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

> Briefs, arguments, decisions, and more.

URL: https://www.frixlaw.com/law-library/documents/fr%3A2024-29463

## Record

- **Collection:** Federal Register
- **Document type:** Rule
- **Published:** December 27, 2024
- **Citation:** 89 FR 105692

## Text

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.

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 (SO
X
), and particulate matter (PM), the EPA is revising the existing secondary sulfur dioxide (SO
2
) 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 SO
2
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:
tennant.ginger@epa.gov.

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, SO
X
and PM

1. N Oxides

2. SO
X

3. Related Actions Addressing Acid Deposition

4. Most Recent Review of the Secondary Standards for N Oxides and SO
X

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 SO
X
, 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 SO
X
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, SO
X
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 SO
X

(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 SO
2
NAAQS

A. Background

B. Interpretation of the Secondary SO
2
Standard

IV. Ambient Air Monitoring Network for SO
2

A. Public Comments

B. Conclusion on the Monitoring Network

V. Clean Air Act Implementation Considerations for the Revised Secondary SO
2
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 SO
2
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 SO
X
, N oxides, and PM. Specifically, this document summarizes the background and rationale for the Administrator's final decisions to revise the secondary SO
2
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 SO
X
, N oxides, and PM NAAQS, the EPA has carefully evaluated the currently available scientific literature on the ecological

effects of SO
X
, 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.

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).

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 SO
2
standard is not requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of SO
X
in ambient air, and that it should be revised to an annual average SO
2
standard, averaged over three years, with a level of 10 ppb to provide the requisite protection for the effects of SO
X
, including those related to atmospheric deposition of sulfur (S) compounds in sensitive ecosystems. The Administrator also decided to retain the secondary nitrogen dioxide (NO
2
) and PM standards, without revision. With regard to the secondary NO
2
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 PM
2.5
standard is warranted and that the existing PM
2.5
secondary standard should be retained without revision.

This document additionally includes revisions related to implementation of the proposed secondary SO
2
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 SO
2
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

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.”

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

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.”

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

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 (μg/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, SO
X
, 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, SO
X
and PM

Secondary NAAQS were first established for N oxides, SO
X
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

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.

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 NO
2
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 NO
2
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).

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.

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 NO
2
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 NO
2
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 SO
X
, which was completed in 2012 (see subsection 4 below).

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.

2. SO
X

The EPA first promulgated secondary NAAQS for SO
X
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 SO
2
,
7

and a 3-hour average standard set at 0.5 ppm SO
2
, 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 SO
2
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 SO
2
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 SO
2
concentrations and other public welfare effects, including effects on materials, visibility, soils, and water, the SO
2
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).

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).

In 1979, the EPA announced initiation of a concurrent review of the air quality criteria for SO
X
and PM and plans for development of a combined AQCD for these pollutants (44 FR 56730, October

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: SO
X
, 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 SO
X
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 SO
X
(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 SO
X
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 SO
X
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 SO
2
emissions, ambient air SO
2
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 SO
2
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 SO
2
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 SO
X

In December 2005, the EPA initiated a joint review
8

of the air quality criteria

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 SO
X
to sensitive aquatic and terrestrial ecosystems (77 FR 20218, April 3, 2012).

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).

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).

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 NO
2
and 3-hour average SO
2
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 NO
2
and SO
2
standards identical to the 1-hour primary NO
2
and SO
2
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 NO
2
standard of 100 ppb, as the 98th percentile of 1-hour daily maximum concentrations, averaged over three years, and a SO
2
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 µg/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 µg/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 SO
X
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 μg/m
3
, as an expected annual arithmetic

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, PM
10
). The new primary and secondary standards included (1) a 24-hour standard of 150 μg/m
3
, in terms of one expected exceedance per year, on average over three years and (2) an annual secondary standard of 50 μg/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 μm (PM
2.5
) as the indicator for fine particles. The new secondary PM
2.5
standards were set equal to the primary PM
2.5
standards, in all respects, as follows: (1) an annual standard with a level of 15.0 μg/m
3
, based on the 3-year average of annual arithmetic mean PM
2.5
concentrations from single or multiple community-oriented monitors,
14

and (2) a 24-hour standard with a level of 65 μg/m
3
, based on the 3-year average of the 98th percentile of 24-hour PM
2.5
concentrations at each monitor within an area. The EPA also retained the primary and secondary annual PM
10
standards, without revision, and revised the form of the 24-hour primary and secondary PM
10
standards to be based on the 99th percentile of 24-hour PM
10
concentrations at each monitor in an area.

