[Federal Register Volume 77, Number 64 (Tuesday, April 3, 2012)]
[Rules and Regulations]
[Pages 20218-20272]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2012-7679]



[[Page 20217]]

Vol. 77

Tuesday,

No. 64

April 3, 2012

Part III





Environmental Protection Agency





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40 CFR Part 50





 Secondary National Ambient Air Quality Standards for Oxides of 
Nitrogen and Sulfur; Final Rule

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

[[Page 20218]]


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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.

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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 
(NO<INF>2</INF>) and sulfur dioxide (SO<INF>2</INF>) 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: <a href="/cdn-cgi/l/email-protection#cdb9a8a3a3aca3b9e3aaa4a3aaa8bf8da8bdace3aaa2bb"><span class="__cf_email__" data-cfemail="6a1e0f04040b041e440d03040d0f182a0f1a0b440d051c">[email protected]</span></a>.

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 
Deposition-Related 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 AAI-Based Standard
    b. Comments on Specific Aspects of an AAI-Based Approach
    2. Comments on 1-Hour NO<INF>2</INF> and SO<INF>2</INF> 
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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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, man-made 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.''
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    \1\ The 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).
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    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 NO<INF>2</INF> 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 
NO<INF>2</INF> 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 NO<INF>2</INF>'' 
(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 NO<INF>2</INF> 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 NO<INF>2</INF> 
(60 FR 52874; October 11, 1995). A year later, the Administrator made a 
final determination not to revise the NAAQS for NO<INF>2</INF> after 
careful evaluation of the comments received on the proposal (61 FR 
52852; October 8, 1996). While the primary NO<INF>2</INF> 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 
NO<INF>2</INF> remains 0.053 ppm (100 micrograms per cubic meter 
[[mu]g/m3] of air), annual arithmetic average, calculated as the 
arithmetic mean of the 1-hour NO<INF>2</INF> concentrations.
2. NAAQS for Oxides of Sulfur
    The EPA promulgated primary and secondary NAAQS for SO<INF>2</INF> 
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

[[Page 20220]]

properly be concluded that the vegetation injury reported resulted from 
the average SO<INF>2</INF> exposure over the growing season, rather 
than from short-term 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 
then-current 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., 
SO<INF>2</INF> 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 SO<INF>2</INF>. This 
proposed decision with regard to the secondary SO<INF>2</INF> 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 SO<INF>2</INF> 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 SO<INF>2</INF> 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 SO<INF>2</INF> 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

[[Page 20221]]

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 
NO<INF>2</INF> and SO<INF>2</INF> 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.
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    \2\ Center for Biological Diversity, et al. v. Johnson, No. 05-
1814 (D.D.C.).
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    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

[[Page 20222]]

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 (O<INF>3</INF>) 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 NO<INF>2</INF> and SO<INF>2</INF> 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 deposition-related 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

[[Page 20223]]

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 
deposition-related 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

[[Page 20224]]

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 
SO<INF>2</INF>, NO<INF>2</INF>, nitric oxide (NO), nitric acid 
(HNO<INF>3</INF>) 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 SO<INF>2</INF> and injury to vegetation (U.S. EPA, 2008, 
section 4.4.1 and 3.4.2.1). Acute foliar injury to vegetation from 
SO<INF>2</INF> 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 
SO<INF>2</INF> exposure concentration and time of exposure.
    The evidence is sufficient to infer a causal relationship between 
exposure to NO, NO<INF>2</INF> and PAN and injury to vegetation (U.S. 
EPA, 2008, section 4.4.2 and 3.4.2.2). At sufficient concentrations, 
NO, NO<INF>2</INF> and PAN can decrease photosynthesis and induce 
visible foliar injury to plants. Evidence is also sufficient to infer a 
causal relationship between exposure to HNO<INF>3</INF> 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 SO<INF>2</INF>, NO, NO<INF>2</INF>, PAN and 
HNO<INF>3</INF> 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 SO<INF>2</INF> 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, SO<INF>2</INF> 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, NO<INF>2</INF>, PAN and HNO<INF>3</INF> 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 HNO<INF>3</INF> 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

[[Page 20225]]

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 
(SO<INF>4</INF><SUP>2-</SUP>) and nitrate (NO<INF>3</INF><SUP>-</SUP>), 
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 
(NH<INF>X</INF>).
    The ISA concluded that deposition of oxides of nitrogen and sulfur 
and NH<INF>X</INF> 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 acidity-related 
stress, in particular, concentrations of hydrogen ion (as pH), calcium 
(Ca<SUP>2+</SUP>) 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 phyto-zooplankton 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 
([mu]eq/L). At annual average ANC levels between 100 and 50 [mu]eq/L, 
the fitness of sensitive species (e.g., brook trout, zooplankton) 
begins to decline. When annual average ANC is <50 [mu]eq/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 [mu]eq/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:

[[Page 20226]]

[GRAPHIC] [TIFF OMITTED] TR03AP12.000

    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 [mu]eq/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 [mu]eq/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 NH<INF>X</INF>, 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 NH<INF>X</INF> 
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

[[Page 20227]]

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 semi-arid 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 NO<INF>3</INF><SUP>-</SUP>, 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 non-atmospheric 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 SO<INF>4</INF><SUP>2-</SUP> and mercury, and where mercury is 
present, increased availability of SO<INF>4</INF><SUP>2-</SUP> 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 SO<INF>4</INF><SUP>2-</SUP> 
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 
SO<INF>2</INF> 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

[[Page 20229]]

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 NO<INF>2</INF>) 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 (N<INF>2</INF>O) 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-of-evidence 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 non-atmospheric 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 SO<INF>2</INF> 
concentrations and emissions, which are also higher in the East. Oxides 
of nitrogen emissions are much greater and generally more widespread 
than ammonia (NH<INF>3</INF>) emissions nationwide; high NH<INF>3</INF> 
emissions tend to be more local (e.g., eastern North Carolina) or sub-
regional (e.g., the upper

[[Page 20230]]

