`[Federal Register Volume 89, Number 248 (Friday, December 27, 2024)]
[Rules and Regulations]
[Pages 105692-105788]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-29463]



[[Page 105691]]

Vol. 89

Friday,

No. 248

December 27, 2024

Part II





Environmental Protection Agency





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40 CFR Part 50





Review of the Secondary National Ambient Air Quality Standards for 
Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter; Final 
Rule

  Federal Register / Vol. 89 , No. 248 / Friday, December 27, 2024 / 
Rules and Regulations  

[[Page 105692]]


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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2014-0128; FRL-5788-05-OAR]
RIN 2060-AS35


Review of the Secondary National Ambient Air Quality Standards 
for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: Based on the Environmental Protection Agency's (EPA's) review 
of the air quality criteria for ecological effects and secondary 
national ambient air quality standards (NAAQS) for oxides of nitrogen 
(N oxides), oxides of sulfur (SOX), and particulate matter 
(PM), the EPA is revising the existing secondary sulfur dioxide 
(SO2) standard to an annual average, averaged over three 
consecutive years, with a level of 10 parts per billion (ppb). 
Additionally, the Agency is retaining the existing secondary standards 
for N oxides and PM, without revision. The EPA is also finalizing 
revisions to the data handling requirements for the secondary 
SO2 NAAQS.

DATES: This final rule is effective on January 27, 2025.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-HQ-OAR-2014-0128. All documents in the docket are 
listed on the https://www.regulations.gov website. Although listed in 
the index, some information is not publicly available, e.g., CBI or 
other information whose disclosure is restricted by statute. Certain 
other material, such as copyrighted material, is not placed on the 
internet and will be publicly available only in hard copy form. 
Publicly available docket materials are available electronically 
through https://www.regulations.gov.

FOR FURTHER INFORMATION CONTACT: Ms. Ginger Tennant, Environmental 
Protection Agency, Health and Environmental Impacts Division, Office of 
Air Quality Planning and Standards (mail code C539-04), Research 
Triangle Park, NC 27711; telephone number: (919) 541-4072; email 
address: [email protected].

SUPPLEMENTARY INFORMATION:

Table of Contents

Executive Summary
I. Background
    A. Legislative Requirements
    B. Related Control Programs
    C. History of the Secondary Standards for N Oxides, 
SOX and PM
    1. N Oxides
    2. SOX
    3. Related Actions Addressing Acid Deposition
    4. Most Recent Review of the Secondary Standards for N Oxides 
and SOX
    5. PM
    D. Current Review
II. Rationale for Decisions
    A. Introduction
    1. Background
    a. Basis for Existing Secondary Standards
    b. Prior Review of Deposition-Related Effects
    c. General Approach for This Review
    2. Overview of Air Quality and Deposition
    a. Sources, Emissions and Atmospheric Processes Affecting 
SOX, N Oxides and PM
    b. Recent Trends in Emissions, Concentrations, and Deposition
    c. Relationships Between Concentrations and Deposition
    3. Overview of Welfare Effects Evidence
    a. Nature of Effects
    (1) Direct Effects of SOX and N Oxides in Ambient Air
    (2) Acid Deposition-Related Ecological Effects
    (3) Nitrogen Enrichment and Associated Ecological Effects
    (4) Other Deposition-Related Effects
    b. Public Welfare Implications
    c. Exposure Conditions and Deposition-Related Metrics
    (1) Acidification and Nitrogen Enrichment in Aquatic Ecosystems
    (2) Deposition-Related Effects in Terrestrial Ecosystems
    (3) Other Effects of N Oxides, SOX and PM in Ambient 
Air
    4. Overview of Exposure and Risk Assessment for Aquatic 
Acidification
    a. Key Design Aspects
    b. Key Limitations and Uncertainties
    c. Summary of Results
    B. Conclusions
    1. Basis for Proposed Decision
    a. Policy-Relevant Evaluations in the Policy Assessment
    (1) Effects Not Related to S and N Deposition
    (2) Evidence of Ecosystem Effects of S and N Deposition
    (3) Sulfur Deposition and SOX
    (4) Nitrogen Deposition and N Oxides and PM
    b. CASAC Advice
    c. Administrator's Proposed Conclusions
    2. Comments on the Proposed Decision
    a. Sulfur Oxides
    (1) Comments Regarding Adequacy of the Existing Standard
    (2) Comments in Support of Proposed Adoption of a New Annual 
Standard
    (3) Comments in Disagreement With Proposed Adoption of a New 
Annual Standard
    (4) Comments Regarding Retaining the Existing Secondary Standard
    b. Nitrogen Oxides and Particulate Matter
    (1) Comments in Support of Proposed Decisions
    (2) Comments in Disagreement With Proposed Decisions
    3. Administrator's Conclusions
    C. Decision on the Secondary Standards
III. Interpretation of the Secondary SO2 NAAQS
    A. Background
    B. Interpretation of the Secondary SO2 Standard
IV. Ambient Air Monitoring Network for SO2
    A. Public Comments
    B. Conclusion on the Monitoring Network
V. Clean Air Act Implementation Considerations for the Revised 
Secondary SO2 Standard
    A. Designation of Areas
    B. Section 110(a)(1) and (2) Infrastructure SIP Requirements
    C. Prevention of Significant Deterioration and Nonattainment New 
Source Review Programs for the Revised Secondary SO2 
Standard
    D. Transportation Conformity Program
    E. General Conformity Program
VI. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 14094: Modernizing Regulatory Review
    B. Paperwork Reduction Act (PRA)
    C. Regulatory Flexibility Act (RFA)
    D. Unfunded Mandates Reform Act (UMRA)
    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 Risks and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use
    I. National Technology Transfer and Advancement Act (NTTAA)
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations and Executive Order 14096: Revitalizing Our Nation's 
Commitment to Environmental Justice for All
    K. Congressional Review Act (CRA)
    L. Judicial Review
VII. References

Executive Summary

    This document presents the Administrator's final decisions in the 
current review of the secondary NAAQS for SOX, N oxides, and 
PM. Specifically, this document summarizes the background and rationale 
for the Administrator's final decisions to revise the secondary 
SO2 standard to an annual average, averaged over three 
consecutive years, with a level of 10 ppb, and to retain the existing 
standards for N oxides and PM. In conducting this review of the 
secondary SOX, N oxides, and PM NAAQS, the EPA has carefully 
evaluated the currently available scientific literature on the 
ecological

[[Page 105693]]

effects of SOX, N oxides, and PM \1\ as described in the 
Integrated Science Assessment (ISA) and conducted quantitative air 
quality, deposition, and risk analyses. The Administrator's final 
decisions are based on his consideration of the characterization of the 
available scientific evidence in the ISA; quantitative and policy 
analyses presented in the Policy Assessment (PA), and related analyses; 
advice from the Clean Air Scientific Advisory Committee (CASAC); and 
public comments on the proposed decision.
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    \1\ Welfare effects of PM considered in the review of the PM 
secondary standards completed in 2020, and reconsidered more 
recently, include effects on visibility and climate and materials 
damage (88 FR 5558, January 27, 2023).
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    Sections 108 and 109 of the Clean Air Act (CAA) require the EPA to 
periodically review the air quality criteria--the science upon which 
the standards are based--and the standards themselves. Under section 
109(b)(2) of the Act, a secondary standard 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.'' As a result 
of the current review, the Administrator concluded that the current 3-
hour secondary SO2 standard is not requisite to protect the 
public welfare from any known or anticipated adverse effects associated 
with the presence of SOX in ambient air, and that it should 
be revised to an annual average SO2 standard, averaged over 
three years, with a level of 10 ppb to provide the requisite protection 
for the effects of SOX, including those related to 
atmospheric deposition of sulfur (S) compounds in sensitive ecosystems. 
The Administrator also decided to retain the secondary nitrogen dioxide 
(NO2) and PM standards, without revision. With regard to the 
secondary NO2 standard, the Administrator finds that the 
evidence related to N oxides does not call into question the adequacy 
of protection provided by the existing standard. Additionally, the 
Administrator concludes that no change to the annual secondary 
PM2.5 standard is warranted and that the existing 
PM2.5 secondary standard should be retained without 
revision.
    This document additionally includes revisions related to 
implementation of the proposed secondary SO2 annual 
standard. Specifically, the EPA is enacting revisions to the data 
handling requirements in appendix T of part 50 to include 
specifications needed for the new annual average standard. This 
document also describes the SO2 monitoring network and its 
adequacy for surveillance for the revised annual standard. Lastly, the 
document discusses implementation processes pertinent to implementation 
of the new standard.

I. Background

A. Legislative Requirements

    Two sections of the CAA govern the establishment and revision of 
the NAAQS. Section 108 (42 U.S.C. 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 
pollutants ``emissions of which, in his 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 
he ``plans to issue air quality criteria . . . .'' (42 U.S.C. 
7408(a)(1)). 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. 7408(a)(2).
    Section 109 of the Act (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 [42 U.S.C. 
7409(a)]. Under section 109(b)(2), a secondary standard 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.'' \2\
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    \2\ Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on 
welfare include, but are not limited to, ``effects on soils, water, 
crops, vegetation, manmade materials, animals, wildlife, weather, 
visibility, and climate, damage to and deterioration of property, 
and hazards to transportation, as well as effects on economic values 
and on personal comfort and well-being.''
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    In setting primary and secondary standards that are ``requisite'' 
to protect public health and welfare, respectively, as provided in 
section 109(b), the EPA's task is to establish standards that are 
neither more nor less stringent than necessary. In so doing, the EPA 
may not consider the costs of implementing the standards. See 
generally, Whitman v. American Trucking Ass'ns, 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 1176, 1185 [D.C. Cir. 1981]). However, courts have 
clarified that in deciding how to revise the NAAQS in the context of 
considering standard levels within the range of reasonable values 
supported by the air quality criteria and judgments of the 
Administrator, EPA may consider ``relative proximity to peak background 
. . . concentrations'' as a factor (American Trucking Ass'ns, v. EPA, 
283 F.3d 355, 379 [D.C. Cir. 2002]).
    Section 109(d)(1) of the Act requires periodic review and, if 
appropriate, revision of existing air quality criteria to reflect 
advances in scientific knowledge on the effects of the pollutant on 
public health and welfare. Under the same provision, the EPA is also to 
periodically review and, if appropriate, revise the NAAQS, based on the 
revised air quality criteria.\3\
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    \3\ This section of the Act requires the Administrator to 
complete these reviews and make any revisions that may be 
appropriate ``at five-year intervals.''
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    Section 109(d)(2) addresses the appointment and advisory functions 
of an independent scientific review committee. Section 109(d)(2)(A) 
requires the Administrator to appoint this committee, which is to be 
composed of ``seven members including at least one member of the 
National Academy of Sciences, one physician, and one person 
representing State air pollution control agencies.'' Section 
109(d)(2)(B) provides that the 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 1980s, this independent review function has been performed by 
the CASAC of the EPA's Science Advisory Board.
    Section 109(b)(2) specifies that ``[a]ny national secondary ambient 
air quality standard prescribed under subsection (a) shall 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 such air pollutant in the 
ambient air.'' Consistent with this statutory direction, EPA has always 
understood the goal of the

[[Page 105694]]

NAAQS is to identify a requisite level of air quality, and the means of 
achieving a specific level of air quality is to set a standard 
expressed as a concentration of a pollutant in the air, such as in 
terms of parts per million (ppm), ppb, or micrograms per cubic meter 
([mu]g/m\3\). Thus, while deposition-related effects are included 
within the ``adverse effects associated with the presence of such air 
pollutant in the ambient air,'' EPA has never found a standard that 
quantifies atmospheric deposition onto surfaces to constitute a 
national secondary ambient air quality standard. Rather, EPA has 
established ambient air quality standards that specify air quality by 
quantifying pollution in the ambient air to address effects of such 
pollution, whether from ambient concentrations or deposition.

B. Related Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once the EPA has 
established them. Under CAA sections 110 and part D, subparts 1, 5, and 
6 for nitrogen and sulfur oxides, and subparts 1, 4, and 6 for PM, and 
related provisions and regulations, States are to submit, for the EPA's 
approval, State implementation plans (SIPs) that provide for the 
attainment and maintenance of such standards through control programs 
directed to sources of the pollutants involved. The States, in 
conjunction with the EPA, also administer the prevention of significant 
deterioration of air quality program that covers these pollutants. See 
42 U.S.C. 7470-7479. In addition, Federal programs provide for or 
result in nationwide reductions in emissions of N oxides, 
SOX, PM and other air pollutants under title II of the Act, 
42 U.S.C. 7521-7574, which involves controls for motor vehicles, 
nonroad engines and equipment, and under the new source performance 
standards in section 111 of the Act, 42 U.S.C. 7411.

C. History of the Secondary Standards for N Oxides, SOX and PM

    Secondary NAAQS were first established for N oxides, SOX 
and PM in 1971 (36 FR 8186, April 30, 1971). Since that time, the EPA 
has periodically reviewed the air quality criteria and secondary 
standards for these pollutants, with the most recent reviews that 
considered the evidence for ecological effects of these pollutants 
being completed in 2012 and 2013 (77 FR 20218, April 3, 2012; 78 FR 
3086, January 15, 2013). The subsections below summarize key 
proceedings from the initial standard setting in 1971 to the last 
reviews in 2012-2013.\4\
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    \4\ Since the late 1970s, each review of the air quality 
criteria and standards has generally involved the development of an 
Air Quality Criteria Document or ISA and a Staff Paper or staff 
Policy Assessment, which is often accompanied by or includes a 
quantitative exposure or risk assessment, prior to the regulatory 
decision-making phase.
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1. N Oxides
    The EPA first promulgated NAAQS for N oxides in April 1971 after 
reviewing the relevant science on the public health and welfare effects 
in the 1971 Air Quality Criteria for Nitrogen Oxides (air quality 
criteria document or AQCD).\5\ With regard to welfare effects, the 1971 
AQCD described effects of NO2 on vegetation and corrosion of 
electrical components linked to particulate nitrate (U.S. EPA, 1971). 
The primary and secondary standards were both set at 0.053 ppm 
NO2 as an annual average (36 FR 8186, April 30, 1971). In 
1982, the EPA published an updated AQCD (U.S. EPA, 1982a). Based on the 
1982 AQCD, the EPA proposed to retain the existing standards in 
February 1984 (49 FR 6866, February 23, 1984). After considering public 
comments, the EPA published the final decision to retain these 
standards in June 1985 (50 FR 25532, June 19, 1985).
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    \5\ In reviews initiated prior to 2007, the AQCD provided the 
scientific foundation (i.e., the air quality criteria) for the 
NAAQS. Since that time, the ISA has replaced the AQCD.
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    The EPA began a second review of the primary and secondary 
standards for oxides of nitrogen in 1987 (52 FR 27580, July 22, 1987). 
In November 1991, the EPA released an updated draft AQCD for CASAC and 
public review and comment (56 FR 59285, November 25, 1991). 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 
provided ``an adequate basis'' for EPA's decision-making in the review 
(Wolff, 1993). The final AQCD was released later in 1993 (U.S. EPA, 
1993). Based on the 1993 AQCD, the EPA's Office of Air Quality Planning 
and Standards (OAQPS) prepared a Staff Paper,\6\ drafts of which were 
reviewed by the CASAC (Wolff, 1995; U.S. EPA, 1995a). In October 1995, 
the EPA proposed not to revise the secondary NO2 NAAQS (60 
FR 52874; October 11, 1995). After consideration of the comments 
received on the proposal, the Administrator finalized the decision not 
to revise the NO2 NAAQS (61 FR 52852; October 8, 1996). The 
subsequent (and most recent) review of the N oxides secondary standard 
was a joint review with the secondary standard for SOX, 
which was completed in 2012 (see subsection 4 below).
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    \6\ Prior to reviews initiated in 2007, the Staff Paper 
summarized and integrated key studies and the scientific evidence, 
and from the 1990s onward, it also assessed potential exposures and 
associated risk. The Staff Paper also presented the EPA staff's 
considerations and conclusions regarding the adequacy of existing 
NAAQS and, when appropriate, the potential alternative standards 
that could be supported by the evidence and information. More recent 
reviews present this information in the Policy Assessment.
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2. SOX
    The EPA first promulgated secondary NAAQS for SOX in 
April 1971 based on the scientific evidence evaluated in the 1969 AQCD 
(U.S. DHEW, 1969a [1969 AQCD]; 36 FR 8186, April 30, 1971). These 
standards, which were established on the basis of evidence of adverse 
effects on vegetation, included an annual arithmetic mean standard, set 
at 0.02 ppm SO2,\7\ and a 3-hour average standard set at 0.5 
ppm SO2, not to be exceeded more than once per year. In 
1973, based on information indicating there to be insufficient data to 
support the finding of a study in the 1969 AQCD concerning vegetation 
injury associated with SO2 exposure over the growing season, 
rather than from short-term peak concentrations, the EPA proposed to 
revoke the annual mean secondary standard (38 FR 11355, May 7, 1973). 
Based on consideration of public comments and external scientific 
review, the EPA released a revised chapter of the AQCD and published 
its final decision to revoke the annual mean secondary standard (U.S. 
EPA, 1973; 38 FR 25678, September 14, 1973). At that time, the EPA 
additionally noted that injury to vegetation was the only type of 
SO2 welfare effect for which the evidence base supported a 
quantitative relationship, stating that although data were not 
available at that time to establish a quantitative relationship between 
SO2 concentrations and other public welfare effects, 
including effects on materials, visibility, soils, and water, the 
SO2 primary standards and the 3-hour secondary standard may 
to some extent mitigate such effects. The EPA also stated it was not 
clear that any such effects, if occurring below the current standards, 
were adverse to the public welfare (38 FR 25679, September 14, 1973).
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    \7\ Established with the annual standard as a guide to be used 
in assessing implementation plans to achieve the annual standard was 
a maximum 24-hour average concentration not to be exceeded more than 
once per year (36 FR 8187, April 30, 1971).
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    In 1979, the EPA announced initiation of a concurrent review of the 
air quality criteria for SOX and PM and plans for 
development of a combined AQCD for these pollutants (44 FR 56730, 
October

[[Page 105695]]

2, 1979). The EPA subsequently released three drafts of a combined AQCD 
for CASAC review and public comment. In these reviews, and in guidance 
provided at the August 20-22, 1980, public meeting of the CASAC on the 
first draft AQCD, the CASAC concluded that acidic deposition was a 
topic of extreme scientific complexity because of the difficulty in 
establishing firm quantitative relationships among emissions of 
relevant pollutants, formation of acidic wet and dry deposition 
products, and effects on terrestrial and aquatic ecosystems (53 FR 
14935, April 26, 1988). The CASAC also noted that a fundamental problem 
of addressing acid deposition in a criteria document is that acid 
deposition is produced by several different criteria pollutants: 
SOX, N oxides, and the fine particulate fraction of 
suspended particles (U.S. EPA, 1982b, pp. 125-126). The CASAC also 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 (53 FR 14936, 
April 26, 1988; Lippman, 1987). For these reasons, CASAC recommended 
that, in addition to including a summary discussion of acid deposition 
in the final AQCD, 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.
    Following CASAC closure on the AQCD for SOX in December 
1981, the EPA released a final AQCD (U.S. EPA, 1982b), and the EPA's 
OAQPS prepared a Staff Paper that was released in November 1982 (U.S. 
EPA, 1982c). The issue of acidic deposition was not, however, assessed 
directly in the OAQPS Staff Paper because the EPA followed the guidance 
given by the CASAC, subsequently preparing 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 14935-36, April 
26, 1988). Although these documents were not considered criteria 
documents and had not undergone CASAC review, they represented the most 
comprehensive summary of scientific information relevant to acid 
deposition completed by the EPA at that point.
    In April 1988, the EPA proposed not to revise the existing 
secondary standards for SOX (53 FR 14926, April 26, 1988). 
The proposed decision reflected 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 (53 FR 14926, April 26, 1988). This review of the secondary 
standard for SOX was concluded in 1993, subsequent to the 
CAA Amendments of 1990 (see section I.C.3.) with the decision not to 
revise the secondary standard. The EPA concluded that revisions to the 
standard to address acidic deposition and related SOX 
welfare effects were not appropriate at that time (58 FR 21351, April 
21, 1993). In describing the decision, the EPA recognized the 
significant reductions in SO2 emissions, ambient air 
SO2 concentrations, and ultimately deposition expected to 
result from implementation of the title IV program, which was expected 
to significantly decrease the acidification of water bodies and damage 
to forest ecosystems and to permit much of the existing damage to be 
reversed with time (58 FR 21357, April 21, 1993). While recognizing 
that further action might be needed to address acidic deposition in the 
longer term, the EPA judged it prudent to await the results of the 
studies and research programs then underway, including those assessing 
the comparative merits of secondary standards, acidic deposition 
standards and other approaches to controlling acidic deposition and 
related effects, and then to determine whether additional control 
measures should be adopted or recommended to Congress (58 FR 21358, 
April 21, 1993).
3. Related Actions Addressing Acid Deposition
    In 1980, Congress created the National Acid Precipitation 
Assessment Program. During the 10-year course of this program, the 
program issued a series of reports, including a final report in 1990 
(NAPAP, 1991). On November 15, 1990, Amendments to the CAA were passed 
by Congress and signed into law by the President. In title IV of these 
Amendments, Congress included a statement of findings including the 
following:

(1) the presence of acidic 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; . . . (3) the problem of acid deposition is of national and 
international significance; . . . (5) current and future generations 
of Americans will be adversely affected by delaying measures to 
remedy the problem[.]

    The goal of title IV was to reduce emissions of SO2 by 
10 million tons and N oxides emissions by 2 million tons from 1980 
emission levels in order to achieve reductions over broad geographic 
regions/areas. In envisioning that further action might be necessary in 
the long term, Congress included section 404 of the 1990 Amendments. 
This section requires 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'' and at the conclusion of the study, submit a report to 
Congress. Five years later, the EPA submitted to Congress its report 
titled Acid Deposition Standard Feasibility Study: Report to Congress 
(U.S. EPA, 1995b) in fulfillment of this requirement. The Report to 
Congress concluded that establishing acid deposition standards for S 
and N deposition might 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.
    The 1990 Amendments also added new language to sections of the CAA 
pertaining to ecosystem effects of criteria pollutants, such as acid 
deposition. For example, a new section 108(g) was inserted, stating 
that ``[t]he Administrator may assess the risks to ecosystems from 
exposure to criteria air pollutants (as identified by the Administrator 
in the Administrator's sole discretion).'' The definition of welfare in 
CAA section 302(h) was expanded to indicate that welfare effects 
include those listed therein, ``whether caused by transformation, 
conversion, or combination with other air pollutants.'' Additionally, 
in response to legislative initiatives such as the 1990 Amendments, the 
EPA and other Federal agencies continued research on the causes and 
effects of acidic deposition and related welfare effects of 
SO2 and implemented an enhanced monitoring program to track 
progress (58 FR 21357, April 21, 1993).
4. Most Recent Review of the Secondary Standards for N Oxides and 
SOX
    In December 2005, the EPA initiated a joint review \8\ of the air 
quality criteria

[[Page 105696]]

and secondary NAAQS for oxides of nitrogen and sulfur (70 FR 73236, 
December 9, 2005). The review focused on the evaluation of the 
protection provided by the standards for two general types of effects: 
(1) direct effects on vegetation of exposure to gaseous oxides of 
nitrogen and sulfur, which are the type of effects that the existing 
standards were developed to protect against, and (2) effects associated 
with the deposition of N oxides and SOX to sensitive aquatic 
and terrestrial ecosystems (77 FR 20218, April 3, 2012).
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    \8\ Although the EPA has historically reviewed separately the 
secondary standards for oxides of nitrogen and oxides of sulfur, the 
EPA conducted a joint review of these standards in recognition of 
the chemical interactions in the atmosphere and associated 
contributions to acid deposition and related environmental effects. 
The joint review was also responsive to a National Research Council 
recommendation that the EPA consider pollutants in combination, as 
appropriate, in considering the NAAQS (NRC, 2004).
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    The Integrated Review Plan (IRP) for the review was released in 
December 2007, after review of a draft IRP by the public and CASAC (72 
FR 57570, October 10, 2007; Russell, 2007; U.S. EPA, 2007). The first 
and second drafts of the ISA were released in December 2007 and August 
2008, respectively, for the CASAC and public review (72 FR 72719, 
December 21, 2007; 73 FR 10243, February 26, 2008; Russell and 
Henderson, 2008; 73 FR 46908, August 12, 2008; 73 FR 53242, September 
15, 2008; Russell and Samet, 2008a). The EPA released a final ISA 
(referred to as 2008 ISA below) in December 2008 (73 FR 75716, December 
12, 2008; U.S. EPA, 2008a). Based on the scientific information in the 
ISA, the EPA planned and developed a quantitative Risk and Exposure 
Assessment (REA),\9\ two drafts of which were made available for public 
comment and reviewed by the CASAC (73 FR 10243, February 26, 2008; 73 
FR 50965, August 29, 2008; Russell and Samet, 2008b; 73 FR 53242, 
September 15, 2008; 74 FR 28698, June 17, 2009; Russell and Samet, 
2009). The final REA was released in September 2009 (U.S. EPA, 2009a; 
74 FR 48543; September 23, 2009).
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    \9\ The REAs for NAAQS reviews may be presented in appendices to 
the PA or in stand-alone documents (e.g., U.S. EPA 2020b, 2020c, and 
PA for current review [U.S. EPA, 2024]).
---------------------------------------------------------------------------

    Drawing on the information in the REA and ISA, the EPA OAQPS 
prepared a PA, two drafts of which were made available for public 
comment and review by the CASAC (75 FR 10479, March 8, 2010; 75 FR 
11877, March 12, 2010; Russell and Samet, 2010b; 75 FR 57463, September 
21, 2010; 75 FR 65480, October 25, 2010; Russell and Samet, 2010a). The 
final PA was released in January 2011 (U.S. EPA, 2011). For the purpose 
of protection against the direct effects on vegetation of exposure to 
gaseous oxides of nitrogen and sulfur, the final PA concluded that 
consideration should be given to retaining the current standards. With 
respect to the effects associated with the deposition of oxides of 
nitrogen and oxides of sulfur to sensitive aquatic and terrestrial 
ecosystems, the 2011 PA focused on the acidifying effects of nitrogen 
and sulfur deposition on sensitive aquatic ecosystems. Based on the 
information in the ISA, the assessments in the REA, and the CASAC 
advice, the 2011 PA concluded that consideration should be given to a 
new multipollutant standard intended to address deposition-related 
effects (details provided in section II.A.1.b. below). Based on 
consideration of the final PA, the CASAC provided additional advice and 
recommendations on the multipollutant, deposition-based standard 
described in the 2011 PA (76 FR 4109, January 24, 2011; 76 FR 16768, 
March 25, 2011; Russell and Samet, 2011).
    On August 1, 2011, the EPA published a proposed decision to retain 
the existing annual average NO2 and 3-hour average 
SO2 secondary standards, recognizing the protection they 
provided from direct effects on vegetation (76 FR 46084, August 1, 
2011). Further, after considering the multipollutant approach to 
establishing secondary standards that was described in the 2011 PA, the 
Administrator proposed not to set such a new multipollutant secondary 
standard in light of a number of uncertainties. Alternatively, the 
Administrator proposed to revise the secondary standards by adopting 
secondary NO2 and SO2 standards identical to the 
1-hour primary NO2 and SO2 standards, both of 
which were set in 2010, noting that these new primary standards, while 
not set based on consideration of atmospheric deposition,\10\ were 
likely to reduce oxides of nitrogen and sulfur emissions and associated 
nitrogen and sulfur deposition in sensitive ecosystems (76 FR 46084, 
August 1, 2011). After consideration of public comments, the EPA 
decided to retain the existing standards (without revision) to address 
the direct effects on vegetation of exposure to gaseous oxides of 
nitrogen and sulfur. At that time, the EPA also described its decision 
that it was not appropriate to set new secondary standards at that time 
to address deposition-related effects associated with oxides of 
nitrogen and sulfur (77 FR 20218, April 3, 2012).
---------------------------------------------------------------------------

    \10\ The 1-hour primary standards set in 2010 were a 
NO2 standard of 100 ppb, as the 98th percentile of 1-hour 
daily maximum concentrations, averaged over three years, and a 
SO2 standard of 75 ppb, as the 99th percentile of daily 
maximum 1-hour concentrations, averaged over three years (75 FR 
6474, February 9, 2010; 75 FR 35520, June 22, 2010).
---------------------------------------------------------------------------

    The EPA's 2012 decision was challenged by the Center for Biological 
Diversity and other environmental groups, who argued that the EPA, 
having decided that the existing standards were not adequate to protect 
against adverse public welfare effects such as damage to sensitive 
ecosystems, was required to identify the requisite level of protection 
for the public welfare and to issue NAAQS to achieve and maintain that 
level of protection. The District of Columbia Circuit (D.C. Circuit) 
disagreed, finding that the EPA acted appropriately in not setting a 
secondary standard given EPA's conclusions that ``the available 
information was insufficient to permit a reasoned judgment about 
whether any proposed standard would be `requisite to protect the public 
welfare . . . '.'' \11\ In reaching this decision, the court noted that 
the EPA had ``explained in great detail'' the profound uncertainties 
associated with setting a secondary NAAQS to protect against aquatic 
acidification.\12\
---------------------------------------------------------------------------

    \11\ Center for Biological Diversity, et al. v. EPA, 749 F.3d 
1079, 1087 (2014).
    \12\ Id. at 1088.
---------------------------------------------------------------------------

5. PM
    The EPA first established a secondary standard for PM in 1971 (36 
FR 8186, April 30, 1971), based on the original AQCD, which described 
the evidence as to effects of PM on visibility, materials, light 
absorption, and vegetation (U.S. DHEW, 1969b). To provide protection 
generally from visibility effects and materials damage, the secondary 
standard was set at 150 [micro]g/m\3\, as a 24-hour average, from total 
suspended particles (TSP), not to be exceeded more than once per year 
(36 FR 8187; April 30, 1971).\13\
---------------------------------------------------------------------------

    \13\ Additionally, a guide to be used in assessing 
implementation plans to achieve the 24-hour standard was set at 60 
[micro]g/m\3\, as an annual geometric mean (36 FR 8187; April 30, 
1971).
---------------------------------------------------------------------------

    In October 1979, the EPA announced the first review of the air 
quality criteria and NAAQS for PM (44 FR 56730, October 2, 1979). A 
combined AQCD for PM and SOX was released in 1982, after 
CASAC and public review of drafts (U.S. EPA, 1982b). Soon after, the 
OAQPS released a Staff Paper (U.S. EPA, 1982d), two drafts of which had 
received public and CASAC review (Friedlander, 1982). In 1984, the EPA 
proposed replacing the secondary standard with an annual TSP standard 
with a level within the range of 70-90 [mu]g/m\3\, as an expected 
annual arithmetic

[[Page 105697]]

mean (49 FR 10408, March 20, 1984). After consideration of public 
comment and review by the CASAC and the public, the OAQPS released an 
Addendum to the Staff Paper in 1986 (Lippman, 1986; U.S. EPA, 1986). In 
1987, the EPA completed the review by adopting two new primary PM NAAQS 
and setting the secondary standards identical to the primary standards 
in all respects, all with a new indicator for PM (particles with a 
nominal mass median diameter of 10 microns, PM10). The new 
primary and secondary standards included (1) a 24-hour standard of 150 
[mu]g/m\3\, in terms of one expected exceedance per year, on average 
over three years and (2) an annual secondary standard of 50 [mu]g/m\3\, 
as an annual arithmetic mean, averaged over three years (52 FR 24634, 
July 1, 1987).
    In April 1994, the EPA initiated the second periodic review of the 
air quality criteria and NAAQS for PM. In developing the AQCD, the 
Agency made available three external review drafts for public and CASAC 
review; the final AQCD was released in 1996 (U.S. EPA, 1996). The OAQPS 
released a Staff Paper in November 1997, after CASAC and public review 
of two drafts (U.S. EPA, 1996; Wolff, 1996). The EPA proposed revisions 
to the PM standards in 1996 and promulgated final standards in 1997 (61 
FR 65738; December 13, 1996; 62 FR 38652, July 18, 1997). With the 1997 
decision, the EPA added new standards, using particles with a nominal 
mean aerodynamic diameter less than or equal to 2.5 [mu]m 
(PM2.5) as the indicator for fine particles. The new 
secondary PM2.5 standards were set equal to the primary 
PM2.5 standards, in all respects, as follows: (1) an annual 
standard with a level of 15.0 [mu]g/m\3\, based on the 3-year average 
of annual arithmetic mean PM2.5 concentrations from single 
or multiple community-oriented monitors,\14\ and (2) a 24-hour standard 
with a level of 65 [mu]g/m\3\, based on the 3-year average of the 98th 
percentile of 24-hour PM2.5 concentrations at each monitor 
within an area. The EPA also retained the primary and secondary annual 
PM10 standards, without revision, and revised the form of 
the 24-hour primary and secondary PM10 standards to be based 
on the 99th percentile of 24-hour PM10 concentrations at 
each monitor in an area.
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    \14\ The 1997 annual PM2.5 standard was compared with 
measurements made at the community-oriented monitoring site 
recording the highest concentration or, if specific constraints were 
met, measurements from multiple community-oriented monitoring sites 
could be averaged (i.e., ``spatial averaging''). In the last review 
(completed in 2012) the EPA replaced the term ``community-oriented'' 
monitor with the term ``area-wide'' monitor. Area-wide monitors are 
those sited at the neighborhood scale or larger, as well as those 
monitors sited at micro- or middle-scales that are representative of 
many such locations in the same core-based statistical area (CBSA) 
(78 FR 3236, January 15, 2013).
---------------------------------------------------------------------------

    Following promulgation of the 1997 PM NAAQS, several parties filed 
petitions for review, raising a broad range of issues. In May 1999, the 
U.S. Court of Appeals for the D.C. Circuit upheld the EPA's decision to 
establish fine particle (PM2.5) standards, (American 
Trucking Ass'ns, Inc. v. EPA, 175 F. 3d 1027, 1055-56 [D.C. Cir. 
1999]). The D.C. Circuit also found ``ample support'' for the EPA's 
decision to regulate coarse particle (PM10) pollution but 
vacated the 1997 PM10 standards, concluding that the EPA had 
not provided a reasonable explanation justifying use of PM10 
as an indicator for coarse particles (id. at 1054-55). Pursuant to the 
D.C. Circuit's decision, the EPA removed the vacated the 1997 
PM10 standards, leaving the pre-existing 1987 
PM10 standards in place (65 FR 80776, December 22, 2000). 
The D.C. Circuit also upheld the EPA's determination not to establish 
more stringent secondary standards for fine particles to address 
effects on visibility (id. at 1027). The D.C. Circuit also addressed 
more general issues related to the NAAQS, including issues related to 
the consideration of costs in setting NAAQS and the EPA's approach to 
establishing the levels of NAAQS.
    In October 1997, the EPA initiated the third periodic review of the 
air quality criteria and NAAQS for PM (62 FR 55201, October 23, 1997). 
The EPA released the final AQCD in October 2004, after the CASAC and 
public review of several drafts (U.S. EPA, 2004a, b). The OAQPS 
released a Staff Paper in December 2005 (U.S. EPA, 2005). Also in 
December 2005, the EPA proposed to revise the PM NAAQS and solicited 
public comment on a broad range of options (71 FR 2620, January 17, 
2006). In September 2006, after consideration of public comment, the 
EPA revised the PM NAAQS, making revisions to the secondary standards 
identical to those for the primary standards, with the decision 
describing the protection provided specifically for visibility and non-
visibility related welfare effects (71 FR 61144, 61203-61210, October 
17, 2006). The EPA revised the level of the 24-hour PM2.5 
standards to 35 [mu]g/m\3\, retained the level of the annual 
PM2.5 standards at 15.0 [mu]g/m\3\, and revised the form of 
the annual PM2.5 standards by narrowing the constraints on 
the optional use of spatial averaging. For PM10, the EPA 
revoked the annual standards and retained the 24-hour standards, both 
with a level of 150 [mu]g/m\3\.
    Several parties filed petitions for review of the 2006 p.m. NAAQS 
decision, with one raising the issue of the secondary PM2.5 
standards being identical to the primary standards. On February 24, 
2009, the D.C. Circuit issued its opinion in American Farm Bureau 
Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009), remanding the 
standards to the EPA stating the Agency had failed to adequately 
explain how setting the secondary standards identical to the primary 
standards provided the required public welfare protection, including 
for visibility impairment (Id. at 528-32). The EPA responded to the 
court's remands as part of the subsequent PM NAAQS review.
    In June 2007, the EPA initiated the fourth periodic review of the 
air quality criteria and the PM NAAQS (72 FR 35462, June 28, 2007). To 
inform planning for the review, the EPA held science/policy issue 
workshops later that year (72 FR 34003, June 20, 2007; 72 FR 34005, 
June 20, 2007). Plans for the review and for welfare assessments were 
developed in 2008 and 2009; the ISA was completed in 2009, an urban-
focused visibility assessment was completed in 2010 and the PA was 
released in 2011 (U.S. EPA, 2008b; U.S. EPA, 2009b; U.S. EPA, 2009c; 
U.S. EPA, 2010; U.S. EPA, 2011). In June 2012, the EPA proposed 
revisions to the PM NAAQS and in December 2012 announced its final 
decisions to revise the primary and secondary PM2.5 annual 
standards (77 FR 38890, June 29, 2012; 78 FR 3086, January 15, 2013). 
With regard to the secondary standards, the EPA retained the 24-hour 
PM2.5 and PM10 standards, with a revision to the 
form of the 24-hour PM2.5, to eliminate the option for 
spatial averaging (78 FR 3086, January 15, 2013). Petitioners 
challenged the EPA's final rule. On judicial review, the revised 
standards and monitoring requirements were upheld in all respects 
(National Association of Manufacturers v. EPA, 750 F.3d 921, [D.C. Cir. 
2014]).
    The subsequent review of the PM secondary standards, completed in 
2020, and its subsequent reconsideration focused on consideration of 
protection provided from visibility effects, materials damage, and 
climate effects (85 FR 82684, December 18, 2020; 89 FR 16202, March 6, 
2024). Those effects--visibility effects, materials damage and climate 
effects--are not addressed in this review. The evidence for ecological 
effects of PM is addressed in the review of the air quality criteria 
and standards described in the PA for this review.

[[Page 105698]]

D. Current Review

    In August 2013, the EPA issued a call for information in the 
Federal Register for information related to the current review of the 
air quality criteria for SOX and N oxides and announced a 
public workshop to discuss policy-relevant scientific information to 
inform the review (78 FR 53452, August 29, 2013). Based in part on the 
information received in response to the call for information, the EPA 
developed a draft IRP, which was made available for consultation with 
the CASAC and for public comment (80 FR 69220, November 9, 2015). 
Comments from the CASAC and the public on the draft IRP were considered 
in preparing the final IRP (Diez Roux and Fernandez, 2016; U.S. EPA, 
2017). In developing the final IRP, the EPA expanded the review to also 
include review of the criteria and standards related to ecological 
effects of PM in recognition of atmospheric transformations and 
deposition involving the three pollutants (N oxides, SOX and 
PM) and associated ecological effects (U.S. EPA, 2017). In so doing, 
the EPA clarified that other effects of PM, including materials damage, 
climate effects and visibility effects are beyond the scope of this 
review (IRP, p. 1-2 and section 2.1).
    In March 2017, the EPA released the first external review draft of 
the Integrated Science Assessment (ISA) for Oxides of Nitrogen, Oxides 
of Sulfur, and Particulate Matter Ecological Criteria (82 FR 15702, 
March 30, 2017), which was then reviewed by the CASAC at public 
meetings in May and August 2017 (82 FR 15701, March 30, 2017; 82 FR 
35200, July 28, 2017; Diez Roux and Fernandez, 2017). A second external 
review draft ISA was released in 2018 and reviewed by the CASAC at 
public meetings in September 2018 and April 2020 (83 FR 2018; July 9, 
2018; 85 FR 16093, March 30, 2020; Cox, Kendall, and Fernandez, 
2020a).\15\ The EPA released the final ISA in October 2020 (85 FR 
66327, October 19, 2020; U.S. EPA, 2020a).
---------------------------------------------------------------------------

    \15\ A change in CASAC membership contributed to an extended 
time period between the two public meetings.
---------------------------------------------------------------------------

    In 2023, the draft PA, including the REA for aquatic acidification 
as an appendix,\16\ was released for review by the CASAC and for public 
comment (88 FR 34852, May 31, 2023). The CASAC conducted its review at 
public meetings in June and September 2023 and conveyed its advice to 
the Administrator on the standards and comments on the draft PA in late 
September 2023 (88 FR 17572, March 23, 2023; 88 FR 45414, July 17, 
2023; Sheppard, 2023). In January 2024, the EPA released the final PA 
(89 FR 2223, January 12, 2024; U.S. EPA, 2024). In April 2024, the EPA 
proposed to revise the secondary SO2 standard and retain the 
secondary standards for N oxides and PM (89 FR 26620, April 15, 2024). 
During the subsequent public comment period, public comments were 
received both orally during a virtual public hearing on May 8, 2024 (89 
FR 26114, April 15, 2024) and in writing to the docket (as discussed in 
section II.B.2. below).\17\ Significant comments received are addressed 
in this preamble to this final action and in the accompanying Response 
to Comments document, which can be found in the docket for this review. 
The schedule for completion of this review has been governed by a 
consent decree that requires the EPA to sign for publication a notice 
of final rulemaking concerning review of the NAAQS for N oxides, 
SOX and PM no later than December 10, 2024 (Center for 
Biological Diversity v. Regan [No. 4:22-cv-02285-HSG (N.D. Cal.)]).
---------------------------------------------------------------------------

    \16\ The planning document for quantitative aquatic 
acidification exposure/risk analyses was also made available for 
public comment and consultation with the CASAC (83 FR 31755, July 9, 
2018; Cox, Kendall, and Fernandez, 2020b; U.S. EPA, 2018; 83 FR 
42497, August 22, 2018).
    \17\ The public hearing transcript and any written testimony 
provided are also in the docket.
---------------------------------------------------------------------------

    Materials upon which the decision in this review is based, 
including the documents described above, are available to the public in 
the docket for this review.\18\ The EPA is basing its decision in this 
review on studies and related information included in the air quality 
criteria, which have undergone CASAC and public review. The studies 
assessed in the ISA and PA, and the integration of the scientific 
evidence presented in them, have undergone extensive critical review by 
the EPA, the CASAC, and the public. The rigor of that review makes 
these studies, and their integrative assessment, the most reliable 
source of scientific information on which to base decisions on the 
NAAQS, decisions that all recognize to be of great import. Decisions on 
the NAAQS can have profound impacts on public health and welfare, and 
NAAQS decisions should be based on studies that have been rigorously 
assessed in an integrated manner not only by the EPA but also by the 
statutorily mandated independent scientific advisory committee, as well 
as the public review that accompanies this process.
---------------------------------------------------------------------------

    \18\ The docket for this review, Docket ID No. EPA-HQ-OAR-2014-
0128, has incorporated the ISA docket (Docket ID No. EPA-HQ-ORD-
2013-0620) by reference. Both are publicly accessible at https://www.regulations.gov.
---------------------------------------------------------------------------

    Some commenters have referred to and discussed individual 
scientific studies on the welfare effects of SOX, N oxides, 
and PM that were not included in the ISA (``new'' studies) and that 
have not gone through this comprehensive review process. In considering 
and responding to comments for which such ``new'' studies were cited in 
support, the EPA has provisionally considered the cited studies in the 
context of the findings of the ISA (Weaver, 2024). The EPA's 
provisional consideration of these studies did not and could not 
provide the kind of in-depth critical review described above, but 
rather was focused on determining whether they warranted reopening the 
review of the air quality criteria to enable the EPA, the CASAC and the 
public to consider them further as part of this review. This approach, 
and the decision to rely on studies and related information included in 
the air quality criteria, which have undergone CASAC and public review, 
is consistent with the EPA's practice in prior NAAQS reviews and its 
interpretation of the requirements of the CAA. Since the 1970 
amendments, the EPA has taken the view that NAAQS decisions are to be 
based on scientific studies and related information that have been 
assessed as a part of the pertinent air quality criteria, and the EPA 
has consistently followed this approach. This longstanding 
interpretation was strengthened by new legislative requirements enacted 
in 1977, which added section 109(d)(2) of the Act concerning CASAC 
review of air quality criteria. See 71 FR 61144, 61148 (October 17, 
2006, final decision on review of NAAQS for particulate matter) for a 
detailed discussion of this issue and the EPA's past practice.
    As discussed in the EPA's 1993 decision not to revise the ozone 
(O3) NAAQS, ``new'' studies may sometimes be of such 
significance that it is appropriate to delay a decision in a NAAQS 
review and to supplement the pertinent air quality criteria so the 
studies can be taken into account (58 FR at 13013-13014, March 9, 
1993). In the present case, the EPA's consideration of ``new'' studies 
concludes that, taken in context, the ``new'' information and findings 
do not materially change any of the broad scientific conclusions made 
in the air quality criteria regarding the health and welfare effects of 
the subject pollutants in ambient air. For this reason, reopening the 
air quality criteria review is not warranted. Accordingly, the EPA is 
basing the final decisions in this review on the studies and related 
information included in the air quality

[[Page 105699]]

criteria that have undergone rigorous review by the EPA, the CASAC, and 
the public. The EPA will consider these ``new'' studies for inclusion 
in the air quality criteria for the next review, which will provide the 
opportunity to fully assess these studies through a more rigorous 
review process involving the EPA, the CASAC, and the public.

II. Rationale for Decisions

    This section presents the rationale for the Administrator's 
decisions in the review of the secondary NAAQS for the ecological 
effects of SOX, N oxides and PM. This rationale is based on 
a thorough review of the full evidence base, including the scientific 
information available since the last reviews of the secondary standards 
for N oxides, SOX and PM. This information on ecological 
effects associated with SOX, N oxides and PM and pertaining 
to their presence in ambient air, which includes studies generally 
published between January 2008 and May 2017 (and considered in the 
ISA), is integrated with the information and conclusions from previous 
assessments and presented in the ISA (ISA, section IS.1.2).\19\ The 
Administrator's rationale also takes into account: (1) the PA 
evaluation of the policy-relevant information in the ISA and 
presentation of quantitative analyses of air quality, exposure and 
aquatic acidification risks; (2) CASAC advice and recommendations, as 
reflected in discussions of drafts of the ISA and PA at public meetings 
and in the CASAC's letters to the Administrator; (3) public comments 
received during the development of these documents; and (4) public 
comments received on the proposed decisions.
---------------------------------------------------------------------------

    \19\ In addition to the review's opening ``Call for 
Information'' (78 FR 53452, August 29, 2013), multiple search 
methodologies were applied to identify relevant scientific findings 
that have emerged since the 2008 ISA. Search techniques for the 
current ISA identified and evaluated studies and reports that have 
undergone scientific peer review and were published or accepted for 
publication between January 2008 (providing some overlap with the 
cutoff date for the 2008 ISA) and May 2017. Studies published after 
the literature cutoff date for this ISA were also considered in the 
ISA if they were submitted in response to the Call for Information 
or identified in subsequent phases of ISA development, particularly 
to the extent that they provide new information that affects key 
scientific conclusions. References that are cited in the ISA, the 
references that were considered for inclusion but not cited, and 
electronic links to bibliographic information and abstracts can be 
found at: https://hero.epa.gov/hero/index.cfm/project/page/project_id/2965 (ISA, section IS.1.2).
---------------------------------------------------------------------------

    Before presenting the rationale for the Administrator's final 
decisions and their foundations, section II.A.1. provides an 
introduction that also summarizes the basis for the existing standards 
(section II.A.1.a.), provides background on the prior review of 
deposition-related effects of N oxides and SOX (section 
II.A.1.b.), and summarizes the general approach in this review (section 
II.A.1.c.). Section II.A.2. provides an overview of the air quality 
information and analyses relating S and N deposition to concentrations 
of SOX, N oxides and PM. Section II.A.3. provides an 
overview of the currently available ecological effects evidence as 
summarized in the ISA, focusing on consideration of key policy-relevant 
aspects, and section II.A.4. provides an overview of the exposure and 
risk information for this review, drawing on the quantitative analyses 
of aquatic acidification risk, presented in the PA. Section II.B.1. 
provides a summary of the Administrator's proposed decisions (section 
II.B.1.c.), which drew on both evidence-based and exposure/risk-based 
considerations from the PA (section II.B.1.a.) and advice from the 
CASAC (section II.B.1.b.). Section II.B.2. discusses comments received 
on the proposed decision, and section II.B.3. presents the 
Administrator's conclusions and associated rationale. The final 
decisions are summarized in section II.C.

A. Introduction

    The Agency's approach in its review of secondary standards is 
consistent with the requirements of the provisions of the CAA related 
to the review of NAAQS and with how the EPA and the courts have 
historically interpreted the CAA. These provisions require the 
Administrator to establish secondary standards that, in the 
Administrator's judgment, are requisite (i.e., neither more nor less 
stringent than necessary) to protect the public welfare from known or 
anticipated adverse effects associated with the presence of the 
pollutant in the ambient air. In so doing, the Administrator considers 
advice from the CASAC and public comment. This approach is based on a 
recognition that the available welfare effects evidence generally 
reflects a range of effects that include ambient air-related exposure 
circumstances for which scientists generally agree that effects are 
likely to occur as well as lower levels at which the likelihood and 
magnitude of response become increasingly uncertain. The CAA does not 
require that standards be set at a zero-risk level, but rather at a 
level that reduces risk sufficiently to protect the public welfare from 
known or anticipated adverse effects.
    The Agency's decisions on the adequacy of the current secondary 
standards and, as appropriate, on any potential alternative standards 
considered in a review, are largely public welfare policy judgments 
made by the Administrator based on the Administrator's informed 
assessment of what constitutes requisite protection against adverse 
effects to the public welfare. A public welfare policy decision draws 
upon scientific information and analyses about welfare effects, 
exposures 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 may be 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 decisions on 
secondary standards, the Administrator seeks to establish standards 
that are neither more nor less stringent than necessary for this 
purpose. In evaluating the public welfare protection afforded by the 
standards, the four basic elements of the NAAQS (indicator, averaging 
time, level, and form) are considered collectively.\20\
---------------------------------------------------------------------------

    \20\ The indicator defines the chemical species or mixture to be 
measured in the ambient air for the purpose of determining whether 
an area attains the standard. The averaging time defines the period 
over which air quality measurements are to be averaged or otherwise 
analyzed. The form of a standard defines the air quality statistic 
that is to be compared to the level of the standard in determining 
whether an area attains the standard. For example, the form of the 
annual NAAQS for fine particulate matter (PM2.5) is the 
average of annual mean concentrations for three consecutive years, 
while the form of the 3-hour secondary NAAQS for SO2 is 
the second highest 3-hour average in a year. The level of the 
standard defines the air quality concentration used for that 
purpose.
---------------------------------------------------------------------------

    Generally, conclusions reached by the Administrator in secondary 
NAAQS reviews on the amount of public welfare protection from the 
presence of the pollutant(s) in ambient air that is appropriate to be 
afforded by a secondary standard take into account a number of 
considerations. Among these considerations are the nature and degree of 
effects of the pollutant, including the Administrator's judgments on 
what constitutes an adverse effect to the public welfare, as well as 
the strengths and limitations of the available and relevant 
information, with its associated uncertainties. Across reviews, it is 
generally recognized that such judgments should neither overstate nor 
understate the strengths and limitations of the evidence and 
information nor the

[[Page 105700]]

appropriate inferences to be drawn as to risks to public welfare, and 
that the choice of the appropriate level of protection is a public 
welfare policy judgment entrusted to the Administrator under the CAA 
taking into account both the available evidence and associated 
uncertainties (80 FR 65404-05, October 26, 2015). Thus, the 
Administrator's final decisions in such reviews draw upon the 
scientific information and analyses about welfare effects, 
environmental exposures and risks, and associated public welfare 
significance, as well as judgments about how to consider the range and 
magnitude of uncertainties that are inherent in the scientific evidence 
and quantitative analyses.
1. Background
    Ecological effects of N oxides, SOX and PM include those 
related to direct contact of the airborne pollutants with plants and 
those related to atmospheric deposition of N- and S-containing 
compounds into sensitive ecosystems. As summarized in section II.A.1.a. 
below, it is the former category of effects (from direct contact) that 
were considered in establishing the existing standards, with those 
effects as the basis for the secondary standards for N oxides and 
SOX. In the last review of those standards, deposition-
related effects were also considered. However, as summarized in section 
II.A.1.b. below, the extent of the uncertainties associated with the 
complex methodology investigated for defining a deposition-based 
standard in that review were found to be so significant that the 
Administrator concluded that the limitations and uncertainties in the 
available information were too great to support establishment of a new 
standard using this methodology that could be concluded to provide the 
requisite protection for such effects under the Act (77 FR 20218, April 
3, 2012). As described in the proposal for the current action, and 
generally summarized in section II.A.1.c. below, in the current review 
we have taken a different approach to considering standards that might 
be expected to provide the appropriate level of protection from 
deposition-related effects.
a. Basis for Existing Secondary Standards
    The existing 3-hour secondary SO2 standard, with its 
level of 0.5 ppm, and the annual secondary NO2 standard, 
with its level of 0.053 ppm were established in 1971 (36 FR 8186, April 
30, 1971). The basis for both the existing SO2 and 
NO2 secondary standards is to provide protection to the 
public welfare related to direct effects on vegetation (U.S. DHEW, 
1969a; U.S. EPA, 1971). There are three secondary PM standards--
established in 1997 (annual PM2.5 standard) and 2006 (24-
hour PM2.5 and PM10 standards)--variously based 
on consideration of materials damage, visibility impacts, climate 
effects and ecological effects.\21\
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    \21\ As noted in section I.D. above, the 2020 review of the PM 
secondary NAAQS and its reconsideration focused on visibility 
effects, materials damage and climate effects, while the ecological 
effects of PM are being addressed in this combined review (89 FR 
16205, March 6, 2024).
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    The welfare effects evidence for SOX in previous reviews 
indicates a relationship between short- and long-term SO2 
exposures and foliar damage to cultivated plants, as well as reductions 
in productivity, species richness, and diversity (U.S. DHEW, 1969a; 
U.S. EPA, 1982c; U.S. EPA, 2008a). At the time the standard was set, 
concentrations of SO2 in the ambient air were also 
associated with other welfare effects, including effects on materials 
and visibility related to sulfate, a particulate transformation product 
of SO2 (U.S. DHEW, 1969a). However, the available data were 
not sufficient to establish a quantitative relationship between 
specific SO2 concentrations and such effects (38 FR 25679, 
September 14, 1973). Accordingly, direct effects of SOX in 
ambient air on vegetation are the basis for the existing secondary 
standard for SOX.
    The welfare effects evidence for N oxides in previous reviews 
includes foliar injury, leaf drop, and reduced yield of some crops 
(U.S. EPA, 1971; U.S. EPA, 1982c; U.S. EPA, 1993; U.S. EPA, 2008a). 
Since it was established in 1971, the secondary standard for N oxides 
has been reviewed three times, in 1985, 1996, and 2012 (50 FR 25532, 
June 19, 1985; 61 FR 52852; October 8, 1996; 77 FR 20218, April 3, 
2012). Although those reviews identified additional effects related to 
N deposition, they all have concluded that the existing NO2 
secondary standard provided adequate protection related to the effects 
of direct contact of airborne N oxides with vegetation on which the 
standard is based.
    In the last review of the secondary PM standards with regard to 
protection from ecological effects, completed in 2013, the EPA retained 
the 24-hour PM2.5 standard, with its level of 35 [micro]g/
m\3\, and the 24-hour PM10 standard, with its level of 150 
[micro]g/m\3\ (78 FR 3228, January 15, 2013). With regard to the annual 
PM2.5 standard, the EPA retained the averaging time and 
level, set at 15 [micro]g/m\3\, while revising the form to remove the 
option for spatial averaging consistent with this change to the primary 
annual PM2.5 standard (78 FR 3225, January 15, 2013). The 
effects considered in that review of the secondary PM standards include 
effects on visibility, materials damage, and climate effects, as well 
as ecological effects; the EPA concluded that those standards provided 
protection for ecological effects (e.g., 78 FR 3225-3226, 3228, January 
15, 2013). In reaching this conclusion, it was noted that the PA for 
the review explicitly excluded discussion of the effects associated 
with deposited PM components of N oxides and SOX and their 
transformation products, which were being addressed in the joint review 
of the secondary NO2 and SO2 NAAQS (78 FR 3202, 
January 15, 2013). The ecological effects of PM considered in the 2013 
review included direct effects on plant foliage as well as effects of 
the ecosystem loading of PM constituents such as metals or organic 
compounds (2009 ISA, section 2.5.3). For all of these effects, the 2013 
decision recognized an absence of information that would support any 
different standards and concluded the existing standards, with the 
revision to the form of the annual PM2.5 standard, provided 
the requisite protection (78 FR 3086, January 15, 2013).
b. Prior Review of Deposition-Related Effects
    In the 2012 review of the NO2 and SO2 
secondary standards, the EPA recognized that a significant increase in 
understanding of the effects of N oxides and SOX had 
occurred since the preceding secondary standards reviews for those 
pollutants (77 FR 20236, April 3, 2012). Considering the extensive 
evidence available in the 2012 review, the Agency concluded that the 
most significant risks of adverse effects of N oxides and 
SOX to the public welfare were those related to deposition 
of N and S compounds in both terrestrial and aquatic ecosystems (77 FR 
20236, April 3, 2012). Accordingly, in addition to evaluating the 
protection provided by the secondary standards for N oxides and 
SOX from effects associated with the airborne pollutants, 
the 2012 review also included extensive analyses of the welfare effects 
associated with atmospheric deposition of N and S compounds in 
sensitive aquatic and terrestrial ecosystems, described in the 2009 REA 
and 2011 PA (77 FR 20218, April 3, 2012).
    The 2009 REA assessed atmospheric deposition of N and S compounds 
and the risks it posed of two categories of ecosystem effects: 
acidification and nutrient enrichment in both terrestrial

[[Page 105701]]

and aquatic ecosystems (U.S. EPA, 2009a). In so doing, however, the 
2009 REA and 2011 PA recognized that the different types of effects 
varied in the strength of the evidence and of the information 
characterizing quantitative linkages between pollutants in ambient air 
and ecosystem responses, and in associated potential public welfare 
implications. The support in the evidence for quantitative assessment 
of aquatic acidification-related effects was strongest and the least 
uncertain.
    With regard to nutrient enrichment-related effects, despite the 
extensive evidence of deleterious effects of excessive ecosystem 
loading of nitrogen, the identification of options to provide 
protection from deposition-related effects was limited by several 
factors. These included the influence in terrestrial ecosystems of 
other air pollutants such as O3, and limiting factors such 
as moisture and other nutrients, and their potential to confound the 
characterization of the effects of changes in any one stressor, such as 
N deposition, in those systems (2011 PA, section 6.3.2). Forest 
management practices were also recognized to have the ability to 
significantly affect nitrogen cycling within a given forest ecosystem 
(2008 ISA section 3.3.2.1 and Annex C, section C.6.3). In aquatic 
systems, appreciable contributions of non-atmospheric sources to 
nutrient loading in most large waterbodies, and limitations in data and 
tools, contributed uncertainties to characterizations of incremental 
adverse impacts of atmospheric N deposition (2011 PA, section 6.3.2). 
With regard to terrestrial acidification effects, data limitations 
contributed uncertainty to identification of appropriate indicator 
reference levels, and the potential for other stressors to confound 
relationships between deposition and terrestrial acidification effects 
was recognized with regard to empirical case studies described in the 
2008 ISA.
    Based on the strong support in the evidence for the relationship 
between atmospheric deposition of acidifying N and S compounds and loss 
of acid neutralizing capacity (ANC) in sensitive ecosystems, with 
associated aquatic acidification effects, the REA analyses for this 
endpoint (aquatic acidification) received greatest emphasis in the 
review relative to other deposition-related effects. This emphasis on 
aquatic acidification-related effects of N oxides and SOX 
also reflected the advice from the CASAC. Accordingly, the 2011 PA 
focused on aquatic acidification effects in identifying policy options 
for providing public welfare protection from deposition-related effects 
of N oxides and SOX, concluding that the available 
information and assessments were only sufficient at that time to 
support development of a standard to address aquatic acidification. 
Consistent with this, the PA concluded it was appropriate to consider a 
secondary standard in the form of an aquatic acidification index (AAI) 
and identified a range of AAI values (which correspond to ANC levels) 
for consideration in establishing such a standard (2011 PA, section 
7.6.2). Conceptually, the AAI is an index that uses the results of 
ecosystem and air quality modeling to estimate waterbody ANC. The 
standard level for an AAI-based standard was conceptually envisioned to 
be a national minimum target ANC for waterbodies in the ecoregions of 
the U.S. for which data were considered adequate for these purposes 
(2011 PA, section 7.6.2).
    While the NAAQS have historically been set in terms of an ambient 
air concentration, an AAI-based standard was envisioned to have a 
single value established for the AAI, but the concentrations of 
SOX and N oxides would be specific to each ecoregion, taking 
into account variation in several factors that influence waterbody ANC, 
and consequently could vary across the U.S. The factors, specific to 
each ecoregion (``F factors''), which it was envisioned would be 
established as part of the standard, include surface water runoff rates 
and ``transference ratios.'' The latter is the term assigned to factors 
applied to deposition values (estimated to achieve the minimum 
specified ANC) to back-calculate or estimate the highest ambient air 
concentrations of SOX and N oxides that would meet the AAI-
based standard level (2011 PA, Chapter 7).\22\ The ecoregion-specific 
values for these factors would be specified based on then-available 
data and simulations of the Community Multiscale Air Quality (CMAQ) 
model and codified as part of such a standard. As part of the standard, 
these factors would be reviewed in the context of each periodic review 
of the NAAQS.
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    \22\ These were among the ecoregion-specific factors that 
comprised the parameters F1 through F4 in the AAI equation (2011 PA, 
p. 7-37). The parameter F2 represented the ecoregion-specific 
estimate of acidifying deposition associated with reduced forms of 
nitrogen, NHX (2011 PA, p. 7-28 and ES-8 to ES-9). The 
2011 PA suggested that this factor could be specified based on a 
2005 CMAQ model simulation over 12-km grid cells or might involve 
the use of monitoring data for NHX applied in dry 
deposition modeling. It was recognized that appreciable spatial 
variability, as well as overall uncertainty, were associated with 
this factor.
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    After consideration of the PA conclusions, the Administrator 
concluded that while the conceptual basis for the AAI was supported by 
the available scientific information, there were limitations in the 
available relevant data and uncertainties associated with specifying 
the elements of the AAI, specifically those based on modeled factors, 
that posed obstacles to establishing such a standard under the CAA. It 
was recognized that the general structure of an AAI-based standard 
addressed the potential for contributions to acid deposition from both 
N oxides and SOX and quantitatively described linkages 
between ambient air concentrations, deposition, and aquatic 
acidification, considering variations in factors affecting these 
linkages across the country. However, the Administrator judged that the 
limitations and uncertainties in the available information were too 
great to support establishment of a new standard that could be 
concluded to provide the requisite protection for such effects under 
the Act (77 FR 20218, April 3, 2012). These uncertainties generally 
related to the quantification of the various elements of the standard 
(the ``F factors'') and their representativeness at an ecoregion scale. 
These uncertainties and the complexities in this approach were 
recognized to be unique to the 2012 review of the NAAQS for N and S 
oxides and were concluded to preclude the characterization and degree 
of protectiveness that would be afforded by an AAI-based standard, 
within the ranges of levels and forms identified in the PA, and the 
representativeness of F factors in the AAI equation described in the 
2011 PA (77 FR 20261, April 3, 2012). As the EPA said:

``[T]he 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.'' [77 FR 20261, April 3, 2012]

    The Administrator concluded that while the existing secondary 
standards were not adequate to provide protection against potentially 
adverse deposition-

[[Page 105702]]

related effects associated with N oxides and SOX, it was not 
appropriate under section 109 of the CAA (given the uncertainties 
summarized immediately above) to set any new or additional standards at 
that time to address effects associated with deposition of N and S 
compounds on sensitive aquatic and terrestrial ecosystems (77 FR 20262-
20263, April 3, 2012). This decision was upheld upon judicial review.
c. General Approach for This Review
    As is the case for all NAAQS reviews, this secondary standards 
review uses the Agency's assessment of the current scientific evidence 
and associated quantitative analyses as a foundation to inform the 
Administrator's judgments regarding secondary standards for 
SOX, N oxides and PM that are requisite to protect the 
public welfare from known or anticipated adverse effects associated 
with that pollutant's presence in the ambient air. The approach for 
this review of the secondary SOX, N oxides, and PM standards 
builds on the last reviews of those pollutants, including the 
substantial assessments and evaluations performed over the course of 
those reviews, and considering the more recent scientific information 
and air quality data now available to inform understanding of the key 
policy-relevant issues in the current review. The EPA's assessments are 
primarily documented in the ISA and PA, both of which received CASAC 
review and public comment, as summarized in section I.D. above.
    This review of the secondary standards for SOX, N 
oxides, and PM assesses the protection provided by the standards from 
two categories of effects: direct contact effects of the airborne 
pollutants and also the effects of the associated S- and N-containing 
compounds (in gaseous and particulate form) deposited in ecosystems. In 
so doing, the review draws on the currently available evidence as 
assessed in the ISA (and prior assessments) and quantitative exposure, 
risk, and air quality information in the PA, including the REA for 
aquatic acidification.
    With regard to direct contact effects, we draw on the currently 
available evidence as assessed in the ISA, including the determinations 
regarding the causal nature of relationships between the airborne 
pollutants and ecological effects, which focus most prominently on 
vegetation, and quantitative exposure and air quality information. 
Based on this information, we consider the policy implications, most 
specifically whether the evidence supports the retention or revision of 
the current NO2 and SO2 secondary standards. With 
regard to the effects of PM, we take a similar approach, based on the 
evidence presented in the current ISA and conclusions from the review 
of the PM NAAQS concluded in 2013 (in which ecological effects were 
last considered) to assess the effectiveness of the current PM standard 
to protect against these types of impacts.
    With regard to deposition-related effects, we consider the evidence 
for the array of effects identified in the ISA (and summarized in 
section II.A.3. below), including both terrestrial and aquatic effects; 
and the limitations in the evidence and associated uncertainties as 
well as the public welfare implications of such effects. The overall 
approach takes into account the nature of the welfare effects and the 
exposure conditions associated with effects in identifying S and N 
deposition levels appropriate to consider in the context of public 
welfare protection. To identify and evaluate metrics relevant to air 
quality standards (and their elements), we have assessed relationships 
developed from air quality measurements near pollutant sources and 
deposition estimates nearby and in downwind ecoregions. In so doing, 
the available quantitative information both on deposition and effects, 
and on ambient air concentrations and deposition, has been assessed 
with regard to the existence of linkages between SOX, N 
oxides, and PM in ambient air and deposition-related effects. These 
assessments, summarized briefly in the sections below (and in detail in 
the PA), inform judgments on the likelihood of occurrence of 
deposition-related effects under air quality that meets the existing 
standards for these pollutants or potential alternatives.
    In considering the information on atmospheric deposition and 
ecological effects, we recognize that the impacts from the dramatically 
higher deposition rates of the past century can affect how ecosystems 
and biota respond to more recent, lower deposition rates, complicating 
interpretation of impacts related to more recent, lower deposition 
levels. This complexity is illustrated by findings of studies that 
compared soil chemistry across intervals of 15 to 30 years (1984-2001 
and 1967-1997). These studies reported that although atmospheric 
deposition in the Northeast declined across those intervals, soil 
acidity increased (ISA, Appendix4, section 4.6.1). As noted in the ISA, 
``[i]n areas where N and S deposition has decreased, chemical recovery 
must first create physical and chemical conditions favorable for 
growth, survival, and reproduction'' (ISA, Appendix 4, section 4.6.1). 
Thus, the extent to which S and N compounds (once deposited) are 
retained in soil matrices (with potential effects on soil chemistry) 
influences the dynamics of the response of the various environmental 
pathways to changes in air quality, including changes in emissions, 
ambient air concentrations and associated deposition.
    The two-pronged approach applied in the PA for deposition-related 
effects includes the consideration of deposition levels that may be 
associated with ecological effects of potential concern and 
consideration of relationships between ambient air concentrations and 
levels of deposition. In considering the ecological effects evidence, 
the focus is on effects for which the evidence is most robust with 
regard to established quantitative relationships between deposition and 
ecosystem effects. Such quantitative information for terrestrial 
ecosystems is derived primarily from analysis of the evidence presented 
in the ISA. For aquatic ecosystems, the primary focus has been given to 
effects related to aquatic acidification, for which we have conducted 
quantitative risk and exposure analyses based on available modeling 
applications that relate acid deposition and acid buffering capability 
in U.S. waterbodies, as summarized in section II.A.4. below (PA, 
section 5.1 and Appendix 5A). Regarding the second prong of the 
approach, we employed several different types of analyses to inform an 
understanding of relationships between ambient air concentrations near 
pollutant sources in terms of metrics relevant to air quality standards 
(and their elements) and ecosystem deposition estimates (as described 
in section II.A.2. below). Interpretation of findings from these 
analyses, in combination with the identified deposition levels of 
interest, and related policy judgments regarding limitations and 
associated uncertainties of the underlying information, informed the 
Administrator's proposed conclusions on the extent to which existing 
standards, or potential alternative standards, might be expected to 
provide protection from these levels and inform the Administrator's 
final decisions in this review, as discussed in section II.B.3. below.
    In summary, the approach to evaluating the standards with regard to 
protection from ecological effects related to ecosystem deposition of N 
and S compounds in this review involves multiple components: (1) review 
of the scientific evidence to identify the ecological effects 
associated with the three pollutants, those related

[[Page 105703]]

both to direct pollutant contact and to ecosystem deposition; (2) 
assessment of the evidence and characterization of the REA results to 
identify deposition levels related to categories of ecosystem effects; 
and (3) analysis of relationships between ambient air concentrations of 
the pollutants and deposition of N and S compounds to understand 
aspects of these relationships that can inform judgments on ambient air 
standards that protect against air concentrations associated with 
direct effects and against deposition associated with deposition-
related effects that are judged adverse to the public welfare. As 
discussed in the PA and the proposal, however, relating ambient air 
concentrations of N oxides and PM to deposition of N compounds is 
particularly complex because N deposition also results from an 
additional air pollutant that is not controlled by NAAQS for N oxides 
and PM. Thus, separate from the evaluation of secondary standards for 
SOX, the evaluation for N oxides and PM also considers 
current information (e.g., spatial and temporal trends) related to the 
additional air pollutant, ammonia (NH3), that contributes to 
N deposition and also related to PM components that do not contribute 
to N deposition. Evaluation of all of this information, together, is 
considered by the Administrator in reaching his decision, as summarized 
in section II.B.3. below.
2. Overview of Air Quality and Deposition
    The three criteria pollutants that are the focus of this review 
(SOX, N oxides, and PM) include both gases and particles. 
Both their physical state and chemical properties, as well as other 
factors, influence their deposition as N- or S-containing compounds. 
The complex pathway from pollutant and precursor emissions (section 
II.A.2.a.) to ambient air concentrations (section II.A.2.b.) and to 
eventual deposition (section II.A.2.c.) varies by pollutant and is 
influenced by a series of atmospheric processes and chemical 
transformations that occur at multiple spatial and temporal scales 
(ISA, Appendix 2; PA, Chapters 2 and 6).
    A complication in the consideration of the influence of these 
criteria pollutants on N deposition and associated ecological effects 
is posed by the contribution of other, non-criteria, pollutants in 
ambient air, specifically NH3. Although emissions of N 
oxides have appreciably declined, NH3 emissions have risen. 
Together, these co-occurring trends have reduced the influence of N 
oxides on total N deposition (PA, sections 6.2.1, 6.4.2 and 7.2.3.3). 
Geographic variability and temporal changes in the percentage of PM 
composed of N- (and S-) containing compounds, are other factors 
affecting decisions in this review.
a. Sources, Emissions and Atmospheric Processes Affecting 
SOX, N Oxides and PM
    Sulfur dioxide is generally present at higher concentrations in the 
ambient air than the other gaseous and highly reactive SOX 
(ISA, Appendix 2, section 2.1) and, as a result, SO2 is the 
indicator for the existing NAAQS for SOX. The main 
anthropogenic source of SO2 emissions is fossil fuel 
combustion (PA, section 2.2.2). Based on the 2020 National Emissions 
Inventory (NEI), the top three emission sources of SO2 in 
the U.S. are coal-fired electricity generating units (48% of total), 
industrial processes (27%), and other stationary source fuel combustion 
(9%).
    Once emitted to the atmosphere, SO2 can either remain as 
SO2 in the gas phase and be transported and/or be dry 
deposited, or it can be oxidized to form sulfate particles 
(SO42-), with modeling studies suggesting that 
oxidation accounts for more than half of SO2 removal 
nationally (PA, section 2.1.1). The rate of SO2 oxidation 
accelerates with greater availability of oxidants, which are generally 
depleted near source stacks. Consequently, oxidization to 
SO42- generally occurs in cleaner air downwind of 
SOX sources (2008 ISA, section 2.6.3.1). As 
SO42- particles are generally within the fine 
particle size range, they are a component of PM2.5 and have 
an atmospheric lifetime ranging from 2 to 10 days (PA, section 2.1.1). 
The areas of highest SO2 and SO42- 
deposition are generally near or downwind of SOX emissions 
sources, with most S deposition occurring in the eastern U.S. (PA, 
section 2.5.3). Geographic variation in precipitation also influences 
the spatial distribution of S wet deposition. In sum, both 
SO2, and the SO42- particles converted 
from SO2, contribute to S deposition, and do so over 
different time and geographic scales, with dry deposition of 
SO2 typically occurring near the source, and wet deposition 
of sulfate particles distributing more regionally.
    The term N oxides refers to all forms of oxidized nitrogen 
compounds, including NO, NO2, nitric acid (HNO3), 
and particulate nitrate (NO3-). Most N oxides 
enter the atmosphere as either NO or NO2, which are 
collectively referred to as NOX (PA, section 2.1.2). 
Anthropogenic sources account for the majority of NOX 
emissions in the U.S., per 2020 NEI estimates, with highway vehicles 
(26% of total), stationary fuel combustion including electric 
generating units (25%), and non-road mobile sources (19%) identified as 
the largest contributors to total emissions (PA, section 2.2.1). Once 
emitted into the atmosphere, NOX can deposit to the surface 
or be chemically converted to other gaseous N oxides, including 
HNO3, as well as to particulate NO3-, 
which may occur in either the fine or coarse particle size range, such 
that not all particulate NO3- is a component of 
PM2.5. In general, gas phase N oxides tend to have shorter 
atmospheric lifetimes, either dry depositing (e.g., as HNO3) 
or quickly converting to particulate NO3-, which 
has a similar atmospheric lifetime as particulate 
SO42- and is generally removed by precipitation 
in wet deposition.
    In addition to N oxides, there is another category of nitrogen 
pollutants, referred to as reduced nitrogen, which also contributes to 
nitrogen deposition. The most common form of reduced N emitted into the 
air is NH3 gas (PA, sections 2.1.3 and 2.2.3), which is not 
a criteria pollutant. The main sources of NH3 emissions 
include livestock waste (49% of total in 2020 NEI), fertilizer 
application (33%) and aggregate fires (11%). Ammonia tends to dry 
deposit near sources, with a fraction of what is emitted being 
converted to particle form, as ammonium (NH4\+\), which can 
be transported away from sources and is most efficiently removed by 
precipitation (PA, section 2.1.3).
    Particulate matter is both emitted to the atmosphere and formed in 
the atmosphere from precursor chemical gases, such as N Oxides, 
SOX and NH3. Accordingly, PM2.5 
contributing to S and N deposition generally results from chemicals 
formed in the atmosphere after being emitted (e.g., particulate 
SO42-, particulate NO3-, 
NH4+). The majority of PM2.5 mass in 
recent periods (e.g., 2019-2021) is composed of materials that do not 
contribute to S and N deposition (PA, section 2.4.3 and 6.4.2). For 
example, at PM2.5 monitoring sites across the U.S., 
SO42- generally comprises no more than about a 
third of PM2.5 mass (in eastern sites), with much lower 
percentages at monitoring sites in much of the West and South (PA 
Figure 2-30 and section 2.4.3). Similarly, nitrogen-containing species 
are also a minority of PM2.5 mass, representing less than 
about 30% and down to about 5% or lower in some areas of South (PA, 
sections 2.4.3 and 6.4.2).
b. Recent Trends in Emissions, Concentrations, and Deposition
    Emissions of SOX, oxides of N, and PM have declined 
dramatically over the past two decades, continuing a longer-