14
The 1997 annual PM
2.5
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 (PM
2.5
) 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 (PM
10
) pollution but vacated the 1997 PM
10
standards, concluding that the EPA had not provided a reasonable explanation justifying use of PM
10
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 PM
10
standards, leaving the pre-existing 1987 PM
10
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 PM
2.5
standards to 35 μg/m
3
, retained the level of the annual PM
2.5
standards at 15.0 μg/m
3
, and revised the form of the annual PM
2.5
standards by narrowing the constraints on the optional use of spatial averaging. For PM
10
, the EPA revoked the annual standards and retained the 24-hour standards, both with a level of 150 μg/m
3
.

Several parties filed petitions for review of the 2006 p.m. NAAQS decision, with one raising the issue of the secondary PM
2.5
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 PM
2.5
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 PM
2.5
and PM
10
standards, with a revision to the form of the 24-hour PM
2.5
, 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.

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 SO
X
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, SO
X
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 SO
2
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, SO
X
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 SO
X
, 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 (O
3
) 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

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 SO
X
, 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, SO
X
and PM. This information on ecological effects associated with SO
X
, 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 SO
X
(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 SO
X
, 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 (PM
2.5
) is the average of annual mean concentrations for three consecutive years, while the form of the 3-hour secondary NAAQS for SO
2
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

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, SO
X
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 SO
X
. 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 SO
2
standard, with its level of 0.5 ppm, and the annual secondary NO
2
standard, with its level of 0.053 ppm were established in 1971 (36 FR 8186, April 30, 1971). The basis for both the existing SO
2
and NO
2
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 PM
2.5
standard) and 2006 (24-hour PM
2.5
and PM
10
standards)—variously based on consideration of materials damage, visibility impacts, climate effects and ecological effects.
21

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).

The welfare effects evidence for SO
X
in previous reviews indicates a relationship between short- and long-term SO
2
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 SO
2
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 SO
2
(U.S. DHEW, 1969a). However, the available data were not sufficient to establish a quantitative relationship between specific SO
2
concentrations and such effects (38 FR 25679, September 14, 1973). Accordingly, direct effects of SO
X
in ambient air on vegetation are the basis for the existing secondary standard for SO
X
.

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 NO
2
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 PM
2.5
standard, with its level of 35 µg/m
3
, and the 24-hour PM
10
standard, with its level of 150 µg/m
3
(78 FR 3228, January 15, 2013). With regard to the annual PM
2.5
standard, the EPA retained the averaging time and level, set at 15 µg/m
3
, while revising the form to remove the option for spatial averaging consistent with this change to the primary annual PM
2.5
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 SO
X
and their transformation products, which were being addressed in the joint review of the secondary NO
2
and SO
2
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 PM
2.5
standard, provided the requisite protection (78 FR 3086, January 15, 2013).

b. Prior Review of Deposition-Related Effects

In the 2012 review of the NO
2
and SO
2
secondary standards, the EPA recognized that a significant increase in understanding of the effects of N oxides and SO
X
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 SO
X
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 SO
X
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

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 O
3
, 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 SO
X
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 SO
X
, 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 SO
X
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 SO
X
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.

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, NH
X
(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 NH
X
applied in dry deposition modeling. It was recognized that appreciable spatial variability, as well as overall uncertainty, were associated with this factor.