Midwest and Plains states). The relative amounts of oxidized versus 
reduced nitrogen deposition are consistent with the relative amounts of 
oxides of nitrogen and NH<INF>3</INF> 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 NH<INF>3</INF>.
    (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, NH<INF>3</INF> and/or 
SO<INF>2</INF>.
    (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 
SO<INF>4</INF><SUP>2-</SUP> and 
NO<INF>3</INF><SUP>-</SUP>concentrations and ANC levels and to affirm 
the understanding that reductions in deposition could influence the 
risk of acidification. Monitoring data from the EPA-administered 
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 
steady-state and dynamic biogeochemical models.
    Although wet deposition rates for SO<INF>2</INF> and oxides of 
nitrogen in the Adirondack Case Study Area have reduced since the mid-
1990s, current concentrations are still well above pre-acidification 
(1860) conditions. For a discussion of the uncertainties of pre-
acidification, see U.S. EPA, 2011, Appendix F. The Model of 
Acidification of Groundwater in Catchments (MAGIC) modeling predicts 
NO<INF>3</INF><SUP>-</SUP> and SO<INF>4</INF><SUP>2-</SUP> are 17- and 
5-fold higher today, respectively. The estimated average ANC for 44 
lakes in the Adirondack Case Study Area is 62.1 [mu]eq/L (<plus-
minus>15.7 [mu]eq/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 [mu]eq/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 NO<INF>3</INF> and 
SO<INF>4</INF><SUP>2-</SUP> concentrations in surface waters. The ANC 
levels increased from about 50 [mu]eq/L in the early 1990s to >75 
[mu]eq/L until 2002, when ANC levels declined back to 1991-1992 levels. 
Current concentrations are still above pre-acidification (1860) 
conditions. The MAGIC modeling predicts surface water concentrations of 
NO<INF>3</INF> and SO<INF>4</INF><SUP>2-</SUP> are 10- and 32-fold 
higher today, respectively. The estimated average ANC for 60 streams in 
the Shenandoah Case Study Area is 57.9 [mu]eq/L (<plus-minus>4.5 
[mu]eq/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 [mu]eq/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.

[[Page 20231]]

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, intermediate-level 
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, fine-root 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 community-level 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, 
SO<INF>4</INF><SUP>2-</SUP>) and increased mercury methylation in 
wetlands and aquatic environments is also included in the REA. On this 
subject the REA concludes that decreases in SO<INF>4</INF><SUP>2-</SUP> 
deposition will likely result in decreases in MeHg concentration; 
however, spatial and biogeochemical variations nationally hinder 
establishing large scale dose-response relationships.
    Several additional issues concerning oxides of nitrogen were 
addressed in the REA. Consideration was also given to N<INF>2</INF>O, a 
potent GHG. The REA concluded that it is most appropriate to analyze 
the role of N<INF>2</INF>O 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 N<INF>2</INF>O production by 
an average of 207 percent across all ecosystems. Nitrogen addition 
ranging from 30 to 240 kg N/ha/yr increased methane (CH<INF>4</INF>) 
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 SO<INF>2</INF>, and 
concentrations of nitric oxide (NO), NO<INF>2</INF> 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

[[Page 20232]]

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

[[Page 20233]]

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 [mu]eq/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 [mu]eq/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 over-enrichment 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

[[Page 20234]]

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 
NO<INF>2</INF> and SO<INF>2</INF> 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 
NO<INF>2</INF> 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 SO<INF>2</INF>. 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 SO<INF>2</INF>, NO, NO<INF>2</INF> 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.
---------------------------------------------------------------------------

    \3\ The current primary NO<INF>2</INF> 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 NO<INF>2</INF>. The current secondary standard remains as it was 
set in 1971.
---------------------------------------------------------------------------

b. Appropriateness and Adequacy of the Current Standards for 
Deposition-related Effects
    This section addresses two concepts necessary to evaluate the 
current standards in the context of deposition-related 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 SO<INF>2</INF> secondary standard (0.5 ppm 
SO<INF>2</INF> 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 SO<INF>2</INF> as measured by the current secondary 
standard. By addressing short-term concentrations, the current 
SO<INF>2</INF> 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 
SO<INF>2</INF> 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 SO<INF>2</INF> 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

[[Page 20235]]

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 NO<INF>2</INF> and SO<INF>2</INF> 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, NO<INF>2</INF> and SO<INF>2</INF>, 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 SO<INF>2</INF> and NO<INF>2</INF> 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, SO<INF>2</INF> and NO<INF>2</INF> 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

[[Page 20236]]

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 SO<INF>4</INF><SUP>2-</SUP> 
followed in decreasing order by a smaller proportion of dry 
SO<INF>2</INF> and a much smaller proportion of deposition as dry 
SO<INF>4</INF><SUP>2-.</SUP>
    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 [mu]eq/L (moderate concern). However, 44 percent of lakes had 
deposition levels exceeding the critical load for an ANC of 50 [mu]eq/L 
(elevated), and 28 percent of lakes had deposition levels exceeding the 
(higher) critical load for an ANC of 20 [mu]eq/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 [mu]eq/L relative to an ANC level at 100 [mu]eq/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 [mu]eq/L and 13 
percent of lakes exceeded the critical load for an ANC of 20 [mu]eq/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 
[mu]eq/L, indicating moderate concern. However, 85 percent of these 
streams had recent deposition exceeding the critical load for an ANC of 
50 [mu]eq/L, and 72 percent exceeded the critical load for an ANC of 20 
[mu]eq/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

[[Page 20237]]

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 NO<INF>2</INF> and SO<INF>2</INF> 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. Short-term 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 non-managed 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,

[[Page 20238]]

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 
SO<INF>4</INF><SUP>2-</SUP> and mercury, and where mercury is present, 
increased availability of SO<INF>4</INF><SUP>2-</SUP> 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 SO<INF>4</INF><SUP>2-</SUP> 
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 SO<INF>2</INF> 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 deposition-related 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 NO<INF>X</INF> and SO<INF>X</INF> can have, 
and are having, adverse environmental impacts. The Panel views that 
the current NO<INF>X</INF> and SO<INF>X</INF> 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 NO<INF>X</INF> and SO<INF>X</INF>. 
Those impacts

[[Page 20239]]

include acidification and undesirable levels of nutrient enrichment 
in some aquatic ecosystems. The levels of the current NO<INF>X</INF> 
and SO<INF>X</INF> 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 multi-
pollutant, 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 NO<INF>2</INF> and SO<INF>2</INF> secondary standards 
with regard to both direct effects on vegetation, as well as on 
deposition-related 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 NO<INF>2</INF> and SO<INF>2</INF> 
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 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. 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 
deposition-related 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 NO<INF>2</INF> 
and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> 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 SO<INF>2</INF>. 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 Deposition-
Related 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 deposition-related effects included 
environmental organizations (Earthjustice, on behalf of the Appalachian 
Mountain Club, National Parks Conservation Association, Sierra Club, 
and Clean Air Council; the Center for Biological Diversity; the Nature 
Conservancy; Adirondack Council; Chesapeake Bay Foundation), the U.S. 
Department of the Interior, NESCAUM, New York Dept. of Environmental 
Conservation, and two tribes. These commenters generally expressed the 
view that the current secondary standards do not provide adequate 
protection from deposition-related effects. More specifically, some of 
these commenters stated that there was overwhelming evidence of 
adversity to sensitive aquatic ecosystems from acidifying deposition. 
These commenters cited a broad range of scientific evidence that 
aquatic acidification was ongoing under current conditions allowed by 
the current secondary standards, and that this acidification 
represented an adverse effect on public welfare. Several commenters 
noted that 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