[[Page 105704]]

term trend (PA, section 2.2). Total SO2 emissions nationwide 
declined by 87% between 2002 and 2022, including reductions of 91% in 
emissions from electricity generating units and 96% in emissions from 
mobile sources. Total anthropogenic NOX emissions also 
trended downward from 2002 to 2022 by 70% nationwide, driven in part by 
large reductions in emissions from highway vehicles (84%) and 
stationary fuel combustion (68%) (PA, section 2.2.1). In contrast with 
these declining 20-year trends in NOX and SOX 
emissions, the annual rate of NH3 emissions increased by 
over 20 percent nationwide between 2002 and 2022 (PA, section 2.2.3). 
The two largest contributors are emissions from livestock waste and 
fertilizer application, which have increased by 11% and 44%, 
respectively. These trends in NOX and NH3 
emissions have had ramifications for N deposition patterns across the 
U.S., as described further below.
    The large reductions in SOX and NOX emissions 
have resulted in substantially lower ambient air concentrations in 
recent years relative to the past. This is true for both 3-hour and 1-
hour average concentrations. With regard to 3-hour SO2 
concentrations, 2021 design values for the existing 3-hour standard at 
all State and Local Air Monitoring Stations (SLAMS) with valid design 
values (n= 333) \23\ are less than the level of the existing secondary 
standard (500 ppb) \24\ and more than 75 percent of the sites have 
design values below 20 ppb (PA, section 2.4.2). This reflects a 
downward trend since 2000, with the median design value declining from 
about 50 ppb to less than 10 ppb in 2021 (PA, Figure 2-27).
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    \23\ A design value is a statistic that summarizes the air 
quality data for a given area in terms of the indicator, averaging 
time, and form of the standard. Design values can be compared to the 
level of the standard and are typically used to designate areas as 
meeting or not meeting the standard and assess progress towards 
meeting the NAAQS. Design values are computed and published annually 
by EPA (https://www.epa.gov/air-trends/air-quality-designvalues).
    \24\ The existing secondary standard for SO2 is 0.5 
ppm (500 ppb), as a 3-hour average, not to be exceeded more than 
once per year.
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    Similarly, design values for the primary SO2 standard 
(annual 99th percentile of daily maximum 1-hour average concentrations, 
averaged over 3 years) have also declined. In the mid-1990s, the median 
value of all sites with valid 1-hour design values often exceeded 75 
ppb (PA, Figure 2-26). Since then, the entire distribution of design 
values (including source-oriented sites) has continued to decline such 
that the median design value for the 1-hour primary standard across the 
network of sites is now between 5 and 10 ppb (PA, Figure 2-26). Annual 
average SO2 concentrations have also declined over this 
period. Additionally, both peak and mean SO2 concentrations 
are higher at source-oriented sites than monitoring locations that are 
not source-oriented.\25\
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    \25\ In the 2019-2021 period, the maximum design value for the 
primary SO2 standard was 376 ppb at a monitoring site 
near an industrial park in southeast Missouri. It is important to 
note that peak and mean SO2 concentrations are higher at 
source-oriented sites than monitoring locations that are not source-
oriented. Additionally, it is not uncommon for there to be high 
SO2 values in areas with recurring volcanic eruptions, 
such as in Hawaii (PA, section 2.4.2).
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    Regarding NO2, design values for the secondary standard 
(annual averages) at all 399 sites with valid design values in 2021 are 
below the 53 ppb level of the existing standard,\26\ and 98% of sites 
have design values below 20 ppb. In 2021, the maximum design value was 
30 ppb,\27\ and the median was 7 ppb, reflecting a downward trend since 
2000 when the median annual design value was 15 ppb.
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    \26\ Sites in the contiguous U.S. have met the existing 
NO2 secondary standard since around 1991 (PA, Figure 2-
22).
    \27\ The maximum annual average NO2 concentrations 
has been at, slightly above, or slightly below 30 ppb since about 
2008, with the highest 3-year average value just above 30 ppb (PA, 
Figures 2-22 and 7-9).
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    Likewise, the median of the annual average PM2.5 
concentrations also decreased substantially from 2000 to 2021, from 
12.8 [mu]g/m\3\ to 8 [mu]g/m\3\. The median of the annual 98th 
percentile 24-hour PM2.5 concentrations at the more than 
1000 sites monitored also decreased, from 32 [mu]g/m\3\ in 2000 to 21 
[mu]g/m\3\ in 2021. Although both the annual average and 98th 
percentile 24-hour PM2.5 concentrations decreased steadily 
from the early 2000s until 2016, these values have fluctuated in recent 
years due to large-scale wildfire events (PA, section 2.4.3; U.S. EPA, 
2023, Figures 23 and 24).
    The changes in emissions and associated concentrations since 2000 
have also contributed to appreciable changes in N and S deposition 
nationwide (PA, sections 2.5.3 and 6.2.1). For S compounds, the 
dramatic reduction in SOX emissions (87% nationwide) 
resulted in concordant reductions in S deposition, 68% on average 
across U.S. (PA, section 6.2.1). This decline is observed across the 
contiguous U.S. (CONUS), with the largest reductions in regions 
downwind of large sources such as electricity generating units. For N 
deposition, the impact of the appreciable reduction in N oxides 
emissions has been offset by deposition arising from increasing 
emissions of reduced forms of nitrogen over the same timeframe.
c. Relationships Between Concentrations and Deposition
    As the NAAQS are set in terms of pollutant concentrations, analyses 
in the PA evaluated relationships between criteria pollutant 
concentrations in ambient air and ecosystem deposition across the U.S. 
These relationships were evaluated over a range of conditions (e.g., 
pollutant, region, time period), and with consideration of deposition 
both near sources and at distance (allowing for pollutant transport and 
associated transformation) using five different approaches (PA, Chapter 
6 and Appendix 6A).
    First, as part of a ``real-world experiment,'' the PA analyses 
leveraged the recent downward trends in NOX and 
SOX emissions and corresponding air quality concentrations 
as well as the trends in deposition to examine the correlation between 
observed decreases in emissions and concentration and observed changes 
in deposition over the past two decades (PA, section 6.2.1). The 
deposition estimates used in these analyses (termed TDep) \28\ are 
based on a hybrid approach that involves a fusion of measured and 
modeled values, where measured values are given more weight at the 
monitoring locations and modeled data are used to fill in spatial gaps 
and provide information on chemical species that are not measured by 
routine monitoring networks (Schwede and Lear, 2014). For the second 
approach, we assessed how ambient air concentrations and associated 
deposition levels are related within the CMAQ \29\ both across the U.S. 
and then at certain Class I areas \30\ (PA, section

[[Page 105705]]

6.2.2.1) where additional monitoring data are collected as part of the 
Clean Air Status and Trends Network (CASTNET) and the Interagency 
Monitoring of Protected Visual Environments (IMPROVE) networks. As a 
third approach, we analyzed the relationships across a limited number 
of monitoring locations (in Class I areas) where both air quality data 
(CASTNET and IMPROVE) and wet deposition of S and N was measured to 
evaluate the associations between concentrations and deposition at a 
local scale (PA, section 6.2.2.2 and 6.2.2.3). The fourth approach also 
considered the associations between the two terms, at the local scale, 
but did so using a broader set of ambient air concentration 
measurements (i.e., all valid SO2, NO2, and 
PM2.5 measurements at SLAMS across the U.S.) and the hybrid 
set of TDep estimates (PA, section 6.2.3).
---------------------------------------------------------------------------

    \28\ Other than the estimates associated with the CMAQ analysis 
(second approach referenced above), the deposition estimates used in 
these analyses are those provided by the National Atmospheric 
Deposition Program, TDep Science Committee. One of the outputs of 
this effort are annual datasets of total deposition estimates in the 
contiguous U.S. (CONUS), which are referred to as the TDep datasets 
(technical updates available from NADP, 2021; ISA, Appendix 2, 
section 2.6). TDep datasets do not currently exist for areas outside 
of the CONUS.
    \29\ The CMAQ is a state of the science photochemical air 
quality model that relies on scientific first principles to simulate 
the concentration of airborne gases and particles and the deposition 
of these pollutants back to Earth's surface under user-prescribed 
scenarios. See https://www.epa.gov/cmaq for more detail.
    \30\ Areas designated as Class I include all international 
parks, national wilderness areas which exceed 5,000 acres in size, 
national memorial parks which exceed 5,000 acres in size, and 
national parks which exceed 6,000 acres in size, provided the park 
or wilderness area was in existence on August 7, 1977. Other areas 
may also be Class I if designated as Class I consistent with the 
CAA.
---------------------------------------------------------------------------

    Finally, in recognition of the fact that air quality at upwind 
locations can also influence downwind deposition, the fifth approach 
used a trajectory model (HYSPLIT--The Hybrid Single-Particle Lagrangian 
Integrated Trajectory model) to identify upwind areas where emissions 
might be expected to influence deposition at downwind ecoregions (PA, 
section 6.2.4 and Appendix 6A).\31\ Once those potential zones of 
influence were established, we evaluated the relationships between air 
quality metrics for the three pollutants \32\ at sites within those 
zones (sites of influence) and deposition estimates in the downwind 
ecoregion, as 3-year averages for five periods: 2001-2003, 2006-2008, 
2010-2012, 2014-2016 and 2018-2020. The metrics, Ecoregion Air Quality 
Metrics (EAQMs), include a weighted-average (EAQM-weighted) and a 
maximum metric (EAQM-max). The EAQM-max is the maximum concentration 
among the upwind monitoring sites identified for each downwind 
ecoregion. For the EAQM-weighted, the value of each site linked to the 
downwind ecoregion was weighted by how often the forward HYSPLIT 
trajectory crossed into the ecoregion, i.e., sites with more frequent 
trajectory intersections with the ecoregion were weighted higher (PA, 
section 6.2.4.1).
---------------------------------------------------------------------------

    \31\ Upwind sites of influence were identified for all 84 
ecoregions (level III categorization) in the contiguous U.S. 
Identification of monitoring sites linked to each downwind ecoregion 
was based on HYSPLIT modeling for a 120-hour period and focusing on 
monitoring site locations estimated to contribute at least 0.5% of 
hits to the downwind ecoregion in the trajectory modeling (PA, 
Appendix 6, section 6A.2).
    \32\ For SO2, there were two sets of metrics: one 
based on an annual average and one based on the 2nd highest 3-hour 
maximum concentration in the year. Both the NO2 and 
PM2.5 metrics are annual averages. For relating to 3-year 
average deposition, all are averaged across three years.
---------------------------------------------------------------------------

    The full set of quantitative results of the characterization of air 
quality and deposition relationships is discussed more thoroughly in 
Chapter 6 and Appendix 6A of the PA. The evaluation of measured air 
quality concentrations (SO2, NO2, and 
PM2.5) and TDep estimates of deposition at all SLAMS 
(generally composed of sites that use either a Federal Reference Method 
[FRM] or a Federal Equivalence Method [FEM]) is a robust analysis 
(i.e., large number of monitors distributed across the U.S.) and 
relevant given that compliance with the current standards (both primary 
and secondary) is judged using design value metrics based on 
measurements at the current SO2, NO2 and 
PM2.5 monitors. As with any assessment, there are 
uncertainties and limitations, as discussed in the PA (PA, sections 6.3 
and 6.4). For example, the SLAMS analyses are site-based comparisons 
that do not account for deposition associated with the transport of 
pollutants emitted some distance upwind. Similarly, the other analyses 
have their own limitations ranging from model uncertainty to 
limitations in geographical scope. In combination, these analyses 
supported the PA conclusion of a strong association between 
SO2 and S deposition. The results and associated information 
for N oxides and PM, however, indicate more variable relationships, 
both between NO2 concentrations and N deposition, and 
between PM2.5 concentrations with either S or N deposition.
    For SO2, annual monitored SO2 concentrations, 
at existing monitors within the SLAMS network, averaged over 3 years at 
the national scale were highly correlated with S deposition estimates 
in the TDep dataset at the local scale (correlation coefficient of 
0.70),\33\ especially in the earlier periods of the record and across 
the eastern U.S. (PA, section 6.2.3). This association is also seen in 
the relationships between SO2 annual values at the 
identified upwind sites of influence and S deposition estimates from 
TDep in downwind ecoregions, especially in those locations where the 
annual average SO2 concentrations are greater than 5 ppb 
(PA, section 6.2.4.2). Finally, we note that the observed declines in 
national levels of S deposition over the past two decades have occurred 
during a period in which emissions of SO2 have also declined 
sharply (PA, sections 6.2.1 and 6.4.1).
---------------------------------------------------------------------------

    \33\ The correlation coefficients reported here, from the PA, 
are based on Spearman's rank correlation coefficient. These 
nonparametric coefficients are generally used with data that are not 
normally distributed to assess how well the relationship between two 
variables can be described via a monotonic function. The term ``r 
value'' is sometimes used as shorthand for this correlation 
coefficient. Higher values indicate that the two variables are 
highly associated with one another (can range from 1.0 to -1.0).
---------------------------------------------------------------------------

    Analyses in the PA also investigated relationships between S 
deposition and air quality metrics other than the current indicator 
species (SO2) in a limited number of circumstances at 
relatively remote sites, generally distant from emissions sources. For 
example, an evaluation of the associations of total S TDep estimates 
with SO42- concentrations and of wet S deposition 
with the sum of SO2 + SO42- at 27 
sites in 27 Class I areas concluded that the correlations for S 
deposition with particulate SO42- and total S 
(i.e., SO2 + SO42-) were lower than 
what was exhibited for S deposition and SO2 concentrations 
at the SLAMS (PA, section 6.2.2). The analyses also found poor 
correlation (correlation coefficient of 0.33) between total S 
deposition estimates (TDep) and PM2.5 mass at IMPROVE sites 
in the 27 Class I areas (PA, sections 2.3.3 and 6.2.2.3). While this 
set of analyses is based on data at a relatively limited number of 
sites (e.g., compared to the SLAMS network), the results do not 
indicate advantages to PM2.5 mass, particulate 
SO42-, or total S (SO42- 
plus SO2) over SO2 (alone) as an indicator for a 
secondary NAAQS to address S deposition-related effects.
    Both NO2 and certain components of PM2.5 
(NO3- and NH4+) contribute 
to N deposition. As is the case for SO2 and S deposition, 
there are multiple pathways for N deposition (dry and wet) and multiple 
scales of N deposition (local and regional). However, there are some 
additional complications to relating ambient air concentrations of 
NO2 and PM2.5 mass to N deposition. First, not 
all N deposition is caused by these pollutants (PA, Chapter 2 and 
section 6.1.1). Ammonia, which is not a criteria pollutant, also 
contributes to N deposition, especially through dry deposition at local 
scales. Second, only certain components of PM2.5 mass 
contribute to N deposition (i.e., NO3- and 
NH4\+\) and these comprise less than about 30% of 
PM2.5 mass across the U.S., below 5% in some regions (PA, 
Figure 6-56). As a result of these two factors, the associations 
between NO2 concentrations and N deposition, and between 
PM2.5 concentrations and N deposition are less robust than 
what is observed for SO2 and S deposition. The multi-faceted 
approach to evaluating these relationships confirmed this expectation. 
For example, there are

[[Page 105706]]

weaker associations of N deposition with NO2 observations at 
SLAMS across the U.S. than what is observed in the similar S deposition 
and SO2 analysis (PA, section 6.4.2). There is little 
correlation for N deposition with NO2 concentrations, as 
evidenced by a Spearman's correlation coefficient of 0.38, compared to 
0.70 for SO2 and S deposition (PA, Table 6-6 and Table 6-4). 
Further, the trajectory-based analyses of the relationships between 
NO2 annual values at the identified upwind sites of 
influence and N deposition estimates from TDep in downwind ecoregions 
indicate negative correlations (PA, Table 6-10). These negative 
correlations are observed for both the EAQM-weighed and EAQM-max 
values. This relative lack of association for NO2 
concentrations with N deposition was confirmed by national trends over 
the past 20 years, where sharp declines in NO2 emissions and 
concentrations are linked in time with sharp declines in oxidized N 
deposition (PA, Table 6-2), but not with trends in total or reduced 
atmospheric N deposition. Since 2010, NO2 concentrations 
have continued to drop while N deposition nationally has remained 
steady (PA, section 6.2.1). As for S deposition and S compound metrics, 
the PA also investigated relationships between N deposition and air 
quality metrics other than the current indicator species 
(NO2) in the 27 Class I areas where collocated data were 
available. Recognizing that such information was not available in 
other, less remote areas of the U.S., including areas where 
contributing emissions are highest or at the regulatory SLAMS monitors, 
no clear advantages of these other parameters (e.g., nitric acid, 
particulate NO3-, and NH4\+\) over 
NO2 or PM2.5 mass were indicated. Across all 
analyses, the evidence indicates NO2 to be a weak indicator 
of total atmospheric N deposition, especially in areas where 
NH3 is prevalent and where PM2.5 mass is 
dominated by species other than NO3- or 
NH4+ (PA, section 6.4.2).
3. Overview of Welfare Effects Evidence
    More than 3,000 welfare effects studies, including approximately 
2,000 studies newly available since the last review, have been 
considered in the ISA.34 35 While expanding the evidence for 
some effect categories, the studies on acid deposition, an important 
category of effects in the last review, are largely consistent with the 
evidence that was previously available. The subsections below briefly 
summarize the nature of welfare effects of S oxides, N oxides and PM 
(section II.A.3.a.), the potential public welfare implications of these 
effects (section II.A.3.b.), and exposure concentrations and 
deposition-related metrics (section II.A.3.c.).
---------------------------------------------------------------------------

    \34\ The ISA builds on evidence and conclusions from previous 
assessments, focusing on synthesizing and integrating the newly 
available evidence (ISA, section IS.1.1). Past assessments are cited 
when providing further details not repeated in newer assessments.
    \35\ The study count and citations are available on the project 
page for the ISA on the Health & Environmental Research Online 
(HERO) website (https://heronet.epa.gov/heronet/index.cfm/project/page/project_id/2965).
---------------------------------------------------------------------------

a. Nature of Effects
    The welfare effects evidence base evaluated in the current review 
includes decades of extensive research on the ecological effects of N 
oxides, SOX and PM. The sections below provide an overview 
of the nature of the direct effects of gas-phase exposure to oxides of 
nitrogen and sulfur (section II.A.3.a.(1)), acid deposition-related 
ecological effects (section II.A.3.a.(2)), N enrichment and associated 
effects (section II.A.3.a.(3)), and other effects (section 
II.A.3.a.(4)).
(1) Direct Effects of SOX and N Oxides in Ambient Air
    A well-established body of scientific evidence has shown that acute 
and chronic exposures to oxides of N and S, such as SO2, 
NO2, NO, HNO3 and peroxyacetyl nitrate (PAN) in 
the air, are associated with negative effects on vegetation. The 
scientific evidence available for these effects in 1971 is the basis 
for the current secondary NAAQS for SOX and N oxides.
    The current scientific evidence continues to be sufficient to infer 
a causal relationship between gas-phase SO2 and injury to 
vegetation (ISA, Appendix 3, section 3.6.1). High concentrations have 
been associated with damage to plant foliage (ISA, Appendix 3, section 
3.2). In addition to foliar injury, which is usually a rapid response, 
and which can vary significantly among species and growth conditions 
(which affect stomatal conductance), SO2 exposures have also 
been documented to reduce plant photosynthesis and growth. As exposures 
have declined in the U.S., some studies in the eastern U.S. have 
reported increased growth in some SO2-sensitive tree species 
(e.g., Thomas et al., 2013). Multiple factors, including reduced 
deposition, buffering and other environmental variables, may play a 
role in such species recovery. (ISA, Appendix 3, section 3.2, Schaberg 
et al., 2014). Some of this evidence seems to suggest a somewhat faster 
recovery than might be expected from deposition-related soil 
acidification alone, which may indicate a relatively greater role for 
changes in ambient air concentrations of SO2, in combination 
with changes in other gases, than was previously understood (ISA, 
Appendix 3, section 3.2 and Appendix 5, section 5.2.1.3). For lichens, 
damage from SO2 exposure has been observed to include 
reduction in metabolic functions that are vital for growth and survival 
(e.g., decreases in photosynthesis and respiration), damage to cellular 
integrity (e.g., leakage of electrolytes), and structural changes (ISA, 
Appendix 3, section 3.2).
    The current scientific evidence also continues to be sufficient to 
infer a causal relationship between gas-phase NO, NO2 and 
PAN and injury to vegetation (ISA, Appendix 3, section 3.6.2). The 
evidence base evaluated in the 1993 Air Quality Criteria Document for 
Oxides of N included evidence of phytotoxic effects of NO, 
NO2, and PAN on plants through decreasing photosynthesis and 
induction of visible foliar injury (U.S. EPA, 1993 [1993 AQCD]). The 
1993 AQCD additionally concluded that concentrations of NO, 
NO2, and PAN in the atmosphere were rarely high enough to 
have phytotoxic effects on vegetation. Little new information is 
available since that time on these phytotoxic effects at concentrations 
currently observed in the U.S. (ISA, Appendix 3, section 3.3).
    With regard to HNO3, the evidence is sufficient to infer 
a causal relationship between exposure to HNO3 and changes 
to vegetation (ISA, Appendix 3, section 3.6.3). The evidence suggests a 
role in observed declines in lichen species in the 1970s in the Los 
Angeles basin (ISA, Appendix 3, section 3.3). A 2008 resampling of 
areas shown to be impacted in the past by HNO3 found 
community shifts, declines in the most pollutant-sensitive lichen 
species, and increases in abundance of nitrogen-tolerant lichen species 
compared to 1976-1977, indicating that these lichen communities have 
not recovered and had experienced additional changes (ISA, Appendix 3, 
section 3.4). The recently available evidence on this topic also 
included a study of six lichen species that reported changes in 
physiology and functioning including decreased chlorophyll content and 
chlorophyll fluorescence, decreased photosynthesis and respiration, and 
increased electrolyte leakage from HNO3 exposures for 2-11 
weeks (daily peak levels near 50 ppb) in controlled chambers. (ISA, 
Appendix 3, section 3.4).
(2) Acid Deposition-Related Ecological Effects
    The connection between SOX and N oxide emissions to 
ambient air,

[[Page 105707]]

atmospheric deposition of S and/or N compounds, and the acidification 
of acid-sensitive soils and surface waters is well documented by many 
decades of evidence, particularly in the eastern U.S. (ISA, section 
IS.5; Appendix 8, section 8.1). Sulfur oxides and N oxides in ambient 
air undergo reactions to form acidic compounds that are removed from 
the atmosphere through deposition. Acidifying deposition can affect 
biogeochemical processes in soils, with ramifications for terrestrial 
biota and for the chemistry and biological functioning of associated 
surface waters (ISA, Appendix 7, section 7.1). These effects depend on 
the magnitude and rate of deposition, as well as multiple 
biogeochemical processes that occur in soils and waterbodies.
    Soil acidification is influenced by the deposition of inorganic 
acids (HNO3 and sulfuric acid 
[H2SO4]), NH4\+\, and by chemical and 
biological processes. When NO3-, or 
SO42- leach from soils to surface waters, an 
equivalent number of positive cations, or countercharge, are also 
transported. If the countercharge is provided by a base cation (e.g., 
calcium, [Ca\2+\], magnesium [Mg2+], sodium 
[Na+], or potassium [K+]), rather than hydrogen 
ions (H+), the leachate is neutralized, but the soil becomes 
more acidic from the hydrogen ions left behind, and the base saturation 
of the soil is reduced by the loss of the base cation. Depending on the 
relative rates of soil processes that contribute to the soil pools of 
H+ and base cations, such as weathering, continued 
SO42- or NO3- leaching can 
deplete the soil base cation pool, which contributes to increased 
acidity of the leaching soil water and by connection, the surface 
water. Accordingly, the ability of a watershed to neutralize acidic 
deposition is determined by a variety of biogeophysical factors 
including weathering rates, bedrock composition, vegetation and 
microbial processes, physical and chemical characteristics of soils, 
and hydrology (ISA Appendix 4, section 4.3).
    Recently available evidence includes some studies describing early 
stages of recovery from soil acidification in some eastern forests. For 
example, studies at the Hubbard Brook Experimental Forest in New 
Hampshire reported indications of acidification recovery in soil 
solution measurements across the period from 1984 to 2011 (ISA, 
Appendix 4, section 4.6.1; Fuss et al., 2015). Another study of 27 
sites in eastern Canada and the northeastern U.S. found reductions in 
wet deposition SO42- were associated with 
increases in soil base saturation and decreases in exchangeable 
aluminum (ISA, Appendix 4, section 4.6.1; Lawrence et al., 2015). 
Recent modeling analyses indicate extended timeframes for recovery are 
likely, as well as delays or lags related to accumulated pools of S in 
forest soils (ISA, Appendix 4, section 4.6.1).
(a) Freshwater Ecosystems
    As was the case in the last review, the body of evidence available 
in this review, including that newly available, is sufficient to infer 
a causal relationship between N and S deposition and the alteration of 
freshwater biogeochemistry (ISA, section IS.6.1). Additionally, based 
on the previously available evidence, the current body of evidence is 
also sufficient to conclude that a causal relationship exists between 
acidifying deposition and changes in biota, including physiological 
impairment and alteration of species richness, community composition, 
and biodiversity in freshwater ecosystems (ISA, section IS.6.3).
    The effects of acid deposition on aquatic systems depend largely 
upon the ability of the system to neutralize additional acidic inputs 
from the environment, whether from the atmosphere or from surface 
inputs. There is a large amount of variability among freshwater systems 
in this regard, which reflects their underlying geology as well as 
their history of acidic inputs. Accordingly, different freshwater 
systems (e.g., in different geographic regions) respond differently to 
similar amounts of acid deposition. The main factor in determining 
sensitivity is the underlying geology of an area and its ability to 
provide soil base cations through weathering to buffer acidic inputs 
(ISA, Appendix 8, section 8.5.1). As noted in the ISA, ``[g]eologic 
formations having low base cation supply, due mainly to low soil and 
bedrock weathering, generally underlie the watersheds of acid-sensitive 
lakes and streams'' (ISA, Appendix 8, p. 8-58).
    Longstanding evidence has well characterized the changes in 
biogeochemical processes and water chemistry caused by N and S 
deposition and the ramifications for biological functioning of 
freshwater ecosystems (ISA, Appendix 8, section 8.1). The more recently 
available scientific research ``reflects incremental improvements in 
scientific knowledge of aquatic biological effects and indicators of 
acidification as compared with knowledge summarized in the 2008 ISA'' 
(ISA, Appendix 8, p. 8-80). Previously and newly available studies 
``indicate that aquatic organisms in sensitive ecosystems have been 
affected by acidification at virtually all trophic levels and that 
these responses have been well characterized for several decades'' 
(ISA, Appendix 8, p. 8-80). For example, information reported in the 
previous 2008 ISA ``showed consistent and coherent evidence for effects 
on aquatic biota, especially algae, benthic invertebrates, and fish 
that are most clearly linked to chemical indicators of acidification'' 
(ISA, Appendix 8, p. 8-80). These indicators are surface water pH, base 
cation ratios, ANC, and inorganic aluminum concentration (ISA, Appendix 
8, Table 8-9).
    The effects of waterbody acidification on fish species are 
especially well documented, with many species (e.g., brown and brook 
trout and Atlantic salmon) experiencing adverse effects from 
acidification and the earliest lifestages being most sensitive (ISA, 
Appendix 8, section 8.3). Many effects of acidic surface waters on 
fish, particularly effects on gill function or structure, relate to low 
pH or the combination of low pH and elevated dissolved aluminum (ISA, 
Appendix 7, section 7.1.2.5 and Appendix 8, sections 8.3.6.1 and 
8.6.4). In general, biological effects in aquatic ecosystems are 
primarily attributable to low pH and high inorganic aluminum 
concentration (ISA, p. ES-14). Waterbody pH largely controls the 
bioavailability of aluminum, which is toxic to fish, and aluminum 
mobilization is largely confined to waters with a pH below about 5.5, 
which the ISA describes as corresponding to an ANC in the range of 
about 10 to 30 microequivalents per liter ([mu]eq/L) in waters of the 
Northeast with low to moderate levels of dissolved organic carbon (ISA, 
Appendix 7, section 7.1.2.6 and Appendix 8, section 8.6.4).
    The parameter ANC is an indicator of the buffering capacity of 
natural waters against acidification. Although ANC does not directly 
affect biota, it is an indicator of acidification that relates to pH 
and aluminum levels (ISA, p. ES-14) or to watershed characteristics 
like base cation weathering (BCw) rate (ISA, Appendix 8, sections 8.1 
and 8.3.6.3). Accordingly, ANC is commonly used to describe the 
potential sensitivity of a freshwater system to acidification-related 
effects. It can be measured in water samples and is also often 
estimated for use in water quality modeling, as is done in the aquatic 
acidification risk assessment for this review (summarized in section 
II.A.4. below). Water quality models are generally better at estimating 
ANC than at estimating other indicators of acidification-related risk, 
such as pH.

[[Page 105708]]

Acid neutralizing capacity is estimated as the molar sum of strong base 
cations minus the molar sum of strong acid anions, specifically 
including SO42- and NO3- 
(e.g., Driscoll et al., 1994). Thus, values below zero indicate a 
deficit in the ability to buffer acidic inputs, and increasing values 
above zero represent increasing buffering capability for acidic inputs 
(ISA, Appendix 7, section 7.1.2.6). In waters with high concentrations 
of naturally occurring organic acids, however, ANC may not be a good 
indicator of risk to biota as those acids can reduce bioavailability of 
aluminum, thus buffering the effects usually associated with low pH and 
high total aluminum concentrations (Waller et al., 2012; ISA, Appendix 
8, section 8.3.6.4).
    In addition to acidity of surface waters quantified over weeks or 
months, waterbodies can also experience spikes in acidity in response 
to episodic precipitation or rapid snowmelt events. In these events 
(hours-days), a surge or pulse of drainage water, containing acidic 
compounds, is routed through upper soil horizons rather than the deeper 
soil horizons that would usually provide buffering for acidic compounds 
(ISA, Appendix 7, section 7.1). While some streams and lakes may have 
chronic or base flow chemistry that provides suitable conditions for 
aquatic biota, they may experience occasional acidic episodes with the 
potential for deleterious consequences to sensitive biota (ISA, 
Appendix 8, section 8.5). For example, in some impacted northeastern 
waterbodies, ANC levels may dip below zero for hours to days or weeks 
in response to such events, while waterbodies labeled chronically 
acidic have ANC levels below zero throughout the year (ISA, Appendix 7, 
section 7.1.1.2; Driscoll et al., 2001). Headwater streams tend to be 
more sensitive to such episodes due to their smaller watersheds and, in 
the East, due to their underlying geology (ISA, Appendix 8, section 
8.5.1).
    National survey data available in the last review, and dating back 
to the early 1980s through 2004, indicated acidifying deposition had 
acidified surface waters in the southwestern Adirondacks, New England 
uplands, eastern portion of the upper Midwest, forested Mid-Atlantic 
highlands, and Mid-Atlantic coastal plain (2008 ISA, section 4.2.2.3; 
ISA, Appendix 8, section 8.5.1). For example, a 1984-1987 survey of 
waterbodies in the Adirondacks found 27% of streams to have ANC values 
below zero, with a minimum value of -134 [mu]eq/L (Sullivan et al., 
2006). Values of ANC below 20 [mu]eq/L in Shenandoah stream sites have 
been reported as having a greater risk of episodic acidification and 
associated reduced populations of sensitive species, such as the native 
brook trout, compared to sites with higher ANC (Bulger et al., 1999; 
Bulger et al., 2000). A more recent study of two groups of Adirondack 
lakes for which water quality data were available from 1982 and 1992, 
respectively, reported significant increases in ANC in the large 
majority of those lakes, with the magnitude of the increases varying 
across the lakes (Driscoll et al., 2016; ISA, Appendix 7, section 
7.1.3.1). As described in the ISA, ``[a]cidic waters were mostly 
restricted to northern New York, New England, the Appalachian Mountain 
chain, upper Midwest, and Florida'' (ISA, Appendix 8, p. 8-60). Despite 
the appreciable reductions in acidifying deposition that have occurred 
in the U.S. since the 1960s and 1970s, aquatic ecosystems across the 
U.S. are still experiencing effects from historical contributions of N 
and S (ISA, Appendix 8, section 8.6).
(b) Terrestrial Ecosystems
    Longstanding evidence, supported and strengthened by evidence newly 
available in this review, describes the changes in soil biogeochemical 
processes caused by acidifying deposition of N and S to terrestrial 
systems that are linked to changes in terrestrial biota, with 
associated impacts on ecosystem characteristics (ISA, Appendix 5, 
section 5.1). Consistent with conclusions in the last review, the 
current body of evidence is sufficient to infer a causal relationship 
between acidifying deposition and alterations of biogeochemistry in 
terrestrial ecosystems. Additionally, and consistent with conclusions 
in the last review, the current body of evidence is sufficient to infer 
a causal relationship between acidifying N and S deposition and the 
alteration of the physiology and growth of terrestrial organisms and 
the productivity of terrestrial ecosystems. The current body of 
evidence is also sufficient to conclude that a causal relationship 
exists between acidifying N and S deposition and alterations of species 
richness, community composition, and biodiversity in terrestrial 
ecosystems (2008 ISA, sections 4.2.1.1 and 4.2.1.2; 2020 ISA, Appendix 
4, section 4.1 and Appendix 5, sections 5.7.1 and 5.7.2).
    Deposition of acidifying compounds to acid-sensitive soils can 
cause soil acidification, increased mobilization of aluminum from soil 
to drainage water, and depletion of the pool of exchangeable base 
cations in the soil (ISA, Appendix 5, section 5.2 and Appendix 4, 
sections 4.3.4 and 4.3.5). Physiological effects of acidification on 
terrestrial biota include slower growth and increased mortality among 
sensitive plant species, which are generally attributable to 
physiological impairment caused by aluminum toxicity (related to 
increased availability of inorganic aluminum in soil water) and a 
reduced ability of plant roots to take up base cations (ISA, Appendix 
4, section 4.3 and Appendix 5, section 5.2).
    The physiological effects of acidifying deposition on terrestrial 
biota can also result in changes in species composition whereby 
sensitive species, such as red spruce and sugar maple, are replaced by 
more tolerant species, or the sensitive species that were dominant in 
the community become a minority. For example, increasing soil cation 
availability (as in Ca2+ addition or gradient experiments) 
has been associated with greater growth and seedling colonization by 
sugar maple, while American beech is more prevalent on soils with lower 
levels of base cations where sugar maple is less often found (ISA, 
Appendix 5, section 5.2.1.3.1; Duchesne and Ouimet, 2009). Soil acid-
base chemistry has also been found to be a predictor of understory 
species composition (ISA, Appendix 5, section 5.2.2.1), and limited 
evidence has indicated an influence of soil acid-base chemistry on 
diversity and composition of soil bacteria, fungi, and nematodes (ISA, 
Appendix 5, section 5.2.4.1). In addition to Ca2+ addition 
experiments, observational gradient studies have also evaluated 
relationships between soil chemistry indicators of acidification (e.g., 
soil pH, base cation to aluminum (Bc:Al) ratio, base saturation, and 
aluminum) and ecosystem biological endpoints, including physiological 
and community responses of trees and other vegetation, lichens, soil 
biota, and fauna (ISA, Appendix 5, Tables 5-2 and 5-6). The 2020 ISA 
also reports on several large observational studies evaluating 
statistical associations between tree growth or survival, as assessed 
at monitoring sites across the U.S., and estimates of average 
deposition of S or N compounds at those sites over time periods on the 
order of 10 years (ISA, Appendix 5, section 5.5.2 and Appendix 6, 
section.6.2.3.1; Dietze and Moorcroft, 2011; Thomas et al., 2010; Horn 
et al., 2018). Negative associations were observed for survival and 
growth in several species or species groups with S deposition metrics; 
positive and negative associations were reported with N deposition (PA, 
sections 5.3.2.3 and 5.3.4 and Appendix 5B).

[[Page 105709]]

    Although there has been no systematic national survey of U.S. 
terrestrial ecosystem soils, the forest ecosystems considered the most 
sensitive to terrestrial acidification from atmospheric deposition 
include forests of the Adirondack Mountains of New York, Green 
Mountains of Vermont, White Mountains of New Hampshire, the Allegheny 
Plateau of Pennsylvania, and mountain top and ridge forest ecosystems 
in the southern Appalachians (2008 ISA, Appendix 3, section 3.2.4.2; 
ISA, Appendix 5, section 5.3). Underlying geology is the principal 
factor governing the sensitivity of both terrestrial and aquatic 
ecosystems to acidification from S and N deposition. Geologic 
formations with low base cation supply (e.g., sandstone, quartzite), 
due mainly to low weathering rates, generally underlie these acid 
sensitive watersheds. Other factors also contribute to the overall 
sensitivity of an area to acidifying nitrogen and sulfur deposition, 
including topography, soil chemistry, land use, and hydrology (ISA, 
Appendix 5, section 5.3). For example, ``[a]cid-sensitive ecosystems 
are mostly located in upland mountainous terrain in the eastern and 
western U.S. and are underlain by bedrock that is resistant to 
weathering, such as granite or quartzite sandstone'' (ISA, Appendix 7, 
p. 7-45). Further, as well documented in the evidence, biogeochemical 
sensitivity to deposition-driven acidification (and eutrophication [see 
following section]) is the ``result of historical loading, geologic/
soil conditions (e.g., mineral weathering and S adsorption), and 
nonanthropogenic sources of N and S loading to the system'' (ISA, 
Appendix 7, p. 7-45 and section 7.1.5).
(3) Nitrogen Enrichment and Associated Ecological Effects
    Ecosystems in the U.S. vary in their sensitivity to N enrichment, 
with organisms in their natural environments commonly adapted to the 
nutrient availability in those environments. Historically, N has been 
the primary limiting nutrient for plants in many ecosystems. In such 
ecosystems, when the limiting nutrient, N, becomes more available, 
whether from atmospheric deposition, runoff, or episodic events, the 
subset of plant species able to most effectively use the higher 
nitrogen levels may out-compete other species, leading to a shift in 
the community composition that may be dominated by a smaller number of 
species, i.e., a community with lower diversity (ISA, sections 
IS.6.1.1.2, IS.6.2.1.1 and IS.7.1.1, Appendix 6, section 6.2.4 and 
Appendix 7, section 7.2.6.6). Thus, change in the availability of 
nitrogen in nitrogen-limited systems can affect growth and 
productivity, with ramifications on relative abundance of different 
species of vegetation and potentially further and broader ramifications 
on ecosystem processes, structure, and function.
    Both N oxides and reduced forms of nitrogen can contribute to N 
enrichment. In addition to atmospheric deposition, other sources of N 
compounds can play relatively greater or lesser roles in ecosystem N 
loading, depending on location. For example, many waterbodies receive 
appreciable amounts of N from agricultural runoff and municipal or 
industrial wastewater discharges. For many aquatic ecosystems, sources 
of N other than atmospheric deposition, including fertilizer and waste 
treatment, contribute more to ecosystem N than atmospheric deposition 
(ISA Appendix 7, sections 7.1 and 7.2). Additionally, the impacts of 
historic N deposition in both aquatic and terrestrial ecosystems pose 
complications to discerning the potential effects of more recent 
deposition rates.
(a) Aquatic and Wetland Ecosystems
    Nitrogen additions to freshwater, estuarine and near-coastal 
ecosystems, including N from atmospheric deposition, can contribute to 
eutrophication, which typically begins with nutrient-stimulated rapid 
algal growth developing into an algal bloom that can, depending on 
various site-specific factors, be followed by anoxic conditions 
associated with the algal die-off (ISA, ES.5.2). Decomposition of the 
plant biomass from the subsequent algal die-off contributes to reduced 
waterbody oxygen, which in turn can affect higher-trophic-level 
species, e.g., contributing to fish mortality (ISA, p. ES-18). The 
extensive body of evidence in this area is sufficient to infer causal 
relationships between N deposition and the alteration of 
biogeochemistry in freshwater, estuarine and near-coastal marine 
systems (ISA, Appendix 7, sections 7.1 and 7.2). Consistent with 
findings in the last review, the current body of evidence is also 
sufficient to infer a causal relationship between N deposition and 
changes in biota, including altered growth and productivity, species 
richness, community composition, and biodiversity due to N enrichment 
in freshwater ecosystems (ISA, Appendix 9, section 9.1). The body of 
evidence is sufficient to infer a causal relationship between N 
deposition and changes in biota, including altered growth, total 
primary production, total algal community biomass, species richness, 
community composition, and biodiversity due to N enrichment in 
estuarine environments (ISA, Appendix 10, section 10.1).
    Evidence newly available in this review provides insights regarding 
N enrichment and its impacts in several types of aquatic systems, 
including freshwater streams and lakes, estuarine and near-coastal 
systems, and wetlands. With regard to freshwaters, for example, studies 
published since the 2008 ISA augment the evidence base for high-
elevation waterbodies where the main N source is atmospheric 
deposition. Recent evidence continues to indicate that N limitation is 
common in oligotrophic waters in the western U.S., with shifts in 
nutrient limitation, from N limitation, to between N and phosphorus (P) 
limitation, or to P limitation, reported in some alpine lake studies 
(ISA, Appendix 9, section 9.1.1.3). Small inputs of N in such water 
bodies have been reported to increase nutrient availability or alter 
the balance of N and P, with the potential to stimulate growth of 
primary producers and contribute to changes in species richness, 
community composition, and diversity.
    Another type of N loading effect in other types of freshwater lakes 
includes a role in the composition of freshwater algal blooms and their 
toxicity (ISA, Appendix 9, section 9.2.6.1). Information in this 
review, including studies in Lake Erie, indicates that growth of some 
harmful algal species, including those that produce microcystin, are 
favored by increased availability of N and its availability in 
dissolved inorganic form (ISA, Appendix 9, p. 9-28; Davis et al., 2015; 
Gobler et al., 2016).
    The relative contribution of N deposition to total N loading varies 
among waterbodies. For example, atmospheric deposition is generally 
considered to be the main source of N inputs to most headwater stream, 
high-elevation lake, and low-order stream watersheds that are far from 
the influence of other N sources like agricultural runoff and 
wastewater effluent (ISA, section ES5.2). In other fresh waterbodies, 
however, agricultural practices and point source discharges have been 
estimated to be larger contributors to total N loading (ISA, Appendix 
7, section 7.1.1.1). Since the 2008 ISA, several long-term monitoring 
studies in the Appalachian Mountains, the Adirondacks, and the Rocky 
Mountains have reported temporal patterns of declines in surface water 
NO3- concentration corresponding to declines in 
atmospheric N deposition (ISA, Appendix 9, section 9.1.1.2).

[[Page 105710]]

Declines in basin wide NO3- concentrations have 
also been reported for the nontidal Potomac River watershed and have 
been attributed to declines in atmospheric N deposition (ISA, Appendix 
7, section 7.1.5.1).
    Nutrient inputs to coastal and estuarine waters are important 
influences on the health of these waterbodies. Continued inputs of N, 
the most common limiting nutrient in estuarine and coastal systems, 
have resulted in N over-enrichment and subsequent alterations to the 
nutrient balance in these systems (ISA, Appendix 10, p. 10-6). For 
example, the rate of N delivery to coastal waters is strongly 
correlated to changes in primary production and phytoplankton biomass 
(ISA, Appendix 10, section 10.1.3). Algal blooms and associated die-
offs can contribute to hypoxic conditions (most common during summer 
months), which can contribute to fish kills and associated reductions 
in marine populations (ISA, Appendix 10). Further, the prevalence and 
health of submerged aquatic vegetation (SAV), which is important 
habitat for many aquatic species, has been identified as a biological 
indicator for N enrichment in estuarine waters (ISA, Appendix 10, 
section 10.2.5). Previously available evidence indicated the role of N 
loading in SAV declines in multiple U.S. estuaries through increased 
production of macroalgae or other algae, which reduce sunlight 
penetration into shallow waters where SAV is found (ISA, Appendix 10, 
section 10.2.3). Newly available studies have reported findings of 
increased SAV populations in two tributaries of the Chesapeake Bay 
corresponding to reduction in total N loading from all sources since 
1990 (ISA, Appendix 10, section 10.2.5). The newly available studies 
also identify other factors threatening SAV, including increasing 
temperature related to climate change (ISA, Appendix 10, section 
10.2.5).
    The degree to which N enrichment and associated ecosystem impacts 
are driven by atmospheric N deposition varies greatly and is largely 
unique to the specific ecosystem. Analyses based on data across two to 
three decades extending from the 1990s through about 2010 estimate that 
most of the analyzed estuaries receive 15-40% of their N inputs from 
atmospheric sources (ISA, section ES 5.2; ISA, Appendix 7, section 
7.2.1), though for specific estuaries contributions can vary more 
widely. In areas along the West Coast, N sources may include coastal 
upwelling from oceanic waters, as well as transport from watersheds. 
Common N inputs to estuaries include those associated with freshwater 
inflows transporting N from agriculture, urban, and wastewater sources, 
in addition to atmospheric deposition across the watershed (ISA, 
section IS 2.2.2; ISA, Appendix 7, section 7.2.1).
    There are estimates of atmospheric N loading to estuaries available 
from several recent modeling studies (ISA, Table 7-9). One analysis of 
estuaries along the Atlantic Coast and the Gulf of Mexico, which 
estimated that 62-81% of N delivered to the eastern U.S coastal zone is 
anthropogenic in source, also reported that atmospheric N deposition to 
freshwater that is subsequently transported to estuaries represents 17-
21% of the total N loading into the coastal zone (McCrackin et al., 
2013; Moore et al., 2011). In the Gulf of Mexico, 26% of the N 
transported to the Gulf in the Mississippi/Atchafalaya River basin was 
estimated to be contributed from atmospheric deposition (which may 
include volatilized losses from natural, urban, and agricultural 
sources) (Robertson and Saad, 2013). Another modeling analysis 
identified atmospheric deposition to watersheds as the dominant source 
of N to the estuaries of the Connecticut, Kennebec, and Penobscot 
rivers. For the entire Northeast and mid-Atlantic coastal region, 
however, it was the third largest source (20%), following agriculture 
(37%) and sewage and population-related sources (28%) (ISA, Appendix 7, 
section 7.2.1). Estimates for West Coast estuaries indicate much 
smaller contribution from atmospheric deposition. For example, analyses 
for Yaquina Bay, Oregon, estimated direct deposition to contribute only 
0.03% of N inputs; estimated N input to the watershed from N-fixing red 
alder (Alnus rubra) trees was a much larger (8%) source (ISA, Appendix 
7, section 7.2.1; Brown and Ozretich, 2009).
    Evidence in coastal waters has recognized that nutrient enrichment 
may play a role in acidification of some coastal waters (ISA, Appendix 
10, section 10.5). More specifically, nutrient-driven algal blooms may 
contribute to ocean acidification, possibly through increased 
decomposition, which lowers dissolved oxygen levels in the water column 
and contributes to lower pH. Such nutrient-enhanced acidification can 
also be exacerbated by warming (associated with increased microbial 
respiration) and changes in buffering capacity (alkalinity) of 
freshwater inputs (ISA, Appendix 10, section 10.5).
    The impact of N additions on wetlands, and whether the wetlands may 
serve as a source, sink, or transformer of atmospherically deposited N 
varies with the type of wetland and other factors, such as physiography 
and local hydrology, as well as climate (ISA, section IS.8.1 and 
Appendix 11, section 11.1). Studies generally show N enrichment to 
decrease the ability of wetlands to retain and store N, which may 
diminish the wetland ecosystem service of improving water quality (ISA, 
section IS.8.1). Consistent with the evidence available in the last 
review, the current body of evidence is sufficient to infer a causal 
relationship between N deposition and the alteration of biogeochemical 
cycling in wetlands. Newly available evidence regarding N inputs and 
plant physiology expands the evidence base related to species 
diversity. The currently available evidence, including that newly 
available, is sufficient to infer a causal relationship between N 
deposition and the alteration of growth and productivity, species 
physiology, species richness, community composition, and biodiversity 
in wetlands (ISA, Appendix 11, section 11.10).
(b) Terrestrial Ecosystems
    It is long established that N enrichment of terrestrial ecosystems 
increases plant productivity (ISA, Appendix 6, section 6.1). Building 
on this, the currently available evidence, including evidence that is 
longstanding, is sufficient to infer a causal relationship between N 
deposition and the alteration of the physiology and growth of 
terrestrial organisms and the productivity of terrestrial ecosystems 
(ISA, Appendix 5, section 5.2 and Appendix 6, section 6.2). Responsive 
ecosystems include those that are N limited and/or contain species that 
have evolved in nutrient-poor environments. In these ecosystems the N-
enrichment changes in plant physiology and growth rates vary among 
species, with species that are adapted to low N supply being readily 
outcompeted by species that require more N. In this manner, the 
relative representation of different vegetation species may be altered, 
and some species may be eliminated altogether, such that community 
composition is changed and species diversity declines (ISA, Appendix 6, 
sections 6.3.2 and 6.3.8). The currently available evidence in this 
area is sufficient to infer a causal relationship between N deposition 
and the alteration of species richness, community composition, and 
biodiversity in terrestrial ecosystems (ISA, section IS.5.3 and 
Appendix 6, section 6.3).
    Previously available evidence described the role of N deposition in 
changing soil carbon and N pools and

[[Page 105711]]

fluxes, as well as altering plant and microbial growth and physiology 
in an array of terrestrial ecosystems (ISA, Appendix 6, section 6.2.1). 
Nitrogen availability is broadly limiting for productivity in many 
terrestrial ecosystems (ISA, Appendix 6, section 6.2.1). Accordingly, N 
additions contribute to increased productivity and can alter 
biodiversity. Eutrophication, one of the mechanisms by which increased 
productivity and changes in biodiversity associated with N addition to 
terrestrial ecosystems can occur, comprises multiple effects that 
include changes to the physiology of individual organisms, alteration 
of the relative growth and abundance of various species, transformation 
of relationships between species, and indirect effects on availability 
of essential resources other than N, such as light, water, and 
nutrients (ISA, Appendix 6, section 6.2.1).
    The currently available evidence for the terrestrial ecosystem 
effects of N enrichment, including eutrophication, includes studies in 
a wide array of systems, including forests (tropical, temperate, and 
boreal), grasslands, arid and semi-arid scrublands, and tundra (PA, 
section 4.1; ISA, Appendix 6). The organisms affected include trees, 
herbs and shrubs, and lichen, as well as fungal, microbial, and 
arthropod communities. Lichen communities, which have important roles 
in hydrologic cycling, nutrient cycling, and as sources of food and 
habitat for other species, are also affected by atmospheric N (PA, 
section 4.1; ISA, Appendix 6). The recently available studies on the 
biological effects of added N in terrestrial ecosystems include 
investigations of plant and microbial physiology, long-term ecosystem-
scale N addition experiments, regional and continental-scale monitoring 
studies, and syntheses.
    The previously available evidence included N addition studies in 
the U.S. and N deposition gradient studies in Europe that reported 
associations of N deposition with reduced species richness and altered 
community composition for grassland plants, forest understory plants, 
and mycorrhizal fungi (soil fungi that have a symbiotic relationship 
with plant roots) (ISA, Appendix 6, section 6.3). Newly available 
evidence for forest communities in this review indicates that N 
deposition alters the physiology and growth of overstory trees, and 
that N deposition has the potential to change the community composition 
of forests (ISA, Appendix 6, section 6.6). Recent studies on forest 
trees include analyses of long-term forest inventory data collected 
from across the U.S. and Europe (ISA, Appendix 6, section 6.2.3.1). The 
recent evidence also includes findings of variation in forest 
understory and non-forest plant communities with atmospheric N 
deposition gradients in the U.S. and in Europe. For example, gradient 
studies in Europe have found higher N deposition to be associated with 
forest understory plant communities with more nutrient-demanding and 
shade-tolerant plant species (ISA, Appendix 6, section 6.3.3.2). A 
recent gradient study in the U.S. found associations between herb and 
shrub species richness and N deposition, that were related to soil pH 
(ISA, Appendix 6, section 6.3.3.2).
    Recent evidence includes associations of variation in lichen 
community composition with N deposition gradients in the U.S. and 
Europe, (ISA, Appendix 6, section 6.3.7; Table 6-23). Differences in 
lichen community composition have been attributed to differences in 
atmospheric N pollution in forests of the West Coast, Rocky Mountains, 
and southeastern Alaska. Differences in epiphytic lichen growth or 
physiology have been observed along atmospheric N deposition gradients 
in the highly impacted area of southern California and in more remote 
locations such as Wyoming and southeastern Alaska (ISA, Appendix 6, 
section 6.3.7). Historical deposition may play a role in observational 
studies of N deposition effects, complicating the disentangling of 
responses that may be related to more recent N loading.
    Newly available findings from N addition experiments expand on the 
understanding of mechanisms for plant and microbial community 
composition effects of increased N availability, indicating that 
competition for resources, such as water in arid and semi-arid 
environments, may exacerbate the effects of N addition on diversity 
(ISA, Appendix 6, section 6.2.6). The newly available studies in arid 
and semiarid ecosystems, particularly in southern California have 
reported changes in plant community composition, in the context of a 
long history of significant N deposition, with fewer observations of 
plant species loss or changes in plant diversity (ISA, Appendix 6, 
section 6.3.6).
    Nitrogen limitation in grasslands and the dominance by fast-growing 
species that can shift in abundance rapidly (in contrast to forest 
trees) contribute to an increased sensitivity of grassland ecosystems 
to N inputs (ISA, Appendix 6, section 6.3.6). Studies in southern 
California coastal sage scrub communities, including studies of the 
long-term history of N deposition, which was appreciably greater in the 
past than recent rates, indicate impacts on community composition and 
species richness in these ecosystems (ISA, Appendix 6, sections 6.2.6 
and 6.3.6). The ability of atmospheric N deposition to override the 
natural spatial heterogeneity in N availability in arid ecosystems, 
such as the Mojave Desert and California coastal sage scrub ecosystems 
in southern California, makes these ecosystems sensitive to N 
deposition (ISA, Appendix 6, section 6.3.8).
    The current evidence includes relatively few studies of N 
enrichment recovery in terrestrial ecosystems. Among N addition studies 
assessing responses after cessation of additions, it has been observed 
that soil nitrate and ammonium concentrations recovered to levels 
observed in untreated controls within 1 to 3 years of the cessation of 
additions, but soil processes such as N mineralization and litter 
decomposition were slower to recover (ISA, Appendix 6, section 6.3.2; 
Stevens, 2016). A range of recovery times have been reported for 
mycorrhizal community composition and abundance from a few years in 
some systems to as long as 28 or 48 years in others (ISA, Appendix 6, 
section 6.3.2; Stevens, 2016; Emmett et al., 1998; Strengbom et al., 
2001). An N addition study in the midwestern U.S. observed that plant 
physiological processes recovered in less than 2 years, although 
grassland communities were slower to recover and still differed from 
controls 20 years after the cessation of N additions (ISA, Appendix 6, 
section 6.3.2; Isbell et al., 2013).
(4) Other Deposition-Related Effects
    Additional categories of effects for which the current evidence is 
sufficient to infer causal relationships with deposition of S or N 
compounds or PM include changes in mercury methylation processes in 
freshwater ecosystems, changes in aquatic biota due to sulfide 
phytotoxicity, and ecological effects from PM deposition other than N 
and S deposition (ISA, Table IS-1). The current evidence, including 
that newly available in this review, is sufficient to infer a causal 
relationship between S deposition and the alteration of mercury 
methylation in surface water, sediment, and soils in wetland and 
freshwater ecosystems (ISA, Table ES-1). The currently available 
evidence is also sufficient to infer a new causal relationship between 
S deposition and changes in biota due to sulfide phytotoxicity, 
including alteration of growth and productivity, species physiology, 
species richness, community composition, and

[[Page 105712]]

biodiversity in wetland and freshwater ecosystems (ISA, section IS.9).
    With regard to PM deposition other than N and S deposition, the 
currently available evidence is sufficient to infer a likely causal 
relationship between deposition of PM and a variety of effects on 
individual organisms and ecosystems (ISA, Appendix 15, section 15.1). 
Particulate matter includes a heterogeneous mixture of particles 
differing in origin, size, and chemical composition. In addition to N 
and S and their transformation products, other PM components, such as 
trace metals and organic compounds, when deposited to ecosystems, may 
affect biota. Material deposited onto leaf surfaces can alter leaf 
processes, and PM components deposited to soils and waterbodies may be 
taken up into biota, with the potential for effects on biological and 
ecosystem processes. Studies involving ambient air PM, however, have 
generally involved conditions that would not be expected to meet the 
current secondary standards for PM. Further, although in some limited 
cases, effects have been attributed to particle size (e.g., soiling of 
leaves by large coarse particles near industrial facilities or unpaved 
roads), ecological effects of PM have been largely attributed more to 
its chemical components, such as trace metals, which can be toxic in 
large amounts (ISA, Appendix 15, sections 15.2 and 15.3.1). The 
evidence largely comes from studies involving areas experiencing 
elevated concentrations of PM, such as near industrial areas or 
historically polluted cities (ISA, Appendix 15, section 15.4).
b. Public Welfare Implications
    In evaluating the public welfare implications of the evidence 
regarding S and N related welfare effects, we must consider the type, 
severity, and geographic extent of the effects. In this section, we 
discuss such factors in light of judgments and conclusions regarding 
effects on the public welfare that have been made in NAAQS reviews.
    As provided in section 109(b)(2) of the CAA, the secondary standard 
is to ``specify a level of air quality the attainment and maintenance 
of which in the judgment of the Administrator . . . is requisite to 
protect the public welfare from any known or anticipated adverse 
effects associated with the presence of such air pollutant in the 
ambient air.'' The secondary standard is not meant to protect against 
all known or anticipated welfare effects related to oxides of N and S, 
and particulate matter, but rather those that are judged to be adverse 
to the public welfare, and a bright-line determination of adversity is 
not required in judging what is ``requisite'' (78 FR 3212, January 15, 
2013; 80 FR 65376, October 26, 2015; see also 73 FR 16496, March 27, 
2008). Thus, the level of protection from known or anticipated adverse 
effects to public welfare that is requisite for the secondary standard 
is a public welfare policy judgment made by the Administrator. The 
Administrator's judgment regarding the available information and 
adequacy of protection provided by an existing standard is generally 
informed by considerations in prior reviews and associated conclusions.
    The categories of effects identified in the CAA to be included 
among welfare effects are quite diverse, and among these categories 
there are many different types of effects that vary broadly with regard 
to specificity and level of resolution. For example, effects on 
vegetation and effects on animals are categories identified in CAA 
section 302(h), and the ISA recognizes effects of N and S deposition at 
the organism, population, community, and ecosystem level, as summarized 
in section II.A.3.a. above (ISA, sections IS.5 to IS.9). As noted in 
the last review of the secondary NAAQS for NOX and 
SOX, while the CAA section 302(h) lists a number of welfare 
effects, ``these effects do not define public welfare in and of 
themselves'' (77 FR 20232, April 3, 2012).
    How important ecological impacts are to the public welfare depends 
on the type, severity and extent of the effects, as well as the 
societal use of the resource and the significance of the resource to 
the public welfare. Such factors can also be considered in the context 
of judgments and conclusions made in some prior reviews regarding 
public welfare effects. For example, in the context of secondary NAAQS 
decisions for O3, judgments regarding public welfare 
significance have given particular attention to effects in areas with 
special federal protections (such as Class I areas), and lands set 
aside by states, Tribes, and public interest groups to provide similar 
benefits to the public welfare (73 FR 16496, March 27, 2008; 80 FR 
65292, October 26, 2015).\36\ In the 2015 O3 NAAQS review, 
the EPA recognized the ``clear public interest in and value of 
maintaining these areas in a condition that does not impair their 
intended use and the fact that many of these lands contain 
O3-sensitive species'' (73 FR 16496, March 27, 2008).
---------------------------------------------------------------------------

    \36\ For example, the fundamental purpose of parks in the 
National Park System ``is to conserve the scenery, natural and 
historic objects, and wildlife in the System units and to provide 
for the enjoyment of the scenery, natural and historic objects, and 
wildlife in such manner and by such means as will leave them 
unimpaired for the enjoyment of future generations'' (54 U.S.C. 
100101). Additionally, the Wilderness Act of 1964 defines designated 
``wilderness areas'' in part as areas ``protected and managed so as 
to preserve [their] natural conditions'' and requires that these 
areas ``shall be administered for the use and enjoyment of the 
American people in such manner as will leave them unimpaired for 
future use and enjoyment as wilderness, and so as to provide for the 
protection of these areas, [and] the preservation of their 
wilderness character . . .'' (16 U.S.C. 1131 (a) and (c)). Other 
lands that benefit the public welfare include national forests which 
are managed for multiple uses including sustained yield management 
in accordance with land management plans (see 16 U.S.C. 1600(1)-(3); 
16 U.S.C. 1601(d)(1)).
---------------------------------------------------------------------------

    Judgments regarding effects on the public welfare can depend on the 
intended use, including conservation, or service (and value) of the 
affected vegetation, ecological receptors, ecosystems and resources and 
the significance of that use to the public welfare (73 FR 16496, March 
27, 2008; 80 FR 65377, October 26, 2015). Uses or services provided by 
areas that have been afforded special protection can flow in part or 
entirely from the vegetation that grows there as well as other natural 
features and resources. Ecosystem services range from those directly 
related to the natural functioning of the ecosystem to ecosystem uses 
for human recreation or profit, such as through the production of 
lumber or fuel (Costanza et al., 2017; ISA, section IS.13). The 
spatial, temporal, and social dimensions of public welfare impacts are 
also influenced by the type of service affected. For example, a 
national park can provide direct recreational services to the thousands 
of visitors that come each year but also provide an indirect value to 
the millions who may not visit but receive satisfaction from knowing it 
exists and is preserved for the future (80 FR 65377, October 26, 2015).
    In the last review of the secondary NAAQS for NOX and 
SOX, ecosystem services were discussed as a method of 
assessing the magnitude and significance to the public of resources 
affected by ambient air concentrations of oxides of nitrogen and sulfur 
and associated deposition in sensitive ecosystems (77 FR 20232, April 
3, 2012). That review recognized that although there is no specific 
definition of adversity to public welfare, one paradigm might involve 
ascribing public welfare significance to disruptions in ecosystem 
structure and function. The concept of considering the extent to which 
a pollutant effect will contribute to such disruptions has been used 
broadly by the EPA in considering effects. An evaluation of adversity 
to public welfare might also consider the

[[Page 105713]]

likelihood, type, magnitude, and spatial scale of the effect, as well 
as the potential for recovery and any uncertainties relating to these 
considerations (77 FR 20218, April 3, 2012).
    The types of effects on aquatic and terrestrial ecosystems 
discussed in section II.A.3.1. above differ with regard to aspects 
important to judging their public welfare significance. For example, in 
the case of effects on timber harvest, such judgments may consider 
aspects such as the heavy management of silviculture in the U.S., while 
judgments for other categories of effects may generally relate to 
considerations regarding natural areas, including specifically those 
areas that are not managed for harvest. Effects on tree growth and 
survival have the potential to be significant to the public welfare 
through impacts in Class I and other areas given special protection in 
their natural/existing state, although they differ in how they might be 
significant.
    In this context, it may be important to consider that S and N 
deposition-related effects, such as changes in growth and survival of 
plant and animal species, could, depending on severity, extent, and 
other factors, lead to effects on a larger scale including changes in 
overall productivity and altered community composition (ISA, section 
IS.2.2.1 and Appendices 5, 6, 8, 9, and 10). Further, effects on 
individual species could contribute to impacts on community composition 
through effects on growth and reproductive success of sensitive species 
in the community, with varying impacts to the system through many 
factors including changes to competitive interactions (ISA, section 
IS.5.2 and Appendix 6, section 6.3.2).
    In acid-impacted surface waters, acidification primarily affects 
the diversity and abundance of fish and other aquatic life and the 
ecosystem services derived from these organisms. (2011 PA, section 
4.4.5). In addition to other types of services, fresh surface waters 
support several cultural services, such as aesthetic, recreational, and 
educational services. The type of service that is likely to be most 
widely and significantly affected by aquatic acidification is 
recreational fishing. Multiple studies have documented the economic 
benefits of recreational fishing. Freshwater rivers and lakes of the 
northeastern United States, surface waters that have been most affected 
by acidification, are not a major source of commercially raised or 
caught fish; they are, however, a source of food for some recreational 
and subsistence fishers and for other consumers (2009 REA, section 
4.2.1.3). It is not known if and how consumption patterns of these 
fishers may have been affected by the historical impacts of surface 
water acidification in the affected systems. Non-use services, which 
include existence (protection and preservation with no expectation of 
direct use) and bequest values, are arguably a significant source of 
benefits from reduced acidification (Banzhaf et al., 2006). Since the 
2012 review, additional approaches and methods have been applied to 
estimate the potential effects of aquatic acidification on uses and 
services of affected aquatic ecosystems; with regard to economic 
impacts, however, ``for many regions and specific services, poorly 
characterized dose-response between deposition, ecological effect, and 
services are the greatest challenge in developing specific data on the 
economic benefits of emission reductions'' (ISA, Appendix 14, p. 14-
23).
    Nitrogen loading in aquatic ecosystems, particularly large 
estuarine and coastal water bodies, has and continues to pose risks to 
the services provided by those ecosystems, with clear implications to 
the public welfare (2011 PA, section 4.4.2; ISA, Appendix 14, section 
14.3.2). For example, the large estuaries of the eastern U.S. are an 
important source of fish and shellfish production, capable of 
supporting large stocks of resident commercial species and serving as 
breeding grounds and interim habitat for several migratory species 
(2009 REA, section 5.2.1.3). These estuaries also provide an important 
and substantial variety of cultural ecosystem services, including 
water-based recreational and aesthetic services. Additionally, as noted 
for fresh waters above, these systems have non-use benefits to the 
public (2011 PA, section 4.4.5). Studies reviewed in the ISA have 
explored both enumeration of the number of ecosystem services that may 
be affected by N loading and the pathways by which this may occur, as 
well as approaches to valuation of such impacts. A finding of one such 
analysis was that ``better quantitative relationships need to be 
established between N and the effects on ecosystems at smaller scales, 
including a better understanding of how N shortages can affect certain 
populations'' (ISA, Appendix 14, sections 14.5 and 14.6). The relative 
contribution of atmospheric deposition to total N loading varies widely 
among estuaries, however, and has declined in some areas in recent 
years (ISA, Appendix 10, section 10.10.1).
    A complication to considering the public welfare implications 
specific to N deposition in terrestrial systems is the potential for N 
to increase growth and yield of plants that, depending on the type of 
plant and its use by human populations (e.g., food for livestock or 
human populations, trees for lumber), could be judged beneficial to the 
public. Such increased growth and yield may be judged and valued 
differently than changes in growth of other species. As noted in 
section II.A.3.a. above, enrichment in natural ecosystems can, by 
increasing growth of N limited plant species, change competitive 
advantages of species in a community, with associated impacts on the 
composition of the ecosystem's plant community. The public welfare 
implications of such effects may vary depending on their severity, 
prevalence, and magnitude. Impacts on some ecosystem characteristics 
(e.g., forest or forest community composition) may be considered of 
greater public welfare significance when occurring in Class I or other 
protected areas, due to the value that the public places on such areas. 
In considering such services in past reviews for secondary standards 
for other pollutants (e.g., O3), the Agency has given 
particular attention to effects in natural ecosystems, indicating that 
a protective standard, based on consideration of effects in natural 
ecosystems in areas afforded special protection, would also ``provide a 
level of protection for other vegetation that is used by the public and 
potentially affected by O3 including timber, produce grown 
for consumption and horticultural plants used for landscaping'' (80 FR 
65403, October 26, 2015).
    Although the welfare effects evidence base describes effects 
related to ecosystem deposition of N and S compounds, the available 
information does not yet provide a framework that can specifically tie 
various magnitudes or prevalences of changes in a biological or 
ecological indicator (e.g., lichen abundance or community composition) 
\37\ to broader effects on the public welfare. The ISA finds that while 
there is an improved understanding from information available in this 
review of the number of pathways by which N and S deposition may affect 
ecosystem services, most of these relationships remain to be quantified 
(ISA, Appendix 14, section 14.6).\38\ This

[[Page 105714]]

gap creates uncertainties when considering the public welfare 
implications of some biological or geochemical responses to ecosystem 
acidification or N enrichment and accordingly complicates judgments on 
the potential for public welfare significance. That notwithstanding, 
while shifts in species abundance or composition of various ecological 
communities may not be easily judged with regard to public welfare 
significance, at some level, such changes, especially if occurring 
broadly in specially protected areas, where the public can be expected 
to place high value, might reasonably be concluded to impact the public 
welfare. An additional complexity in the current review with regard to 
assessment of effects associated with existing deposition rates is that 
the current, much-improved air quality and associated reduced 
deposition is within the context of a longer history that included 
appreciably greater deposition in the middle of the last century, the 
environmental impacts of which may remain, affecting ecosystem 
responses.
---------------------------------------------------------------------------

    \37\ As recognized in section II.A.3.a.(3)(b) above, lichen 
communities have important roles in ecosystem function, such as in 
hydrologic cycling, nutrient cycling, and as sources of food and 
habitat for other species (ISA, Appendix 6).
    \38\ While ``there is evidence that N and S emissions/deposition 
have a range of effects on U.S. ecosystem services and their social 
value'' and ``there are some economic studies that demonstrate such 
effects in broad terms,'' ``it remains methodologically difficult to 
derive economic costs and benefits associated with specific 
regulatory decisions/standards'' (ISA, Appendix 14, pp. 14-23 to 14-
24).
---------------------------------------------------------------------------

    In summary, several considerations are important to judgments on 
the public welfare significance of given welfare effects under 
different exposures. These include uncertainties and limitations that 
must be taken into account regarding the magnitude of key effects that 
might be concluded to be adverse to ecosystem health and associated 
services. Additionally, there are numerous locations vulnerable to 
public welfare impacts from S or N deposition-related effects on 
terrestrial and aquatic ecosystems and their associated services. Other 
important considerations include the exposure circumstances that may 
elicit effects and the potential for the significance of the effects to 
vary in specific situations due to differences in sensitivity of the 
exposed species, the severity and associated significance of the 
observed or predicted effect, the role that the species plays in the 
ecosystem, the intended use of the affected species and its associated 
ecosystem and services, the presence of other co-occurring predisposing 
or mitigating factors, and associated uncertainties and limitations.
c. Exposure Conditions and Deposition-Related Metrics
    The ecological effects identified in section II.A.3.a. above vary 
widely in their extent and the resolution of the available information 
that describes the exposure circumstances under which they occur. The 
information for direct effects of SOX, N oxides and PM in 
ambient air is somewhat more straight-forward to consider as it is 
generally presented in terms of concentrations in air. For deposition-
related effects, the information may be about S and N compounds in soil 
or water or may be for metrics intended to represent atmospheric 
deposition of those compounds. For the latter, as recognized in section 
II.A.1.c. above, we face the challenge of relating that information to 
patterns of ambient air concentrations.
    With regard to the more complex consideration of deposition-related 
effects such as ecosystem acidification and N enrichment, there is also 
wide variation in the extent and level of detail of the evidence 
available to describe the ecosystem characteristics (e.g., physical, 
chemical, and geological characteristics, as well as atmospheric 
deposition history) that influences the degree to which deposition of N 
and S associated with the oxides of S and N and PM in ambient air may 
be linked to ecological effects. One reason for this relates to the 
contribution of many decades of uncontrolled atmospheric deposition 
before the establishment of NAAQS for PM, oxides of S and oxides of N 
(in 1971), followed by the subsequent decades of continued deposition 
as standards were implemented and updated. The impacts of this 
deposition history remain in soils of many parts of the U.S. today 
(e.g., in the Northeast and portions of the Appalachian Mountains in 
both hardwood and coniferous forests, as well as areas in and near the 
Los Angeles Basin), with recent signs of recovery in some areas (ISA, 
Appendix 4, section 4.6.1; 2008 ISA, section 3.2.1.1). This backdrop 
and associated site-specific characteristics are among the challenges 
faced in identifying deposition targets that might be expected to 
provide protection going forward from the range of effects for which we 
have evidence as a result of the deposition of the past.
    Critical loads (CLs) are frequently used in studies that 
investigate associations between various chemical, biological, 
ecological and ecosystem characteristics and a variety of N or S 
deposition-related metrics. The term critical load, which refers to an 
amount (or a rate of addition) of a pollutant to an ecosystem that is 
estimated to be at (or just below) that which would result in an 
ecological effect of interest, has multiple interpretations and 
applications (ISA, p. IS-14). The dynamic nature of ecosystem pollutant 
processing and the broad array of factors that influence it adds 
complications to critical load identification and interpretation. Time 
is an important dimension, which is sometimes unstated (e.g., in 
empirical or observational analyses) and is sometimes explicit (e.g., 
in steady-state or dynamic modeling analyses) (ISA, section IS.2.2.4). 
Further, this variety in meanings stems in part from differing 
judgments and associated identifications regarding the ecological 
effect (both type and level of severity) on which the critical load 
focuses and judgment of its significance or meaning.
    Studies, based on which CLs are often identified, vary widely with 
regard to the specific ecosystem characteristics being evaluated, as 
well as the benchmarks selected for judging them. The specific details 
of these various judgments influence the strengths and limitations, and 
associated uncertainty, of using critical load information from such 
studies for different applications. The summary that follows is 
intended to reach beyond individual critical loads developed over a 
variety of studies and ecosystems and consider the underlying study 
findings about key aspects of the environmental conditions and 
ecological characteristics studied. A more quantitative variation of 
this is the methodology developed for the aquatic acidification REA in 
this review, presented in the PA and summarized in section II.A.4. 
below. In those analyses, the concept of a critical load is employed 
with steady-state modeling that relates deposition to waterbody acid 
neutralizing capacity.
    While recognizing the inherent connections between watersheds and 
waterbodies, such as lakes and streams, the organization of this 
section recognizes the more established state of the information, 
tools, and data for aquatic ecosystems for characterizing relationships 
between atmospheric deposition and acidification and/or nutrient 
enrichment effects under air quality associated with the current 
standards (PA, Chapter 5).\39\ Further, we

