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

Federal RegisterApr 15, 2024

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2014-0128; FRL-5788-02-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:

Proposed rule.

SUMMARY:

Based on the Environmental Protection Agency's (EPA's) review of the air quality criteria and national ambient air quality standards (NAAQS) for oxides of nitrogen (N oxides), oxides of sulfur (SO

), and particulate matter (PM), the Environmental Protection Agency (EPA) proposes to revise the existing secondary sulfur dioxide (SO

) standard to an annual average, averaged over three consecutive years, with a level within the range from 10 to 15 parts per billion (ppb). Additionally, the Agency proposes to retain the existing secondary standards for N oxides and PM, without revision. The EPA also proposes revisions to the data handling requirements for the proposed secondary SO

NAAQS.

DATES:

Comments must be received on or before June 14, 2024.

Public Hearings:

The EPA will hold a virtual public hearing on this proposed rule. This hearing will be announced in a separate

Federal Register

notice that provides details, including specific dates, times, and contact information for these hearings.

ADDRESSES:

You may submit comments, identified by Docket ID No. EPA-HQ-OAR-2014-0128, by any of the following means:

•

Federal eRulemaking Portal: https://www.regulations.gov/

(our preferred method). Follow the online instructions for submitting comments.

•

Email: a-and-r-Docket@epa.gov.

Include the Docket ID No. EPA-HQ-OAR-2014-0128 in the subject line of the message.

•

Mail:

U.S. Environmental Protection Agency, EPA Docket Center, Air and Radiation Docket, Mail Code 28221T, 1200 Pennsylvania Avenue NW, Washington, DC 20460.

•

Hand Delivery or Courier (by scheduled appointment only):

EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution Avenue NW, Washington, DC 20004. The Docket Center's hours of operations are 8:30 a.m.-4:30 p.m., Monday-Friday (except Federal Holidays).

Instructions:

All submissions received must include the Docket ID No. for this document. Comments received may be posted without change to

https://www.regulations.gov,

including any personal information provided. For detailed instructions on sending comments and additional information on the rulemaking process, see the

SUPPLEMENTARY INFORMATION

section of this document.

FOR FURTHER INFORMATION CONTACT:

Ms. Ginger Tennant, Health and Environmental Impacts Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Mail Code C539-04, Research Triangle Park, NC 27711; telephone: (919) 541-4072; email:

tennant.ginger@epa.gov.

SUPPLEMENTARY INFORMATION:

General Information

Preparing Comments for the EPA

Follow the online instructions for submitting comments. Once submitted to the Federal eRulemaking Portal, comments cannot be edited or withdrawn. The EPA may publish any comment received to its public docket. Do not submit electronically any information you consider to be Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Multimedia submissions (audio, video, etc.) must be accompanied by a written submission. The written comment is considered the official comment and should include discussion of all points you wish to make. The EPA will generally not consider comments or comment contents located outside of the primary submission (

i.e.,

on the web, the cloud, or other file sharing system). For additional submission methods, the full EPA public comment policy, information about CBI or multimedia submissions, and general guidance on making effective comments, please visit

https://www.epa.gov/dockets/commenting-epa-dockets.

When submitting comments, remember to:

• Identify the action by docket number and other identifying information (subject heading,

Federal Register

date and page number).

• Explain why you agree or disagree, suggest alternatives, and substitute language for your requested changes.

• Describe any assumptions and provide any technical information and/or data that you used.

• Provide specific examples to illustrate your concerns and suggest alternatives.

• Explain your views as clearly as possible, avoiding the use of profanity or personal threats.

• Make sure to submit your comments by the comment period deadline identified.

Availability of Information Related to This Action

All documents in the dockets pertaining to this action are listed on the

www.regulations.gov

website. This includes documents in the docket for the proposed decision (Docket ID No. EPA-HQ-OAR-2014-0128) and a separate docket, established for the Integrated Science Assessment (ISA) (Docket ID No. EPA-HQ-ORD-2013-0620) that has been incorporated by reference into the docket for this proposed decision. 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 may be viewed with prior arrangement with the EPA Docket Center. Additionally, a number of the documents that are relevant to this proposed decision are available through the EPA's website at

https://www.epa.gov/naaqs/.

These documents include the Integrated Science Assessment for Oxides of Nitrogen, Oxides of Sulfur and Particulate Matter Ecological Criteria (U.S. EPA, 2020a), available at

https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=349473,

and the Policy Assessment for the Review of the Secondary National Ambient Air Quality Standards for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter, (U.S. EPA, 2024), available at

https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards.

Table of Contents

The following topics are discussed in this preamble:

Executive Summary

I. Background

A. Legislative Requirements

B. Related Control Programs

C. History of the Secondary Standards for N Oxides, SO

and PM

1. N Oxides

2. SO

3. Related Actions Addressing Acid Deposition

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

5. PM

D. Current Review

II. Rationale for Proposed Decisions

A. Introduction

1. Basis for Existing Secondary Standards

2. Prior Review of Deposition-Related Effects

3. General Approach for This Review

B. Air Quality and Deposition

1. Sources, Emissions and Atmospheric Processes Affecting SO

, N Oxides and PM

2. Recent Trends in Emissions, Concentrations and Deposition

3. Relationships Between Concentrations and Deposition

C. Welfare Effects Evidence

1. Nature of Effects

a. Direct Effects of SO

and N Oxides

b. Acid Deposition-Related Ecological Effects

c. Nitrogen Enrichment and Associated Ecological Effects

d. Other Deposition-Related Effects

2. Public Welfare Implications

3. Exposure Conditions and Deposition-Related Metrics

a. Acidification and Nitrogen Enrichment in Aquatic Ecosystems

b. Deposition-Related Effects in Terrestrial Ecosystems

c. Direct Effects of N Oxides, SO

and PM in Ambient Air

D. Quantitative Exposure and Risk Assessment for Aquatic Acidification

1. Key Design Aspects

2. Key Limitations and Uncertainties

3. Summary of Results

E. Proposed Conclusions

1. Evidence and Exposure/Risk-Based Considerations in the Policy Assessment

a. Direct Effects on Biota

b. Evidence of Ecosystem Effects of S and N Deposition

c. Sulfur Deposition and SO

d. Nitrogen Deposition and N Oxides and PM

2. CASAC Advice and Public Comments

3. Administrator's Proposed Conclusions

F. Proposed Decision on the Secondary Standards

III. Interpretation of the Secondary SO

Standard

A. Background

B. Interpretation of the Secondary SO

Standard

IV. Ambient Air Monitoring Network for SO

V. Clean Air Act Implementation Requirements for Proposed Secondary SO

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 Proposed Secondary SO

Standard

D. Alternative PSD Compliance Demonstration Approach for the Proposed Secondary SO

Standard

E. Transportation Conformity Program

F. General Conformity Program

VI. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

B. Paperwork Reduction Act (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

References

Executive Summary

This document presents the Administrator's proposed decisions in the current review of the secondary NAAQS for SO

, N oxides, and PM. The existing secondary standards are: for SO

, 0.5 ppm as a 3-hour average not to be exceeded more than once in a year; for NO

, 53 ppb as an annual average; for PM

2.5

, 15.0 µg/m

as the 3-year average of annual averages, and 35 µg/m

as the 3-year average of annual 98th percentile 24-hour averages; and, for PM

, 150 µg/m

as a 24-hour average, not to be exceeded more than once per year on average over three years. Sections 108 and 109 of the Clean Air Act (CAA, the Act) 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.” In conducting this review of the secondary SO

, N oxides, and PM NAAQS, the EPA has carefully evaluated the currently available scientific literature on the ecological effects of SO

, N oxides, and PM,

focusing particularly on the new literature available since the conclusion of the previous reviews in 2012 and 2013, respectively, as described in the Integrated Science Assessment (ISA). The ecological effects addressed in this review include direct effects of N oxides and SO

, and PM loading, on vegetation surfaces, as well as ecological effects related to atmospheric deposition of S and N compounds in sensitive ecosystems.

The ecological effects of PM that are the focus of this action were not considered in EPA's recently completed reconsideration of the primary and secondary NAAQS for PM. In the review of the PM secondary standards completed in 2020, and reconsidered more recently, the EPA considered effects on visibility and climate and materials damage, but did not consider the ecological effects that are addressed here (89 FR 16202, March 6, 2024).

Sulfur oxides and N oxides, their transformation products (which include particulate compounds), and N- and S-containing components of PM in ambient air can contribute to atmospheric deposition of S and N compounds. Emissions of SO

, N oxides, PM and PM precursors have declined dramatically over the past two decades, continuing a longer-term trend. In response to the reductions in S- and N-containing compounds, levels of S and N deposition have also been reduced, although the declining trend in N deposition in the last decade has slowed and, in some areas, reversed, due to increasing ammonia emissions.

The Administrator's proposed decision in this review is to revise the existing secondary SO

standard and to retain the existing secondary standards for N oxides and PM. In this document, the EPA summarizes the background and rationale for the Administrator's proposed decisions in this review. The EPA solicits comment on the proposed decisions described here and on a number of alternate options, and requests commenters also provide the rationales supporting the views articulated in submitted comments.

The Administrator's proposed 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 advice from the Clean Air Scientific Advisory Committee (CASAC).

In conveying its advice in this review, the CASAC provided consensus advice that the existing SO

and NO

secondary standards were adequate to protect against direct effects of S and N oxides on plants and lichens. With regard to deposition-related effects and SO

, the majority of CASAC recommended an annual secondary standard of 10-15 ppb, and the minority recommended a secondary standard identical to the existing primary standard. In consideration of deposition-related effects and the NO

and PM

2.5

secondary standards, the

CASAC majority recommended revision of the levels of the existing annual NO

and PM

2.5

secondary standards, and the minority recommended adopting secondary standards identical to the existing annual NO

and PM

2.5

primary standards.