[[Page 20240]]

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 
NO<INF>2</INF> and SO<INF>2</INF>, 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 [mu]eq/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

[[Page 20241]]

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 
NH<INF>X</INF> 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 NO<INF>2</INF>, and the current secondary standard for 
oxides of sulfur at 0.5 ppm, 3-hour average concentration, measured in 
the ambient air as SO<INF>2</INF>.
---------------------------------------------------------------------------

    \4\ The 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 NO<INF>2</INF> primary NAAQS (75 FR 6531; 
February 9, 2010).
---------------------------------------------------------------------------

    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

[[Page 20242]]

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 deposition-related 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 AAI-based 
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 
NO<INF>2</INF> and SO<INF>2</INF> 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 multi-pollutant 
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 multi-pollutant 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 
multi-pollutant 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 multi-pollutant standard developed in the 
PA would employ (1) Total reactive oxidized nitrogen (NO<INF>y</INF>) 
and oxides of sulfur (SO<INF>X</INF>) 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

[[Page 20243]]

concentrations of NO<INF>y</INF> and SO<INF>X</INF> 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 NO<INF>y</INF> and SO<INF>X</INF> 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 
NO<INF>2</INF> and SO<INF>2</INF> 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, NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF>, NO, HNO<INF>3</INF> or PAN is identical, as is the 
potential for each molecule of SO<INF>2</INF> or ion of particulate 
sulfate (p-SO<INF>4</INF>). 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), SO<INF>2</INF>, sulfur 
trioxide (SO<INF>3</INF>), disulfur monoxide (S<INF>2</INF>O), and 
particulate-phase sulfur compounds (referred to as SO<INF>4</INF>) that 
result from gas-phase sulfur oxides interacting with particles. 
However, the sum of SO<INF>2</INF> and SO<INF>4</INF> 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 
SO<INF>2</INF> and particulate SO<INF>4</INF>. The PA concludes that 
the sum of SO<INF>2</INF> and SO<INF>4</INF>, referred to as 
SO<INF>X</INF>, 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 SO<INF>2</INF> and SO<INF>4</INF>.
b. Oxides of Nitrogen
    As discussed in the PA (U.S. EPA, 2011, section 7.1.2), 
NO<INF>y</INF>, as defined in chapter 2 of the PA, incorporates 
basically all of the oxidized nitrogen species that have acidifying 
potential and as such, NO<INF>y</INF> should be considered as an 
appropriate indicator for oxides of nitrogen. Total reactive oxidized 
nitrogen is an aggregate measure of NO and NO<INF>2</INF> and all of 
the reactive oxidized products of NO and NO<INF>2</INF>. That is, 
NO<INF>y</INF> 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 NO<INF>y</INF> 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 NO<INF>y</INF> and SO<INF>X</INF>, 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.
[GRAPHIC] [TIFF OMITTED] TR03AP12.001


[[Page 20244]]


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 NO<INF>y</INF> and 
SO<INF>X</INF> 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 NO<INF>y</INF> and 
SO<INF>X</INF>.
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 NO<INF>y</INF> and 
SO<INF>X</INF> deposition, the PA concludes that ANC is an appropriate 
ecological indicator for relating adverse aquatic ecosystem effects to 
acidifying atmospheric deposition of SO<INF>X</INF> and NO<INF>y</INF>, 
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 steady-state 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:
---------------------------------------------------------------------------

    \5\ This 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 NO<INF>y</INF> and 
SO<INF>X</INF> and deposition of nitrogen and sulfur.

---------------------------------------------------------------------------

[[Page 20245]]

[GRAPHIC] [TIFF OMITTED] TR03AP12.002

Where:

CL<INF>ANClim</INF>(N + S) is the critical load of deposition, with 
units of equivalent charge/(area-time);
[BC]<INF>0</INF>* is the natural contribution of base cations from 
weathering, soil processes and preindustrial deposition, with units 
of equivalent charge/volume;
[ANC<INF>lim</INF>] 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, CL<INF>ANClim</INF>(N + S), that is associated with a 
sustainable long-term ANC target, [ANC<INF>lim</INF>], given the 
capacity of the natural system to generate ANC, [BC]<INF>0</INF>*, 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 Q<INF>r</INF> and CL<INF>r,</INF> 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.
---------------------------------------------------------------------------

    \6\ Because 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.
---------------------------------------------------------------------------

    As presented above, the terms S and N in the CL<INF>ANClim</INF> (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 SO<INF>X</INF> 
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, NO<INF>y</INF>, in addition to 
the reduced nitrogen species, ammonia and ammonium ion, referred to as 
NH<INF>X</INF>. The NH<INF>X</INF> 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 NH<INF>X</INF>, 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, NO<INF>y</INF> and 
SO<INF>X</INF>. 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 
NO<INF>y</INF> and SO<INF>X</INF> in terms of NO<INF>y</INF> and 
SO<INF>X</INF> ambient concentrations, two transference ratios were 
defined, where T<INF>SOx</INF> equals the ratio of the combined dry and 
wet deposition of SO<INF>X</INF> to the ambient air concentration of 
SO<INF>X</INF>, and T<INF>NOy</INF> equals the ratio of the combined 
dry and wet deposition of NO<INF>y</INF> to the ambient air 
concentration of NO<INF>y</INF>.
    As described in chapter 7 of the PA, reduced forms of nitrogen 
(NH<INF>X</INF>) are included in total nitrogen in the critical load 
equation, III-1. Reduced forms of nitrogen are treated separately, as 
are NO<INF>y</INF> and SO<INF>X</INF>, and the transference ratios are 
applied. This results in the following critical load expression that is 
defined explicitly in terms of the indicators NO<INF>y</INF> and 
SO<INF>X</INF>:
[GRAPHIC] [TIFF OMITTED] TR03AP12.003


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:
[GRAPHIC] [TIFF OMITTED] TR03AP12.004


[[Page 20246]]