[[Page 105715]]

recognize the generally greater role of atmospheric deposition in 
waterbodies impacted by aquatic acidification compared to its role in 
eutrophication-related impacts of surface waters, particularly rivers 
and estuaries in and downstream of populated watersheds, to which 
direct discharges have also long contributed, as recognized in section 
II.A.3.a(3) above (ISA, Appendix 13, section 13.1.3.1; ISA, Appendix 7, 
section 7.1.1.1; 2008 ISA, section 3.2). Therefore, with regard to 
deposition-related effects, we focus first on the quantitative 
information for aquatic ecosystem effects in sections II.A.3.c.(1) 
below. Section II.A.3.c.(2) discusses the available evidence regarding 
relationships between deposition-related exposures and the occurrence 
and severity of effects on trees and understory communities in 
terrestrial ecosystems. Section II.A.3.c.(3) discusses the currently 
available information related to consideration of exposure 
concentrations associated with other welfare effects of nitrogen and 
sulfur oxides and PM in ambient air.
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    \39\ With regard to other deposition-related effects of S 
compounds, quantitative tools or approaches for relating S 
deposition to ecosystem impacts are not currently well developed. As 
summarized in section II.A.3.a.(4) above, these effects, in wetland 
and freshwater ecosystems, include the alteration of Hg methylation 
in surface water, sediment, and soils; and changes in biota due to 
sulfide phytotoxicity including alteration of growth and 
productivity, species physiology, species richness, community 
composition, and biodiversity. No studies are in the available 
evidence regarding the estimation of critical loads for 
SOX deposition related to these non-acidifying effects of 
S deposition into these ecosystems (ISA, Appendix 12, section 12.6).
---------------------------------------------------------------------------

(1) Acidification and Nitrogen Enrichment in Aquatic Ecosystems
    Prior to the peak in S deposition levels that occurred in the 1970s 
and early 1980s, when deposition likely exceeded 30 kg S/ha-yr in some 
areas (PA, Appendix B, Figure 5B-9), surface water 
SO42- concentrations were increasing in response 
to the extremely high S deposition of the preceding years. 
Subsequently, and especially more recently, surface water 
SO42- concentrations have generally decreased, 
particularly in the Northeast (Robinson et al., 2008; ISA, section 
7.1.5.1.4). Some studies of long-term projections in some waterbodies 
(e.g., in the Blue Ridge Mountains region in Virginia), however, 
continue to indicate little or slow reduction in acidic ions, even as 
emissions have declined. This is an example of the competing role of 
changes in S adsorption on soils and the release of historically 
deposited S from soils into surface water,\40\ which some modeling has 
suggested will delay chemical recovery in those water bodies (ISA, 
Appendix 7, sections 7.1.2.2 and 7.1.5.1).
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    \40\ Some modeling studies in some areas have indicated the 
potential for a lagged response even as emissions and deposition 
decline; this lag reflects a reduction in soil absorption of 
SO4-2 and leaching of previously accumulated S 
from watersheds (ISA, Appendix 7, section 7.1.2.2).
---------------------------------------------------------------------------

    In the 2012 review of the oxides of N and S, quantitative analyses 
relating deposition in recent times (e.g., since 2000) to ecosystem 
acidification, and particularly aquatic acidification, were generally 
considered to be less uncertain, and the ability of those analyses to 
inform NAAQS policy judgments more robust, than analyses related to 
deposition and ecosystem nutrient enrichment or eutrophication (2011 
PA). While quantitative assessment approaches for aquatic 
eutrophication as a result of total N loading are also well 
established, and the evidence base regarding atmospheric deposition and 
nutrient enrichment has expanded since the 2012 review, the 
significance of non-air N loading to rivers, estuaries and coastal 
waters (as recognized in section II.A.3.a. above) continues to 
complicate the assessment of nutrient enrichment-related risks 
specifically related to atmospheric N deposition. Accordingly, the REA 
analyses developed in this review focus on aquatic acidification. The 
REA and its findings regarding deposition rates associated with 
different levels of aquatic acidification risk are summarized in 
section II.A.4. below. Thus, the paragraphs below focus on available 
quantitative information regarding atmospheric deposition and N 
enrichment in aquatic ecosystems.\41\ The overview provided here draws 
on the summary in the PA of the evidence as characterized in the ISA 
with regard to deposition level estimates that studies have related to 
various degrees of different effects with associated differences in 
potential for or clarity in public welfare significance (PA, section 
5.2).
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    \41\ Separate quantitative analyses have not been performed in 
this review for N enrichment-related effects in these waterbodies in 
recognition of a number of factors, including modeling and 
assessment complexities, and site- or waterbody-specific data 
requirements, as well as, in some cases, issues of apportionment of 
atmospheric sources separate from other influential sources.
---------------------------------------------------------------------------

    The eutrophication of wetlands and other aquatic systems is 
primarily associated with nitrogen inputs, whether from deposition or 
other sources. Atmospheric deposition is the main source of new N 
inputs to some freshwater wetlands and fresh waterbodies, such as 
headwater streams and high-elevation lakes, while other N inputs, such 
as agricultural runoff and wastewater effluent, can be significant 
contributors to waterbodies in agricultural and populated areas (ISA, 
Appendix 9, section 9.1 and Appendix 11, section 11.3.1). Rates of 
total N deposition associated with eutrophication-related effects in 
aquatic systems ranges from a few kilograms per hectare per year (kg/
ha-yr) for differences in diatom community composition in high 
elevation lakes to over 500 kg N/ha-yr for some effects in saltwater 
wetlands. While the evidence for these effects contributes to ISA 
causal determinations, it is often very location-specific and less 
informative for other uses, such as in quantitative assessments 
relating deposition to waterbody response across broad geographic 
areas.
    In estuaries and coastal systems, the well-established 
relationships between N loading and algal blooms and associated water 
quality impacts have been the focus of numerous water quality modeling 
projects that have quantified eutrophication processes across a wide 
variety of U.S. ecosystems. These projects, which have generally 
involved quantification of N loading and association with various water 
quality indicators, have informed management decision-making in 
multiple estuaries, including Chesapeake Bay, Narraganset Bay, Tampa 
Bay, Neuse River Estuary and Waquoit Bay (ISA, Appendix 7, section 
7.2). The indicators of nutrient enrichment employed include 
chlorophyll a, dissolved oxygen, and reduced abundance of submerged 
aquatic vegetation, among others (ISA, section IS.7.3 and Appendix 10, 
section 10.6).
    The decision-making in these projects generally focuses on 
identification of total N loading targets for purposes of attaining 
water quality standards, informed by modeling work that includes 
apportionment of sources, which vary by system. We note that the 
assignment of targets to different source types (e.g., groundwater, 
surface water runoff, and atmospheric deposition) in different 
waterbodies and watersheds varies for both practical and policy 
reasons. Further, during the multi-decade time period across which 
these activities have occurred, atmospheric deposition of N in coastal 
areas has declined. In general, however, atmospheric deposition targets 
for N for the large systems summarized above have been approximately 10 
kg/ha-yr.
    The establishment of target N loads to surface waterbodies is in 
many areas related to implementation of the total maximum daily load 
(TMDL) requirements of section 303(d) of the Clean Water Act.\42\ 
Nutrient load allocation and reduction activities in some large 
estuaries predate

[[Page 105716]]

development of CWA 303(d) TMDLs. The multiple Chesapeake Bay Agreements 
signed by the U.S. EPA, District of Columbia, and states of Virginia, 
Maryland, and Pennsylvania first established the voluntary government 
partnership that directs and manages bay cleanup efforts and 
subsequently included commitments for reduction of N and phosphorus 
loading to the bay. Efforts prior to 2000 focused largely on point-
source discharges, with slower progress for nonpoint-source reductions 
via strategies such as adoption of better agricultural practices, 
reduction of atmospheric N deposition, enhancement of wetlands and 
other nutrient sinks, and control of urban sprawl (2008 ISA, section 
3.3.8.3). Studies since 2000 estimate atmospheric deposition as a major 
N source in the overall N budget for the Chesapeake Bay \43\ (ISA, 
section 7.2.1; Howarth, 2008; Boyer et al., 2002). The TMDL established 
for the Chesapeake Bay in 2010, under requirements of section 303(d) of 
the Clean Water Act, included a loading allocation for atmospheric 
deposition of N directly to tidal waters, which was projected to be 
achieved by 2020 based on air quality progress under existing CAA 
regulations and programs (U.S. EPA, 2010).\44\
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    \42\ Under the CWA, section 303(d), every two years, states and 
other jurisdictions are required to list impaired waterbodies not 
meeting water quality standards. For waterbodies on the list, a TMDL 
must be developed that identifies the maximum amount of pollutant a 
waterbody can receive and still meet water quality standards, e.g., 
standards for dissolved oxygen and chlorophyll a (which are 
indicators of eutrophication).
    \43\ For example, a 2011 analysis estimated atmospheric 
deposition to the Chesapeake Bay watershed to account for 
approximately 25% of total N inputs to the estuary (ISA, Appendix 7, 
section 7.2.1).
    \44\ As recognized on the EPA web page describing this activity, 
the TMDL, formally established in December 2010 ``is designed to 
ensure that all pollution control measures needed to fully restore 
the Bay and its tidal rivers are in place by 2025.'' The website 
also indicates that ``EPA expects practices in place by 2017 to meet 
60 percent of the necessary reductions,'' and for some areas to 
recover before others, but for it to take years after 2025 for the 
Bay and its tributaries to fully recover (https://www.epa.gov/chesapeake-bay-tmdl/frequent-questions-about-chesapeake-bay-tmdl).
---------------------------------------------------------------------------

    Jurisdictions for other U.S. estuaries have also developed TMDLs to 
address nutrient loading causing eutrophication. For example, 
atmospheric deposition in 2000 was identified as the third largest 
source of N loading to Narragansett Bay via the watershed and directly 
to the Bay, at 20% of the total (ISA, Appendix 7, section 7.2.1). 
Similarly, atmospheric deposition was estimated to account for 
approximately a third of N input to several small- to medium-sized 
estuaries of southern New England, with the percentage varying widely 
for individual estuaries (ISA, Appendix 7, section 7.2.1; Latimer and 
Charpentier, 2010).\45\ Another modeling study in the Waquoit Bay 
estuaries in Cape Cod, Massachusetts, using data since 1990, estimated 
atmospheric deposition to have decreased by about 41% while wastewater 
inputs increased 80%, with a net result that total loads were concluded 
to not have changed over that time period (ISA, Appendix 7, section 
7.2.1). Another well-studied estuarine system is Tampa Bay, for which a 
2013 study estimated atmospheric sources to account for more than 70% 
of total N loading based on 2002 data (ISA, Appendix 7, section 7.2.1). 
The TMDL for Tampa Bay allocates 11.8 kg/ha-yr N loading to atmospheric 
deposition (ISA, Appendix 16, section 16.4.2; Janicki Environmental, 
2013). The Neuse River Estuary is another for which modeling work has 
investigated the role of N loading from multiple sources on nutrient 
enrichment \46\ and associated water quality indicators, including 
chlorophyll a (ISA, Appendix 10, section 10.2).
---------------------------------------------------------------------------

    \45\ For example, across the 74 estuaries in the 3-state coastal 
region studied, N from atmospheric deposition to estuary watersheds 
was generally estimated to account for less than 25% of total N 
inputs, while estimates for a few small estuaries in CT were higher 
than 51% (but below 75%) (Latimer and Charpentier, 2010).
    \46\ One evaluation of progress in achieving mandated N 
reductions in the Neuse River Basin in NC found that flow-normalized 
N loading from NO3- decreased beginning in the 
1992-1996 period (ISA Appendix 10, section 10.2; Lebo et al., 2012).
---------------------------------------------------------------------------

    Nitrogen loading to estuaries has also been considered specifically 
for impacts on submerged aquatic vegetation. For example, eelgrass 
coverage was estimated to be markedly reduced in shallow New England 
estuaries with N loading at or above 100 kg N/ha-yr (ISA, Appendix 10, 
section 10.2.5). Another study estimated loading rates above 50 kg/ha-
yr as a threshold at which habitat extent may be impacted (ISA, 
Appendix 10, section 10.2.5; Latimer and Rego, 2010). Factors that 
influence the impact of N loading on submerged vegetation include 
flushing and drainage in estuaries (ISA, Appendix 10, section 10.6).
(2) Deposition-Related Effects in Terrestrial Ecosystems
    The subsections below describe the available information for 
quantitative relationships between atmospheric deposition rates and 
acidification and N enrichment-related effects in terrestrial systems. 
In the 2012 review, analyses included a critical load-based 
quantitative modeling analysis focused on BC:Al ratios in soils for 
terrestrial acidification and a qualitative characterization of 
nutrient enrichment (2009 REA). The more qualitative approach taken for 
nutrient enrichment in the 2012 review involved describing deposition 
ranges identified from observational or modeling research as associated 
with potential effects/changes in species, communities, and ecosystems, 
with recognition of uncertainties associated with quantitative analysis 
of these depositional effects (2011 PA, section 3.2.3). In this review, 
rather than performing new quantitative analyses focused on terrestrial 
ecosystems, we draw on analyses in the 2009 REA and on more recent 
published studies recognized in the ISA that provide information 
pertaining to deposition levels associated with effects related to 
terrestrial acidification and N enrichment.
    Several recent publications have added to the information available 
in the last review including analyses of large datasets from field 
assessments of tree growth and survival, as well as analyses of 
understory plant community richness, containing estimates of 
atmospheric N and/or S deposition (ISA, Appendix 6, section 6.5). The 
understory plant studies investigate the existence of associations of 
variations in plant community structure and other metrics including 
species richness, growth, and survival with variations in deposition 
during an overlapping time period, generally of a decade or two in 
duration. Soil acidification modeling and observational studies, as 
well as experimental addition studies, each with their various design 
features and associated strengths and limitations (as noted immediately 
below), inform consideration of N and S deposition levels of interest 
in the review.
    In general, observational or gradient studies differ from the 
chemical mass balance modeling approach in a number of ways that are 
relevant to their consideration and use for our purposes in this 
review. One difference of note is the extent to which their findings 
address the ecosystem impacts of historical deposition. Observational 
studies describe variation in indicators in the current context, which 
may include stores of historically deposited chemicals. In these 
studies, such historical loading, and its associated impacts, can 
contribute to effects quantified by the study ecological metrics, yet 
the metric values are assessed in relation to estimates of more recent 
deposition. Mass balance modeling for steady-state conditions is 
commonly used for estimating critical loads for acidification risk but 
does not usually address the complication of historical deposition 
impacts that can play a significant role in timing of system recovery. 
In this type of modeling, timelines of the various processes are not 
addressed. While this provides a simple approach that may facilitate 
consideration unrelated to

[[Page 105717]]

recovery timelines, it cannot address the potential for changes in 
influential factors that may occur over time with different or changed 
deposition patterns. Thus, while observational studies contribute to 
the evidence base on the potential for N/S deposition to contribute to 
ecosystem effects (and thus are important evidence in the ISA 
determinations regarding causality), their uncertainties (and 
underlying assumptions) differ from those of modeling analyses, and 
they may be somewhat less informative with regard to identification of 
specific N and S deposition levels that may elicit ecosystem impacts of 
interest. Both types of studies, as well as N addition experiments, 
which are not generally confounded by exposure changes beyond those 
assessed (yet may have other limitations), have been considered, with 
key findings summarized below.
(a) Deposition and Risks to Trees
    The 2009 REA performed a steady-state modeling analysis to estimate 
the annual amounts of S and N acidifying deposition at or below which 
one of three BC:Al target values would be met in a 24-state area in 
which the acid-sensitive species, red spruce and sugar maple, occur. A 
range of acid deposition was estimated for each of the three target 
values. Recent estimates of total S and N deposition in regions of the 
U.S. appear to meet all but the most restrictive of these targets, for 
which the uncertainty is greatest (e.g., ISA, Appendix 2, sections 2.6 
and 2.7).\47\
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    \47\ Uncertainties associated with the 2009 REA analyses include 
those associated with the limited dataset of laboratory-generated 
data on which the BC:Al target values are based (PA, section 5.3.2) 
as well as in the steady-state modeling parameters, most prominently 
those related to base cation weathering and acid neutralizing 
capacity (2009 REA, section 4.3.9). A new approach to estimating 
weathering has more recently been employed and reported to reduce 
the uncertainty associated with this parameter (e.g., Phelan et al., 
2014; McDonnell et al., 2012; ISA, Appendix 4, sections 4.6.2.1 and 
4.8.4 and Appendix 5, section 5.4).
---------------------------------------------------------------------------

    Experimental addition studies of S, or S plus N have been performed 
in eastern locations, focusing on a small set of tree species, and 
generally involving S and N additions greater than 20 kg/ha-yr, in 
combination with appreciable background deposition at the time, and 
have generally not reported growth reductions (PA, Appendix 5B, Table 
5B-1; ISA, Appendix 5, section 5.5.1). Uncertainties associated with 
these analyses include the extent to which the studies reflect steady-
state conditions. Given the variability in the durations across these 
studies and the relatively short durations for some (e.g., less than 
five years), it might be expected that steady-state conditions have not 
been reached, such that the S/N loading is within the buffering 
capacity of the soils. With regard to N addition alone, the available 
studies have reported mixed results for growth and survival (PA, Table 
5B-1; Magill et al., 2004; McNulty et al., 2005; Pregitzer et al., 
2008; Wallace et al., 2007). It is not clear the extent to which such 
findings may be influenced by species-specific sensitivities or soils 
and trees already impacted by historic deposition, or other 
environmental factors.
    With regard to S deposition, two large observational studies that 
analyzed growth and/or survival measurements in tree species at sites 
in the eastern U.S. or across the country reported negative 
associations of tree survival for 9 of the 10 species' functional type 
groupings with the S deposition metric and of tree survival and growth 
for nearly half of the species individually (Dietze and Moorcroft, 
2011; Horn et al., 2018).\48\ Interestingly, survival for the same 9 
species groups was also negatively associated with long-term average 
O3 (Dietze and Moorcroft, 2011). The S deposition metrics 
for the two studies were mean annual average deposition estimates for 
total S or sulfate (wet deposition) during different, but overlapping, 
time periods of roughly 10-year durations. The full range of average 
SO42- deposition estimated for the 1994-2005 
period assessed by Dietze and Moorcroft (2011) for the eastern U.S. 
study area was 4 to 30 kg S/ha-yr. The second study covered the more 
recent time period (2000-2013) and 71 species distributed across the 
U.S. To draw on this study with regard to S deposition levels of 
interest, the distribution of S deposition estimates for each species 
were considered in the PA; the range of median S deposition for sites 
of those species for which negative associations with growth or 
survival were reported was 5 to 12 kg S/ha-yr, with few exceptions 
(Appendix 5B, section 5B.2 and Attachments 2A and 2B; Horn et al., 
2018).\49\
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    \48\ The study by Horn et al. (2018) constrained the S analyses 
to preclude a positive association with S.
    \49\ This range is for median S deposition estimates (based on 
measurement interval average, occurring within the years 2000-2013) 
of nonwestern species with negative associations with growth or 
survival ranged (Horn et al., 2018).
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    Regarding N deposition, the three large observational studies that 
analyzed growth and/or survival measurements in tree species samples at 
sites in the northeastern or eastern U.S., or across the country, 
reported associations of tree survival and growth with several N 
deposition metrics (Dietze and Moorcroft, 2011; Thomas et al., 2010; 
Horn et al., 2018). Estimates of average N deposition across the full 
set of sites analyzed by Thomas et al. (2010) in 19 states in the 
northeastern quadrant of the U.S. ranged from 3 to 11 kg N/ha-yr for 
the period 2000-2004. The N deposition metrics for these three studies 
were mean annual average deposition estimates for total N or nitrate 
(wet deposition) during different, but overlapping, time periods that 
varied from 5 to more than 10 years. The full range of average 
NO3- deposition estimated for the 1994-2005 
period assessed by Dietze and Moorcroft (2011) for the eastern U.S. 
study area was 6 to 16 kg N/ha-yr. Median N deposition estimated 
(measurement interval average [falling within the years 2000-2013]) at 
sites of nonwestern species for which associations with growth or 
survival were negative (either over full range or at median for 
species) ranged from 7 to 12 kg N/ha-yr (Horn et al., 2018).
    In considering what can be drawn from these studies with regard to 
deposition levels of potential interest for tree species effects, such 
as the ranges identified above, a number of uncertainties are 
recognized. For example, several factors were not accounted for that 
have potential to influence tree growth and survival. Although 
O3 was analyzed in one of the three studies, soil 
characteristics and other factors with potential to impact tree growth 
and survival (other than climate) were not assessed, contributing 
uncertainty to their interpretations. Also, the influence of historical 
deposition patterns and associated impacts is unknown.\50\ Further, 
differences in findings for the various species (or species' groups) 
may relate to differences in geographic distribution of sampling 
locations, which may

[[Page 105718]]

contribute to differences in ranges of deposition history, geochemistry 
etc.
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    \50\ The influence of historically higher deposition (e.g., 
versus deposition over the measurement interval) on observations is 
unknown. Given the influence of deposition on soil conditions that 
affect tree growth and survival, and generally similar geographic 
variation for recent and historic deposition, a quantitative 
interpretation of uncertainty is the extent to which similarity of 
the two studies' findings indicate a potential for both metrics to 
reflect geographic variation in impacts stemming from historic 
deposition. Although geographic deposition patterns have changed 
little across the time period of the studies, annual S and N 
deposition rates have changed appreciably (e.g., PA, Appendix 5B, 
Figures 5B-9 through 5B-12), which may also contribute uncertainty 
to interpretation of specific deposition rates associated with 
patterns of tree growth and survival. Few studies on recovery in 
historically impacted areas that might address such uncertainties 
are available (e.g., ISA, section IS.11).
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(b) Deposition Studies of Herbs, Shrubs and Lichens
    Studies evaluating the effects of N addition on herbs, shrubs and 
lichens include observational studies of herbaceous species richness at 
sites in a multi-state study area and of grassland or coastal sage 
scrub communities in southern California, and experimental addition 
studies in several western herb or shrub ecosystems. The experimental 
addition studies indicate effects on community composition associated 
with annual N additions of 10 kg N/ha-yr (in the context of background 
deposition on the order of 6 kg N/ha-yr [PA, Appendix 5B, Table 5B-7]) 
and higher (PA, sections 5.3.3.1 and 5.3.4.2; ISA, Appendix 6, section 
6.3.6). Experiments involving additions of 5 kg N/ha-yr variously 
reported no response or increased cover for one species (in context of 
background deposition estimated at 5 kg N/ha-yr). The landscape-level 
analysis of coastal sage scrub community history in southern California 
observed a greater likelihood of recovery of sites with relatively low 
levels of exotic invasive grasses when the N deposition metric level 
was below 11 kg N/ha-yr. Lastly, the multi-state analysis of herbaceous 
species richness reported a negative association with N deposition 
metric values above 8.7 kg N/ha-yr at open-canopy sites and above 6.5 
kg/ha-yr and low pH sites. In forested sites, negative associations 
were found above 11.6 kg N/ha-yr in sites with acidic soil pH at or 
above 4.5 (PA, section 5.3.3).
    Limitations and associated uncertainties vary between the two types 
of studies (experimental addition and observational), but both are 
limited with regard to consideration of the impacts of long-term 
deposition. Such studies are necessarily limited in scope with regard 
to species and ecosystem, and while there are some experimental 
addition studies lasting more than 20 years, many are for fewer than 10 
years. In the case of observational studies, these studies generally 
have not accounted for the influence of historical pollution (including 
decades of S and N deposition and elevated concentrations of 
O3 and N oxides) on the associations observed with more 
recent deposition metrics. Further, there is uncertainty associated 
with the extent to which the exposure metric utilized reflects the 
particular conditions that may be eliciting the ecosystem response 
quantified by the ecosystem metric.
    The few studies of lichen species diversity and deposition-related 
metrics, while contributing to the evidence that relates deposition to 
relative abundance of different lichen species, are more limited in the 
extent to which they inform an understanding of specific exposure 
conditions in terms of deposition rates that may elicit specific 
responses. Related factors include uncertainties related to the methods 
employed to represent N deposition, the potential role of other 
unaccounted-for environmental factors (including O3, 
SO2, S deposition and historical air quality and associated 
deposition), and uncertainty concerning the independence of any effect 
of deposition levels from residual effects of past patterns of 
deposition (PA, section 5.3.3.2). Information on exposure conditions 
associated with effects of oxides of N such as HNO3 on 
lichen species is also addressed in section II.A.3.c.(3) below.
(3) Other Effects of N Oxides, SOX and PM in Ambient Air
    The evidence related to exposure conditions for other effects of 
SOX, N oxides and PM in ambient air includes concentrations 
of SO2 and NO2 associated with effects on plants, 
concentrations of NO2 and HNO3 associated with 
effects on plants and lichens, and concentrations of PM mass or PM 
loading (much higher than those associated with the existing standard) 
that affect plant photosynthesis. With regard to oxides of N and S, we 
note that some effects described as direct may be related to dry 
deposition of SO2 and HNO3 onto plant and lichen 
surfaces, exposure pathways that would be captured in observational 
studies and could also be captured in some fumigation experiments.
    With regard to SO2, the evidence primarily includes 
field studies for the higher concentrations associated with visible 
foliar injury and laboratory studies for other effects, e.g., depressed 
photosynthesis and reduced growth or yield (ISA, Appendix 3, section 
3.2; 1982 AQCD, section 8.3). The recently available information also 
includes observational studies reporting increased tree growth in 
association with reductions in SO2 emissions, although these 
studies do not generally report the SO2 concentrations in 
ambient air or account for the influence of changes in concentrations 
of co-occurring pollutants such as O3 (ISA, Appendix 3, 
section 3.2). With regard to foliar injury, the current ISA states 
there to be limited research since the 1982 AQCD and ``no clear 
evidence of acute foliar injury below the level of the current 
standard'' (ISA, p. IS-37). Few studies report yield effects from acute 
exposures, with the available ones reporting relatively high 
concentrations, such as multiple hours with concentrations above 1 ppm 
or 1000 ppb (1982 AQCD, section 8.3). Effects have also been reported 
on photosynthesis and other functions in a few lichen species groups, 
although recovery of these functions was observed from short, multi-
hour exposures to concentrations below about 1 ppm (ISA, Appendix 3, 
section 3.2).
    With regard to oxides of N, the evidence indicates that effects on 
plants and lichens occur at much lower exposures to HNO3 
(than to NO2). The laboratory and field studies of oxides of 
N vary regarding their limitations; field studies are limited regarding 
identification of threshold exposures for the reported effects, and 
uncertainties associated with controlled experiments include whether 
the conditions under which the observed effects occur would be expected 
in the field. Plant studies reported in the ISA did not report effects 
on photosynthesis and growth resulting from exposures of NO2 
concentrations below 0.1 ppm (ISA, Appendix 3, section 3.3).
    With regard to the HNO3, the elevated concentrations of 
NO2 and HNO3 in the Los Angeles area in the 
1970s-90s are well documented as is the decline of lichen species in 
the Los Angeles Basin during that time, although such an analysis is 
not available elsewhere in the U.S. (PA, section 5.4.2; ISA, Appendix 
3).\51\ Other evidence specific to HNO3, which can deposit 
on and bind to leaf or needle surfaces, includes controlled exposure 
studies describing foliar effects on several tree species. Studies of 
ponderosa pine, white fir, California black oak and canyon live oak 
involving continuous chamber exposure over a month to 24-hour average 
HNO3 concentrations generally ranging from 10 to 18 
[micro]g/m\3\ (moderate treatment) or 18 to 42 [micro]g/m\3\ (high 
treatment), with the average of the highest 10% of concentrations 
generally ranging from 18 to 42 [micro]g/m\3\ (30-60 [micro]g/m\3\ 
peak) or 89 to 155 [micro]g/m\3\ (95-160 [micro]g/m\3\ peak), resulted 
in damage to foliar surfaces of the 1 to 2-year old plants (ISA, 
Appendix 3, section 3.4; Padgett et al., 2009). Available evidence for 
lichens

[[Page 105719]]

also includes a recent laboratory study of daily HNO3 
exposures for 18 to 78 days, with daily peaks near 50 ppb (~75 
[micro]g/m\3\) that reported decreased photosynthesis, among other 
effects (ISA, Appendix 6, section 6.2.3.3; Riddell et al., 2012). Based 
on studies extending back to the 1980s, HNO3 has been 
suspected to have had an important role in the dramatic declines of 
lichen communities that occurred in the Los Angeles basin (ISA, 
Appendix 3, section 3.4; Nash and Sigal, 1999; Riddell et al., 2008; 
Riddell et al., 2012). In more recent studies, variation in eutrophic 
lichen abundance has been associated with variation in N deposition 
metrics (ISA, Appendix 6, section 6.2.3.3), although the extent to 
which these associations are influenced by residual impacts of historic 
air quality is unclear and the extent to which similar atmospheric 
conditions and ecological relationships exist in other locations in the 
U.S. is uncertain.
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    \51\ For example, concentrations of HNO3 reported in 
forested areas of California in the 1980s ranged up to 33 ug/m\3\, 
and annual average NO2 concentrations in the Los Angeles 
area ranged from 0.078 ppm in 1979 to 0.053 ppm in the early 1990s 
(PA, section 5.4.2). Ambient air concentrations of HNO3 
in the Los Angeles metropolitan area have declined markedly, as 
shown in Figure 2-23 of the PA, which compares concentrations at 
CASTNET monitoring sites between 2019 and 1996 (PA, section 2.4.1).
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    Little information is available on welfare effects of airborne PM 
at concentrations commonly occurring in the U.S. today, and the 
available information does not indicate effects to occur under such 
conditions. The concentrations at which PM has been reported to affect 
vegetation (e.g., through effects on leaf surfaces, which may affect 
function, or through effects on gas exchange processes) are generally 
higher than those associated with conditions meeting the current 
standards and may be focused on specific particulate chemicals rather 
than on the mixture of chemicals in PM occurring in ambient air (ISA, 
Appendix 15, sections 15.4.3 and 15.4.6). Studies involving ambient air 
PM have generally involved conditions that are much higher than those 
common to the U.S. today (ISA, Appendix 15, sections 15.4.3 and 
15.4.4).
4. Overview of Exposure and Risk Assessment for Aquatic Acidification
    Our consideration of the scientific evidence available in the 
current review is informed by results from quantitative analyses of 
estimated acidic deposition and associated risk of aquatic 
acidification (PA, section 5.1 and Appendix 5A). These REA analyses, 
like those in the last review, make use of well-established modeling 
tools and assessment approaches for this endpoint. Other categories of 
effects of S and N deposition have been the subject of quantitative 
analyses, both in the last review (e.g., terrestrial acidification) and 
in other contexts (e.g., eutrophication of large rivers and estuaries), 
each with associated complexities and specificity. The PA, while 
focusing the new analyses on aquatic acidification risks, as summarized 
here, also draws on findings of available analyses for the other 
categories of effects.
    The REA analyses, summarized here and presented in detail in 
Appendix 5A of the PA, have focused on ANC as an indicator of aquatic 
acidification risk (PA, section 5.1 and Appendix 5A). This focus is 
consistent with such analyses performed in the 2012 review and with the 
longstanding evidence that continues to demonstrate a causal 
relationship between S and N deposition and alteration of freshwater 
biogeochemistry and between acidifying S and N deposition and changes 
in biota, including physiological impairment and alteration of species 
richness, community composition, and biodiversity in freshwater 
ecosystems (ISA, Table ES-1), as summarized in section II.A.3 above.
    Section II.A.4.a. summarizes key aspects of the assessment design, 
including the conceptual approach and tools, indicator reference or 
benchmark concentrations, the assessment scales, study areas and 
waterbodies analyzed, and exposure and risk metrics derived. Key 
limitations and uncertainties associated with the assessment are 
identified in section II.A.4.b. and the exposure and risk estimates are 
summarized in section II.A.4.c. An overarching focus of these analyses 
is characterization of aquatic acidification risk in sensitive 
ecoregions associated with different deposition conditions.
a. Key Design Aspects
    The REA for this review entailed a multi-scale analysis of 
waterbodies in the contiguous U.S. that assessed waterbody-specific 
aquatic acidification at three spatial scales: national, ecoregion, and 
case study area (PA, Appendix 5A). The assessment involved evaluation 
of deposition and water quality response (ANC) at the waterbody site 
level. The results are then summarized at the national, ecoregion, and 
case study level. The national-scale analysis included all waterbody 
sites across the U.S. for which relevant data were available.\52\ The 
ecoregion-scale analysis focused on waterbodies with relevant data in a 
set of 25 ecoregions generally characterized as acid-sensitive; and the 
more localized case study-scale analysis focused on such waterbodies in 
five case study areas across the U.S., within each of which were Class 
I areas.
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    \52\ The national-scale analysis focused on the contiguous U.S. 
as there are insufficient data available for Hawaii, Alaska, and the 
territories. Of the four hierarchical levels of ecoregion 
categorization, the REA utilized level III which divides the 
contiguous U.S. into 84 ecoregions (Omernik and Griffith, 2014). The 
69 of these 84 ecoregions in which there was at least one site with 
sufficient data comprised the national scale.
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    The impact of acidifying S or N deposition estimated for five 
different time periods (2001-03, 2006-08, 2010-12, 2014-16 and 2018-20) 
was evaluated using a CL approach that relied on comparison of 
waterbody location-specific deposition estimates to waterbody location-
specific CL estimates derived for other applications and available in 
the National Critical Loads Database (NCLD) \53\ (PA, Appendix 5A). The 
CL estimates used in the assessment were largely based on steady-state 
modeling, and the modeling applications focused on ANC, producing CL 
estimates (acidifying deposition in terms of kg/ha-yr or meq/m\2\-yr 
[milliequivalents per square meter per year] for S and N compounds) for 
different target or threshold ANC concentrations (also termed 
benchmarks). Of the 84 ecoregions in the contiguous U.S., 64 have at 
least one waterbody site with a CL estimate (PA, Appendix 5A). Given 
its common use in categorizing waterbody sensitivity, ANC was used as 
the indicator of acidification risk in this assessment (PA, section 
5.1.2.2). Deposition estimates, as 3-year averages of annual TDep 
estimates for each site, were compared to the CL estimates for three 
different ANC benchmark concentrations (targets or thresholds), in 
recognition of the watershed variability and associated uncertainties, 
as an approach for characterizing aquatic acidification risk (PA, 
section 5.1).
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    \53\ The NCLD is comprised of CLs calculated from several common 
models: (1) steady-state mass-balance models such as the Steady-
State Water Chemistry (SSWC), (2) dynamic models such as Model of 
Acidification of Groundwater In Catchments (MAGIC) (Cosby et al., 
1985) or Photosynthesis EvapoTranspiration Biogeochemical model 
(PnET-BGC) (Zhou et. al., 2015) run out to year 2100 or 3000 to 
model steady-state conditions and (3) regional regression models 
that use results from dynamic models to extrapolate to other 
waterbodies (McDonnell et. al., 2012; Sullivan et al., 2012a). Data 
and CL estimates in the NCLD are generally focused on waterbodies 
impacted by deposition-driven acidification and are described in 
documentation for the database version (PA, section 5.1.2.3; Lynch 
et al., 2022).
---------------------------------------------------------------------------

    The available evidence and scientific judgments were considered in 
identifying the three ANC benchmark concentrations: 20 [mu]eq/L, 30 
[mu]eq/L, 50 [mu]eq/L (PA, section 5.1.2.2). Selection of these 
benchmark ANC concentrations reflects several considerations. For 
example, most aquatic CL studies conducted in the U.S. since 2010 use 
an ANC of 20 and/or 50 [mu]eq/L, because 20 [mu]eq/L has been suggested 
to provide

[[Page 105720]]

protection for a ``natural'' or ``historical'' \54\ range of ANC, and 
50 [mu]eq/L to provide greater protection, particularly from episodic 
acidification events \55\ (Dupont et al., 2005; Fakhraei et al., 2014; 
Lawrence et al., 2015; Lynch et al., 2022; McDonnell et al., 2012, 
2014; Sullivan et al., 2012a, 2012b). For example, levels below 20 
[mu]eq/L have been associated with fish species reductions in some 
sensitive waterbodies of the Shenandoah and Adirondack Mountains. 
Levels of ANC ranging from 30 to 40 [mu]eq/L have been reported to 
provide sufficient buffering to withstand acidic inputs associated with 
episodic springtime rain or snowmelt events. An ANC value of 50 [mu]eq/
L has often been cited in the literature as a target for many areas, 
and in the 2012 review, ANC values at or above 50 [mu]eq/L were 
described as providing an additional level of protection although with 
increasingly greater uncertainty for values at/above 75 [mu]eq/L \56\ 
(2011 PA, pp. 7-47 to 7-48). In the western U.S., lakes and streams 
vulnerable to deposition-driven aquatic acidification are often found 
in the mountains where surface water ANC levels are naturally low and 
typically vary between 0 and 30 [mu]eq/L (Williams and Labou, 2017; 
Shaw et al., 2014). For these reasons, this assessment also develops 
results for an ANC threshold of 50 [mu]eq/L for sites in the East and 
20 [mu]eq/L for sites in the West (denoted as ``50/20'' [mu]eq/L).\57\ 
Thus, the set of benchmark concentrations used in this REA includes ANC 
concentrations that are naturally occurring in many areas and also 
includes concentrations that, depending on watershed characteristics, 
may provide additional buffering in times of episodic acidification 
events.
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    \54\ For example, dynamic modeling simulations in acid-sensitive 
streams of the southern Blue Ridge Mountains have predicted all 
streams to have pre-industrial time ANC levels above 20 [mu]eq/L, 
while also predicting more than a third of the streams to have pre-
industrial ANC levels below 50 [mu]eq/L (Sullivan et al., 2011).
    \55\ As noted in section II.A.3.a. above, events such as spring 
snowmelt and heavy rain events can contribute to episodic 
acidification events. For example, in some impacted northeastern 
waterbodies, particularly headwater streams, ANC levels may dip 
below zero for hours to days or weeks in response to such events, 
while waterbodies labeled chronically acidic have ANC levels below 
zero throughout the year (ISA, Appendix 6, section 6.1.1.1; Driscoll 
et al., 2001).
    \56\ In considering higher ANC levels (e.g., up to 80 [mu]eq/L 
and higher), it was also recognized that many waterbodies, 
particularly in acid-sensitive regions of the contiguous U.S., never 
had an ANC that high and would never reach an ANC that high 
naturally (Williams and Labou 2017; Shaw et al., 2014; PA, section 
5.1.2.2). Additionally, in conveying its advice in the 2012 review, 
the CASAC expressed its view that ``[l]evels of 50 [mu]eq/L and 
higher would provide additional protection, but the Panel has less 
confidence in the significance of the incremental benefits as the 
level increases above 50 [mu]eq/L'' (Russell and Samet, 2010a; pp. 
15-16).
    \57\ This approach is also used in multiple studies and the NCLD 
(PA, section 5.1.2.2).
---------------------------------------------------------------------------

    Since acidification of waterbodies is controlled by local factors 
such as geology, hydrology, and other landscape factors, aquatic CLs 
for acidification were determined at the waterbody level (based on 
site-specific data) and then summarized at the national, ecoregion, and 
case study level. National-scale analyses were performed using two 
approaches: one considering acid deposition of N and S compounds 
combined and one for S deposition only. Findings from these analyses 
indicated that across the five different time periods analyzed, the 
percent of waterbodies exceeding their CLs was similar for the two 
approaches (PA, Appendix 5A, sections 5A.1.6.2 and 5A.2.1). Thus, to 
facilitate interpretation of the results, further analysis of the 
results focused on the findings for S only deposition.
    Critical load estimates for specific waterbody sites across the 
contiguous U.S. were drawn from the NCLD (version 3.2.1) \58\ for 
comparison to total deposition estimates in the same locations for the 
five time periods. Comparisons were only performed for sites at which 
CL estimates were greater than zero, indicating that achievement of the 
associated ANC benchmark concentration would be feasible.\59\ The 
results of these analyses are summarized with regard to the spatial 
extent and severity of deposition-related acidification effects and the 
protection from these effects associated with a range of annual S 
deposition.
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    \58\ A waterbody is represented as a single CL value. In many 
cases, a waterbody has more than one CL value calculated for it 
because different studies determined a value for the same waterbody. 
When more than one CL exists, the CL from the most recent study was 
selected, while the CL values were averaged when the publications 
are from the same timeframe (PA, Appendix 5A, section 5A.1.5).
    \59\ Critical load estimates are estimates of the S deposition 
rate at which a particular waterbody site is estimated to be able to 
achieve a specified ANC level. A CL estimate at or below zero would 
indicate that no S deposition estimate would provide for such a 
result.
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    The ecoregion-scale analyses focused on 25 ecoregions,\60\ 18 in 
the East and 7 in the West. Ecoregions are areas of similarity 
regarding patterns in vegetation, aquatic, and terrestrial ecosystem 
components. The 25 ecoregions in this analysis each had more than 50 
waterbody sites (or locations) for which a prior modeling application 
had developed a CL estimate, which was available in the NCLD (PA, 
section 5A.2.2.2). Although a total of 32 ecoregions had more than 50 
CL sites,\61\ four in the West were excluded as having very low 
deposition that resulted in no CL exceedances across the complete 20-
year analysis period. An additional three ecoregions (i.e., 
Southeastern Plains, Southern Coastal Plain, and Atlantic Coastal Pine 
Barrens) were excluded as they are known to have naturally acidic 
surface waters, and the low CL estimates for these ecoregions (and 
resulting CL exceedances) are likely driven by natural acidity linked 
to high levels of dissolved organic carbon, hydrology, and natural 
biogeochemical processes rather than atmospheric deposition (2008 ISA, 
section 3.2.4.2; Baker et al., 1991; Herlihy et al., 1991).
---------------------------------------------------------------------------

    \60\ The ecoregion classification scheme used to group waterbody 
sites into ecoregions is based on that described in Omernik (1987), 
which classifies regions through the analysis of the patterns and 
the composition of biotic and abiotic characteristics that affect or 
reflect differences in ecosystem quality and integrity (e.g., 
geology, physiography, vegetation, climate, soils, land use, 
wildlife, and hydrology).
    \61\ In light of the size of the level III ecoregions, 50 was 
identified as an appropriate minimum number of CL sites within an 
ecoregion to include it in the analysis.
---------------------------------------------------------------------------

    The case study scale represents the smallest scale at which CLs and 
their comparison to deposition estimates were summarized and is 
intended to give some insight into potential local impacts of aquatic 
acidification. Five case study areas across the U.S. were examined: 
Shenandoah Valley Area, White Mountain National Forest, Northern 
Minnesota, Sierra Nevada Mountains, and Rocky Mountain National Park 
(details presented in PA, section 5.1.3.3 and Appendix 5A, section 
5A.2.1). These areas include a number of national parks and forests 
that vary in their sensitivity to acidification but represent high 
value or protected ecosystems, such as Class 1 areas, wilderness, and 
national forests (PA, Appendix 5A, section 5A.2.1). The most well 
studied of these, the Shenandoah Valley Area case study, includes the 
Class I area, Shenandoah National Park, and waterbodies in each of 
three ecoregions. The number of waterbody sites with CLs available in 
the NCLD for the Shenandoah study area (4,977 sites) is nearly an order 
of magnitude greater than the total for the four other areas combined 
(524 sites).
    The analyses at different scales differed in how results were 
summarized and evaluated. For example, at the national scale, 
percentages of water bodies with deposition estimates exceeding their 
CLs (for the different ANC benchmarks) were reported for each of the 
five time periods for which deposition was assessed (PA, Table 5-1). 
From the case

[[Page 105721]]

study scale analyses, we focused primarily on the distribution of CL 
estimates in each study area. In so doing, the CLs for each case study 
area were characterized in terms of the average and two lower 
percentiles (e.g., the 30th percentile CL, which is the value below 70% 
of the CL estimates for that study area, and the 10th percentile).
    In the ecoregion-scale analyses, percentages of waterbody sites per 
ecoregion that exceeded their estimated CLs and percentages of 
waterbody sites that fell at or below them--for each of the three ANC 
benchmarks--were summarized by ecoregion for each of the five time 
periods: 2001-2003, 2006-2008, 2010-2012, 2014-2016 and 2018-2020 (PA, 
section 5.1.3.2 and Appendix 5A, section 5A.2.2). Percentages of 
waterbody sites that did not exceed their estimated CLs were described 
as achieving the associated ANC benchmark (or target). These results of 
the site-specific ANC modeling were then considered in two ways. The 
first is based on a binning of this dataset of percentages of 
waterbodies per ecoregion-time period combinations that were estimated 
to achieve each of the ANC targets by the median deposition for that 
ecoregion during that time period (e.g., percentage achieving ANC 
target of 20 [mu]eq/L when ecoregion median deposition was at/below 5 
kg/ha-yr).\62\ The second approach involved summarizing ecoregion-
specific trends in percentage of waterbodies per ecoregion estimated to 
achieve the three threshold or target ANC values (or estimated to 
exceed the associated CLs).
---------------------------------------------------------------------------

    \62\ The percentages of waterbodies in an ecoregion with 
estimated ANC at/above a target ANC is paired with the median 
deposition for that ecoregion. The percentages are then binned by 
the median deposition values.
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b. Key Limitations and Uncertainties
    The nature and magnitude of associated uncertainties and their 
impact on the REA estimates are characterized with a mainly qualitative 
approach, informed by several quantitative sensitivity analyses (PA, 
Appendix 5A, section 5A.3). The mainly qualitative approach used to 
characterize uncertainty here and in quantitative analyses in other 
NAAQS reviews is described by World Health Organization (WHO, 2008). 
Briefly, with this approach, we have identified key aspects of the 
assessment approach that may contribute to uncertainty in the 
conclusions and provided the rationale for their inclusion. Then, we 
characterized the magnitude and direction of the influence on the 
assessment for each of these identified sources of uncertainty. 
Consistent with the WHO (2008) guidance, we scaled the overall impact 
of the uncertainty by considering the degree of uncertainty as implied 
by the relationship between the source of uncertainty and the exposure 
and risk estimates. A qualitative characterization of low, moderate, 
and high was assigned to the magnitude of influence and knowledge base 
uncertainty descriptors, using quantitative observations relating to 
understanding the uncertainty, where possible. The direction of 
influence, whether the source of uncertainty was judged to potentially 
over-estimate (``over''), under-estimate (``under''), or have an 
unknown impact to exposure/risk estimates was also characterized. Two 
types of quantitative analyses of the variability and uncertainty 
associated with the CL estimates used in the REA support the overall 
uncertainty characterization. The first type of analysis is a 
sensitivity analysis using Monte Carlo techniques to quantify CL 
estimate uncertainty associated with several model inputs, and the 
second is an analysis of the variation in CL estimates among the three 
primary modeling approaches on which the CLs used in this assessment 
were based.
    As overarching observations regarding uncertainty associated with 
this REA, we note two overarching aspects of the assessment. The first 
relates to interpretation of specific thresholds of ANC, and the second 
to our understanding of the biogeochemical linkages between deposition 
of S and N compounds and waterbody ANC, and the associated estimation 
of CLs. While ANC is an established indicator of aquatic acidification 
risk, there is uncertainty in our understanding of relationships 
between ANC and risk to native biota, particularly in waterbodies in 
geologic regions prone to waterbody acidity. Such uncertainties relate 
to the varying influences of site-specific factors other than ANC, such 
as soil type. Uncertainty associated with our understanding of the 
biogeochemical linkages between deposition and ANC and the 
determination of steady-state CLs is difficult to characterize and 
assess. Uncertainty in CL estimates is associated with parameters used 
in the steady-state CL models. While the Steady-State Water Chemistry 
(SSWC) and other CL models are well conceived and based on a 
substantial amount of research and applications available in the peer-
reviewed literature, there is uncertainty associated with the 
availability of the necessary data to support certain model components.
    The strength of the CL estimates and the exceedance calculation 
rely on the ability of models to estimate the catchment-average base-
cation supply (i.e., input of base cations from weathering of bedrock 
and soils and air), runoff, and surface water chemistry. The 
uncertainty associated with runoff and surface water parameters relates 
to availability of measurements; however, the ability to accurately 
estimate the catchment supply of base cations to a water body is still 
difficult and uncertain (PA, Appendix 5A, section 5A.3). This area of 
uncertainty is important because the catchment supply of base cations 
from the weathering of bedrock and soils is the factor with the 
greatest influence on the CL calculation and has the largest 
uncertainty (Li and McNulty, 2007). For example, the well-established 
models generally rely on input or simulated values for BCw rate, a 
parameter the ISA notes to be ``one of the most influential yet 
difficult to estimate parameters in the calculation of critical acid 
loads of N and S deposition for protection against terrestrial 
acidification'' (ISA, section IS.14.2.2.1). Obtaining accurate 
estimates of weathering rates is difficult because weathering is a 
process that occurs over very long periods of time, and the estimates 
on an ecosystem's ability to buffer acid deposition rely on accurate 
estimates of weathering. Although the approach to estimate base-cation 
supply for the national case study (e.g., F-factor approach) has been 
widely published and analyzed in Canada and Europe and has been applied 
in the U.S. (e.g., Dupont et al., 2005 and others), the uncertainty in 
this estimate is unclear and could be large in some cases.
    In light of the significant contribution of this input to the CL 
estimates, a quantitative uncertainty analysis of CL estimates based on 
state-steady CL modeling was performed (PA, Appendix 5A, section 
5A.3.1). This analysis, involving many model simulations for the more 
than 14,000 waterbodies, drawing on Monte Carlo sampling, provided a 
description of the uncertainty around the CL estimate in terms of the 
confidence interval for each waterbody mean result. The size of the 
confidence interval for S CL estimates ranged from 0.1 kg S/ha-yr at 
the 5th percentile to 5.3 kg S/ha-yr at the 95th percentile. Smaller 
confidence intervals were associated with CLs determined with long-term 
water quality data and low variability in runoff measurements. 
Estimates of CL determined by one or very few water quality 
measurements, and in areas where runoff is quite variable (e.g., the 
western U.S.), had larger confidence intervals, indicating greater 
uncertainty. Critical load estimates with the lowest uncertainty

[[Page 105722]]

were for waterbody sites in the eastern U.S., particularly along the 
Appalachian Mountains, in the Upper Midwest, and in the Rocky 
Mountains, which are areas for which there are relatively larger site-
specific datasets (e.g., for water quality parameters). Greater 
uncertainty is associated with CLs in the Midwest and South and along 
the California to Washington coast. This uncertainty in the Midwest is 
associated with most of the CLs in waterbodies in this area being based 
on one or a few water quality measurements, while the high uncertainty 
for sites along the California and Washington coasts relates to 
variability in runoff values. On average, the size of the confidence 
interval for the vast majority of CLs (those based on the widely used 
steady-state water chemistry model) was 7.68 meq S/m\2\-yr or 1.3 kg S/
ha-yr, giving a confidence interval of 3.84 meq/m\2\-yr or 
0.65 kg S/ha-yr. While a comprehensive analysis of 
uncertainty had not been completed for these estimates prior to this 
assessment, judgment by EPA experts suggested the uncertainty for 
combined N and S CLs to be on average about 0.5 kg/ha-yr 
(3.125 meq/m\2\-yr), which is generally consistent with the range of 
uncertainty determined from this quantitative uncertainty analysis (PA, 
Appendix 5A, section 5A.3).
    At the ecoregion scale, 51 ecoregions had sufficient data to 
calculate the 5th to 95th percentile (PA, Appendix 5A, Table 5A-56). 
Smaller confidence intervals around the mean CL (i.e., lower 
uncertainty CLs) were associated with ecoregions in the Appalachian 
Mountains (e.g., Northern Appalachian and Atlantic Maritime Highlands, 
Blue Ridge, Northern Lakes and Forests, and North Central Appalachians) 
and Rockies (e.g., Sierra Nevada, Southern Rockies, and Idaho 
Batholith). Ecoregions with more uncertain CLs included the 
Northeastern Coastal Zone, Cascades, Coast Range, Interior Plateau, and 
Klamath Mountains/California High North Coast Range.
    Although the vast majority of CLs in this assessment were based on 
the SSWC model, an analysis was conducted to understand differences in 
the CLs calculated with the different methods. There are three main CL 
approaches, all based on the watershed mass-balance approach where 
acid-base inputs are balanced. The three approaches include: (1) SSWC 
model and F-Factor that is based on quantitative relationships to water 
chemistry (Dupont et al., 2005; Scheffe et al., 2014; Lynch et al., 
2022), (2) Statistical Regression Model that extrapolated weathering 
rates across the landscape using water quality or landscape factors 
(Sullivan et al., 2012b; McDonnell et al., 2014), and (3) Dynamic 
Models (Model of Acidification of Groundwater In Catchments [MAGIC)] or 
Photosynthesis EvapoTranspiration Biogeochemcial model [Pnet-BGC]). 
Critical load values were compared between these models to determine 
model biases. Results from the comparison between different CL methods 
that were used to calculate the critical loads in the NCLD are 
summarized in PA Appendix 5A, section 5A.3.1, for lakes in New England 
and the Adirondacks and streams in the Appalachian Mountains. Overall, 
good agreement was found between the three methods used to calculate 
CLs, indicating there was not a systematic bias between the methods and 
that they should produce comparable results when used together as they 
were in these analyses (PA, Appendix 5A, section 5A.3).
c. Summary of Results
    The findings from the aquatic acidification REA are summarized in 
terms of S deposition due to the finding of a negligible additional 
influence of N deposition compared to S deposition on acidification in 
this assessment \63\ (PA, Appendix 5A, section 5A.2.1). As summarized 
more fully below, the analyses of five case study areas, including the 
acidification-impacted Shenandoah Valley area, indicate that with 
annual average S deposition below 12 and 10 kg/ha yr, the average 
waterbody in each area (average as to acid-sensitivity) would be 
estimated to achieve the ANC benchmarks of 20 and 50 [micro]eq/L, 
respectively. Seventy percent of waterbodies in each area would be 
estimated to achieve these benchmarks with deposition below 10 and 7 
kg/ha-yr, respectively. At the ecoregion-scale, the results from the 
analysis of 25 ecoregions, dominated by acid-sensitive waterbodies, 
indicate acid buffering capabilities to have improved substantially 
over the past 20 years, and particularly between the first and second 
decades of the period. By the 2010-2012 period, the percentages of 
waterbodies achieving the three ANC benchmarks in all 25 ecoregions 
exceeded 80%, 80% and 70% (for 20, 30 and 50 [micro]eq/L, 
respectively). By the subsequent analysis period (2014-2016), these 
percentages were 90%, 80% and 80%. The ecoregion median annual average 
deposition in all 25 ecoregions was below 8 kg/ha-yr for 2010-2012 and 
below 5 kg/ha-yr for 2014-2016. An alternate approach to analyzing 
these estimates (for the 25 ecoregions across all five time periods) 
suggested that the three ANC benchmarks could be met in more than 80%, 
80% and 70% (for 20, 30 and 50 [micro]eq/L, respectively) of 
waterbodies per ecoregion in all ecoregions and time periods for which 
annual average ecoregion median deposition is estimated to be at or 
below 7 kg/ha-yr.
---------------------------------------------------------------------------

    \63\ More specifically, the percentage of waterbodies across the 
contiguous U.S. estimated to exceed a CL for combined total S and N 
are very similar or just slightly higher (e.g., by 1-2%) than S only 
percentages of the waterbodies estimated to not meet the ANC 
benchmarks. This indicates that most of the N deposition entering 
the watershed is retained within the watershed and/or converted to 
gaseous N (PA, Appendix 5A, section 5A.2.1).
---------------------------------------------------------------------------

    Between the three-year period of 2000-2002, which was the analysis 
year for the 2009 REA, and 2018-2020, the latest period considered in 
the REA for this review, national average sulfur deposition has 
declined appreciably across the U.S. This decline in deposition is 
reflected in the very different aquatic acidification impact estimates 
for the two periods. Unlike the findings for 2000-2002 in the 2009 REA, 
in the national-scale analysis of the current REA, few waterbody sites 
are estimated to be receiving deposition in excess of their CLs for 
relevant ANC targets under recent S deposition levels. While 
recognizing inherent limitations and associated uncertainties of any 
such analysis, the national-scale assessment performed as part of the 
current review indicates that under deposition scenarios for the 2018-
2020 period, the percentage of waterbodies nationwide that might not be 
able to maintain an ANC of 50 [micro]eq/L is less than 5% (table 1; PA, 
Table 5-1).

[[Page 105723]]



Table 1--Percentage of Waterbodies Nationally for Which Annual Average S Deposition During the Five Time Periods
         Assessed Exceed the Waterbody CL (for CLs Greater Than 0) for Each of the Specified ANC Targets
----------------------------------------------------------------------------------------------------------------
        ANC ([micro]eq/L)          2001-2003  %    2006-2008  %    2010-2012  %    2014-2016  %    2018-2020  %
----------------------------------------------------------------------------------------------------------------
20..............................              22              16               5               3               1
30..............................              25              19               7               4               2
50..............................              28              24              11               6               4
50/20 *.........................              28              23              10               6               4
----------------------------------------------------------------------------------------------------------------
* This combination refers to the use of a target of 50 [micro]eq/L in eastern ecoregions and 20 [micro]eq/L in
  western ecoregions.

    The case study analyses provide estimates of S deposition (with 
associated uncertainties) that might be expected to allow these 
geographically diverse locations to meet the three ANC targets (PA, 
Table 5-6). Focusing on the three eastern case studies, the CL modeling 
indicates that at an annual average S deposition of 9-10 kg/ha-yr, the 
sites in these areas, on average,\64\ might be expected to achieve an 
ANC at or above 50 [micro]eq/L. At an annual average S deposition of 
about 6-9 kg/ha-yr, 70% of the sites in the areas are estimated to 
achieve an ANC at or above 20 [micro]eq/L and at about 5-8 kg S/ha-yr, 
70% are estimated to achieve an ANC at or above 30 [micro]eq/L. Lower S 
deposition values are estimated to achieve higher ANC across more 
sites. Across the three eastern areas, the CL estimates for each ANC 
target are lowest for the White Mountains National Forest study area, 
and highest for the Shenandoah Valley study area.
---------------------------------------------------------------------------

    \64\ The term ``average'' here refers to the average CL 
estimated for the specified ANC across all sites with CL estimates 
in each case study area (PA, Table 5-6).
---------------------------------------------------------------------------

    The ecoregion-level analyses of 25 acid-sensitive ecoregions for 
the five periods from 2001-2003 through 2018-2020 illustrate the 
spatial variability and magnitude of the findings for the three target 
ANC levels and the temporal changes across the 20-year period, as 
described in the PA, section 5.1.3.2. For example, during the two most 
recent 3-year periods, the median S deposition estimates for each of 
the 25 ecoregions were all below 5 kg/ha-yr in 2014-2016 and all below 
4 kg/ha-yr in 2018-2020 (table 2). Across all five time periods, the 
range of ecoregion median S deposition extended from below 2 kg/ha-yr 
up to nearly 18 kg/ha-yr, with the higher values occurring in the 
eastern ecoregions (table 2).

     Table 2--Summary of Ecoregion Medians Derived as Median of S Deposition Estimates at CL Sites Within an
                                                    Ecoregion
----------------------------------------------------------------------------------------------------------------
                                                     Ecoregion median * total sulfur deposition (kg S/ha-yr)
                                                ----------------------------------------------------------------
                                                   2001-03      2006-08      2010-12      2014-16      2018-20
----------------------------------------------------------------------------------------------------------------
                                         All 25 Ecoregions:
----------------------------------------------------------------------------------------------------------------
    Minimum....................................         1.18         1.22         1.02         1.08         0.62
    Maximum....................................        17.27        14.44         7.25         4.58         3.88
    Median.....................................         7.77         6.50         3.71         2.32         1.73
----------------------------------------------------------------------------------------------------------------
                                       18 Eastern Ecoregions:
----------------------------------------------------------------------------------------------------------------
    Minimum....................................         4.01         3.10         2.34         1.88         1.31
    Maximum....................................        17.27        14.44         7.25         4.58         3.88
    Median.....................................        11.08         9.36         4.76         2.97         2.04
----------------------------------------------------------------------------------------------------------------
                                       7 Western Ecoregions:
----------------------------------------------------------------------------------------------------------------
    Minimum....................................         1.18         1.22         1.02         1.08         0.62
    Maximum....................................         1.94         1.83         1.47         1.56         1.19
    Median.....................................         1.40         1.52         1.29         1.17         0.87
----------------------------------------------------------------------------------------------------------------
* The ecoregion medians for which descriptive statistics are presented here are medians of the deposition
  estimates across each ecoregion's waterbody sites with CL estimates.

    The ecoregion-scale results (e.g., percentage of waterbodies per 
ecoregion estimated to achieve the various ANC targets, or 
alternatively to exceed the associated CLs) for the 18 eastern and 7 
western ecoregions are summarized in two ways. One approach, summarized 
further below, is framed by the temporal trends in median S deposition 
per ecoregion, and the second approach is in terms of ecoregion-time 
period combinations, using ecoregion S deposition estimates (medians of 
deposition estimates at waterbodies with CLs in each ecoregion) as the 
organizing parameter. For example, table 3 presents the percentages of 
waterbody sites per ecoregion estimated to achieve the three ANC target 
levels, summarized by bins for different magnitudes of ecoregion median 
annual average S deposition (regardless of the 3-year period in which 
it occurred). For the 18 eastern ecoregions and five time periods, 
there are 90 ecoregion-time period combinations, and for each of these, 
there are waterbody percentages for each of the three ANC targets. In 
table 3, the three percentages (for the three ANC targets) for each of 
the 18

[[Page 105724]]

eastern ecoregions in each of the five time periods are grouped in the 
bins describing the median S deposition in that ecoregion and time 
period. As can be seen from this table, fewer than half of the eastern 
ecoregion-time period combinations had an ecoregion median S deposition 
estimate at or below 4 kg/ha-yr.\65\ Table 3 indicates that lower 
levels of S deposition at the ecoregion scale are associated with 
improved ANC values and greater percentages of waterbodies expected to 
reach ANC targets. Across the ecoregion-time period dataset of CL 
exceedances for the three ANC targets for all 90 eastern ecoregion-time 
period combinations (for which ecoregion median S deposition was at or 
below 18 kg/ha-yr), 73% of the combinations had at least 90% of 
waterbodies per ecoregion estimated to achieve ANC at or above 20 
[micro]eq/L, and 60% had at least 90% of the waterbodies estimated to 
achieve ANC at or above 50 [micro]eq/L (table 3). For ecoregion median 
S deposition estimates at or below 9 kg/ha-yr (approximately three 
quarters of the combinations), at least 90% of all waterbodies per 
ecoregion were estimated to achieve ANC at or above 20, 30 and 50 
[micro]eq/L in 87%, 81% and 72% of combinations, respectively. For S 
deposition estimates at or below 5 kg S/ha-yr (the lowest ecoregion 
median deposition bin that includes at least half of the full dataset), 
these values are 96%, 92% and 82% of combinations. For the 75 western 
ecoregion-time period combinations, all of which had ecoregion median S 
deposition estimates below 4 kg/ha-yr, at least 90% of waterbodies per 
ecoregion were estimated to achieve an ANC at or above 50 [micro]eg/L 
(PA, Table 5-5).
---------------------------------------------------------------------------

    \65\ The ecoregion median S deposition in all seven of the 
western ecoregions in all five time periods were at or below 2 kg/
ha-yr (PA, Table 5-4).

 Table 3--Percentage of Ecoregion-Time Periods Combinations With at Least 90, 85, 80, 75 and 70% of Waterbodies Estimated To Achieve an ANC at/Above the
 ANC Targets of 20, 30 and 50 [micro]eq/L as a Function of Annual Average S Deposition for 18 Eastern Ecoregions (90 Ecoregion-Time Period Combinations)
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                  Number of                       % Waterbodies per ecoregion-time period meeting specified ANC target
Total sulfur deposition (kg S/   ecoregion-   ----------------------------------------------------------------------------------------------------------
       ha-yr) at/below:         time  periods   90%    85%    80%    75%     70%    90%    85%    80%    75%     70%    90%    85%    80%    75%    70%
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                  ANC target of 20 [micro]eq/L
                                      ANC target of 30 [micro]eq/L
                                      ANC target of 50 [micro]eq/L
--------------------------------------------------------------------------------------------------------------------------------------------------------
2............................              10    100    100    100    100    100     100    100    100    100    100     100    100    100    100    100
3............................              29    100    100    100    100    100     100    100    100    100    100      97    100    100    100    100
4............................              41    100    100    100    100    100      95    100    100    100    100      93     98    100    100    100
5............................              51     96     98    100    100    100      92     98    100    100    100      82     94     96     98    100
6............................              59     93     98    100    100    100      88     98    100    100    100      78     93     97     98    100
7............................              63     92     98    100    100    100      87     97    100    100    100      78     92     95     98    100
8............................              67     87     94    100    100    100      82     91     99    100    100      73     87     93     96    100
9............................              69     87     94    100    100    100      81     91     99    100    100      72     87     93     96    100
10...........................              73     85     92     99     99     99      78     89     97     99     99      70     85     92     95     99
11...........................              76     83     91     97     99     99      76     88     96     99     99      68     83     91     95     99
12...........................              79     81     89     95     96     97      73     86     94     96     96      66     81     89     92     96
13...........................              81     80     88     95     96     98      73     85     94     96     96      65     80     88     93     96
14...........................              84     77     86     93     95     96      70     83     92     94     95      63     79     86     90     94
15...........................              86     76     84     91     93     95      69     81     90     92     93      62     77     84     88     92
16...........................              88     75     83     90     92     94      68     81     89     91     92      61     76     83     88     91
17...........................              88     75     83     90     92     94      68     81     89     91     92      61     76     83     88     91
18...........................              90     73     81     88     90     92      67     79     87     89     90      60     74     81     86     89
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Given the decreasing temporal trend in S deposition across all 
ecoregions, we also analyzed the aquatic acidification results at the 
ecoregion scale across the 20 years represented by the five time 
periods (2001-03, 2006-08, 2010-12, 2014-16, 2018-20) from a temporal 
perspective. With regard to percentages of waterbodies per ecoregion 
estimated to achieve the three ANC targets, an appreciable improvement 
is observed for the latter three time periods compared to the initial 
two time periods (e.g., PA, Figure 5-13). By the 2010-2012 time period, 
more than 70% of waterbodies in all 25 ecoregions are estimated to 
achieve an ANC at or above 50 [micro]eq/L, and at least 85% are able to 
achieve an ANC at or above 20 [micro]eq/L (figure 1; PA, Table 7-2). By 
the 2014-2016 period, the percentages are 85% and nearly 90%, 
respectively. The median deposition for the CL sites in each of the 18 
eastern ecoregions during the latter three time periods ranges from 1.3 
kg S/h-yr to 7.3 kg S/h-yr, and with each reduction in S deposition in 
each subsequent time period, more waterbodies in each of the eastern 
ecoregions are estimated to be able to achieve the ANC targets. Nearly 
90% of the 18 eastern ecoregions are estimated to have at least 90% of 
their waterbodies achieving an ANC of 20 [micro]eq/L in the 2010-12 
period and achieving an ANC of 50 [micro]eq/L in the 2014-16 period. 
When the 7 western ecoregions are included in a summary based on ANC 
targets of 20 [micro]eq/L for the West and 50 [micro]eq/L for the 
East,\66\ over 70% of the full set of ecoregions are estimated to have 
at least 90% of their waterbodies achieving the ANC targets by the 
2010-12 period. More than 90% of the ecoregions are estimated to have 
at least 90% of their waterbodies achieving the ANC targets by the 
2014-16 period (figure 1; \67\ PA, Table 7-2).
---------------------------------------------------------------------------

    \66\ This combination of targets recognizes the naturally and 
typically low ANC levels observed in western waterbodies while also 
including a higher target for the East (as described in the PA, 
section 5.1.2.2).
    \67\ The right panel of this figure has been corrected from the 
version that was in the proposal. The right panel of this figure in 
the proposal (89 FR 26656, April 15, 2024) had a few extraneous 
datapoints in the space between the 2006-2008 and 2010-2012 vertical 
lines. These extraneous datapoints are also in the right panel of an 
earlier version of this figure in the PA (PA, Figure 7-1). Also, in 
the left panel of the PA, Figure 7-1, the datapoints for the 2018-
2020 period were placed to the left of the 2018-2020 vertical line.
---------------------------------------------------------------------------

BILLING CODE 6560-50-P

[[Page 105725]]

[GRAPHIC] [TIFF OMITTED] TR27DE24.000


[[Page 105726]]


BILLING CODE 6560-50-C

B. Conclusions

1. Basis for Proposed Decision
    In reaching his proposed decision on the existing secondary 
standards for SOX, N oxides and PM (presented in section 
II.B.1.c.), the Administrator took into account the available evidence 
in the ISA, along with the policy-relevant, evidence-based and air 
quality-, exposure- and risk-based considerations discussed in the PA 
(summarized in section II.B.1.a.), as well as advice from the CASAC 
(section II.B.1.b.). In general, the role of the PA is to help ``bridge 
the gap'' between the Agency's assessment of the current evidence and 
quantitative analyses of air quality, exposure and risk, and the 
judgments required of the Administrator in determining whether it is 
appropriate to retain or revise the NAAQS. Evidence-based 
considerations draw upon the EPA's integrated assessment of the 
scientific evidence presented in the ISA (summarized in section II.A.3. 
above) to address key policy-relevant questions in the review. 
Similarly, the air quality-, exposure- and risk-based considerations 
draw upon our assessment of air quality, exposure, and associated risk 
(summarized in section II.A.4. above).
    This approach to reviewing the secondary standards is consistent 
with requirements of the provisions of the CAA related to the review of 
the NAAQS and with how the EPA and the courts have historically 
interpreted the CAA. As discussed in section I.A. above, these 
provisions require the Administrator to establish secondary standards 
that, in the Administrator's judgment, are requisite (i.e., neither 
more nor less stringent than necessary) to protect the public welfare 
from known or anticipated adverse effects associated with the presence 
of the pollutant in the ambient air. Consistent with the Agency's 
approach across all NAAQS reviews, the EPA's approach to informing 
these judgments is based on a recognition that the available welfare 
effects evidence generally reflects a continuum that includes ambient 
air-related exposures for which scientists generally agree that effects 
are likely to occur, through lower levels at which the likelihood and 
magnitude of response become increasingly uncertain. The CAA does not 
require the Administrator to establish secondary standards at a zero-
risk level, but rather at levels that reduce risk sufficiently so as to 
protect the public welfare from known or anticipated adverse effects. 
The proposed decision on the secondary standards for SOX, N 
oxides and PM described below is a public welfare policy judgment by 
the Administrator that draws upon the scientific evidence for welfare 
effects, quantitative analyses of air quality, exposure, and risks, as 
available, and judgments about how to consider the uncertainties and 
limitations that are inherent in the scientific evidence and 
quantitative analyses. The four basic elements of the NAAQS (i.e., 
indicator, averaging time, form, and level) have been considered 
collectively in evaluating the public welfare protection afforded by 
the current standards. The Administrator's final decision additionally 
considers public comments received on this proposed decision.
a. Policy-Relevant Evaluations in the Policy Assessment
    The PA presented an evaluation of the evidence and quantitative 
analyses of air quality, exposure and potential risk related to 
ecological effects of SOX, N oxides and PM. These ecological 
effects include both direct effects of the three criteria pollutants on 
biota and ecological effects of ecosystem deposition of N and S 
associated with these pollutants. The PA identified an array of policy 
options for consideration by the Administrator. For SOX, the 
PA identified options for adoption of an annual average SO2 
standard, averaged over three years, with a level within the range 
extending below 15 ppb and down to 5 ppb. For N oxides and 
PM2.5, the PA identified options for retention of the 
existing standards, without revision, and options for revision, 
although with recognition of appreciable associated uncertainty. The PA 
also considered the potential for establishment of a revised secondary 
standard or suite of standards with alternate indicator(s) that might 
target specific N or S containing chemicals (e.g., particulate 
NO3-, SO4\2\-, 
NH4\+\), but recognized there to be a number of associated 
uncertainties and complications, including uncertainties in how to 
interpret air measurements and deposition estimates from remote areas 
in the context of concentrations near sources, without finding there to 
be a clear advantage to this approach. The PA additionally recognized 
that, in secondary NAAQS reviews in general, decisions by the 
Administrator on the adequacy of existing standards or the 
appropriateness of new or revised standards depend in part on public 
welfare policy judgments, science policy judgments regarding aspects of 
the evidence and exposure/risk estimates, and judgments about the level 
of public welfare protection that is requisite under the CAA.
    In its evaluation of policy options, the PA considered the 
evidence, as evaluated in both the current and prior reviews, with 
regard to the EPA's overall conclusions on the ecological effects of 
SOX, N oxides and PM in ambient air and once deposited into 
ecosystems. The PA also considers the available information related to 
the general approach or framework in which to evaluate public welfare 
protection of the standard and the currently available quantitative 
information on environmental exposures likely to occur in areas of the 
U.S. where the standards are met. In so doing, the PA takes into 
account associated limitations and uncertainties, as well as the 
significance of these exposures with regard to the potential for 
effects, their potential severity and any associated public welfare 
implications. The PA also considers judgments about the uncertainties 
in the scientific evidence and quantitative analyses that are integral 
to consideration of whether the currently available information 
supports or calls into question the adequacy of the current secondary 
standards.
(1) Effects Not Related to S and N Deposition
    In considering the currently available evidence and quantitative 
information pertaining to ecological effects of SOX, N 
oxides and PM in ambient air, other than those associated with 
ecosystem deposition of S and N, the PA focused on the extent to which 
the newly available information alters our scientific understanding of 
the ecological effects of SOX, N oxides and PM in ambient 
air; the extent to which the currently available information indicates 
the potential for exposures associated with ecological effects under 
air quality meeting the existing standards and whether such effects 
might be of sufficient magnitude, severity, extent and/or frequency 
such that they might reasonably be judged to be adverse to public 
welfare; and to what extent important uncertainties identified in past 
reviews have been reduced and/or whether new uncertainties emerged. 
These considerations are summarized below, first for SOX, 
followed by N oxides and then PM.
(a) Sulfur Oxides
    Most of the available evidence for the direct effects of 
SOX on vegetation is not new to the current review. Among 
the gaseous SOX--which include SO, SO2, sulfur 
trioxide, and disulfur monoxide--only SO2 is present in the 
lower troposphere at concentrations