Over the course of this review, the EPA developed planning documents, an ISA and a PA, drafts of which were made available for public comment and reviewed by the CASAC Oxides of Nitrogen, Oxides of Sulfide and Particulate Matter Secondary NAAQS Panel (

https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards

Based on his consideration of the ecological effects evidence in the ISA, the evaluations and quantitative information in the PA, including the quantitative REA for aquatic acidification, and advice from the CASAC, the Administrator is proposing that the current short-term secondary SO

standard is not requisite to protect the public welfare from known or anticipated adverse effects associated with the presence of SO

in ambient air, including particularly deposition-related effects, and that it should be revised to also provide such protection against effects related to deposition of sulfur (S) compounds to ecosystems. Specifically, the EPA is proposing to revise the existing standard to be an annual average standard, averaged over three years, with a level within the range from 10 to 15 parts per billion (ppb) based on the Administrator's proposed judgment that a standard in this range would provide protection for both direct effects on vegetation surfaces and ecosystem deposition-related effects. The EPA solicits comments on this proposal, including the averaging time, form and range of levels for the revised standard. The EPA also solicits comments on a number of alternative options for a new secondary SO

standard. The EPA solicits comment on setting the level for a new annual average standard (averaged over three years) in the range from 5 to 10 ppb, and on revising the existing secondary standard to be identical to the existing primary standard in all respects. Further, the EPA solicits comments on retaining the existing 3-hour standard, in addition to establishing a new annual secondary standard.

The Administrator is also proposing to retain the secondary nitrogen dioxide (NO

) and PM standards, without revision. With regard to the secondary NO

standard, the Administrator finds that the evidence related to the N oxides, NO

and nitrogen oxide (NO), does not clearly call into question the adequacy of protection provided by the existing standard for direct effects or for deposition-related effects (in light of the currently diminished role of N oxides in total N deposition, particularly in areas with highest deposition), such that revision is not warranted. The EPA solicits comment on the proposed decision to retain the existing secondary NO

standard, without revision, and also on the alternative approach of revising the form of the existing standard to a 3-year average and the level to a value within the range from 35 to 40 ppb.

With regard to PM, the Administrator proposes to conclude that the current evidence does not call into question the adequacy of the existing PM standards with regard to direct effects and deposition of pollutants other than S and N compounds. Further, he judges that protection of sensitive ecosystems from S deposition is more effectively achieved through a revised SO

standard than a PM standard, and that a revised PM standard is not warranted to provide public welfare protection against adverse effects related to S or N deposition. The Administrator additionally proposes to conclude that PM

2.5

is not an appropriate indicator for a secondary standard intended to provide protection of the public welfare from adverse effects related to S or N deposition. Thus, based on consideration of the PA analyses and conclusions, and consideration of CASAC advice, the Administrator proposes to conclude that no change to the annual PM

2.5

secondary standard is warranted and he proposes to retain the existing PM

2.5

secondary standard, without revision. The EPA solicits comment on the proposed decision. Additionally, in recognizing that there may be alternate views with regard to whether and to what extent a standard with a PM

2.5

indicator might be expected to provide control of N deposition, and in light of the rationale provided by the CASAC minority, the EPA also solicits comment on the alternative approach of revising the secondary PM

2.5

(with PM

2.5

referring to particles with a nominal mean aerodynamic diameter less than or equal to 2.5 micrometers) annual standard to a level of 12 micrograms per meter cubed (µg/m

). With regard to other PM secondary standards, based on evaluations and conclusions of the PA, including consideration of recommendations from the CASAC, the Administrator proposes to retain the existing 24-hour secondary PM

2.5

standard, without revision. Further, based on the lack of evidence calling into question the adequacy of the secondary PM

standards for protection of ecological effects, he also proposes to retain the secondary PM

standards without revision.

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

annual standard. Specifically, the EPA is proposing revisions to the data handling requirements in appendix T of 40 CFR part 50 to include specifications needed for the proposed new annual average standard. This document also describes the SO

monitoring network and its adequacy for surveillance for the proposed annual standard. Lastly, the document discusses implementation processes pertinent to implementation of the proposed 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.”

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

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

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,

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.

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), parts per billion (ppb), or micrograms per cubic meter (μg/m

). 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.

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

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 the new source performance standards under section 111 of the Act, 42 U.S.C. 7411.

C. History of the Secondary Standards for N Oxides, SO

and PM

Secondary NAAQS were first established for N oxides, SO

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.

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).

With regard to welfare effects, the 1971 AQCD described effects of NO

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 parts per million (ppm) NO

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).

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,

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

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

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

, which was completed in 2012 (see subsection 4 below).

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

The EPA first promulgated secondary NAAQS for sulfur oxides 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

and a 3-hour average standard set at 0.5 ppm SO

, 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

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

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

concentrations and other public welfare effects, including effects on materials, visibility, soils, and water, the SO

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).

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 oxides of sulfur 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: oxides of sulfur, oxides of nitrogen, 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 oxides of sulfur 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

(53 FR 14926, April 26, 1988). This proposed decision with regard to the secondary SO

NAAQS was due to the Administrator's conclusions that: (1) based upon the then-current scientific understanding of the acid deposition problem, it would be premature and unwise to prescribe any regulatory control program at that time; and (2) when the fundamental scientific uncertainties had been decreased through ongoing research efforts, the EPA would draft and support an appropriate set of control measures (53 FR 14926, April 26, 1988). This review of the secondary standard for SO

was concluded in 1993, subsequent to the Clean Air Act Amendments of 1990 (see section I.C.3). The EPA decided not to revise the secondary standard, concluding that revisions to the standard to address acidic deposition and related SO

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

emissions, ambient air SO

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 (NAPAP). 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

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 Sand 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

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

In December 2005, the EPA initiated a joint review

of the air quality criteria for oxides of nitrogen and sulfur and the secondary NAAQS for NO

and SO

(70 FR 73236, December 9, 2005).

The review focused on the evaluation of the protection provided by the secondary standards for oxides of nitrogen and oxides of sulfur 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 NO

and SO

secondary standards were developed to protect against, and (2) effects associated with the deposition of oxides of nitrogen and sulfur to sensitive aquatic and terrestrial ecosystems (77 FR 20218, April 3, 2012).

Although the EPA has historically adopted separate secondary standards for oxides of nitrogen and oxides of sulfur, the EPA conducted a joint review of these standards because oxides of nitrogen and sulfur and their associated transformation products are linked from an atmospheric chemistry perspective, as well as from an environmental effects perspective. The joint review was also responsive to the National Research Council (NRC) recommendation for the EPA to consider multiple pollutants, as appropriate, in forming the scientific basis for the NAAQS (NRC, 2004).

The review was conducted under a schedule specified by consent decree entered into by the EPA with the Center for Biological Diversity and four other plaintiffs. The schedule, which was revised on October 22, 2009, provided that the EPA sign notices of proposed and final rulemaking concerning its review of the oxides of nitrogen and oxides of sulfur NAAQS no later than July 12, 2011, and March 20, 2012, respectively.

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 final ISA (referred to as 2008 ISA below) was released 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),

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).

Although the REA for the 2012 review was presented in its own separate document, the REA for a NAAQS review may be presented in its own separate document or as one or more appendices in the PA (

e.g.,

U.S. EPA 2020b, 2020c, and PA for current review [U.S. EPA, 2024]).

Drawing on the information in the final 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). Based on additional discussion subsequent to release 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).

For the purpose of protection against the direct effects on vegetation of exposure to gaseous oxides of nitrogen and sulfur, the 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.

On August 1, 2011, the EPA published a proposed decision to retain the existing annual average NO

and 3-hour average SO

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. Additionally, the Administrator proposed to revise the secondary standards by adding secondary NO

and SO

standards identical to the 1-hour primary NO

and SO

standards, both set in 2010, noting that these new primary standards

would result in reductions in oxides of nitrogen and sulfur that would likely reduce nitrogen and sulfur deposition to sensitive

ecosystems (76 FR 46084, August 1, 2011). After consideration of public comments, the final decision in the review was to retain the existing standards to address the direct effects on vegetation of exposure to gaseous oxides of nitrogen and sulfur and to not set additional standards particular to effects associated with deposition of oxides of nitrogen and sulfur on sensitive aquatic and terrestrial ecosystems at that time (77 FR 20218, April 3, 2012).

The 1-hour primary standards set in 2010 included the NO

standard of 100 ppb, as the 98th percentile of 1-hour daily maximum concentrations, averaged over three years, and the SO

standard of 75 ppb, as the 99th percentile of 1-hour daily maximum 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. The petitioners argued that having decided that the existing standards were not adequate to protect against adverse public welfare effects such as damage to sensitive ecosystems, the Administrator was required to identify the requisite level of protection for the public welfare and to issue a 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 the 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 . . .'.”

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.

Center for Biological Diversity, et al.

EPA,

749 F.3d 1079, 1087 (2014).

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

, 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).

Additionally, a guide to be used in assessing implementation plans to achieve the 24-hour standard was set at 60 µg/m

, as an annual geometric mean (36 FR 8187; April 30, 1971).

In October 1979, the EPA announced the first periodic review of the air quality criteria and NAAQS for PM (44 FR 56730, October 2, 1979). As summarized in subsection 2 above, the EPA developed a new AQCD for PM and SO

, drafts of which were reviewed by the CASAC (U.S. EPA, 1982b). Subsequently, the EPA OAQPS developed a Staff Paper (U.S. EPA, 1982d), two drafts of which were reviewed by the CASAC (Friedlander, 1982). Further, the EPA OAQPS prepared an Addendum to the 1982 Staff Paper, which also received CASAC review (Lippman, 1986; U.S. EPA, 1986). After consideration of public comments on a proposed decision, the final decision in that review revised the indicator for PM NAAQS from TSP to particulate matter with mass median diameter of 10 microns (PM

) (49 FR 10408, March 20, 1984; 52 FR 24634, July 1, 1987). With an indicator of PM

, two secondary standards were established to be the same as the primary standards. A 24-hour secondary standard was set at 150 µg/m

, with the form of one expected exceedance per year, on average over three years. Additionally, an annual secondary standard was set at 50 µg/m

, with a form of 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 EPA's OAQPS prepared a Staff Paper that was released in November 1997, after CASAC and public review of two drafts (U.S. EPA, 1996; Wolff, 1996). Revisions to the PM standards were proposed in 1996, and in 1997 the EPA promulgated final revisions (61 FR 65738; December 13, 1996; 62 FR 38652, July 18, 1997). With the 1997 decision, the EPA added new standards, using PM

2.5

as the indicator for fine particles. The new secondary standards were set equal to the primary standards, in all respects, as follows: (1) an annual standard with a level of 15.0 µg/m

, based on the 3-year average of annual arithmetic mean PM

2.5

concentrations from single or multiple community-oriented monitors;

and (2) a 24-hour standard with a level of 65 µg/m

, based on the 3-year average of the 98th percentile of 24-hour PM

2.5

concentrations at each monitor within an area. Further, the EPA retained the annual PM

standard, without revision, and revised the form of the 24-hour PM

standard to be based on the 99th percentile of 24-hour PM

concentrations at each monitor in an area.