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 SO
X
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-

related effects associated with N oxides and SO
X
, 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 SO
X
, 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 SO
X
, 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 SO
X
, 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 NO
2
and SO
2
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 SO
X
, 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

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 SO
X
, the evaluation for N oxides and PM also considers current information (
e.g.,
spatial and temporal trends) related to the additional air pollutant, ammonia (NH
3
), 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 (SO
X
, 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 NH
3
. Although emissions of N oxides have appreciably declined, NH
3
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 SO
X
, N Oxides and PM

Sulfur dioxide is generally present at higher concentrations in the ambient air than the other gaseous and highly reactive SO
X
(ISA, Appendix 2, section 2.1) and, as a result, SO
2
is the indicator for the existing NAAQS for SO
X
. The main anthropogenic source of SO
2
emissions is fossil fuel combustion (PA, section 2.2.2). Based on the 2020 National Emissions Inventory (NEI), the top three emission sources of SO
2
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, SO
2
can either remain as SO
2
in the gas phase and be transported and/or be dry deposited, or it can be oxidized to form sulfate particles (SO
4
2−
), with modeling studies suggesting that oxidation accounts for more than half of SO
2
removal nationally (PA, section 2.1.1). The rate of SO
2
oxidation accelerates with greater availability of oxidants, which are generally depleted near source stacks. Consequently, oxidization to SO
4
2−
generally occurs in cleaner air downwind of SO
X
sources (2008 ISA, section 2.6.3.1). As SO
4
2−
particles are generally within the fine particle size range, they are a component of PM
2.5
and have an atmospheric lifetime ranging from 2 to 10 days (PA, section 2.1.1). The areas of highest SO
2
and SO
4
2−
deposition are generally near or downwind of SO
X
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 SO
2
, and the SO
4
2−
particles converted from SO
2
, contribute to S deposition, and do so over different time and geographic scales, with dry deposition of SO
2
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, NO
2
, nitric acid (HNO
3
), and particulate nitrate (NO
3
−
). Most N oxides enter the atmosphere as either NO or NO
2
, which are collectively referred to as NO
X
(PA, section 2.1.2). Anthropogenic sources account for the majority of NO
X
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, NO
X
can deposit to the surface or be chemically converted to other gaseous N oxides, including HNO
3
, as well as to particulate NO
3
−
, which may occur in either the fine or coarse particle size range, such that not all particulate NO
3
−
is a component of PM
2.5
. In general, gas phase N oxides tend to have shorter atmospheric lifetimes, either dry depositing (
e.g.,
as HNO
3
) or quickly converting to particulate NO
3
−
, which has a similar atmospheric lifetime as particulate SO
4
2−
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 NH
3
gas (PA, sections 2.1.3 and 2.2.3), which is not a criteria pollutant. The main sources of NH
3
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 (NH
4
+
), 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, SO
X
and NH
3
. Accordingly, PM
2.5
contributing to S and N deposition generally results from chemicals formed in the atmosphere after being emitted (
e.g.,
particulate SO
4
2−
, particulate NO
3
−
, NH
4
+
). The majority of PM
2.5
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 PM
2.5
monitoring sites across the U.S., SO
4
2−
generally comprises no more than about a third of PM
2.5
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 PM
2.5
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 SO
X
, oxides of N, and PM have declined dramatically over the past two decades, continuing a longer-

term trend (PA, section 2.2). Total SO
2
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 NO
X
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 NO
X
and SO
X
emissions, the annual rate of NH
3
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 NO
X
and NH
3
emissions have had ramifications for N deposition patterns across the U.S., as described further below.

The large reductions in SO
X
and NO
X
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 SO
2
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).

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 SO
2
is 0.5 ppm (500 ppb), as a 3-hour average, not to be exceeded more than once per year.

Similarly, design values for the primary SO
2
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 SO
2
concentrations have also declined over this period. Additionally, both peak and mean SO
2
concentrations are higher at source-oriented sites than monitoring locations that are not source-oriented.
25

25
In the 2019-2021 period, the maximum design value for the primary SO
2
standard was 376 ppb at a monitoring site near an industrial park in southeast Missouri. It is important to note that peak and mean SO
2
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 SO
2
values in areas with recurring volcanic eruptions, such as in Hawaii (PA, section 2.4.2).

Regarding NO
2
, 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.