[[Page 105727]]

relevant for environmental considerations (ISA, Appendix 2, section 
2.1). The available evidence is focused primarily on the effects of 
SO2 on vegetation, including foliar injury, depressed 
photosynthesis and reduced growth or yield (ISA, Appendix 3, section 
3.2). The newer studies continue to support the determination that the 
evidence is sufficient to infer a causal relationship between gas-phase 
SO2 and injury to vegetation (ISA, section 3.6.1). In 
general, direct effects on plants, including foliar injury, occur at 
SO2 exposures higher than a 3-hour average concentration of 
0.5 ppm (500 ppb).
    Uncertainties associated with the current information relate to 
limitations in reflecting the natural environment and in untangling 
effects of SO2 from those of other pollutants that may have 
influenced the analyzed effects. Even with these uncertainties, the 
evidence indicates effects are generally associated with air 
concentrations and durations not expected to occur when the existing 
standard (0.5 ppm, as a 3-hour average, not to be exceeded more than 
once per year) is met (PA, section 7.1.1; ISA, Appendix 2, section 
2.1).
(b) Nitrogen Oxides
    The currently available information on direct effects of gaseous N 
oxides in ambient air on plants and lichens is composed predominantly 
of studies of NO2, HNO3, and PAN. The very few 
studies newly available in this review do not alter our prior 
understanding of effects of these N oxides, which include visible 
foliar injury, as well as effects on photosynthesis and growth at 
exposures much higher than current levels in ambient air (ISA, section 
3.3). Thus, as in the last review, the body of evidence is sufficient 
to infer a causal relationship between gas-phase NO, NO2, 
and PAN and injury to vegetation (ISA, section IS.4.2).
    Information is limited regarding the potential for exposure levels 
associated with ecological effects to occur under air quality meeting 
the existing NO2 secondary standard. With regard to the risk 
posed by N oxides, and particularly HNO3, the evidence 
summarized in the ISA indicates the potential for effects on lichen 
species related to air quality occurring during periods when the 
current secondary standard was not met. Evidence is more limited for 
consideration of effects under conditions meeting the current standard 
(PA, section 7.1.2). Uncertainties also remain in our interpretation of 
the evidence, including those related to limitations and uncertainties 
of the various study types.
(c) Particulate Matter
    The evidence for ecological effects of PM is consistent with that 
available in the last review and focused on effects associated with PM 
loading (e.g., to leaf surfaces), rather than direct effects of PM 
suspended in ambient air. In this review, as in the last one, the 
ecological effects evidence was found to be sufficient to conclude 
there is likely to exist a causal relationship between deposition of PM 
(other than N and S deposition) and a variety of effects on individual 
organisms and ecosystems (ISA, Appendix 15; 2012 p.m. ISA, section 
9.4). While some uncertainties remain, new uncertainties have not 
emerged since the last review. There is little information available on 
effects of PM concentrations likely to occur under conditions meeting 
the current secondary standards, and the limited available information 
does not indicate effects to occur under those conditions (PA, section 
7.1.3).
(2) Evidence of Ecosystem Effects of S and N Deposition
    The evidence base of ecological effects related to atmospheric 
deposition of N and S compounds has expanded since the last review with 
regard to acidic deposition in aquatic and terrestrial ecosystems and 
regarding ecosystem N enrichment. Both S and N compounds have 
contributed to ecosystem acidification, with relative contributions 
varying with emissions, air concentrations, and atmospheric chemistry, 
among other factors. Ecological effects have been documented 
comprehensively in waterbodies of the Adirondack and Appalachian 
Mountains, and in forests of the Northeast, at the organism to 
ecosystem scale. With regard to N enrichment, research on its effects 
in estuaries and large river systems across the U.S. extends back at 
least four decades, and there is longstanding evidence of effects in 
estuaries along the East and Gulf Coasts of the U.S., as summarized in 
more detail in Chapters 4 and 5 of the PA (ISA, Appendix 7, section 
7.2.9; 2008 ISA, section 3.3.2.4; Officer et al., 1984). Information on 
the effects of N enrichment in terrestrial ecosystems, primarily in 
grassland and forested ecosystems, augmented in the current review, 
also includes evidence that was available in the last review (e.g., 
2008 ISA, sections 3.3.3 and 3.3.5; ISA, Appendix 6).
    With regard to uncertainties, some that were associated with the 
evidence available in the 2012 review remain, and some additional 
important uncertainties have been identified. In addition to 
uncertainties related to the specific air quality circumstances 
associated with effects (e.g., magnitude, duration, and frequency of 
concentrations associated with effects), there are also uncertainties 
associated with the effects of N and S deposition expected under 
changing environmental circumstances. Such uncertainties include 
atmospheric loading that has declined since 2000, with associated 
changes to soil and waterbody biogeochemistry and meteorological 
changes associated with changing climate (ISA, section IS.12; PA 
section, 7.2.1). The PA also recognizes important uncertainties 
associated with the various assessment approaches employed by different 
study types (PA, sections 5.3 and 7.2.1). Additionally, there are 
uncertainties contributed by variation in physical, chemical, and 
ecological responses to N and S deposition and by the potential 
influence of unaccounted-for stressors on response measures.
    In sum, a wealth of scientific evidence, spanning many decades, 
demonstrates effects of acidifying deposition associated with N and S 
compounds in aquatic and terrestrial ecosystems (ISA, sections ES.5.1, 
IS.5.1, IS.5.3, IS.6.1 and IS.6.3; 2008 ISA, section 3.2; U.S. EPA, 
1982b, Chapter 7). This evidence base supports conclusions also reached 
in the 2008 ISA (for the review completed in 2012) of causal 
relationships between N and S deposition and alteration of soil and 
aquatic biogeochemistry, alteration of the physiology and growth of 
terrestrial organisms and of associated productivity, changes in 
aquatic biota, including physiological impairment, and alteration of 
species richness, community composition, and biodiversity in both 
aquatic and terrestrial ecosystems (ISA, Table ES-1). Similarly, a 
robust evidence base demonstrates effects of N enrichment in both 
estuarine and freshwater ecosystems, supporting conclusions also 
reached in the last review of a causal relationship between N 
deposition and changes in biota, including altered growth and 
productivity, and alteration of species richness, community composition 
and biodiversity due to N enrichment (ISA, sections ES.5.2, IS.6, and 
IS.7, and Table ES-1). Additional effects of N deposition in wetlands, 
also recognized in the last review, include alteration of 
biogeochemical cycling, growth, productivity, species physiology, 
species richness, community composition, and biodiversity (ISA, Table 
ES-1).
    In terrestrial ecosystems, as in the last review, the now expanded 
evidence

[[Page 105728]]

base supports determination of a causal relationship between N 
deposition and alteration of species richness, community composition, 
and biodiversity (ISA, Table ES-1). The ISA additionally determines 
there to be a causal relationship for alteration of the physiology and 
growth of terrestrial organisms and associated productivity, a category 
of effects not included in the 2008 ISA (ISA, Table ES-1). Other 
evidence of effects causally associated with S deposition in wetland 
and freshwater ecosystems includes that related to chemical 
transformation and associated toxicity, most specifically alteration of 
mercury methylation, which was also recognized in the last review. The 
other category of effects, not included in the last review, is related 
to sulfide phytotoxicity and its associated effects in wetland and 
freshwater ecosystems (ISA, Table ES-1).
    Thus, while an array of effects is associated with S and N 
deposition, information important for quantitative analysis varies 
across the array. For some categories of effects (e.g., sulfide 
phytotoxicity) the information regarding environmental levels that 
relate to effects is limited and/or quite variable across locations, 
thus hindering analysis. For other effect categories, the information 
on linkages to criteria pollutants is limited and/or quite variable. 
The information with clearest implications to NAAQS decisions pertains 
to SOX and S deposition-related ecosystem acidification. 
While the information regarding effects associated with N loading to 
ecosystems is extensive, information to support quantitative analysis 
to inform NAAQS decisions regarding N oxides and PM is not clear, with 
multiple complicating factors. Such factors include contributions from 
other, non-criteria pollutants (such as NH3) and challenges 
in assessing N deposition-related effects of ambient air concentrations 
of N oxides and PM. While the role of N deposition in aquatic 
acidification is evaluated in the REA, the available information does 
not provide effective support for analysis of other N deposition-
related effects of N oxides and PM independent of effects from other 
(non-criteria) pollutants or, in some cases, from other (non-air) 
sources.
(3) Sulfur Deposition and SOX
    Evidence- and exposure/risk-based considerations discussed in the 
PA pertaining to S deposition and SOX in ambient air are 
summarized in the subsections below. These considerations reflect 
discussion in the PA, which draws on the available welfare effects 
evidence described in the current ISA, the 2008 NOX/
SOX ISA, the 2009 p.m. ISA, and past AQCDs, as well as 
information available from quantitative analyses (summarized in 
Chapters 5 and 6 of the PA), both analyses developed in this review and 
those available from the 2009 REA.
    In considering potential public welfare protection from S 
deposition-related acidification effects in aquatic ecosystems and 
forested areas, the PA recognizes the public welfare implications of 
various effects of acidifying deposition on the natural resources in 
these areas, including the differences in response between waterbodies 
and trees, as well as the severity and extent of such effects. Given 
the more extensive quantitative analyses for aquatic acidification in 
this review, the PA discusses the public welfare implications of S 
deposition-related effects in aquatic ecosystems with an eye toward 
their prominence for decision-making in this review (PA, sections 4.5 
and 7.2.2.2). In its consideration of options for S deposition-related 
effects and in recognizing linkages between watershed soils and 
waterbody acidification, as well as terrestrial effects, the PA conveys 
that focusing on public welfare protection from aquatic acidification-
related effects may reasonably be expected to also contribute 
protection for terrestrial effects (PA, section 7.4).
    The PA notes that, as also recognized in the 2012 review, aquatic 
ecosystems provide a number of services important to the public 
welfare, ranging from recreational and commercial fisheries to 
recreational activities engaged in by the public (77 FR 20232, April 3, 
2012). Because aquatic acidification affects the diversity and 
abundance of aquatic biota, it also affects the ecosystem services that 
are derived from the fish and other aquatic life found in these surface 
waters (PA, section 4.5; ISA, Appendix 14, section 14.3.1). Fresh 
surface waters support several cultural services, such as aesthetic and 
educational services; the type of service that is likely to be most 
widely and significantly affected by aquatic acidification is 
recreational fishing, with associated economic and other benefits. 
Other potentially affected services include provision of food for some 
recreational and subsistence fishers and for other consumers, as well 
as non-use services, including existence (protection and preservation 
with no expectation of direct use) and bequest values (PA, section 
4.5).
    The PA recognizes that some level of S deposition and associated 
risk of aquatic acidification, including those associated with past 
decades of acidifying deposition in the Northeast, can impact the 
public welfare and thus might reasonably be judged adverse to the 
public welfare. Depending on magnitude and associated impacts, there 
are many locations in which S deposition and associated aquatic 
acidification can adversely affect the public welfare. For example, 
there is evidence in some waterbodies that aquatic acidification 
resulting in reduced acid buffering capacity can adversely affect 
waterbodies and associated fisheries, which in addition to any 
commercial ramifications can have ramifications on recreational 
enjoyment of affected areas (PA, sections 5.1.1 and 4.5).
    In other secondary NAAQS reviews, the EPA's consideration of the 
public welfare significance of the associated effects has recognized a 
particular importance of Class I areas and other similarly protected 
areas. Accordingly, we note that waterbodies that have been most 
affected by acidic deposition are in the eastern U.S., including in 
several Class I areas and other national and State parks and forests 
(PA, section 5.1.2.1),\68\ with two such areas included as case studies 
in the aquatic acidification REA (PA, section 5.1.3.3). Assuring 
continued improvement of affected waterbodies throughout the U.S. 
(e.g., through lower S deposition than the levels of the past) may 
reasonably be considered to be of public welfare importance and may be 
particularly important in Class I and similarly protected areas. In 
this review, in considering the potential public welfare significance 
of aquatic acidification effects of differing levels of S deposition, 
the PA summarizes the REA ecoregion-scale results in terms of 
percentages of ecoregions in which differing percentages of waterbodies 
are estimated to achieve the three acid buffering capacity targets. The 
PA summarized results in this way to inform identification of S 
deposition estimates in the context of potential policy options.
---------------------------------------------------------------------------

    \68\ A comparison of Figures 4-4 and 5-6 of the PA indicates 
multiple Class I areas in ecoregions considered acid sensitive.
---------------------------------------------------------------------------

    The first subsection below, II.B.1.a.(3)(a), focuses on the aquatic 
acidification REA analyses (summarized in section II.A.4. above), 
considering first the use of ANC as an indicator of acidification risk, 
then evaluating the risk estimates as to what they indicate about 
acidification risks in freshwater streams and lakes of the contiguous 
U.S. for S deposition rates estimated to have occurred over the past 
two decades

[[Page 105729]]

(much of which is newly assessed in this review),\69\ and lastly 
identifying important uncertainties associated with the estimates. 
Section II.B.1.a.(3)(b) considers the evidence and quantitative 
exposure/risk information from a public welfare protection perspective, 
focusing first on what might be indicated regarding deposition 
conditions under which waterbodies in acid-sensitive ecoregions might 
be expected to achieve acid buffering capacity of interest and what the 
available information indicates pertaining to the consideration of 
public welfare protection from S deposition related effects in aquatic 
ecosystems. Section II.B.1.a.(3)(b) also considers what the published 
quantitative information regarding S deposition and terrestrial 
acidification indicates regarding deposition levels of potential 
concern, along with associated uncertainties in this information. 
Section II.B.1.a.(3)(c) then summarizes considerations in relating 
SOX air quality metrics to deposition of S compounds.
---------------------------------------------------------------------------

    \69\ Aquatic acidification risk analyses in the last review 
considered deposition estimates for 2002 and 2006 derived from CMAQ 
modeling, 2002 emissions estimates (2009 REA, Appendix 1).
---------------------------------------------------------------------------

(a) Quantitative Information for Ecosystem Risks Associated With S 
Deposition
    As in the last review, the PA gives primary attention to the 
quantitative assessment of aquatic acidification (including 
particularly that attributable to S deposition) and recognizes these 
results to be informative to the identification of S deposition levels 
associated with potential for aquatic acidification effects of concern, 
as summarized below. This assessment of quantitative linkages between S 
deposition and potential for aquatic acidification is one component of 
the approach implemented in the PA for informing judgments on the 
likelihood of occurrence of such effects under differing air quality 
conditions. Although the approaches and tools for assessing aquatic 
acidification have often been applied for S and N deposition in 
combination, the REA approach for this review focused on S deposition. 
This focus is supported by analyses in the PA indicating the relatively 
greater contribution of S deposition than N deposition to aquatic 
acidification risk under the more recent air quality conditions that 
are the focus of this review (PA, Appendix 5A). As summarized in 
section II.A.4. above, the aquatic acidification REA relied on well-
established site-specific water quality modeling applications with a 
widely recognized indicator of aquatic acidification, ANC.
    Quantitative tools are also available for the assessment of 
terrestrial acidification related to S deposition (PA, section 5.3.2.1; 
2009 REA, section 4.3).\70\ In the last review, analyses that related 
estimated atmospheric deposition of acidic N and S compounds (during 
the early 2000s) to terrestrial effects, or indicators of terrestrial 
ecosystem risk, were generally considered to be more uncertain than 
conceptually similar modeling analyses for aquatic ecosystems (2009 
REA, section 7.5; 2011 PA, section 1.3). The PA for this review also 
notes that quantitative tools and approaches are not well developed for 
other ecological effects associated with atmospheric deposition of S 
compounds, such as mercury methylation and sulfide toxicity in aquatic 
systems (PA, sections 4.2.3.1 and 4.2.3.2).
---------------------------------------------------------------------------

    \70\ Given findings from the 2009 REA that aquatic acidification 
provided a more sensitive measure for use in assessing deposition 
related to ecosystem acidification, and consideration of recent 
information not likely to result in a different finding, the REA for 
the current review focused on aquatic acidification.
---------------------------------------------------------------------------

    As described in sections II.A.3.a.(2)(a) and II.A.4. above, ANC is 
an indicator of susceptibility or risk of acidification-related effects 
in waterbodies, with lower levels indicating relatively higher 
potential for acidification and related waterbody effects. The PA 
recognized strong support in the evidence for use of ANC for purposes 
of making judgments regarding risk to aquatic biota in streams impacted 
by acidifying deposition and for consideration of the set of targets 
analyzed in the aquatic acidification REA: 20, 30, and 50 [micro]eq/L 
(PA, section 5.1). There is longstanding evidence of an array of 
impacts on aquatic biota and species richness reported in surface 
waters with ANC values below zero and in some historically impacted 
waterbodies with ANC values below 20 [micro]eq/L (PA, section 5.1.2.2). 
The severity of impacts is greatest at the lowest ANC levels. This 
evidence derives primarily from lakes and streams of the Adirondack 
Mountains and areas along the Appalachian Mountains. As recognized in 
the 2012 review, in addition to providing protection during base flow 
situations, ANC is a water quality characteristic that affords 
protection against the likelihood of decreased pH from episodic events 
in impacted watersheds. For example, some waterbodies with ANC below 20 
[micro]eq/L have been associated with increased probability of low pH 
events, that, depending on other factors as noted above, may have 
potential for reduced survival or loss of fitness of sensitive biota or 
lifestages (2008 ISA, section 5.1.2.1). As noted in the ISA, 
``[s]treams that are designated as episodically acidic (chronic ANC 
from 0 to 20 [mu]eq/L) are considered marginal for brook trout because 
acidic episodes are likely'' (ISA, Appendix 8, p. 8-26). In general, 
the higher the ANC level above zero, the lower the risk presented by 
episodic acidity. In summarizing and considering the acidification risk 
estimates for the different scales of analysis (national, ecoregion and 
case study) and using the water quality modeling-based CLs derived for 
three different ANC targets (20, 30 and 50 [micro]eq/L), the PA 
recognizes both the differing risk that might be ascribed to the 
different ANC targets and the variation in ANC response across 
waterbodies that may be reasonable to expect with differences in 
geology, history of acidifying deposition, and patterns of S 
deposition.
    The PA also recognizes limitations and uncertainties in the use of 
ANC as an indicator for model-based risk assessments (PA, section 
7.2.2.1). The support is strongest in aquatic systems low in organic 
material such as historically affected waterbodies in the eastern U.S. 
(e.g., in the Adirondack Mountains) and Canada. In waterbodies with 
relatively higher levels of dissolved organic material, the presence of 
organic acid anions contributes to reduced pH, but these organic acids 
can also create complexes with dissolved aluminum that protect resident 
biota against aluminum toxicity such that biota in such systems 
tolerate lower ANC values (and pH) than biota in waterbodies with low 
dissolved organic carbon (ISA, Appendix 8, section 8.3.6.2; PA, section 
7.2.2.1). Thus, while the evidence generally supports the use of ANC as 
an acidification indicator and as a useful metric for judging the 
potential for ecosystem acidification effects to occur, the 
relationship between ANC and potential risk varies depending on the 
presence of naturally occurring organic acids, which can affect the 
responsiveness of ANC to acidifying deposition. For these reasons, ANC 
is less well supported as an indicator for acidic deposition-related 
effects (and waterbodies are less responsive to changes in acidic 
deposition) due to dissolved organic material in some areas, including 
the Middle Atlantic Coastal Plain, Southern Coastal Plains, and 
Atlantic Coastal Pine Barrens ecoregions (PA, section 5.1.2.2).
    The REA national-scale analysis of more than 13,000 waterbody sites 
in 69 ecoregions demonstrated an appreciable

[[Page 105730]]

reduction in risk over the 20-year period of analysis (PA, section 
5.1.3) with the percentage of waterbodies unable to achieve an ANC of 
20 [micro]eq/L or greater declining from 20% for the 2001-2003 period 
to 1% by the 2018-20 period (table 1). The 25 ecoregions included in 
the ecoregion-scale analyses (i.e., 18 in the East and 7 in the West in 
which there are at least 50 waterbody sites with CL estimates) are 
dominated by ecoregions categorized as acid sensitive (PA, Table 5A-5) 
and exclude the three ecoregions identified above as having natural 
acidity related to organic acids (PA, section 5.1.2.1). Due to the 
dominance of the acid-sensitive ecoregions among the 25 ecoregions 
analyzed, the percentages of waterbodies not able to meet the ANC 
targets are higher than the national percentages. Specifically, in the 
most affected ecoregion (Central Appalachians), more than 50% of 
waterbodies were estimated to be unable to achieve an ANC of 20 
[micro]eq/L or greater based on S deposition estimates for the 2001-
2003 period (figure 1 above, and PA, Figure 5-13). By the 2018-2020 
period, less than 10% of waterbodies in any of the 25 ecoregions (and 
less than 5% in all but one) were estimated to be unable to achieve an 
ANC of 20 [micro]eq/L, and less than 15% of waterbodies in the most 
affected ecoregion were estimated to be unable to achieve an ANC of 50 
[micro]eq/L (figure 1 above and PA, Figure 5-13).
    The PA recognizes uncertainty associated with two overarching 
aspects of the aquatic acidification assessment of effects (PA, section 
5.1.4 and Appendix 5A, section 5A.3). The first relates to 
interpretation of specific thresholds or benchmark concentrations of 
ANC with regard to aquatic acidification risk to aquatic biota. While 
ANC is a well-established indicator of aquatic acidification risk, 
uncertainty remains in our understanding of relationships between ANC 
and risk to native biota, particularly in waterbodies in geologic 
regions prone to waterbody acidity. Such uncertainties relate to the 
varying influences of site-specific factors, such as the prevalence of 
organic acids in the watershed, and to historical loading to watershed 
soils that can influence acidity of episodic high-flow events (PA, 
sections 5.1.4 and 7.2.2.1 and Appendix 5A, section 5A.3). The second 
overarching aspect of uncertainty relates to our understanding of the 
biogeochemical model linkages between deposition of S and N compounds 
and waterbody ANC, which is reflected in the modeling employed, and the 
associated estimation of CLs, as described in section II.A.4.b. above. 
Although the approaches to estimate base-cation supply in the REA 
(e.g., the F-factor approach) have been widely published and analyzed 
in Canada and Europe, and have been applied in the U.S. (e.g., Dupont 
et al., 2005), the magnitude of uncertainty in the base-cation supply 
estimate is unclear and could be large in some cases. The REA's 
quantitative analysis of uncertainty in CL estimates indicates lower 
uncertainty associated with CLs estimated for sites with more extensive 
and longer-term water quality datasets and relatively low variability 
in the runoff measurements, such as CLs for waterbody sites in the 
eastern U.S. (PA, Appendix 5A, section 5A.3.1).
(b) General Approach for Considering Public Welfare Protection
    In discussing key considerations in judging public welfare 
protection from S deposition associated with the secondary standard for 
SOX, the PA first focused on what the aquatic acidification 
REA indicated about deposition conditions under which waterbodies in 
sensitive ecoregions might be expected to achieve ANC levels of 
interest. Particular focus was given to the ecoregion and case-study 
analyses, which use the waterbody-specific comparisons of estimated 
deposition and waterbody CLs to provide ecoregion wide and cross-
ecoregion summaries of estimated waterbody responses to ecoregion 
estimates of deposition. The PA also considered the extent to which 
waterbodies in each ecoregion analyzed were estimated to achieve or 
exceed the three target ANC levels in the context of the variation in 
ANC response reasonably expected across waterbodies in an ecoregion due 
to differences in watershed sensitivity to S deposition impacts and 
different spatial or geographic patterns of S deposition.
    Based on the array of CL-based analyses, the PA provides a general 
sense of the ANC values that waterbodies in sensitive regions across 
the continental U.S. may be able to achieve, including for areas 
heavily affected by a long history of acidifying deposition, such as 
waterbodies in the well-studied Shenandoah Valley area (4,977 sites 
distributed across three ecoregions). For the other case study areas 
(White Mountain National Forest, Northern Minnesota, Sierra Nevada 
Mountains and Rocky Mountain National Park), there are appreciably 
fewer waterbody sites for which modeling has been performed to estimate 
CLs, and accordingly greater uncertainty. Yet, the case study area 
averages of waterbody CLs for achieving ANC at or above each of the 
three targets (20, 30 or 50 [micro]eq/L) are quite similar across the 
five case studies (PA, Table 5-6). The PA found the case study 
estimates to suggest that a focus on S deposition below 10 kg/ha-yr may 
be appropriate.
    Findings from the ecoregion-scale analyses of 25 ecoregions (18 
East and 7 West), nearly all of which are considered acid sensitive, 
indicated ranges of deposition (summarized in terms of ecoregion 
medians) associated with high percentages of waterbodies estimated to 
achieve the three ANC targets that are similar to the case study 
results immediately above. This was true when considering the 
ecoregion-scale analysis results in both of the ways they were 
presented: (1) in terms of ecoregion median deposition regardless of 
time period or ecoregion (ecoregion-time period combinations), and (2) 
in terms of temporal trends in S deposition and waterbody percentages 
achieving ANC targets. In total, the ecoregion-time periods 
presentation indicates the likelihood of appreciably more waterbodies 
achieving the acid buffering capacity targets among the combinations 
with ecoregion median deposition at or below 9 kg/ha-yr (and for the 
bins for lower values) in eastern ecoregions compared to the estimates 
of waterbodies achieving acid buffering targets based on the full 
dataset that includes ecoregion median deposition estimates up to 18 
kg/ha-yr (table 4 below). For example, in the ecoregion-time period 
combinations presentation, at least 90% of waterbody sites in 87% of 
the eastern ecoregion-time period combinations are estimated to be able 
to achieve an ANC at or above 20 [micro]eq/L with ecoregion median S 
deposition at or below 9 kg/ha-yr and in 96% of those combinations for 
ecoregion median S deposition at or below 5 kg/ha-yr (table 4). 
Additionally, these percentages increase across the bins for the lower 
deposition estimates, although they are also based on smaller 
proportions of the supporting dataset (i.e., fewer ecoregion-time 
period combinations in each subsequently lower deposition bin) 
contributing to increased uncertainty for those results.

[[Page 105731]]



Table 4--Summary of the Eastern Ecoregion and Time Period Combinations Achieving Different ANC Targets With Estimated S Deposition at or Below Different
                                                                         Values
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                        % of Eastern ecoregion-time period combinations ** with at least 90%, 80% or 70%
                                                                                         waterbodies per ecoregion achieving ANC target
                                                            % of      ----------------------------------------------------------------------------------
              S deposition (kg/ha-yr) *                 combinations       >90% of waterbodies         >80% of waterbodies        >70% of waterbodies
                                                          included    ----------------------------------------------------------------------------------
                                                                          20       30       50        20       30       50        20       30       50
--------------------------------------------------------------------------------------------------------------------------------------------------------
ANC ([micro]eq/L) at/below:
    <=18.............................................             100       73       67       60        88       87       81        92       90       89
    <=13.............................................              90       80       73       65        95       94       88        98       96       96
    <=11.............................................              84       83       76       68        97       96       91        99       99       99
    <=9..............................................              77       87       81       72       100       99       93       100      100      100
    <=7..............................................              70       92       87       78       100      100       95       100      100      100
    <=6..............................................              66       93       88       78       100      100       97       100      100      100
    <=5..............................................              57       96       92       82       100      100       96       100      100      100
--------------------------------------------------------------------------------------------------------------------------------------------------------
* These values are ecoregion median estimates across all waterbody sites in an ecoregion with a CL estimate.
** These percentages are from the more extensive presentation of results in PA, Table 5-5.

    The PA observes that estimates from the temporal trend perspective 
similarly indicate appreciable percentages of waterbodies per ecoregion 
being estimated to achieve the acid buffering capacity targets with 
ecoregion median deposition below a range of approximately 5 to 8 kg/
ha-yr. For example, by the 2010-2012 period, by which time all 25 
ecoregions are estimated to have more than 70% of waterbodies able to 
achieve an ANC at or above 50 [micro]eq/L (and at least 85% able to 
achieve an ANC at or above 20 [micro]eq/L), median deposition in the 
ecoregions analyzed was below 8 kg S/ha-yr, ranging from 1.3 to 7.3 kg 
S/ha-yr (PA, Table 7-2). As shown in table 5 below, with each reduction 
in S deposition in each subsequent time period, more waterbodies in 
each of the eastern ecoregions are estimated to be able to achieve the 
ANC targets. Nearly 90% of the 18 eastern ecoregions are estimated to 
have at least 90% of their waterbodies achieving an ANC of 20 
[micro]eq/L in the 2010-12 period and achieving an ANC of 50 [micro]eq/
L in the 2014-16 period. When the 7 western ecoregions are included in 
a summary based on ANC targets of 20 [micro]eq/L for the West and 50 
[micro]eq/L for the East,\71\ over 70% of the full set of ecoregions 
are estimated to have at least 90% of their waterbodies achieving the 
ANC targets by the 2010-12 period (table 5). By the 2014-2016 and 2018-
2020 periods, 24 of the 25 ecoregions were estimated to have more than 
90% of waterbodies able to achieve an ANC at/above 50 [micro]eq/L, and 
median S deposition in all 25 ecoregions was below 5 kg/ha-yr (table 
5).
---------------------------------------------------------------------------

    \71\ This combination of targets recognizes the naturally and 
typically low ANC levels observed in western waterbodies while also 
including a higher target for the East, as described in section 
5.1.2.2 of the PA.

                       Table 5--Ecoregions Estimated To Have Different Percentages of Waterbodies Achieving Different ANC Targets for the Five Deposition Periods Analyzed
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                              % (n) of ecoregions with specified percentage of waterbodies per ecoregion achieving specified ANC
                                                            ------------------------------------------------------------------------------------------------------------------------------------
                                                                       ANC:                      20 [micro]eq/L                      30 [micro]eq/L                      50 [micro]eq/L
                                                            ------------------------------------------------------------------------------------------------------------------------------------
                        Time period                             Ecoregion median S         Percent of waterbodies per          Percent of waterbodies per          Percent of waterbodies per
                                                               deposition (kg/ha-yr)                ecoregion                           ecoregion                          ecoregion
                                                            ------------------------------------------------------------------------------------------------------------------------------------
                                                                 Min          Max         90%         80%         70%         90%         80%         70%         90%         80%         70%
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                       East
                                                                                                       Of 18 Eastern Ecoregions
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
2001-03....................................................          4.0         17.3    39% (7)    67% (12)    72% (13)     28% (5)    61% (11)    72% (13)     22% (4)     50% (9)    72% (13)
2006-08....................................................          3.1         14.4     44 (8)     72 (13)     89 (16)      33 (6)     72 (13)     78 (14)      33 (6)     67 (12)     72 (13)
2010-12....................................................          2.3          7.3    89 (16)    100 (18)    100 (18)     83 (15)    100 (18)    100 (18)     61 (11)     89 (16)    100 (18)
2014-16....................................................          1.9          4.6    94 (17)    100 (18)    100 (18)     94 (17)    100 (18)    100 (18)     89 (16)    100 (18)    100 (18)
2018-20....................................................          1.3          3.9   100 (18)    100 (18)    100 (18)     94 (17)    100 (18)    100 (18)     94 (17)    100 (18)    100 (18)
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                        All
                                                                                                  Of 25 Ecoregions (18 East, 7 West)
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
2001-03....................................................          1.2         17.3    56 (14)     76 (19)     80 (20)     48 (12)     72 (18)     80 (20)     44 (11)     64 (16)     80 (20)
2006-08....................................................          1.2         14.4    60 (15)     80 (20)     92 (23)     52 (13)     80 (20)     84 (21)     52 (13)     76 (19)     80 (20)
2010-12....................................................          1.0          7.3    92 (23)    100 (25)    100 (25)     88 (22)    100 (25)    100 (25)     72 (18)     92 (23)    100 (25)
2014-16....................................................          1.1          4.6    96 (24)    100 (25)    100 (25)     96 (24)    100 (25)    100 (25)     92 (23)    100 (25)    100 (25)
2018-20....................................................         0.62          3.9   100 (25)    100 (25)    100 (25)     96 (24)    100 (25)    100 (25)     96 (24)    100 (25)    100 (25)
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note: Estimates for ANC of 50 [micro]eq/L (East) and 20 [micro]eq/L (West) are identical to those for 50 in all 25 ecoregions.

    The temporal trends in percentage of waterbodies estimated to 
achieve the target ANC levels for each of the 25 individual ecoregions 
document a large difference between the time periods prior to 2010 and 
subsequent time periods (figure 1 above). For the S deposition 
estimated for the 2010-2012 period, more than 70% of waterbodies are 
estimated to be able to achieve an ANC of 50 ueq/L in all 25 ecoregions 
(figure 1, left panel), and 85% to 100% of waterbodies in all 
ecoregions are estimated to be able to achieve an ANC of 20 ueq/L 
(figure 1, right panel).
    Given the dependency of the ANC estimates on S deposition 
estimates, this distinction between the period prior to 2010 and the 
subsequent decade is also seen in the ecoregion deposition

[[Page 105732]]

estimates for the 25 REA ecoregions (figure 2; PA, Figure 7-2).\72\ The 
distribution of deposition estimates at waterbody sites assessed in 
each ecoregion, and particularly the temporal pattern for the upper 
percentiles, illustrates the deposition estimates that are driving 
temporal pattern in the REA estimates.\73\ For example, across the 25 
ecoregions (figure 2, left panel), the median of the ecoregion 90th 
percentiles \74\ of S deposition during the two earliest periods ranged 
from approximately 14 to 17 kg/ha-yr and the highest ecoregion 90th 
percentile values were above 20 kg/ha-yr. In contrast, during the 
latter three periods (2010-2020), the median of ecoregion 90th 
percentile values ranged from approximately 2 to 5 kg/ha-yr and all 
ecoregion 90th percentile estimates were below approximately 8 kg/ha-yr 
(figure 2). The contrast is less sharp for the ecoregion medians, as 
the median is a statistic less influenced by changes in the magnitude 
of values at the upper end of the distribution (figure 2). Overall, 
this indicates the significant reduction in the highest levels of 
deposition within each ecoregion over the time periods analyzed.
---------------------------------------------------------------------------

    \72\ In Figure 7-2 of the PA (which is an earlier version of 
figure 2), the box and whiskers presented for the medians were 
incorrect. They are correct in figure 2 here, and they were also 
correct in figure 2 of the proposal.
    \73\ Figure 2 presents temporal trends for three different 
statistics for deposition within the REA ecoregions. For example, 
the leftmost box and whiskers among the set of three presents the 
distribution of values that are the 90th percentile deposition 
estimates (at REA assessed waterbodies) in the 25 ecoregions. The 
rightmost box and whiskers presents the distribution of median 
deposition estimates for these ecoregions (figure 2, left panel).
    \74\ The median of the ecoregion 90th percentiles is the 
horizontal line in the leftmost box of the set of three. This is a 
measure of the central tendency of the 90th percentile deposition 
(across REA sites) in the 25 assessed ecoregions.
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BILLING CODE 6560-50-P

[[Page 105733]]

[GRAPHIC] [TIFF OMITTED] TR27DE24.001

BILLING CODE 6560-50-C
    Thus, in considering identification of S deposition levels that may 
be associated with a desired level of ecosystem protection for a 
SOX

[[Page 105734]]

standard, the PA took note of the increased percentages of waterbodies 
estimated to achieve more protective ANC levels across the five time 
periods. The pattern of estimated improving water quality over the 20-
year study period is paralleled by the pattern of declining deposition 
(figure 2). This temporal pattern indicates an appreciable reduction in 
ecoregion S deposition between the first and second decades of the 
period with associated reduction in aquatic acidification risk. As 
noted immediately above, the risk estimates associated with the 
deposition estimates of the second decade indicate generally high 
percentages of waterbodies per ecoregion as able to achieve or exceed 
the three ANC targets. Similarly, the ecoregion-time period binning 
summary also indicates generally high percentages of waterbodies 
achieving ANC targets for ecoregion median S deposition at or below 
about 8 or 9 kg/ha-yr (table 4). Thus, in light of these 
observations,\75\ the PA describes S deposition, on an areawide basis 
(i.e., ecoregion median), that falls at or below approximately 5 to 9 
(differing slightly depending on the supporting analysis), as being 
associated with the potential to achieve acid buffering capacity levels 
of interest in an appreciable portion of sensitive areas.
---------------------------------------------------------------------------

    \75\ The PA also suggested, based on the case study CL 
estimates, a focus on deposition below 10 kg/ha-yr, although the 
deposition estimates discussed in the case study analysis are 
smaller scale, e.g., site-level (PA, section 5.1.3.3).
---------------------------------------------------------------------------

    In considering what the quantitative information for S deposition 
and terrestrial acidification indicates regarding deposition levels of 
potential concern for acidification-related effects (and the associated 
uncertainties), the PA considers soil chemistry modeling analyses (both 
in published studies and in the 2009 REA), studies involving 
experimental additions of S compounds to defined forestry plots, and 
observational studies of potential relationships between terrestrial 
biota assessments and metrics for S deposition (PA, section 5.3). With 
regard to soil chemistry modeling analyses performed in the last 
review, the PA found the 2009 soil acidification modeling to indicate 
that a focus on aquatic acidification might reasonably be expected to 
also provide protection from soil acidification effects on terrestrial 
biota. With regard to studies involving S additions to experimental 
forested areas, the PA observes that effects on the sensitive tree 
species analyzed have not been reported with S additions below 20 kg/
ha-yr (which is in addition to the atmospheric deposition occurring 
during the experiment).
    The PA also considers the recently available quantitative 
information on S deposition and terrestrial acidification drawn from 
recent observational studies that report associations of tree growth 
and/or survival metrics with various air quality or S deposition 
metrics (PA, section 5.3.2.3 and Appendix 5B, section 5B.3.2). The 
metrics used in the two largest studies include site-specific estimates 
of average SO42- deposition and of average total 
S deposition over the interval between tree measurements, generally on 
the order of 10 years (Dietze and Moorcroft, 2011; Horn et al., 2018). 
In the study that used SO42- as the indicator of 
acidic S deposition, and for which the study area was the eastern half 
of the contiguous U.S., site-specific average 
SO42- deposition (1994-2005) ranged from a 
minimum of 4 kg/ha-yr to a maximum of 30 kg/ha-yr (Dietze and 
Moorcroft, 2011). Review of the study area for this study and a map 
indicating geographic patterns of deposition during the period of the 
deposition data indicate the lowest deposition areas to be west of the 
Mississippi River, northern New England (e.g., Maine) and southern 
Georgia and Florida (in which S deposition in the 2000-2002 period was 
estimated to fall below 8 kg/ha-yr), and the highest deposition areas 
to be a large area extending from New York through the Ohio River 
valley (PA, Appendix 5B, Figures 5B-1 and 5B-11). In the second study, 
deposition at the sites with species for which growth or survival was 
negatively associated with S deposition ranged from a minimum below 5 
kg/ha-yr to a site maximum above 40 kg/ha-yr, with medians for these 
species generally ranging from around 5 to 12 kg S/ha-yr (PA, Appendix 
5B, section 5B.3.2.3; Horn et al., 2018).
    In considering these study observations, the PA notes the history 
of appreciable acidic deposition in the eastern U.S., with its 
associated impacts on soil chemistry, that has the potential to be 
exerting a legacy influence on tree growth and survival more recently 
(PA, section 5.3.2 and Appendix 5B). Further, the PA notes that, at a 
national scale, the geographic deposition patterns (e.g., locations of 
relatively greater versus relatively lesser deposition) in more recent 
times appear to be somewhat similar to those of several decades ago 
(e.g., PA, sections 2.5.4 and 6.2.1). This similarity in patterns is 
recognized to have the potential to influence findings of observational 
studies that assess associations between variation in tree growth and 
survival with variation in levels of a metric for recent deposition at 
the tree locations, and to contribute uncertainty with regard to 
interpretation of these studies as to a specific magnitude of 
deposition that might be expected to elicit specific tree responses, 
such as those for which associations have been found. The PA notes 
that, as recognized in the study by Dietze and Moorcroft (2011), which 
grouped species into plant functional groups, acidification impacts on 
tree mortality rates are the result of cumulative long-term deposition, 
and patterns reported by their study should be interpreted with that in 
mind (PA, section 5.3.1 and Appendix 5B).
(c) Relating Air Quality Metrics to S Deposition
    In considering what the available information and air quality 
analyses indicate regarding relationships between air quality metrics 
and S deposition, the PA evaluated trends over the past two decades as 
well as a series of analyses of relationships between S deposition and 
ambient air concentrations of SO2 (in terms of 3-year 
averages of the existing SO2 standard and of an annual 
average),\76\ and between S deposition and ambient air concentrations 
of other S compounds (e.g., SO42- or the sum of 
SO42- and SO2) at 27 Class I area 
sites (collocated CASTNET and IMPROVE network sites), as summarized in 
section II.B. above. With regard to the latter, lower correlations were 
observed for total S deposition estimates collocated with ambient air 
concentrations of S-containing pollutants (SO42- 
and the sum of S in SO2 and SO42-) in 
27 Class I areas than between S deposition and annual average 
SO2 concentrations (averaged over three years) at SLAMS 
monitors (PA, Figure 6-31, center and right panels, and Table 6-4). 
Thus, while information for S compounds other than SO2 are 
available at the Class I area sites, the analyses based on data from 
SLAMS are considered particularly relevant given that those sites are 
primarily in areas of higher SO2 concentrations (near 
emissions sources) and collect FRM/FEM measurements for

[[Page 105735]]

existing NAAQS monitoring. Data from these monitoring sites informed 
the PA consideration of how changes in SO2 emissions, 
reflected in ambient air concentrations, may relate to changes in 
deposition and, correspondingly, what secondary standard options might 
best relate to ambient air concentrations such that deposition in areas 
of interest is maintained at or below range of levels identified above 
(PA, section 7.2.2.3).
---------------------------------------------------------------------------

    \76\ The air quality metrics include one based on the current 
secondary SO2 NAAQS, which is the second highest 3-hour 
daily maximum in a year, as well as an annual average SO2 
air quality metric (averaged over three years). Since many factors 
contribute variability to S deposition, the analyses focus on a 3-
year average of all of the air quality and deposition metrics and 
include multiple years of data, generally on the order of 20 years 
and covering a period of declining concentrations and deposition. Of 
the two air quality metrics analyzed, the PA focused primarily on 
the annual average of SO2 concentrations, averaged over 3 
years, given the focus on control of long-term S deposition and the 
greater stability of the metric (PA, section 7.2.2.3).
---------------------------------------------------------------------------

    Together the air quality and deposition data and analyses in the PA 
indicate a significant association of S deposition with SO2 
concentrations, with statistically significant correlation coefficients 
ranging from approximately 0.5 to 0.7 from the trajectory-based and 
SLAMS analyses for the five 3-year time periods (during 2001-2020) 
across all ecoregions. Higher correlations were observed for dry S 
deposition and at sites in the eastern U.S. (PA, section 7.2.2.3). As 
summarized in section II.A.2. above, S deposition is generally higher 
in the East and dry S deposition is generally higher near 
SO2 emissions sources. A strength of the analyses for 
concentrations and deposition estimates at SLAMS locations is the 
capturing of near-source deposition, while a strength of the 
trajectory-based analyses is accounting for the role of transport and 
transformation in contributing to downwind deposition.
    While recognizing the significant correlations between 
SO2 concentrations and S deposition, the PA additionally 
took note of the variability in, and uncertainty associated with, these 
relationships. The variability derives from the complexity of the 
atmospheric chemistry, pollutant transport, and deposition processes 
(PA, sections 2.1.1 and 2.5). The uncertainty in these relationships 
relates to a number of factors, including uncertainty in our estimates 
of S deposition (PA, section 2.5.2) and spatial distribution of monitor 
sites, including the representation of significant SO2 
emissions sources, as well as elements of the trajectory-based 
analysis, e.g., inclusion criteria for identifying monitoring sites of 
influence (PA, section 6.3 and Table 6-13). The PA concluded that it is 
unclear how much and in what way each of these various uncertainties in 
the data and analyses, and the inherent variability of the physical and 
chemical processes involved, might impact the conclusions concerning 
ambient air SO2 concentrations related to S deposition 
estimates at different scales (PA, section 7.2.2.3). In light of such 
uncertainty and variability, the REA aquatic acidification analyses and 
discussion of S deposition levels focused on statistics for deposition 
estimates representing large areas (e.g., at the ecoregion median and 
75th or 90th percentile, and case study area average or 70th and 90th 
percentile CLs). While uncertainty may be greater for relating 
concentrations to higher points on the distribution of deposition in an 
ecoregion, the PA recognized that it is the higher deposition 
estimates, if focused on individual waterbodies, that will contribute 
most to aquatic acidification risk. The PA additionally observed that 
the distribution of S deposition estimates within ecoregions has 
narrowed in more recent years, with 90th percentile estimates falling 
much closer to the medians than in the first decade of the 20-year 
period (figure 2 above).
    In identifying levels for consideration for a potential annual 
average SO2 standard, the PA first considered SO2 
concentrations at SLAMs and associated S deposition levels, focusing on 
the most recent of the five time periods analyzed (i.e., since 2010) 
when the REA indicated appreciably improved levels of acid buffering 
capability in the waterbodies of the 25 analyzed ecoregions (when ANC 
targets were met or exceeded in a high percentage of water bodies 
across a high percentage of ecoregions). Since 2010 (when ecoregion 
median and 90th percentile S deposition estimates for the 25 REA 
ecoregions were below 10 kg/ha-yr), the highest 3-year average annual 
SO2 concentrations were generally somewhat below 10 ppb 
(with some exceptions during the 2019-2021 period) (PA, Figure 7-5, 
left panel).\77\ The PA also considered SO2 concentrations 
at monitoring sites of influence identified in the trajectory-based 
analyses across different ranges of downwind ecoregion S deposition 
estimates. Across all 84 ecoregions in the contiguous U.S., the maximum 
annual average SO2 concentrations, as 3-year averages, at 
sites of influence to downwind ecoregions with median S deposition 
below 9 kg down to 6 kg/ha-yr,\78\ were all below 15 ppb, and 75% of 
the concentrations at these sites were at or below 10 ppb (PA, Figure 
7-3).\79\ In the 25 REA ecoregions, for the ecoregion median S 
deposition below 9 down to 6 kg/ha-yr, the concentrations for the 
metric based on maximum concentration at upwind sites of influence 
(EAQM-max) range as high as 15 ppb, with more than half below 10 ppb 
(PA, Figure 7-4, left panel). The EAQM-max concentrations associated 
with ecoregion median S deposition below 6 kg/ha-yr were all below 10 
ppb. This PA presentation further indicates that for the 25 REA 
ecoregions, when the highest EAQM-max concentration is at approximately 
11 or 10 ppb, both the median and 90th percentile deposition are both 
below 9 kg/ha-yr, with the overwhelming majority below 6 kg/ha-yr (PA, 
Figure 7-4).
---------------------------------------------------------------------------

    \77\ The similar pattern observed for annual average 
SO2 concentrations as 3-year averages suggests little 
year-to-year variability in this metric (PA, Figure 7-5).
    \78\ The bin for ``<9-6 kg/ha-yr'' is discussed here as it is 
the bin closest to the deposition target range of 10 or 8 to 5 kg/
ha-yr identified above.
    \79\ Figure 7-3 of the PA presents the pairs of median 
deposition estimates and associated upwind sites of influence EAQM-
max SO2 concentrations from the trajectory-based analysis 
in section 6.2.4 of the PA (specifically, the combined datasets 
presented in PA, Figure 6-41).
---------------------------------------------------------------------------

    In its use of the trajectory-based analyses to identify a range of 
annual average SO2 EAQM-max concentrations associated with 
an ecoregion median S deposition target range, the PA recognizes 
several important considerations. First, monitor concentrations of 
SO2 can vary substantially across the U.S., complicating 
consideration of the relationship between maximum contributing monitors 
identified in the trajectory-based analysis and S deposition levels in 
downwind ecosystems. Additionally, the substantial scatter in the 
relationship between S deposition estimates and measured SO2 
concentrations with ecoregion median S deposition values below about 5 
kg/ha-yr contributes increased uncertainty to conclusions regarding 
potential secondary standard SO2 metric levels intended to 
relate to ecoregion median deposition levels at or below 5 kg/ha-yr 
(PA, section 7.2.2.3). The PA additionally discusses limitations in the 
context of the two metrics (weighted and max). Between these metrics, 
somewhat stronger correlations were found for the annual average 
SO2 weighted EAQM (which provides for proportional weighting 
of air concentrations from locations projected to contribute more 
heavily to a particular ecoregion), compared to the EAQM-max, 
particularly for the first two to three time periods of the 20-year 
period. This difference is related to the extent to which monitor 
concentrations can be indicative of atmospheric loading. The weighted 
EAQM is intended to more closely represent the atmospheric loading for 
the locations (and associated sources) of the contributing (sites of 
influence) monitors than a single contributing monitor can. However, 
the weighted metric is not directly translatable to a standard level 
(which is an upper limit

[[Page 105736]]

on concentrations in individual locations).
    The PA also considered relationships between S deposition and 
PM2.5, noting the poor correlations for total S deposition 
estimates with PM2.5 at the 27 Class I area sites (r = 0.33, 
PA, Figure 6-31), and not much stronger correlations for ecoregion S 
deposition estimates with PM2.5 at upwind sites of influence 
from the trajectory-based analysis (r = -0.22 and 0.48, PA, Table 6-
12). The PA also considered relationships between total S deposition 
and ambient air SO42- concentrations noting that 
they are focused on remote locations (Class I areas), distant from 
sources of SO2 emissions, and that the relationship is not 
stronger than that for SO2 at the SLAMS, which are generally 
near sources monitoring SO2 (the source for atmospheric 
SO42-). In light of these considerations, the PA 
found that the available analyses did not indicate an advantage for an 
indicator based on SO42- measurements (or 
SO42- and SO2 combined), such as is 
currently collected at CASTNET sites, or PM2.5 mass over 
options for a potential annual average standard metric focused on 
SO2 concentrations (based on FRM/FEMs).\80\
---------------------------------------------------------------------------

    \80\ It is also of note that use of SO42- 
measurements, alone or in combination with SO2 
concentrations, as an indicator of a new standard would entail 
development of sample collection and analysis FRM/FEMs and of a 
surveillance network.
---------------------------------------------------------------------------

(4) Nitrogen Deposition and N Oxides and PM
    The evidence and exposure/risk-based considerations of the PA 
pertaining to N deposition and concentrations of N oxides and PM in 
ambient air draw on the available welfare effects evidence described in 
the current ISA (as well as prior ISAs and AQCDs), and discussed in 
Chapters 4, 5 and 6 of the PA. The focus of these considerations is 
primarily on N deposition and effects other than aquatic acidification 
(PA, sections 4.3, 5.2 and 5.3). As recognized in section II.A.4. 
above, the PA finds S deposition to be the dominant influence on 
aquatic acidification risk in the 20-year period analyzed (2001-2020), 
based on the finding that the inclusion of acidic N deposition to the 
aquatic acidification risk analyses did not appreciably change patterns 
and percentages of waterbodies estimated to exceed CLs for the three 
ANC targets (PA, section 5.1.2.4).
    In considering potential public welfare protection from N 
deposition-related effects (in light of the evidence summarized in 
sections II.A.3. and II.A.3.c. above), the PA recognizes the potential 
public welfare implications of the effects of N deposition in both 
aquatic and terrestrial ecosystems (PA, section 7.2.3.2). For example, 
the public welfare significance of eutrophication in large estuaries 
and coastal waters of the eastern U.S. related to decades of N loading 
is illustrated by the broad state, local and national government 
engagement in activities aimed at assessing and reducing the loading 
(PA, section 5.2.3). This significance relates both to the severity of 
the effects and the wide-ranging public uses dependent on these waters, 
including as important sources of fish and shellfish production, 
providing support for large stocks of resident commercial species, 
serving as breeding grounds and interim habitat for several migratory 
species, and providing an important and substantial variety of cultural 
ecosystem services. The public also benefits from water-based 
recreational uses and aesthetic values placed on aquatic systems. Many 
impacts of eutrophication relate to reduced waterbody oxygen, which 
contributes to fish mortality, and changes in aquatic habitat related 
to changes in resident plant and animal species, with associated 
ecosystem effects (PA, section 4.3; ISA, Appendix 7).
    The relative contribution of atmospheric deposition to total N 
loading, however, varies widely among estuaries and has declined in 
recent years, contributing a complexity to considerations in this 
review. While N loading in smaller, more isolated fresh waterbodies is 
primarily from atmospheric deposition, the evidence with regard to 
public welfare significance of any small deposition-related effects in 
these systems is less clear and well established. For example, the 
public welfare implications of relatively subtle effects of N 
enrichment in aquatic systems, such as shifts in phytoplankton species 
communities in remote alpine lakes, are not clear. Additionally, the 
public welfare implications of HNO3 effects on lichens 
(which might be considered to be ``direct'' effects or the result of 
deposition onto plant surfaces) are also not clear and might depend on 
the extent to which they impact whole communities, other biota, or 
ecosystem structure and function (PA, section 7.2.3.2).
    The effects of N enrichment in terrestrial ecosystems may vary with 
regard to public welfare implications. As noted above with regard to 
impacts of aquatic acidification, the PA recognizes that some level of 
N deposition and associated effects on terrestrial ecosystems can 
impact the public welfare and thus might reasonably be judged adverse 
to the public welfare. Depending on magnitude and the associated 
impacts, there are situations in which N deposition and associated 
nutrient enrichment-related impacts might reasonably be concluded to be 
significant to the public welfare, such as N deposition that alters 
forest ecosystem community structures in ways that appreciably affect 
use and enjoyment of those areas by the public (PA, section 7.2.3.2). A 
complication to consideration of public welfare implications that is 
specific to N deposition in terrestrial systems is its potential to 
increase growth and yield of plants that, depending on the plant and 
its use by human populations (e.g., trees for lumber, food for 
livestock or human populations), may be considered beneficial to the 
public. Nitrogen enrichment in natural ecosystems can, by increasing 
growth of N limited plant species, change competitive advantages of 
species in a community, with associated impacts on the composition of 
the ecosystem's plant community. The public welfare implications of 
such effects may vary depending on their severity, prevalence or 
magnitude. For example, only those rising to a particular severity 
(e.g., with associated significant impact on key ecosystem functions or 
other services), magnitude or prevalence may be considered of public 
welfare significance (PA, section 7.2.3.2).
(a) Quantitative Information for Ecosystem Risks Associated With N 
Deposition
    The PA considers the available information regarding air quality 
and atmospheric deposition and risk or likelihood of occurrence of 
ecosystem effects under differing conditions. In so doing, the PA notes 
the varying directionality of some of the N enrichment-related effects 
in terrestrial ecosystems, such that some effects can, in particular 
ecosystems and for particular species, seem beneficial (e.g., to growth 
or survival of those species), although in a multispecies system, 
effects are more complex with potential for alteration of community 
composition. The information is also considered with regard to the key 
limitations and associated uncertainties of this evidence.
    Beginning with the appreciable evidence base documenting 
assessments of N loading to waterbodies across the U.S., the PA notes 
the waterbody-specific nature of such responses and the relative role 
played by atmospheric deposition, among other N sources. For example, 
the relative contribution to such loading from atmospheric

[[Page 105737]]

deposition compared to other sources (e.g., agricultural runoff and 
wastewater discharges) varies among waterbody types and locations, 
which can be a complicating factor in quantitative analyses. 
Additionally, characteristics of resident biota populations and other 
environmental factors are influential in waterbody responses to N 
loading, e.g., temperature, organic microbial community structure, and 
aquatic habitat type, among others (ISA, Appendix 7). Based on 
identification of eutrophication as a factor in impacts on important 
fisheries in some estuaries across the U.S., multiple government and 
nongovernment organizations have engaged in research and water quality 
management activities over the past several decades in large and small 
estuaries and coastal waters across the U.S. These activities have 
generally involved quantitative modeling of relationships between N 
loading and water quality parameters such as dissolved oxygen (ISA, 
Appendix 7, section 7.2). This research documents both the impacts of N 
enrichment in these waterbodies and the relationships between effects 
on waterbody biota, ecosystem processes and functions, and N loading 
(PA, section 5.2.3). The evidence base recognizes N loading to have 
contributions from multiple types of sources to these large waterbodies 
and their associated watersheds, including surface and ground water 
discharges, as well as atmospheric deposition. Accordingly, loading 
targets or reduction targets identified for these systems have 
generally been identified in light of policy and management 
considerations related to the different source types, as discussed 
further in section II.B.1.(4)(b) below.
    Focused assessments in freshwater lakes, including alpine lakes, 
where atmospheric deposition may be the dominant or only source of N 
loading, also provide evidence linking N loading with seemingly subtle 
changes, such as whether P or N is the nutrient limiting phytoplankton 
growth (and productivity) and shifts in phytoplankton community 
composition (PA, section 5.2.2); public welfare implications of such 
changes are less clear (PA, section 7.2.3.1).
    With regard to terrestrial ecosystems and effects on trees and 
other plants, the PA recognizes the complexity, referenced above, that 
poses challenges to approaches for simulating terrestrial ecosystem 
responses to N deposition across areas diverse in geography, geology, 
native vegetation, deposition history, and site-specific aspects of 
other environmental characteristics. In its consideration of the 
different types of quantitative analysis, the PA recognizes limitations 
particular to each and associated uncertainties. Uncertainties 
associated with the soil acidification modeling analyses in the last 
review include those associated with the limited dataset of laboratory-
generated data on which the BC:Al targets are based, as well as the 
steady-state modeling parameters, most prominently those related to 
base cation weathering and acid-neutralizing capacity (PA, section 
5.3.4.1). Uncertainties associated with experimental addition analyses 
include the extent to which the studies reflect steady-state 
conditions, as well as a lack of information regarding historic 
deposition at the study locations (PA, section 5.3.4.1). Several 
aspects of observational or gradient studies of tree growth and 
survival (or of species richness for herbs, shrubs and lichens) 
contribute uncertainties to identification of deposition levels of 
potential concern for tree species effects, including unaccounted-for 
factors with potential influence on tree growth and survival (e.g., 
ozone and soil characteristics), as well as the extent to which 
associations may reflect the influence of historical deposition 
patterns and associated impact. Thus, while the evidence is robust as 
to ecological effects of ecosystem N loading, a variety of factors, 
including the history of deposition and variability of response across 
the landscape, complicate our ability to quantitatively relate specific 
N deposition rates, associated with various air quality conditions, to 
N enrichment-related risks of harm to forests and other plant 
communities in areas across the U.S. (PA, section 5.3.4).
(b) General Approach for Considering Public Welfare Protection
    In considering public welfare protection with regard to N 
enrichment, the PA notes, as an initial matter, that the effects of 
acidification on plant growth and survival, at the individual level, 
are generally directionally harmful, including reduced growth and 
survival. In contrast, the effects of N enrichment can, in particular 
ecosystems and for particular species, be beneficial or harmful (e.g., 
to growth or survival of those species). Accordingly, the PA recognizes 
added complexity to risk management policy decisions for this category 
of effects, including the lack of established risk management targets 
or objectives, particularly in light of historical deposition and its 
associated effects that have influenced the current status of 
terrestrial ecosystems and their biota, structure, and function.
    Further, the PA recognizes the complication posed by the 
contribution to N deposition of atmospheric pollutants other than the 
criteria pollutants N oxides and PM, most significantly the 
contribution of NH3 (PA, section 6.2.1). In light of the 
contrasting temporal trends for emissions of oxidized and reduced N 
compounds, the PA observes a declining influence of ambient air 
concentrations of N oxides and PM on N deposition over the past 20 
years, complicating consideration of the protection from N deposition-
related effects that can be provided by secondary NAAQS for these 
pollutants. This declining trend in N oxides emissions and associated 
oxidized N deposition coincides with increases in NH3 
emissions and deposition of reduced N compounds, such that reduced N 
deposition has generally been more than half of total N deposition at 
CASTNET sites since 2015 (PA, Figures 6-3, 6-17, 6-18 and 6-19). In 
2021, estimated dry deposition of NH3 was as much as 65% of 
total N deposition across the 92 CASTNET sites (PA, Figure 6-19). At 
25% of the CASTNET sites, more than 30% of N deposition is from dry 
deposition of NH3 (PA, Figure 6-19), a noteworthy 
observation given the preponderance of CASTNET sites in the West and 
relatively few in the areas of highest NH3 emissions where 
the percentage would be expected to be higher still (PA, Figures 2-9 
and 2-17). In light of this information, the PA finds that 
NH3, which is not a criteria pollutant, and its contribution 
to total N deposition, particularly in parts of the U.S. where N 
deposition is highest, are complicating factors in considering policy 
options related to NAAQS for addressing ecological effects related to N 
deposition (e.g., PA, Figure 6-18 and 6-13).
    In considering what the currently available quantitative 
information regarding terrestrial ecosystem responses to N deposition 
indicates about levels of N deposition that may be associated with 
increased concern for adverse effects, the PA focuses first on the 
evidence for effects of N deposition on trees that is derived from 
experimental addition studies and observational studies of potential 
relationships between tree growth and survival and metrics for N 
deposition. With regard to the experimental addition studies, while 
recognizing study limitations and associated uncertainties, the PA 
notes that the lowest N addition that elicited forest effects was 15 
kg/ha-yr over the 14 years from 1988 to 2002 (PA, sections 5.3.2

[[Page 105738]]

and 7.2.3.2 and Appendix 5B, Table 5B-1; McNulty et al., 2005). Based 
on the estimates from several observational studies, the PA observed 
that N deposition ranging from 7 to 12 kg/ha-yr, on a large area basis, 
reflects conditions for which statistical associations have been 
reported for terrestrial effects, such as reduced tree growth and 
survival.\81\ (PA, sections 5.3.4 and 7.2.3.2).
---------------------------------------------------------------------------

    \81\ The largest study reported associations of tree survival 
and growth with N deposition that varied from positive to negative 
across the range of deposition at the measurement plots for some 
species, and also varied among species (PA, section 5.3.2, Appendix 
5B, section 5B.3.2.3; Horn et al., 2018). Among the species for 
which the association varied from negative to positive across 
deposition levels, this is the range for those species for which the 
association was negative at the median deposition value (PA, section 
5.3.4). This also excluded species for which sample sites were 
limited to the western U.S. based on recognition by the study 
authors of greater uncertainty in the west (Horn et al., 2018).
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    With regard to studies of herb and shrub community metrics, the PA 
considered several recently available addition experiments, recent 
gradient studies of coastal sage scrub in southern California, and a 
larger observational study of herb and shrub species richness in open- 
and closed-canopy communities. As summarized in section II.A.3.c.(2)(b) 
above, N deposition estimates ranging from 6.5 kg/ha-yr to 11.6 kg/ha-
yr were identified from these studies as reflecting conditions for 
which statistical associations have indicated potential for effects in 
herb and shrub communities (PA, section 5.3.3.1 and Appendix 5B, 
sections 5B.3.1 and 5B.3.2; Cox et al., 2014; Fenn et al., 2010). 
Lastly, the PA notes the observational studies that have analyzed 
variation in lichen community composition in relation to indicators of 
N deposition, but recognize limitations with regard to interpretation, 
as well as uncertainties such as alternate methods for utilizing N 
deposition estimates as well as the potential influence of unaccounted-
for environmental factors, e.g., ozone, SO2, and historical 
air quality and associated deposition (PA, section 5.3.3.2 and Appendix 
5B, section 5B.4.2).
    With regard to the evidence for effects of N deposition in aquatic 
ecosystems, the PA recognizes several different types of information 
including the observational studies utilizing statistical modeling to 
estimate critical loads, such as those related to subtle shifts in the 
composition of phytoplankton species communities in western lakes. 
There are also many decades of research on the impacts and causes of 
eutrophication in large rivers and estuaries. As noted above, the 
public attention, including government expenditures, that has been 
given to N loading and eutrophication in multiple estuarine and coastal 
systems are indicative of the recognized public welfare implications of 
related impacts. In large aquatic systems across the U.S., the 
relationship between N loading and algal blooms, and associated water 
quality impacts (both short- and longer-term), has led to numerous 
water quality modeling projects to inform water quality management 
decision-making in multiple estuaries, including the Chesapeake Bay, 
Narraganset Bay, Tampa Bay, Neuse River Estuary and Waquoit Bay (ISA, 
Appendix 7, section 7.2). These projects often use indicators of 
nutrient enrichment, such as chlorophyll a, dissolved oxygen, and 
abundance of submerged aquatic vegetation (ISA, section IS.7.3 and 
Appendix 10, section 10.6). For these estuaries, the available 
information regarding atmospheric deposition and the establishment of 
associated target loads varies across estuaries (ISA, Appendix 7, Table 
7-9), and in many cases atmospheric loading has decreased since the 
initial modeling analyses.
    As summarized in section II.A.3.c.(1) above, analyses in multiple 
East Coast estuaries--including the Chesapeake Bay, Tampa Bay, Neuse 
River Estuary and Waquoit Bay--have addressed atmospheric deposition as 
a source of N loading (ISA, Appendix 7, section 7.2.1). Total estuary 
loading or loading reductions were established in TMDLs developed under 
the Clean Water Act for these estuaries. Levels identified for 
allocation of atmospheric N loading in the first three of these 
estuaries were 6.1, 11.8 and 6.9 kg/ha-yr, respectively, and 
atmospheric loading estimated to be occurring in the fourth was below 5 
kg/ha-yr (PA, section 7.3). This information, combined with the 
information from terrestrial studies summarized above, led to the PA 
identifying 7-12 kg/ha-yr as an appropriate N deposition range on which 
to focus in considering policy options (PA, section 7.2.3.2).
(c) Relating Air Quality Metrics to N Deposition Associated With N 
Oxides and PM
    In exploring how well various air quality metrics relate to N 
deposition, the PA finds the analyses utilizing data from monitors 
using FRM/FEM to collect ambient air concentration data for evaluation 
with the NAAQS (e.g., to identify violations) to be particularly 
relevant given that the current standards are judged using design 
values derived from FRM/FEM measurements at existing SLAMS (PA, section 
7.2.3.3). Given their role in monitoring for compliance with the NAAQS, 
most or many of these monitors are located in areas of relatively 
higher pollutant concentrations, such as near large sources of 
NO2 or PM. Accordingly, the PA recognized the information 
from these monitoring sites as having potential for informing how 
changes in NO2 and/or PM emissions, reflected in ambient air 
concentrations, may relate to changes in deposition and, 
correspondingly, for informing consideration of secondary standard 
options that might best regulate ambient air concentrations such that 
deposition in sensitive ecosystems of interest is maintained at or 
below levels of potential concern.
    In considering the information and findings of these analyses of N 
deposition and N oxides and PM in ambient air, the PA notes, as an 
initial matter, that relationships between N deposition and 
NO2 and PM air quality are affected by NH3 
emissions and non-N-containing components of PM (PA, section 6.4.2). 
The PA further notes that the influence of these factors on the 
relationships has varied across the 20-year evaluation period and 
varies across different regions of the U.S. (PA, section 6.2.1). Both 
factors (NH3 emissions and non-N-containing components of 
PM) are recognized to influence relationships between total N 
deposition and NO2 and PM air quality metrics. For example, 
for total N deposition estimated for TDep grid cells with collocated 
SLAMS monitors, the correlations with annual average NO2 
concentrations, averaged over three years, are generally low across all 
sites and particularly in the East (PA, Table 6-6). This likely 
reflects the relatively greater role of NH3 in N deposition 
in the East, which for purposes of the analyses in this PA extends 
across the Midwest (PA, section 6.4.2). The correlation between 
estimates of total N deposition in eastern ecoregions and annual 
average NO2 concentrations at upwind monitor sites of 
influence for the five periods from 2001-2020 is low to moderate, with 
the earlier part of the 20-year period, when NO2 
concentrations were higher and NH3 emissions were lower (as 
indicated by Figures 6-6 and 6-5 of the PA), having relatively higher 
correlation than the later part (e.g., correlation coefficients below 
0.4, except for EAQM-weighted in 2001-03 [PA, Table 6-10]). The 
correlation is negative or near zero for the western ecoregions (PA, 
section 6.2.4).
    Based on the decreasing trends in NO2 emissions and 
oxidized N deposition in

[[Page 105739]]

the past 10 years, and coincident trend of increased NH3 
emissions and deposition of reduced N (NH3 and 
NH4\+\), most particularly in areas of the Midwest, Texas, 
Florida and North Carolina (PA, Figures 6-16 and 6-17), the PA finds 
NO2 emissions to have much less influence on total N 
deposition now than in the past (PA, sections 6.2.1 and 6.4). In terms 
of ecoregion median statistics, the PA observes the decreasing trend in 
ecoregion median total N deposition across the period from 2001 through 
2012, while taking note that from 2012 onward, total N deposition 
increases, most particularly in ecoregions where most of the total 
deposition is from reduced N (PA, Figure 7-6). The PA also considers 
the impact of increasing deposition of reduced N on the 20-year trend 
in total N deposition as illustrated by TDep estimates at the 92 
CASTNET sites. At these sites, the median percentage of total N 
deposition comprised by oxidized N species, which is driven 
predominantly by N oxides, has declined from more than 70% to less than 
45% (PA, Figure 6-19). Based on examination of the trends for 
components of reduced N deposition, the PA notes that the greatest 
influence on the parallel increase in N deposition percentage composed 
of reduced N is the increasing role of NH3 dry deposition. 
The percentage of total N deposition at the CASTNET sites that is from 
NH3 has increased, from a median below 10% in 2000 to a 
median somewhat above 25% in 2021 (PA, Figure 6-19).
    Recognizing limitations in the extent to which CASTNET sites can 
provide information representative of the U.S. as a whole, the PA also 
analyzed TDep estimates across the U.S. for the most recent period 
assessed (2018-2020). In areas with ecoregion median total N deposition 
above 9 kg/ha-yr (PA, Figure 7-7, upper panel), the ecoregion median 
percentage of total N deposition composed of reduced N is greater than 
60% (PA, Figure 7-7, lower panel). The 2019-2021 TDep estimates across 
individual TDep grid cells similarly show that the areas of the U.S. 
where total N deposition is highest and greater than potential N 
deposition targets (identified in section 7.2.3.2 of the PA) are also 
the areas with the greatest deposition of NH3 (PA, Figure 7-
8), comprising more than 30% of total N deposition. That is, the PA 
finds that NH3 driven deposition is greatest in regions of 
the U.S. where total deposition is greatest (PA, section 7.2.3.3).
    Turning to PM2.5, the PA notes that the correlation for 
ecoregion median N deposition and PM2.5 concentrations at 
upwind sites of influence is poor and negative or moderate (r=0.45) 
depending on the metric (PA, section 6.2.4). For total N deposition and 
PM2.5 concentrations at SLAMS, a low to moderate correlation 
is observed (PA, section 6.2.3). In considering NH3 
emissions and non-N containing components of PM, the PA notes that some 
NH3 transforms to NH4+, which is a 
component of PM2.5, while also noting that, in the areas of 
greatest N deposition, the portion represented by deposition of gaseous 
NH3 generally exceeds 30%. Additionally, while 
NH3 emissions have been increasing over the past 20 years, 
the proportion of PM2.5 that is composed of N compounds has 
declined. The median percentage of PM2.5 comprised by N 
compounds has declined from about 25% in 2006-2008 to about 17% in 
2020-2022 and the highest percentage across sites declined from over 
50% to 30% (PA, section 6.4.2 and Figure 6-56). Further, the 
percentages vary regionally, with sites in the nine southeast states 
having less than 10% of PM2.5 mass composed of N compounds 
(PA, Figure 6-56).
    In summary, the PA concludes that in recent years, NH3 
contributes appreciably to total N deposition, particularly in parts of 
the country where N deposition is highest (as illustrated by comparison 
of Figures 6-13 and 6-18 of the PA). The PA finds that this situation--
of an increasing, and spatially variable, portion of N deposition not 
being derived from N oxides or PM--complicates assessment of policy 
options for protection against ecological effects related to N 
deposition associated with N oxides and PM, and for secondary standards 
for those pollutants that may be associated with a desired level of 
welfare protection. The PA recognizes that the available information as 
a whole also suggests the potential for future reductions in N oxide-
related N deposition to be negated by increasing reduced N deposition. 
Further, the PA notes that the results also suggest that while the 
PM2.5 annual average standard may provide some control of N 
deposition associated with PM and N oxides, PM2.5 monitors 
also capture other non-S and non-N related pollutants (e.g., organic 
and elemental carbon) as part of the PM2.5 mass (PA, section 
7.2.3.3). The amounts of each category of compounds vary regionally 
(and seasonally), and as noted above, N compounds generally comprise 
less than 30% of total PM2.5 mass (PA, section 6.3 and 6.4).
    In considering relationships between air quality metrics based on 
indicators other than those of the existing standards and N deposition 
(and associated uncertainties), the PA drew on the analyses of 
relationships for collocated measurements and modeled estimates of N 
compounds other than NO2 with N deposition in a subset of 27 
CASTNET sites located in 27 Class I areas, the majority of which (21 of 
27) are located in the western U.S. (PA, sections 6.2.2, 6.3 and 
6.4.2). The analyses indicate that total N deposition in these rural 
areas has a moderate correlation with air concentrations of nitric acid 
and particulate nitrate for the 20-year dataset (2000-2020) (PA, Figure 
6-32). The correlations are comparable to the correlation of 
NO2 with total N deposition at western SLAMS, a not 
unexpected observation given that more than 75% of the 27 CASTNET sites 
are in the West. A much lower correlation was observed at SLAMS in the 
East, and with the trajectory-based dataset. The PA notes that 
deposition at the western U.S sites is generally less affected by 
NH3 (PA, section 6.4.2). Further, the observed trend of 
increasing contribution to N deposition of NH3 emissions 
over the past decade suggests that such correlations of N deposition 
with oxidized N may be still further reduced in the future. Thus, the 
PA concludes that the evidence does not provide support for the 
oxidized N compounds (as analyzed at the 27 Class I sites) as 
indicators of total atmospheric N deposition, especially in areas where 
NH3 is prevalent (PA, section 7.2.3.3).
    The analyses involving N deposition and N-containing PM components 
at the 27 Class I area sites do not yield higher correlation 
coefficients than those for N deposition (TDep) and PM2.5 at 
SLAMS monitors (PA, section 7.2.3.3 and Figures 6-33, 6-39 [upper 
panel], and 6-32 [left panel]). Further, the graphs of total N 
deposition estimates versus total particulate N in ambient air at the 
27 Class I area sites indicate the calculated correlations (and slopes) 
likely to be appreciably influenced by the higher concentrations 
occurring in the first decade of the 20-year timeframe (PA, Figure 6-
33). Thus, the PA concludes that the available analyses of N-containing 
PM2.5 components at the small dataset of sites remote from 
sources also do not indicate an overall benefit or advantage of N-
containing PM2.5 components over consideration of 
PM2.5 (PA, section 7.4). As a whole, the PA finds that the 
limited dataset with varying analytical methods and monitor locations, 
generally distant from sources, does not clearly support a conclusion 
that such alternative indicators might provide better control of N 
deposition related to N oxides and

[[Page 105740]]

PM over those used for the existing standards (PA, section 7.2.3.3). 
The PA also notes that use of the NO3- or 
particulate N measurements analyzed with deposition estimates at the 27 
Class I area sites, alone or in combination with NO2, as an 
indicator for a new standard would entail development of sample 
collection and analysis FRM/FEMs \82\ and of a surveillance network.
---------------------------------------------------------------------------