    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, 
NO<INF>y</INF> and SO<INF>X</INF>, 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 NO<INF>y</INF> and SO<INF>X</INF>. The calculated ANC 
will differ from the target ANC (ANClim) depending on how much the 
nitrogen and sulfur deposition associated with NO<INF>y</INF>, 
SO<INF>X</INF>, and NH<INF>X</INF> differs from the critical load 
associated with just achieving the target ANC.
---------------------------------------------------------------------------

    \7\ Because NH<INF>X</INF> is characterized directly as 
deposition, not as an ambient concentration in this equation, no 
transference ratio is needed for this term.
---------------------------------------------------------------------------

    Based on equation III-3, the PA defines an AAI that is more simply 
stated using terms that highlight the ambient air indicators:
[GRAPHIC] [TIFF OMITTED] TR03AP12.005


where the AAI represents the long-term (or steady-state) ANC level 
associated with ambient air concentrations of NO<INF>y</INF> and 
SO<INF>X</INF>. 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-to-concentration translators for NO<INF>y</INF> 
(F3) and SO<INF>X</INF> (F4).
Specifically:
F1 = ANClim + CL<INF>r</INF>/Q<INF>r</INF> ;
F2 = NH<INF>X</INF>/Q<INF>r</INF> = NH<INF>X</INF> deposition 
divided by Q<INF>r</INF>;
F3 = T<INF>NOy</INF>/Q<INF>r</INF> ; T<INF>NOy</INF> is the 
transference ratio that converts ambient air concentrations of 
NO<INF>y</INF> to deposition of NO<INF>y</INF>; and
F4 = T<INF>SOx</INF>/Q<INF>r</INF> ; T<INF>SOx</INF> is the 
transference ratio that converts ambient air concentrations of 
SO<INF>X</INF> to deposition of SO<INF>X</INF>.

All of these factors include representative Q<INF>r</INF> 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]*<INF>0</INF>) 
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, {NH<INF>X</INF> + 
F3[NO<INF>y</INF>]{time} , being greater than Neco. In rare cases where 
Neco is greater than {NH<INF>X</INF> + F3[NO<INF>y</INF>]{time} , 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 SO<INF>X</INF>, 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, BC<INF>0</INF>* 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 over-protection 
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 deposition-related effects (e.g., terrestrial 
acidification, nutrient enrichment) that link atmospheric 
concentrations to an aquatic or terrestrial ecological indicator.
---------------------------------------------------------------------------

    \8\ We note that an 85th area within Omernik's Ecoregion Level 
III is currently being developed for California.
---------------------------------------------------------------------------

    The PA used Omernik's original alkalinity data (U.S. EPA, 2011, 
section

[[Page 20247]]

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 over-protection 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 Acid-Sensitive Areas
------------------------------------------------------------------------
                                                               Ecoregion
                       Ecoregion name                           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 + CL<INF>r</INF>/Q<INF>r</INF>. The value of F1 for 
each ecoregion would be based on a calculated critical load used to 
represent the ecoregion (CL<INF>r</INF>) associated with a single 
national target ANC level (ANClim, discussed below in section III.D), 
as well as on a runoff rate (Q<INF>r</INF>) to represent the region. To 
specify ecoregion-specific values for the term Q<INF>r</INF>, 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 CL<INF>r</INF> 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.
---------------------------------------------------------------------------

    \9\ The 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.
    \10\ The PA judged the data to be sufficient for this purpose if 
data are available from more than 10 water bodies in an ecoregion.
---------------------------------------------------------------------------

    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 NO<INF>y</INF> and SO<INF>X</INF> 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

[[Page 20248]]

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 acid-sensitive 
ecoregions. Consideration of a lower percentile would avoid 
representing a relatively non-acid sensitive ecoregion by a critical 
load associated with relatively more acid-sensitive 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 = NH<INF>X</INF>/
Q<INF>r</INF>. The PA calculated the representative runoff rate, 
Q<INF>r</INF>, 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 NH<INF>X</INF> for each ecoregion. The NH<INF>X</INF> 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 NO<INF>y</INF> and SO<INF>X</INF> to the 
associated deposition, and are defined as follows: F3 = 
T<INF>NOy</INF>/Q<INF>r</INF> and F4 = T<INF>SOx</INF>/Q<INF>r</INF>. 
T<INF>NOy</INF> is the transference ratio that converts ambient air 
concentrations of NO<INF>y</INF> to deposition of NO<INF>y</INF> and 
T<INF>SOx</INF> is the transference ratio that converts ambient air 
concentrations of SO<INF>X</INF> to deposition of SO<INF>X</INF>. 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 NO<INF>y</INF> or SO<INF>X</INF> spatially averaged 
across the ecoregion and divided by the annual ambient air 
concentration of NO<INF>y</INF> or SO<INF>X</INF>, 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 acid-sensitive 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 NO<INF>y</INF> and SO<INF>X</INF>. 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 
NO<INF>y</INF> and SO<INF>X</INF>.
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 NO<INF>y</INF> and SO<INF>X</INF> 
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 NO<INF>y</INF> and SO<INF>X</INF> 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.
---------------------------------------------------------------------------

    \11\ Unlike 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.
---------------------------------------------------------------------------

    Based on associations between pH levels and target ANC levels and

[[Page 20249]]

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 [mu]eq/L. The available evidence indicates that 
target ANC levels below 20 [mu]eq/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 [mu]eq/L in some 
sensitive ecosystems, the degree and nature of those effects are less 
significant than at levels below 20 [mu]eq/L. Levels at and above 50 
[mu]eq/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 [mu]eq/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 SO<INF>X</INF>, NO<INF>y</INF>, 
and NH<INF>X</INF> as well as the critical load-related 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 
NO<INF>y</INF> and SO<INF>X</INF> 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 pre-industrial base cation production within 
the AAI and ecoregion frameworks would be achieved.
    While characterization of NH<INF>X</INF> deposition has been 
evolving over the last decade, the high uncertainty in characterizing 
NH<INF>X</INF> deposition is due to both the lack of field measurements 
and the inherent complexity of characterizing NH<INF>X</INF> 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 NH<INF>X</INF> 
concentrations and deposition patterns is limited. While direct 
measurement of NH<INF>X</INF> 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 semi-volatile nature of ammonia 
results in re-entrainment 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 NH<INF>X</INF> 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 
NH<INF>X</INF> deposition.
---------------------------------------------------------------------------

    \12\ Field measurements of NH<INF>X</INF> have been extremely 
limited, but have begun to be enhanced through the NADP's passive 
ammonia network (AMoN).
---------------------------------------------------------------------------