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 p.m. 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 standards, (

American Trucking Ass'ns, Inc.

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 pollution, but vacated the 1997 PM

standards, concluding that the EPA had not provided a reasonable explanation justifying use of PM

as an indicator for coarse particles (

at 1054-55). Pursuant to the D.C. Circuit's decision, the EPA removed the vacated 1997 PM

standards, leaving the pre-existing 1987 PM

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 (

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). After the CASAC and public review of several drafts of the AQCD, the EPA released the final AQCD in October 2004 (U.S. EPA, 2004a, b). The EPA's OAQPS finalized the Staff Paper in December 2005 (U.S. EPA, 2005). On December 20, 2005, the EPA announced its proposed decision to revise the NAAQS for PM and solicited public comment on a broad range of options (71 FR 2620, January 17, 2006). On September 21, 2006, the EPA announced its final decisions to revise the PM NAAQS to provide increased protection of public health and welfare (71 FR 61144, October 17, 2006). Revisions to the secondary standards were 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 61203-61210, October 17, 2006). With regard to the standards for fine particles, the EPA revised the level of

the 24-hour PM

2.5

standards to 35 µg/m

, retained the level of the annual PM

2.5

standards at 15.0 µg/m

, and revised the form of the annual PM

2.5

standards by narrowing the constraints on the optional use of spatial averaging. With regard to the standards for PM

, the EPA retained the 24-hour standards, with levels at 150 µg/m

, and revoked the annual standards.

Several parties filed petitions for review of the EPA's 2006 p.m. NAAQS decision. One of these petitions raised the issue of setting the secondary PM

2.5

standards identical to the primary standards. On February 24, 2009, the D.C. Circuit issued its opinion in

American Farm Bureau Federation

EPA,

559 F. 3d 512 (D.C. Cir. 2009) and remanded the standards to the EPA because the Agency failed to adequately explain why setting the secondary PM standards identical to the primary standards provided the required protection for public welfare, including protection from visibility impairment (

Id.

at 528-32). The EPA responded to the court's remands as part of the subsequent review of the PM NAAQS, which was initiated in 2007.

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). Based on the NAAQS review process, as revised in 2008 and again in 2009, the EPA held science/policy issue workshops on the primary and secondary PM NAAQS (72 FR 34003, June 20, 2007; 72 FR 34005, June 20, 2007), and prepared and released the planning and assessment documents that are part of the review process (

i.e.,

IRP [U.S. EPA, 2008b], ISA [U.S. EPA, 2009b], REA planning document for welfare [U.S. EPA, 2009c], and an urban-focused visibility assessment [U.S. EPA, 2010], and PA [U.S. EPA, 2011]). In June 2012, the EPA announced its proposed decision to revise the NAAQS for PM (77 FR 38890, June 29, 2012). In December 2012, the EPA announced its final decisions to revise the primary and secondary PM

2.5

annual standards (78 FR 3086, January 15, 2013). With regard to the secondary standards, the EPA retained the 24-hour PM

2.5

and PM

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

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 newly initiated review of the air quality criteria for oxides of sulfur and oxides of nitrogen 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 linkages between these pollutants (oxides of nitrogen, oxides of sulfur and PM) with respect to atmospheric transformation of N and S oxides into particulate compounds, deposition of N and S compounds and associated ecological effects (U.S. EPA, 2017). Addressing the pollutants together enables a comprehensive consideration of the nature and interactions of the pollutants, which is important for ensuring thorough evaluation of the scientific information relevant to ecological effects of N and S deposition.

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 a public meeting on May 24-25, 2017 (82 FR 15701, March 30, 2017) and August 31, 2017 (82 FR 35200, July 28, 2017; Diez Roux and Fernandez, 2017). With consideration of comments from the CASAC and the public, the EPA released a second external review draft (83 FR 29786, June 26, 2018), which the CASAC reviewed at public meetings on September 5-6, 2018 (83 FR 2018; July 9, 2018) and April 27, 2020 (85 FR 16093, March 30, 2020; Cox, Kendall, and Fernandez, 2020a).

The EPA released the final ISA in October 2020 (85 FR 66327, October 19, 2020; U.S. EPA, 2020a). In planning for quantitative aquatic acidification exposure/risk analyses for consideration in the PA, the EPA solicited public comment and consulted with the CASAC (83 FR 31755, July 9, 2018; Cox, Kendall, and Fernandez, 2020b; U.S. EPA, 2018; 83 FR 42497, August 22, 2018).

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

The draft PA, including the REA for aquatic acidification as an appendix, was completed in May 2023 and was made available for review by the CASAC and for public comment (88 FR 34852, May 31, 2023). The CASAC review was conducted at public meetings held on June 28-29, 2023 (88 FR 17572, March 23, 2023), and September 5-6, 2023 (88 FR 45414, July 17, 2023). The CASAC conveyed advice on the standards and comments on the draft PA in its September 27, 2023, letter to the Administrator (Sheppard, 2023). The final PA was completed in January 2024 (89 FR 2223, January 12, 2024).

Materials upon which this proposed decision is based, including the documents described above, are available to the public in the docket for this review.

The timeline for the remainder of this review is governed by a consent decree that requires the EPA to sign a notice of proposed decision by April 9, 2024, and a final decision notice by December 10, 2024 (

Center for Biological Diversity

Regan

[No. 4:22-cv-02285-HSG (N.D. Cal.]).

The docket for this review, EPA-HQ-OAR-2014-0128, has incorporated the ISA docket (EPA-HQ-ORD-2013-0620) by reference. Both are publicly accessible at

www.regulations.gov.

II. Rationale for Proposed Decisions

This section presents the rationale for the Administrator's proposed decisions in the review of the secondary standards for the ecological effects of SO

, 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 review of the secondary standards for N oxides and SO

, which is generally published between January 2008 and May 2017 (and considered in the ISA), as well as more recent studies identified during peer review or by public

comments (ISA, section IS.1.2),

integrated with the information and conclusions from previous assessments and presented in the ISA, on ecological effects associated with SO

, N oxides and PM and pertaining to their presence in ambient air. 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; and (3) public comments received during the development of these documents.

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 last ISA) and May 2017. Studies published after the literature cutoff date for this ISA were also considered 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).

In presenting the rationale for the Administrator's proposed decisions and their foundations, section II.A provides background on the general approach in this review, including a summary of the basis for the existing standards (section II.A.1), a summary of the prior review of the SO

and N oxides standards for deposition-related effects (section II.A.2) and the general approach for the current review (section II.A.3).

Section II.B summarizes air quality information and analyses relating S and N deposition to concentrations of SO

, N oxides and PM. Section II.C summarizes the currently available ecological effects evidence as summarized in the ISA, focusing on consideration of key policy-relevant aspects. Section II.D summarizes the exposure and risk information for this review, drawing on the quantitative analyses of aquatic acidification risk, presented in the PA. Section II.E presents the Administrator's proposed conclusions on the current standards and potential alternatives (section II.E.3), drawing on both evidence-based and exposure/risk-based considerations from the PA (section II.E.1) and advice from the CASAC (section II.E.2).

A. Introduction

As is the case for all such reviews, this review is based, most fundamentally, on using the Agency's assessments of the current scientific evidence and associated quantitative analyses to inform the Administrator's judgment regarding secondary standards for SO

, 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 EPA's assessments are primarily documented in the ISA and PA, both of which have received CASAC review and public comment (82 FR 15702, March 30, 2017; 82 FR 15701, March 30, 2018; 83 FR 29786; June 26, 2018; 83 FR 31755, July 9, 2018; 85 FR 16093; March 20, 2020; 88 FR 34852, May 31, 2023; 88 FR 17572, March 23, 2023; 88 FR 45414, July 17, 2023). In bridging the gap between the scientific assessments of the ISA and the judgments required of the Administrator in his decisions on the current standard, the PA evaluates policy implications of the assessment of the current evidence in the ISA and the quantitative exposure and risk analyses and information documented in the PA. In evaluating the public welfare protection afforded by the current standard, the four basic elements of the NAAQS (indicator, averaging time, level, and form) are considered collectively.

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

is the second highest 3-hour average in a year. The level of the standard defines the air quality concentration used for that purpose.

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 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 so as 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 public welfare. A public welfare policy decision draws upon scientific information and analyses about welfare effects, exposure and risks, as well as judgments about the appropriate response to the range of uncertainties that are inherent in the scientific evidence and analyses. The ultimate determination as to what level of damage to ecosystems and the services provided by those ecosystems is adverse to public welfare is not wholly a scientific question, although it is informed by scientific studies linking ecosystem damage to losses in ecosystem services and information on the value of those losses of ecosystem services. In reaching such decisions, the Administrator seeks to establish standards that are neither more nor less stringent than necessary for this purpose.

Thus, in general, 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 the 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. Basis for Existing Secondary Standards

In the last review of the secondary standards for SO

and N oxides, completed in 2012, the EPA retained the existing 3-hour SO

standard, with its level of 0.5 ppm, and the annual NO

standard, with its level of 0.053 ppm (77 FR 20218, April 3, 2012). Both of these secondary standards were established in 1971 (36 FR 8186, April 30, 1971). The basis for both the existing SO

and NO

secondary standard is to provide protection to the public welfare related to direct effects on vegetation (U.S. DHEW, 1969a; U.S. EPA, 1971).

The welfare effects evidence for SO

in previous reviews indicates a relationship between short- and long-term SO

exposures and foliar damage to cultivated plants, reductions in productivity, species richness, and diversity (U.S. DHEW, 1969a; U.S. EPA, 1982c; U.S. EPA, 2008). At the time the standard was set, concentrations of SO

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

(U.S. DHEW, 1969a). However, the available data were not sufficient to establish a quantitative relationship between specific SO

concentrations and such effects (38 FR 25679, September 14, 1973). Accordingly, direct effects of SO

in ambient air on vegetation is the basis for the existing secondary standard for SO

. Effects on materials and visibility (which relate to particles in air, including sulfates) have more recently been considered in the PM secondary NAAQS reviews (

e.g.,

85 FR 82684, December 18, 2020).

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

secondary standard provided adequate protection related to the “direct” effects of airborne N oxides on 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

, and the 24-hour PM

standard, with its level of 150 µg/m

(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

, 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 2013 review considered the PM standards with regard to protection for an array of effects that include effects on visibility, materials damage, and climate effects, as well as ecological effects, and 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

and their transformation products, which were being addressed in the joint review of the secondary NO

and SO

NAAQS (78 FR 3202, January 15, 2013). The ecological effects of PM considered include 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).