    \82\ For example, sampling challenges have long been recognized 
for particulate NH4+ (e.g., ISA, Appendix 2, 
sections 2.4.5; 2008 ISA, section 2.7.3).
---------------------------------------------------------------------------

b. CASAC Advice
    The CASAC provided advice and recommendations regarding the 
standards review based on the CASAC's review of the draft PA. In the 
letter conveying its advice, the CASAC first recognized that 
``translation of deposition-based effects to an ambient concentration 
in air is fraught with difficulties and complexities'' (Sheppard, 2023, 
pp. 1-2). Further, the CASAC expressed its view that, based on its 
interpretation of the CAA, NAAQS could be in terms of atmospheric 
deposition, which it concluded ``would be a cleaner, more 
scientifically defensible approach to standard setting.'' Accordingly, 
the CASAC recommended that direct atmospheric deposition standards be 
considered in future reviews (Sheppard, 2023, pp. 2 and 5). The CASAC 
then, as summarized below, provided recommendations regarding standards 
based on air concentrations.
    With regard to protection from effects other than those associated 
with ecosystem deposition of S and N compounds, the CASAC concluded 
that the existing SO2 and NO2 secondary standards 
provide adequate protection for direct effects of those pollutants on 
plants and lichens, providing consensus recommendations that these 
standards should be retained without revision for this purpose 
(Sheppard, 2023, p. 5 of letter and p. 23 of Response to Charge 
Questions). With regard to deposition-related effects of S and N 
compounds, the CASAC members did not reach consensus, with their advice 
divided between a majority opinion and a minority opinion. Advice 
conveyed from both the majority and minority groups of members 
concerning deposition-related effects is summarized here.
    With regard to deposition-related effects of S and standards for 
SOX, the majority of CASAC members recommended a new annual 
SO2 standard with a level in the range of 10 to 15 ppb,\83\ 
which these members concluded would generally maintain ecoregion median 
S deposition below 5 kg/ha-yr \84\ based on consideration of the 
trajectory-based SO2 analyses (and associated figures) in 
the draft PA (Sheppard, 2023, Response to Charge Questions, p. 25). 
They concluded that such a level of S deposition would afford 
protection for tree and lichen species \85\ and aquatic ecosystems. 
Regarding aquatic ecosystems, these members cited the ecoregion-scale 
estimates (from the aquatic acidification REA analyses) associated with 
median S deposition bins for the 90 ecoregion-time period combinations 
(PA, section 5.1.3.2) in conveying that for S deposition below 5 kg/ha-
yr, 80%, 80% and 70% of waterbodies per ecoregion are estimated to 
achieve an ANC at or above 20, 30 and 50 [micro]eq/L, respectively, in 
all ecoregion-time period combinations (Sheppard, 2023, Response to 
Charge Questions, p. 25).\86\ In recommending an annual SO2 
standard with a level in the range of 10 to 15 ppb, these members 
stated that such a standard would ``preclude the possibility of 
returning to deleterious deposition values as observed associated with 
the emergence of high annual average SO2 concentrations near 
industrial sources in 2019, 2020, and 2021,'' citing Figure 2-25 of the 
draft PA \87\ (Sheppard, 2023, Response to Charge Questions, p. 24).
---------------------------------------------------------------------------

    \83\ Although the CASAC letter does not specify the form for 
such a new annual standard, the justification provided for this 
recommendation cites two figures in the draft PA (Figures 6-17 and 
6-18) which presented annual average SO2 concentrations 
averaged over three consecutive years (Sheppard, 2023, Response to 
Charge Questions, p. 25). Therefore, we are interpreting the CASAC 
majority recommendation to be for an annual standard, averaged over 
three years.
    \84\ Although the CASAC letter does not specify the statistic 
for the 5 kg/ha-yr value, the draft PA analyses referenced in citing 
that value, both the trajectory analyses and the ecoregion-scale 
summary of aquatic acidification results, focus on ecoregion 
medians. Further, the draft PA presentations of ecoregion 
percentages of waterbodies achieving the three ANC targets were for 
bins at or below specific deposition values (e.g., ``at/below'' 5, 6 
or 7 kg/ha-yr [draft PA, table 5-4]). Therefore, we are interpreting 
the CASAC advice on this point to pertain to ecoregion median at or 
below 5 kg/ha-yr.
    \85\ In making this statement, these CASAC members cite two 
observational data studies with national-scale study areas published 
after the literature cut-off date for the ISA: one study is on 
lichen species richness and abundance and the second is on tree 
growth and mortality (Geiser et al., 2019; Pavlovic et al., 2023). 
The lichen study by Geiser et al. (2019) relies on lichen community 
surveys conducted at U.S. Forest Service sites from 1990 to 2012. 
The tree study by Pavlovic et al. (2023) uses machine learning 
models with the dataset from the observational study by Horn et al. 
(2018) to estimate confidence intervals for CLs for growth and 
survival for 108 species based on the dataset first analyzed by Horn 
et al. (2018).
    \86\ As seen in tables 3 and 4 in this preamble, these levels of 
protection are also achieved in ecoregion-time period combinations 
for which the ecoregion median S deposition estimate is at or below 
7 kg/ha-yr (PA, section 7.2.2.2 and Table 7-1).
    \87\ The figure cited by the CASAC majority is the prior version 
of Figure 2-28 in section 2.4.2 of the final PA. The figure presents 
temporal trend in distribution (box and whiskers) of annual average 
SO2 concentrations since 2000 at SLAMS.
---------------------------------------------------------------------------

    One CASAC member dissented from this recommendation for an annual 
SO2 standard \88\ and instead recommended adoption of a new 
1-hour SO2 secondary standard identical in form, averaging 
time, and level to the existing primary standard based on the 
conclusion that the ecoregion 3-year average S deposition estimates for 
the most recent periods are generally below 5 kg/ha-yr and that those 
periods correspond to the timing of implementation of the existing 
primary SO2 standard (established in 2010), indicating the 
more recent lower deposition to be a product of current regulatory 
requirements (Sheppard, 2023, Appendix A, p. A-2).\89\
---------------------------------------------------------------------------

    \88\ Also dissenting from this advice was a member of the CASAC 
Oxides of Nitrogen, Oxides of Sulfur and Particulate Matter 
Secondary NAAQS Panel who was not also a member of the CASAC 
(Sheppard, 2023, Response to Charge Questions, p. 23). The former is 
a Panel formed for this review, while the latter is the standing 
Committee specified in the CAA.
    \89\ This member stated that the existing primary NAAQS for the 
three pollutants were significantly more restrictive than the 
existing secondary standards and provide adequate protection for 
deposition-related effects (Sheppard, 2023, Appendix A).
---------------------------------------------------------------------------

    With regard to N oxides and protection against deposition-related 
welfare effects of N, the majority of CASAC members recommended 
revision of the existing annual NO2 standard to a level 
``<10-20 ppb'' (Sheppard, 2023, Response to Charge Questions, p.24). 
The justification these members provided was related to their 
consideration of the relationship presented in the draft PA of median 
ecosystem N deposition with the weighted \90\ annual average 
NO2 metric concentrations, averaged over three years, at 
monitoring sites linked to the ecosystems by trajectory-based analyses 
and a focus on total N deposition estimates at or below 10 kg/ha-yr 
\91\ (Sheppard, 2023, Response to Charge Questions, p. 24). These 
members

[[Page 105741]]

additionally recognized, however, that ``when considering all 
ecoregions, there is no correlation between annual average 
NO2 and N deposition'' (Sheppard, 2023, Response to Charge 
Questions, p. 24). Their focus on total N deposition estimates at or 
below 10 kg/ha-yr appears to relate to consideration of TMDL analyses 
in four East Coast estuaries: Chesapeake Bay, Tampa Bay, Neuse River 
Estuary and Waquoit Bay (Sheppard, 2023, Response to Charge Questions, 
pp. 12-14 and 29). Levels identified for allocation of atmospheric N 
loading in the first three of these estuaries were 6.1, 11.8 and 6.9 
\92\ kg/ha-yr, respectively, and atmospheric loading estimated in the 
fourth was below 5 kg/ha-yr (Sheppard, 2023, Response to Charge 
Questions, pp. 12-14). These members also concluded that 10 kg N/ha-yr 
is ``at the middle to upper end of the N critical load threshold for 
numerous species effects (e.g., richness) and ecosystem effects (e.g., 
tree growth) in U.S. forests grasslands, deserts, and shrublands (e.g., 
Pardo et al., 2011; Simkin et al., 2016) and thus 10 kg N/ha-yr 
provides a good benchmark for assessing the deposition-related effects 
of NO2 in ambient air'' (Sheppard, 2023, Response to Charge 
Questions, p. 23).
---------------------------------------------------------------------------

    \90\ The weighted metric is constructed by applying weighting to 
concentrations to the monitors identified as sites of influence, 
with the weighting equal to the relative contribution of air from 
the monitor location to the downwind ecoregion based on the 
trajectory analysis (PA, section 6.2.4). Values of this metric are 
not directly translatable to individual monitor concentrations or to 
potential standard levels.
    \91\ The metric for N deposition in these analyses is the median 
of the TDep estimates across each ecoregion (PA, section 6.2.4).
    \92\ The CASAC letter states that the Neuse River Estuary TMDL 
specified a 30% reduction from the 1991-1995 loading estimate of 9.8 
kg/ha-yr, yielding a remaining atmospheric load target of 6.9 kg/ha-
yr (Sheppard, 2023, Response to Charge Questions, p. 13).
---------------------------------------------------------------------------

    One CASAC member disagreed with revision of the existing annual 
NO2 standard and instead recommended adoption of a new 1-
hour NO2 secondary standard identical in form, averaging 
time and level to the existing primary standard based on the conclusion 
that the N deposition estimates for the most recent periods generally 
reflect reduced deposition that is a product of current regulatory 
requirements, including implementation of the existing primary 
standards for NO2 and PM (Sheppard, 2023, Appendix A). This 
member additionally noted that bringing into attainment the areas still 
out of attainment with the 2013 primary annual PM2.5 
standard (12.0 [micro]g/m\3\) will provide further reductions in N 
deposition. This member also noted his analysis of NO2 
annual and 1-hour design values for the past 10 years (2013-2022) as 
indicating that the current primary NO2 standard provides 
protection for annual average NO2 concentrations below 31 
ppb (Sheppard, 2023, Appendix A).
    With regard to PM and effects related to deposition of N and S, the 
CASAC focused on the PM2.5 standards and made no 
recommendations regarding the PM10 standard. In considering 
the annual PM2.5 standard, the majority of CASAC members 
recommended revision of the annual secondary PM2.5 standard 
to a level of 6 to 10 [micro]g/m\3\. In their justification for this 
range, these members focus on rates of total N deposition at or below 
10 kg/ha-yr and total S deposition at or below 5 kg/ha-yr that they 
state would ``afford an adequate level of protection to several species 
and ecosystems across the U.S.'' (Sheppard, 2023, Response to Charge 
Questions, p. 23). In reaching this conclusion for protection from N 
deposition, the CASAC majority cited studies of U.S. forests, 
grasslands, deserts and shrublands that are included in the ISA. For S 
deposition, the CASAC majority notes the Pavlovic et al. (2023) 
analysis of the dataset used by Horn et al. (2018). Conclusions of the 
latter study (Horn et al., 2018), which is characterized in the ISA and 
discussed in sections 5.3.2.3 and 7.2.2.2 of the PA (in noting median 
deposition of 5-12 kg S/ha-yr in ranges of species for which survival 
and/or growth was observed to be negatively associated with S 
deposition), are consistent with the more recent analysis in the 2023 
publication (ISA, Appendix 6, sections 6.2.3 and 6.3.3).
    As justification for their recommended range of annual 
PM2.5 levels (6-10 [micro]g/m\3\), this group of CASAC 
members provided several statements, without further explanation, 
regarding PM2.5 annual concentrations and estimates of S and 
N deposition for which they cited several figures in the draft PA. 
Citing figures in the draft PA with TDep deposition estimates and 
IMPROVE and CASTNET monitoring data, they stated that ``[i]n remote 
areas, IMPROVE PM2.5 concentrations in the range of 2-8 
[micro]g/m\3\ for the periods 2014-2016 and 2017-2019 correspond with 
total S deposition levels <5 kg/ha-yr (Figure 6-12), with levels 
generally below 3 kg/ha-yr, and with total N deposition levels <=10 kg/
ha-yr (Figure 6-13)'' (Sheppard, 2023, Response to Charge Questions, p. 
23). With regard to S deposition, these members additionally cited a 
figure in the draft PA as indicating ecosystem median S deposition 
estimates at/below 5 kg/ha-yr occurring with PM2.5 EAQM-max 
values in the range of 6 to 12 [micro]g/m\3\ (Sheppard, 2023, Response 
to Charge Questions, pp. 23-24). With regard to N deposition, these 
members additionally cited figures in the draft PA as indicating that 
areas of 2019-2021 total N deposition estimates greater than 15 kg/ha-
yr (``in California, the Midwest, and the East'') correspond with areas 
where the annual PM2.5 design values for 2019-2021 range 
from 6 to 12 [micro]g/m\3\,\93\ and other figures (based on trajectory 
analyses) as indicating ecosystem median N deposition estimates below 
10 kg N/ha-yr occurring only with PM2.5 weighted EAQM values 
below 6 [micro]g/m\3\,\94\ and PM2.5 EAQM-max values below 8 
[micro]g/m\3\ (Sheppard, 2023, Response to Charge Questions, pp. 23-
24). The CASAC also noted the correlation coefficient for N deposition 
with the EAQM weighted metric (which was a moderate value of about 
0.5), while also recognizing that the correlation coefficient for the 
EAQM-max was ``minimal.'' The bases for the N and S deposition levels 
targeted in this CASAC majority recommendation are described in the 
paragraphs earlier in this section.
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    \93\ We note, however, that the design value figure cited by 
these members indicate California sites to have design values as 
high as 17.8 [micro]g/m\3\, i.e., violating the current 
PM2.5 secondary standard (draft PA, Figure 2-27; PA, 
Figure 2-31).
    \94\ As noted earlier in this section, weighted EAQM values are 
not directly translatable to concentrations at individual monitors 
or to potential standard levels.
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    One CASAC member recommended revision of the annual secondary 
PM2.5 standard to a level of 12 [micro]g/m\3\ based on his 
interpretation of figures in the draft PA that present S and N 
deposition estimates for five different 3-year time periods from 2001 
to 2020. This member observed that these figures indicate ecoregion 
median S and N deposition estimates in the last 10 years below 5 and 10 
kg/ha-yr, respectively. This member concluded this to indicate that the 
2013 primary annual PM2.5 standard of 12.0 [micro]g/m\3\ 
provides adequate protection against long-term annual S and N 
deposition-related effects (Sheppard, 2023, Appendix A).
    Regarding the existing 24-hour PM2.5 secondary standard, 
the majority of CASAC members recommended revision of the level to 25 
ug/m\3\ or revision of the indicator and level to deciviews \95\ and 20 
to 25, respectively (Sheppard, 2023, Response to Charge Questions, p 
25). These members variously cited ``seasonal variabilities'' of 
``[e]cological sensitivities,'' describing sensitive lichen species to 
be influenced by fog or cloud water from which they state S and N 
contributions to be highly episodic, and visibility impairment 
(Sheppard, 2023, Response to Charge Questions, p 25). These members did 
not provide further specificity regarding their reference to lichen 
species and fog or cloud water. With regard to visibility impairment, 
these members described

[[Page 105742]]

the EPA solicitation of comments that occurred with the separate EPA 
action to reconsider the 2020 decision to retain the existing 
PM2.5 standards as the basis for their recommendations on 
the secondary 24-hr PM2.5 standard (Sheppard, 2023, Response 
to Charge Questions, p 25; 88 FR 5562-5663, January 27, 2023).\96\ One 
CASAC member dissented from this view and supported retention of the 
existing secondary 24-hr PM2.5 standard.
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    \95\ Deciviews, units derived from light extinction, are 
frequently used in the scientific and regulatory literature to 
assess visibility (U.S. EPA 2019, section 13.2).
    \96\ Protection from impairment of visibility effects was one of 
the welfare effects within the scope of the PM NAAQS reconsideration 
rather than the scope of this review (U.S. EPA, 2016, 2017). In that 
action, the Administrator proposed not to change the 24-hour 
secondary PM NAAQS for visibility protection and also solicited 
comment on revising the level of the current secondary 24-hour 
PM2.5 standard to a level as low as 25 [micro]g/m\3\; in 
the final action, the Administrator concluded that the current 
secondary PM standards provide requisite protection against PM-
related visibility effects and retained the existing standards 
without revision (88 FR 5558, January 27, 2023; 89 FR 16202, March 
6, 2024).
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    Among the CASAC comments on the draft PA \97\ was the comment that 
substantial new evidence has been published since development of the 
2020 ISA that supports changes to the draft PA conclusions on N 
deposition effects. Accordingly, in the final PA, a number of aspects 
of Chapters 4 and 5 were revised from the draft PA; these changes took 
into account the information emphasized by the CASAC while also 
referring to the ISA and studies considered in it (PA, section 7.3). 
More recent studies cited by the CASAC generally concerned effects 
described in the ISA based on studies available at that time. While the 
newer studies include additional analyses and datasets, the ISA and 
studies in it also generally support the main points raised and 
observations made by the CASAC (PA, section 7.3).
---------------------------------------------------------------------------

    \97\ Consideration of CASAC comments and areas of the PA in 
which revisions have been made between the draft and this final 
document are described in section 1.4 of the PA.
---------------------------------------------------------------------------

c. Administrator's Proposed Conclusions
    In reaching his proposed conclusions on the adequacy of the 
existing secondary standards for SOX, N oxides, and PM, and 
on what revisions or alternatives may be appropriate, the Administrator 
drew on the ISA conclusions regarding the weight of the evidence for 
both the direct effects of SOX, N oxides, and PM in ambient 
air and for effects associated with ecosystem deposition of N and S 
compounds, and associated areas of uncertainty; quantitative analyses 
of aquatic acidification risk and of air quality and deposition 
estimates, and associated limitations and uncertainties; staff 
evaluations of the evidence, exposure/risk information, and air quality 
information in the PA; CASAC advice; and public comments received by 
that time. The Administrator recognized the evidence of direct 
biological effects associated with elevated short-term concentrations 
of SOX and N oxides that formed the basis for the existing 
secondary SO2 and NO2 standards, the evidence of 
ecological effects of PM in ambient air, primarily associated with 
loading on vegetation surfaces, and also the extensive evidence of 
ecological effects associated with atmospheric deposition of N and S 
compounds into sensitive ecosystems. The Administrator also took note 
of the quantitative analyses and policy evaluations documented in the 
PA that, with CASAC advice, informed his judgments in reaching his 
proposed decisions in this review.
    With regard to the secondary standard for SOX and the 
adequacy of the existing standard for providing protection of the 
public welfare from direct effects on biota and from ecological effects 
related to ecosystem deposition of S compounds, the Administrator 
considered the evidence regarding direct effects, as described in the 
ISA and evaluated in the PA, which is focused on SO2. He 
took note of the PA finding that the evidence indicates SO2 
concentrations associated with direct effects to be higher than those 
allowed by the existing SO2 secondary standard (PA sections 
5.4.1, 7.1.1 and 7.4). Additionally, he took note of the CASAC 
unanimous conclusion that the existing standard provides protection 
from direct effects of SOX in ambient air, as summarized in 
section II.B.1.b. above. Based on all of these considerations, he 
judged the existing secondary SO2 standard to provide the 
needed protection from direct effects of SOX.
    The Administrator next considered the ISA findings for ecological 
effects related to ecosystem deposition of S compounds. He first 
recognized the long-standing evidence of the role of SOX in 
ecosystem acidification and related ecological effects. While he 
additionally noted the ISA determinations of causality for S deposition 
with two other categories of effects related to mercury methylation and 
sulfide phytotoxicity (ISA, Table ES-1; PA, section 4.4), he recognized 
that quantitative assessment tools and approaches are not well 
developed for ecological effects associated with atmospheric deposition 
of S other than ecosystem acidification (PA, section 7.2.2.1). 
Accordingly, he gave primary attention to effects related to acidifying 
deposition, given the robust evidence base and available quantitative 
tools, as well as the longstanding recognition of impacts in acid-
sensitive ecosystems across the U.S. In so doing, the Administrator 
focused on the findings of the aquatic acidification REA and related 
policy evaluations in the PA. The range of ecoregion deposition 
estimates across the contiguous U.S. analyzed during the 20-year period 
from 2001 through 2020 extended up to as high as 20 kg S/ha-yr,\98\ and 
design values for the existing SO2 standard (second highest 
3-hour average in a year), in all States except Hawaii,\99\ were below 
its current level of 500 ppb, and generally well below (PA, section 
6.2.1). The Administrator took note of the aquatic acidification risk 
estimates that indicate that the pattern of S deposition, estimated to 
have occurred during periods when the existing standard was met (e.g., 
2001-2003), is associated with 20% to more than half of waterbody sites 
in each affected eastern ecoregion \100\ being unable to achieve even 
the lowest of the three acid buffering capacity targets or benchmarks 
(ANC of 20 [micro]eq/L), and he judged such risks to be of public 
welfare significance. The Administrator also considered the advice from 
both the majority and the minority of CASAC that recommended adoption 
of a new SO2 standard for this purpose in light of 
conclusions that the existing standard did not provide such needed 
protection. Thus, based on the findings of the REA, associated policy 
evaluations in the PA with regard to S deposition and acidification-
related effects in sensitive ecosystems, and in consideration of advice 
from the CASAC, the Administrator proposed to judge that the current 
SO2 secondary standard is not requisite to protect the 
public welfare from adverse effects associated with acidic deposition 
of S compounds in sensitive ecosystems.
---------------------------------------------------------------------------

    \98\ During 2001-2003, the 90th percentile S deposition per 
ecoregion of sites assessed in the REA was at or above 15 kg/ha-yr 
in half of the 18 eastern ecoregions and ranged up above 20 kg/ha-yr 
(figure 2).
    \99\ This analysis excluded Hawaii where it is not uncommon for 
there to be high SO2 values in areas with recurring 
volcanic eruptions (PA, section 2.4.2).
    \100\ Aquatic acidification risk estimates for the 2001-2020 
deposition estimates in the eight western ecoregions indicated ANC 
levels achieving all three targets in at least 90% of all sites 
assessed in each ecoregion (PA, Table 5-4). Ecoregion median 
deposition estimates were at or below 2 kg/ha-yr in all eight 
western ecoregions (PA, Table 5-3).
---------------------------------------------------------------------------

    Having reached this proposed conclusion that the existing secondary 
SO2 standard does not provide the

[[Page 105743]]

requisite protection of the public welfare from adverse S deposition-
related effects, most prominently those associated with aquatic 
acidification, the Administrator then considered options for a 
secondary standard that would provide the requisite protection from S 
deposition-related effects (i.e., a standard that is neither more nor 
less stringent than necessary, as discussed in section II.A. above). In 
so doing, the Administrator turned first to the policy evaluations and 
staff conclusions in the PA, and the quantitative analyses and 
information described in Chapter 5 of the PA, for purposes of 
identifying S deposition rates that might be judged to provide an 
appropriate level of public welfare protection from acidification-
related effects. In this context, he took note of the PA focus on the 
aquatic acidification risk estimates and recognition of linkages 
between watershed soils and waterbody acidification, as well as 
terrestrial effects. He concurred with the PA view regarding such 
linkages and what they indicate with regard to the potential for a 
focus on protecting waterbodies from reduced acid buffering capacity 
(with ANC as the indicator) to also provide protection for watershed 
soils and terrestrial effects. Accordingly, he focused on the PA 
evaluation of the risk estimates in terms of waterbodies estimated to 
achieve the three acid buffering capacity benchmarks (20, 30 and 50 
[micro]eq/L). In so doing, he concurred with the PA consideration of 
the ecosystem-scale estimates as appropriate for his purposes in 
identifying conditions that provide the requisite protection of the 
public welfare.
    The Administrator gave particular attention to the findings of the 
aquatic acidification REA for the 18 well-studied, acid-sensitive 
eastern ecoregions, and considered the PA evaluation of ecoregion 
median S deposition values at and below which the risk estimates 
indicated a high proportion of waterbodies in a high proportion of 
ecoregions would achieve ANC values at or above the three benchmarks 
(20, 30 and 50 [micro]eq/L), as summarized in Tables 7-1 and 5-5 of the 
PA. In so doing, he recognized a number of factors, as described in the 
PA, which contribute variability and uncertainty to waterbody estimates 
of ANC and to interpretation of acidification risk associated with 
different values of ANC (PA, section 5.1.4 and Appendix 5A, section 
5A.3). The Administrator additionally took note of the approach taken 
by the CASAC majority in considering the ecoregion-scale risk 
estimates. These members considered the summary of results for the 
ecoregion-scale analysis of ecoregion median deposition bins (in the 
draft PA) \101\ and focused on the results with acid buffering capacity 
at or above the three ANC benchmarks in 80% (for ANC of 20 and 30 
[micro]eq/L) or 70% (for ANC of 50 [micro]eq/L) of waterbodies in all 
ecoregion-time period combinations \102\ (Sheppard, p. 25 of the 
Response to Charge Questions). As recognized in the PA, these results 
are observed for median S-deposition at or below 7 kg/ha-yr for all 
time periods for the 18 eastern ecoregions. When considering all 25 
analyzed ecoregions, somewhat higher percentages are achieved (as seen 
in tables 4 and 5 above).\103\ The Administrator additionally 
considered the PA evaluation of the temporal trend (or pattern) of 
ecoregion-scale risk estimates across the five time periods in relation 
to the declining S deposition estimates for those periods. Based on the 
PA observation of appreciably improved acid buffering capacity (i.e., 
increased ANC) estimates by the third time period (2010-2012), the PA 
focused on the REA risk and deposition estimates for this and 
subsequent periods. By 2010-2012, ecoregion median S deposition (across 
CL sites) ranged from 2.3 to 7.3 kg/ha-yr in the 18 eastern ecoregions 
(with the highest ecoregion 90th percentile at approximately 8 kg/ha-
yr) and more than 70% of waterbodies per ecoregion were estimated to be 
able to achieve an ANC of 50 [micro]eq/L in all 25 ecoregions, and more 
than 80% of waterbodies per ecoregion in all ecoregions were estimated 
to be able to achieve an ANC of 20 [micro]eq/L (table 5 and figures 1 
and 2 above). The Administrator observed that these estimates of acid 
buffering capacity achievement for the 2010-12 period deposition--
achieving the ANC benchmarks in at least 70% to 80% (depending on the 
specific benchmark) of waterbodies per ecoregion--are consistent with 
the objectives identified by the CASAC majority (in considering 
estimates from the ecoregion-scale analysis). By the 2014-2016 period, 
when deposition estimates were somewhat lower, the ANC benchmarks were 
estimated to be achieved in 80% to 90% of waterbodies per ecoregion. In 
his consideration of these ANC achievement percentages identified by 
the CASAC majority, while noting the variation across the U.S. 
waterbodies with regard to site-specific factors that affect acid 
buffering (as summarized in sections II.A.3.a.(2) and II.A.4. above and 
section 5.1.4 of the PA), the Administrator concurred with the PA 
conclusion on considering ecoregion-scale ANC achievement results of 
70% to 80% and 80% to 90% with regard to acid buffering capacity 
objectives for the purposes of protecting ecoregions from aquatic 
acidification risk of a magnitude with potential to be considered of 
public welfare significance.
---------------------------------------------------------------------------

    \101\ While the final PA provides additional presentations of 
aquatic acidification risk estimates, including those at the 
ecoregion-scale, the estimates are unchanged from those in the draft 
PA (PA, section 5.1.3).
    \102\ The presentation of such percentages in the draft PA 
(reviewed by the CASAC) were specific to the 90 ecoregion-time 
period combinations for the 18 eastern ecoregions. Inclusion of the 
7 western ecoregions yields higher percentages, as more than 90% of 
waterbodies in those ecoregions were estimated to achieve all three 
ANC concentration in all time periods (PA, Table 5-4).
    \103\ Ecoregion median deposition was below 2 kg S/ha-yr in all 
35 ecoregion-time period combinations for the eight western 
ecoregions (PA, Table 5-4).
---------------------------------------------------------------------------

    With regard to the variation in deposition across areas within 
ecoregions, the Administrator noted the PA observation that the sites 
estimated to receive the higher levels of deposition are those most 
influencing the extent to which the potential objectives for aquatic 
acidification protection are or are not met. He further noted the PA 
observation of an appreciable reduction across the 20-year analysis 
period in the 90th percentile deposition estimates, as well as the 
median, for REA sites in the 25 ecoregions analyzed (figure 2 above). 
In this context, the Administrator took note of the PA findings that 
the ecoregion-scale acid buffering objectives identified by the CASAC 
(more than 70% to 80% of waterbody sites in all ecoregions assessed 
achieving or exceeding the set of ANC benchmarks) might be expected to 
be met when ecoregion median and upper (90th) percentile deposition 
estimates at sensitive ecoregions are generally at and below about 5 to 
8 kg/ha-yr. He also took note of the PA identification of deposition 
rates at and below about 5 to 8 or 10 kg/ha-yr \104\ as associated with 
a potential to achieve acid buffering capacity benchmarks in an 
appreciable portion of acid sensitive areas based on consideration of 
uncertainties associated with the deposition estimates and associated 
aquatic acidification risk estimates at individual waterbody sites

[[Page 105744]]

(PA, section 5.1.4), as well as the REA case study analysis estimates.
---------------------------------------------------------------------------

    \104\ The PA's consideration of the case study analyses as well 
as the ecoregion-scale results for both the ecoregion-time period 
groups and the temporal perspectives indicated a range of S 
deposition below approximately 5 to 8 or 10 kg/ha-yr to be 
associated with a potential to achieve acid buffering capacity 
levels of interest in an appreciable portion of acid sensitive areas 
(PA, section 7.4).
---------------------------------------------------------------------------

    Based on all of the above considerations, the Administrator focused 
on identification of a secondary standard that might be associated with 
S deposition of such a magnitude. In so doing he recognized the 
complexity of identifying a NAAQS focused on protection of the public 
welfare from adverse effects associated with national patterns of 
atmospheric deposition (rather than on protection from national 
patterns of ambient air concentrations directly). In light of the 
influence of emissions from multiple, distributed sources, atmospheric 
chemistry and transport on air concentrations and the influence of air 
concentrations and other factors on atmospheric deposition (ecosystem 
loading), the Administrator concurred with the PA judgment that 
consideration of the location of source emissions and expected 
pollutant transport (in addition to the influence of physical and 
chemical processes) is important to understanding relationships between 
SO2 concentrations at ambient air monitors and S deposition 
rates in sensitive ecosystems of interest. Accordingly, the 
Administrator concurred with the PA that to achieve a desired level of 
protection from aquatic acidification effects associated with S 
deposition in sensitive ecosystems, SO2 emissions must be 
controlled at their sources, and that associated NAAQS compliance 
monitoring includes regulatory SO2 monitors generally sited 
near large SO2 emissions sources.
    The Administrator considered findings of the PA analyses of 
relationships between ambient air concentrations and S deposition 
estimates, conducted in recognition of the variation across the U.S. in 
the source locations and magnitude of SOX emissions, as well 
as the processes that govern transport and transformation of 
SOX to eventual deposition of S compounds. Recognizing the 
linkages connecting SOX emissions and S deposition-related 
effects, the Administrator considered the current information with 
regard to support for SO2 as the indicator for a new or 
revised standard for SOX that would be expected to provide 
protection from aquatic acidification-related risks of S deposition in 
sensitive ecoregions. The Administrator noted the PA analyses 
demonstrated there to be an association between SO2 
concentrations and nearby or downwind S deposition (PA, section 7.4) 
based on the general association of higher local S deposition estimates 
with higher annual average SO2 concentrations at SLAMS, in 
addition to the correlations observed for ecoregion median S deposition 
with upwind SO2 monitoring sites of influence in the EAQM 
analyses (PA, sections 6.4.1 and 7.4). He additionally took note of the 
PA findings of parallel trends of SO2 emissions and S 
deposition in the U.S. over the past 20 years, including the sharp 
declines, that indicate the strong influence of SO2 in 
ambient air on S deposition (PA, sections 6.4.1 and 7.4), and of the PA 
finding of parallel temporal trends of ecoregion S deposition estimates 
and REA aquatic acidification risk estimates across the five time 
periods analyzed. In light of all of these considerations, the 
Administrator judged SO2 to be the appropriate indicator for 
a standard addressing S deposition-related effects.
    With regard to the appropriate averaging time and form for such a 
standard, the Administrator took note of the PA focus on a year's 
averaging time based on the recognition that longer-term averages (such 
as over a year) most appropriately relate to ecosystem deposition and 
associated effects, and of the recommendation from the CASAC majority 
for an annual average standard. The quantitative analyses of air 
quality and deposition in the PA also used a 3-year average form based 
on a recognition in the NAAQS program that such a form affords a 
stability to air quality management programs that contributes to 
effective environmental protection.\105\ Similarly, the CASAC majority 
recommendation focused on a 3-year average form. In consideration of 
these conclusions of the PA and the CASAC majority, the Administrator 
focused on annual average SO2 concentrations, averaged over 
three years, as the appropriate averaging time and form for a revised 
standard providing public welfare protection from adverse effects 
associated with long-term atmospheric deposition of S compounds.
---------------------------------------------------------------------------

    \105\ A 3-year form, common to recently adopted NAAQS, provides 
a desired stability to the air quality management programs which is 
considered to contribute to improved public health and welfare 
protection (e.g., 78 FR 3198, January 15, 2013; 80 FR 65352, October 
26, 2015; 85 FR 87267, December 31, 2020).
---------------------------------------------------------------------------

    In considering a level for such a standard, the Administrator again 
noted the complexity associated with identifying a NAAQS focused on 
protection from national patterns of atmospheric deposition. As 
discussed further in the PA and the proposal, in identifying a standard 
to provide a pattern of ambient air concentrations that together 
contribute to deposition across the U.S., it is important to consider 
the distribution of air concentrations to which the standard will 
apply.\106\ In identifying an appropriate range of concentrations for a 
standard level, the Administrator considered the evaluations and 
associated findings of the PA and advice from the CASAC. In so doing, 
he considered the two PA options of somewhat below 15 ppb to a level of 
10 ppb and a level ranging below 10 ppb to 5 ppb, with a 3-year average 
form. He additionally recognized that uncertainties in aspects of the 
aquatic acidification risk modeling contribute uncertainty to the 
resulting estimates, and that uncertainty in the significance of 
aquatic acidification risk is greater with lower deposition levels (PA, 
section 5.1.4). Accordingly, the Administrator took note of additional 
and appreciably greater uncertainty associated with consideration of a 
standard level below 10 ppb, including uncertainties in the 
relationships between S deposition and annual average SO2 
concentrations below 10 ppb (PA, Chapter 6, section 7.4). Thus, the 
Administrator recognized there to be, on the whole across the various 
linkages, increased uncertainty for lower SO2 concentrations 
and S deposition rates. The Administrator additionally considered the 
CASAC majority recommended range of 10 to 15 ppb for an annual average 
SO2 standard to address S deposition-related ecological 
effects, as described in section II.B.1.b. above. These members 
indicated that this range of levels was ``generally'' associated with S 
deposition ``at <5 kg/ha-yr'' in the two most recent trajectory 
analysis periods in the PA, and that a standard level in this range 
would afford protection against ecological effects in terrestrial 
ecosystems as well as aquatic ecosystems. These members also stated 
that such a standard would ``preclude the possibility of returning to 
deleterious deposition values'' (Sheppard, Response to Charge 
Questions, pp. 24-25). Thus, based on analyses and evaluations in the 
PA, including judgments related to uncertainties in relating ambient 
air concentrations to deposition estimates for the purpose of 
identifying a standard level associated with a desired level of 
ecological protection, and based on advice from the CASAC majority, the

[[Page 105745]]

Administrator judged that a level within the range from 10 to 15 ppb 
would be appropriate for an annual average SO2 standard 
requisite to protect the public welfare from adverse effects related to 
S deposition.
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    \106\ As recognized in section II.B.1.a. above, the trajectory 
analyses relate contributions from individual monitor locations to 
deposition in receiving ecosystems (without explicitly addressing 
the multiple factors at play), with the somewhat higher correlations 
of the EAQM-weighted than the EAQM-max metric likely reflecting the 
weighting of concentrations across multiple upwind monitors to 
represent relative loading.
---------------------------------------------------------------------------

    The Administrator also considered the extent to which a new annual 
average standard might be expected to control short-term concentrations 
(e.g., of three hours duration) and accordingly provide protection from 
direct effects that is currently provided by the existing 3-hour 
secondary standard. In this context, he noted the analyses and 
conclusions of the PA with regard to the extent of control for short-
term concentrations (e.g., of three hours duration) that might be 
expected to be provided by an annual secondary SO2 standard. 
These analyses indicate that in areas and periods when the annual 
SO2 concentration (annual average, averaged over three 
years) is below 15 ppb, design values for the existing 3-hour standard 
are well below the existing secondary standard level of 0.5 ppm 
SO2 (PA, Figure 2-29). Based on these findings of the PA, 
the Administrator proposed that it is appropriate to consider revision 
of the existing secondary SO2 standard to an annual 
standard, with a 3-year average form and a level in the range from 10 
to 15 ppb.
    The Administrator also took note of the recommendation from the 
CASAC minority to establish a 1-hour SO2 secondary standard, 
identical to the primary standard (section II.B.1.b. above; Sheppard, 
2023, p. A-2), based on its observation that most of the S deposition 
estimates for the last 10 years are less than 5 kg/ha-yr and a judgment 
that this indicates that the existing 1-hour primary SO2 
standard adequately protects against long-term annual S deposition-
related effects. The Administrator preliminarily concluded an annual 
standard to be a more appropriate form to address deposition-related 
effects, but also recognized that greater weight could be given to 
consideration of the effectiveness of the existing 1-hour primary 
standard in controlling emissions and associated deposition. In light 
of these considerations, the EPA solicited comment on such an alternate 
option for the secondary SO2 standard.
    In summary, based on all of the considerations identified above, 
including the currently available evidence in the ISA, the quantitative 
and policy evaluations in the PA, and the CASAC advice, the 
Administrator proposed to revise the existing secondary SO2 
standard to an annual average standard, with a 3-year average form and 
a level within the range from 10 to 15 ppb as requisite to protect the 
public welfare. The EPA also solicited comment on a lower level for a 
new annual standard down to 5 ppb, as well as on whether the existing 
3-hour secondary standard should be retained in addition to 
establishing a new annual SO2 standard. The EPA also 
solicited comment on the option of revising the existing secondary 
SO2 standard to be equal to the current primary standard in 
all respects.
    With regard to the secondary PM standards, the Administrator 
considered the available information and the PA evaluations and 
conclusions regarding S deposition-related effects. In so doing, he 
took note of the information indicating the variation in 
PM2.5 composition across the U.S. (PA, section 2.4.3), with 
non-S containing compounds typically comprising more than 70% of total 
annual PM2.5 mass in much of the country. Further, he 
considered the PA findings of appreciable variation in associations, 
and generally low correlations, between S deposition and 
PM2.5, as summarized in section II.A.2. above (PA, sections 
6.2.2.3 and 6.2.4.2). He also took note of the discussion above in 
support of his decision regarding a revised secondary SO2 
standard, including the atmospheric chemistry information which 
indicates the dependency of S deposition on airborne SOX, as 
evidenced by the parallel trends of SO2 emissions and S 
deposition. Based on all of these considerations, the Administrator 
judged that protection of sensitive ecosystems from S deposition is 
more effectively achieved through a revised SO2 standard 
than a standard for PM, and that a revised PM standard is not warranted 
to provide protection against the effects of S deposition.
    Based on his consideration of the secondary standards for N oxides 
and PM with regard to the protection afforded from direct ecological 
effects and from ecological effects related to ecosystem N deposition, 
the Administrator proposed to retain the existing NO2 and PM 
standards. With regard to protection from direct effects of N oxides in 
ambient air, the Administrator noted that the evidence of welfare 
effects at the time this standard was established in 1971 indicated the 
direct effects of N oxides on vegetation and that the currently 
available information continues to document such effects, as summarized 
in section II.B.1.a.(1) above (ISA, Appendix 3, sections 3.3 and 3.4; 
PA, sections 4.1 and 5.4.2). With regard to the direct effects of 
NO2 and NO, the Administrator concurred with the PA 
conclusion that the evidence does not call into question the adequacy 
of protection provided by the existing standard. With regard to the N 
oxide, HNO3, consistent with the conclusion in the PA, the 
Administrator judged the limited evidence to lack a clear basis for 
concluding that effects associated with air concentrations and 
associated HNO3 dry deposition on plant and lichen surfaces 
might have been elicited by air quality that met the secondary 
NO2 standard. Thus, the Administrator recognized the 
limitations of the evidence for these effects, and associated 
uncertainties, and judges them too great to provide support to a 
revised secondary NO2 standard, additionally taking note of 
the unanimous view of the CASAC that the existing secondary 
NO2 standard provides protection from direct effects of N 
oxides (section II.B.1.b. above).
    The Administrator next turned to consideration of the larger 
information base of effects related to N deposition in ecosystems. In 
so doing, he recognized the complexities and challenges associated with 
quantitative characterization of N enrichment-related effects in 
terrestrial or aquatic ecosystems across the U.S. that might be 
expected to occur due to specific rates of atmospheric deposition of N 
over prolonged periods, and the associated uncertainties (PA, section 
7.2.3). The Administrator also found there to be substantially more 
significant limitations and uncertainties associated with the evidence 
base for ecosystem effects related to N deposition associated with N 
oxides and PM, and with the available air quality information related 
to the limited potential for control of N deposition in areas across 
the U.S., in light of the impacts of other pollutants (i.e., 
NH3) on N deposition. The first set of limitations and 
uncertainties relates to quantitative relationships between N 
deposition and ecosystem effects, based on which differing judgments 
may be made in decisions regarding protection of the public welfare. In 
the case of protection of the public welfare from adverse effects 
associated with nutrient enrichment, there is also complexity 
associated with identification of appropriate protection objectives in 
the context of changing conditions in aquatic and terrestrial systems 
as recent deposition has declined from the historical rates of loading. 
The second set of limitations and uncertainties relates to the 
emergence of NH3, which is not a criteria pollutant, as a 
greater influence on N deposition than N oxides

[[Page 105746]]

and PM over the more recent years,\107\ and the variation in PM 
composition across the U.S.
---------------------------------------------------------------------------

    \107\ Further, this influence appears to be exerted in areas 
with some of the highest N deposition estimates for those years.
---------------------------------------------------------------------------

    Additionally, the Administrator recognized additional complexities 
in risk management and policy judgments, including with regard to 
identifying risk management objectives for public welfare protection 
from an ecosystem stressor like N enrichment, for which as the CASAC 
recognized, in terrestrial systems, there are both ``benefits and 
disbenefits'' (Sheppard, 2023, p. 8). As noted in the PA, the existence 
of benefits complicates the judgment of effects that may be considered 
adverse to the public welfare (PA, section 7.4). For aquatic systems, 
identification of appropriate public welfare protection objectives is 
further complicated by N contributions to many of these systems from 
multiple sources other than atmospheric deposition, as well as by the 
effects of historical deposition that have influenced the current 
status of soils, surface waters, associated biota, and ecosystem 
structure and function.
    In considering the evidence and air quality information related to 
N deposition, the Administrator took note of the fact that ecosystem N 
deposition is influenced by air pollutants other than N oxides, 
particularly, NH3, which is not a CAA criteria pollutant 
(PA, sections 6.1, 6.2.1 and 7.2.3.3). As noted above, the extent of 
this contribution varies appreciably across the U.S. and has increased 
during the past 20 years, with the areas of highest N deposition 
appearing to correspond to the areas with the greatest deposition of 
NH3 (PA, Figure 7-8).\108\ The Administrator concurred with 
the PA conclusion that this information complicates his consideration 
of the currently available information with regard to protection from N 
deposition-related effects that might be afforded by the secondary 
standard for N oxides, particularly when considering the information 
since 2010 (and in more localized areas prior to that). That is, while 
the information regarding recent rates of ecoregion N deposition may in 
some individual areas (particularly those for which reduced N, 
specifically NH3, has a larger role) indicate rates greater 
than the range of values identified in the PA for consideration (e.g., 
7-12 kg/ha-yr based on the considerations in section 7.2.3 of the PA 
and the benchmark of 10 kg/ha-yr, as conveyed in the advice from the 
CASAC), the PA notes that the extent to which this occurrence relates 
to the existing NO2 secondary standard is unclear. The lack 
of clarity is both because of uncertainties in relating ambient air 
NO2 concentrations to rates of deposition, and because of 
the increasing contribution of NH3 to N deposition.
---------------------------------------------------------------------------

    \108\ This associated lessening influence of N oxides on total N 
deposition is also evidenced by the lower correlations between N 
deposition and annual average NO2 concentrations than 
observed for S deposition and SO2 concentrations (PA, 
sections 6.2.3 and 6.2.4), which may be related to increasing 
emissions of NH3 in more recent years and at eastern 
sites (PA, section 2.2.3 and Figure 6-5).
---------------------------------------------------------------------------

    The Administrator additionally noted the PA finding that the 
temporal trend in ecoregion N deposition differs for ecoregions in 
which N deposition is driven by reduced N compared to those where 
reduced N comprises less of the total (e.g., PA, Figures 7-6 and 7-7). 
In light of the PA evaluations of N deposition and relative 
contribution from reduced and oxidized N compounds, the Administrator 
concurred with the PA conclusion that, based on the current air quality 
and deposition information and trends, a secondary standard for N 
oxides cannot be expected to effectively control total N deposition 
(PA, section 7.4).
    The Administrator additionally considered the two sets of advice 
from the CASAC regarding an NO2 annual standard in 
consideration of N deposition effects (section II.B.1.b. above). The 
CASAC majority recommended revision of the existing annual 
NO2 standard level to a value ``<10 to 20 ppb'' (Sheppard, 
2023, p. 24). The basis for this advice, however, relates to a graph in 
the draft PA of the dataset of results from the trajectory-based 
analyses for the weighted annual NO2 metric (annual 
NO2 EAQM-weighted), which, as noted in section II.B.1.b. 
above, is not directly translatable to concentrations at individual 
monitors or to potential standard levels. Additionally, these results 
found no correlation between the ecoregion deposition and the EAQM-
weighted or EAQM-max values at upwind locations, as also recognized by 
CASAC members and indicated in the final PA (PA, Table 6-10). 
Accordingly, based on the lack of a correlation for N deposition with 
the EAQMs, as well as the lack of translatability of the EAQM-weighted 
values to monitor concentrations or standard levels, the PA did not 
find the information highlighted by the CASAC majority for relating N 
deposition levels to ambient air concentrations to provide scientific 
support for their recommended levels. In light of this, the 
Administrator did not find agreement with the CASAC majority 
recommendations on revisions to the annual NO2 standard.
    The CASAC minority recommended revision of the secondary 
NO2 standard to be identical to the primary standard based 
on their conclusion that the recent N deposition levels meet its 
desired objectives and that the primary standard is currently the 
controlling standard (Sheppard, 2023, Appendix A). As noted in the PA, 
among the NO2 primary and secondary NAAQS, the 1-hour 
primary standard (established in 2010) may currently be the controlling 
standard for ambient air concentrations, and annual average 
NO2 concentrations, averaged over three years, in areas that 
meet the current 1-hour primary standard, have generally been below 
approximately 35 to 40 ppb.\109\ The Administrator also considered the 
PA revision option (i.e., revision to a level below the current level 
of 53 ppb to as low as 35 to 40 ppb [PA, section 7.4]), taking note of 
the PA characterization that support for this option is ``not strong'' 
(PA, section 7.4). He further noted the PA conclusion that while the 
option may have potential to provide some level of protection from N 
deposition related to N oxides, there is significant uncertainty as to 
the level of protection that would be provided, with this uncertainty 
relating most prominently to the influence of NH3 on total N 
deposition separate from that of N oxides (PA, section 7.2.3.3). The 
Administrator further recognized the PA statement that the extent to 
which the relative roles of these two pollutants (N oxides and 
NH3) may change in the future is not known. As evaluated in 
the PA, these factors together affect the extent of support for, and 
contribute significant uncertainty to, a judgment as to a level of N 
oxides in ambient air that might be expected to provide requisite 
protection from N deposition-related effects on the public welfare.
---------------------------------------------------------------------------

    \109\ The air quality information regarding annual average 
NO2 concentrations at SLAMS monitors indicates more 
recent NO2 concentrations are well below the existing 
standard level of 53 ppb. As noted in the PA, the temporal trend 
figures indicate that, subsequent to 2011-2012, when median N 
deposition levels in 95% of the eastern ecoregions of the 
continental U.S. have generally been at or below 11 kg N/ha-yr, 
annual average NO2 concentrations, averaged across three 
years, have been at or below 35 ppb (PA, section 7.2.3.3).
---------------------------------------------------------------------------

    In light of the considerations recognized above, the Administrator 
found that the existing evidence does not clearly call into question 
the adequacy of the existing secondary NO2 standard, 
additionally noting that recent median N deposition estimates are below 
the N deposition benchmark identified by the CASAC majority of 10 kg/
ha-yr in ecoregions for which approximately half or more of recent 
total N deposition is estimated to be

[[Page 105747]]

oxidized N, driven by N oxides (PA, section 7.2.3.3). In addition to 
the substantial uncertainty described above regarding the need for 
control of N deposition from N oxides that might be provided by a 
secondary standard for N oxides, the PA found there to be substantial 
uncertainty about the effect of a secondary standard for N oxides on 
the control of N deposition, such that it is also not clear whether the 
available information provides a sufficient basis for a revised 
standard that might be judged to provide the requisite protection. In 
light of this PA finding, the current information on air quality and N 
deposition, and all of the above considerations, the Administrator 
proposed to also judge that the available evidence in this review is 
sufficient to conclude a revision to the secondary annual 
NO2 standard is not warranted. Based on all of these 
considerations, he proposed to retain the existing secondary 
NO2 standard, without revision. The EPA also solicited 
comments on the alternative of revising the level and form of the 
existing secondary NO2 standard to a level within the range 
from 35 to 40 ppb with a 3-year average form.
    Lastly, the Administrator considered the existing standards for PM. 
He took note of the PA discussion and conclusion that the available 
information does not call into question the adequacy of protection 
afforded by the secondary PM2.5 standards from direct 
effects and deposition of pollutants other than S and N compounds (PA, 
sections 7.1.3 and 7.4). The evidence characterized in the ISA and 
summarized in the PA indicates such effects to be associated with 
conditions associated with concentrations much higher than the existing 
standards. Thus, the Administrator proposed to conclude that the 
current evidence does not call into question the adequacy of the 
existing PM standards with regard to direct effects and deposition of 
pollutants other than S and N compounds.
    With regard to N deposition and PM2.5, the Administrator 
considered the analyses and evaluations in the PA, as well as advice 
from the CASAC. He took note of the substantial and significant 
limitations and uncertainties associated with the evidence base for 
ecosystem effects related to N deposition associated with PM and with 
the available air quality information related to the limited potential 
for control of N deposition in areas across the U.S. in light of the 
impacts of NH3 on N deposition, and the variation in PM 
composition across the U.S., as summarized earlier. For example, as 
noted in the PA, the variable composition of PM2.5 across 
the U.S. contributes to geographic variability in the relationship 
between N deposition and PM2.5 concentrations, and there is 
an appreciable percentage of PM2.5 mass that does not 
contribute to N deposition. The PA further notes that this variability 
in percentage of PM2.5 represented by N (or S) containing 
pollutants contributes a high level of uncertainty to our understanding 
of the potential effect of a PM2.5 standard on patterns of N 
deposition.
    In considering the advice from the CASAC for revision of the 
existing annual secondary PM2.5 standard, the Administrator 
noted that the CASAC provided two different recommendations for 
revising the level of the standard: one for a level in the range from 6 
to 10 [micro]g/m\3\ and the second for a level of 12 [micro]g/m\3\. As 
summarized in the PA, the specific rationale for the range from 6 to 10 
ug/m\3\ is unclear, with levels within this range described as both 
relating to N deposition in a preferred range (at or below 10 kg N/ha-
yr) and relating to deposition above that range.\110\ The PA noted that 
this ``overlap'' illustrates the weakness and variability of 
relationships of PM2.5 with N deposition across the U.S. 
(PA, section 7.4). Further, the PA notes the low correlation for total 
N deposition estimates with annual average PM2.5 design 
values in the last 10 years at SLAMS (PA, Table 6-7). The second 
recommendation, from the CASAC minority, was based on their conclusion 
that the recent N (and S) deposition levels meet their desired targets 
and that the primary annual PM2.5 standard, which has been 
12 [micro]g/m\3\ since 2013, has been the controlling standard for 
annual PM2.5 concentrations (Sheppard, 2023, Appendix A).
---------------------------------------------------------------------------

    \110\ For example, the justification provided for the range of 
levels recommended by the CASAC majority for a revised 
PM2.5 annual standard (6 to 10 [micro]g/m\3\) refers both 
to annual average PM2.5 concentrations (3-yr averages) 
ranging from 2 to 8 [micro]g/m\3\ in 27 Class I areas (as 
corresponding to N deposition estimates at or below 10 kg/ha-yr) and 
to annual average PM2.5 concentrations (3-year averages) 
ranging from 6 to 12 [micro]g/m\3\ (at design value sites in areas 
of N deposition estimates greater than 15 kg/ha-yr), as summarized 
in section II.B.1.b. above.
---------------------------------------------------------------------------

    Based on the currently available information, taking into account 
its limitations and associated uncertainties, and in consideration of 
all of the above, the Administrator proposed to conclude that 
PM2.5 is not an appropriate indicator for a secondary 
standard intended to provide protection of the public welfare from 
adverse effects related to N deposition. In reaching this proposed 
conclusion, the Administrator focused in particular on the weak 
correlation between annual average PM2.5 design values and N 
deposition estimates in recent time periods, and additionally noted the 
PA conclusion that the available evidence, as evaluated in the PA, is 
reasonably judged insufficient to provide a basis for revising the 
PM2.5 annual standard with regard to effects of N deposition 
related to PM. Thus, based on consideration of the PA analyses and 
conclusions, as well as consideration of advice from the CASAC, the 
Administrator further proposed to conclude that no change to the annual 
secondary PM2.5 standard is warranted, and he proposed to 
retain the existing PM2.5 secondary standard, without 
revision. The EPA solicited comment on this proposed decision and also 
solicited comment on revising the existing standard level to a level of 
12 [micro]g/m\3\, in light of the recommendation and associated 
rationale provided by the CASAC minority.
    With regard to other PM standards, the Administrator concurred with 
the PA's finding of a lack of information that would call into question 
the adequacy of protection afforded by the existing PM10 
secondary standard for ecological effects, and thus concluded it is 
appropriate to propose retaining this standard without revision. With 
regard to the 24-hour PM2.5 standard, the Administrator took 
note of the PA conclusion that the evidence available in this review, 
as documented in the ISA, or cited by the CASAC,\111\ does not call 
into question the adequacy of protection provided by the 24-hour 
PM2.5 standard from ecological effects (PA, section 7.4). 
The Administrator also considered the comments of the CASAC majority 
and recommendations for revision of this standard to a lower level or 
to an indicator of deciviews, as summarized in section II.B.1.b. The 
Administrator noted the PA consideration of the lack of quantitative 
information in the ISA related to the specific type of N deposition 
raised by the CASAC comments. Further, the specific revision options 
recommended by the CASAC majority were based on visibility 
considerations, although the adequacy of protection provided by the 
secondary PM2.5 standard from visibility effects has been 
addressed in the

[[Page 105748]]

reconsideration of the 2020 p.m. NAAQS decision (89 FR 16202, March 6, 
2024) and is not included in this review. The Administrator 
additionally noted the recommendation from the CASAC minority to retain 
the existing 24-hour secondary PM2.5 standard without 
revision. Based on all of these considerations, the Administrator 
proposed to retain the existing 24-hour secondary PM2.5 
standard, without revision. Additionally, based on the lack of evidence 
calling into question the adequacy of the secondary PM10 
standards, he also proposed to retain the secondary PM10 
standards without revision.
---------------------------------------------------------------------------

    \111\ As summarized in section II.B.1.b. above, the CASAC 
majority, in its recommendation for revision of the existing 
standard, did not provide specificity regarding the basis for its 
statements on lichen species and fog or cloud water, and the 
available evidence as characterized in the ISA does not provide 
estimates of this deposition or describe associated temporal 
variability, or specifically describe related effects on biota (ISA, 
Appendix 2).
---------------------------------------------------------------------------

    In reaching the proposed conclusions regarding protection of the 
public welfare from ecological effects associated with ecosystem 
deposition of N and S compounds, the Administrator also noted the PA 
consideration of the potential for indicators different from those for 
the current standards that may target specific chemicals that deposit N 
and S, e.g., NO3-, SO42-, 
NH4+ (PA, sections 7.2.2.3, 7.2.3.3 and 7.4). In 
so doing, however, he recognized a number of uncertainties and gaps in 
the available information important to such consideration. Based on 
these, the Administrator judged that the currently available 
information does not support standards based on such indicators at this 
time. In so doing, he also recognized that additional data collection 
and analysis is needed to develop the required evidence base to inform 
more comprehensive consideration of such alternatives.
2. Comments on the Proposed Decisions
    Over 27,000 individuals and organizations indicated their views in 
public comments on the proposed decision. Nearly all of these are 
associated with mass mail campaigns or petitions. Approximately 20 
separate submissions were also received from individuals, 
organizations, or groups of organizations. Many of the individual 
commenters made a general recommendation to ``strengthen'' the 
standards under review, emphasizing giving attention to the scientific 
information and recommendations from the CASAC, and protection of 
natural ecosystems and associated wildlife. Among the organizations 
commenting were State and federal agencies, a Tribal organization, 
environmental protection advocacy organizations, industry organizations 
and regulatory policy-focused organizations.
    Some commentors expressed the overarching view that none of the 
standards for the three pollutants in this review should be revised, 
generally stating that the implementation work by State agencies 
associated with new standards would be for no environmental gain in 
light of the emissions reductions and ``dramatic improvements'' in 
associated air quality that have already occurred since 2000. While the 
EPA recognizes that air quality has improved over the last two decades, 
we note that the existence of such trends and the fact of the CAA 
requirements for implementation of NAAQS, alone or in combination, are 
not appropriate bases for the Administrator's decision under section 
109 of the Act. Accordingly, in finding that revision to the existing 
SOX standard is necessary to provide the requisite public 
welfare protection for SOX, while revisions to the N oxides 
and PM standards are not necessary to provide the requisite public 
welfare protection for those pollutants, the Administrator has based 
his decisions on the evidence of welfare effects, air quality 
information and the extent of public welfare protection provided by the 
existing standards, as described in section II.B.3. below. Other 
comments on the proposed decisions in the review of the secondary 
standards for protection of ecological effects of SOX, N 
oxides and PM are addressed below.
    Comments regarding the proposed decision to revise the secondary 
standard for SOX are addressed in section II.B.2.a., and 
those regarding the proposed decision to retain the secondary standards 
for N oxides and PM are addressed in sections II.B.2.b. and II.B.2.c., 
respectively. Other comments, including comments related to other 
legal, procedural, or administrative issues, those related to issues 
not germane to this review, and comments related to the Endangered 
Species Act are addressed in the separate Response to Comments 
document.
a. Sulfur Oxides
(1) Comments Regarding Adequacy of the Existing Secondary Standard
    With regard to welfare effects associated with SOX in 
ambient air, including those related to deposition of S compounds, in 
consideration of the welfare effects evidence, quantitative analyses of 
ecosystem exposure and risk and advice from the CASAC, the 
Administrator proposed to judge that the existing 3-hour secondary 
SO2 standard is not requisite to protect the public welfare 
from adverse effects associated with acidic deposition of S compounds 
in sensitive ecosystems. An array of comments was received regarding 
the Administrator's proposed decision to address this insufficiency in 
protection through revision to an annual average standard. These 
comments are addressed in the following section.
(2) Comments in Support of Proposed Adoption of a New Annual Standard
    In consideration of the welfare effects evidence, quantitative 
analyses of ecosystem exposure and risk, and advice from the CASAC 
majority to adopt an annual standard with a level within the range of 
10 to 15 ppb to address the deposition-related effects of 
SOX, the Administrator proposed revision of the existing 
standard to be an annual standard, as summarized in section II.B.1.c. 
above. Commenters expressed several views concerning the level of such 
a standard; these comments are addressed in the subsections below.
(a) Comments Agreeing With a Level Within the Proposed Range
    The EPA received multiple comments in support of the proposed 
establishment of an annual standard, with a 3-year form and level 
within the proposed range. Some of these comments concurred with the 
full range of levels as proposed, while some recommended a range of 
levels that overlapped with the lower end of the proposed range and 
also extended below it. The commenters in agreement with the full 
proposed range variously cited, concurred with, and expanded upon 
information discussed in the proposal, in addition to noting 
consistency of the proposed decision with recommendations from the 
majority of CASAC. In so doing, one commenter expressed the view that 
the proposed new standard would provide protection for direct 
vegetation effects and ecosystem deposition-related effects including 
aquatic acidification, which they noted affects the diversity and 
abundance of fish and aquatic life, thus providing support to cultural 
services and recreational fishing, which have long-term societal and 
economic benefits. Another comment expressed the view that the new 
standard would support Tribal efforts to protect lakes and streams from 
deposition-related effects including potential impacts to cultural 
fishing practices. One comment, in advocating for a level within the 
range of 5 to 10 ppb (which overlaps with the proposed range at a level 
of 10 ppb), expressed the view that ``to meet statutory requirements 
and act rationally and respond to CASAC consensus scientific expertise, 
EPA must,'' among several recommendations, ``[s]et an annual secondary 
SO2 standard of 5-10 ppb to

[[Page 105749]]

protect against deposition effects and maintain total sulfur deposition 
at <5 kg/ha on an annual basis.''
    The EPA agrees with the comment that a new annual standard with a 
level in the proposed range (of 10-15 ppb) would be expected to provide 
protection for direct effects on vegetation and for ecosystem 
deposition-related effects, including specifically those associated 
with aquatic acidification. The EPA also agrees that such a standard, 
by protecting against acidifying atmospheric deposition in aquatic and 
terrestrial ecosystems, can be expected to impact an array of societal 
and economic benefits from this protection. As summarized in section 
II.A.3.b. above and recognized in the Administrator's conclusions in 
section II.B.3. below, such benefits include providing protection for 
recreational and subsistence fisheries, as well as for recreational 
uses of sensitive forests and protected waterbodies.
    Additionally, with regard to the lower end of the proposed range 
and its overlap with the commenter-recommended-range of 5 to 10 ppb, 
the EPA agrees with the commenter that a standard with a level of 10 
ppb would generally be associated with S deposition at or below 5 kg/ha 
annually in sensitive ecosystems, consistent with comments by the CASAC 
majority in its rationale for recommending a new annual standard with a 
level in the range of 10 to 15 ppb, which it described as ``generally'' 
maintaining S deposition below 5 kg/ha-yr (as summarized in section 
II.B.1.b. above). The CASAC majority based its conclusion regarding 
annual SO2 standard levels associated with S deposition at/
below 5 kg/ha-yr on analyses in the draft PA, as described in section 
II.B.1.b.
    In reaching his proposed decision for a level in the range of 10 to 
15 ppb, the Administrator considered the expanded analyses and 
conclusions in the final PA. In reaching his final decision, as 
described in section II.B.3. below, the Administrator also considered 
additional analyses in a technical memorandum to the docket that extend 
the PA air quality and deposition analyses (Sales et al., 2024). These 
ecoregion-based analyses of air quality and deposition from five 3-year 
time periods from 2001 through 2020 indicate that when annual average 
SO2 concentrations (as a 3-year average) are at or below 10 
ppb, median S deposition in associated downwind ecoregions is generally 
at or below 5 kg/ha-yr. Specifically, more than 85% of associated 
downwind ecoregions are at or below 5 kg/ha-yr, with 95% below about 6 
kg/ha-yr and all below about 8 kg/ha-yr. This analysis additionally 
found that in every instance of the upwind maximum annual 
SO2 concentration above 10 ppb, the associated downwind 
ecoregion median deposition was greater than 5 kg/ha-yr, ranging from 
about 6 kg/ha-yr up to about 18 kg/ha-yr and with 75% of occurrences 
greater than 9 kg/ha-yr (Sales et al., 2024). In consideration of these 
findings, among other considerations, the Administrator judged a level 
of 10 ppb to provide the requisite protection of public welfare for the 
new annual secondary SO2 standard, as described in section 
II.B.3.
(b) Comments in Support of a Level Below the Proposed Range
    Three comments indicated support or potential support for a new 
annual standard with a level below 10 ppb (i.e., below the proposed 
range). In addition to the comment referenced above that expressed 
support for a level in the range from 5 to 10 ppb, a second comment, 
that expressed support for an annual standard with a level within the 
proposed range of 10 to 15 ppb, additionally expressed support for a 
level as low as 5 ppb to the extent it could ``be supported by the 
current science.'' A third comment expressed support for an annual 
standard level of 5 ppb, stating the view that such a standard could 
provide necessary protection for the public welfare and for resources 
managed by the U.S. National Park Service. Beyond a statement by one of 
these comments (also discussed in section II.B.2.a.(2)(a) above) that 
their recommended range of 5 to 10 ppb was needed to ``maintain sulfur 
deposition at <5 kg/ha on an annual basis,'' none of these commenters 
presented a specific scientific rationale for a specific standard level 
below 10 ppb. One comment stated that 71% of national parks are 
experiencing wet deposition of S greater than 1 kg/ha-yr and suggested 
that this indicates harmful impacts to park soil, waterbodies, and 
associated wildlife.
    With regard to the latter comment regarding wet S deposition above 
1 kg/ha-yr, the commenter did not provide evidence to support their 
conclusion of harmful impacts for such a level, and the EPA has not 
found the available evidence to support such a finding in this review. 
In describing the 1 kg/ha-yr value (for wet deposition of both S and 
N), the comment cited two papers that are focused on N deposition as a 
basis for the conclusion that conditions of wet deposition below 1 kg/
ha-yr are ``good'' while greater levels indicate acidification 
conditions. These papers--Baron et al. (2011) and Sheibley et al. 
(2014)--are summarized in addressing another comment in section 
II.B.2.b.(2)(b) below. Neither paper, however, addresses S deposition. 
Based on this and consideration of the evidence and quantitative 
analyses available in this review, the EPA does not find that wet S 
deposition greater than 1 kg/ha-yr in national parks indicates adverse 
impacts to the public welfare.
    We note that the phrase regarding maintaining S deposition ``at <5 
kg/ha'' on an annual basis is consistent with the phrase used by the 
CASAC majority in its justification for its recommended range of 10-15 
ppb, for which it cited analyses in the draft PA. As summarized above, 
and discussed in section II.B.3. below, the Administrator has 
considered the CASAC advice and the findings of the analyses in the 
final PA, in combination with additional presentations in Sales et al 
(2024), which he judged to provide support for his decision to adopt an 
annual SO2 standard with a level of 10 ppb, a value within 
the commenter-supported range of 5 to 10 ppb.
    The commenter that recommended a level of 5 ppb additionally 
expressed their view that a standard with a higher level (within the 
proposed range of 10 to 15 ppb) would not prevent effects of S 
deposition in Class I areas that they described as harmful, improve air 
quality, or reduce S deposition in Class I areas. Based on this view 
and their judgment that a further reduction in ambient air 
concentrations is needed, this commenter recommended that EPA set the 
level for a new annual standard below recent annual average 
SO2 concentrations, stating that a standard level of 5 ppb 
``could'' reduce S deposition from current levels. However, this 
commenter did not elaborate as to what magnitude of S deposition would 
be expected to be associated with a standard level of 5 ppb or why such 
a magnitude would provide an appropriate level for protection of the 
public welfare from S deposition-related effects. As a basis for their 
conclusion that harmful effects of S deposition are associated with 
current S deposition rates in national parks that are Class I areas, 
this commenter referred to National Park Services analyses that assign 
grades or ``conditions'' to these areas based on S deposition estimates 
and ``park-specific critical loads'' and stated that current S 
deposition levels in National Park Service managed Class I areas are 
above these loads for multiple ecosystem components. This commenter 
indicated that these analyses show that natural

[[Page 105750]]

resources in these parks are in fair or poor condition and that a 
standard with a level around 5 ppb ``could improve air quality and 
reduce S deposition levels'' in areas that the commenter states are 
already experiencing S deposition impacts.
    Although the commenter provided tables listing numbers of areas 
that they stated are in poor or fair condition for various ecosystem 
components (e.g., aquatic systems, trees) and potential threats (e.g., 
acidification by S deposition, growth effects and S deposition), the 
commenter submitted no information (beyond their statement that there 
are critical load exceedances) on how they reach such conclusions. As 
support for the general statement that the term critical load describes 
the amount of pollution above which harmful changes in sensitive 
ecosystems occur, the commenter cited a publication that discusses the 
concept of critical loads and the potential for their usefulness in 
natural resources management. We note, however, that this publication 
does not provide details (e.g., specific deposition rates associated 
with specific types of effect in specific types of ecosystems) that 
might inform the EPA's consideration of the type, severity and 
prevalence of particular effects that would be expected from specific 
levels of deposition. Such information, as that provided by the aquatic 
acidification REA and the evidence underlying it, is needed in 
judgments regarding deposition levels and deposition-related effects of 
public welfare significance, which are integral to the Administrator's 
decision on the secondary standard for SOX. Further, the 
commenter did not provide or refer to evidence relating a standard 
level of 5 ppb to expected S deposition levels. As discussed in section 
II.B.3. below, the Administrator has based his decision for an annual 
secondary SO2 standard with a level of 10 ppb on his 
consideration of the available evidence and quantitative analyses 
supporting the Agency's understanding of relationships between S 
deposition-related effects and S deposition levels and SO2 
concentrations, and also on his judgments regarding the public welfare 
significance of the S deposition-related effects assessed in his 
decision.
    As we describe in section II.A.3.c. above, the term critical load 
has multiple interpretations and applications (ISA, p. IS-14). The 
variety in meanings stems in part from differing judgments and 
associated identifications regarding the ecological effect (both type 
and level of severity) on which the critical load focuses and from 
judgment of its significance or meaning. Accordingly, all CLs are not 
comparable with regard to severity or significance of harm or, as is 
more pertinent to decision-making in this review, with regard to 
potential for adversity to the public welfare. Rather, science policy 
judgments in these areas are required in order to reach conclusions 
regarding impacts for which secondary standards should be established. 
For example, the analysis in the PA which utilized CLs--the aquatic 
acidification REA--described their basis in detail. Further, in the 
Administrator's consideration of the REA results, he recognized the 
variation and uncertainty associated in the CLs and their relevance to 
different waterbodies. Thus, while we appreciate the comment, we find 
the information provided by the commenter to be insufficient for 
reaching judgments as to the significance and strength of the various 
CLs in their technical analysis, and likewise insufficient for 
concluding that reduced deposition levels are necessary to avoid 
adverse public welfare effects in Class I areas (or for assessing what 
level of deposition would be associated with a 5 ppb standard).
(3) Comments in Disagreement With Proposed Adoption of a New Annual 
Standard
    Several public comments expressed disagreement with the proposed 
adoption of a new annual secondary standard to address S deposition-
related effects of SOX in ambient air. These comments cited 
a variety of reasons in support of this position, including the view 
that the EPA lacks authority to set a secondary standard to address 
public welfare effects of acid deposition. This comment is addressed in 
section II.B.2.a.(3)(a) below. Other reasons described in some comments 
advocating this position include the view that the proposed standard 
has no ``benefits'' and is therefore not ``necessary'' or 
``requisite.'' Some other comments variously cite implementation 
burdens (e.g., SIP preparation), uncertainties in the scientific basis, 
and a lack of CASAC consensus. Another commenter expressed the view 
that the proposal did not adequately discuss how effects are adverse to 
the public welfare and additionally stated that the ANC targets used in 
reaching conclusions on the need for protection from acid deposition 
relied on the judgments of others, rather than EPA. These other 
comments are addressed in section II.B.2.a.(3)(b) Some comments in 
opposition to a new annual standard expressed support for a secondary 
standard identical in all respects to the primary standard. Those 
comments are addressed in section II.B.2.a.(3)(c).
(a) Authority for a Secondary Standard Based on Acid Deposition
    A few commenters that disagreed with the proposed decision to adopt 
a new annual standard to address deposition-related effects expressed 
the view that the EPA lacks authority to set a secondary standard based 
on acid deposition, stating that the specific focus of the Acid Rain 
Program (CAA, title IV) on acidification preempts action on the same 
issue through the secondary NAAQS.\112\ These commenters argue that the 
enactment of title IV of the CAA in 1990 displaced the EPA's authority 
to address acidification through the setting of NAAQS, contending that 
the existence of a specific regulatory program to address the 
acidification effects of oxides of nitrogen and sulfur, that was 
established subsequent to the establishment of the NAAQS program in 
1970, supplants the EPA's general authority under the Act. In support 
of this contention, the commenters cite a Supreme Court decision 
pertaining to regulation of tobacco by the FDA (Food & Drug Admin. v. 
Brown & Williamson Tobacco Corp., 529 U.S. 120 (2000)) and also claim 
that their view regarding a lack of authority for the NAAQS program is 
demonstrated by the legislative history and a close reading of section 
404 of the Act, 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 report 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. Commenters also claimed that the EPA has in 
the past recognized that the NAAQS program does not provide an 
effective mechanism for addressing acid deposition and has not 
adequately explained its change in position. These commenters 
additionally cite comments from the CASAC, made in its review of the 
draft PA for this NAAQS review, regarding challenges in identifying a 
concentration-based standard to address deposition-related effects as 
supporting

[[Page 105751]]

the commenter's view that the CASAC also recognized a mismatch between 
the NAAQS program and regulation of acid deposition.
---------------------------------------------------------------------------

    \112\ One comment additionally cited the CASAC statement (in its 
advice to the Administrator in this review, summarized in section 
II.B.1.b.) that the CASAC's view was that a standard in terms of 
atmospheric deposition would be a more appropriate means of 
addressing deposition-related effects as indicative of a lack of 
CASAC support for a revised SO2 standard to address 
deposition-related effects of SOX.
---------------------------------------------------------------------------