    In characterizing uncertainties with respect to available 
measurement data and the use of ecological and

[[Page 20250]]

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 AAI-based standard are modeled 
using the CMAQ atmospheric model and steady-state 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 
n<SUP>th</SUP> 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 NO<INF>Y</INF> and SO<INF>X</INF> 
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 clay-based 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 
SO<INF>X</INF> and NO<INF>y</INF> 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

[[Page 20251]]

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, multi-media, 
environmentally modified, geographically variable nature of 
SO<INF>X</INF>/NO<INF>y</INF> 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 [mu]eq/L. CASAC also recognized that the level and the form of 
any AAI-based 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 ([mu]eq/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

[[Page 20252]]

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 acid-sensitive 
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 AAI-based 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 
NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>y</INF> 
and SO<INF>X</INF> deposition to sensitive ecosystems. The 
Administrator proposed to revise the secondary standards by adding 
secondary standards identical to the NO<INF>2</INF> and SO<INF>2</INF> 
primary 1-hour standards set in 2010, including a 1-hour secondary 
NO<INF>2</INF> standard set at a level of 100 ppb and a 1-hour 
secondary SO<INF>2</INF> 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 deposition-related 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 NO<INF>2</INF> and 
SO<INF>2</INF> 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 implementation-related 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

[[Page 20253]]

standard in this review. These commenters generally focused on the 
limitations and uncertainties in the scientific evidence used by the 
EPA as a basis for its consideration of an AAI-based standard, 
expressing the view that these limitations and uncertainties were so 
great as to preclude setting such a standard at this time. Several 
industry commenters felt the uncertainties were of sufficient magnitude 
as to invalidate the AAI approach for use in the NAAQS, while others 
agreed with the EPA's finding that further information and analysis is 
needed, and further noted that this work should be completed before the 
EPA could propose a new multi-pollutant standard. More fundamentally, 
some commenters in this group expressed the view that any consideration 
of such a standard is inconsistent with various provisions of the CAA 
and thus unlawful.
    With regard to their views on the underlying scientific 
information, many of these commenters focused on what they asserted 
were areas of substantial uncertainty in the AAI approach including 
uncertainties in the individual F factors of the AAI, air deposition 
modeling, critical loads modeling, and available water quality and 
watershed data. Several commenters felt a more rigorous uncertainty and 
variability analysis of the AAI, beyond the analyses that the EPA 
presented in the PA, would be needed if the EPA were to consider such a 
standard in the future.
    Some commenters expressed concerns with specific aspects of the 
AAI, such as the adequacy of the Omernik ecoregion approach as a method 
of waterbody aggregation for critical 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 (NH<INF>x</INF>). 
Other commenters were also critical of the reliance of the AAI on 
modeling, and expressed the view that CMAQ would require intensive 
deposition-focused evaluation.
    A second group of commenters, including several environmental 
groups (e.g., Center for Biological Diversity, Earthjustice, and 
Adirondack Council), the U.S. Department of Interior and the National 
Park Service, the New York Department of Environmental Conservation, 
and two tribes (Fond du Lac Band and Potawatomi) strongly disagreed 
with the EPA's proposed decision not to set an AAI-based standard in 
this review. These commenters generally focused on the strengths of the 
evidence of deposition-related effects, the extent to which analyses 
presented in the PA addressed uncertainties and limitations in the 
evidence, and on information regarding the adversity of such effects as 
a basis for their views that such a standard was warranted at this 
time. Many of these commenters pointed to CASAC's review of the 
underlying scientific evidence and its support for moving forward with 
an AAI-based standard at this time as support for their views.
    In general, the environmental group commenters expressed the view 
that the current standards are clearly not adequate and that a combined 
NO<INF>X</INF>/SO<INF>X</INF> 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 deposition-related effects. They also noted that the EPA has 
worked for decades to solve the acid deposition problem and that in 
their view the AAI represents an elegant solution to that problem.
    With regard to their views on the underlying scientific 
information, these commenters generally agreed with the EPA's proposed 
conclusions that there are well-established water quality and 
biological indicators of aquatic deposition and well-established models 
that address air deposition, water quality impacts, and effects on 
biota. Many of these commenters expressed the view that the 
uncertainties and limitations in the scientific evidence were 
adequately addressed in the PA, which was reviewed by CASAC. Many of 
these commenters pointed to CASAC's support for adopting an AAI-based 
standard in this review while concurrently conducting additional field 
monitoring and longer-term research that might reduce uncertainties in 
future reviews of secondary NAAQS for oxides of nitrogen and sulfur.
    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 
NO<INF>X</INF> and SO<INF>X</INF> 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 
NO<INF>X</INF> and SO<INF>X</INF> 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 
NO<INF>y</INF> and SO<INF>X</INF> as well-justified indicators of 
atmospheric concentrations of oxides of nitrogen and sulfur, (3) the 
use of Omernik Level III ecoregions, (4) the division of ecoregions 
into sensitive and non-sensitive categories, (5) the use of a 3 to 5 
year averaging time, and (6) the appropriateness of an AAI level 
between 20 to 75 [mu]eq/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 AAI-based standard 
while continuing to rely solely on a form of the standard that is 
inadequate and allows serious harms to the public welfare to continue. 
When confronted with scientific uncertainties and incomplete data, they 
feel the EPA must act in a precautionary manner that errs toward 
stronger protections. Further, they believe that the EPA's reliance on 
scientific uncertainty as a basis for its inaction is unsupportable in 
light of CASAC's advice and the EPA staff's conclusions in the ISA, REA 
and PA.
    In addition to the two broad groups of commenters discussed above, 
a few other commenters offered more general

[[Page 20254]]

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 deposition-related 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 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 (U.S. EPA, 2011, chapter 7).
    The EPA also agrees with the second group of commenters and CASAC 
that ANC would be an appropriate ecological indicator, reflecting the 
acidifying effects of deposition of nitrogen and sulfur (U.S. EPA, 
2011, chapter 7.2 and Russell and Samet, 2011a). Further, the EPA 
agrees that the structure of an AAI-based standard is well-grounded in 
science and would address the combined effects of deposition from 
oxides of nitrogen and sulfur by characterizing the linkages between 
ambient concentrations, deposition, and aquatic acidification, and that 
the structure of the standard takes into account relevant variations in 
these linkages across the country (section III.B. 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 NO<INF>y</INF> 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 non-sensitive ecoregions, attempting to be neither over-protective 
nor under-protective 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

[[Page 20255]]