2. Prior Review of Deposition-Related Effects

In the 2012 review of the NO

and SO

secondary standards, the EPA recognized that a significant increase in understanding of the effects of N oxides and SO

had occurred since the prior secondary standards reviews for those pollutants (77 FR 20236, April 3, 2012). Considering the extensive evidence available at that time, the Agency concluded that the most significant risks of adverse effects of N oxides and SO

to public welfare were those related to deposition of N and S compounds to 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

from effects associated with the airborne pollutants, the 2012 review also included extensive analyses of the welfare effects associated with nitrogen and sulfur deposition to sensitive aquatic and terrestrial ecosystems (77 FR 20218, April 3, 2012).

Based on the available evidence, the risks of atmospheric deposition analyzed in the 2009 REA related to two categories of ecosystem effects: acidification and nutrient enrichment (U.S. EPA, 2009a). The analyses included assessment of risks of both types of effects in both terrestrial and aquatic ecosystems. While the available evidence supported conclusions regarding the role of atmospheric deposition of S and N compounds in acidification and nutrient enrichment of aquatic and terrestrial ecosystems, there was variation in the strength of the evidence and of the information supporting multiple quantitative linkages between pollutants in ambient air and ecosystem responses and potential public welfare implications.

While there is extensive evidence of deleterious effects of excessive nitrogen loadings to terrestrial and aquatic ecosystems, consideration of the nutrient enrichment-related effects of atmospheric N and S deposition with regard to identification of options to provide protection for deposition-related effects was limited by several factors. For example, the co-stressors affecting forests, including other air pollutants such as ozone, and limiting factors such as moisture and other nutrients, confound the assessment of marginal changes in any one stressor or nutrient in a forest ecosystem, limiting the information on the effects of changes in N deposition on forestlands and other terrestrial ecosystems (2011 PA, section 6.3.2). Further, only a fraction of the deposited N was reported to be taken up by the forests, with most of the N retained in the soils, such that forest management practices can significantly affect the nitrogen cycling within a forest ecosystem (2008 ISA section 3.3.2.1 and Annex C, section C.6.3). Factors affecting consideration of aquatic eutrophication effects included the appreciable contributions of non-atmospheric sources to waterbody nutrient loading, which affected our attribution of specific effects to

atmospheric sources of N, and limitations in the ability of the available data and models to characterize incremental adverse impacts of atmospheric N deposition (2011 PA, section 6.3.2).

The linkages between terrestrial acidification and atmospheric deposition of N and S compounds were also limited by the sparseness of available data for identifying appropriate assessment levels for terrestrial acidification indicators and uncertainties with regard to empirical case studies in the ISA (

e.g.,

the potential for other stressors to confound relationships between deposition and terrestrial acidification effects). However, the evidence in the 2008 ISA and the REA analyses of aquatic acidification provided strong support to the evidence for a relationship between atmospheric deposition of N and S compounds and loss of acid neutralizing capacity (ANC) in sensitive ecosystems, with associated aquatic acidification effects.

In light of the evidence and findings of these analyses and advice from the CASAC, the PA concluded it was appropriate to place greatest confidence in findings related to the aquatic acidification-related effects of N oxides and SO

relative to other deposition-related effects. Therefore, the PA focused on aquatic acidification effects from deposition of N and S compounds in identifying policy options for providing public welfare protection from deposition-related effects of N oxides and SO

, 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 minimum ANC levels) for consideration (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

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 so-called “transference ratios,” which are factors applied to back-calculate or estimate the concentrations of SO

and N oxides corresponding to target deposition values that would meet the AAI-based standard level, which is also the target minimum ANC (2011 PA, Chapter 7).

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.

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

(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 monitoring might involve the use of monitoring data for NH

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

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).

. . . the Administrator recognizes that characterization of the uncertainties in the AAI equation as a whole represents a unique challenge in this review primarily as a result of the complexity in the structure of an AAI based standard. In this case, the very nature of some of the uncertainties is fundamentally different than uncertainties that have been relevant in other NAAQS reviews. She notes, for example, some of the uncertainties uniquely associated with the quantification of various elements of the AAI result from limitations in the extent to which ecological and atmospheric models, which have not been used to define other NAAQS, have been evaluated. Another important type of uncertainty relates to limitations in the extent to which the representativeness of various factors can be determined at an ecoregion scale, which has not been a consideration in other NAAQS.” [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

, it was not appropriate under section 109 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).

3. 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 that are requisite to protect the public welfare from known or anticipated adverse effects. The approach for this review of the secondary SO

, 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.

This review of the secondary standards for SO

, N oxides, and PM assesses the protection provided by the standards from two categories of effects: direct effects of the airborne pollutants and indirect 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 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

and SO

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.B 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

, N oxides, and PM in ambient air and deposition-related effects. These assessments then 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 deposition and 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 15-30-year intervals (1984-2001 and 1967-1997) and reported that although atmospheric deposition in the Northeast declined across those intervals, soil acidity increased (ISA, Appendix 4, 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, in addition to the influences of emissions, ambient air concentrations and associated deposition.

The two-pronged approach to this review's consideration of deposition-related effects based on the available information in the ISA (summarized in section II.C and II.D below) includes the consideration of deposition levels that may be associated with ecological effects of potential concern. In this step, we consider and strive to focus on effects for which the evidence is most robust with regard to established quantitative relationships between deposition and ecosystem effects. The information for terrestrial ecosystems is derived primarily from analysis of the evidence presented in the ISA. For aquatic ecosystems, primary focus is 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.D below (PA, section 5.1 and Appendix 5A).

In parallel fashion to identification of deposition levels for consideration, air quality and deposition analyses have been employed 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.B below, several different types of analyses have been performed in this review for this purpose. 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, inform 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.

In summary, our approach to evaluating the standards with regard to protection from ecological effects related to ecosystem deposition of N and S compounds (presented in the sections that follow) involves multiple components: (1) review of the scientific evidence to identify the ecological effects associated with the three pollutants, both those related 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; (3) analysis of relationships between ambient air concentrations of the three pollutants and deposition of N and S compounds to understand key aspects of these relationships that can inform the Administrator's decisions on policy options for ambient air standards to protect against air concentrations associated with direct effects and with deposition-related effects that are judged adverse to the public welfare. As is described in sections II.B and II.E, for two of the pollutants, N oxides and PM, relating ambient air concentrations 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 standards for SO

, 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

), that contributes to N deposition and 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 proposed decision, as summarized in section II.E.

B. Air Quality and Deposition

The three criteria pollutants that are the focus of this review (SO

, 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 emissions of these pollutants and their precursors to eventual deposition varies by pollutant and is influenced by a series of atmospheric processes and chemical transformations that occur at multiple spatial and temporal scales (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

. As summarized below, although there is a decreasing temporal trend in emissions of N oxides, the coincident increasing trend in NH

emissions has reduced the influence of N oxides on N deposition (PA, sections 6.2.1, 6.4.2 and 7.2.3.3). Variability and temporal changes in the composition of PM, including with regard to N- (and S-) containing compounds, is another factor affecting decisions in this review (as discussed in sections II.1.d(3)) and II.3 below).

This section includes a brief summary of the major emission sources of SO

, N oxides, and PM (section II.B.1). This is followed by a description of how those emissions are transported and transformed within the atmosphere to eventually contribute to S and N deposition (section II.B.1). Available information on current levels of emissions and air concentrations of these three pollutants across the U.S. and their trends is summarized in section II.B.2, accompanied by a description of estimated deposition levels across the U.S. and how they have changed over the past two decades. Finally, while many of the ecological effects examined in this review are associated with deposition of N and S, the NAAQS are set in terms of pollutant concentrations. To that end, section II.B.3 discusses the findings of analyses performed to relate ambient air concentrations of the relevant pollutants and S or N deposition, over a range of conditions (

e.g.,

pollutant, region, time period), and summarizes key observations that may inform the Administrator's judgments in this review.

1. Sources, Emissions and Atmospheric Processes Affecting SO

, N Oxides and PM

Sulfur dioxide is one of a small group of highly reactive gases collectively known as SO

. Sulfur dioxide is generally present at higher concentrations in the ambient air than the other gaseous SO

species (ISA, Appendix 2, section 2.1) and, as a result, SO

is the indicator for the existing NAAQS for SO

. The main anthropogenic source of SO

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

in the U.S. are: coal-fired electrical generating units (48% of total), industrial processes (27%), and other stationary source fuel combustion (9%).

Once emitted to the atmosphere, the atmospheric lifetime of SO

is typically less than 1-2 days; it can either remain in the gas phase or be oxidized to form sulfate particles (SO

2−

). Modeling studies suggest that oxidation accounts for more than half of SO

removal on a national basis (PA, section 2.1.1). The rate of SO

oxidation accelerates with greater availability of oxidants. Oxidants are generally depleted near source stacks, so that more SO

is oxidized to SO

2−

in cleaner air downwind of SO

sources (2008 ISA, section 2.6.3.1). The atmospheric lifetime of SO

2−

particles is longer, ranging from 2 to 10 days. As SO

2−

particles are generally within the fine particle size range, they are a component of PM

2.5

(PA, section 2.1.1). The spatial distribution of both SO

and SO

2−

deposition reflects the distribution of SO

emissions (

i.e.,

most S deposition is in the eastern U.S.; PA, section 2.5.3) and wind patterns. Precipitation variability also modulates the spatial distribution of S wet deposition. In sum, both SO

, and the SO

2−

particles converted from SO

, contribute to S deposition but do so over different time and geographic scales, with dry deposition of SO

typically occurring near the source, and wet deposition of sulfate particles being more regional in nature.

The term N oxides refers to all forms of oxidized nitrogen compounds, including nitric oxide (NO), NO

, nitric acid (HNO

), and particulate nitrate (NO

−

). Most N oxides enter the atmosphere as either NO or NO

, which are collectively referred to as NO

(PA, section 2.1.2). Anthropogenic sources account for the majority of NO

emissions in the U.S., per the 2020 NEI, with highway vehicles (26% of total), stationary fuel combustion which includes electric generating units (25%), and non-road mobile sources (19%) identified as the largest contributors to total emissions. Other anthropogenic NO

sources include agricultural field burning, prescribed fires, and various industrial processes such as cement manufacturing and oil and gas production (PA, section 2.2.1).

