105692 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 50 

[EPA–HQ–OAR–2014–0128; FRL–5788–05– 

OAR] 

RIN 2060–AS35 

Review of the Secondary National 

Ambient Air Quality Standards for 

Oxides of Nitrogen, Oxides of Sulfur, 

and Particulate Matter 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final rule. 

SUMMARY: Based on the Environmental 

Protection Agency’s (EPA’s) review of 

the air quality criteria for ecological 

effects and secondary national ambient 

air quality standards (NAAQS) for 

oxides of nitrogen (N oxides), oxides of 

sulfur (SOX), and particulate matter 

(PM), the EPA is revising the existing 

secondary sulfur dioxide (SO2) standard 

to an annual average, averaged over 

three consecutive years, with a level of 

10 parts per billion (ppb). Additionally, 

the Agency is retaining the existing 

secondary standards for N oxides and 

PM, without revision. The EPA is also 

finalizing revisions to the data handling 

requirements for the secondary SO2

NAAQS. 

DATES: This final rule is effective on 

January 27, 2025. 

ADDRESSES: The EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2014–0128. All 

documents in the docket are listed on 

the https://www.regulations.gov 

website. Although listed in the index, 

some information is not publicly 

available, e.g., CBI or other information 

whose disclosure is restricted by statute. 

Certain other material, such as 

copyrighted material, is not placed on 

the internet and will be publicly 

available only in hard copy form. 

Publicly available docket materials are 

available electronically through https:// 

www.regulations.gov. 

FOR FURTHER INFORMATION CONTACT: Ms. 

Ginger Tennant, Environmental 

Protection Agency, Health and 

Environmental Impacts Division, Office 

of Air Quality Planning and Standards 

(mail code C539–04), Research Triangle 

Park, NC 27711; telephone number: 

(919) 541–4072; email address: 

tennant.ginger@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Table of Contents 

Executive Summary 

I. Background 

A. Legislative Requirements 

B. Related Control Programs 

C. History of the Secondary Standards for 

N Oxides, SOX and PM 

1. N Oxides 

2. SOX

3. Related Actions Addressing Acid 

Deposition 

4. Most Recent Review of the Secondary 

Standards for N Oxides and SOX

5. PM 

D. Current Review 

II. Rationale for Decisions 

A. Introduction 

1. Background 

a. Basis for Existing Secondary Standards 

b. Prior Review of Deposition-Related 

Effects 

c. General Approach for This Review 

2. Overview of Air Quality and Deposition 

a. Sources, Emissions and Atmospheric 

Processes Affecting SOX, N Oxides and 

PM 

b. Recent Trends in Emissions, 

Concentrations, and Deposition 

c. Relationships Between Concentrations 

and Deposition 

3. Overview of Welfare Effects Evidence 

a. Nature of Effects 

(1) Direct Effects of SOX and N Oxides in 

Ambient Air 

(2) Acid Deposition-Related Ecological 

Effects 

(3) Nitrogen Enrichment and Associated 

Ecological Effects 

(4) Other Deposition-Related Effects 

b. Public Welfare Implications 

c. Exposure Conditions and DepositionRelated Metrics 

(1) Acidification and Nitrogen Enrichment 

in Aquatic Ecosystems 

(2) Deposition-Related Effects in Terrestrial 

Ecosystems 

(3) Other Effects of N Oxides, SOX and PM 

in Ambient Air 

4. Overview of Exposure and Risk 

Assessment for Aquatic Acidification 

a. Key Design Aspects 

b. Key Limitations and Uncertainties 

c. Summary of Results 

B. Conclusions 

1. Basis for Proposed Decision 

a. Policy-Relevant Evaluations in the 

Policy Assessment 

(1) Effects Not Related to S and N 

Deposition 

(2) Evidence of Ecosystem Effects of S and 

N Deposition 

(3) Sulfur Deposition and SOX

(4) Nitrogen Deposition and N Oxides and 

PM 

b. CASAC Advice 

c. Administrator’s Proposed Conclusions 

2. Comments on the Proposed Decision 

a. Sulfur Oxides 

(1) Comments Regarding Adequacy of the 

Existing Standard 

(2) Comments in Support of Proposed 

Adoption of a New Annual Standard 

(3) Comments in Disagreement With 

Proposed Adoption of a New Annual 

Standard 

(4) Comments Regarding Retaining the 

Existing Secondary Standard 

b. Nitrogen Oxides and Particulate Matter 

(1) Comments in Support of Proposed 

Decisions 

(2) Comments in Disagreement With 

Proposed Decisions 

3. Administrator’s Conclusions 

C. Decision on the Secondary Standards 

III. Interpretation of the Secondary SO2

NAAQS 

A. Background 

B. Interpretation of the Secondary SO2

Standard 

IV. Ambient Air Monitoring Network for SO2

A. Public Comments 

B. Conclusion on the Monitoring Network 

V. Clean Air Act Implementation 

Considerations for the Revised 

Secondary SO2 Standard 

A. Designation of Areas 

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

SIP Requirements 

C. Prevention of Significant Deterioration 

and Nonattainment New Source Review 

Programs for the Revised Secondary SO2

Standard 

D. Transportation Conformity Program 

E. General Conformity Program 

VI. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 14094: Modernizing Regulatory 

Review 

B. Paperwork Reduction Act (PRA) 

C. Regulatory Flexibility Act (RFA) 

D. Unfunded Mandates Reform Act 

(UMRA) 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution or Use 

I. National Technology Transfer and 

Advancement Act (NTTAA) 

J. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations and Executive Order 14096: 

Revitalizing Our Nation’s Commitment 

to Environmental Justice for All 

K. Congressional Review Act (CRA) 

L. Judicial Review 

VII. References 

Executive Summary 

This document presents the 

Administrator’s final decisions in the 

current review of the secondary NAAQS 

for SOX, N oxides, and PM. Specifically, 

this document summarizes the 

background and rationale for the 

Administrator’s final decisions to revise 

the secondary SO2 standard to an 

annual average, averaged over three 

consecutive years, with a level of 10 

ppb, and to retain the existing standards 

for N oxides and PM. In conducting this 

review of the secondary SOX, N oxides, 

and PM NAAQS, the EPA has carefully 

evaluated the currently available 

scientific literature on the ecological 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105693 

1Welfare effects of PM considered in the review 

of the PM secondary standards completed in 2020, 

and reconsidered more recently, include effects on 

visibility and climate and materials damage (88 FR 

5558, January 27, 2023). 

2Under 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 wellbeing.’’ 

3This section of the Act requires the 

Administrator to complete these reviews and make 

any revisions that may be appropriate ‘‘at five-year 

intervals.’’ 

effects of SOX, N oxides, and PM 1 as 

described in the Integrated Science 

Assessment (ISA) and conducted 

quantitative air quality, deposition, and 

risk analyses. The Administrator’s final 

decisions are based on his consideration 

of the characterization of the available 

scientific evidence in the ISA; 

quantitative and policy analyses 

presented in the Policy Assessment 

(PA), and related analyses; advice from 

the Clean Air Scientific Advisory 

Committee (CASAC); and public 

comments on the proposed decision. 

Sections 108 and 109 of the Clean Air 

Act (CAA) require the EPA to 

periodically review the air quality 

criteria—the science upon which the 

standards are based—and the standards 

themselves. Under section 109(b)(2) of 

the Act, a secondary standard must 

‘‘specify a level of air quality the 

attainment and maintenance of which, 

in the judgment of the Administrator, 

based on such criteria, is requisite to 

protect the public welfare from any 

known or anticipated adverse effects 

associated with the presence of [the] 

pollutant in the ambient air.’’ As a result 

of the current review, the Administrator 

concluded that the current 3-hour 

secondary SO2 standard is not requisite 

to protect the public welfare from any 

known or anticipated adverse effects 

associated with the presence of SOX in 

ambient air, and that it should be 

revised to an annual average SO2

standard, averaged over three years, 

with a level of 10 ppb to provide the 

requisite protection for the effects of 

SOX, including those related to 

atmospheric deposition of sulfur (S) 

compounds in sensitive ecosystems. 

The Administrator also decided to 

retain the secondary nitrogen dioxide 

(NO2) and PM standards, without 

revision. With regard to the secondary 

NO2 standard, the Administrator finds 

that the evidence related to N oxides 

does not call into question the adequacy 

of protection provided by the existing 

standard. Additionally, the 

Administrator concludes that no change 

to the annual secondary PM2.5 standard 

is warranted and that the existing PM2.5

secondary standard should be retained 

without revision. 

This document additionally includes 

revisions related to implementation of 

the proposed secondary SO2 annual 

standard. Specifically, the EPA is 

enacting revisions to the data handling 

requirements in appendix T of part 50 

to include specifications needed for the 

new annual average standard. This 

document also describes the SO2

monitoring network and its adequacy 

for surveillance for the revised annual 

standard. Lastly, the document 

discusses implementation processes 

pertinent to implementation of the new 

standard. 

I. Background 

A. Legislative Requirements 

Two sections of the CAA govern the 

establishment and revision of the 

NAAQS. Section 108 (42 U.S.C. 7408) 

directs the Administrator to identify and 

list certain air pollutants and then to 

issue air quality criteria for those 

pollutants. The Administrator is to list 

those pollutants ‘‘emissions of which, in 

his judgment, cause or contribute to air 

pollution which may reasonably be 

anticipated to endanger public health or 

welfare’’; ‘‘the presence of which in the 

ambient air results from numerous or 

diverse mobile or stationary sources’’; 

and for which he ‘‘plans to issue air 

quality criteria . . . .’’ (42 U.S.C. 

7408(a)(1)). Air quality criteria are 

intended to ‘‘accurately reflect the latest 

scientific knowledge useful in 

indicating the kind and extent of all 

identifiable effects on public health or 

welfare which may be expected from the 

presence of [a] pollutant in the ambient 

air . . . .’’ 42 U.S.C. 7408(a)(2). 

Section 109 of the Act (42 U.S.C. 

7409) directs the Administrator to 

propose and promulgate ‘‘primary’’ and 

‘‘secondary’’ NAAQS for pollutants for 

which air quality criteria are issued [42 

U.S.C. 7409(a)]. Under section 109(b)(2), 

a secondary standard must ‘‘specify a 

level of air quality the attainment and 

maintenance of which in the judgment 

of the Administrator, based on such 

criteria, is requisite to protect the public 

welfare from any known or anticipated 

adverse effects associated with the 

presence of [the] pollutant in the 

ambient air.’’ 2

In setting primary and secondary 

standards that are ‘‘requisite’’ to protect 

public health and welfare, respectively, 

as provided in section 109(b), the EPA’s 

task is to establish standards that are 

neither more nor less stringent than 

necessary. In so doing, the EPA may not 

consider the costs of implementing the 

standards. See generally, Whitman v. 

American Trucking Ass’ns, 531 U.S. 

457, 465–472, 475–76 (2001). Likewise, 

‘‘[a]ttainability and technological 

feasibility are not relevant 

considerations in the promulgation of 

national ambient air quality standards’’ 

(American Petroleum Institute v. Costle, 

665 F.2d 1176, 1185 [D.C. Cir. 1981]). 

However, courts have clarified that in 

deciding how to revise the NAAQS in 

the context of considering standard 

levels within the range of reasonable 

values supported by the air quality 

criteria and judgments of the 

Administrator, EPA may consider 

‘‘relative proximity to peak background 

. . . concentrations’’ as a factor 

(American Trucking Ass’ns, v. EPA, 283 

F.3d 355, 379 [D.C. Cir. 2002]). 

Section 109(d)(1) of the Act requires 

periodic review and, if appropriate, 

revision of existing air quality criteria to 

reflect advances in scientific knowledge 

on the effects of the pollutant on public 

health and welfare. Under the same 

provision, the EPA is also to 

periodically review and, if appropriate, 

revise the NAAQS, based on the revised 

air quality criteria.3

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 


khammond on DSK9W7S144PROD with RULES2
105694 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

4Since 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. 

5 In reviews initiated prior to 2007, the AQCD 

provided the scientific foundation (i.e., the air 

quality criteria) for the NAAQS. Since that time, the 

ISA has replaced the AQCD. 

6Prior 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. 

7Established 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). 

NAAQS is to identify a requisite level 

of air quality, and the means of 

achieving a specific level of air quality 

is to set a standard expressed as a 

concentration of a pollutant in the air, 

such as in terms of parts per million 

(ppm), ppb, or micrograms per cubic 

meter (mg/m3). Thus, while depositionrelated effects are included within the 

‘‘adverse effects associated with the 

presence of such air pollutant in the 

ambient air,’’ EPA has never found a 

standard that quantifies atmospheric 

deposition onto surfaces to constitute a 

national secondary ambient air quality 

standard. Rather, EPA has established 

ambient air quality standards that 

specify air quality by quantifying 

pollution in the ambient air to address 

effects of such pollution, whether from 

ambient concentrations or deposition. 

B. Related Control Programs 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once the 

EPA has established them. Under CAA 

sections 110 and part D, subparts 1, 5, 

and 6 for nitrogen and sulfur oxides, 

and subparts 1, 4, and 6 for PM, and 

related provisions and regulations, 

States are to submit, for the EPA’s 

approval, State implementation plans 

(SIPs) that provide for the attainment 

and maintenance of such standards 

through control programs directed to 

sources of the pollutants involved. The 

States, in conjunction with the EPA, 

also administer the prevention of 

significant deterioration of air quality 

program that covers these pollutants. 

See 42 U.S.C. 7470–7479. In addition, 

Federal programs provide for or result 

in nationwide reductions in emissions 

of N oxides, SOX, PM and other air 

pollutants under title II of the Act, 42 

U.S.C. 7521–7574, which involves 

controls for motor vehicles, nonroad 

engines and equipment, and under the 

new source performance standards in 

section 111 of the Act, 42 U.S.C. 7411. 

C. History of the Secondary Standards 

for N Oxides, SOX and PM 

Secondary NAAQS were first 

established for N oxides, SOX and PM 

in 1971 (36 FR 8186, April 30, 1971). 

Since that time, the EPA has 

periodically reviewed the air quality 

criteria and secondary standards for 

these pollutants, with the most recent 

reviews that considered the evidence for 

ecological effects of these pollutants 

being completed in 2012 and 2013 (77 

FR 20218, April 3, 2012; 78 FR 3086, 

January 15, 2013). The subsections 

below summarize key proceedings from 

the initial standard setting in 1971 to 

the last reviews in 2012–2013.4

1. N Oxides 

The EPA first promulgated NAAQS 

for N oxides in April 1971 after 

reviewing the relevant science on the 

public health and welfare effects in the 

1971 Air Quality Criteria for Nitrogen 

Oxides (air quality criteria document or 

AQCD).5 With regard to welfare effects, 

the 1971 AQCD described effects of NO2

on vegetation and corrosion of electrical 

components linked to particulate nitrate 

(U.S. EPA, 1971). The primary and 

secondary standards were both set at 

0.053 ppm NO2 as an annual average (36 

FR 8186, April 30, 1971). In 1982, the 

EPA published an updated AQCD (U.S. 

EPA, 1982a). Based on the 1982 AQCD, 

the EPA proposed to retain the existing 

standards in February 1984 (49 FR 6866, 

February 23, 1984). After considering 

public comments, the EPA published 

the final decision to retain these 

standards in June 1985 (50 FR 25532, 

June 19, 1985). 

The EPA began a second review of the 

primary and secondary standards for 

oxides of nitrogen in 1987 (52 FR 27580, 

July 22, 1987). In November 1991, the 

EPA released an updated draft AQCD 

for CASAC and public review and 

comment (56 FR 59285, November 25, 

1991). The CASAC reviewed the draft 

document at a meeting held on July 1, 

1993, and concluded in a closure letter 

to the Administrator that the document 

provided ‘‘an adequate basis’’ for EPA’s 

decision-making in the review (Wolff, 

1993). The final AQCD was released 

later in 1993 (U.S. EPA, 1993). Based on 

the 1993 AQCD, the EPA’s Office of Air 

Quality Planning and Standards 

(OAQPS) prepared a Staff Paper,6 drafts 

of which were reviewed by the CASAC 

(Wolff, 1995; U.S. EPA, 1995a). In 

October 1995, the EPA proposed not to 

revise the secondary NO2 NAAQS (60 

FR 52874; October 11, 1995). After 

consideration of the comments received 

on the proposal, the Administrator 

finalized the decision not to revise the 

NO2 NAAQS (61 FR 52852; October 8, 

1996). The subsequent (and most recent) 

review of the N oxides secondary 

standard was a joint review with the 

secondary standard for SOX, which was 

completed in 2012 (see subsection 4 

below). 

2. SOX

The EPA first promulgated secondary 

NAAQS for SOX in April 1971 based on 

the scientific evidence evaluated in the 

1969 AQCD (U.S. DHEW, 1969a [1969 

AQCD]; 36 FR 8186, April 30, 1971). 

These standards, which were 

established on the basis of evidence of 

adverse effects on vegetation, included 

an annual arithmetic mean standard, set 

at 0.02 ppm SO2,7 and a 3-hour average 

standard set at 0.5 ppm SO2, not to be 

exceeded more than once per year. In 

1973, based on information indicating 

there to be insufficient data to support 

the finding of a study in the 1969 AQCD 

concerning vegetation injury associated 

with SO2 exposure over the growing 

season, rather than from short-term peak 

concentrations, the EPA proposed to 

revoke the annual mean secondary 

standard (38 FR 11355, May 7, 1973). 

Based on consideration of public 

comments and external scientific 

review, the EPA released a revised 

chapter of the AQCD and published its 

final decision to revoke the annual 

mean secondary standard (U.S. EPA, 

1973; 38 FR 25678, September 14, 

1973). At that time, the EPA 

additionally noted that injury to 

vegetation was the only type of SO2

welfare effect for which the evidence 

base supported a quantitative 

relationship, stating that although data 

were not available at that time to 

establish a quantitative relationship 

between SO2 concentrations and other 

public welfare effects, including effects 

on materials, visibility, soils, and water, 

the SO2 primary standards and the 3- 

hour secondary standard may to some 

extent mitigate such effects. The EPA 

also stated it was not clear that any such 

effects, if occurring below the current 

standards, were adverse to the public 

welfare (38 FR 25679, September 14, 

1973). 

In 1979, the EPA announced initiation 

of a concurrent review of the air quality 

criteria for SOX and PM and plans for 

development of a combined AQCD for 

these pollutants (44 FR 56730, October 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105695 

8Although the EPA has historically reviewed 

separately the secondary standards for oxides of 

nitrogen and oxides of sulfur, the EPA conducted 

a joint review of these standards in recognition of 

the chemical interactions in the atmosphere and 

Continued 

2, 1979). The EPA subsequently released 

three drafts of a combined AQCD for 

CASAC review and public comment. In 

these reviews, and in guidance provided 

at the August 20–22, 1980, public 

meeting of the CASAC on the first draft 

AQCD, the CASAC concluded that 

acidic deposition was a topic of extreme 

scientific complexity because of the 

difficulty in establishing firm 

quantitative relationships among 

emissions of relevant pollutants, 

formation of acidic wet and dry 

deposition products, and effects on 

terrestrial and aquatic ecosystems (53 

FR 14935, April 26, 1988). The CASAC 

also noted that a fundamental problem 

of addressing acid deposition in a 

criteria document is that acid deposition 

is produced by several different criteria 

pollutants: SOX, N oxides, and the fine 

particulate fraction of suspended 

particles (U.S. EPA, 1982b, pp. 125– 

126). The CASAC also felt that any 

document on this subject should 

address both wet and dry deposition, 

since dry deposition was believed to 

account for a substantial portion of the 

total acid deposition problem (53 FR 

14936, April 26, 1988; Lippman, 1987). 

For these reasons, CASAC 

recommended that, in addition to 

including a summary discussion of acid 

deposition in the final AQCD, a 

separate, comprehensive document on 

acid deposition be prepared prior to any 

consideration of using the NAAQS as a 

regulatory mechanism for the control of 

acid deposition. 

Following CASAC closure on the 

AQCD for SOX in December 1981, the 

EPA released a final AQCD (U.S. EPA, 

1982b), and the EPA’s OAQPS prepared 

a Staff Paper that was released in 

November 1982 (U.S. EPA, 1982c). The 

issue of acidic deposition was not, 

however, assessed directly in the 

OAQPS Staff Paper because the EPA 

followed the guidance given by the 

CASAC, subsequently preparing the 

following documents to address acid 

deposition: The Acidic Deposition 

Phenomenon and Its Effects: Critical 

Assessment Review Papers, Volumes I 

and II (U.S. EPA, 1984a, b) and The 

Acidic Deposition Phenomenon and Its 

Effects: Critical Assessment Document 

(U.S. EPA, 1985) (53 FR 14935–36, April 

26, 1988). Although these documents 

were not considered criteria documents 

and had not undergone CASAC review, 

they represented the most 

comprehensive summary of scientific 

information relevant to acid deposition 

completed by the EPA at that point. 

In April 1988, the EPA proposed not 

to revise the existing secondary 

standards for SOX (53 FR 14926, April 

26, 1988). The proposed decision 

reflected the Administrator’s 

conclusions that: (1) based upon the 

then-current scientific understanding of 

the acid deposition problem, it would 

be premature and unwise to prescribe 

any regulatory control program at that 

time; and (2) when the fundamental 

scientific uncertainties had been 

decreased through ongoing research 

efforts, the EPA would draft and support 

an appropriate set of control measures 

(53 FR 14926, April 26, 1988). This 

review of the secondary standard for 

SOX was concluded in 1993, subsequent 

to the CAA Amendments of 1990 (see 

section I.C.3.) with the decision not to 

revise the secondary standard. The EPA 

concluded that revisions to the standard 

to address acidic deposition and related 

SOX welfare effects were not 

appropriate at that time (58 FR 21351, 

April 21, 1993). In describing the 

decision, the EPA recognized the 

significant reductions in SO2 emissions, 

ambient air SO2 concentrations, and 

ultimately deposition expected to result 

from implementation of the title IV 

program, which was expected to 

significantly decrease the acidification 

of water bodies and damage to forest 

ecosystems and to permit much of the 

existing damage to be reversed with 

time (58 FR 21357, April 21, 1993). 

While recognizing that further action 

might be needed to address acidic 

deposition in the longer term, the EPA 

judged it prudent to await the results of 

the studies and research programs then 

underway, including those assessing the 

comparative merits of secondary 

standards, acidic deposition standards 

and other approaches to controlling 

acidic deposition and related effects, 

and then to determine whether 

additional control measures should be 

adopted or recommended to Congress 

(58 FR 21358, April 21, 1993). 

3. Related Actions Addressing Acid 

Deposition 

In 1980, Congress created the National 

Acid Precipitation Assessment Program. 

During the 10-year course of this 

program, the program issued a series of 

reports, including a final report in 1990 

(NAPAP, 1991). On November 15, 1990, 

Amendments to the CAA were passed 

by Congress and signed into law by the 

President. In title IV of these 

Amendments, Congress included a 

statement of findings including the 

following: 

(1) the presence of acidic compounds and 

their precursors in the atmosphere and in 

deposition from the atmosphere represents a 

threat to natural resources, ecosystems, 

materials, visibility, and public health; . . . 

(3) the problem of acid deposition is of 

national and international significance; . . . 

(5) current and future generations of 

Americans will be adversely affected by 

delaying measures to remedy the problem[.] 

The goal of title IV was to reduce 

emissions of SO2 by 10 million tons and 

N oxides emissions by 2 million tons 

from 1980 emission levels in order to 

achieve reductions over broad 

geographic regions/areas. In envisioning 

that further action might be necessary in 

the long term, Congress included 

section 404 of the 1990 Amendments. 

This section requires the EPA to 

conduct a study on the feasibility and 

effectiveness of an acid deposition 

standard or standards to protect 

‘‘sensitive and critically sensitive 

aquatic and terrestrial resources’’ and at 

the conclusion of the study, submit a 

report to Congress. Five years later, the 

EPA submitted to Congress its report 

titled Acid Deposition Standard 

Feasibility Study: Report to Congress 

(U.S. EPA, 1995b) in fulfillment of this 

requirement. The Report to Congress 

concluded that establishing acid 

deposition standards for S and N 

deposition might at some point in the 

future be technically feasible although 

appropriate deposition loads for these 

acidifying chemicals could not be 

defined with reasonable certainty at that 

time. 

The 1990 Amendments also added 

new language to sections of the CAA 

pertaining to ecosystem effects of 

criteria pollutants, such as acid 

deposition. For example, a new section 

108(g) was inserted, stating that ‘‘[t]he 

Administrator may assess the risks to 

ecosystems from exposure to criteria air 

pollutants (as identified by the 

Administrator in the Administrator’s 

sole discretion).’’ The definition of 

welfare in CAA section 302(h) was 

expanded to indicate that welfare effects 

include those listed therein, ‘‘whether 

caused by transformation, conversion, 

or combination with other air 

pollutants.’’ Additionally, in response to 

legislative initiatives such as the 1990 

Amendments, the EPA and other 

Federal agencies continued research on 

the causes and effects of acidic 

deposition and related welfare effects of 

SO2 and implemented an enhanced 

monitoring program to track progress 

(58 FR 21357, April 21, 1993). 

4. Most Recent Review of the Secondary 

Standards for N Oxides and SOX

In December 2005, the EPA initiated 

a joint review 8 of the air quality criteria 


khammond on DSK9W7S144PROD with RULES2
105696 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

associated contributions to acid deposition and 

related environmental effects. The joint review was 

also responsive to a National Research Council 

recommendation that the EPA consider pollutants 

in combination, as appropriate, in considering the 

NAAQS (NRC, 2004). 

9The REAs for NAAQS reviews may be presented 

in appendices to the PA or in stand-alone 

documents (e.g., U.S. EPA 2020b, 2020c, and PA for 

current review [U.S. EPA, 2024]). 

10The 1-hour primary standards set in 2010 were 

a NO2 standard of 100 ppb, as the 98th percentile 

of 1-hour daily maximum concentrations, averaged 

over three years, and a SO2 standard of 75 ppb, as 

the 99th percentile of daily maximum 1-hour 

concentrations, averaged over three years (75 FR 

6474, February 9, 2010; 75 FR 35520, June 22, 

2010). 

11Center for Biological Diversity, et al. v. EPA, 

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

12 Id. at 1088. 

13Additionally, a guide to be used in assessing 

implementation plans to achieve the 24-hour 

standard was set at 60 mg/m3, as an annual 

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

and secondary NAAQS for oxides of 

nitrogen and sulfur (70 FR 73236, 

December 9, 2005). The review focused 

on the evaluation of the protection 

provided by the standards for two 

general types of effects: (1) direct effects 

on vegetation of exposure to gaseous 

oxides of nitrogen and sulfur, which are 

the type of effects that the existing 

standards were developed to protect 

against, and (2) effects associated with 

the deposition of N oxides and SOX to 

sensitive aquatic and terrestrial 

ecosystems (77 FR 20218, April 3, 

2012). 

The Integrated Review Plan (IRP) for 

the review was released in December 

2007, after review of a draft IRP by the 

public and CASAC (72 FR 57570, 

October 10, 2007; Russell, 2007; U.S. 

EPA, 2007). The first and second drafts 

of the ISA were released in December 

2007 and August 2008, respectively, for 

the CASAC and public review (72 FR 

72719, December 21, 2007; 73 FR 10243, 

February 26, 2008; Russell and 

Henderson, 2008; 73 FR 46908, August 

12, 2008; 73 FR 53242, September 15, 

2008; Russell and Samet, 2008a). The 

EPA released a final ISA (referred to as 

2008 ISA below) in December 2008 (73 

FR 75716, December 12, 2008; U.S. EPA, 

2008a). Based on the scientific 

information in the ISA, the EPA 

planned and developed a quantitative 

Risk and Exposure Assessment (REA),9

two drafts of which were made available 

for public comment and reviewed by the 

CASAC (73 FR 10243, February 26, 

2008; 73 FR 50965, August 29, 2008; 

Russell and Samet, 2008b; 73 FR 53242, 

September 15, 2008; 74 FR 28698, June 

17, 2009; Russell and Samet, 2009). The 

final REA was released in September 

2009 (U.S. EPA, 2009a; 74 FR 48543; 

September 23, 2009). 

Drawing on the information in the 

REA and ISA, the EPA OAQPS prepared 

a PA, two drafts of which were made 

available for public comment and 

review by the CASAC (75 FR 10479, 

March 8, 2010; 75 FR 11877, March 12, 

2010; Russell and Samet, 2010b; 75 FR 

57463, September 21, 2010; 75 FR 

65480, October 25, 2010; Russell and 

Samet, 2010a). The final PA was 

released in January 2011 (U.S. EPA, 

2011). For the purpose of protection 

against the direct effects on vegetation 

of exposure to gaseous oxides of 

nitrogen and sulfur, the final PA 

concluded that consideration should be 

given to retaining the current standards. 

With respect to the effects associated 

with the deposition of oxides of 

nitrogen and oxides of sulfur to 

sensitive aquatic and terrestrial 

ecosystems, the 2011 PA focused on the 

acidifying effects of nitrogen and sulfur 

deposition on sensitive aquatic 

ecosystems. Based on the information in 

the ISA, the assessments in the REA, 

and the CASAC advice, the 2011 PA 

concluded that consideration should be 

given to a new multipollutant standard 

intended to address deposition-related 

effects (details provided in section 

II.A.1.b. below). Based on consideration 

of the final PA, the CASAC provided 

additional advice and recommendations 

on the multipollutant, deposition-based 

standard described in the 2011 PA (76 

FR 4109, January 24, 2011; 76 FR 16768, 

March 25, 2011; Russell and Samet, 

2011). 

On August 1, 2011, the EPA 

published a proposed decision to retain 

the existing annual average NO2 and 3- 

hour average SO2 secondary standards, 

recognizing the protection they 

provided from direct effects on 

vegetation (76 FR 46084, August 1, 

2011). Further, after considering the 

multipollutant approach to establishing 

secondary standards that was described 

in the 2011 PA, the Administrator 

proposed not to set such a new 

multipollutant secondary standard in 

light of a number of uncertainties. 

Alternatively, the Administrator 

proposed to revise the secondary 

standards by adopting secondary NO2

and SO2 standards identical to the 1- 

hour primary NO2 and SO2 standards, 

both of which were set in 2010, noting 

that these new primary standards, while 

not set based on consideration of 

atmospheric deposition,10 were likely to 

reduce oxides of nitrogen and sulfur 

emissions and associated nitrogen and 

sulfur deposition in sensitive 

ecosystems (76 FR 46084, August 1, 

2011). After consideration of public 

comments, the EPA decided to retain 

the existing standards (without revision) 

to address the direct effects on 

vegetation of exposure to gaseous oxides 

of nitrogen and sulfur. At that time, the 

EPA also described its decision that it 

was not appropriate to set new 

secondary standards at that time to 

address deposition-related effects 

associated with oxides of nitrogen and 

sulfur (77 FR 20218, April 3, 2012). 

The EPA’s 2012 decision was 

challenged by the Center for Biological 

Diversity and other environmental 

groups, who argued that the EPA, 

having decided that the existing 

standards were not adequate to protect 

against adverse public welfare effects 

such as damage to sensitive ecosystems, 

was required to identify the requisite 

level of protection for the public welfare 

and to issue NAAQS to achieve and 

maintain that level of protection. The 

District of Columbia Circuit (D.C. 

Circuit) disagreed, finding that the EPA 

acted appropriately in not setting a 

secondary standard given EPA’s 

conclusions that ‘‘the available 

information was insufficient to permit a 

reasoned judgment about whether any 

proposed standard would be ‘requisite 

to protect the public welfare . . . ’.’’ 11

In reaching this decision, the court 

noted that the EPA had ‘‘explained in 

great detail’’ the profound uncertainties 

associated with setting a secondary 

NAAQS to protect against aquatic 

acidification.12

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 mg/m3, as a 24- 

hour average, from total suspended 

particles (TSP), not to be exceeded more 

than once per year (36 FR 8187; April 

30, 1971).13

In October 1979, the EPA announced 

the first review of the air quality criteria 

and NAAQS for PM (44 FR 56730, 

October 2, 1979). A combined AQCD for 

PM and SOX was released in 1982, after 

CASAC and public review of drafts 

(U.S. EPA, 1982b). Soon after, the 

OAQPS released a Staff Paper (U.S. 

EPA, 1982d), two drafts of which had 

received public and CASAC review 

(Friedlander, 1982). In 1984, the EPA 

proposed replacing the secondary 

standard with an annual TSP standard 

with a level within the range of 70–90 

mg/m3, as an expected annual arithmetic 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105697 

14The 1997 annual PM2.5 standard was compared 

with measurements made at the communityoriented 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. Areawide 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). 

mean (49 FR 10408, March 20, 1984). 

After consideration of public comment 

and review by the CASAC and the 

public, the OAQPS released an 

Addendum to the Staff Paper in 1986 

(Lippman, 1986; U.S. EPA, 1986). In 

1987, the EPA completed the review by 

adopting two new primary PM NAAQS 

and setting the secondary standards 

identical to the primary standards in all 

respects, all with a new indicator for PM 

(particles with a nominal mass median 

diameter of 10 microns, PM10). The new 

primary and secondary standards 

included (1) a 24-hour standard of 150 

mg/m3, in terms of one expected 

exceedance per year, on average over 

three years and (2) an annual secondary 

standard of 50 mg/m3, as an annual 

arithmetic mean, averaged over three 

years (52 FR 24634, July 1, 1987). 

In April 1994, the EPA initiated the 

second periodic review of the air quality 

criteria and NAAQS for PM. In 

developing the AQCD, the Agency made 

available three external review drafts for 

public and CASAC review; the final 

AQCD was released in 1996 (U.S. EPA, 

1996). The OAQPS released a Staff 

Paper in November 1997, after CASAC 

and public review of two drafts (U.S. 

EPA, 1996; Wolff, 1996). The EPA 

proposed revisions to the PM standards 

in 1996 and promulgated final standards 

in 1997 (61 FR 65738; December 13, 

1996; 62 FR 38652, July 18, 1997). With 

the 1997 decision, the EPA added new 

standards, using particles with a 

nominal mean aerodynamic diameter 

less than or equal to 2.5 mm (PM2.5) as 

the indicator for fine particles. The new 

secondary PM2.5 standards were set 

equal to the primary PM2.5 standards, in 

all respects, as follows: (1) an annual 

standard with a level of 15.0 mg/m3, 

based on the 3-year average of annual 

arithmetic mean PM2.5 concentrations 

from single or multiple communityoriented monitors,14 and (2) a 24-hour 

standard with a level of 65 mg/m3, based 

on the 3-year average of the 98th 

percentile of 24-hour PM2.5

concentrations at each monitor within 

an area. The EPA also retained the 

primary and secondary annual PM10

standards, without revision, and revised 

the form of the 24-hour primary and 

secondary PM10 standards to be based 

on the 99th percentile of 24-hour PM10

concentrations at each monitor in an 

area. 

Following promulgation of the 1997 

PM NAAQS, several parties filed 

petitions for review, raising a broad 

range of issues. In May 1999, the U.S. 

Court of Appeals for the D.C. Circuit 

upheld the EPA’s decision to establish 

fine particle (PM2.5) standards, 

(American Trucking Ass’ns, Inc. v. EPA, 

175 F. 3d 1027, 1055–56 [D.C. Cir. 

1999]). The D.C. Circuit also found 

‘‘ample support’’ for the EPA’s decision 

to regulate coarse particle (PM10) 

pollution but vacated the 1997 PM10

standards, concluding that the EPA had 

not provided a reasonable explanation 

justifying use of PM10 as an indicator for 

coarse particles (id. at 1054–55). 

Pursuant to the D.C. Circuit’s decision, 

the EPA removed the vacated the 1997 

PM10 standards, leaving the pre-existing 

1987 PM10 standards in place (65 FR 

80776, December 22, 2000). The D.C. 

Circuit also upheld the EPA’s 

determination not to establish more 

stringent secondary standards for fine 

particles to address effects on visibility 

(id. at 1027). The D.C. Circuit also 

addressed more general issues related to 

the NAAQS, including issues related to 

the consideration of costs in setting 

NAAQS and the EPA’s approach to 

establishing the levels of NAAQS. 

In October 1997, the EPA initiated the 

third periodic review of the air quality 

criteria and NAAQS for PM (62 FR 

55201, October 23, 1997). The EPA 

released the final AQCD in October 

2004, after the CASAC and public 

review of several drafts (U.S. EPA, 

2004a, b). The OAQPS released a Staff 

Paper in December 2005 (U.S. EPA, 

2005). Also in December 2005, the EPA 

proposed to revise the PM NAAQS and 

solicited public comment on a broad 

range of options (71 FR 2620, January 

17, 2006). In September 2006, after 

consideration of public comment, the 

EPA revised the PM NAAQS, making 

revisions to the secondary standards 

identical to those for the primary 

standards, with the decision describing 

the protection provided specifically for 

visibility and non-visibility related 

welfare effects (71 FR 61144, 61203– 

61210, October 17, 2006). The EPA 

revised the level of the 24-hour PM2.5

standards to 35 mg/m3, retained the level 

of the annual PM2.5 standards at 15.0 mg/ 

m3, and revised the form of the annual 

PM2.5 standards by narrowing the 

constraints on the optional use of spatial 

averaging. For PM10, the EPA revoked 

the annual standards and retained the 

24-hour standards, both with a level of 

150 mg/m3. 

Several parties filed petitions for 

review of the 2006 p.m. NAAQS 

decision, with one raising the issue of 

the secondary PM2.5 standards being 

identical to the primary standards. On 

February 24, 2009, the D.C. Circuit 

issued its opinion in American Farm 

Bureau Federation v. EPA, 559 F. 3d 

512 (D.C. Cir. 2009), remanding the 

standards to the EPA stating the Agency 

had failed to adequately explain how 

setting the secondary standards 

identical to the primary standards 

provided the required public welfare 

protection, including for visibility 

impairment (Id. at 528–32). The EPA 

responded to the court’s remands as part 

of the subsequent PM NAAQS review. 

In June 2007, the EPA initiated the 

fourth periodic review of the air quality 

criteria and the PM NAAQS (72 FR 

35462, June 28, 2007). To inform 

planning for the review, the EPA held 

science/policy issue workshops later 

that year (72 FR 34003, June 20, 2007; 

72 FR 34005, June 20, 2007). Plans for 

the review and for welfare assessments 

were developed in 2008 and 2009; the 

ISA was completed in 2009, an urbanfocused visibility assessment was 

completed in 2010 and the PA was 

released in 2011 (U.S. EPA, 2008b; U.S. 

EPA, 2009b; U.S. EPA, 2009c; U.S. EPA, 

2010; U.S. EPA, 2011). In June 2012, the 

EPA proposed revisions to the PM 

NAAQS and in December 2012 

announced its final decisions to revise 

the primary and secondary PM2.5 annual 

standards (77 FR 38890, June 29, 2012; 

78 FR 3086, January 15, 2013). With 

regard to the secondary standards, the 

EPA retained the 24-hour PM2.5 and 

PM10 standards, with a revision to the 

form of the 24-hour PM2.5, to eliminate 

the option for spatial averaging (78 FR 

3086, January 15, 2013). Petitioners 

challenged the EPA’s final rule. On 

judicial review, the revised standards 

and monitoring requirements were 

upheld in all respects (National 

Association of Manufacturers v. EPA, 

750 F.3d 921, [D.C. Cir. 2014]). 

The subsequent review of the PM 

secondary standards, completed in 

2020, and its subsequent 

reconsideration focused on 

consideration of protection provided 

from visibility effects, materials damage, 

and climate effects (85 FR 82684, 

December 18, 2020; 89 FR 16202, March 

6, 2024). Those effects—visibility 

effects, materials damage and climate 

effects—are not addressed in this 

review. The evidence for ecological 

effects of PM is addressed in the review 

of the air quality criteria and standards 

described in the PA for this review. 


khammond on DSK9W7S144PROD with RULES2
105698 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

15A change in CASAC membership contributed 

to an extended time period between the two public 

meetings. 

16The planning document for quantitative 

aquatic acidification exposure/risk analyses was 

also made available for public comment and 

consultation with the CASAC (83 FR 31755, July 9, 

2018; Cox, Kendall, and Fernandez, 2020b; U.S. 

EPA, 2018; 83 FR 42497, August 22, 2018). 

17The public hearing transcript and any written 

testimony provided are also in the docket. 

18The docket for this review, Docket ID No. EPA– 

HQ–OAR–2014–0128, has incorporated the ISA 

docket (Docket ID No. EPA–HQ–ORD–2013–0620) 

by reference. Both are publicly accessible at https:// 

www.regulations.gov. 

D. Current Review 

In August 2013, the EPA issued a call 

for information in the Federal Register 

for information related to the current 

review of the air quality criteria for SOX

and N oxides and announced a public 

workshop to discuss policy-relevant 

scientific information to inform the 

review (78 FR 53452, August 29, 2013). 

Based in part on the information 

received in response to the call for 

information, the EPA developed a draft 

IRP, which was made available for 

consultation with the CASAC and for 

public comment (80 FR 69220, 

November 9, 2015). Comments from the 

CASAC and the public on the draft IRP 

were considered in preparing the final 

IRP (Diez Roux and Fernandez, 2016; 

U.S. EPA, 2017). In developing the final 

IRP, the EPA expanded the review to 

also include review of the criteria and 

standards related to ecological effects of 

PM in recognition of atmospheric 

transformations and deposition 

involving the three pollutants (N oxides, 

SOX and PM) and associated ecological 

effects (U.S. EPA, 2017). In so doing, the 

EPA clarified that other effects of PM, 

including materials damage, climate 

effects and visibility effects are beyond 

the scope of this review (IRP, p. 1–2 and 

section 2.1). 

In March 2017, the EPA released the 

first external review draft of the 

Integrated Science Assessment (ISA) for 

Oxides of Nitrogen, Oxides of Sulfur, 

and Particulate Matter Ecological 

Criteria (82 FR 15702, March 30, 2017), 

which was then reviewed by the CASAC 

at public meetings in May and August 

2017 (82 FR 15701, March 30, 2017; 82 

FR 35200, July 28, 2017; Diez Roux and 

Fernandez, 2017). A second external 

review draft ISA was released in 2018 

and reviewed by the CASAC at public 

meetings in September 2018 and April 

2020 (83 FR 2018; July 9, 2018; 85 FR 

16093, March 30, 2020; Cox, Kendall, 

and Fernandez, 2020a).15 The EPA 

released the final ISA in October 2020 

(85 FR 66327, October 19, 2020; U.S. 

EPA, 2020a). 

In 2023, the draft PA, including the 

REA for aquatic acidification as an 

appendix,16 was released for review by 

the CASAC and for public comment (88 

FR 34852, May 31, 2023). The CASAC 

conducted its review at public meetings 

in June and September 2023 and 

conveyed its advice to the 

Administrator on the standards and 

comments on the draft PA in late 

September 2023 (88 FR 17572, March 

23, 2023; 88 FR 45414, July 17, 2023; 

Sheppard, 2023). In January 2024, the 

EPA released the final PA (89 FR 2223, 

January 12, 2024; U.S. EPA, 2024). In 

April 2024, the EPA proposed to revise 

the secondary SO2 standard and retain 

the secondary standards for N oxides 

and PM (89 FR 26620, April 15, 2024). 

During the subsequent public comment 

period, public comments were received 

both orally during a virtual public 

hearing on May 8, 2024 (89 FR 26114, 

April 15, 2024) and in writing to the 

docket (as discussed in section II.B.2. 

below).17 Significant comments 

received are addressed in this preamble 

to this final action and in the 

accompanying Response to Comments 

document, which can be found in the 

docket for this review. The schedule for 

completion of this review has been 

governed by a consent decree that 

requires the EPA to sign for publication 

a notice of final rulemaking concerning 

review of the NAAQS for N oxides, SOX

and PM no later than December 10, 2024 

(Center for Biological Diversity v. Regan 

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

Materials upon which the decision in 

this review is based, including the 

documents described above, are 

available to the public in the docket for 

this review.18 The EPA is basing its 

decision in this review on studies and 

related information included in the air 

quality criteria, which have undergone 

CASAC and public review. The studies 

assessed in the ISA and PA, and the 

integration of the scientific evidence 

presented in them, have undergone 

extensive critical review by the EPA, the 

CASAC, and the public. The rigor of 

that review makes these studies, and 

their integrative assessment, the most 

reliable source of scientific information 

on which to base decisions on the 

NAAQS, decisions that all recognize to 

be of great import. Decisions on the 

NAAQS can have profound impacts on 

public health and welfare, and NAAQS 

decisions should be based on studies 

that have been rigorously assessed in an 

integrated manner not only by the EPA 

but also by the statutorily mandated 

independent scientific advisory 

committee, as well as the public review 

that accompanies this process. 

Some commenters have referred to 

and discussed individual scientific 

studies on the welfare effects of SOX, N 

oxides, and PM that were not included 

in the ISA (‘‘new’’ studies) and that 

have not gone through this 

comprehensive review process. In 

considering and responding to 

comments for which such ‘‘new’’ 

studies were cited in support, the EPA 

has provisionally considered the cited 

studies in the context of the findings of 

the ISA (Weaver, 2024). The EPA’s 

provisional consideration of these 

studies did not and could not provide 

the kind of in-depth critical review 

described above, but rather was focused 

on determining whether they warranted 

reopening the review of the air quality 

criteria to enable the EPA, the CASAC 

and the public to consider them further 

as part of this review. This approach, 

and the decision to rely on studies and 

related information included in the air 

quality criteria, which have undergone 

CASAC and public review, is consistent 

with the EPA’s practice in prior NAAQS 

reviews and its interpretation of the 

requirements of the CAA. Since the 

1970 amendments, the EPA has taken 

the view that NAAQS decisions are to 

be based on scientific studies and 

related information that have been 

assessed as a part of the pertinent air 

quality criteria, and the EPA has 

consistently followed this approach. 

This longstanding interpretation was 

strengthened by new legislative 

requirements enacted in 1977, which 

added section 109(d)(2) of the Act 

concerning CASAC review of air quality 

criteria. See 71 FR 61144, 61148 

(October 17, 2006, final decision on 

review of NAAQS for particulate matter) 

for a detailed discussion of this issue 

and the EPA’s past practice. 

As discussed in the EPA’s 1993 

decision not to revise the ozone (O3) 

NAAQS, ‘‘new’’ studies may sometimes 

be of such significance that it is 

appropriate to delay a decision in a 

NAAQS review and to supplement the 

pertinent air quality criteria so the 

studies can be taken into account (58 FR 

at 13013–13014, March 9, 1993). In the 

present case, the EPA’s consideration of 

‘‘new’’ studies concludes that, taken in 

context, the ‘‘new’’ information and 

findings do not materially change any of 

the broad scientific conclusions made in 

the air quality criteria regarding the 

health and welfare effects of the subject 

pollutants in ambient air. For this 

reason, reopening the air quality criteria 

review is not warranted. Accordingly, 

the EPA is basing the final decisions in 

this review on the studies and related 

information included in the air quality 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105699 

19 In addition to the review’s opening ‘‘Call for 

Information’’ (78 FR 53452, August 29, 2013), 

multiple search methodologies were applied to 

identify relevant scientific findings that have 

emerged since the 2008 ISA. Search techniques for 

the current ISA identified and evaluated studies 

and reports that have undergone scientific peer 

review and were published or accepted for 

publication between January 2008 (providing some 

overlap with the cutoff date for the 2008 ISA) and 

May 2017. Studies published after the literature 

cutoff date for this ISA were also considered in the 

ISA if they were submitted in response to the Call 

for Information or identified in subsequent phases 

of ISA development, particularly to the extent that 

they provide new information that affects key 

scientific conclusions. References that are cited in 

the ISA, the references that were considered for 

inclusion but not cited, and electronic links to 

bibliographic information and abstracts can be 

found at: https://hero.epa.gov/hero/index.cfm/ 

project/page/project_id/2965 (ISA, section IS.1.2). 

20The indicator defines the chemical species or 

mixture to be measured in the ambient air for the 

purpose of determining whether an area attains the 

standard. The averaging time defines the period 

over which air quality measurements are to be 

averaged or otherwise analyzed. The form of a 

standard defines the air quality statistic that is to 

be compared to the level of the standard in 

determining whether an area attains the standard. 

For example, the form of the annual NAAQS for 

fine particulate matter (PM2.5) is the average of 

annual mean concentrations for three consecutive 

years, while the form of the 3-hour secondary 

NAAQS for SO2 is the second highest 3-hour 

average in a year. The level of the standard defines 

the air quality concentration used for that purpose. 

criteria that have undergone rigorous 

review by the EPA, the CASAC, and the 

public. The EPA will consider these 

‘‘new’’ studies for inclusion in the air 

quality criteria for the next review, 

which will provide the opportunity to 

fully assess these studies through a 

more rigorous review process involving 

the EPA, the CASAC, and the public. 

II. Rationale for Decisions 

This section presents the rationale for 

the Administrator’s decisions in the 

review of the secondary NAAQS for the 

ecological effects of SOX, N oxides and 

PM. This rationale is based on a 

thorough review of the full evidence 

base, including the scientific 

information available since the last 

reviews of the secondary standards for 

N oxides, SOX and PM. This 

information on ecological effects 

associated with SOX, N oxides and PM 

and pertaining to their presence in 

ambient air, which includes studies 

generally published between January 

2008 and May 2017 (and considered in 

the ISA), is integrated with the 

information and conclusions from 

previous assessments and presented in 

the ISA (ISA, section IS.1.2).19 The 

Administrator’s rationale also takes into 

account: (1) the PA evaluation of the 

policy-relevant information in the ISA 

and presentation of quantitative 

analyses of air quality, exposure and 

aquatic acidification risks; (2) CASAC 

advice and recommendations, as 

reflected in discussions of drafts of the 

ISA and PA at public meetings and in 

the CASAC’s letters to the 

Administrator; (3) public comments 

received during the development of 

these documents; and (4) public 

comments received on the proposed 

decisions. 

Before presenting the rationale for the 

Administrator’s final decisions and their 

foundations, section II.A.1. provides an 

introduction that also summarizes the 

basis for the existing standards (section 

II.A.1.a.), provides background on the 

prior review of deposition-related 

effects of N oxides and SOX (section 

II.A.1.b.), and summarizes the general 

approach in this review (section 

II.A.1.c.). Section II.A.2. provides an 

overview of the air quality information 

and analyses relating S and N 

deposition to concentrations of SOX, N 

oxides and PM. Section II.A.3. provides 

an overview of the currently available 

ecological effects evidence as 

summarized in the ISA, focusing on 

consideration of key policy-relevant 

aspects, and section II.A.4. provides an 

overview of the exposure and risk 

information for this review, drawing on 

the quantitative analyses of aquatic 

acidification risk, presented in the PA. 

Section II.B.1. provides a summary of 

the Administrator’s proposed decisions 

(section II.B.1.c.), which drew on both 

evidence-based and exposure/risk-based 

considerations from the PA (section 

II.B.1.a.) and advice from the CASAC 

(section II.B.1.b.). Section II.B.2. 

discusses comments received on the 

proposed decision, and section II.B.3. 

presents the Administrator’s 

conclusions and associated rationale. 

The final decisions are summarized in 

section II.C. 

A. Introduction 

The Agency’s approach in its review 

of secondary standards is consistent 

with the requirements of the provisions 

of the CAA related to the review of 

NAAQS and with how the EPA and the 

courts have historically interpreted the 

CAA. These provisions require the 

Administrator to establish secondary 

standards that, in the Administrator’s 

judgment, are requisite (i.e., neither 

more nor less stringent than necessary) 

to protect the public welfare from 

known or anticipated adverse effects 

associated with the presence of the 

pollutant in the ambient air. In so doing, 

the Administrator considers advice from 

the CASAC and public comment. This 

approach is based on a recognition that 

the available welfare effects evidence 

generally reflects a range of effects that 

include ambient air-related exposure 

circumstances for which scientists 

generally agree that effects are likely to 

occur as well as lower levels at which 

the likelihood and magnitude of 

response become increasingly uncertain. 

The CAA does not require that 

standards be set at a zero-risk level, but 

rather at a level that reduces risk 

sufficiently to protect the public welfare 

from known or anticipated adverse 

effects. 

The Agency’s decisions on the 

adequacy of the current secondary 

standards and, as appropriate, on any 

potential alternative standards 

considered in a review, are largely 

public welfare policy judgments made 

by the Administrator based on the 

Administrator’s informed assessment of 

what constitutes requisite protection 

against adverse effects to the public 

welfare. A public welfare policy 

decision draws upon scientific 

information and analyses about welfare 

effects, exposures and risks, as well as 

judgments about the appropriate 

response to the range of uncertainties 

that are inherent in the scientific 

evidence and analyses. The ultimate 

determination as to what level of 

damage to ecosystems and the services 

provided by those ecosystems is adverse 

to public welfare is not wholly a 

scientific question, although it may be 

informed by scientific studies linking 

ecosystem damage to losses in 

ecosystem services and information on 

the value of those losses of ecosystem 

services. In reaching decisions on 

secondary standards, the Administrator 

seeks to establish standards that are 

neither more nor less stringent than 

necessary for this purpose. In evaluating 

the public welfare protection afforded 

by the standards, the four basic 

elements of the NAAQS (indicator, 

averaging time, level, and form) are 

considered collectively.20

Generally, conclusions reached by the 

Administrator in secondary NAAQS 

reviews on the amount of public welfare 

protection from the presence of the 

pollutant(s) in ambient air that is 

appropriate to be afforded by a 

secondary standard take into account a 

number of considerations. Among these 

considerations are the nature and degree 

of effects of the pollutant, including the 

Administrator’s judgments on what 

constitutes an adverse effect to the 

public welfare, as well as the strengths 

and limitations of the available and 

relevant information, with its associated 

uncertainties. Across reviews, it is 

generally recognized that such 

judgments should neither overstate nor 

understate the strengths and limitations 

of the evidence and information nor the 


khammond on DSK9W7S144PROD with RULES2
105700 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

21As noted in section I.D. above, the 2020 review 

of the PM secondary NAAQS and its 

reconsideration focused on visibility effects, 

materials damage and climate effects, while the 

ecological effects of PM are being addressed in this 

combined review (89 FR 16205, March 6, 2024). 

appropriate inferences to be drawn as to 

risks to public welfare, and that the 

choice of the appropriate level of 

protection is a public welfare policy 

judgment entrusted to the Administrator 

under the CAA taking into account both 

the available evidence and associated 

uncertainties (80 FR 65404–05, October 

26, 2015). Thus, the Administrator’s 

final decisions in such reviews draw 

upon the scientific information and 

analyses about welfare effects, 

environmental exposures and risks, and 

associated public welfare significance, 

as well as judgments about how to 

consider the range and magnitude of 

uncertainties that are inherent in the 

scientific evidence and quantitative 

analyses. 

1. Background 

Ecological effects of N oxides, SOX

and PM include those related to direct 

contact of the airborne pollutants with 

plants and those related to atmospheric 

deposition of N- and S-containing 

compounds into sensitive ecosystems. 

As summarized in section II.A.1.a. 

below, it is the former category of effects 

(from direct contact) that were 

considered in establishing the existing 

standards, with those effects as the basis 

for the secondary standards for N oxides 

and SOX. In the last review of those 

standards, deposition-related effects 

were also considered. However, as 

summarized in section II.A.1.b. below, 

the extent of the uncertainties 

associated with the complex 

methodology investigated for defining a 

deposition-based standard in that 

review were found to be so significant 

that the Administrator concluded that 

the limitations and uncertainties in the 

available information were too great to 

support establishment of a new standard 

using this methodology that could be 

concluded to provide the requisite 

protection for such effects under the Act 

(77 FR 20218, April 3, 2012). As 

described in the proposal for the current 

action, and generally summarized in 

section II.A.1.c. below, in the current 

review we have taken a different 

approach to considering standards that 

might be expected to provide the 

appropriate level of protection from 

deposition-related effects. 

a. Basis for Existing Secondary 

Standards 

The existing 3-hour secondary SO2

standard, with its level of 0.5 ppm, and 

the annual secondary NO2 standard, 

with its level of 0.053 ppm were 

established in 1971 (36 FR 8186, April 

30, 1971). The basis for both the existing 

SO2 and NO2 secondary standards is to 

provide protection to the public welfare 

related to direct effects on vegetation 

(U.S. DHEW, 1969a; U.S. EPA, 1971). 

There are three secondary PM 

standards—established in 1997 (annual 

PM2.5 standard) and 2006 (24-hour PM2.5

and PM10 standards)—variously based 

on consideration of materials damage, 

visibility impacts, climate effects and 

ecological effects.21

The welfare effects evidence for SOX

in previous reviews indicates a 

relationship between short- and longterm SO2 exposures and foliar damage 

to cultivated plants, as well as 

reductions in productivity, species 

richness, and diversity (U.S. DHEW, 

1969a; U.S. EPA, 1982c; U.S. EPA, 

2008a). At the time the standard was set, 

concentrations of SO2 in the ambient air 

were also associated with other welfare 

effects, including effects on materials 

and visibility related to sulfate, a 

particulate transformation product of 

SO2 (U.S. DHEW, 1969a). However, the 

available data were not sufficient to 

establish a quantitative relationship 

between specific SO2 concentrations 

and such effects (38 FR 25679, 

September 14, 1973). Accordingly, 

direct effects of SOX in ambient air on 

vegetation are the basis for the existing 

secondary standard for SOX. 

The welfare effects evidence for N 

oxides in previous reviews includes 

foliar injury, leaf drop, and reduced 

yield of some crops (U.S. EPA, 1971; 

U.S. EPA, 1982c; U.S. EPA, 1993; U.S. 

EPA, 2008a). Since it was established in 

1971, the secondary standard for N 

oxides has been reviewed three times, in 

1985, 1996, and 2012 (50 FR 25532, 

June 19, 1985; 61 FR 52852; October 8, 

1996; 77 FR 20218, April 3, 2012). 

Although those reviews identified 

additional effects related to N 

deposition, they all have concluded that 

the existing NO2 secondary standard 

provided adequate protection related to 

the effects of direct contact of airborne 

N oxides with vegetation on which the 

standard is based. 

In the last review of the secondary PM 

standards with regard to protection from 

ecological effects, completed in 2013, 

the EPA retained the 24-hour PM2.5

standard, with its level of 35 mg/m3, and 

the 24-hour PM10 standard, with its 

level of 150 mg/m3 (78 FR 3228, January 

15, 2013). With regard to the annual 

PM2.5 standard, the EPA retained the 

averaging time and level, set at 15 mg/ 

m3, while revising the form to remove 

the option for spatial averaging 

consistent with this change to the 

primary annual PM2.5 standard (78 FR 

3225, January 15, 2013). The effects 

considered in that review of the 

secondary PM standards include effects 

on visibility, materials damage, and 

climate effects, as well as ecological 

effects; the EPA concluded that those 

standards provided protection for 

ecological effects (e.g., 78 FR 3225– 

3226, 3228, January 15, 2013). In 

reaching this conclusion, it was noted 

that the PA for the review explicitly 

excluded discussion of the effects 

associated with deposited PM 

components of N oxides and SOX and 

their transformation products, which 

were being addressed in the joint review 

of the secondary NO2 and SO2 NAAQS 

(78 FR 3202, January 15, 2013). The 

ecological effects of PM considered in 

the 2013 review included direct effects 

on plant foliage as well as effects of the 

ecosystem loading of PM constituents 

such as metals or organic compounds 

(2009 ISA, section 2.5.3). For all of these 

effects, the 2013 decision recognized an 

absence of information that would 

support any different standards and 

concluded the existing standards, with 

the revision to the form of the annual 

PM2.5 standard, provided the requisite 

protection (78 FR 3086, January 15, 

2013). 

b. Prior Review of Deposition-Related 

Effects 

In the 2012 review of the NO2 and 

SO2 secondary standards, the EPA 

recognized that a significant increase in 

understanding of the effects of N oxides 

and SOX had occurred since the 

preceding secondary standards reviews 

for those pollutants (77 FR 20236, April 

3, 2012). Considering the extensive 

evidence available in the 2012 review, 

the Agency concluded that the most 

significant risks of adverse effects of N 

oxides and SOX to the public welfare 

were those related to deposition of N 

and S compounds in both terrestrial and 

aquatic ecosystems (77 FR 20236, April 

3, 2012). Accordingly, in addition to 

evaluating the protection provided by 

the secondary standards for N oxides 

and SOX from effects associated with 

the airborne pollutants, the 2012 review 

also included extensive analyses of the 

welfare effects associated with 

atmospheric deposition of N and S 

compounds in sensitive aquatic and 

terrestrial ecosystems, described in the 

2009 REA and 2011 PA (77 FR 20218, 

April 3, 2012). 

The 2009 REA assessed atmospheric 

deposition of N and S compounds and 

the risks it posed of two categories of 

ecosystem effects: acidification and 

nutrient enrichment in both terrestrial 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105701 

22These were among the ecoregion-specific 

factors that comprised the parameters F1 through 

F4 in the AAI equation (2011 PA, p. 7–37). The 

parameter F2 represented the ecoregion-specific 

estimate of acidifying deposition associated with 

reduced forms of nitrogen, NHX (2011 PA, p. 7–28 

and ES–8 to ES–9). The 2011 PA suggested that this 

factor could be specified based on a 2005 CMAQ 

model simulation over 12-km grid cells or might 

involve the use of monitoring data for NHX applied 

in dry deposition modeling. It was recognized that 

appreciable spatial variability, as well as overall 

uncertainty, were associated with this factor. 

and aquatic ecosystems (U.S. EPA, 

2009a). In so doing, however, the 2009 

REA and 2011 PA recognized that the 

different types of effects varied in the 

strength of the evidence and of the 

information characterizing quantitative 

linkages between pollutants in ambient 

air and ecosystem responses, and in 

associated potential public welfare 

implications. The support in the 

evidence for quantitative assessment of 

aquatic acidification-related effects was 

strongest and the least uncertain. 

With regard to nutrient enrichmentrelated effects, despite the extensive 

evidence of deleterious effects of 

excessive ecosystem loading of nitrogen, 

the identification of options to provide 

protection from deposition-related 

effects was limited by several factors. 

These included the influence in 

terrestrial ecosystems of other air 

pollutants such as O3, and limiting 

factors such as moisture and other 

nutrients, and their potential to 

confound the characterization of the 

effects of changes in any one stressor, 

such as N deposition, in those systems 

(2011 PA, section 6.3.2). Forest 

management practices were also 

recognized to have the ability to 

significantly affect nitrogen cycling 

within a given forest ecosystem (2008 

ISA section 3.3.2.1 and Annex C, 

section C.6.3). In aquatic systems, 

appreciable contributions of nonatmospheric sources to nutrient loading 

in most large waterbodies, and 

limitations in data and tools, 

contributed uncertainties to 

characterizations of incremental adverse 

impacts of atmospheric N deposition 

(2011 PA, section 6.3.2). With regard to 

terrestrial acidification effects, data 

limitations contributed uncertainty to 

identification of appropriate indicator 

reference levels, and the potential for 

other stressors to confound 

relationships between deposition and 

terrestrial acidification effects was 

recognized with regard to empirical case 

studies described in the 2008 ISA. 

Based on the strong support in the 

evidence for the relationship between 

atmospheric deposition of acidifying N 

and S compounds and loss of acid 

neutralizing capacity (ANC) in sensitive 

ecosystems, with associated aquatic 

acidification effects, the REA analyses 

for this endpoint (aquatic acidification) 

received greatest emphasis in the review 

relative to other deposition-related 

effects. This emphasis on aquatic 

acidification-related effects of N oxides 

and SOX also reflected the advice from 

the CASAC. Accordingly, the 2011 PA 

focused on aquatic acidification effects 

in identifying policy options for 

providing public welfare protection 

from deposition-related effects of N 

oxides and SOX, concluding that the 

available information and assessments 

were only sufficient at that time to 

support development of a standard to 

address aquatic acidification. Consistent 

with this, the PA concluded it was 

appropriate to consider a secondary 

standard in the form of an aquatic 

acidification index (AAI) and identified 

a range of AAI values (which 

correspond to ANC levels) for 

consideration in establishing such a 

standard (2011 PA, section 7.6.2). 

Conceptually, the AAI is an index that 

uses the results of ecosystem and air 

quality modeling to estimate waterbody 

ANC. The standard level for an AAIbased standard was conceptually 

envisioned to be a national minimum 

target ANC for waterbodies in the 

ecoregions of the U.S. for which data 

were considered adequate for these 

purposes (2011 PA, section 7.6.2). 

While the NAAQS have historically 

been set in terms of an ambient air 

concentration, an AAI-based standard 

was envisioned to have a single value 

established for the AAI, but the 

concentrations of SOX and N oxides 

would be specific to each ecoregion, 

taking into account variation in several 

factors that influence waterbody ANC, 

and consequently could vary across the 

U.S. The factors, specific to each 

ecoregion (‘‘F factors’’), which it was 

envisioned would be established as part 

of the standard, include surface water 

runoff rates and ‘‘transference ratios.’’ 

The latter is the term assigned to factors 

applied to deposition values (estimated 

to achieve the minimum specified ANC) 

to back-calculate or estimate the highest 

ambient air concentrations of SOX and 

N oxides that would meet the AAI-based 

standard level (2011 PA, Chapter 7).22

The ecoregion-specific values for these 

factors would be specified based on 

then-available data and simulations of 

the Community Multiscale Air Quality 

(CMAQ) model and codified as part of 

such a standard. As part of the standard, 

these factors would be reviewed in the 

context of each periodic review of the 

NAAQS. 

After consideration of the PA 

conclusions, the Administrator 

concluded that while the conceptual 

basis for the AAI was supported by the 

available scientific information, there 

were limitations in the available 

relevant data and uncertainties 

associated with specifying the elements 

of the AAI, specifically those based on 

modeled factors, that posed obstacles to 

establishing such a standard under the 

CAA. It was recognized that the general 

structure of an AAI-based standard 

addressed the potential for 

contributions to acid deposition from 

both N oxides and SOX and 

quantitatively described linkages 

between ambient air concentrations, 

deposition, and aquatic acidification, 

considering variations in factors 

affecting these linkages across the 

country. However, the Administrator 

judged that the limitations and 

uncertainties in the available 

information were too great to support 

establishment of a new standard that 

could be concluded to provide the 

requisite protection for such effects 

under the Act (77 FR 20218, April 3, 

2012). These uncertainties generally 

related to the quantification of the 

various elements of the standard (the ‘‘F 

factors’’) and their representativeness at 

an ecoregion scale. These uncertainties 

and the complexities in this approach 

were recognized to be unique to the 

2012 review of the NAAQS for N and S 

oxides and were concluded to preclude 

the characterization and degree of 

protectiveness that would be afforded 

by an AAI-based standard, within the 

ranges of levels and forms identified in 

the PA, and the representativeness of F 

factors in the AAI equation described in 

the 2011 PA (77 FR 20261, April 3, 

2012). As the EPA said: 

‘‘[T]he Administrator recognizes that 

characterization of the uncertainties in the 

AAI equation as a whole represents a unique 

challenge in this review primarily as a result 

of the complexity in the structure of an AAI 

based standard. In this case, the very nature 

of some of the uncertainties is fundamentally 

different than uncertainties that have been 

relevant in other NAAQS reviews. She notes, 

for example, some of the uncertainties 

uniquely associated with the quantification 

of various elements of the AAI result from 

limitations in the extent to which ecological 

and atmospheric models, which have not 

been used to define other NAAQS, have been 

evaluated. Another important type of 

uncertainty relates to limitations in the 

extent to which the representativeness of 

various factors can be determined at an 

ecoregion scale, which has not been a 

consideration in other NAAQS.’’ [77 FR 

20261, April 3, 2012] 

The Administrator concluded that 

while the existing secondary standards 

were not adequate to provide protection 

against potentially adverse depositionVerDate Sep<11>2014 20:39 Dec 26, 2024 Jkt 265001 PO 00000 Frm 00011 Fmt 4701 Sfmt 4700 E:\FR\FM\27DER2.SGM 27DER2

khammond on DSK9W7S144PROD with RULES2
105702 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

related effects associated with N oxides 

and SOX, it was not appropriate under 

section 109 of the CAA (given the 

uncertainties summarized immediately 

above) to set any new or additional 

standards at that time to address effects 

associated with deposition of N and S 

compounds on sensitive aquatic and 

terrestrial ecosystems (77 FR 20262– 

20263, April 3, 2012). This decision was 

upheld upon judicial review. 

c. General Approach for This Review 

As is the case for all NAAQS reviews, 

this secondary standards review uses 

the Agency’s assessment of the current 

scientific evidence and associated 

quantitative analyses as a foundation to 

inform the Administrator’s judgments 

regarding secondary standards for SOX, 

N oxides and PM that are requisite to 

protect the public welfare from known 

or anticipated adverse effects associated 

with that pollutant’s presence in the 

ambient air. The approach for this 

review of the secondary SOX, N oxides, 

and PM standards builds on the last 

reviews of those pollutants, including 

the substantial assessments and 

evaluations performed over the course 

of those reviews, and considering the 

more recent scientific information and 

air quality data now available to inform 

understanding of the key policy-relevant 

issues in the current review. The EPA’s 

assessments are primarily documented 

in the ISA and PA, both of which 

received CASAC review and public 

comment, as summarized in section I.D. 

above. 

This review of the secondary 

standards for SOX, N oxides, and PM 

assesses the protection provided by the 

standards from two categories of effects: 

direct contact effects of the airborne 

pollutants and also the effects of the 

associated S- and N-containing 

compounds (in gaseous and particulate 

form) deposited in ecosystems. In so 

doing, the review draws on the 

currently available evidence as assessed 

in the ISA (and prior assessments) and 

quantitative exposure, risk, and air 

quality information in the PA, including 

the REA for aquatic acidification. 

With regard to direct contact effects, 

we draw on the currently available 

evidence as assessed in the ISA, 

including the determinations regarding 

the causal nature of relationships 

between the airborne pollutants and 

ecological effects, which focus most 

prominently on vegetation, and 

quantitative exposure and air quality 

information. Based on this information, 

we consider the policy implications, 

most specifically whether the evidence 

supports the retention or revision of the 

current NO2 and SO2 secondary 

standards. With regard to the effects of 

PM, we take a similar approach, based 

on the evidence presented in the current 

ISA and conclusions from the review of 

the PM NAAQS concluded in 2013 (in 

which ecological effects were last 

considered) to assess the effectiveness of 

the current PM standard to protect 

against these types of impacts. 

With regard to deposition-related 

effects, we consider the evidence for the 

array of effects identified in the ISA 

(and summarized in section II.A.3. 

below), including both terrestrial and 

aquatic effects; and the limitations in 

the evidence and associated 

uncertainties as well as the public 

welfare implications of such effects. The 

overall approach takes into account the 

nature of the welfare effects and the 

exposure conditions associated with 

effects in identifying S and N deposition 

levels appropriate to consider in the 

context of public welfare protection. To 

identify and evaluate metrics relevant to 

air quality standards (and their 

elements), we have assessed 

relationships developed from air quality 

measurements near pollutant sources 

and deposition estimates nearby and in 

downwind ecoregions. In so doing, the 

available quantitative information both 

on deposition and effects, and on 

ambient air concentrations and 

deposition, has been assessed with 

regard to the existence of linkages 

between SOX, N oxides, and PM in 

ambient air and deposition-related 

effects. These assessments, summarized 

briefly in the sections below (and in 

detail in the PA), inform judgments on 

the likelihood of occurrence of 

deposition-related effects under air 

quality that meets the existing standards 

for these pollutants or potential 

alternatives. 

In considering the information on 

atmospheric deposition and ecological 

effects, we recognize that the impacts 

from the dramatically higher deposition 

rates of the past century can affect how 

ecosystems and biota respond to more 

recent, lower deposition rates, 

complicating interpretation of impacts 

related to more recent, lower deposition 

levels. This complexity is illustrated by 

findings of studies that compared soil 

chemistry across intervals of 15 to 30 

years (1984–2001 and 1967–1997). 

These studies reported that although 

atmospheric deposition in the Northeast 

declined across those intervals, soil 

acidity increased (ISA, Appendix4, 

section 4.6.1). As noted in the ISA, ‘‘[i]n 

areas where N and S deposition has 

decreased, chemical recovery must first 

create physical and chemical conditions 

favorable for growth, survival, and 

reproduction’’ (ISA, Appendix 4, 

section 4.6.1). Thus, the extent to which 

S and N compounds (once deposited) 

are retained in soil matrices (with 

potential effects on soil chemistry) 

influences the dynamics of the response 

of the various environmental pathways 

to changes in air quality, including 

changes in emissions, ambient air 

concentrations and associated 

deposition. 