    The EPA does not agree with commenters that the enactment of title 
IV of the Act displaced the EPA's authority under section 109 to adopt 
NAAQS to address adverse effects on public welfare associated with 
deposition of SOX from the ambient air. We note that the 
purpose of title IV ``is to reduce the adverse effects of acid 
deposition'' by reducing sulfur dioxide emissions by 10 million tons 
(and NOX emissions by 2 million tons) from 1980 levels (CAA 
section 401(b)). By contrast, section 109 directs the Administrator to 
set a standard that is ``requisite to protect public welfare from any 
known or anticipated adverse effects,'' based on the air quality 
criteria (CAA section 109(b)(2)). Congress explicitly requires the air 
quality criteria and standards be reviewed every five years, and has 
thus required secondary standards to reflect the latest scientific 
information (CAA section 109(d)(1)). There is no reason to believe that 
a Congressional effort to achieve 10 million tons in reductions of 
SO2 was intended to supersede EPA's ongoing obligations to 
assess the impact of SO2 on public welfare. See Whitman v. 
Am. Trucking Ass'ns, 531 U.S. 457, 468 (2001) (``Congress, we have 
held, does not alter the fundamental details of a regulatory scheme in 
vague terms or ancillary provisions--it does not, one might say, hide 
elephants in mouseholes.'').
    These two provisions are not in conflict, but represent the 
combined approach often taken by Congress to address the frequently 
complex problems of air pollution. There is nothing unusual about the 
CAA relying on multiple approaches to improve air quality, and in 
particular relying on the NAAQS to identify the requisite level of air 
quality and relying on both State implementation plans as well as 
federal CAA programs to control emissions of criteria pollutants in 
order to attain and maintain the NAAQS. For example, the existence of 
title II of the Act (Emission Standards for Moving Sources) does not 
divest the EPA of authority to set a NAAQS for ozone, despite the fact 
that many mobile source controls are adopted to control ozone 
precursors and indeed may be sufficient in some areas to attain and 
maintain the ozone NAAQS. Had Congress wanted to channel the EPA's 
authority to address acidification exclusively through title IV it 
could have done so explicitly. For example, it generally excluded 
criteria pollutants from regulation under section 111(d) and 112. 
Instead, at the same time that it enacted title IV, Congress also added 
section 108(g) to the CAA, specifying that the air quality criteria 
used for setting the NAAQS ``may assess the risks to ecosystems from 
exposure to criteria air pollutants.''
    In adding title IV to the CAA, Congress created a new program to 
reduce the emissions of SO2 and NOX from electric 
generating units, the most significant sources of acidifying pollution 
in 1990. Nothing in the text or the legislative history of title IV of 
the Act indicates that in creating additional authority Congress 
intended to foreclose the EPA's authority to address acid deposition 
through the NAAQS process. Indeed, to the extent that Congress 
addressed the impact of title IV on other provisions of the CAA, it 
made clear that title IV had no impact on the compliance obligations of 
covered sources under other CAA provisions. See CAA section 413, 
``Except as expressly provided, compliance with the requirements of 
this subchapter shall not exempt or exclude the owner or operator of 
any source subject to this subchapter from compliance with any other 
applicable requirements of this chapter.''
    The legislative history of the title IV program makes clear that 
Congress was acting to provide the EPA with additional tools to address 
the problem of acidification more effectively. See, e.g., S. Rep. 
No.101-228, at 289-291 (1989). Congress did not conclude that the EPA 
lacked the regulatory authority to address acidification but rather 
concluded that ``a major acid deposition control program [was] 
warranted . . . because of the evidence of damage that had already 
occurred as well the likelihood of further damage in the absence of 
Congressional action'' (H.R. Rep. No 101-490, at 360 (1990)). The 
Senate Report made it clear that while the EPA envisioned CAA section 
109 as providing authority to adopt a secondary NAAQS to address the 
effects of acid deposition, the EPA remained concerned about the 
effectiveness of this and other regulatory approaches (S. Rep. No. 101-
228, at 290-291). Congress addressed these issues by adding the new 
authorities found in title IV but made no mention of supplanting the 
EPA's authority under section 109 to address acidification effects. 
There is no discussion in the legislative history of title IV of 
curtailing the EPA's authority under the NAAQS program.
    As such, the requirement in 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 
demonstrate that Congress concluded that an amendment to the CAA would 
be necessary to give the EPA the authority to issue standards 
addressing acidification under section 109. See CAA section 401. The 
significance of the report required by section 404 can be understood in 
the overall context of (1) the history of Congress' and the EPA's 
attempts to understand and to address the causes and effects of acid 
deposition; (2) the distinction between an acid deposition standard 
(expressed as kg/ha-yr) and an ambient air quality standard addressing 
effects of deposition (expressed as ppb); \113\ and (3) the EPA's 
proposed conclusion in 1988 that the scientific uncertainties 
associated with acid deposition were too great to allow the Agency to 
establish a secondary NAAQS at that time to address those effects. The 
EPA notes that it was clear at the time of the 1990 CAA Amendments that 
a program to address acid deposition was needed and that the primary 
and most important of these provisions is title IV of the Act, 
establishing the Acid Rain Program. The Report required under section 
404 of the Amendments reflects this concern and requires an evaluation 
of an acid deposition standard and a comparison of its effectiveness to 
the effectiveness of various other regulatory authorities under the 
Act, including the authority for a secondary NAAQS under section 109 
(CAA Amendments, Public Law 101-549, 104 Stat. 2399, 2632 (1990) 
(describing that ``Reports'' under CAA 404 (42 U.S.C. 7651), should 
include ``(6) . . . other control strategies including ambient air 
quality standards'')). This indicates the existence of an ongoing 
authority under section 109. Likewise, in preparing the Report itself, 
EPA concluded that ``[i]t may be possible to set acid deposition 
standards under existing statutory authority'' (U.S. EPA, 1995b, at 
100).
---------------------------------------------------------------------------

    \113\ For example, the 1995 Report discusses potential ranges 
for an acid deposition standard as measured by kg/ha/year (e.g., 
U.S. EPA [1995b] at 118).
---------------------------------------------------------------------------

    For these reasons, the commenters' analogy to tobacco regulation, 
at issue in FDA v. Brown & Williamson Tobacco Corp., 529 U.S. 120 
(2000), is entirely inapt. The issue before the Supreme Court in that 
case was whether the FDA had authority to regulate tobacco at all, and 
the Court held that where the FDA consistently took the position it did 
not have such authority, and Congress enacted multiple statutes 
consistent with that position, Congress had ratified the FDA's 
understanding of its authority and had created a separate regulatory 
structure. By contrast, while the EPA has on multiple occasions noted 
the

[[Page 105752]]

scientific difficulties associated with identifying a standard to 
protect against acid deposition, EPA has engaged with those scientific 
difficulties because the EPA's longstanding interpretation of section 
109 is that acid deposition is within the scope of adverse effects on 
public welfare to be addressed under section 109. There is no reason to 
understand Congressional action to establish programs to reduce 
emissions of SOX under title IV as depriving EPA of 
authority to specify a level of air quality the attainment and 
maintenance of which is requisite to protect the public welfare against 
effects of SOX under section 109. See Massachusetts v. EPA, 
549 U.S. 497, 530 (2007) (distinguishing Brown & Williamson where EPA 
jurisdiction would not lead to extreme results, was not 
counterintuitive and EPA had never disavowed its authority).
    The EPA now concludes, as discussed in section II.B.2.b.(2)(a) 
below, that it does not have the authority to set a deposition standard 
under the existing CAA, and the EPA is not adopting a deposition 
standard in this action. Rather, consistent with the Agency's 
longstanding approach, the EPA has concluded that it must consider the 
effects of acid deposition in setting an air quality standard. Section 
109 of the Act requires the Administrator to set an ambient air quality 
standard the attainment of which protects against ``any known or 
anticipated adverse effects associated with the presence of [the] air 
pollutant in the ambient air.'' The EPA has concluded that the best 
interpretation of this language is that a deposition standard is not an 
``air quality'' standard because a deposition standard focuses not on 
concentrations of the pollutant in the ambient air but rather on 
quantities deposited on surfaces (as discussed in section 
II.B.2.b.(2)(a) below). Rather, the EPA has consistently viewed the 
best interpretation of this language to require consideration of the 
adverse effects that can be anticipated from presence of the pollutant 
in the ambient air, including via deposition of the pollutant to 
aquatic and other ecosystems. The CASAC indicated in its comments to 
the Administrator (as summarized in section II.B.1.b. above) that a 
deposition standard would be more scientifically appropriate, and it 
may be that Congress will at some point revisit the question of whether 
the EPA should also have authority to adopt an acid deposition 
standard, but such a question is independent of the scope of the 
authority, and obligation, the EPA currently has under section 109.
    In assessing the import of section 404, the EPA has noted in the 
past 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 21356, April 21, 1993; 77 FR 
20223, April 3, 2012). Such reservation of judgment by Congress 
concerned whether Congress should adopt additional statutory provisions 
to address the effects of acid deposition, as it did in 1990. It does 
not indicate a view that the EPA lacked authority under CAA section 109 
to establish a secondary NAAQS to address acid deposition.
    The EPA's decision in both the 1993 and 2012 reviews reflects the 
view that there is ongoing authority to address the effects of acid 
deposition under section 109 of the Act and does not indicate that the 
EPA believed that title IV implicitly amended the CAA and removed all 
such regulatory authority outside of title IV. In both the 1993 and 
2012 decisions on the question of whether to revise the secondary NAAQS 
to address acid deposition-related effects, the EPA decided not to 
adopt a standard targeting deposition-related effects. The EPA noted 
the consistency of this decision with Congress' actions in the 1990 
amendments but nowhere indicated that Congress' actions meant the EPA 
no longer had the authority to adopt a secondary NAAQS to address acid 
deposition. Instead, in the 1993 and 2012 decisions, the EPA stated 
that due to scientific uncertainty, the Agency would not at those times 
adopt a secondary NAAQS targeting deposition-related effects but would 
instead gather additional data and perform research and would determine 
in the future what further action to take under CAA section 109 (77 FR 
20263, April 3, 2012; 75 FR 28157-58, April 21, 1993).
    Although substantial progress was made between the 1993 and 2012 
reviews addressing some areas of uncertainty, the Administrator again 
concluded in 2012 that uncertainties associated with setting a NAAQS to 
address acidification were too substantial to allow her to set a 
standard that in her judgment would be requisite to protect the welfare 
from such effects. More than 10 years later, the evidence base on air 
quality, deposition and deposition-related effects has progressed 
substantially. That evidence base and associated quantitative analyses 
developed in the current review provide the foundation for the current 
decision for a NAAQS to protect against acid deposition. Thus, although 
we recognize the CASAC's view to be that a deposition standard would be 
a more appropriate means of addressing deposition-related effects, we 
find that for SO2 the relationship between ambient air 
concentrations and deposition is sufficiently well established to 
support a revised secondary SO2 NAAQS.\114\
---------------------------------------------------------------------------

    \114\ We have explained in section II.B.2.b.(2)(a), below, why 
we do not view section 109 as authorizing a deposition standard.
---------------------------------------------------------------------------

    We do not understand the CASAC as suggesting that, in the absence 
of a deposition standard, the EPA should decline to set an air quality 
standard to address deposition-related effects. Rather, contrary to the 
implication of the commenter that the CASAC did not support a NAAQS to 
address deposition, the CASAC expressed strong consensus support for 
the EPA setting a NAAQS for this purpose and recommended concentration-
based standards to the EPA for consideration. In summary, the EPA 
disagrees with the commenters' interpretation of the information cited 
and does not agree that the Administrator lacks the authority to set a 
secondary standard to address acid deposition-related effects.
(b) Other Comments in Opposition to the Proposed Annual Standard
    In addition to the view discussed immediately above regarding the 
EPA's authority to set a NAAQS to address effects related to 
atmospheric deposition, some commenters cited other reasons in 
opposition to the proposed annual secondary SO2 standard. 
For example, based on the EPA's analyses indicating that the proposed 
revision of the secondary standard would not require emissions 
reductions beyond those needed to meet the primary standard, some 
commenters stated that revision of secondary standard has no 
``benefits'' and is therefore not ``necessary'' and not ``requisite.'' 
Some additionally cited implementation requirements on States (e.g., 
SIP preparation) as a reason that the standard should not be revised, 
in light of the view that current air quality conditions do not pose a 
risk of adverse welfare effects. Some commenters expressed the view 
that the uncertainties are too great and the scientific basis for a 
standard to address acid deposition-related effects is lacking. One 
commenter stated that the EPA should thoroughly review the scientific 
studies published since the cut-off publication date for studies 
included in the ISA, and that to allow for this, the EPA should retain 
the existing standard pending that review

[[Page 105753]]

and the associated creation of an up-to-date record in the next NAAQS 
review. One commenter additionally noted the lack of CASAC consensus on 
recommendations for a standard to address deposition-related effects 
and stated the view that this lack of consensus further weakens support 
for such a new standard. One comment expressed the view that the 
proposal did not adequately discuss how effects are adverse to public 
welfare and additionally stated that the ANC targets used in reaching 
conclusions regarding the need for protection from acid deposition 
relied on the judgments of others, rather than the EPA.
    Regarding the view that a new annual standard to address 
deposition-related effects is not ``necessary'' or ``requisite,'' the 
EPA disagrees that simply because current or projected air quality in 
areas that meet the existing primary standard is expected to achieve 
the new standard, the current standard is already requisite to protect 
the public welfare, and a revised standard is unnecessary. The CAA 
requires secondary NAAQS to be set at the level of air quality 
requisite to protect the public welfare from known or anticipated 
adverse effects (CAA, section 109(b)(2)). The EPA recognizes the clear 
evidence, the CASAC consensus conclusions, and the Administrator's 
judgment, described in section II.B.3. below, that the current 
secondary standard does not provide protection for deposition-related 
effects of SOX and is therefore not requisite. Accordingly, 
based on the available information and CASAC advice, the Administrator 
proposed to revise the existing standard to reflect a level of air 
quality that would provide the needed protection (89 FR 26620, April 
15, 2024). Such a revision is ``necessary'' to address the requirements 
of the Act. In adopting a new annual standard, as described in section 
II.B.3. below, the Administrator has considered a range of options for 
limiting deposition-related effects with an air quality standard and 
identified such a standard that, in his judgment, is neither more nor 
less stringent than necessary to achieve the desired level of 
protection from welfare effects, most particularly those associated 
with atmospheric deposition of S compounds in sensitive ecosystems.
    With regard to implementation requirements, while the 
Administrator's decision on revision of the secondary standard to 
provide the requisite public welfare protection is not expected to 
result in changes to existing air quality, he has not considered 
implementation requirements in reaching his decision on the revised 
standard. Consistent with the CAA requirements described in section 
I.A. above, the Administrator is barred by CAA section 109 from 
considering costs of implementation in judging the adequacy of a 
standards, and he has not done so.
    The EPA additionally disagrees with the view that the secondary 
SO2 standard should not be revised because a revised 
standard would not be expected to require emissions reductions beyond 
those already required for meeting the primary SO2 standard, 
such that there would be little or no emissions reductions. As the D.C. 
Circuit has held in a prior challenge to SO2 NAAQS, 
``Nothing in the CAA requires EPA to give the current air quality such 
a controlling role in setting NAAQS'' (Nat'l Envtl. Dev. Association's 
Clean Air Project v. EPA, 686 F.3d 803, 813 ([D.C. Cir. 2012]). In this 
review, the EPA is engaged in the task of identifying a secondary 
standard that provides the requisite public welfare protection under 
the Act. The fact that the existing primary SO2 standard is 
expected, based on recent data, to control air quality such that the 
new annual secondary SO2 standard may also be met does not 
satisfy the requirements of CAA section 109(b)(2) or a priori make the 
secondary standard not requisite or without benefit. The benefit is 
assurance of the protection of the public welfare that is required of 
the secondary standard separate from the protection of the public 
health that is required of the primary standard. Further, the CAA 
requires the establishment of secondary standards requisite to protect 
against known or anticipated effects, and that requirement is separate 
and independent of the obligation to establish primary standards to 
protect the public health with an adequate margin of safety. The 
implication of the comment is that when the EPA next revises the 
primary NAAQS for SOX, the Administrator would be required 
to consider the effect of any revisions to the primary NAAQS on both 
public health and welfare, a consideration inconsistent with the entire 
purpose of having distinct standards, as well as the text of section 
109.
    Furthermore, while air quality is currently expected to meet the 
new annual secondary standard when the primary standard is met, 
patterns of SO2 concentrations may change in some areas in 
the future, such that both the new annual secondary standard and the 
existing primary standard are violated or such that the secondary 
standard could be violated without a violation of the primary standard. 
The analyses of SO2 concentrations described in the PA 
illustrate how SO2 concentration patterns have changed over 
the past two decades in response to various changes in the largest 
emissions sources and in emissions controls implemented on such 
sources. Thus, sometimes changes occur over the long term in the 
multiple factors that influence air quality, that can contribute to 
future air quality patterns that may differ from those prevalent 
currently. Regardless, we recognize that section 109 of the Act does 
not only require establishment of standards that will result in changes 
in existing air quality. Rather, the Act specifies that there be 
secondary standards in place that will provide the requisite protection 
in the face of current and future air quality. And, as discussed above 
and in section II.B.3. below, the existing secondary SO2 
standard does not provide the requisite protection from known or 
anticipated adverse effects on the public welfare related to 
atmospheric deposition of S compounds associated with SOX in 
ambient air. The Administrator's decision is therefore to revise the 
standard to one that in his judgment will provide that protection, as 
described in section II.B.3. below.
    The EPA disagrees with the comment stating that the Agency should 
retain the existing secondary SO2 standard pending review of 
the scientific studies that have been published since the cut-off date 
for studies considered in the ISA. Given the need for thorough 
consideration and CASAC review of studies that are part of the air 
quality criteria on which NAAQS must be based, there is always a cut-
off date for studies to be considered in the ISA, and there are always 
studies published after the cut-off date. The NAAQS are subject to 
regular review precisely to allow for EPA to base its review of the 
standards on the latest available science and to also revisit the 
standards in the future based on additional scientific information. As 
noted in section I.D. above, in consideration of public comments 
received on this action, the EPA has provisionally considered all such 
``new'' studies cited in comments and concluded that they do not 
materially change the broad scientific conclusions of the ISA (Weaver, 
2024). Thus, the EPA has concluded that reopening the air quality 
criteria is not warranted. Therefore, as discussed in section II.B.3. 
below, the Administrator has considered the available evidence, as 
summarized in the ISA, the quantitative and policy evaluations in the 
PA, and the related additional analyses (Sales et al., 2024), as well 
as CASAC advice and public comment on

[[Page 105754]]

the proposed decision and judged this an appropriate basis for his 
decision in the current review.
    The EPA also disagrees with commenters' claims that the 
uncertainties are too great to provide the necessary scientific support 
for a new annual secondary standard or that consensus advice is needed 
from the CASAC. With regard to the advice from the CASAC, we disagree 
that consensus is needed before the Administrator can make a decision 
in a NAAQS review. The CAA does not require the CASAC to reach 
consensus in its advice on revisions to the standards. The EPA has made 
decisions on NAAQS in multiple reviews in which the CASAC did not reach 
consensus on its advice for the standards (e.g., 85 FR 87256, December 
31, 2020 and 89 FR 16202, March 6, 2024). In reaching his decision in 
this review, as described in section II.B.3. below, the Administrator 
has considered advice provided from both the majority and the minority 
of the CASAC.
    In support of their claim that uncertainties are too great, 
commenters list statements from the proposal that recognize specific 
technical areas of uncertainty in our understanding of deposition-
related effects of SO2 in ambient air. We note that many of 
these statements are simply recognizing aspects of the evidence base 
that illustrate the complexity of addressing deposition-related 
effects. For example, one statement cited by commenters as indicative 
of significant uncertainty that should preclude action in this review 
recognized that there is not a simple one-to-one relationship between 
ambient air concentrations and any one indicator of S or N deposition. 
This statement simply recognizes the complexity inherent in analyses 
supporting this review. This complexity relates in part to the complex 
atmospheric chemistry and meteorology as well as aspects of ambient air 
monitoring and deposition estimation datasets (ISA, Appendix 2; PA, 
Chapters 2 and 6). In light of these factors, as summarized in the 
proposal and in section II.A.2. and II.B.1.a. above, we analyzed 
multiple datasets that investigate relationships between concentrations 
for different metrics in different types of locations.
    While we recognize the uncertainties and complexities of the 
evidence base and quantitative information, we have taken them into 
account in our evaluations, and we disagree that the available 
information is insufficient to permit a reasoned judgment about a 
secondary SO2 standard that may be considered to provide the 
appropriate protection from adverse effects on the public welfare. For 
example, some of the areas cited by commenters relate to uncertainty in 
how quickly sensitive ecosystems might respond to the already reduced 
deposition. While we recognize there to be uncertainty in estimates 
related to ecosystem response times, the EPA does not find predictions 
of this to be necessary in this decision, and accordingly has not 
considered timing of future recoveries as a factor in determining the 
standard that would provide the desired level of protection. Other 
areas cited by commenters simply recognize the inherent variability of 
environmental response to varying patterns of SO2 
concentrations. The Agency has recognized this variability in its focus 
on a year's averaging time for the new standard, which will not be 
affected by short-term variability, and in its focus on medians in 
characterizing ecosystem deposition targets.
    Lastly, the commenters noted uncertainty associated with the 
trajectory-based analysis (or EAQM approach), citing areas of 
uncertainty identified in the PA or proposal, and comments by the CASAC 
in its review of the draft PA, which stated that the description in the 
draft PA was insufficiently detailed and that sensitivity analyses were 
needed to characterize associated uncertainty. In addition to CASAC 
comments, these public comments quoted statements by three individual 
members of the CASAC Panel for this review that state there are 
uncertainties and shortcomings of the EAQM approach, state that there 
are poor correlations of S deposition with ambient air concentrations 
and suggest a need for peer review. With regard to correlations, we 
disagree that the correlation coefficients for the two SO2 
EAQMs in the final PA analyses (0.49 and 0.56 when considering the full 
dataset in the final PA), which are statistically significant at the 
0.05 level, are fairly characterized as ``poor'' (PA, Table 6-8). That 
said, the use of such relationships in this review is not for the 
development of a function to generate precise predictions of S 
deposition associated with individual monitor air concentrations. 
Rather, the analyses and the statistical significance of the 
deposition-to-EAQM value associations support the conclusion that 
higher upwind SO2 concentrations contribute to higher 
downwind S deposition. With this support, they also inform judgments 
regarding standard levels through consideration of the patterns of 
downwind deposition rates that have occurred during periods associated 
with different maximum upwind SO2 concentrations.
    With regard to peer review, in addition to noting the scientific 
peer review provided by the CASAC Panel for this review which resulted 
in substantial improvements in the analyses from the draft to the final 
PA, we also note that the trajectory analyses are based on a well-
established and peer-reviewed model, HYSPLIT (Stein et al., 2015). This 
model, as described further in the PA, is commonly used to compute 
simple air parcel trajectories using historical meteorological data and 
to simulate the trajectories of air parcels as they are transported 
through the atmosphere for a given set of meteorological conditions 
(PA, Appendix 6A).
    In consideration of the robust scientific and technical peer review 
provided by the CASAC and its Oxides of Nitrogen, Oxides of Sulfur and 
Particulate Matter Secondary National Ambient Air Quality Standards 
Panel in their review of the draft PA, several improvements were 
implemented. For example, sensitivity analyses were conducted to judge 
the influence of key aspects of the approach employed (e.g., duration 
of the trajectory simulations and criteria used to identify influential 
upwind monitors), and findings from these analyses informed development 
of the trajectory-based approach for the final PA. As a result, the 
final PA includes substantially more detail in describing the approach 
and in the presentation of results, including for the various 
sensitivity analyses. Thus, as noted in the final PA, analyses 
presented in that document were revised and additional information 
added to address the CASAC concerns (PA, section 1.4).
    While the PA includes multiple approaches for analyzing 
relationships between ambient air concentrations and ecosystem 
deposition of S compounds, the trajectory-based approach is the only 
one that accounts for pollutant transport, which is integral to how 
SO2 emissions and associated concentrations contribute to 
acidic precipitation and acidification of ecosystems many miles 
away.\115\ Such transport modeling has been used for years, with its 
use verified twenty years ago by a study documenting the movement of 
air

[[Page 105755]]

masses containing elevated concentrations of 
SO42 - from the Ohio River Valley to the eastern 
U.S. and Canada (Hennigan et al., 2006), where acid-sensitive 
waterbodies have been impacted by acidification (ISA, Appendix 16, 
section 16.2). Thus, consideration of the trajectory-based analyses by 
the Administrator in reaching his proposed and final conclusions rely 
on different analyses (from those described in the draft PA) that have 
been improved to address comments by the CASAC, and consideration of 
these analyses (in addition to the other approaches) presented in the 
final PA is important to identifying a secondary standard that accounts 
for pollutant transport to downwind sensitive ecosystems.
---------------------------------------------------------------------------

    \115\ The importance of this transport, with co-occurring 
transformation of SO2 to SO42 -, in 
contributing to ecosystem acidification was recognized decades ago 
in the 1982 AQCD for PM and SOX which stated that 
``[b]ecause of long range transport, acidic precipitation in a 
particular state or region can be the result of emissions from 
sources in states or regions many miles away, rather than from local 
sources'' (1982 p.m. and SOX AQCD, p. 7-2; Altshuller 
1976).
---------------------------------------------------------------------------

    With regard to our recognition of the uncertainties associated with 
issues in this review, we note that Congress and the courts have 
recognized that some uncertainties in assessing the effects of air 
pollution are inevitable, and the Administrator is required to exercise 
his judgment in the face of imperfect information. See, e.g., Lead 
Indus. Ass'n, Inc. v. EPA, 647 F.2d 1130, 1155 & n.50 (D.C. Cir. 1980) 
(quoting H.R. Rep. No. 95-294, at 50). Only when the Administrator 
judges that the uncertainties are so great as to preclude the ability 
to identify a standard that would be expected to provide the requisite 
protection do uncertainties justify a decision to not act. See, Center 
for Biological Diversity v. EPA, 749 F.3d 1079, 1087 (D.C. Cir. 2014). 
As discussed further in section II.B.3. below, that is not the case for 
this standard. Thus, the EPA's judgment is that the available 
information, including evidence of the effect of SOX on 
sensitive ecosystems and the analyses of transport of pollutants across 
airsheds, is sufficient to allow the Administrator to make a reasoned 
judgment about where to set a revised SO2 NAAQS, while 
recognizing that substantial uncertainties remain.\116\
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    \116\ As recognized in section II.A. above, the Administrator's 
decisions in secondary NAAQS reviews draw upon scientific 
information and analyses about welfare effects, exposures and risks, 
as well as judgments about the appropriate response to the range of 
uncertainties that are inherent in the scientific evidence and 
analyses. As described in section II.B.3. below, the Administrator's 
decision reflects these considerations.
---------------------------------------------------------------------------

    Regarding the comment that the proposal insufficiently evaluated or 
discussed how the effects to be addressed by the new annual secondary 
standard are adverse to public welfare, we note the evidence of aquatic 
acidification and its effects on fisheries in lakes and streams across 
the northeast and Appalachian Mountains. This evidence was evaluated 
and documented in the current and last ISA and prior AQCDs (e.g., ISA, 
Appendix 8, section 8.5.2 and Appendix 16, section 16.2.3.2.1; 2008 
ISA, sections 3.2.4.4 and 3.2.4.5; 1982 AQCD, section 7.1.1.1). For 
example, acidified aquatic habitats have a lower number of species 
(species richness) of fishes, including culturally and recreationally 
important species, as well as shifts in biodiversity of both flora and 
fauna. This evidence and the findings of the quantitative aquatic 
acidification REA, as well as the analyses of relationships between air 
quality and S deposition, and advice from the CASAC were considered by 
the Administrator in reaching his proposed decision that the existing 
SO2 standard does not provide the requisite protection of 
the public welfare from known or anticipated adverse effect. This 
information, and public comments, have also been considered in his 
decision on revisions to the SO2 standard, as discussed 
further in section II.B.3. below. Further, the public welfare 
implications of aquatic acidification-related effects, including the 
influence of their severity and geographic extent, on harm posed to the 
public welfare, are described in the PA, the proposal and section 
II.A.3.b. above (PA, section 4.5; 89 FR 26641-26644, April 15, 2024). 
In reaching his decision on the existing standard and on the revisions 
that would provide the requisite protection, the Administrator has 
considered these factors (severity and geographic extent of 
acidification-related effects), as well as the evidence of varying 
sensitivity of ecoregions across the U.S. In the end, as noted in 
sections I.A. and II.A. above, the CAA recognizes that judgments on 
effects to the public welfare that are adverse are within the purview 
of the Administrator in reaching his decision on secondary standards.
    In judging the existing standard to not provide the requisite 
protection of the public welfare, the Administrator has considered the 
evidence, evaluations in the PA, strengths and uncertainties in the 
evidence, and quantitative analyses. In so doing, he focused 
particularly on the REA findings for aquatic acidification risk 
estimates for the earliest part of the 20-year assessment period. With 
the pattern of deposition estimated for this period (when the existing 
standard was met), the REA found more than a third of waterbody sites 
in the five most affected ecoregions unable to achieve even the lowest 
of the three acid buffering capacity benchmarks used as risk indicators 
(below which the increased risk of episodic acidification events may 
threaten survival of sensitive aquatic species), and more than half of 
waterbody sites unable to meet this benchmark in the single most 
affected ecoregion. The Administrator judged that this level of aquatic 
acidification risk, associated with deposition levels estimated to have 
occurred when the existing standard was met, can be anticipated to 
cause adverse effects on the public welfare.
    Lastly, we disagree with the view of one commenter that the ANC 
benchmarks used in reaching conclusions regarding the need for 
protection from acid deposition relied on the judgments of others, 
rather than the EPA. Rather, as described in the PA and summarized in 
section II.A.4. above, the quantitative REA employed an array of ANC 
benchmarks in recognition of variation among waterbodies in their 
ability to achieve different benchmarks and in the associated risk to 
fisheries, to specifically avoid putting undue weight on a single 
value. In characterizing risk and levels of protection associated with 
different S deposition circumstances in the REA, we reported the 
percentages of waterbodies per ecoregion estimated to achieve the 
different benchmarks. The PA focused on this pattern of percentages in 
characterizing risk and the CASAC majority also considered this pattern 
in expressing its recommendations for a revised standard. Similarly, in 
weighing the evidence and the REA findings, the Administrator also 
considered these patterns and the weight to place on different 
benchmarks for ANC as an indicator of acidification risk, as well as 
the CASAC majority consideration of them in its recommendation of a 
range of standards expected to achieve a desired level of public 
welfare protection. In so doing, as described in section II.B.3. below, 
he judged it appropriate to consider patterns of ANC across ecoregion 
waterbodies, rather than limiting his judgment to consideration of a 
single ANC benchmark in all areas. Thus, contrary to the view of the 
commenter, the Administrator made all relevant judgments on the weight 
to place on different tools for indicating acidification risk, 
including ANC benchmarks in reaching a decision on the secondary 
SO2 standard.
(c) Comments Recommending Revision To Be Identical to the Primary 
Standard
    In disagreeing with the EPA's proposal to revise the 3-hour 
secondary SO2 standard to an annual standard for the reasons 
discussed in the two

[[Page 105756]]

sections above, a few commenters additionally expressed support for an 
alternate revision that would set the secondary standard to be 
identical to the primary standard, in all respects. One commenter 
stated that this option would be supported by a finding of no locations 
in the U.S. that would not achieve an annual standard with a level at 
the low end of the proposed range. The other commenter cited comments 
from the minority of CASAC that also recommended this option based on a 
judgment that the 1-hour primary standard is currently controlling of 
air quality and the view that most deposition values during the period 
since the primary standard was established have been less than 5 kg/ha-
yr. This commenter additionally quoted the EPA's March 9, 2024, 
technical memorandum \117\ regarding the highest annual average 
concentrations observed during the period 2017-2022 in areas that do 
not violate the primary standard. Additionally, one commenter expressed 
support for ``any alternatives,'' including revising the secondary 
standard to be identical to the primary standard in all respects, 
``that can be supported by the current science,'' without providing 
further elaboration.
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    \117\ This memorandum in the docket (Docket ID No. EPA-HQ-OAR-
2014-0128-0039) describes the basis for the EPA's decision that a 
Regulatory Impact Analysis was not warranted for the proposed 
decision (89 FR 26692, April 15, 2024).
---------------------------------------------------------------------------

    While the EPA agrees with the commenters regarding the air quality 
and deposition estimates in recent years, the EPA considered the 
available quantitative analyses, including the additional analyses 
presented in the technical memorandum to the docket (Sales et al., 
2024), and finds that a secondary standard identical to the existing 
primary standard (75 ppb, as the annual 99th percentile daily maximum 
1-hour concentration, averaged over three consecutive years) would be 
expected to provide a greater stringency in SO2 
concentrations than required to generally maintain S deposition levels 
of interest. As indicated by the additional analyses, a higher level 
(e.g., of 120 ppb) for a 1-hour standard, with averaging time and form 
identical to the primary standard, is associated with downwind 
ecoregion median S deposition levels more like those associated with an 
annual SO2 standard of 10 ppb than is such a 1-hour standard 
with a level of 75 ppb (Sales et al., 2024). Thus, the EPA disagrees 
with these commenters that a 1-hour secondary standard identical in all 
respects to the existing primary standard would provide the requisite 
protection of the public welfare, noting that it may provide more 
control than necessary to achieve the desired protection. As described 
in section II.B.3. below, the Administrator judges that an annual 
average standard, averaged over three years, with a level of 10 ppb can 
be expected to provide the needed protection of the public welfare.
(4) Comments Regarding Retaining the Existing Secondary Standard
    The very few comments that addressed the issue of retaining the 
existing 0.5 ppm (500 ppb) 3-hour standard recommended retention, 
variously noting that this standard is important for short-term direct 
impacts of SO2, that such a standard would prevent peak 
episodic events, and that in the past this standard was the controlling 
standard for many areas and its retention would ensure those areas 
maintain adequate protections. With regard to protection from the 
short-term direct impacts of SO2 in ambient air, the EPA 
agrees that the existing standard provides such protection, as 
concluded by the Administrator in the proposal and by the CASAC. We 
further note, however, that the additional air quality analyses 
conducted in response to public comments indicate that in areas with 
SO2 concentrations from 2000 through 2021 that would meet an 
annual standard of 10 ppb (excluding Hawaii),\118\ virtually all 3-hour 
standard design values (the second highest annual 3-hour concentration 
at regulatory monitors) are less than 0.25 ppm (Sales et al., 2024, 
Figure 10). These analyses further indicate that more than 99% of the 
highest 3-hour concentrations at monitored sites in each of the more 
recent years of the analysis period (2011-2021) are below 0.2 ppm 
(Sales et al., 2024, Table 6). Reflecting the evidence in the ISA and 
prior AQCDs for SOX, the PA summary of the lowest short-term 
concentrations (e.g., over a few hours) associated with effects on 
plants or lichens does not include any concentrations below 0.25 ppm 
(PA, section 5.4.2; ISA, Appendix 3, section 3.2; 1982 AQCD, section 
8.3). Together this information indicates that short-term 
concentrations in areas that would be expected to meet an annual 
standard of 10 ppb are well below those that have been associated with 
effects on plants or lichens. In light of information such as this, as 
described in section II.B.3. below, the Administrator judges that 
short-term peak concentrations of potential concern for welfare effects 
are adequately controlled by an annual average standard of 10 ppb, such 
that revision of the secondary standard to this annual standard 
provides requisite protection from both short-term effects of 
SO2 in the ambient air and effects related to the deposition 
of S compounds in sensitive ecosystems.
---------------------------------------------------------------------------

    \118\ This analysis excluded Hawaii where it is not uncommon for 
there to be high SO2 values in areas with recurring 
volcanic eruptions (PA, section 2.4.2).
---------------------------------------------------------------------------

b. Nitrogen Oxides and Particulate Matter
(1) Comments in Support of the Proposed Decisions
(a) Nitrogen Oxides
    Among the few comments received on the proposed judgment that the 
existing secondary NO2 standard provides the needed 
protection from direct effects of N oxides in ambient air on plant and 
lichen surfaces, all expressed support. In the context of ecological 
effects of N oxides more broadly, including deposition-related effects, 
several public comments expressed support for the proposed decision to 
retain the existing standard, which was based on the Administrator's 
proposed judgment that the available evidence does not clearly call 
into question the adequacy of the existing standard. In expressing 
support for the proposed decision, commenters raised several 
uncertainties, referencing the discussion in the proposal. These 
uncertainties include those related to the weak relationship between 
NO2 concentrations and N deposition; the increasing 
contribution of NH3 to N deposition; the expected impacts of 
current deposition levels absent residual historic impacts and the 
identification of appropriate protection objectives in this context of 
changing conditions; and the role of N benefits and disbenefits. We 
agree that these are important uncertainties in the evidence base, and, 
as discussed in section II.B.3. below, these factors are among those 
the EPA considered in reaching the decision to retain the existing 
NO2 standard.
    Some other commenters, in support of their position that the N 
oxides standard should not be revised, further expressed the view that 
N oxides emissions is one of the principal sources of acidic compounds 
and that the EPA lacks authority to set standards based on acidic 
deposition, citing CAA section 401(a). As discussed in section 
II.B.2.a.(3)(a) above, the EPA disagrees with the view that NAAQS 
cannot be established to provide protection for acidic deposition-
related effects. We additionally note the REA conclusion, however, that 
under current air quality and based on the current information, as 
discussed in section 5.1.2.4 and

[[Page 105757]]

Appendix 5A, section 5A.2.1 of the PA, the contribution of N compounds 
to acidification is negligible.
(b) Particulate Matter
    Among the public comments on the proposed decisions to retain the 
current secondary PM standards, only a few were received on the 
proposed judgment that the existing secondary PM standards provide the 
needed protection from the effects of PM in ambient air associated with 
direct contact with and loading onto plant and lichen surfaces. All of 
these comments expressed support for that judgment. In the context of 
ecological effects of PM more broadly, including deposition-related 
effects, comments received in support of the Administrator's proposed 
decision to retain the current secondary PM standards, without 
revision, generally noted aspects of the rationale presented in the 
proposal. For example, some comments noted uncertainties in the 
relationship between concentrations of PM2.5 and deposition 
of N or S compounds. One comment, focused on the PM10 
standard, expressed the view that the scientific information does not 
support revision of the PM10 standard. The EPA agrees with 
the view that the available information does not support revision of 
the PM NAAQS.
    In support of their position that the PM standards should not be 
revised, one commenter, noting a PA statement regarding PM components 
that may contribute to ecosystem acidification risk, expressed the view 
that the EPA lacks authority to set standards based on acidic 
deposition. As discussed in section II.B.2.a.(3)(a) above, the EPA 
disagrees with the view that NAAQS cannot be established to provide 
protection from acidic deposition-related effects. Accordingly, as 
discussed in section II.B.3. below, the decision to retain the existing 
PM standards without revision is not based on such a premise.
(2) Comments in Disagreement With the Proposed Decisions
    Most of the comments received in opposition to the proposed 
decisions to retain the existing secondary NO2 and PM 
standards, without revision, expressed the view that the standards 
should be revised to address N deposition and associated effects. Some 
of these comments additionally take note of the information indicating 
that the contribution of reduced N compounds has increased such that 
NH4\+\ is a greater contributor to N deposition than in the 
past. Further, some commenters expressed the views that the CAA 
supports a standard in terms of N deposition and that the CAA also 
supports consideration of NH3 as a criteria pollutant.
(a) Nitrogen Deposition
    Most of the commenters that disagreed with the proposed decisions 
on the secondary standards for N oxides and PM focus on N deposition 
and related effects in describing their rationales. Some commenters 
expressed the view that current N deposition is having impacts on 
resources in national parks (including parks that are also Class I 
areas); this comment is addressed in section II.B.2.b.(2)(b) below. 
These commenters also generally expressed the view that setting a 
deposition standard would be the best and/or a more scientifically 
defensible approach to standard setting, noting the CASAC advice in 
this regard. In so doing, one group of commenters noted the increasing 
role of NH3 in N deposition in recent times and expressed 
the view that the most effective way to address the NH3 
contribution to N deposition and associated effects would be to set a 
standard in terms of total N deposition. Some other commenters 
expressed disagreement with the CASAC advice regarding establishment of 
a deposition standard under section 109 of the CAA, stating that given 
the EPA's definition of ambient air as a portion of the atmosphere, an 
ambient air standard cannot be defined in terms of deposition rate.
    As also discussed in section II.B.2.a.(3)(a) above, we disagree 
with the premise that the CAA supports setting a NAAQS in terms of 
rates of deposition of a pollutant from the air onto surfaces. In 
addition, it is important to note that the criteria pollutants under 
review are PM and oxides of nitrogen, not nitrogen. Thus, the EPA is 
reviewing the standards intended to address the anticipated effects 
resulting from the presence of PM and N oxides in the ambient air, not 
the anticipated effects of NH3 in the ambient air, nor the 
effects of total N deposition in aquatic and terrestrial ecosystems 
generally. With regard to setting a NAAQS in terms of deposition rate, 
the commenters note the view of the CASAC in claiming the Act does not 
prevent the EPA from setting a standard in terms of atmospheric 
deposition rates. In so claiming, and in expressing their view on 
interpretation of the term ``level of air quality,'' the commenters 
indicate that the term might variously (depending on the impact a 
pollutant has on the public welfare) be interpreted as ``the pollution 
carried in the air that is deposited,'' or the pollutant suspended in 
the air. Without further explanation, the commenters cite section 108 
of the CAA as providing support for such a view.
    We disagree with the commenter's interpretation of the Act. The EPA 
agrees that under section 108 the air quality criteria shall ``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 such pollutant in the ambient 
air.'' However, (as noted in section I.A above) section 109(b)(2) of 
the Act specifies that ``[a]ny national secondary ambient air quality 
standard prescribed under subsection (a) shall 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 such air pollutant in the ambient air.'' 
Consistent with this statutory direction, the EPA has always understood 
the goal of the NAAQS is to identify a requisite level of air quality, 
and the means of achieving a specific level of air quality is to set a 
standard expressed as a concentration of a pollutant in the ambient 
air, such as in terms of parts per million (ppm), parts per billion 
(ppb), or micrograms per cubic meter ([mu]g/m\3\). Additionally, as 
noted by some other commenters, the definition of ambient air in 40 CFR 
50.1(e) describes ambient air as a portion of ``the atmosphere'' 
(``external to buildings, to which the general public has 
access'').\119\ Thus, taking section 108 and section 109 together, the 
EPA concludes that deposition-related effects are included within the 
``adverse effects associated with the presence of such air pollutant in 
the ambient air,'' but the standard itself must define a level of air 
quality. The EPA disagrees that a standard that quantifies atmospheric

[[Page 105758]]

deposition onto surfaces qualifies as such an air quality standard.
---------------------------------------------------------------------------

    \119\ In expressing their disagreement with the CASAC position 
that a NAAQS in terms of deposition rate is supported by the Act, 
some commenters emphasize that deposition is a process rather than a 
``level of air quality'' as specified by section 109 of the CAA, and 
also cite the definition of ambient air under 40 CFR 50.1(e). These 
commenters additionally express the view that if the CASAC's 
position were correct and the Act supported NAAQS in terms of 
deposition rate, then Congress would not have adopted title IV of 
the Act to address control of acid deposition. We do not agree with 
this latter view. Regardless of the role of NAAQS or of a potential 
role of acid deposition standards, as discussed more fully in 
section II.B.2.a.(3)(a), the action of Congress in adopting title IV 
into the Act simply provided the EPA with additional tools to 
address the problem of acid deposition more effectively.
---------------------------------------------------------------------------

    In support of their disagreement with the EPA's proposed decisions 
to retain the NO2 and PM2.5 standards without 
revision, commenters claim that the EPA's ``approach to N deposition'' 
is unlawful and arbitrary because in their view if NH3 is a 
precursor to PM then, under the definition of ``air pollutant'' in CAA 
section 302(g), NH3 ``effectively'' becomes a criteria 
pollutant. The EPA disagrees that precursors to criteria pollutants 
should be themselves treated as criteria air pollutants for all 
purposes. Section 108 of the Act is quite explicit that only air 
pollutants that have been listed by the Administrator are criteria air 
pollutants, and the Administrator has never listed NH3 as a 
criteria pollutant. Of course, criteria air pollutants may have 
precursors and in considering strategies to attain and maintain the 
NAAQS, it is important to understand whether criteria pollutants are 
emitted into the air or formed in the atmosphere from precursor 
pollutants. However, those precursors are controlled to attain and 
maintain the NAAQS for the criteria pollutants--not because they 
themselves ``effectively'' become criteria pollutants that must be 
controlled.\120\ For example, in some areas, ozone formation is 
NOX limited, such that controls on VOC emissions may have 
little or no impact on ozone formation. State implementation plans for 
such an area will differ from those in an area where ozone formation is 
VOC-limited, because control of precursors is a means to the end of 
controlling ozone.\121\ It would be unnecessary to require controls on 
both VOCs and NOX in every area simply to control ozone. 
Thus, EPA disagrees that it should treat every precursor, including 
NH3, as a criteria pollutant.
---------------------------------------------------------------------------

    \120\ To the extent CAA section 302(g) is relevant it simply 
provides discretion to the Administrator to treat precursors as 
pollutants where appropriate. While treating precursors as 
pollutants may be appropriate in some circumstances when 
implementing the NAAQS, the Administrator does not find it 
appropriate to treat precursors as criteria pollutants for purposes 
of reviewing and revising the NAAQS.
    \121\ Additionally, precursors may be regulated in their own 
right as pollutants. For example, oxides of nitrogen are both a 
criteria pollutant and precursors to ozone, and VOCs may be 
regulated both as NESHAP and as ozone precursors. See CAA section 
112(b)(2). However, in those cases the pollutant has independently 
satisfied the prerequisites for regulation under the relevant 
programs.
---------------------------------------------------------------------------

(b) Nitrogen Oxides
    The public comments that disagreed with the proposed decision to 
retain the secondary NO2 standard, without revision, 
expressed support for revision of the standard level to a value within 
the range that was recommended by the CASAC majority, with some 
commenters additionally citing the CASAC majority comments on the draft 
PA. In support of the position that the NO2 standard should 
be revised as recommended by the CASAC majority, commenters variously 
claimed that in not revising the standard, the EPA is not fully 
considering CASAC recommendations, or that the scientific evidence for 
N deposition demonstrates ``harmful'' or concerning impacts of current 
N deposition in national parks. Also, some of the commenters that 
support revision of the NO2 standard to a level within the 
range recommended by the CASAC majority (``<10-20 ppb'') stated that 
the existing standard does not include all forms of nitrogen that 
contribute to acidification, eutrophication, or nutrient enrichment, 
and the standard would need to be much lower in consideration of 
relationships with total nitrogen deposition. One comment also 
expressed support for both retaining the existing standard and for 
revising the standard to a level of 35-40 ppb, averaged over three 
years, ``as supported by the scientific evidence,'' without 
elaboration. Another comment recommended revision of the indicator of 
the existing standard to include nitric oxide (NO) in addition to 
NO2, while recommending no other revisions.
    We disagree with the commenters' position that the NO2 
standard needs revision to provide public welfare protection from total 
N deposition. As an initial matter, we note that, as discussed in 
section II.B.2.b.(2)(a) above, not all nitrogen compounds are criteria 
pollutants and accordingly, the CAA does not require the consideration 
of NAAQS for all N compounds or for total N deposition. Further, the 
secondary standard for N oxides is not required by the Act to address 
pollutants other than N oxides. Additionally, the air quality and 
deposition analyses developed in this review (e.g., PA, Chapter 6 and 
Sales et al., 2024) describe appreciable geographic (and temporal) 
variation in the portion of total N deposition contributed by N oxides, 
potentially explaining the poor or lack of correlation between 
NO2 concentrations and total N deposition observed in the PA 
analyses,\122\ which indicates that a NO2 standard would 
have little likelihood of efficacy in such a use.
---------------------------------------------------------------------------

    \122\ For example, as recognized by the CASAC majority ``when 
considering all ecoregions, there is no correlation between annual 
average NO2 and N deposition'' (Sheppard, 2023, Response 
to Charge Questions, p. 24). The final PA reported negative 
correlation coefficients for both NO2 EAQMs and a 
coefficient below 0.4 for SLAMS NO2 concentrations.
---------------------------------------------------------------------------

    One commenter stated that the current N deposition is resulting in 
harm to national park resources and expressed the view that the 
scientific evidence of N deposition adverse effects outweighs 
uncertainties associated with N critical loads. In so doing, the 
commenter claimed that justifications described in the PA for the 
option of retaining the NO2 standard, without revision, 
included (1) a lack of clarity of the role of current and legacy 
deposition in causing harm, and (2) the position that CLs involve 
designations of harm based on ``arbitrary'' levels of change. In so 
stating, the commenter conveyed their view that CLs are often based on 
studies that they stated demonstrate that reducing N deposition 
improves the resource condition even if N deposition continues to 
exceed a resource-specific CL.
    As an initial matter, the EPA disagrees that the PA conclusions 
relied on a judgment that critical loads are ``arbitrary'' to support 
the option of retaining the NO2 standard and notes that is 
also not part of the basis for the proposed decision to retain this 
standard. As described in sections II.A.3.c. and II.B.2.a.(2)(a), the 
EPA recognizes the usefulness of the CL concept in appropriate contexts 
and has utilized CLs in the aquatic acidification REA. The findings 
from the REA, based on the use of CLs for a set of ANC benchmarks, are 
a critical aspect of the Administrator's decision on the secondary 
SOX standard, as discussed in section II.B.3. below. Thus, 
while this concept can inform decision-making in NAAQS reviews, the 
science policy judgments associated with secondary NAAQS decisions, 
including those regarding risk levels associated with CL values and the 
weight to place on the evidence supporting them (with its various 
limitations and associated uncertainties), are to be made by the 
Administrator. The EPA does not agree with the view that a deposition 
rate identified as a CL is necessarily synonymous with environmental 
loading anticipated to elicit effects that are adverse to public 
welfare. Simply being labelled a CL does not confer such a status on a 
level of ecosystem loading without, for example, consideration of the 
strength of the evidence on which the CL is based, and a 
characterization of the ecological response (including severity and 
scale) for which it is estimated.

[[Page 105759]]

    In making their statement that assignment of a ``poor'' or ``fair'' 
conditions rating indicates impacts on national park resources, the 
commenter referred to a National Park Service technical analysis of 
``park-specific critical loads'' and deposition, without providing that 
analysis or describing the basis for their judgments of harm for 
instances when estimated deposition in a specific area exceeds the 
critical loads they have derived.\123\ In addition, the commenter also 
did not provide any evidence specific to N oxides or deposition of 
oxidized N to support their claim regarding the N oxides standard. 
Rather the comment implied the view that impacts associated with total 
N deposition are attributable to N oxides. We disagree with the 
commenter's view that deposition from N oxides under the existing 
standard is causing harm. As described in the proposal (section 
II.E.3.), in the PA, and, in greater detail, in the additional analyses 
presented in Sales et al. (2024), for the areas of highest total N 
deposition, such as areas where average total N deposition is above 10 
kg/ha-yr, which is the benchmark emphasized by the CASAC in making its 
recommendations regarding standards to address the ecological effects 
of N compounds (as described in section II.B.1.b. above), oxidized N is 
no longer playing the leading role. Rather, reduced N contributes the 
majority of N deposition in these areas.\124\ Unlike the situation in 
2000-2002, when oxidized N deposition accounted for up to approximately 
80% of total N deposition, on average, in States with average total N 
deposition greater than 10 kg/ha-yr, oxidized N deposition is now 
approximately half or less of total N deposition (Sales et al., 2024, 
Table 5). In fact, in the most recent period analyzed (2019-2021), mean 
oxidized N deposition is below 5 kg/ha-yr in all States of the CONUS; 
this is also the case for median oxidized N deposition in all CONUS 
ecoregions (Sales et al., 2024).
---------------------------------------------------------------------------

    \123\ The comment did not discuss why this approach to assigning 
a ``poor,'' or other than ``good,'' rating is evidence of N 
deposition-related impacts that could be addressed by revision of 
the NO2 or PM2.5 national ambient air quality 
standards or that indicates a potential for adverse effect to the 
public welfare.
    \124\ For example, in the 14 ecoregions with median N deposition 
estimated to be above 10 kg/ha-yr in the 2019-2021 period, reduced N 
comprises more than 50% of total N deposition (Sales et al., 2024, 
Table 3).
---------------------------------------------------------------------------

    Another group of commenters also referenced the National Park 
Service descriptions of park conditions related to N (and S) deposition 
in stating that 95% of parks are experiencing wet deposition of N 
greater than 1 kg/ha-yr. They claimed that the occurrence of this level 
of deposition indicates harmful impacts to park soil, waterbodies and 
associated wildlife and indicated that such an occurrence supports 
their position that the secondary NO2 (and PM) standards 
should be revised as recommended by the CASAC majority. As support for 
the 1 kg/ha-yr benchmark below which a ``good'' condition is assigned 
(and above which is assigned a ``fair'' or ``poor'' rating which the 
commenters characterized as indicative of harm), the commenters cited 
two studies.
    The EPA notes, however, that the cited studies are limited in scope 
(to a lake in Washington State and a group of high-altitude lakes in 
some western and eastern regions) and include judgments by the authors 
of specific measures on which the authors base their CLs. One of the 
two studies actually identifies CLs ranging up to 8 kg/ha-yr (Baron et 
al., 2011).\125\ Yet, the comment focuses on 1 kg/ha-yr, without 
consideration of 8 kg/ha-yr. In light of the limited scope of these 
studies, and the fact that a number of the identified CLs exceed 1 kg/
ha-yr, among other factors, the EPA does not agree that these studies 
provide a basis for concluding that adverse public welfare effects are 
occurring in 95% of parks based on estimated deposition at/above 1 kg/
ha-yr (a level far below the level referenced by the CASAC majority in 
advice regarding protective standards). These commenters also did not 
indicate how the National Park Service assignments of conditions in 
parks support the position that the NO2 standard should be 
revised to a level of <10-20 ppb, and we are unaware of any linkage. 
Further, as noted above, an appreciable amount of total N deposition is 
deposition of reduced N which is not influenced by N oxides in ambient 
air and consequently would not be affected by changes in a NAAQS for N 
oxides.
---------------------------------------------------------------------------

    \125\ This study estimates multiple CLs that differ for nutrient 
enrichment- and acidification-related effects and for eastern and 
western lakes, relying on data generally dating from 1997 to 2006 
(Baron et al., 2011). The second study uses a lake sediment core 
indicating a period of changed phytoplankton composition, estimated 
to be around 1969-75, and N deposition estimates for the 1969-75 
period (Sheibley et al., 2014).
---------------------------------------------------------------------------

    With regard to acidification risk posed by deposition of N 
compounds, we additionally note the REA finding that recent deposition 
conditions indicate negligible contribution of N compounds to aquatic 
acidification risk. Accordingly, as discussed in section II.B.3. below, 
the decision to revise the SO2 standard is intended to 
address the main contributor to ecosystem acidification, S compounds 
associated with ambient air concentrations of SOX. Thus, in 
consideration of the preceding discussion and other factors further 
discussed in section II.B.3. below, the Administrator judges that, 
based on the available evidence in this review, revision to the 
secondary annual NO2 standard is not warranted.
    The commenter recommending revision of the standard indicator to 
include NO, in addition to NO2, expressed the view that the 
EPA should not assume that effects reported to be associated with 
short-term NO2 concentrations in ambient air have no 
relationship to NO, which the commenter stated is also present in 
ambient air. In so doing, the commenter cited a controlled human 
exposure study of diesel exhaust and brain function indicator changes, 
additionally cites an epidemiologic study that reports an association 
of health care costs with ambient air concentrations of NO2 
and NO and noted that NO concentrations are higher than NO2 
concentrations (in terms of ppb) in areas near traffic or oil and 
methane gas extraction activities. The EPA disagrees with the commenter 
that the effects on which the commenter focused--subtle changes in 
cellular activity in a specific region of human brain as reported in a 
controlled human exposure study of short-duration diesel exhaust 
exposures (in which NO2 [but not NO] was one of the 
components analyzed) and health care costs--are welfare effects; thus, 
their relevance for this review is unclear.
    Further, in support of their statement that NO2 
concentrations in ambient air have no relationship to NO 
concentrations,\126\ the commenter simply referenced tables of hourly 
NO and NO2 concentrations available from Colorado Department 
of Public Health and Environment, which are clearly labeled as data 
collected in real-time that ``have not been corrected nor validated.'' 
We note that, although the data have not been validated, they generally 
illustrate the expected diurnal pattern for these pollutants near 
combustion sources (e.g., with NO initially increasing with morning 
traffic, and then declining as it is converted to NO2 [1971 
AQCD, p. 6-1]). While recognizing these common patterns in the 
relationship between the two

[[Page 105760]]

chemicals, we further note that the form of the existing standard is an 
annual average, and the commenter did not provide validated data or 
analyses that might assess the existence of a, or support their view 
that there is no, relationship between annual average concentrations of 
NO and NO2.
---------------------------------------------------------------------------

    \126\ In describing their position that the indicator should be 
revised, the commenter also suggests that the NAAQS ambient air 
monitoring system is inadequate. The commenter provided no evidence 
in support of this suggestion, and we continue to find that the 
current ambient air monitoring network for NAAQS is appropriate.
---------------------------------------------------------------------------

    The comment also does not include any information related to 
concentrations of either pollutant, or both in combination, at which 
welfare effects of concern may occur and relate that to ambient air 
concentrations associated with the existing secondary NO2 
standard. The evidence in 1971 when the existing standard was set 
describes the conversion of NO to NO2 in the presence of 
oxygen, with NO2 being the more stable air pollutant away 
from sites of combustion and the one for which analytical methodology 
existed for its direct analysis at that time (1971 AQCD). While there 
is a study from the mid-1980s for effects of NO on photosynthesis, 
which indicates a potential for greater toxicity of NO to some plant 
species, the NO concentrations reported for this study are nearly two 
orders of magnitude greater than those found in ambient air. Further, 
the vegetation effects evidence base is much more extensive (with 
regard to species and specific effects studied) for NO2 and 
includes studies that investigate both NO and NO2 together 
(ISA, Appendix 3, section 3.3). The NO2 standard is intended 
to provide protection from anticipated effects of oxides of nitrogen, 
including NO and NO2, but the commenter does not provide a 
basis for concluding that an annual average NO2 standard is 
insufficient to provide the requisite protection. Thus, we find no 
support in the available information in this review that might support 
their claim that the existing standard should be revised to be an 
annual average concentration of 53 ppb, in terms of the sum of NO and 
NO2.
(c) Particulate Matter
    Comments opposed to the proposed decision to retain the secondary 
PM standards generally focused on PM2.5 and called for more 
stringent secondary standards. In so doing, these commenters cited the 
specific PM2.5 standard revisions recommended by the CASAC 
majority, summarized in II.B.1.b. above. With regard to the annual 
PM2.5 standard, these commenters also discussed analyses 
presented in the PA, which they stated provide support to the use of 
the annual PM2.5 standard to address total N deposition. In 
support of a revision to the PM2.5 standard, some commenters 
noted the increased role of NH3 in total N deposition, 
including in estuaries and coastal waters where eutrophication has been 
reported or in national parks. These commenters expressed the view that 
the contribution of NH3 to N deposition and related effects 
can be addressed through revisions to the PM2.5 standard. In 
so doing, they further stated that the EPA's proposed decision to 
retain the existing standard is based on uncertainties and complexities 
related to NH3 and that such uncertainties and complexities 
are an insufficient basis for retaining the existing standard, 
additionally citing a 2002 court decision regarding EPA acting when it 
has enough information to do so (Am. Trucking Ass'ns v. EPA, 283 F.3d 
355, 380 [D.C. Cir. 2002]). In support of their position, the 
commenters stated that the EPA must act when enough information is 
available to anticipate such effect, and deciding not to revise is 
inconsistent with the Act's protective direction. Commenters 
additionally suggest that the EPA inappropriately imposed limits on its 
consideration of the trajectory-based analyses so as to provide support 
for the EPA conclusion that the NO2 and PM2.5 
metrics do not provide adequate vehicles for regulating N deposition. 
Another commenter, in support of their position that the existing 
PM2.5 standards should be revised as recommended by the 
CASAC majority, expressed the view that reduced N deposition has become 
the dominant form of N deposition, which they stated is impacting 
national park resources in many areas of the U.S. such that a revised 
standard would help to reduce such pollutants. Additionally, a comment 
recommending revision of the PM2.5 standard stated that the 
range of revised levels suggested by the CASAC majority would keep S 
deposition below 5 kg/ha-yr and N deposition at or below 10 kg/ha-yr 
and stated that the CASAC majority range was based on NADP and IMPROVE 
monitoring data and modeled results, without further explanation. 
Another comment recommended revision of the annual PM2.5 
standard to 12 [mu]g/m\3\,\127\ based on their view that it would add 
no additional requirements and could streamline implementation plan 
development and compliance. Lastly, some commenters additionally 
expressed that the 24-hour PM2.5 standard should be revised, 
again citing recommendations from the CASAC majority and protection 
against short-term episodic deposition and visibility impairment.
---------------------------------------------------------------------------

    \127\ This was also the advice of the CASAC minority, with 12 
[mu]g/m\3\ being the level of the annual primary standard when CASAC 
provided its advice.
---------------------------------------------------------------------------

    For the reasons stated below, elsewhere in section II.B.2., in 
section II.B.3. and in the Response to Comments document, the EPA 
disagrees that these comments provide a sufficient justification for 
revising the PM secondary standards. In support of their position that 
the PM2.5 standard is an appropriate tool for controlling 
particulate N and should be revised to a value within the range of 6 to 
10 [mu]g/m\3\ recommended by the CASAC majority, some commenters state 
that NH4\+\ has been increasing in cloud water and in 
PM2.5 and reference statistically significant correlation 
coefficients for total N deposition estimates and concentrations of 
PM2.5 mass (and N components) in remote Class I areas (PA, 
Figure 6-32), which they suggest supports their view that use of 
PM2.5 ``as an ambient air quality indicator to total 
nitrogen deposition is not unreasonable.'' They also claim that Figure 
6-32 in the final PA, and Figure 6-33 presenting total N deposition 
estimates versus total particulate N and NH4\+\ at 27 Class 
I area sites, provide support for the CASAC majority recommendation on 
revising the PM2.5 standard, which they endorse.
    As an initial matter, we disagree with the view that effects of 
total N deposition (from all contributing pollutants) are a 
determinative consideration in judging the adequacy of the secondary 
PM2.5 standard, as discussed in section II.B.2.b.(2)(a) 
above. Further, we disagree that NH4\+\ in PM2.5 
has been increasing, finding instead that the contribution of 
NH4\+\ to PM2.5 mass at sites across the U.S. has 
been decreasing over the past decade (Sales et al., 2024). Further, to 
the extent the commenters are claiming the CASAC majority recommended 
range of annual PM2.5 standard levels, which they endorse, 
to be supported by the pattern of PM2.5 concentrations and 
total N deposition estimates at 27 Class I area IMPROVE monitoring 
sites (in either draft PA Figure 6-13 or final PA Figure 6-32), we 
disagree that this information provides a basis for decisions on the 
standard. The commenters are overlooking several relevant aspects of 
the available information.\128\

[[Page 105761]]

Particularly important is that the monitoring sites represented by 
these figures comprise just a small subset of the more than a thousand 
PM2.5 monitoring sites across the U.S., and this subset of 
monitors is in remote areas. Accordingly, these monitors are not in 
areas where PM2.5 concentrations are highest. Thus, the 
PM2.5 concentrations in the remote area figure are not 
representative of PM2.5 concentrations that would need to be 
controlled to limit deposition across the U.S., including in these 
areas. Such deposition is necessarily related to atmospheric transport, 
among other factors, and a focus solely on remote areas cannot be 
expected to identify the level for a PM2.5 standard (that 
would need to be met across the U.S.) with the potential to yield the 
desired deposition rate in these areas. This is because at the time of 
the deposition levels observed in these areas, the PM2.5 
concentrations are higher in areas not represented in the figure that 
may contribute to deposition at the sites in the figure (and at other 
sites).\129\ Further, the PA analyses of N deposition and 
PM2.5 concentrations at SLAMs also do not provide a basis 
for identifying 3-year average annual PM2.5 concentrations 
that might be expected to constrain nearby N deposition below certain 
target levels (e.g., PA, Figure 6-39).\130\ For all of these and 
related reasons, the Administrator, in making his proposed and final 
judgments regarding the secondary PM standards, did not find the CASAC 
majority focus on remote area analyses to be informative in making 
decisions on the annual PM2.5 standard.
---------------------------------------------------------------------------

    \128\ One aspect overlooked is that the PA Figure 6-32 cited by 
the commenter in referencing correlation coefficients presents a 
different metric than the figure in the draft PA cited by the CASAC 
majority in conveying its PM2.5 standard recommendations. 
Figure 6-13 in the draft PA that was cited by the CASAC majority 
presents 3-year average concentrations of data from 2002 to 2019 
(using different 3-year periods than those used throughout the rest 
of the PA), while the final figure in the final PA presents annual 
averages from 2000 to 2019 (PA, Figure 6-32).
    \129\ In the period from 2014 through 2019 (the period 
emphasized in the CASAC majority justification that relied on draft 
PA Figure 6-13) when TDep estimated N deposition is at/below 10 kg/
ha-yr and annual average PM2.5 concentrations are at/
below 10 [mu]g/m\3\ at the 27 Class I area sites, annual average 
PM2.5 concentrations are much higher in other areas of 
the U.S. that are more fully represented in the regulatory 
monitoring network (PA, Figure 2-37). As indicated by recent 
PM2.5 design values, the highest concentrations sites are 
generally in the far west of the country, which given prevailing 
wind patterns, are generally upwind from the Class I areas (PA, 
Figures 2-31 and 2-32).
    \130\ Concentrations at SLAMS from just above 15 [mu]g/m\3\ down 
to approximately 4 [mu]g/m\3\ since 2010 had nearby total N 
deposition (in same grid cell) both above and at/below 10 kg/ha-yr 
(PA, Figure 6-39), and the SLAMS analyses did not provide 
information on ecoregion median deposition for the ecoregion of 
SLAMS monitor.
---------------------------------------------------------------------------

    Regarding the commenters' criticism of the EPA's consideration of 
the trajectory-based analyses for N deposition and the PM2.5 
metric, we note that the commenters do not identify a technical flaw in 
EPA's considerations or state what they conclude from the trajectory-
based analyses and how they do so. The EPA has fully considered the 
trajectory-based analysis results presented in the PA (PA, section 
6.2.4.2, 6.4, 7.2.3.3 and 7.4) and summarized in section II.A.2. above. 
We note that, while, when considering the full dataset, there is a 
positive correlation of downwind total N deposition and upwind values 
of the EAQM-weighted metric, with a low-moderate coefficient value, the 
correlation coefficient value is essentially zero in the most recent 
time period (PA, Table 6-11). And, importantly, there is a poor and 
negative correlation for the EAQM-max metric; this correlation is 
negative both for the overall dataset inclusive of all five time 
periods and for each of the five time periods individually (PA, Table 
6-11). Thus, we disagree with commenters that we have inappropriately 
or inadequately considered the trajectory-based analyses for 
PM2.5 and N deposition. Also, rather than limiting 
consideration of these results to a narrow temporal window, as claimed 
by the commenters, we have considered multiple aspects of the full 
results. As described in section II.B.3. below, these considerations 
were part of the basis for the Administrator's conclusion on the PM 
standards.
    Also overlooked by the commenters is the fact, as discussed in 
section II.A.2. above, that the percentage of PM2.5 mass 
comprised of N compounds is no higher than about 30% in the recent 
period, and ranges down to less than 5% across the U.S., varying widely 
from region to region (PA, Figure 6-56 [upper panel]; Sales et al., 
2024). We note that focus by the commenters (and the CASAC majority) on 
a small subset of the PM2.5 monitors across the U.S. (i.e., 
monitors in 27 Class I area sites [PA Figure 6-32]) would not 
necessarily reflect the variability of PM2.5 mass 
composition occurring across the U.S. Nonetheless, the percentage of 
PM2.5 mass comprised of N compounds affects the extent to 
which a particular level for an annual secondary PM2.5 
standard levels can be expected to control N deposition rates to meet a 
particular objective for protection from deposition-related effects. As 
described in section I.A. above, the Administrator is required to set a 
NAAQS that is neither more stringent nor less stringent that necessary. 
Given the fact that up to 95% of PM2.5 in some regions of 
the U.S. (and no more than 70% in others) is not N compounds, we are 
unable to make a reasoned judgment about levels of N deposition that 
would result from control measures to reduce PM2.5 
concentrations to any particular level. In fact, based on the 
information available, annual average PM2.5 concentrations 
could be reduced in some areas, e.g., to meet a lower standard, without 
reducing concentrations of the N components of PM2.5 and, 
therefore, without affecting N deposition derived from 
PM2.5. Thus, contrary to the commenters' claims, including 
that revision to a level within the CASAC majority recommended range 
would keep N deposition at or below 10 kg/ha-yr, the current 
information indicates that a PM2.5 standard would not be 
expected to provide effective control of particulate N compounds.
    With regard to the comment that the EPA should revise the 
PM2.5 standard to address the effects of N deposition 
contributed by NH3, we first note that while some 
NH3 (a gas) transforms to NH4\+\ (a particulate N 
compound in PM2.5), some NH3 is directly 
deposited in dry deposition. Further, some NH3 is captured 
in raindrops, where it transforms into NH4\+\ as it is 
deposited in wet deposition (PA, section 2.5.2; Sales et al., 2024). We 
additionally note, as discussed in section II.B.2.b.(2)(a), that 
NH3 is not a criteria pollutant. As described above and 
discussed in section II.B.3., the Administrator has considered the 
PM2.5 standard with regard to ecological effects of N 
deposition associated with PM and protection of the public welfare from 
such effects. In so doing, he has understood that the percentage of 
PM2.5 relevant to such effects ranges from 30% down to 5% or 
less that is N compounds, and that this percentage varies across the 
U.S. In light of this and other relevant factors, the Administrator has 
judged that the PM2.5 standard would be ineffective with 
regard to control of deposition of particulate N compounds, and, as 
discussed more fully in section II.B.3., has decided to retain the 
existing standard, without revision.
    The EPA also disagrees with the view that the uncertainties and 
complexities (and limitations) associated with the evidence base and 
air quality information that were cited by the EPA in its proposed 
decision to retain the PM standards are an insufficient basis for 
retaining the existing standard. Although these uncertainties and 
complexities include those related to NH3, they are not, as 
the commenter suggests, limited to those related to NH3. In 
support of the commenters' view, they note that the EPA must act when 
enough information is available to anticipate such effect and then 
assert that to not revise the secondary PM2.5 standards ``is 
inconsistent with the Act's protective direction.'' While we agree

[[Page 105762]]

that the EPA must act when enough information is available to 
anticipate effects, and we recognize that revising the NAAQS generally 
requires acting in the face of uncertainties to provide necessary 
protection (as the Administrator is doing in setting a new 
SO2 standard), the Administrator cannot set a standard if he 
lacks any ability to make a reasoned judgment about the effect of the 
standard. As recognized above and discussed in section II.B.3. below, 
the uncertainties and limitations of the information with regard to 
support for a PM2.5 standard that can be concluded to 
provide control for deposition-related effects of particulate N 
compounds, including NH4\+\, preclude our ability to 
characterize the extent of control that can be expected.
    In addition, the EPA disagrees with commenters who support revising 
the PM2.5 standard based on their view that this would 
maintain S deposition generally at/below 5 kg/ha-yr. First, we find 
that the PM2.5 indicator is not an appropriate tool and 
cannot be expected to be an effective tool for controlling S deposition 
in light of the fact that, in recent periods, 
SO42- (the predominant particulate S compound) is 
not the dominant component of PM2.5 across the U.S. and is a 
small component in many areas (ISA, Appendix 2, Figure 2-5 [panel B, 
2013-2015]; PA, Figure 2-30 [2019-2021]). The variability in the 
fraction of PM2.5 comprised of S compounds likely 
contributes to the PA findings on correlations of S deposition with 
PM2.5 concentrations (PA, Chapter 6). The correlation 
coefficients for this relationship in the trajectory-based analyses are 
lower than those for the relationship between S deposition and 
SO2 concentrations, with the correlation for the 
PM2.5 EAQM-max actually being negative (PA, Tables 6-12 and 
6-8). In light of such findings, the Administrator has not found 
PM2.5 to be an appropriate indicator for a secondary 
standard to provide protection from ecosystem effects of S compound 
deposition. Rather, as discussed in section II.B.3. below, based on the 
available information and analyses, the Administrator has judged that a 
new annual secondary SO2 standard of 10 ppb can be expected 
to achieve the target identified by the CASAC majority of generally 
maintaining S deposition at/below approximately 5 kg/ha-yr. This new 
SO2 standard provides a much more explicit and precise 
approach for controlling S deposition-related effects of SOX 
and particulate S compounds.
    The comment that recommended revision of the annual 
PM2.5 standard to be 12 [mu]g/m\3\, based on the view that 
it would not present additional requirements and could streamline 
implementation plan development and compliance, provided no information 
related to the extent of public welfare protection that might be 
provided by such a revision, or information indicating that the 
existing standard does not provide adequate protection. As explained in 
section II.B.3. below, the EPA disagrees with the commenter's 
recommendation for such a revision, and the Administrator finds that 
the available information supports retaining the current standard.
    The comment regarding revision of the 24-hour PM2.5 
standard to address short-term episodic deposition and visibility 
impairment expresses support for the CASAC majority recommendation on 
this. Beyond this reference to the CASAC majority recommendation, the 
comment provided no evidence to support their view that there are 
adverse effects of episodic deposition that would be appropriately 
addressed by revision of the standard level to 25 [mu]g/m\3\ (from 35 
[mu]g/m\3\). As described in section II.B.1.b. above, the CASAC 
majority recommendation, while alluding to a potential for seasonal 
variability in deposition and in sensitivity of some species, did not 
provide evidence for such potentials or evidence to support the 
conclusion that a revised standard is needed to protect against adverse 
ecological effects on the public welfare, and the EPA is not aware of 
such evidence. Thus, as described in section II.B.3. below, the 
Administrator has decided to retain the existing 24-hour secondary 
PM2.5 standard.
    Regarding visibility impairment, as conveyed in the IRP, PA and 
proposed decision document for this review, PM2.5 effects on 
visibility are outside the scope and are not being addressed in this 
review because they were addressed in the recently completed PM NAAQS 
review, which also revised the primary NAAQS for PM2.5 (89 
FR 16202, March 6, 2024). The commenters advocating for consideration 
of visibility here erroneously state that these effects were addressed 
in setting the primary PM2.5 NAAQS and further state that 
this is not a reason for excluding them from consideration in this 
review. We note, however, that the primary PM2.5 NAAQS are 
not intended to address visibility impairment. Rather, the recently 
completed review covered both the primary PM2.5 NAAQS as 
well as review of the secondary NAAQS for visibility, materials damage 
and climate effects. See 89 FR 16202 at 16311-16343 (rationale for 
decisions on the secondary NAAQS). Thus, visibility is a welfare effect 
that has been addressed in assessing the protection provided for the 
public welfare by the secondary PM2.5 standard in the 2020 
PM NAAQS decision and the reconsideration of that decision which was 
completed earlier this year (89 FR 16202, March 6, 2024) and is outside 
the scope of this review.
3. Administrator's Conclusions
    Having carefully considered the public comments, as discussed 
above, the Administrator believes that the fundamental scientific 
conclusions on the ecological effects of SOX, N oxides, and 
PM reached in the ISA and summarized in the PA and in section II.C. of 
the proposal remain valid. Additionally, the Administrator believes 
that the judgments he reached in the proposal (section II.E.3.) with 
regard to consideration of the evidence and quantitative assessments 
and advice from the CASAC remain appropriate. Thus, as described below, 
the Administrator concludes that the current secondary SO2 
standard is not requisite to protect the public welfare from known and 
anticipated adverse effects associated with the presence of 
SOX in the ambient air and that the standard should be 
revised. Further, based on the information available in this review and 
summarized in the proposal, including advice from the CASAC, as well as 
public comment and additional analyses developed in consideration of 
public comments, the Administrator concludes that revision of the 
existing 3-hour secondary SO2 standard to an annual standard 
of 10 ppb, averaged over three years, is required to provide additional 
needed protection from atmospheric deposition-related effects. He 
additionally concludes that it is appropriate to retain the existing 
secondary standards for N oxides and PM.
    In his consideration of the adequacy of the existing secondary 
standards for SOX, N oxides, and PM, and what revisions or 
alternatives are appropriate, the Administrator has carefully 
considered the available evidence and conclusions contained in the ISA 
regarding the weight of the evidence for both the direct effects of 
SOX, N oxides, and PM on plants and lichens and for effects 
related to atmosphere deposition in ecosystems of N and S compounds 
associated with the presence of these pollutants in ambient air, and 
associated areas of uncertainty. In so doing, he recognizes the 
evidence of direct biological effects associated with elevated short-
term concentrations of SOX and N oxides that formed the 
basis for the existing secondary SO2 and NO2 
standards, the evidence of ecological effects of PM in ambient air, 
primarily