Once emitted into the atmosphere, NO

can deposit to the surface or be chemically converted to other gaseous N oxides, including HNO

, as well as to particulate NO

−

. Unlike particulate SO

2−

, which exists almost entirely in the fine particle range, NO

−

particles may occur either in the fine or coarse size range, such that not all particulate NO

−

is a component of PM

2.5

. Each form of oxidized N is removed from the atmosphere at different rates by both dry and wet deposition. As a general rule, the gas phase species tend to have shorter atmospheric lifetimes, either dry depositing (

e.g.,

as HNO

) or quickly converting to particulate NO

−

. Particulate NO

−

is more efficiently removed by precipitation (wet deposition) and has a similar atmospheric lifetime as particulate SO

2−

(2-10 days).

In addition to N oxides, there is another category of nitrogen pollutants, referred to as reduced nitrogen, which is distinct from N oxides but also contributes to nitrogen deposition. The most common form of reduced N in the air is ammonia gas (NH

). Sources of NH

emissions include livestock waste (49% of total in 2020 NEI), fertilizer application (33%) and aggregate fires (11%). Ammonia tends to dry deposit near sources (PA, section 2.1.3). It can also be converted to particle form, as ammonium (NH

), which can be transported farther distances and is most efficiently removed by precipitation (PA, section 2.1.3). Ammonia, unlike N oxides or PM

2.5

, is not a criteria pollutant and is not directly regulated under CAA section 109.

In sum, particulate matter is both emitted to the atmosphere and can be formed in the atmosphere from precursor chemical gases (such as is the case for NO

and SO

). The components of PM

2.5

mass that contribute to S and N deposition are secondary products formed in the atmosphere after being emitted (

e.g.,

particulate sulfate, particulate NO

−

, NH

). There are other components of PM

2.5

mass that do not contribute to S and N deposition,

e.g.,

black carbon, organic carbon, dust (PA, section 2.4.3).

2. Recent Trends in Emissions, Concentrations, and Deposition

Emissions of SO

, oxides of N, and PM have declined dramatically over the past two decades, continuing a longer-term trend (PA, section 2.2). NEI data indicate an 87% decrease in total SO

emissions between 2002 and 2022, including reductions of 91% in emissions from electricity generating units and 96% in emissions from mobile

sources. Total anthropogenic NO

emissions have also trended downward across the U.S. between 2002 and 2022 at only slightly smaller percentages than SO

. Nationwide estimates indicate a 70% decrease in anthropogenic NO

emissions over this time period, driven in part by large emission reductions in the highway vehicle sector (84%) and from stationary fuel combustion (68%) (PA, section 2.2.1). In contrast with these declining 20-year trends in NO

and SO

emissions, the annual rate of NH

emissions has increased by over 20 percent since 2002 (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, from 2002 to 2022. These trends in emissions have had ramifications for N deposition patterns across the U.S., as described further below.

As expected, the large reductions in SO

and NO

emissions have resulted in substantially lower ambient air concentrations in recent years relative to what was observed in previous periods. The State and Local Air Monitoring Stations (SLAMS) network supports the implementation of the NAAQS. In 2021, all ambient monitoring sites with valid SO

design values (n=333)

are less than the level of the existing secondary standard (500 ppb)

and more than 75 percent of the sites have design values less than 20 ppb (PA, section 2.4.2). These values reflect a downward trend over the past two decades with median 3-hour secondary SO

values down substantially from 2000 levels (from ~50 ppb to ~10 ppb).

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

The existing secondary standard for SO

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

Similar trends are evident in the data for the primary SO

standard (annual 99th percentile of 1-hour daily maximum concentrations, averaged over 3 years with a level of 75 ppb). In the 2019-2021 period, the maximum design value for the primary SO

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

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

values in areas with recurring volcanic eruptions (

e.g.,

Hawaii). In the mid-1990s, the median value of all sites with valid 1-hour SO

design values often exceeded 75 ppb (PA, Figure 2-26). Since then, the entire distribution of values (including source-oriented sites) has continued to decline such that the median value across the network of sites is now between 5 and 10 ppb (PA, Figure 2-26). The EPA also evaluated trends in annual average SO

data from 2000-2021 and observed improving trends of similar magnitude with the longer-term (annual) averaging time. It is important to note that both peak and mean SO

concentrations are higher at source-oriented sites than monitoring locations that are not source-oriented.

Regarding NO

, design values at all 399 sites with valid secondary NO

design values (annual average concentrations) in 2021 are less than the 53 ppb level of the existing secondary standard,

and the majority of sites (98 percent) have design values that are less than 20 ppb. In 2021, the maximum was 30 ppb,

and the median was 7 ppb. As with SO

, the more recent NO

design values also reflect a downward trend over the past two decades. Median annual NO

design values across the U.S. decreased by ~50% between 2000 and 2021 (15 ppb to 7 ppb).

Sites in the contiguous U.S. have met the existing NO

secondary standard since around 1991 (PA, Figure 2-22).

The maximum annual average NO

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 decreased substantially from 2000 to 2021 (from 12.8 µg/m

to 8 µg/m

). 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

in 2000 to 21 µg/m

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).

These emission reductions and subsequent downward trends in air concentrations have also contributed to a nationwide decrease in N and S deposition (PA, sections 2.5.3 and 6.2.1). Total S deposition and N deposition declined by 68% and 15%, respectively, calculated as a nationwide, three-year average between 2000-2002 and 2019-2021 (PA, section 6.2.1). The trend in S deposition is more robust than for N because of the offsetting influence of increasing emissions of reduced forms of nitrogen over the same timeframe. The largest reductions in total S and N deposition are seen in regions downwind of point sources and transportation corridors related to emission reductions from electricity generating units and mobile sources.

3. 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. We examined these relationships over a range of conditions (

e.g.,

pollutant, region, time period), and considered deposition both near sources and at distance (allowing for pollutant transport and associated transformation). The findings of these analyses, described in detail in Chapter 6 and Appendix 6A of the PA, have informed consideration of indicators and levels for potential secondary standards based on consideration of deposition-related effects (PA, Chapter 7).

As is evident from the air quality-deposition analyses, relating ecosystem deposition to ambient air concentrations is not straightforward. Deposition rates vary across ecosystems nationally, and there is not a simple one-to-one relationship between ambient air concentrations of any one indicator and S or N deposition. As discussed above, the atmospheric processes that lead from pollutant emissions loading to eventual deposition to the earth's surface are complex. Multiple chemicals, both gaseous and particulate, from multiple types of sources contribute to S and N deposition. Further, both criteria pollutants and non-criteria pollutants contribute to N deposition. There are also multiple deposition pathways (

i.e.,

dry deposition and wet deposition) that can influence the spatial and temporal scales at which deposition occurs, which vary by pollutant and pollutant phase.

In light of these challenges, the PA employed five different approaches for considering relationships between S and N deposition rates and ambient air concentrations. First, as part of a “real-world experiment,” the PA analyses leveraged the recent downward trends in NO

and SO

emissions and corresponding air quality concentrations as well as the trends in deposition estimates (TDep or total 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 TDep estimates used in these analyses 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 air quality concentrations and associated deposition levels are related within a chemical-transport model (CMAQ

) both nationally and then at certain Class I areas

(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 local 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

, NO

, and PM

2.5

measurements at SLAMS across the U.S.) and a hybrid set of deposition estimates (TDep) (PA, section 6.2.3).

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 U.S., which are referred to as the TDep datasets (technical updates available from NADP, 2021; ISA, appendix 2, section 2.6).

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.

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).

Once those potential zones of influence were established, we evaluated the relationships between air quality metrics for the three pollutants

at sites within those zones with 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 metric value among the sites linked to the downwind ecoregion and, 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 are weighted higher (PA, section 6.2.4.1).

Upwind sites of influence were identified for all 84 ecoregions (level III categorization) in the contiguous U.S.

For SO

, 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

and PM

2.5

metrics are annual averages. For relating to 3-year average deposition, all are averaged across three years.

As with any assessment, there are uncertainties and limitations associated with the analyses summarized above. These are more fully discussed in the PA (PA, sections 6.3 and 6.4). The evaluation of measured air quality concentrations (SO

, NO

, 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 particularly 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

, NO

and PM

2.5

monitors. However, these site-based comparisons do not account for deposition associated with the transport of pollutants emitted some distance upwind. Each of the other analyses completed to bolster this analysis have their own limitations ranging from model uncertainty to limited geographical scope.

The full set of quantitative results of the characterization of air quality and deposition relationships are discussed more thoroughly in Chapter 6 and Appendix 6A of the PA. In combination, these analyses supported the PA conclusion of a strong association between SO

and S deposition. Regarding N oxides and PM, however, the results, and associated information, indicated more variable relationships between NO

concentrations and N deposition, and PM

2.5

concentrations with either S or N deposition.

For SO

, annual monitored SO

concentrations, at existing monitors within the SLAMS network, averaged over 3 years at the national scale were highly correlated to S deposition estimates in the TDep dataset at the local scale (correlation coefficient of 0.70),

especially in the earlier periods of the record and across the eastern U.S. (PA, section 6.2.3). This association was confirmed by the relationships between SO

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

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

have also declined sharply (PA, sections 6.2.1 and 6.4.1).