The two-pronged approach applied in 

the PA for deposition-related effects 

includes the consideration of deposition 

levels that may be associated with 

ecological effects of potential concern 

and consideration of relationships 

between ambient air concentrations and 

levels of deposition. In considering the 

ecological effects evidence, the focus is 

on effects for which the evidence is 

most robust with regard to established 

quantitative relationships between 

deposition and ecosystem effects. Such 

quantitative information for terrestrial 

ecosystems is derived primarily from 

analysis of the evidence presented in 

the ISA. For aquatic ecosystems, the 

primary focus has been given to effects 

related to aquatic acidification, for 

which we have conducted quantitative 

risk and exposure analyses based on 

available modeling applications that 

relate acid deposition and acid buffering 

capability in U.S. waterbodies, as 

summarized in section II.A.4. below 

(PA, section 5.1 and Appendix 5A). 

Regarding the second prong of the 

approach, we employed several 

different types of analyses to inform an 

understanding of relationships between 

ambient air concentrations near 

pollutant sources in terms of metrics 

relevant to air quality standards (and 

their elements) and ecosystem 

deposition estimates (as described in 

section II.A.2. below). Interpretation of 

findings from these analyses, in 

combination with the identified 

deposition levels of interest, and related 

policy judgments regarding limitations 

and associated uncertainties of the 

underlying information, informed the 

Administrator’s proposed conclusions 

on the extent to which existing 

standards, or potential alternative 

standards, might be expected to provide 

protection from these levels and inform 

the Administrator’s final decisions in 

this review, as discussed in section 

II.B.3. below. 

In summary, the approach to 

evaluating the standards with regard to 

protection from ecological effects 

related to ecosystem deposition of N 

and S compounds in this review 

involves multiple components: (1) 

review of the scientific evidence to 

identify the ecological effects associated 

with the three pollutants, those related 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105703 

both to direct pollutant contact and to 

ecosystem deposition; (2) assessment of 

the evidence and characterization of the 

REA results to identify deposition levels 

related to categories of ecosystem 

effects; and (3) analysis of relationships 

between ambient air concentrations of 

the pollutants and deposition of N and 

S compounds to understand aspects of 

these relationships that can inform 

judgments on ambient air standards that 

protect against air concentrations 

associated with direct effects and 

against deposition associated with 

deposition-related effects that are 

judged adverse to the public welfare. As 

discussed in the PA and the proposal, 

however, relating ambient air 

concentrations of N oxides and PM to 

deposition of N compounds is 

particularly complex because N 

deposition also results from an 

additional air pollutant that is not 

controlled by NAAQS for N oxides and 

PM. Thus, separate from the evaluation 

of secondary standards for SOX, the 

evaluation for N oxides and PM also 

considers current information (e.g., 

spatial and temporal trends) related to 

the additional air pollutant, ammonia 

(NH3), that contributes to N deposition 

and also related to PM components that 

do not contribute to N deposition. 

Evaluation of all of this information, 

together, is considered by the 

Administrator in reaching his decision, 

as summarized in section II.B.3. below. 

2. Overview of Air Quality and 

Deposition 

The three criteria pollutants that are 

the focus of this review (SOX, N oxides, 

and PM) include both gases and 

particles. Both their physical state and 

chemical properties, as well as other 

factors, influence their deposition as Nor S-containing compounds. The 

complex pathway from pollutant and 

precursor emissions (section II.A.2.a.) to 

ambient air concentrations (section 

II.A.2.b.) and to eventual deposition 

(section II.A.2.c.) varies by pollutant 

and is influenced by a series of 

atmospheric processes and chemical 

transformations that occur at multiple 

spatial and temporal scales (ISA, 

Appendix 2; PA, Chapters 2 and 6). 

A complication in the consideration 

of the influence of these criteria 

pollutants on N deposition and 

associated ecological effects is posed by 

the contribution of other, non-criteria, 

pollutants in ambient air, specifically 

NH3. Although emissions of N oxides 

have appreciably declined, NH3

emissions have risen. Together, these 

co-occurring trends have reduced the 

influence of N oxides on total N 

deposition (PA, sections 6.2.1, 6.4.2 and 

7.2.3.3). Geographic variability and 

temporal changes in the percentage of 

PM composed of N- (and S-) containing 

compounds, are other factors affecting 

decisions in this review. 

a. Sources, Emissions and Atmospheric 

Processes Affecting SOX, N Oxides and 

PM 

Sulfur dioxide is generally present at 

higher concentrations in the ambient air 

than the other gaseous and highly 

reactive SOX (ISA, Appendix 2, section 

2.1) and, as a result, SO2 is the indicator 

for the existing NAAQS for SOX. The 

main anthropogenic source of SO2

emissions is fossil fuel combustion (PA, 

section 2.2.2). Based on the 2020 

National Emissions Inventory (NEI), the 

top three emission sources of SO2 in the 

U.S. are coal-fired electricity generating 

units (48% of total), industrial processes 

(27%), and other stationary source fuel 

combustion (9%). 

Once emitted to the atmosphere, SO2

can either remain as SO2 in the gas 

phase and be transported and/or be dry 

deposited, or it can be oxidized to form 

sulfate particles (SO4

2¥), with modeling 

studies suggesting that oxidation 

accounts for more than half of SO2

removal nationally (PA, section 2.1.1). 

The rate of SO2 oxidation accelerates 

with greater availability of oxidants, 

which are generally depleted near 

source stacks. Consequently, oxidization 

to SO4

2¥ generally occurs in cleaner air 

downwind of SOX sources (2008 ISA, 

section 2.6.3.1). As SO4

2¥ particles are 

generally within the fine particle size 

range, they are a component of PM2.5

and have an atmospheric lifetime 

ranging from 2 to 10 days (PA, section 

2.1.1). The areas of highest SO2 and 

SO4

2¥ deposition are generally near or 

downwind of SOX emissions sources, 

with most S deposition occurring in the 

eastern U.S. (PA, section 2.5.3). 

Geographic variation in precipitation 

also influences the spatial distribution 

of S wet deposition. In sum, both SO2, 

and the SO4

2¥ particles converted from 

SO2, contribute to S deposition, and do 

so over different time and geographic 

scales, with dry deposition of SO2

typically occurring near the source, and 

wet deposition of sulfate particles 

distributing more regionally. 

The term N oxides refers to all forms 

of oxidized nitrogen compounds, 

including NO, NO2, nitric acid (HNO3), 

and particulate nitrate (NO3

¥). Most N 

oxides enter the atmosphere as either 

NO or NO2, which are collectively 

referred to as NOX (PA, section 2.1.2). 

Anthropogenic sources account for the 

majority of NOX emissions in the U.S., 

per 2020 NEI estimates, with highway 

vehicles (26% of total), stationary fuel 

combustion including electric 

generating units (25%), and non-road 

mobile sources (19%) identified as the 

largest contributors to total emissions 

(PA, section 2.2.1). Once emitted into 

the atmosphere, NOX can deposit to the 

surface or be chemically converted to 

other gaseous N oxides, including 

HNO3, as well as to particulate NO3

¥, 

which may occur in either the fine or 

coarse particle size range, such that not 

all particulate NO3

¥ is a component of 

PM2.5. In general, gas phase N oxides 

tend to have shorter atmospheric 

lifetimes, either dry depositing (e.g., as 

HNO3) or quickly converting to 

particulate NO3

¥, which has a similar 

atmospheric lifetime as particulate 

SO4

2¥ and is generally removed by 

precipitation in wet deposition. 

In addition to N oxides, there is 

another category of nitrogen pollutants, 

referred to as reduced nitrogen, which 

also contributes to nitrogen deposition. 

The most common form of reduced N 

emitted into the air is NH3 gas (PA, 

sections 2.1.3 and 2.2.3), which is not a 

criteria pollutant. The main sources of 

NH3 emissions include livestock waste 

(49% of total in 2020 NEI), fertilizer 

application (33%) and aggregate fires 

(11%). Ammonia tends to dry deposit 

near sources, with a fraction of what is 

emitted being converted to particle 

form, as ammonium (NH4

+), which can 

be transported away from sources and is 

most efficiently removed by 

precipitation (PA, section 2.1.3). 

Particulate matter is both emitted to 

the atmosphere and formed in the 

atmosphere from precursor chemical 

gases, such as N Oxides, SOX and NH3. 

Accordingly, PM2.5 contributing to S 

and N deposition generally results from 

chemicals formed in the atmosphere 

after being emitted (e.g., particulate 

SO4

2¥, particulate NO3

¥, NH4∂). The 

majority of PM2.5 mass in recent periods 

(e.g., 2019–2021) is composed of 

materials that do not contribute to S and 

N deposition (PA, section 2.4.3 and 

6.4.2). For example, at PM2.5 monitoring 

sites across the U.S., SO4

2¥ generally 

comprises no more than about a third of 

PM2.5 mass (in eastern sites), with much 

lower percentages at monitoring sites in 

much of the West and South (PA Figure 

2–30 and section 2.4.3). Similarly, 

nitrogen-containing species are also a 

minority of PM2.5 mass, representing 

less than about 30% and down to about 

5% or lower in some areas of South (PA, 

sections 2.4.3 and 6.4.2). 

b. Recent Trends in Emissions, 

Concentrations, and Deposition 

Emissions of SOX, oxides of N, and 

PM have declined dramatically over the 

past two decades, continuing a longerVerDate Sep<11>2014 20:39 Dec 26, 2024 Jkt 265001 PO 00000 Frm 00013 Fmt 4701 Sfmt 4700 E:\FR\FM\27DER2.SGM 27DER2

khammond on DSK9W7S144PROD with RULES2
105704 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

23A 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-qualitydesignvalues). 

24The existing secondary standard for SO2 is 0.5 

ppm (500 ppb), as a 3-hour average, not to be 

exceeded more than once per year. 

25 In the 2019–2021 period, the maximum design 

value for the primary SO2 standard was 376 ppb at 

a monitoring site near an industrial park in 

southeast Missouri. It is important to note that peak 

and mean SO2 concentrations are higher at sourceoriented sites than monitoring locations that are not 

source-oriented. Additionally, it is not uncommon 

for there to be high SO2 values in areas with 

recurring volcanic eruptions, such as in Hawaii 

(PA, section 2.4.2). 

26Sites in the contiguous U.S. have met the 

existing NO2 secondary standard since around 1991 

(PA, Figure 2–22). 

27The maximum annual average NO2

concentrations has been at, slightly above, or 

slightly below 30 ppb since about 2008, with the 

highest 3-year average value just above 30 ppb (PA, 

Figures 2–22 and 7–9). 

28Other than the estimates associated with the 

CMAQ analysis (second approach referenced 

above), the deposition estimates used in these 

analyses are those provided by the National 

Atmospheric Deposition Program, TDep Science 

Committee. One of the outputs of this effort are 

annual datasets of total deposition estimates in the 

contiguous U.S. (CONUS), which are referred to as 

the TDep datasets (technical updates available from 

NADP, 2021; ISA, Appendix 2, section 2.6). TDep 

datasets do not currently exist for areas outside of 

the CONUS. 

29The 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. 

30Areas 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 

term trend (PA, section 2.2). Total SO2

emissions nationwide declined by 87% 

between 2002 and 2022, including 

reductions of 91% in emissions from 

electricity generating units and 96% in 

emissions from mobile sources. Total 

anthropogenic NOX emissions also 

trended downward from 2002 to 2022 

by 70% nationwide, driven in part by 

large reductions in emissions from 

highway vehicles (84%) and stationary 

fuel combustion (68%) (PA, section 

2.2.1). In contrast with these declining 

20-year trends in NOX and SOX

emissions, the annual rate of NH3

emissions increased by over 20 percent 

nationwide between 2002 and 2022 (PA, 

section 2.2.3). The two largest 

contributors are emissions from 

livestock waste and fertilizer 

application, which have increased by 

11% and 44%, respectively. These 

trends in NOX and NH3 emissions have 

had ramifications for N deposition 

patterns across the U.S., as described 

further below. 

The large reductions in SOX and NOX

emissions have resulted in substantially 

lower ambient air concentrations in 

recent years relative to the past. This is 

true for both 3-hour and 1-hour average 

concentrations. With regard to 3-hour 

SO2 concentrations, 2021 design values 

for the existing 3-hour standard at all 

State and Local Air Monitoring Stations 

(SLAMS) with valid design values (n= 

333) 23 are less than the level of the 

existing secondary standard (500 ppb) 24

and more than 75 percent of the sites 

have design values below 20 ppb (PA, 

section 2.4.2). This reflects a downward 

trend since 2000, with the median 

design value declining from about 50 

ppb to less than 10 ppb in 2021 (PA, 

Figure 2–27). 

Similarly, design values for the 

primary SO2 standard (annual 99th 

percentile of daily maximum 1-hour 

average concentrations, averaged over 3 

years) have also declined. In the mid1990s, the median value of all sites with 

valid 1-hour design values often 

exceeded 75 ppb (PA, Figure 2–26). 

Since then, the entire distribution of 

design values (including sourceoriented sites) has continued to decline 

such that the median design value for 

the 1-hour primary standard across the 

network of sites is now between 5 and 

10 ppb (PA, Figure 2–26). Annual 

average SO2 concentrations have also 

declined over this period. Additionally, 

both peak and mean SO2 concentrations 

are higher at source-oriented sites than 

monitoring locations that are not sourceoriented.25

Regarding NO2, design values for the 

secondary standard (annual averages) at 

all 399 sites with valid design values in 

2021 are below the 53 ppb level of the 

existing standard,26 and 98% of sites 

have design values below 20 ppb. In 

2021, the maximum design value was 30 

ppb,27 and the median was 7 ppb, 

reflecting a downward trend since 2000 

when the median annual design value 

was 15 ppb. 

Likewise, the median of the annual 

average PM2.5 concentrations also 

decreased substantially from 2000 to 

2021, from 12.8 mg/m3 to 8 mg/m3. The 

median of the annual 98th percentile 

24-hour PM2.5 concentrations at the 

more than 1000 sites monitored also 

decreased, from 32 mg/m3 in 2000 to 21 

mg/m3 in 2021. Although both the 

annual average and 98th percentile 24- 

hour PM2.5 concentrations decreased 

steadily from the early 2000s until 2016, 

these values have fluctuated in recent 

years due to large-scale wildfire events 

(PA, section 2.4.3; U.S. EPA, 2023, 

Figures 23 and 24). 

The changes in emissions and 

associated concentrations since 2000 

have also contributed to appreciable 

changes in N and S deposition 

nationwide (PA, sections 2.5.3 and 

6.2.1). For S compounds, the dramatic 

reduction in SOX emissions (87% 

nationwide) resulted in concordant 

reductions in S deposition, 68% on 

average across U.S. (PA, section 6.2.1). 

This decline is observed across the 

contiguous U.S. (CONUS), with the 

largest reductions in regions downwind 

of large sources such as electricity 

generating units. For N deposition, the 

impact of the appreciable reduction in 

N oxides emissions has been offset by 

deposition arising from increasing 

emissions of reduced forms of nitrogen 

over the same timeframe. 

c. Relationships Between 

Concentrations and Deposition 

As the NAAQS are set in terms of 

pollutant concentrations, analyses in the 

PA evaluated relationships between 

criteria pollutant concentrations in 

ambient air and ecosystem deposition 

across the U.S. These relationships were 

evaluated over a range of conditions 

(e.g., pollutant, region, time period), and 

with consideration of deposition both 

near sources and at distance (allowing 

for pollutant transport and associated 

transformation) using five different 

approaches (PA, Chapter 6 and 

Appendix 6A). 

First, as part of a ‘‘real-world 

experiment,’’ the PA analyses leveraged 

the recent downward trends in NOX and 

SOX emissions and corresponding air 

quality concentrations as well as the 

trends in deposition to examine the 

correlation between observed decreases 

in emissions and concentration and 

observed changes in deposition over the 

past two decades (PA, section 6.2.1). 

The deposition estimates used in these 

analyses (termed TDep) 28 are based on 

a hybrid approach that involves a fusion 

of measured and modeled values, where 

measured values are given more weight 

at the monitoring locations and modeled 

data are used to fill in spatial gaps and 

provide information on chemical 

species that are not measured by routine 

monitoring networks (Schwede and 

Lear, 2014). For the second approach, 

we assessed how ambient air 

concentrations and associated 

deposition levels are related within the 

CMAQ 29 both across the U.S. and then 

at certain Class I areas 30 (PA, section 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105705 

be Class I if designated as Class I consistent with 

the CAA. 

31Upwind sites of influence were identified for 

all 84 ecoregions (level III categorization) in the 

contiguous U.S. Identification of monitoring sites 

linked to each downwind ecoregion was based on 

HYSPLIT modeling for a 120-hour period and 

focusing on monitoring site locations estimated to 

contribute at least 0.5% of hits to the downwind 

ecoregion in the trajectory modeling (PA, Appendix 

6, section 6A.2). 

32For SO2, there were two sets of metrics: one 

based on an annual average and one based on the 

2nd highest 3-hour maximum concentration in the 

year. Both the NO2 and PM2.5 metrics are annual 

averages. For relating to 3-year average deposition, 

all are averaged across three years. 

33The correlation coefficients reported here, from 

the PA, are based on Spearman’s rank correlation 

coefficient. These nonparametric coefficients are 

generally used with data that are not normally 

distributed to assess how well the relationship 

between two variables can be described via a 

monotonic function. The term ‘‘r value’’ is 

sometimes used as shorthand for this correlation 

coefficient. Higher values indicate that the two 

variables are highly associated with one another 

(can range from 1.0 to ¥1.0). 

6.2.2.1) where additional monitoring 

data are collected as part of the Clean 

Air Status and Trends Network 

(CASTNET) and the Interagency 

Monitoring of Protected Visual 

Environments (IMPROVE) networks. As 

a third approach, we analyzed the 

relationships across a limited number of 

monitoring locations (in Class I areas) 

where both air quality data (CASTNET 

and IMPROVE) and wet deposition of S 

and N was measured to evaluate the 

associations between concentrations 

and deposition at a local scale (PA, 

section 6.2.2.2 and 6.2.2.3). The fourth 

approach also considered the 

associations between the two terms, at 

the local scale, but did so using a 

broader set of ambient air concentration 

measurements (i.e., all valid SO2, NO2, 

and PM2.5 measurements at SLAMS 

across the U.S.) and the hybrid set of 

TDep estimates (PA, section 6.2.3). 

Finally, in recognition of the fact that 

air quality at upwind locations can also 

influence downwind deposition, the 

fifth approach used a trajectory model 

(HYSPLIT—The Hybrid Single-Particle 

Lagrangian Integrated Trajectory model) 

to identify upwind areas where 

emissions might be expected to 

influence deposition at downwind 

ecoregions (PA, section 6.2.4 and 

Appendix 6A).31 Once those potential 

zones of influence were established, we 

evaluated the relationships between air 

quality metrics for the three 

pollutants 32 at sites within those zones 

(sites of influence) and deposition 

estimates in the downwind ecoregion, 

as 3-year averages for five periods: 

2001–2003, 2006–2008, 2010–2012, 

2014–2016 and 2018–2020. The metrics, 

Ecoregion Air Quality Metrics (EAQMs), 

include a weighted-average (EAQMweighted) and a maximum metric 

(EAQM-max). The EAQM-max is the 

maximum concentration among the 

upwind monitoring sites identified for 

each downwind ecoregion. For the 

EAQM-weighted, the value of each site 

linked to the downwind ecoregion was 

weighted by how often the forward 

HYSPLIT trajectory crossed into the 

ecoregion, i.e., sites with more frequent 

trajectory intersections with the 

ecoregion were weighted higher (PA, 

section 6.2.4.1). 

The full set of quantitative results of 

the characterization of air quality and 

deposition relationships is discussed 

more thoroughly in Chapter 6 and 

Appendix 6A of the PA. The evaluation 

of measured air quality concentrations 

(SO2, NO2, and PM2.5) and TDep 

estimates of deposition at all SLAMS 

(generally composed of sites that use 

either a Federal Reference Method 

[FRM] or a Federal Equivalence Method 

[FEM]) is a robust analysis (i.e., large 

number of monitors distributed across 

the U.S.) and relevant given that 

compliance with the current standards 

(both primary and secondary) is judged 

using design value metrics based on 

measurements at the current SO2, NO2

and PM2.5 monitors. As with any 

assessment, there are uncertainties and 

limitations, as discussed in the PA (PA, 

sections 6.3 and 6.4). For example, the 

SLAMS analyses are site-based 

comparisons that do not account for 

deposition associated with the transport 

of pollutants emitted some distance 

upwind. Similarly, the other analyses 

have their own limitations ranging from 

model uncertainty to limitations in 

geographical scope. In combination, 

these analyses supported the PA 

conclusion of a strong association 

between SO2 and S deposition. The 

results and associated information for N 

oxides and PM, however, indicate more 

variable relationships, both between 

NO2 concentrations and N deposition, 

and between PM2.5 concentrations with 

either S or N deposition. 

For SO2, annual monitored SO2

concentrations, at existing monitors 

within the SLAMS network, averaged 

over 3 years at the national scale were 

highly correlated with S deposition 

estimates in the TDep dataset at the 

local scale (correlation coefficient of 

0.70),33 especially in the earlier periods 

of the record and across the eastern U.S. 

(PA, section 6.2.3). This association is 

also seen in the relationships between 

SO2 annual values at the identified 

upwind sites of influence and S 

deposition estimates from TDep in 

downwind ecoregions, especially in 

those locations where the annual 

average SO2 concentrations are greater 

than 5 ppb (PA, section 6.2.4.2). Finally, 

we note that the observed declines in 

national levels of S deposition over the 

past two decades have occurred during 

a period in which emissions of SO2 have 

also declined sharply (PA, sections 6.2.1 

and 6.4.1). 

Analyses in the PA also investigated 

relationships between S deposition and 

air quality metrics other than the 

current indicator species (SO2) in a 

limited number of circumstances at 

relatively remote sites, generally distant 

from emissions sources. For example, an 

evaluation of the associations of total S 

TDep estimates with SO4

2¥

concentrations and of wet S deposition 

with the sum of SO2 + SO4

2¥ at 27 sites 

in 27 Class I areas concluded that the 

correlations for S deposition with 

particulate SO4

2¥ and total S (i.e., SO2

+ SO4

2¥) were lower than what was 

exhibited for S deposition and SO2

concentrations at the SLAMS (PA, 

section 6.2.2). The analyses also found 

poor correlation (correlation coefficient 

of 0.33) between total S deposition 

estimates (TDep) and PM2.5 mass at 

IMPROVE sites in the 27 Class I areas 

(PA, sections 2.3.3 and 6.2.2.3). While 

this set of analyses is based on data at 

a relatively limited number of sites (e.g., 

compared to the SLAMS network), the 

results do not indicate advantages to 

PM2.5 mass, particulate SO4

2¥, or total 

S (SO4

2¥ plus SO2) over SO2 (alone) as 

an indicator for a secondary NAAQS to 

address S deposition-related effects. 

Both NO2 and certain components of 

PM2.5 (NO3

¥ and NH4∂) contribute to N 

deposition. As is the case for SO2 and 

S deposition, there are multiple 

pathways for N deposition (dry and wet) 

and multiple scales of N deposition 

(local and regional). However, there are 

some additional complications to 

relating ambient air concentrations of 

NO2 and PM2.5 mass to N deposition. 

First, not all N deposition is caused by 

these pollutants (PA, Chapter 2 and 

section 6.1.1). Ammonia, which is not a 

criteria pollutant, also contributes to N 

deposition, especially through dry 

deposition at local scales. Second, only 

certain components of PM2.5 mass 

contribute to N deposition (i.e., NO3

¥

and NH4

+) and these comprise less than 

about 30% of PM2.5 mass across the 

U.S., below 5% in some regions (PA, 

Figure 6–56). As a result of these two 

factors, the associations between NO2

concentrations and N deposition, and 

between PM2.5 concentrations and N 

deposition are less robust than what is 

observed for SO2 and S deposition. The 

multi-faceted approach to evaluating 

these relationships confirmed this 

expectation. For example, there are 


khammond on DSK9W7S144PROD with RULES2
105706 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

34The 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. 

35The study count and citations are available on 

the project page for the ISA on the Health & 

Environmental Research Online (HERO) website 

(https://heronet.epa.gov/heronet/index.cfm/project/ 

page/project_id/2965). 

weaker associations of N deposition 

with NO2 observations at SLAMS across 

the U.S. than what is observed in the 

similar S deposition and SO2 analysis 

(PA, section 6.4.2). There is little 

correlation for N deposition with NO2

concentrations, as evidenced by a 

Spearman’s correlation coefficient of 

0.38, compared to 0.70 for SO2 and S 

deposition (PA, Table 6–6 and Table 6– 

4). Further, the trajectory-based analyses 

of the relationships between NO2 annual 

values at the identified upwind sites of 

influence and N deposition estimates 

from TDep in downwind ecoregions 

indicate negative correlations (PA, Table 

6–10). These negative correlations are 

observed for both the EAQM-weighed 

and EAQM-max values. This relative 

lack of association for NO2

concentrations with N deposition was 

confirmed by national trends over the 

past 20 years, where sharp declines in 

NO2 emissions and concentrations are 

linked in time with sharp declines in 

oxidized N deposition (PA, Table 6–2), 

but not with trends in total or reduced 

atmospheric N deposition. Since 2010, 

NO2 concentrations have continued to 

drop while N deposition nationally has 

remained steady (PA, section 6.2.1). As 

for S deposition and S compound 

metrics, the PA also investigated 

relationships between N deposition and 

air quality metrics other than the 

current indicator species (NO2) in the 27 

Class I areas where collocated data were 

available. Recognizing that such 

information was not available in other, 

less remote areas of the U.S., including 

areas where contributing emissions are 

highest or at the regulatory SLAMS 

monitors, no clear advantages of these 

other parameters (e.g., nitric acid, 

particulate NO3

¥, and NH4

+) over NO2

or PM2.5 mass were indicated. Across all 

analyses, the evidence indicates NO2 to 

be a weak indicator of total atmospheric 

N deposition, especially in areas where 

NH3 is prevalent and where PM2.5 mass 

is dominated by species other than 

NO3

¥ or NH4∂ (PA, section 6.4.2). 

3. Overview of Welfare Effects Evidence 

More than 3,000 welfare effects 

studies, including approximately 2,000 

studies newly available since the last 

review, have been considered in the 

ISA.34 35 While expanding the evidence 

for some effect categories, the studies on 

acid deposition, an important category 

of effects in the last review, are largely 

consistent with the evidence that was 

previously available. The subsections 

below briefly summarize the nature of 

welfare effects of S oxides, N oxides and 

PM (section II.A.3.a.), the potential 

public welfare implications of these 

effects (section II.A.3.b.), and exposure 

concentrations and deposition-related 

metrics (section II.A.3.c.). 

a. Nature of Effects 

The welfare effects evidence base 

evaluated in the current review includes 

decades of extensive research on the 

ecological effects of N oxides, SOX and 

PM. The sections below provide an 

overview of the nature of the direct 

effects of gas-phase exposure to oxides 

of nitrogen and sulfur (section 

II.A.3.a.(1)), acid deposition-related 

ecological effects (section II.A.3.a.(2)), N 

enrichment and associated effects 

(section II.A.3.a.(3)), and other effects 

(section II.A.3.a.(4)). 

(1) Direct Effects of SOX and N Oxides 

in Ambient Air 

A well-established body of scientific 

evidence has shown that acute and 

chronic exposures to oxides of N and S, 

such as SO2, NO2, NO, HNO3 and 

peroxyacetyl nitrate (PAN) in the air, are 

associated with negative effects on 

vegetation. The scientific evidence 

available for these effects in 1971 is the 

basis for the current secondary NAAQS 

for SOX and N oxides. 

The current scientific evidence 

continues to be sufficient to infer a 

causal relationship between gas-phase 

SO2 and injury to vegetation (ISA, 

Appendix 3, section 3.6.1). High 

concentrations have been associated 

with damage to plant foliage (ISA, 

Appendix 3, section 3.2). In addition to 

foliar injury, which is usually a rapid 

response, and which can vary 

significantly among species and growth 

conditions (which affect stomatal 

conductance), SO2 exposures have also 

been documented to reduce plant 

photosynthesis and growth. As 

exposures have declined in the U.S., 

some studies in the eastern U.S. have 

reported increased growth in some SO2- 

sensitive tree species (e.g., Thomas et 

al., 2013). Multiple factors, including 

reduced deposition, buffering and other 

environmental variables, may play a 

role in such species recovery. (ISA, 

Appendix 3, section 3.2, Schaberg et al., 

2014). Some of this evidence seems to 

suggest a somewhat faster recovery than 

might be expected from depositionrelated soil acidification alone, which 

may indicate a relatively greater role for 

changes in ambient air concentrations of 

SO2, in combination with changes in 

other gases, than was previously 

understood (ISA, Appendix 3, section 

3.2 and Appendix 5, section 5.2.1.3). 

For lichens, damage from SO2 exposure 

has been observed to include reduction 

in metabolic functions that are vital for 

growth and survival (e.g., decreases in 

photosynthesis and respiration), damage 

to cellular integrity (e.g., leakage of 

electrolytes), and structural changes 

(ISA, Appendix 3, section 3.2). 

The current scientific evidence also 

continues to be sufficient to infer a 

causal relationship between gas-phase 

NO, NO2 and PAN and injury to 

vegetation (ISA, Appendix 3, section 

3.6.2). The evidence base evaluated in 

the 1993 Air Quality Criteria Document 

for Oxides of N included evidence of 

phytotoxic effects of NO, NO2, and PAN 

on plants through decreasing 

photosynthesis and induction of visible 

foliar injury (U.S. EPA, 1993 [1993 

AQCD]). The 1993 AQCD additionally 

concluded that concentrations of NO, 

NO2, and PAN in the atmosphere were 

rarely high enough to have phytotoxic 

effects on vegetation. Little new 

information is available since that time 

on these phytotoxic effects at 

concentrations currently observed in the 

U.S. (ISA, Appendix 3, section 3.3). 

With regard to HNO3, the evidence is 

sufficient to infer a causal relationship 

between exposure to HNO3 and changes 

to vegetation (ISA, Appendix 3, section 

3.6.3). The evidence suggests a role in 

observed declines in lichen species in 

the 1970s in the Los Angeles basin (ISA, 

Appendix 3, section 3.3). A 2008 

resampling of areas shown to be 

impacted in the past by HNO3 found 

community shifts, declines in the most 

pollutant-sensitive lichen species, and 

increases in abundance of nitrogentolerant lichen species compared to 

1976–1977, indicating that these lichen 

communities have not recovered and 

had experienced additional changes 

(ISA, Appendix 3, section 3.4). The 

recently available evidence on this topic 

also included a study of six lichen 

species that reported changes in 

physiology and functioning including 

decreased chlorophyll content and 

chlorophyll fluorescence, decreased 

photosynthesis and respiration, and 

increased electrolyte leakage from HNO3

exposures for 2–11 weeks (daily peak 

levels near 50 ppb) in controlled 

chambers. (ISA, Appendix 3, section 

3.4). 

(2) Acid Deposition-Related Ecological 

Effects 

The connection between SOX and N 

oxide emissions to ambient air, 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105707 

atmospheric deposition of S and/or N 

compounds, and the acidification of 

acid-sensitive soils and surface waters is 

well documented by many decades of 

evidence, particularly in the eastern 

U.S. (ISA, section IS.5; Appendix 8, 

section 8.1). Sulfur oxides and N oxides 

in ambient air undergo reactions to form 

acidic compounds that are removed 

from the atmosphere through 

deposition. Acidifying deposition can 

affect biogeochemical processes in soils, 

with ramifications for terrestrial biota 

and for the chemistry and biological 

functioning of associated surface waters 

(ISA, Appendix 7, section 7.1). These 

effects depend on the magnitude and 

rate of deposition, as well as multiple 

biogeochemical processes that occur in 

soils and waterbodies. 

Soil acidification is influenced by the 

deposition of inorganic acids (HNO3 and 

sulfuric acid [H2SO4]), NH4

+, and by 

chemical and biological processes. 

When NO3

¥, or SO42¥ leach from soils 

to surface waters, an equivalent number 

of positive cations, or countercharge, are 

also transported. If the countercharge is 

provided by a base cation (e.g., calcium, 

[Ca2+], magnesium [Mg2∂], sodium 

[Na∂], or potassium [K∂]), rather than 

hydrogen ions (H∂), the leachate is 

neutralized, but the soil becomes more 

acidic from the hydrogen ions left 

behind, and the base saturation of the 

soil is reduced by the loss of the base 

cation. Depending on the relative rates 

of soil processes that contribute to the 

soil pools of H∂ and base cations, such 

as weathering, continued SO4

2¥ or 

NO3

¥ leaching can deplete the soil base 

cation pool, which contributes to 

increased acidity of the leaching soil 

water and by connection, the surface 

water. Accordingly, the ability of a 

watershed to neutralize acidic 

deposition is determined by a variety of 

biogeophysical factors including 

weathering rates, bedrock composition, 

vegetation and microbial processes, 

physical and chemical characteristics of 

soils, and hydrology (ISA Appendix 4, 

section 4.3). 

Recently available evidence includes 

some studies describing early stages of 

recovery from soil acidification in some 

eastern forests. For example, studies at 

the Hubbard Brook Experimental Forest 

in New Hampshire reported indications 

of acidification recovery in soil solution 

measurements across the period from 

1984 to 2011 (ISA, Appendix 4, section 

4.6.1; Fuss et al., 2015). Another study 

of 27 sites in eastern Canada and the 

northeastern U.S. found reductions in 

wet deposition SO4

2¥ were associated 

with increases in soil base saturation 

and decreases in exchangeable 

aluminum (ISA, Appendix 4, section 

4.6.1; Lawrence et al., 2015). Recent 

modeling analyses indicate extended 

timeframes for recovery are likely, as 

well as delays or lags related to 

accumulated pools of S in forest soils 

(ISA, Appendix 4, section 4.6.1). 

(a) Freshwater Ecosystems 

As was the case in the last review, the 

body of evidence available in this 

review, including that newly available, 

is sufficient to infer a causal 

relationship between N and S 

deposition and the alteration of 

freshwater biogeochemistry (ISA, 

section IS.6.1). Additionally, based on 

the previously available evidence, the 

current body of evidence is also 

sufficient to conclude that a causal 

relationship exists between acidifying 

deposition and changes in biota, 

including physiological impairment and 

alteration of species richness, 

community composition, and 

biodiversity in freshwater ecosystems 

(ISA, section IS.6.3). 

The effects of acid deposition on 

aquatic systems depend largely upon 

the ability of the system to neutralize 

additional acidic inputs from the 

environment, whether from the 

atmosphere or from surface inputs. 

There is a large amount of variability 

among freshwater systems in this 

regard, which reflects their underlying 

geology as well as their history of acidic 

inputs. Accordingly, different 

freshwater systems (e.g., in different 

geographic regions) respond differently 

to similar amounts of acid deposition. 

The main factor in determining 

sensitivity is the underlying geology of 

an area and its ability to provide soil 

base cations through weathering to 

buffer acidic inputs (ISA, Appendix 8, 

section 8.5.1). As noted in the ISA, 

‘‘[g]eologic formations having low base 

cation supply, due mainly to low soil 

and bedrock weathering, generally 

underlie the watersheds of acidsensitive lakes and streams’’ (ISA, 

Appendix 8, p. 8–58). 

Longstanding evidence has well 

characterized the changes in 

biogeochemical processes and water 

chemistry caused by N and S deposition 

and the ramifications for biological 

functioning of freshwater ecosystems 

(ISA, Appendix 8, section 8.1). The 

more recently available scientific 

research ‘‘reflects incremental 

improvements in scientific knowledge 

of aquatic biological effects and 

indicators of acidification as compared 

with knowledge summarized in the 

2008 ISA’’ (ISA, Appendix 8, p. 8–80). 

Previously and newly available studies 

‘‘indicate that aquatic organisms in 

sensitive ecosystems have been affected 

by acidification at virtually all trophic 

levels and that these responses have 

been well characterized for several 

decades’’ (ISA, Appendix 8, p. 8–80). 

For example, information reported in 

the previous 2008 ISA ‘‘showed 

consistent and coherent evidence for 

effects on aquatic biota, especially algae, 

benthic invertebrates, and fish that are 

most clearly linked to chemical 

indicators of acidification’’ (ISA, 

Appendix 8, p. 8–80). These indicators 

are surface water pH, base cation ratios, 

ANC, and inorganic aluminum 

concentration (ISA, Appendix 8, Table 

8–9). 

The effects of waterbody acidification 

on fish species are especially well 

documented, with many species (e.g., 

brown and brook trout and Atlantic 

salmon) experiencing adverse effects 

from acidification and the earliest 

lifestages being most sensitive (ISA, 

Appendix 8, section 8.3). Many effects 

of acidic surface waters on fish, 

particularly effects on gill function or 

structure, relate to low pH or the 

combination of low pH and elevated 

dissolved aluminum (ISA, Appendix 7, 

section 7.1.2.5 and Appendix 8, sections 

8.3.6.1 and 8.6.4). In general, biological 

effects in aquatic ecosystems are 

primarily attributable to low pH and 

high inorganic aluminum concentration 

(ISA, p. ES–14). Waterbody pH largely 

controls the bioavailability of 

aluminum, which is toxic to fish, and 

aluminum mobilization is largely 

confined to waters with a pH below 

about 5.5, which the ISA describes as 

corresponding to an ANC in the range 

of about 10 to 30 microequivalents per 

liter (meq/L) in waters of the Northeast 

with low to moderate levels of dissolved 

organic carbon (ISA, Appendix 7, 

section 7.1.2.6 and Appendix 8, section 

8.6.4). 

The parameter ANC is an indicator of 

the buffering capacity of natural waters 

against acidification. Although ANC 

does not directly affect biota, it is an 

indicator of acidification that relates to 

pH and aluminum levels (ISA, p. ES–14) 

or to watershed characteristics like base 

cation weathering (BCw) rate (ISA, 

Appendix 8, sections 8.1 and 8.3.6.3). 

Accordingly, ANC is commonly used to 

describe the potential sensitivity of a 

freshwater system to acidificationrelated effects. It can be measured in 

water samples and is also often 

estimated for use in water quality 

modeling, as is done in the aquatic 

acidification risk assessment for this 

review (summarized in section II.A.4. 

below). Water quality models are 

generally better at estimating ANC than 

at estimating other indicators of 

acidification-related risk, such as pH. 


khammond on DSK9W7S144PROD with RULES2
105708 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

Acid neutralizing capacity is estimated 

as the molar sum of strong base cations 

minus the molar sum of strong acid 

anions, specifically including SO4

2¥

and NO3

¥ (e.g., Driscoll et al., 1994). 

Thus, values below zero indicate a 

deficit in the ability to buffer acidic 

inputs, and increasing values above zero 

represent increasing buffering capability 

for acidic inputs (ISA, Appendix 7, 

section 7.1.2.6). In waters with high 

concentrations of naturally occurring 

organic acids, however, ANC may not be 

a good indicator of risk to biota as those 

acids can reduce bioavailability of 

aluminum, thus buffering the effects 

usually associated with low pH and 

high total aluminum concentrations 

(Waller et al., 2012; ISA, Appendix 8, 

section 8.3.6.4). 

In addition to acidity of surface 

waters quantified over weeks or months, 

waterbodies can also experience spikes 

in acidity in response to episodic 

precipitation or rapid snowmelt events. 

In these events (hours-days), a surge or 

pulse of drainage water, containing 

acidic compounds, is routed through 

upper soil horizons rather than the 

deeper soil horizons that would usually 

provide buffering for acidic compounds 

(ISA, Appendix 7, section 7.1). While 

some streams and lakes may have 

chronic or base flow chemistry that 

provides suitable conditions for aquatic 

biota, they may experience occasional 

acidic episodes with the potential for 

deleterious consequences to sensitive 

biota (ISA, Appendix 8, section 8.5). For 

example, in some impacted northeastern 

waterbodies, ANC levels may dip below 

zero for hours to days or weeks in 

response to such events, while 

waterbodies labeled chronically acidic 

have ANC levels below zero throughout 

the year (ISA, Appendix 7, section 

7.1.1.2; Driscoll et al., 2001). Headwater 

streams tend to be more sensitive to 

such episodes due to their smaller 

watersheds and, in the East, due to their 

underlying geology (ISA, Appendix 8, 

section 8.5.1). 

National survey data available in the 

last review, and dating back to the early 

1980s through 2004, indicated 

acidifying deposition had acidified 

surface waters in the southwestern 

Adirondacks, New England uplands, 

eastern portion of the upper Midwest, 

forested Mid-Atlantic highlands, and 

Mid-Atlantic coastal plain (2008 ISA, 

section 4.2.2.3; ISA, Appendix 8, 

section 8.5.1). For example, a 1984– 

1987 survey of waterbodies in the 

Adirondacks found 27% of streams to 

have ANC values below zero, with a 

minimum value of ¥134 meq/L 

(Sullivan et al., 2006). Values of ANC 

below 20 meq/L in Shenandoah stream 

sites have been reported as having a 

greater risk of episodic acidification and 

associated reduced populations of 

sensitive species, such as the native 

brook trout, compared to sites with 

higher ANC (Bulger et al., 1999; Bulger 

et al., 2000). A more recent study of two 

groups of Adirondack lakes for which 

water quality data were available from 

1982 and 1992, respectively, reported 

significant increases in ANC in the large 

majority of those lakes, with the 

magnitude of the increases varying 

across the lakes (Driscoll et al., 2016; 

ISA, Appendix 7, section 7.1.3.1). As 

described in the ISA, ‘‘[a]cidic waters 

were mostly restricted to northern New 

York, New England, the Appalachian 

Mountain chain, upper Midwest, and 

Florida’’ (ISA, Appendix 8, p. 8–60). 

Despite the appreciable reductions in 

acidifying deposition that have occurred 

in the U.S. since the 1960s and 1970s, 

aquatic ecosystems across the U.S. are 

still experiencing effects from historical 

contributions of N and S (ISA, 

Appendix 8, section 8.6). 

(b) Terrestrial Ecosystems 

Longstanding evidence, supported 

and strengthened by evidence newly 

available in this review, describes the 

changes in soil biogeochemical 

processes caused by acidifying 

deposition of N and S to terrestrial 

systems that are linked to changes in 

terrestrial biota, with associated impacts 

on ecosystem characteristics (ISA, 

Appendix 5, section 5.1). Consistent 

with conclusions in the last review, the 

current body of evidence is sufficient to 

infer a causal relationship between 

acidifying deposition and alterations of 

biogeochemistry in terrestrial 

ecosystems. Additionally, and 

consistent with conclusions in the last 

review, the current body of evidence is 

sufficient to infer a causal relationship 

between acidifying N and S deposition 

and the alteration of the physiology and 

growth of terrestrial organisms and the 

productivity of terrestrial ecosystems. 

The current body of evidence is also 

sufficient to conclude that a causal 

relationship exists between acidifying N 

and S deposition and alterations of 

species richness, community 

composition, and biodiversity in 

terrestrial ecosystems (2008 ISA, 

sections 4.2.1.1 and 4.2.1.2; 2020 ISA, 

Appendix 4, section 4.1 and Appendix 

5, sections 5.7.1 and 5.7.2). 

Deposition of acidifying compounds 

to acid-sensitive soils can cause soil 

acidification, increased mobilization of 

aluminum from soil to drainage water, 

and depletion of the pool of 

exchangeable base cations in the soil 

(ISA, Appendix 5, section 5.2 and 

Appendix 4, sections 4.3.4 and 4.3.5). 

Physiological effects of acidification on 

terrestrial biota include slower growth 

and increased mortality among sensitive 

plant species, which are generally 

attributable to physiological impairment 

caused by aluminum toxicity (related to 

increased availability of inorganic 

aluminum in soil water) and a reduced 

ability of plant roots to take up base 

cations (ISA, Appendix 4, section 4.3 

and Appendix 5, section 5.2). 

The physiological effects of acidifying 

deposition on terrestrial biota can also 

result in changes in species composition 

whereby sensitive species, such as red 

spruce and sugar maple, are replaced by 

more tolerant species, or the sensitive 

species that were dominant in the 

community become a minority. For 

example, increasing soil cation 

availability (as in Ca2∂ addition or 

gradient experiments) has been 

associated with greater growth and 

seedling colonization by sugar maple, 

while American beech is more prevalent 

on soils with lower levels of base 

cations where sugar maple is less often 

found (ISA, Appendix 5, section 

5.2.1.3.1; Duchesne and Ouimet, 2009). 

Soil acid-base chemistry has also been 

found to be a predictor of understory 

species composition (ISA, Appendix 5, 

section 5.2.2.1), and limited evidence 

has indicated an influence of soil acidbase chemistry on diversity and 

composition of soil bacteria, fungi, and 

nematodes (ISA, Appendix 5, section 

5.2.4.1). In addition to Ca2∂ addition 

experiments, observational gradient 

studies have also evaluated 

relationships between soil chemistry 

indicators of acidification (e.g., soil pH, 

base cation to aluminum (Bc:Al) ratio, 

base saturation, and aluminum) and 

ecosystem biological endpoints, 

including physiological and community 

responses of trees and other vegetation, 

lichens, soil biota, and fauna (ISA, 

Appendix 5, Tables 5–2 and 5–6). The 

2020 ISA also reports on several large 

observational studies evaluating 

statistical associations between tree 

growth or survival, as assessed at 

monitoring sites across the U.S., and 

estimates of average deposition of S or 

N compounds at those sites over time 

periods on the order of 10 years (ISA, 

Appendix 5, section 5.5.2 and Appendix 

6, section.6.2.3.1; Dietze and Moorcroft, 

2011; Thomas et al., 2010; Horn et al., 

2018). Negative associations were 

observed for survival and growth in 

several species or species groups with S 

deposition metrics; positive and 

negative associations were reported 

with N deposition (PA, sections 5.3.2.3 

and 5.3.4 and Appendix 5B). 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105709 

Although there has been no 

systematic national survey of U.S. 

terrestrial ecosystem soils, the forest 

ecosystems considered the most 

sensitive to terrestrial acidification from 

atmospheric deposition include forests 

of the Adirondack Mountains of New 

York, Green Mountains of Vermont, 

White Mountains of New Hampshire, 

the Allegheny Plateau of Pennsylvania, 

and mountain top and ridge forest 

ecosystems in the southern 

Appalachians (2008 ISA, Appendix 3, 

section 3.2.4.2; ISA, Appendix 5, 

section 5.3). Underlying geology is the 

principal factor governing the 

sensitivity of both terrestrial and aquatic 

ecosystems to acidification from S and 

N deposition. Geologic formations with 

low base cation supply (e.g., sandstone, 

quartzite), due mainly to low 

weathering rates, generally underlie 

these acid sensitive watersheds. Other 

factors also contribute to the overall 

sensitivity of an area to acidifying 

nitrogen and sulfur deposition, 

including topography, soil chemistry, 

land use, and hydrology (ISA, Appendix 

5, section 5.3). For example, ‘‘[a]cidsensitive ecosystems are mostly located 

in upland mountainous terrain in the 

eastern and western U.S. and are 

underlain by bedrock that is resistant to 

weathering, such as granite or quartzite 

sandstone’’ (ISA, Appendix 7, p. 7–45). 

Further, as well documented in the 

evidence, biogeochemical sensitivity to 

deposition-driven acidification (and 

eutrophication [see following section]) 

is the ‘‘result of historical loading, 

geologic/soil conditions (e.g., mineral 

weathering and S adsorption), and 

nonanthropogenic sources of N and S 

loading to the system’’ (ISA, Appendix 

7, p. 7–45 and section 7.1.5). 

(3) Nitrogen Enrichment and Associated 

Ecological Effects 

Ecosystems in the U.S. vary in their 

sensitivity to N enrichment, with 

organisms in their natural environments 

commonly adapted to the nutrient 

availability in those environments. 

Historically, N has been the primary 

limiting nutrient for plants in many 

ecosystems. In such ecosystems, when 

the limiting nutrient, N, becomes more 

available, whether from atmospheric 

deposition, runoff, or episodic events, 

the subset of plant species able to most 

effectively use the higher nitrogen levels 

may out-compete other species, leading 

to a shift in the community composition 

that may be dominated by a smaller 

number of species, i.e., a community 

with lower diversity (ISA, sections 

IS.6.1.1.2, IS.6.2.1.1 and IS.7.1.1, 

Appendix 6, section 6.2.4 and Appendix 

7, section 7.2.6.6). Thus, change in the 

availability of nitrogen in nitrogenlimited systems can affect growth and 

productivity, with ramifications on 

relative abundance of different species 

of vegetation and potentially further and 

broader ramifications on ecosystem 

processes, structure, and function. 

Both N oxides and reduced forms of 

nitrogen can contribute to N 

enrichment. In addition to atmospheric 

deposition, other sources of N 

compounds can play relatively greater 

or lesser roles in ecosystem N loading, 

depending on location. For example, 

many waterbodies receive appreciable 

amounts of N from agricultural runoff 

and municipal or industrial wastewater 

discharges. For many aquatic 

ecosystems, sources of N other than 

atmospheric deposition, including 

fertilizer and waste treatment, 

contribute more to ecosystem N than 

atmospheric deposition (ISA Appendix 

7, sections 7.1 and 7.2). Additionally, 

the impacts of historic N deposition in 

both aquatic and terrestrial ecosystems 

pose complications to discerning the 

potential effects of more recent 

deposition rates. 

(a) Aquatic and Wetland Ecosystems 

Nitrogen additions to freshwater, 

estuarine and near-coastal ecosystems, 

including N from atmospheric 

deposition, can contribute to 

eutrophication, which typically begins 

with nutrient-stimulated rapid algal 

growth developing into an algal bloom 

that can, depending on various sitespecific factors, be followed by anoxic 

conditions associated with the algal dieoff (ISA, ES.5.2). Decomposition of the 

plant biomass from the subsequent algal 

die-off contributes to reduced 

waterbody oxygen, which in turn can 

affect higher-trophic-level species, e.g., 

contributing to fish mortality (ISA, p. 

ES–18). The extensive body of evidence 

in this area is sufficient to infer causal 

relationships between N deposition and 

the alteration of biogeochemistry in 

freshwater, estuarine and near-coastal 

marine systems (ISA, Appendix 7, 

sections 7.1 and 7.2). Consistent with 

findings in the last review, the current 

body of evidence is also sufficient to 

infer a causal relationship between N 

deposition and changes in biota, 

including altered growth and 

productivity, species richness, 

community composition, and 

biodiversity due to N enrichment in 

freshwater ecosystems (ISA, Appendix 

9, section 9.1). The body of evidence is 

sufficient to infer a causal relationship 

between N deposition and changes in 

biota, including altered growth, total 

primary production, total algal 

community biomass, species richness, 

community composition, and 

biodiversity due to N enrichment in 

estuarine environments (ISA, Appendix 

10, section 10.1). 

Evidence newly available in this 

review provides insights regarding N 

enrichment and its impacts in several 

types of aquatic systems, including 

freshwater streams and lakes, estuarine 

and near-coastal systems, and wetlands. 

With regard to freshwaters, for example, 

studies published since the 2008 ISA 

augment the evidence base for highelevation 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, highelevation lake, and low-order stream 

watersheds that are far from the 

influence of other N sources like 

agricultural runoff and wastewater 

effluent (ISA, section ES5.2). In other 

fresh waterbodies, however, agricultural 

practices and point source discharges 

have been estimated to be larger 

contributors to total N loading (ISA, 

Appendix 7, section 7.1.1.1). Since the 

2008 ISA, several long-term monitoring 

studies in the Appalachian Mountains, 

the Adirondacks, and the Rocky 

Mountains have reported temporal 

patterns of declines in surface water 

NO3

- concentration corresponding to 

declines in atmospheric N deposition 

(ISA, Appendix 9, section 9.1.1.2). 


khammond on DSK9W7S144PROD with RULES2
105710 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

Declines in basin wide NO3

-

concentrations have also been reported 

for the nontidal Potomac River 

watershed and have been attributed to 

declines in atmospheric N deposition 

(ISA, Appendix 7, section 7.1.5.1). 

Nutrient inputs to coastal and 

estuarine waters are important 

influences on the health of these 

waterbodies. Continued inputs of N, the 

most common limiting nutrient in 

estuarine and coastal systems, have 

resulted in N over-enrichment and 

subsequent alterations to the nutrient 

balance in these systems (ISA, 

Appendix 10, p. 10–6). For example, the 

rate of N delivery to coastal waters is 

strongly correlated to changes in 

primary production and phytoplankton 

biomass (ISA, Appendix 10, section 

10.1.3). Algal blooms and associated 

die-offs can contribute to hypoxic 

conditions (most common during 

summer months), which can contribute 

to fish kills and associated reductions in 

marine populations (ISA, Appendix 10). 

Further, the prevalence and health of 

submerged aquatic vegetation (SAV), 

which is important habitat for many 

aquatic species, has been identified as a 

biological indicator for N enrichment in 

estuarine waters (ISA, Appendix 10, 

section 10.2.5). Previously available 

evidence indicated the role of N loading 

in SAV declines in multiple U.S. 

estuaries through increased production 

of macroalgae or other algae, which 

reduce sunlight penetration into 

shallow waters where SAV is found 

(ISA, Appendix 10, section 10.2.3). 

Newly available studies have reported 

findings of increased SAV populations 

in two tributaries of the Chesapeake Bay 

corresponding to reduction in total N 

loading from all sources since 1990 

(ISA, Appendix 10, section 10.2.5). The 

newly available studies also identify 

other factors threatening SAV, including 

increasing temperature related to 

climate change (ISA, Appendix 10, 

section 10.2.5). 

The degree to which N enrichment 

and associated ecosystem impacts are 

driven by atmospheric N deposition 

varies greatly and is largely unique to 

the specific ecosystem. Analyses based 

on data across two to three decades 

extending from the 1990s through about 

2010 estimate that most of the analyzed 

estuaries receive 15–40% of their N 

inputs from atmospheric sources (ISA, 

section ES 5.2; ISA, Appendix 7, section 

7.2.1), though for specific estuaries 

contributions can vary more widely. In 

areas along the West Coast, N sources 

may include coastal upwelling from 

oceanic waters, as well as transport from 

watersheds. Common N inputs to 

estuaries include those associated with 

freshwater inflows transporting N from 

agriculture, urban, and wastewater 

sources, in addition to atmospheric 

deposition across the watershed (ISA, 

section IS 2.2.2; ISA, Appendix 7, 

section 7.2.1). 

There are estimates of atmospheric N 

loading to estuaries available from 

several recent modeling studies (ISA, 

Table 7–9). One analysis of estuaries 

along the Atlantic Coast and the Gulf of 

Mexico, which estimated that 62¥81% 

of N delivered to the eastern U.S coastal 

zone is anthropogenic in source, also 

reported that atmospheric N deposition 

to freshwater that is subsequently 

transported to estuaries represents 

17¥21% of the total N loading into the 

coastal zone (McCrackin et al., 2013; 

Moore et al., 2011). In the Gulf of 

Mexico, 26% of the N transported to the 

Gulf in the Mississippi/Atchafalaya 

River basin was estimated to be 

contributed from atmospheric 

deposition (which may include 

volatilized losses from natural, urban, 

and agricultural sources) (Robertson and 

Saad, 2013). Another modeling analysis 

identified atmospheric deposition to 

watersheds as the dominant source of N 

to the estuaries of the Connecticut, 

Kennebec, and Penobscot rivers. For the 

entire Northeast and mid-Atlantic 

coastal region, however, it was the third 

largest source (20%), following 

agriculture (37%) and sewage and 

population-related sources (28%) (ISA, 

Appendix 7, section 7.2.1). Estimates for 

West Coast estuaries indicate much 

smaller contribution from atmospheric 

deposition. For example, analyses for 

Yaquina Bay, Oregon, estimated direct 

deposition to contribute only 0.03% of 

N inputs; estimated N input to the 

watershed from N-fixing red alder 

(Alnus rubra) trees was a much larger 

(8%) source (ISA, Appendix 7, section 

7.2.1; Brown and Ozretich, 2009). 

Evidence in coastal waters has 

recognized that nutrient enrichment 

may play a role in acidification of some 

coastal waters (ISA, Appendix 10, 

section 10.5). More specifically, 

nutrient-driven algal blooms may 

contribute to ocean acidification, 

possibly through increased 

decomposition, which lowers dissolved 

oxygen levels in the water column and 

contributes to lower pH. Such nutrientenhanced acidification can also be 

exacerbated by warming (associated 

with increased microbial respiration) 

and changes in buffering capacity 

(alkalinity) of freshwater inputs (ISA, 

Appendix 10, section 10.5). 

The impact of N additions on 

wetlands, and whether the wetlands 

may serve as a source, sink, or 

transformer of atmospherically 

deposited N varies with the type of 

wetland and other factors, such as 

physiography and local hydrology, as 

well as climate (ISA, section IS.8.1 and 

Appendix 11, section 11.1). Studies 

generally show N enrichment to 

decrease the ability of wetlands to retain 

and store N, which may diminish the 

wetland ecosystem service of improving 

water quality (ISA, section IS.8.1). 

Consistent with the evidence available 

in the last review, the current body of 

evidence is sufficient to infer a causal 

relationship between N deposition and 

the alteration of biogeochemical cycling 

in wetlands. Newly available evidence 

regarding N inputs and plant physiology 

expands the evidence base related to 

species diversity. The currently 

available evidence, including that newly 

available, is sufficient to infer a causal 

relationship between N deposition and 

the alteration of growth and 

productivity, species physiology, 

species richness, community 

composition, and biodiversity in 

wetlands (ISA, Appendix 11, section 

11.10). 

(b) Terrestrial Ecosystems 

It is long established that N 

enrichment of terrestrial ecosystems 

increases plant productivity (ISA, 

Appendix 6, section 6.1). Building on 

this, the currently available evidence, 

including evidence that is longstanding, 

is sufficient to infer a causal 

relationship between N deposition and 

the alteration of the physiology and 

growth of terrestrial organisms and the 

productivity of terrestrial ecosystems 

(ISA, Appendix 5, section 5.2 and 

Appendix 6, section 6.2). Responsive 

ecosystems include those that are N 

limited and/or contain species that have 

evolved in nutrient-poor environments. 

In these ecosystems the N-enrichment 

changes in plant physiology and growth 

rates vary among species, with species 

that are adapted to low N supply being 

readily outcompeted by species that 

require more N. In this manner, the 

relative representation of different 

vegetation species may be altered, and 

some species may be eliminated 

altogether, such that community 

composition is changed and species 

diversity declines (ISA, Appendix 6, 

sections 6.3.2 and 6.3.8). The currently 

available evidence in this area is 

sufficient to infer a causal relationship 

between N deposition and the alteration 

of species richness, community 

composition, and biodiversity in 

terrestrial ecosystems (ISA, section 

IS.5.3 and Appendix 6, section 6.3). 

Previously available evidence 

described the role of N deposition in 

changing soil carbon and N pools and 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105711 

fluxes, as well as altering plant and 

microbial growth and physiology in an 

array of terrestrial ecosystems (ISA, 

Appendix 6, section 6.2.1). Nitrogen 

availability is broadly limiting for 

productivity in many terrestrial 

ecosystems (ISA, Appendix 6, section 

6.2.1). Accordingly, N additions 

contribute to increased productivity and 

can alter biodiversity. Eutrophication, 

one of the mechanisms by which 

increased productivity and changes in 

biodiversity associated with N addition 

to terrestrial ecosystems can occur, 

comprises multiple effects that include 

changes to the physiology of individual 

organisms, alteration of the relative 

growth and abundance of various 

species, transformation of relationships 

between species, and indirect effects on 

availability of essential resources other 

than N, such as light, water, and 

nutrients (ISA, Appendix 6, section 

6.2.1). 

The currently available evidence for 

the terrestrial ecosystem effects of N 

enrichment, including eutrophication, 

includes studies in a wide array of 

systems, including forests (tropical, 

temperate, and boreal), grasslands, arid 

and semi-arid scrublands, and tundra 

(PA, section 4.1; ISA, Appendix 6). The 

organisms affected include trees, herbs 

and shrubs, and lichen, as well as 

fungal, microbial, and arthropod 

communities. Lichen communities, 

which have important roles in 

hydrologic cycling, nutrient cycling, 

and as sources of food and habitat for 

other species, are also affected by 

atmospheric N (PA, section 4.1; ISA, 

Appendix 6). The recently available 

studies on the biological effects of 

added N in terrestrial ecosystems 

include investigations of plant and 

microbial physiology, long-term 

ecosystem-scale N addition 

experiments, regional and continentalscale monitoring studies, and syntheses. 

The previously available evidence 

included N addition studies in the U.S. 

and N deposition gradient studies in 

Europe that reported associations of N 

deposition with reduced species 

richness and altered community 

composition for grassland plants, forest 

understory plants, and mycorrhizal 

fungi (soil fungi that have a symbiotic 

relationship with plant roots) (ISA, 

Appendix 6, section 6.3). Newly 

available evidence for forest 

communities in this review indicates 

that N deposition alters the physiology 

and growth of overstory trees, and that 

N deposition has the potential to change 

the community composition of forests 

(ISA, Appendix 6, section 6.6). Recent 

studies on forest trees include analyses 

of long-term forest inventory data 

collected from across the U.S. and 

Europe (ISA, Appendix 6, section 

6.2.3.1). The recent evidence also 

includes findings of variation in forest 

understory and non-forest plant 

communities with atmospheric N 

deposition gradients in the U.S. and in 

Europe. For example, gradient studies in 

Europe have found higher N deposition 

to be associated with forest understory 

plant communities with more nutrientdemanding and shade-tolerant plant 

species (ISA, Appendix 6, section 

6.3.3.2). A recent gradient study in the 

U.S. found associations between herb 

and shrub species richness and N 

deposition, that were related to soil pH 

(ISA, Appendix 6, section 6.3.3.2). 

Recent evidence includes associations 

of variation in lichen community 

composition with N deposition 

gradients in the U.S. and Europe, (ISA, 

Appendix 6, section 6.3.7; Table 6–23). 

Differences in lichen community 

composition have been attributed to 

differences in atmospheric N pollution 

in forests of the West Coast, Rocky 

Mountains, and southeastern Alaska. 

Differences in epiphytic lichen growth 

or physiology have been observed along 

atmospheric N deposition gradients in 

the highly impacted area of southern 

California and in more remote locations 

such as Wyoming and southeastern 

Alaska (ISA, Appendix 6, section 6.3.7). 

Historical deposition may play a role in 

observational studies of N deposition 

effects, complicating the disentangling 

of responses that may be related to more 

recent N loading. 

Newly available findings from N 

addition experiments expand on the 

understanding of mechanisms for plant 

and microbial community composition 

effects of increased N availability, 

indicating that competition for 

resources, such as water in arid and 

semi-arid environments, may exacerbate 

the effects of N addition on diversity 

(ISA, Appendix 6, section 6.2.6). The 

newly available studies in arid and 

semiarid ecosystems, particularly in 

southern California have reported 

changes in plant community 

composition, in the context of a long 

history of significant N deposition, with 

fewer observations of plant species loss 

or changes in plant diversity (ISA, 

Appendix 6, section 6.3.6). 

Nitrogen limitation in grasslands and 

the dominance by fast-growing species 

that can shift in abundance rapidly (in 

contrast to forest trees) contribute to an 

increased sensitivity of grassland 

ecosystems to N inputs (ISA, Appendix 

6, section 6.3.6). Studies in southern 

California coastal sage scrub 

communities, including studies of the 

long-term history of N deposition, 

which was appreciably greater in the 

past than recent rates, indicate impacts 

on community composition and species 

richness in these ecosystems (ISA, 

Appendix 6, sections 6.2.6 and 6.3.6). 

The ability of atmospheric N deposition 

to override the natural spatial 

heterogeneity in N availability in arid 

ecosystems, such as the Mojave Desert 

and California coastal sage scrub 

ecosystems in southern California, 

makes these ecosystems sensitive to N 

deposition (ISA, Appendix 6, section 

6.3.8). 

The current evidence includes 

relatively few studies of N enrichment 

recovery in terrestrial ecosystems. 

Among N addition studies assessing 

responses after cessation of additions, it 

has been observed that soil nitrate and 

ammonium concentrations recovered to 

levels observed in untreated controls 

within 1 to 3 years of the cessation of 

additions, but soil processes such as N 

mineralization and litter decomposition 

were slower to recover (ISA, Appendix 

6, section 6.3.2; Stevens, 2016). A range 

of recovery times have been reported for 

mycorrhizal community composition 

and abundance from a few years in 

some systems to as long as 28 or 48 

years in others (ISA, Appendix 6, 

section 6.3.2; Stevens, 2016; Emmett et 

al., 1998; Strengbom et al., 2001). An N 

addition study in the midwestern U.S. 

observed that plant physiological 

processes recovered in less than 2 years, 

although grassland communities were 

slower to recover and still differed from 

controls 20 years after the cessation of 

N additions (ISA, Appendix 6, section 

6.3.2; Isbell et al., 2013). 

(4) Other Deposition-Related Effects 

Additional categories of effects for 

which the current evidence is sufficient 

to infer causal relationships with 

deposition of S or N compounds or PM 

include changes in mercury methylation 

processes in freshwater ecosystems, 

changes in aquatic biota due to sulfide 

phytotoxicity, and ecological effects 

from PM deposition other than N and S 

deposition (ISA, Table IS–1). The 

current evidence, including that newly 

available in this review, is sufficient to 

infer a causal relationship between S 

deposition and the alteration of mercury 

methylation in surface water, sediment, 

and soils in wetland and freshwater 

ecosystems (ISA, Table ES–1). The 

currently available evidence is also 

sufficient to infer a new causal 

relationship between S deposition and 

changes in biota due to sulfide 

phytotoxicity, including alteration of 

growth and productivity, species 

physiology, species richness, 

community composition, and 


khammond on DSK9W7S144PROD with RULES2
105712 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

36For example, the fundamental purpose of parks 

in the National Park System ‘‘is to conserve the 

scenery, natural and historic objects, and wildlife 

in the System units and to provide for the 

enjoyment of the scenery, natural and historic 

objects, and wildlife in such manner and by such 

means as will leave them unimpaired for the 

enjoyment of future generations’’ (54 U.S.C. 

100101). Additionally, the Wilderness Act of 1964 

defines designated ‘‘wilderness areas’’ in part as 

areas ‘‘protected and managed so as to preserve 

[their] natural conditions’’ and requires that these 

areas ‘‘shall be administered for the use and 

enjoyment of the American people in such manner 

as will leave them unimpaired for future use and 

enjoyment as wilderness, and so as to provide for 

the protection of these areas, [and] the preservation 

of their wilderness character . . .’’ (16 U.S.C. 1131 

(a) and (c)). Other lands that benefit the public 

welfare include national forests which are managed 

for multiple uses including sustained yield 

management in accordance with land management 

plans (see 16 U.S.C. 1600(1)–(3); 16 U.S.C. 

1601(d)(1)). 

biodiversity in wetland and freshwater 

ecosystems (ISA, section IS.9). 

With regard to PM deposition other 

than N and S deposition, the currently 

available evidence is sufficient to infer 

a likely causal relationship between 

deposition of PM and a variety of effects 

on individual organisms and ecosystems 

(ISA, Appendix 15, section 15.1). 

Particulate matter includes a 

heterogeneous mixture of particles 

differing in origin, size, and chemical 

composition. In addition to N and S and 

their transformation products, other PM 

components, such as trace metals and 

organic compounds, when deposited to 

ecosystems, may affect biota. Material 

deposited onto leaf surfaces can alter 

leaf processes, and PM components 

deposited to soils and waterbodies may 

be taken up into biota, with the 

potential for effects on biological and 

ecosystem processes. Studies involving 

ambient air PM, however, have 

generally involved conditions that 

would not be expected to meet the 

current secondary standards for PM. 

Further, although in some limited cases, 

effects have been attributed to particle 

size (e.g., soiling of leaves by large 

coarse particles near industrial facilities 

or unpaved roads), ecological effects of 

PM have been largely attributed more to 

its chemical components, such as trace 

metals, which can be toxic in large 

amounts (ISA, Appendix 15, sections 

15.2 and 15.3.1). The evidence largely 

comes from studies involving areas 

experiencing elevated concentrations of 

PM, such as near industrial areas or 

historically polluted cities (ISA, 

Appendix 15, section 15.4). 

b. Public Welfare Implications 

In evaluating the public welfare 

implications of the evidence regarding S 

and N related welfare effects, we must 

consider the type, severity, and 

geographic extent of the effects. In this 

section, we discuss such factors in light 

of judgments and conclusions regarding 

effects on the public welfare that have 

been made in NAAQS reviews. 

As provided in section 109(b)(2) of 

the CAA, the secondary standard is to 

‘‘specify a level of air quality the 

attainment and maintenance of which in 

the judgment of the Administrator . . . 

is requisite to protect the public welfare 

from any known or anticipated adverse 

effects associated with the presence of 

such air pollutant in the ambient air.’’ 

The secondary standard is not meant to 

protect against all known or anticipated 

welfare effects related to oxides of N 

and S, and particulate matter, but rather 

those that are judged to be adverse to 

the public welfare, and a bright-line 

determination of adversity is not 

required in judging what is ‘‘requisite’’ 

(78 FR 3212, January 15, 2013; 80 FR 

65376, October 26, 2015; see also 73 FR 

16496, March 27, 2008). Thus, the level 

of protection from known or anticipated 

adverse effects to public welfare that is 

requisite for the secondary standard is a 

public welfare policy judgment made by 

the Administrator. The Administrator’s 

judgment regarding the available 

information and adequacy of protection 

provided by an existing standard is 

generally informed by considerations in 

prior reviews and associated 

conclusions. 

The categories of effects identified in 

the CAA to be included among welfare 

effects are quite diverse, and among 

these categories there are many different 

types of effects that vary broadly with 

regard to specificity and level of 

resolution. For example, effects on 

vegetation and effects on animals are 

categories identified in CAA section 

302(h), and the ISA recognizes effects of 

N and S deposition at the organism, 

population, community, and ecosystem 

level, as summarized in section II.A.3.a. 

above (ISA, sections IS.5 to IS.9). As 

noted in the last review of the secondary 

NAAQS for NOX and SOX, while the 

CAA section 302(h) lists a number of 

welfare effects, ‘‘these effects do not 

define public welfare in and of 

themselves’’ (77 FR 20232, April 3, 

2012). 

How important ecological impacts are 

to the public welfare depends on the 

type, severity and extent of the effects, 

as well as the societal use of the 

resource and the significance of the 

resource to the public welfare. Such 

factors can also be considered in the 

context of judgments and conclusions 

made in some prior reviews regarding 

public welfare effects. For example, in 

the context of secondary NAAQS 

decisions for O3, judgments regarding 

public welfare significance have given 

particular attention to effects in areas 

with special federal protections (such as 

Class I areas), and lands set aside by 

states, Tribes, and public interest groups 

to provide similar benefits to the public 

welfare (73 FR 16496, March 27, 2008; 

80 FR 65292, October 26, 2015).36 In the 

2015 O3 NAAQS review, the EPA 

recognized the ‘‘clear public interest in 

and value of maintaining these areas in 

a condition that does not impair their 

intended use and the fact that many of 

these lands contain O3-sensitive 

species’’ (73 FR 16496, March 27, 2008). 

Judgments regarding effects on the 

public welfare can depend on the 

intended use, including conservation, or 

service (and value) of the affected 

vegetation, ecological receptors, 

ecosystems and resources and the 

significance of that use to the public 

welfare (73 FR 16496, March 27, 2008; 

80 FR 65377, October 26, 2015). Uses or 

services provided by areas that have 

been afforded special protection can 

flow in part or entirely from the 

vegetation that grows there as well as 

other natural features and resources. 

Ecosystem services range from those 

directly related to the natural 

functioning of the ecosystem to 

ecosystem uses for human recreation or 

profit, such as through the production of 

lumber or fuel (Costanza et al., 2017; 

ISA, section IS.13). The spatial, 

temporal, and social dimensions of 

public welfare impacts are also 

influenced by the type of service 

affected. For example, a national park 

can provide direct recreational services 

to the thousands of visitors that come 

each year but also provide an indirect 

value to the millions who may not visit 

but receive satisfaction from knowing it 

exists and is preserved for the future (80 

FR 65377, October 26, 2015). 

In the last review of the secondary 

NAAQS for NOX and SOX, ecosystem 

services were discussed as a method of 

assessing the magnitude and 

significance to the public of resources 

affected by ambient air concentrations 

of oxides of nitrogen and sulfur and 

associated deposition in sensitive 

ecosystems (77 FR 20232, April 3, 

2012). That review recognized that 

although there is no specific definition 

of adversity to public welfare, one 

paradigm might involve ascribing public 

welfare significance to disruptions in 

ecosystem structure and function. The 

concept of considering the extent to 

which a pollutant effect will contribute 

to such disruptions has been used 

broadly by the EPA in considering 

effects. An evaluation of adversity to 

public welfare might also consider the 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105713 

37As recognized in section II.A.3.a.(3)(b) above, 

lichen communities have important roles in 

ecosystem function, such as in hydrologic cycling, 

nutrient cycling, and as sources of food and habitat 

for other species (ISA, Appendix 6). 