26
Sites in the contiguous U.S. have met the existing NO
2
secondary standard since around 1991 (PA, Figure 2-22).

27
The maximum annual average NO
2
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).

Likewise, the median of the annual average PM
2.5
concentrations also decreased substantially from 2000 to 2021, from 12.8 μg/m
3
to 8 μg/m
3
. The median of the annual 98th percentile 24-hour PM
2.5
concentrations at the more than 1000 sites monitored also decreased, from 32 μg/m
3
in 2000 to 21 μg/m
3
in 2021. Although both the annual average and 98th percentile 24-hour PM
2.5
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 SO
X
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 NO
X
and SO
X
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

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 SO
2
, NO
2
, and PM
2.5
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 SO
2
, 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 NO
2
and PM
2.5
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 (SO
2
, NO
2
, and PM
2.5
) 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 SO
2
, NO
2
and PM
2.5
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 SO
2
and S deposition. The results and associated information for N oxides and PM, however, indicate more variable relationships, both between NO
2
concentrations and N deposition, and between PM
2.5
concentrations with either S or N deposition.

For SO
2
, annual monitored SO
2
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 SO
2
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 SO
2
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 SO
2
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 (SO
2
) 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 SO
4
2−
concentrations and of wet S deposition with the sum of SO
2
+ SO
4
2−
at 27 sites in 27 Class I areas concluded that the correlations for S deposition with particulate SO
4
2−
and total S (
i.e.,
SO
2
+ SO
4
2−
) were lower than what was exhibited for S deposition and SO
2
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 PM
2.5
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 PM
2.5
mass, particulate SO
4
2−
, or total S (SO
4
2−
plus SO
2
) over SO
2
(alone) as an indicator for a secondary NAAQS to address S deposition-related effects.

Both NO
2
and certain components of PM
2.5
(NO
3
−
and NH
4
+
) contribute to N deposition. As is the case for SO
2
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 NO
2
and PM
2.5
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 PM
2.5
mass contribute to N deposition (
i.e.,
NO
3
−
and NH
4
+
) and these comprise less than about 30% of PM
2.5
mass across the U.S., below 5% in some regions (PA, Figure 6-56). As a result of these two factors, the associations between NO
2
concentrations and N deposition, and between PM
2.5
concentrations and N deposition are less robust than what is observed for SO
2
and S deposition. The multi-faceted approach to evaluating these relationships confirmed this expectation. For example, there are

weaker associations of N deposition with NO
2
observations at SLAMS across the U.S. than what is observed in the similar S deposition and SO
2
analysis (PA, section 6.4.2). There is little correlation for N deposition with NO
2
concentrations, as evidenced by a Spearman's correlation coefficient of 0.38, compared to 0.70 for SO
2
and S deposition (PA, Table 6-6 and Table 6-4). Further, the trajectory-based analyses of the relationships between NO
2
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 NO
2
concentrations with N deposition was confirmed by national trends over the past 20 years, where sharp declines in NO
2
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, NO
2
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 (NO
2
) 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 NO
3
−
, and NH
4
+
) over NO
2
or PM
2.5
mass were indicated. Across all analyses, the evidence indicates NO
2
to be a weak indicator of total atmospheric N deposition, especially in areas where NH
3
is prevalent and where PM
2.5
mass is dominated by species other than NO
3
−
or NH
4
+
(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, SO
X
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 SO
X
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 SO
2
, NO
2
, NO, HNO
3
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 SO
X
and N oxides.

The current scientific evidence continues to be sufficient to infer a causal relationship between gas-phase SO
2
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), SO
2
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 SO
2
-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 SO
2
, 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 SO
2
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, NO
2
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, NO
2,
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, NO
2
, 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 HNO
3
, the evidence is sufficient to infer a causal relationship between exposure to HNO
3
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 HNO
3
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 HNO
3
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 SO
X
and N oxide emissions to ambient air,

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 (HNO
3
and sulfuric acid [H
2
SO
4
]), NH
4
+
, and by chemical and biological processes. When NO
3
−
, or SO
4
2
−
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 [Mg
2+
], 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 SO
4
2−
or NO
3
−
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 SO
4
2−
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 indicato

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A2024-29463. Public record. Not legal advice.