis that they would directionally 

provide some degree of additional 

protection. At that time, the 

Administrator reasoned that setting 

such standards would be consistent 

with her conclusions that the current 

NO2 and SO2 secondary standards are 

neither sufficiently protective nor 

appropriate in form, and that it is not 

appropriate to set a new, ecologically 

relevant multi-pollutant secondary 

standard at this time. 

In reconsidering this proposal, the 

Administrator first notes that although 

the ISA, REA, and PA did not directly 

consider secondary standards set 

identical to the 1-hour NO2 and SO2

primary standards, the information and 

conclusions in those documents provide 

strong support for the judgment that 

such short-term, peak standards are not 

ecologically relevant to address 

deposition-related effects associated 

with long-term deposition from ambient 

concentrations of oxides of nitrogen and 

sulfur. The Administrator notes that 

commenters on this aspect of the 

proposal broadly and strongly 

supported this view. The Administrator 

also recognizes that the Agency has not 

presented in these documents or 

elsewhere any analysis of the degree of 

protectiveness that would likely be 

afforded by such standards with regard 

to deposition-related effects in general 

or aquatic acidification effects in 

particular. She also recognizes, as 

discussed above in response to 

comments on this issue, that such an 

analysis cannot be done since there is 

no demonstrable linkage between 1- 

hour average concentrations of NO2 and 

SO2 in the ambient air and the impact 

of longer-term 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, as 

in the case of an AAI-based standard as 

discussed above, the Administrator 

concludes that there is no basis to make 

a reasoned judgment as to what levels 

of 1-hour NO2 and SO2 standards would 

be requisite to protect public welfare, 

such that the EPA cannot demonstrate a 

reasoned basis for judging that the 

proposed standards would be sufficient 

but not more stringent than necessary to 

protect against adverse depositionrelated effects to sensitive ecosystems. 

With regard to considering the views 

of CASAC, the Administrator notes that 

the PA did not discuss the alternative of 

setting secondary standards that are 

identical to the 1-hour NO2 and SO2

primary standards. As a consequence, 

this alternative was not presented for 

consideration by CASAC and therefore 

CASAC has not expressed its views on 

this alternative set of standards. 

In light of the above considerations, 

and taking into consideration public 

comments, the Administrator has 

further considered whether it is 

necessary or appropriate under Section 

109 of the CAA to set such 1-hour NO2

and SO2 secondary standards, having 

concluded that the current NO2 and SO2

secondary standards are neither 

adequate nor appropriate to address 

potentially adverse deposition-related 

effects on sensitive ecosystems 

associated with oxides of nitrogen and 

sulfur. In reaching this decision, the 

Administrator concludes that it is 

appropriate to apply the same reasoning 

as she did in reaching the decision that 

it is premature and not appropriate 

under Section 109(b) to set a new AAIbased standard at this time. In 

considering such 1-hour standards, the 

Administrator judges that the 

uncertainties are likely even 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. In addition, with 

respect to areas that are not acid 

sensitive, and unlike an AAI standard, 

it is likely that the proposed 1-hour 

standards directionally would provide 

more protection than is warranted. 

Therefore, the Administrator now 

concludes that it is neither necessary 

nor appropriate to set 1-hour NO2 and 

SO2 secondary standards, since in her 

judgment setting such standards cannot 

reasonably be judged to provide 

requisite protection of public welfare. 

In summary, for the reasons discussed 

above, and taking into account 

information and assessments presented 

in the ISA, REA, and PA, the advice and 

recommendations of CASAC, and the 

public comments on the proposal, the 

Administrator has decided that it is not 

appropriate under Section 109(b) to set 

any new secondary standards at this 

time to address potentially adverse 

deposition-related effects associated 

with oxides of nitrogen and sulfur. 

Further, as discussed above in section 

II.D, she has also decided to retain the 

current NO2 and SO2 secondary 

standards to address direct effects of 


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gaseous NO2 and SO2 on vegetation. 

Thus, taken together, the Administrator 

has decided to retain and not revise the 

current NO2 and SO2 secondary 

standards. Specifically these secondary 

standards include an NO2 standard set 

at a level of 0.053 ppm, annual 

arithmetic average, and an SO2 standard 

set at a level of 0.5 ppm, 3-hour average, 

not to be exceeded more than once per 

year. 

IV. Field Pilot Program and Ambient 

Monitoring 

This section discusses elements of a 

field pilot program and the evaluation of 

monitoring methods for ambient air 

indicators of NOy and SOX that could be 

conducted to implement the 

Administrator’s decision to undertake 

such a field monitoring program in 

conjunction with her decision not to set 

a new multi-pollutant secondary 

standard in this review, as discussed 

above in section III.E. The PA included 

considerations related to monitoring 

methods and network design that could 

support an AAI-based standard, which 

were reviewed by the CASAC Ambient 

Monitoring Methods Subcommittee 

(AMMS) (Russell and Samet, 2011b). As 

discussed below, the CASAC AMMS 

supported the approach of basing a 

potential future air monitoring network 

on the existing Clean Air Status and 

Trends Network (CASTNET) program. 

In addition, the CASAC AMMS 

supported the use of the CASTNET filter 

packs (CFPs) as appropriate methods to 

measure the oxides of sulfur indictor, 

SOX, and the use of commercially 

available NOy instruments to measure 

the oxides of nitrogen indicator, NOy. 

CASAC AMMS also supported the 

inclusion of complementary 

measurements in any future field 

monitoring program that would support 

the evaluation of the monitoring 

methods and air quality models upon 

which the AAI developed in the PA was 

based. 