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 NO<INF>y</INF> and 
SO<INF>X</INF> 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 
NO<INF>y</INF> and SO<INF>X</INF> 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 acid-sensitive 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 AAI-based 
standard at this time. Although the Administrator has concluded that 
the current secondary standards are neither appropriate nor adequate to 
protect against potentially adverse deposition-related effects 
associated with ambient concentrations of oxides of nitrogen and 
sulfur, such a conclusion does not require the EPA to adopt a new NAAQS 
where the Administrator cannot reasonably judge that it would meet the 
criteria for a secondary NAAQS.
    The Administrator judges that the current limitations in relevant 
data and the uncertainties associated with specifying the elements of a 
new AAI-based NAAQS defined in terms of modeled factors are of such 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 NO<INF>2</INF> and 
SO<INF>2</INF> 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.
---------------------------------------------------------------------------

    \13\ No 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.
---------------------------------------------------------------------------

    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

[[Page 20256]]

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 AAI-based 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 (NH<INF>X</INF>), in addition to oxides of nitrogen, in the 
AAI equation; (2) whether such a standard would be appropriately 
construed as a national standard versus a regional standard; (3) 
whether such a standard would be appropriately construed as an ambient 
air quality standard versus a water quality standard, and (4) whether 
the EPA has authority under the CAA to set a multi-pollutant NAAQS.
    (1) As described above in section III.A, the AAI equation contains 
a separate factor that accounts for the acidifying potential of 
NH<INF>X</INF>, 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 
NH<INF>X</INF> should be treated the same as NO<INF>X</INF> in the AAI, 
while others felt it should not be included at all in the AAI. Several 
commenters expressed the view that accounting for NH<INF>X</INF> 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 
NH<INF>X</INF> 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 NH<INF>X</INF> in the AAI equation, 
even though it is not a listed pollutant, since aquatic ecosystems 
respond to inputs of NH<INF>X</INF> to create acidity just like they do 
with inputs of NO<INF>X</INF> and SO<INF>X</INF>.
    The EPA has included NH<INF>X</INF> 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 
NH<INF>X</INF> 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 NH<INF>X</INF> 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 SO<INF>X</INF>) and 
nitrogen (as NO<INF>y</INF>) are accounted for separately from the 
deposition of NH<INF>X</INF> 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 NO<INF>y</INF> and SO<INF>X</INF> to determine an 
area's attainment status. Conversely, there would be no requirement to 
measure concentrations of NH<INF>X</INF> to determine compliance with 
an AAI-based standard. Rather, ecoregion-specific values of 
NH<INF>X</INF> deposition would be determined by modeling and would be 
specified by the EPA in conjunction with setting such a

[[Page 20257]]

standard, and would not be a variable in the AAI equation as would 
SO<INF>X</INF> and NO<INF>y</INF>. 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 
(NO<INF>y</INF> and SO<INF>X</INF>) and the associated degree of 
aquatic acidification. Section 108 requires the air quality criteria to 
evaluate to the extent practicable the variable factors such as 
atmospheric conditions that affect the impact of the ambient air 
pollutant (in this case oxides of nitrogen and sulfur) on the public 
welfare. In this review, such variable factors include the deposition 
of reduced nitrogen in an ecoregion, as well as all of the other 
elements reflected in the factors F1 to F4, and the designation of an 
area as acid-sensitive or not acid-sensitive. Section 109 calls for the 
EPA to base the NAAQS on the air quality criteria, and accounting for 
the role of reduced nitrogen deposition in the AAI reflects this.
    In considering this aspect of an AAI-based standard, the EPA took 
into account that in applying the AAI equation, all factors, including 
NH<INF>X</INF> 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, NH<INF>X</INF> 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 NH<INF>X</INF> 
deposition occur from one review to the next, the ecoregion-specific F2 
factors would be updated to reflect such changes. To the extent that 
NH<INF>X</INF> 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 NO<INF>y</INF> and 
SO<INF>X</INF> that would meet a specific AAI-based standard; 
conversely, increased levels of NH<INF>X</INF> deposition would allow 
for potentially lower levels of NO<INF>y</INF> and SO<INF>X</INF>. 
Meeting a specific AAI-based standard would only require that the 
combined levels of NO<INF>y</INF> and SO<INF>X</INF> 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 NH<INF>X</INF> 
deposition would be accounted for, NH<INF>X</INF> emissions would not 
be regulated through the implementation of an AAI-based standard. 
NH<INF>X</INF> 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 NH<INF>X</INF> in the AAI equation would not 
result in de facto regulation of NH<INF>X</INF> 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 NO<INF>y</INF> and SO<INF>X</INF>, 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, NO<INF>y</INF> and SO<INF>X</INF>. 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 NO<INF>y</INF> and SO<INF>X</INF> 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 PM<INF>10</INF> standard with a 
single numerical level, which then allowed varying levels of coarse PM, 
a subset of PM<INF>10</INF>. The PM<INF>10</INF> 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 PM<INF>10</INF> allowed

[[Page 20258]]

variable levels of coarse PM, with higher allowable levels where there 
was less evidence of risk and lower allowable levels where the evidence 
of risk was greater. This approach was upheld in American Farm Bur. 
Fed. v. EPA, 559 F.3d 512, 533-536 (D.C. Cir. 2009).
    In conjunction with consideration of an AAI-based standard, the EPA 
has recognized that the nation includes some relatively acid-sensitive 
and some relatively non-acid sensitive ecoregions. This delineation 
allows for an appropriate application of the AAI equation that 
increases its relevancy from a national perspective as it avoids 
creating more than requisite protection in areas that are not acid 
sensitive. The AAI equation and the selected level of such a standard 
would be applicable everywhere; however, factors in the AAI equation 
are appropriately dependent on the sensitive and non-sensitive 
ecoregion classification. Therefore, the delineation of sensitive and 
non-sensitive regions allows for a nationally consistent application of 
the AAI equation as it targets protection on those areas most likely to 
benefit from reductions in acidifying deposition of oxides of nitrogen 
and sulfur, and avoids more than requisite protection in areas that 
would not benefit from such reductions.
    (3) 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 NO<INF>y</INF> and 
SO<INF>X</INF>, 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 NO<INF>2</INF> and SO<INF>2</INF> Secondary 
Standards
    Comments received on the proposal related to setting new 1-hour 
NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and 
SO<INF>2</INF> secondary standards identical to the 1-hour 
NO<INF>2</INF> and SO<INF>2</INF> 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 
NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> 
and SO<INF>2</INF> secondary standards identical to the 1-hour 
NO<INF>2</INF> and SO<INF>2</INF> primary standards is unlawful, 
commenters asserted that the EPA's