The correlation coefficients are based on Spearman's rank correlation coefficient. These coefficients are generally used 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

) in a limited number of circumstances. For example, an evaluation of the association between SO

2−

and total S deposition across 27 Class I areas where data for both parameters were available, concluded that the correlations between particle sulfate and total sulfate (

i.e.,

+ SO

2−

) with total S deposition (correlation coefficients of 0.55 and 0.61, respectively) was lower than what was exhibited for SO

and S deposition at the SLAMS (PA, section 6.2.2). The analyses also concluded that there was poor correlation (correlation coefficient of 0.33) between PM

2.5

mass, as measured at IMPROVE sites, with total S deposition estimates for those sites (PA, sections 2.3.3 and 6.2.2.3). While these analyses are based on data at a relatively limited number of sites, as compared to the SLAMS network, the

results suggest that there are no clear advantages to considering PM

2.5

mass, particle sulfate, or total sulfate as an indicator for a secondary NAAQS, over using SO

Both NO

and certain components of PM

2.5

can contribute to N deposition. As is the case for SO

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 in the consideration of how air quality concentrations (

i.e.,

and PM

2.5

mass) are associated with eventual N deposition. First, not all N deposition is caused by the criteria pollutants (PA, Chapter 2 and section 6.1.1). Ammonia emissions also lead 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.,

−

and NH

). As a result of these two factors, the association between NO

concentrations and N deposition, and PM

2.5

concentrations and N deposition is less robust than what is observed for SO

. Our multi-faceted approach to evaluating these relationships confirmed this expectation. For example, when comparing NO

observations at SLAMS across the U.S. against the N deposition estimates from TDep, there are weaker associations than what is observed in the similar SO

comparisons (PA, section 6.4.2). There is little correlation for N deposition with concentrations of NO

, as evidenced by a Spearman's correlation coefficient of 0.38, compared to 0.70 for SO

and S deposition (PA, Table 6-6 and Table 6-4). Further, the trajectory-based analyses of the relationships between NO

annual values in the identified upwind zones 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 was confirmed by considering national trends over the past 20 years, where sharp declines in NO

emissions and concentrations are linked in time with sharp declines in oxidized N deposition (PA, Table 6-2), but not associated with recent trends in total or reduced atmospheric N deposition. Since 2010, NO

concentrations have continued to drop while N deposition has remained steady (PA, section 6.2.1). As noted for S deposition and S compound metrics above, the PA also investigated relationships between N deposition and air quality metrics other than the current indicator species (NO

). Across the 27 Class I areas where collocated data were available, the PA evaluated the relationships between several air quality parameters (

e.g.,

nitric acid, particulate NO

−

, and NH

) and, as for S deposition and S compound metrics, the PA concluded there were no clear advantages over the consideration of NO

or PM

2.5

mass. In sum, the evidence suggests that NO

would be a weak indicator of total atmospheric N deposition, especially in areas where ammonia is prevalent or where PM

2.5

mass is dominated by species other than NO

−

or NH

(PA, section 6.4.2).

C. Welfare Effects Evidence

The information summarized here is based on our scientific assessment of the welfare effects evidence available in this review; this assessment is documented in the ISA

and its policy implications are further discussed in the PA (and summarized in section II.E.1 below). More than 3,000 studies are newly available since the last review and considered in the ISA.

While expanding the evidence for some effect categories, studies on acid deposition, a key group of effects from the last review, are largely consistent with the evidence that was previously available. The subsections below briefly summarize the following aspects of the evidence: the nature of welfare effects of S oxides, N oxides and PM (section II.C.1); the potential public welfare implications (section II.C.2); and exposure concentrations and deposition-related metrics (section II.C.3).

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.

The study count and citations are available on the project page for the ISA on the Health & Environmental Research Online (HERO) website documents these studies (

https://heronet.epa.gov/heronet/index.cfm/project/page/project_id/2965

1. Nature of Effects

This welfare effects evidence base available in the current review includes decades of extensive research on the ecological effects oxides of nitrogen, oxides of sulfur and PM. In the sections below we summarize the nature of the direct effects of gas-phase exposure to oxides of nitrogen and sulfur (section II.C.1.a), acid deposition-related ecological effects (section II.C.1.b), N enrichment and associated effects (section II.C.1.c), and other effects (section II.C.1.d).

a. Direct Effects of SO

and N Oxides

There is a well-established body of scientific evidence that has shown that acute and chronic exposures to oxides of N and S, such as SO

, NO

, NO, HNO

and peroxyacetyl nitrate (PAN) in the air, are associated with negative effects on vegetation. Such scientific evidence, as was available in 1971, was the basis for the current secondary NAAQS for oxides of sulfur and oxides of nitrogen. The current scientific evidence continues to demonstrate such effects, with the ISA specifically concluding that the evidence is sufficient to infer a causal relationship between gas-phase SO

and injury to vegetation (ISA, Appendix 3, section 3.6.1), and between gas-phase NO, NO

and PAN and injury to vegetation (ISA, Appendix 3, section 3.6.2). The ISA additionally concluded the evidence to be sufficient to infer a causal relationship between exposure to HNO

and changes to vegetation, noting that experimental exposure can damage leaf cuticle of tree seedlings and HNO

concentrations have been reported to have contributed to declines in lichen species in the Los Angeles basin (ISA, Appendix 3, section 3.6.3).

Specifically for SO

, high concentrations in the first half of the twentieth century have been blamed for severe damage to plant foliage that occurred near large ore smelters during that time (ISA, Appendix 3, section 3.2). In addition to foliar injury, which is usually a rapid response, SO

exposures have also been documented to reduce plant photosynthesis and growth. The appearance of foliar injury can vary significantly among species and growth conditions (which affect stomatal conductance). For lichens, damage from SO

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; Belnap et al., 1993; Farmer et al., 1992, Hutchinson et al., 1996).

Although there is evidence of plant injury associated with SO

exposures dating back more than a century (ISA, Appendix 3, section 3.2), as exposures have declined in the U.S., some studies in the eastern U.S. have reported increased growth in some SO

-sensitive tree species (

e.g.,

Thomas et al., 2013). Although the authors attributed the growth response to reductions in SO

-associated acid deposition, and related recovery from soil acidification, the relative roles of different pathways are

unclear as a historical deposition record was not available (ISA, Appendix 3, section 3.2). Other researchers have suggested that the observed response was related to the fact that the trees were growing on a limestone outcrop that could be well buffered from soil acidification (Schaberg et al., 2014). This seems to suggest a somewhat faster recovery than might be expected from deposition-related soil acidification, which may indicate a relatively greater role for changes in ambient air concentrations of SO

, 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).

The evidence base evaluated in the 1993

Air Quality Criteria Document for Oxides of N

included evidence of phytotoxic effects of NO, NO

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

, 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).

The evidence indicates that HNO

had 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

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

exposures for 2-11 weeks (daily peak levels near 50 ppb) in controlled chambers. (ISA, Appendix 3, section 3.4).

b. Acid Deposition-Related Ecological Effects

The connection between SO

and N oxide emissions to ambient air, atmospheric deposition of N and/or S, and the acidification of acid-sensitive soils and surface waters is well documented with many decades of evidence, particularly in the eastern U.S. (ISA, section IS.5; Appendix 8, section 8.1). In the atmosphere, SO

and N oxides undergo reactions to form various 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

and H

), and by chemical and biological processes. When NO

−

, or SO

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

], magnesium [Mg

], 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

2−

or NO

−

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).

(1) 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 between 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 to surface waters and their watersheds and the ramifications for biological functioning of freshwater ecosystems (ISA, Appendix 8, section 8.1). The 2020 ISA found that the newly available scientific research “reflects incremental improvements in scientific knowledge of aquatic biological effects and indicators of acidification as compared with knowledge summarized in the 2008 ISA” (ISA, Appendix 8, p. 8-80). Previously and newly available studies “indicate that aquatic organisms in sensitive ecosystems have been affected by acidification at virtually all trophic levels and that these responses have been well characterized for several decades” (ISA, Appendix 8, p. 8-80). For example, information reported in the previous 2008 ISA “showed consistent and coherent evidence for effects on aquatic biota, especially algae, benthic invertebrates, and fish that are most clearly linked to chemical indicators of acidification” (ISA, Appendix 8, p. 8-80). These indicators are surface water pH, base cation ratios, ANC, and inorganic aluminum (Al) concentration (ISA, Appendix 8, Table 8-9).

The effects of waterbody acidification on fish species are especially well understood in the scientific literature, and many species (

e.g.,

brown and brook trout and Atlantic salmon) have been documented to have experienced adverse effects from acidification (ISA, Appendix 8, section 8.3). Among these species, the earliest lifestages are most

sensitive to acidic conditions. Many effects of acidic surface waters on fish, particularly effects on gill function or structure, relate to the combination of low pH and elevated dissolved Al (ISA, Appendix 8, section 8.3.6.1). In general, biological effects in aquatic ecosystems are primarily attributable to low pH and high inorganic aluminum concentration (ISA, p. ES-14). Waterbody pH largely controls the bioavailability of Al, which is toxic to fish, and aluminum mobilization is largely confined to waters with a pH below about 5.5, which the ISA describes as corresponding to an ANC in the range of about 10 to 30 µeq/L in low to moderate DOC waters of the Northeast (ISA, Appendix 7, section 7.1.2.6 and Appendix 8, section 8.6.4).

The parameter ANC is an indicator of the buffering capacity of natural waters against acidification. Although ANC does not directly affect biota, it is an indicator of acidification that relates to pH and aluminum levels (ISA, p. ES-14), or watershed characteristics like base cation weathering (BCw) rate (ISA, Appendix 8, sections 8.1 and 8.3.6.3). Accordingly, ANC is commonly used to describe the potential sensitivity of a freshwater system to acidification-related effects. It can be measured in water samples and is also often estimated for use in water quality modeling, as is done in the aquatic acidification risk assessment for this review, as summarized in section II.D below. Water quality models are generally better at estimating ANC than at estimating other indicators of acidification-related risk, such as pH. Acid neutralizing capacity is estimated as the molar sum of strong base cations minus the molar sum of strong acid anions, specifically including SO

2−

and NO

−

(

e.g.,

Driscoll et al., 1994). Thus, values below zero indicate a deficit in the ability to buffer acidic inputs, and increasing values above zero represent increasing buffering capability for acidic inputs. Further, across waterbodies within impacted areas of Shenandoah National Park streams and Adirondack Mountain lakes, a positive relationship has often been observed between ANC and number of fish species, at least for the ANC range from about zero to 50 µeq/L (ISA, Appendix 7, section 7.1.2.6; Cosby et al., 2006; Sullivan et al., 2006, Bulger et al., 1999).

Values of ANC can also be influenced by high concentrations of naturally occurring organic acids, which can reduce bioavailability of Al, buffering effects usually associated with low pH and high total Al concentrations (Waller et al., 2012; ISA, Appendix 8, section 8.3.6.4); in waters where that occurs, ANC may not be a good indicator of risk to biota.