Section IV.A below outlines the 

objectives, scope, and key elements of 

the field pilot program as presented in 

the proposal and section IV.B 

summarizes the EPA’s proposed 

approach to evaluating monitoring 

methods. These approaches reflect 

consideration of the advice of the 

CASAC AMMS. Public comments on 

the field pilot program and evaluation of 

monitor methods are discussed below in 

section IV.C. These comments have 

been helpful in shaping the process that 

the EPA is now undertaking to develop 

the field pilot program and monitoring 

methods evaluation. 

The following sections provide 

insight into the EPA’s current ideas 

about what could be incorporated into 

the pilot program, but the EPA has not 

made any final decisions on what will 

be included. These ideas will be 

discussed further in a draft white paper 

to be made available later this year for 

public comment. The draft white paper 

will present more detailed plans for the 

field pilot program and monitoring 

methods evaluation. The draft white 

paper is intended to serve as both a draft 

work plan and a vehicle for continued 

input from outside interests. Taking into 

consideration comments received on the 

draft white paper, the EPA will prepare 

a final white paper that will serve as a 

program management and 

communication document to facilitate 

engagement with interested 

stakeholders and convey the EPA’s final 

plans. 

A. Overview of Proposed Field Pilot 

Program 

As discussed in the proposal, the 

primary goal of this field pilot program, 

and the related monitoring methods 

evaluation summarized below in section 

IV.B, is to enhance our understanding of 

the degree of protectiveness that would 

likely be afforded by a standard based 

on the AAI, as described above in 

section III.A. This program is intended 

to aid the Agency in considering in 

future reviews an appropriate multipollutant standard that would be 

requisite to protect public welfare 

consistent with Section 109 of the CAA, 

through the following objectives: 

(1) Evaluate measurement methods for 

the ambient air indicators of NOy and 

SOX and consider designation of such 

methods as Federal Reference Methods 

(FRMs); 

(2) Examine the variability and 

improve characterization of 

concentration and deposition patterns of 

NOy and SOX, as well as reduced forms 

of nitrogen, within and across a number 

of sensitive ecoregions across the 

country; 

(3) Develop updated ecoregionspecific factors (i.e., F1 through F4) for 

the AAI equation based in part on new 

observed air quality data within the 

sample ecoregions as well as on updated 

nationwide air quality model results 

and expanded critical load data bases, 

and explore alternative approaches for 

developing such representative factors; 

(4) Calculate ecoregion-specific AAI 

values using observed NOy and SOX

data and updated ecoregion-specific 

factors to examine the extent to which 

the sample ecoregions would meet a set 

of alternative AAI-based standards; 

(5) Develop air monitoring network 

design criteria for an AAI-based 

standard; 

(6) Assess the use of total nitrate 

measurements as a potential alternative 

indicator for NOy; 

(7) Support related longer-term 

research efforts, including 

enhancements to and evaluation of 

modeled dry deposition algorithms; and 

(8) Facilitate stakeholder engagement 

in addressing implementation issues 

associated with possible future adoption 

of an AAI-based standard. 

The EPA proposed to use CASTNET 

sites (Figure IV–1) in selected acidsensitive ecoregions to serve as the 

platform for this pilot program, 

potentially starting in late 2012 and 

extending through 2018. The CASTNET 

sites in three to five acid-sensitive 

ecoregions would collect NOy and SOX

(i.e., SO2 and p-SO4) measurements over 

a 5-year period. The initial step in 

developing a data base of observed 

ambient air indicators for oxides of 

nitrogen and sulfur requires the 

addition of NOy samplers at the pilot 

study sites so that a full complement of 

indicator measurements are available to 

calculate AAI values. These CASTNET 

sites would also be used to make 

supplemental observations useful for 

evaluation of CMAQ’s characterization 

of factors F2–F4 in the AAI equation. 

The selected ecoregions would 

account for geographic variability by 

including regions from across the 

United States, including the east, upper 

midwest, and west. Each selected 

ecoregion would have at least two 

existing CASTNET sites. 


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Over the course of this 5-year pilot 

program, the most current national air 

quality modeling, based on the most 

current national emissions inventory, 

would be used to develop an updated 

set of F2–F4 factors. A parallel multiagency national critical load data base 

development effort would be used as the 

basis for calculating updated F1 factors. 

As discussed above in section III.A, 

these factors would be based on average 

parameter values across an ecoregion. 

Using this new set of F factors, 

observations of NOy and SOX derived 

from the field pilot program, averaged 

across each ecoregion, would be used to 

calculate AAI values in the sample 

ecoregions. The data from the field pilot 

program would also be used to examine 

alternative approaches to generating 

representative air quality values, such as 

examining the appropriateness of spatial 

averaging in areas of high spatial 

variability. 

Beyond this basic overview of the 

field pilot program, the following 

sections highlight complementary 

measurements that may be performed as 

part of the program (section IV.A.1), 

complementary areas of related research 

(section IV.A.2), a discussion of 

implementation challenges that would 

be addressed during the course of the 

field pilot program (section IV.A.3), and 

plans for program development and 

stakeholder participation (section 

IV.A.4). 

1. Complementary Measurements 

Complementary measurements may 

be performed at some sites in the pilot 

network to reduce uncertainties in the 

recommended methods for measuring 

ambient oxides of nitrogen and sulfur 

and to better characterize model 

performance and application to the AAI. 

The CASAC AMMS advised the EPA 

that such supplemental measurements 

were of critical importance in a field 

measurement program related to an 

AAI-based standard (Russell and Samet, 

2011b). 