[[Page 20259]]

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 NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 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 AAI-based standard, which is expressly 
designed to address important differences in ecosystem sensitivities, 
in part on uncertainties and limitations in relevant information that 
were of such nature and degree as to prevent the Administrator from 
reaching a reasoned decision at this time as to what level and form of 
such a standard would provide a particular degree of protection. This 
commenter asserts that the proposed decision to set new 1-hour 
NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 
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 NO<INF>2</INF> 
and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 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 
NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 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 AAI-based standard, the 1-hour 
standards would tend to provide more protection than is warranted in 
areas that are not acid-sensitive.
    Further, the EPA recognizes that any benefits that would accrue as 
a result of actions taken to meet the 2010 1-hour NO<INF>2</INF> and 
SO<INF>2</INF> 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

[[Page 20260]]

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 acid-sensitive 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 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.\14\
---------------------------------------------------------------------------

    \14\ Thus, as discussed above, EPA's disagreement with 
commenters concerning adoption of an AAI-based standard at this time 
appears to stem from differing views on whether or not there is an 
adequate scientific or technical basis for judging the degree of 
protection which an AAI-based standard would afford. There does not 
appear to be a disagreement with the view that EPA should not adopt 
a standard absent such a scientific or technical basis.
---------------------------------------------------------------------------

    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 NO<INF>2</INF> and SO<INF>2</INF> 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 AAI-based standard to provide protection against potentially 
adverse deposition-related effects associated with oxides of nitrogen 
and sulfur, or setting new secondary standards identical to the current 
1-hour NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 
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.
---------------------------------------------------------------------------

    \15\ Section 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.''
---------------------------------------------------------------------------

    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

[[Page 20261]]

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 NO<INF>y</INF> 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 AAI-
based 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 AAI-based 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 model-
derived 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

[[Page 20262]]

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 AAI-based 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 multi-media 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

[[Page 20263]]

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 NO<INF>2</INF> and 
SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> primary standards. In the 
proposal, the Administrator recognized that the new NO<INF>2</INF> and 
SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> 
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 NO<INF>2</INF> and SO<INF>2</INF> 
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 NO<INF>2</INF> and SO<INF>2</INF> 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 NO<INF>2</INF> and 
SO<INF>2</INF> 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 deposition-related 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 NO<INF>2</INF> and 
SO<INF>2</INF> 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 NO<INF>2</INF> and SO<INF>2</INF> secondary standards, having 
concluded that the current NO<INF>2</INF> and SO<INF>2</INF> 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 AAI-based 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 NO<INF>2</INF> and SO<INF>2</INF> 
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 NO<INF>2</INF> and 
SO<INF>2</INF> secondary standards to address direct effects of

[[Page 20264]]

gaseous NO<INF>2</INF> and SO<INF>2</INF> on vegetation. Thus, taken 
together, the Administrator has decided to retain and not revise the 
current NO<INF>2</INF> and SO<INF>2</INF> secondary standards. 
Specifically these secondary standards include an NO<INF>2</INF> 
standard set at a level of 0.053 ppm, annual arithmetic average, and an 
SO<INF>2</INF> 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 
NO<INF>y</INF> and SO<INF>X</INF> 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, SO<INF>X</INF>, and the use of commercially 
available NO<INF>y</INF> instruments to measure the oxides of nitrogen 
indicator, NO<INF>y</INF>. 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 multi-
pollutant 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 
NO<INF>y</INF> and SO<INF>X</INF> and consider designation of such 
methods as Federal Reference Methods (FRMs);
    (2) Examine the variability and improve characterization of 
concentration and deposition patterns of NO<INF>y</INF> and 
SO<INF>X</INF>, as well as reduced forms of nitrogen, within and across 
a number of sensitive ecoregions across the country;
    (3) Develop updated ecoregion-specific 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 
NO<INF>y</INF> and SO<INF>X</INF> 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 NO<INF>y</INF>;
    (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 
acid-sensitive 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 NO<INF>y</INF> and SO<INF>X</INF> (i.e., SO<INF>2</INF> and p-
SO<INF>4</INF>) 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 NO<INF>y</INF> 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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[GRAPHIC] [TIFF OMITTED] TR03AP12.006

    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 multi-agency 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 NO<INF>y</INF> and SO<INF>X</INF> 
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 SO<INF>2</INF> 
and speciated PM<INF>2.5</INF> measurements. The co-located deployment 
of a continuous SO<INF>2</INF> analyzer with the CFP for SO<INF>2</INF> 
will provide test data for determining suitability of continuous 
SO<INF>2</INF> 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 SO<INF>2</INF> standard. 
Conversely, because of the relatively low SO<INF>2</INF> concentrations 
associated with many acid-sensitive ecoregions, existing SO<INF>2</INF> 
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 PM<INF>2.5</INF> 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 p-
SO<INF>4</INF> and the speciation samplers used throughout the state 
and local air quality networks. The EPA notes that CASTNET already has 
several co-

[[Page 20266]]

located IMPROVE chemical speciation samplers. Because the AAI equation 
is based in part on the concentration of p-SO<INF>4</INF>, the original 
motivation for capturing all particle size fractions is not as 
important relative to simply capturing the concentration of total p-
SO<INF>4</INF>.
    Candidate measurements to complement oxidized nitrogen 
measurements, in addition to the CFP, include a mix of continuous and 
periodic sampling for the dominant NO<INF>y</INF> species, namely NO, 
directly measured NO<INF>2</INF>, PAN, HNO<INF>3</INF>, and particulate 
nitrate, p-NO<INF>3</INF>. The CASAC AMMS (Russell and Samet, 2011b) 
recommended that the EPA consider the use of total nitrate (t-
NO<INF>3</INF>) obtained from CASTNET sampling as an indicator for 
NO<INF>y</INF>, reasoning that t-NO<INF>3</INF> 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-NO<INF>3</INF> as the indicator of oxides of nitrogen for use in an 
AAI-based secondary standard.
    The CASAC AMMS also recommended that total NH<INF>X</INF> 
(NH<INF>3</INF> and particulate ammonium (p-NH<INF>4</INF>)) be 
considered as a proxy for reduced nitrogen species, reasoning that the 
subsequent partitioning to NH<INF>3</INF> and p-NH<INF>4</INF> 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 NH<INF>X</INF> measurements and calculated values of 
NH<INF>3</INF> and ammonium (NH<INF>4</INF>) to the AAI.
    The additional supplemental measurements of speciated 
NO<INF>y</INF>, continuous SO<INF>2</INF> and NH<INF>X</INF> 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 SO<INF>2</INF> instruments in rural environments, and 
using the speciated NO<INF>y</INF> data to assess NO<INF>y</INF> 
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 n\th\ 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, regionally-dispersed pollutants and 
relatively complex compliance determinations based on regionally 
variable levels of NO<INF>y</INF> and SO<INF>X</INF> 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