38While ‘‘there is evidence that N and S 

emissions/deposition have a range of effects on U.S. 

ecosystem services and their social value’’ and 

Continued 

likelihood, type, magnitude, and spatial 

scale of the effect, as well as the 

potential for recovery and any 

uncertainties relating to these 

considerations (77 FR 20218, April 3, 

2012). 

The types of effects on aquatic and 

terrestrial ecosystems discussed in 

section II.A.3.1. above differ with regard 

to aspects important to judging their 

public welfare significance. For 

example, in the case of effects on timber 

harvest, such judgments may consider 

aspects such as the heavy management 

of silviculture in the U.S., while 

judgments for other categories of effects 

may generally relate to considerations 

regarding natural areas, including 

specifically those areas that are not 

managed for harvest. Effects on tree 

growth and survival have the potential 

to be significant to the public welfare 

through impacts in Class I and other 

areas given special protection in their 

natural/existing state, although they 

differ in how they might be significant. 

In this context, it may be important to 

consider that S and N deposition-related 

effects, such as changes in growth and 

survival of plant and animal species, 

could, depending on severity, extent, 

and other factors, lead to effects on a 

larger scale including changes in overall 

productivity and altered community 

composition (ISA, section IS.2.2.1 and 

Appendices 5, 6, 8, 9, and 10). Further, 

effects on individual species could 

contribute to impacts on community 

composition through effects on growth 

and reproductive success of sensitive 

species in the community, with varying 

impacts to the system through many 

factors including changes to competitive 

interactions (ISA, section IS.5.2 and 

Appendix 6, section 6.3.2). 

In acid-impacted surface waters, 

acidification primarily affects the 

diversity and abundance of fish and 

other aquatic life and the ecosystem 

services derived from these organisms. 

(2011 PA, section 4.4.5). In addition to 

other types of services, fresh surface 

waters support several cultural services, 

such as aesthetic, recreational, and 

educational services. The type of service 

that is likely to be most widely and 

significantly affected by aquatic 

acidification is recreational fishing. 

Multiple studies have documented the 

economic benefits of recreational 

fishing. Freshwater rivers and lakes of 

the northeastern United States, surface 

waters that have been most affected by 

acidification, are not a major source of 

commercially raised or caught fish; they 

are, however, a source of food for some 

recreational and subsistence fishers and 

for other consumers (2009 REA, section 

4.2.1.3). It is not known if and how 

consumption patterns of these fishers 

may have been affected by the historical 

impacts of surface water acidification in 

the affected systems. Non-use services, 

which include existence (protection and 

preservation with no expectation of 

direct use) and bequest values, are 

arguably a significant source of benefits 

from reduced acidification (Banzhaf et 

al., 2006). Since the 2012 review, 

additional approaches and methods 

have been applied to estimate the 

potential effects of aquatic acidification 

on uses and services of affected aquatic 

ecosystems; with regard to economic 

impacts, however, ‘‘for many regions 

and specific services, poorly 

characterized dose-response between 

deposition, ecological effect, and 

services are the greatest challenge in 

developing specific data on the 

economic benefits of emission 

reductions’’ (ISA, Appendix 14, p. 14– 

23). 

Nitrogen loading in aquatic 

ecosystems, particularly large estuarine 

and coastal water bodies, has and 

continues to pose risks to the services 

provided by those ecosystems, with 

clear implications to the public welfare 

(2011 PA, section 4.4.2; ISA, Appendix 

14, section 14.3.2). For example, the 

large estuaries of the eastern U.S. are an 

important source of fish and shellfish 

production, capable of supporting large 

stocks of resident commercial species 

and serving as breeding grounds and 

interim habitat for several migratory 

species (2009 REA, section 5.2.1.3). 

These estuaries also provide an 

important and substantial variety of 

cultural ecosystem services, including 

water-based recreational and aesthetic 

services. Additionally, as noted for fresh 

waters above, these systems have nonuse benefits to the public (2011 PA, 

section 4.4.5). Studies reviewed in the 

ISA have explored both enumeration of 

the number of ecosystem services that 

may be affected by N loading and the 

pathways by which this may occur, as 

well as approaches to valuation of such 

impacts. A finding of one such analysis 

was that ‘‘better quantitative 

relationships need to be established 

between N and the effects on 

ecosystems at smaller scales, including 

a better understanding of how N 

shortages can affect certain 

populations’’ (ISA, Appendix 14, 

sections 14.5 and 14.6). The relative 

contribution of atmospheric deposition 

to total N loading varies widely among 

estuaries, however, and has declined in 

some areas in recent years (ISA, 

Appendix 10, section 10.10.1). 

A complication to considering the 

public welfare implications specific to 

N deposition in terrestrial systems is the 

potential for N to increase growth and 

yield of plants that, depending on the 

type of plant and its use by human 

populations (e.g., food for livestock or 

human populations, trees for lumber), 

could be judged beneficial to the public. 

Such increased growth and yield may be 

judged and valued differently than 

changes in growth of other species. As 

noted in section II.A.3.a. above, 

enrichment in natural ecosystems can, 

by increasing growth of N limited plant 

species, change competitive advantages 

of species in a community, with 

associated impacts on the composition 

of the ecosystem’s plant community. 

The public welfare implications of such 

effects may vary depending on their 

severity, prevalence, and magnitude. 

Impacts on some ecosystem 

characteristics (e.g., forest or forest 

community composition) may be 

considered of greater public welfare 

significance when occurring in Class I 

or other protected areas, due to the 

value that the public places on such 

areas. In considering such services in 

past reviews for secondary standards for 

other pollutants (e.g., O3), the Agency 

has given particular attention to effects 

in natural ecosystems, indicating that a 

protective standard, based on 

consideration of effects in natural 

ecosystems in areas afforded special 

protection, would also ‘‘provide a level 

of protection for other vegetation that is 

used by the public and potentially 

affected by O3 including timber, 

produce grown for consumption and 

horticultural plants used for 

landscaping’’ (80 FR 65403, October 26, 

2015). 

Although the welfare effects evidence 

base describes effects related to 

ecosystem deposition of N and S 

compounds, the available information 

does not yet provide a framework that 

can specifically tie various magnitudes 

or prevalences of changes in a biological 

or ecological indicator (e.g., lichen 

abundance or community 

composition) 37 to broader effects on the 

public welfare. The ISA finds that while 

there is an improved understanding 

from information available in this 

review of the number of pathways by 

which N and S deposition may affect 

ecosystem services, most of these 

relationships remain to be quantified 

(ISA, Appendix 14, section 14.6).38 This 


khammond on DSK9W7S144PROD with RULES2
105714 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

‘‘there are some economic studies that demonstrate 

such effects in broad terms,’’ ‘‘it remains 

methodologically difficult to derive economic costs 

and benefits associated with specific regulatory 

decisions/standards’’ (ISA, Appendix 14, pp. 14–23 

to 14–24). 

39With regard to other deposition-related effects 

of S compounds, quantitative tools or approaches 

for relating S deposition to ecosystem impacts are 

not currently well developed. As summarized in 

section II.A.3.a.(4) above, these effects, in wetland 

and freshwater ecosystems, include the alteration of 

Hg methylation in surface water, sediment, and 

soils; and changes in biota due to sulfide 

phytotoxicity including alteration of growth and 

productivity, species physiology, species richness, 

community composition, and biodiversity. No 

studies are in the available evidence regarding the 

estimation of critical loads for SOX deposition 

related to these non-acidifying effects of S 

deposition into these ecosystems (ISA, Appendix 

12, section 12.6). 

gap creates uncertainties when 

considering the public welfare 

implications of some biological or 

geochemical responses to ecosystem 

acidification or N enrichment and 

accordingly complicates judgments on 

the potential for public welfare 

significance. That notwithstanding, 

while shifts in species abundance or 

composition of various ecological 

communities may not be easily judged 

with regard to public welfare 

significance, at some level, such 

changes, especially if occurring broadly 

in specially protected areas, where the 

public can be expected to place high 

value, might reasonably be concluded to 

impact the public welfare. An 

additional complexity in the current 

review with regard to assessment of 

effects associated with existing 

deposition rates is that the current, 

much-improved air quality and 

associated reduced deposition is within 

the context of a longer history that 

included appreciably greater deposition 

in the middle of the last century, the 

environmental impacts of which may 

remain, affecting ecosystem responses. 

In summary, several considerations 

are important to judgments on the 

public welfare significance of given 

welfare effects under different 

exposures. These include uncertainties 

and limitations that must be taken into 

account regarding the magnitude of key 

effects that might be concluded to be 

adverse to ecosystem health and 

associated services. Additionally, there 

are numerous locations vulnerable to 

public welfare impacts from S or N 

deposition-related effects on terrestrial 

and aquatic ecosystems and their 

associated services. Other important 

considerations include the exposure 

circumstances that may elicit effects and 

the potential for the significance of the 

effects to vary in specific situations due 

to differences in sensitivity of the 

exposed species, the severity and 

associated significance of the observed 

or predicted effect, the role that the 

species plays in the ecosystem, the 

intended use of the affected species and 

its associated ecosystem and services, 

the presence of other co-occurring 

predisposing or mitigating factors, and 

associated uncertainties and limitations. 

c. Exposure Conditions and DepositionRelated Metrics 

The ecological effects identified in 

section II.A.3.a. above vary widely in 

their extent and the resolution of the 

available information that describes the 

exposure circumstances under which 

they occur. The information for direct 

effects of SOX, N oxides and PM in 

ambient air is somewhat more straightforward to consider as it is generally 

presented in terms of concentrations in 

air. For deposition-related effects, the 

information may be about S and N 

compounds in soil or water or may be 

for metrics intended to represent 

atmospheric deposition of those 

compounds. For the latter, as recognized 

in section II.A.1.c. above, we face the 

challenge of relating that information to 

patterns of ambient air concentrations. 

With regard to the more complex 

consideration of deposition-related 

effects such as ecosystem acidification 

and N enrichment, there is also wide 

variation in the extent and level of 

detail of the evidence available to 

describe the ecosystem characteristics 

(e.g., physical, chemical, and geological 

characteristics, as well as atmospheric 

deposition history) that influences the 

degree to which deposition of N and S 

associated with the oxides of S and N 

and PM in ambient air may be linked to 

ecological effects. One reason for this 

relates to the contribution of many 

decades of uncontrolled atmospheric 

deposition before the establishment of 

NAAQS for PM, oxides of S and oxides 

of N (in 1971), followed by the 

subsequent decades of continued 

deposition as standards were 

implemented and updated. The impacts 

of this deposition history remain in soils 

of many parts of the U.S. today (e.g., in 

the Northeast and portions of the 

Appalachian Mountains in both 

hardwood and coniferous forests, as 

well as areas in and near the Los 

Angeles Basin), with recent signs of 

recovery in some areas (ISA, Appendix 

4, section 4.6.1; 2008 ISA, section 

3.2.1.1). This backdrop and associated 

site-specific characteristics are among 

the challenges faced in identifying 

deposition targets that might be 

expected to provide protection going 

forward from the range of effects for 

which we have evidence as a result of 

the deposition of the past. 

Critical loads (CLs) are frequently 

used in studies that investigate 

associations between various chemical, 

biological, ecological and ecosystem 

characteristics and a variety of N or S 

deposition-related metrics. The term 

critical load, which refers to an amount 

(or a rate of addition) of a pollutant to 

an ecosystem that is estimated to be at 

(or just below) that which would result 

in an ecological effect of interest, has 

multiple interpretations and 

applications (ISA, p. IS–14). The 

dynamic nature of ecosystem pollutant 

processing and the broad array of factors 

that influence it adds complications to 

critical load identification and 

interpretation. Time is an important 

dimension, which is sometimes 

unstated (e.g., in empirical or 

observational analyses) and is 

sometimes explicit (e.g., in steady-state 

or dynamic modeling analyses) (ISA, 

section IS.2.2.4). Further, this variety in 

meanings stems in part from differing 

judgments and associated 

identifications regarding the ecological 

effect (both type and level of severity) 

on which the critical load focuses and 

judgment of its significance or meaning. 

Studies, based on which CLs are often 

identified, vary widely with regard to 

the specific ecosystem characteristics 

being evaluated, as well as the 

benchmarks selected for judging them. 

The specific details of these various 

judgments influence the strengths and 

limitations, and associated uncertainty, 

of using critical load information from 

such studies for different applications. 

The summary that follows is intended to 

reach beyond individual critical loads 

developed over a variety of studies and 

ecosystems and consider the underlying 

study findings about key aspects of the 

environmental conditions and 

ecological characteristics studied. A 

more quantitative variation of this is the 

methodology developed for the aquatic 

acidification REA in this review, 

presented in the PA and summarized in 

section II.A.4. below. In those analyses, 

the concept of a critical load is 

employed with steady-state modeling 

that relates deposition to waterbody 

acid neutralizing capacity. 

While recognizing the inherent 

connections between watersheds and 

waterbodies, such as lakes and streams, 

the organization of this section 

recognizes the more established state of 

the information, tools, and data for 

aquatic ecosystems for characterizing 

relationships between atmospheric 

deposition and acidification and/or 

nutrient enrichment effects under air 

quality associated with the current 

standards (PA, Chapter 5).39 Further, we 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105715 

40Some modeling studies in some areas have 

indicated the potential for a lagged response even 

as emissions and deposition decline; this lag 

reflects a reduction in soil absorption of SO4

¥2 and 

leaching of previously accumulated S from 

watersheds (ISA, Appendix 7, section 7.1.2.2). 

41Separate quantitative analyses have not been 

performed in this review for N enrichment-related 

effects in these waterbodies in recognition of a 

number of factors, including modeling and 

assessment complexities, and site- or waterbodyspecific data requirements, as well as, in some 

cases, issues of apportionment of atmospheric 

sources separate from other influential sources. 

42Under the CWA, section 303(d), every two 

years, states and other jurisdictions are required to 

list impaired waterbodies not meeting water quality 

standards. For waterbodies on the list, a TMDL 

must be developed that identifies the maximum 

amount of pollutant a waterbody can receive and 

still meet water quality standards, e.g., standards for 

dissolved oxygen and chlorophyll a (which are 

indicators of eutrophication). 

recognize the generally greater role of 

atmospheric deposition in waterbodies 

impacted by aquatic acidification 

compared to its role in eutrophicationrelated impacts of surface waters, 

particularly rivers and estuaries in and 

downstream of populated watersheds, to 

which direct discharges have also long 

contributed, as recognized in section 

II.A.3.a(3) above (ISA, Appendix 13, 

section 13.1.3.1; ISA, Appendix 7, 

section 7.1.1.1; 2008 ISA, section 3.2). 

Therefore, with regard to depositionrelated effects, we focus first on the 

quantitative information for aquatic 

ecosystem effects in sections II.A.3.c.(1) 

below. Section II.A.3.c.(2) discusses the 

available evidence regarding 

relationships between depositionrelated exposures and the occurrence 

and severity of effects on trees and 

understory communities in terrestrial 

ecosystems. Section II.A.3.c.(3) 

discusses the currently available 

information related to consideration of 

exposure concentrations associated with 

other welfare effects of nitrogen and 

sulfur oxides and PM in ambient air. 

(1) Acidification and Nitrogen 

Enrichment in Aquatic Ecosystems 

Prior to the peak in S deposition 

levels that occurred in the 1970s and 

early 1980s, when deposition likely 

exceeded 30 kg S/ha-yr in some areas 

(PA, Appendix B, Figure 5B–9), surface 

water SO4

2¥ concentrations were 

increasing in response to the extremely 

high S deposition of the preceding 

years. Subsequently, and especially 

more recently, surface water SO4

2¥

concentrations have generally 

decreased, particularly in the Northeast 

(Robinson et al., 2008; ISA, section 

7.1.5.1.4). Some studies of long-term 

projections in some waterbodies (e.g., in 

the Blue Ridge Mountains region in 

Virginia), however, continue to indicate 

little or slow reduction in acidic ions, 

even as emissions have declined. This is 

an example of the competing role of 

changes in S adsorption on soils and the 

release of historically deposited S from 

soils into surface water,40 which some 

modeling has suggested will delay 

chemical recovery in those water bodies 

(ISA, Appendix 7, sections 7.1.2.2 and 

7.1.5.1). 

In the 2012 review of the oxides of N 

and S, quantitative analyses relating 

deposition in recent times (e.g., since 

2000) to ecosystem acidification, and 

particularly aquatic acidification, were 

generally considered to be less 

uncertain, and the ability of those 

analyses to inform NAAQS policy 

judgments more robust, than analyses 

related to deposition and ecosystem 

nutrient enrichment or eutrophication 

(2011 PA). While quantitative 

assessment approaches for aquatic 

eutrophication as a result of total N 

loading are also well established, and 

the evidence base regarding atmospheric 

deposition and nutrient enrichment has 

expanded since the 2012 review, the 

significance of non-air N loading to 

rivers, estuaries and coastal waters (as 

recognized in section II.A.3.a. above) 

continues to complicate the assessment 

of nutrient enrichment-related risks 

specifically related to atmospheric N 

deposition. Accordingly, the REA 

analyses developed in this review focus 

on aquatic acidification. The REA and 

its findings regarding deposition rates 

associated with different levels of 

aquatic acidification risk are 

summarized in section II.A.4. below. 

Thus, the paragraphs below focus on 

available quantitative information 

regarding atmospheric deposition and N 

enrichment in aquatic ecosystems.41

The overview provided here draws on 

the summary in the PA of the evidence 

as characterized in the ISA with regard 

to deposition level estimates that 

studies have related to various degrees 

of different effects with associated 

differences in potential for or clarity in 

public welfare significance (PA, section 

5.2). 

The eutrophication of wetlands and 

other aquatic systems is primarily 

associated with nitrogen inputs, 

whether from deposition or other 

sources. Atmospheric deposition is the 

main source of new N inputs to some 

freshwater wetlands and fresh 

waterbodies, such as headwater streams 

and high-elevation lakes, while other N 

inputs, such as agricultural runoff and 

wastewater effluent, can be significant 

contributors to waterbodies in 

agricultural and populated areas (ISA, 

Appendix 9, section 9.1 and Appendix 

11, section 11.3.1). Rates of total N 

deposition associated with 

eutrophication-related effects in aquatic 

systems ranges from a few kilograms per 

hectare per year (kg/ha-yr) for 

differences in diatom community 

composition in high elevation lakes to 

over 500 kg N/ha-yr for some effects in 

saltwater wetlands. While the evidence 

for these effects contributes to ISA 

causal determinations, it is often very 

location-specific and less informative 

for other uses, such as in quantitative 

assessments relating deposition to 

waterbody response across broad 

geographic areas. 

In estuaries and coastal systems, the 

well-established relationships between 

N loading and algal blooms and 

associated water quality impacts have 

been the focus of numerous water 

quality modeling projects that have 

quantified eutrophication processes 

across a wide variety of U.S. 

ecosystems. These projects, which have 

generally involved quantification of N 

loading and association with various 

water quality indicators, have informed 

management decision-making in 

multiple estuaries, including 

Chesapeake Bay, Narraganset Bay, 

Tampa Bay, Neuse River Estuary and 

Waquoit Bay (ISA, Appendix 7, section 

7.2). The indicators of nutrient 

enrichment employed include 

chlorophyll a, dissolved oxygen, and 

reduced abundance of submerged 

aquatic vegetation, among others (ISA, 

section IS.7.3 and Appendix 10, section 

10.6). 

The decision-making in these projects 

generally focuses on identification of 

total N loading targets for purposes of 

attaining water quality standards, 

informed by modeling work that 

includes apportionment of sources, 

which vary by system. We note that the 

assignment of targets to different source 

types (e.g., groundwater, surface water 

runoff, and atmospheric deposition) in 

different waterbodies and watersheds 

varies for both practical and policy 

reasons. Further, during the multidecade time period across which these 

activities have occurred, atmospheric 

deposition of N in coastal areas has 

declined. In general, however, 

atmospheric deposition targets for N for 

the large systems summarized above 

have been approximately 10 kg/ha-yr. 

The establishment of target N loads to 

surface waterbodies is in many areas 

related to implementation of the total 

maximum daily load (TMDL) 

requirements of section 303(d) of the 

Clean Water Act.42 Nutrient load 

allocation and reduction activities in 

some large estuaries predate 


khammond on DSK9W7S144PROD with RULES2
105716 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

43For example, a 2011 analysis estimated 

atmospheric deposition to the Chesapeake Bay 

watershed to account for approximately 25% of 

total N inputs to the estuary (ISA, Appendix 7, 

section 7.2.1). 

44As recognized on the EPA web page describing 

this activity, the TMDL, formally established in 

December 2010 ‘‘is designed to ensure that all 

pollution control measures needed to fully restore 

the Bay and its tidal rivers are in place by 2025.’’ 

The website also indicates that ‘‘EPA expects 

practices in place by 2017 to meet 60 percent of the 

necessary reductions,’’ and for some areas to 

recover before others, but for it to take years after 

2025 for the Bay and its tributaries to fully recover 

(https://www.epa.gov/chesapeake-bay-tmdl/ 

frequent-questions-about-chesapeake-bay-tmdl). 

45For example, across the 74 estuaries in the 3- 

state coastal region studied, N from atmospheric 

deposition to estuary watersheds was generally 

estimated to account for less than 25% of total N 

inputs, while estimates for a few small estuaries in 

CT were higher than 51% (but below 75%) (Latimer 

and Charpentier, 2010). 

46One evaluation of progress in achieving 

mandated N reductions in the Neuse River Basin in 

NC found that flow-normalized N loading from 

NO3

¥ decreased beginning in the 1992–1996 period 

(ISA Appendix 10, section 10.2; Lebo et al., 2012). 

development of CWA 303(d) TMDLs. 

The multiple Chesapeake Bay 

Agreements signed by the U.S. EPA, 

District of Columbia, and states of 

Virginia, Maryland, and Pennsylvania 

first established the voluntary 

government partnership that directs and 

manages bay cleanup efforts and 

subsequently included commitments for 

reduction of N and phosphorus loading 

to the bay. Efforts prior to 2000 focused 

largely on point-source discharges, with 

slower progress for nonpoint-source 

reductions via strategies such as 

adoption of better agricultural practices, 

reduction of atmospheric N deposition, 

enhancement of wetlands and other 

nutrient sinks, and control of urban 

sprawl (2008 ISA, section 3.3.8.3). 

Studies since 2000 estimate atmospheric 

deposition as a major N source in the 

overall N budget for the Chesapeake 

Bay 43 (ISA, section 7.2.1; Howarth, 

2008; Boyer et al., 2002). The TMDL 

established for the Chesapeake Bay in 

2010, under requirements of section 

303(d) of the Clean Water Act, included 

a loading allocation for atmospheric 

deposition of N directly to tidal waters, 

which was projected to be achieved by 

2020 based on air quality progress under 

existing CAA regulations and programs 

(U.S. EPA, 2010).44

Jurisdictions for other U.S. estuaries 

have also developed TMDLs to address 

nutrient loading causing eutrophication. 

For example, atmospheric deposition in 

2000 was identified as the third largest 

source of N loading to Narragansett Bay 

via the watershed and directly to the 

Bay, at 20% of the total (ISA, Appendix 

7, section 7.2.1). Similarly, atmospheric 

deposition was estimated to account for 

approximately a third of N input to 

several small- to medium-sized estuaries 

of southern New England, with the 

percentage varying widely for 

individual estuaries (ISA, Appendix 7, 

section 7.2.1; Latimer and Charpentier, 

2010).45 Another modeling study in the 

Waquoit Bay estuaries in Cape Cod, 

Massachusetts, using data since 1990, 

estimated atmospheric deposition to 

have decreased by about 41% while 

wastewater inputs increased 80%, with 

a net result that total loads were 

concluded to not have changed over that 

time period (ISA, Appendix 7, section 

7.2.1). Another well-studied estuarine 

system is Tampa Bay, for which a 2013 

study estimated atmospheric sources to 

account for more than 70% of total N 

loading based on 2002 data (ISA, 

Appendix 7, section 7.2.1). The TMDL 

for Tampa Bay allocates 11.8 kg/ha-yr N 

loading to atmospheric deposition (ISA, 

Appendix 16, section 16.4.2; Janicki 

Environmental, 2013). The Neuse River 

Estuary is another for which modeling 

work has investigated the role of N 

loading from multiple sources on 

nutrient enrichment 46 and associated 

water quality indicators, including 

chlorophyll a (ISA, Appendix 10, 

section 10.2). 

Nitrogen loading to estuaries has also 

been considered specifically for impacts 

on submerged aquatic vegetation. For 

example, eelgrass coverage was 

estimated to be markedly reduced in 

shallow New England estuaries with N 

loading at or above 100 kg N/ha-yr (ISA, 

Appendix 10, section 10.2.5). Another 

study estimated loading rates above 50 

kg/ha-yr as a threshold at which habitat 

extent may be impacted (ISA, Appendix 

10, section 10.2.5; Latimer and Rego, 

2010). Factors that influence the impact 

of N loading on submerged vegetation 

include flushing and drainage in 

estuaries (ISA, Appendix 10, section 

10.6). 

(2) Deposition-Related Effects in 

Terrestrial Ecosystems 

The subsections below describe the 

available information for quantitative 

relationships between atmospheric 

deposition rates and acidification and N 

enrichment-related effects in terrestrial 

systems. In the 2012 review, analyses 

included a critical load-based 

quantitative modeling analysis focused 

on BC:Al ratios in soils for terrestrial 

acidification and a qualitative 

characterization of nutrient enrichment 

(2009 REA). The more qualitative 

approach taken for nutrient enrichment 

in the 2012 review involved describing 

deposition ranges identified from 

observational or modeling research as 

associated with potential effects/ 

changes in species, communities, and 

ecosystems, with recognition of 

uncertainties associated with 

quantitative analysis of these 

depositional effects (2011 PA, section 

3.2.3). In this review, rather than 

performing new quantitative analyses 

focused on terrestrial ecosystems, we 

draw on analyses in the 2009 REA and 

on more recent published studies 

recognized in the ISA that provide 

information pertaining to deposition 

levels associated with effects related to 

terrestrial acidification and N 

enrichment. 

Several recent publications have 

added to the information available in 

the last review including analyses of 

large datasets from field assessments of 

tree growth and survival, as well as 

analyses of understory plant community 

richness, containing estimates of 

atmospheric N and/or S deposition 

(ISA, Appendix 6, section 6.5). The 

understory plant studies investigate the 

existence of associations of variations in 

plant community structure and other 

metrics including species richness, 

growth, and survival with variations in 

deposition during an overlapping time 

period, generally of a decade or two in 

duration. Soil acidification modeling 

and observational studies, as well as 

experimental addition studies, each 

with their various design features and 

associated strengths and limitations (as 

noted immediately below), inform 

consideration of N and S deposition 

levels of interest in the review. 

In general, observational or gradient 

studies differ from the chemical mass 

balance modeling approach in a number 

of ways that are relevant to their 

consideration and use for our purposes 

in this review. One difference of note is 

the extent to which their findings 

address the ecosystem impacts of 

historical deposition. Observational 

studies describe variation in indicators 

in the current context, which may 

include stores of historically deposited 

chemicals. In these studies, such 

historical loading, and its associated 

impacts, can contribute to effects 

quantified by the study ecological 

metrics, yet the metric values are 

assessed in relation to estimates of more 

recent deposition. Mass balance 

modeling for steady-state conditions is 

commonly used for estimating critical 

loads for acidification risk but does not 

usually address the complication of 

historical deposition impacts that can 

play a significant role in timing of 

system recovery. In this type of 

modeling, timelines of the various 

processes are not addressed. While this 

provides a simple approach that may 

facilitate consideration unrelated to 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105717 

47Uncertainties associated with the 2009 REA 

analyses include those associated with the limited 

dataset of laboratory-generated data on which the 

BC:Al target values are based (PA, section 5.3.2) as 

well as in the steady-state modeling parameters, 

most prominently those related to base cation 

weathering and acid neutralizing capacity (2009 

REA, section 4.3.9). A new approach to estimating 

weathering has more recently been employed and 

reported to reduce the uncertainty associated with 

this parameter (e.g., Phelan et al., 2014; McDonnell 

et al., 2012; ISA, Appendix 4, sections 4.6.2.1 and 

4.8.4 and Appendix 5, section 5.4). 

48The study by Horn et al. (2018) constrained the 

S analyses to preclude a positive association with 

S. 

49This range is for median S deposition estimates 

(based on measurement interval average, occurring 

within the years 2000–2013) of nonwestern species 

with negative associations with growth or survival 

ranged (Horn et al., 2018). 

50The influence of historically higher deposition 

(e.g., versus deposition over the measurement 

interval) on observations is unknown. Given the 

influence of deposition on soil conditions that 

affect tree growth and survival, and generally 

similar geographic variation for recent and historic 

deposition, a quantitative interpretation of 

uncertainty is the extent to which similarity of the 

two studies’ findings indicate a potential for both 

metrics to reflect geographic variation in impacts 

stemming from historic deposition. Although 

geographic deposition patterns have changed little 

across the time period of the studies, annual S and 

N deposition rates have changed appreciably (e.g., 

PA, Appendix 5B, Figures 5B–9 through 5B–12), 

which may also contribute uncertainty to 

interpretation of specific deposition rates associated 

with patterns of tree growth and survival. Few 

studies on recovery in historically impacted areas 

that might address such uncertainties are available 

(e.g., ISA, section IS.11). 

recovery timelines, it cannot address the 

potential for changes in influential 

factors that may occur over time with 

different or changed deposition 

patterns. Thus, while observational 

studies contribute to the evidence base 

on the potential for N/S deposition to 

contribute to ecosystem effects (and 

thus are important evidence in the ISA 

determinations regarding causality), 

their uncertainties (and underlying 

assumptions) differ from those of 

modeling analyses, and they may be 

somewhat less informative with regard 

to identification of specific N and S 

deposition levels that may elicit 

ecosystem impacts of interest. Both 

types of studies, as well as N addition 

experiments, which are not generally 

confounded by exposure changes 

beyond those assessed (yet may have 

other limitations), have been 

considered, with key findings 

summarized below. 

(a) Deposition and Risks to Trees 

The 2009 REA performed a steadystate modeling analysis to estimate the 

annual amounts of S and N acidifying 

deposition at or below which one of 

three BC:Al target values would be met 

in a 24-state area in which the acidsensitive species, red spruce and sugar 

maple, occur. A range of acid deposition 

was estimated for each of the three 

target values. Recent estimates of total S 

and N deposition in regions of the U.S. 

appear to meet all but the most 

restrictive of these targets, for which the 

uncertainty is greatest (e.g., ISA, 

Appendix 2, sections 2.6 and 2.7).47

Experimental addition studies of S, or 

S plus N have been performed in eastern 

locations, focusing on a small set of tree 

species, and generally involving S and 

N additions greater than 20 kg/ha-yr, in 

combination with appreciable 

background deposition at the time, and 

have generally not reported growth 

reductions (PA, Appendix 5B, Table 

5B–1; ISA, Appendix 5, section 5.5.1). 

Uncertainties associated with these 

analyses include the extent to which the 

studies reflect steady-state conditions. 

Given the variability in the durations 

across these studies and the relatively 

short durations for some (e.g., less than 

five years), it might be expected that 

steady-state conditions have not been 

reached, such that the S/N loading is 

within the buffering capacity of the 

soils. With regard to N addition alone, 

the available studies have reported 

mixed results for growth and survival 

(PA, Table 5B–1; Magill et al., 2004; 

McNulty et al., 2005; Pregitzer et al., 

2008; Wallace et al., 2007). It is not clear 

the extent to which such findings may 

be influenced by species-specific 

sensitivities or soils and trees already 

impacted by historic deposition, or 

other environmental factors. 

With regard to S deposition, two large 

observational studies that analyzed 

growth and/or survival measurements in 

tree species at sites in the eastern U.S. 

or across the country reported negative 

associations of tree survival for 9 of the 

10 species’ functional type groupings 

with the S deposition metric and of tree 

survival and growth for nearly half of 

the species individually (Dietze and 

Moorcroft, 2011; Horn et al., 2018).48

Interestingly, survival for the same 9 

species groups was also negatively 

associated with long-term average O3

(Dietze and Moorcroft, 2011). The S 

deposition metrics for the two studies 

were mean annual average deposition 

estimates for total S or sulfate (wet 

deposition) during different, but 

overlapping, time periods of roughly 10- 

year durations. The full range of average 

SO4

2¥ deposition estimated for the 

1994–2005 period assessed by Dietze 

and Moorcroft (2011) for the eastern 

U.S. study area was 4 to 30 kg S/ha-yr. 

The second study covered the more 

recent time period (2000–2013) and 71 

species distributed across the U.S. To 

draw on this study with regard to S 

deposition levels of interest, the 

distribution of S deposition estimates 

for each species were considered in the 

PA; the range of median S deposition for 

sites of those species for which negative 

associations with growth or survival 

were reported was 5 to 12 kg S/ha-yr, 

with few exceptions (Appendix 5B, 

section 5B.2 and Attachments 2A and 

2B; Horn et al., 2018).49

Regarding N deposition, the three 

large observational studies that analyzed 

growth and/or survival measurements in 

tree species samples at sites in the 

northeastern or eastern U.S., or across 

the country, reported associations of 

tree survival and growth with several N 

deposition metrics (Dietze and 

Moorcroft, 2011; Thomas et al., 2010; 

Horn et al., 2018). Estimates of average 

N deposition across the full set of sites 

analyzed by Thomas et al. (2010) in 19 

states in the northeastern quadrant of 

the U.S. ranged from 3 to 11 kg N/hayr for the period 2000–2004. The N 

deposition metrics for these three 

studies were mean annual average 

deposition estimates for total N or 

nitrate (wet deposition) during different, 

but overlapping, time periods that 

varied from 5 to more than 10 years. The 

full range of average NO3

¥ deposition 

estimated for the 1994–2005 period 

assessed by Dietze and Moorcroft (2011) 

for the eastern U.S. study area was 6 to 

16 kg N/ha-yr. Median N deposition 

estimated (measurement interval 

average [falling within the years 2000– 

2013]) at sites of nonwestern species for 

which associations with growth or 

survival were negative (either over full 

range or at median for species) ranged 

from 7 to 12 kg N/ha-yr (Horn et al., 

2018). 

In considering what can be drawn 

from these studies with regard to 

deposition levels of potential interest for 

tree species effects, such as the ranges 

identified above, a number of 

uncertainties are recognized. For 

example, several factors were not 

accounted for that have potential to 

influence tree growth and survival. 

Although O3 was analyzed in one of the 

three studies, soil characteristics and 

other factors with potential to impact 

tree growth and survival (other than 

climate) were not assessed, contributing 

uncertainty to their interpretations. 

Also, the influence of historical 

deposition patterns and associated 

impacts is unknown.50 Further, 

differences in findings for the various 

species (or species’ groups) may relate 

to differences in geographic distribution 

of sampling locations, which may 


khammond on DSK9W7S144PROD with RULES2
105718 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

51For example, concentrations of HNO3 reported 

in forested areas of California in the 1980s ranged 

up to 33 ug/m3, and annual average NO2

concentrations in the Los Angeles area ranged from 

0.078 ppm in 1979 to 0.053 ppm in the early 1990s 

(PA, section 5.4.2). Ambient air concentrations of 

HNO3 in the Los Angeles metropolitan area have 

declined markedly, as shown in Figure 2–23 of the 

PA, which compares concentrations at CASTNET 

monitoring sites between 2019 and 1996 (PA, 

section 2.4.1). 

contribute to differences in ranges of 

deposition history, geochemistry etc. 

(b) Deposition Studies of Herbs, Shrubs 

and Lichens 

Studies evaluating the effects of N 

addition on herbs, shrubs and lichens 

include observational studies of 

herbaceous species richness at sites in a 

multi-state study area and of grassland 

or coastal sage scrub communities in 

southern California, and experimental 

addition studies in several western herb 

or shrub ecosystems. The experimental 

addition studies indicate effects on 

community composition associated with 

annual N additions of 10 kg N/ha-yr (in 

the context of background deposition on 

the order of 6 kg N/ha-yr [PA, Appendix 

5B, Table 5B–7]) and higher (PA, 

sections 5.3.3.1 and 5.3.4.2; ISA, 

Appendix 6, section 6.3.6). Experiments 

involving additions of 5 kg N/ha-yr 

variously reported no response or 

increased cover for one species (in 

context of background deposition 

estimated at 5 kg N/ha-yr). The 

landscape-level analysis of coastal sage 

scrub community history in southern 

California observed a greater likelihood 

of recovery of sites with relatively low 

levels of exotic invasive grasses when 

the N deposition metric level was below 

11 kg N/ha-yr. Lastly, the multi-state 

analysis of herbaceous species richness 

reported a negative association with N 

deposition metric values above 8.7 kg 

N/ha-yr at open-canopy sites and above 

6.5 kg/ha-yr and low pH sites. In 

forested sites, negative associations 

were found above 11.6 kg N/ha-yr in 

sites with acidic soil pH at or above 4.5 

(PA, section 5.3.3). 

Limitations and associated 

uncertainties vary between the two 

types of studies (experimental addition 

and observational), but both are limited 

with regard to consideration of the 

impacts of long-term deposition. Such 

studies are necessarily limited in scope 

with regard to species and ecosystem, 

and while there are some experimental 

addition studies lasting more than 20 

years, many are for fewer than 10 years. 

In the case of observational studies, 

these studies generally have not 

accounted for the influence of historical 

pollution (including decades of S and N 

deposition and elevated concentrations 

of O3 and N oxides) on the associations 

observed with more recent deposition 

metrics. Further, there is uncertainty 

associated with the extent to which the 

exposure metric utilized reflects the 

particular conditions that may be 

eliciting the ecosystem response 

quantified by the ecosystem metric. 

The few studies of lichen species 

diversity and deposition-related metrics, 

while contributing to the evidence that 

relates deposition to relative abundance 

of different lichen species, are more 

limited in the extent to which they 

inform an understanding of specific 

exposure conditions in terms of 

deposition rates that may elicit specific 

responses. Related factors include 

uncertainties related to the methods 

employed to represent N deposition, the 

potential role of other unaccounted-for 

environmental factors (including O3, 

SO2, S deposition and historical air 

quality and associated deposition), and 

uncertainty concerning the 

independence of any effect of 

deposition levels from residual effects of 

past patterns of deposition (PA, section 

5.3.3.2). Information on exposure 

conditions associated with effects of 

oxides of N such as HNO3 on lichen 

species is also addressed in section 

II.A.3.c.(3) below. 

(3) Other Effects of N Oxides, SOX and 

PM in Ambient Air 

The evidence related to exposure 

conditions for other effects of SOX, N 

oxides and PM in ambient air includes 

concentrations of SO2 and NO2

associated with effects on plants, 

concentrations of NO2 and HNO3

associated with effects on plants and 

lichens, and concentrations of PM mass 

or PM loading (much higher than those 

associated with the existing standard) 

that affect plant photosynthesis. With 

regard to oxides of N and S, we note that 

some effects described as direct may be 

related to dry deposition of SO2 and 

HNO3 onto plant and lichen surfaces, 

exposure pathways that would be 

captured in observational studies and 

could also be captured in some 

fumigation experiments. 

With regard to SO2, the evidence 

primarily includes field studies for the 

higher concentrations associated with 

visible foliar injury and laboratory 

studies for other effects, e.g., depressed 

photosynthesis and reduced growth or 

yield (ISA, Appendix 3, section 3.2; 

1982 AQCD, section 8.3). The recently 

available information also includes 

observational studies reporting 

increased tree growth in association 

with reductions in SO2 emissions, 

although these studies do not generally 

report the SO2 concentrations in 

ambient air or account for the influence 

of changes in concentrations of cooccurring pollutants such as O3 (ISA, 

Appendix 3, section 3.2). With regard to 

foliar injury, the current ISA states there 

to be limited research since the 1982 

AQCD and ‘‘no clear evidence of acute 

foliar injury below the level of the 

current standard’’ (ISA, p. IS–37). Few 

studies report yield effects from acute 

exposures, with the available ones 

reporting relatively high concentrations, 

such as multiple hours with 

concentrations above 1 ppm or 1000 

ppb (1982 AQCD, section 8.3). Effects 

have also been reported on 

photosynthesis and other functions in a 

few lichen species groups, although 

recovery of these functions was 

observed from short, multi-hour 

exposures to concentrations below 

about 1 ppm (ISA, Appendix 3, section 

3.2). 

With regard to oxides of N, the 

evidence indicates that effects on plants 

and lichens occur at much lower 

exposures to HNO3 (than to NO2). The 

laboratory and field studies of oxides of 

N vary regarding their limitations; field 

studies are limited regarding 

identification of threshold exposures for 

the reported effects, and uncertainties 

associated with controlled experiments 

include whether the conditions under 

which the observed effects occur would 

be expected in the field. Plant studies 

reported in the ISA did not report 

effects on photosynthesis and growth 

resulting from exposures of NO2

concentrations below 0.1 ppm (ISA, 

Appendix 3, section 3.3). 

With regard to the HNO3, the elevated 

concentrations of NO2 and HNO3 in the 

Los Angeles area in the 1970s–90s are 

well documented as is the decline of 

lichen species in the Los Angeles Basin 

during that time, although such an 

analysis is not available elsewhere in 

the U.S. (PA, section 5.4.2; ISA, 

Appendix 3).51 Other evidence specific 

to HNO3, which can deposit on and 

bind to leaf or needle surfaces, includes 

controlled exposure studies describing 

foliar effects on several tree species. 

Studies of ponderosa pine, white fir, 

California black oak and canyon live oak 

involving continuous chamber exposure 

over a month to 24-hour average HNO3

concentrations generally ranging from 

10 to 18 mg/m3 (moderate treatment) or 

18 to 42 mg/m3 (high treatment), with 

the average of the highest 10% of 

concentrations generally ranging from 

18 to 42 mg/m3 (30–60 mg/m3 peak) or 

89 to 155 mg/m3 (95–160 mg/m3 peak), 

resulted in damage to foliar surfaces of 

the 1 to 2-year old plants (ISA, 

Appendix 3, section 3.4; Padgett et al., 

2009). Available evidence for lichens 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105719 

52The national-scale analysis focused on the 

contiguous U.S. as there are insufficient data 

available for Hawaii, Alaska, and the territories. Of 

the four hierarchical levels of ecoregion 

categorization, the REA utilized level III which 

divides the contiguous U.S. into 84 ecoregions 

(Omernik and Griffith, 2014). The 69 of these 84 

ecoregions in which there was at least one site with 

sufficient data comprised the national scale. 

53The NCLD is comprised of CLs calculated from 

several common models: (1) steady-state massbalance models such as the Steady-State Water 

Chemistry (SSWC), (2) dynamic models such as 

Model of Acidification of Groundwater In 

Catchments (MAGIC) (Cosby et al., 1985) or 

Photosynthesis EvapoTranspiration Biogeochemical 

model (PnET–BGC) (Zhou et. al., 2015) run out to 

year 2100 or 3000 to model steady-state conditions 

and (3) regional regression models that use results 

from dynamic models to extrapolate to other 

waterbodies (McDonnell et. al., 2012; Sullivan et 

al., 2012a). Data and CL estimates in the NCLD are 

generally focused on waterbodies impacted by 

deposition-driven acidification and are described in 

documentation for the database version (PA, section 

5.1.2.3; Lynch et al., 2022). 

also includes a recent laboratory study 

of daily HNO3 exposures for 18 to 78 

days, with daily peaks near 50 ppb (∼75 

mg/m3) that reported decreased 

photosynthesis, among other effects 

(ISA, Appendix 6, section 6.2.3.3; 

Riddell et al., 2012). Based on studies 

extending back to the 1980s, HNO3 has 

been suspected to have had an 

important role in the dramatic declines 

of lichen communities that occurred in 

the Los Angeles basin (ISA, Appendix 3, 

section 3.4; Nash and Sigal, 1999; 

Riddell et al., 2008; Riddell et al., 2012). 

In more recent studies, variation in 

eutrophic lichen abundance has been 

associated with variation in N 

deposition metrics (ISA, Appendix 6, 

section 6.2.3.3), although the extent to 

which these associations are influenced 

by residual impacts of historic air 

quality is unclear and the extent to 

which similar atmospheric conditions 

and ecological relationships exist in 

other locations in the U.S. is uncertain. 

Little information is available on 

welfare effects of airborne PM at 

concentrations commonly occurring in 

the U.S. today, and the available 

information does not indicate effects to 

occur under such conditions. The 

concentrations at which PM has been 

reported to affect vegetation (e.g., 

through effects on leaf surfaces, which 

may affect function, or through effects 

on gas exchange processes) are generally 

higher than those associated with 

conditions meeting the current 

standards and may be focused on 

specific particulate chemicals rather 

than on the mixture of chemicals in PM 

occurring in ambient air (ISA, Appendix 

15, sections 15.4.3 and 15.4.6). Studies 

involving ambient air PM have generally 

involved conditions that are much 

higher than those common to the U.S. 

today (ISA, Appendix 15, sections 

15.4.3 and 15.4.4). 

4. Overview of Exposure and Risk 

Assessment for Aquatic Acidification 

Our consideration of the scientific 

evidence available in the current review 

is informed by results from quantitative 

analyses of estimated acidic deposition 

and associated risk of aquatic 

acidification (PA, section 5.1 and 

Appendix 5A). These REA analyses, like 

those in the last review, make use of 

well-established modeling tools and 

assessment approaches for this 

endpoint. Other categories of effects of 

S and N deposition have been the 

subject of quantitative analyses, both in 

the last review (e.g., terrestrial 

acidification) and in other contexts (e.g., 

eutrophication of large rivers and 

estuaries), each with associated 

complexities and specificity. The PA, 

while focusing the new analyses on 

aquatic acidification risks, as 

summarized here, also draws on 

findings of available analyses for the 

other categories of effects. 

The REA analyses, summarized here 

and presented in detail in Appendix 5A 

of the PA, have focused on ANC as an 

indicator of aquatic acidification risk 

(PA, section 5.1 and Appendix 5A). This 

focus is consistent with such analyses 

performed in the 2012 review and with 

the longstanding evidence that 

continues to demonstrate a causal 

relationship between S and N 

deposition and alteration of freshwater 

biogeochemistry and between acidifying 

S and N deposition and changes in 

biota, including physiological 

impairment and alteration of species 

richness, community composition, and 

biodiversity in freshwater ecosystems 

(ISA, Table ES–1), as summarized in 

section II.A.3 above. 

Section II.A.4.a. summarizes key 

aspects of the assessment design, 

including the conceptual approach and 

tools, indicator reference or benchmark 

concentrations, the assessment scales, 

study areas and waterbodies analyzed, 

and exposure and risk metrics derived. 

Key limitations and uncertainties 

associated with the assessment are 

identified in section II.A.4.b. and the 

exposure and risk estimates are 

summarized in section II.A.4.c. An 

overarching focus of these analyses is 

characterization of aquatic acidification 

risk in sensitive ecoregions associated 

with different deposition conditions. 

a. Key Design Aspects 

The REA for this review entailed a 

multi-scale analysis of waterbodies in 

the contiguous U.S. that assessed 

waterbody-specific aquatic acidification 

at three spatial scales: national, 

ecoregion, and case study area (PA, 

Appendix 5A). The assessment involved 

evaluation of deposition and water 

quality response (ANC) at the waterbody 

site level. The results are then 

summarized at the national, ecoregion, 

and case study level. The national-scale 

analysis included all waterbody sites 

across the U.S. for which relevant data 

were available.52 The ecoregion-scale 

analysis focused on waterbodies with 

relevant data in a set of 25 ecoregions 

generally characterized as acidsensitive; and the more localized case 

study-scale analysis focused on such 

waterbodies in five case study areas 

across the U.S., within each of which 

were Class I areas. 

The impact of acidifying S or N 

deposition estimated for five different 

time periods (2001–03, 2006–08, 2010– 

12, 2014–16 and 2018–20) was 

evaluated using a CL approach that 

relied on comparison of waterbody 

location-specific deposition estimates to 

waterbody location-specific CL 

estimates derived for other applications 

and available in the National Critical 

Loads Database (NCLD) 53 (PA, 

Appendix 5A). The CL estimates used in 

the assessment were largely based on 

steady-state modeling, and the modeling 

applications focused on ANC, 

producing CL estimates (acidifying 

deposition in terms of kg/ha-yr or meq/ 

m2-yr [milliequivalents per square meter 

per year] for S and N compounds) for 

different target or threshold ANC 

concentrations (also termed 

benchmarks). Of the 84 ecoregions in 

the contiguous U.S., 64 have at least one 

waterbody site with a CL estimate (PA, 

Appendix 5A). Given its common use in 

categorizing waterbody sensitivity, ANC 

was used as the indicator of 

acidification risk in this assessment (PA, 

section 5.1.2.2). Deposition estimates, as 

3-year averages of annual TDep 

estimates for each site, were compared 

to the CL estimates for three different 

ANC benchmark concentrations (targets 

or thresholds), in recognition of the 

watershed variability and associated 

uncertainties, as an approach for 

characterizing aquatic acidification risk 

(PA, section 5.1). 

The available evidence and scientific 

judgments were considered in 

identifying the three ANC benchmark 

concentrations: 20 meq/L, 30 meq/L, 50 

meq/L (PA, section 5.1.2.2). Selection of 

these benchmark ANC concentrations 

reflects several considerations. For 

example, most aquatic CL studies 

conducted in the U.S. since 2010 use an 

ANC of 20 and/or 50 meq/L, because 20 

meq/L has been suggested to provide 


khammond on DSK9W7S144PROD with RULES2
105720 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

54For example, dynamic modeling simulations in 

acid-sensitive streams of the southern Blue Ridge 

Mountains have predicted all streams to have preindustrial time ANC levels above 20 meq/L, while 

also predicting more than a third of the streams to 

have pre-industrial ANC levels below 50 meq/L 

(Sullivan et al., 2011). 55As noted in section II.A.3.a. above, events such 

as spring snowmelt and heavy rain events can 

contribute to episodic acidification events. For 

example, in some impacted northeastern 

waterbodies, particularly headwater streams, ANC 

levels may dip below zero for hours to days or 

weeks in response to such events, while 

waterbodies labeled chronically acidic have ANC 

levels below zero throughout the year (ISA, 

Appendix 6, section 6.1.1.1; Driscoll et al., 2001). 

56 In considering higher ANC levels (e.g., up to 80 

meq/L and higher), it was also recognized that many 

waterbodies, particularly in acid-sensitive regions 

of the contiguous U.S., never had an ANC that high 

and would never reach an ANC that high naturally 

(Williams and Labou 2017; Shaw et al., 2014; PA, 

section 5.1.2.2). Additionally, in conveying its 

advice in the 2012 review, the CASAC expressed its 

view that ‘‘[l]evels of 50 meq/L and higher would 

provide additional protection, but the Panel has less 

confidence in the significance of the incremental 

benefits as the level increases above 50 meq/L’’ 

(Russell and Samet, 2010a; pp. 15–16). 

57This approach is also used in multiple studies 

and the NCLD (PA, section 5.1.2.2). 

58A waterbody is represented as a single CL 

value. In many cases, a waterbody has more than 

one CL value calculated for it because different 

studies determined a value for the same waterbody. 

When more than one CL exists, the CL from the 

most recent study was selected, while the CL values 

were averaged when the publications are from the 

same timeframe (PA, Appendix 5A, section 5A.1.5). 

59Critical load estimates are estimates of the S 

deposition rate at which a particular waterbody site 

is estimated to be able to achieve a specified ANC 

level. A CL estimate at or below zero would 

indicate that no S deposition estimate would 

provide for such a result. 

60The ecoregion classification scheme used to 

group waterbody sites into ecoregions is based on 

that described in Omernik (1987), which classifies 

regions through the analysis of the patterns and the 

composition of biotic and abiotic characteristics 

that affect or reflect differences in ecosystem quality 

and integrity (e.g., geology, physiography, 

vegetation, climate, soils, land use, wildlife, and 

hydrology). 

61 In light of the size of the level III ecoregions, 

50 was identified as an appropriate minimum 

number of CL sites within an ecoregion to include 

it in the analysis. 

protection for a ‘‘natural’’ or 

‘‘historical’’ 54 range of ANC, and 50 

meq/L to provide greater protection, 

particularly from episodic acidification 

events 55 (Dupont et al., 2005; Fakhraei 

et al., 2014; Lawrence et al., 2015; 

Lynch et al., 2022; McDonnell et al., 

2012, 2014; Sullivan et al., 2012a, 

2012b). For example, levels below 20 

meq/L have been associated with fish 

species reductions in some sensitive 

waterbodies of the Shenandoah and 

Adirondack Mountains. Levels of ANC 

ranging from 30 to 40 meq/L have been 

reported to provide sufficient buffering 

to withstand acidic inputs associated 

with episodic springtime rain or 

snowmelt events. An ANC value of 50 

meq/L has often been cited in the 

literature as a target for many areas, and 

in the 2012 review, ANC values at or 

above 50 meq/L were described as 

providing an additional level of 

protection although with increasingly 

greater uncertainty for values at/above 

75 meq/L 56 (2011 PA, pp. 7–47 to 7–48). 

In the western U.S., lakes and streams 

vulnerable to deposition-driven aquatic 

acidification are often found in the 

mountains where surface water ANC 

levels are naturally low and typically 

vary between 0 and 30 meq/L (Williams 

and Labou, 2017; Shaw et al., 2014). For 

these reasons, this assessment also 

develops results for an ANC threshold 

of 50 meq/L for sites in the East and 20 

meq/L for sites in the West (denoted as 

‘‘50/20’’ meq/L).57 Thus, the set of 

benchmark concentrations used in this 

REA includes ANC concentrations that 

are naturally occurring in many areas 

and also includes concentrations that, 

depending on watershed characteristics, 

may provide additional buffering in 

times of episodic acidification events. 

Since acidification of waterbodies is 

controlled by local factors such as 

geology, hydrology, and other landscape 

factors, aquatic CLs for acidification 

were determined at the waterbody level 

(based on site-specific data) and then 

summarized at the national, ecoregion, 

and case study level. National-scale 

analyses were performed using two 

approaches: one considering acid 

deposition of N and S compounds 

combined and one for S deposition 

only. Findings from these analyses 

indicated that across the five different 

time periods analyzed, the percent of 

waterbodies exceeding their CLs was 

similar for the two approaches (PA, 

Appendix 5A, sections 5A.1.6.2 and 

5A.2.1). Thus, to facilitate interpretation 

of the results, further analysis of the 

results focused on the findings for S 

only deposition. 

Critical load estimates for specific 

waterbody sites across the contiguous 

U.S. were drawn from the NCLD 

(version 3.2.1) 58 for comparison to total 

deposition estimates in the same 

locations for the five time periods. 

Comparisons were only performed for 

sites at which CL estimates were greater 

than zero, indicating that achievement 

of the associated ANC benchmark 

concentration would be feasible.59 The 

results of these analyses are summarized 

with regard to the spatial extent and 

severity of deposition-related 

acidification effects and the protection 

from these effects associated with a 

range of annual S deposition. 

The ecoregion-scale analyses focused 

on 25 ecoregions,60 18 in the East and 

7 in the West. Ecoregions are areas of 

similarity regarding patterns in 

vegetation, aquatic, and terrestrial 

ecosystem components. The 25 

ecoregions in this analysis each had 

more than 50 waterbody sites (or 

locations) for which a prior modeling 

application had developed a CL 

estimate, which was available in the 

NCLD (PA, section 5A.2.2.2). Although 

a total of 32 ecoregions had more than 

50 CL sites,61 four in the West were 

excluded as having very low deposition 

that resulted in no CL exceedances 

across the complete 20-year analysis 

period. An additional three ecoregions 

(i.e., Southeastern Plains, Southern 

Coastal Plain, and Atlantic Coastal Pine 

Barrens) were excluded as they are 

known to have naturally acidic surface 

waters, and the low CL estimates for 

these ecoregions (and resulting CL 

exceedances) are likely driven by 

natural acidity linked to high levels of 

dissolved organic carbon, hydrology, 

and natural biogeochemical processes 

rather than atmospheric deposition 

(2008 ISA, section 3.2.4.2; Baker et al., 

1991; Herlihy et al., 1991). 

The case study scale represents the 

smallest scale at which CLs and their 

comparison to deposition estimates 

were summarized and is intended to 

give some insight into potential local 

impacts of aquatic acidification. Five 

case study areas across the U.S. were 

examined: Shenandoah Valley Area, 

White Mountain National Forest, 

Northern Minnesota, Sierra Nevada 

Mountains, and Rocky Mountain 

National Park (details presented in PA, 

section 5.1.3.3 and Appendix 5A, 

section 5A.2.1). These areas include a 

number of national parks and forests 

that vary in their sensitivity to 

acidification but represent high value or 

protected ecosystems, such as Class 1 

areas, wilderness, and national forests 

(PA, Appendix 5A, section 5A.2.1). The 

most well studied of these, the 

Shenandoah Valley Area case study, 

includes the Class I area, Shenandoah 

National Park, and waterbodies in each 

of three ecoregions. The number of 

waterbody sites with CLs available in 

the NCLD for the Shenandoah study 

area (4,977 sites) is nearly an order of 

magnitude greater than the total for the 

four other areas combined (524 sites). 

The analyses at different scales 

differed in how results were 

summarized and evaluated. For 

example, at the national scale, 

percentages of water bodies with 

deposition estimates exceeding their 

CLs (for the different ANC benchmarks) 

were reported for each of the five time 

periods for which deposition was 

assessed (PA, Table 5–1). From the case 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105721 

62The percentages of waterbodies in an ecoregion 

with estimated ANC at/above a target ANC is paired 

with the median deposition for that ecoregion. The 

percentages are then binned by the median 

deposition values. 

study scale analyses, we focused 

primarily on the distribution of CL 

estimates in each study area. In so 

doing, the CLs for each case study area 

were characterized in terms of the 

average and two lower percentiles (e.g., 

the 30th percentile CL, which is the 

value below 70% of the CL estimates for 

that study area, and the 10th percentile). 

In the ecoregion-scale analyses, 

percentages of waterbody sites per 

ecoregion that exceeded their estimated 

CLs and percentages of waterbody sites 

that fell at or below them—for each of 

the three ANC benchmarks—were 

summarized by ecoregion for each of the 

five time periods: 2001–2003, 2006– 

2008, 2010–2012, 2014–2016 and 2018– 

2020 (PA, section 5.1.3.2 and Appendix 

5A, section 5A.2.2). Percentages of 

waterbody sites that did not exceed 

their estimated CLs were described as 

achieving the associated ANC 

benchmark (or target). These results of 

the site-specific ANC modeling were 

then considered in two ways. The first 

is based on a binning of this dataset of 

percentages of waterbodies per 

ecoregion-time period combinations that 

were estimated to achieve each of the 

ANC targets by the median deposition 

for that ecoregion during that time 

period (e.g., percentage achieving ANC 

target of 20 meq/L when ecoregion 

median deposition was at/below 5 kg/ 

ha-yr).62 The second approach involved 

summarizing ecoregion-specific trends 

in percentage of waterbodies per 

ecoregion estimated to achieve the three 

threshold or target ANC values (or 

estimated to exceed the associated CLs). 

b. Key Limitations and Uncertainties 

The nature and magnitude of 

associated uncertainties and their 

impact on the REA estimates are 

characterized with a mainly qualitative 

approach, informed by several 

quantitative sensitivity analyses (PA, 

Appendix 5A, section 5A.3). The mainly 

qualitative approach used to 

characterize uncertainty here and in 

quantitative analyses in other NAAQS 

reviews is described by World Health 

Organization (WHO, 2008). Briefly, with 

this approach, we have identified key 

aspects of the assessment approach that 

may contribute to uncertainty in the 

conclusions and provided the rationale 

for their inclusion. Then, we 

characterized the magnitude and 

direction of the influence on the 

assessment for each of these identified 

sources of uncertainty. Consistent with 

the WHO (2008) guidance, we scaled the 

overall impact of the uncertainty by 

considering the degree of uncertainty as 

implied by the relationship between the 

source of uncertainty and the exposure 

and risk estimates. A qualitative 

characterization of low, moderate, and 

high was assigned to the magnitude of 

influence and knowledge base 

uncertainty descriptors, using 

quantitative observations relating to 

understanding the uncertainty, where 

possible. The direction of influence, 

whether the source of uncertainty was 

judged to potentially over-estimate 

(‘‘over’’), under-estimate (‘‘under’’), or 

have an unknown impact to exposure/ 

risk estimates was also characterized. 

Two types of quantitative analyses of 

the variability and uncertainty 

associated with the CL estimates used in 

the REA support the overall uncertainty 

characterization. The first type of 

analysis is a sensitivity analysis using 

Monte Carlo techniques to quantify CL 

estimate uncertainty associated with 

several model inputs, and the second is 

an analysis of the variation in CL 

estimates among the three primary 

modeling approaches on which the CLs 

used in this assessment were based. 

As overarching observations regarding 

uncertainty associated with this REA, 

we note two overarching aspects of the 

assessment. The first relates to 

interpretation of specific thresholds of 

ANC, and the second to our 

understanding of the biogeochemical 

linkages between deposition of S and N 

compounds and waterbody ANC, and 

the associated estimation of CLs. While 

ANC is an established indicator of 

aquatic acidification risk, there is 

uncertainty in our understanding of 

relationships between ANC and risk to 

native biota, particularly in waterbodies 

in geologic regions prone to waterbody 

acidity. Such uncertainties relate to the 

varying influences of site-specific 

factors other than ANC, such as soil 

type. Uncertainty associated with our 

understanding of the biogeochemical 

linkages between deposition and ANC 

and the determination of steady-state 

CLs is difficult to characterize and 

assess. Uncertainty in CL estimates is 

associated with parameters used in the 

steady-state CL models. While the 

Steady-State Water Chemistry (SSWC) 

and other CL models are well conceived 

and based on a substantial amount of 

research and applications available in 

the peer-reviewed literature, there is 

uncertainty associated with the 

availability of the necessary data to 

support certain model components. 

The strength of the CL estimates and 

the exceedance calculation rely on the 

ability of models to estimate the 

catchment-average base-cation supply 

(i.e., input of base cations from 

weathering of bedrock and soils and 

air), runoff, and surface water 

chemistry. The uncertainty associated 

with runoff and surface water 

parameters relates to availability of 

measurements; however, the ability to 

accurately estimate the catchment 

supply of base cations to a water body 

is still difficult and uncertain (PA, 

Appendix 5A, section 5A.3). This area 

of uncertainty is important because the 

catchment supply of base cations from 

the weathering of bedrock and soils is 

the factor with the greatest influence on 

the CL calculation and has the largest 

uncertainty (Li and McNulty, 2007). For 

example, the well-established models 

generally rely on input or simulated 

values for BCw rate, a parameter the ISA 

notes to be ‘‘one of the most influential 

yet difficult to estimate parameters in 

the calculation of critical acid loads of 

N and S deposition for protection 

against terrestrial acidification’’ (ISA, 

section IS.14.2.2.1). Obtaining accurate 

estimates of weathering rates is difficult 

because weathering is a process that 

occurs over very long periods of time, 

and the estimates on an ecosystem’s 

ability to buffer acid deposition rely on 

accurate estimates of weathering. 

Although the approach to estimate basecation supply for the national case study 

(e.g., F-factor approach) has been widely 

published and analyzed in Canada and 

Europe and has been applied in the U.S. 

(e.g., Dupont et al., 2005 and others), the 

uncertainty in this estimate is unclear 

and could be large in some cases. 

In light of the significant contribution 

of this input to the CL estimates, a 

quantitative uncertainty analysis of CL 

estimates based on state-steady CL 

modeling was performed (PA, Appendix 

5A, section 5A.3.1). This analysis, 

involving many model simulations for 

the more than 14,000 waterbodies, 

drawing on Monte Carlo sampling, 

provided a description of the 

uncertainty around the CL estimate in 

terms of the confidence interval for each 

waterbody mean result. The size of the 

confidence interval for S CL estimates 

ranged from 0.1 kg S/ha-yr at the 5th 

percentile to 5.3 kg S/ha-yr at the 95th 

percentile. Smaller confidence intervals 

were associated with CLs determined 

with long-term water quality data and 

low variability in runoff measurements. 

Estimates of CL determined by one or 

very few water quality measurements, 

and in areas where runoff is quite 

variable (e.g., the western U.S.), had 

larger confidence intervals, indicating 

greater uncertainty. Critical load 

estimates with the lowest uncertainty 


khammond on DSK9W7S144PROD with RULES2
105722 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

63More specifically, the percentage of 

waterbodies across the contiguous U.S. estimated to 

exceed a CL for combined total S and N are very 

similar or just slightly higher (e.g., by 1–2%) than 

S only percentages of the waterbodies estimated to 

not meet the ANC benchmarks. This indicates that 

most of the N deposition entering the watershed is 

retained within the watershed and/or converted to 

gaseous N (PA, Appendix 5A, section 5A.2.1). 

were for waterbody sites in the eastern 

U.S., particularly along the Appalachian 

Mountains, in the Upper Midwest, and 

in the Rocky Mountains, which are 

areas for which there are relatively 

larger site-specific datasets (e.g., for 

water quality parameters). Greater 

uncertainty is associated with CLs in the 

Midwest and South and along the 

California to Washington coast. This 

uncertainty in the Midwest is associated 

with most of the CLs in waterbodies in 

this area being based on one or a few 

water quality measurements, while the 

high uncertainty for sites along the 

California and Washington coasts relates 

to variability in runoff values. On 

average, the size of the confidence 

interval for the vast majority of CLs 

(those based on the widely used steadystate water chemistry model) was 7.68 

meq S/m2-yr or 1.3 kg S/ha-yr, giving a 

confidence interval of ±3.84 meq/m2-yr 

or ±0.65 kg S/ha-yr. While a 

comprehensive analysis of uncertainty 

had not been completed for these 

estimates prior to this assessment, 

judgment by EPA experts suggested the 

uncertainty for combined N and S CLs 

to be on average about ±0.5 kg/ha-yr 

(3.125 meq/m2-yr), which is generally 

consistent with the range of uncertainty 

determined from this quantitative 

uncertainty analysis (PA, Appendix 5A, 

section 5A.3). 

At the ecoregion scale, 51 ecoregions 

had sufficient data to calculate the 5th 

to 95th percentile (PA, Appendix 5A, 

Table 5A–56). Smaller confidence 

intervals around the mean CL (i.e., 

lower uncertainty CLs) were associated 

with ecoregions in the Appalachian 

Mountains (e.g., Northern Appalachian 

and Atlantic Maritime Highlands, Blue 

Ridge, Northern Lakes and Forests, and 

North Central Appalachians) and 

Rockies (e.g., Sierra Nevada, Southern 

Rockies, and Idaho Batholith). 

Ecoregions with more uncertain CLs 

included the Northeastern Coastal Zone, 

Cascades, Coast Range, Interior Plateau, 

and Klamath Mountains/California High 

North Coast Range. 

Although the vast majority of CLs in 

this assessment were based on the 

SSWC model, an analysis was 

conducted to understand differences in 

the CLs calculated with the different 

methods. There are three main CL 

approaches, all based on the watershed 

mass-balance approach where acid-base 

inputs are balanced. The three 

approaches include: (1) SSWC model 

and F-Factor that is based on 

quantitative relationships to water 

chemistry (Dupont et al., 2005; Scheffe 

et al., 2014; Lynch et al., 2022), (2) 

Statistical Regression Model that 

extrapolated weathering rates across the 

landscape using water quality or 

landscape factors (Sullivan et al., 2012b; 

McDonnell et al., 2014), and (3) 

Dynamic Models (Model of 

Acidification of Groundwater In 

Catchments [MAGIC)] or Photosynthesis 

EvapoTranspiration Biogeochemcial 

model [Pnet-BGC]). Critical load values 

were compared between these models to 

determine model biases. Results from 

the comparison between different CL 

methods that were used to calculate the 

critical loads in the NCLD are 

summarized in PA Appendix 5A, 

section 5A.3.1, for lakes in New England 

and the Adirondacks and streams in the 

Appalachian Mountains. Overall, good 

agreement was found between the three 

methods used to calculate CLs, 

indicating there was not a systematic 

bias between the methods and that they 

should produce comparable results 

when used together as they were in 

these analyses (PA, Appendix 5A, 

section 5A.3). 

c. Summary of Results 

The findings from the aquatic 

acidification REA are summarized in 

terms of S deposition due to the finding 

of a negligible additional influence of N 

deposition compared to S deposition on 

acidification in this assessment 63 (PA, 

Appendix 5A, section 5A.2.1). As 

summarized more fully below, the 

analyses of five case study areas, 

including the acidification-impacted 

Shenandoah Valley area, indicate that 

with annual average S deposition below 

12 and 10 kg/ha yr, the average 

waterbody in each area (average as to 

acid-sensitivity) would be estimated to 

achieve the ANC benchmarks of 20 and 

50 meq/L, respectively. Seventy percent 

of waterbodies in each area would be 

estimated to achieve these benchmarks 

with deposition below 10 and 7 kg/hayr, respectively. At the ecoregion-scale, 

the results from the analysis of 25 

ecoregions, dominated by acid-sensitive 

waterbodies, indicate acid buffering 

capabilities to have improved 

substantially over the past 20 years, and 

particularly between the first and 

second decades of the period. By the 

2010–2012 period, the percentages of 

waterbodies achieving the three ANC 

benchmarks in all 25 ecoregions 

exceeded 80%, 80% and 70% (for 20, 30 

and 50 meq/L, respectively). By the 

subsequent analysis period (2014–2016), 

these percentages were 90%, 80% and 

80%. The ecoregion median annual 

average deposition in all 25 ecoregions 

was below 8 kg/ha-yr for 2010–2012 and 

below 5 kg/ha-yr for 2014–2016. An 

alternate approach to analyzing these 

estimates (for the 25 ecoregions across 

all five time periods) suggested that the 

three ANC benchmarks could be met in 

more than 80%, 80% and 70% (for 20, 

30 and 50 meq/L, respectively) of 

waterbodies per ecoregion in all 

ecoregions and time periods for which 

annual average ecoregion median 

deposition is estimated to be at or below 

7 kg/ha-yr. 

Between the three-year period of 

2000–2002, which was the analysis year 

for the 2009 REA, and 2018–2020, the 

latest period considered in the REA for 

this review, national average sulfur 

deposition has declined appreciably 

across the U.S. This decline in 

deposition is reflected in the very 

different aquatic acidification impact 

estimates for the two periods. Unlike the 

findings for 2000–2002 in the 2009 REA, 

in the national-scale analysis of the 

current REA, few waterbody sites are 

estimated to be receiving deposition in 

excess of their CLs for relevant ANC 

targets under recent S deposition levels. 

While recognizing inherent limitations 

and associated uncertainties of any such 

analysis, the national-scale assessment 

performed as part of the current review 

indicates that under deposition 

scenarios for the 2018–2020 period, the 

percentage of waterbodies nationwide 

that might not be able to maintain an 

ANC of 50 meq/L is less than 5% (table 

1; PA, Table 5–1). 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105723 

64The term ‘‘average’’ here refers to the average 

CL estimated for the specified ANC across all sites 

with CL estimates in each case study area (PA, 

Table 5–6). 

TABLE 1—PERCENTAGE OF WATERBODIES NATIONALLY FOR WHICH ANNUAL AVERAGE S DEPOSITION DURING THE FIVE

TIME PERIODS ASSESSED EXCEED THE WATERBODY CL (FOR CLS GREATER THAN 0) FOR EACH OF THE SPECIFIED

ANC TARGETS

ANC 

(μeq/L) 

2001–2003 

% 

2006–2008 

% 

2010–2012 

% 

2014–2016 

% 

2018–2020 

% 

20 ......................................................................................... 22 16 5 3 1 

30 ......................................................................................... 25 19 7 4 2 

50 ......................................................................................... 28 24 11 6 4 

50/20 * .................................................................................. 28 23 10 6 4 

* This combination refers to the use of a target of 50 μeq/L in eastern ecoregions and 20 μeq/L in western ecoregions. 

The case study analyses provide 

estimates of S deposition (with 

associated uncertainties) that might be 

expected to allow these geographically 

diverse locations to meet the three ANC 

targets (PA, Table 5–6). Focusing on the 

three eastern case studies, the CL 

modeling indicates that at an annual 

average S deposition of 9–10 kg/ha-yr, 

the sites in these areas, on average,64

might be expected to achieve an ANC at 

or above 50 meq/L. At an annual average 

S deposition of about 6–9 kg/ha-yr, 70% 

of the sites in the areas are estimated to 

achieve an ANC at or above 20 meq/L 

and at about 5–8 kg S/ha-yr, 70% are 

estimated to achieve an ANC at or above 

30 meq/L. Lower S deposition values are 

estimated to achieve higher ANC across 

more sites. Across the three eastern 

areas, the CL estimates for each ANC 

target are lowest for the White 

Mountains National Forest study area, 

and highest for the Shenandoah Valley 

study area. 