Candidate complementary 

measurements to address sulfur, in 

addition to those provided by CFPs, 

include trace gas continuous SO2 and 

speciated PM2.5 measurements. The colocated deployment of a continuous SO2

analyzer with the CFP for SO2 will 

provide test data for determining 

suitability of continuous SO2

measurements as a Federal Equivalent 

Method (FEM) for an AAI-based 

standard, as well as producing valuable 

time-series data for model evaluation 

purposes. The weekly averaging time 

provided by the CFP adequately 

addresses the annual-average basis of an 

AAI-based secondary standard, but 

would not be applicable to short-term 

(i.e., 1-hour) averages associated with 

the primary SO2 standard. Conversely, 

because of the relatively low SO2

concentrations associated with many 

acid-sensitive ecoregions, existing SO2

FRMs designated for use in determining 

compliance with the primary standard, 

which typically are used in higher 

concentration environments, would not 

necessarily be appropriate for use in 

conjunction with an AAI-based 

secondary standard. 

Co-locating the PM2.5 sampler used in 

the EPA Chemical Speciation Network 

and the Interagency Monitoring of 

Protected Visual Environments 

(IMPROVE) network at pilot network 

sites would allow for characterizing the 

relationship between the CFP-derived pSO4 and the speciation samplers used 

throughout the state and local air 

quality networks. The EPA notes that 

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located IMPROVE chemical speciation 

samplers. Because the AAI equation is 

based in part on the concentration of pSO4, the original motivation for 

capturing all particle size fractions is 

not as important relative to simply 

capturing the concentration of total pSO4. 

Candidate measurements to 

complement oxidized nitrogen 

measurements, in addition to the CFP, 

include a mix of continuous and 

periodic sampling for the dominant NOy

species, namely NO, directly measured 

NO2, PAN, HNO3, and particulate 

nitrate, p-NO3. The CASAC AMMS 

(Russell and Samet, 2011b) 

recommended that the EPA consider the 

use of total nitrate (t-NO3) obtained from 

CASTNET sampling as an indicator for 

NOy, reasoning that t-NO3 is typically a 

significant fraction of deposited 

oxidized nitrogen in rural environments 

and CASTNET measurements are 

widely available. Collection of these 

data would support further 

consideration of using the CFP for t-NO3

as the indicator of oxides of nitrogen for 

use in an AAI-based secondary 

standard. 

The CASAC AMMS also 

recommended that total NHX (NH3 and 

particulate ammonium (p-NH4)) be 

considered as a proxy for reduced 

nitrogen species, reasoning that the 

subsequent partitioning to NH3 and pNH4 may be estimated using 

equilibrium chemistry calculations. 

Reduced nitrogen measurements are 

used to evaluate air quality modeling 

that is used in generating factor F2. 

Additional studies are needed to 

determine the applicability of NHX

measurements and calculated values of 

NH3 and ammonium (NH4) to the AAI. 

The additional supplemental 

measurements of speciated NOy, 

continuous SO2 and NHX will be used 

in future air quality modeling 

evaluation efforts. Because there often is 

significant lag in the availability of 

contemporary emissions data to drive 

air quality modeling, the complete use 

of these data sets will extend beyond the 

5-year collection period of the pilot 

program. Consequently, the immediate 

application of those data will address 

instrument performance comparisons 

that explore the feasibility of using 

continuous SO2 instruments in rural 

environments, and using the speciated 

NOy data to assess NOy instrument 

performance. Although contemporary 

air quality modeling will lag behind 

measurement data availability, the 

observations can be used in deposition 

models to compare observed 

transference ratios with the previously 

calculated transference ratios to test 

temporal stability of the ratios. 

An extended water quality sampling 

effort that would parallel the air quality 

measurement program would help to 

address some of the uncertainties 

related to factor F1 and the 

representativeness of the nth percentile 

critical load, as discussed in section 

III.B.5.b.i of the proposal. The objective 

of the water quality sampling would be 

to develop a larger data base of critical 

loads in each of the pilot ecoregions 

such that the nth percentile can 

adequately be characterized in terms of 

representing all water bodies. 

Opportunities to leverage and perhaps 

enhance existing ecosystem modeling 

efforts enabling more advanced critical 

load modeling and improved methods 

to estimate base cation production could 

be pursued. For example, areas with 

ongoing research studies producing data 

for dynamic critical load modeling 

could be considered when selecting the 

pilot ecoregions. 

2. Complementary Areas of Research 

The EPA recognizes that a source of 

uncertainty in an AAI-based secondary 

standard that would not be directly 

addressed in the pilot program stems 

from the uncertainty in the model used 

to link atmospheric concentrations to 

dry deposition fluxes. Currently, there 

are no ongoing direct dry deposition 

measurement studies at CASTNET sites 

that can be used to evaluate modeled 

results. It was strongly recommended by 

CASAC AMMS that a comprehensive 

sampling-intensive study be conducted 

in at least one, preferably two sites in 

different ecoregions to assess 

characterization of dry deposition of 

sulfur and nitrogen. These sites would 

be the same as those for the 

complementary measurements 

described above, but they would afford 

an opportunity to also complement dry 

deposition process research that benefits 

from the ambient air measurements 

collected in the pilot program. The 

concerns regarding uncertainties 

underlying an AAI-based secondary 

standard suggest that research that 

includes dry deposition measurements 

and evaluation of dry deposition models 

would be a high priority. 