[[Page 105763]]

associated with loading on vegetation surfaces, and also the extensive 
evidence of ecological effects associated with atmospheric deposition 
of N and S compounds into sensitive ecosystems. He has also considered 
the quantitative analyses of aquatic acidification risk and of air 
quality and deposition estimates, with associated limitations and 
uncertainties; policy evaluations of the evidence, exposure/risk 
information, and air quality information in the PA; and the related 
additional analyses (Sales et al., 2024). Together, these conclusions, 
analyses, and evaluations, along with CASAC advice and public comments, 
inform his judgments in reaching his decisions on secondary standards 
for SOX, N oxides, and PM that provide the requisite 
protection under the CAA.
    In recognizing that a prominent part of this review is the 
consideration of secondary NAAQS with regard to ecological effects 
related to deposition of S and N compounds, the Administrator notes the 
view of the CASAC regarding deposition standards. In its advice to the 
Administrator in this review, the CASAC expressed the view that the CAA 
does not preclude the establishment of a NAAQS in terms of atmospheric 
deposition (section II.B.1.b. above). As discussed in sections 
II.B.2.b.(2)(a) and II.B.2.b.(3)(a) above, the EPA disagrees with this 
view. Rather, the EPA concludes that it does not have the authority to 
set a deposition standard under the existing CAA, and the EPA is not 
adopting a deposition standard in this action.
    With regard to the adequacy of public welfare protection provided 
by the existing secondary SO2 standard, the Administrator 
first considers the adequacy of protection the existing standard 
provides for ecological effects related to ecosystem deposition of S 
compounds associated with the presence of SOX in ambient 
air. As an initial matter, the Administrator recognizes the long-
standing evidence of the role of SOX in ecosystem 
acidification and related ecological effects. While he also notes the 
ISA determinations of causality for S deposition with two other 
categories of effects related to mercury methylation and sulfide 
phytotoxicity (ISA, Table ES-1; PA, section 4.4), he recognizes, as 
noted in section II.A.3.c. above, that quantitative tools and 
approaches are not well developed for ecological effects associated 
with atmospheric deposition of S other than ecosystem acidification 
(PA, section 7.2.2.1).\131\ In this context, he notes that the current 
evidence does not indicate such effects to be associated with S 
deposition at lower rates than those posing risks of ecosystem 
acidification, and judges that a decision focused on providing the 
requisite protection for acidification-related effects will also 
contribute protection for other effects. Thus, he gives primary 
consideration to effects related to acidifying deposition, given the 
robust evidence base and available quantitative tools, as well as the 
longstanding recognition of historical impacts in acid-sensitive 
ecosystems across the U.S.
---------------------------------------------------------------------------

    \131\ For example, there are no studies in the available 
evidence investigating linkages of S deposition, in terms of 
quantitative estimates, such as CLs, with other non-acidifying 
effects (ISA, Appendix 12, section 12.6); these effects, in wetland 
and freshwater ecosystems, include the alteration of Hg methylation 
in surface water, sediment, and soils; and changes in biota due to 
sulfide phytotoxicity including alteration of growth and 
productivity, species physiology, species richness, community 
composition, and biodiversity (ISA, Appendix 12, section 12.7).
---------------------------------------------------------------------------

    As an initial matter, the Administrator notes that, during the 20-
year period from 2001 through 2020, the range of median S deposition 
estimates for the 84 ecoregions in the contiguous U.S. extend up to 20 
kg S/ha-yr (PA, Appendix 5A, Table 5A-11) and that during this period 
the existing secondary SO2 standard was met (Sales et al., 
2024). Over this 20-year period in the contiguous U.S., design values 
for the existing secondary SO2 standard (second highest 3-
hour average in a year) were generally well below the standard level of 
500 ppb (PA, section 6.2.1). For example, in the earliest 3-yr period 
analyzed (2001-2003), when median total S deposition was estimated to 
be approximately 20 kg/ha-yr in the Western Allegheny Plateau ecoregion 
(which includes the Ohio River Valley) and just over 16 kg/ha-yr in the 
Central Appalachians ecoregion (PA, Appendix 5A, Table 5A-11), 
virtually all design values for the existing 3-hour secondary standard 
were below 400 ppb (across the CONUS) and the 75th percentile of 3-hour 
design values was below 100 ppb (PA, Figure 2-27). With regard to the 
18 eastern ecoregions assessed in the REA, the Administrator notes that 
during this period, the ecoregion median deposition ranged above 15 kg/
ha-yr and the 90th percentile \132\ S deposition estimates for half of 
these 18 ecoregions were at or above 15 kg/ha-yr, ranging up above 20 
kg/ha-yr in the highest ecoregion (figure 2 above).
---------------------------------------------------------------------------

    \132\ This refers to the 90th percentile in the distribution of 
S deposition estimates for TDep grid cells in each ecoregion in 
which there were waterbody sites assessed in the REA.
---------------------------------------------------------------------------

    In considering the extent to which this magnitude of estimated S 
deposition (summarized immediately above) indicates a potential for 
effects on the public welfare, the Administrator turns to consideration 
of the aquatic acidification risk indicated for such estimates by the 
REA. Specifically, he takes note of the REA estimates of aquatic 
acidification risk associated with the S deposition estimated to have 
occurred in 2001-2003, when the existing standard was met. In this time 
period, the REA finds that across the 18 acid-sensitive ecoregions 
analyzed, the pattern of S deposition in the five most affected 
ecoregions is associated with more than about a third of waterbody 
sites in the ecoregions being unable to achieve even the lowest of the 
three acid buffering capacity benchmarks used as risk indicators (ANC 
of 20 [mu]eq/L). And, in the single most affected ecoregion, more than 
half of waterbody sites are unable to meet this benchmark. In 
considering these results, the Administrator recognizes the use of ANC 
as an indicator of aquatic acidification risk and as a quantitative 
tool within a larger framework of considerations pertaining to the 
public welfare significance of acid deposition-related effects. In this 
framework, he takes note of the PA description of the three benchmarks 
used in the REA, with the value of 20 [mu]eq/L considered to represent 
a level of acid buffering capacity consistent with a natural or 
historically occurring ANC range and 50 [mu]eq/L to provide greater 
protection, particularly from episodic acidification events, 
additionally recognizing that ANC levels below 20 [mu]eq/L have been 
associated with reductions in number of fish species (and species 
population sizes) in some sensitive waterbodies of the Shenandoah and 
Adirondack Mountains (as summarized in section II.A.4.a. above).\133\
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    \133\ Effects of elevated acid deposition have been evident for 
decades in the Adirondack region of New York, USA (Driscoll et al 
2016). Fisheries surveys by NY DEC in the 1980s indicated reductions 
in fish populations in Adirondack lakes which researchers indicate 
may relate to acidification in these lakes (Baker and Schofield, 
1985). For example, a survey of 1469 Adirondack lakes conducted in 
1984-87 found chronic acidity (ANC below 0 [mu]eq/L) in 27% of lakes 
(Kretser et al., 1989). An additional 21% of Adirondack lakes were 
found to have summertime ANC values between 0 and 50 [mu]eq/L, 
indicating a potential for ANC to dip to values near or below 0 
[mu]eq/L during periods of high discharge, such as snowmelt or 
precipitation events (Kretser et al., 1989).
---------------------------------------------------------------------------

    The Administrator also takes note of the PA discussion of the 
potential public welfare impacts of aquatic acidification that can 
include reductions in recreational and subsistence fisheries, and 
related reductions in recreational and cultural usage of these areas by 
the public, summarized in sections II.A.3.b. and II.B.1.a.(3) above. 
For example, he

[[Page 105764]]

recognizes that aquatic acidification affects the diversity and 
abundance of fish and other aquatic biota in the affected waters, and 
consequently also affects the array of public uses of these 
waterbodies. With this in mind, he focuses on the prevalence of 
elevated aquatic acidification risk across multiple waterbodies in 
multiple ecoregions (with ANC as the acidification risk indicator) 
recognizing that the significance of aquatic acidification-related 
impacts on the public welfare (e.g., associated with reductions in 
public usage of aquatic ecosystems in which fisheries have been 
affected by acidification) increases with greater prevalence of 
affected waterbodies and ecoregions. In this context, the Administrator 
judges that the prevalence of waterbodies concluded to be unable to 
achieve the lowest ANC benchmark (below which the increased risk of 
episodic acidification events may threaten survival of sensitive 
aquatic species) during the 2001-2003 period--extending from more than 
30% to just over 50% in the five most affected eastern ecoregions 
(figure 1 above)--can be anticipated to cause adverse effects on the 
public welfare. The Administrator also considers the advice from the 
CASAC in considering deposition-related effects of S compounds, noting 
the CASAC consensus that the existing standard does not provide 
protection from such effects. Lastly, he notes the lack of public 
comments conveying the position that the existing standard provides 
protection from deposition-related effects (section II.B.2.a. above). 
Thus, based on the findings of the REA, associated policy evaluations 
in the PA with regard to S deposition and acidification-related effects 
in sensitive ecosystems, and in consideration of advice from the CASAC 
and public comments on the proposed decision, the Administrator judges 
that the current SO2 secondary standard is not requisite to 
protect the public welfare from adverse effects associated with acidic 
deposition of S compounds in sensitive ecosystems.
    Having reached this conclusion that the existing secondary 
SO2 standard does not provide the requisite protection of 
the public welfare from adverse S deposition-related effects, most 
prominently those associated with aquatic acidification, the 
Administrator next turns to identification of a secondary standard to 
provide such protection. In so doing, consistent with the approach 
employed in the PA, he focuses first on identifying S deposition rates 
that might be judged to provide an appropriate level of public welfare 
protection from deposition-related effects. As in reaching his proposed 
decision, the Administrator focuses primarily on the aquatic 
acidification risk estimates as presented and evaluated in the PA (PA, 
sections 5.1, 7.1 and 7.3, and Appendix 5A) and summarized in sections 
II.A.4. and II.B.1.a.(3) above. In this context and consistent with his 
consideration of these estimates in judging the existing SO2 
standard to be inadequate, he finds the PA evaluation of the risk 
estimates in terms of waterbodies estimated to achieve the three acid 
buffering capacity benchmarks (20, 30 and 50 [mu]eq/L) to be an 
appropriate basis for his consideration of levels of protection. 
Further, he judges that a focus on the ecosystem-scale estimates, in 
particular, is appropriate for his purposes in identifying conditions 
that provide the requisite protection of the public welfare.
    The Administrator recognizes that the CAA requires the 
establishment of secondary standards that are, in the Administrator's 
judgment, requisite (i.e., neither more nor less stringent than 
necessary) to protect the public welfare from known or anticipated 
adverse effects associated with the presence of the pollutant in the 
ambient air. As in all NAAQS reviews, the EPA's approach to informing 
these judgments is based on a recognition that the available welfare 
effects evidence generally reflects a continuum that includes ambient 
air-related exposures for which scientists generally agree that effects 
are likely to occur, through lower levels at which the likelihood and 
magnitude of response become increasingly uncertain. The Administrator 
recognizes that the CAA does not require establishment of secondary 
standards at a zero-risk level, but rather at levels that reduce risk 
sufficiently so as to protect the public welfare from known or 
anticipated adverse effects. Thus, the Administrator recognizes that 
his decision on the secondary standard for SOX is inherently 
a public welfare policy judgment that draws upon the scientific 
evidence for welfare effects, quantitative analyses of air quality, 
exposure, and risks, as available, and judgments about how to consider 
the uncertainties and limitations that are inherent in the scientific 
evidence and quantitative analyses.
    In his consideration of deposition conditions that provide the 
requisite protection of the public welfare, as in reaching his proposed 
decision, the Administrator focuses on the ecoregion-scale findings of 
the aquatic acidification REA, with particular attention to the 
waterbody-specific risk estimates summarized in the PA for each of the 
18 well-studied, acid-sensitive eastern ecoregions and the five time 
periods. The PA summarizes the percentages of waterbodies per ecoregion 
estimated to achieve (i.e., to meet or exceed) the three ANC benchmarks 
in each time period in terms of the ecoregion median S deposition value 
for that time period, which are grouped into bins (e.g., percentages 
for ecoregion-time period combinations with median ecoregion S 
deposition at/below 10 kg/ha-yr, or 8 kg/ha-yr or 5 kg/ha-yr). The 
Administrator considers particularly the ecoregion median S deposition 
values at and below which the associated waterbody-specific risk 
estimates indicated a high proportion of waterbodies in a high 
proportion of ecoregions would achieve ANC values at or above the three 
acid buffering capacity benchmarks (as summarized in tables 3 and 4 
above). In so doing, he recognizes a number of factors, as described in 
the PA, which contribute variability and uncertainty to waterbody 
estimates of ANC and to interpretation of acidification risk associated 
with different values of ANC (PA, section 5.1.4 and Appendix 5A, 
section 5A.3). In light of these factors, rather than focusing on REA 
ecoregion-scale results for a single ANC benchmark, he finds it 
appropriate to consider the pattern of REA results across all three 
benchmarks, as evaluated in the PA and considered by the CASAC majority 
(summarized in section II.B.1.b. above).
    In considering the summary of results for the ecoregion-scale 
analysis of ecoregion median deposition bins (in the draft PA),\134\ 
the CASAC majority focused on a level of S deposition estimated to 
achieve acid buffering capacity at or above the three ANC benchmarks in 
80% (for ANC of 20 and 30 [mu]eq/L) or 70% (for ANC of 50 [mu]eq/L) of 
waterbodies in all ecoregion-time period combinations \135\ (Sheppard, 
p. 25 of the Response to Charge

[[Page 105765]]

Questions). The CASAC majority identify S deposition levels 
``generally'' at or below 5 kg/ha-yr as associated with this pattern of 
acid buffering. The Administrator notes that, as recognized in the PA 
and the proposal, the REA found ecoregion median S deposition at or 
below 7 kg/ha-yr in the 18 eastern ecoregions also yields these 
percentages of waterbodies achieving the three ANC benchmarks (as seen 
in tables 3 and 4 above).\136\
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    \134\ While the final PA provides additional presentations of 
aquatic acidification risk estimates, including those at the 
ecoregion-scale, the estimates for percentages of waterbodies per 
ecoregion achieving ANC targets at or below different S deposition 
values are unchanged from those in the draft PA (PA, section 5.1.3; 
Table 5-5 [draft PA, Table 5-4]).
    \135\ The presentation of such percentages in the draft PA 
(reviewed by the CASAC) were specific to the 90 ecoregion-time 
period combinations for the 18 eastern ecoregions (draft PA, Table 
5-4; PA, Table 5-5). Inclusion of the 7 western ecoregions would 
yield higher percentages, as more than 90% of waterbodies in those 
ecoregions were estimated to achieve all three ANC concentration in 
all time periods (PA, Table 5-4).
    \136\ The results for median S deposition at or below 7 kg/ha-yr 
further indicate that 90% of waterbodies per ecoregion achieve ANC 
at or above 20, 30 and 50 [mu]eq/L in 96%, 92% and 82%, 
respectively, of eastern ecoregion-time period combinations (as 
summarized in section II.A.4.c.).
---------------------------------------------------------------------------

    The Administrator additionally takes note of the PA evaluation of 
the temporal trend of the ecoregion-scale risk estimates across the 
five time periods, in the 20 years analyzed, which shows a decline in 
response to the declining S deposition estimates for those periods. As 
summarized in the PA and the proposal, the vast majority of the decline 
occurred across the first decade of the 20-year period. The S 
deposition estimated to be occurring in the 2010-2012 period included 
ecoregion medians (across CL sites) ranging from 2.3 to 7.3 kg/ha-yr in 
the 18 eastern ecoregions (and lower in the 7 western ecoregions), and 
the highest ecoregion 90th percentile was approximately 8 kg/ha-yr 
(table 5 and figure 2 above). For this pattern of deposition, the REA 
estimated more than 70% of waterbodies in all 25 ecoregions assessed to 
be able to achieve an ANC of 50 ueq/L (figure 1, left panel, above), 
and more than 80% of waterbodies in all ecoregions to be able to 
achieve an ANC of 20 ueq/L (figure 1, right panel). The Administrator 
observes that these estimates of acid buffering capacity achievement 
for the 2010-12 period deposition--achieving the ANC benchmarks in at 
least 70% to 80% (depending on the specific benchmark) of waterbodies 
per ecoregion--are consistent with the objectives identified by the 
CASAC majority (in emphasizing ecoregion ANC achievement estimates of 
70%, 80% and 80% for ANC benchmarks of 50, 30 and 20 [mu]eq/L, 
respectively). Based on these evaluations of the REA estimates in the 
PA and advice from the CASAC majority, the Administrator judges that 
these ecoregion-scale ANC achievement estimates for the three ANC 
benchmarks (70%, 80% and 80% for ANC benchmarks of 50, 30 and 20 
[mu]eq/L, respectively) are reasonable acid buffering capacity 
objectives for the purposes of protecting ecoregions from aquatic 
acidification risk of a magnitude of potential public welfare 
significance. Further, as discussed earlier in this section, the 
Administrator recognizes that the significance of aquatic 
acidification-related impacts on the public welfare, including those 
associated with reductions in public usage of aquatic ecosystems with 
fisheries affected by acidification, increases with greater prevalence 
of affected waterbodies and ecoregions. Thus, he finds the CASAC-
identified percentages of waterbodies per ecoregion that meet (or 
exceed) the three ANC benchmarks to be appropriate minimum percentages 
(for each ANC benchmark) for ecoregions across the U.S. for use in his 
identification of a secondary NAAQS that will provide the appropriate 
level of protection against risks of potential public welfare 
significance. In so doing, he additionally notes that these percentages 
are met (or exceeded) for the most recent time periods analyzed in the 
REA (through 2018-2020).
    In turning to his consideration of S deposition levels that might 
be expected to maintain such a level of protection from aquatic 
acidification risk, the Administrator considers the CASAC majority 
recommended range of annual average secondary SO2 standard 
levels (i.e., 10-15 ppb) that, in the view of these members, would 
generally maintain S deposition at or below 5 kg/ha-yr. As recognized 
in the PA, the CASAC majority reference to S deposition associated with 
their acid buffering objectives was in terms of ecoregion median values 
in the REA ecoregion-scale analysis.\137\ The Administrator 
additionally takes note of the PA observation of an appreciable 
reduction in S deposition across the 20-year analysis period in the 25 
REA ecoregions, both in terms of the 90th percentile across REA sites 
in each ecoregion and in terms of the median such that in the second 
decade of the period (since 2010), the difference in S deposition value 
between the ecoregion median and 90th percentile is much reduced from 
what it was in the 2001-2003 period. Although the ecoregion 90th 
percentile and median estimates for the REA ecoregions ranged up to 
approximately 22 and 17 kg/ha-yr, respectively, in the 2001-2003 
period, both types of estimates fall below approximately 7 to 8 kg/ha-
yr by the 2010-2012 period (figure 2 above). In light of this trend, as 
well as the temporal trend in the REA estimates, and also while 
recognizing the uncertainties associated with the deposition estimates 
at individual waterbody sites and with the associated estimates of 
aquatic acidification risk (PA, section 5.1.4), the Administrator 
concurs with the PA findings that the ecoregion-scale acid buffering 
objectives identified above (more than 70% to 80% of waterbody sites in 
all ecoregions assessed achieving or exceeding the set of ANC 
benchmarks) can be expected to be met when the median and upper (90th) 
percentile deposition estimates for sensitive ecoregions are generally 
at and below about 5 kg/ha-yr with a few occurrences as high as about 8 
kg/ha-yr. Thus, he considers it appropriate to focus on S deposition 
generally at or below about 5 kg/ha-yr, with infrequent occurrences as 
high as about 8 kg/ha-yr. Based on all of these considerations, the 
Administrator judges that a secondary standard that would generally 
maintain a pattern of ecoregion median S deposition consistent with 
these objectives (at or below 5 kg/ha-yr, with only infrequent 
occurrences as high as 8 kg/ha-yr) would provide the appropriate level 
of public welfare protection from aquatic acidification risk.
---------------------------------------------------------------------------

    \137\ While the REA ecoregion-scale analysis summarizes risk 
estimates for each ecoregion in terms of the ecoregion median of the 
sites analyzed in each ecoregion, the PA notes that the sites 
estimated to receive the higher levels of deposition are those most 
influencing the extent to which the potential objectives for aquatic 
acidification protection are or are not met.
---------------------------------------------------------------------------

    In his consideration of deposition levels that might provide for 
protection from aquatic acidification consistent with his identified 
objectives, the Administrator also considers protection of terrestrial 
ecosystems from effects related to S deposition. In so doing, he notes 
that in primarily focusing on the aquatic acidification risk estimates 
in its evaluation of options for a standard to address deposition-
related effects, the PA recognized the linkages between watershed soils 
and waterbody acidification, suggesting that such linkages indicate 
that protecting waterbodies from reduced acid buffering capacity (with 
ANC as the indicator) will also, necessarily, provide protection for 
watershed soils (PA, section 7.4).\138\ The Administrator also notes 
that a revised standard that would be associated with lower S 
deposition in sensitive ecoregions than the existing standard 
(consistent with his decision reached above) would necessarily be 
associated with lower S deposition in both terrestrial and aquatic 
ecosystems.

[[Page 105766]]

He also notes the PA evaluation of the current evidence, particularly 
with regard to terrestrial plants, including the PA's identification of 
S deposition levels extending from 5 kg/ha-yr (up to 12 kg/ha-yr), as 
summarized in section II.A.3.c.(2) above.\139\ He further recognizes 
that this range includes the benchmark referenced by the CASAC majority 
(generally at or below 5 kg/ha-yr) as affording protection to various 
tree and lichen species (as summarized in section II.B.1.b. above). In 
so doing, he recognizes the overlap of these values with his objectives 
identified above (S deposition generally at or below about 5 kg/ha-hr, 
with infrequent higher occurrences). Thus, based on the PA, and in 
consideration of CASAC advice and public comments, the Administrator 
judges that his focus on aquatic acidification risk and on a pattern of 
ecoregion median S deposition consistent with his objectives identified 
above will also provide protection for terrestrial ecosystems, such 
that a different standard is not needed to provide protection for 
terrestrial effects.
---------------------------------------------------------------------------

    \138\ The PA additionally considered the terrestrial 
acidification risk analyses in the last review which found that 
total deposition estimates in recent years appear to meet all but 
the most restrictive of acid deposition target values, with which 
the PA observed uncertainties to be the greatest (PA, section 
5.3.2.1).
    \139\ This range of S deposition levels reflects the PA analysis 
of studies of effects on terrestrial biota (PA, section 5.3.4 and 
Appendix 5B). For example, from the most recent observational study 
evaluated in the ISA and PA, for the non-western tree species that 
were reported to have a negative association of growth or survival 
with S deposition, this encompasses the species-specific median 
deposition estimates for the sites where these species were assessed 
(PA, section 5.3.4.1 and Appendix 5B, sections 5B.2.2.3 and 5B.2.3).
---------------------------------------------------------------------------

    The Administrator next turns to identification of a secondary 
standard that can be expected to generally maintain a pattern of 
ecoregion median S deposition at or below 5 kg/ha-yr, with potentially 
very few occurrences up to about 8 kg/ha-yr. In so doing, he recognizes 
the complexity of identifying a national ambient air quality standard 
focused on protection of the public welfare from adverse effects 
associated with national patterns of atmospheric deposition, 
particularly given the degree to which those patterns are influenced by 
transport and chemical transformation of emissions. As more 
specifically described in the PA, atmospheric deposition (ecosystem 
loading) of S is, in a simple sense, the product of atmospheric 
concentrations of S compounds, factors affecting S transfer from air to 
surfaces, and time. Further, atmospheric concentrations in an ecosystem 
are, themselves, the result of emissions from multiple, distributed 
sources both near and far, atmospheric chemistry, and transport. 
Accordingly, the Administrator concurs with the PA that consideration 
of the location of source emissions and expected pollutant transport, 
in addition to the influence of physical and chemical processes, is 
important to understanding relationships between SO2 
concentrations at ambient air monitors and S deposition rates in 
sensitive ecosystems of interest.
    Based on these considerations, the Administrator concurs with the 
PA conclusion that to achieve the requisite level of protection from 
aquatic acidification effects associated with S deposition in sensitive 
ecosystems, SO2 emissions must be controlled at their 
sources. Accordingly, the Administrator considers findings of the PA 
analyses of relationships between S deposition estimates and 
SO2 concentrations near SO2 monitors, including 
at NAAQS regulatory monitors, which are often near large sources of 
SO2 emissions. To account for the relationship between 
upwind concentrations near sources and deposition in downwind areas, 
the Administrator also considers PA analyses of relationships between 
ecoregion S deposition estimates and SO2 concentrations at 
upwind sites of influence, identified by trajectory analyses (sections 
II.A.2. and II.B.1.a.(3) above, and PA, sections 6.2.2 through 6.2.4). 
As evidence of the influence of SO2 in ambient air on S 
deposition, all of these analyses demonstrated a positive association 
between SO2 concentrations and nearby or downwind S 
deposition (PA, section 7.4).
    With regard to an indicator for a standard to address the effects 
of S deposition associated with SOX in ambient air, the 
Administrator finds his proposed decision for an SO2 
indicator to be appropriate. He reaches this decision based on 
consideration of the PA evaluations of the linkages connecting 
SOX emissions and S deposition-related effects, including 
the parallel trends of SO2 emissions and S deposition in the 
U.S. over the past 20 years that indicate the strong influence of 
SO2 in ambient air on S deposition (PA, sections 6.4.1 and 
7.4) and the PA finding of SO2 as a good indicator for a 
secondary standard to address S deposition (PA, sections 6.4.1 and 
7.4). Specific aspects of the PA findings include the declining trend 
of S deposition that is consistent with and parallel to the sharp 
declines in annual average SO2 emissions across the 20-year 
period, as well as the general association of higher annual average 
SO2 concentrations (averaged over three years) at SLAMS with 
higher local S deposition estimates, in addition to the statistically 
significant positive correlations observed for ecoregion median S 
deposition with SO2 concentrations at upwind monitoring 
sites of influence in the EAQM analyses. In reaching this decision, the 
Administrator also notes the CASAC consensus advice and public comments 
that recommended a standard with SO2 as an indicator to 
address ecosystem effects of sulfur deposition.
    The Administrator has also considered PM2.5 with regard 
to its potential to be an effective indicator for a standard providing 
public welfare protection from S deposition-related effects. In so 
doing, he recognizes that the S species that deposits in ecosystems, 
SO42-, is a component of PM2.5. 
However, he also recognizes that SO42- 
constitutes less than half of PM2.5, by mass, across the 
country, with non-S containing compounds most typically comprising more 
than 70% of the total annual PM2.5 mass in the East and even 
more in the West (PA, section 2.4.3). He finds that this generally low 
presence of SO42- in PM2.5 and the 
extent to which it varies across the country inhibit his ability to 
identify a PM2.5 standard level that might be expected to 
provide the desired level of protection from S deposition related 
effects, an inhibition that does not exist in his use of the 
SO2 standard for this purpose. In addition, he takes note of 
the discussion above in support of his decision regarding a revised 
secondary SO2 standard, including the atmospheric chemistry 
information which indicates the dependency of S deposition on airborne 
SOX, as evidenced by the parallel trends of SO2 
emissions and S deposition. Based on all of these considerations, the 
Administrator judges that protection of sensitive ecosystems from S 
deposition-related effects is more effectively achieved through a 
revised SO2 standard than a standard for PM. Thus, the 
Administrator judges SO2 to be the appropriate indicator for 
a standard addressing S deposition-related effects.
    With regard to averaging time and form, the Administrator continues 
to find his proposed decision (for an averaging time of a year and a 
form that averages the annual values across three consecutive years) to 
be appropriate, based on consideration of the PA findings and related 
analyses, advice from the CASAC majority, and public comments. Among 
the public commenters that supported adoption of a standard to address 
deposition-related effects, none objected to the conclusion of the PA 
that an annual standard would be appropriate for this purpose, although 
some commenters did support a secondary standard with the same 
averaging time, form and level of the primary standard, apparently for 
implementation reasons (discussed in

[[Page 105767]]

section II.B.2.a.(3)(c) above).\140\ In the quantitative analyses of 
air quality and deposition, the PA generally focused on a year's 
averaging time based on the recognition that longer-term averages (such 
as over a year, compared to one or a few hours) most appropriately 
relate to deposition and associated ecosystem effects. The PA analyses 
also used a 3-year form based on a recognition in the NAAQS program 
that such a form affords stability to the associated air quality 
management programs that contributes to effective environmental 
protection. Similarly, in the advice of the CASAC majority on a 
standard addressing S deposition, these members recommended an annual 
average standard, and, while these members did not explicitly address 
form, the information cited in the justification for their 
recommendation focused on a 3-year form (section II.B.1.b. above). In 
consideration of these conclusions of the PA and the CASAC majority, 
and public comments (as discussed in section II.B.2.a. above), the 
Administrator judges an averaging time and form in terms of annual 
average SO2 concentrations, averaged over three years,\141\ 
to be appropriate for a secondary standard providing public welfare 
protection from adverse effects associated with long-term atmospheric 
deposition of S compounds.
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    \140\ As discussed further below, the EPA is not adopting such a 
standard identical to the existing primary standard because such as 
standard would be more stringent than necessary.
    \141\ A 3-year form is common to NAAQS adopted over the more 
recent past. This form provides a desired stability to the air 
quality management programs which is considered to contribute to 
improved public health and welfare protection (e.g., 78 FR 3198, 
January 15, 2013; 80 FR 65352, October 26, 2015; 85 FR 87267, 
December 31, 2020).
---------------------------------------------------------------------------

    In turning to consideration of a level for such a standard, as an 
initial matter, the Administrator again notes the complexity associated 
with identifying a national ambient air quality standard focused on 
protection from national patterns of atmospheric deposition, and the 
associated uncertainty, as described in section II.E.3. of the 
proposal. Particularly in this case of identifying a standard to 
provide a pattern of ambient air concentrations that as a whole 
contributes to deposition across the U.S., it is important to consider 
the distribution of air concentrations to which the standard will 
apply. The Administrator considers the evaluations and associated 
findings of the PA, as well as findings of the related additional 
analyses, advice from the CASAC, and public comments on the proposed 
decision for a level within the range of 10 to 15 ppb.
    With regard to the advice from the CASAC, the Administrator notes 
that, as described in section II.B.1.b. above, the majority of the 
CASAC recommended adoption of an annual SO2 standard with a 
level within the range of 10 to 15 ppb. These members indicated their 
view that this range of levels ``generally maintains'' S deposition at 
or below 5 kg/ha-yr (based on their consideration of the draft 
PA).\142\ The CASAC majority further conveyed that a standard level in 
this range would afford protection to tree and lichen species, as well 
as achieve the acid buffering targets in waterbodies of sensitive 
ecoregions (described above), and further stated that such a standard 
would ``preclude the possibility of returning to deleterious deposition 
values'' (Sheppard, Response to Charge Questions, pp. 24-25).
---------------------------------------------------------------------------

    \142\ As noted in section II.B.1.b. above, the PA analyses the 
CASAC majority cited were in terms of ecoregion median S deposition 
at/below values. Accordingly, the PA and the Administrator, in his 
judgments here, focuses on consideration of S deposition values in 
terms of such ecoregion medians.
---------------------------------------------------------------------------

    The Administrator also takes note of the air quality and deposition 
analyses described in the PA and summarized in sections II.A.2. and 
II.B.1.a.(3) above. In so doing, the Administrator focused particularly 
on the results of the PA's trajectory-based analyses for the EAQM-max 
metric, including the related additional analyses developed in 
consideration of public comments (Sales et al., 2024). He notes that 
these results indicate that when the maximum upwind annual 
SO2 concentration (3-year average) was no higher than 10 
ppb, median deposition in the downwind ecoregion was below 5 kg/ha-yr 
in more than 90% of the ecoregion-time period combinations in the 
analysis and below about 6 kg/ha-yr in at least 95% of combinations, 
with deposition in the remaining few combinations no higher than about 
8 kg/ha-yr. Further, he notes the analysis finding that in every 
instance of upwind maximum annual SO2 concentrations 
(averaged over three years) above 10 ppb, the associated estimates of 
downwind ecoregion median S deposition are all above 5 kg/ha-yr, 
extending from about 6 kg/ha-yr to as high as approximately 18 kg/ha-yr 
with 75% of the occurrences above 9 kg/ha-yr (Sales et al., 2024). He 
judges this magnitude of ecoregion S deposition associated with 
standard levels above 10 ppb to be well above his objectives. Thus, he 
finds that a standard level greater than 10 ppb would provide 
insufficient control of S deposition and related effects and 
accordingly would not provide the requisite public welfare protection. 
With regard to a level of 10 ppb, however, the Administrator finds 
these analyses to indicate that such a level is associated with a 
pattern of ecoregion median deposition consistent with his previously 
identified objectives of ecoregion median deposition generally below 
about 5 kg/ha-yr, with few occurrences of higher levels up to or below 
about 8 kg/ha-yr. The Administrator additionally finds a level of 10 
ppb and the ecoregion median estimates of associated S deposition to be 
in general agreement with the advice from the CASAC majority including 
their recommended range of 10-15 ppb for an annual standard level, and 
their characterization of ``generally'' maintaining S deposition at or 
below 5 kg/ha-yr.
    Before reaching his decision on a standard that in his judgment 
would provide the requisite protection from deposition-related effects, 
the Administrator also considered the protection that might be afforded 
by an annual SO2 standard, averaged over three years, with a 
level below 10 ppb. In so doing, he focused on consideration of the 
level of 5 ppb that was raised in public comment, as discussed in 
section II.B.2.a.(2) above, considering the findings of the additional 
analyses of the PA trajectory-based dataset that summarize the 
ecoregion median S deposition associated with maximum annual average 
concentrations, averaged over three years, no higher than 5 ppb at 
upwind sites of influence (Sales et al., 2024). The Administrator notes 
that for a maximum upwind annual average concentration no higher than 5 
ppb, the trajectory-based analyses indicate downwind ecoregions to have 
ecoregion median S deposition appreciably below his objectives, which 
as noted above are for such deposition generally at or below 5 kg/ha-
yr, with infrequent higher occurrences, very rarely as high as about 8 
kg/ha-yr. Specifically, the analyses indicate ecoregion median 
deposition below approximately 4.5 kg/ha-yr in all of the ecoregion-
time period combinations, with 75% below approximately 2.5 kg/ha-yr. 
The Administrator judges this magnitude of ecoregion S deposition 
associated with a standard level of 5 ppb to be well below his 
identified objectives. Thus, in light of his judgments, described 
above, regarding the pattern of ecoregion deposition associated with 
his and the CASAC majority's acidification protection targets, the 
Administrator finds an annual SO2 standard, averaged over 
three years, with a level below 10 ppb, to be associated with air 
quality

[[Page 105768]]

more stringent than necessary to provide the requisite protection of 
the public welfare under the Act.
    Further, in consideration of public comments and the recommendation 
from the CASAC minority, the Administrator additionally considered the 
public welfare protection that might be afforded by an alternate 
secondary standard in terms of a standard identical to the existing 
primary standard in all respects. In so doing, he notes the PA 
observations that most of the ecoregion median S deposition estimates 
for the last 10 years are less than 5 kg/ha-yr, and he notes the views 
expressed by the CASAC minority and in public comments that this 
indicates that the existing 1-hour primary SO2 standard 
adequately protects against long-term annual S deposition-related 
effects. He additionally notes the additional analyses related to the 
PA trajectory-based analyses that indicate the stringency, with regard 
to expected control of associated S deposition, associated with a 1-
hour standard identical to the primary standard (Sales et al., 2024, 
section 4.2). As discussed in II.B.2.a.(3)(c) above, such a standard is 
associated with ecoregion median S deposition well below the 
Administrator's objectives (summarized above). Specifically, the 
trajectory analyses indicate that for upwind sites of influence at or 
below 75 ppb, in terms of the existing primary standard (3-year average 
of 99th percentile daily maximum 1-hour average concentrations), the 
downwind ecoregion median S deposition estimates for all ecoregion-time 
period combinations are below 3 kg/ha-yr, with 95% of them below 2 kg/
ha-yr. Thus, he judges such a standard would be more stringent than 
necessary and accordingly not provide the requisite protection of the 
public welfare.
    In light of all of the above, along with analyses and evaluations 
in the PA, including judgments related to uncertainties in relating 
ambient air concentrations to deposition estimates for the purpose of 
identifying a standard level associated with a desired level of 
ecological protection, advice from the CASAC majority, and 
consideration of public comment, the Administrator judges that a 
SO2 standard in terms of an annual average, averaged over 
three years, with a level of 10 ppb would provide the requisite 
protection of the public welfare from adverse effects related to S 
deposition.
    The Administrator also considered the extent to which a new annual 
average standard might be expected to control short-term SO2 
concentrations (e.g., of three hours duration) and accordingly also 
provide the necessary protection from direct effects of SOX 
that is currently provided by the existing 3-hour secondary standard. 
In this context, he notes the analyses and conclusions of the PA, and 
particularly the related additional analyses, with regard to the extent 
of control for short-term concentrations that might be expected to be 
provided by an annual secondary standard (Sales et al., 2024). The 
Administrator also notes that these analyses are of air quality data 
from across the U.S. collected over the past 20 years, thus capturing a 
broad array of air quality conditions and their influences on 
relationships between the short-term and annual air quality metrics. As 
also discussed in section II.B.2.a.(4) above, these analyses indicate 
that in areas and periods when the annual SO2 concentration 
(annual average, averaged over three years) is at or below 10 ppb, 
design values for the existing 3-hour standard are well below the 
existing secondary standard level of 0.5 ppm SO2 and short-
term SO2 concentrations are below those associated with 
direct effects on vegetation or lichens (PA, Figure 2-29; Sales et al., 
2024). Based on these findings, the Administrator judges that revision 
of the existing standard to a new annual standard, with a 3-year 
average form and a level of 10 ppb, will provide the necessary 
protection for direct effects of SOX on plants and lichens, 
as well as effects associated with longer-term deposition of S 
compounds in ecosystems. Thus, based on all of the considerations 
identified above, including the currently available evidence in the 
ISA, the quantitative and policy evaluations in the PA, related 
analyses, the advice from the CASAC, and public comment, the 
Administrator judges it appropriate to revise the existing secondary 
SO2 standard, to be an annual average standard, with a 3-
year average form and a level of 10 ppb in order to provide the 
requisite protection of the public welfare from known or anticipated 
adverse effects.
    Having reached his decision with regard to the welfare effects of 
SOX, including those related to deposition of S compounds in 
sensitive ecosystems, the Administrator now turns to consideration of 
the secondary standards for N oxides and PM. As described below, the 
Administrator has decided to retain the existing NO2 and PM 
standards. These decisions are based on his consideration of the 
welfare effects evidence as characterized in the ISA and evaluated in 
the PA; the public welfare implications of these effects; the 
quantitative information concerning N oxides, PM and N deposition 
presented in the ISA and PA, and additional analyses developed in 
consideration of public comments (e.g., Sales et al., 2024); the 
majority and minority advice from the CASAC; and public comments (as 
discussed in section II.B.2.b. above and in the Response to Comments 
document).
    With regard to the secondary standard for N oxides, the 
Administrator turns first to consideration of the protection afforded 
for effects of N oxides associated with direct contact on surfaces of 
plants and lichens. In so doing, he notes that the evidence of such 
effects was the basis for the establishment of the existing standard in 
1971, and that the currently available information, summarized in 
section II.A.3.a.(1) above, continues to document such effects (ISA, 
Appendix 3, sections 3.3 and 3.4; PA, sections 4.1, 5.4.2 and 7.4). 
With regard to the adequacy of the existing standard in protecting 
against such effects, the Administrator's conclusions reflect those in 
the proposal, which he notes are consistent with the unanimous view of 
the CASAC (summarized in section II.B.1.b. above). Specifically, he 
finds that the evidence for NO2 and NO does not indicate 
effects associated with ambient air concentrations allowed by the 
existing standard. With regard to the N oxide, HNO3, he 
considered the PA evaluation of the evidence of effects associated with 
air concentrations and associated HNO3 dry deposition on 
plant and lichen surfaces, and uncertainty as to the extent to which 
exposures associated with such effects may be allowed by the existing 
secondary NO2 standard (PA sections 7.1.2 and 5.4.2, and 
Appendix 5B, section 5B.4). In so doing, the Administrator judges that 
the limited evidence, with associated uncertainties, are insufficient 
to conclude that air quality that meets the secondary NO2 
standard will nevertheless elicit such effects. Thus, he concludes that 
the existing standard continues to provide the needed protection from 
the direct effects of N oxides.
    The Administrator next turns to consideration of the welfare 
effects related to atmospheric N deposition and the contribution of N 
oxides to such effects. In so doing, he notes that the information for 
N deposition and N oxides includes substantially more significant 
complexities, limitations of the available information, and related 
uncertainties than is the case for S deposition and S oxides. These 
complexities and limitations are generally technical or science policy 
in

[[Page 105769]]

nature, or both. Those of a technical nature include the untangling of 
historic N deposition impacts (e.g., in terrestrial ecosystems) from 
impacts that might be expected from specific annual deposition rates 
absent that history, and also the complexity--more prominent for many 
aquatic systems, including those receiving some of the highest N 
loading--associated with estimating the portion of N inputs, and 
associated contribution to effects, derived from atmospheric sources 
(and specifically sources of N oxides). The science policy-related 
complexities relate to judgments regarding the implications of N 
deposition-related biological or ecological effects in the context of 
the Administrator's judgments concerning protection of the public 
welfare from adverse effects. Lastly, both technical and science policy 
challenges are presented by the coincidence in this review of the 
substantially reduced influence of N oxides on N deposition and the 
emergence of NH3, which is not a criteria pollutant, as a 
major N deposition influence, particularly in areas with some of the 
highest N deposition estimates.
    With regard to science policy judgments, the Administrator 
recognizes particular complexity associated with judging the requisite 
public welfare protection for an ecosystem stressor like N enrichment, 
for which as the CASAC recognized, in terrestrial systems there are 
both ``benefits and disbenefits'' (Sheppard, 2023, p. 8). As noted by 
the CASAC, ``[b]enefits include fertilization of crops and trees and 
the potential for improved sequestration of carbon in soils and plant 
biomass'' (Sheppard, 2023, p. 8). As noted in the PA, this also 
complicates conclusions regarding the extent to which some ecological 
effects may be judged adverse to the public welfare (PA, section 7.4). 
In many aquatic systems, identification of appropriate public welfare 
protection objectives is further complicated by N contributions to 
these systems from multiple sources other than atmospheric 
deposition,\143\ as well as by the effects of historical deposition 
that have influenced the current status of soils, surface waters, 
associated biota, and ecosystem structure and function. For example, 
changes to ecosystems that have resulted from past, appreciably higher 
levels of atmospheric deposition in those areas have the potential to 
affect how the ecosystem responds to current, lower levels of 
deposition or to different N inputs in the future.
---------------------------------------------------------------------------

    \143\ For example, a study of the Chesapeake Bay and its sources 
of N loading concluded that `` `about one-third' of the total N load 
for the Bay is the result of direct deposition to the Bay or 
deposition to the watershed which is transported to the Bay'' (U.S. 
EPA, 2010, p. 4-33), indicating that two thirds of N loading comes 
from non-air sources.
---------------------------------------------------------------------------

    Further, the Administrator notes that his decision under the Act 
regarding the secondary NAAQS for N oxides is necessarily based on his 
judgments related to protection from the effects associated with N 
oxides. Yet, he recognizes that there are contributions to ecosystem N 
deposition, and related effects, from pollutants other than--and not 
derived from--N oxides in ambient air, most prominently NH3. 
He additionally notes that the influence of NH3 on N 
deposition varies appreciably across the U.S. and has grown over the 
past 20 years, while the contribution of N oxides to N deposition has 
declined. In a related manner, he takes note of the findings of the PA 
and the additional analyses that indicate ecoregions and States with 
highest N deposition (e.g., above 10 kg/ha-yr) include areas with some 
of the highest deposition rates for reduced N and NH3 (PA, 
Figure 7-8; Sales et al., 2024). This associated lessening influence of 
N oxides on total N deposition is also evidenced by the generally poor 
(r<0.4) or negative correlations between N deposition and annual 
average NO2 concentrations, in the SLAMS and full 
trajectory-based datasets, respectively,, and also in the most recent 
period analyzed, 2018-2020(PA, sections 6.2.3 and 6.2.4). While low-
moderate positive correlations are observed in both sets of analysis 
for eastern sites when including all time periods, correlations are 
only statistically significant in the earlier periods, prior to 2014, 
which may be related to increasing emissions of NH3 in more 
recent years (PA, section 2.2.3 and Figure 6-5).
    More specifically, the analyses of N deposition over the years 
since 2002 period \144\ document the reductions in N deposition that 
correspond to reductions in emissions of N oxides, while additionally 
documenting the increased role of NH3 in N deposition and 
the co-occurring and associated tempering of total N deposition 
reductions nationwide. For example, in all 14 ecoregions with median 
total N deposition in 2019-2021 greater than 10 kg/ha-yr, deposition of 
NH3 has increased since 2000 (Sales et al., 2024).\145\ And, 
in five of these 14 ecoregions, the increases in NH3 
deposition and associated NH4\+\ deposition are greater than 
the reductions in oxidized N deposition such that overall N deposition, 
in terms of ecoregion median, has increased. In the 14 ecoregions with 
total N deposition greater than 10 kg/ha-yr, the N deposition arising 
directly from N oxides (oxidized N deposition) constitutes the minority 
(approximately 23 to 42%) of total N deposition (Sales et al., 2024, 
Table 3). Across the other 70 ecoregions in CONUS \146\ with median 
total N deposition below 10 kg/ha-yr in 2019-2021, ecoregion median 
oxidized N deposition, on average, declined (from 4.7 to 2.4 kg N/ha-
yr) while ecoregion median NH3 deposition, on average, more 
than doubled (from 0.7 to 1.6 kg N/ha-yr) (Sales et al., 2024, Table 
4). At a State-level scale, average rates of oxidized N deposition have 
also declined in all 48 States of the CONUS, including where total N 
deposition has increased as a result of increased deposition from 
reduced N compounds associated with NH3. In the most recent 
period, oxidized N deposition, in terms of Statewide average, is below 
5 kg N/ha-yr in all 48 States (Sales et al., 2024). And in the six 
States with average total N deposition above 10 kg/ha-yr in the 2019-
2021 period, oxidized N deposition comprises less than 40% (Sales et 
al., 2024, Table 5). The Administrator recognizes that these findings 
augment those of the PA analyses and indicate a much lower influence of 
N oxides on total N deposition relative to the influence of reduced N 
compounds in areas of the U.S. where N deposition is currently the 
highest (PA, section 7.2.3.3).
---------------------------------------------------------------------------

    \144\ Modeling estimates of N deposition in 2002 were the basis 
for the risk analyses in the 2013 review (2009 REA, sections 3.2.3 
and 3.3.3). After also considering estimates and wet deposition 
measurements for 2003-2005, the 2009 REA concluded ``overall, for 
each case study area, the amount of nitrogen deposition in 2002 is 
generally representative of current conditions'' (2009 REA, p. 3-
30). The total deposition estimates at that time relied on a 
different and less advanced modeling approach than that used in the 
current review (PA, section 2.5).
    \145\ Ecoregion median NH3 deposition has also 
increased since 2002 in 68 of the other 70 CONUS ecoregions; in the 
remaining two ecoregions, it is unchanged (Sales et al., 2024).
    \146\ The TDep estimates of N deposition are only available for 
the CONUS and not for parts of the U.S. outside of the CONUS.
---------------------------------------------------------------------------

    The Administrator also considers both the majority and minority 
advice from the CASAC regarding an NO2 annual standard in 
consideration of total N deposition effects. In so doing, he notes that 
in considering the justification provided by the CASAC majority for its 
recommendation, the PA did not find the information highlighted by the 
CASAC for relating total N deposition levels to ambient air 
concentrations of NO2 to provide scientific support for 
their recommended revision. The Administrator additionally notes that, 
as summarized in section II.B.1.b. above,

[[Page 105770]]

notwithstanding the CASAC majority recognition of a lack of correlation 
between NO2 concentrations and ecoregion total N deposition, 
these members recommend an annual NO2 standard with a level 
of ``<10-20 ppb'' based on their objective of N deposition below 10 kg/
ha-yr based on studies of total N deposition. He finds their 
recommendation less than persuasive because for an NO2 
standard to exert control of N deposition, there would need to be a 
significant positive relationship (e.g., correlation) between 
NO2 concentrations and N deposition. As discussed above, the 
correlations reported in the PA between NO2 concentrations 
and downwind ecoregions are generally low or negative, particularly in 
recent periods. Further, the justification provided by the CASAC 
majority for its recommended revision focuses on the results of the 
trajectory-based analysis in the draft PA, about which they also 
expressed concerns, with a focus on the EAQM-weighted metric, although, 
as described in section II.B.1.a.(2), concentrations of this metric are 
not directly translatable to potential standard levels due to the 
weighting across multiple monitors. In light of these limitations in 
the CASAC majority advice and based on current air quality and 
deposition information and trends as summarized above, the 
Administrator judges that, a secondary standard for N oxides cannot be 
expected to effectively control total N deposition.
    With regard to the minority CASAC recommendation to revise the 
secondary standard to be identical to the primary NO2 
standard in all respects, the Administrator notes the justification 
provided by the minority CASAC, which observed that the primary 
standard has been met over the last 10 years and indicated that ``most 
of the N deposition values within the last 10 years'' are less than 10 
kg/ha-yr. The Administrator does not find this rationale sufficient to 
support a decision for revision as the CASAC minority recommended. The 
fact that N deposition has declined in many locations to less than 10 
kg/ha-yr and that all areas meet the current primary standard does not 
signify that a secondary standard set equal to the primary would be 
effective in controlling total N deposition, given the rise in reduced 
N deposition just discussed, or that such a standard would be requisite 
for protection of the public welfare.
    In this context, the Administrator considers the implications of N 
deposition directly related to N oxides with regard to welfare effects. 
In so doing, he notes that the information available at the time of 
proposal (presented in the PA) was unclear with regard to the extent to 
which occurrences of ecoregion median N deposition greater than the 
total N deposition values identified by the CASAC majority (10 kg/ha-
yr) and in section 7.2.3. of the PA (7-12 kg/ha-yr) may relate to the 
existing NO2 secondary standard (89 FR 26682, April 15, 
2024). However, the more recent additional analyses (developed in 
consideration of public comments) now provide clarification. These 
additional analyses indicate that ecoregion median levels of oxidized N 
(the component of total N deposition directly related to N oxides) are 
well below the PA-identified range of values (Sales et al., 2024). 
Specifically, median oxidized N deposition in all ecoregions of the 
CONUS is below 5 kg N/ha-yr, less than half of the N deposition 
benchmark considered by the CASAC (and below the lower end of the N 
deposition range [7-12 kg/ha-yr] identified by the PA), with the 
majority of ecoregions (45 of 84) having a median below 3 kg N/ha-yr 
(Sales et al., 2024). These analyses further indicate that the 
Statewide averages of oxidized N deposition in all 50 States are below 
the CASAC identified N deposition benchmark and the PA identified 
range, with the average across States well below half these values 
(Sales et al., 2024, Table 5).
    In light of all of the considerations above, the Administrator 
notes first that the N deposition benchmark identified by the CASAC 
majority, and the range of levels identified in the PA for 
consideration, are in terms of total N deposition. He notes that most 
ecoregions have total N deposition levels below the CASAC majority and 
PA identified levels (that might be considered appropriate levels of 
protection for effects associated with total N deposition) but that 
some areas have higher total N deposition with levels above such 
benchmarks of potential public welfare significance. He notes that in 
areas with total N deposition above the CASAC majority and PA 
identified levels, available evidence indicates the level of total N 
deposition is predominantly the result of deposition from reduced N, 
which is increasing, while deposition of oxides of N is playing a 
notably smaller role (with such contributions decreasing over recent 
years). Based on these patterns and the current analyses, he notes his 
conclusion above, that, based on the information available in this 
review, a secondary standard for N oxides cannot be expected to 
effectively control total N deposition. Further, he notes that recent 
levels of oxidized N deposition (N deposition derived from N oxides in 
ambient air) are well below the CASAC majority and PA identified 
levels. With respect to the adequacy of protection for effects related 
to oxidized N deposition, he does not find a basis in the evidence for 
concluding that revisions to the current ambient air standard for N 
oxides are necessary. Therefore, based on all the considerations above, 
including the minority contribution of N oxides to total N deposition 
and the general lack of correlation between ambient air NO2 
concentrations and such deposition, the Administrator finds that the 
existing evidence does not call into question the adequacy of 
protection of the existing secondary NO2 standard with 
regard to deposition-related effects of N oxides. Further, based on the 
findings of the PA and additional analyses of recent information on air 
quality and N deposition, and all the above considerations, the 
Administrator judges, based on the available evidence in this review, 
that revision to the secondary annual NO2 standard is not 
warranted and the existing secondary NO2 standard should be 
retained, without revision.
    Lastly, the Administrator turns to consideration of the existing 
secondary standards for PM. As an initial matter, he takes note of the 
PA discussion and conclusion that the available information does not 
call into question the adequacy of protection afforded by the secondary 
PM2.5 standards from direct effects and deposition of 
pollutants other than S and N compounds (PA, sections 7.1.3 and 7.4). 
As also discussed in the proposal, the evidence characterized in the 
ISA and summarized in the PA indicates such effects to be associated 
with conditions associated with concentrations much higher than those 
associated with the existing standards. Thus, as in the proposal, the 
Administrator judges that the current evidence does not call into 
question the adequacy of the existing PM standards with regard to 
direct effects and deposition of pollutants other than S and N 
compounds.
    With regard to S deposition and PM, as noted earlier in this 
section, the Administrator judges that protection of sensitive 
ecosystems from S deposition-related effects is more effectively 
achieved through a revised SO2 standard than a standard for 
PM. Accordingly, as discussed above, the Administrator has decided to 
revise the existing secondary SO2 standard to provide for 
such protection. Thus, the Administrator judges that revising one or 
more of the secondary PM standards

[[Page 105771]]

in consideration of protection of the public welfare from effects 
related to S deposition is not warranted.
    With regard to N deposition and adequacy of the secondary PM 
standards, the Administrator considers the analyses and evaluations in 
the PA, related analyses conducted in consideration of public comments, 
advice from the CASAC, and public comments. As an initial matter, the 
Administrator takes note of the substantial and significant limitations 
and uncertainties associated with the evidence base for ecosystem 
effects related to N deposition associated with PM (similar to those 
recognized above for N oxides). With regard to limitations and 
associated uncertainties of the current information related to N 
deposition arising from PM, the Administrator notes, as an initial 
matter, the PA findings, based on the full 20-year dataset, of negative 
to barely moderate correlations between N deposition estimates and 
annual average PM2.5 concentrations at upwind locations, 
with low or a negative correlation in the most recent time period (PA, 
sections 6.2.4 and 7.2.3.3). Across the SLAMS sites, the strength of a 
N deposition estimates with nearby PM2.5 concentrations is 
also seen to consistently decline across the five time periods analyzed 
since 2001 (PA, Table 6-7).\147\ As discussed in the PA, these findings 
are likely related to both the increased impacts of NH3 on N 
deposition (as summarized earlier), and the declining presence of N 
compounds in PM (specifically in PM2.5) over the past two 
decades, as well as the current relatively low and variable 
representation of N compounds in PM (PA, section 6.4.2).
---------------------------------------------------------------------------

    \147\ Further, as noted in section II.B.2.b.(2)(c) above, the PA 
analysis of N deposition and PM2.5 concentrations at 
SLAMs does not provide a basis for identifying 3-year average annual 
PM2.5 concentrations that might be expected to constrain 
nearby N deposition below certain levels, such as an ecoregion 
median of 10 kg/ha-yr (e.g., PA, Figure 6-39).
---------------------------------------------------------------------------

    While the Administrator recognizes that NH4\+\, a 
transformation product of NH3, exists in particles and is a 
component of PM2.5, he also recognizes that the combined 
presence of all N-containing compounds in PM2.5 constitutes 
less than 30% of total PM2.5 mass at sites across the U.S. 
(PA, section 6.2.4; Sales et al., 2024). The Administrator additionally 
takes note of the finding that the composition of PM2.5 
across the U.S. varies appreciably. Specifically, the percentage of 
PM2.5 represented by N compounds at the 120 CSN sites in the 
2020-2022 period (that inform our current understanding for the various 
regions across the U.S.) ranges from a high of about 30% down to 5 to 
15% across the South and Northwest and just below 5% in some areas (PA, 
section 6.4.2; Sales et al., 2024). As discussed in the PA, this 
contributes to geographic variability in the relationship between N 
deposition and annual PM2.5 concentrations (PA, section 
6.4.2; Sales et al., 2024). The Administrator recognizes these findings 
together to indicate that an appreciable percentage of PM2.5 
mass does not contribute to N deposition, and that the contributing 
amount varies across regions of the U.S. He further recognizes that 
this indicates that PM2.5 concentrations can be controlled 
or reduced without necessarily having any effect on concentrations of 
particulate N compounds. The Administrator also takes note that while 
deposition of the particulate N species associated with NH3 
emissions (i.e., NH4\+\) has increased since 2000-2002, the 
percentage of PM2.5 mass comprised by nitrogen compounds has 
declined, as has the percentage comprised by NH4\+\, alone 
(Sales et al., 2024). In this context, he additionally notes that 
deposition of NH3 (which is not particulate) is estimated to 
be more than a third of total N deposition in some ecoregions and 
States, including those the highest total deposition (Sales et al., 
2024). The Administrator concludes that collectively, this information 
indicates that a PM mass standard is unlikely to achieve a predictable 
or specified amount of control on N deposition across the U.S.
    In considering the advice from the CASAC for revision of the annual 
PM2.5 secondary standard, the Administrator notes that, as 
discussed in the PA, summarized in section II.B.1.b. above and 
recognized in reaching his proposed decision, the specific rationale 
for the range of standard levels recommended by the CASAC majority is 
unclear. The EPA does not find the CASAC majority observations 
regarding PM2.5 concentrations in remote areas or in areas 
of higher concentrations in 2019-2021 or in the trajectory-based 
analyses to demonstrate that an annual PM2.5 standard, with 
a level of 6 to 10 [micro]g/m\3\, would be expected to control total N 
deposition at or below 10 kg/ha-yr. As recognized in the proposal, in 
the CASAC majority comments, PM2.5 concentrations within its 
recommended range were both described as relating to N deposition at/
below its recommended benchmark (10 kg N/ha-yr) and relating to 
deposition above that range (as summarized in II.B.1.c. above). 
Additionally, as discussed in section II.B.2.b.(2)(c) above, the EPA 
disagrees that the PA analyses of PM2.5 concentrations and N 
deposition estimates in remote areas, without consideration of 
information for areas where PM2.5 is emitted or produced, 
are informative in this regard.\148\ Regarding the trajectory-based 
analyses, as discussed in section II.B.1.b. above, and noted above, the 
correlation coefficient for N deposition with PM2.5 
concentrations at the maximum upwind monitor (the EAQM-Max metric) does 
not indicate a positive relationship. In light of these limitations in 
the information cited by the CASAC majority and based on the broader 
consideration above of the variability of PM2.5 composition 
across the U.S., including with regard to N components, among other 
factors, the Administrator disagrees with the CASAC majority's 
recommendation on revision of the annual PM2.5 standard. In 
so doing, he also notes that the recommendation by these members to 
consider a new total N PM2.5 indicator, based on their view 
that it would achieve a better measure of total reactive N deposition, 
was offered in the context of such consideration ``in the next review'' 
(Sheppard, 2023, Letter, p. 5), and notes that the record in this 
review does not provide a basis for considering, much less adopting, a 
new indicator in the current review.
---------------------------------------------------------------------------

    \148\ The CASAC majority reference to concentrations in non-
remote areas was with regard to the range of recent design values 
observed in areas where N deposition estimates ranged above 15 kg/
ha-yr in California, the Midwest and the East; although not noted in 
the justification, design values at California sites were as high as 
17.3 [micro]g/m\3\ (as summarized in section II.B.1.c. above), and 
the justification does not address how this may relate to a 
relationship of these concentrations to N deposition.
---------------------------------------------------------------------------

    The CASAC minority recommendation, based on a conclusion that the 
2013 annual primary PM2.5 standard was controlling N 
deposition as needed since its establishment (as described in section 
II.B.1.b. above), cited scatterplots in the draft PA of N deposition 
estimates and annual average PM2.5 concentrations and did 
not address the issue of variable PM composition or lack of analyses 
for a 1-hour metric. As described earlier, the Administrator finds the 
issue of variability in PM2.5 composition to be an important 
consideration in his decision and accordingly, he finds the minority 
CASAC recommendation to not be well supported by the full record at 
this time in this review.
    Based on the currently available information, taking into account 
its limitations and associated uncertainties, and in consideration of 
all of the above,

[[Page 105772]]

the Administrator concludes that given the variable composition of 
PM2.5 across the U.S., the relatively low percentage of 
PM2.5 represented by N compounds (lower now than in the 
past), and the contributors to total N deposition that are not PM 
components, a PM2.5 standard could not, as discussed above, 
be expected to provide predictable and effective control of total N 
deposition. Accordingly, he judges that PM2.5 is not an 
appropriate indicator for a secondary standard intended to provide 
protection of the public welfare from adverse effects related to N 
deposition. Additionally, he notes that while it is unclear whether any 
PM standard would provide an appropriate indicator for consideration of 
N deposition-related effects, this issue may warrant evaluation in 
future reviews.
    Further, as in his decision for N oxides above, the Administrator 
recognizes the factors identified here to contribute appreciable 
uncertainty to an understanding of the level of protection from N 
deposition-related effects associated with PM that might be afforded by 
the existing or an alternate secondary standard for PM2.5. 
Thus, he is unable to identify a standard that would provide requisite 
protection from known or anticipated adverse N-deposition-related 
effects to the public welfare associated with the presence of PM in the 
ambient air. In summary, based on all these considerations, the 
Administrator concludes after considering the available evidence as 
assessed in the ISA, the quantitative analyses and associated 
evaluations in the PA and related more recent additional analyses, that 
no change to the annual secondary PM2.5 standard is 
warranted and he is retaining the existing PM2.5 secondary 
standard, without revision.
    With regard to the 24-hour PM2.5 standard, the 
Administrator takes note of the PA conclusion that the evidence 
available in this review, as documented in the ISA, does not call into 
question the adequacy of protection provided by the 24-hour 
PM2.5 standard from ecological effects (PA, section 7.4). He 
additionally notes the agreement of this finding with the 
recommendation of the CASAC minority to retain the existing standard. 
The Administrator also considers the comments of the CASAC majority and 
recommendations for revision of this standard to a lower level or to an 
indicator of deciviews (with a level of 20 to-25 deciviews), based on 
the CASAC majority's consideration of visibility impairment and short-
term fog or cloud-related deposition events that these members indicate 
may threaten sensitive lichen species, as summarized in section 
II.B.1.b. above. With regard to short-term fog or cloud-related events, 
the Administrator considers the PA finding in evaluating these 
recommendations, that, while the available evidence in the ISA 
recognizes there to be N deposition associated with cloud water or fog, 
it does not provide estimates of this deposition, describe associated 
temporal variability, or present evidence of effects on biota from such 
events (ISA, Appendix 2; PA, section 7.3).\149\ Thus, he does not find 
a basis in the evidence base for this review for the CASAC majority 
revisions or their stated intention of addressing short-term events and 
lichen sensitivity. Further, the justification of the specific revision 
options recommended by the CASAC majority focuses on consideration of 
visibility impairment, and the Administrator notes that the adequacy of 
protection provided by the secondary PM2.5 standard from 
visibility effects has been addressed in his reconsideration of the 
2020 p.m. NAAQS decision (89 FR 16202, March 6, 2024) and is not 
included in this review. Thus, based on his judgment that the evidence 
does not call the existing standard into question, the Administrator 
retains the existing 24-hour secondary PM2.5 standard, 
without revision.
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    \149\ As noted in the PA and summarized in section II.B.1.b. 
above, the CASAC majority, in its justification for revision of the 
existing standard, did not identify studies in support of its 
statements related to lichen species and fog or cloud water.
---------------------------------------------------------------------------

    Regarding the PM10 standard, the Administrator concurs 
with the PA's finding of a lack of information that calls into question 
the adequacy of protection afforded by the existing PM10 
secondary standard for ecological effects. Thus, he also retains the 
secondary PM10 standard without revision.

C. Decision on the Secondary Standards

    For the reasons discussed above and considering the evidence 
assessed in the ISA, the qualitative assessments and policy evaluations 
presented in the PA and associated technical memorandum, the advice and 
recommendations of the CASAC, and the public comments, the 
Administrator is revising the secondary standard for SOX to 
provide the requisite protection of the public welfare from known and 
anticipated adverse effects. More specifically, the Administrator is 
revising the secondary SO2 standard to be an annual average, 
averaged over three years, with a level of 10 ppb SO2. With 
this decision, the Agency is also making corresponding revisions to 
data handling conventions are specified in revisions to appendix T, 
discussed in section III. below.
    With regard to the secondary standards for N oxides and PM, based 
on the evidence assessed in the ISA, the qualitative assessments and 
policy evaluations presented in the PA and associated technical 
memorandum, the advice and recommendations of the CASAC, and the public 
comments, and for the reasons discussed above, the Administrator 
concludes that no changes are warranted, and is retaining the existing 
standards, without revision.

III. Interpretation of the Secondary SO2 NAAQS

    The EPA received no comments regarding the proposed data handling 
procedures for SO2 monitoring data for purposes of 
determining when the new annual secondary SO2 NAAQS is met. 
Therefore, the EPA is finalizing the proposed revisions to appendix T 
to 40 CFR part 50, Interpretation of the Primary National Ambient Air 
Quality Standards for Oxides of Sulfur, to establish data handling 
procedures for the new annual secondary SO2 standard. The 
regulatory text at 40 CFR 50.21, which sets the averaging period, 
level, indicator, and form of the annual standard, refers to this 
appendix T. The revised appendix T details the computations necessary 
for determining when the annual secondary SO2 NAAQS is met. 
The revised appendix T also addresses data reporting, data completeness 
considerations, and rounding conventions.

A. Background

    The general purpose of a data interpretation appendix is to provide 
the practical details on how to make a comparison between multi-day and 
possibly multi-monitor ambient air concentration data and the level of 
the NAAQS, so that determinations of attainment and nonattainment are 
as objective as possible. Data interpretation guidelines also provide 
criteria for determining whether there are sufficient data to make a 
NAAQS level comparison at all. The regulatory language for the 
secondary SO2 NAAQS adopted in 1971 does not contain 
detailed data interpretation instructions. This situation contrasts 
with the primary NO2, ozone, PM2.5, 
PM10, lead, and primary SO2 NAAQS regulations, 
for which there are detailed data interpretation appendices in 40 CFR 
part 50 addressing issues that can arise in comparing monitoring data 
to the NAAQS. The existing appendix T includes these detailed data 
interpretation requirements for the 1-hour primary SO2 
NAAQS, thus the

[[Page 105773]]

revision provides similar information for the new annual secondary 
SO2 NAAQS. The EPA has used its experience developing and 
applying this data interpretation appendix to develop the revisions to 
the text in appendix T to address the new annual secondary 
SO2 standard.

B. Interpretation of the Secondary SO2 Standard

    The purpose of the data interpretation provisions for the secondary 
SO2 NAAQS is to give effect to the form, level, averaging 
time, and indicator specified in the regulatory text at 40 CFR 50.21, 
anticipating and resolving in advance various future situations that 
could occur. The revised appendix T provides definitions and 
requirements that apply to the annual secondary standard for 
SO2. The requirements clarify how ambient air data are to be 
reported, what ambient air data are to be used for comparisons with the 
SO2 NAAQS, and how to calculate design values for 
comparisons with the SO2 NAAQS. The data already required to 
be reported by ambient air SO2 monitors for use in 
calculating design values for the current 1-hour primary SO2 
NAAQS are also sufficient for use in calculating design values for the 
new annual secondary SO2 NAAQS.
    The revised appendix T specifies that the annual secondary 
SO2 NAAQS is met at an ambient air quality monitoring site 
when the valid annual secondary standard design value is less than or 
equal to 10 ppb. The annual secondary standard design value for an 
ambient air quality monitoring site is described as the mean of the 
annual means for three consecutive years, with the annual mean derived 
as the annual average of daily means, with rounding and data 
completeness specified as described below. The use of a daily mean 
value in deriving the design value is consistent with the existing data 
handling requirements for the current 1-hour primary SO2 
NAAQS.
    Data completeness requirements for the annual secondary standard in 
the revised appendix T follow past EPA practice for other NAAQS 
pollutants by requiring that in general at least 75% of the monitoring 
data that should have resulted from following the planned monitoring 
schedule in a period must be available for the key air quality 
statistic from that period to be considered valid. These data 
completeness requirements are consistent with the current data 
completeness requirements for the 1-hour primary SO2 NAAQS 
in appendix T, and the revised appendix T does not change those 
requirements. For the annual secondary SO2 NAAQS, the key 
air quality statistics are the annual average of daily mean (24-hour 
average, midnight-to-midnight) concentrations in three successive 
years. It is important that daily means are representative of the 24-
hour period and that all seasons of the year are well represented. 
Hence, the 75% requirement is applied at the daily and quarterly 
levels. These completeness requirements, including the calculation of 
the daily mean, are consistent with existing completeness requirements 
for the current 1-hour primary SO2 NAAQS.
    Recognizing that there may be years with incomplete data, the text 
provides that a design value derived from incomplete data will 
nevertheless be considered valid if at least 75 percent of the days in 
each quarter of each of three consecutive years have at least one 
reported hourly value, and the 3-year annual average design value 
calculated according to the procedures specified in the revised 
appendix T is above the level of the secondary annual standard. 
Additionally, following provisions in the revised appendix T, a 
substitution test may be used to demonstrate validity of incomplete 
design values above the level of the standard by substituting a ``low'' 
daily mean value from the same calendar quarter in the 3-year design 
value period. Similarly, another substitution test may be used to 
demonstrate validity of incomplete design values below the level of the 
standard by substituting a ``high'' daily mean value from the same 
calendar quarter in the 3-year design value period. These substitution 
tests are consistent with existing substitution tests for the current 
1-hour primary SO2 NAAQS.
    It should be noted that one possible outcome of applying the 
substitution test is that a year with incomplete data may nevertheless 
be determined to not have a valid design value and thus to be unusable 
in making annual secondary NAAQS compliance determinations for that 3-
year period. However, the intention of the substitution test is to 
reduce the frequency of such occurrences.
    The EPA Administrator has general discretion to use incomplete 
monitoring data to calculate design values that would be treated as 
valid for comparison to the NAAQS despite the incompleteness, either at 
the request of a State or at the Administrator's own initiative. 
Similar provisions exist already for the PM2.5, 
NO2, lead, and 1-hour primary SO2 NAAQS. The EPA 
may consider monitoring site closures/moves, monitoring diligence, and 
nearby concentrations in determining whether to use such data.
    The rounding conventions for the new annual secondary 
SO2 NAAQS are consistent with rounding conventions used for 
the current 1-hour primary SO2 NAAQS. Specifically, hourly 
SO2 measurement data shall be reported to EPA's regulatory 
database in units of ppb, to at most one place after the decimal, with 
additional digits to the right being truncated with no further 
rounding. Daily mean values and the annual mean of those daily values 
are not rounded. Further, the annual secondary standard design value is 
calculated pursuant to the revised appendix T and then rounded to the 
nearest whole number or 1 ppb (decimals 0.5 and greater are rounded up 
to the nearest whole number, and any decimal lower than 0.5 is rounded 
down to the nearest whole number).

IV. Ambient Air Monitoring Network for SO2

    In the NPRM, the EPA did not propose any changes to the minimum 
monitoring requirements as part of the proposal to revise the secondary 
SO2 NAAQS. Based on a review of the network history, current 
network design, reported data, and monitoring objectives (Watkins et 
al., 2024), and in recognition of the network's adaptability and 
flexibility provided in 40 CFR part 58, the Agency proposed and took 
comment on its determination that the current network is adequate to 
provide the data needed to implement the new secondary SO2 
standard. The EPA also concluded that the Agency, along with State, 
local, Tribal, and industry stakeholders, have the authority and 
ability to adjust monitoring efforts and redirect resources needed to 
ensure that the monitoring objectives of the SO2 network 
continue to be met, and thus no changes to minimum monitoring 
requirements are necessary.

A. Public Comments

    The EPA received a few comments related to the ambient air 
monitoring network design prescribed by the minimum monitoring 
requirements in 40 CFR part 58, section 4.4 as it relates to supporting 
the implementation of the new standard. The commenters recognized the 
value and importance of the network, with one stating that they support 
the use of ambient air quality monitoring data in designation 
activities, and that they believe ``the existing monitoring network is 
adequate for making attainment decisions.'' Another commenter expressed 
the view that ``EPA must maintain a ground monitoring network that 
supports science-based decision making in the NAAQS standard setting 
process, as

[[Page 105774]]

well as for compliance with a standard once it is set,'' and concurred 
with a CASAC comment that monitoring networks, including the SLAMS, 
which are required through 40 CFR part 58, are ``essential to provide 
the scientific basis for this review'' (Sheppard, 2023).
    Another commenter recommended that EPA ``[i]ncrease monitoring in 
high-risk areas and ensure strict enforcement of the NAAQS,'' including 
by deploying monitors in areas the commenter calls ``frontline and 
fence-line communities,'' and making the data publicly accessible. With 
regard to this comment, the EPA notes that the current network already 
has a significant subset of sites with monitoring objectives that 
provide for measurements in areas of higher SO2 emissions 
and in locations of expected maximum concentrations. Measurements from 
monitors with those objectives provide the data needed to support the 
new standard. However, the same monitors, sited in locations of 
expected maximum concentrations, can also be in ``frontline and fence-
line communities.'' Further, all monitoring conducted by State, local, 
and Tribal air agencies, as well as data from industry that fulfill the 
requirements of 40 CFR parts 50, 53, and 58, the regulations that set 
out minimum monitoring requirements, and other requirements are 
publicly available through various means. These include but are not 
limited to obtaining the data directly from the air monitoring agencies 
themselves, from EPA's Air Data website, or from EPA's Air Quality 
System (AQS) database.

B. Conclusion on the Monitoring Network

    The EPA stated in the proposal that it believes that the current 
ambient air SO2 monitoring network design, deployment, and 
monitoring objectives are adequate to provide the data needed to 
implement the new secondary SO2 NAAQS. After consideration 
of public comments, and with reliance on EPA's assessment of the 
monitoring network provided as part of the proposal for this review, 
the Agency still asserts that the network is adequate and that no 
network design changes are necessary because EPA, State, local, Tribal, 
and industry stakeholders have the authority and ability to adjust 
monitoring efforts and redirect resources as needed to ensure that the 
monitoring objectives of the SO2 network continue to be met. 
The Administrator has therefore chosen to retain the existing minimum 
monitoring requirements for SO2 without modification, as 
currently prescribed, operated, and maintained in accordance with 40 
CFR parts 50, 53, and 58, as proposed.