The ecoregion-level analyses of 25 

acid-sensitive ecoregions for the five 

periods from 2001–2003 through 2018– 

2020 illustrate the spatial variability and 

magnitude of the findings for the three 

target ANC levels and the temporal 

changes across the 20-year period, as 

described in the PA, section 5.1.3.2. For 

example, during the two most recent 3- 

year periods, the median S deposition 

estimates for each of the 25 ecoregions 

were all below 5 kg/ha-yr in 2014–2016 

and all below 4 kg/ha-yr in 2018–2020 

(table 2). Across all five time periods, 

the range of ecoregion median S 

deposition extended from below 2 kg/ 

ha-yr up to nearly 18 kg/ha-yr, with the 

higher values occurring in the eastern 

ecoregions (table 2). 

TABLE 2—SUMMARY OF ECOREGION MEDIANS DERIVED AS MEDIAN OF S DEPOSITION ESTIMATES AT CL SITES WITHIN AN

ECOREGION

Ecoregion median * total sulfur deposition (kg S/ha-yr) 

2001–03 2006–08 2010–12 2014–16 2018–20 

All 25 Ecoregions: 

Minimum ........................................................................................... 1.18 1.22 1.02 1.08 0.62 

Maximum .......................................................................................... 17.27 14.44 7.25 4.58 3.88 

Median .............................................................................................. 7.77 6.50 3.71 2.32 1.73 

18 Eastern Ecoregions: 

Minimum ........................................................................................... 4.01 3.10 2.34 1.88 1.31 

Maximum .......................................................................................... 17.27 14.44 7.25 4.58 3.88 

Median .............................................................................................. 11.08 9.36 4.76 2.97 2.04 

7 Western Ecoregions: 

Minimum ........................................................................................... 1.18 1.22 1.02 1.08 0.62 

Maximum .......................................................................................... 1.94 1.83 1.47 1.56 1.19 

Median .............................................................................................. 1.40 1.52 1.29 1.17 0.87 

* The ecoregion medians for which descriptive statistics are presented here are medians of the deposition estimates across each ecoregion’s 

waterbody sites with CL estimates. 

The ecoregion-scale results (e.g., 

percentage of waterbodies per ecoregion 

estimated to achieve the various ANC 

targets, or alternatively to exceed the 

associated CLs) for the 18 eastern and 7 

western ecoregions are summarized in 

two ways. One approach, summarized 

further below, is framed by the temporal 

trends in median S deposition per 

ecoregion, and the second approach is 

in terms of ecoregion-time period 

combinations, using ecoregion S 

deposition estimates (medians of 

deposition estimates at waterbodies 

with CLs in each ecoregion) as the 

organizing parameter. For example, 

table 3 presents the percentages of 

waterbody sites per ecoregion estimated 

to achieve the three ANC target levels, 

summarized by bins for different 

magnitudes of ecoregion median annual 

average S deposition (regardless of the 

3-year period in which it occurred). For 

the 18 eastern ecoregions and five time 

periods, there are 90 ecoregion-time 

period combinations, and for each of 

these, there are waterbody percentages 

for each of the three ANC targets. In 

table 3, the three percentages (for the 

three ANC targets) for each of the 18 


khammond on DSK9W7S144PROD with RULES2
105724 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

65The ecoregion median S deposition in all seven 

of the western ecoregions in all five time periods 

were at or below 2 kg/ha-yr (PA, Table 5–4). 66This combination of targets recognizes the 

naturally and typically low ANC levels observed in 

western waterbodies while also including a higher 

target for the East (as described in the PA, section 

5.1.2.2). 

67The right panel of this figure has been corrected 

from the version that was in the proposal. The right 

panel of this figure in the proposal (89 FR 26656, 

April 15, 2024) had a few extraneous datapoints in 

the space between the 2006–2008 and 2010–2012 

vertical lines. These extraneous datapoints are also 

in the right panel of an earlier version of this figure 

in the PA (PA, Figure 7–1). Also, in the left panel 

of the PA, Figure 7–1, the datapoints for the 2018– 

2020 period were placed to the left of the 2018– 

*COM007*2020 vertical line. 

eastern ecoregions in each of the five 

time periods are grouped in the bins 

describing the median S deposition in 

that ecoregion and time period. As can 

be seen from this table, fewer than half 

of the eastern ecoregion-time period 

combinations had an ecoregion median 

S deposition estimate at or below 4 kg/ 

ha-yr.65 Table 3 indicates that lower 

levels of S deposition at the ecoregion 

scale are associated with improved ANC 

values and greater percentages of 

waterbodies expected to reach ANC 

targets. Across the ecoregion-time 

period dataset of CL exceedances for the 

three ANC targets for all 90 eastern 

ecoregion-time period combinations (for 

which ecoregion median S deposition 

was at or below 18 kg/ha-yr), 73% of the 

combinations had at least 90% of 

waterbodies per ecoregion estimated to 

achieve ANC at or above 20 meq/L, and 

60% had at least 90% of the 

waterbodies estimated to achieve ANC 

at or above 50 meq/L (table 3). For 

ecoregion median S deposition 

estimates at or below 9 kg/ha-yr 

(approximately three quarters of the 

combinations), at least 90% of all 

waterbodies per ecoregion were 

estimated to achieve ANC at or above 

20, 30 and 50 meq/L in 87%, 81% and 

72% of combinations, respectively. For 

S deposition estimates at or below 5 kg 

S/ha-yr (the lowest ecoregion median 

deposition bin that includes at least half 

of the full dataset), these values are 

96%, 92% and 82% of combinations. 

For the 75 western ecoregion-time 

period combinations, all of which had 

ecoregion median S deposition 

estimates below 4 kg/ha-yr, at least 90% 

of waterbodies per ecoregion were 

estimated to achieve an ANC at or above 

50 meg/L (PA, Table 5–5). 

TABLE 3—PERCENTAGE OF ECOREGION-TIME PERIODS COMBINATIONS WITH AT LEAST 90, 85, 80, 75 AND 70% OF

WATERBODIES ESTIMATED TO ACHIEVE AN ANC AT/ABOVE THE ANC TARGETS OF 20, 30 AND 50 μEQ/L AS A FUNCTION OF ANNUAL AVERAGE S DEPOSITION FOR 18 EASTERN ECOREGIONS (90 ECOREGION-TIME PERIOD COMBINATIONS) 

Total sulfur deposition 

(kg S/ha-yr) at/below: 

Number of 

ecoregiontime 

periods 

% Waterbodies per ecoregion-time period meeting specified ANC target 

90% 85% 80% 75% 70% 90% 85% 80% 75% 70% 90% 85% 80% 75% 70% 

ANC target of 20 μeq/L ANC target of 30 μeq/L ANC target of 50 μeq/L 

2 ....................................................... 10 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 

3 ....................................................... 29 100 100 100 100 100 100 100 100 100 100 97 100 100 100 100 

4 ....................................................... 41 100 100 100 100 100 95 100 100 100 100 93 98 100 100 100 

5 ....................................................... 51 96 98 100 100 100 92 98 100 100 100 82 94 96 98 100 

6 ....................................................... 59 93 98 100 100 100 88 98 100 100 100 78 93 97 98 100 

7 ....................................................... 63 92 98 100 100 100 87 97 100 100 100 78 92 95 98 100 

8 ....................................................... 67 87 94 100 100 100 82 91 99 100 100 73 87 93 96 100 

9 ....................................................... 69 87 94 100 100 100 81 91 99 100 100 72 87 93 96 100 

10 ..................................................... 73 85 92 99 99 99 78 89 97 99 99 70 85 92 95 99 

11 ..................................................... 76 83 91 97 99 99 76 88 96 99 99 68 83 91 95 99 

12 ..................................................... 79 81 89 95 96 97 73 86 94 96 96 66 81 89 92 96 

13 ..................................................... 81 80 88 95 96 98 73 85 94 96 96 65 80 88 93 96 

14 ..................................................... 84 77 86 93 95 96 70 83 92 94 95 63 79 86 90 94 

15 ..................................................... 86 76 84 91 93 95 69 81 90 92 93 62 77 84 88 92 

16 ..................................................... 88 75 83 90 92 94 68 81 89 91 92 61 76 83 88 91 

17 ..................................................... 88 75 83 90 92 94 68 81 89 91 92 61 76 83 88 91 

18 ..................................................... 90 73 81 88 90 92 67 79 87 89 90 60 74 81 86 89 

Given the decreasing temporal trend 

in S deposition across all ecoregions, we 

also analyzed the aquatic acidification 

results at the ecoregion scale across the 

20 years represented by the five time 

periods (2001–03, 2006–08, 2010–12, 

2014–16, 2018–20) from a temporal 

perspective. With regard to percentages 

of waterbodies per ecoregion estimated 

to achieve the three ANC targets, an 

appreciable improvement is observed 

for the latter three time periods 

compared to the initial two time periods 

(e.g., PA, Figure 5–13). By the 2010– 

2012 time period, more than 70% of 

waterbodies in all 25 ecoregions are 

estimated to achieve an ANC at or above 

50 meq/L, and at least 85% are able to 

achieve an ANC at or above 20 meq/L 

(figure 1; PA, Table 7–2). By the 2014– 

2016 period, the percentages are 85% 

and nearly 90%, respectively. The 

median deposition for the CL sites in 

each of the 18 eastern ecoregions during 

the latter three time periods ranges from 

1.3 kg S/h-yr to 7.3 kg S/h-yr, and with 

each reduction in S deposition in each 

subsequent time period, more 

waterbodies in each of the eastern 

ecoregions are estimated to be able to 

achieve the ANC targets. Nearly 90% of 

the 18 eastern ecoregions are estimated 

to have at least 90% of their waterbodies 

achieving an ANC of 20 meq/L in the 

2010–12 period and achieving an ANC 

of 50 meq/L in the 2014–16 period. 

When the 7 western ecoregions are 

included in a summary based on ANC 

targets of 20 meq/L for the West and 50 

meq/L for the East,66 over 70% of the full 

set of ecoregions are estimated to have 

at least 90% of their waterbodies 

achieving the ANC targets by the 2010– 

12 period. More than 90% of the 

ecoregions are estimated to have at least 

90% of their waterbodies achieving the 

ANC targets by the 2014–16 period 

(figure 1; 67 PA, Table 7–2). 

BILLING CODE 6560–50–P 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105725 


khammond on DSK9W7S144PROD with RULES2ER27DE24.000</GPH>
105726 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

BILLING CODE 6560–50–C 

B. Conclusions 

1. Basis for Proposed Decision 

In reaching his proposed decision on 

the existing secondary standards for 

SOX, N oxides and PM (presented in 

section II.B.1.c.), the Administrator took 

into account the available evidence in 

the ISA, along with the policy-relevant, 

evidence-based and air quality-, 

exposure- and risk-based considerations 

discussed in the PA (summarized in 

section II.B.1.a.), as well as advice from 

the CASAC (section II.B.1.b.). In general, 

the role of the PA is to help ‘‘bridge the 

gap’’ between the Agency’s assessment 

of the current evidence and quantitative 

analyses of air quality, exposure and 

risk, and the judgments required of the 

Administrator in determining whether it 

is appropriate to retain or revise the 

NAAQS. Evidence-based considerations 

draw upon the EPA’s integrated 

assessment of the scientific evidence 

presented in the ISA (summarized in 

section II.A.3. above) to address key 

policy-relevant questions in the review. 

Similarly, the air quality-, exposure- and 

risk-based considerations draw upon 

our assessment of air quality, exposure, 

and associated risk (summarized in 

section II.A.4. above). 

This approach to reviewing the 

secondary standards is consistent with 

requirements of the provisions of the 

CAA related to the review of the 

NAAQS and with how the EPA and the 

courts have historically interpreted the 

CAA. As discussed in section I.A. 

above, these provisions require the 

Administrator to establish secondary 

standards that, in the Administrator’s 

judgment, are requisite (i.e., neither 

more nor less stringent than necessary) 

to protect the public welfare from 

known or anticipated adverse effects 

associated with the presence of the 

pollutant in the ambient air. Consistent 

with the Agency’s approach across all 

NAAQS reviews, the EPA’s approach to 

informing these judgments is based on 

a recognition that the available welfare 

effects evidence generally reflects a 

continuum that includes ambient airrelated exposures for which scientists 

generally agree that effects are likely to 

occur, through lower levels at which the 

likelihood and magnitude of response 

become increasingly uncertain. The 

CAA does not require the Administrator 

to establish secondary standards at a 

zero-risk level, but rather at levels that 

reduce risk sufficiently so as to protect 

the public welfare from known or 

anticipated adverse effects. The 

proposed decision on the secondary 

standards for SOX, N oxides and PM 

described below is a public welfare 

policy judgment by the Administrator 

that draws upon the scientific evidence 

for welfare effects, quantitative analyses 

of air quality, exposure, and risks, as 

available, and judgments about how to 

consider the uncertainties and 

limitations that are inherent in the 

scientific evidence and quantitative 

analyses. The four basic elements of the 

NAAQS (i.e., indicator, averaging time, 

form, and level) have been considered 

collectively in evaluating the public 

welfare protection afforded by the 

current standards. The Administrator’s 

final decision additionally considers 

public comments received on this 

proposed decision. 

a. Policy-Relevant Evaluations in the 

Policy Assessment 

The PA presented an evaluation of the 

evidence and quantitative analyses of 

air quality, exposure and potential risk 

related to ecological effects of SOX, N 

oxides and PM. These ecological effects 

include both direct effects of the three 

criteria pollutants on biota and 

ecological effects of ecosystem 

deposition of N and S associated with 

these pollutants. The PA identified an 

array of policy options for consideration 

by the Administrator. For SOX, the PA 

identified options for adoption of an 

annual average SO2 standard, averaged 

over three years, with a level within the 

range extending below 15 ppb and 

down to 5 ppb. For N oxides and PM2.5, 

the PA identified options for retention 

of the existing standards, without 

revision, and options for revision, 

although with recognition of 

appreciable associated uncertainty. The 

PA also considered the potential for 

establishment of a revised secondary 

standard or suite of standards with 

alternate indicator(s) that might target 

specific N or S containing chemicals 

(e.g., particulate NO3

¥, SO4

2·, NH4+), 

but recognized there to be a number of 

associated uncertainties and 

complications, including uncertainties 

in how to interpret air measurements 

and deposition estimates from remote 

areas in the context of concentrations 

near sources, without finding there to be 

a clear advantage to this approach. The 

PA additionally recognized that, in 

secondary NAAQS reviews in general, 

decisions by the Administrator on the 

adequacy of existing standards or the 

appropriateness of new or revised 

standards depend in part on public 

welfare policy judgments, science 

policy judgments regarding aspects of 

the evidence and exposure/risk 

estimates, and judgments about the level 

of public welfare protection that is 

requisite under the CAA. 

In its evaluation of policy options, the 

PA considered the evidence, as 

evaluated in both the current and prior 

reviews, with regard to the EPA’s 

overall conclusions on the ecological 

effects of SOX, N oxides and PM in 

ambient air and once deposited into 

ecosystems. The PA also considers the 

available information related to the 

general approach or framework in 

which to evaluate public welfare 

protection of the standard and the 

currently available quantitative 

information on environmental 

exposures likely to occur in areas of the 

U.S. where the standards are met. In so 

doing, the PA takes into account 

associated limitations and uncertainties, 

as well as the significance of these 

exposures with regard to the potential 

for effects, their potential severity and 

any associated public welfare 

implications. The PA also considers 

judgments about the uncertainties in the 

scientific evidence and quantitative 

analyses that are integral to 

consideration of whether the currently 

available information supports or calls 

into question the adequacy of the 

current secondary standards. 

(1) Effects Not Related to S and N 

Deposition 

In considering the currently available 

evidence and quantitative information 

pertaining to ecological effects of SOX, 

N oxides and PM in ambient air, other 

than those associated with ecosystem 

deposition of S and N, the PA focused 

on the extent to which the newly 

available information alters our 

scientific understanding of the 

ecological effects of SOX, N oxides and 

PM in ambient air; the extent to which 

the currently available information 

indicates the potential for exposures 

associated with ecological effects under 

air quality meeting the existing 

standards and whether such effects 

might be of sufficient magnitude, 

severity, extent and/or frequency such 

that they might reasonably be judged to 

be adverse to public welfare; and to 

what extent important uncertainties 

identified in past reviews have been 

reduced and/or whether new 

uncertainties emerged. These 

considerations are summarized below, 

first for SOX, followed by N oxides and 

then PM. 

(a) Sulfur Oxides 

Most of the available evidence for the 

direct effects of SOX on vegetation is not 

new to the current review. Among the 

gaseous SOX—which include SO, SO2, 

sulfur trioxide, and disulfur 

monoxide—only SO2 is present in the 

lower troposphere at concentrations 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105727 

relevant for environmental 

considerations (ISA, Appendix 2, 

section 2.1). The available evidence is 

focused primarily on the effects of SO2

on vegetation, including foliar injury, 

depressed photosynthesis and reduced 

growth or yield (ISA, Appendix 3, 

section 3.2). The newer studies continue 

to support the determination that the 

evidence is sufficient to infer a causal 

relationship between gas-phase SO2 and 

injury to vegetation (ISA, section 3.6.1). 

In general, direct effects on plants, 

including foliar injury, occur at SO2

exposures higher than a 3-hour average 

concentration of 0.5 ppm (500 ppb). 

Uncertainties associated with the 

current information relate to limitations 

in reflecting the natural environment 

and in untangling effects of SO2 from 

those of other pollutants that may have 

influenced the analyzed effects. Even 

with these uncertainties, the evidence 

indicates effects are generally associated 

with air concentrations and durations 

not expected to occur when the existing 

standard (0.5 ppm, as a 3-hour average, 

not to be exceeded more than once per 

year) is met (PA, section 7.1.1; ISA, 

Appendix 2, section 2.1). 

(b) Nitrogen Oxides 

The currently available information 

on direct effects of gaseous N oxides in 

ambient air on plants and lichens is 

composed predominantly of studies of 

NO2, HNO3, and PAN. The very few 

studies newly available in this review 

do not alter our prior understanding of 

effects of these N oxides, which include 

visible foliar injury, as well as effects on 

photosynthesis and growth at exposures 

much higher than current levels in 

ambient air (ISA, section 3.3). Thus, as 

in the last review, the body of evidence 

is sufficient to infer a causal 

relationship between gas-phase NO, 

NO2, and PAN and injury to vegetation 

(ISA, section IS.4.2). 

Information is limited regarding the 

potential for exposure levels associated 

with ecological effects to occur under 

air quality meeting the existing NO2

secondary standard. With regard to the 

risk posed by N oxides, and particularly 

HNO3, the evidence summarized in the 

ISA indicates the potential for effects on 

lichen species related to air quality 

occurring during periods when the 

current secondary standard was not met. 

Evidence is more limited for 

consideration of effects under 

conditions meeting the current standard 

(PA, section 7.1.2). Uncertainties also 

remain in our interpretation of the 

evidence, including those related to 

limitations and uncertainties of the 

various study types. 

(c) Particulate Matter 

The evidence for ecological effects of 

PM is consistent with that available in 

the last review and focused on effects 

associated with PM loading (e.g., to leaf 

surfaces), rather than direct effects of 

PM suspended in ambient air. In this 

review, as in the last one, the ecological 

effects evidence was found to be 

sufficient to conclude there is likely to 

exist a causal relationship between 

deposition of PM (other than N and S 

deposition) and a variety of effects on 

individual organisms and ecosystems 

(ISA, Appendix 15; 2012 p.m. ISA, 

section 9.4). While some uncertainties 

remain, new uncertainties have not 

emerged since the last review. There is 

little information available on effects of 

PM concentrations likely to occur under 

conditions meeting the current 

secondary standards, and the limited 

available information does not indicate 

effects to occur under those conditions 

(PA, section 7.1.3). 

(2) Evidence of Ecosystem Effects of S 

and N Deposition 

The evidence base of ecological 

effects related to atmospheric deposition 

of N and S compounds has expanded 

since the last review with regard to 

acidic deposition in aquatic and 

terrestrial ecosystems and regarding 

ecosystem N enrichment. Both S and N 

compounds have contributed to 

ecosystem acidification, with relative 

contributions varying with emissions, 

air concentrations, and atmospheric 

chemistry, among other factors. 

Ecological effects have been 

documented comprehensively in 

waterbodies of the Adirondack and 

Appalachian Mountains, and in forests 

of the Northeast, at the organism to 

ecosystem scale. With regard to N 

enrichment, research on its effects in 

estuaries and large river systems across 

the U.S. extends back at least four 

decades, and there is longstanding 

evidence of effects in estuaries along the 

East and Gulf Coasts of the U.S., as 

summarized in more detail in Chapters 

4 and 5 of the PA (ISA, Appendix 7, 

section 7.2.9; 2008 ISA, section 3.3.2.4; 

Officer et al., 1984). Information on the 

effects of N enrichment in terrestrial 

ecosystems, primarily in grassland and 

forested ecosystems, augmented in the 

current review, also includes evidence 

that was available in the last review 

(e.g., 2008 ISA, sections 3.3.3 and 3.3.5; 

ISA, Appendix 6). 

With regard to uncertainties, some 

that were associated with the evidence 

available in the 2012 review remain, 

and some additional important 

uncertainties have been identified. In 

addition to uncertainties related to the 

specific air quality circumstances 

associated with effects (e.g., magnitude, 

duration, and frequency of 

concentrations associated with effects), 

there are also uncertainties associated 

with the effects of N and S deposition 

expected under changing environmental 

circumstances. Such uncertainties 

include atmospheric loading that has 

declined since 2000, with associated 

changes to soil and waterbody 

biogeochemistry and meteorological 

changes associated with changing 

climate (ISA, section IS.12; PA section, 

7.2.1). The PA also recognizes important 

uncertainties associated with the 

various assessment approaches 

employed by different study types (PA, 

sections 5.3 and 7.2.1). Additionally, 

there are uncertainties contributed by 

variation in physical, chemical, and 

ecological responses to N and S 

deposition and by the potential 

influence of unaccounted-for stressors 

on response measures. 

In sum, a wealth of scientific 

evidence, spanning many decades, 

demonstrates effects of acidifying 

deposition associated with N and S 

compounds in aquatic and terrestrial 

ecosystems (ISA, sections ES.5.1, IS.5.1, 

IS.5.3, IS.6.1 and IS.6.3; 2008 ISA, 

section 3.2; U.S. EPA, 1982b, Chapter 7). 

This evidence base supports 

conclusions also reached in the 2008 

ISA (for the review completed in 2012) 

of causal relationships between N and S 

deposition and alteration of soil and 

aquatic biogeochemistry, alteration of 

the physiology and growth of terrestrial 

organisms and of associated 

productivity, changes in aquatic biota, 

including physiological impairment, 

and alteration of species richness, 

community composition, and 

biodiversity in both aquatic and 

terrestrial ecosystems (ISA, Table ES–1). 

Similarly, a robust evidence base 

demonstrates effects of N enrichment in 

both estuarine and freshwater 

ecosystems, supporting conclusions also 

reached in the last review of a causal 

relationship between N deposition and 

changes in biota, including altered 

growth and productivity, and alteration 

of species richness, community 

composition and biodiversity due to N 

enrichment (ISA, sections ES.5.2, IS.6, 

and IS.7, and Table ES–1). Additional 

effects of N deposition in wetlands, also 

recognized in the last review, include 

alteration of biogeochemical cycling, 

growth, productivity, species 

physiology, species richness, 

community composition, and 

biodiversity (ISA, Table ES–1). 

In terrestrial ecosystems, as in the last 

review, the now expanded evidence 


khammond on DSK9W7S144PROD with RULES2
105728 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

68A comparison of Figures 4–4 and 5–6 of the PA 

indicates multiple Class I areas in ecoregions 

considered acid sensitive. 

base supports determination of a causal 

relationship between N deposition and 

alteration of species richness, 

community composition, and 

biodiversity (ISA, Table ES–1). The ISA 

additionally determines there to be a 

causal relationship for alteration of the 

physiology and growth of terrestrial 

organisms and associated productivity, 

a category of effects not included in the 

2008 ISA (ISA, Table ES–1). Other 

evidence of effects causally associated 

with S deposition in wetland and 

freshwater ecosystems includes that 

related to chemical transformation and 

associated toxicity, most specifically 

alteration of mercury methylation, 

which was also recognized in the last 

review. The other category of effects, not 

included in the last review, is related to 

sulfide phytotoxicity and its associated 

effects in wetland and freshwater 

ecosystems (ISA, Table ES–1). 

Thus, while an array of effects is 

associated with S and N deposition, 

information important for quantitative 

analysis varies across the array. For 

some categories of effects (e.g., sulfide 

phytotoxicity) the information regarding 

environmental levels that relate to 

effects is limited and/or quite variable 

across locations, thus hindering 

analysis. For other effect categories, the 

information on linkages to criteria 

pollutants is limited and/or quite 

variable. The information with clearest 

implications to NAAQS decisions 

pertains to SOX and S deposition-related 

ecosystem acidification. While the 

information regarding effects associated 

with N loading to ecosystems is 

extensive, information to support 

quantitative analysis to inform NAAQS 

decisions regarding N oxides and PM is 

not clear, with multiple complicating 

factors. Such factors include 

contributions from other, non-criteria 

pollutants (such as NH3) and challenges 

in assessing N deposition-related effects 

of ambient air concentrations of N 

oxides and PM. While the role of N 

deposition in aquatic acidification is 

evaluated in the REA, the available 

information does not provide effective 

support for analysis of other N 

deposition-related effects of N oxides 

and PM independent of effects from 

other (non-criteria) pollutants or, in 

some cases, from other (non-air) 

sources. 

(3) Sulfur Deposition and SOX

Evidence- and exposure/risk-based 

considerations discussed in the PA 

pertaining to S deposition and SOX in 

ambient air are summarized in the 

subsections below. These considerations 

reflect discussion in the PA, which 

draws on the available welfare effects 

evidence described in the current ISA, 

the 2008 NOX/SOX ISA, the 2009 p.m. 

ISA, and past AQCDs, as well as 

information available from quantitative 

analyses (summarized in Chapters 5 and 

6 of the PA), both analyses developed in 

this review and those available from the 

2009 REA. 

In considering potential public 

welfare protection from S depositionrelated acidification effects in aquatic 

ecosystems and forested areas, the PA 

recognizes the public welfare 

implications of various effects of 

acidifying deposition on the natural 

resources in these areas, including the 

differences in response between 

waterbodies and trees, as well as the 

severity and extent of such effects. 

Given the more extensive quantitative 

analyses for aquatic acidification in this 

review, the PA discusses the public 

welfare implications of S depositionrelated effects in aquatic ecosystems 

with an eye toward their prominence for 

decision-making in this review (PA, 

sections 4.5 and 7.2.2.2). In its 

consideration of options for S 

deposition-related effects and in 

recognizing linkages between watershed 

soils and waterbody acidification, as 

well as terrestrial effects, the PA 

conveys that focusing on public welfare 

protection from aquatic acidificationrelated effects may reasonably be 

expected to also contribute protection 

for terrestrial effects (PA, section 7.4). 

The PA notes that, as also recognized 

in the 2012 review, aquatic ecosystems 

provide a number of services important 

to the public welfare, ranging from 

recreational and commercial fisheries to 

recreational activities engaged in by the 

public (77 FR 20232, April 3, 2012). 

Because aquatic acidification affects the 

diversity and abundance of aquatic 

biota, it also affects the ecosystem 

services that are derived from the fish 

and other aquatic life found in these 

surface waters (PA, section 4.5; ISA, 

Appendix 14, section 14.3.1). Fresh 

surface waters support several cultural 

services, such as aesthetic and 

educational services; the type of service 

that is likely to be most widely and 

significantly affected by aquatic 

acidification is recreational fishing, with 

associated economic and other benefits. 

Other potentially affected services 

include provision of food for some 

recreational and subsistence fishers and 

for other consumers, as well as non-use 

services, including existence (protection 

and preservation with no expectation of 

direct use) and bequest values (PA, 

section 4.5). 

The PA recognizes that some level of 

S deposition and associated risk of 

aquatic acidification, including those 

associated with past decades of 

acidifying deposition in the Northeast, 

can impact the public welfare and thus 

might reasonably be judged adverse to 

the public welfare. Depending on 

magnitude and associated impacts, there 

are many locations in which S 

deposition and associated aquatic 

acidification can adversely affect the 

public welfare. For example, there is 

evidence in some waterbodies that 

aquatic acidification resulting in 

reduced acid buffering capacity can 

adversely affect waterbodies and 

associated fisheries, which in addition 

to any commercial ramifications can 

have ramifications on recreational 

enjoyment of affected areas (PA, 

sections 5.1.1 and 4.5). 

In other secondary NAAQS reviews, 

the EPA’s consideration of the public 

welfare significance of the associated 

effects has recognized a particular 

importance of Class I areas and other 

similarly protected areas. Accordingly, 

we note that waterbodies that have been 

most affected by acidic deposition are in 

the eastern U.S., including in several 

Class I areas and other national and 

State parks and forests (PA, section 

5.1.2.1),68 with two such areas included 

as case studies in the aquatic 

acidification REA (PA, section 5.1.3.3). 

Assuring continued improvement of 

affected waterbodies throughout the 

U.S. (e.g., through lower S deposition 

than the levels of the past) may 

reasonably be considered to be of public 

welfare importance and may be 

particularly important in Class I and 

similarly protected areas. In this review, 

in considering the potential public 

welfare significance of aquatic 

acidification effects of differing levels of 

S deposition, the PA summarizes the 

REA ecoregion-scale results in terms of 

percentages of ecoregions in which 

differing percentages of waterbodies are 

estimated to achieve the three acid 

buffering capacity targets. The PA 

summarized results in this way to 

inform identification of S deposition 

estimates in the context of potential 

policy options. 

The first subsection below, 

II.B.1.a.(3)(a), focuses on the aquatic 

acidification REA analyses (summarized 

in section II.A.4. above), considering 

first the use of ANC as an indicator of 

acidification risk, then evaluating the 

risk estimates as to what they indicate 

about acidification risks in freshwater 

streams and lakes of the contiguous U.S. 

for S deposition rates estimated to have 

occurred over the past two decades 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105729 

69Aquatic acidification risk analyses in the last 

review considered deposition estimates for 2002 

and 2006 derived from CMAQ modeling, 2002 

emissions estimates (2009 REA, Appendix 1). 

70 Given findings from the 2009 REA that aquatic 

acidification provided a more sensitive measure for 

use in assessing deposition related to ecosystem 

acidification, and consideration of recent 

information not likely to result in a different 

finding, the REA for the current review focused on 

aquatic acidification. 

(much of which is newly assessed in 

this review),69 and lastly identifying 

important uncertainties associated with 

the estimates. Section II.B.1.a.(3)(b) 

considers the evidence and quantitative 

exposure/risk information from a public 

welfare protection perspective, focusing 

first on what might be indicated 

regarding deposition conditions under 

which waterbodies in acid-sensitive 

ecoregions might be expected to achieve 

acid buffering capacity of interest and 

what the available information indicates 

pertaining to the consideration of public 

welfare protection from S deposition 

related effects in aquatic ecosystems. 

Section II.B.1.a.(3)(b) also considers 

what the published quantitative 

information regarding S deposition and 

terrestrial acidification indicates 

regarding deposition levels of potential 

concern, along with associated 

uncertainties in this information. 

Section II.B.1.a.(3)(c) then summarizes 

considerations in relating SOX air 

quality metrics to deposition of S 

compounds. 

(a) Quantitative Information for 

Ecosystem Risks Associated With S 

Deposition 

As in the last review, the PA gives 

primary attention to the quantitative 

assessment of aquatic acidification 

(including particularly that attributable 

to S deposition) and recognizes these 

results to be informative to the 

identification of S deposition levels 

associated with potential for aquatic 

acidification effects of concern, as 

summarized below. This assessment of 

quantitative linkages between S 

deposition and potential for aquatic 

acidification is one component of the 

approach implemented in the PA for 

informing judgments on the likelihood 

of occurrence of such effects under 

differing air quality conditions. 

Although the approaches and tools for 

assessing aquatic acidification have 

often been applied for S and N 

deposition in combination, the REA 

approach for this review focused on S 

deposition. This focus is supported by 

analyses in the PA indicating the 

relatively greater contribution of S 

deposition than N deposition to aquatic 

acidification risk under the more recent 

air quality conditions that are the focus 

of this review (PA, Appendix 5A). As 

summarized in section II.A.4. above, the 

aquatic acidification REA relied on 

well-established site-specific water 

quality modeling applications with a 

widely recognized indicator of aquatic 

acidification, ANC. 

Quantitative tools are also available 

for the assessment of terrestrial 

acidification related to S deposition 

(PA, section 5.3.2.1; 2009 REA, section 

4.3).70 In the last review, analyses that 

related estimated atmospheric 

deposition of acidic N and S 

compounds (during the early 2000s) to 

terrestrial effects, or indicators of 

terrestrial ecosystem risk, were 

generally considered to be more 

uncertain than conceptually similar 

modeling analyses for aquatic 

ecosystems (2009 REA, section 7.5; 2011 

PA, section 1.3). The PA for this review 

also notes that quantitative tools and 

approaches are not well developed for 

other ecological effects associated with 

atmospheric deposition of S 

compounds, such as mercury 

methylation and sulfide toxicity in 

aquatic systems (PA, sections 4.2.3.1 

and 4.2.3.2). 

As described in sections II.A.3.a.(2)(a) 

and II.A.4. above, ANC is an indicator 

of susceptibility or risk of acidificationrelated effects in waterbodies, with 

lower levels indicating relatively higher 

potential for acidification and related 

waterbody effects. The PA recognized 

strong support in the evidence for use 

of ANC for purposes of making 

judgments regarding risk to aquatic 

biota in streams impacted by acidifying 

deposition and for consideration of the 

set of targets analyzed in the aquatic 

acidification REA: 20, 30, and 50 meq/ 

L (PA, section 5.1). There is 

longstanding evidence of an array of 

impacts on aquatic biota and species 

richness reported in surface waters with 

ANC values below zero and in some 

historically impacted waterbodies with 

ANC values below 20 meq/L (PA, section 

5.1.2.2). The severity of impacts is 

greatest at the lowest ANC levels. This 

evidence derives primarily from lakes 

and streams of the Adirondack 

Mountains and areas along the 

Appalachian Mountains. As recognized 

in the 2012 review, in addition to 

providing protection during base flow 

situations, ANC is a water quality 

characteristic that affords protection 

against the likelihood of decreased pH 

from episodic events in impacted 

watersheds. For example, some 

waterbodies with ANC below 20 meq/L 

have been associated with increased 

probability of low pH events, that, 

depending on other factors as noted 

above, may have potential for reduced 

survival or loss of fitness of sensitive 

biota or lifestages (2008 ISA, section 

5.1.2.1). As noted in the ISA, ‘‘[s]treams 

that are designated as episodically 

acidic (chronic ANC from 0 to 20 meq/ 

L) are considered marginal for brook 

trout because acidic episodes are likely’’ 

(ISA, Appendix 8, p. 8–26). In general, 

the higher the ANC level above zero, the 

lower the risk presented by episodic 

acidity. In summarizing and considering 

the acidification risk estimates for the 

different scales of analysis (national, 

ecoregion and case study) and using the 

water quality modeling-based CLs 

derived for three different ANC targets 

(20, 30 and 50 meq/L), the PA recognizes 

both the differing risk that might be 

ascribed to the different ANC targets 

and the variation in ANC response 

across waterbodies that may be 

reasonable to expect with differences in 

geology, history of acidifying 

deposition, and patterns of S deposition. 

The PA also recognizes limitations 

and uncertainties in the use of ANC as 

an indicator for model-based risk 

assessments (PA, section 7.2.2.1). The 

support is strongest in aquatic systems 

low in organic material such as 

historically affected waterbodies in the 

eastern U.S. (e.g., in the Adirondack 

Mountains) and Canada. In waterbodies 

with relatively higher levels of 

dissolved organic material, the presence 

of organic acid anions contributes to 

reduced pH, but these organic acids can 

also create complexes with dissolved 

aluminum that protect resident biota 

against aluminum toxicity such that 

biota in such systems tolerate lower 

ANC values (and pH) than biota in 

waterbodies with low dissolved organic 

carbon (ISA, Appendix 8, section 

8.3.6.2; PA, section 7.2.2.1). Thus, while 

the evidence generally supports the use 

of ANC as an acidification indicator and 

as a useful metric for judging the 

potential for ecosystem acidification 

effects to occur, the relationship 

between ANC and potential risk varies 

depending on the presence of naturally 

occurring organic acids, which can 

affect the responsiveness of ANC to 

acidifying deposition. For these reasons, 

ANC is less well supported as an 

indicator for acidic deposition-related 

effects (and waterbodies are less 

responsive to changes in acidic 

deposition) due to dissolved organic 

material in some areas, including the 

Middle Atlantic Coastal Plain, Southern 

Coastal Plains, and Atlantic Coastal Pine 

Barrens ecoregions (PA, section 5.1.2.2). 

The REA national-scale analysis of 

more than 13,000 waterbody sites in 69 

ecoregions demonstrated an appreciable 


khammond on DSK9W7S144PROD with RULES2
105730 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

reduction in risk over the 20-year period 

of analysis (PA, section 5.1.3) with the 

percentage of waterbodies unable to 

achieve an ANC of 20 meq/L or greater 

declining from 20% for the 2001–2003 

period to 1% by the 2018–20 period 

(table 1). The 25 ecoregions included in 

the ecoregion-scale analyses (i.e., 18 in 

the East and 7 in the West in which 

there are at least 50 waterbody sites 

with CL estimates) are dominated by 

ecoregions categorized as acid sensitive 

(PA, Table 5A–5) and exclude the three 

ecoregions identified above as having 

natural acidity related to organic acids 

(PA, section 5.1.2.1). Due to the 

dominance of the acid-sensitive 

ecoregions among the 25 ecoregions 

analyzed, the percentages of 

waterbodies not able to meet the ANC 

targets are higher than the national 

percentages. Specifically, in the most 

affected ecoregion (Central 

Appalachians), more than 50% of 

waterbodies were estimated to be unable 

to achieve an ANC of 20 meq/L or greater 

based on S deposition estimates for the 

2001–2003 period (figure 1 above, and 

PA, Figure 5–13). By the 2018–2020 

period, less than 10% of waterbodies in 

any of the 25 ecoregions (and less than 

5% in all but one) were estimated to be 

unable to achieve an ANC of 20 meq/L, 

and less than 15% of waterbodies in the 

most affected ecoregion were estimated 

to be unable to achieve an ANC of 50 

meq/L (figure 1 above and PA, Figure 5– 

13). 

The PA recognizes uncertainty 

associated with two overarching aspects 

of the aquatic acidification assessment 

of effects (PA, section 5.1.4 and 

Appendix 5A, section 5A.3). The first 

relates to interpretation of specific 

thresholds or benchmark concentrations 

of ANC with regard to aquatic 

acidification risk to aquatic biota. While 

ANC is a well-established indicator of 

aquatic acidification risk, uncertainty 

remains in our understanding of 

relationships between ANC and risk to 

native biota, particularly in waterbodies 

in geologic regions prone to waterbody 

acidity. Such uncertainties relate to the 

varying influences of site-specific 

factors, such as the prevalence of 

organic acids in the watershed, and to 

historical loading to watershed soils that 

can influence acidity of episodic highflow events (PA, sections 5.1.4 and 

7.2.2.1 and Appendix 5A, section 5A.3). 

The second overarching aspect of 

uncertainty relates to our understanding 

of the biogeochemical model linkages 

between deposition of S and N 

compounds and waterbody ANC, which 

is reflected in the modeling employed, 

and the associated estimation of CLs, as 

described in section II.A.4.b. above. 

Although the approaches to estimate 

base-cation supply in the REA (e.g., the 

F-factor approach) have been widely 

published and analyzed in Canada and 

Europe, and have been applied in the 

U.S. (e.g., Dupont et al., 2005), the 

magnitude of uncertainty in the basecation supply estimate is unclear and 

could be large in some cases. The REA’s 

quantitative analysis of uncertainty in 

CL estimates indicates lower 

uncertainty associated with CLs 

estimated for sites with more extensive 

and longer-term water quality datasets 

and relatively low variability in the 

runoff measurements, such as CLs for 

waterbody sites in the eastern U.S. (PA, 

Appendix 5A, section 5A.3.1). 

(b) General Approach for Considering 

Public Welfare Protection 

In discussing key considerations in 

judging public welfare protection from S 

deposition associated with the 

secondary standard for SOX, the PA first 

focused on what the aquatic 

acidification REA indicated about 

deposition conditions under which 

waterbodies in sensitive ecoregions 

might be expected to achieve ANC 

levels of interest. Particular focus was 

given to the ecoregion and case-study 

analyses, which use the waterbodyspecific comparisons of estimated 

deposition and waterbody CLs to 

provide ecoregion wide and crossecoregion summaries of estimated 

waterbody responses to ecoregion 

estimates of deposition. The PA also 

considered the extent to which 

waterbodies in each ecoregion analyzed 

were estimated to achieve or exceed the 

three target ANC levels in the context of 

the variation in ANC response 

reasonably expected across waterbodies 

in an ecoregion due to differences in 

watershed sensitivity to S deposition 

impacts and different spatial or 

geographic patterns of S deposition. 

Based on the array of CL-based 

analyses, the PA provides a general 

sense of the ANC values that 

waterbodies in sensitive regions across 

the continental U.S. may be able to 

achieve, including for areas heavily 

affected by a long history of acidifying 

deposition, such as waterbodies in the 

well-studied Shenandoah Valley area 

(4,977 sites distributed across three 

ecoregions). For the other case study 

areas (White Mountain National Forest, 

Northern Minnesota, Sierra Nevada 

Mountains and Rocky Mountain 

National Park), there are appreciably 

fewer waterbody sites for which 

modeling has been performed to 

estimate CLs, and accordingly greater 

uncertainty. Yet, the case study area 

averages of waterbody CLs for achieving 

ANC at or above each of the three targets 

(20, 30 or 50 meq/L) are quite similar 

across the five case studies (PA, Table 

5–6). The PA found the case study 

estimates to suggest that a focus on S 

deposition below 10 kg/ha-yr may be 

appropriate. 

Findings from the ecoregion-scale 

analyses of 25 ecoregions (18 East and 

7 West), nearly all of which are 

considered acid sensitive, indicated 

ranges of deposition (summarized in 

terms of ecoregion medians) associated 

with high percentages of waterbodies 

estimated to achieve the three ANC 

targets that are similar to the case study 

results immediately above. This was 

true when considering the ecoregionscale analysis results in both of the ways 

they were presented: (1) in terms of 

ecoregion median deposition regardless 

of time period or ecoregion (ecoregiontime period combinations), and (2) in 

terms of temporal trends in S deposition 

and waterbody percentages achieving 

ANC targets. In total, the ecoregion-time 

periods presentation indicates the 

likelihood of appreciably more 

waterbodies achieving the acid buffering 

capacity targets among the combinations 

with ecoregion median deposition at or 

below 9 kg/ha-yr (and for the bins for 

lower values) in eastern ecoregions 

compared to the estimates of 

waterbodies achieving acid buffering 

targets based on the full dataset that 

includes ecoregion median deposition 

estimates up to 18 kg/ha-yr (table 4 

below). For example, in the ecoregiontime period combinations presentation, 

at least 90% of waterbody sites in 87% 

of the eastern ecoregion-time period 

combinations are estimated to be able to 

achieve an ANC at or above 20 meq/L 

with ecoregion median S deposition at 

or below 9 kg/ha-yr and in 96% of those 

combinations for ecoregion median S 

deposition at or below 5 kg/ha-yr (table 

4). Additionally, these percentages 

increase across the bins for the lower 

deposition estimates, although they are 

also based on smaller proportions of the 

supporting dataset (i.e., fewer ecoregiontime period combinations in each 

subsequently lower deposition bin) 

contributing to increased uncertainty for 

those results. 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105731 

71This combination of targets recognizes the 

naturally and typically low ANC levels observed in 

western waterbodies while also including a higher target for the East, as described in section 5.1.2.2 

of the PA. 

TABLE 4—SUMMARY OF THE EASTERN ECOREGION AND TIME PERIOD COMBINATIONS ACHIEVING DIFFERENT ANC 

TARGETS WITH ESTIMATED S DEPOSITION AT OR BELOW DIFFERENT VALUES

S deposition (kg/ha-yr) * 

% of 

combinations 

included 

% of Eastern ecoregion-time period combinations ** with at least 90%, 80% or 70% 

waterbodies per ecoregion achieving ANC target 

>90% of waterbodies >80% of waterbodies >70% of waterbodies 

20 30 50 20 30 50 20 30 50 

ANC (μeq/L) at/below: 

≤18 ................................................................................ 100 73 67 60 88 87 81 92 90 89 

≤13 ................................................................................ 90 80 73 65 95 94 88 98 96 96 

≤11 ................................................................................ 84 83 76 68 97 96 91 99 99 99 

≤9 .................................................................................. 77 87 81 72 100 99 93 100 100 100 

≤7 .................................................................................. 70 92 87 78 100 100 95 100 100 100 

≤6 .................................................................................. 66 93 88 78 100 100 97 100 100 100 

≤5 .................................................................................. 57 96 92 82 100 100 96 100 100 100 

* These values are ecoregion median estimates across all waterbody sites in an ecoregion with a CL estimate. 

** These percentages are from the more extensive presentation of results in PA, Table 5–5. 

The PA observes that estimates from 

the temporal trend perspective similarly 

indicate appreciable percentages of 

waterbodies per ecoregion being 

estimated to achieve the acid buffering 

capacity targets with ecoregion median 

deposition below a range of 

approximately 5 to 8 kg/ha-yr. For 

example, by the 2010–2012 period, by 

which time all 25 ecoregions are 

estimated to have more than 70% of 

waterbodies able to achieve an ANC at 

or above 50 meq/L (and at least 85% able 

to achieve an ANC at or above 20 meq/ 

L), median deposition in the ecoregions 

analyzed was below 8 kg S/ha-yr, 

ranging from 1.3 to 7.3 kg S/ha-yr (PA, 

Table 7–2). As shown in table 5 below, 

with each reduction in S deposition in 

each subsequent time period, more 

waterbodies in each of the eastern 

ecoregions are estimated to be able to 

achieve the ANC targets. Nearly 90% of 

the 18 eastern ecoregions are estimated 

to have at least 90% of their waterbodies 

achieving an ANC of 20 meq/L in the 

2010–12 period and achieving an ANC 

of 50 meq/L in the 2014–16 period. 

When the 7 western ecoregions are 

included in a summary based on ANC 

targets of 20 meq/L for the West and 50 

meq/L for the East,71 over 70% of the full 

set of ecoregions are estimated to have 

at least 90% of their waterbodies 

achieving the ANC targets by the 2010– 

12 period (table 5). By the 2014–2016 

and 2018–2020 periods, 24 of the 25 

ecoregions were estimated to have more 

than 90% of waterbodies able to achieve 

an ANC at/above 50 meq/L, and median 

S deposition in all 25 ecoregions was 

below 5 kg/ha-yr (table 5). 

TABLE 5—ECOREGIONS ESTIMATED TO HAVE DIFFERENT PERCENTAGES OF WATERBODIES ACHIEVING DIFFERENT ANC 

TARGETS FOR THE FIVE DEPOSITION PERIODS ANALYZED

Time period 

% (n) of ecoregions with specified percentage of waterbodies per ecoregion achieving specified ANC 

ANC: 20 μeq/L 30 μeq/L 50 μeq/L 

Ecoregion median S deposition 

(kg/ha-yr) 

Percent of waterbodies per 

ecoregion 

Percent of waterbodies per 

ecoregion 

Percent of waterbodies per 

ecoregion 

Min Max 90% 80% 70% 90% 80% 70% 90% 80% 70% 

East Of 18 Eastern Ecoregions 

2001–03 ........... 4.0 17.3 39% (7) 67% (12) 72% (13) 28% (5) 61% (11) 72% (13) 22% (4) 50% (9) 72% (13) 

2006–08 ........... 3.1 14.4 44 (8) 72 (13) 89 (16) 33 (6) 72 (13) 78 (14) 33 (6) 67 (12) 72 (13) 

2010–12 ........... 2.3 7.3 89 (16) 100 (18) 100 (18) 83 (15) 100 (18) 100 (18) 61 (11) 89 (16) 100 (18) 

2014–16 ........... 1.9 4.6 94 (17) 100 (18) 100 (18) 94 (17) 100 (18) 100 (18) 89 (16) 100 (18) 100 (18) 

2018–20 ........... 1.3 3.9 100 (18) 100 (18) 100 (18) 94 (17) 100 (18) 100 (18) 94 (17) 100 (18) 100 (18) 

All Of 25 Ecoregions (18 East, 7 West) 

2001–03 ........... 1.2 17.3 56 (14) 76 (19) 80 (20) 48 (12) 72 (18) 80 (20) 44 (11) 64 (16) 80 (20) 

2006–08 ........... 1.2 14.4 60 (15) 80 (20) 92 (23) 52 (13) 80 (20) 84 (21) 52 (13) 76 (19) 80 (20) 

2010–12 ........... 1.0 7.3 92 (23) 100 (25) 100 (25) 88 (22) 100 (25) 100 (25) 72 (18) 92 (23) 100 (25) 

2014–16 ........... 1.1 4.6 96 (24) 100 (25) 100 (25) 96 (24) 100 (25) 100 (25) 92 (23) 100 (25) 100 (25) 

2018–20 ........... 0.62 3.9 100 (25) 100 (25) 100 (25) 96 (24) 100 (25) 100 (25) 96 (24) 100 (25) 100 (25) 

Note: Estimates for ANC of 50 μeq/L (East) and 20 μeq/L (West) are identical to those for 50 in all 25 ecoregions. 

The temporal trends in percentage of 

waterbodies estimated to achieve the 

target ANC levels for each of the 25 

individual ecoregions document a large 

difference between the time periods 

prior to 2010 and subsequent time 

periods (figure 1 above). For the S 

deposition estimated for the 2010–2012 

period, more than 70% of waterbodies 

are estimated to be able to achieve an 

ANC of 50 ueq/L in all 25 ecoregions 

(figure 1, left panel), and 85% to 100% 

of waterbodies in all ecoregions are 

estimated to be able to achieve an ANC 

of 20 ueq/L (figure 1, right panel). 

Given the dependency of the ANC 

estimates on S deposition estimates, this 

distinction between the period prior to 

2010 and the subsequent decade is also 

seen in the ecoregion deposition 


khammond on DSK9W7S144PROD with RULES2
105732 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

72 In Figure 7–2 of the PA (which is an earlier 

version of figure 2), the box and whiskers presented 

for the medians were incorrect. They are correct in 

figure 2 here, and they were also correct in figure 

2 of the proposal. 

73Figure 2 presents temporal trends for three 

different statistics for deposition within the REA 

ecoregions. For example, the leftmost box and 

whiskers among the set of three presents the 

distribution of values that are the 90th percentile 

deposition estimates (at REA assessed waterbodies) 

in the 25 ecoregions. The rightmost box and 

whiskers presents the distribution of median 

deposition estimates for these ecoregions (figure 2, 

left panel). 

74The median of the ecoregion 90th percentiles 

is the horizontal line in the leftmost box of the set 

of three. This is a measure of the central tendency 

of the 90th percentile deposition (across REA sites) 

in the 25 assessed ecoregions. 

estimates for the 25 REA ecoregions 

(figure 2; PA, Figure 7–2).72 The 

distribution of deposition estimates at 

waterbody sites assessed in each 

ecoregion, and particularly the temporal 

pattern for the upper percentiles, 

illustrates the deposition estimates that 

are driving temporal pattern in the REA 

estimates.73 For example, across the 25 

ecoregions (figure 2, left panel), the 

median of the ecoregion 90th 

percentiles 74 of S deposition during the 

two earliest periods ranged from 

approximately 14 to 17 kg/ha-yr and the 

highest ecoregion 90th percentile values 

were above 20 kg/ha-yr. In contrast, 

during the latter three periods (2010– 

2020), the median of ecoregion 90th 

percentile values ranged from 

approximately 2 to 5 kg/ha-yr and all 

ecoregion 90th percentile estimates 

were below approximately 8 kg/ha-yr 

(figure 2). The contrast is less sharp for 

the ecoregion medians, as the median is 

a statistic less influenced by changes in 

the magnitude of values at the upper 

end of the distribution (figure 2). 

Overall, this indicates the significant 

reduction in the highest levels of 

deposition within each ecoregion over 

the time periods analyzed. 

BILLING CODE 6560–50–P 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105733 

BILLING CODE 6560–50–C Thus, in considering identification of 

S deposition levels that may be 

associated with a desired level of 

ecosystem protection for a SOX


khammond on DSK9W7S144PROD with RULES2ER27DE24.001</GPH>
105734 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

75The PA also suggested, based on the case study 

CL estimates, a focus on deposition below 10 kg/ 

ha-yr, although the deposition estimates discussed 

in the case study analysis are smaller scale, e.g., 

site-level (PA, section 5.1.3.3). 

76The air quality metrics include one based on 

the current secondary SO2 NAAQS, which is the 

second highest 3-hour daily maximum in a year, as 

well as an annual average SO2 air quality metric 

(averaged over three years). Since many factors 

contribute variability to S deposition, the analyses 

focus on a 3-year average of all of the air quality 

and deposition metrics and include multiple years 

of data, generally on the order of 20 years and 

covering a period of declining concentrations and 

deposition. Of the two air quality metrics analyzed, 

the PA focused primarily on the annual average of 

SO2 concentrations, averaged over 3 years, given the 

focus on control of long-term S deposition and the 

greater stability of the metric (PA, section 7.2.2.3). 

standard, the PA took note of the 

increased percentages of waterbodies 

estimated to achieve more protective 

ANC levels across the five time periods. 

The pattern of estimated improving 

water quality over the 20-year study 

period is paralleled by the pattern of 

declining deposition (figure 2). This 

temporal pattern indicates an 

appreciable reduction in ecoregion S 

deposition between the first and second 

decades of the period with associated 

reduction in aquatic acidification risk. 

As noted immediately above, the risk 

estimates associated with the deposition 

estimates of the second decade indicate 

generally high percentages of 

waterbodies per ecoregion as able to 

achieve or exceed the three ANC targets. 

Similarly, the ecoregion-time period 

binning summary also indicates 

generally high percentages of 

waterbodies achieving ANC targets for 

ecoregion median S deposition at or 

below about 8 or 9 kg/ha-yr (table 4). 

Thus, in light of these observations,75

the PA describes S deposition, on an 

areawide basis (i.e., ecoregion median), 

that falls at or below approximately 5 to 

9 (differing slightly depending on the 

supporting analysis), as being associated 

with the potential to achieve acid 

buffering capacity levels of interest in 

an appreciable portion of sensitive 

areas. 

In considering what the quantitative 

information for S deposition and 

terrestrial acidification indicates 

regarding deposition levels of potential 

concern for acidification-related effects 

(and the associated uncertainties), the 

PA considers soil chemistry modeling 

analyses (both in published studies and 

in the 2009 REA), studies involving 

experimental additions of S compounds 

to defined forestry plots, and 

observational studies of potential 

relationships between terrestrial biota 

assessments and metrics for S 

deposition (PA, section 5.3). With 

regard to soil chemistry modeling 

analyses performed in the last review, 

the PA found the 2009 soil acidification 

modeling to indicate that a focus on 

aquatic acidification might reasonably 

be expected to also provide protection 

from soil acidification effects on 

terrestrial biota. With regard to studies 

involving S additions to experimental 

forested areas, the PA observes that 

effects on the sensitive tree species 

analyzed have not been reported with S 

additions below 20 kg/ha-yr (which is in 

addition to the atmospheric deposition 

occurring during the experiment). 

The PA also considers the recently 

available quantitative information on S 

deposition and terrestrial acidification 

drawn from recent observational studies 

that report associations of tree growth 

and/or survival metrics with various air 

quality or S deposition metrics (PA, 

section 5.3.2.3 and Appendix 5B, 

section 5B.3.2). The metrics used in the 

two largest studies include site-specific 

estimates of average SO4

2¥ deposition 

and of average total S deposition over 

the interval between tree measurements, 

generally on the order of 10 years 

(Dietze and Moorcroft, 2011; Horn et al., 

2018). In the study that used SO4

2¥ as 

the indicator of acidic S deposition, and 

for which the study area was the eastern 

half of the contiguous U.S., site-specific 

average SO4

2¥ deposition (1994–2005) 

ranged from a minimum of 4 kg/ha-yr to 

a maximum of 30 kg/ha-yr (Dietze and 

Moorcroft, 2011). Review of the study 

area for this study and a map indicating 

geographic patterns of deposition during 

the period of the deposition data 

indicate the lowest deposition areas to 

be west of the Mississippi River, 

northern New England (e.g., Maine) and 

southern Georgia and Florida (in which 

S deposition in the 2000–2002 period 

was estimated to fall below 8 kg/ha-yr), 

and the highest deposition areas to be a 

large area extending from New York 

through the Ohio River valley (PA, 

Appendix 5B, Figures 5B–1 and 5B–11). 

In the second study, deposition at the 

sites with species for which growth or 

survival was negatively associated with 

S deposition ranged from a minimum 

below 5 kg/ha-yr to a site maximum 

above 40 kg/ha-yr, with medians for 

these species generally ranging from 

around 5 to 12 kg S/ha-yr (PA, 

Appendix 5B, section 5B.3.2.3; Horn et 

al., 2018). 

In considering these study 

observations, the PA notes the history of 

appreciable acidic deposition in the 

eastern U.S., with its associated impacts 

on soil chemistry, that has the potential 

to be exerting a legacy influence on tree 

growth and survival more recently (PA, 

section 5.3.2 and Appendix 5B). 

Further, the PA notes that, at a national 

scale, the geographic deposition 

patterns (e.g., locations of relatively 

greater versus relatively lesser 

deposition) in more recent times appear 

to be somewhat similar to those of 

several decades ago (e.g., PA, sections 

2.5.4 and 6.2.1). This similarity in 

patterns is recognized to have the 

potential to influence findings of 

observational studies that assess 

associations between variation in tree 

growth and survival with variation in 

levels of a metric for recent deposition 

at the tree locations, and to contribute 

uncertainty with regard to interpretation 

of these studies as to a specific 

magnitude of deposition that might be 

expected to elicit specific tree 

responses, such as those for which 

associations have been found. The PA 

notes that, as recognized in the study by 

Dietze and Moorcroft (2011), which 

grouped species into plant functional 

groups, acidification impacts on tree 

mortality rates are the result of 

cumulative long-term deposition, and 

patterns reported by their study should 

be interpreted with that in mind (PA, 

section 5.3.1 and Appendix 5B). 

(c) Relating Air Quality Metrics to S 

Deposition 

In considering what the available 

information and air quality analyses 

indicate regarding relationships 

between air quality metrics and S 

deposition, the PA evaluated trends 

over the past two decades as well as a 

series of analyses of relationships 

between S deposition and ambient air 

concentrations of SO2 (in terms of 3-year 

averages of the existing SO2 standard 

and of an annual average),76 and 

between S deposition and ambient air 

concentrations of other S compounds 

(e.g., SO4

2¥ or the sum of SO42¥ and 

SO2) at 27 Class I area sites (collocated 

CASTNET and IMPROVE network 

sites), as summarized in section II.B. 

above. With regard to the latter, lower 

correlations were observed for total S 

deposition estimates collocated with 

ambient air concentrations of Scontaining pollutants (SO4

2¥ and the 

sum of S in SO2 and SO4

2¥) in 27 Class 

I areas than between S deposition and 

annual average SO2 concentrations 

(averaged over three years) at SLAMS 

monitors (PA, Figure 6–31, center and 

right panels, and Table 6–4). Thus, 

while information for S compounds 

other than SO2 are available at the Class 

I area sites, the analyses based on data 

from SLAMS are considered particularly 

relevant given that those sites are 

primarily in areas of higher SO2

concentrations (near emissions sources) 

and collect FRM/FEM measurements for 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105735 

77The similar pattern observed for annual average 

SO2 concentrations as 3-year averages suggests little 

year-to-year variability in this metric (PA, Figure 7– 

5). 

78The bin for ‘‘<9–6 kg/ha-yr’’ is discussed here 

as it is the bin closest to the deposition target range 

of 10 or 8 to 5 kg/ha-yr identified above. 

79Figure 7–3 of the PA presents the pairs of 

median deposition estimates and associated 

upwind sites of influence EAQM-max SO2

concentrations from the trajectory-based analysis in 

section 6.2.4 of the PA (specifically, the combined 

datasets presented in PA, Figure 6–41). 

existing NAAQS monitoring. Data from 

these monitoring sites informed the PA 

consideration of how changes in SO2

emissions, reflected in ambient air 

concentrations, may relate to changes in 

deposition and, correspondingly, what 

secondary standard options might best 

relate to ambient air concentrations 

such that deposition in areas of interest 

is maintained at or below range of levels 

identified above (PA, section 7.2.2.3). 

Together the air quality and 

deposition data and analyses in the PA 

indicate a significant association of S 

deposition with SO2 concentrations, 

with statistically significant correlation 

coefficients ranging from approximately 

0.5 to 0.7 from the trajectory-based and 

SLAMS analyses for the five 3-year time 

periods (during 2001–2020) across all 

ecoregions. Higher correlations were 

observed for dry S deposition and at 

sites in the eastern U.S. (PA, section 

7.2.2.3). As summarized in section 

II.A.2. above, S deposition is generally 

higher in the East and dry S deposition 

is generally higher near SO2 emissions 

sources. A strength of the analyses for 

concentrations and deposition estimates 

at SLAMS locations is the capturing of 

near-source deposition, while a strength 

of the trajectory-based analyses is 

accounting for the role of transport and 

transformation in contributing to 

downwind deposition. 

While recognizing the significant 

correlations between SO2 concentrations 

and S deposition, the PA additionally 

took note of the variability in, and 

uncertainty associated with, these 

relationships. The variability derives 

from the complexity of the atmospheric 

chemistry, pollutant transport, and 

deposition processes (PA, sections 2.1.1 

and 2.5). The uncertainty in these 

relationships relates to a number of 

factors, including uncertainty in our 

estimates of S deposition (PA, section 

2.5.2) and spatial distribution of 

monitor sites, including the 

representation of significant SO2

emissions sources, as well as elements 

of the trajectory-based analysis, e.g., 

inclusion criteria for identifying 

monitoring sites of influence (PA, 

section 6.3 and Table 6–13). The PA 

concluded that it is unclear how much 

and in what way each of these various 

uncertainties in the data and analyses, 

and the inherent variability of the 

physical and chemical processes 

involved, might impact the conclusions 

concerning ambient air SO2

concentrations related to S deposition 

estimates at different scales (PA, section 

7.2.2.3). In light of such uncertainty and 

variability, the REA aquatic 

acidification analyses and discussion of 

S deposition levels focused on statistics 

for deposition estimates representing 

large areas (e.g., at the ecoregion median 

and 75th or 90th percentile, and case 

study area average or 70th and 90th 

percentile CLs). While uncertainty may 

be greater for relating concentrations to 

higher points on the distribution of 

deposition in an ecoregion, the PA 

recognized that it is the higher 

deposition estimates, if focused on 

individual waterbodies, that will 

contribute most to aquatic acidification 

risk. The PA additionally observed that 

the distribution of S deposition 

estimates within ecoregions has 

narrowed in more recent years, with 

90th percentile estimates falling much 

closer to the medians than in the first 

decade of the 20-year period (figure 2 

above). 

In identifying levels for consideration 

for a potential annual average SO2

standard, the PA first considered SO2

concentrations at SLAMs and associated 

S deposition levels, focusing on the 

most recent of the five time periods 

analyzed (i.e., since 2010) when the 

REA indicated appreciably improved 

levels of acid buffering capability in the 

waterbodies of the 25 analyzed 

ecoregions (when ANC targets were met 

or exceeded in a high percentage of 

water bodies across a high percentage of 

ecoregions). Since 2010 (when 

ecoregion median and 90th percentile S 

deposition estimates for the 25 REA 

ecoregions were below 10 kg/ha-yr), the 

highest 3-year average annual SO2

concentrations were generally 

somewhat below 10 ppb (with some 

exceptions during the 2019–2021 

period) (PA, Figure 7–5, left panel).77

The PA also considered SO2

concentrations at monitoring sites of 

influence identified in the trajectorybased analyses across different ranges of 

downwind ecoregion S deposition 

estimates. Across all 84 ecoregions in 

the contiguous U.S., the maximum 

annual average SO2 concentrations, as 3- 

year averages, at sites of influence to 

downwind ecoregions with median S 

deposition below 9 kg down to 6 kg/hayr,78 were all below 15 ppb, and 75% 

of the concentrations at these sites were 

at or below 10 ppb (PA, Figure 7–3).79

In the 25 REA ecoregions, for the 

ecoregion median S deposition below 9 

down to 6 kg/ha-yr, the concentrations 

for the metric based on maximum 

concentration at upwind sites of 

influence (EAQM-max) range as high as 

15 ppb, with more than half below 10 

ppb (PA, Figure 7–4, left panel). The 

EAQM-max concentrations associated 

with ecoregion median S deposition 

below 6 kg/ha-yr were all below 10 ppb. 

This PA presentation further indicates 

that for the 25 REA ecoregions, when 

the highest EAQM-max concentration is 

at approximately 11 or 10 ppb, both the 

median and 90th percentile deposition 

are both below 9 kg/ha-yr, with the 

overwhelming majority below 6 kg/hayr (PA, Figure 7–4). 

In its use of the trajectory-based 

analyses to identify a range of annual 

average SO2 EAQM-max concentrations 

associated with an ecoregion median S 

deposition target range, the PA 

recognizes several important 

considerations. First, monitor 

concentrations of SO2 can vary 

substantially across the U.S., 

complicating consideration of the 

relationship between maximum 

contributing monitors identified in the 

trajectory-based analysis and S 

deposition levels in downwind 

ecosystems. Additionally, the 

substantial scatter in the relationship 

between S deposition estimates and 

measured SO2 concentrations with 

ecoregion median S deposition values 

below about 5 kg/ha-yr contributes 

increased uncertainty to conclusions 

regarding potential secondary standard 

SO2 metric levels intended to relate to 

ecoregion median deposition levels at or 

below 5 kg/ha-yr (PA, section 7.2.2.3). 

The PA additionally discusses 

limitations in the context of the two 

metrics (weighted and max). Between 

these metrics, somewhat stronger 

correlations were found for the annual 

average SO2 weighted EAQM (which 

provides for proportional weighting of 

air concentrations from locations 

projected to contribute more heavily to 

a particular ecoregion), compared to the 

EAQM-max, particularly for the first 

two to three time periods of the 20-year 

period. This difference is related to the 

extent to which monitor concentrations 

can be indicative of atmospheric 

loading. The weighted EAQM is 

intended to more closely represent the 

atmospheric loading for the locations 

(and associated sources) of the 

contributing (sites of influence) 

monitors than a single contributing 

monitor can. However, the weighted 

metric is not directly translatable to a 

standard level (which is an upper limit 


khammond on DSK9W7S144PROD with RULES2
105736 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

80 It is also of note that use of SO4

2¥

measurements, alone or in combination with SO2

concentrations, as an indicator of a new standard 

would entail development of sample collection and 

analysis FRM/FEMs and of a surveillance network. 

on concentrations in individual 

locations). 

The PA also considered relationships 

between S deposition and PM2.5, noting 

the poor correlations for total S 

deposition estimates with PM2.5 at the 

27 Class I area sites (r = 0.33, PA, Figure 

6–31), and not much stronger 

correlations for ecoregion S deposition 

estimates with PM2.5 at upwind sites of 

influence from the trajectory-based 

analysis (r = ¥0.22 and 0.48, PA, Table 

6–12). The PA also considered 

relationships between total S deposition 

and ambient air SO4

2¥ concentrations 

noting that they are focused on remote 

locations (Class I areas), distant from 

sources of SO2 emissions, and that the 

relationship is not stronger than that for 

SO2 at the SLAMS, which are generally 

near sources monitoring SO2 (the source 

for atmospheric SO4

2¥). In light of these 

considerations, the PA found that the 

available analyses did not indicate an 

advantage for an indicator based on 

SO4

2¥ measurements (or SO42¥ and SO2

combined), such as is currently 

collected at CASTNET sites, or PM2.5

mass over options for a potential annual 

average standard metric focused on SO2

concentrations (based on FRM/FEMs).80

(4) Nitrogen Deposition and N Oxides 

and PM 

The evidence and exposure/risk-based 

considerations of the PA pertaining to N 

deposition and concentrations of N 

oxides and PM in ambient air draw on 

the available welfare effects evidence 

described in the current ISA (as well as 

prior ISAs and AQCDs), and discussed 

in Chapters 4, 5 and 6 of the PA. The 

focus of these considerations is 

primarily on N deposition and effects 

other than aquatic acidification (PA, 

sections 4.3, 5.2 and 5.3). As recognized 

in section II.A.4. above, the PA finds S 

deposition to be the dominant influence 

on aquatic acidification risk in the 20- 

year period analyzed (2001–2020), 

based on the finding that the inclusion 

of acidic N deposition to the aquatic 

acidification risk analyses did not 

appreciably change patterns and 

percentages of waterbodies estimated to 

exceed CLs for the three ANC targets 

(PA, section 5.1.2.4). 

In considering potential public 

welfare protection from N depositionrelated effects (in light of the evidence 

summarized in sections II.A.3. and 

II.A.3.c. above), the PA recognizes the 

potential public welfare implications of 

the effects of N deposition in both 

aquatic and terrestrial ecosystems (PA, 

section 7.2.3.2). For example, the public 

welfare significance of eutrophication in 

large estuaries and coastal waters of the 

eastern U.S. related to decades of N 

loading is illustrated by the broad state, 

local and national government 

engagement in activities aimed at 

assessing and reducing the loading (PA, 

section 5.2.3). This significance relates 

both to the severity of the effects and the 

wide-ranging public uses dependent on 

these waters, including as important 

sources of fish and shellfish production, 

providing support for large stocks of 

resident commercial species, serving as 

breeding grounds and interim habitat for 

several migratory species, and providing 

an important and substantial variety of 

cultural ecosystem services. The public 

also benefits from water-based 

recreational uses and aesthetic values 

placed on aquatic systems. Many 

impacts of eutrophication relate to 

reduced waterbody oxygen, which 

contributes to fish mortality, and 

changes in aquatic habitat related to 

changes in resident plant and animal 

species, with associated ecosystem 

effects (PA, section 4.3; ISA, Appendix 

7). 

The relative contribution of 

atmospheric deposition to total N 

loading, however, varies widely among 

estuaries and has declined in recent 

years, contributing a complexity to 

considerations in this review. While N 

loading in smaller, more isolated fresh 

waterbodies is primarily from 

atmospheric deposition, the evidence 

with regard to public welfare 

significance of any small depositionrelated effects in these systems is less 

clear and well established. For example, 

the public welfare implications of 

relatively subtle effects of N enrichment 

in aquatic systems, such as shifts in 

phytoplankton species communities in 

remote alpine lakes, are not clear. 

Additionally, the public welfare 

implications of HNO3 effects on lichens 

(which might be considered to be 

‘‘direct’’ effects or the result of 

deposition onto plant surfaces) are also 

not clear and might depend on the 

extent to which they impact whole 

communities, other biota, or ecosystem 

structure and function (PA, section 

7.2.3.2). 

The effects of N enrichment in 

terrestrial ecosystems may vary with 

regard to public welfare implications. 

As noted above with regard to impacts 

of aquatic acidification, the PA 

recognizes that some level of N 

deposition and associated effects on 

terrestrial ecosystems can impact the 

public welfare and thus might 

reasonably be judged adverse to the 

public welfare. Depending on 

magnitude and the associated impacts, 

there are situations in which N 

deposition and associated nutrient 

enrichment-related impacts might 

reasonably be concluded to be 

significant to the public welfare, such as 

N deposition that alters forest ecosystem 

community structures in ways that 

appreciably affect use and enjoyment of 

those areas by the public (PA, section 

7.2.3.2). A complication to 

consideration of public welfare 

implications that is specific to N 

deposition in terrestrial systems is its 

potential to increase growth and yield of 

plants that, depending on the plant and 

its use by human populations (e.g., trees 

for lumber, food for livestock or human 

populations), may be considered 

beneficial to the public. Nitrogen 

enrichment in natural ecosystems can, 

by increasing growth of N limited plant 

species, change competitive advantages 

of species in a community, with 

associated impacts on the composition 

of the ecosystem’s plant community. 

The public welfare implications of such 

effects may vary depending on their 

severity, prevalence or magnitude. For 

example, only those rising to a 

particular severity (e.g., with associated 

significant impact on key ecosystem 

functions or other services), magnitude 

or prevalence may be considered of 

public welfare significance (PA, section 

7.2.3.2). 