Similar leveraging could be pursued 

with respect to ecosystem research 

activities. For example, studies that 

capture a suite of soil, vegetation, 

hydrological, and water quality 

properties that can help evaluate more 

advanced critical load models would 

complement the atmospheric-based 

pilot program. In concept, such studies 

could provide the infrastructure for true 

multi-pollutant, multimedia ‘‘super’’ 

sites assuming the planning, 

coordination, and resource facets can be 

aligned. While this discussion 

emphasizes the opportunity of 

leveraging ongoing research efforts, 

consideration could be given to 

explicitly including related research 

components directly in the pilot 

program. 

3. Implementation Challenges 

The CAA requires that once a NAAQS 

is established, designation and 

implementation must move forward. 

With a standard as innovative as the 

AAI-based standard considered in this 

review, the Administrator believes that 

should such a standard be adopted in 

the future, its success would be greatly 

improved if, while additional data are 

being collected to reduce the 

uncertainties discussed above, the 

implementing agencies and other 

stakeholders have an opportunity to 

discuss and thoroughly understand how 

such a standard would work. And since, 

as noted above, emissions reductions 

that are directionally correct to reduce 

aquatic acidification will be occurring 

as a result of other CAA programs, the 

Administrator believes that this period 

of further discussion will enable 

agencies to implement a multi-pollutant 

standard to address aquatic acidification 

if one is adopted in a future review. 

Consideration of an AAI-based 

secondary standard for oxides of 

nitrogen and sulfur would present 

significant implementation challenges 

because it involves multiple, regionallydispersed pollutants and relatively 

complex compliance determinations 

based on regionally variable levels of 

NOy and SOX concentrations that would 

be necessary to achieve a national ANC 

target. The anticipated implementation 

challenges fall into three main 

categories: monitoring and compliance 

determinations for area designations, 

pre-construction permit application 

analyses of individual source impacts, 

and State Implementation Plan (SIP) 

development. Several overarching 

implementation questions that we 

anticipate will be addressed in parallel 

with the field pilot program’s five-year 

data collection period include: 

(1) What are the appropriate 

monitoring network density and siting 

requirements to support a compliance 

system based on ecoregions? 

(2) Given the unique spatial nature of 

the secondary standard (e.g., 

ecoregions), what are the appropriate 

parameters for establishing 

nonattainment areas? 

(3) How can new or modified major 

sources of oxides of nitrogen and oxides 

of sulfur emissions assess their ambient 


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impacts on the standard and 

demonstrate that they are not causing or 

contributing to a violation of the 

NAAQS for preconstruction permitting? 

To what extent does the fact that a 

single source may be impacting multiple 

areas, with different acid sensitivities 

and variable levels of NOy and SOX

concentrations that would be necessary 

to achieve a national ANC target, 

complicate this assessment and how can 

these additional complexities best be 

addressed? 

(4) What additional tools, 

information, and planning structures are 

needed to assist states with SIP 

development, including the assessment 

of interstate pollutant transport and 

deposition? 

(5) Would transportation conformity 

apply in nonattainment and 

maintenance areas for this secondary 

standard, and, if it does, would 

satisfying requirements that apply for 

related primary standards (e.g., ozone, 

PM2.5, and NO2) be demonstrated to 

satisfy requirements for this secondary 

standard? 

4. Monitoring Plan Development and 

Stakeholder Participation 

The existing CASTNET sampling site 

infrastructure provides an effective 

means of quickly and efficiently 

deploying a monitoring program to 

support potential implementation of an 

AAI-based secondary standard, and also 

provides an additional opportunity for 

federally managed networks to 

collaborate and support the states, local 

agencies and tribes (SLT) in determining 

compliance with a secondary standard. 

A collaborative effort would help to 

optimize limited federal and SLT 

monitoring funds and would be 

beneficial to all involved. The 

CASTNET is already a stakeholderbased program with over 20 participants 

and contributors, including federal, 

state and tribal partners. 

The CASAC AMMS generally 

endorsed the technical approaches used 

in CASTNET, but concerns were raised 

by individual representatives of state 

agencies concerning the perception of 

the EPA-controlled management aspects 

of CASTNET and data ownership. 

Potential approaches to resolve these 

issues will be developed and evaluated 

in existing National Association of 

Clean Air Agencies (NACAA)/EPA 

ambient air monitoring and National 

Atmospheric Deposition Program 

(NADP) science committees. The EPA 

Office of Air and Radiation (which 

includes the Office of Air Quality 

Planning Standards, OAQPS; and the 

Office of Atmospheric Program’s Clean 

Air Markets Division, OAP–CAMD), and 

their partners on the NACAA monitor 

steering committee will work to develop 

a prioritized plan that identifies three to 

five ecoregions and specific 

instrumentation to be deployed. 

Although this pilot program is focused 

on data collection, the plan will also 

include data analysis approaches as 

well as a process to facilitate 

engagement by those within the EPA 

and the SLTs to foster progress on the 

implementation questions noted above. 

B. Summary of Proposed Evaluation of 

Monitoring Methods 

This section provides a brief overview 

of the EPA’s plans for evaluating 

monitoring methods of NOy and SOX, as 

discussed in section IV.B of the 

proposal. The EPA generally relies on 

monitoring methods that have been 

designated as FRMs or FEMs for the 

purpose of determining the attainment 

status of areas with regard to existing 

NAAQS. Such FRMs or FEMs are 

generally required to measure the air 

quality indicators that are compared to 

the level of a standard to assess 

compliance with a NAAQS. Prior to 

their designation by the EPA as FRM/ 

FEMs through a rulemaking process, 

these methods must be determined to be 

applicable for routine field use and need 

to have been experimentally validated 

by meeting or exceeding specific 

accuracy, reproducibility, and reliability 

criteria established by the EPA for this 

purpose. As discussed above in section 

III.A, the ambient air indicators being 

considered for use in an AAI-based 

standard include SO2, p-SO4, and NOy. 