V. Clean Air Act Implementation Considerations for the Revised 
Secondary SO2 Standard

    The EPA's revision to the secondary SO2 NAAQS will 
trigger a number of implementation-related activities that were 
described in the proposal. The two most immediate implementation 
impacts following a final new or revised NAAQS are related to 
stationary source permitting and the initial area designations process. 
Permitting implications are discussed in section V.C., and designation 
implications are discussed in section V.A. The Agency is finalizing an 
action retaining the secondary NO2 and PM NAAQS. Retention 
of existing secondary standards does not trigger any new implementation 
actions. Additional implementation information is available in the 
proposal preamble in section V.
    At the outset, promulgation of a new or revised NAAQS triggers a 
process through which States \150\ would make recommendations to the 
Administrator regarding initial area designations. States also would be 
required to make a new SIP submission to establish that they meet the 
necessary structural requirements for such new or revised NAAQS 
pursuant to CAA section 110(a)(1) and (2), also referred to as the 
``infrastructure SIP submission'' (more on this submission below). This 
section provides background information for understanding the 
implementation implications of the secondary SO2 NAAQS 
changes and describes the EPA's intentions for providing guidance 
regarding implementation.
---------------------------------------------------------------------------

    \150\ This and all subsequent references to ``state'' are meant 
to include State, local and Tribal agencies responsible for the 
implementation of a SO2 control program.
---------------------------------------------------------------------------

A. Designation of Areas

    As described in section II.B.3., the EPA is revising the secondary 
SO2 NAAQS to 10 ppb, as an annual average, averaged over 
three consecutive years. After the EPA establishes a new or revised 
NAAQS (primary or secondary), the CAA requires the EPA and States to 
take steps to ensure that the new or revised NAAQS is met. The timeline 
for initial area designations begins with promulgation of the new 
NAAQS, as stated in CAA section 107(d)(1)(A). Initial area designations 
involve identifying areas of the country that either meet or do not 
meet the new or revised NAAQS, along with the nearby areas contributing 
to NAAQS violations. The following includes additional information 
regarding the designations process described in the CAA.
    Section 107(d)(1)(A) of the CAA states that, ``By such date as the 
Administrator may reasonably require, but not later than 1 year after 
promulgation of a new or revised [NAAQS] for any pollutant under 
[section 109], the Governor of each State shall . . . submit to the 
Administrator a list of all areas (or portions thereof) in the State'' 
and make recommendations for whether the EPA should designate those 
areas as ``nonattainment,'' ``attainment,'' or ``unclassifiable.'' 
\151\ A nonattainment area is any area that does not meet (or that 
contributes to ambient air quality in a nearby area that does not meet) 
a NAAQS; an attainment area is any area (other than an area identified 
as a nonattainment area) that meets a NAAQS; and an unclassifiable area 
is any area that cannot be classified on the basis of available 
information as meeting or not meeting a NAAQS.\152\ The CAA provides 
the EPA with discretion to require States to submit their designations 
recommendations within a reasonable amount of time not exceeding 1 year 
after the promulgation of a new or revised NAAQS. CAA section 
107(d)(1)(B)(a) also stipulates that ``the Administrator may not 
require the Governor to submit the required list sooner than 120 days 
after promulgating a new or revised [NAAQS].'' This same section 
further provides, ``Upon promulgation or revision of a [NAAQS], the 
Administrator shall promulgate the designations of all areas (or 
portions thereof) . . . as expeditiously as practicable, but in no case 
later than 2 years from the date of promulgation . . . . Such period 
may be extended for up to one year in the event the Administrator has 
insufficient information to promulgate the designations.'' With respect 
to the NAAQS setting process, courts have interpreted the term 
``promulgation'' to be signature and widespread dissemination of a 
final rule.\153\
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    \151\ While the CAA says ``designating'' with respect to the 
Governor's letter, in the full context of the CAA section it is 
clear that the Governor makes a recommendation to which the EPA must 
respond via a specified process if the EPA does not accept it.
    \152\ See 42 U.S.C. 7407(d)(1)(A)(i)-(iii).
    \153\ API v. Costle, 609 F.2d 20 (D.C. Cir. 1979).
---------------------------------------------------------------------------

    If the EPA agrees that the State's designations recommendations are 
consistent with all relevant CAA requirements, then the EPA may proceed 
to promulgate the designations for such areas. However, if the EPA 
disagrees that a State's recommendation is consistent with all relevant 
CAA requirements, then the EPA may make

[[Page 105775]]

modifications to the recommended designations by following the process 
outlined in the CAA. By no later than 120 days prior to promulgating 
the final designations, the EPA is required to notify States of any 
intended modifications to the designations of any areas or portions 
thereof, including the boundaries of areas, as the EPA may deem 
necessary. States then have an opportunity to comment on the EPA's 
intended designations decisions. If a State elects not to provide 
designations recommendations, then the EPA must timely promulgate the 
designations that it deems appropriate. CAA section 107(d)(1)(B)(ii).
    While section 107(d) of the CAA specifically addresses the 
designations process for States, the EPA intends to follow the same 
process for Tribes to the extent practicable, pursuant to section 
301(d) of the CAA regarding Tribal authority, and the Tribal Authority 
Rule (63 FR 7254, February 12, 1998). To provide clarity and 
consistency in doing so, the EPA issued a guidance memorandum to our 
Regional Offices on working with Tribes during the designations 
process.\154\
---------------------------------------------------------------------------

    \154\ ``Guidance to Regions for Working with Tribes during the 
National Ambient Air Quality Standards (NAAQS) Designations 
Process,'' December 20, 2011, Memorandum from Stephen D. Page to 
Regional Air Directors, Regions 1-X available at https://www.epa.gov/sites/default/files/2017-02/documents/12-20-11_guidance_to_regions_for_working_with_tribes_naaqs_designations.pdf
.
---------------------------------------------------------------------------

    Consistent with the process used in previous initial area 
designations efforts for both primary and secondary standards, the EPA 
will employ a nationally consistent framework and approach to evaluate 
each State's designations recommendations. Section 107(d) of the CAA 
explicitly requires that the EPA designate as nonattainment not only 
the area that is violating the pertinent standard, but also those 
nearby areas that contribute to ambient air quality in the violating 
area. Consistent with past practice, the EPA plans to address issues 
relevant to the initial area designations more fully in a separate 
designations-specific memorandum.
    The EPA intends to issue the designations for the secondary 
SO2 NAAQS based on the most recent 3 years of complete, 
certified, and valid air quality monitoring data in the areas where 
monitors are installed and operating. The EPA intends to use such 
available air quality monitoring data from the current SO2 
monitoring network. For further information on the adequacy of the 
monitoring network, refer to the memorandum in the docket for this 
action titled ``Ambient Air SO2 Monitoring Network Review 
and Background'' (Watkins et al., 2024). Monitoring data are currently 
available from existing FEM and FRM monitors sited and operated in 
accordance with 40 CFR parts 50 and 58 to determine compliance with the 
revised secondary SO2 NAAQS.
    State or Tribal air agencies may flag air quality data for certain 
days in the Air Quality System (AQS) database due to potential impacts 
from exceptional events. CAA section 319(b) defines an exceptional 
event as an event that (i) affects air quality; (ii) is not reasonably 
controllable or preventable; (iii) is an event caused by human activity 
that is unlikely to recur at a particular location or a natural event; 
and (iv) is determined by the Administrator through the process 
established in the regulations to be an exceptional event (e.g., 
volcanic activity for SO2). For emissions affecting air 
quality to be considered an exceptional event, there must be a clear 
causal relationship between the specific event and the monitored 
exceedance or violation. Air quality monitoring data affected by 
exceptional events may be excluded from use in determinations of 
exceedances or violations if the data meet the criteria for exclusion 
under CAA section 319(b) and EPA's ``Treatment of Data Influenced by 
Exceptional Events'' Final Rule (81 FR 68216; October 3, 2016) 
(Exceptional Events Rule) codified at 40 CFR 50.1, 50.14, and 51.930. 
For events affecting initial area designations, the air agency is 
required to follow the exceptional events demonstration submission 
deadlines that are identified in table 2 to 40 CFR 50.14(c)(2)(vi), 
``Schedule for Initial Notification and Demonstration Submission for 
Data Influenced by Exceptional Events for Use in Initial Area 
Designations.'' The EPA encourages air agencies to work collaboratively 
with the appropriate EPA Regional office after identifying any 
exceptional event influencing ambient air quality concentrations in a 
way that could affect area designations for the annual SO2 
secondary NAAQS.

B. Section 110(a)(1) and (2) Infrastructure SIP Requirements

    As discussed in the proposal preamble section V.B., the CAA directs 
States to address basic SIP requirements to implement, maintain, and 
enforce the NAAQS. Under CAA sections 110(a)(1) and (2), States are 
required to have State implementation plans that provide the necessary 
air quality management infrastructure including, among other things, 
enforceable emissions limitations, an ambient air monitoring program, 
an enforcement program, air quality modeling capabilities, and adequate 
personnel, resources, and legal authority to carry out the 
implementation of the SIP. After the EPA promulgates a new or revised 
NAAQS, States are required to make a new SIP submission to establish 
that they meet the necessary structural requirements for such new or 
revised NAAQS or make changes to do so. The EPA refers to this type of 
SIP submission as an ``infrastructure SIP submission.'' Under CAA 
section 110(a)(1), all States are required to make these infrastructure 
SIP submissions within 3 years after promulgation of a new or revised 
standard. While the CAA authorizes the EPA to set a shorter time for 
States to make these SIP submissions, the EPA is requiring submission 
of infrastructure SIPs within 3 years of the promulgation date of this 
revised secondary SO2 NAAQS. Section 110(b) of the CAA also 
provides that the EPA may extend the deadline for the 
``infrastructure'' SIP submission for a revised secondary NAAQS by up 
to 18 months beyond the initial 3 years. If a state requests an 
extension pursuant to CAA section 110(b) and 40 CFR 51.341 and the 
Administrator determines an extension is necessary, the EPA will set 
additional time for that state for the infrastructure SIP submittal in 
a separate action from this final rule. The EPA does not anticipate 
that extensions will be necessary as most, if not all, states' existing 
infrastructure SIPs may already be sufficient to satisfy the 
infrastructure SIP requirements for this revised secondary 
SO2 NAAQS, and those states can reiterate that they have met 
the requirements in their infrastructure SIP submissions.
    Under CAA sections 110(a)(1) and (2), States are required to make 
SIP submissions that address requirements pertaining to implementation, 
maintenance, and enforcement of a new or revised NAAQS. The specific 
subsections in CAA section 110(a)(2) require States to address a number 
of requirements, as applicable: (A) emissions limits and other control 
measures; (B) ambient air quality monitoring/data system; (C) programs 
for enforcement of control measures and for construction or 
modification of stationary sources; (D)(i) interstate pollution 
transport and (ii) interstate and international pollution abatement; 
(E) adequate resources and authority, conflict of interest, and 
oversight of local governments and regional agencies; (F) stationary 
source monitoring and reporting; (G) emergency powers; (H) SIP 
revisions; (I)

[[Page 105776]]

plan revisions for nonattainment areas; (J) consultation with 
government officials, public notification, Prevention of Significant 
Deterioration (PSD) and visibility protection; (K) air quality modeling 
and submission of modeling data; (L) permitting fees; and (M) 
consultation and participation by affected local entities. These 
requirements apply to all SIP submissions in general, but the EPA has 
provided specific guidance to States concerning its interpretation of 
these requirements in the specific context of infrastructure SIP 
submissions for a new or revised NAAQS.\155\
---------------------------------------------------------------------------

    \155\ See ``Guidance on Infrastructure State Implementation Plan 
(SIP) Elements under Clean Air Act sections 110(a)(1) and 
110(a)(2)'' September 2013, Memorandum from Stephen D. Page to 
Regional Air Directors, Regions 1-10.
---------------------------------------------------------------------------

    As a reminder, States are not required to address nonattainment 
plan requirements for purposes of the revised secondary SO2 
NAAQS on the same schedule as infrastructure SIP requirements. For the 
reasons explained below, the EPA interprets the CAA such that (1) the 
portion of CAA section 110(a)(2)(C), programs for enforcement of 
control measures and for construction or modification of sources that 
applies to permit programs applicable in designated nonattainment 
areas, (known as ``nonattainment new source review'') under part D; and 
(2) CAA section 110(a)(2)(I) in its entirety are not subject to the 3-
year submission deadline of CAA section 110(a)(1), and thus States are 
not required to address them in the context of an infrastructure SIP 
submission. Accordingly, the EPA does not expect States to address the 
requirement for a new or revised NAAQS in the infrastructure SIP 
submissions to include regulations or emissions limits developed 
specifically for attaining the relevant standard in areas designated 
nonattainment for the revised secondary SO2 NAAQS. States 
are required to submit infrastructure SIP submissions for the secondary 
SO2 NAAQS before they are required to submit nonattainment 
plan SIP submissions to demonstrate attainment with the same NAAQS. As 
a general matter, states would be required to submit nonattainment 
plans to provide for attainment and maintenance of the revised 
secondary SO2 NAAQS within 3 years from the effective date 
of nonattainment area designations as required under CAA section 
172(b). In addition, because this NAAQS is a secondary standard, CAA 
section 110(b) also provides that the EPA may extend the deadline for 
the nonattainment plan for up to 18 months beyond the initial 3 years. 
If a state requests an extension pursuant to CAA section 110(b) and 40 
CFR 51.341 and the Administrator determines an extension is necessary, 
the EPA will set additional time for the nonattainment plan submittal 
in a separate action from this final rule. The EPA reviews and acts 
upon these later SIP submissions through a separate process. For these 
reasons, the EPA does not expect States to address new nonattainment 
area emissions controls per CAA section 110(a)(2)(I) in their 
infrastructure SIP submissions.
    Another required infrastructure SIP element is that each State's 
SIP must contain adequate provisions to prohibit, consistent with the 
provisions of title I of the CAA, emissions from within the State that 
will significantly contribute to nonattainment in, or interfere with 
maintenance by, any other State of the primary or secondary NAAQS.\156\ 
This element is often referred to as the ``good neighbor'' or 
``interstate transport'' provision.\157\ The provision has two prongs: 
significant contribution to nonattainment (prong 1) and interference 
with maintenance (prong 2). The EPA and States must give independent 
significance to prong 1 and prong 2 when evaluating downwind air 
quality problems under CAA section 110(a)(2)(D)(i)(I).\158\ Further, 
case law has established that the EPA and States must implement 
requirements to meet interstate transport obligations in alignment with 
the applicable statutory attainment schedule of the downwind areas 
impacted by upwind-State emissions.\159\ The EPA anticipates 
coordinating with States with respect to the requirements of CAA 
section 110(a)(2)(D)(i)(I) for implementation of the secondary 
SO2 NAAQS.
---------------------------------------------------------------------------

    \156\ CAA section 110(a)(2)(D)(i)(I).
    \157\ CAA section 110(a)(2)(D)(i)(II) also addresses certain 
interstate effects that States must address and thus is also 
sometimes referred to as relating to ``interstate transport.''
    \158\ See North Carolina v. EPA, 531 F.3d 896, 909-11 (D.C. Cir. 
2008).
    \159\ See id. at 911-13. See also Wisconsin v. EPA, 938 F.3d 
303, 313-20 (D.C. Cir. 2019); Maryland v. EPA, 958 F.3d 1185, 1203-
04 (D.C. Cir. 2020).
---------------------------------------------------------------------------

    Each State has the authority and responsibility to review its air 
quality management program's existing SIP provisions in light of each 
new or revised NAAQS to determine whether any revisions to the State's 
regulations or program are necessary to implement a new or revised 
NAAQS. Most States have revised and updated their SIPs in recent years 
to address requirements associated with other revised NAAQS. For some 
States, it may be the case that, for a number of infrastructure 
elements, the State may believe it already has adequate State 
regulations adopted and approved into the SIP to address a particular 
requirement with respect to any new or revised NAAQS. For such portions 
of the State's infrastructure SIP submission, the State could provide 
an explanation of how its existing SIP provisions are adequate.
    If a State determines that existing SIP-approved provisions, such 
as those approved for the 1-hour primary SO2 NAAQS, remain 
adequate in light of the new annual secondary SO2 NAAQS with 
respect to a given infrastructure SIP element (or sub-element), then 
the State may make a SIP submission containing relevant supporting 
information ``certifying'' that the existing SIP contains provisions 
that address those requirements of the specific CAA section 110(a)(2) 
infrastructure elements.\160\ In the case of such a certification 
submission, the State would not have to include a copy of the relevant 
provision (e.g., rule or statute) itself. Rather, this certification 
submission should provide citations to the EPA-approved State statutes, 
regulations, or non-regulatory measures, as appropriate, in or 
referenced by the already EPA-approved SIP that meet particular 
infrastructure SIP element requirements. The State's infrastructure SIP 
submission should also include an explanation as to how the State has 
determined that those existing provisions meet the relevant 
requirements.
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    \160\ A ``certification'' approach would not be appropriate for 
the interstate pollution control requirements of CAA section 
110(a)(2)(D)(i).
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    Like any other SIP submission, that State can make such an 
infrastructure SIP submission certifying that it has already met some 
or all of the applicable requirements only after it has provided 
reasonable notice and opportunity for public hearing. This ``reasonable 
notice and opportunity for public hearing'' requirement for 
infrastructure SIP submissions is to meet the requirements of CAA 
sections 110(a) and 110(l). Under the EPA's regulations at 40 CFR part 
51, if a public hearing is held, an infrastructure SIP submittal must 
include a certification by the State that the public hearing was held 
in accordance with the EPA's procedural requirements for public 
hearings. See 40 CFR part 51, appendix V, 2.1(g); and see 40 CFR 
51.102.
    In consultation with its EPA Regional Office, a State should follow 
all applicable EPA regulations governing infrastructure SIP submissions 
in 40 CFR part 51--e.g., subpart I (Review of New Sources and 
Modifications), subpart J (Ambient Air Quality

[[Page 105777]]

Surveillance), subpart K (Source Surveillance), subpart L (Legal 
Authority), subpart M (Intergovernmental Consultation), subpart O 
(Miscellaneous Plan Content Requirements), subpart P (Protection of 
Visibility), and subpart Q (Reports). For the EPA's general criteria 
for infrastructure SIP submissions, refer to 40 CFR part 51, appendix 
V, Criteria for Determining the Completeness of Plan Submissions. The 
EPA recommends that States electronically submit their infrastructure 
SIPs to the EPA through the State Plan Electronic Collaboration System 
(SPeCS),\161\ an online system available through the EPA's Central Data 
Exchange. The EPA acknowledges that the timeline for submission of 
infrastructure SIPs for the secondary SO2 NAAQS may overlap 
in part with the timeline for submission of infrastructure SIPs for the 
recently revised primary PM2.5 NAAQS. Air Agencies may elect 
to streamline their infrastructure SIP submittal and development by 
combining the two distinct infrastructure SIP submissions for both 
NAAQS into one submission. The EPA appreciates the obligations may 
differ for some infrastructure elements, and simply notes that this 
option may represent a more streamlined approach for some areas.
---------------------------------------------------------------------------

    \161\ https://cdx.epa.gov/.
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C. Prevention of Significant Deterioration and Nonattainment New Source 
Review Programs for the Revised Secondary SO2 Standard

    The CAA, at parts C and D of title I, contains preconstruction 
review and permitting programs applicable to new major stationary 
sources and major modifications of existing major sources. The 
preconstruction review of each new major stationary source and major 
modification applies on a pollutant-specific basis, and the 
requirements that apply for each pollutant depend on whether the area 
in which the source is situated is designated as attainment (or 
unclassifiable) or nonattainment for that pollutant. In areas 
designated attainment or unclassifiable for a pollutant, the PSD 
requirements under part C apply to construction at major sources. In 
areas designated nonattainment for a pollutant, the Nonattainment New 
Source Review (NNSR) requirements under part D apply to major source 
construction. Collectively, those two sets of permit requirements are 
commonly referred to as the ``major New Source Review'' or ``major 
NSR'' programs.
    The statutory requirements for a PSD permit program set forth under 
part C of title I of the CAA (sections 160 through 169) are implemented 
through the EPA's PSD regulations found at 40 CFR 51.166 (minimum 
requirements for an approvable PSD SIP) and 40 CFR 52.21 (PSD 
permitting program for permits issued under the EPA's Federal 
permitting authority). Whenever a proposed new major source or major 
modification triggers PSD requirements for SO2, either 40 
CFR 52.21 or State regulations based on 40 CFR 51.166 will apply for 
undesignated areas and for areas that are designated as attainment or 
as unclassifiable for the revised secondary SO2 NAAQS.
    For PSD, a ``major stationary source'' is one with the potential to 
emit 250 tons per year (tpy) or more of any regulated NSR pollutant, 
unless the new or modified source is classified under a list of 28 
source categories contained in the statutory definition of ``major 
emitting facility'' in CAA section 169(1). For those 28 source 
categories, a ``major stationary source'' is one with the potential to 
emit 100 tpy or more of any regulated NSR pollutant. A ``major 
modification'' is a physical change or a change in the method of 
operation of an existing major stationary source that results, first, 
in a significant emissions increase of a regulated NSR pollutant and, 
second, in a significant net emissions increase of that pollutant. See 
40 CFR 51.166(b)(2)(i), 52.21(b)(2)(i). The EPA PSD regulations define 
the term ``regulated NSR pollutant'' to include any pollutant for which 
a NAAQS has been promulgated and any pollutant identified in the EPA 
regulations as a constituent or precursor to such pollutant. See 40 CFR 
51.166(b)(49), 52.21(b)(50). Thus, the PSD program currently requires 
the review and control of emissions of SO2, as applicable. 
Among other things, for each regulated NSR pollutant emitted or 
increased in a significant amount, the PSD program requires a new major 
stationary source or a major modification to apply the ``best available 
control technology'' (BACT) and to conduct an air quality impact 
analysis to demonstrate that the proposed major stationary source or 
major modification will not cause or contribute to a violation of any 
NAAQS or PSD increment.\162\ See CAA section 165(a)(3)-(4), 40 CFR 
51.166(j) and (k), 52.21(j) and (k). The PSD requirements may also 
include, in appropriate cases, an analysis of potential adverse impacts 
on Class I areas. See CAA sections 162(a) and 165, 40 CFR 51.166(p), 
52.21(p)).\163\
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    \162\ By establishing the maximum allowable level of ambient air 
pollutant concentration increase in a particular area, an increment 
defines ``significant deterioration'' of air quality in that area. 
Increments are defined by the CAA as maximum allowable increases in 
ambient air concentrations above a baseline concentration and are 
specified in the PSD regulations by pollutant and area 
classification (Class I, II and III). 40 CFR 51.166(c), 52.21(c); 75 
FR 64864; October 20, 2010. The EPA has developed the Guideline on 
Air Quality Models and other documents to, among other things, 
provide methods and guidance for demonstrating compliance the NAAQS 
and PSD increments including the annual SO2 standard. See 
40 CFR part 51, appendix W; 82 FR 5182, January 17, 2017.
    \163\ Congress established certain Class I areas in section 
162(a) of the CAA, including international parks, national 
wilderness areas, and national parks that meet certain criteria. 
Such Class I areas, known as mandatory Federal Class I areas, are 
afforded special protection under the CAA. In addition, states and 
Tribal governments may establish Class I areas within their own 
political jurisdictions to provide similar special air quality 
protection.
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    With regard to nonattainment NSR, the EPA's regulations for the 
NNSR programs are contained in 40 CFR 51.165, 40 CFR 52.24, and 40 CFR 
part 51, appendix S. Specifically, the EPA developed minimum program 
requirements for a NNSR program that is approvable in a SIP, and those 
requirements, which include requirements for SO2, are 
contained in 40 CFR 51.165. In addition, 40 CFR part 51, appendix S 
contains requirements constituting an interim NNSR program. This 
program enables NNSR permitting in nonattainment areas by States that 
lack a SIP-approved NNSR permitting program (or a program that does not 
apply to the relevant pollutant) during the time between the date of 
the relevant designation and the date that the EPA approves into the 
SIP a NNSR program. See 40 CFR part 51, appendix S, part I; 40 CFR 
52.24(k). Any new NNSR requirements for SO2 associated with 
the revised secondary standard would become applicable upon the 
effective date of any nonattainment designation for the final standard.
    As stated in the proposal section V.C., the EPA is not making any 
changes to the NSR program regulations to implement the revised 
secondary SO2 NAAQS. Under the PSD program, any permit 
issued on and after the effective date of the new annual secondary 
SO2 NAAQS will require a demonstration that the emissions 
from the proposed major stationary source or major modification would 
not cause or contribute to violation of that standard. The EPA has 
regulations, models, guidance, and other tools for making this showing, 
and anticipates that sources and reviewing authorities will be able to 
use most of these existing tools to demonstrate compliance with the 
revised secondary SO2 NAAQS. However, as provided in the 
NPRM, the EPA developed a separate technical

[[Page 105778]]

document (Tillerson et al., 2024),\164\ which provides a technical 
justification for how a demonstration of compliance with the 1-hour 
primary SO2 standard can suffice to demonstrate compliance 
with the new SO2 secondary standard. The EPA has determined 
that this alternative compliance demonstration approach is technically 
justified and can provide for streamlined implementation of the new 
secondary SO2 NAAQS under the PSD program in all areas of 
the country. Accordingly, the EPA plans to issue a memorandum that 
explains how permit applicants and permitting authorities may use this 
alternative compliance demonstration approach and supporting technical 
analysis in making the required demonstration for the new secondary 
SO2 NAAQS. The EPA intends to issue this memorandum close in 
time to the effective date of the new secondary SO2 NAAQS to 
help provide for a smooth transition to implementing the revised 
secondary standard under the PSD program.
---------------------------------------------------------------------------

    \164\ This technical memo (Tillerson et al., 2024) is available 
in the docket for this NAAQS review (Docket ID No. EPA-HQ-OAR-2014-
0128-0041).
---------------------------------------------------------------------------

D. Transportation Conformity Program

    As discussed in the proposal section V.E., transportation 
conformity is required under CAA section 176(c) (42 U.S.C. 7506(c)) to 
ensure that federally supported highway and transit activities are 
consistent with (``conform to'') the purpose of the SIP. Transportation 
conformity applies to areas that are designated as nonattainment areas 
and to nonattainment areas that have been redesignated to attainment 
with an approved CAA section 175A maintenance plan (i.e., maintenance 
areas) for transportation-related criteria pollutants: carbon monoxide, 
ozone, NO2, PM2.5, and PM10. Motor 
vehicles are not significant sources of SO2, and thus 
transportation conformity does not apply to any SO2 NAAQS 
(40 CFR 93.102(b)(1)), either the existing NAAQS or this revised 
secondary SO2 NAAQS.\165\ Therefore, this final rule does 
not affect the transportation conformity rule (40 CFR 51.390 and 40 CFR 
part 93, subpart A).
---------------------------------------------------------------------------

    \165\ See ``VII. Description of the Proposal'' in ``Criteria and 
Procedures for Determining Conformity to State or Federal 
Implementation Plans of Transportation Plans, Programs, and Projects 
funded or Approved Under Title 23 U.S.C. or the Federal Transit 
Act.'' (58 FR 3768, January 11, 1993). The EPA finalized the 
original transportation conformity regulations on November 24, 1993 
(58 FR 62188). The rule has subsequently been revised and the 
current provisions of the transportation conformity rule are found 
at 40 CFR part 93, subpart A.
---------------------------------------------------------------------------

E. General Conformity Program

    The General Conformity program applies to federal activities that 
cause emissions of the criteria or precursor pollutants to originate 
within designated nonattainment areas \166\ or redesignated attainment 
areas that operate under approved CAA section 175A maintenance plans 
(i.e., maintenance areas). The General Conformity program requirements 
at 40 CFR part 93, subpart B establish criteria and procedures for 
determining conformity as required under CAA section 176(c),\167\ which 
prohibits a Federal agency from taking an action that would interfere 
with the ability of a State or Tribe to attain or maintain the NAAQS. 
General Conformity applies only to Federal activities not defined as 
transportation plans, programs, or projects under 40 CFR 93.102. The 
program requirements apply to emissions of all six criteria pollutants 
and their precursors, including NOX, SOX, and PM, 
per 40 CFR 93.153(b)(1) and (2), but only to the extent the emissions 
can be characterized as ``direct emissions'' or ``indirect emissions'' 
as defined under 40 CFR 93.152. General federal activities that cause 
emissions of SO2 are subject to General Conformity; however, 
no change to the regulations is necessary to accommodate any changes to 
the secondary SO2 NAAQS made by this rulemaking.
---------------------------------------------------------------------------

    \166\ Applicability of the General Conformity program to any 
newly designated nonattainment area for a specific NAAQS begins one 
year following the effective date of the final nonattainment 
designation, as allowed under CAA section 176(c)(6) and 40 CFR 
93.153(k).
    \167\ Under CAA section 176(c)(1), Federal agencies have the 
affirmative responsibility to assure their actions achieve 
conformity to the purpose of an implementation plan, where the term 
``conformity to an implementation plan'' is defined at CAA sections 
176(c)(1)(A) and 176(c)(1)(B). Under CAA section 176(c)(4), the EPA 
is required to establish criteria and procedures for determining 
conformity.
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VI. Statutory and Executive Order Reviews

    Additional information about these statutes and Executive orders 
can be found at https://www.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive 
Order 14094: Modernizing Regulatory Review

    This action is a ``significant regulatory action'' as defined in 
Executive Order 12866, as amended by Executive Order 14094. 
Accordingly, EPA submitted this action to the Office of Management and 
Budget (OMB) for Executive Order 12866 review. Documentation of any 
changes made in response to the Executive Order 12866 review is 
available in the docket. The EPA prepared an analysis to determine if 
additional emission reductions would be needed to meet the revised 
secondary SO2 NAAQS. This analysis is contained in the 
document ``Air Quality Analyses Using Sulfur Dioxide (SO2) 
Air Quality Data, Updated'' which is available in the docket for this 
NAAQS review (ID No. EPA-HQ-OAR-2014-0128). The analysis concluded that 
no additional emissions reductions beyond any needed to meet the 
current 1-hour primary SO2 NAAQS are expected to be 
necessary to meet the new annual secondary SO2 NAAQS of 10 
ppb, averaged over three years. Thus, there are no pollution controls 
expected to be necessary, and accordingly no costs or monetized 
benefits associated with this NAAQS revision. Accordingly, no 
regulatory impact analysis has been prepared for this final rule.

B. Paperwork Reduction Act (PRA)

    This action does not impose any new information collection burden 
under the PRA. The OMB has previously approved the information 
collection activities contained in the existing regulations and has 
assigned OMB control number 2060-0084. The data collected through the 
information collection activities in the existing regulations consist 
of ambient air concentration measurements for the seven air pollutants 
with national ambient air quality standards (i.e., ozone, sulfur 
dioxide, nitrogen dioxide, lead, carbon monoxide, PM2.5 and 
PM10), ozone precursors, air toxics, meteorological 
variables at a select number of sites, and other supporting 
measurements. Accompanying the pollutant concentration data are quality 
assurance/quality control data and air monitoring network design 
information. The EPA and others (e.g., State and local air quality 
management agencies, Tribal entities, environmental organizations, 
academic institutions, and industrial groups) use the ambient air 
quality data for many purposes including informing the public and other 
interested parties of an area's air quality, judging an area's air 
quality in comparison with the established health or welfare standards, 
evaluating an air quality management agency's progress in achieving or 
maintaining air pollutant levels below the national and local 
standards, developing and revising State Implementation Plans (SIPs), 
evaluating air pollutant control strategies, developing or revising 
national control policies, providing data for air quality model 
development and validation,

[[Page 105779]]

supporting enforcement actions, documenting episodes and initiating 
episode controls, assessing air quality trends, and conducting air 
pollution research.

C. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic 
impact on a substantial number of small entities under the RFA. This 
action will not impose any requirements on small entities. Rather, this 
final rule establishes national standards for allowable annual average 
concentrations of SO2 in ambient air as required by section 
109 of the CAA. See also American Trucking Associations v. EPA, 175 
F.3d 1027, 1044-45 (D.C. Cir. 1999) (NAAQS do not have significant 
impacts upon small entities because NAAQS themselves impose no 
regulations upon small entities), rev'd in part on other grounds, 
Whitman v. American Trucking Associations, 531 U.S. 457 (2001).

D. Unfunded Mandates Reform Act (UMRA)

    This action does not contain an unfunded mandate as described in 
UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely affect 
small governments. 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 the RIA pursuant to the Unfunded Mandates Reform Act would not 
furnish any information that the court could consider in reviewing the 
NAAQS).

E. Executive Order 13132: Federalism

    This action 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. However, 
the EPA recognizes that states will have a substantial interest in this 
action and any future revisions to associated requirements.

F. Executive Order 13175: Consultation and Coordination With Indian 
Tribal Governments

    This action does not have Tribal implications, as specified in 
Executive Order 13175. It does not have a substantial direct effect on 
one or more Indian Tribes as Tribes are not obligated to adopt or 
implement any NAAQS. In addition, Tribes are not obligated to conduct 
ambient monitoring for SO2 or to adopt the ambient air 
monitoring requirements of 40 CFR part 58. Thus, Executive Order 13175 
does not apply to this action. However, consistent with the EPA Policy 
on Consultation and Coordination with Indian Tribes, the EPA offered 
consultation to all 574 Federally Recognized Tribes during the 
development of this action. Although no Tribes requested consultation, 
the EPA provided informational meetings and provided information on the 
monthly National Tribal Air Association calls, and during the public 
comment period we received comments on the proposed rule from this 
Tribal organization.

G. Executive Order 13045: Protection of Children From Environmental 
Health Risks and Safety Risks

    EPA interprets Executive Order 13045 as applying only to those 
regulatory actions that concern environmental health or safety risks 
that EPA has reason to believe may disproportionately affect children, 
per the definition of ``covered regulatory action'' in section 2-202 of 
the Executive order.
    Therefore, this action is not subject to Executive Order 13045 
because it does not concern an environmental health risk or safety 
risk. Since this action does not concern human health, EPA's Policy on 
Children's Health also does not apply.

H. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use

    This action is not a ``significant energy action'' because it is 
not likely to have a significant adverse effect on the supply, 
distribution, or use of energy. The purpose of this action is to revise 
the existing secondary SO2 standard, and also to retain the 
current secondary standards for NO2, PM2.5 and 
PM10. The action does not prescribe specific pollution 
control strategies by which these ambient air standards and monitoring 
revisions will be met. Such strategies will be developed by states on a 
case-by-case basis, and the EPA cannot predict whether the control 
options selected by states will include regulations on energy 
suppliers, distributors, or users. Thus, the EPA concludes that this 
action does not constitute a significant energy action as defined in 
Executive Order 13211.

I. National Technology Transfer and Advancement Act (NTTAA)

    This action involves environmental monitoring or measurements. The 
EPA has decided to use the existing indicator, SO2, for 
measurements in support of this action and is not revising the 
SO2 FRMs or FEMs for measurement of this air pollutant. The 
EPA employs a Performance-Based Measurement System (PBMS) when 
designating monitoring methods as either FRM or FEM, which does not 
require the use of specific, prescribed analytic methods. This 
performance-based assessment of candidate methods is described in 40 
CFR part 50 and the reference and equivalency criteria described in 40 
CFR part 53. The EPA does not preclude the use of other methods, 
whether it constitutes a voluntary consensus standard or not, as long 
as it meets the specified performance criteria defined in 40 CFR part 
53 and is approved by EPA as an FRM or FEM. Our approach in the past 
has resulted in multiple brands of monitors being approved as FRM for 
SO2, and we expect this trend to continue.

J. Executive Order 12898: Federal Actions To Address Environmental 
Justice in Minority Populations and Low-Income Populations and 
Executive Order 14096: Revitalizing Our Nation's Commitment to 
Environmental Justice for All

    The EPA believes that the human health and environmental conditions 
that exist prior to this action do not result in disproportionate and 
adverse effects on communities with Environmental Justice (EJ) 
concerns. As discussed in sections II.A.4. and II.B. above, and 
chapters 5 and 7 of the PA, the acid buffering capacity of waterbodies 
in key acid-sensitive ecoregions in recent years is estimated to meet 
protection targets in high percentages. As discussed in section 
II.A.3.b. above, impacts on acid-sensitive waterbodies, if sufficiently 
severe, would have the potential to impact the public welfare through 
impacts to fisheries. Although recent conditions do not indicate such a 
level of severity, to the extent local communities relied on such 
fisheries disproportionately to their representation in the population, 
such effects of the past (e.g., effects associated with acidification 
risks of 20 or more years ago) would have had the potential

[[Page 105780]]

for disproportionate impacts. Recent conditions do not indicate risk of 
aquatic acidification to such a level of severity, and the available 
information for recent acid buffering capacity levels does not include 
evidence of disproportionate and adverse impacts on communities with EJ 
concerns. As the action is to establish a new, more stringent standard 
to protect acid-sensitive waterbodies to recent levels and protect 
against recurrence of acidification effects from the past, for which 
the potential for disproportionate and adverse effects on local 
communities is unknown, the EPA believes that this action is not likely 
to result in new disproportionate and adverse effects on communities 
with EJ concerns. The information supporting this Executive order 
review is contained in the PA for this review and sections II.A.3., 
II.A.4., II.B.1. and II.B.3. of this document.

K. Congressional Review Act (CRA)

    This action is subject to the CRA, and the EPA will submit a rule 
report to each House of the Congress and to the Comptroller General of 
the United States. This action is not a ``major rule'' as defined by 5 
U.S.C. 804(2).

L. Judicial Review

    Under section 307(b)(1) of the CAA, this final action is 
``nationally applicable'' and petitions for judicial review of this 
action must be filed in the U.S. Court of Appeals for the District of 
Columbia Circuit within 60 days from the date this final action is 
published in the Federal Register. Filing a petition for 
reconsideration by the Administrator of this final action does not 
affect the finality of the action for the purposes of judicial review, 
nor does it extend the time within which a petition for judicial review 
must be filed and shall not postpone the effectiveness of such action.

VII. References

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sulfur dioxide to sulfates in the United States. J Air Poll Contr 
Assoc 26: 318-324.
Baker, LA, Herlihy, AT, Kaufmann, PR, and Eilers, JM (1991). Acidic 
lakes and streams in the United States: the role of acidic 
deposition. Science 252: 1151-1154.
Baker JP; Schofield CL. (1985). Acidification impacts on fish 
populations: A review. In: Adams D; Page WP (Eds.), Acid deposition: 
environmental, economic, and policy issues (pp. 183-221). New York 
and London: Plenum Press.
Banzhaf, S, Burtraw, D, Evans, D and Krupnick, A (2006). Valuation 
of Natural Resource Improvements in the Adirondacks. Land Econ 82: 
445-464.
Baron, JS, Driscoll, CT, Stoddard, JL and Richer, EE (2011). 
Empirical critical loads of atmospheric nitrogen deposition for 
nutrient enrichment and acidification of sensitive US lakes. 
Bioscience 61: 602-613.
Boyer, EW, Goodale, CL, Jaworski, NA and Howarth, RW (2002). 
Anthropogenic nitrogen sources and relationships to riverine 
nitrogen export in the northeastern USA. Biogeochemistry 57: 137-
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Brown, CA and Ozretich, RJ (2009). Coupling between the coastal 
ocean and Yaquina Bay, Oregon: Importance of oceanic inputs relative 
to other nitrogen sources. Estuaries Coasts 32: 219-237.
Bulger, AJ, Cosby, BJ, Dolloff, CA, Eshleman, KN, Webb, JR and 
Galloway, JN (1999). SNP:FISH. Shenandoah National Park: Fish in 
sensitive habitats. Project Final Report-Volume 1-4. 
Charlottesville, VA, University of Virginia.
Bulger, AJ, Cosby, BJ and Webb, JR (2000). Current, reconstructed 
past, and projected future status of brook trout (Salvelinus 
fontinalis) streams in Virginia. Can J Fish Aquat Sci 57(7): 1515-
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Cosby, BJ, Hornberger, GM, Galloway, JN and Wright, RF (1985). 
Modeling the effects of acid deposition: Assessment of a lumped 
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Costanza, R, De Groot, R, Braat, L, Kubiszewski, I, Fioramonti, L, 
Sutton, P, Farber, S and Grasso, M (2017). Twenty years of ecosystem 
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Cox, L, Kendall, R and Fernandez, I (2020a). Letter from Louis Cox, 
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Sulfur and Ivan Fernandez, Immediate Past Chair, Secondary NAAQS 
Review Panel for Oxides of Nitrogen and Sulfur to the Administrator 
Andrew R. Wheeler, Re: CASAC Review of the EPA's Integrated Science 
Assessment for Oxides of Nitrogen, Oxides of Sulfur, and Particulate 
Matter--Ecological Criteria (Second External Review Draft--June 
2018). May 5, 2020. EPA-CASAC-20-004. Office of the Administrator, 
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Cox, L, Kendall, R and Fernandez, I (2020b). Letter from Louis Cox, 
Chair, Clean Air Scientific Advisory Committee, Ronald Kendall, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur and Ivan Fernandez, Immediate Past Chair, Secondary NAAQS 
Review Panel for Oxides of Nitrogen and Sulfur to Administrator 
Andrew R. Wheeler, Re: Consultation on the EPA's Review of the 
Secondary Standards for Ecological Effects of Oxides of Nitrogen, 
Oxides of Sulfur, and Particulate Matter: Risk and Exposure 
Assessment Planning Document (August--2018). May 5, 2020. EPA-CASAC-
20-005. Office of the Administrator, Science Advisory Board, 
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Cox, RD, Preston, KL, Johnson, RF, Minnich, RA and Allen, EB (2014). 
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exotic annual grassland in southern California, USA. Glob Ecol 
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Davis, TW, Bullerjahn, GS, Tuttle, T, Mckay, RM and Watson, SB 
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blooms in Sandusky Bay, Lake Erie. Environ Sci Technol 49: 7197-
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Dietze, MC and Moorcroft, PR (2011). Tree mortality in the eastern 
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Diez Roux, A and Fernandez, I (2016). Letter from Anna Diez Roux, 
Chair, Clean Air Scientific Advisory Committee and Ivan Fernandez, 
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Sulfur, to Administrator Gina McCarthy, Re: CASAC Review of the 
EPA's Draft Integrated Review Plan for the National Ambient Air 
Quality Standards for Oxides of Nitrogen and Oxides of Sulfur. April 
1, 2016. EPA-CASAC-16-001. Office of the Administrator, Science 
Advisory Board, Washington, DC Available at: https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Diez Roux, A and Fernandez, I (2017). Letter from Anna Diez Roux, 
Chair, Clean Air Scientific Advisory Committee and Ivan Fernandez, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur, to the Honorable Gina McCarthy, Administrator, Re: CASAC 
Review of the EPA's Integrated Science Assessment for Oxides of 
Nitrogen, Oxides of Sulfur, and Particulate Matter--Ecological 
Criteria (First External Review Draft--February 2017). September 28, 
2017. EPA-CASAC-17-004. Office of the Administrator, Science 
Advisory Board, Washington, DC Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100X9FA.PDF.
Driscoll, CT, Driscoll, KM, Fakhraei, H, Civerolo, K (2016). Long-
term temporal trends and spatial patterns in the acid-base chemistry 
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Standards, Research Triangle Park, NC. EPA-452/R-95-005. September 
1995. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=00002UBE.PDF.
U.S. EPA (1995b). Acid Deposition Standard Feasibility Study: Report 
to Congress. Office of Air and Radiation, Acid Rain Division, 
Washington, DC. EPA-430-R-95-001a. October 1995. Available at: 
https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000WTGY.PDF.
U.S. EPA (1996). Review of the National Ambient Air Quality 
Standards for Particulate Matter: Policy Assessment of Scientific 
and Technical Information (OAQPS Staff Paper). Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452\R-96-
013. July 1996. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000DLIE.PDF.
U.S. EPA (2004a). Air Quality Criteria for Particulate Matter. (Vol 
I of II). Office of Research and Development, Research Triangle 
Park, NC. EPA-600/P-99-002aF. October 2004. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100LFIQ.PDF.
U.S. EPA (2004b). Air Quality Criteria for Particulate Matter. (Vol 
II of II). Office of Research and Development, Research Triangle 
Park, NC. EPA-600/P-99-002bF. October 2004. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100LG7Q.PDF.
U.S. EPA (2005). Review of the National Ambient Air Quality 
Standards for Particulate Matter: Policy Assessment of Scientific 
and Technical Information, OAQPS Staff Paper. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-05-
005a. December 2005. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1009MZM.PDF.
U.S. EPA (2007). Integrated Review Plan for the Secondary National 
Ambient Air Quality Standards for Nitrogen Dioxide and Sulfur 
Dioxide. Office of Research and Development, Research Triangle Park, 
NC, EPA-452/R-08-006. December 2007. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1001FDM.PDF.
U.S. EPA (2008a). Integrated Science Assessment (ISA) for Oxides of 
Nitrogen and Sulfur Ecological Criteria. Office of Research and 
Development, Research Triangle Park, NC. EPA/600/R-08/082F. December 
2008. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100R7MG.PDF.
U.S. EPA (2008b). Integrated Review Plan for the National Ambient 
Air Quality Standards for Particulate Matter. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA 452/R-08-
004. March 2008. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1001FB9.PDF.
U.S. EPA (2009a). Risk and Exposure Assessment for Review of the 
Secondary National Ambient Air Quality Standards for Oxides of 
Nitrogen and Oxides of Sulfur (Main Content). Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-09-
008a. September 2009. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100FNQV.PDF.
U.S. EPA (2009b). Integrated Science Assessment for Particulate 
Matter. Office of Research and Development, Research Triangle Park, 
NC. EPA/600/R-08/139F. December 2009. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P10060Z4.PDF.
U.S. EPA (2009c). Particulate Matter National Ambient Air Quality 
Standards (NAAQS): Scope and Methods Plan for Urban Visibility 
Impact Assessment. Office of Air Quality Planning and Standards, 
Research Triangle Park, NC. EPA-452/P-09-001. February 2009. 
Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100FLUX.PDF.
U.S. EPA (2010). Particulate Matter Urban-Focused Visibility 
Assessment--Final Document. Office of Air Quality Planning and 
Standards, Research Triangle Park, NC. EPA-452/R- 10-004. July 2010. 
Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100FO5D.PDF.
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. Office of Air Quality Planning and Standards, 
Research Triangle Park, NC. EPA-452/R-11-005a, b. February 2011. 
Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1009R7U.PDF https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1009RHY.PDF.
U.S. EPA (2016). Integrated Review Plan for the National Ambient Air 
Quality Standards for Particulate Matter. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-16-
005. December 2016. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100R5VE.PDF.
U.S. EPA (2017). Integrated Review Plan for the Secondary NAAQS for 
Oxides of Nitrogen and Oxides of Sulfur and Particulate Matter--
Final. Office of Air Quality Planning and Standards, Research 
Triangle Park, NC. EPA-452/R-17-002. January 2017. Available at: 
https://nepis.epa.gov/Exe/ZyPDf.cgi?Dockey=P100R607.PDF.
U.S. EPA (2018). Review of the Secondary Standards for Ecological 
Effects of Oxides of Nitrogen, Oxides of Sulfur, and Particulate 
Matter: Risk and Exposure Assessment Planning Document. Office of 
Air Quality Planning and Standards, Research Triangle Park, NC. EPA-
452/D-18-001. August 2018. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100V7JA.PDF.
U.S. EPA (2019). Integrated Science Assessment (ISA) for Particulate 
Matter (Final Report, Dec 2019). U.S. Environmental Protection 
Agency, Washington, DC, EPA/600/R-19/188, 2019.
U.S. EPA (2020a). Integrated Science Assessment (ISA) for Oxides of 
Nitrogen, Oxides of Sulfur and Particulate Matter Ecological 
Criteria (Final Report, 2020). Office of Air Quality Planning and 
Standards, Research Triangle Park, NC. EPA/600/R-20/278. September 
2020. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1010WR3.PDF.
U.S. EPA (2023). Overview of Particulate Matter (PM) Air Quality in 
the United States. Available at: https://www.epa.gov/air-quality-analysis/particulate-matter-naaqs-review-analyses-and-data-sets.
U.S. EPA (2024). Policy Assessment for the Review of the Secondary 
National Ambient Air Quality Standards for Oxides of Nitrogen, 
Oxides of Sulfur and Particulate Matter. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-24-
003. January 2024. Available at: https://www.epa.gov/system/files/documents/2024-01/noxsoxpm-final.pdf.
Wallace, ZP, Lovett, GM, Hart, JE and Machona, B (2007). Effects of 
nitrogen saturation on tree growth and death in a mixed-oak forest. 
For Ecol Manage 243: 210-218.
Waller, K, Driscoll, C, Lynch, J, Newcomb, D and Roy, K (2012). 
Long-term recovery of lakes in the Adirondack region of New York to 
decreases in acidic deposition. Atmos Environ 46: 56-64.
Watkins, N, Boyette, L and Jager, D (2024). Memorandum to Secondary 
NOX/SOX/PM NAAQS Review Docket (Docket ID No. 
EPA-HQ-OAR-2014-0128). Ambient Air SO2 Monitoring Network 
Review and Background (January 2024). January 18, 2024. Office of 
Air Quality Planning and Standards, Research Triangle Park, NC. 
Available at https://www.regulations.gov. Document Identifier EPA-
HQ-OAR-2014-0128-0040.
Weaver, C (2024). Memorandum to Secondary NOX/
SOX/PM NAAQS Review Docket (Docket ID No. EPA-HQ-OAR-
2014-0128). List of Studies Identified by Public Commenters That 
Have Been Provisionally Considered in the Context of the Conclusions 
of the 2020 Integrated Science Assessment for the Secondary National 
Ambient Air Quality Standard review of Oxides of Nitrogen, Oxides of 
Sulfur and Particulate Matter. October 16, 2024. Office of Research 
and Development, Research Triangle Park, NC. Available at https://www.regulations.gov. Docket ID No. EPA-HQ-OAR-2014-0128.
WHO (World Health Organization) (2008). WHO/IPCS Harmonization 
Project Document No. 6. Part 1: Guidance Document on Characterizing 
and Communicating Uncertainty in Exposure Assessment. International 
Programme on Chemical Safety. World Health Organization. Geneva, 
Switzerland. Available at: https://www.who.int/ipcs/methods/harmonization/areas/exposure/en/.
Williams, J and Labou, S (2017). A database of georeferenced 
nutrient chemistry data

[[Page 105785]]

for mountain lakes of the Western United States. Sci Data 4: 170069.
Wolff, GT (1993). Letter from George T. Wolff, Chair, Clean Air 
Scientific Advisory Committee to the Honorable Carol M. Browner, 
Administrator, U.S. EPA. Re: Clean Air Scientific Advisory Committee 
Closure on the Air Quality Criteria Document for Oxides of Nitrogen. 
September 30, 1993. EPA-SAB-CASAC-LTR-93-015. Office of the 
Administrator, Science Advisory Board, Washington, DC. Available at: 
https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Wolff, GT (1995). Letter from George T. Wolff, Chair, Clean Air 
Scientific Advisory Committee to the Honorable Carol M. Browner, 
Administrator, Re: CASAC Review of the Staff Paper for the Review of 
the National Ambient Air Quality Standards for Nitrogen Dioxide: 
Assessment of Scientific and Technical Information. August 22, 1995. 
EPA-SAB-CASAC-LTR-95-004. Office of the Administrator, Science 
Advisory Board, Washington, DC. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100FL6Q.PDF.
Wolff, GT (1996). Letter from George T. Wolff, Chair, Clean Air 
Scientific Advisory Committee to the Honorable Carol M. Browner, 
Administrator, Re: Closure by the Clean Air Scientific Advisory 
Committee (CASAC) on the Staff Paper for Particulate Matter. June 
13, 1996. EPA-SAB-CASAC-LTR-96-008. Office of the Administrator, 
Science Advisory Board, Washington, DC. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=9100TTBM.PDF.
Zhou, Q, Driscoll, CT and Sullivan, TJ (2015). Responses of 20 lake-
watersheds in the Adirondack region of New York to historical and 
potential future acidic deposition. Sci Total Environ 511: 186-194.

List of Subjects in 40 CFR Part 50

    Environmental protection, Air pollution control, Nitrogen dioxide, 
Particulate matter, Sulfur oxides.

Michael S. Regan,
Administrator.
    For the reasons set forth in the preamble, the Environmental 
Protection Agency is amending chapter I of title 40 of the Code of 
Federal Regulations as follows:

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY 
STANDARDS

0
1. The authority citation for part 50 continues to read as follows:

    Authority: 42 U.S.C. 7401, et seq.


0
2. Add Sec.  50.21 to read as follows:


Sec.  50.21  National secondary ambient air quality standards for 
sulfur oxides (sulfur dioxide).

    (a) The level of the annual secondary national ambient air quality 
standard for oxides of sulfur is 10 parts per billion (ppb), measured 
in the ambient air as sulfur dioxide (SO2) by a reference 
method based on appendix A-1 and appendix A-2 of this part, or by a 
Federal Equivalent Method (FEM) designated in accordance with part 53 
of this chapter.
    (b) The annual secondary standard is met when the 3-year average of 
the annual SO2 concentration is less than or equal to 10 
ppb, as determined in accordance with appendix T of this part.

0
3. Revise appendix T to part 50 to read as follows:

Appendix T to Part 50--Interpretation of the Primary and Secondary 
National Ambient Air Quality Standards for Oxides of Sulfur (Sulfur 
Dioxide)

1. General

    (a) This appendix explains the data handling conventions and 
computations necessary for determining when the primary and 
secondary national ambient air quality standards for Oxides of 
Sulfur as measured by Sulfur Dioxide (``SO2 NAAQS'') 
specified in Sec.  50.17 are met at an ambient air quality 
monitoring site. Sulfur dioxide (SO2) is measured in the 
ambient air by a Federal reference method (FRM) based on appendix A-
1 or A-2 to this part or by a Federal equivalent method (FEM) 
designated in accordance with part 53 of this chapter. Data handling 
and computation procedures to be used in making comparisons between 
reported SO2 concentrations and the levels of the 
SO2 NAAQS are specified in the following sections.
    (b) Decisions to exclude, retain, or make adjustments to the 
data affected by exceptional events, including natural events, are 
made according to the requirements and process deadlines specified 
in Sec. Sec.  50.1, 50.14 and 51.930 of this chapter.
    (c) The terms used in this appendix are defined as follows:
    Annual mean refers to the annual average of all the daily mean 
values as defined in section 5.2 of this appendix.
    Daily maximum 1-hour values for SO2 refers to the 
maximum 1-hour SO2 concentration values measured from 
midnight to midnight (local standard time) that are used in NAAQS 
computations.
    Daily mean values for SO2 refers to the 24-hour 
average of 1-hour SO2 concentration values measured from 
midnight to midnight (local standard time) that are used in NAAQS 
computations.
    Design values are the metrics (i.e., statistics) that are 
compared to the NAAQS levels to determine compliance, calculated as 
specified in section 5 of this appendix. The design value for the 
primary 1-hour NAAQS is the 3-year average of annual 99th percentile 
daily maximum 1-hour values for a monitoring site (referred to as 
the ``1-hour primary standard design value''). The design value for 
the secondary annual NAAQS is the 3-year average of the annual mean 
of daily mean values for a monitoring site (referred to as the 
``annual secondary standard'').
    99th percentile daily maximum 1-hour value is the value below 
which nominally 99 percent of all daily maximum 1-hour concentration 
values fall, using the ranking and selection method specified in 
section 5.1 of this appendix.
    Pollutant Occurrence Code (POC) refers to a numerical code (1, 
2, 3, etc.) used to distinguish the data from two or more monitors 
for the same parameter at a single monitoring site.
    Quarter refers to a calendar quarter.
    Year refers to a calendar year.

2. Requirements for Data Used for Comparisons With the SO2 NAAQS and 
Data Reporting Considerations

    (a) All valid FRM/FEM SO2 hourly data required to be 
submitted to EPA's Air Quality System (AQS), or otherwise available 
to EPA, meeting the requirements of part 58 of this chapter 
including appendices A, C, and E shall be used in design value 
calculations. Multi-hour average concentration values collected by 
wet chemistry methods shall not be used.
    (b) Data from two or more monitors from the same year at the 
same site reported to EPA under distinct Pollutant Occurrence Codes 
shall not be combined in an attempt to meet data completeness 
requirements. The Administrator will combine annual 99th percentile 
daily maximum concentration values from different monitors in 
different years, selected as described here, for the purpose of 
developing a valid 1-hour primary standard design value. If more 
than one of the monitors meets the completeness requirement for all 
four quarters of a year, the steps specified in section 5.1(a) of 
this appendix shall be applied to the data from the monitor with the 
highest average of the four quarterly completeness values to derive 
a valid annual 99th percentile daily maximum concentration. If no 
monitor is complete for all four quarters in a year, the steps 
specified in sections 3.1(c) and 5.1(a) of this appendix shall be 
applied to the data from the monitor with the highest average of the 
four quarterly completeness values in an attempt to derive a valid 
annual 99th percentile daily maximum concentration. Similarly, the 
Administrator will combine annual means from different monitors in 
different years, selected as described here, for the purpose of 
developing a valid annual secondary standard design value. If more 
than one of the monitors meets the completeness requirement for all 
four quarters of a year, the steps specified in section 5.2(a) of 
this appendix shall be applied to the data from the monitor with the 
highest average of the four quarterly completeness values to derive 
a valid annual mean. If no monitor is complete for all four quarters 
in a year, the steps specified in sections 3.2(c) and 5.2(a) of this 
appendix shall be applied to the data from the monitor with the 
highest average of the four quarterly completeness values in an 
attempt to derive a valid annual mean. This paragraph does not

[[Page 105786]]

prohibit a monitoring agency from making a local designation of one 
physical monitor as the primary monitor for a Pollutant Occurrence 
Code and substituting the 1-hour data from a second physical monitor 
whenever a valid concentration value is not obtained from the 
primary monitor; if a monitoring agency substitutes data in this 
manner, each substituted value must be accompanied by an AQS 
qualifier code indicating that substitution with a value from a 
second physical monitor has taken place.
    (c) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.

3. Comparisons With the NAAQS

3.1 Comparisons With the 1-Hour Primary SO2 NAAQS

    (a) The 1-hour primary SO2 NAAQS is met at an ambient 
air quality monitoring site when the valid 1-hour primary standard 
design value is less than or equal to 75 parts per billion (ppb).
    (b) An SO2 1-hour primary standard design value is 
valid if it encompasses three consecutive calendar years of complete 
data. A year meets data completeness requirements when all four 
quarters are complete. A quarter is complete when at least 75 
percent of the sampling days for each quarter have complete data. A 
sampling day has complete data if 75 percent of the hourly 
concentration values, including State-flagged data affected by 
exceptional events which have been approved for exclusion by the 
Administrator, are reported.
    (c) In the case of one, two, or three years that do not meet the 
completeness requirements of section 3.1(b) of this appendix and 
thus would normally not be useable for the calculation of a valid 3-
year 1-hour primary standard design value, the 3-year 1-hour primary 
standard design value shall nevertheless be considered valid if one 
of the following conditions is true.
    (i) At least 75 percent of the days in each quarter of each of 
three consecutive years have at least one reported hourly value, and 
the design value calculated according to the procedures specified in 
section 5.1 is above the level of the primary 1-hour standard.
    (ii)(A) A 1-hour primary standard design value that is equal to 
or below the level of the NAAQS can be validated if the substitution 
test in section 3.1(c)(ii)(B) of this appendix results in a ``test 
design value'' that is below the level of the NAAQS. The test 
substitutes actual ``high'' reported daily maximum 1-hour values 
from the same site at about the same time of the year (specifically, 
in the same calendar quarter) for unknown values that were not 
successfully measured. Note that the test is merely diagnostic in 
nature, intended to confirm that there is a very high likelihood 
that the original design value (the one with less than 75 percent 
data capture of hours by day and of days by quarter) reflects the 
true under-NAAQS-level status for that 3-year period; the result of 
this data substitution test (the ``test design value,'' as defined 
in section 3.1(c)(ii)(B) of this appendix) is not considered the 
actual design value. For this test, substitution is permitted only 
if there are at least 200 days across the three matching quarters of 
the three years under consideration (which is about 75 percent of 
all possible daily values in those three quarters) for which 75 
percent of the hours in the day, including State-flagged data 
affected by exceptional events which have been approved for 
exclusion by the Administrator, have reported concentrations. 
However, maximum 1-hour values from days with less than 75 percent 
of the hours reported shall also be considered in identifying the 
high value to be used for substitution.
    (B) The substitution test is as follows: Data substitution will 
be performed in all quarter periods that have less than 75 percent 
data capture but at least 50 percent data capture, including State-
flagged data affected by exceptional events which have been approved 
for exclusion by the Administrator; if any quarter has less than 50 
percent data capture then this substitution test cannot be used. 
Identify for each quarter (e.g., January-March) the highest reported 
daily maximum 1-hour value for that quarter, excluding State-flagged 
data affected by exceptional events which have been approved for 
exclusion by the Administrator, looking across those three months of 
all three years under consideration. All daily maximum 1-hour values 
from all days in the quarter period shall be considered when 
identifying this highest value, including days with less than 75 
percent data capture. If after substituting the highest reported 
daily maximum 1-hour value for a quarter for as much of the missing 
daily data in the matching deficient quarter(s) as is needed to make 
them 100 percent complete, the procedure in section 5 yields a 
recalculated 3-year 1-hour standard ``test design value'' less than 
or equal to the level of the standard, then the 1-hour primary 
standard design value is deemed to have passed the diagnostic test 
and is valid, and the level of the standard is deemed to have been 
met in that 3-year period. As noted in section 3.1(c)(i) of this 
appendix, in such a case, the 3-year design value based on the data 
actually reported, not the ``test design value,'' shall be used as 
the valid design value.
    (iii)(A) A 1-hour primary standard design value that is above 
the level of the NAAQS can be validated if the substitution test in 
section 3.1(c)(iii)(B) of this appendix results in a ``test design 
value'' that is above the level of the NAAQS. The test substitutes 
actual ``low'' reported daily maximum 1-hour values from the same 
site at about the same time of the year (specifically, in the same 
three months of the calendar) for unknown hourly values that were 
not successfully measured. Note that the test is merely diagnostic 
in nature, intended to confirm that there is a very high likelihood 
that the original design value (the one with less than 75 percent 
data capture of hours by day and of days by quarter) reflects the 
true above-NAAQS-level status for that 3-year period; the result of 
this data substitution test (the ``test design value,'' as defined 
in section 3.1(c)(iii)(B) of this appendix) is not considered the 
actual design value. For this test, substitution is permitted only 
if there are a minimum number of available daily data points from 
which to identify the low quarter-specific daily maximum 1-hour 
values, specifically if there are at least 200 days across the three 
matching quarters of the three years under consideration (which is 
about 75 percent of all possible daily values in those three 
quarters) for which 75 percent of the hours in the day have reported 
concentrations. Only days with at least 75 percent of the hours 
reported shall be considered in identifying the low value to be used 
for substitution.
    (B) The substitution test is as follows: Data substitution will 
be performed in all quarter periods that have less than 75 percent 
data capture. Identify for each quarter (e.g., January-March) the 
lowest reported daily maximum 1-hour value for that quarter, looking 
across those three months of all three years under consideration. 
All daily maximum 1-hour values from all days with at least 75 
percent capture in the quarter period shall be considered when 
identifying this lowest value. If after substituting the lowest 
reported daily maximum 1-hour value for a quarter for as much of the 
missing daily data in the matching deficient quarter(s) as is needed 
to make them 75 percent complete, the procedure in section 5.1 of 
this appendix yields a recalculated 3-year 1-hour standard ``test 
design value'' above the level of the standard, then the 1-hour 
primary standard design value is deemed to have passed the 
diagnostic test and is valid, and the level of the standard is 
deemed to have been exceeded in that 3-year period. As noted in 
section 3.1(c)(i) of this appendix, in such a case, the 3-year 
design value based on the data actually reported, not the ``test 
design value'', shall be used as the valid design value.
    (d) A 1-hour primary standard design value based on data that do 
not meet the completeness criteria stated in section 3.1(b) of this 
appendix and also do not satisfy section 3.1(c) of this appendix, 
may also be considered valid with the approval of, or at the 
initiative of, the Administrator, who may consider factors such as 
monitoring site closures/moves, monitoring diligence, the 
consistency and levels of the valid concentration measurements that 
are available, and nearby concentrations in determining whether to 
use such data.
    (e) The procedures for calculating the 1-hour primary standard 
design values are given in section 5.1 of this appendix.

3.2 Comparisons With the Annual Secondary SO2 NAAQS

    (a) The annual secondary SO2 NAAQS is met at an 
ambient air quality monitoring site when the valid annual secondary 
standard design value is less than or equal to 10 parts per billion 
(ppb).
    (b) An SO2 annual secondary standard design value is 
valid if it encompasses three consecutive calendar years of complete 
data. A year meets data completeness requirements when all four 
quarters are complete. A quarter is complete when at least 75 
percent of the sampling days for each quarter have complete data. A 
sampling day has complete data if 75 percent of the hourly 
concentration values, including State-flagged data affected

[[Page 105787]]

by exceptional events which have been approved for exclusion by the 
Administrator, are reported.
    (c) In the case of one, two, or three years that do not meet the 
completeness requirements of section 3.2(b) of this appendix and 
thus would normally not be useable for the calculation of a valid 3-
year annual secondary standard design value, the 3-year annual 
secondary standard design value shall nevertheless be considered 
valid if one of the following conditions is true.
    (i) At least 75 percent of the days in each quarter of each of 
three consecutive years have at least one reported hourly value, and 
the design value calculated according to the procedures specified in 
section 5.2 of this appendix is above the level of the secondary 
annual standard.
    (ii)(A) An annual secondary standard design value that is equal 
to or below the level of the NAAQS can be validated if the 
substitution test in section 3.2(c)(ii)(B) of this appendix results 
in a ``test design value'' that is below the level of the NAAQS. The 
test substitutes actual ``high'' reported daily mean values from the 
same site at about the same time of the year (specifically, in the 
same calendar quarter) for unknown or incomplete (less than 75 
percent of hours reported) daily mean values. Note that the test is 
merely diagnostic in nature, intended to confirm that there is a 
very high likelihood that the original design value (the one with 
less than 75 percent data capture of hours by day and of days by 
quarter) reflects the true under-NAAQS-level status for that 3-year 
period; the result of this data substitution test (the ``test design 
value,'' as defined in section 3.2(c)(ii)(B)) of this appendix is 
not considered the actual design value. For this test, substitution 
is permitted only if there are at least 200 days across the three 
matching quarters of the three years under consideration (which is 
about 75 percent of all possible daily values in those three 
quarters) for which 75 percent of the hours in the day, including 
State-flagged data affected by exceptional events which have been 
approved for exclusion by the Administrator, have reported 
concentrations. However, daily mean values from days with less than 
75 percent of the hours reported shall also be considered in 
identifying the high daily mean value to be used for substitution.
    (B) The substitution test is as follows: Data substitution will 
be performed in all quarter periods that have less than 75 percent 
data capture but at least 50 percent data capture, including State-
flagged data affected by exceptional events which have been approved 
for exclusion by the Administrator; if any quarter has less than 50 
percent data capture then this substitution test cannot be used. 
Identify for each quarter (e.g., January-March) the highest reported 
daily mean value for that quarter, excluding State-flagged data 
affected by exceptional events which have been approved for 
exclusion by the Administrator, looking across those three months of 
all three years under consideration. All daily mean values from all 
days in the quarter period shall be considered when identifying this 
highest value, including days with less than 75 percent data 
capture. If after substituting the highest daily mean value for a 
quarter for as much of the missing daily data in the matching 
deficient quarter(s) as is needed to make them 100 percent complete, 
the procedure in section 5 of this appendix yields a recalculated 3-
year annual standard ``test design value'' less than or equal to the 
level of the standard, then the annual secondary standard design 
value is deemed to have passed the diagnostic test and is valid, and 
the level of the standard is deemed to have been met in that 3-year 
period. As noted in section 3.2(c)(i) of this appendix, in such a 
case, the 3-year design value based on the data actually reported, 
not the ``test design value,'' shall be used as the valid design 
value.
    (iii)(A) An annual secondary standard design value that is above 
the level of the NAAQS can be validated if the substitution test in 
section 3.2(c)(iii)(B) of this appendix results in a ``test design 
value'' that is above the level of the NAAQS. The test substitutes 
actual ``low'' reported daily mean values from the same site at 
about the same time of the year (specifically, in the same three 
months of the calendar) for unknown or incomplete (less than 75 
percent of hours reported) daily mean values. Note that the test is 
merely diagnostic in nature, intended to confirm that there is a 
very high likelihood that the original design value (the one with 
less than 75 percent data capture of hours by day and of days by 
quarter) reflects the true above-NAAQS-level status for that 3-year 
period; the result of this data substitution test (the ``test design 
value,'' as defined in section 3.2(c)(iii)(B) of this appendix) is 
not considered the actual design value. For this test, substitution 
is permitted only if there are a minimum number of valid daily mean 
values from which to identify the low quarter-specific daily mean 
values, specifically if there are at least 200 days across the three 
matching quarters of the three years under consideration (which is 
about 75 percent of all possible daily values in those three 
quarters) for which 75 percent of the hours in the day have reported 
concentrations. Only days with at least 75 percent of the hours 
reported shall be considered in identifying the low daily mean value 
to be used for substitution.
    (B) The substitution test is as follows: Data substitution will 
be performed in all quarter periods that have less than 75 percent 
data capture. Identify for each quarter (e.g., January-March) the 
lowest reported daily mean value for that quarter, looking across 
those three months of all three years under consideration. All daily 
mean values from all days with at least 75 percent capture in the 
quarter period shall be considered when identifying this lowest 
value. If after substituting the lowest reported daily mean value 
for a quarter for as much of the missing daily data in the matching 
deficient quarter(s) as is needed to make them 75 percent complete, 
the procedure in section 5.2 of this appendix yields a recalculated 
3-year annual standard ``test design value'' above the level of the 
standard, then the annual secondary standard design value is deemed 
to have passed the diagnostic test and is valid, and the level of 
the standard is deemed to have been exceeded in that 3-year period. 
As noted in section 3.2(c)(i) of this appendix, in such a case, the 
3-year design value based on the data actually reported, not the 
``test design value,'' shall be used as the valid design value.
    (d) An annual secondary standard design value based on data that 
do not meet the completeness criteria stated in section 3.2(b) of 
this appendix and also do not satisfy section 3.2(c) of this 
appendix, may also be considered valid with the approval of, or at 
the initiative of, the Administrator, who may consider factors such 
as monitoring site closures/moves, monitoring diligence, the 
consistency and levels of the valid concentration measurements that 
are available, and nearby concentrations in determining whether to 
use such data.
    (e) The procedures for calculating the annual secondary standard 
design values are given in section 5.2 of this appendix.

4. Rounding Conventions

4.1 Rounding Conventions for the 1-Hour Primary SO2 NAAQS

    (a) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.
    (b) Daily maximum 1-hour values and, therefore, the annual 99th 
percentile of those daily values are not rounded.
    (c) The 1-hour primary standard design value is calculated 
pursuant to section 5.1 of this appendix and then rounded to the 
nearest whole number or 1 ppb (decimals 0.5 and greater are rounded 
up to the nearest whole number, and any decimal lower than 0.5 is 
rounded down to the nearest whole number).

4.2 Rounding Conventions for the Annual Secondary SO2 NAAQS

    (a) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.
    (b) Daily mean values and the annual mean of those daily values 
are not rounded.
    (c) The annual secondary standard design value is calculated 
pursuant to section 5.2 of this appendix and then rounded to the 
nearest whole number or 1 ppb (decimals 0.5 and greater are rounded 
up to the nearest whole number, and any decimal lower than 0.5 is 
rounded down to the nearest whole number).

5. Calculation Procedures

5.1 Calculation Procedures for the 1-Hour Primary SO2 NAAQS

    (a) Procedure for identifying annual 99th percentile values. 
When the data for a particular ambient air quality monitoring site 
and year meet the data completeness requirements in section 3.1(b) 
of this appendix, or if one of the conditions of section 3.1(c) of 
this appendix is met, or if the Administrator exercises the 
discretionary authority in section 3.1(d) of this appendix, 
identification of annual 99th percentile value is accomplished as 
follows.

[[Page 105788]]

    (i) The annual 99th percentile value for a year is the higher of 
the two values resulting from the following two procedures.
    (A) Procedure 1. For the year, determine the number of days with 
at least 75 percent of the hourly values reported.
    (1) For the year, determine the number of days with at least 75 
percent of the hourly values reported including State-flagged data 
affected by exceptional events which have been approved for 
exclusion by the Administrator.
    (2) For the year, from only the days with at least 75 percent of 
the hourly values reported, select from each day the maximum hourly 
value excluding State-flagged data affected by exceptional events 
which have been approved for exclusion by the Administrator.
    (3) Sort all these daily maximum hourly values from a particular 
site and year by descending value. (For example: (x[1], x[2], x[3], 
. . . x[n]). In this case, x[1] is the largest number and x[n] is 
the smallest value.) The 99th percentile is determined from this 
sorted series of daily values which is ordered from the highest to 
the lowest number. Using the left column of table 1, determine the 
appropriate range (i.e., row) for the annual number of days with 
valid data for year y (cny). The corresponding ``n'' 
value in the right column identifies the rank of the annual 99th 
percentile value in the descending sorted list of daily site values 
for year y. Thus, P0.99, y = the nth largest value.
    (B) Procedure 2. For the year, determine the number of days with 
at least one hourly value reported.
    (1) For the year, determine the number of days with at least one 
hourly value reported including State-flagged data affected by 
exceptional events which have been approved for exclusion by the 
Administrator.
    (2) For the year, from all the days with at least one hourly 
value reported, select from each day the maximum hourly value 
excluding State-flagged data affected by exceptional events which 
have been approved for exclusion by the Administrator.
    (3) Sort all these daily maximum values from a particular site 
and year by descending value. (For example: (x[1], x[2], x[3], . . . 
x[n]). In this case, x[1] is the largest number and x[n] is the 
smallest value.) The 99th percentile is determined from this sorted 
series of daily values which is ordered from the highest to the 
lowest number. Using the left column of table 1, determine the 
appropriate range (i.e., row) for the annual number of days with 
valid data for year y (cny). The corresponding ``n'' 
value in the right column identifies the rank of the annual 99th 
percentile value in the descending sorted list of daily site values 
for year y. Thus, P0.99,y = the nth largest value.
    (b) The 1-hour primary standard design value for an ambient air 
quality monitoring site is mean of the three annual 99th percentile 
values, rounded according to the conventions in section 4.1 of this 
appendix.

                                 Table 1
------------------------------------------------------------------------
                                                    P0.99,y is the nth
 Annual number of days with valid data for year    maximum value of the
                  ``y'' (cny)                      year, where n is the
                                                      listed number
------------------------------------------------------------------------
1-100..........................................                        1
101-200........................................                        2
201-300........................................                        3
301-366........................................                        4
------------------------------------------------------------------------

5.2 Calculation Procedures for the Annual Secondary SO2 NAAQS

    (a) When the data for a site and year meet the data completeness 
requirements in section 3.2(b) of this appendix, or if the 
Administrator exercises the discretionary authority in section 
3.2(c), the annual mean is simply the arithmetic average of all the 
daily mean values.
    (b) The annual secondary standard design value for an ambient 
air quality monitoring site is the mean of the annual means for 
three consecutive years, rounded according to the conventions in 
section 4.2 of this appendix.

[FR Doc. 2024-29463 Filed 12-26-24; 8:45 am]
BILLING CODE 6560-50-P`