(a) Quantitative Information for 

Ecosystem Risks Associated With N 

Deposition 

The PA considers the available 

information regarding air quality and 

atmospheric deposition and risk or 

likelihood of occurrence of ecosystem 

effects under differing conditions. In so 

doing, the PA notes the varying 

directionality of some of the N 

enrichment-related effects in terrestrial 

ecosystems, such that some effects can, 

in particular ecosystems and for 

particular species, seem beneficial (e.g., 

to growth or survival of those species), 

although in a multispecies system, 

effects are more complex with potential 

for alteration of community 

composition. The information is also 

considered with regard to the key 

limitations and associated uncertainties 

of this evidence. 

Beginning with the appreciable 

evidence base documenting assessments 

of N loading to waterbodies across the 

U.S., the PA notes the waterbodyspecific nature of such responses and 

the relative role played by atmospheric 

deposition, among other N sources. For 

example, the relative contribution to 

such loading from atmospheric 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105737 

deposition compared to other sources 

(e.g., agricultural runoff and wastewater 

discharges) varies among waterbody 

types and locations, which can be a 

complicating factor in quantitative 

analyses. Additionally, characteristics of 

resident biota populations and other 

environmental factors are influential in 

waterbody responses to N loading, e.g., 

temperature, organic microbial 

community structure, and aquatic 

habitat type, among others (ISA, 

Appendix 7). Based on identification of 

eutrophication as a factor in impacts on 

important fisheries in some estuaries 

across the U.S., multiple government 

and nongovernment organizations have 

engaged in research and water quality 

management activities over the past 

several decades in large and small 

estuaries and coastal waters across the 

U.S. These activities have generally 

involved quantitative modeling of 

relationships between N loading and 

water quality parameters such as 

dissolved oxygen (ISA, Appendix 7, 

section 7.2). This research documents 

both the impacts of N enrichment in 

these waterbodies and the relationships 

between effects on waterbody biota, 

ecosystem processes and functions, and 

N loading (PA, section 5.2.3). The 

evidence base recognizes N loading to 

have contributions from multiple types 

of sources to these large waterbodies 

and their associated watersheds, 

including surface and ground water 

discharges, as well as atmospheric 

deposition. Accordingly, loading targets 

or reduction targets identified for these 

systems have generally been identified 

in light of policy and management 

considerations related to the different 

source types, as discussed further in 

section II.B.1.(4)(b) below. 

Focused assessments in freshwater 

lakes, including alpine lakes, where 

atmospheric deposition may be the 

dominant or only source of N loading, 

also provide evidence linking N loading 

with seemingly subtle changes, such as 

whether P or N is the nutrient limiting 

phytoplankton growth (and 

productivity) and shifts in 

phytoplankton community composition 

(PA, section 5.2.2); public welfare 

implications of such changes are less 

clear (PA, section 7.2.3.1). 

With regard to terrestrial ecosystems 

and effects on trees and other plants, the 

PA recognizes the complexity, 

referenced above, that poses challenges 

to approaches for simulating terrestrial 

ecosystem responses to N deposition 

across areas diverse in geography, 

geology, native vegetation, deposition 

history, and site-specific aspects of 

other environmental characteristics. In 

its consideration of the different types of 

quantitative analysis, the PA recognizes 

limitations particular to each and 

associated uncertainties. Uncertainties 

associated with the soil acidification 

modeling analyses in the last review 

include those associated with the 

limited dataset of laboratory-generated 

data on which the BC:Al targets are 

based, as well as the steady-state 

modeling parameters, most prominently 

those related to base cation weathering 

and acid-neutralizing capacity (PA, 

section 5.3.4.1). Uncertainties associated 

with experimental addition analyses 

include the extent to which the studies 

reflect steady-state conditions, as well 

as a lack of information regarding 

historic deposition at the study 

locations (PA, section 5.3.4.1). Several 

aspects of observational or gradient 

studies of tree growth and survival (or 

of species richness for herbs, shrubs and 

lichens) contribute uncertainties to 

identification of deposition levels of 

potential concern for tree species 

effects, including unaccounted-for 

factors with potential influence on tree 

growth and survival (e.g., ozone and soil 

characteristics), as well as the extent to 

which associations may reflect the 

influence of historical deposition 

patterns and associated impact. Thus, 

while the evidence is robust as to 

ecological effects of ecosystem N 

loading, a variety of factors, including 

the history of deposition and variability 

of response across the landscape, 

complicate our ability to quantitatively 

relate specific N deposition rates, 

associated with various air quality 

conditions, to N enrichment-related 

risks of harm to forests and other plant 

communities in areas across the U.S. 

(PA, section 5.3.4). 

(b) General Approach for Considering 

Public Welfare Protection 

In considering public welfare 

protection with regard to N enrichment, 

the PA notes, as an initial matter, that 

the effects of acidification on plant 

growth and survival, at the individual 

level, are generally directionally 

harmful, including reduced growth and 

survival. In contrast, the effects of N 

enrichment can, in particular 

ecosystems and for particular species, 

be beneficial or harmful (e.g., to growth 

or survival of those species). 

Accordingly, the PA recognizes added 

complexity to risk management policy 

decisions for this category of effects, 

including the lack of established risk 

management targets or objectives, 

particularly in light of historical 

deposition and its associated effects that 

have influenced the current status of 

terrestrial ecosystems and their biota, 

structure, and function. 

Further, the PA recognizes the 

complication posed by the contribution 

to N deposition of atmospheric 

pollutants other than the criteria 

pollutants N oxides and PM, most 

significantly the contribution of NH3

(PA, section 6.2.1). In light of the 

contrasting temporal trends for 

emissions of oxidized and reduced N 

compounds, the PA observes a declining 

influence of ambient air concentrations 

of N oxides and PM on N deposition 

over the past 20 years, complicating 

consideration of the protection from N 

deposition-related effects that can be 

provided by secondary NAAQS for 

these pollutants. This declining trend in 

N oxides emissions and associated 

oxidized N deposition coincides with 

increases in NH3 emissions and 

deposition of reduced N compounds, 

such that reduced N deposition has 

generally been more than half of total N 

deposition at CASTNET sites since 2015 

(PA, Figures 6–3, 6–17, 6–18 and 6–19). 

In 2021, estimated dry deposition of 

NH3 was as much as 65% of total N 

deposition across the 92 CASTNET sites 

(PA, Figure 6–19). At 25% of the 

CASTNET sites, more than 30% of N 

deposition is from dry deposition of 

NH3 (PA, Figure 6–19), a noteworthy 

observation given the preponderance of 

CASTNET sites in the West and 

relatively few in the areas of highest 

NH3 emissions where the percentage 

would be expected to be higher still 

(PA, Figures 2–9 and 2–17). In light of 

this information, the PA finds that NH3, 

which is not a criteria pollutant, and its 

contribution to total N deposition, 

particularly in parts of the U.S. where 

N deposition is highest, are 

complicating factors in considering 

policy options related to NAAQS for 

addressing ecological effects related to 

N deposition (e.g., PA, Figure 6–18 and 

6–13). 

In considering what the currently 

available quantitative information 

regarding terrestrial ecosystem 

responses to N deposition indicates 

about levels of N deposition that may be 

associated with increased concern for 

adverse effects, the PA focuses first on 

the evidence for effects of N deposition 

on trees that is derived from 

experimental addition studies and 

observational studies of potential 

relationships between tree growth and 

survival and metrics for N deposition. 

With regard to the experimental 

addition studies, while recognizing 

study limitations and associated 

uncertainties, the PA notes that the 

lowest N addition that elicited forest 

effects was 15 kg/ha-yr over the 14 years 

from 1988 to 2002 (PA, sections 5.3.2 


khammond on DSK9W7S144PROD with RULES2
105738 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

81The largest study reported associations of tree 

survival and growth with N deposition that varied 

from positive to negative across the range of 

deposition at the measurement plots for some 

species, and also varied among species (PA, section 

5.3.2, Appendix 5B, section 5B.3.2.3; Horn et al., 

2018). Among the species for which the association 

varied from negative to positive across deposition 

levels, this is the range for those species for which 

the association was negative at the median 

deposition value (PA, section 5.3.4). This also 

excluded species for which sample sites were 

limited to the western U.S. based on recognition by 

the study authors of greater uncertainty in the west 

(Horn et al., 2018). 

and 7.2.3.2 and Appendix 5B, Table 5B– 

1; McNulty et al., 2005). Based on the 

estimates from several observational 

studies, the PA observed that N 

deposition ranging from 7 to 12 kg/hayr, on a large area basis, reflects 

conditions for which statistical 

associations have been reported for 

terrestrial effects, such as reduced tree 

growth and survival.81 (PA, sections 

5.3.4 and 7.2.3.2). 

With regard to studies of herb and 

shrub community metrics, the PA 

considered several recently available 

addition experiments, recent gradient 

studies of coastal sage scrub in southern 

California, and a larger observational 

study of herb and shrub species richness 

in open- and closed-canopy 

communities. As summarized in section 

II.A.3.c.(2)(b) above, N deposition 

estimates ranging from 6.5 kg/ha-yr to 

11.6 kg/ha-yr were identified from these 

studies as reflecting conditions for 

which statistical associations have 

indicated potential for effects in herb 

and shrub communities (PA, section 

5.3.3.1 and Appendix 5B, sections 

5B.3.1 and 5B.3.2; Cox et al., 2014; Fenn 

et al., 2010). Lastly, the PA notes the 

observational studies that have analyzed 

variation in lichen community 

composition in relation to indicators of 

N deposition, but recognize limitations 

with regard to interpretation, as well as 

uncertainties such as alternate methods 

for utilizing N deposition estimates as 

well as the potential influence of 

unaccounted-for environmental factors, 

e.g., ozone, SO2, and historical air 

quality and associated deposition (PA, 

section 5.3.3.2 and Appendix 5B, 

section 5B.4.2). 

With regard to the evidence for effects 

of N deposition in aquatic ecosystems, 

the PA recognizes several different types 

of information including the 

observational studies utilizing statistical 

modeling to estimate critical loads, such 

as those related to subtle shifts in the 

composition of phytoplankton species 

communities in western lakes. There are 

also many decades of research on the 

impacts and causes of eutrophication in 

large rivers and estuaries. As noted 

above, the public attention, including 

government expenditures, that has been 

given to N loading and eutrophication 

in multiple estuarine and coastal 

systems are indicative of the recognized 

public welfare implications of related 

impacts. In large aquatic systems across 

the U.S., the relationship between N 

loading and algal blooms, and 

associated water quality impacts (both 

short- and longer-term), has led to 

numerous water quality modeling 

projects to inform water quality 

management decision-making in 

multiple estuaries, including the 

Chesapeake Bay, Narraganset Bay, 

Tampa Bay, Neuse River Estuary and 

Waquoit Bay (ISA, Appendix 7, section 

7.2). These projects often use indicators 

of nutrient enrichment, such as 

chlorophyll a, dissolved oxygen, and 

abundance of submerged aquatic 

vegetation (ISA, section IS.7.3 and 

Appendix 10, section 10.6). For these 

estuaries, the available information 

regarding atmospheric deposition and 

the establishment of associated target 

loads varies across estuaries (ISA, 

Appendix 7, Table 7–9), and in many 

cases atmospheric loading has 

decreased since the initial modeling 

analyses. 

As summarized in section II.A.3.c.(1) 

above, analyses in multiple East Coast 

estuaries—including the Chesapeake 

Bay, Tampa Bay, Neuse River Estuary 

and Waquoit Bay—have addressed 

atmospheric deposition as a source of N 

loading (ISA, Appendix 7, section 

7.2.1). Total estuary loading or loading 

reductions were established in TMDLs 

developed under the Clean Water Act 

for these estuaries. Levels identified for 

allocation of atmospheric N loading in 

the first three of these estuaries were 

6.1, 11.8 and 6.9 kg/ha-yr, respectively, 

and atmospheric loading estimated to be 

occurring in the fourth was below 5 kg/ 

ha-yr (PA, section 7.3). This 

information, combined with the 

information from terrestrial studies 

summarized above, led to the PA 

identifying 7–12 kg/ha-yr as an 

appropriate N deposition range on 

which to focus in considering policy 

options (PA, section 7.2.3.2). 

(c) Relating Air Quality Metrics to N 

Deposition Associated With N Oxides 

and PM 

In exploring how well various air 

quality metrics relate to N deposition, 

the PA finds the analyses utilizing data 

from monitors using FRM/FEM to 

collect ambient air concentration data 

for evaluation with the NAAQS (e.g., to 

identify violations) to be particularly 

relevant given that the current standards 

are judged using design values derived 

from FRM/FEM measurements at 

existing SLAMS (PA, section 7.2.3.3). 

Given their role in monitoring for 

compliance with the NAAQS, most or 

many of these monitors are located in 

areas of relatively higher pollutant 

concentrations, such as near large 

sources of NO2 or PM. Accordingly, the 

PA recognized the information from 

these monitoring sites as having 

potential for informing how changes in 

NO2 and/or PM emissions, reflected in 

ambient air concentrations, may relate 

to changes in deposition and, 

correspondingly, for informing 

consideration of secondary standard 

options that might best regulate ambient 

air concentrations such that deposition 

in sensitive ecosystems of interest is 

maintained at or below levels of 

potential concern. 

In considering the information and 

findings of these analyses of N 

deposition and N oxides and PM in 

ambient air, the PA notes, as an initial 

matter, that relationships between N 

deposition and NO2 and PM air quality 

are affected by NH3 emissions and nonN-containing components of PM (PA, 

section 6.4.2). The PA further notes that 

the influence of these factors on the 

relationships has varied across the 20- 

year evaluation period and varies across 

different regions of the U.S. (PA, section 

6.2.1). Both factors (NH3 emissions and 

non-N-containing components of PM) 

are recognized to influence 

relationships between total N deposition 

and NO2 and PM air quality metrics. For 

example, for total N deposition 

estimated for TDep grid cells with 

collocated SLAMS monitors, the 

correlations with annual average NO2

concentrations, averaged over three 

years, are generally low across all sites 

and particularly in the East (PA, Table 

6–6). This likely reflects the relatively 

greater role of NH3 in N deposition in 

the East, which for purposes of the 

analyses in this PA extends across the 

Midwest (PA, section 6.4.2). The 

correlation between estimates of total N 

deposition in eastern ecoregions and 

annual average NO2 concentrations at 

upwind monitor sites of influence for 

the five periods from 2001–2020 is low 

to moderate, with the earlier part of the 

20-year period, when NO2

concentrations were higher and NH3

emissions were lower (as indicated by 

Figures 6–6 and 6–5 of the PA), having 

relatively higher correlation than the 

later part (e.g., correlation coefficients 

below 0.4, except for EAQM-weighted 

in 2001–03 [PA, Table 6–10]). The 

correlation is negative or near zero for 

the western ecoregions (PA, section 

6.2.4). 

Based on the decreasing trends in NO2

emissions and oxidized N deposition in 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105739 

the past 10 years, and coincident trend 

of increased NH3 emissions and 

deposition of reduced N (NH3 and 

NH4

+), most particularly in areas of the 

Midwest, Texas, Florida and North 

Carolina (PA, Figures 6–16 and 6–17), 

the PA finds NO2 emissions to have 

much less influence on total N 

deposition now than in the past (PA, 

sections 6.2.1 and 6.4). In terms of 

ecoregion median statistics, the PA 

observes the decreasing trend in 

ecoregion median total N deposition 

across the period from 2001 through 

2012, while taking note that from 2012 

onward, total N deposition increases, 

most particularly in ecoregions where 

most of the total deposition is from 

reduced N (PA, Figure 7–6). The PA also 

considers the impact of increasing 

deposition of reduced N on the 20-year 

trend in total N deposition as illustrated 

by TDep estimates at the 92 CASTNET 

sites. At these sites, the median 

percentage of total N deposition 

comprised by oxidized N species, which 

is driven predominantly by N oxides, 

has declined from more than 70% to 

less than 45% (PA, Figure 6–19). Based 

on examination of the trends for 

components of reduced N deposition, 

the PA notes that the greatest influence 

on the parallel increase in N deposition 

percentage composed of reduced N is 

the increasing role of NH3 dry 

deposition. The percentage of total N 

deposition at the CASTNET sites that is 

from NH3 has increased, from a median 

below 10% in 2000 to a median 

somewhat above 25% in 2021 (PA, 

Figure 6–19). 

Recognizing limitations in the extent 

to which CASTNET sites can provide 

information representative of the U.S. as 

a whole, the PA also analyzed TDep 

estimates across the U.S. for the most 

recent period assessed (2018–2020). In 

areas with ecoregion median total N 

deposition above 9 kg/ha-yr (PA, Figure 

7–7, upper panel), the ecoregion median 

percentage of total N deposition 

composed of reduced N is greater than 

60% (PA, Figure 7–7, lower panel). The 

2019–2021 TDep estimates across 

individual TDep grid cells similarly 

show that the areas of the U.S. where 

total N deposition is highest and greater 

than potential N deposition targets 

(identified in section 7.2.3.2 of the PA) 

are also the areas with the greatest 

deposition of NH3 (PA, Figure 7–8), 

comprising more than 30% of total N 

deposition. That is, the PA finds that 

NH3 driven deposition is greatest in 

regions of the U.S. where total 

deposition is greatest (PA, section 

7.2.3.3). 

Turning to PM2.5, the PA notes that 

the correlation for ecoregion median N 

deposition and PM2.5 concentrations at 

upwind sites of influence is poor and 

negative or moderate (r=0.45) depending 

on the metric (PA, section 6.2.4). For 

total N deposition and PM2.5

concentrations at SLAMS, a low to 

moderate correlation is observed (PA, 

section 6.2.3). In considering NH3

emissions and non-N containing 

components of PM, the PA notes that 

some NH3 transforms to NH4

∂, which is 

a component of PM2.5, while also noting 

that, in the areas of greatest N 

deposition, the portion represented by 

deposition of gaseous NH3 generally 

exceeds 30%. Additionally, while NH3

emissions have been increasing over the 

past 20 years, the proportion of PM2.5

that is composed of N compounds has 

declined. The median percentage of 

PM2.5 comprised by N compounds has 

declined from about 25% in 2006–2008 

to about 17% in 2020–2022 and the 

highest percentage across sites declined 

from over 50% to 30% (PA, section 

6.4.2 and Figure 6–56). Further, the 

percentages vary regionally, with sites 

in the nine southeast states having less 

than 10% of PM2.5 mass composed of N 

compounds (PA, Figure 6–56). 

In summary, the PA concludes that in 

recent years, NH3 contributes 

appreciably to total N deposition, 

particularly in parts of the country 

where N deposition is highest (as 

illustrated by comparison of Figures 6– 

13 and 6–18 of the PA). The PA finds 

that this situation—of an increasing, and 

spatially variable, portion of N 

deposition not being derived from N 

oxides or PM—complicates assessment 

of policy options for protection against 

ecological effects related to N deposition 

associated with N oxides and PM, and 

for secondary standards for those 

pollutants that may be associated with 

a desired level of welfare protection. 

The PA recognizes that the available 

information as a whole also suggests the 

potential for future reductions in N 

oxide-related N deposition to be negated 

by increasing reduced N deposition. 

Further, the PA notes that the results 

also suggest that while the PM2.5 annual 

average standard may provide some 

control of N deposition associated with 

PM and N oxides, PM2.5 monitors also 

capture other non-S and non-N related 

pollutants (e.g., organic and elemental 

carbon) as part of the PM2.5 mass (PA, 

section 7.2.3.3). The amounts of each 

category of compounds vary regionally 

(and seasonally), and as noted above, N 

compounds generally comprise less 

than 30% of total PM2.5 mass (PA, 

section 6.3 and 6.4). 

In considering relationships between 

air quality metrics based on indicators 

other than those of the existing 

standards and N deposition (and 

associated uncertainties), the PA drew 

on the analyses of relationships for 

collocated measurements and modeled 

estimates of N compounds other than 

NO2 with N deposition in a subset of 27 

CASTNET sites located in 27 Class I 

areas, the majority of which (21 of 27) 

are located in the western U.S. (PA, 

sections 6.2.2, 6.3 and 6.4.2). The 

analyses indicate that total N deposition 

in these rural areas has a moderate 

correlation with air concentrations of 

nitric acid and particulate nitrate for the 

20-year dataset (2000–2020) (PA, Figure 

6–32). The correlations are comparable 

to the correlation of NO2 with total N 

deposition at western SLAMS, a not 

unexpected observation given that more 

than 75% of the 27 CASTNET sites are 

in the West. A much lower correlation 

was observed at SLAMS in the East, and 

with the trajectory-based dataset. The 

PA notes that deposition at the western 

U.S sites is generally less affected by 

NH3 (PA, section 6.4.2). Further, the 

observed trend of increasing 

contribution to N deposition of NH3

emissions over the past decade suggests 

that such correlations of N deposition 

with oxidized N may be still further 

reduced in the future. Thus, the PA 

concludes that the evidence does not 

provide support for the oxidized N 

compounds (as analyzed at the 27 Class 

I sites) as indicators of total atmospheric 

N deposition, especially in areas where 

NH3 is prevalent (PA, section 7.2.3.3). 

The analyses involving N deposition 

and N-containing PM components at the 

27 Class I area sites do not yield higher 

correlation coefficients than those for N 

deposition (TDep) and PM2.5 at SLAMS 

monitors (PA, section 7.2.3.3 and 

Figures 6–33, 6–39 [upper panel], and 

6–32 [left panel]). Further, the graphs of 

total N deposition estimates versus total 

particulate N in ambient air at the 27 

Class I area sites indicate the calculated 

correlations (and slopes) likely to be 

appreciably influenced by the higher 

concentrations occurring in the first 

decade of the 20-year timeframe (PA, 

Figure 6–33). Thus, the PA concludes 

that the available analyses of Ncontaining PM2.5 components at the 

small dataset of sites remote from 

sources also do not indicate an overall 

benefit or advantage of N-containing 

PM2.5 components over consideration of 

PM2.5 (PA, section 7.4). As a whole, the 

PA finds that the limited dataset with 

varying analytical methods and monitor 

locations, generally distant from 

sources, does not clearly support a 

conclusion that such alternative 

indicators might provide better control 

of N deposition related to N oxides and 


khammond on DSK9W7S144PROD with RULES2
105740 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

82For example, sampling challenges have long 

been recognized for particulate NH4

∂ (e.g., ISA, 

Appendix 2, sections 2.4.5; 2008 ISA, section 2.7.3). 

83Although the CASAC letter does not specify the 

form for such a new annual standard, the 

justification provided for this recommendation cites 

two figures in the draft PA (Figures 6–17 and 6–18) 

which presented annual average SO2 concentrations 

averaged over three consecutive years (Sheppard, 

2023, Response to Charge Questions, p. 25). 

Therefore, we are interpreting the CASAC majority 

recommendation to be for an annual standard, 

averaged over three years. 

84Although the CASAC letter does not specify the 

statistic for the 5 kg/ha-yr value, the draft PA 

analyses referenced in citing that value, both the 

trajectory analyses and the ecoregion-scale 

summary of aquatic acidification results, focus on 

ecoregion medians. Further, the draft PA 

presentations of ecoregion percentages of 

waterbodies achieving the three ANC targets were 

for bins at or below specific deposition values (e.g., 

‘‘at/below’’ 5, 6 or 7 kg/ha-yr [draft PA, table 5–4]). 

Therefore, we are interpreting the CASAC advice on 

this point to pertain to ecoregion median at or 

below 5 kg/ha-yr. 

85 In making this statement, these CASAC 

members cite two observational data studies with 

national-scale study areas published after the 

literature cut-off date for the ISA: one study is on 

lichen species richness and abundance and the 

second is on tree growth and mortality (Geiser et 

al., 2019; Pavlovic et al., 2023). The lichen study 

by Geiser et al. (2019) relies on lichen community 

surveys conducted at U.S. Forest Service sites from 

1990 to 2012. The tree study by Pavlovic et al. 

(2023) uses machine learning models with the 

dataset from the observational study by Horn et al. 

(2018) to estimate confidence intervals for CLs for 

growth and survival for 108 species based on the 

dataset first analyzed by Horn et al. (2018). 

86As seen in tables 3 and 4 in this preamble, 

these levels of protection are also achieved in 

ecoregion-time period combinations for which the 

ecoregion median S deposition estimate is at or 

below 7 kg/ha-yr (PA, section 7.2.2.2 and Table 7– 

1). 

87The figure cited by the CASAC majority is the 

prior version of Figure 2–28 in section 2.4.2 of the 

final PA. The figure presents temporal trend in 

distribution (box and whiskers) of annual average 

SO2 concentrations since 2000 at SLAMS. 

88Also dissenting from this advice was a member 

of the CASAC Oxides of Nitrogen, Oxides of Sulfur 

and Particulate Matter Secondary NAAQS Panel 

who was not also a member of the CASAC 

(Sheppard, 2023, Response to Charge Questions, p. 

23). The former is a Panel formed for this review, 

while the latter is the standing Committee specified 

in the CAA. 

89This member stated that the existing primary 

NAAQS for the three pollutants were significantly 

more restrictive than the existing secondary 

standards and provide adequate protection for 

deposition-related effects (Sheppard, 2023, 

Appendix A). 

90The weighted metric is constructed by applying 

weighting to concentrations to the monitors 

identified as sites of influence, with the weighting 

equal to the relative contribution of air from the 

monitor location to the downwind ecoregion based 

on the trajectory analysis (PA, section 6.2.4). Values 

of this metric are not directly translatable to 

individual monitor concentrations or to potential 

standard levels. 

91The metric for N deposition in these analyses 

is the median of the TDep estimates across each 

ecoregion (PA, section 6.2.4). 

PM over those used for the existing 

standards (PA, section 7.2.3.3). The PA 

also notes that use of the NO3

¥ or 

particulate N measurements analyzed 

with deposition estimates at the 27 

Class I area sites, alone or in 

combination with NO2, as an indicator 

for a new standard would entail 

development of sample collection and 

analysis FRM/FEMs 82 and of a 

surveillance network. 

b. CASAC Advice 

The CASAC provided advice and 

recommendations regarding the 

standards review based on the CASAC’s 

review of the draft PA. In the letter 

conveying its advice, the CASAC first 

recognized that ‘‘translation of 

deposition-based effects to an ambient 

concentration in air is fraught with 

difficulties and complexities’’ 

(Sheppard, 2023, pp. 1–2). Further, the 

CASAC expressed its view that, based 

on its interpretation of the CAA, 

NAAQS could be in terms of 

atmospheric deposition, which it 

concluded ‘‘would be a cleaner, more 

scientifically defensible approach to 

standard setting.’’ Accordingly, the 

CASAC recommended that direct 

atmospheric deposition standards be 

considered in future reviews (Sheppard, 

2023, pp. 2 and 5). The CASAC then, as 

summarized below, provided 

recommendations regarding standards 

based on air concentrations. 

With regard to protection from effects 

other than those associated with 

ecosystem deposition of S and N 

compounds, the CASAC concluded that 

the existing SO2 and NO2 secondary 

standards provide adequate protection 

for direct effects of those pollutants on 

plants and lichens, providing consensus 

recommendations that these standards 

should be retained without revision for 

this purpose (Sheppard, 2023, p. 5 of 

letter and p. 23 of Response to Charge 

Questions). With regard to depositionrelated effects of S and N compounds, 

the CASAC members did not reach 

consensus, with their advice divided 

between a majority opinion and a 

minority opinion. Advice conveyed 

from both the majority and minority 

groups of members concerning 

deposition-related effects is summarized 

here. 

With regard to deposition-related 

effects of S and standards for SOX, the 

majority of CASAC members 

recommended a new annual SO2

standard with a level in the range of 10 

to 15 ppb,83 which these members 

concluded would generally maintain 

ecoregion median S deposition below 5 

kg/ha-yr 84 based on consideration of the 

trajectory-based SO2 analyses (and 

associated figures) in the draft PA 

(Sheppard, 2023, Response to Charge 

Questions, p. 25). They concluded that 

such a level of S deposition would 

afford protection for tree and lichen 

species 85 and aquatic ecosystems. 

Regarding aquatic ecosystems, these 

members cited the ecoregion-scale 

estimates (from the aquatic acidification 

REA analyses) associated with median S 

deposition bins for the 90 ecoregiontime period combinations (PA, section 

5.1.3.2) in conveying that for S 

deposition below 5 kg/ha-yr, 80%, 80% 

and 70% of waterbodies per ecoregion 

are estimated to achieve an ANC at or 

above 20, 30 and 50 meq/L, respectively, 

in all ecoregion-time period 

combinations (Sheppard, 2023, 

Response to Charge Questions, p. 25).86

In recommending an annual SO2

standard with a level in the range of 10 

to 15 ppb, these members stated that 

such a standard would ‘‘preclude the 

possibility of returning to deleterious 

deposition values as observed 

associated with the emergence of high 

annual average SO2 concentrations near 

industrial sources in 2019, 2020, and 

2021,’’ citing Figure 2–25 of the draft 

PA 87 (Sheppard, 2023, Response to 

Charge Questions, p. 24). 

One CASAC member dissented from 

this recommendation for an annual SO2

standard 88 and instead recommended 

adoption of a new 1-hour SO2 secondary 

standard identical in form, averaging 

time, and level to the existing primary 

standard based on the conclusion that 

the ecoregion 3-year average S 

deposition estimates for the most recent 

periods are generally below 5 kg/ha-yr 

and that those periods correspond to the 

timing of implementation of the existing 

primary SO2 standard (established in 

2010), indicating the more recent lower 

deposition to be a product of current 

regulatory requirements (Sheppard, 

2023, Appendix A, p. A–2).89

With regard to N oxides and 

protection against deposition-related 

welfare effects of N, the majority of 

CASAC members recommended 

revision of the existing annual NO2

standard to a level ‘‘<10–20 ppb’’ 

(Sheppard, 2023, Response to Charge 

Questions, p.24). The justification these 

members provided was related to their 

consideration of the relationship 

presented in the draft PA of median 

ecosystem N deposition with the 

weighted 90 annual average NO2 metric 

concentrations, averaged over three 

years, at monitoring sites linked to the 

ecosystems by trajectory-based analyses 

and a focus on total N deposition 

estimates at or below 10 kg/ha-yr 91

(Sheppard, 2023, Response to Charge 

Questions, p. 24). These members 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105741 

92The CASAC letter states that the Neuse River 

Estuary TMDL specified a 30% reduction from the 

1991–1995 loading estimate of 9.8 kg/ha-yr, 

yielding a remaining atmospheric load target of 6.9 

kg/ha-yr (Sheppard, 2023, Response to Charge 

Questions, p. 13). 

93We note, however, that the design value figure 

cited by these members indicate California sites to 

have design values as high as 17.8 mg/m3, i.e., 

violating the current PM2.5 secondary standard 

(draft PA, Figure 2–27; PA, Figure 2–31). 

94As noted earlier in this section, weighted 

EAQM values are not directly translatable to 

concentrations at individual monitors or to 

potential standard levels. 

95 Deciviews, units derived from light extinction, 

are frequently used in the scientific and regulatory 

literature to assess visibility (U.S. EPA 2019, section 

13.2). 

additionally recognized, however, that 

‘‘when considering all ecoregions, there 

is no correlation between annual 

average NO2 and N deposition’’ 

(Sheppard, 2023, Response to Charge 

Questions, p. 24). Their focus on total N 

deposition estimates at or below 10 kg/ 

ha-yr appears to relate to consideration 

of TMDL analyses in four East Coast 

estuaries: Chesapeake Bay, Tampa Bay, 

Neuse River Estuary and Waquoit Bay 

(Sheppard, 2023, Response to Charge 

Questions, pp. 12–14 and 29). Levels 

identified for allocation of atmospheric 

N loading in the first three of these 

estuaries were 6.1, 11.8 and 6.9 92 kg/hayr, respectively, and atmospheric 

loading estimated in the fourth was 

below 5 kg/ha-yr (Sheppard, 2023, 

Response to Charge Questions, pp. 12– 

14). These members also concluded that 

10 kg N/ha-yr is ‘‘at the middle to upper 

end of the N critical load threshold for 

numerous species effects (e.g., richness) 

and ecosystem effects (e.g., tree growth) 

in U.S. forests grasslands, deserts, and 

shrublands (e.g., Pardo et al., 2011; 

Simkin et al., 2016) and thus 10 kg N/ 

ha-yr provides a good benchmark for 

assessing the deposition-related effects 

of NO2 in ambient air’’ (Sheppard, 2023, 

Response to Charge Questions, p. 23). 

One CASAC member disagreed with 

revision of the existing annual NO2

standard and instead recommended 

adoption of a new 1-hour NO2

secondary standard identical in form, 

averaging time and level to the existing 

primary standard based on the 

conclusion that the N deposition 

estimates for the most recent periods 

generally reflect reduced deposition that 

is a product of current regulatory 

requirements, including implementation 

of the existing primary standards for 

NO2 and PM (Sheppard, 2023, 

Appendix A). This member additionally 

noted that bringing into attainment the 

areas still out of attainment with the 

2013 primary annual PM2.5 standard 

(12.0 mg/m3) will provide further 

reductions in N deposition. This 

member also noted his analysis of NO2

annual and 1-hour design values for the 

past 10 years (2013–2022) as indicating 

that the current primary NO2 standard 

provides protection for annual average 

NO2 concentrations below 31 ppb 

(Sheppard, 2023, Appendix A). 

With regard to PM and effects related 

to deposition of N and S, the CASAC 

focused on the PM2.5 standards and 

made no recommendations regarding 

the PM10 standard. In considering the 

annual PM2.5 standard, the majority of 

CASAC members recommended 

revision of the annual secondary PM2.5

standard to a level of 6 to 10 mg/m3. In 

their justification for this range, these 

members focus on rates of total N 

deposition at or below 10 kg/ha-yr and 

total S deposition at or below 5 kg/hayr that they state would ‘‘afford an 

adequate level of protection to several 

species and ecosystems across the U.S.’’ 

(Sheppard, 2023, Response to Charge 

Questions, p. 23). In reaching this 

conclusion for protection from N 

deposition, the CASAC majority cited 

studies of U.S. forests, grasslands, 

deserts and shrublands that are 

included in the ISA. For S deposition, 

the CASAC majority notes the Pavlovic 

et al. (2023) analysis of the dataset used 

by Horn et al. (2018). Conclusions of the 

latter study (Horn et al., 2018), which is 

characterized in the ISA and discussed 

in sections 5.3.2.3 and 7.2.2.2 of the PA 

(in noting median deposition of 5–12 kg 

S/ha-yr in ranges of species for which 

survival and/or growth was observed to 

be negatively associated with S 

deposition), are consistent with the 

more recent analysis in the 2023 

publication (ISA, Appendix 6, sections 

6.2.3 and 6.3.3). 

As justification for their 

recommended range of annual PM2.5

levels (6–10 mg/m3), this group of 

CASAC members provided several 

statements, without further explanation, 

regarding PM2.5 annual concentrations 

and estimates of S and N deposition for 

which they cited several figures in the 

draft PA. Citing figures in the draft PA 

with TDep deposition estimates and 

IMPROVE and CASTNET monitoring 

data, they stated that ‘‘[i]n remote areas, 

IMPROVE PM2.5 concentrations in the 

range of 2–8 mg/m3 for the periods 

2014–2016 and 2017–2019 correspond 

with total S deposition levels <5 kg/hayr (Figure 6–12), with levels generally 

below 3 kg/ha-yr, and with total N 

deposition levels ≤10 kg/ha-yr (Figure 

6–13)’’ (Sheppard, 2023, Response to 

Charge Questions, p. 23). With regard to 

S deposition, these members 

additionally cited a figure in the draft 

PA as indicating ecosystem median S 

deposition estimates at/below 5 kg/ha-yr 

occurring with PM2.5 EAQM-max values 

in the range of 6 to 12 mg/m3 (Sheppard, 

2023, Response to Charge Questions, pp. 

23–24). With regard to N deposition, 

these members additionally cited figures 

in the draft PA as indicating that areas 

of 2019–2021 total N deposition 

estimates greater than 15 kg/ha-yr (‘‘in 

California, the Midwest, and the East’’) 

correspond with areas where the annual 

PM2.5 design values for 2019–2021 range 

from 6 to 12 mg/m3,93 and other figures 

(based on trajectory analyses) as 

indicating ecosystem median N 

deposition estimates below 10 kg N/hayr occurring only with PM2.5 weighted 

EAQM values below 6 mg/m3,94 and 

PM2.5 EAQM-max values below 8 mg/m3

(Sheppard, 2023, Response to Charge 

Questions, pp. 23–24). The CASAC also 

noted the correlation coefficient for N 

deposition with the EAQM weighted 

metric (which was a moderate value of 

about 0.5), while also recognizing that 

the correlation coefficient for the 

EAQM-max was ‘‘minimal.’’ The bases 

for the N and S deposition levels 

targeted in this CASAC majority 

recommendation are described in the 

paragraphs earlier in this section. 

One CASAC member recommended 

revision of the annual secondary PM2.5

standard to a level of 12 mg/m3 based on 

his interpretation of figures in the draft 

PA that present S and N deposition 

estimates for five different 3-year time 

periods from 2001 to 2020. This member 

observed that these figures indicate 

ecoregion median S and N deposition 

estimates in the last 10 years below 5 

and 10 kg/ha-yr, respectively. This 

member concluded this to indicate that 

the 2013 primary annual PM2.5 standard 

of 12.0 mg/m3 provides adequate 

protection against long-term annual S 

and N deposition-related effects 

(Sheppard, 2023, Appendix A). 

Regarding the existing 24-hour PM2.5

secondary standard, the majority of 

CASAC members recommended 

revision of the level to 25 ug/m3 or 

revision of the indicator and level to 

deciviews 95 and 20 to 25, respectively 

(Sheppard, 2023, Response to Charge 

Questions, p 25). These members 

variously cited ‘‘seasonal variabilities’’ 

of ‘‘[e]cological sensitivities,’’ describing 

sensitive lichen species to be influenced 

by fog or cloud water from which they 

state S and N contributions to be highly 

episodic, and visibility impairment 

(Sheppard, 2023, Response to Charge 

Questions, p 25). These members did 

not provide further specificity regarding 

their reference to lichen species and fog 

or cloud water. With regard to visibility 

impairment, these members described 


khammond on DSK9W7S144PROD with RULES2
105742 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

96Protection from impairment of visibility effects 

was one of the welfare effects within the scope of 

the PM NAAQS reconsideration rather than the 

scope of this review (U.S. EPA, 2016, 2017). In that 

action, the Administrator proposed not to change 

the 24-hour secondary PM NAAQS for visibility 

protection and also solicited comment on revising 

the level of the current secondary 24-hour PM2.5

standard to a level as low as 25 mg/m3; in the final 

action, the Administrator concluded that the 

current secondary PM standards provide requisite 

protection against PM-related visibility effects and 

retained the existing standards without revision (88 

FR 5558, January 27, 2023; 89 FR 16202, March 6, 

2024). 

97Consideration of CASAC comments and areas 

of the PA in which revisions have been made 

between the draft and this final document are 

described in section 1.4 of the PA. 

98 During 2001–2003, the 90th percentile S 

deposition per ecoregion of sites assessed in the 

REA was at or above 15 kg/ha-yr in half of the 18 

eastern ecoregions and ranged up above 20 kg/hayr (figure 2). 

99This analysis excluded Hawaii where it is not 

uncommon for there to be high SO2 values in areas 

with recurring volcanic eruptions (PA, section 

2.4.2). 

100Aquatic acidification risk estimates for the 

2001–2020 deposition estimates in the eight 

western ecoregions indicated ANC levels achieving 

all three targets in at least 90% of all sites assessed 

in each ecoregion (PA, Table 5–4). Ecoregion 

median deposition estimates were at or below 2 kg/ 

ha-yr in all eight western ecoregions (PA, Table 5– 

3). 

the EPA solicitation of comments that 

occurred with the separate EPA action 

to reconsider the 2020 decision to retain 

the existing PM2.5 standards as the basis 

for their recommendations on the 

secondary 24-hr PM2.5 standard 

(Sheppard, 2023, Response to Charge 

Questions, p 25; 88 FR 5562–5663, 

January 27, 2023).96 One CASAC 

member dissented from this view and 

supported retention of the existing 

secondary 24-hr PM2.5 standard. 

Among the CASAC comments on the 

draft PA 97 was the comment that 

substantial new evidence has been 

published since development of the 

2020 ISA that supports changes to the 

draft PA conclusions on N deposition 

effects. Accordingly, in the final PA, a 

number of aspects of Chapters 4 and 5 

were revised from the draft PA; these 

changes took into account the 

information emphasized by the CASAC 

while also referring to the ISA and 

studies considered in it (PA, section 

7.3). More recent studies cited by the 

CASAC generally concerned effects 

described in the ISA based on studies 

available at that time. While the newer 

studies include additional analyses and 

datasets, the ISA and studies in it also 

generally support the main points raised 

and observations made by the CASAC 

(PA, section 7.3). 

c. Administrator’s Proposed 

Conclusions 

In reaching his proposed conclusions 

on the adequacy of the existing 

secondary standards for SOX, N oxides, 

and PM, and on what revisions or 

alternatives may be appropriate, the 

Administrator drew on the ISA 

conclusions regarding the weight of the 

evidence for both the direct effects of 

SOX, N oxides, and PM in ambient air 

and for effects associated with 

ecosystem deposition of N and S 

compounds, and associated areas of 

uncertainty; quantitative analyses of 

aquatic acidification risk and of air 

quality and deposition estimates, and 

associated limitations and uncertainties; 

staff evaluations of the evidence, 

exposure/risk information, and air 

quality information in the PA; CASAC 

advice; and public comments received 

by that time. The Administrator 

recognized the evidence of direct 

biological effects associated with 

elevated short-term concentrations of 

SOX and N oxides that formed the basis 

for the existing secondary SO2 and NO2

standards, the evidence of ecological 

effects of PM in ambient air, primarily 

associated with loading on vegetation 

surfaces, and also the extensive 

evidence of ecological effects associated 

with atmospheric deposition of N and S 

compounds into sensitive ecosystems. 

The Administrator also took note of the 

quantitative analyses and policy 

evaluations documented in the PA that, 

with CASAC advice, informed his 

judgments in reaching his proposed 

decisions in this review. 

With regard to the secondary standard 

for SOX and the adequacy of the existing 

standard for providing protection of the 

public welfare from direct effects on 

biota and from ecological effects related 

to ecosystem deposition of S 

compounds, the Administrator 

considered the evidence regarding 

direct effects, as described in the ISA 

and evaluated in the PA, which is 

focused on SO2. He took note of the PA 

finding that the evidence indicates SO2

concentrations associated with direct 

effects to be higher than those allowed 

by the existing SO2 secondary standard 

(PA sections 5.4.1, 7.1.1 and 7.4). 

Additionally, he took note of the 

CASAC unanimous conclusion that the 

existing standard provides protection 

from direct effects of SOX in ambient 

air, as summarized in section II.B.1.b. 

above. Based on all of these 

considerations, he judged the existing 

secondary SO2 standard to provide the 

needed protection from direct effects of 

SOX. 

The Administrator next considered 

the ISA findings for ecological effects 

related to ecosystem deposition of S 

compounds. He first recognized the 

long-standing evidence of the role of 

SOX in ecosystem acidification and 

related ecological effects. While he 

additionally noted the ISA 

determinations of causality for S 

deposition with two other categories of 

effects related to mercury methylation 

and sulfide phytotoxicity (ISA, Table 

ES–1; PA, section 4.4), he recognized 

that quantitative assessment tools and 

approaches are not well developed for 

ecological effects associated with 

atmospheric deposition of S other than 

ecosystem acidification (PA, section 

7.2.2.1). Accordingly, he gave primary 

attention to effects related to acidifying 

deposition, given the robust evidence 

base and available quantitative tools, as 

well as the longstanding recognition of 

impacts in acid-sensitive ecosystems 

across the U.S. In so doing, the 

Administrator focused on the findings 

of the aquatic acidification REA and 

related policy evaluations in the PA. 

The range of ecoregion deposition 

estimates across the contiguous U.S. 

analyzed during the 20-year period from 

2001 through 2020 extended up to as 

high as 20 kg S/ha-yr,98 and design 

values for the existing SO2 standard 

(second highest 3-hour average in a 

year), in all States except Hawaii,99 were 

below its current level of 500 ppb, and 

generally well below (PA, section 6.2.1). 

The Administrator took note of the 

aquatic acidification risk estimates that 

indicate that the pattern of S deposition, 

estimated to have occurred during 

periods when the existing standard was 

met (e.g., 2001–2003), is associated with 

20% to more than half of waterbody 

sites in each affected eastern 

ecoregion 100 being unable to achieve 

even the lowest of the three acid 

buffering capacity targets or benchmarks 

(ANC of 20 meq/L), and he judged such 

risks to be of public welfare 

significance. The Administrator also 

considered the advice from both the 

majority and the minority of CASAC 

that recommended adoption of a new 

SO2 standard for this purpose in light of 

conclusions that the existing standard 

did not provide such needed protection. 

Thus, based on the findings of the REA, 

associated policy evaluations in the PA 

with regard to S deposition and 

acidification-related effects in sensitive 

ecosystems, and in consideration of 

advice from the CASAC, the 

Administrator proposed to judge that 

the current SO2 secondary standard is 

not requisite to protect the public 

welfare from adverse effects associated 

with acidic deposition of S compounds 

in sensitive ecosystems. 

Having reached this proposed 

conclusion that the existing secondary 

SO2 standard does not provide the 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105743 

101While the final PA provides additional 

presentations of aquatic acidification risk estimates, 

including those at the ecoregion-scale, the estimates 

are unchanged from those in the draft PA (PA, 

section 5.1.3). 102The presentation of such percentages in the 

draft PA (reviewed by the CASAC) were specific to 

the 90 ecoregion-time period combinations for the 

18 eastern ecoregions. Inclusion of the 7 western 

ecoregions yields higher percentages, as more than 

90% of waterbodies in those ecoregions were 

estimated to achieve all three ANC concentration in 

all time periods (PA, Table 5–4). 

103Ecoregion median deposition was below 2 kg 

S/ha-yr in all 35 ecoregion-time period 

combinations for the eight western ecoregions (PA, 

Table 5–4). 

104The PA’s consideration of the case study 

analyses as well as the ecoregion-scale results for 

both the ecoregion-time period groups and the 

temporal perspectives indicated a range of S 

deposition below approximately 5 to 8 or 10 kg/hayr to be associated with a potential to achieve acid 

buffering capacity levels of interest in an 

appreciable portion of acid sensitive areas (PA, 

section 7.4). 

requisite protection of the public 

welfare from adverse S depositionrelated effects, most prominently those 

associated with aquatic acidification, 

the Administrator then considered 

options for a secondary standard that 

would provide the requisite protection 

from S deposition-related effects (i.e., a 

standard that is neither more nor less 

stringent than necessary, as discussed in 

section II.A. above). In so doing, the 

Administrator turned first to the policy 

evaluations and staff conclusions in the 

PA, and the quantitative analyses and 

information described in Chapter 5 of 

the PA, for purposes of identifying S 

deposition rates that might be judged to 

provide an appropriate level of public 

welfare protection from acidificationrelated effects. In this context, he took 

note of the PA focus on the aquatic 

acidification risk estimates and 

recognition of linkages between 

watershed soils and waterbody 

acidification, as well as terrestrial 

effects. He concurred with the PA view 

regarding such linkages and what they 

indicate with regard to the potential for 

a focus on protecting waterbodies from 

reduced acid buffering capacity (with 

ANC as the indicator) to also provide 

protection for watershed soils and 

terrestrial effects. Accordingly, he 

focused on the PA evaluation of the risk 

estimates in terms of waterbodies 

estimated to achieve the three acid 

buffering capacity benchmarks (20, 30 

and 50 meq/L). In so doing, he concurred 

with the PA consideration of the 

ecosystem-scale estimates as 

appropriate for his purposes in 

identifying conditions that provide the 

requisite protection of the public 

welfare. 

The Administrator gave particular 

attention to the findings of the aquatic 

acidification REA for the 18 wellstudied, acid-sensitive eastern 

ecoregions, and considered the PA 

evaluation of ecoregion median S 

deposition values at and below which 

the risk estimates indicated a high 

proportion of waterbodies in a high 

proportion of ecoregions would achieve 

ANC values at or above the three 

benchmarks (20, 30 and 50 meq/L), as 

summarized in Tables 7–1 and 5–5 of 

the PA. In so doing, he recognized a 

number of factors, as described in the 

PA, which contribute variability and 

uncertainty to waterbody estimates of 

ANC and to interpretation of 

acidification risk associated with 

different values of ANC (PA, section 

5.1.4 and Appendix 5A, section 5A.3). 

The Administrator additionally took 

note of the approach taken by the 

CASAC majority in considering the 

ecoregion-scale risk estimates. These 

members considered the summary of 

results for the ecoregion-scale analysis 

of ecoregion median deposition bins (in 

the draft PA) 101 and focused on the 

results with acid buffering capacity at or 

above the three ANC benchmarks in 

80% (for ANC of 20 and 30 meq/L) or 

70% (for ANC of 50 meq/L) of 

waterbodies in all ecoregion-time period 

combinations 102 (Sheppard, p. 25 of the 

Response to Charge Questions). As 

recognized in the PA, these results are 

observed for median S-deposition at or 

below 7 kg/ha-yr for all time periods for 

the 18 eastern ecoregions. When 

considering all 25 analyzed ecoregions, 

somewhat higher percentages are 

achieved (as seen in tables 4 and 5 

above).103 The Administrator 

additionally considered the PA 

evaluation of the temporal trend (or 

pattern) of ecoregion-scale risk estimates 

across the five time periods in relation 

to the declining S deposition estimates 

for those periods. Based on the PA 

observation of appreciably improved 

acid buffering capacity (i.e., increased 

ANC) estimates by the third time period 

(2010–2012), the PA focused on the REA 

risk and deposition estimates for this 

and subsequent periods. By 2010–2012, 

ecoregion median S deposition (across 

CL sites) ranged from 2.3 to 7.3 kg/hayr in the 18 eastern ecoregions (with the 

highest ecoregion 90th percentile at 

approximately 8 kg/ha-yr) and more 

than 70% of waterbodies per ecoregion 

were estimated to be able to achieve an 

ANC of 50 meq/L in all 25 ecoregions, 

and more than 80% of waterbodies per 

ecoregion in all ecoregions were 

estimated to be able to achieve an ANC 

of 20 meq/L (table 5 and figures 1 and 

2 above). The Administrator observed 

that these estimates of acid buffering 

capacity achievement for the 2010–12 

period deposition—achieving the ANC 

benchmarks in at least 70% to 80% 

(depending on the specific benchmark) 

of waterbodies per ecoregion—are 

consistent with the objectives identified 

by the CASAC majority (in considering 

estimates from the ecoregion-scale 

analysis). By the 2014–2016 period, 

when deposition estimates were 

somewhat lower, the ANC benchmarks 

were estimated to be achieved in 80% 

to 90% of waterbodies per ecoregion. In 

his consideration of these ANC 

achievement percentages identified by 

the CASAC majority, while noting the 

variation across the U.S. waterbodies 

with regard to site-specific factors that 

affect acid buffering (as summarized in 

sections II.A.3.a.(2) and II.A.4. above 

and section 5.1.4 of the PA), the 

Administrator concurred with the PA 

conclusion on considering ecoregionscale ANC achievement results of 70% 

to 80% and 80% to 90% with regard to 

acid buffering capacity objectives for the 

purposes of protecting ecoregions from 

aquatic acidification risk of a magnitude 

with potential to be considered of 

public welfare significance. 

With regard to the variation in 

deposition across areas within 

ecoregions, the Administrator noted the 

PA observation that the sites estimated 

to receive the higher levels of deposition 

are those most influencing the extent to 

which the potential objectives for 

aquatic acidification protection are or 

are not met. He further noted the PA 

observation of an appreciable reduction 

across the 20-year analysis period in the 

90th percentile deposition estimates, as 

well as the median, for REA sites in the 

25 ecoregions analyzed (figure 2 above). 

In this context, the Administrator took 

note of the PA findings that the 

ecoregion-scale acid buffering objectives 

identified by the CASAC (more than 

70% to 80% of waterbody sites in all 

ecoregions assessed achieving or 

exceeding the set of ANC benchmarks) 

might be expected to be met when 

ecoregion median and upper (90th) 

percentile deposition estimates at 

sensitive ecoregions are generally at and 

below about 5 to 8 kg/ha-yr. He also 

took note of the PA identification of 

deposition rates at and below about 5 to 

8 or 10 kg/ha-yr 104 as associated with a 

potential to achieve acid buffering 

capacity benchmarks in an appreciable 

portion of acid sensitive areas based on 

consideration of uncertainties 

associated with the deposition estimates 

and associated aquatic acidification risk 

estimates at individual waterbody sites 


khammond on DSK9W7S144PROD with RULES2
105744 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

105A 3-year form, common to recently adopted 

NAAQS, provides a desired stability to the air 

quality management programs which is considered 

to contribute to improved public health and welfare 

protection (e.g., 78 FR 3198, January 15, 2013; 80 

FR 65352, October 26, 2015; 85 FR 87267, 

December 31, 2020). 

106As recognized in section II.B.1.a. above, the 

trajectory analyses relate contributions from 

individual monitor locations to deposition in 

receiving ecosystems (without explicitly addressing 

the multiple factors at play), with the somewhat 

higher correlations of the EAQM-weighted than the 

EAQM-max metric likely reflecting the weighting of 

concentrations across multiple upwind monitors to 

represent relative loading. 

(PA, section 5.1.4), as well as the REA 

case study analysis estimates. 

Based on all of the above 

considerations, the Administrator 

focused on identification of a secondary 

standard that might be associated with 

S deposition of such a magnitude. In so 

doing he recognized the complexity of 

identifying a NAAQS focused on 

protection of the public welfare from 

adverse effects associated with national 

patterns of atmospheric deposition 

(rather than on protection from national 

patterns of ambient air concentrations 

directly). In light of the influence of 

emissions from multiple, distributed 

sources, atmospheric chemistry and 

transport on air concentrations and the 

influence of air concentrations and 

other factors on atmospheric deposition 

(ecosystem loading), the Administrator 

concurred with the PA judgment that 

consideration of the location of source 

emissions and expected pollutant 

transport (in addition to the influence of 

physical and chemical processes) is 

important to understanding 

relationships between SO2

concentrations at ambient air monitors 

and S deposition rates in sensitive 

ecosystems of interest. Accordingly, the 

Administrator concurred with the PA 

that to achieve a desired level of 

protection from aquatic acidification 

effects associated with S deposition in 

sensitive ecosystems, SO2 emissions 

must be controlled at their sources, and 

that associated NAAQS compliance 

monitoring includes regulatory SO2

monitors generally sited near large SO2

emissions sources. 

The Administrator considered 

findings of the PA analyses of 

relationships between ambient air 

concentrations and S deposition 

estimates, conducted in recognition of 

the variation across the U.S. in the 

source locations and magnitude of SOX

emissions, as well as the processes that 

govern transport and transformation of 

SOX to eventual deposition of S 

compounds. Recognizing the linkages 

connecting SOX emissions and S 

deposition-related effects, the 

Administrator considered the current 

information with regard to support for 

SO2 as the indicator for a new or revised 

standard for SOX that would be 

expected to provide protection from 

aquatic acidification-related risks of S 

deposition in sensitive ecoregions. The 

Administrator noted the PA analyses 

demonstrated there to be an association 

between SO2 concentrations and nearby 

or downwind S deposition (PA, section 

7.4) based on the general association of 

higher local S deposition estimates with 

higher annual average SO2

concentrations at SLAMS, in addition to 

the correlations observed for ecoregion 

median S deposition with upwind SO2

monitoring sites of influence in the 

EAQM analyses (PA, sections 6.4.1 and 

7.4). He additionally took note of the PA 

findings of parallel trends of SO2

emissions and S deposition in the U.S. 

over the past 20 years, including the 

sharp declines, that indicate the strong 

influence of SO2 in ambient air on S 

deposition (PA, sections 6.4.1 and 7.4), 

and of the PA finding of parallel 

temporal trends of ecoregion S 

deposition estimates and REA aquatic 

acidification risk estimates across the 

five time periods analyzed. In light of all 

of these considerations, the 

Administrator judged SO2 to be the 

appropriate indicator for a standard 

addressing S deposition-related effects. 

With regard to the appropriate 

averaging time and form for such a 

standard, the Administrator took note of 

the PA focus on a year’s averaging time 

based on the recognition that longerterm averages (such as over a year) most 

appropriately relate to ecosystem 

deposition and associated effects, and of 

the recommendation from the CASAC 

majority for an annual average standard. 

The quantitative analyses of air quality 

and deposition in the PA also used a 3- 

year average form based on a 

recognition in the NAAQS program that 

such a form affords a stability to air 

quality management programs that 

contributes to effective environmental 

protection.105 Similarly, the CASAC 

majority recommendation focused on a 

3-year average form. In consideration of 

these conclusions of the PA and the 

CASAC majority, the Administrator 

focused on annual average SO2

concentrations, averaged over three 

years, as the appropriate averaging time 

and form for a revised standard 

providing public welfare protection 

from adverse effects associated with 

long-term atmospheric deposition of S 

compounds. 

In considering a level for such a 

standard, the Administrator again noted 

the complexity associated with 

identifying a NAAQS focused on 

protection from national patterns of 

atmospheric deposition. As discussed 

further in the PA and the proposal, in 

identifying a standard to provide a 

pattern of ambient air concentrations 

that together contribute to deposition 

across the U.S., it is important to 

consider the distribution of air 

concentrations to which the standard 

will apply.106 In identifying an 

appropriate range of concentrations for 

a standard level, the Administrator 

considered the evaluations and 

associated findings of the PA and advice 

from the CASAC. In so doing, he 

considered the two PA options of 

somewhat below 15 ppb to a level of 10 

ppb and a level ranging below 10 ppb 

to 5 ppb, with a 3-year average form. He 

additionally recognized that 

uncertainties in aspects of the aquatic 

acidification risk modeling contribute 

uncertainty to the resulting estimates, 

and that uncertainty in the significance 

of aquatic acidification risk is greater 

with lower deposition levels (PA, 

section 5.1.4). Accordingly, the 

Administrator took note of additional 

and appreciably greater uncertainty 

associated with consideration of a 

standard level below 10 ppb, including 

uncertainties in the relationships 

between S deposition and annual 

average SO2 concentrations below 10 

ppb (PA, Chapter 6, section 7.4). Thus, 

the Administrator recognized there to 

be, on the whole across the various 

linkages, increased uncertainty for 

lower SO2 concentrations and S 

deposition rates. The Administrator 

additionally considered the CASAC 

majority recommended range of 10 to 15 

ppb for an annual average SO2 standard 

to address S deposition-related 

ecological effects, as described in 

section II.B.1.b. above. These members 

indicated that this range of levels was 

‘‘generally’’ associated with S 

deposition ‘‘at <5 kg/ha-yr’’ in the two 

most recent trajectory analysis periods 

in the PA, and that a standard level in 

this range would afford protection 

against ecological effects in terrestrial 

ecosystems as well as aquatic 

ecosystems. These members also stated 

that such a standard would ‘‘preclude 

the possibility of returning to 

deleterious deposition values’’ 

(Sheppard, Response to Charge 

Questions, pp. 24–25). Thus, based on 

analyses and evaluations in the PA, 

including judgments related to 

uncertainties in relating ambient air 

concentrations to deposition estimates 

for the purpose of identifying a standard 

level associated with a desired level of 

ecological protection, and based on 

advice from the CASAC majority, the 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105745 

Administrator judged that a level within 

the range from 10 to 15 ppb would be 

appropriate for an annual average SO2

standard requisite to protect the public 

welfare from adverse effects related to S 

deposition. 

The Administrator also considered 

the extent to which a new annual 

average standard might be expected to 

control short-term concentrations (e.g., 

of three hours duration) and accordingly 

provide protection from direct effects 

that is currently provided by the 

existing 3-hour secondary standard. In 

this context, he noted the analyses and 

conclusions of the PA with regard to the 

extent of control for short-term 

concentrations (e.g., of three hours 

duration) that might be expected to be 

provided by an annual secondary SO2

standard. These analyses indicate that 

in areas and periods when the annual 

SO2 concentration (annual average, 

averaged over three years) is below 15 

ppb, design values for the existing 3- 

hour standard are well below the 

existing secondary standard level of 0.5 

ppm SO2 (PA, Figure 2–29). Based on 

these findings of the PA, the 

Administrator proposed that it is 

appropriate to consider revision of the 

existing secondary SO2 standard to an 

annual standard, with a 3-year average 

form and a level in the range from 10 

to 15 ppb. 

The Administrator also took note of 

the recommendation from the CASAC 

minority to establish a 1-hour SO2

secondary standard, identical to the 

primary standard (section II.B.1.b. 

above; Sheppard, 2023, p. A–2), based 

on its observation that most of the S 

deposition estimates for the last 10 years 

are less than 5 kg/ha-yr and a judgment 

that this indicates that the existing 1- 

hour primary SO2 standard adequately 

protects against long-term annual S 

deposition-related effects. The 

Administrator preliminarily concluded 

an annual standard to be a more 

appropriate form to address depositionrelated effects, but also recognized that 

greater weight could be given to 

consideration of the effectiveness of the 

existing 1-hour primary standard in 

controlling emissions and associated 

deposition. In light of these 

considerations, the EPA solicited 

comment on such an alternate option for 

the secondary SO2 standard. 

In summary, based on all of the 

considerations identified above, 

including the currently available 

evidence in the ISA, the quantitative 

and policy evaluations in the PA, and 

the CASAC advice, the Administrator 

proposed to revise the existing 

secondary SO2 standard to an annual 

average standard, with a 3-year average 

form and a level within the range from 

10 to 15 ppb as requisite to protect the 

public welfare. The EPA also solicited 

comment on a lower level for a new 

annual standard down to 5 ppb, as well 

as on whether the existing 3-hour 

secondary standard should be retained 

in addition to establishing a new annual 

SO2 standard. The EPA also solicited 

comment on the option of revising the 

existing secondary SO2 standard to be 

equal to the current primary standard in 

all respects. 

With regard to the secondary PM 

standards, the Administrator considered 

the available information and the PA 

evaluations and conclusions regarding S 

deposition-related effects. In so doing, 

he took note of the information 

indicating the variation in PM2.5

composition across the U.S. (PA, section 

2.4.3), with non-S containing 

compounds typically comprising more 

than 70% of total annual PM2.5 mass in 

much of the country. Further, he 

considered the PA findings of 

appreciable variation in associations, 

and generally low correlations, between 

S deposition and PM2.5, as summarized 

in section II.A.2. above (PA, sections 

6.2.2.3 and 6.2.4.2). He also took note of 

the discussion above in support of his 

decision regarding a revised secondary 

SO2 standard, including the 

atmospheric chemistry information 

which indicates the dependency of S 

deposition on airborne SOX, as 

evidenced by the parallel trends of SO2

emissions and S deposition. Based on 

all of these considerations, the 

Administrator judged that protection of 

sensitive ecosystems from S deposition 

is more effectively achieved through a 

revised SO2 standard than a standard for 

PM, and that a revised PM standard is 

not warranted to provide protection 

against the effects of S deposition. 

Based on his consideration of the 

secondary standards for N oxides and 

PM with regard to the protection 

afforded from direct ecological effects 

and from ecological effects related to 

ecosystem N deposition, the 

Administrator proposed to retain the 

existing NO2 and PM standards. With 

regard to protection from direct effects 

of N oxides in ambient air, the 

Administrator noted that the evidence 

of welfare effects at the time this 

standard was established in 1971 

indicated the direct effects of N oxides 

on vegetation and that the currently 

available information continues to 

document such effects, as summarized 

in section II.B.1.a.(1) above (ISA, 

Appendix 3, sections 3.3 and 3.4; PA, 

sections 4.1 and 5.4.2). With regard to 

the direct effects of NO2 and NO, the 

Administrator concurred with the PA 

conclusion that the evidence does not 

call into question the adequacy of 

protection provided by the existing 

standard. With regard to the N oxide, 

HNO3, consistent with the conclusion in 

the PA, the Administrator judged the 

limited evidence to lack a clear basis for 

concluding that effects associated with 

air concentrations and associated HNO3

dry deposition on plant and lichen 

surfaces might have been elicited by air 

quality that met the secondary NO2

standard. Thus, the Administrator 

recognized the limitations of the 

evidence for these effects, and 

associated uncertainties, and judges 

them too great to provide support to a 

revised secondary NO2 standard, 

additionally taking note of the 

unanimous view of the CASAC that the 

existing secondary NO2 standard 

provides protection from direct effects 

of N oxides (section II.B.1.b. above). 

The Administrator next turned to 

consideration of the larger information 

base of effects related to N deposition in 

ecosystems. In so doing, he recognized 

the complexities and challenges 

associated with quantitative 

characterization of N enrichment-related 

effects in terrestrial or aquatic 

ecosystems across the U.S. that might be 

expected to occur due to specific rates 

of atmospheric deposition of N over 

prolonged periods, and the associated 

uncertainties (PA, section 7.2.3). The 

Administrator also found there to be 

substantially more significant 

limitations and uncertainties associated 

with the evidence base for ecosystem 

effects related to N deposition 

associated with N oxides and PM, and 

with the available air quality 

information related to the limited 

potential for control of N deposition in 

areas across the U.S., in light of the 

impacts of other pollutants (i.e., NH3) on 

N deposition. The first set of limitations 

and uncertainties relates to quantitative 

relationships between N deposition and 

ecosystem effects, based on which 

differing judgments may be made in 

decisions regarding protection of the 

public welfare. In the case of protection 

of the public welfare from adverse 

effects associated with nutrient 

enrichment, there is also complexity 

associated with identification of 

appropriate protection objectives in the 

context of changing conditions in 

aquatic and terrestrial systems as recent 

deposition has declined from the 

historical rates of loading. The second 

set of limitations and uncertainties 

relates to the emergence of NH3, which 

is not a criteria pollutant, as a greater 

influence on N deposition than N oxides 


khammond on DSK9W7S144PROD with RULES2
105746 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

107Further, this influence appears to be exerted 

in areas with some of the highest N deposition 

estimates for those years. 

108This associated lessening influence of N 

oxides on total N deposition is also evidenced by 

the lower correlations between N deposition and 

annual average NO2 concentrations than observed 

for S deposition and SO2 concentrations (PA, 

sections 6.2.3 and 6.2.4), which may be related to 

increasing emissions of NH3 in more recent years 

and at eastern sites (PA, section 2.2.3 and Figure 6– 

5). 

109The air quality information regarding annual 

average NO2 concentrations at SLAMS monitors 

indicates more recent NO2 concentrations are well 

below the existing standard level of 53 ppb. As 

noted in the PA, the temporal trend figures indicate 

that, subsequent to 2011–2012, when median N 

deposition levels in 95% of the eastern ecoregions 

of the continental U.S. have generally been at or 

below 11 kg N/ha-yr, annual average NO2

concentrations, averaged across three years, have 

been at or below 35 ppb (PA, section 7.2.3.3). 

and PM over the more recent years,107

and the variation in PM composition 

across the U.S. 

Additionally, the Administrator 

recognized additional complexities in 

risk management and policy judgments, 

including with regard to identifying risk 

management objectives for public 

welfare protection from an ecosystem 

stressor like N enrichment, for which as 

the CASAC recognized, in terrestrial 

systems, there are both ‘‘benefits and 

disbenefits’’ (Sheppard, 2023, p. 8). As 

noted in the PA, the existence of 

benefits complicates the judgment of 

effects that may be considered adverse 

to the public welfare (PA, section 7.4). 

For aquatic systems, identification of 

appropriate public welfare protection 

objectives is further complicated by N 

contributions to many of these systems 

from multiple sources other than 

atmospheric deposition, as well as by 

the effects of historical deposition that 

have influenced the current status of 

soils, surface waters, associated biota, 

and ecosystem structure and function. 

In considering the evidence and air 

quality information related to N 

deposition, the Administrator took note 

of the fact that ecosystem N deposition 

is influenced by air pollutants other 

than N oxides, particularly, NH3, which 

is not a CAA criteria pollutant (PA, 

sections 6.1, 6.2.1 and 7.2.3.3). As noted 

above, the extent of this contribution 

varies appreciably across the U.S. and 

has increased during the past 20 years, 

with the areas of highest N deposition 

appearing to correspond to the areas 

with the greatest deposition of NH3 (PA, 

Figure 7–8).108 The Administrator 

concurred with the PA conclusion that 

this information complicates his 

consideration of the currently available 

information with regard to protection 

from N deposition-related effects that 

might be afforded by the secondary 

standard for N oxides, particularly when 

considering the information since 2010 

(and in more localized areas prior to 

that). That is, while the information 

regarding recent rates of ecoregion N 

deposition may in some individual 

areas (particularly those for which 

reduced N, specifically NH3, has a larger 

role) indicate rates greater than the 

range of values identified in the PA for 

consideration (e.g., 7–12 kg/ha-yr based 

on the considerations in section 7.2.3 of 

the PA and the benchmark of 10 kg/hayr, as conveyed in the advice from the 

CASAC), the PA notes that the extent to 

which this occurrence relates to the 

existing NO2 secondary standard is 

unclear. The lack of clarity is both 

because of uncertainties in relating 

ambient air NO2 concentrations to rates 

of deposition, and because of the 

increasing contribution of NH3 to N 

deposition. 

The Administrator additionally noted 

the PA finding that the temporal trend 

in ecoregion N deposition differs for 

ecoregions in which N deposition is 

driven by reduced N compared to those 

where reduced N comprises less of the 

total (e.g., PA, Figures 7–6 and 7–7). In 

light of the PA evaluations of N 

deposition and relative contribution 

from reduced and oxidized N 

compounds, the Administrator 

concurred with the PA conclusion that, 

based on the current air quality and 

deposition information and trends, a 

secondary standard for N oxides cannot 

be expected to effectively control total N 

deposition (PA, section 7.4). 

The Administrator additionally 

considered the two sets of advice from 

the CASAC regarding an NO2 annual 

standard in consideration of N 

deposition effects (section II.B.1.b. 

above). The CASAC majority 

recommended revision of the existing 

annual NO2 standard level to a value 

‘‘<10 to 20 ppb’’ (Sheppard, 2023, p. 

24). The basis for this advice, however, 

relates to a graph in the draft PA of the 

dataset of results from the trajectorybased analyses for the weighted annual 

NO2 metric (annual NO2 EAQMweighted), which, as noted in section 

II.B.1.b. above, is not directly 

translatable to concentrations at 

individual monitors or to potential 

standard levels. Additionally, these 

results found no correlation between the 

ecoregion deposition and the EAQMweighted or EAQM-max values at 

upwind locations, as also recognized by 

CASAC members and indicated in the 

final PA (PA, Table 6–10). Accordingly, 

based on the lack of a correlation for N 

deposition with the EAQMs, as well as 

the lack of translatability of the EAQMweighted values to monitor 

concentrations or standard levels, the 

PA did not find the information 

highlighted by the CASAC majority for 

relating N deposition levels to ambient 

air concentrations to provide scientific 

support for their recommended levels. 

In light of this, the Administrator did 

not find agreement with the CASAC 

majority recommendations on revisions 

to the annual NO2 standard. 

The CASAC minority recommended 

revision of the secondary NO2 standard 

to be identical to the primary standard 

based on their conclusion that the 

recent N deposition levels meet its 

desired objectives and that the primary 

standard is currently the controlling 

standard (Sheppard, 2023, Appendix A). 

As noted in the PA, among the NO2

primary and secondary NAAQS, the 1- 

hour primary standard (established in 

2010) may currently be the controlling 

standard for ambient air concentrations, 

and annual average NO2 concentrations, 

averaged over three years, in areas that 

meet the current 1-hour primary 

standard, have generally been below 

approximately 35 to 40 ppb.109 The 

Administrator also considered the PA 

revision option (i.e., revision to a level 

below the current level of 53 ppb to as 

low as 35 to 40 ppb [PA, section 7.4]), 

taking note of the PA characterization 

that support for this option is ‘‘not 

strong’’ (PA, section 7.4). He further 

noted the PA conclusion that while the 

option may have potential to provide 

some level of protection from N 

deposition related to N oxides, there is 

significant uncertainty as to the level of 

protection that would be provided, with 

this uncertainty relating most 

prominently to the influence of NH3 on 

total N deposition separate from that of 

N oxides (PA, section 7.2.3.3). The 

Administrator further recognized the PA 

statement that the extent to which the 

relative roles of these two pollutants (N 

oxides and NH3) may change in the 

future is not known. As evaluated in the 

PA, these factors together affect the 

extent of support for, and contribute 

significant uncertainty to, a judgment as 

to a level of N oxides in ambient air that 

might be expected to provide requisite 

protection from N deposition-related 

effects on the public welfare. 