The CASTNET provides a wellestablished infrastructure that would 

meet the basic location and 

measurement requirements of an AAIbased secondary standard given the 

rural placement of sites in acid sensitive 

areas. In addition, CFPs currently 

provide very economical weekly, 

integrated average concentration 

measurements of SO2, p-SO4, NH4 and 

t-NO3, the sum of HNO3 and p-NO3. 

While routinely operated instruments 

that measure SO2, p-SO4, NOy and/or 

t-NO3 exist, instruments that measure pSO4, NOy, t-NO3, or the CFP for SO2

have not been designated by the EPA as 

FRMs or FEMs. The EPA’s Office of 

Research and Development has initiated 

work that will support future FRM 

designations by the EPA for SO2 and pSO4 measurements based on the CFP. 

Such a designation by the EPA could be 

done for the purpose of facilitating 

consistent research related to an AAIbased standard and/or in conjunction 

with setting and supporting an AAIbased secondary standard. 

Based on extensive review of 

literature and available data, the EPA 

has identified potential methods that 

appear suitable for measuring each of 

the three components of the indicators. 

As discussed more fully in section IV.B 

of the proposal, these methods are being 

considered as new FRMs to be used for 

measuring the ambient concentrations 

of the three components (SO2, p-SO4

and NOy) that would be needed to 

determine compliance with an AAIbased secondary standard. 

For the SO2 and p-SO4 measurements, 

the EPA is considering the CFP method, 

which provides weekly average 

concentration measurements for SO2

and p-SO4. This method has been used 

in the EPA’s CASTNET monitoring 

network for 15 years, and experience 

with this method strongly indicates that 

it will meet the requirements for use as 

an FRM for the SO2 and p-SO4

concentrations for an AAI-based 

secondary standard. 

Although the CFP method would 

provide measurements of both the SO2

and p-SO4 components in a unified 

sampling and analysis procedure, 

individual FRMs will be considered for 

each. The EPA recognizes that an 

existing FRM to measure SO2

concentrations using ultra-violet 

fluorescence (UVF) exists (40 CFR part 

50, appendix A–1) for the purpose of 

monitoring compliance for the primary 

SO2 NAAQS. However, several factors 

suggest that the CFP method would be 

superior to the UVF FRM for monitoring 

compliance with an AAI-based 

secondary standard. 

For monitoring the NOy component, a 

continuous analyzer for measuring NOy

is commercially available and is 

considered by the EPA to be likely 

suitable for use as an FRM. This method 

is similar in design to the existing NO2

FRM (described in 40 CFR part 50, 

appendix F), which is based on the 

ozone chemiluminescence measurement 

technique. The method is adapted to 

and further optimized to measure all 

NOy. However, this NOy method 

requires further evaluation before it can 

be fully confirmed as a suitable FRM. 

The EPA is currently completing a full 

scientific assessment of the NOy method 

to determine whether it would be 

appropriate to consider for designation 

by the EPA as an FRM. 

On February 16, 2011, the EPA 

presented this set of potential FRMs to 

the CASAC AMMS for their 

consideration and comment. In 

response, the CASAC AMMS stated 

that, overall, it believes that the EPA’s 

planned evaluation of methods for 

measuring NOy, SO2 and p-SO4 as 

ambient air indicators is a suitable 


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approach in concept. On supporting the 

CFP method as a potential FRM for SO2, 

CASAC stated that they felt that the CFP 

is adequate for measuring long-term 

average SO2 gas concentrations in rural 

areas with low levels (less than 5 parts 

per billion by volume (ppbv)) and is 

therefore suitable for consideration as 

an FRM. For p-SO4, CASAC AMMS 

generally supports the use of the CFP as 

a potential FRM for measuring p-SO4 for 

an AAI-based secondary standard. The 

method has been relatively wellcharacterized and evaluated, and it has 

a documented, long-term track record of 

successful use in a field network 

designed to assess spatial patterns and 

long-term trends. On supporting the 

photometric NOy method as a potential 

FRM, CASAC AMMS concluded that 

the existing NOy method is generally an 

appropriate approach for the indicator 

of an AAI-based standard. However, 

CASAC AMMS agreed that additional 

characterization and research is needed 

to fully understand the method in order 

to designate it as an FRM. 

C. Comments on Field Pilot Program 

and Monitoring Methods Evaluation 

Public comments on the EPA’s 

proposed plans for a field pilot program 

and related evaluation of monitoring 

methods generally fell into the 

following four topic areas: (1) Goals, 

objectives, and scope; (2) monitoring 

network and site selection; (3) 

complementary measurements and 

instrumentation; and (4) collaboration 

and stakeholder participation. An 

overview of these comments and the 

EPA’s responses are discussed below. In 

addition, many commenters generally 

requested that the EPA provide 

clarification of its plans regarding the 

field pilot program. 

1. Goals, Objectives, and Scope of Field 

Pilot Program 

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., 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 as 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 the EPA intended to 

consider such a standard in future 

reviews, the field pilot program would 

need to expand in coverage and 

incorporate a much 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 the 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. 

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 undersampled 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. 