[[Page 20267]]

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 NO<INF>y</INF> and SO<INF>X</INF> 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, PM<INF>2.5</INF>, and NO<INF>2</INF>) 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 stakeholder-based 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 NO<INF>y</INF> and SO<INF>X</INF>, 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 
SO<INF>2</INF>, p-SO<INF>4</INF>, and NO<INF>y</INF>.
    The CASTNET provides a well-established infrastructure that would 
meet the basic location and measurement requirements of an AAI-based 
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 SO<INF>2</INF>, p-
SO<INF>4</INF>, NH<INF>4</INF> and t-NO<INF>3</INF>, the sum of 
HNO<INF>3</INF> and p-NO<INF>3</INF>.
    While routinely operated instruments that measure SO<INF>2</INF>, 
p-SO<INF>4</INF>, NO<INF>y</INF> and/or t-NO<INF>3</INF> exist, 
instruments that measure p-SO<INF>4</INF>, NO<INF>y</INF>, t-
NO<INF>3</INF>, or the CFP for SO<INF>2</INF> 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 SO<INF>2</INF> and p-SO<INF>4</INF> 
measurements based on the CFP. Such a designation by the EPA could be 
done for the purpose of facilitating consistent research related to an 
AAI-based standard and/or in conjunction with setting and supporting an 
AAI-based 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 (SO<INF>2</INF>, p-SO<INF>4</INF> and NO<INF>y</INF>) 
that would be needed to determine compliance with an AAI-based 
secondary standard.
    For the SO<INF>2</INF> and p-SO<INF>4</INF> measurements, the EPA 
is considering the CFP method, which provides weekly average 
concentration measurements for SO<INF>2</INF> and p-SO<INF>4</INF>. 
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 SO<INF>2</INF> and 
p-SO<INF>4</INF> concentrations for an AAI-based secondary standard.
    Although the CFP method would provide measurements of both the 
SO<INF>2</INF> and p-SO<INF>4</INF> components in a unified sampling 
and analysis procedure, individual FRMs will be considered for each. 
The EPA recognizes that an existing FRM to measure SO<INF>2</INF> 
concentrations using ultra-violet fluorescence (UVF) exists (40 CFR 
part 50, appendix A-1) for the purpose of monitoring compliance for the 
primary SO<INF>2</INF> 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 NO<INF>y</INF> component, a continuous analyzer 
for measuring NO<INF>y</INF> 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 NO<INF>2</INF> 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 NO<INF>y</INF>. However, this 
NO<INF>y</INF> 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 NO<INF>y</INF> 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 NO<INF>y</INF>, SO<INF>2</INF> and 
p-SO<INF>4</INF> as ambient air indicators is a suitable

[[Page 20268]]

approach in concept. On supporting the CFP method as a potential FRM 
for SO<INF>2</INF>, CASAC stated that they felt that the CFP is 
adequate for measuring long-term average SO<INF>2</INF> 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-SO<INF>4</INF>, CASAC AMMS generally supports the use 
of the CFP as a potential FRM for measuring p-SO<INF>4</INF> for an 
AAI-based secondary standard. The method has been relatively well-
characterized 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 
NO<INF>y</INF> method as a potential FRM, CASAC AMMS concluded that the 
existing NO<INF>y</INF> 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 NH<INF>X</INF> 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 under-sampled areas in other parts of 
country such as the mountainous West. Also, some commenters requested 
that relatively non-acid sensitive areas be included in the field pilot 
program in the interest of broader national applicability or, as one 
state agency suggested, the availability of a rich data base in the 
Chesapeake Bay region. Some commenters also expressed the view that 
results from the field pilot program would not be available for the 
next periodic review of the secondary standards for oxides of nitrogen 
and sulfur.
    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

[[Page 20269]]

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 Long-Term Monitoring (TIME/LTM) water 
sampling programs. These water sampling programs should also be 
extended to other under-sampled areas of the country that are acid 
sensitive. Some industry commenters 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 NO<INF>X</INF> and SO<INF>X</INF> to sensitive 
ecosystems, which is especially relevant for the AAI applications. 
NCore was designed as a more generalized network to collect 
measurements in a variety of geographical areas, with no specific focus 
on acid-sensitive ecosystems. Moreover, CASTNET has established a track 
record over the last two decades of providing quality measurements, 
whereas NCore is a relatively new network that has been fully deployed 
for less than two years and therefore not been subjected to review and 
analysis commensurate with the CASTNET program. Nevertheless, as some 
states suggested, this pilot program should afford an opportunity to 
explore the use of existing rural NCore sites in acid-sensitive 
ecoregions. 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

[[Page 20270]]

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 
SO<INF>X</INF> 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 SO<INF>X</INF> 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 NO<INF>y</INF> 
instruments used to measure the indicators, NO<INF>y</INF> and 
SO<INF>X</INF>. 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 
SO<INF>2</INF> and speciated PM<INF>2.5</INF> measurements in the field 
pilot program to provide test data for determining the suitability of 
continuous SO<INF>2</INF> 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 NO<INF>y</INF> species. Several commenters 
suggested adding NADP wet deposition samplers.
    Several commenters supported the development of an FRM for 
NO<INF>y</INF> and CFP-based SO<INF>2</INF> and sulfate measurements. 
Greater attention was addressed to NO<INF>y</INF> 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 NO<INF>y</INF> 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 NO<INF>y</INF> and SO<INF>X</INF>. 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 NO<INF>y</INF> FRM development should be 
decoupled from the pilot studies, while recognizing that separate 
NO<INF>y</INF> measurements are an important component of the pilot 
study. Although NO<INF>y</INF> 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.

[[Page 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 cost-
benefit 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 cost-
effective 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 cost-
effective 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

[[Page 20272]]

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 NO<INF>X</INF> and SO<INF>X</INF>: 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 NO<INF>X</INF> and SO<INF>X</INF>: 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 NO<INF>X</INF> and SO<INF>X</INF>: FINAL. EPA-CASAC-11-
005.
Russell and Samet, 2011b Review of EPA Draft Documents on Monitoring 
and Methods for Oxides of Nitrogen (NO<INF>X</INF>) and Sulfur 
(SO<INF>X</INF>). 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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