In light of the considerations 

recognized above, the Administrator 

found that the existing evidence does 

not clearly call into question the 

adequacy of the existing secondary NO2

standard, additionally noting that recent 

median N deposition estimates are 

below the N deposition benchmark 

identified by the CASAC majority of 10 

kg/ha-yr in ecoregions for which 

approximately half or more of recent 

total N deposition is estimated to be 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105747 

110For example, the justification provided for the 

range of levels recommended by the CASAC 

majority for a revised PM2.5 annual standard (6 to 

10 mg/m3) refers both to annual average PM2.5

concentrations (3-yr averages) ranging from 2 to 8 

mg/m3 in 27 Class I areas (as corresponding to N 

deposition estimates at or below 10 kg/ha-yr) and 

to annual average PM2.5 concentrations (3-year 

averages) ranging from 6 to 12 mg/m3 (at design 

value sites in areas of N deposition estimates greater 

than 15 kg/ha-yr), as summarized in section II.B.1.b. 

above. 

111As summarized in section II.B.1.b. above, the 

CASAC majority, in its recommendation for 

revision of the existing standard, did not provide 

specificity regarding the basis for its statements on 

lichen species and fog or cloud water, and the 

available evidence as characterized in the ISA does 

not provide estimates of this deposition or describe 

associated temporal variability, or specifically 

describe related effects on biota (ISA, Appendix 2). 

oxidized N, driven by N oxides (PA, 

section 7.2.3.3). In addition to the 

substantial uncertainty described above 

regarding the need for control of N 

deposition from N oxides that might be 

provided by a secondary standard for N 

oxides, the PA found there to be 

substantial uncertainty about the effect 

of a secondary standard for N oxides on 

the control of N deposition, such that it 

is also not clear whether the available 

information provides a sufficient basis 

for a revised standard that might be 

judged to provide the requisite 

protection. In light of this PA finding, 

the current information on air quality 

and N deposition, and all of the above 

considerations, the Administrator 

proposed to also judge that the available 

evidence in this review is sufficient to 

conclude a revision to the secondary 

annual NO2 standard is not warranted. 

Based on all of these considerations, he 

proposed to retain the existing 

secondary NO2 standard, without 

revision. The EPA also solicited 

comments on the alternative of revising 

the level and form of the existing 

secondary NO2 standard to a level 

within the range from 35 to 40 ppb with 

a 3-year average form. 

Lastly, the Administrator considered 

the existing standards for PM. He took 

note of the PA discussion and 

conclusion that the available 

information does not call into question 

the adequacy of protection afforded by 

the secondary PM2.5 standards from 

direct effects and deposition of 

pollutants other than S and N 

compounds (PA, sections 7.1.3 and 7.4). 

The evidence characterized in the ISA 

and summarized in the PA indicates 

such effects to be associated with 

conditions associated with 

concentrations much higher than the 

existing standards. Thus, the 

Administrator proposed to conclude 

that the current evidence does not call 

into question the adequacy of the 

existing PM standards with regard to 

direct effects and deposition of 

pollutants other than S and N 

compounds. 

With regard to N deposition and 

PM2.5, the Administrator considered the 

analyses and evaluations in the PA, as 

well as advice from the CASAC. He took 

note of the substantial and significant 

limitations and uncertainties associated 

with the evidence base for ecosystem 

effects related to N deposition 

associated with PM and with the 

available air quality information related 

to the limited potential for control of N 

deposition in areas across the U.S. in 

light of the impacts of NH3 on N 

deposition, and the variation in PM 

composition across the U.S., as 

summarized earlier. For example, as 

noted in the PA, the variable 

composition of PM2.5 across the U.S. 

contributes to geographic variability in 

the relationship between N deposition 

and PM2.5 concentrations, and there is 

an appreciable percentage of PM2.5 mass 

that does not contribute to N deposition. 

The PA further notes that this variability 

in percentage of PM2.5 represented by N 

(or S) containing pollutants contributes 

a high level of uncertainty to our 

understanding of the potential effect of 

a PM2.5 standard on patterns of N 

deposition. 

In considering the advice from the 

CASAC for revision of the existing 

annual secondary PM2.5 standard, the 

Administrator noted that the CASAC 

provided two different 

recommendations for revising the level 

of the standard: one for a level in the 

range from 6 to 10 mg/m3 and the second 

for a level of 12 mg/m3. As summarized 

in the PA, the specific rationale for the 

range from 6 to 10 ug/m3 is unclear, 

with levels within this range described 

as both relating to N deposition in a 

preferred range (at or below 10 kg N/hayr) and relating to deposition above that 

range.110 The PA noted that this 

‘‘overlap’’ illustrates the weakness and 

variability of relationships of PM2.5 with 

N deposition across the U.S. (PA, 

section 7.4). Further, the PA notes the 

low correlation for total N deposition 

estimates with annual average PM2.5

design values in the last 10 years at 

SLAMS (PA, Table 6–7). The second 

recommendation, from the CASAC 

minority, was based on their conclusion 

that the recent N (and S) deposition 

levels meet their desired targets and that 

the primary annual PM2.5 standard, 

which has been 12 mg/m3 since 2013, 

has been the controlling standard for 

annual PM2.5 concentrations (Sheppard, 

2023, Appendix A). 

Based on the currently available 

information, taking into account its 

limitations and associated uncertainties, 

and in consideration of all of the above, 

the Administrator proposed to conclude 

that PM2.5 is not an appropriate 

indicator for a secondary standard 

intended to provide protection of the 

public welfare from adverse effects 

related to N deposition. In reaching this 

proposed conclusion, the Administrator 

focused in particular on the weak 

correlation between annual average 

PM2.5 design values and N deposition 

estimates in recent time periods, and 

additionally noted the PA conclusion 

that the available evidence, as evaluated 

in the PA, is reasonably judged 

insufficient to provide a basis for 

revising the PM2.5 annual standard with 

regard to effects of N deposition related 

to PM. Thus, based on consideration of 

the PA analyses and conclusions, as 

well as consideration of advice from the 

CASAC, the Administrator further 

proposed to conclude that no change to 

the annual secondary PM2.5 standard is 

warranted, and he proposed to retain 

the existing PM2.5 secondary standard, 

without revision. The EPA solicited 

comment on this proposed decision and 

also solicited comment on revising the 

existing standard level to a level of 12 

mg/m3, in light of the recommendation 

and associated rationale provided by the 

CASAC minority. 

With regard to other PM standards, 

the Administrator concurred with the 

PA’s finding of a lack of information 

that would call into question the 

adequacy of protection afforded by the 

existing PM10 secondary standard for 

ecological effects, and thus concluded it 

is appropriate to propose retaining this 

standard without revision. With regard 

to the 24-hour PM2.5 standard, the 

Administrator took note of the PA 

conclusion that the evidence available 

in this review, as documented in the 

ISA, or cited by the CASAC,111 does not 

call into question the adequacy of 

protection provided by the 24-hour 

PM2.5 standard from ecological effects 

(PA, section 7.4). The Administrator 

also considered the comments of the 

CASAC majority and recommendations 

for revision of this standard to a lower 

level or to an indicator of deciviews, as 

summarized in section II.B.1.b. The 

Administrator noted the PA 

consideration of the lack of quantitative 

information in the ISA related to the 

specific type of N deposition raised by 

the CASAC comments. Further, the 

specific revision options recommended 

by the CASAC majority were based on 

visibility considerations, although the 

adequacy of protection provided by the 

secondary PM2.5 standard from visibility 

effects has been addressed in the 


khammond on DSK9W7S144PROD with RULES2
105748 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

reconsideration of the 2020 p.m. 

NAAQS decision (89 FR 16202, March 

6, 2024) and is not included in this 

review. The Administrator additionally 

noted the recommendation from the 

CASAC minority to retain the existing 

24-hour secondary PM2.5 standard 

without revision. Based on all of these 

considerations, the Administrator 

proposed to retain the existing 24-hour 

secondary PM2.5 standard, without 

revision. Additionally, based on the lack 

of evidence calling into question the 

adequacy of the secondary PM10

standards, he also proposed to retain the 

secondary PM10 standards without 

revision. 

In reaching the proposed conclusions 

regarding protection of the public 

welfare from ecological effects 

associated with ecosystem deposition of 

N and S compounds, the Administrator 

also noted the PA consideration of the 

potential for indicators different from 

those for the current standards that may 

target specific chemicals that deposit N 

and S, e.g., NO3

¥, SO4

2¥, NH4

∂ (PA, 

sections 7.2.2.3, 7.2.3.3 and 7.4). In so 

doing, however, he recognized a number 

of uncertainties and gaps in the 

available information important to such 

consideration. Based on these, the 

Administrator judged that the currently 

available information does not support 

standards based on such indicators at 

this time. In so doing, he also 

recognized that additional data 

collection and analysis is needed to 

develop the required evidence base to 

inform more comprehensive 

consideration of such alternatives. 

2. Comments on the Proposed Decisions 

Over 27,000 individuals and 

organizations indicated their views in 

public comments on the proposed 

decision. Nearly all of these are 

associated with mass mail campaigns or 

petitions. Approximately 20 separate 

submissions were also received from 

individuals, organizations, or groups of 

organizations. Many of the individual 

commenters made a general 

recommendation to ‘‘strengthen’’ the 

standards under review, emphasizing 

giving attention to the scientific 

information and recommendations from 

the CASAC, and protection of natural 

ecosystems and associated wildlife. 

Among the organizations commenting 

were State and federal agencies, a Tribal 

organization, environmental protection 

advocacy organizations, industry 

organizations and regulatory policyfocused organizations. 

Some commentors expressed the 

overarching view that none of the 

standards for the three pollutants in this 

review should be revised, generally 

stating that the implementation work by 

State agencies associated with new 

standards would be for no 

environmental gain in light of the 

emissions reductions and ‘‘dramatic 

improvements’’ in associated air quality 

that have already occurred since 2000. 

While the EPA recognizes that air 

quality has improved over the last two 

decades, we note that the existence of 

such trends and the fact of the CAA 

requirements for implementation of 

NAAQS, alone or in combination, are 

not appropriate bases for the 

Administrator’s decision under section 

109 of the Act. Accordingly, in finding 

that revision to the existing SOX

standard is necessary to provide the 

requisite public welfare protection for 

SOX, while revisions to the N oxides 

and PM standards are not necessary to 

provide the requisite public welfare 

protection for those pollutants, the 

Administrator has based his decisions 

on the evidence of welfare effects, air 

quality information and the extent of 

public welfare protection provided by 

the existing standards, as described in 

section II.B.3. below. Other comments 

on the proposed decisions in the review 

of the secondary standards for 

protection of ecological effects of SOX, 

N oxides and PM are addressed below. 

Comments regarding the proposed 

decision to revise the secondary 

standard for SOX are addressed in 

section II.B.2.a., and those regarding the 

proposed decision to retain the 

secondary standards for N oxides and 

PM are addressed in sections II.B.2.b. 

and II.B.2.c., respectively. Other 

comments, including comments related 

to other legal, procedural, or 

administrative issues, those related to 

issues not germane to this review, and 

comments related to the Endangered 

Species Act are addressed in the 

separate Response to Comments 

document. 

a. Sulfur Oxides 

(1) Comments Regarding Adequacy of 

the Existing Secondary Standard 

With regard to welfare effects 

associated with SOX in ambient air, 

including those related to deposition of 

S compounds, in consideration of the 

welfare effects evidence, quantitative 

analyses of ecosystem exposure and risk 

and advice from the CASAC, the 

Administrator proposed to judge that 

the existing 3-hour secondary SO2

standard is not requisite to protect the 

public welfare from adverse effects 

associated with acidic deposition of S 

compounds in sensitive ecosystems. An 

array of comments was received 

regarding the Administrator’s proposed 

decision to address this insufficiency in 

protection through revision to an annual 

average standard. These comments are 

addressed in the following section. 

(2) Comments in Support of Proposed 

Adoption of a New Annual Standard 

In consideration of the welfare effects 

evidence, quantitative analyses of 

ecosystem exposure and risk, and 

advice from the CASAC majority to 

adopt an annual standard with a level 

within the range of 10 to 15 ppb to 

address the deposition-related effects of 

SOX, the Administrator proposed 

revision of the existing standard to be an 

annual standard, as summarized in 

section II.B.1.c. above. Commenters 

expressed several views concerning the 

level of such a standard; these 

comments are addressed in the 

subsections below. 

(a) Comments Agreeing With a Level 

Within the Proposed Range 

The EPA received multiple comments 

in support of the proposed 

establishment of an annual standard, 

with a 3-year form and level within the 

proposed range. Some of these 

comments concurred with the full range 

of levels as proposed, while some 

recommended a range of levels that 

overlapped with the lower end of the 

proposed range and also extended 

below it. The commenters in agreement 

with the full proposed range variously 

cited, concurred with, and expanded 

upon information discussed in the 

proposal, in addition to noting 

consistency of the proposed decision 

with recommendations from the 

majority of CASAC. In so doing, one 

commenter expressed the view that the 

proposed new standard would provide 

protection for direct vegetation effects 

and ecosystem deposition-related effects 

including aquatic acidification, which 

they noted affects the diversity and 

abundance of fish and aquatic life, thus 

providing support to cultural services 

and recreational fishing, which have 

long-term societal and economic 

benefits. Another comment expressed 

the view that the new standard would 

support Tribal efforts to protect lakes 

and streams from deposition-related 

effects including potential impacts to 

cultural fishing practices. One 

comment, in advocating for a level 

within the range of 5 to 10 ppb (which 

overlaps with the proposed range at a 

level of 10 ppb), expressed the view that 

‘‘to meet statutory requirements and act 

rationally and respond to CASAC 

consensus scientific expertise, EPA 

must,’’ among several 

recommendations, ‘‘[s]et an annual 

secondary SO2 standard of 5–10 ppb to 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105749 

protect against deposition effects and 

maintain total sulfur deposition at <5 

kg/ha on an annual basis.’’ 

The EPA agrees with the comment 

that a new annual standard with a level 

in the proposed range (of 10–15 ppb) 

would be expected to provide protection 

for direct effects on vegetation and for 

ecosystem deposition-related effects, 

including specifically those associated 

with aquatic acidification. The EPA also 

agrees that such a standard, by 

protecting against acidifying 

atmospheric deposition in aquatic and 

terrestrial ecosystems, can be expected 

to impact an array of societal and 

economic benefits from this protection. 

As summarized in section II.A.3.b. 

above and recognized in the 

Administrator’s conclusions in section 

II.B.3. below, such benefits include 

providing protection for recreational 

and subsistence fisheries, as well as for 

recreational uses of sensitive forests and 

protected waterbodies. 

Additionally, with regard to the lower 

end of the proposed range and its 

overlap with the commenterrecommended-range of 5 to 10 ppb, the 

EPA agrees with the commenter that a 

standard with a level of 10 ppb would 

generally be associated with S 

deposition at or below 5 kg/ha annually 

in sensitive ecosystems, consistent with 

comments by the CASAC majority in its 

rationale for recommending a new 

annual standard with a level in the 

range of 10 to 15 ppb, which it 

described as ‘‘generally’’ maintaining S 

deposition below 5 kg/ha-yr (as 

summarized in section II.B.1.b. above). 

The CASAC majority based its 

conclusion regarding annual SO2

standard levels associated with S 

deposition at/below 5 kg/ha-yr on 

analyses in the draft PA, as described in 

section II.B.1.b. 

In reaching his proposed decision for 

a level in the range of 10 to 15 ppb, the 

Administrator considered the expanded 

analyses and conclusions in the final 

PA. In reaching his final decision, as 

described in section II.B.3. below, the 

Administrator also considered 

additional analyses in a technical 

memorandum to the docket that extend 

the PA air quality and deposition 

analyses (Sales et al., 2024). These 

ecoregion-based analyses of air quality 

and deposition from five 3-year time 

periods from 2001 through 2020 

indicate that when annual average SO2

concentrations (as a 3-year average) are 

at or below 10 ppb, median S deposition 

in associated downwind ecoregions is 

generally at or below 5 kg/ha-yr. 

Specifically, more than 85% of 

associated downwind ecoregions are at 

or below 5 kg/ha-yr, with 95% below 

about 6 kg/ha-yr and all below about 8 

kg/ha-yr. This analysis additionally 

found that in every instance of the 

upwind maximum annual SO2

concentration above 10 ppb, the 

associated downwind ecoregion median 

deposition was greater than 5 kg/ha-yr, 

ranging from about 6 kg/ha-yr up to 

about 18 kg/ha-yr and with 75% of 

occurrences greater than 9 kg/ha-yr 

(Sales et al., 2024). In consideration of 

these findings, among other 

considerations, the Administrator 

judged a level of 10 ppb to provide the 

requisite protection of public welfare for 

the new annual secondary SO2 standard, 

as described in section II.B.3. 

(b) Comments in Support of a Level 

Below the Proposed Range 

Three comments indicated support or 

potential support for a new annual 

standard with a level below 10 ppb (i.e., 

below the proposed range). In addition 

to the comment referenced above that 

expressed support for a level in the 

range from 5 to 10 ppb, a second 

comment, that expressed support for an 

annual standard with a level within the 

proposed range of 10 to 15 ppb, 

additionally expressed support for a 

level as low as 5 ppb to the extent it 

could ‘‘be supported by the current 

science.’’ A third comment expressed 

support for an annual standard level of 

5 ppb, stating the view that such a 

standard could provide necessary 

protection for the public welfare and for 

resources managed by the U.S. National 

Park Service. Beyond a statement by one 

of these comments (also discussed in 

section II.B.2.a.(2)(a) above) that their 

recommended range of 5 to 10 ppb was 

needed to ‘‘maintain sulfur deposition 

at <5 kg/ha on an annual basis,’’ none 

of these commenters presented a 

specific scientific rationale for a specific 

standard level below 10 ppb. One 

comment stated that 71% of national 

parks are experiencing wet deposition of 

S greater than 1 kg/ha-yr and suggested 

that this indicates harmful impacts to 

park soil, waterbodies, and associated 

wildlife. 

With regard to the latter comment 

regarding wet S deposition above 1 kg/ 

ha-yr, the commenter did not provide 

evidence to support their conclusion of 

harmful impacts for such a level, and 

the EPA has not found the available 

evidence to support such a finding in 

this review. In describing the 1 kg/hayr value (for wet deposition of both S 

and N), the comment cited two papers 

that are focused on N deposition as a 

basis for the conclusion that conditions 

of wet deposition below 1 kg/ha-yr are 

‘‘good’’ while greater levels indicate 

acidification conditions. These papers— 

Baron et al. (2011) and Sheibley et al. 

(2014)—are summarized in addressing 

another comment in section 

II.B.2.b.(2)(b) below. Neither paper, 

however, addresses S deposition. Based 

on this and consideration of the 

evidence and quantitative analyses 

available in this review, the EPA does 

not find that wet S deposition greater 

than 1 kg/ha-yr in national parks 

indicates adverse impacts to the public 

welfare. 

We note that the phrase regarding 

maintaining S deposition ‘‘at <5 kg/ha’’ 

on an annual basis is consistent with the 

phrase used by the CASAC majority in 

its justification for its recommended 

range of 10–15 ppb, for which it cited 

analyses in the draft PA. As summarized 

above, and discussed in section II.B.3. 

below, the Administrator has 

considered the CASAC advice and the 

findings of the analyses in the final PA, 

in combination with additional 

presentations in Sales et al (2024), 

which he judged to provide support for 

his decision to adopt an annual SO2

standard with a level of 10 ppb, a value 

within the commenter-supported range 

of 5 to 10 ppb. 

The commenter that recommended a 

level of 5 ppb additionally expressed 

their view that a standard with a higher 

level (within the proposed range of 10 

to 15 ppb) would not prevent effects of 

S deposition in Class I areas that they 

described as harmful, improve air 

quality, or reduce S deposition in Class 

I areas. Based on this view and their 

judgment that a further reduction in 

ambient air concentrations is needed, 

this commenter recommended that EPA 

set the level for a new annual standard 

below recent annual average SO2

concentrations, stating that a standard 

level of 5 ppb ‘‘could’’ reduce S 

deposition from current levels. 

However, this commenter did not 

elaborate as to what magnitude of S 

deposition would be expected to be 

associated with a standard level of 5 

ppb or why such a magnitude would 

provide an appropriate level for 

protection of the public welfare from S 

deposition-related effects. As a basis for 

their conclusion that harmful effects of 

S deposition are associated with current 

S deposition rates in national parks that 

are Class I areas, this commenter 

referred to National Park Services 

analyses that assign grades or 

‘‘conditions’’ to these areas based on S 

deposition estimates and ‘‘park-specific 

critical loads’’ and stated that current S 

deposition levels in National Park 

Service managed Class I areas are above 

these loads for multiple ecosystem 

components. This commenter indicated 

that these analyses show that natural 


khammond on DSK9W7S144PROD with RULES2
105750 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

112One comment additionally cited the CASAC 

statement (in its advice to the Administrator in this 

review, summarized in section II.B.1.b.) that the 

CASAC’s view was that a standard in terms of 

atmospheric deposition would be a more 

appropriate means of addressing deposition-related 

effects as indicative of a lack of CASAC support for 

a revised SO2 standard to address depositionrelated effects of SOX. 

resources in these parks are in fair or 

poor condition and that a standard with 

a level around 5 ppb ‘‘could improve air 

quality and reduce S deposition levels’’ 

in areas that the commenter states are 

already experiencing S deposition 

impacts. 

Although the commenter provided 

tables listing numbers of areas that they 

stated are in poor or fair condition for 

various ecosystem components (e.g., 

aquatic systems, trees) and potential 

threats (e.g., acidification by S 

deposition, growth effects and S 

deposition), the commenter submitted 

no information (beyond their statement 

that there are critical load exceedances) 

on how they reach such conclusions. As 

support for the general statement that 

the term critical load describes the 

amount of pollution above which 

harmful changes in sensitive ecosystems 

occur, the commenter cited a 

publication that discusses the concept 

of critical loads and the potential for 

their usefulness in natural resources 

management. We note, however, that 

this publication does not provide details 

(e.g., specific deposition rates associated 

with specific types of effect in specific 

types of ecosystems) that might inform 

the EPA’s consideration of the type, 

severity and prevalence of particular 

effects that would be expected from 

specific levels of deposition. Such 

information, as that provided by the 

aquatic acidification REA and the 

evidence underlying it, is needed in 

judgments regarding deposition levels 

and deposition-related effects of public 

welfare significance, which are integral 

to the Administrator’s decision on the 

secondary standard for SOX. Further, 

the commenter did not provide or refer 

to evidence relating a standard level of 

5 ppb to expected S deposition levels. 

As discussed in section II.B.3. below, 

the Administrator has based his 

decision for an annual secondary SO2

standard with a level of 10 ppb on his 

consideration of the available evidence 

and quantitative analyses supporting the 

Agency’s understanding of relationships 

between S deposition-related effects and 

S deposition levels and SO2

concentrations, and also on his 

judgments regarding the public welfare 

significance of the S deposition-related 

effects assessed in his decision. 

As we describe in section II.A.3.c. 

above, the term critical load has 

multiple interpretations and 

applications (ISA, p. IS–14). The variety 

in meanings stems in part from differing 

judgments and associated 

identifications regarding the ecological 

effect (both type and level of severity) 

on which the critical load focuses and 

from judgment of its significance or 

meaning. Accordingly, all CLs are not 

comparable with regard to severity or 

significance of harm or, as is more 

pertinent to decision-making in this 

review, with regard to potential for 

adversity to the public welfare. Rather, 

science policy judgments in these areas 

are required in order to reach 

conclusions regarding impacts for 

which secondary standards should be 

established. For example, the analysis in 

the PA which utilized CLs—the aquatic 

acidification REA—described their basis 

in detail. Further, in the Administrator’s 

consideration of the REA results, he 

recognized the variation and uncertainty 

associated in the CLs and their 

relevance to different waterbodies. 

Thus, while we appreciate the 

comment, we find the information 

provided by the commenter to be 

insufficient for reaching judgments as to 

the significance and strength of the 

various CLs in their technical analysis, 

and likewise insufficient for concluding 

that reduced deposition levels are 

necessary to avoid adverse public 

welfare effects in Class I areas (or for 

assessing what level of deposition 

would be associated with a 5 ppb 

standard). 

(3) Comments in Disagreement With 

Proposed Adoption of a New Annual 

Standard 

Several public comments expressed 

disagreement with the proposed 

adoption of a new annual secondary 

standard to address S deposition-related 

effects of SOX in ambient air. These 

comments cited a variety of reasons in 

support of this position, including the 

view that the EPA lacks authority to set 

a secondary standard to address public 

welfare effects of acid deposition. This 

comment is addressed in section 

II.B.2.a.(3)(a) below. Other reasons 

described in some comments advocating 

this position include the view that the 

proposed standard has no ‘‘benefits’’ 

and is therefore not ‘‘necessary’’ or 

‘‘requisite.’’ Some other comments 

variously cite implementation burdens 

(e.g., SIP preparation), uncertainties in 

the scientific basis, and a lack of CASAC 

consensus. Another commenter 

expressed the view that the proposal did 

not adequately discuss how effects are 

adverse to the public welfare and 

additionally stated that the ANC targets 

used in reaching conclusions on the 

need for protection from acid deposition 

relied on the judgments of others, rather 

than EPA. These other comments are 

addressed in section II.B.2.a.(3)(b) Some 

comments in opposition to a new 

annual standard expressed support for a 

secondary standard identical in all 

respects to the primary standard. Those 

comments are addressed in section 

II.B.2.a.(3)(c). 

(a) Authority for a Secondary Standard 

Based on Acid Deposition 

A few commenters that disagreed 

with the proposed decision to adopt a 

new annual standard to address 

deposition-related effects expressed the 

view that the EPA lacks authority to set 

a secondary standard based on acid 

deposition, stating that the specific 

focus of the Acid Rain Program (CAA, 

title IV) on acidification preempts action 

on the same issue through the secondary 

NAAQS.112 These commenters argue 

that the enactment of title IV of the CAA 

in 1990 displaced the EPA’s authority to 

address acidification through the setting 

of NAAQS, contending that the 

existence of a specific regulatory 

program to address the acidification 

effects of oxides of nitrogen and sulfur, 

that was established subsequent to the 

establishment of the NAAQS program in 

1970, supplants the EPA’s general 

authority under the Act. In support of 

this contention, the commenters cite a 

Supreme Court decision pertaining to 

regulation of tobacco by the FDA (Food 

& Drug Admin. v. Brown & Williamson 

Tobacco Corp., 529 U.S. 120 (2000)) and 

also claim that their view regarding a 

lack of authority for the NAAQS 

program is demonstrated by the 

legislative history and a close reading of 

section 404 of the Act, which required 

the EPA to report to Congress on the 

feasibility of developing an acid 

deposition standard and the actions that 

would be required to integrate such a 

program into the CAA. The required 

report described in section 404, 

commenters argue, demonstrates that 

Congress had concluded that the EPA 

lacked the authority under section 109 

of the CAA to establish a secondary 

NAAQS to address acid deposition. 

Commenters also claimed that the EPA 

has in the past recognized that the 

NAAQS program does not provide an 

effective mechanism for addressing acid 

deposition and has not adequately 

explained its change in position. These 

commenters additionally cite comments 

from the CASAC, made in its review of 

the draft PA for this NAAQS review, 

regarding challenges in identifying a 

concentration-based standard to address 

deposition-related effects as supporting 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105751 

113For example, the 1995 Report discusses 

potential ranges for an acid deposition standard as 

measured by kg/ha/year (e.g., U.S. EPA [1995b] at 

118). 

the commenter’s view that the CASAC 

also recognized a mismatch between the 

NAAQS program and regulation of acid 

deposition. 

The EPA does not agree with 

commenters that the enactment of title 

IV of the Act displaced the EPA’s 

authority under section 109 to adopt 

NAAQS to address adverse effects on 

public welfare associated with 

deposition of SOX from the ambient air. 

We note that the purpose of title IV ‘‘is 

to reduce the adverse effects of acid 

deposition’’ by reducing sulfur dioxide 

emissions by 10 million tons (and NOX

emissions by 2 million tons) from 1980 

levels (CAA section 401(b)). By contrast, 

section 109 directs the Administrator to 

set a standard that is ‘‘requisite to 

protect public welfare from any known 

or anticipated adverse effects,’’ based on 

the air quality criteria (CAA section 

109(b)(2)). Congress explicitly requires 

the air quality criteria and standards be 

reviewed every five years, and has thus 

required secondary standards to reflect 

the latest scientific information (CAA 

section 109(d)(1)). There is no reason to 

believe that a Congressional effort to 

achieve 10 million tons in reductions of 

SO2 was intended to supersede EPA’s 

ongoing obligations to assess the impact 

of SO2 on public welfare. See Whitman 

v. Am. Trucking Ass’ns, 531 U.S. 457, 

468 (2001) (‘‘Congress, we have held, 

does not alter the fundamental details of 

a regulatory scheme in vague terms or 

ancillary provisions—it does not, one 

might say, hide elephants in 

mouseholes.’’). 

These two provisions are not in 

conflict, but represent the combined 

approach often taken by Congress to 

address the frequently complex 

problems of air pollution. There is 

nothing unusual about the CAA relying 

on multiple approaches to improve air 

quality, and in particular relying on the 

NAAQS to identify the requisite level of 

air quality and relying on both State 

implementation plans as well as federal 

CAA programs to control emissions of 

criteria pollutants in order to attain and 

maintain the NAAQS. For example, the 

existence of title II of the Act (Emission 

Standards for Moving Sources) does not 

divest the EPA of authority to set a 

NAAQS for ozone, despite the fact that 

many mobile source controls are 

adopted to control ozone precursors and 

indeed may be sufficient in some areas 

to attain and maintain the ozone 

NAAQS. Had Congress wanted to 

channel the EPA’s authority to address 

acidification exclusively through title IV 

it could have done so explicitly. For 

example, it generally excluded criteria 

pollutants from regulation under section 

111(d) and 112. Instead, at the same 

time that it enacted title IV, Congress 

also added section 108(g) to the CAA, 

specifying that the air quality criteria 

used for setting the NAAQS ‘‘may assess 

the risks to ecosystems from exposure to 

criteria air pollutants.’’ 

In adding title IV to the CAA, 

Congress created a new program to 

reduce the emissions of SO2 and NOX

from electric generating units, the most 

significant sources of acidifying 

pollution in 1990. Nothing in the text or 

the legislative history of title IV of the 

Act indicates that in creating additional 

authority Congress intended to foreclose 

the EPA’s authority to address acid 

deposition through the NAAQS process. 

Indeed, to the extent that Congress 

addressed the impact of title IV on other 

provisions of the CAA, it made clear 

that title IV had no impact on the 

compliance obligations of covered 

sources under other CAA provisions. 

See CAA section 413, ‘‘Except as 

expressly provided, compliance with 

the requirements of this subchapter 

shall not exempt or exclude the owner 

or operator of any source subject to this 

subchapter from compliance with any 

other applicable requirements of this 

chapter.’’ 

The legislative history of the title IV 

program makes clear that Congress was 

acting to provide the EPA with 

additional tools to address the problem 

of acidification more effectively. See, 

e.g., S. Rep. No.101–228, at 289–291 

(1989). Congress did not conclude that 

the EPA lacked the regulatory authority 

to address acidification but rather 

concluded that ‘‘a major acid deposition 

control program [was] warranted . . . 

because of the evidence of damage that 

had already occurred as well the 

likelihood of further damage in the 

absence of Congressional action’’ (H.R. 

Rep. No 101–490, at 360 (1990)). The 

Senate Report made it clear that while 

the EPA envisioned CAA section 109 as 

providing authority to adopt a 

secondary NAAQS to address the effects 

of acid deposition, the EPA remained 

concerned about the effectiveness of this 

and other regulatory approaches (S. Rep. 

No. 101–228, at 290–291). Congress 

addressed these issues by adding the 

new authorities found in title IV but 

made no mention of supplanting the 

EPA’s authority under section 109 to 

address acidification effects. There is no 

discussion in the legislative history of 

title IV of curtailing the EPA’s authority 

under the NAAQS program. 

As such, the requirement in section 

404 of the 1990 CAA Amendments that 

the EPA send to Congress ‘‘a report on 

the feasibility and effectiveness of an 

acid deposition standard or standards’’ 

does not demonstrate that Congress 

concluded that an amendment to the 

CAA would be necessary to give the 

EPA the authority to issue standards 

addressing acidification under section 

109. See CAA section 401. The 

significance of the report required by 

section 404 can be understood in the 

overall context of (1) the history of 

Congress’ and the EPA’s attempts to 

understand and to address the causes 

and effects of acid deposition; (2) the 

distinction between an acid deposition 

standard (expressed as kg/ha-yr) and an 

ambient air quality standard addressing 

effects of deposition (expressed as 

ppb); 113 and (3) the EPA’s proposed 

conclusion in 1988 that the scientific 

uncertainties associated with acid 

deposition were too great to allow the 

Agency to establish a secondary NAAQS 

at that time to address those effects. The 

EPA notes that it was clear at the time 

of the 1990 CAA Amendments that a 

program to address acid deposition was 

needed and that the primary and most 

important of these provisions is title IV 

of the Act, establishing the Acid Rain 

Program. The Report required under 

section 404 of the Amendments reflects 

this concern and requires an evaluation 

of an acid deposition standard and a 

comparison of its effectiveness to the 

effectiveness of various other regulatory 

authorities under the Act, including the 

authority for a secondary NAAQS under 

section 109 (CAA Amendments, Public 

Law 101–549, 104 Stat. 2399, 2632 

(1990) (describing that ‘‘Reports’’ under 

CAA 404 (42 U.S.C. 7651), should 

include ‘‘(6) . . . other control strategies 

including ambient air quality 

standards’’)). This indicates the 

existence of an ongoing authority under 

section 109. Likewise, in preparing the 

Report itself, EPA concluded that ‘‘[i]t 

may be possible to set acid deposition 

standards under existing statutory 

authority’’ (U.S. EPA, 1995b, at 100). 

For these reasons, the commenters’ 

analogy to tobacco regulation, at issue in 

FDA v. Brown & Williamson Tobacco 

Corp., 529 U.S. 120 (2000), is entirely 

inapt. The issue before the Supreme 

Court in that case was whether the FDA 

had authority to regulate tobacco at all, 

and the Court held that where the FDA 

consistently took the position it did not 

have such authority, and Congress 

enacted multiple statutes consistent 

with that position, Congress had ratified 

the FDA’s understanding of its authority 

and had created a separate regulatory 

structure. By contrast, while the EPA 

has on multiple occasions noted the 


khammond on DSK9W7S144PROD with RULES2
105752 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

114We have explained in section II.B.2.b.(2)(a), 

below, why we do not view section 109 as 

authorizing a deposition standard. 

scientific difficulties associated with 

identifying a standard to protect against 

acid deposition, EPA has engaged with 

those scientific difficulties because the 

EPA’s longstanding interpretation of 

section 109 is that acid deposition is 

within the scope of adverse effects on 

public welfare to be addressed under 

section 109. There is no reason to 

understand Congressional action to 

establish programs to reduce emissions 

of SOX under title IV as depriving EPA 

of authority to specify a level of air 

quality the attainment and maintenance 

of which is requisite to protect the 

public welfare against effects of SOX

under section 109. See Massachusetts v. 

EPA, 549 U.S. 497, 530 (2007) 

(distinguishing Brown & Williamson 

where EPA jurisdiction would not lead 

to extreme results, was not 

counterintuitive and EPA had never 

disavowed its authority). 

The EPA now concludes, as discussed 

in section II.B.2.b.(2)(a) below, that it 

does not have the authority to set a 

deposition standard under the existing 

CAA, and the EPA is not adopting a 

deposition standard in this action. 

Rather, consistent with the Agency’s 

longstanding approach, the EPA has 

concluded that it must consider the 

effects of acid deposition in setting an 

air quality standard. Section 109 of the 

Act requires the Administrator to set an 

ambient air quality standard the 

attainment of which protects against 

‘‘any known or anticipated adverse 

effects associated with the presence of 

[the] air pollutant in the ambient air.’’ 

The EPA has concluded that the best 

interpretation of this language is that a 

deposition standard is not an ‘‘air 

quality’’ standard because a deposition 

standard focuses not on concentrations 

of the pollutant in the ambient air but 

rather on quantities deposited on 

surfaces (as discussed in section 

II.B.2.b.(2)(a) below). Rather, the EPA 

has consistently viewed the best 

interpretation of this language to require 

consideration of the adverse effects that 

can be anticipated from presence of the 

pollutant in the ambient air, including 

via deposition of the pollutant to 

aquatic and other ecosystems. The 

CASAC indicated in its comments to the 

Administrator (as summarized in 

section II.B.1.b. above) that a deposition 

standard would be more scientifically 

appropriate, and it may be that Congress 

will at some point revisit the question 

of whether the EPA should also have 

authority to adopt an acid deposition 

standard, but such a question is 

independent of the scope of the 

authority, and obligation, the EPA 

currently has under section 109. 

In assessing the import of section 404, 

the EPA has noted in the past that 

‘‘Congress reserved judgment as to 

whether further action might be 

necessary or appropriate in the longer 

term’’ to address any problems 

remaining after implementation of the 

title IV program, and ‘‘if so, what form 

it should take’’ (58 FR 21356, April 21, 

1993; 77 FR 20223, April 3, 2012). Such 

reservation of judgment by Congress 

concerned whether Congress should 

adopt additional statutory provisions to 

address the effects of acid deposition, as 

it did in 1990. It does not indicate a 

view that the EPA lacked authority 

under CAA section 109 to establish a 

secondary NAAQS to address acid 

deposition. 

The EPA’s decision in both the 1993 

and 2012 reviews reflects the view that 

there is ongoing authority to address the 

effects of acid deposition under section 

109 of the Act and does not indicate that 

the EPA believed that title IV implicitly 

amended the CAA and removed all such 

regulatory authority outside of title IV. 

In both the 1993 and 2012 decisions on 

the question of whether to revise the 

secondary NAAQS to address acid 

deposition-related effects, the EPA 

decided not to adopt a standard 

targeting deposition-related effects. The 

EPA noted the consistency of this 

decision with Congress’ actions in the 

1990 amendments but nowhere 

indicated that Congress’ actions meant 

the EPA no longer had the authority to 

adopt a secondary NAAQS to address 

acid deposition. Instead, in the 1993 

and 2012 decisions, the EPA stated that 

due to scientific uncertainty, the Agency 

would not at those times adopt a 

secondary NAAQS targeting depositionrelated effects but would instead gather 

additional data and perform research 

and would determine in the future what 

further action to take under CAA section 

109 (77 FR 20263, April 3, 2012; 75 FR 

28157–58, April 21, 1993). 

Although substantial progress was 

made between the 1993 and 2012 

reviews addressing some areas of 

uncertainty, the Administrator again 

concluded in 2012 that uncertainties 

associated with setting a NAAQS to 

address acidification were too 

substantial to allow her to set a standard 

that in her judgment would be requisite 

to protect the welfare from such effects. 

More than 10 years later, the evidence 

base on air quality, deposition and 

deposition-related effects has progressed 

substantially. That evidence base and 

associated quantitative analyses 

developed in the current review provide 

the foundation for the current decision 

for a NAAQS to protect against acid 

deposition. Thus, although we recognize 

the CASAC’s view to be that a 

deposition standard would be a more 

appropriate means of addressing 

deposition-related effects, we find that 

for SO2 the relationship between 

ambient air concentrations and 

deposition is sufficiently well 

established to support a revised 

secondary SO2 NAAQS.114

We do not understand the CASAC as 

suggesting that, in the absence of a 

deposition standard, the EPA should 

decline to set an air quality standard to 

address deposition-related effects. 

Rather, contrary to the implication of 

the commenter that the CASAC did not 

support a NAAQS to address 

deposition, the CASAC expressed strong 

consensus support for the EPA setting a 

NAAQS for this purpose and 

recommended concentration-based 

standards to the EPA for consideration. 

In summary, the EPA disagrees with the 

commenters’ interpretation of the 

information cited and does not agree 

that the Administrator lacks the 

authority to set a secondary standard to 

address acid deposition-related effects. 

(b) Other Comments in Opposition to 

the Proposed Annual Standard 

In addition to the view discussed 

immediately above regarding the EPA’s 

authority to set a NAAQS to address 

effects related to atmospheric 

deposition, some commenters cited 

other reasons in opposition to the 

proposed annual secondary SO2

standard. For example, based on the 

EPA’s analyses indicating that the 

proposed revision of the secondary 

standard would not require emissions 

reductions beyond those needed to meet 

the primary standard, some commenters 

stated that revision of secondary 

standard has no ‘‘benefits’’ and is 

therefore not ‘‘necessary’’ and not 

‘‘requisite.’’ Some additionally cited 

implementation requirements on States 

(e.g., SIP preparation) as a reason that 

the standard should not be revised, in 

light of the view that current air quality 

conditions do not pose a risk of adverse 

welfare effects. Some commenters 

expressed the view that the 

uncertainties are too great and the 

scientific basis for a standard to address 

acid deposition-related effects is 

lacking. One commenter stated that the 

EPA should thoroughly review the 

scientific studies published since the 

cut-off publication date for studies 

included in the ISA, and that to allow 

for this, the EPA should retain the 

existing standard pending that review 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105753 

and the associated creation of an up-todate record in the next NAAQS review. 

One commenter additionally noted the 

lack of CASAC consensus on 

recommendations for a standard to 

address deposition-related effects and 

stated the view that this lack of 

consensus further weakens support for 

such a new standard. One comment 

expressed the view that the proposal did 

not adequately discuss how effects are 

adverse to public welfare and 

additionally stated that the ANC targets 

used in reaching conclusions regarding 

the need for protection from acid 

deposition relied on the judgments of 

others, rather than the EPA. 

Regarding the view that a new annual 

standard to address deposition-related 

effects is not ‘‘necessary’’ or ‘‘requisite,’’ 

the EPA disagrees that simply because 

current or projected air quality in areas 

that meet the existing primary standard 

is expected to achieve the new standard, 

the current standard is already requisite 

to protect the public welfare, and a 

revised standard is unnecessary. The 

CAA requires secondary NAAQS to be 

set at the level of air quality requisite to 

protect the public welfare from known 

or anticipated adverse effects (CAA, 

section 109(b)(2)). The EPA recognizes 

the clear evidence, the CASAC 

consensus conclusions, and the 

Administrator’s judgment, described in 

section II.B.3. below, that the current 

secondary standard does not provide 

protection for deposition-related effects 

of SOX and is therefore not requisite. 

Accordingly, based on the available 

information and CASAC advice, the 

Administrator proposed to revise the 

existing standard to reflect a level of air 

quality that would provide the needed 

protection (89 FR 26620, April 15, 

2024). Such a revision is ‘‘necessary’’ to 

address the requirements of the Act. In 

adopting a new annual standard, as 

described in section II.B.3. below, the 

Administrator has considered a range of 

options for limiting deposition-related 

effects with an air quality standard and 

identified such a standard that, in his 

judgment, is neither more nor less 

stringent than necessary to achieve the 

desired level of protection from welfare 

effects, most particularly those 

associated with atmospheric deposition 

of S compounds in sensitive 

ecosystems. 

With regard to implementation 

requirements, while the Administrator’s 

decision on revision of the secondary 

standard to provide the requisite public 

welfare protection is not expected to 

result in changes to existing air quality, 

he has not considered implementation 

requirements in reaching his decision 

on the revised standard. Consistent with 

the CAA requirements described in 

section I.A. above, the Administrator is 

barred by CAA section 109 from 

considering costs of implementation in 

judging the adequacy of a standards, 

and he has not done so. 

The EPA additionally disagrees with 

the view that the secondary SO2

standard should not be revised because 

a revised standard would not be 

expected to require emissions 

reductions beyond those already 

required for meeting the primary SO2

standard, such that there would be little 

or no emissions reductions. As the D.C. 

Circuit has held in a prior challenge to 

SO2 NAAQS, ‘‘Nothing in the CAA 

requires EPA to give the current air 

quality such a controlling role in setting 

NAAQS’’ (Nat’l Envtl. Dev. 

Association’s Clean Air Project v. EPA, 

686 F.3d 803, 813 ([D.C. Cir. 2012]). In 

this review, the EPA is engaged in the 

task of identifying a secondary standard 

that provides the requisite public 

welfare protection under the Act. The 

fact that the existing primary SO2

standard is expected, based on recent 

data, to control air quality such that the 

new annual secondary SO2 standard 

may also be met does not satisfy the 

requirements of CAA section 109(b)(2) 

or a priori make the secondary standard 

not requisite or without benefit. The 

benefit is assurance of the protection of 

the public welfare that is required of the 

secondary standard separate from the 

protection of the public health that is 

required of the primary standard. 

Further, the CAA requires the 

establishment of secondary standards 

requisite to protect against known or 

anticipated effects, and that requirement 

is separate and independent of the 

obligation to establish primary 

standards to protect the public health 

with an adequate margin of safety. The 

implication of the comment is that 

when the EPA next revises the primary 

NAAQS for SOX, the Administrator 

would be required to consider the effect 

of any revisions to the primary NAAQS 

on both public health and welfare, a 

consideration inconsistent with the 

entire purpose of having distinct 

standards, as well as the text of section 

109. 

Furthermore, while air quality is 

currently expected to meet the new 

annual secondary standard when the 

primary standard is met, patterns of SO2

concentrations may change in some 

areas in the future, such that both the 

new annual secondary standard and the 

existing primary standard are violated 

or such that the secondary standard 

could be violated without a violation of 

the primary standard. The analyses of 

SO2 concentrations described in the PA 

illustrate how SO2 concentration 

patterns have changed over the past two 

decades in response to various changes 

in the largest emissions sources and in 

emissions controls implemented on 

such sources. Thus, sometimes changes 

occur over the long term in the multiple 

factors that influence air quality, that 

can contribute to future air quality 

patterns that may differ from those 

prevalent currently. Regardless, we 

recognize that section 109 of the Act 

does not only require establishment of 

standards that will result in changes in 

existing air quality. Rather, the Act 

specifies that there be secondary 

standards in place that will provide the 

requisite protection in the face of 

current and future air quality. And, as 

discussed above and in section II.B.3. 

below, the existing secondary SO2

standard does not provide the requisite 

protection from known or anticipated 

adverse effects on the public welfare 

related to atmospheric deposition of S 

compounds associated with SOX in 

ambient air. The Administrator’s 

decision is therefore to revise the 

standard to one that in his judgment 

will provide that protection, as 

described in section II.B.3. below. 

The EPA disagrees with the comment 

stating that the Agency should retain the 

existing secondary SO2 standard 

pending review of the scientific studies 

that have been published since the cutoff date for studies considered in the 

ISA. Given the need for thorough 

consideration and CASAC review of 

studies that are part of the air quality 

criteria on which NAAQS must be 

based, there is always a cut-off date for 

studies to be considered in the ISA, and 

there are always studies published after 

the cut-off date. The NAAQS are subject 

to regular review precisely to allow for 

EPA to base its review of the standards 

on the latest available science and to 

also revisit the standards in the future 

based on additional scientific 

information. As noted in section I.D. 

above, in consideration of public 

comments received on this action, the 

EPA has provisionally considered all 

such ‘‘new’’ studies cited in comments 

and concluded that they do not 

materially change the broad scientific 

conclusions of the ISA (Weaver, 2024). 

Thus, the EPA has concluded that 

reopening the air quality criteria is not 

warranted. Therefore, as discussed in 

section II.B.3. below, the Administrator 

has considered the available evidence, 

as summarized in the ISA, the 

quantitative and policy evaluations in 

the PA, and the related additional 

analyses (Sales et al., 2024), as well as 

CASAC advice and public comment on 


khammond on DSK9W7S144PROD with RULES2
105754 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

115The importance of this transport, with cooccurring transformation of SO2 to SO4

2 ¥, in 

contributing to ecosystem acidification was 

recognized decades ago in the 1982 AQCD for PM 

and SOX which stated that ‘‘[b]ecause of long range 

transport, acidic precipitation in a particular state 

or region can be the result of emissions from 

sources in states or regions many miles away, rather 

than from local sources’’ (1982 p.m. and SOX

AQCD, p. 7–2; Altshuller 1976). 

the proposed decision and judged this 

an appropriate basis for his decision in 

the current review. 

The EPA also disagrees with 

commenters’ claims that the 

uncertainties are too great to provide the 

necessary scientific support for a new 

annual secondary standard or that 

consensus advice is needed from the 

CASAC. With regard to the advice from 

the CASAC, we disagree that consensus 

is needed before the Administrator can 

make a decision in a NAAQS review. 

The CAA does not require the CASAC 

to reach consensus in its advice on 

revisions to the standards. The EPA has 

made decisions on NAAQS in multiple 

reviews in which the CASAC did not 

reach consensus on its advice for the 

standards (e.g., 85 FR 87256, December 

31, 2020 and 89 FR 16202, March 6, 

2024). In reaching his decision in this 

review, as described in section II.B.3. 

below, the Administrator has 

considered advice provided from both 

the majority and the minority of the 

CASAC. 

In support of their claim that 

uncertainties are too great, commenters 

list statements from the proposal that 

recognize specific technical areas of 

uncertainty in our understanding of 

deposition-related effects of SO2 in 

ambient air. We note that many of these 

statements are simply recognizing 

aspects of the evidence base that 

illustrate the complexity of addressing 

deposition-related effects. For example, 

one statement cited by commenters as 

indicative of significant uncertainty that 

should preclude action in this review 

recognized that there is not a simple 

one-to-one relationship between 

ambient air concentrations and any one 

indicator of S or N deposition. This 

statement simply recognizes the 

complexity inherent in analyses 

supporting this review. This complexity 

relates in part to the complex 

atmospheric chemistry and meteorology 

as well as aspects of ambient air 

monitoring and deposition estimation 

datasets (ISA, Appendix 2; PA, Chapters 

2 and 6). In light of these factors, as 

summarized in the proposal and in 

section II.A.2. and II.B.1.a. above, we 

analyzed multiple datasets that 

investigate relationships between 

concentrations for different metrics in 

different types of locations. 

While we recognize the uncertainties 

and complexities of the evidence base 

and quantitative information, we have 

taken them into account in our 

evaluations, and we disagree that the 

available information is insufficient to 

permit a reasoned judgment about a 

secondary SO2 standard that may be 

considered to provide the appropriate 

protection from adverse effects on the 

public welfare. For example, some of 

the areas cited by commenters relate to 

uncertainty in how quickly sensitive 

ecosystems might respond to the already 

reduced deposition. While we recognize 

there to be uncertainty in estimates 

related to ecosystem response times, the 

EPA does not find predictions of this to 

be necessary in this decision, and 

accordingly has not considered timing 

of future recoveries as a factor in 

determining the standard that would 

provide the desired level of protection. 

Other areas cited by commenters simply 

recognize the inherent variability of 

environmental response to varying 

patterns of SO2 concentrations. The 

Agency has recognized this variability 

in its focus on a year’s averaging time 

for the new standard, which will not be 

affected by short-term variability, and in 

its focus on medians in characterizing 

ecosystem deposition targets. 

Lastly, the commenters noted 

uncertainty associated with the 

trajectory-based analysis (or EAQM 

approach), citing areas of uncertainty 

identified in the PA or proposal, and 

comments by the CASAC in its review 

of the draft PA, which stated that the 

description in the draft PA was 

insufficiently detailed and that 

sensitivity analyses were needed to 

characterize associated uncertainty. In 

addition to CASAC comments, these 

public comments quoted statements by 

three individual members of the CASAC 

Panel for this review that state there are 

uncertainties and shortcomings of the 

EAQM approach, state that there are 

poor correlations of S deposition with 

ambient air concentrations and suggest 

a need for peer review. With regard to 

correlations, we disagree that the 

correlation coefficients for the two SO2

EAQMs in the final PA analyses (0.49 

and 0.56 when considering the full 

dataset in the final PA), which are 

statistically significant at the 0.05 level, 

are fairly characterized as ‘‘poor’’ (PA, 

Table 6–8). That said, the use of such 

relationships in this review is not for 

the development of a function to 

generate precise predictions of S 

deposition associated with individual 

monitor air concentrations. Rather, the 

analyses and the statistical significance 

of the deposition-to-EAQM value 

associations support the conclusion that 

higher upwind SO2 concentrations 

contribute to higher downwind S 

deposition. With this support, they also 

inform judgments regarding standard 

levels through consideration of the 

patterns of downwind deposition rates 

that have occurred during periods 

associated with different maximum 

upwind SO2 concentrations. 

With regard to peer review, in 

addition to noting the scientific peer 

review provided by the CASAC Panel 

for this review which resulted in 

substantial improvements in the 

analyses from the draft to the final PA, 

we also note that the trajectory analyses 

are based on a well-established and 

peer-reviewed model, HYSPLIT (Stein 

et al., 2015). This model, as described 

further in the PA, is commonly used to 

compute simple air parcel trajectories 

using historical meteorological data and 

to simulate the trajectories of air parcels 

as they are transported through the 

atmosphere for a given set of 

meteorological conditions (PA, 

Appendix 6A). 

In consideration of the robust 

scientific and technical peer review 

provided by the CASAC and its Oxides 

of Nitrogen, Oxides of Sulfur and 

Particulate Matter Secondary National 

Ambient Air Quality Standards Panel in 

their review of the draft PA, several 

improvements were implemented. For 

example, sensitivity analyses were 

conducted to judge the influence of key 

aspects of the approach employed (e.g., 

duration of the trajectory simulations 

and criteria used to identify influential 

upwind monitors), and findings from 

these analyses informed development of 

the trajectory-based approach for the 

final PA. As a result, the final PA 

includes substantially more detail in 

describing the approach and in the 

presentation of results, including for the 

various sensitivity analyses. Thus, as 

noted in the final PA, analyses 

presented in that document were 

revised and additional information 

added to address the CASAC concerns 

(PA, section 1.4). 

While the PA includes multiple 

approaches for analyzing relationships 

between ambient air concentrations and 

ecosystem deposition of S compounds, 

the trajectory-based approach is the only 

one that accounts for pollutant 

transport, which is integral to how SO2

emissions and associated concentrations 

contribute to acidic precipitation and 

acidification of ecosystems many miles 

away.115 Such transport modeling has 

been used for years, with its use verified 

twenty years ago by a study 

documenting the movement of air 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105755 

116As recognized in section II.A. above, the 

Administrator’s decisions in secondary NAAQS 

reviews draw upon scientific information and 

analyses about welfare effects, exposures and risks, 

as well as judgments about the appropriate response 

to the range of uncertainties that are inherent in the 

scientific evidence and analyses. As described in 

section II.B.3. below, the Administrator’s decision 

reflects these considerations. 

masses containing elevated 

concentrations of SO4

2 ¥ from the Ohio 

River Valley to the eastern U.S. and 

Canada (Hennigan et al., 2006), where 

acid-sensitive waterbodies have been 

impacted by acidification (ISA, 

Appendix 16, section 16.2). Thus, 

consideration of the trajectory-based 

analyses by the Administrator in 

reaching his proposed and final 

conclusions rely on different analyses 

(from those described in the draft PA) 

that have been improved to address 

comments by the CASAC, and 

consideration of these analyses (in 

addition to the other approaches) 

presented in the final PA is important 

to identifying a secondary standard that 

accounts for pollutant transport to 

downwind sensitive ecosystems. 

With regard to our recognition of the 

uncertainties associated with issues in 

this review, we note that Congress and 

the courts have recognized that some 

uncertainties in assessing the effects of 

air pollution are inevitable, and the 

Administrator is required to exercise his 

judgment in the face of imperfect 

information. See, e.g., Lead Indus. 

Ass’n, Inc. v. EPA, 647 F.2d 1130, 1155 

& n.50 (D.C. Cir. 1980) (quoting H.R. 

Rep. No. 95–294, at 50). Only when the 

Administrator judges that the 

uncertainties are so great as to preclude 

the ability to identify a standard that 

would be expected to provide the 

requisite protection do uncertainties 

justify a decision to not act. See, Center 

for Biological Diversity v. EPA, 749 F.3d 

1079, 1087 (D.C. Cir. 2014). As 

discussed further in section II.B.3. 

below, that is not the case for this 

standard. Thus, the EPA’s judgment is 

that the available information, including 

evidence of the effect of SOX on 

sensitive ecosystems and the analyses of 

transport of pollutants across airsheds, 

is sufficient to allow the Administrator 

to make a reasoned judgment about 

where to set a revised SO2 NAAQS, 

while recognizing that substantial 

uncertainties remain.116

Regarding the comment that the 

proposal insufficiently evaluated or 

discussed how the effects to be 

addressed by the new annual secondary 

standard are adverse to public welfare, 

we note the evidence of aquatic 

acidification and its effects on fisheries 

in lakes and streams across the 

northeast and Appalachian Mountains. 

This evidence was evaluated and 

documented in the current and last ISA 

and prior AQCDs (e.g., ISA, Appendix 8, 

section 8.5.2 and Appendix 16, section 

16.2.3.2.1; 2008 ISA, sections 3.2.4.4 

and 3.2.4.5; 1982 AQCD, section 

7.1.1.1). For example, acidified aquatic 

habitats have a lower number of species 

(species richness) of fishes, including 

culturally and recreationally important 

species, as well as shifts in biodiversity 

of both flora and fauna. This evidence 

and the findings of the quantitative 

aquatic acidification REA, as well as the 

analyses of relationships between air 

quality and S deposition, and advice 

from the CASAC were considered by the 

Administrator in reaching his proposed 

decision that the existing SO2 standard 

does not provide the requisite 

protection of the public welfare from 

known or anticipated adverse effect. 

This information, and public comments, 

have also been considered in his 

decision on revisions to the SO2

standard, as discussed further in section 

II.B.3. below. Further, the public welfare 

implications of aquatic acidificationrelated effects, including the influence 

of their severity and geographic extent, 

on harm posed to the public welfare, are 

described in the PA, the proposal and 

section II.A.3.b. above (PA, section 4.5; 

89 FR 26641–26644, April 15, 2024). In 

reaching his decision on the existing 

standard and on the revisions that 

would provide the requisite protection, 

the Administrator has considered these 

factors (severity and geographic extent 

of acidification-related effects), as well 

as the evidence of varying sensitivity of 

ecoregions across the U.S. In the end, as 

noted in sections I.A. and II.A. above, 

the CAA recognizes that judgments on 

effects to the public welfare that are 

adverse are within the purview of the 

Administrator in reaching his decision 

on secondary standards. 

In judging the existing standard to not 

provide the requisite protection of the 

public welfare, the Administrator has 

considered the evidence, evaluations in 

the PA, strengths and uncertainties in 

the evidence, and quantitative analyses. 

In so doing, he focused particularly on 

the REA findings for aquatic 

acidification risk estimates for the 

earliest part of the 20-year assessment 

period. With the pattern of deposition 

estimated for this period (when the 

existing standard was met), the REA 

found more than a third of waterbody 

sites in the five most affected ecoregions 

unable to achieve even the lowest of the 

three acid buffering capacity 

benchmarks used as risk indicators 

(below which the increased risk of 

episodic acidification events may 

threaten survival of sensitive aquatic 

species), and more than half of 

waterbody sites unable to meet this 

benchmark in the single most affected 

ecoregion. The Administrator judged 

that this level of aquatic acidification 

risk, associated with deposition levels 

estimated to have occurred when the 

existing standard was met, can be 

anticipated to cause adverse effects on 

the public welfare. 

Lastly, we disagree with the view of 

one commenter that the ANC 

benchmarks used in reaching 

conclusions regarding the need for 

protection from acid deposition relied 

on the judgments of others, rather than 

the EPA. Rather, as described in the PA 

and summarized in section II.A.4. 

above, the quantitative REA employed 

an array of ANC benchmarks in 

recognition of variation among 

waterbodies in their ability to achieve 

different benchmarks and in the 

associated risk to fisheries, to 

specifically avoid putting undue weight 

on a single value. In characterizing risk 

and levels of protection associated with 

different S deposition circumstances in 

the REA, we reported the percentages of 

waterbodies per ecoregion estimated to 

achieve the different benchmarks. The 

PA focused on this pattern of 

percentages in characterizing risk and 

the CASAC majority also considered 

this pattern in expressing its 

recommendations for a revised 

standard. Similarly, in weighing the 

evidence and the REA findings, the 

Administrator also considered these 

patterns and the weight to place on 

different benchmarks for ANC as an 

indicator of acidification risk, as well as 

the CASAC majority consideration of 

them in its recommendation of a range 

of standards expected to achieve a 

desired level of public welfare 

protection. In so doing, as described in 

section II.B.3. below, he judged it 

appropriate to consider patterns of ANC 

across ecoregion waterbodies, rather 

than limiting his judgment to 

consideration of a single ANC 

benchmark in all areas. Thus, contrary 

to the view of the commenter, the 

Administrator made all relevant 

judgments on the weight to place on 

different tools for indicating 

acidification risk, including ANC 

benchmarks in reaching a decision on 

the secondary SO2 standard. 

(c) Comments Recommending Revision 

To Be Identical to the Primary Standard 

In disagreeing with the EPA’s 

proposal to revise the 3-hour secondary 

SO2 standard to an annual standard for 

the reasons discussed in the two 


khammond on DSK9W7S144PROD with RULES2
105756 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

117This memorandum in the docket (Docket ID 

No. EPA–HQ–OAR–2014–0128–0039) describes the 

basis for the EPA’s decision that a Regulatory 

Impact Analysis was not warranted for the 

proposed decision (89 FR 26692, April 15, 2024). 

118This analysis excluded Hawaii where it is not 

uncommon for there to be high SO2 values in areas 

with recurring volcanic eruptions (PA, section 

2.4.2). 

sections above, a few commenters 

additionally expressed support for an 

alternate revision that would set the 

secondary standard to be identical to the 

primary standard, in all respects. One 

commenter stated that this option 

would be supported by a finding of no 

locations in the U.S. that would not 

achieve an annual standard with a level 

at the low end of the proposed range. 

The other commenter cited comments 

from the minority of CASAC that also 

recommended this option based on a 

judgment that the 1-hour primary 

standard is currently controlling of air 

quality and the view that most 

deposition values during the period 

since the primary standard was 

established have been less than 5 kg/hayr. This commenter additionally quoted 

the EPA’s March 9, 2024, technical 

memorandum 117 regarding the highest 

annual average concentrations observed 

during the period 2017–2022 in areas 

that do not violate the primary standard. 

Additionally, one commenter expressed 

support for ‘‘any alternatives,’’ 

including revising the secondary 

standard to be identical to the primary 

standard in all respects, ‘‘that can be 

supported by the current science,’’ 

without providing further elaboration. 

While the EPA agrees with the 

commenters regarding the air quality 

and deposition estimates in recent 

years, the EPA considered the available 

quantitative analyses, including the 

additional analyses presented in the 

technical memorandum to the docket 

(Sales et al., 2024), and finds that a 

secondary standard identical to the 

existing primary standard (75 ppb, as 

the annual 99th percentile daily 

maximum 1-hour concentration, 

averaged over three consecutive years) 

would be expected to provide a greater 

stringency in SO2 concentrations than 

required to generally maintain S 

deposition levels of interest. As 

indicated by the additional analyses, a 

higher level (e.g., of 120 ppb) for a 1- 

hour standard, with averaging time and 

form identical to the primary standard, 

is associated with downwind ecoregion 

median S deposition levels more like 

those associated with an annual SO2

standard of 10 ppb than is such a 1-hour 

standard with a level of 75 ppb (Sales 

et al., 2024). Thus, the EPA disagrees 

with these commenters that a 1-hour 

secondary standard identical in all 

respects to the existing primary 

standard would provide the requisite 

protection of the public welfare, noting 

that it may provide more control than 

necessary to achieve the desired 

protection. As described in section 

II.B.3. below, the Administrator judges 

that an annual average standard, 

averaged over three years, with a level 

of 10 ppb can be expected to provide the 

needed protection of the public welfare. 

(4) Comments Regarding Retaining the 

Existing Secondary Standard 

The very few comments that 

addressed the issue of retaining the 

existing 0.5 ppm (500 ppb) 3-hour 

standard recommended retention, 

variously noting that this standard is 

important for short-term direct impacts 

of SO2, that such a standard would 

prevent peak episodic events, and that 

in the past this standard was the 

controlling standard for many areas and 

its retention would ensure those areas 

maintain adequate protections. With 

regard to protection from the short-term 

direct impacts of SO2 in ambient air, the 

EPA agrees that the existing standard 

provides such protection, as concluded 

by the Administrator in the proposal 

and by the CASAC. We further note, 

however, that the additional air quality 

analyses conducted in response to 

public comments indicate that in areas 

with SO2 concentrations from 2000 

through 2021 that would meet an 

annual standard of 10 ppb (excluding 

Hawaii),118 virtually all 3-hour standard 

design values (the second highest 

annual 3-hour concentration at 

regulatory monitors) are less than 0.25 

ppm (Sales et al., 2024, Figure 10). 

These analyses further indicate that 

more than 99% of the highest 3-hour 

concentrations at monitored sites in 

each of the more recent years of the 

analysis period (2011–2021) are below 

0.2 ppm (Sales et al., 2024, Table 6). 

Reflecting the evidence in the ISA and 

prior AQCDs for SOX, the PA summary 

of the lowest short-term concentrations 

(e.g., over a few hours) associated with 

effects on plants or lichens does not 

include any concentrations below 0.25 

ppm (PA, section 5.4.2; ISA, Appendix 

3, section 3.2; 1982 AQCD, section 8.3). 

Together this information indicates that 

short-term concentrations in areas that 

would be expected to meet an annual 

standard of 10 ppb are well below those 

that have been associated with effects 

on plants or lichens. In light of 

information such as this, as described in 

section II.B.3. below, the Administrator 

judges that short-term peak 

concentrations of potential concern for 

welfare effects are adequately controlled 

by an annual average standard of 10 

ppb, such that revision of the secondary 

standard to this annual standard 

provides requisite protection from both 

short-term effects of SO2 in the ambient 

air and effects related to the deposition 

of S compounds in sensitive 

ecosystems. 

b. Nitrogen Oxides and Particulate 

Matter 

(1) Comments in Support of the 

Proposed Decisions 

(a) Nitrogen Oxides 

Among the few comments received on 

the proposed judgment that the existing 

secondary NO2 standard provides the 

needed protection from direct effects of 

N oxides in ambient air on plant and 

lichen surfaces, all expressed support. 

In the context of ecological effects of N 

oxides more broadly, including 

deposition-related effects, several public 

comments expressed support for the 

proposed decision to retain the existing 

standard, which was based on the 

Administrator’s proposed judgment that 

the available evidence does not clearly 

call into question the adequacy of the 

existing standard. In expressing support 

for the proposed decision, commenters 

raised several uncertainties, referencing 

the discussion in the proposal. These 

uncertainties include those related to 

the weak relationship between NO2

concentrations and N deposition; the 

increasing contribution of NH3 to N 

deposition; the expected impacts of 

current deposition levels absent residual 

historic impacts and the identification 

of appropriate protection objectives in 

this context of changing conditions; and 

the role of N benefits and disbenefits. 

We agree that these are important 

uncertainties in the evidence base, and, 

as discussed in section II.B.3. below, 

these factors are among those the EPA 

considered in reaching the decision to 

retain the existing NO2 standard. 

Some other commenters, in support of 

their position that the N oxides standard 

should not be revised, further expressed 

the view that N oxides emissions is one 

of the principal sources of acidic 

compounds and that the EPA lacks 

authority to set standards based on 

acidic deposition, citing CAA section 

401(a). As discussed in section 

II.B.2.a.(3)(a) above, the EPA disagrees 

with the view that NAAQS cannot be 

established to provide protection for 

acidic deposition-related effects. We 

additionally note the REA conclusion, 

however, that under current air quality 

and based on the current information, as 

discussed in section 5.1.2.4 and 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105757 

119 In expressing their disagreement with the 

CASAC position that a NAAQS in terms of 

deposition rate is supported by the Act, some 

commenters emphasize that deposition is a process 

rather than a ‘‘level of air quality’’ as specified by 

section 109 of the CAA, and also cite the definition 

of ambient air under 40 CFR 50.1(e). These 

commenters additionally express the view that if 

the CASAC’s position were correct and the Act 

supported NAAQS in terms of deposition rate, then 

Congress would not have adopted title IV of the Act 

to address control of acid deposition. We do not 

agree with this latter view. Regardless of the role 

of NAAQS or of a potential role of acid deposition 

standards, as discussed more fully in section 

II.B.2.a.(3)(a), the action of Congress in adopting 

title IV into the Act simply provided the EPA with 

additional tools to address the problem of acid 

deposition more effectively. 

Appendix 5A, section 5A.2.1 of the PA, 

the contribution of N compounds to 

acidification is negligible. 

(b) Particulate Matter 

Among the public comments on the 

proposed decisions to retain the current 

secondary PM standards, only a few 

were received on the proposed 

judgment that the existing secondary 

PM standards provide the needed 

protection from the effects of PM in 

ambient air associated with direct 

contact with and loading onto plant and 

lichen surfaces. All of these comments 

expressed support for that judgment. In 

the context of ecological effects of PM 

more broadly, including depositionrelated effects, comments received in 

support of the Administrator’s proposed 

decision to retain the current secondary 

PM standards, without revision, 

generally noted aspects of the rationale 

presented in the proposal. For example, 

some comments noted uncertainties in 

the relationship between concentrations 

of PM2.5 and deposition of N or S 

compounds. One comment, focused on 

the PM10 standard, expressed the view 

that the scientific information does not 

support revision of the PM10 standard. 

The EPA agrees with the view that the 

available information does not support 

revision of the PM NAAQS. 

In support of their position that the 

PM standards should not be revised, one 

commenter, noting a PA statement 

regarding PM components that may 

contribute to ecosystem acidification 

risk, expressed the view that the EPA 

lacks authority to set standards based on 

acidic deposition. As discussed in 

section II.B.2.a.(3)(a) above, the EPA 

disagrees with the view that NAAQS 

cannot be established to provide 

protection from acidic depositionrelated effects. Accordingly, as 

discussed in section II.B.3. below, the 

decision to retain the existing PM 

standards without revision is not based 

on such a premise. 

(2) Comments in Disagreement With the 

Proposed Decisions 

Most of the comments received in 

opposition to the proposed decisions to 

retain the existing secondary NO2 and 

PM standards, without revision, 

expressed the view that the standards 

should be revised to address N 

deposition and associated effects. Some 

of these comments additionally take 

note of the information indicating that 

the contribution of reduced N 

compounds has increased such that 

NH4

+ is a greater contributor to N 

deposition than in the past. Further, 

some commenters expressed the views 

that the CAA supports a standard in 

terms of N deposition and that the CAA 

also supports consideration of NH3 as a 

criteria pollutant. 

(a) Nitrogen Deposition 

Most of the commenters that 

disagreed with the proposed decisions 

on the secondary standards for N oxides 

and PM focus on N deposition and 

related effects in describing their 

rationales. Some commenters expressed 

the view that current N deposition is 

having impacts on resources in national 

parks (including parks that are also 

Class I areas); this comment is 

addressed in section II.B.2.b.(2)(b) 

below. These commenters also generally 

expressed the view that setting a 

deposition standard would be the best 

and/or a more scientifically defensible 

approach to standard setting, noting the 

CASAC advice in this regard. In so 

doing, one group of commenters noted 

the increasing role of NH3 in N 

deposition in recent times and 

expressed the view that the most 

effective way to address the NH3

contribution to N deposition and 

associated effects would be to set a 

standard in terms of total N deposition. 

Some other commenters expressed 

disagreement with the CASAC advice 

regarding establishment of a deposition 

standard under section 109 of the CAA, 

stating that given the EPA’s definition of 

ambient air as a portion of the 

atmosphere, an ambient air standard 

cannot be defined in terms of deposition 

rate. 

As also discussed in section 

II.B.2.a.(3)(a) above, we disagree with 

the premise that the CAA supports 

setting a NAAQS in terms of rates of 

deposition of a pollutant from the air 

onto surfaces. In addition, it is 

important to note that the criteria 

pollutants under review are PM and 

oxides of nitrogen, not nitrogen. Thus, 

the EPA is reviewing the standards 

intended to address the anticipated 

effects resulting from the presence of 

PM and N oxides in the ambient air, not 

the anticipated effects of NH3 in the 

ambient air, nor the effects of total N 

deposition in aquatic and terrestrial 

ecosystems generally. With regard to 

setting a NAAQS in terms of deposition 

rate, the commenters note the view of 

the CASAC in claiming the Act does not 

prevent the EPA from setting a standard 

in terms of atmospheric deposition 

rates. In so claiming, and in expressing 

their view on interpretation of the term 

‘‘level of air quality,’’ the commenters 

indicate that the term might variously 

(depending on the impact a pollutant 

has on the public welfare) be interpreted 

as ‘‘the pollution carried in the air that 

is deposited,’’ or the pollutant 

suspended in the air. Without further 

explanation, the commenters cite 

section 108 of the CAA as providing 

support for such a view. 