Having considered these comments 

contending that the scope of the field 

pilot program is too limited spatially 

and not sufficiently comprehensive, the 

EPA maintains that the purpose and 

scope of the pilot studies program as 

presented in the proposal remain 

appropriate. As summarized above in 

section IV.A, 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. The 

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. 

The 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 


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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 the 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 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. For example, the 

EPA plans to 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, the 

EPA plans to 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, the 

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, 

the EPA plans to 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 AAI-based standard 

would be intended to protect. 

Nevertheless, the 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. The 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 the 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. 

2. Network Design and Role of 

CASTNET 

Most commenters expressed the view 

that CASTNET was an appropriate 

program to support the field pilot 

program and a potential AAI-based 

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 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 the EPA. 

Environmental groups also supported 

the use of CASTNET and encouraged 

the EPA to adopt the multiple 

stakeholder process of the NCLDB 

program and to align CASTNET sites 

with the Temporally Integrated 

Monitoring of Ecosystems and LongTerm 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 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. Relatedly, the NPS 

and environmental groups encouraged 

the EPA to make this effort a priority for 

funding. 

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 acidsensitive ecoregions. The 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 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 the EPA, the agency has 


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aggressively supported the user 

community management approach 

adopted in the 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 the EPA 

will try to leverage existing water 

quality monitoring programs such as 

TIME/LTM in selecting field pilot 

program site locations. The 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, the 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. 

3. Complementary Measurements and 

Instrumentation 

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 an 

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. 

Several commenters supported the 

development of an 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 the EPA’s 

approach of using the EPA’s research 

office to conduct instrument evaluation 

as a related but separate program from 

the field pilot program. Some 

commenters also recommended testing 

NOy at locations with extreme 

temperature and relative humidity 

regimes. 

The EPA appreciates the support 

expressed by commenters regarding the 

use of complementary measurements. 

While the 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. The 

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. 

4. Collaboration 

Most commenters agreed with the 

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 the 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. 

The EPA is encouraged by 

commenters’ interest in the field pilot 

program. While the EPA’s Office of Air 

and Radiation (OAR) will assume 

primary leadership of this program, 

OAR will take several actions to 

promote collaboration across the 

internal EPA research programs and 

other government agencies. Paralleling 

this effort, the EPA will solicit comment 

on a draft white paper to enable ongoing 

review and input from the public. 

These pilot studies afford 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 these pilot 

studies, the EPA will engage other 

federal agencies (U.S. Geological 

Survey, NPS, U.S. Forest Service) and 

state and local agencies primarily 

through existing NADP and NACAA 

committee structures. 

V. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

Under Executive Order 12866 (58 FR 

51735, October 4, 1993), this action is a 

‘‘significant regulatory action.’’ 

Accordingly, the EPA submitted this 

action to the Office of Management and 

Budget (OMB) for review under 

Executive Orders 12866 and 13563 (76 

FR 3821, January 21, 2011), and any 

changes made in response to Office of 

Management and Budget (OMB) 

recommendations have been 

documented in the docket for this 

action. 

B. Paperwork Reduction Act 

This action does not impose an 

information collection burden under the 

provisions of the Paperwork Reduction 

Act, 44 U.S.C. 3501 et seq. Burden is 

defined at 5 CFR 1320.3(b). There are no 

information collection requirements 

directly associated with the 

establishment of a NAAQS under 

Section 109 of the CAA and this 

rulemaking will retain current standards 

and will not establish any new 

standards. 


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Federal Register / Vol. 77, No. 64 / Tuesday, April 3, 2012 / Rules and Regulations 20271 

C. Regulatory Flexibility Act 

For purposes of assessing the impacts 

of today’s rule on small entities, small 

entity is defined as: (1) A small business 

that is a small industrial entity as 

defined by the Small Business 

Administration’s (SBA) regulations at 13 

CFR 121.201; (2) a small governmental 

jurisdiction that is a government of a 

city, county, town, school district or 

special district with a population of less 

than 50,000; and (3) a small 

organization that is any not-for-profit 

enterprise which is independently 

owned and operated and is not 

dominant in its field. 

After considering the economic 

impacts of today’s final rule on small 

entities, I certify that this action will not 

have a significant economic impact on 

a substantial number of small entities. 

This final rule will not impose any 

requirements on small entities. Rather, 

this rule will retain the current 

secondary standards and does not 

establish any new national standards. 

See also American Trucking 

Associations v. EPA. 175 F. 3d at 1044– 

45 (NAAQS do not have significant 

impacts upon small entities because 

NAAQS themselves impose no 

regulations upon small entities). 

D. Unfunded Mandates Reform Act 

Title II of the Unfunded Mandates 

Reform Act of 1995 (UMRA), Public 

Law 104–4, establishes requirements for 

Federal agencies to assess the effects of 

their regulatory actions on State, local, 

and Tribal governments and the private 

sector. Under Section 202 of the UMRA, 

the EPA generally must prepare a 

written statement, including a costbenefit analysis, for proposed and final 

rules with ‘‘Federal mandates’’ that may 

result in expenditures to state, local, 

and tribal governments, in the aggregate, 

or to the private sector, of $100 million 

or more in any 1 year. Before 

promulgating an EPA rule for which a 

written statement is needed, Section 205 

of the UMRA generally requires the EPA 

to identify and consider a reasonable 

number of regulatory alternatives and to 

adopt the least costly, most costeffective or least burdensome alternative 

that achieves the objectives of the rule. 