In addition to acidity of surface waters quantified over weeks or months, waterbodies can also experience spikes in acidity in response to episodic precipitation or rapid snowmelt events. In these events (hours-days), a surge or pulse of drainage water, containing acidic compounds, is routed through upper soil horizons rather than the deeper soil horizons that would usually provide buffering for acidic compounds (ISA, Appendix 7, section 7.1). While some streams and lakes may have chronic or base flow chemistry that provides suitable conditions for aquatic biota, they may experience occasional acidic episodes with the potential for deleterious consequences to sensitive biota (ISA, Appendix 8, section 8.5). For example, in some impacted northeastern waterbodies, ANC levels may dip below zero for hours to days or weeks in response to such events, while waterbodies labeled chronically acidic have ANC levels below zero throughout the year (ISA, Appendix 7, section 7.1.1.2; Driscoll et al., 2001). Accordingly, headwater streams in both the eastern and western U.S. tend to be more sensitive to such episodes due to their smaller watersheds and, in the east, their underlying geology (ISA, Appendix 8, section 8.5.1).

National survey data dating back to the early 1980s through 2004, that were available for the 2008 ISA, indicated acidifying deposition had acidified surface waters in the southwestern Adirondacks, New England uplands, eastern portion of the upper Midwest, forested Mid-Atlantic highlands, and Mid-Atlantic coastal plain (2008 ISA, section 4.2.2.3; ISA, Appendix 8, section 8.5.1). For example, a survey of waterbodies in the Adirondacks in 1984-1987 found 27% of streams to have ANC values below zero, with a minimum value of −134 microequivalents per liter (μeq/L) (Sullivan et al., 2006). Values of ANC below 20 μeq/L in Shenandoah stream sites were associated with fewer fish of sensitive species compared to sites with higher ANC (Bulger et al., 1999). A more recent study of two groups of Adirondack lakes for which water quality data were available from 1982 and 1992, respectively, reported significant increases in ANC in the large majority of those lakes, with the magnitude of the increases varying across the lakes (Driscoll et al., 2016; ISA, Appendix 7, section 7.1.3.1). As described in the ISA, “[a]cidic waters were mostly restricted to northern New York, New England, the Appalachian Mountain chain, upper Midwest, and Florida” (ISA, Appendix 8, p. 8-60). Despite the appreciable reductions in acidifying deposition that have occurred in the U.S. since the 1960s and 1970s, aquatic ecosystems across the U.S. are still experiencing effects from historical contributions of N and S (ISA, Appendix 8, section 8.6).

(2) Terrestrial Ecosystems

There is longstanding evidence that changes in soil biogeochemical processes caused by acidifying deposition of N and S to terrestrial systems are linked to changes in terrestrial biota, with associated impacts on ecosystem characteristics. The currently available evidence, including that newly available in this review, supports and strengthens this understanding (ISA, Appendix 5, section 5.1). Consistent with conclusions in the last review, the current body of evidence is sufficient to infer a causal relationship between acidifying deposition and alterations of biogeochemistry in terrestrial ecosystems. Additionally, and consistent with conclusions in the last review, the current body of evidence is sufficient to infer a causal relationship between acidifying N and S deposition and the alteration of the physiology and growth of terrestrial organisms and the productivity of terrestrial ecosystems. The current body of evidence is also sufficient to conclude that a causal relationship exists between acidifying N and S deposition and alterations of species richness, community composition, and biodiversity in terrestrial ecosystems (2008 ISA, sections 4.2.1.1 and 4.2.1.2; 2020 ISA, Appendix 4, section 4.1 and Appendix 5, sections 5.7.1 and 5.7.2).

Deposition of acidifying compounds to acid-sensitive soils can cause soil acidification, increased mobilization of Al from soil to drainage water, and deplete the pool of exchangeable base cations in the soil (ISA, Appendix 5, section 5.2 and Appendix 4, sections 4.3.4 and 4.3.5). The physiological effects of acidification on terrestrial biota include slower growth and increased mortality among sensitive plant species, which are generally attributable to physiological impairment caused by Al toxicity (related to increased availability of inorganic Al in soil water) and a reduced ability of plant roots to take up base cations (ISA, Appendix 4, section 4.3 and Appendix 5, section 5.2). The U.S. tree species most studied with regard to effects of acid deposition are red spruce and sugar maple, although there is also evidence for other tree species such as flowering

dogwood (ISA, Appendix 5, section 5.2.1).

The physiological effects of acidifying deposition on terrestrial biota can also result in changes in species composition whereby sensitive species are replaced by more tolerant species, or the sensitive species that were dominant in the community become a minority. For example, increasing soil cation availability (as in Ca

addition or gradient experiments) was associated with greater growth and seedling colonization for sugar maple while American beech was more prevalent on soils with lower levels of base cations where sugar maple is less often found (ISA, Appendix 5, section 5.2.1.3.1; Duchesne and Ouimet, 2009). In a study of understory species composition, soil acid-base chemistry was found to be a predictor of understory species composition (ISA, Appendix 5, section 5.2.2.1). Additionally, limited evidence, including a recent S addition study and agricultural soil gradient study, indicated that soil acid-base chemistry predicted and was correlated with diversity and composition of soil bacteria, fungi, and nematodes (ISA, Appendix 5, section 5.2.4.1).

In addition to Ca

addition experiments, the recently available evidence also includes addition or gradient studies evaluating relationships between soil chemistry indicators of acidification (

e.g.,

soil pH, base cation to aluminum (Bc:Al) ratio, base saturation, and Al) and ecosystem biological endpoints, including physiological and community responses of trees and other vegetation, lichens, soil biota, and fauna (ISA, Appendix 5, Tables 5-2 and 5-6). Further, the 2020 ISA reports on several large observational studies evaluating statistical associations between tree growth or survival, as assessed at monitoring sites across the U.S. and estimates of average deposition of S or N compounds at those sites over time periods on the order of 10 years (ISA, Appendix 5, section 5.5.2 and Appendix 6, section.6.2.3.1; Dietze and Moorcroft, 2011; Thomas et al., 2010; Horn et al., 2018). Negative associations were observed for survival and growth in several species or species groups with S deposition metrics; positive and negative associations were reported with N deposition (PA, sections 5.3.2.3 and 5.3.4 and Appendix 5B).

Although there has been no systematic national survey of U.S. terrestrial ecosystem soils, the forest ecosystems considered the most sensitive to terrestrial acidification from atmospheric deposition include forests of the Adirondack Mountains of New York, Green Mountains of Vermont, White Mountains of New Hampshire, the Allegheny Plateau of Pennsylvania, and mountain top and ridge forest ecosystems in the southern Appalachians (2008 ISA, Appendix 3, section 3.2.4.2; ISA, Appendix 5, section 5.3). Underlying geology is the principal factor governing the sensitivity of both terrestrial and aquatic ecosystems to acidification from S and N deposition. Geologic formations with low base cation supply (

e.g.,

sandstone, quartzite), due mainly to low weathering rates, generally underlie these acid sensitive watersheds. Other factors also contribute to the overall sensitivity of an area to acidifying nitrogen and sulfur deposition, including topography, soil chemistry, land use, and hydrology (ISA, Appendix 5, section 5.3). For example, “[a]cid-sensitive ecosystems are mostly located in upland mountainous terrain in the eastern and western U.S. and are underlain by bedrock that is resistant to weathering, such as granite or quartzite sandstone” (ISA, Appendix 7, p. 7-45). Further, as well documented in the evidence, biogeochemical sensitivity to deposition-driven acidification (and eutrophication [see section 4.3 below]) is the result of historical loading, geologic/soil conditions (

e.g.,

mineral weathering and S adsorption), and nonanthropogenic sources of N and S loading to the system (ISA, Appendix 7, section 7.1.5).

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

2−

were associated with increases in soil base saturation and decreases in exchangeable Al (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).

c. Nitrogen Enrichment and Associated Ecological Effects

The numerous ecosystem types that occur across the U.S. have a broad range of sensitivity to N enrichment. Organisms in their natural environments are commonly adapted to the nutrient availability in those environments. Historically, N has been the primary limiting nutrient for plants in many ecosystems. In such ecosystems, when the limiting nutrient, N, becomes more available, whether from atmospheric deposition, runoff, or episodic events, the subset of plant species able to most effectively use the higher nitrogen levels may out-compete other species, leading to a shift in the community composition that may be dominated by a smaller number of species,

i.e.,

a community with lower diversity (ISA, sections IS.6.1.1.2, IS.6.2.1.1 and IS.7.1.1, Appendix 6, section 6.2.4 and Appendix 7, section 7.2.6.6). Thus, change in the availability of nitrogen in nitrogen-limited systems can affect growth and productivity, with ramifications on relative abundance of different species of vegetation, and potentially further and broader ramifications on ecosystem processes, structure, and function.

Both N oxides and reduced forms of nitrogen (NH

) can contribute to N enrichment. In addition to atmospheric deposition, other sources of S and N can play relatively greater or lesser roles in contributing to S and N inputs, depending on location. For example, many waterbodies receive appreciable amounts of N from agricultural runoff and municipal or industrial wastewater discharges. For many terrestrial and freshwater ecosystems, sources of N other than atmospheric deposition, including fertilizer and waste treatment, contribute to ecosystem total N with contributions that can be larger than that from atmospheric deposition (ISA Appendix 7, sections 7.1 and 7.2). Additionally, the impacts of historic deposition in both aquatic and terrestrial ecosystems pose complications to discerning the potential effects of more recent lower deposition rates.

(1) Aquatic and Wetland Ecosystems

Nitrogen additions, including from atmospheric deposition, to freshwater, estuarine and near-coastal ecosystems can contribute to eutrophication, which typically begins with nutrient-stimulated rapid algal growth developing into an algal bloom that can, depending on various site-specific factors, be followed by anoxic conditions associated with the algal die-off (ISA, ES.5.2). Decomposition of the plant biomass from the subsequent algal die-off contributes to reduced waterbody oxygen, which in turn can affect higher-trophic-level species,

e.g.,

contributing to fish mortality (ISA, p. ES-18).

The extensive body of evidence in this area is sufficient to infer causal relationships between N deposition and

the alteration of biogeochemistry in freshwater, estuarine and near-coastal marine systems (ISA, Appendix 7, sections 7.1 and 7.2). Further, consistent with findings in the last review, the current body of evidence is sufficient to infer a causal relationship between N deposition and changes in biota, including altered growth and productivity, species richness, community composition, and biodiversity due to N enrichment in freshwater ecosystems (ISA, Appendix 9, section 9.1). The body of evidence is sufficient to infer a causal relationship between N deposition and changes in biota, including altered growth, total primary production, total algal community biomass, species richness, community composition, and biodiversity due to N enrichment in estuarine environments (ISA, Appendix 10, section 10.1).