We disagree with the commenter’s 

interpretation of the Act. The EPA 

agrees that under section 108 the air 

quality criteria shall ‘‘reflect the latest 

scientific knowledge useful in 

indicating the kind and extent of all 

identifiable effects on public health or 

welfare which may be expected from the 

presence of such pollutant in the 

ambient air.’’ However, (as noted in 

section I.A above) section 109(b)(2) of 

the Act specifies that ‘‘[a]ny national 

secondary ambient air quality standard 

prescribed under subsection (a) shall 

specify a level of air quality the 

attainment and maintenance of which in 

the judgment of the Administrator, 

based on such criteria, is requisite to 

protect the public welfare from any 

known or anticipated adverse effects 

associated with the presence of such air 

pollutant in the ambient air.’’ Consistent 

with this statutory direction, the EPA 

has always understood the goal of the 

NAAQS is to identify a requisite level 

of air quality, and the means of 

achieving a specific level of air quality 

is to set a standard expressed as a 

concentration of a pollutant in the 

ambient air, such as in terms of parts 

per million (ppm), parts per billion 

(ppb), or micrograms per cubic meter 

(mg/m3). Additionally, as noted by some 

other commenters, the definition of 

ambient air in 40 CFR 50.1(e) describes 

ambient air as a portion of ‘‘the 

atmosphere’’ (‘‘external to buildings, to 

which the general public has 

access’’).119 Thus, taking section 108 

and section 109 together, the EPA 

concludes that deposition-related effects 

are included within the ‘‘adverse effects 

associated with the presence of such air 

pollutant in the ambient air,’’ but the 

standard itself must define a level of air 

quality. The EPA disagrees that a 

standard that quantifies atmospheric 


khammond on DSK9W7S144PROD with RULES2
105758 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

120To the extent CAA section 302(g) is relevant 

it simply provides discretion to the Administrator 

to treat precursors as pollutants where appropriate. 

While treating precursors as pollutants may be 

appropriate in some circumstances when 

implementing the NAAQS, the Administrator does 

not find it appropriate to treat precursors as criteria 

pollutants for purposes of reviewing and revising 

the NAAQS. 

121Additionally, precursors may be regulated in 

their own right as pollutants. For example, oxides 

of nitrogen are both a criteria pollutant and 

precursors to ozone, and VOCs may be regulated 

both as NESHAP and as ozone precursors. See CAA 

section 112(b)(2). However, in those cases the 

pollutant has independently satisfied the 

prerequisites for regulation under the relevant 

programs. 

122For example, as recognized by the CASAC 

majority ‘‘when considering all ecoregions, there is 

no correlation between annual average NO2 and N 

deposition’’ (Sheppard, 2023, Response to Charge 

Questions, p. 24). The final PA reported negative 

correlation coefficients for both NO2 EAQMs and a 

coefficient below 0.4 for SLAMS NO2

concentrations. 

deposition onto surfaces qualifies as 

such an air quality standard. 

In support of their disagreement with 

the EPA’s proposed decisions to retain 

the NO2 and PM2.5 standards without 

revision, commenters claim that the 

EPA’s ‘‘approach to N deposition’’ is 

unlawful and arbitrary because in their 

view if NH3 is a precursor to PM then, 

under the definition of ‘‘air pollutant’’ 

in CAA section 302(g), NH3

‘‘effectively’’ becomes a criteria 

pollutant. The EPA disagrees that 

precursors to criteria pollutants should 

be themselves treated as criteria air 

pollutants for all purposes. Section 108 

of the Act is quite explicit that only air 

pollutants that have been listed by the 

Administrator are criteria air pollutants, 

and the Administrator has never listed 

NH3 as a criteria pollutant. Of course, 

criteria air pollutants may have 

precursors and in considering strategies 

to attain and maintain the NAAQS, it is 

important to understand whether 

criteria pollutants are emitted into the 

air or formed in the atmosphere from 

precursor pollutants. However, those 

precursors are controlled to attain and 

maintain the NAAQS for the criteria 

pollutants—not because they 

themselves ‘‘effectively’’ become criteria 

pollutants that must be controlled.120

For example, in some areas, ozone 

formation is NOX limited, such that 

controls on VOC emissions may have 

little or no impact on ozone formation. 

State implementation plans for such an 

area will differ from those in an area 

where ozone formation is VOC-limited, 

because control of precursors is a means 

to the end of controlling ozone.121 It 

would be unnecessary to require 

controls on both VOCs and NOX in 

every area simply to control ozone. 

Thus, EPA disagrees that it should treat 

every precursor, including NH3, as a 

criteria pollutant. 

(b) Nitrogen Oxides 

The public comments that disagreed 

with the proposed decision to retain the 

secondary NO2 standard, without 

revision, expressed support for revision 

of the standard level to a value within 

the range that was recommended by the 

CASAC majority, with some 

commenters additionally citing the 

CASAC majority comments on the draft 

PA. In support of the position that the 

NO2 standard should be revised as 

recommended by the CASAC majority, 

commenters variously claimed that in 

not revising the standard, the EPA is not 

fully considering CASAC 

recommendations, or that the scientific 

evidence for N deposition demonstrates 

‘‘harmful’’ or concerning impacts of 

current N deposition in national parks. 

Also, some of the commenters that 

support revision of the NO2 standard to 

a level within the range recommended 

by the CASAC majority (‘‘<10–20 ppb’’) 

stated that the existing standard does 

not include all forms of nitrogen that 

contribute to acidification, 

eutrophication, or nutrient enrichment, 

and the standard would need to be 

much lower in consideration of 

relationships with total nitrogen 

deposition. One comment also 

expressed support for both retaining the 

existing standard and for revising the 

standard to a level of 35–40 ppb, 

averaged over three years, ‘‘as supported 

by the scientific evidence,’’ without 

elaboration. Another comment 

recommended revision of the indicator 

of the existing standard to include nitric 

oxide (NO) in addition to NO2, while 

recommending no other revisions. 

We disagree with the commenters’ 

position that the NO2 standard needs 

revision to provide public welfare 

protection from total N deposition. As 

an initial matter, we note that, as 

discussed in section II.B.2.b.(2)(a) 

above, not all nitrogen compounds are 

criteria pollutants and accordingly, the 

CAA does not require the consideration 

of NAAQS for all N compounds or for 

total N deposition. Further, the 

secondary standard for N oxides is not 

required by the Act to address 

pollutants other than N oxides. 

Additionally, the air quality and 

deposition analyses developed in this 

review (e.g., PA, Chapter 6 and Sales et 

al., 2024) describe appreciable 

geographic (and temporal) variation in 

the portion of total N deposition 

contributed by N oxides, potentially 

explaining the poor or lack of 

correlation between NO2 concentrations 

and total N deposition observed in the 

PA analyses,122 which indicates that a 

NO2 standard would have little 

likelihood of efficacy in such a use. 

One commenter stated that the 

current N deposition is resulting in 

harm to national park resources and 

expressed the view that the scientific 

evidence of N deposition adverse effects 

outweighs uncertainties associated with 

N critical loads. In so doing, the 

commenter claimed that justifications 

described in the PA for the option of 

retaining the NO2 standard, without 

revision, included (1) a lack of clarity of 

the role of current and legacy deposition 

in causing harm, and (2) the position 

that CLs involve designations of harm 

based on ‘‘arbitrary’’ levels of change. In 

so stating, the commenter conveyed 

their view that CLs are often based on 

studies that they stated demonstrate that 

reducing N deposition improves the 

resource condition even if N deposition 

continues to exceed a resource-specific 

CL. 

As an initial matter, the EPA disagrees 

that the PA conclusions relied on a 

judgment that critical loads are 

‘‘arbitrary’’ to support the option of 

retaining the NO2 standard and notes 

that is also not part of the basis for the 

proposed decision to retain this 

standard. As described in sections 

II.A.3.c. and II.B.2.a.(2)(a), the EPA 

recognizes the usefulness of the CL 

concept in appropriate contexts and has 

utilized CLs in the aquatic acidification 

REA. The findings from the REA, based 

on the use of CLs for a set of ANC 

benchmarks, are a critical aspect of the 

Administrator’s decision on the 

secondary SOX standard, as discussed in 

section II.B.3. below. Thus, while this 

concept can inform decision-making in 

NAAQS reviews, the science policy 

judgments associated with secondary 

NAAQS decisions, including those 

regarding risk levels associated with CL 

values and the weight to place on the 

evidence supporting them (with its 

various limitations and associated 

uncertainties), are to be made by the 

Administrator. The EPA does not agree 

with the view that a deposition rate 

identified as a CL is necessarily 

synonymous with environmental 

loading anticipated to elicit effects that 

are adverse to public welfare. Simply 

being labelled a CL does not confer such 

a status on a level of ecosystem loading 

without, for example, consideration of 

the strength of the evidence on which 

the CL is based, and a characterization 

of the ecological response (including 

severity and scale) for which it is 

estimated. 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105759 

123The comment did not discuss why this 

approach to assigning a ‘‘poor,’’ or other than 

‘‘good,’’ rating is evidence of N deposition-related 

impacts that could be addressed by revision of the 

NO2 or PM2.5 national ambient air quality standards 

or that indicates a potential for adverse effect to the 

public welfare. 

124For example, in the 14 ecoregions with 

median N deposition estimated to be above 10 kg/ 

ha-yr in the 2019–2021 period, reduced N 

comprises more than 50% of total N deposition 

(Sales et al., 2024, Table 3). 

125This study estimates multiple CLs that differ 

for nutrient enrichment- and acidification-related 

effects and for eastern and western lakes, relying on 

data generally dating from 1997 to 2006 (Baron et 

al., 2011). The second study uses a lake sediment 

core indicating a period of changed phytoplankton 

composition, estimated to be around 1969–75, and 

N deposition estimates for the 1969–75 period 

(Sheibley et al., 2014). 

126 In describing their position that the indicator 

should be revised, the commenter also suggests that 

the NAAQS ambient air monitoring system is 

inadequate. The commenter provided no evidence 

in support of this suggestion, and we continue to 

find that the current ambient air monitoring 

network for NAAQS is appropriate. 

In making their statement that 

assignment of a ‘‘poor’’ or ‘‘fair’’ 

conditions rating indicates impacts on 

national park resources, the commenter 

referred to a National Park Service 

technical analysis of ‘‘park-specific 

critical loads’’ and deposition, without 

providing that analysis or describing the 

basis for their judgments of harm for 

instances when estimated deposition in 

a specific area exceeds the critical loads 

they have derived.123 In addition, the 

commenter also did not provide any 

evidence specific to N oxides or 

deposition of oxidized N to support 

their claim regarding the N oxides 

standard. Rather the comment implied 

the view that impacts associated with 

total N deposition are attributable to N 

oxides. We disagree with the 

commenter’s view that deposition from 

N oxides under the existing standard is 

causing harm. As described in the 

proposal (section II.E.3.), in the PA, and, 

in greater detail, in the additional 

analyses presented in Sales et al. (2024), 

for the areas of highest total N 

deposition, such as areas where average 

total N deposition is above 10 kg/ha-yr, 

which is the benchmark emphasized by 

the CASAC in making its 

recommendations regarding standards 

to address the ecological effects of N 

compounds (as described in section 

II.B.1.b. above), oxidized N is no longer 

playing the leading role. Rather, 

reduced N contributes the majority of N 

deposition in these areas.124 Unlike the 

situation in 2000–2002, when oxidized 

N deposition accounted for up to 

approximately 80% of total N 

deposition, on average, in States with 

average total N deposition greater than 

10 kg/ha-yr, oxidized N deposition is 

now approximately half or less of total 

N deposition (Sales et al., 2024, Table 

5). In fact, in the most recent period 

analyzed (2019–2021), mean oxidized N 

deposition is below 5 kg/ha-yr in all 

States of the CONUS; this is also the 

case for median oxidized N deposition 

in all CONUS ecoregions (Sales et al., 

2024). 

Another group of commenters also 

referenced the National Park Service 

descriptions of park conditions related 

to N (and S) deposition in stating that 

95% of parks are experiencing wet 

deposition of N greater than 1 kg/ha-yr. 

They claimed that the occurrence of this 

level of deposition indicates harmful 

impacts to park soil, waterbodies and 

associated wildlife and indicated that 

such an occurrence supports their 

position that the secondary NO2 (and 

PM) standards should be revised as 

recommended by the CASAC majority. 

As support for the 1 kg/ha-yr benchmark 

below which a ‘‘good’’ condition is 

assigned (and above which is assigned 

a ‘‘fair’’ or ‘‘poor’’ rating which the 

commenters characterized as indicative 

of harm), the commenters cited two 

studies. 

The EPA notes, however, that the 

cited studies are limited in scope (to a 

lake in Washington State and a group of 

high-altitude lakes in some western and 

eastern regions) and include judgments 

by the authors of specific measures on 

which the authors base their CLs. One 

of the two studies actually identifies 

CLs ranging up to 8 kg/ha-yr (Baron et 

al., 2011).125 Yet, the comment focuses 

on 1 kg/ha-yr, without consideration of 

8 kg/ha-yr. In light of the limited scope 

of these studies, and the fact that a 

number of the identified CLs exceed 1 

kg/ha-yr, among other factors, the EPA 

does not agree that these studies provide 

a basis for concluding that adverse 

public welfare effects are occurring in 

95% of parks based on estimated 

deposition at/above 1 kg/ha-yr (a level 

far below the level referenced by the 

CASAC majority in advice regarding 

protective standards). These 

commenters also did not indicate how 

the National Park Service assignments 

of conditions in parks support the 

position that the NO2 standard should 

be revised to a level of <10–20 ppb, and 

we are unaware of any linkage. Further, 

as noted above, an appreciable amount 

of total N deposition is deposition of 

reduced N which is not influenced by 

N oxides in ambient air and 

consequently would not be affected by 

changes in a NAAQS for N oxides. 

With regard to acidification risk posed 

by deposition of N compounds, we 

additionally note the REA finding that 

recent deposition conditions indicate 

negligible contribution of N compounds 

to aquatic acidification risk. 

Accordingly, as discussed in section 

II.B.3. below, the decision to revise the 

SO2 standard is intended to address the 

main contributor to ecosystem 

acidification, S compounds associated 

with ambient air concentrations of SOX. 

Thus, in consideration of the preceding 

discussion and other factors further 

discussed in section II.B.3. below, the 

Administrator judges that, based on the 

available evidence in this review, 

revision to the secondary annual NO2

standard is not warranted. 

The commenter recommending 

revision of the standard indicator to 

include NO, in addition to NO2, 

expressed the view that the EPA should 

not assume that effects reported to be 

associated with short-term NO2

concentrations in ambient air have no 

relationship to NO, which the 

commenter stated is also present in 

ambient air. In so doing, the commenter 

cited a controlled human exposure 

study of diesel exhaust and brain 

function indicator changes, additionally 

cites an epidemiologic study that 

reports an association of health care 

costs with ambient air concentrations of 

NO2 and NO and noted that NO 

concentrations are higher than NO2

concentrations (in terms of ppb) in areas 

near traffic or oil and methane gas 

extraction activities. The EPA disagrees 

with the commenter that the effects on 

which the commenter focused—subtle 

changes in cellular activity in a specific 

region of human brain as reported in a 

controlled human exposure study of 

short-duration diesel exhaust exposures 

(in which NO2 [but not NO] was one of 

the components analyzed) and health 

care costs—are welfare effects; thus, 

their relevance for this review is 

unclear. 

Further, in support of their statement 

that NO2 concentrations in ambient air 

have no relationship to NO 

concentrations,126 the commenter 

simply referenced tables of hourly NO 

and NO2 concentrations available from 

Colorado Department of Public Health 

and Environment, which are clearly 

labeled as data collected in real-time 

that ‘‘have not been corrected nor 

validated.’’ We note that, although the 

data have not been validated, they 

generally illustrate the expected diurnal 

pattern for these pollutants near 

combustion sources (e.g., with NO 

initially increasing with morning traffic, 

and then declining as it is converted to 

NO2 [1971 AQCD, p. 6–1]). While 

recognizing these common patterns in 

the relationship between the two 


khammond on DSK9W7S144PROD with RULES2
105760 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

127This was also the advice of the CASAC 

minority, with 12 mg/m3 being the level of the 

annual primary standard when CASAC provided its 

advice. 

128One aspect overlooked is that the PA Figure 

6–32 cited by the commenter in referencing 

correlation coefficients presents a different metric 

than the figure in the draft PA cited by the CASAC 

majority in conveying its PM2.5 standard 

recommendations. Figure 6–13 in the draft PA that 

was cited by the CASAC majority presents 3-year 

average concentrations of data from 2002 to 2019 

(using different 3-year periods than those used 

throughout the rest of the PA), while the final figure 

chemicals, we further note that the form 

of the existing standard is an annual 

average, and the commenter did not 

provide validated data or analyses that 

might assess the existence of a, or 

support their view that there is no, 

relationship between annual average 

concentrations of NO and NO2. 

The comment also does not include 

any information related to 

concentrations of either pollutant, or 

both in combination, at which welfare 

effects of concern may occur and relate 

that to ambient air concentrations 

associated with the existing secondary 

NO2 standard. The evidence in 1971 

when the existing standard was set 

describes the conversion of NO to NO2

in the presence of oxygen, with NO2

being the more stable air pollutant away 

from sites of combustion and the one for 

which analytical methodology existed 

for its direct analysis at that time (1971 

AQCD). While there is a study from the 

mid-1980s for effects of NO on 

photosynthesis, which indicates a 

potential for greater toxicity of NO to 

some plant species, the NO 

concentrations reported for this study 

are nearly two orders of magnitude 

greater than those found in ambient air. 

Further, the vegetation effects evidence 

base is much more extensive (with 

regard to species and specific effects 

studied) for NO2 and includes studies 

that investigate both NO and NO2

together (ISA, Appendix 3, section 3.3). 

The NO2 standard is intended to 

provide protection from anticipated 

effects of oxides of nitrogen, including 

NO and NO2, but the commenter does 

not provide a basis for concluding that 

an annual average NO2 standard is 

insufficient to provide the requisite 

protection. Thus, we find no support in 

the available information in this review 

that might support their claim that the 

existing standard should be revised to 

be an annual average concentration of 

53 ppb, in terms of the sum of NO and 

NO2. 

(c) Particulate Matter 

Comments opposed to the proposed 

decision to retain the secondary PM 

standards generally focused on PM2.5

and called for more stringent secondary 

standards. In so doing, these 

commenters cited the specific PM2.5

standard revisions recommended by the 

CASAC majority, summarized in 

II.B.1.b. above. With regard to the 

annual PM2.5 standard, these 

commenters also discussed analyses 

presented in the PA, which they stated 

provide support to the use of the annual 

PM2.5 standard to address total N 

deposition. In support of a revision to 

the PM2.5 standard, some commenters 

noted the increased role of NH3 in total 

N deposition, including in estuaries and 

coastal waters where eutrophication has 

been reported or in national parks. 

These commenters expressed the view 

that the contribution of NH3 to N 

deposition and related effects can be 

addressed through revisions to the PM2.5

standard. In so doing, they further stated 

that the EPA’s proposed decision to 

retain the existing standard is based on 

uncertainties and complexities related 

to NH3 and that such uncertainties and 

complexities are an insufficient basis for 

retaining the existing standard, 

additionally citing a 2002 court decision 

regarding EPA acting when it has 

enough information to do so (Am. 

Trucking Ass’ns v. EPA, 283 F.3d 355, 

380 [D.C. Cir. 2002]). In support of their 

position, the commenters stated that the 

EPA must act when enough information 

is available to anticipate such effect, and 

deciding not to revise is inconsistent 

with the Act’s protective direction. 

Commenters additionally suggest that 

the EPA inappropriately imposed limits 

on its consideration of the trajectorybased analyses so as to provide support 

for the EPA conclusion that the NO2 and 

PM2.5 metrics do not provide adequate 

vehicles for regulating N deposition. 

Another commenter, in support of their 

position that the existing PM2.5

standards should be revised as 

recommended by the CASAC majority, 

expressed the view that reduced N 

deposition has become the dominant 

form of N deposition, which they stated 

is impacting national park resources in 

many areas of the U.S. such that a 

revised standard would help to reduce 

such pollutants. Additionally, a 

comment recommending revision of the 

PM2.5 standard stated that the range of 

revised levels suggested by the CASAC 

majority would keep S deposition below 

5 kg/ha-yr and N deposition at or below 

10 kg/ha-yr and stated that the CASAC 

majority range was based on NADP and 

IMPROVE monitoring data and modeled 

results, without further explanation. 

Another comment recommended 

revision of the annual PM2.5 standard to 

12 mg/m3,127 based on their view that it 

would add no additional requirements 

and could streamline implementation 

plan development and compliance. 

Lastly, some commenters additionally 

expressed that the 24-hour PM2.5

standard should be revised, again citing 

recommendations from the CASAC 

majority and protection against shortterm episodic deposition and visibility 

impairment. 

For the reasons stated below, 

elsewhere in section II.B.2., in section 

II.B.3. and in the Response to Comments 

document, the EPA disagrees that these 

comments provide a sufficient 

justification for revising the PM 

secondary standards. In support of their 

position that the PM2.5 standard is an 

appropriate tool for controlling 

particulate N and should be revised to 

a value within the range of 6 to 10 mg/ 

m3 recommended by the CASAC 

majority, some commenters state that 

NH4

+ has been increasing in cloud water 

and in PM2.5 and reference statistically 

significant correlation coefficients for 

total N deposition estimates and 

concentrations of PM2.5 mass (and N 

components) in remote Class I areas 

(PA, Figure 6–32), which they suggest 

supports their view that use of PM2.5 ‘‘as 

an ambient air quality indicator to total 

nitrogen deposition is not 

unreasonable.’’ They also claim that 

Figure 6–32 in the final PA, and Figure 

6–33 presenting total N deposition 

estimates versus total particulate N and 

NH4

+ at 27 Class I area sites, provide 

support for the CASAC majority 

recommendation on revising the PM2.5

standard, which they endorse. 

As an initial matter, we disagree with 

the view that effects of total N 

deposition (from all contributing 

pollutants) are a determinative 

consideration in judging the adequacy 

of the secondary PM2.5 standard, as 

discussed in section II.B.2.b.(2)(a) 

above. Further, we disagree that NH4

+ in 

PM2.5 has been increasing, finding 

instead that the contribution of NH4

+ to 

PM2.5 mass at sites across the U.S. has 

been decreasing over the past decade 

(Sales et al., 2024). Further, to the extent 

the commenters are claiming the 

CASAC majority recommended range of 

annual PM2.5 standard levels, which 

they endorse, to be supported by the 

pattern of PM2.5 concentrations and total 

N deposition estimates at 27 Class I area 

IMPROVE monitoring sites (in either 

draft PA Figure 6–13 or final PA Figure 

6–32), we disagree that this information 

provides a basis for decisions on the 

standard. The commenters are 

overlooking several relevant aspects of 

the available information.128


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105761 

in the final PA presents annual averages from 2000 

to 2019 (PA, Figure 6–32). 

129 In the period from 2014 through 2019 (the 

period emphasized in the CASAC majority 

justification that relied on draft PA Figure 6–13) 

when TDep estimated N deposition is at/below 10 

kg/ha-yr and annual average PM2.5 concentrations 

are at/below 10 mg/m3 at the 27 Class I area sites, 

annual average PM2.5 concentrations are much 

higher in other areas of the U.S. that are more fully 

represented in the regulatory monitoring network 

(PA, Figure 2–37). As indicated by recent PM2.5

design values, the highest concentrations sites are 

generally in the far west of the country, which given 

prevailing wind patterns, are generally upwind 

from the Class I areas (PA, Figures 2–31 and 2–32). 

130Concentrations at SLAMS from just above 15 

mg/m3 down to approximately 4 mg/m3 since 2010 

had nearby total N deposition (in same grid cell) 

both above and at/below 10 kg/ha-yr (PA, Figure 6– 

39), and the SLAMS analyses did not provide 

information on ecoregion median deposition for the 

ecoregion of SLAMS monitor. 

Particularly important is that the 

monitoring sites represented by these 

figures comprise just a small subset of 

the more than a thousand PM2.5

monitoring sites across the U.S., and 

this subset of monitors is in remote 

areas. Accordingly, these monitors are 

not in areas where PM2.5 concentrations 

are highest. Thus, the PM2.5

concentrations in the remote area figure 

are not representative of PM2.5

concentrations that would need to be 

controlled to limit deposition across the 

U.S., including in these areas. Such 

deposition is necessarily related to 

atmospheric transport, among other 

factors, and a focus solely on remote 

areas cannot be expected to identify the 

level for a PM2.5 standard (that would 

need to be met across the U.S.) with the 

potential to yield the desired deposition 

rate in these areas. This is because at the 

time of the deposition levels observed in 

these areas, the PM2.5 concentrations are 

higher in areas not represented in the 

figure that may contribute to deposition 

at the sites in the figure (and at other 

sites).129 Further, the PA analyses of N 

deposition and PM2.5 concentrations at 

SLAMs also do not provide a basis for 

identifying 3-year average annual PM2.5

concentrations that might be expected to 

constrain nearby N deposition below 

certain target levels (e.g., PA, Figure 6– 

39).130 For all of these and related 

reasons, the Administrator, in making 

his proposed and final judgments 

regarding the secondary PM standards, 

did not find the CASAC majority focus 

on remote area analyses to be 

informative in making decisions on the 

annual PM2.5 standard. 

Regarding the commenters’ criticism 

of the EPA’s consideration of the 

trajectory-based analyses for N 

deposition and the PM2.5 metric, we 

note that the commenters do not 

identify a technical flaw in EPA’s 

considerations or state what they 

conclude from the trajectory-based 

analyses and how they do so. The EPA 

has fully considered the trajectory-based 

analysis results presented in the PA 

(PA, section 6.2.4.2, 6.4, 7.2.3.3 and 7.4) 

and summarized in section II.A.2. 

above. We note that, while, when 

considering the full dataset, there is a 

positive correlation of downwind total 

N deposition and upwind values of the 

EAQM-weighted metric, with a lowmoderate coefficient value, the 

correlation coefficient value is 

essentially zero in the most recent time 

period (PA, Table 6–11). And, 

importantly, there is a poor and negative 

correlation for the EAQM-max metric; 

this correlation is negative both for the 

overall dataset inclusive of all five time 

periods and for each of the five time 

periods individually (PA, Table 6–11). 

Thus, we disagree with commenters that 

we have inappropriately or inadequately 

considered the trajectory-based analyses 

for PM2.5 and N deposition. Also, rather 

than limiting consideration of these 

results to a narrow temporal window, as 

claimed by the commenters, we have 

considered multiple aspects of the full 

results. As described in section II.B.3. 

below, these considerations were part of 

the basis for the Administrator’s 

conclusion on the PM standards. 

Also overlooked by the commenters is 

the fact, as discussed in section II.A.2. 

above, that the percentage of PM2.5 mass 

comprised of N compounds is no higher 

than about 30% in the recent period, 

and ranges down to less than 5% across 

the U.S., varying widely from region to 

region (PA, Figure 6–56 [upper panel]; 

Sales et al., 2024). We note that focus by 

the commenters (and the CASAC 

majority) on a small subset of the PM2.5

monitors across the U.S. (i.e., monitors 

in 27 Class I area sites [PA Figure 6–32]) 

would not necessarily reflect the 

variability of PM2.5 mass composition 

occurring across the U.S. Nonetheless, 

the percentage of PM2.5 mass comprised 

of N compounds affects the extent to 

which a particular level for an annual 

secondary PM2.5 standard levels can be 

expected to control N deposition rates to 

meet a particular objective for 

protection from deposition-related 

effects. As described in section I.A. 

above, the Administrator is required to 

set a NAAQS that is neither more 

stringent nor less stringent that 

necessary. Given the fact that up to 95% 

of PM2.5 in some regions of the U.S. (and 

no more than 70% in others) is not N 

compounds, we are unable to make a 

reasoned judgment about levels of N 

deposition that would result from 

control measures to reduce PM2.5

concentrations to any particular level. In 

fact, based on the information available, 

annual average PM2.5 concentrations 

could be reduced in some areas, e.g., to 

meet a lower standard, without reducing 

concentrations of the N components of 

PM2.5 and, therefore, without affecting N 

deposition derived from PM2.5. Thus, 

contrary to the commenters’ claims, 

including that revision to a level within 

the CASAC majority recommended 

range would keep N deposition at or 

below 10 kg/ha-yr, the current 

information indicates that a PM2.5

standard would not be expected to 

provide effective control of particulate 

N compounds. 

With regard to the comment that the 

EPA should revise the PM2.5 standard to 

address the effects of N deposition 

contributed by NH3, we first note that 

while some NH3 (a gas) transforms to 

NH4

+ (a particulate N compound in 

PM2.5), some NH3 is directly deposited 

in dry deposition. Further, some NH3 is 

captured in raindrops, where it 

transforms into NH4

+ as it is deposited 

in wet deposition (PA, section 2.5.2; 

Sales et al., 2024). We additionally note, 

as discussed in section II.B.2.b.(2)(a), 

that NH3 is not a criteria pollutant. As 

described above and discussed in 

section II.B.3., the Administrator has 

considered the PM2.5 standard with 

regard to ecological effects of N 

deposition associated with PM and 

protection of the public welfare from 

such effects. In so doing, he has 

understood that the percentage of PM2.5

relevant to such effects ranges from 30% 

down to 5% or less that is N 

compounds, and that this percentage 

varies across the U.S. In light of this and 

other relevant factors, the Administrator 

has judged that the PM2.5 standard 

would be ineffective with regard to 

control of deposition of particulate N 

compounds, and, as discussed more 

fully in section II.B.3., has decided to 

retain the existing standard, without 

revision. 

The EPA also disagrees with the view 

that the uncertainties and complexities 

(and limitations) associated with the 

evidence base and air quality 

information that were cited by the EPA 

in its proposed decision to retain the 

PM standards are an insufficient basis 

for retaining the existing standard. 

Although these uncertainties and 

complexities include those related to 

NH3, they are not, as the commenter 

suggests, limited to those related to NH3. 

In support of the commenters’ view, 

they note that the EPA must act when 

enough information is available to 

anticipate such effect and then assert 

that to not revise the secondary PM2.5

standards ‘‘is inconsistent with the Act’s 

protective direction.’’ While we agree 


khammond on DSK9W7S144PROD with RULES2
105762 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

that the EPA must act when enough 

information is available to anticipate 

effects, and we recognize that revising 

the NAAQS generally requires acting in 

the face of uncertainties to provide 

necessary protection (as the 

Administrator is doing in setting a new 

SO2 standard), the Administrator cannot 

set a standard if he lacks any ability to 

make a reasoned judgment about the 

effect of the standard. As recognized 

above and discussed in section II.B.3. 

below, the uncertainties and limitations 

of the information with regard to 

support for a PM2.5 standard that can be 

concluded to provide control for 

deposition-related effects of particulate 

N compounds, including NH4

+, 

preclude our ability to characterize the 

extent of control that can be expected. 

In addition, the EPA disagrees with 

commenters who support revising the 

PM2.5 standard based on their view that 

this would maintain S deposition 

generally at/below 5 kg/ha-yr. First, we 

find that the PM2.5 indicator is not an 

appropriate tool and cannot be expected 

to be an effective tool for controlling S 

deposition in light of the fact that, in 

recent periods, SO4

2¥ (the predominant 

particulate S compound) is not the 

dominant component of PM2.5 across the 

U.S. and is a small component in many 

areas (ISA, Appendix 2, Figure 2–5 

[panel B, 2013–2015]; PA, Figure 2–30 

[2019–2021]). The variability in the 

fraction of PM2.5 comprised of S 

compounds likely contributes to the PA 

findings on correlations of S deposition 

with PM2.5 concentrations (PA, Chapter 

6). The correlation coefficients for this 

relationship in the trajectory-based 

analyses are lower than those for the 

relationship between S deposition and 

SO2 concentrations, with the correlation 

for the PM2.5 EAQM-max actually being 

negative (PA, Tables 6–12 and 6–8). In 

light of such findings, the Administrator 

has not found PM2.5 to be an appropriate 

indicator for a secondary standard to 

provide protection from ecosystem 

effects of S compound deposition. 

Rather, as discussed in section II.B.3. 

below, based on the available 

information and analyses, the 

Administrator has judged that a new 

annual secondary SO2 standard of 10 

ppb can be expected to achieve the 

target identified by the CASAC majority 

of generally maintaining S deposition 

at/below approximately 5 kg/ha-yr. This 

new SO2 standard provides a much 

more explicit and precise approach for 

controlling S deposition-related effects 

of SOX and particulate S compounds. 

The comment that recommended 

revision of the annual PM2.5 standard to 

be 12 mg/m3, based on the view that it 

would not present additional 

requirements and could streamline 

implementation plan development and 

compliance, provided no information 

related to the extent of public welfare 

protection that might be provided by 

such a revision, or information 

indicating that the existing standard 

does not provide adequate protection. 

As explained in section II.B.3. below, 

the EPA disagrees with the commenter’s 

recommendation for such a revision, 

and the Administrator finds that the 

available information supports retaining 

the current standard. 

The comment regarding revision of 

the 24-hour PM2.5 standard to address 

short-term episodic deposition and 

visibility impairment expresses support 

for the CASAC majority 

recommendation on this. Beyond this 

reference to the CASAC majority 

recommendation, the comment 

provided no evidence to support their 

view that there are adverse effects of 

episodic deposition that would be 

appropriately addressed by revision of 

the standard level to 25 mg/m3 (from 35 

mg/m3). As described in section II.B.1.b. 

above, the CASAC majority 

recommendation, while alluding to a 

potential for seasonal variability in 

deposition and in sensitivity of some 

species, did not provide evidence for 

such potentials or evidence to support 

the conclusion that a revised standard is 

needed to protect against adverse 

ecological effects on the public welfare, 

and the EPA is not aware of such 

evidence. Thus, as described in section 

II.B.3. below, the Administrator has 

decided to retain the existing 24-hour 

secondary PM2.5 standard. 

Regarding visibility impairment, as 

conveyed in the IRP, PA and proposed 

decision document for this review, 

PM2.5 effects on visibility are outside the 

scope and are not being addressed in 

this review because they were addressed 

in the recently completed PM NAAQS 

review, which also revised the primary 

NAAQS for PM2.5 (89 FR 16202, March 

6, 2024). The commenters advocating 

for consideration of visibility here 

erroneously state that these effects were 

addressed in setting the primary PM2.5

NAAQS and further state that this is not 

a reason for excluding them from 

consideration in this review. We note, 

however, that the primary PM2.5

NAAQS are not intended to address 

visibility impairment. Rather, the 

recently completed review covered both 

the primary PM2.5 NAAQS as well as 

review of the secondary NAAQS for 

visibility, materials damage and climate 

effects. See 89 FR 16202 at 16311–16343 

(rationale for decisions on the secondary 

NAAQS). Thus, visibility is a welfare 

effect that has been addressed in 

assessing the protection provided for the 

public welfare by the secondary PM2.5

standard in the 2020 PM NAAQS 

decision and the reconsideration of that 

decision which was completed earlier 

this year (89 FR 16202, March 6, 2024) 

and is outside the scope of this review. 

3. Administrator’s Conclusions 

Having carefully considered the 

public comments, as discussed above, 

the Administrator believes that the 

fundamental scientific conclusions on 

the ecological effects of SOX, N oxides, 

and PM reached in the ISA and 

summarized in the PA and in section 

II.C. of the proposal remain valid. 

Additionally, the Administrator believes 

that the judgments he reached in the 

proposal (section II.E.3.) with regard to 

consideration of the evidence and 

quantitative assessments and advice 

from the CASAC remain appropriate. 

Thus, as described below, the 

Administrator concludes that the 

current secondary SO2 standard is not 

requisite to protect the public welfare 

from known and anticipated adverse 

effects associated with the presence of 

SOX in the ambient air and that the 

standard should be revised. Further, 

based on the information available in 

this review and summarized in the 

proposal, including advice from the 

CASAC, as well as public comment and 

additional analyses developed in 

consideration of public comments, the 

Administrator concludes that revision of 

the existing 3-hour secondary SO2

standard to an annual standard of 10 

ppb, averaged over three years, is 

required to provide additional needed 

protection from atmospheric depositionrelated effects. He additionally 

concludes that it is appropriate to retain 

the existing secondary standards for N 

oxides and PM. 

In his consideration of the adequacy 

of the existing secondary standards for 

SOX, N oxides, and PM, and what 

revisions or alternatives are appropriate, 

the Administrator has carefully 

considered the available evidence and 

conclusions contained in the ISA 

regarding the weight of the evidence for 

both the direct effects of SOX, N oxides, 

and PM on plants and lichens and for 

effects related to atmosphere deposition 

in ecosystems of N and S compounds 

associated with the presence of these 

pollutants in ambient air, and associated 

areas of uncertainty. In so doing, he 

recognizes the evidence of direct 

biological effects associated with 

elevated short-term concentrations of 

SOX and N oxides that formed the basis 

for the existing secondary SO2 and NO2

standards, the evidence of ecological 

effects of PM in ambient air, primarily 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105763 

131For example, there are no studies in the 

available evidence investigating linkages of S 

deposition, in terms of quantitative estimates, such 

as CLs, with other non-acidifying effects (ISA, 

Appendix 12, section 12.6); these effects, in 

wetland and freshwater ecosystems, include the 

alteration of Hg methylation in surface water, 

sediment, and soils; and changes in biota due to 

sulfide phytotoxicity including alteration of growth 

and productivity, species physiology, species 

richness, community composition, and biodiversity 

(ISA, Appendix 12, section 12.7). 

132This refers to the 90th percentile in the 

distribution of S deposition estimates for TDep grid 

cells in each ecoregion in which there were 

waterbody sites assessed in the REA. 

133Effects of elevated acid deposition have been 

evident for decades in the Adirondack region of 

New York, USA (Driscoll et al 2016). Fisheries 

surveys by NY DEC in the 1980s indicated 

reductions in fish populations in Adirondack lakes 

which researchers indicate may relate to 

acidification in these lakes (Baker and Schofield, 

1985). For example, a survey of 1469 Adirondack 

lakes conducted in 1984–87 found chronic acidity 

(ANC below 0 meq/L) in 27% of lakes (Kretser et 

al., 1989). An additional 21% of Adirondack lakes 

were found to have summertime ANC values 

between 0 and 50 meq/L, indicating a potential for 

ANC to dip to values near or below 0 meq/L during 

periods of high discharge, such as snowmelt or 

precipitation events (Kretser et al., 1989). 

associated with loading on vegetation 

surfaces, and also the extensive 

evidence of ecological effects associated 

with atmospheric deposition of N and S 

compounds into sensitive ecosystems. 

He has also considered the quantitative 

analyses of aquatic acidification risk 

and of air quality and deposition 

estimates, with associated limitations 

and uncertainties; policy evaluations of 

the evidence, exposure/risk information, 

and air quality information in the PA; 

and the related additional analyses 

(Sales et al., 2024). Together, these 

conclusions, analyses, and evaluations, 

along with CASAC advice and public 

comments, inform his judgments in 

reaching his decisions on secondary 

standards for SOX, N oxides, and PM 

that provide the requisite protection 

under the CAA. 

In recognizing that a prominent part 

of this review is the consideration of 

secondary NAAQS with regard to 

ecological effects related to deposition 

of S and N compounds, the 

Administrator notes the view of the 

CASAC regarding deposition standards. 

In its advice to the Administrator in this 

review, the CASAC expressed the view 

that the CAA does not preclude the 

establishment of a NAAQS in terms of 

atmospheric deposition (section II.B.1.b. 

above). As discussed in sections 

II.B.2.b.(2)(a) and II.B.2.b.(3)(a) above, 

the EPA disagrees with this view. 

Rather, the EPA concludes that it does 

not have the authority to set a 

deposition standard under the existing 

CAA, and the EPA is not adopting a 

deposition standard in this action. 

With regard to the adequacy of public 

welfare protection provided by the 

existing secondary SO2 standard, the 

Administrator first considers the 

adequacy of protection the existing 

standard provides for ecological effects 

related to ecosystem deposition of S 

compounds associated with the 

presence of SOX in ambient air. As an 

initial matter, the Administrator 

recognizes the long-standing evidence of 

the role of SOX in ecosystem 

acidification and related ecological 

effects. While he also notes the ISA 

determinations of causality for S 

deposition with two other categories of 

effects related to mercury methylation 

and sulfide phytotoxicity (ISA, Table 

ES–1; PA, section 4.4), he recognizes, as 

noted in section II.A.3.c. above, that 

quantitative tools and approaches are 

not well developed for ecological effects 

associated with atmospheric deposition 

of S other than ecosystem acidification 

(PA, section 7.2.2.1).131 In this context, 

he notes that the current evidence does 

not indicate such effects to be associated 

with S deposition at lower rates than 

those posing risks of ecosystem 

acidification, and judges that a decision 

focused on providing the requisite 

protection for acidification-related 

effects will also contribute protection 

for other effects. Thus, he gives primary 

consideration to effects related to 

acidifying deposition, given the robust 

evidence base and available quantitative 

tools, as well as the longstanding 

recognition of historical impacts in acidsensitive ecosystems across the U.S. 

As an initial matter, the Administrator 

notes that, during the 20-year period 

from 2001 through 2020, the range of 

median S deposition estimates for the 

84 ecoregions in the contiguous U.S. 

extend up to 20 kg S/ha-yr (PA, 

Appendix 5A, Table 5A–11) and that 

during this period the existing 

secondary SO2 standard was met (Sales 

et al., 2024). Over this 20-year period in 

the contiguous U.S., design values for 

the existing secondary SO2 standard 

(second highest 3-hour average in a 

year) were generally well below the 

standard level of 500 ppb (PA, section 

6.2.1). For example, in the earliest 3-yr 

period analyzed (2001–2003), when 

median total S deposition was estimated 

to be approximately 20 kg/ha-yr in the 

Western Allegheny Plateau ecoregion 

(which includes the Ohio River Valley) 

and just over 16 kg/ha-yr in the Central 

Appalachians ecoregion (PA, Appendix 

5A, Table 5A–11), virtually all design 

values for the existing 3-hour secondary 

standard were below 400 ppb (across 

the CONUS) and the 75th percentile of 

3-hour design values was below 100 ppb 

(PA, Figure 2–27). With regard to the 18 

eastern ecoregions assessed in the REA, 

the Administrator notes that during this 

period, the ecoregion median deposition 

ranged above 15 kg/ha-yr and the 90th 

percentile 132 S deposition estimates for 

half of these 18 ecoregions were at or 

above 15 kg/ha-yr, ranging up above 20 

kg/ha-yr in the highest ecoregion (figure 

2 above). 

In considering the extent to which 

this magnitude of estimated S 

deposition (summarized immediately 

above) indicates a potential for effects 

on the public welfare, the Administrator 

turns to consideration of the aquatic 

acidification risk indicated for such 

estimates by the REA. Specifically, he 

takes note of the REA estimates of 

aquatic acidification risk associated 

with the S deposition estimated to have 

occurred in 2001–2003, when the 

existing standard was met. In this time 

period, the REA finds that across the 18 

acid-sensitive ecoregions analyzed, the 

pattern of S deposition in the five most 

affected ecoregions is associated with 

more than about a third of waterbody 

sites in the ecoregions being unable to 

achieve even the lowest of the three acid 

buffering capacity benchmarks used as 

risk indicators (ANC of 20 meq/L). And, 

in the single most affected ecoregion, 

more than half of waterbody sites are 

unable to meet this benchmark. In 

considering these results, the 

Administrator recognizes the use of 

ANC as an indicator of aquatic 

acidification risk and as a quantitative 

tool within a larger framework of 

considerations pertaining to the public 

welfare significance of acid depositionrelated effects. In this framework, he 

takes note of the PA description of the 

three benchmarks used in the REA, with 

the value of 20 meq/L considered to 

represent a level of acid buffering 

capacity consistent with a natural or 

historically occurring ANC range and 50 

meq/L to provide greater protection, 

particularly from episodic acidification 

events, additionally recognizing that 

ANC levels below 20 meq/L have been 

associated with reductions in number of 

fish species (and species population 

sizes) in some sensitive waterbodies of 

the Shenandoah and Adirondack 

Mountains (as summarized in section 

II.A.4.a. above).133

The Administrator also takes note of 

the PA discussion of the potential 

public welfare impacts of aquatic 

acidification that can include reductions 

in recreational and subsistence fisheries, 

and related reductions in recreational 

and cultural usage of these areas by the 

public, summarized in sections II.A.3.b. 

and II.B.1.a.(3) above. For example, he 


khammond on DSK9W7S144PROD with RULES2
105764 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

134While the final PA provides additional 

presentations of aquatic acidification risk estimates, 

including those at the ecoregion-scale, the estimates 

for percentages of waterbodies per ecoregion 

achieving ANC targets at or below different S 

deposition values are unchanged from those in the 

draft PA (PA, section 5.1.3; Table 5–5 [draft PA, 

Table 5–4]). 

135The presentation of such percentages in the 

draft PA (reviewed by the CASAC) were specific to 

the 90 ecoregion-time period combinations for the 

18 eastern ecoregions (draft PA, Table 5–4; PA, 

Table 5–5). Inclusion of the 7 western ecoregions 

would yield higher percentages, as more than 90% 

of waterbodies in those ecoregions were estimated 

to achieve all three ANC concentration in all time 

periods (PA, Table 5–4). 

recognizes that aquatic acidification 

affects the diversity and abundance of 

fish and other aquatic biota in the 

affected waters, and consequently also 

affects the array of public uses of these 

waterbodies. With this in mind, he 

focuses on the prevalence of elevated 

aquatic acidification risk across 

multiple waterbodies in multiple 

ecoregions (with ANC as the 

acidification risk indicator) recognizing 

that the significance of aquatic 

acidification-related impacts on the 

public welfare (e.g., associated with 

reductions in public usage of aquatic 

ecosystems in which fisheries have been 

affected by acidification) increases with 

greater prevalence of affected 

waterbodies and ecoregions. In this 

context, the Administrator judges that 

the prevalence of waterbodies 

concluded to be unable to achieve the 

lowest ANC benchmark (below which 

the increased risk of episodic 

acidification events may threaten 

survival of sensitive aquatic species) 

during the 2001–2003 period— 

extending from more than 30% to just 

over 50% in the five most affected 

eastern ecoregions (figure 1 above)—can 

be anticipated to cause adverse effects 

on the public welfare. The 

Administrator also considers the advice 

from the CASAC in considering 

deposition-related effects of S 

compounds, noting the CASAC 

consensus that the existing standard 

does not provide protection from such 

effects. Lastly, he notes the lack of 

public comments conveying the 

position that the existing standard 

provides protection from depositionrelated effects (section II.B.2.a. above). 

Thus, based on the findings of the REA, 

associated policy evaluations in the PA 

with regard to S deposition and 

acidification-related effects in sensitive 

ecosystems, and in consideration of 

advice from the CASAC and public 

comments on the proposed decision, the 

Administrator judges that the current 

SO2 secondary standard is not requisite 

to protect the public welfare from 

adverse effects associated with acidic 

deposition of S compounds in sensitive 

ecosystems. 

Having reached this conclusion that 

the existing secondary SO2 standard 

does not provide the requisite 

protection of the public welfare from 

adverse S deposition-related effects, 

most prominently those associated with 

aquatic acidification, the Administrator 

next turns to identification of a 

secondary standard to provide such 

protection. In so doing, consistent with 

the approach employed in the PA, he 

focuses first on identifying S deposition 

rates that might be judged to provide an 

appropriate level of public welfare 

protection from deposition-related 

effects. As in reaching his proposed 

decision, the Administrator focuses 

primarily on the aquatic acidification 

risk estimates as presented and 

evaluated in the PA (PA, sections 5.1, 

7.1 and 7.3, and Appendix 5A) and 

summarized in sections II.A.4. and 

II.B.1.a.(3) above. In this context and 

consistent with his consideration of 

these estimates in judging the existing 

SO2 standard to be inadequate, he finds 

the PA evaluation of the risk estimates 

in terms of waterbodies estimated to 

achieve the three acid buffering capacity 

benchmarks (20, 30 and 50 meq/L) to be 

an appropriate basis for his 

consideration of levels of protection. 

Further, he judges that a focus on the 

ecosystem-scale estimates, in particular, 

is appropriate for his purposes in 

identifying conditions that provide the 

requisite protection of the public 

welfare. 

The Administrator recognizes that the 

CAA requires the establishment of 

secondary standards that are, in the 

Administrator’s judgment, requisite (i.e., 

neither more nor less stringent than 

necessary) to protect the public welfare 

from known or anticipated adverse 

effects associated with the presence of 

the pollutant in the ambient air. As in 

all NAAQS reviews, the EPA’s approach 

to informing these judgments is based 

on a recognition that the available 

welfare effects evidence generally 

reflects a continuum that includes 

ambient air-related exposures for which 

scientists generally agree that effects are 

likely to occur, through lower levels at 

which the likelihood and magnitude of 

response become increasingly uncertain. 

The Administrator recognizes that the 

CAA does not require establishment of 

secondary standards at a zero-risk level, 

but rather at levels that reduce risk 

sufficiently so as to protect the public 

welfare from known or anticipated 

adverse effects. Thus, the Administrator 

recognizes that his decision on the 

secondary standard for SOX is 

inherently a public welfare policy 

judgment that draws upon the scientific 

evidence for welfare effects, quantitative 

analyses of air quality, exposure, and 

risks, as available, and judgments about 

how to consider the uncertainties and 

limitations that are inherent in the 

scientific evidence and quantitative 

analyses. 

In his consideration of deposition 

conditions that provide the requisite 

protection of the public welfare, as in 

reaching his proposed decision, the 

Administrator focuses on the ecoregionscale findings of the aquatic 

acidification REA, with particular 

attention to the waterbody-specific risk 

estimates summarized in the PA for 

each of the 18 well-studied, acidsensitive eastern ecoregions and the five 

time periods. The PA summarizes the 

percentages of waterbodies per 

ecoregion estimated to achieve (i.e., to 

meet or exceed) the three ANC 

benchmarks in each time period in 

terms of the ecoregion median S 

deposition value for that time period, 

which are grouped into bins (e.g., 

percentages for ecoregion-time period 

combinations with median ecoregion S 

deposition at/below 10 kg/ha-yr, or 8 

kg/ha-yr or 5 kg/ha-yr). The 

Administrator considers particularly the 

ecoregion median S deposition values at 

and below which the associated 

waterbody-specific risk estimates 

indicated a high proportion of 

waterbodies in a high proportion of 

ecoregions would achieve ANC values 

at or above the three acid buffering 

capacity benchmarks (as summarized in 

tables 3 and 4 above). In so doing, he 

recognizes a number of factors, as 

described in the PA, which contribute 

variability and uncertainty to waterbody 

estimates of ANC and to interpretation 

of acidification risk associated with 

different values of ANC (PA, section 

5.1.4 and Appendix 5A, section 5A.3). 

In light of these factors, rather than 

focusing on REA ecoregion-scale results 

for a single ANC benchmark, he finds it 

appropriate to consider the pattern of 

REA results across all three benchmarks, 

as evaluated in the PA and considered 

by the CASAC majority (summarized in 

section II.B.1.b. above). 

In considering the summary of results 

for the ecoregion-scale analysis of 

ecoregion median deposition bins (in 

the draft PA),134 the CASAC majority 

focused on a level of S deposition 

estimated to achieve acid buffering 

capacity at or above the three ANC 

benchmarks in 80% (for ANC of 20 and 

30 meq/L) or 70% (for ANC of 50 meq/ 

L) of waterbodies in all ecoregion-time 

period combinations 135 (Sheppard, p. 

25 of the Response to Charge 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105765 

136The results for median S deposition at or 

below 7 kg/ha-yr further indicate that 90% of 

waterbodies per ecoregion achieve ANC at or above 

20, 30 and 50 meq/L in 96%, 92% and 82%, 

respectively, of eastern ecoregion-time period 

combinations (as summarized in section II.A.4.c.). 

137While the REA ecoregion-scale analysis 

summarizes risk estimates for each ecoregion in 

terms of the ecoregion median of the sites analyzed 

in each ecoregion, the PA notes that the sites 

estimated to receive the higher levels of deposition 

are those most influencing the extent to which the 

potential objectives for aquatic acidification 

protection are or are not met. 

138The PA additionally considered the terrestrial 

acidification risk analyses in the last review which 

found that total deposition estimates in recent years 

appear to meet all but the most restrictive of acid 

deposition target values, with which the PA 

observed uncertainties to be the greatest (PA, 

section 5.3.2.1). 

Questions). The CASAC majority 

identify S deposition levels ‘‘generally’’ 

at or below 5 kg/ha-yr as associated with 

this pattern of acid buffering. The 

Administrator notes that, as recognized 

in the PA and the proposal, the REA 

found ecoregion median S deposition at 

or below 7 kg/ha-yr in the 18 eastern 

ecoregions also yields these percentages 

of waterbodies achieving the three ANC 

benchmarks (as seen in tables 3 and 4 

above).136

The Administrator additionally takes 

note of the PA evaluation of the 

temporal trend of the ecoregion-scale 

risk estimates across the five time 

periods, in the 20 years analyzed, which 

shows a decline in response to the 

declining S deposition estimates for 

those periods. As summarized in the PA 

and the proposal, the vast majority of 

the decline occurred across the first 

decade of the 20-year period. The S 

deposition estimated to be occurring in 

the 2010–2012 period included 

ecoregion medians (across CL sites) 

ranging from 2.3 to 7.3 kg/ha-yr in the 

18 eastern ecoregions (and lower in the 

7 western ecoregions), and the highest 

ecoregion 90th percentile was 

approximately 8 kg/ha-yr (table 5 and 

figure 2 above). For this pattern of 

deposition, the REA estimated more 

than 70% of waterbodies in all 25 

ecoregions assessed to be able to achieve 

an ANC of 50 ueq/L (figure 1, left panel, 

above), and more than 80% of 

waterbodies in all ecoregions to be able 

to achieve an ANC of 20 ueq/L (figure 

1, right panel). The Administrator 

observes that these estimates of acid 

buffering capacity achievement for the 

2010–12 period deposition—achieving 

the ANC benchmarks in at least 70% to 

80% (depending on the specific 

benchmark) of waterbodies per 

ecoregion—are consistent with the 

objectives identified by the CASAC 

majority (in emphasizing ecoregion 

ANC achievement estimates of 70%, 

80% and 80% for ANC benchmarks of 

50, 30 and 20 meq/L, respectively). 

Based on these evaluations of the REA 

estimates in the PA and advice from the 

CASAC majority, the Administrator 

judges that these ecoregion-scale ANC 

achievement estimates for the three 

ANC benchmarks (70%, 80% and 80% 

for ANC benchmarks of 50, 30 and 20 

meq/L, respectively) are reasonable acid 

buffering capacity objectives for the 

purposes of protecting ecoregions from 

aquatic acidification risk of a magnitude 

of potential public welfare significance. 

Further, as discussed earlier in this 

section, the Administrator recognizes 

that the significance of aquatic 

acidification-related impacts on the 

public welfare, including those 

associated with reductions in public 

usage of aquatic ecosystems with 

fisheries affected by acidification, 

increases with greater prevalence of 

affected waterbodies and ecoregions. 

Thus, he finds the CASAC-identified 

percentages of waterbodies per 

ecoregion that meet (or exceed) the three 

ANC benchmarks to be appropriate 

minimum percentages (for each ANC 

benchmark) for ecoregions across the 

U.S. for use in his identification of a 

secondary NAAQS that will provide the 

appropriate level of protection against 

risks of potential public welfare 

significance. In so doing, he 

additionally notes that these 

percentages are met (or exceeded) for 

the most recent time periods analyzed 

in the REA (through 2018–2020). 

In turning to his consideration of S 

deposition levels that might be expected 

to maintain such a level of protection 

from aquatic acidification risk, the 

Administrator considers the CASAC 

majority recommended range of annual 

average secondary SO2 standard levels 

(i.e., 10–15 ppb) that, in the view of 

these members, would generally 

maintain S deposition at or below 5 kg/ 

ha-yr. As recognized in the PA, the 

CASAC majority reference to S 

deposition associated with their acid 

buffering objectives was in terms of 

ecoregion median values in the REA 

ecoregion-scale analysis.137 The 

Administrator additionally takes note of 

the PA observation of an appreciable 

reduction in S deposition across the 20- 

year analysis period in the 25 REA 

ecoregions, both in terms of the 90th 

percentile across REA sites in each 

ecoregion and in terms of the median 

such that in the second decade of the 

period (since 2010), the difference in S 

deposition value between the ecoregion 

median and 90th percentile is much 

reduced from what it was in the 2001– 

2003 period. Although the ecoregion 

90th percentile and median estimates 

for the REA ecoregions ranged up to 

approximately 22 and 17 kg/ha-yr, 

respectively, in the 2001–2003 period, 

both types of estimates fall below 

approximately 7 to 8 kg/ha-yr by the 

2010–2012 period (figure 2 above). In 

light of this trend, as well as the 

temporal trend in the REA estimates, 

and also while recognizing the 

uncertainties associated with the 

deposition estimates at individual 

waterbody sites and with the associated 

estimates of aquatic acidification risk 

(PA, section 5.1.4), the Administrator 

concurs with the PA findings that the 

ecoregion-scale acid buffering objectives 

identified above (more than 70% to 80% 

of waterbody sites in all ecoregions 

assessed achieving or exceeding the set 

of ANC benchmarks) can be expected to 

be met when the median and upper 

(90th) percentile deposition estimates 

for sensitive ecoregions are generally at 

and below about 5 kg/ha-yr with a few 

occurrences as high as about 8 kg/ha-yr. 

Thus, he considers it appropriate to 

focus on S deposition generally at or 

below about 5 kg/ha-yr, with infrequent 

occurrences as high as about 8 kg/ha-yr. 

Based on all of these considerations, the 

Administrator judges that a secondary 

standard that would generally maintain 

a pattern of ecoregion median S 

deposition consistent with these 

objectives (at or below 5 kg/ha-yr, with 

only infrequent occurrences as high as 

8 kg/ha-yr) would provide the 

appropriate level of public welfare 

protection from aquatic acidification 

risk. 

In his consideration of deposition 

levels that might provide for protection 

from aquatic acidification consistent 

with his identified objectives, the 

Administrator also considers protection 

of terrestrial ecosystems from effects 

related to S deposition. In so doing, he 

notes that in primarily focusing on the 

aquatic acidification risk estimates in its 

evaluation of options for a standard to 

address deposition-related effects, the 

PA recognized the linkages between 

watershed soils and waterbody 

acidification, suggesting that such 

linkages indicate that protecting 

waterbodies from reduced acid buffering 

capacity (with ANC as the indicator) 

will also, necessarily, provide 

protection for watershed soils (PA, 

section 7.4).138 The Administrator also 

notes that a revised standard that would 

be associated with lower S deposition in 

sensitive ecoregions than the existing 

standard (consistent with his decision 

reached above) would necessarily be 

associated with lower S deposition in 

both terrestrial and aquatic ecosystems. 


khammond on DSK9W7S144PROD with RULES2
105766 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

139This range of S deposition levels reflects the 

PA analysis of studies of effects on terrestrial biota 

(PA, section 5.3.4 and Appendix 5B). For example, 

from the most recent observational study evaluated 

in the ISA and PA, for the non-western tree species 

that were reported to have a negative association of 

growth or survival with S deposition, this 

encompasses the species-specific median 

deposition estimates for the sites where these 

species were assessed (PA, section 5.3.4.1 and 

Appendix 5B, sections 5B.2.2.3 and 5B.2.3). 

He also notes the PA evaluation of the 

current evidence, particularly with 

regard to terrestrial plants, including the 

PA’s identification of S deposition 

levels extending from 5 kg/ha-yr (up to 

12 kg/ha-yr), as summarized in section 

II.A.3.c.(2) above.139 He further 

recognizes that this range includes the 

benchmark referenced by the CASAC 

majority (generally at or below 5 kg/hayr) as affording protection to various 

tree and lichen species (as summarized 

in section II.B.1.b. above). In so doing, 

he recognizes the overlap of these 

values with his objectives identified 

above (S deposition generally at or 

below about 5 kg/ha-hr, with infrequent 

higher occurrences). Thus, based on the 

PA, and in consideration of CASAC 

advice and public comments, the 

Administrator judges that his focus on 

aquatic acidification risk and on a 

pattern of ecoregion median S 

deposition consistent with his 

objectives identified above will also 

provide protection for terrestrial 

ecosystems, such that a different 

standard is not needed to provide 

protection for terrestrial effects. 

The Administrator next turns to 

identification of a secondary standard 

that can be expected to generally 

maintain a pattern of ecoregion median 

S deposition at or below 5 kg/ha-yr, 

with potentially very few occurrences 

up to about 8 kg/ha-yr. In so doing, he 

recognizes the complexity of identifying 

a national ambient air quality standard 

focused on protection of the public 

welfare from adverse effects associated 

with national patterns of atmospheric 

deposition, particularly given the degree 

to which those patterns are influenced 

by transport and chemical 

transformation of emissions. As more 

specifically described in the PA, 

atmospheric deposition (ecosystem 

loading) of S is, in a simple sense, the 

product of atmospheric concentrations 

of S compounds, factors affecting S 

transfer from air to surfaces, and time. 

Further, atmospheric concentrations in 

an ecosystem are, themselves, the result 

of emissions from multiple, distributed 

sources both near and far, atmospheric 

chemistry, and transport. Accordingly, 

the Administrator concurs with the PA 

that consideration of the location of 

source emissions and expected 

pollutant transport, in addition to the 

influence of physical and chemical 

processes, is important to understanding 

relationships between SO2

concentrations at ambient air monitors 

and S deposition rates in sensitive 

ecosystems of interest. 

Based on these considerations, the 

Administrator concurs with the PA 

conclusion that to achieve the requisite 

level of protection from aquatic 

acidification effects associated with S 

deposition in sensitive ecosystems, SO2

emissions must be controlled at their 

sources. Accordingly, the Administrator 

considers findings of the PA analyses of 

relationships between S deposition 

estimates and SO2 concentrations near 

SO2 monitors, including at NAAQS 

regulatory monitors, which are often 

near large sources of SO2 emissions. To 

account for the relationship between 

upwind concentrations near sources and 

deposition in downwind areas, the 

Administrator also considers PA 

analyses of relationships between 

ecoregion S deposition estimates and 

SO2 concentrations at upwind sites of 

influence, identified by trajectory 

analyses (sections II.A.2. and II.B.1.a.(3) 

above, and PA, sections 6.2.2 through 

6.2.4). As evidence of the influence of 

SO2 in ambient air on S deposition, all 

of these analyses demonstrated a 

positive association between SO2

concentrations and nearby or downwind 

S deposition (PA, section 7.4). 

With regard to an indicator for a 

standard to address the effects of S 

deposition associated with SOX in 

ambient air, the Administrator finds his 

proposed decision for an SO2 indicator 

to be appropriate. He reaches this 

decision based on consideration of the 

PA evaluations of the linkages 

connecting SOX emissions and S 

deposition-related effects, including the 

parallel trends of SO2 emissions and S 

deposition in the U.S. over the past 20 

years that indicate the strong influence 

of SO2 in ambient air on S deposition 

(PA, sections 6.4.1 and 7.4) and the PA 

finding of SO2 as a good indicator for a 

secondary standard to address S 

deposition (PA, sections 6.4.1 and 7.4). 

Specific aspects of the PA findings 

include the declining trend of S 

deposition that is consistent with and 

parallel to the sharp declines in annual 

average SO2 emissions across the 20- 

year period, as well as the general 

association of higher annual average 

SO2 concentrations (averaged over three 

years) at SLAMS with higher local S 

deposition estimates, in addition to the 

statistically significant positive 

correlations observed for ecoregion 

median S deposition with SO2

concentrations at upwind monitoring 

sites of influence in the EAQM analyses. 

In reaching this decision, the 

Administrator also notes the CASAC 

consensus advice and public comments 

that recommended a standard with SO2

as an indicator to address ecosystem 

effects of sulfur deposition. 

The Administrator has also 

considered PM2.5 with regard to its 

potential to be an effective indicator for 

a standard providing public welfare 

protection from S deposition-related 

effects. In so doing, he recognizes that 

the S species that deposits in 

ecosystems, SO4

2¥, is a component of 

PM2.5. However, he also recognizes that 

SO4

2¥ constitutes less than half of 

PM2.5, by mass, across the country, with 

non-S containing compounds most 

typically comprising more than 70% of 

the total annual PM2.5 mass in the East 

and even more in the West (PA, section 

2.4.3). He finds that this generally low 

presence of SO4

2¥ in PM2.5 and the 

extent to which it varies across the 

country inhibit his ability to identify a 

PM2.5 standard level that might be 

expected to provide the desired level of 

protection from S deposition related 

effects, an inhibition that does not exist 

in his use of the SO2 standard for this 

purpose. In addition, he takes note of 

the discussion above in support of his 

decision regarding a revised secondary 

SO2 standard, including the 

atmospheric chemistry information 

which indicates the dependency of S 

deposition on airborne SOX, as 

evidenced by the parallel trends of SO2

emissions and S deposition. Based on 

all of these considerations, the 

Administrator judges that protection of 

sensitive ecosystems from S depositionrelated effects is more effectively 

achieved through a revised SO2

standard than a standard for PM. Thus, 

the Administrator judges SO2 to be the 

appropriate indicator for a standard 

addressing S deposition-related effects. 

With regard to averaging time and 

form, the Administrator continues to 

find his proposed decision (for an 

averaging time of a year and a form that 

averages the annual values across three 

consecutive years) to be appropriate, 

based on consideration of the PA 

findings and related analyses, advice 

from the CASAC majority, and public 

comments. Among the public 

commenters that supported adoption of 

a standard to address deposition-related 

effects, none objected to the conclusion 

of the PA that an annual standard would 

be appropriate for this purpose, 

although some commenters did support 

a secondary standard with the same 

averaging time, form and level of the 

primary standard, apparently for 

implementation reasons (discussed in 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105767 

140As discussed further below, the EPA is not 

adopting such a standard identical to the existing 

primary standard because such as standard would 

be more stringent than necessary. 

141A 3-year form is common to NAAQS adopted 

over the more recent past. This form provides a 

desired stability to the air quality management 

programs which is considered to contribute to 

improved public health and welfare protection (e.g., 

78 FR 3198, January 15, 2013; 80 FR 65352, October 

26, 2015; 85 FR 87267, December 31, 2020). 

142As noted in section II.B.1.b. above, the PA 

analyses the CASAC majority cited were in terms 

of ecoregion median S deposition at/below values. 

Accordingly, the PA and the Administrator, in his 

judgments here, focuses on consideration of S 

deposition values in terms of such ecoregion 

medians. 

section II.B.2.a.(3)(c) above).140 In the 

quantitative analyses of air quality and 

deposition, the PA generally focused on 

a year’s averaging time based on the 

recognition that longer-term averages 

(such as over a year, compared to one 

or a few hours) most appropriately 

relate to deposition and associated 

ecosystem effects. The PA analyses also 

used a 3-year form based on a 

recognition in the NAAQS program that 

such a form affords stability to the 

associated air quality management 

programs that contributes to effective 

environmental protection. Similarly, in 

the advice of the CASAC majority on a 

standard addressing S deposition, these 

members recommended an annual 

average standard, and, while these 

members did not explicitly address 

form, the information cited in the 

justification for their recommendation 

focused on a 3-year form (section 

II.B.1.b. above). In consideration of 

these conclusions of the PA and the 

CASAC majority, and public comments 

(as discussed in section II.B.2.a. above), 

the Administrator judges an averaging 

time and form in terms of annual 

average SO2 concentrations, averaged 

over three years,141 to be appropriate for 

a secondary standard providing public 

welfare protection from adverse effects 

associated with long-term atmospheric 

deposition of S compounds. 

In turning to consideration of a level 

for such a standard, as an initial matter, 

the Administrator again notes the 

complexity associated with identifying a 

national ambient air quality standard 

focused on protection from national 

patterns of atmospheric deposition, and 

the associated uncertainty, as described 

in section II.E.3. of the proposal. 

Particularly in this case of identifying a 

standard to provide a pattern of ambient 

air concentrations that as a whole 

contributes to deposition across the 

U.S., it is important to consider the 

distribution of air concentrations to 

which the standard will apply. The 

Administrator considers the evaluations 

and associated findings of the PA, as 

well as findings of the related additional 

analyses, advice from the CASAC, and 

public comments on the proposed 

decision for a level within the range of 

10 to 15 ppb. 

With regard to the advice from the 

CASAC, the Administrator notes that, as 

described in section II.B.1.b. above, the 

majority of the CASAC recommended 

adoption of an annual SO2 standard 

with a level within the range of 10 to 15 

ppb. These members indicated their 

view that this range of levels ‘‘generally 

maintains’’ S deposition at or below 5 

kg/ha-yr (based on their consideration of 

the draft PA).142 The CASAC majority 

further conveyed that a standard level in 

this range would afford protection to 

tree and lichen species, as well as 

achieve the acid buffering targets in 

waterbodies of sensitive ecoregions 

(described above), and further stated 

that such a standard would ‘‘preclude 

the possibility of returning to 

deleterious deposition values’’ 

(Sheppard, Response to Charge 

Questions, pp. 24–25). 

The Administrator also takes note of 

the air quality and deposition analyses 

described in the PA and summarized in 

sections II.A.2. and II.B.1.a.(3) above. In 

so doing, the Administrator focused 

particularly on the results of the PA’s 

trajectory-based analyses for the EAQMmax metric, including the related 

additional analyses developed in 

consideration of public comments (Sales 

et al., 2024). He notes that these results 

indicate that when the maximum 

upwind annual SO2 concentration (3- 

year average) was no higher than 10 

ppb, median deposition in the 

downwind ecoregion was below 5 kg/ 

ha-yr in more than 90% of the 

ecoregion-time period combinations in 

the analysis and below about 6 kg/ha-yr 

in at least 95% of combinations, with 

deposition in the remaining few 

combinations no higher than about 8 kg/ 

ha-yr. Further, he notes the analysis 

finding that in every instance of upwind 

maximum annual SO2 concentrations 

(averaged over three years) above 10 

ppb, the associated estimates of 

downwind ecoregion median S 

deposition are all above 5 kg/ha-yr, 

extending from about 6 kg/ha-yr to as 

high as approximately 18 kg/ha-yr with 

75% of the occurrences above 9 kg/hayr (Sales et al., 2024). He judges this 

magnitude of ecoregion S deposition 

associated with standard levels above 10 

ppb to be well above his objectives. 

Thus, he finds that a standard level 

greater than 10 ppb would provide 

insufficient control of S deposition and 

related effects and accordingly would 

not provide the requisite public welfare 

protection. With regard to a level of 10 

ppb, however, the Administrator finds 

these analyses to indicate that such a 

level is associated with a pattern of 

ecoregion median deposition consistent 

with his previously identified objectives 

of ecoregion median deposition 

generally below about 5 kg/ha-yr, with 

few occurrences of higher levels up to 

or below about 8 kg/ha-yr. The 

Administrator additionally finds a level 

of 10 ppb and the ecoregion median 

estimates of associated S deposition to 

be in general agreement with the advice 

from the CASAC majority including 

their recommended range of 10–15 ppb 

for an annual standard level, and their 

characterization of ‘‘generally’’ 

maintaining S deposition at or below 5 

kg/ha-yr. 

Before reaching his decision on a 

standard that in his judgment would 

provide the requisite protection from 

deposition-related effects, the 

Administrator also considered the 

protection that might be afforded by an 

annual SO2 standard, averaged over 

three years, with a level below 10 ppb. 

In so doing, he focused on consideration 

of the level of 5 ppb that was raised in 

public comment, as discussed in section 

II.B.2.a.(2) above, considering the 

findings of the additional analyses of 

the PA trajectory-based dataset that 

summarize the ecoregion median S 

deposition associated with maximum 

annual average concentrations, averaged 

over three years, no higher than 5 ppb 

at upwind sites of influence (Sales et al., 

2024). The Administrator notes that for 

a maximum upwind annual average 

concentration no higher than 5 ppb, the 

trajectory-based analyses indicate 

downwind ecoregions to have ecoregion 

median S deposition appreciably below 

his objectives, which as noted above are 

for such deposition generally at or 

below 5 kg/ha-yr, with infrequent higher 

occurrences, very rarely as high as about 

8 kg/ha-yr. Specifically, the analyses 

indicate ecoregion median deposition 

below approximately 4.5 kg/ha-yr in all 

of the ecoregion-time period 

combinations, with 75% below 

approximately 2.5 kg/ha-yr. The 

Administrator judges this magnitude of 

ecoregion S deposition associated with 

a standard level of 5 ppb to be well 

below his identified objectives. Thus, in 

light of his judgments, described above, 

regarding the pattern of ecoregion 

deposition associated with his and the 

CASAC majority’s acidification 

protection targets, the Administrator 

finds an annual SO2 standard, averaged 

over three years, with a level below 10 

ppb, to be associated with air quality 


khammond on DSK9W7S144PROD with RULES2
105768 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

more stringent than necessary to 

provide the requisite protection of the 

public welfare under the Act. 