The provisions of Section 205 do not 

apply when they are inconsistent with 

applicable law. Moreover, Section 205 

allows the EPA to adopt an alternative 

other than the least costly, most costeffective or least burdensome alternative 

if the Administrator publishes with the 

final rule an explanation why that 

alternative was not adopted. Before the 

EPA establishes any regulatory 

requirements that may significantly or 

uniquely affect small governments, 

including tribal governments, it must 

have developed under Section 203 of 

the UMRA a small government agency 

plan. The plan must provide for 

notifying potentially affected small 

governments, enabling officials of 

affected small governments to have 

meaningful and timely input in the 

development of the EPA regulatory 

proposals with significant Federal 

intergovernmental mandates, and 

informing, educating, and advising 

small governments on compliance with 

the regulatory requirements. 

This action contains no Federal 

mandates under the provisions of Title 

II of the Unfunded Mandates Reform 

Act of 1995 (UMRA), 2 U.S.C. 1531– 

1538 for state, local, or tribal 

governments or the private sector. 

Therefore, this action is not subject to 

the requirements of Sections 202 or 205. 

Furthermore, as indicated previously, in 

setting a NAAQS the EPA cannot 

consider the economic or technological 

feasibility of attaining ambient air 

quality standards; although such factors 

may be considered to a degree in the 

development of state plans to 

implement the standards. See also 

American Trucking Associations v. 

EPA, 175 F. 3d at 1043 (noting that 

because the EPA is precluded from 

considering costs of implementation in 

establishing NAAQS, preparation of a 

Regulatory Impact Analysis pursuant to 

the Unfunded Mandates Reform Act 

would not furnish any information 

which the court could consider in 

reviewing the NAAQS). Accordingly, 

the EPA has determined that the 

provisions of Sections 202, 203, and 205 

of the UMRA do not apply to this final 

decision not to establish new standards. 

E. Executive Order 13132: Federalism 

This final rule does not have 

federalism implications. It will not have 

substantial direct effects on the states, 

on the relationship between the national 

government and the states, or on the 

distribution of power and 

responsibilities among the various 

levels of government, as specified in 

Executive Order 13132 because it does 

not contain legally binding 

requirements. Thus, the requirements of 

Executive Order 13132 do not apply to 

this rule. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

Executive Order 13175, entitled 

‘‘Consultation and Coordination with 

Indian Tribal Governments’’ (65 FR 

67249, November 9, 2000), requires the 

EPA to develop an accountable process 

to ensure ‘‘meaningful and timely input 

by tribal officials in the development of 

regulatory policies that have tribal 

implications.’’ This rule concerns the 

establishment of national standards to 

address the public welfare effects of 

oxides of nitrogen and sulfur. 

This action does not have tribal 

implications, as specified in Executive 

Order 13175 (65 FR 67249, November 9, 

2000) as tribes are not obligated to adopt 

or implement any NAAQS. We 

recognize, however, that this rule does 

concern resources of special interest to 

the tribes. Accordingly, 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 consultation call 

with the Forest County Potawatomi 

Community, with the participation of 

four other tribes (Fond du Lac 

Reservation, Southern Ute, Fort 

Belknap, and San Juan Southern Paiute). 

The EPA also received public comment 

from two tribes on this rule. The EPA 

has responded to the tribal comments in 

its Response to Comments Document. 

G. Executive Order 13045: Protection of 

Children From Environmental Health & 

Safety Risks 

This action is not subject to EO 13045 

because it is not an economically 

significant rule as defined in EO 12866. 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

This action is not a ‘‘significant 

energy action’’ as defined in Executive 

Order 13211 (66 FR 28355, May 22, 

2001), because it will not have a 

significant adverse effect on the supply, 

distribution, or use of energy. This 

action does not establish new national 

standards to address the public welfare 

effects of oxides of nitrogen and sulfur. 

I. National Technology Transfer and 

Advancement Act 

Section 12(d) of the National 

Technology Transfer and Advancement 

Act of 1995 (NTTAA), Public Law 104– 

113, 12(d) (15 U.S.C. 272 note) directs 

the EPA to use voluntary consensus 

standards in its regulatory activities 

unless to do so would be inconsistent 

with applicable law or otherwise 

impractical. Voluntary consensus 

standards are technical standards (e.g., 

materials specifications, test methods, 

sampling procedures, and business 

practices) that are developed or adopted 

by voluntary consensus standards 

bodies. The NTTAA directs the EPA to 

provide Congress, through OMB, 

explanations when the Agency decides 


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20272 Federal Register / Vol. 77, No. 64 / Tuesday, April 3, 2012 / Rules and Regulations 

not to use available and applicable 

voluntary consensus standards. 

The EPA is not aware of any 

voluntary consensus standards that are 

relevant to the provisions of this final 

rule. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

Executive Order 12898 (59 FR 7629 

(Feb. 16, 1994)) establishes federal 

executive policy on environmental 

justice. Its main provision directs 

federal agencies, to the greatest extent 

practicable and permitted by law, to 

make environmental justice part of their 

mission by identifying and addressing, 

as appropriate, disproportionately high 

and adverse human health or 

environmental effects of their programs, 

policies, and activities on minority 

populations, low-income populations, 

or indigenous populations in the United 

States. 

The EPA has determined that this 

final rule will not have 

disproportionately high and adverse 

human health or environmental effects 

on minority, low-income populations, 

or indigenous populations because it 

retains the level of environmental 

protection for all affected populations 

without having any disproportionately 

high and adverse human health or 

environmental effects on any 

population, including any minority, 

low-income population, or indigenous 

population. 