The impact of N additions on wetlands, and whether they may serve as a source, sink, or transformer of atmospherically deposited N, is extremely variable and depends on the type of wetland and other factors, such as physiography, and local hydrology, as well as climate (ISA, section IS.8.1 and Appendix 11, section 11.1). Studies generally show N enrichment to decrease the ability of wetlands to retain and store N, which may diminish the wetland ecosystem service of improving water quality (ISA, section IS.8.1). Consistent with the evidence available in the last review, the current body of evidence is sufficient to infer a causal relationship between N deposition and the alteration of biogeochemical cycling in wetlands. Newly available evidence regarding N inputs and plant physiology expands the evidence base related to species diversity. The currently available evidence, including that newly available, is sufficient to infer a causal relationship between N deposition and the alteration of growth and productivity, species physiology, species richness, community composition, and biodiversity in wetlands (ISA, Appendix 11, section 11.10).

The relative contribution of atmospheric deposition to total wetland N loading varies with wetland type, with bogs receiving the greatest contribution and accordingly being most vulnerable to nutrient enrichment effects of N deposition (ISA, Appendix 11, section 11.1). For example, bogs, which receive 70-100% of hydrological input from rainfall, are more sensitive to N deposition than fens (55-83% as rainfall), which are more sensitive than coastal wetlands (10-20% as rainfall) (ISA, Appendix 11, section 11.10). For freshwater fens, marshes, and swamps, inputs from ground and surface water are often of similar order of magnitude as that from precipitation, while estuarine and coastal wetlands receive water from multiple sources, with precipitation being among the smaller of those sources (ISA, Appendix 11, section 11.1).

Nitrogen loading and other factors contribute to nutrient enrichment, which contributes to eutrophication. Such nitrogen-driven eutrophication alters freshwater biogeochemistry and can impact physiology, survival, and biodiversity of sensitive aquatic biota. Evidence newly available in this review provides insights regarding N enrichment and its impacts in several types of aquatic systems, including freshwater streams and lakes, estuarine and near-coastal systems, and wetlands. With regard to freshwaters, for example, studies published since the 2008 ISA augment the evidence base for high-elevation waterbodies where the main N source is atmospheric deposition. Recent evidence continues to indicate that N limitation is common in oligotrophic waters in the western U.S., with shifts in nutrient limitation, from N limitation, to between N and phosphorus (P) limitation, or to P limitation, reported in some alpine lake studies (ISA, Appendix 9, section 9.1.1.3). Small inputs of N in such water bodies have been reported to increase nutrient availability or alter the balance of N and P, with the potential to stimulate growth of primary producers and contribute to changes in species richness, community composition, and diversity.

Another type of N loading effect in other types of freshwater lakes includes a role in the composition of freshwater algal blooms and their toxicity (ISA, Appendix 9, section 9.2.6.1). Information in this review, including studies in Lake Erie, indicates that growth of some harmful algal species, including those that produce microcystin, are favored by increased availability of N and its availability in dissolved inorganic form (ISA, Appendix 9, p. 9-28; Davis et al., 2015; Gobler et al., 2016).

The relative contribution of N deposition to total N loading varies among waterbodies. For example, atmospheric deposition is generally considered to be the main source of N inputs to most headwater stream, high-elevation lake, and low-order stream watersheds that are far from the influence of other N sources like agricultural runoff and wastewater effluent (ISA, section ES5.2). In other fresh waterbodies, however, agricultural practices and point source discharges have been estimated to be larger contributors to total N loading (ISA, Appendix 7, section 7.1.1.1). Since the 2008 ISA, several long-term monitoring studies in the Appalachian Mountains, the Adirondacks, and the Rocky Mountains have reported temporal patterns of declines in surface water NO

−

concentration corresponding to declines in atmospheric N deposition (ISA, Appendix 9, section 9.1.1.2). Declines in basin wide NO

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concentrations have also been reported for the nontidal Potomac River watershed and have been attributed to declines in atmospheric N deposition (ISA, Appendix 7, section 7.1.5.1).

Nutrient inputs to coastal and estuarine waters are important influences on the health of these waterbodies. Continued inputs of N, the most common limiting nutrient in estuarine and coastal systems, have resulted in N over-enrichment and subsequent alterations to the nutrient balance in these systems (ISA, Appendix 10, p. 10-6). For example, the rate of N delivery to coastal waters is strongly correlated to changes in primary production and phytoplankton biomass (ISA, Appendix 10, section 10.1.3). Algal blooms and associated die-offs can contribute to hypoxic conditions (most common during summer months), which can contribute to fish kills and associated reductions in marine populations (ISA, Appendix 10). Further, the prevalence and health of submerged aquatic vegetation (SAV), which is important habitat for many aquatic species, has been identified as a biological indicator for N enrichment in estuarine waters (ISA, Appendix 10, section 10.2.5). Previously available evidence indicated the role of N loading in SAV declines in multiple U.S. estuaries through increased production of macroalgae or other algae, which reduce sunlight penetration into shallow waters where SAV is found (ISA, Appendix 10, section 10.2.3). Newly available studies have reported findings of increased SAV populations in two tributaries of the Chesapeake Bay corresponding to reduction in total N loading from all sources since 1990 (ISA, Appendix 10, section 10.2.5). The newly available studies also identify other factors threatening SAV, including increasing temperature related to climate change (ISA, Appendix 10, section 10.2.5).

The degree to which N enrichment and associated ecosystem impacts are driven by atmospheric N deposition varies greatly and is largely unique to the specific ecosystem. Analyses based on data across two to three decades

extending from the 1990s through about 2010 estimate that most of the analyzed estuaries receive 15-40% of their N inputs from atmospheric sources (ISA, section ES 5.2; ISA, Appendix 7, section 7.2.1), though for specific estuaries contributions can vary more widely. In areas along the West Coast, N sources may include coastal upwelling from oceanic waters, as well as transport from watersheds. Common N inputs to estuaries include those associated with freshwater inflows transporting N from agriculture, urban, and wastewater sources, in addition to atmospheric deposition across the watershed (ISA, section IS 2.2.2; ISA, Appendix 7, section 7.2.1).

There are estimates of atmospheric N loading to estuaries available from several recent modeling studies (ISA, Table 7-9). One analysis of estuaries along the Atlantic Coast and the Gulf of Mexico, which estimated that 62-81% of N delivered to the eastern U.S. coastal zone is anthropogenic in source, also reported that atmospheric N deposition to freshwater that is subsequently transported to estuaries represents 17-21% of the total N loading into the coastal zone (McCrackin et al., 2013; Moore et al., 2011). In the Gulf of Mexico, 26% of the N transported to the Gulf in the Mississippi/Atchafalaya River basin was estimated to be contributed from atmospheric deposition (which may include volatilized losses from natural, urban, and agricultural sources) (Robertson and Saad, 2013). Another modeling analysis identified atmospheric deposition to watersheds as the dominant source of N to the estuaries of the Connecticut, Kennebec, and Penobscot rivers. For the entire Northeast and mid-Atlantic coastal region, however, it was the third largest source (20%), following agriculture (37%) and sewage and population-related sources (28%) (ISA, Appendix 7, section 7.2.1). Estimates for West Coast estuaries indicate much smaller contribution from atmospheric deposition. For example, analyses for Yaquina Bay, Oregon, estimated direct deposition to contribute only 0.03% of N inputs; estimated N input to the watershed from N fixing red alder (

Alnus rubra

) trees was a much larger (8%) source (ISA, Appendix 7, section 7.2.1; Brown and Ozretich, 2009).

Evidence in coastal waters has recognized a role of nutrient enrichment in acidification of some coastal waters (ISA, Appendix 10, section 10.5). More specifically, nutrient-driven algal blooms may contribute to ocean acidification, possibly through increased decomposition, which lowers dissolved oxygen levels in the water column and contributes to lower pH. Such nutrient-enhanced acidification can also be exacerbated by warming (associated with increased microbial respiration) and changes in buffering capacity (alkalinity) of freshwater inputs (ISA, Appendix 10, section 10.5).

(2) Terrestrial Ecosystems

It is long established that N enrichment of terrestrial ecosystems increases plant productivity (ISA, Appendix 6, section 6.1). Building on this, the currently available evidence, including evidence that is longstanding, is sufficient to infer a causal relationship between N deposition and the alteration of the physiology and growth of terrestrial organisms and the productivity of terrestrial ecosystems (ISA, Appendix 5, section 5.2 and Appendix 6, section 6.2). Responsive ecosystems include those that are N limited and/or contain species that have evolved in nutrient-poor environments. In these ecosystems the N-enrichment changes in plant physiology and growth rates vary among species, with species that are adapted to low N supply being readily outcompeted by species that require more N. In this manner, the relative representation of different vegetation species may be altered, and some species may be eliminated altogether, such that community composition is changed and species diversity declines (ISA, Appendix 6, sections 6.3.2 and 6.3.8). The currently available evidence in this area is sufficient to infer a causal relationship between N deposition and the alteration of species richness, community composition, and biodiversity in terrestrial ecosystems (ISA, section IS.5.3 and Appendix 6, section 6.3).

Previously available evidence described the role of N deposition in changing soil carbon and N pools and fluxes, as well as altering plant and microbial growth and physiology in an array of terrestrial ecosystems (ISA, Appendix 6, section 6.2.1). Nitrogen availability is broadly limiting for productivity in many terrestrial ecosystems (ISA, Appendix 6, section 6.2.1). Accordingly, N additions contribute to increased productivity and can alter biodiversity. Eutrophication, one of the mechanisms by which increased productivity and changes in biodiversity associated with N addition to terrestrial ecosystems can occur, comprises multiple effects that include changes to the physiology of individual organisms, alteration of the relative growth and abundance of various species, transformation of relationships between species, and indirect effects on availability of essential resources other than N, such as light, water, and nutrients (ISA, Appendix 6, section 6.2.1).

The currently available evidence for the terrestrial ecosystem effects of N enrichment, including eutrophication, includes studies in a wide array of systems, including forests (tropical, temperate, and boreal), grasslands, arid and semi-arid scrublands, and tundra (PA, section 4.1; ISA, Appendix 6). The organisms affected include trees, herbs and shrubs, and lichen, as well as fungal, microbial, and arthropod comm

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