K. Congressional Review Act 

The Congressional Review Act, 5 

U.S.C. 801, et seq., as added by the 

SBREFA of 1996, generally provides 

that before a rule may take effect, the 

agency promulgating the rule must 

submit a rule report, which includes a 

copy of the rule, to each House of the 

Congress and to the Comptroller General 

of the United States. The EPA will 

submit a report containing this final rule 

and other required information to the 

United States Senate, the United States 

House of Representatives and the 

Comptroller General of the United 

States prior to publication of the rule in 

the Federal Register. A major rule 

cannot take effect until 60 days after it 

is published in the Federal Register. 

This action is not a ‘‘major rule’’ as 

defined by 5 U.S.C. 804(2). This rule 

will be effective June 4, 2012. 

References 

Banzhaf, S., D. Burtraw, D. Evans, and A. 

Krupnick. 2006. ‘‘Valuation of Natural 

Resource Improvements in the 

Adirondacks.’’ Land Economics 82:445– 

464. 

NAPAP. 1990. Acid Deposition: State of 

Science and Technology. National Acid 

Precipitation Assessment Program. 

Office of the Director, Washington, DC. 

NAPAP. 2005. National acid precipitation 

assessment program report to Congress: 

An integrated assessment. http://www.

esrl.noaa.gov/csd/aqrs/reports/

napapreport05.pdf. Silver Spring, MD: 

National Acid Precipitation Assessment 

Program (NAPAP); Committee on 

Environment and Natural Resources 

(CENR) of the National Science and 

Technology Council (NSTC). 

NRC (National Research Council). 2004. Air 

quality management in the United States. 

Washington, DC: National Research 

Council (NRC); The National Academies 

Press. 

Russell, A and J.M. Samet, 2010a. Review of 

the Policy Assessment for the Review of 

the Secondary National Ambient Air 

Quality Standard for NOX and SOX: First 

Draft. EPA–CASAC–10–014. 

Russell, A and J.M. Samet, 2010b. Review of 

the Policy Assessment for the Review of 

the Secondary National Ambient Air 

Quality Standard for NOX and SOX: 

Second Draft. EPA–CASAC–11–003. 

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. EPA–CASAC–11–005. 

Russell and Samet, 2011b Review of EPA 

Draft Documents on Monitoring and 

Methods for Oxides of Nitrogen (NOX) 

and Sulfur (SOX). http://yosemite.epa.

gov/sab/sabpeople.nsf/WebCommittees/

CASAC. 

U.S. EPA, 1973. ‘‘Effects of Sulfur Oxide in 

the Atmosphere on Vegetation.’’ Revised 

Chapter 5 of Air Quality Criteria For 

Sulfur Oxides. U.S. Environmental 

Protection Agency. Research Triangle 

Park, N.C. EPA–R3–73–030. 

U.S. EPA. 1982. Review of the National 

Ambient Air Quality Standards for 

Sulfur Oxides: Assessment of Scientific 

and Technical Information. OAQPS Staff 

Paper. EPA–450/5–82–007. U.S. 

Environmental Protection Agency, Office 

of Air Quality Planning and Standards, 

Research Triangle Park, NC. 

U.S. EPA, 1984a. The Acidic Deposition 

Phenomenon and Its Effects: Critical 

Assessment Review Papers. Volume I 

Atmospheric Sciences. EPA–600/8–83– 

016AF. Office of Research and 

Development, Washington, DC. 

U.S. EPA, 1984b. The Acidic Deposition 

Phenomenon and Its Effects: Critical 

Assessment Review Papers. Volume II 

Effects Sciences. EPA–600/8–83–016BF. 

Office of Research and Development, 

Washington, DC. 

U.S. EPA, 1985. The Acidic Deposition 

Phenomenon and Its Effects: Critical 

Assessment Document. EPA–600/8–85/ 

001. Office of Research and 

Development, Washington, DC. 

U.S. EPA. 1995a. Review of the National 

Ambient Air Quality Standards for 

Nitrogen Dioxide: Assessment of 

Scientific and Technical Information. 

OAQPS Staff Paper. EPA–452/R–95–005. 

U.S. Environmental Protection Agency, 

Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. 

September. 

U.S. EPA. 1995b. Acid Deposition Standard 

Feasibility Study Report to Congress. 

U.S. Environmental Protection Agency, 

Washington, DC. EPA–430/R–95–001a. 

U.S. EPA 2007. Integrated Review Plan for 

the Secondary National Ambient Air 

Quality Standards for Nitrogen Dioxide 

and Sulfur Dioxide. U.S. Environmental 

Protection Agency, Research Triangle 

Park, NC, EPA–452/R–08–006. 

U.S. EPA 2008. Integrated Science 

Assessment (ISA) for Oxides of Nitrogen 

and Sulfur Ecological Criteria (Final 

Report). U.S. Environmental Protection 

Agency, Washington, DC, 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, DC, 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, DC, EPA–452/R–11–005a. 

Wolff, G.T. 1993. CASAC closure letter for 

the 1993 Criteria Document for Oxides of 

Nitrogen addressed to U.S. EPA 

Administrator Carol M. Browner dated 

September 30, 1993. 

Wolff, G.T. 1995. CASAC closure letter for 

the 1995 OAQPS Staff Paper addressed 

to U.S. EPA Administrator Carol M. 

Browner dated August 22, 1995. 

List of Subjects in 40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

Dated: March 20, 2012. 

Lisa P. Jackson, 

Administrator. 

[FR Doc. 2012–7679 Filed 4–2–12; 8:45 am] 

BILLING CODE 6560–50–P 


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