Further, in consideration of public 

comments and the recommendation 

from the CASAC minority, the 

Administrator additionally considered 

the public welfare protection that might 

be afforded by an alternate secondary 

standard in terms of a standard identical 

to the existing primary standard in all 

respects. In so doing, he notes the PA 

observations that most of the ecoregion 

median S deposition estimates for the 

last 10 years are less than 5 kg/ha-yr, 

and he notes the views expressed by the 

CASAC minority and in public 

comments that this indicates that the 

existing 1-hour primary SO2 standard 

adequately protects against long-term 

annual S deposition-related effects. He 

additionally notes the additional 

analyses related to the PA trajectorybased analyses that indicate the 

stringency, with regard to expected 

control of associated S deposition, 

associated with a 1-hour standard 

identical to the primary standard (Sales 

et al., 2024, section 4.2). As discussed 

in II.B.2.a.(3)(c) above, such a standard 

is associated with ecoregion median S 

deposition well below the 

Administrator’s objectives (summarized 

above). Specifically, the trajectory 

analyses indicate that for upwind sites 

of influence at or below 75 ppb, in terms 

of the existing primary standard (3-year 

average of 99th percentile daily 

maximum 1-hour average 

concentrations), the downwind 

ecoregion median S deposition 

estimates for all ecoregion-time period 

combinations are below 3 kg/ha-yr, with 

95% of them below 2 kg/ha-yr. Thus, he 

judges such a standard would be more 

stringent than necessary and 

accordingly not provide the requisite 

protection of the public welfare. 

In light of all of the above, along with 

analyses and evaluations in the PA, 

including judgments related to 

uncertainties in relating ambient air 

concentrations to deposition estimates 

for the purpose of identifying a standard 

level associated with a desired level of 

ecological protection, advice from the 

CASAC majority, and consideration of 

public comment, the Administrator 

judges that a SO2 standard in terms of 

an annual average, averaged over three 

years, with a level of 10 ppb would 

provide the requisite protection of the 

public welfare from adverse effects 

related to S deposition. 

The Administrator also considered 

the extent to which a new annual 

average standard might be expected to 

control short-term SO2 concentrations 

(e.g., of three hours duration) and 

accordingly also provide the necessary 

protection from direct effects of SOX

that is currently provided by the 

existing 3-hour secondary standard. In 

this context, he notes the analyses and 

conclusions of the PA, and particularly 

the related additional analyses, with 

regard to the extent of control for shortterm concentrations that might be 

expected to be provided by an annual 

secondary standard (Sales et al., 2024). 

The Administrator also notes that these 

analyses are of air quality data from 

across the U.S. collected over the past 

20 years, thus capturing a broad array of 

air quality conditions and their 

influences on relationships between the 

short-term and annual air quality 

metrics. As also discussed in section 

II.B.2.a.(4) above, these analyses 

indicate that in areas and periods when 

the annual SO2 concentration (annual 

average, averaged over three years) is at 

or below 10 ppb, design values for the 

existing 3-hour standard are well below 

the existing secondary standard level of 

0.5 ppm SO2 and short-term SO2

concentrations are below those 

associated with direct effects on 

vegetation or lichens (PA, Figure 2–29; 

Sales et al., 2024). Based on these 

findings, the Administrator judges that 

revision of the existing standard to a 

new annual standard, with a 3-year 

average form and a level of 10 ppb, will 

provide the necessary protection for 

direct effects of SOX on plants and 

lichens, as well as effects associated 

with longer-term deposition of S 

compounds in ecosystems. Thus, based 

on all of the considerations identified 

above, including the currently available 

evidence in the ISA, the quantitative 

and policy evaluations in the PA, 

related analyses, the advice from the 

CASAC, and public comment, the 

Administrator judges it appropriate to 

revise the existing secondary SO2

standard, to be an annual average 

standard, with a 3-year average form 

and a level of 10 ppb in order to provide 

the requisite protection of the public 

welfare from known or anticipated 

adverse effects. 

Having reached his decision with 

regard to the welfare effects of SOX, 

including those related to deposition of 

S compounds in sensitive ecosystems, 

the Administrator now turns to 

consideration of the secondary 

standards for N oxides and PM. As 

described below, the Administrator has 

decided to retain the existing NO2 and 

PM standards. These decisions are 

based on his consideration of the 

welfare effects evidence as characterized 

in the ISA and evaluated in the PA; the 

public welfare implications of these 

effects; the quantitative information 

concerning N oxides, PM and N 

deposition presented in the ISA and PA, 

and additional analyses developed in 

consideration of public comments (e.g., 

Sales et al., 2024); the majority and 

minority advice from the CASAC; and 

public comments (as discussed in 

section II.B.2.b. above and in the 

Response to Comments document). 

With regard to the secondary standard 

for N oxides, the Administrator turns 

first to consideration of the protection 

afforded for effects of N oxides 

associated with direct contact on 

surfaces of plants and lichens. In so 

doing, he notes that the evidence of 

such effects was the basis for the 

establishment of the existing standard in 

1971, and that the currently available 

information, summarized in section 

II.A.3.a.(1) above, continues to 

document such effects (ISA, Appendix 

3, sections 3.3 and 3.4; PA, sections 4.1, 

5.4.2 and 7.4). With regard to the 

adequacy of the existing standard in 

protecting against such effects, the 

Administrator’s conclusions reflect 

those in the proposal, which he notes 

are consistent with the unanimous view 

of the CASAC (summarized in section 

II.B.1.b. above). Specifically, he finds 

that the evidence for NO2 and NO does 

not indicate effects associated with 

ambient air concentrations allowed by 

the existing standard. With regard to the 

N oxide, HNO3, he considered the PA 

evaluation of the evidence of effects 

associated with air concentrations and 

associated HNO3 dry deposition on 

plant and lichen surfaces, and 

uncertainty as to the extent to which 

exposures associated with such effects 

may be allowed by the existing 

secondary NO2 standard (PA sections 

7.1.2 and 5.4.2, and Appendix 5B, 

section 5B.4). In so doing, the 

Administrator judges that the limited 

evidence, with associated uncertainties, 

are insufficient to conclude that air 

quality that meets the secondary NO2

standard will nevertheless elicit such 

effects. Thus, he concludes that the 

existing standard continues to provide 

the needed protection from the direct 

effects of N oxides. 

The Administrator next turns to 

consideration of the welfare effects 

related to atmospheric N deposition and 

the contribution of N oxides to such 

effects. In so doing, he notes that the 

information for N deposition and N 

oxides includes substantially more 

significant complexities, limitations of 

the available information, and related 

uncertainties than is the case for S 

deposition and S oxides. These 

complexities and limitations are 

generally technical or science policy in 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105769 

143For example, a study of the Chesapeake Bay 

and its sources of N loading concluded that ‘‘ ‘about 

one-third’ of the total N load for the Bay is the 

result of direct deposition to the Bay or deposition 

to the watershed which is transported to the Bay’’ 

(U.S. EPA, 2010, p. 4–33), indicating that two thirds 

of N loading comes from non-air sources. 

144Modeling estimates of N deposition in 2002 

were the basis for the risk analyses in the 2013 

review (2009 REA, sections 3.2.3 and 3.3.3). After 

also considering estimates and wet deposition 

measurements for 2003–2005, the 2009 REA 

concluded ‘‘overall, for each case study area, the 

amount of nitrogen deposition in 2002 is generally 

representative of current conditions’’ (2009 REA, p. 

3–30). The total deposition estimates at that time 

relied on a different and less advanced modeling 

approach than that used in the current review (PA, 

section 2.5). 

145Ecoregion median NH3 deposition has also 

increased since 2002 in 68 of the other 70 CONUS 

ecoregions; in the remaining two ecoregions, it is 

unchanged (Sales et al., 2024). 

146The TDep estimates of N deposition are only 

available for the CONUS and not for parts of the 

U.S. outside of the CONUS. 

nature, or both. Those of a technical 

nature include the untangling of historic 

N deposition impacts (e.g., in terrestrial 

ecosystems) from impacts that might be 

expected from specific annual 

deposition rates absent that history, and 

also the complexity—more prominent 

for many aquatic systems, including 

those receiving some of the highest N 

loading—associated with estimating the 

portion of N inputs, and associated 

contribution to effects, derived from 

atmospheric sources (and specifically 

sources of N oxides). The science 

policy-related complexities relate to 

judgments regarding the implications of 

N deposition-related biological or 

ecological effects in the context of the 

Administrator’s judgments concerning 

protection of the public welfare from 

adverse effects. Lastly, both technical 

and science policy challenges are 

presented by the coincidence in this 

review of the substantially reduced 

influence of N oxides on N deposition 

and the emergence of NH3, which is not 

a criteria pollutant, as a major N 

deposition influence, particularly in 

areas with some of the highest N 

deposition estimates. 

With regard to science policy 

judgments, the Administrator recognizes 

particular complexity associated with 

judging the requisite public welfare 

protection for an ecosystem stressor like 

N enrichment, for which as the CASAC 

recognized, in terrestrial systems there 

are both ‘‘benefits and disbenefits’’ 

(Sheppard, 2023, p. 8). As noted by the 

CASAC, ‘‘[b]enefits include fertilization 

of crops and trees and the potential for 

improved sequestration of carbon in 

soils and plant biomass’’ (Sheppard, 

2023, p. 8). As noted in the PA, this also 

complicates conclusions regarding the 

extent to which some ecological effects 

may be judged adverse to the public 

welfare (PA, section 7.4). In many 

aquatic systems, identification of 

appropriate public welfare protection 

objectives is further complicated by N 

contributions to these systems from 

multiple sources other than atmospheric 

deposition,143 as well as by the effects 

of historical deposition that have 

influenced the current status of soils, 

surface waters, associated biota, and 

ecosystem structure and function. For 

example, changes to ecosystems that 

have resulted from past, appreciably 

higher levels of atmospheric deposition 

in those areas have the potential to 

affect how the ecosystem responds to 

current, lower levels of deposition or to 

different N inputs in the future. 

Further, the Administrator notes that 

his decision under the Act regarding the 

secondary NAAQS for N oxides is 

necessarily based on his judgments 

related to protection from the effects 

associated with N oxides. Yet, he 

recognizes that there are contributions 

to ecosystem N deposition, and related 

effects, from pollutants other than—and 

not derived from—N oxides in ambient 

air, most prominently NH3. He 

additionally notes that the influence of 

NH3 on N deposition varies appreciably 

across the U.S. and has grown over the 

past 20 years, while the contribution of 

N oxides to N deposition has declined. 

In a related manner, he takes note of the 

findings of the PA and the additional 

analyses that indicate ecoregions and 

States with highest N deposition (e.g., 

above 10 kg/ha-yr) include areas with 

some of the highest deposition rates for 

reduced N and NH3 (PA, Figure 7–8; 

Sales et al., 2024). This associated 

lessening influence of N oxides on total 

N deposition is also evidenced by the 

generally poor (r<0.4) or negative 

correlations between N deposition and 

annual average NO2 concentrations, in 

the SLAMS and full trajectory-based 

datasets, respectively,, and also in the 

most recent period analyzed, 2018– 

2020(PA, sections 6.2.3 and 6.2.4). 

While low-moderate positive 

correlations are observed in both sets of 

analysis for eastern sites when 

including all time periods, correlations 

are only statistically significant in the 

earlier periods, prior to 2014, which 

may be related to increasing emissions 

of NH3 in more recent years (PA, section 

2.2.3 and Figure 6–5). 

More specifically, the analyses of N 

deposition over the years since 2002 

period 144 document the reductions in N 

deposition that correspond to 

reductions in emissions of N oxides, 

while additionally documenting the 

increased role of NH3 in N deposition 

and the co-occurring and associated 

tempering of total N deposition 

reductions nationwide. For example, in 

all 14 ecoregions with median total N 

deposition in 2019–2021 greater than 10 

kg/ha-yr, deposition of NH3 has 

increased since 2000 (Sales et al., 

2024).145 And, in five of these 14 

ecoregions, the increases in NH3

deposition and associated NH4

+

deposition are greater than the 

reductions in oxidized N deposition 

such that overall N deposition, in terms 

of ecoregion median, has increased. In 

the 14 ecoregions with total N 

deposition greater than 10 kg/ha-yr, the 

N deposition arising directly from N 

oxides (oxidized N deposition) 

constitutes the minority (approximately 

23 to 42%) of total N deposition (Sales 

et al., 2024, Table 3). Across the other 

70 ecoregions in CONUS 146 with 

median total N deposition below 10 kg/ 

ha-yr in 2019–2021, ecoregion median 

oxidized N deposition, on average, 

declined (from 4.7 to 2.4 kg N/ha-yr) 

while ecoregion median NH3

deposition, on average, more than 

doubled (from 0.7 to 1.6 kg N/ha-yr) 

(Sales et al., 2024, Table 4). At a Statelevel scale, average rates of oxidized N 

deposition have also declined in all 48 

States of the CONUS, including where 

total N deposition has increased as a 

result of increased deposition from 

reduced N compounds associated with 

NH3. In the most recent period, oxidized 

N deposition, in terms of Statewide 

average, is below 5 kg N/ha-yr in all 48 

States (Sales et al., 2024). And in the six 

States with average total N deposition 

above 10 kg/ha-yr in the 2019–2021 

period, oxidized N deposition 

comprises less than 40% (Sales et al., 

2024, Table 5). The Administrator 

recognizes that these findings augment 

those of the PA analyses and indicate a 

much lower influence of N oxides on 

total N deposition relative to the 

influence of reduced N compounds in 

areas of the U.S. where N deposition is 

currently the highest (PA, section 

7.2.3.3). 

The Administrator also considers both 

the majority and minority advice from 

the CASAC regarding an NO2 annual 

standard in consideration of total N 

deposition effects. In so doing, he notes 

that in considering the justification 

provided by the CASAC majority for its 

recommendation, the PA did not find 

the information highlighted by the 

CASAC for relating total N deposition 

levels to ambient air concentrations of 

NO2 to provide scientific support for 

their recommended revision. The 

Administrator additionally notes that, as 

summarized in section II.B.1.b. above, 


khammond on DSK9W7S144PROD with RULES2
105770 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

notwithstanding the CASAC majority 

recognition of a lack of correlation 

between NO2 concentrations and 

ecoregion total N deposition, these 

members recommend an annual NO2

standard with a level of ‘‘<10–20 ppb’’ 

based on their objective of N deposition 

below 10 kg/ha-yr based on studies of 

total N deposition. He finds their 

recommendation less than persuasive 

because for an NO2 standard to exert 

control of N deposition, there would 

need to be a significant positive 

relationship (e.g., correlation) between 

NO2 concentrations and N deposition. 

As discussed above, the correlations 

reported in the PA between NO2

concentrations and downwind 

ecoregions are generally low or negative, 

particularly in recent periods. Further, 

the justification provided by the CASAC 

majority for its recommended revision 

focuses on the results of the trajectorybased analysis in the draft PA, about 

which they also expressed concerns, 

with a focus on the EAQM-weighted 

metric, although, as described in section 

II.B.1.a.(2), concentrations of this metric 

are not directly translatable to potential 

standard levels due to the weighting 

across multiple monitors. In light of 

these limitations in the CASAC majority 

advice and based on current air quality 

and deposition information and trends 

as summarized above, the Administrator 

judges that, a secondary standard for N 

oxides cannot be expected to effectively 

control total N deposition. 

With regard to the minority CASAC 

recommendation to revise the secondary 

standard to be identical to the primary 

NO2 standard in all respects, the 

Administrator notes the justification 

provided by the minority CASAC, 

which observed that the primary 

standard has been met over the last 10 

years and indicated that ‘‘most of the N 

deposition values within the last 10 

years’’ are less than 10 kg/ha-yr. The 

Administrator does not find this 

rationale sufficient to support a decision 

for revision as the CASAC minority 

recommended. The fact that N 

deposition has declined in many 

locations to less than 10 kg/ha-yr and 

that all areas meet the current primary 

standard does not signify that a 

secondary standard set equal to the 

primary would be effective in 

controlling total N deposition, given the 

rise in reduced N deposition just 

discussed, or that such a standard 

would be requisite for protection of the 

public welfare. 

In this context, the Administrator 

considers the implications of N 

deposition directly related to N oxides 

with regard to welfare effects. In so 

doing, he notes that the information 

available at the time of proposal 

(presented in the PA) was unclear with 

regard to the extent to which 

occurrences of ecoregion median N 

deposition greater than the total N 

deposition values identified by the 

CASAC majority (10 kg/ha-yr) and in 

section 7.2.3. of the PA (7–12 kg/ha-yr) 

may relate to the existing NO2

secondary standard (89 FR 26682, April 

15, 2024). However, the more recent 

additional analyses (developed in 

consideration of public comments) now 

provide clarification. These additional 

analyses indicate that ecoregion median 

levels of oxidized N (the component of 

total N deposition directly related to N 

oxides) are well below the PA-identified 

range of values (Sales et al., 2024). 

Specifically, median oxidized N 

deposition in all ecoregions of the 

CONUS is below 5 kg N/ha-yr, less than 

half of the N deposition benchmark 

considered by the CASAC (and below 

the lower end of the N deposition range 

[7–12 kg/ha-yr] identified by the PA), 

with the majority of ecoregions (45 of 

84) having a median below 3 kg N/hayr (Sales et al., 2024). These analyses 

further indicate that the Statewide 

averages of oxidized N deposition in all 

50 States are below the CASAC 

identified N deposition benchmark and 

the PA identified range, with the 

average across States well below half 

these values (Sales et al., 2024, Table 5). 

In light of all of the considerations 

above, the Administrator notes first that 

the N deposition benchmark identified 

by the CASAC majority, and the range 

of levels identified in the PA for 

consideration, are in terms of total N 

deposition. He notes that most 

ecoregions have total N deposition 

levels below the CASAC majority and 

PA identified levels (that might be 

considered appropriate levels of 

protection for effects associated with 

total N deposition) but that some areas 

have higher total N deposition with 

levels above such benchmarks of 

potential public welfare significance. He 

notes that in areas with total N 

deposition above the CASAC majority 

and PA identified levels, available 

evidence indicates the level of total N 

deposition is predominantly the result 

of deposition from reduced N, which is 

increasing, while deposition of oxides of 

N is playing a notably smaller role (with 

such contributions decreasing over 

recent years). Based on these patterns 

and the current analyses, he notes his 

conclusion above, that, based on the 

information available in this review, a 

secondary standard for N oxides cannot 

be expected to effectively control total N 

deposition. Further, he notes that recent 

levels of oxidized N deposition (N 

deposition derived from N oxides in 

ambient air) are well below the CASAC 

majority and PA identified levels. With 

respect to the adequacy of protection for 

effects related to oxidized N deposition, 

he does not find a basis in the evidence 

for concluding that revisions to the 

current ambient air standard for N 

oxides are necessary. Therefore, based 

on all the considerations above, 

including the minority contribution of N 

oxides to total N deposition and the 

general lack of correlation between 

ambient air NO2 concentrations and 

such deposition, the Administrator 

finds that the existing evidence does not 

call into question the adequacy of 

protection of the existing secondary NO2

standard with regard to depositionrelated effects of N oxides. Further, 

based on the findings of the PA and 

additional analyses of recent 

information on air quality and N 

deposition, and all the above 

considerations, the Administrator 

judges, based on the available evidence 

in this review, that revision to the 

secondary annual NO2 standard is not 

warranted and the existing secondary 

NO2 standard should be retained, 

without revision. 

Lastly, the Administrator turns to 

consideration of the existing secondary 

standards for PM. As an initial matter, 

he takes note of the PA discussion and 

conclusion that the available 

information does not call into question 

the adequacy of protection afforded by 

the secondary PM2.5 standards from 

direct effects and deposition of 

pollutants other than S and N 

compounds (PA, sections 7.1.3 and 7.4). 

As also discussed in the proposal, the 

evidence characterized in the ISA and 

summarized in the PA indicates such 

effects to be associated with conditions 

associated with concentrations much 

higher than those associated with the 

existing standards. Thus, as in the 

proposal, the Administrator judges that 

the current evidence does not call into 

question the adequacy of the existing 

PM standards with regard to direct 

effects and deposition of pollutants 

other than S and N compounds. 

With regard to S deposition and PM, 

as noted earlier in this section, the 

Administrator judges that protection of 

sensitive ecosystems from S depositionrelated effects is more effectively 

achieved through a revised SO2

standard than a standard for PM. 

Accordingly, as discussed above, the 

Administrator has decided to revise the 

existing secondary SO2 standard to 

provide for such protection. Thus, the 

Administrator judges that revising one 

or more of the secondary PM standards 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105771 

147Further, as noted in section II.B.2.b.(2)(c) 

above, the PA analysis of N deposition and PM2.5

concentrations at SLAMs does not provide a basis 

for identifying 3-year average annual PM2.5

concentrations that might be expected to constrain 

nearby N deposition below certain levels, such as 

an ecoregion median of 10 kg/ha-yr (e.g., PA, Figure 

6–39). 

148The CASAC majority reference to 

concentrations in non-remote areas was with regard 

to the range of recent design values observed in 

areas where N deposition estimates ranged above 15 

kg/ha-yr in California, the Midwest and the East; 

although not noted in the justification, design 

values at California sites were as high as 17.3 mg/ 

m3 (as summarized in section II.B.1.c. above), and 

the justification does not address how this may 

relate to a relationship of these concentrations to N 

deposition. 

in consideration of protection of the 

public welfare from effects related to S 

deposition is not warranted. 

With regard to N deposition and 

adequacy of the secondary PM 

standards, the Administrator considers 

the analyses and evaluations in the PA, 

related analyses conducted in 

consideration of public comments, 

advice from the CASAC, and public 

comments. As an initial matter, the 

Administrator takes note of the 

substantial and significant limitations 

and uncertainties associated with the 

evidence base for ecosystem effects 

related to N deposition associated with 

PM (similar to those recognized above 

for N oxides). With regard to limitations 

and associated uncertainties of the 

current information related to N 

deposition arising from PM, the 

Administrator notes, as an initial matter, 

the PA findings, based on the full 20- 

year dataset, of negative to barely 

moderate correlations between N 

deposition estimates and annual average 

PM2.5 concentrations at upwind 

locations, with low or a negative 

correlation in the most recent time 

period (PA, sections 6.2.4 and 7.2.3.3). 

Across the SLAMS sites, the strength of 

a N deposition estimates with nearby 

PM2.5 concentrations is also seen to 

consistently decline across the five time 

periods analyzed since 2001 (PA, Table 

6–7).147 As discussed in the PA, these 

findings are likely related to both the 

increased impacts of NH3 on N 

deposition (as summarized earlier), and 

the declining presence of N compounds 

in PM (specifically in PM2.5) over the 

past two decades, as well as the current 

relatively low and variable 

representation of N compounds in PM 

(PA, section 6.4.2). 

While the Administrator recognizes 

that NH4

+, a transformation product of 

NH3, exists in particles and is a 

component of PM2.5, he also recognizes 

that the combined presence of all Ncontaining compounds in PM2.5

constitutes less than 30% of total PM2.5

mass at sites across the U.S. (PA, section 

6.2.4; Sales et al., 2024). The 

Administrator additionally takes note of 

the finding that the composition of 

PM2.5 across the U.S. varies appreciably. 

Specifically, the percentage of PM2.5

represented by N compounds at the 120 

CSN sites in the 2020–2022 period (that 

inform our current understanding for 

the various regions across the U.S.) 

ranges from a high of about 30% down 

to 5 to 15% across the South and 

Northwest and just below 5% in some 

areas (PA, section 6.4.2; Sales et al., 

2024). As discussed in the PA, this 

contributes to geographic variability in 

the relationship between N deposition 

and annual PM2.5 concentrations (PA, 

section 6.4.2; Sales et al., 2024). The 

Administrator recognizes these findings 

together to indicate that an appreciable 

percentage of PM2.5 mass does not 

contribute to N deposition, and that the 

contributing amount varies across 

regions of the U.S. He further recognizes 

that this indicates that PM2.5

concentrations can be controlled or 

reduced without necessarily having any 

effect on concentrations of particulate N 

compounds. The Administrator also 

takes note that while deposition of the 

particulate N species associated with 

NH3 emissions (i.e., NH4

+) has increased 

since 2000–2002, the percentage of 

PM2.5 mass comprised by nitrogen 

compounds has declined, as has the 

percentage comprised by NH4

+, alone 

(Sales et al., 2024). In this context, he 

additionally notes that deposition of 

NH3 (which is not particulate) is 

estimated to be more than a third of 

total N deposition in some ecoregions 

and States, including those the highest 

total deposition (Sales et al., 2024). The 

Administrator concludes that 

collectively, this information indicates 

that a PM mass standard is unlikely to 

achieve a predictable or specified 

amount of control on N deposition 

across the U.S. 

In considering the advice from the 

CASAC for revision of the annual PM2.5

secondary standard, the Administrator 

notes that, as discussed in the PA, 

summarized in section II.B.1.b. above 

and recognized in reaching his proposed 

decision, the specific rationale for the 

range of standard levels recommended 

by the CASAC majority is unclear. The 

EPA does not find the CASAC majority 

observations regarding PM2.5

concentrations in remote areas or in 

areas of higher concentrations in 2019– 

2021 or in the trajectory-based analyses 

to demonstrate that an annual PM2.5

standard, with a level of 6 to 10 mg/m3, 

would be expected to control total N 

deposition at or below 10 kg/ha-yr. As 

recognized in the proposal, in the 

CASAC majority comments, PM2.5

concentrations within its recommended 

range were both described as relating to 

N deposition at/below its recommended 

benchmark (10 kg N/ha-yr) and relating 

to deposition above that range (as 

summarized in II.B.1.c. above). 

Additionally, as discussed in section 

II.B.2.b.(2)(c) above, the EPA disagrees 

that the PA analyses of PM2.5

concentrations and N deposition 

estimates in remote areas, without 

consideration of information for areas 

where PM2.5 is emitted or produced, are 

informative in this regard.148 Regarding 

the trajectory-based analyses, as 

discussed in section II.B.1.b. above, and 

noted above, the correlation coefficient 

for N deposition with PM2.5

concentrations at the maximum upwind 

monitor (the EAQM-Max metric) does 

not indicate a positive relationship. In 

light of these limitations in the 

information cited by the CASAC 

majority and based on the broader 

consideration above of the variability of 

PM2.5 composition across the U.S., 

including with regard to N components, 

among other factors, the Administrator 

disagrees with the CASAC majority’s 

recommendation on revision of the 

annual PM2.5 standard. In so doing, he 

also notes that the recommendation by 

these members to consider a new total 

N PM2.5 indicator, based on their view 

that it would achieve a better measure 

of total reactive N deposition, was 

offered in the context of such 

consideration ‘‘in the next review’’ 

(Sheppard, 2023, Letter, p. 5), and notes 

that the record in this review does not 

provide a basis for considering, much 

less adopting, a new indicator in the 

current review. 

The CASAC minority 

recommendation, based on a conclusion 

that the 2013 annual primary PM2.5

standard was controlling N deposition 

as needed since its establishment (as 

described in section II.B.1.b. above), 

cited scatterplots in the draft PA of N 

deposition estimates and annual average 

PM2.5 concentrations and did not 

address the issue of variable PM 

composition or lack of analyses for a 1- 

hour metric. As described earlier, the 

Administrator finds the issue of 

variability in PM2.5 composition to be an 

important consideration in his decision 

and accordingly, he finds the minority 

CASAC recommendation to not be well 

supported by the full record at this time 

in this review. 

Based on the currently available 

information, taking into account its 

limitations and associated uncertainties, 

and in consideration of all of the above, 


khammond on DSK9W7S144PROD with RULES2
105772 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

149As noted in the PA and summarized in section 

II.B.1.b. above, the CASAC majority, in its 

justification for revision of the existing standard, 

did not identify studies in support of its statements 

related to lichen species and fog or cloud water. 

the Administrator concludes that given 

the variable composition of PM2.5 across 

the U.S., the relatively low percentage of 

PM2.5 represented by N compounds 

(lower now than in the past), and the 

contributors to total N deposition that 

are not PM components, a PM2.5

standard could not, as discussed above, 

be expected to provide predictable and 

effective control of total N deposition. 

Accordingly, he judges that PM2.5 is not 

an appropriate indicator for a secondary 

standard intended to provide protection 

of the public welfare from adverse 

effects related to N deposition. 

Additionally, he notes that while it is 

unclear whether any PM standard 

would provide an appropriate indicator 

for consideration of N depositionrelated effects, this issue may warrant 

evaluation in future reviews. 

Further, as in his decision for N 

oxides above, the Administrator 

recognizes the factors identified here to 

contribute appreciable uncertainty to an 

understanding of the level of protection 

from N deposition-related effects 

associated with PM that might be 

afforded by the existing or an alternate 

secondary standard for PM2.5. Thus, he 

is unable to identify a standard that 

would provide requisite protection from 

known or anticipated adverse Ndeposition-related effects to the public 

welfare associated with the presence of 

PM in the ambient air. In summary, 

based on all these considerations, the 

Administrator concludes after 

considering the available evidence as 

assessed in the ISA, the quantitative 

analyses and associated evaluations in 

the PA and related more recent 

additional analyses, that no change to 

the annual secondary PM2.5 standard is 

warranted and he is retaining the 

existing PM2.5 secondary standard, 

without revision. 

With regard to the 24-hour PM2.5

standard, the Administrator takes note 

of the PA conclusion that the evidence 

available in this review, as documented 

in the ISA, does not call into question 

the adequacy of protection provided by 

the 24-hour PM2.5 standard from 

ecological effects (PA, section 7.4). He 

additionally notes the agreement of this 

finding with the recommendation of the 

CASAC minority to retain the existing 

standard. The Administrator also 

considers the comments of the CASAC 

majority and recommendations for 

revision of this standard to a lower level 

or to an indicator of deciviews (with a 

level of 20 to-25 deciviews), based on 

the CASAC majority’s consideration of 

visibility impairment and short-term fog 

or cloud-related deposition events that 

these members indicate may threaten 

sensitive lichen species, as summarized 

in section II.B.1.b. above. With regard to 

short-term fog or cloud-related events, 

the Administrator considers the PA 

finding in evaluating these 

recommendations, that, while the 

available evidence in the ISA recognizes 

there to be N deposition associated with 

cloud water or fog, it does not provide 

estimates of this deposition, describe 

associated temporal variability, or 

present evidence of effects on biota from 

such events (ISA, Appendix 2; PA, 

section 7.3).149 Thus, he does not find 

a basis in the evidence base for this 

review for the CASAC majority 

revisions or their stated intention of 

addressing short-term events and lichen 

sensitivity. Further, the justification of 

the specific revision options 

recommended by the CASAC majority 

focuses on consideration of visibility 

impairment, and the Administrator 

notes that the adequacy of protection 

provided by the secondary PM2.5

standard from visibility effects has been 

addressed in his reconsideration of the 

2020 p.m. NAAQS decision (89 FR 

16202, March 6, 2024) and is not 

included in this review. Thus, based on 

his judgment that the evidence does not 

call the existing standard into question, 

the Administrator retains the existing 

24-hour secondary PM2.5 standard, 

without revision. 

Regarding the PM10 standard, the 

Administrator concurs with the PA’s 

finding of a lack of information that 

calls into question the adequacy of 

protection afforded by the existing PM10

secondary standard for ecological 

effects. Thus, he also retains the 

secondary PM10 standard without 

revision. 

C. Decision on the Secondary Standards 

For the reasons discussed above and 

considering the evidence assessed in the 

ISA, the qualitative assessments and 

policy evaluations presented in the PA 

and associated technical memorandum, 

the advice and recommendations of the 

CASAC, and the public comments, the 

Administrator is revising the secondary 

standard for SOX to provide the 

requisite protection of the public 

welfare from known and anticipated 

adverse effects. More specifically, the 

Administrator is revising the secondary 

SO2 standard to be an annual average, 

averaged over three years, with a level 

of 10 ppb SO2. With this decision, the 

Agency is also making corresponding 

revisions to data handling conventions 

are specified in revisions to appendix T, 

discussed in section III. below. 

With regard to the secondary 

standards for N oxides and PM, based 

on the evidence assessed in the ISA, the 

qualitative assessments and policy 

evaluations presented in the PA and 

associated technical memorandum, the 

advice and recommendations of the 

CASAC, and the public comments, and 

for the reasons discussed above, the 

Administrator concludes that no 

changes are warranted, and is retaining 

the existing standards, without revision. 

III. Interpretation of the Secondary SO2

NAAQS 

The EPA received no comments 

regarding the proposed data handling 

procedures for SO2 monitoring data for 

purposes of determining when the new 

annual secondary SO2 NAAQS is met. 

Therefore, the EPA is finalizing the 

proposed revisions to appendix T to 40 

CFR part 50, Interpretation of the 

Primary National Ambient Air Quality 

Standards for Oxides of Sulfur, to 

establish data handling procedures for 

the new annual secondary SO2 standard. 

The regulatory text at 40 CFR 50.21, 

which sets the averaging period, level, 

indicator, and form of the annual 

standard, refers to this appendix T. The 

revised appendix T details the 

computations necessary for determining 

when the annual secondary SO2

NAAQS is met. The revised appendix T 

also addresses data reporting, data 

completeness considerations, and 

rounding conventions. 

A. Background 

The general purpose of a data 

interpretation appendix is to provide 

the practical details on how to make a 

comparison between multi-day and 

possibly multi-monitor ambient air 

concentration data and the level of the 

NAAQS, so that determinations of 

attainment and nonattainment are as 

objective as possible. Data interpretation 

guidelines also provide criteria for 

determining whether there are sufficient 

data to make a NAAQS level 

comparison at all. The regulatory 

language for the secondary SO2 NAAQS 

adopted in 1971 does not contain 

detailed data interpretation instructions. 

This situation contrasts with the 

primary NO2, ozone, PM2.5, PM10, lead, 

and primary SO2 NAAQS regulations, 

for which there are detailed data 

interpretation appendices in 40 CFR 

part 50 addressing issues that can arise 

in comparing monitoring data to the 

NAAQS. The existing appendix T 

includes these detailed data 

interpretation requirements for the 1- 

hour primary SO2 NAAQS, thus the 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105773 

revision provides similar information 

for the new annual secondary SO2

NAAQS. The EPA has used its 

experience developing and applying 

this data interpretation appendix to 

develop the revisions to the text in 

appendix T to address the new annual 

secondary SO2 standard. 

B. Interpretation of the Secondary SO2

Standard 

The purpose of the data interpretation 

provisions for the secondary SO2

NAAQS is to give effect to the form, 

level, averaging time, and indicator 

specified in the regulatory text at 40 

CFR 50.21, anticipating and resolving in 

advance various future situations that 

could occur. The revised appendix T 

provides definitions and requirements 

that apply to the annual secondary 

standard for SO2. The requirements 

clarify how ambient air data are to be 

reported, what ambient air data are to be 

used for comparisons with the SO2

NAAQS, and how to calculate design 

values for comparisons with the SO2

NAAQS. The data already required to be 

reported by ambient air SO2 monitors 

for use in calculating design values for 

the current 1-hour primary SO2 NAAQS 

are also sufficient for use in calculating 

design values for the new annual 

secondary SO2 NAAQS. 

The revised appendix T specifies that 

the annual secondary SO2 NAAQS is 

met at an ambient air quality monitoring 

site when the valid annual secondary 

standard design value is less than or 

equal to 10 ppb. The annual secondary 

standard design value for an ambient air 

quality monitoring site is described as 

the mean of the annual means for three 

consecutive years, with the annual 

mean derived as the annual average of 

daily means, with rounding and data 

completeness specified as described 

below. The use of a daily mean value in 

deriving the design value is consistent 

with the existing data handling 

requirements for the current 1-hour 

primary SO2 NAAQS. 

Data completeness requirements for 

the annual secondary standard in the 

revised appendix T follow past EPA 

practice for other NAAQS pollutants by 

requiring that in general at least 75% of 

the monitoring data that should have 

resulted from following the planned 

monitoring schedule in a period must be 

available for the key air quality statistic 

from that period to be considered valid. 

These data completeness requirements 

are consistent with the current data 

completeness requirements for the 1- 

hour primary SO2 NAAQS in appendix 

T, and the revised appendix T does not 

change those requirements. For the 

annual secondary SO2 NAAQS, the key 

air quality statistics are the annual 

average of daily mean (24-hour average, 

midnight-to-midnight) concentrations in 

three successive years. It is important 

that daily means are representative of 

the 24-hour period and that all seasons 

of the year are well represented. Hence, 

the 75% requirement is applied at the 

daily and quarterly levels. These 

completeness requirements, including 

the calculation of the daily mean, are 

consistent with existing completeness 

requirements for the current 1-hour 

primary SO2 NAAQS. 

Recognizing that there may be years 

with incomplete data, the text provides 

that a design value derived from 

incomplete data will nevertheless be 

considered valid if at least 75 percent of 

the days in each quarter of each of three 

consecutive years have at least one 

reported hourly value, and the 3-year 

annual average design value calculated 

according to the procedures specified in 

the revised appendix T is above the 

level of the secondary annual standard. 

Additionally, following provisions in 

the revised appendix T, a substitution 

test may be used to demonstrate validity 

of incomplete design values above the 

level of the standard by substituting a 

‘‘low’’ daily mean value from the same 

calendar quarter in the 3-year design 

value period. Similarly, another 

substitution test may be used to 

demonstrate validity of incomplete 

design values below the level of the 

standard by substituting a ‘‘high’’ daily 

mean value from the same calendar 

quarter in the 3-year design value 

period. These substitution tests are 

consistent with existing substitution 

tests for the current 1-hour primary SO2

NAAQS. 

It should be noted that one possible 

outcome of applying the substitution 

test is that a year with incomplete data 

may nevertheless be determined to not 

have a valid design value and thus to be 

unusable in making annual secondary 

NAAQS compliance determinations for 

that 3-year period. However, the 

intention of the substitution test is to 

reduce the frequency of such 

occurrences. 

The EPA Administrator has general 

discretion to use incomplete monitoring 

data to calculate design values that 

would be treated as valid for 

comparison to the NAAQS despite the 

incompleteness, either at the request of 

a State or at the Administrator’s own 

initiative. Similar provisions exist 

already for the PM2.5, NO2, lead, and 1- 

hour primary SO2 NAAQS. The EPA 

may consider monitoring site closures/ 

moves, monitoring diligence, and 

nearby concentrations in determining 

whether to use such data. 

The rounding conventions for the new 

annual secondary SO2 NAAQS are 

consistent with rounding conventions 

used for the current 1-hour primary SO2

NAAQS. Specifically, hourly SO2

measurement data shall be reported to 

EPA’s regulatory database in units of 

ppb, to at most one place after the 

decimal, with additional digits to the 

right being truncated with no further 

rounding. Daily mean values and the 

annual mean of those daily values are 

not rounded. Further, the annual 

secondary standard design value is 

calculated pursuant to the revised 

appendix T and then rounded to the 

nearest whole number or 1 ppb 

(decimals 0.5 and greater are rounded 

up to the nearest whole number, and 

any decimal lower than 0.5 is rounded 

down to the nearest whole number). 

IV. Ambient Air Monitoring Network 

for SO2

In the NPRM, the EPA did not 

propose any changes to the minimum 

monitoring requirements as part of the 

proposal to revise the secondary SO2

NAAQS. Based on a review of the 

network history, current network 

design, reported data, and monitoring 

objectives (Watkins et al., 2024), and in 

recognition of the network’s adaptability 

and flexibility provided in 40 CFR part 

58, the Agency proposed and took 

comment on its determination that the 

current network is adequate to provide 

the data needed to implement the new 

secondary SO2 standard. The EPA also 

concluded that the Agency, along with 

State, local, Tribal, and industry 

stakeholders, have the authority and 

ability to adjust monitoring efforts and 

redirect resources needed to ensure that 

the monitoring objectives of the SO2

network continue to be met, and thus no 

changes to minimum monitoring 

requirements are necessary. 

A. Public Comments 

The EPA received a few comments 

related to the ambient air monitoring 

network design prescribed by the 

minimum monitoring requirements in 

40 CFR part 58, section 4.4 as it relates 

to supporting the implementation of the 

new standard. The commenters 

recognized the value and importance of 

the network, with one stating that they 

support the use of ambient air quality 

monitoring data in designation 

activities, and that they believe ‘‘the 

existing monitoring network is adequate 

for making attainment decisions.’’ 

Another commenter expressed the view 

that ‘‘EPA must maintain a ground 

monitoring network that supports 

science-based decision making in the 

NAAQS standard setting process, as 


khammond on DSK9W7S144PROD with RULES2
105774 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

150This and all subsequent references to ‘‘state’’ 

are meant to include State, local and Tribal agencies 

responsible for the implementation of a SO2 control 

program. 

151While the CAA says ‘‘designating’’ with 

respect to the Governor’s letter, in the full context 

of the CAA section it is clear that the Governor 

makes a recommendation to which the EPA must 

respond via a specified process if the EPA does not 

accept it. 

152See 42 U.S.C. 7407(d)(1)(A)(i)–(iii). 

153API v. Costle, 609 F.2d 20 (D.C. Cir. 1979). 

well as for compliance with a standard 

once it is set,’’ and concurred with a 

CASAC comment that monitoring 

networks, including the SLAMS, which 

are required through 40 CFR part 58, are 

‘‘essential to provide the scientific basis 

for this review’’ (Sheppard, 2023). 

Another commenter recommended 

that EPA ‘‘[i]ncrease monitoring in highrisk areas and ensure strict enforcement 

of the NAAQS,’’ including by deploying 

monitors in areas the commenter calls 

‘‘frontline and fence-line communities,’’ 

and making the data publicly accessible. 

With regard to this comment, the EPA 

notes that the current network already 

has a significant subset of sites with 

monitoring objectives that provide for 

measurements in areas of higher SO2

emissions and in locations of expected 

maximum concentrations. 

Measurements from monitors with those 

objectives provide the data needed to 

support the new standard. However, the 

same monitors, sited in locations of 

expected maximum concentrations, can 

also be in ‘‘frontline and fence-line 

communities.’’ Further, all monitoring 

conducted by State, local, and Tribal air 

agencies, as well as data from industry 

that fulfill the requirements of 40 CFR 

parts 50, 53, and 58, the regulations that 

set out minimum monitoring 

requirements, and other requirements 

are publicly available through various 

means. These include but are not 

limited to obtaining the data directly 

from the air monitoring agencies 

themselves, from EPA’s Air Data 

website, or from EPA’s Air Quality 

System (AQS) database. 

B. Conclusion on the Monitoring 

Network 

The EPA stated in the proposal that it 

believes that the current ambient air SO2

monitoring network design, 

deployment, and monitoring objectives 

are adequate to provide the data needed 

to implement the new secondary SO2

NAAQS. After consideration of public 

comments, and with reliance on EPA’s 

assessment of the monitoring network 

provided as part of the proposal for this 

review, the Agency still asserts that the 

network is adequate and that no 

network design changes are necessary 

because EPA, State, local, Tribal, and 

industry stakeholders have the authority 

and ability to adjust monitoring efforts 

and redirect resources as needed to 

ensure that the monitoring objectives of 

the SO2 network continue to be met. 

The Administrator has therefore chosen 

to retain the existing minimum 

monitoring requirements for SO2

without modification, as currently 

prescribed, operated, and maintained in 

accordance with 40 CFR parts 50, 53, 

and 58, as proposed. 

V. Clean Air Act Implementation 

Considerations for the Revised 

Secondary SO2 Standard 

The EPA’s revision to the secondary 

SO2 NAAQS will trigger a number of 

implementation-related activities that 

were described in the proposal. The two 

most immediate implementation 

impacts following a final new or revised 

NAAQS are related to stationary source 

permitting and the initial area 

designations process. Permitting 

implications are discussed in section 

V.C., and designation implications are 

discussed in section V.A. The Agency is 

finalizing an action retaining the 

secondary NO2 and PM NAAQS. 

Retention of existing secondary 

standards does not trigger any new 

implementation actions. Additional 

implementation information is available 

in the proposal preamble in section V. 

At the outset, promulgation of a new 

or revised NAAQS triggers a process 

through which States 150 would make 

recommendations to the Administrator 

regarding initial area designations. 

States also would be required to make 

a new SIP submission to establish that 

they meet the necessary structural 

requirements for such new or revised 

NAAQS pursuant to CAA section 

110(a)(1) and (2), also referred to as the 

‘‘infrastructure SIP submission’’ (more 

on this submission below). This section 

provides background information for 

understanding the implementation 

implications of the secondary SO2

NAAQS changes and describes the 

EPA’s intentions for providing guidance 

regarding implementation. 

A. Designation of Areas 

As described in section II.B.3., the 

EPA is revising the secondary SO2

NAAQS to 10 ppb, as an annual average, 

averaged over three consecutive years. 

After the EPA establishes a new or 

revised NAAQS (primary or secondary), 

the CAA requires the EPA and States to 

take steps to ensure that the new or 

revised NAAQS is met. The timeline for 

initial area designations begins with 

promulgation of the new NAAQS, as 

stated in CAA section 107(d)(1)(A). 

Initial area designations involve 

identifying areas of the country that 

either meet or do not meet the new or 

revised NAAQS, along with the nearby 

areas contributing to NAAQS violations. 

The following includes additional 

information regarding the designations 

process described in the CAA. 

Section 107(d)(1)(A) of the CAA states 

that, ‘‘By such date as the Administrator 

may reasonably require, but not later 

than 1 year after promulgation of a new 

or revised [NAAQS] for any pollutant 

under [section 109], the Governor of 

each State shall . . . submit to the 

Administrator a list of all areas (or 

portions thereof) in the State’’ and make 

recommendations for whether the EPA 

should designate those areas as 

‘‘nonattainment,’’ ‘‘attainment,’’ or 

‘‘unclassifiable.’’ 151 A nonattainment 

area is any area that does not meet (or 

that contributes to ambient air quality in 

a nearby area that does not meet) a 

NAAQS; an attainment area is any area 

(other than an area identified as a 

nonattainment area) that meets a 

NAAQS; and an unclassifiable area is 

any area that cannot be classified on the 

basis of available information as 

meeting or not meeting a NAAQS.152

The CAA provides the EPA with 

discretion to require States to submit 

their designations recommendations 

within a reasonable amount of time not 

exceeding 1 year after the promulgation 

of a new or revised NAAQS. CAA 

section 107(d)(1)(B)(a) also stipulates 

that ‘‘the Administrator may not require 

the Governor to submit the required list 

sooner than 120 days after promulgating 

a new or revised [NAAQS].’’ This same 

section further provides, ‘‘Upon 

promulgation or revision of a [NAAQS], 

the Administrator shall promulgate the 

designations of all areas (or portions 

thereof) . . . as expeditiously as 

practicable, but in no case later than 2 

years from the date of promulgation 

. . . . Such period may be extended for 

up to one year in the event the 

Administrator has insufficient 

information to promulgate the 

designations.’’ With respect to the 

NAAQS setting process, courts have 

interpreted the term ‘‘promulgation’’ to 

be signature and widespread 

dissemination of a final rule.153

If the EPA agrees that the State’s 

designations recommendations are 

consistent with all relevant CAA 

requirements, then the EPA may 

proceed to promulgate the designations 

for such areas. However, if the EPA 

disagrees that a State’s recommendation 

is consistent with all relevant CAA 

requirements, then the EPA may make 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105775 

154 ‘‘Guidance to Regions for Working with Tribes 

during the National Ambient Air Quality Standards 

(NAAQS) Designations Process,’’ December 20, 

2011, Memorandum from Stephen D. Page to 

Regional Air Directors, Regions 1–X available at 

https://www.epa.gov/sites/default/files/2017-02/ 

documents/12-20-11_guidance_to_regions_for_

working_with_tribes_naaqs_designations.pdf. 

modifications to the recommended 

designations by following the process 

outlined in the CAA. By no later than 

120 days prior to promulgating the final 

designations, the EPA is required to 

notify States of any intended 

modifications to the designations of any 

areas or portions thereof, including the 

boundaries of areas, as the EPA may 

deem necessary. States then have an 

opportunity to comment on the EPA’s 

intended designations decisions. If a 

State elects not to provide designations 

recommendations, then the EPA must 

timely promulgate the designations that 

it deems appropriate. CAA section 

107(d)(1)(B)(ii). 

While section 107(d) of the CAA 

specifically addresses the designations 

process for States, the EPA intends to 

follow the same process for Tribes to the 

extent practicable, pursuant to section 

301(d) of the CAA regarding Tribal 

authority, and the Tribal Authority Rule 

(63 FR 7254, February 12, 1998). To 

provide clarity and consistency in doing 

so, the EPA issued a guidance 

memorandum to our Regional Offices on 

working with Tribes during the 

designations process.154

Consistent with the process used in 

previous initial area designations efforts 

for both primary and secondary 

standards, the EPA will employ a 

nationally consistent framework and 

approach to evaluate each State’s 

designations recommendations. Section 

107(d) of the CAA explicitly requires 

that the EPA designate as nonattainment 

not only the area that is violating the 

pertinent standard, but also those 

nearby areas that contribute to ambient 

air quality in the violating area. 

Consistent with past practice, the EPA 

plans to address issues relevant to the 

initial area designations more fully in a 

separate designations-specific 

memorandum. 

The EPA intends to issue the 

designations for the secondary SO2

NAAQS based on the most recent 3 

years of complete, certified, and valid 

air quality monitoring data in the areas 

where monitors are installed and 

operating. The EPA intends to use such 

available air quality monitoring data 

from the current SO2 monitoring 

network. For further information on the 

adequacy of the monitoring network, 

refer to the memorandum in the docket 

for this action titled ‘‘Ambient Air SO2

Monitoring Network Review and 

Background’’ (Watkins et al., 2024). 

Monitoring data are currently available 

from existing FEM and FRM monitors 

sited and operated in accordance with 

40 CFR parts 50 and 58 to determine 

compliance with the revised secondary 

SO2 NAAQS. 

State or Tribal air agencies may flag 

air quality data for certain days in the 

Air Quality System (AQS) database due 

to potential impacts from exceptional 

events. CAA section 319(b) defines an 

exceptional event as an event that (i) 

affects air quality; (ii) is not reasonably 

controllable or preventable; (iii) is an 

event caused by human activity that is 

unlikely to recur at a particular location 

or a natural event; and (iv) is 

determined by the Administrator 

through the process established in the 

regulations to be an exceptional event 

(e.g., volcanic activity for SO2). For 

emissions affecting air quality to be 

considered an exceptional event, there 

must be a clear causal relationship 

between the specific event and the 

monitored exceedance or violation. Air 

quality monitoring data affected by 

exceptional events may be excluded 

from use in determinations of 

exceedances or violations if the data 

meet the criteria for exclusion under 

CAA section 319(b) and EPA’s 

‘‘Treatment of Data Influenced by 

Exceptional Events’’ Final Rule (81 FR 

68216; October 3, 2016) (Exceptional 

Events Rule) codified at 40 CFR 50.1, 

50.14, and 51.930. For events affecting 

initial area designations, the air agency 

is required to follow the exceptional 

events demonstration submission 

deadlines that are identified in table 2 

to 40 CFR 50.14(c)(2)(vi), ‘‘Schedule for 

Initial Notification and Demonstration 

Submission for Data Influenced by 

Exceptional Events for Use in Initial 

Area Designations.’’ The EPA 

encourages air agencies to work 

collaboratively with the appropriate 

EPA Regional office after identifying 

any exceptional event influencing 

ambient air quality concentrations in a 

way that could affect area designations 

for the annual SO2 secondary NAAQS. 

B. Section 110(a)(1) and (2) 

Infrastructure SIP Requirements 

As discussed in the proposal 

preamble section V.B., the CAA directs 

States to address basic SIP requirements 

to implement, maintain, and enforce the 

NAAQS. Under CAA sections 110(a)(1) 

and (2), States are required to have State 

implementation plans that provide the 

necessary air quality management 

infrastructure including, among other 

things, enforceable emissions 

limitations, an ambient air monitoring 

program, an enforcement program, air 

quality modeling capabilities, and 

adequate personnel, resources, and legal 

authority to carry out the 

implementation of the SIP. After the 

EPA promulgates a new or revised 

NAAQS, States are required to make a 

new SIP submission to establish that 

they meet the necessary structural 

requirements for such new or revised 

NAAQS or make changes to do so. The 

EPA refers to this type of SIP 

submission as an ‘‘infrastructure SIP 

submission.’’ Under CAA section 

110(a)(1), all States are required to make 

these infrastructure SIP submissions 

within 3 years after promulgation of a 

new or revised standard. While the CAA 

authorizes the EPA to set a shorter time 

for States to make these SIP 

submissions, the EPA is requiring 

submission of infrastructure SIPs within 

3 years of the promulgation date of this 

revised secondary SO2 NAAQS. Section 

110(b) of the CAA also provides that the 

EPA may extend the deadline for the 

‘‘infrastructure’’ SIP submission for a 

revised secondary NAAQS by up to 18 

months beyond the initial 3 years. If a 

state requests an extension pursuant to 

CAA section 110(b) and 40 CFR 51.341 

and the Administrator determines an 

extension is necessary, the EPA will set 

additional time for that state for the 

infrastructure SIP submittal in a 

separate action from this final rule. The 

EPA does not anticipate that extensions 

will be necessary as most, if not all, 

states’ existing infrastructure SIPs may 

already be sufficient to satisfy the 

infrastructure SIP requirements for this 

revised secondary SO2 NAAQS, and 

those states can reiterate that they have 

met the requirements in their 

infrastructure SIP submissions. 

Under CAA sections 110(a)(1) and (2), 

States are required to make SIP 

submissions that address requirements 

pertaining to implementation, 

maintenance, and enforcement of a new 

or revised NAAQS. The specific 

subsections in CAA section 110(a)(2) 

require States to address a number of 

requirements, as applicable: (A) 

emissions limits and other control 

measures; (B) ambient air quality 

monitoring/data system; (C) programs 

for enforcement of control measures and 

for construction or modification of 

stationary sources; (D)(i) interstate 

pollution transport and (ii) interstate 

and international pollution abatement; 

(E) adequate resources and authority, 

conflict of interest, and oversight of 

local governments and regional 

agencies; (F) stationary source 

monitoring and reporting; (G) 

emergency powers; (H) SIP revisions; (I) 


khammond on DSK9W7S144PROD with RULES2
105776 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

155See ‘‘Guidance on Infrastructure State 

Implementation Plan (SIP) Elements under Clean 

Air Act sections 110(a)(1) and 110(a)(2)’’ September 

2013, Memorandum from Stephen D. Page to 

Regional Air Directors, Regions 1–10. 

156CAA section 110(a)(2)(D)(i)(I). 

157CAA section 110(a)(2)(D)(i)(II) also addresses 

certain interstate effects that States must address 

and thus is also sometimes referred to as relating 

to ‘‘interstate transport.’’ 

158See North Carolina v. EPA, 531 F.3d 896, 909– 

11 (D.C. Cir. 2008). 

159See id. at 911–13. See also Wisconsin v. EPA, 

938 F.3d 303, 313–20 (D.C. Cir. 2019); Maryland v. 

EPA, 958 F.3d 1185, 1203–04 (D.C. Cir. 2020). 

160A ‘‘certification’’ approach would not be 

appropriate for the interstate pollution control 

requirements of CAA section 110(a)(2)(D)(i). 

plan revisions for nonattainment areas; 

(J) consultation with government 

officials, public notification, Prevention 

of Significant Deterioration (PSD) and 

visibility protection; (K) air quality 

modeling and submission of modeling 

data; (L) permitting fees; and (M) 

consultation and participation by 

affected local entities. These 

requirements apply to all SIP 

submissions in general, but the EPA has 

provided specific guidance to States 

concerning its interpretation of these 

requirements in the specific context of 

infrastructure SIP submissions for a new 

or revised NAAQS.155

As a reminder, States are not required 

to address nonattainment plan 

requirements for purposes of the revised 

secondary SO2 NAAQS on the same 

schedule as infrastructure SIP 

requirements. For the reasons explained 

below, the EPA interprets the CAA such 

that (1) the portion of CAA section 

110(a)(2)(C), programs for enforcement 

of control measures and for construction 

or modification of sources that applies 

to permit programs applicable in 

designated nonattainment areas, (known 

as ‘‘nonattainment new source review’’) 

under part D; and (2) CAA section 

110(a)(2)(I) in its entirety are not subject 

to the 3-year submission deadline of 

CAA section 110(a)(1), and thus States 

are not required to address them in the 

context of an infrastructure SIP 

submission. Accordingly, the EPA does 

not expect States to address the 

requirement for a new or revised 

NAAQS in the infrastructure SIP 

submissions to include regulations or 

emissions limits developed specifically 

for attaining the relevant standard in 

areas designated nonattainment for the 

revised secondary SO2 NAAQS. States 

are required to submit infrastructure SIP 

submissions for the secondary SO2

NAAQS before they are required to 

submit nonattainment plan SIP 

submissions to demonstrate attainment 

with the same NAAQS. As a general 

matter, states would be required to 

submit nonattainment plans to provide 

for attainment and maintenance of the 

revised secondary SO2 NAAQS within 3 

years from the effective date of 

nonattainment area designations as 

required under CAA section 172(b). In 

addition, because this NAAQS is a 

secondary standard, CAA section 110(b) 

also provides that the EPA may extend 

the deadline for the nonattainment plan 

for up to 18 months beyond the initial 

3 years. If a state requests an extension 

pursuant to CAA section 110(b) and 40 

CFR 51.341 and the Administrator 

determines an extension is necessary, 

the EPA will set additional time for the 

nonattainment plan submittal in a 

separate action from this final rule. The 

EPA reviews and acts upon these later 

SIP submissions through a separate 

process. For these reasons, the EPA does 

not expect States to address new 

nonattainment area emissions controls 

per CAA section 110(a)(2)(I) in their 

infrastructure SIP submissions. 

Another required infrastructure SIP 

element is that each State’s SIP must 

contain adequate provisions to prohibit, 

consistent with the provisions of title I 

of the CAA, emissions from within the 

State that will significantly contribute to 

nonattainment in, or interfere with 

maintenance by, any other State of the 

primary or secondary NAAQS.156 This 

element is often referred to as the ‘‘good 

neighbor’’ or ‘‘interstate transport’’ 

provision.157 The provision has two 

prongs: significant contribution to 

nonattainment (prong 1) and 

interference with maintenance (prong 

2). The EPA and States must give 

independent significance to prong 1 and 

prong 2 when evaluating downwind air 

quality problems under CAA section 

110(a)(2)(D)(i)(I).158 Further, case law 

has established that the EPA and States 

must implement requirements to meet 

interstate transport obligations in 

alignment with the applicable statutory 

attainment schedule of the downwind 

areas impacted by upwind-State 

emissions.159 The EPA anticipates 

coordinating with States with respect to 

the requirements of CAA section 

110(a)(2)(D)(i)(I) for implementation of 

the secondary SO2 NAAQS. 

Each State has the authority and 

responsibility to review its air quality 

management program’s existing SIP 

provisions in light of each new or 

revised NAAQS to determine whether 

any revisions to the State’s regulations 

or program are necessary to implement 

a new or revised NAAQS. Most States 

have revised and updated their SIPs in 

recent years to address requirements 

associated with other revised NAAQS. 

For some States, it may be the case that, 

for a number of infrastructure elements, 

the State may believe it already has 

adequate State regulations adopted and 

approved into the SIP to address a 

particular requirement with respect to 

any new or revised NAAQS. For such 

portions of the State’s infrastructure SIP 

submission, the State could provide an 

explanation of how its existing SIP 

provisions are adequate. 

If a State determines that existing SIPapproved provisions, such as those 

approved for the 1-hour primary SO2

NAAQS, remain adequate in light of the 

new annual secondary SO2 NAAQS 

with respect to a given infrastructure 

SIP element (or sub-element), then the 

State may make a SIP submission 

containing relevant supporting 

information ‘‘certifying’’ that the 

existing SIP contains provisions that 

address those requirements of the 

specific CAA section 110(a)(2) 

infrastructure elements.160 In the case of 

such a certification submission, the 

State would not have to include a copy 

of the relevant provision (e.g., rule or 

statute) itself. Rather, this certification 

submission should provide citations to 

the EPA-approved State statutes, 

regulations, or non-regulatory measures, 

as appropriate, in or referenced by the 

already EPA-approved SIP that meet 

particular infrastructure SIP element 

requirements. The State’s infrastructure 

SIP submission should also include an 

explanation as to how the State has 

determined that those existing 

provisions meet the relevant 

requirements. 

Like any other SIP submission, that 

State can make such an infrastructure 

SIP submission certifying that it has 

already met some or all of the applicable 

requirements only after it has provided 

reasonable notice and opportunity for 

public hearing. This ‘‘reasonable notice 

and opportunity for public hearing’’ 

requirement for infrastructure SIP 

submissions is to meet the requirements 

of CAA sections 110(a) and 110(l). 

Under the EPA’s regulations at 40 CFR 

part 51, if a public hearing is held, an 

infrastructure SIP submittal must 

include a certification by the State that 

the public hearing was held in 

accordance with the EPA’s procedural 

requirements for public hearings. See 40 

CFR part 51, appendix V, 2.1(g); and see 

40 CFR 51.102. 

In consultation with its EPA Regional 

Office, a State should follow all 

applicable EPA regulations governing 

infrastructure SIP submissions in 40 

CFR part 51—e.g., subpart I (Review of 

New Sources and Modifications), 

subpart J (Ambient Air Quality 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105777 

161https://cdx.epa.gov/. 

162By establishing the maximum allowable level 

of ambient air pollutant concentration increase in 

a particular area, an increment defines ‘‘significant 

deterioration’’ of air quality in that area. Increments 

are defined by the CAA as maximum allowable 

increases in ambient air concentrations above a 

baseline concentration and are specified in the PSD 

regulations by pollutant and area classification 

(Class I, II and III). 40 CFR 51.166(c), 52.21(c); 75 

FR 64864; October 20, 2010. The EPA has 

developed the Guideline on Air Quality Models and 

other documents to, among other things, provide 

methods and guidance for demonstrating 

compliance the NAAQS and PSD increments 

including the annual SO2 standard. See 40 CFR part 

51, appendix W; 82 FR 5182, January 17, 2017. 

163Congress established certain Class I areas in 

section 162(a) of the CAA, including international 

parks, national wilderness areas, and national parks 

that meet certain criteria. Such Class I areas, known 

as mandatory Federal Class I areas, are afforded 

special protection under the CAA. In addition, 

states and Tribal governments may establish Class 

I areas within their own political jurisdictions to 

provide similar special air quality protection. 

Surveillance), subpart K (Source 

Surveillance), subpart L (Legal 

Authority), subpart M 

(Intergovernmental Consultation), 

subpart O (Miscellaneous Plan Content 

Requirements), subpart P (Protection of 

Visibility), and subpart Q (Reports). For 

the EPA’s general criteria for 

infrastructure SIP submissions, refer to 

40 CFR part 51, appendix V, Criteria for 

Determining the Completeness of Plan 

Submissions. The EPA recommends that 

States electronically submit their 

infrastructure SIPs to the EPA through 

the State Plan Electronic Collaboration 

System (SPeCS),161 an online system 

available through the EPA’s Central Data 

Exchange. The EPA acknowledges that 

the timeline for submission of 

infrastructure SIPs for the secondary 

SO2 NAAQS may overlap in part with 

the timeline for submission of 

infrastructure SIPs for the recently 

revised primary PM2.5 NAAQS. Air 

Agencies may elect to streamline their 

infrastructure SIP submittal and 

development by combining the two 

distinct infrastructure SIP submissions 

for both NAAQS into one submission. 

The EPA appreciates the obligations 

may differ for some infrastructure 

elements, and simply notes that this 

option may represent a more 

streamlined approach for some areas. 

C. Prevention of Significant 

Deterioration and Nonattainment New 

Source Review Programs for the Revised 

Secondary SO2 Standard 

The CAA, at parts C and D of title I, 

contains preconstruction review and 

permitting programs applicable to new 

major stationary sources and major 

modifications of existing major sources. 

The preconstruction review of each new 

major stationary source and major 

modification applies on a pollutantspecific basis, and the requirements that 

apply for each pollutant depend on 

whether the area in which the source is 

situated is designated as attainment (or 

unclassifiable) or nonattainment for that 

pollutant. In areas designated 

attainment or unclassifiable for a 

pollutant, the PSD requirements under 

part C apply to construction at major 

sources. In areas designated 

nonattainment for a pollutant, the 

Nonattainment New Source Review 

(NNSR) requirements under part D 

apply to major source construction. 

Collectively, those two sets of permit 

requirements are commonly referred to 

as the ‘‘major New Source Review’’ or 

‘‘major NSR’’ programs. 

The statutory requirements for a PSD 

permit program set forth under part C of 

title I of the CAA (sections 160 through 

169) are implemented through the EPA’s 

PSD regulations found at 40 CFR 51.166 

(minimum requirements for an 

approvable PSD SIP) and 40 CFR 52.21 

(PSD permitting program for permits 

issued under the EPA’s Federal 

permitting authority). Whenever a 

proposed new major source or major 

modification triggers PSD requirements 

for SO2, either 40 CFR 52.21 or State 

regulations based on 40 CFR 51.166 will 

apply for undesignated areas and for 

areas that are designated as attainment 

or as unclassifiable for the revised 

secondary SO2 NAAQS. 

For PSD, a ‘‘major stationary source’’ 

is one with the potential to emit 250 

tons per year (tpy) or more of any 

regulated NSR pollutant, unless the new 

or modified source is classified under a 

list of 28 source categories contained in 

the statutory definition of ‘‘major 

emitting facility’’ in CAA section 169(1). 

For those 28 source categories, a ‘‘major 

stationary source’’ is one with the 

potential to emit 100 tpy or more of any 

regulated NSR pollutant. A ‘‘major 

modification’’ is a physical change or a 

change in the method of operation of an 

existing major stationary source that 

results, first, in a significant emissions 

increase of a regulated NSR pollutant 

and, second, in a significant net 

emissions increase of that pollutant. See 

40 CFR 51.166(b)(2)(i), 52.21(b)(2)(i). 

The EPA PSD regulations define the 

term ‘‘regulated NSR pollutant’’ to 

include any pollutant for which a 

NAAQS has been promulgated and any 

pollutant identified in the EPA 

regulations as a constituent or precursor 

to such pollutant. See 40 CFR 

51.166(b)(49), 52.21(b)(50). Thus, the 

PSD program currently requires the 

review and control of emissions of SO2,

as applicable. Among other things, for 

each regulated NSR pollutant emitted or 

increased in a significant amount, the 

PSD program requires a new major 

stationary source or a major 

modification to apply the ‘‘best 

available control technology’’ (BACT) 

and to conduct an air quality impact 

analysis to demonstrate that the 

proposed major stationary source or 

major modification will not cause or 

contribute to a violation of any NAAQS 

or PSD increment.162 See CAA section 

165(a)(3)–(4), 40 CFR 51.166(j) and (k), 

52.21(j) and (k). The PSD requirements 

may also include, in appropriate cases, 

an analysis of potential adverse impacts 

on Class I areas. See CAA sections 

162(a) and 165, 40 CFR 51.166(p), 

52.21(p)).163

With regard to nonattainment NSR, 

the EPA’s regulations for the NNSR 

programs are contained in 40 CFR 

51.165, 40 CFR 52.24, and 40 CFR part 

51, appendix S. Specifically, the EPA 

developed minimum program 

requirements for a NNSR program that 

is approvable in a SIP, and those 

requirements, which include 

requirements for SO2, are contained in 

40 CFR 51.165. In addition, 40 CFR part 

51, appendix S contains requirements 

constituting an interim NNSR program. 

This program enables NNSR permitting 

in nonattainment areas by States that 

lack a SIP-approved NNSR permitting 

program (or a program that does not 

apply to the relevant pollutant) during 

the time between the date of the 

relevant designation and the date that 

the EPA approves into the SIP a NNSR 

program. See 40 CFR part 51, appendix 

S, part I; 40 CFR 52.24(k). Any new 

NNSR requirements for SO2 associated 

with the revised secondary standard 

would become applicable upon the 

effective date of any nonattainment 

designation for the final standard. 

As stated in the proposal section V.C., 

the EPA is not making any changes to 

the NSR program regulations to 

implement the revised secondary SO2

NAAQS. Under the PSD program, any 

permit issued on and after the effective 

date of the new annual secondary SO2

NAAQS will require a demonstration 

that the emissions from the proposed 

major stationary source or major 

modification would not cause or 

contribute to violation of that standard. 

The EPA has regulations, models, 

guidance, and other tools for making 

this showing, and anticipates that 

sources and reviewing authorities will 

be able to use most of these existing 

tools to demonstrate compliance with 

the revised secondary SO2 NAAQS. 

However, as provided in the NPRM, the 

EPA developed a separate technical 


khammond on DSK9W7S144PROD with RULES2
105778 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

164This technical memo (Tillerson et al., 2024) is 

available in the docket for this NAAQS review 

(Docket ID No. EPA–HQ–OAR–2014–0128–0041). 

165See ‘‘VII. Description of the Proposal’’ in 

‘‘Criteria and Procedures for Determining 

Conformity to State or Federal Implementation 

Plans of Transportation Plans, Programs, and 

Projects funded or Approved Under Title 23 U.S.C. 

or the Federal Transit Act.’’ (58 FR 3768, January 

11, 1993). The EPA finalized the original 

transportation conformity regulations on November 

24, 1993 (58 FR 62188). The rule has subsequently 

been revised and the current provisions of the 

transportation conformity rule are found at 40 CFR 

part 93, subpart A. 

166Applicability of the General Conformity 

program to any newly designated nonattainment 

area for a specific NAAQS begins one year 

following the effective date of the final 

nonattainment designation, as allowed under CAA 

section 176(c)(6) and 40 CFR 93.153(k). 

167Under CAA section 176(c)(1), Federal agencies 

have the affirmative responsibility to assure their 

actions achieve conformity to the purpose of an 

implementation plan, where the term ‘‘conformity 

to an implementation plan’’ is defined at CAA 

sections 176(c)(1)(A) and 176(c)(1)(B). Under CAA 

section 176(c)(4), the EPA is required to establish 

criteria and procedures for determining conformity. 

document (Tillerson et al., 2024),164

which provides a technical justification 

for how a demonstration of compliance 

with the 1-hour primary SO2 standard 

can suffice to demonstrate compliance 

with the new SO2 secondary standard. 

The EPA has determined that this 

alternative compliance demonstration 

approach is technically justified and can 

provide for streamlined implementation 

of the new secondary SO2 NAAQS 

under the PSD program in all areas of 

the country. Accordingly, the EPA plans 

to issue a memorandum that explains 

how permit applicants and permitting 

authorities may use this alternative 

compliance demonstration approach 

and supporting technical analysis in 

making the required demonstration for 

the new secondary SO2 NAAQS. The 

EPA intends to issue this memorandum 

close in time to the effective date of the 

new secondary SO2 NAAQS to help 

provide for a smooth transition to 

implementing the revised secondary 

standard under the PSD program. 

D. Transportation Conformity Program 

As discussed in the proposal section 

V.E., transportation conformity is 

required under CAA section 176(c) (42 

U.S.C. 7506(c)) to ensure that federally 

supported highway and transit activities 

are consistent with (‘‘conform to’’) the 

purpose of the SIP. Transportation 

conformity applies to areas that are 

designated as nonattainment areas and 

to nonattainment areas that have been 

redesignated to attainment with an 

approved CAA section 175A 

maintenance plan (i.e., maintenance 

areas) for transportation-related criteria 

pollutants: carbon monoxide, ozone, 

NO2, PM2.5, and PM10. Motor vehicles 

are not significant sources of SO2, and 

thus transportation conformity does not 

apply to any SO2 NAAQS (40 CFR 

93.102(b)(1)), either the existing NAAQS 

or this revised secondary SO2

NAAQS.165 Therefore, this final rule 

does not affect the transportation 

conformity rule (40 CFR 51.390 and 40 

CFR part 93, subpart A). 

E. General Conformity Program 

The General Conformity program 

applies to federal activities that cause 

emissions of the criteria or precursor 

pollutants to originate within 

designated nonattainment areas 166 or 

redesignated attainment areas that 

operate under approved CAA section 

175A maintenance plans (i.e., 

maintenance areas). The General 

Conformity program requirements at 40 

CFR part 93, subpart B establish criteria 

and procedures for determining 

conformity as required under CAA 

section 176(c),167 which prohibits a 

Federal agency from taking an action 

that would interfere with the ability of 

a State or Tribe to attain or maintain the 

NAAQS. General Conformity applies 

only to Federal activities not defined as 

transportation plans, programs, or 

projects under 40 CFR 93.102. The 

program requirements apply to 

emissions of all six criteria pollutants 

and their precursors, including NOX, 

SOX, and PM, per 40 CFR 93.153(b)(1) 

and (2), but only to the extent the 

emissions can be characterized as 

‘‘direct emissions’’ or ‘‘indirect 

emissions’’ as defined under 40 CFR 

93.152. General federal activities that 

cause emissions of SO2 are subject to 

General Conformity; however, no 

change to the regulations is necessary to 

accommodate any changes to the 

secondary SO2 NAAQS made by this 

rulemaking. 

VI. Statutory and Executive Order 

Reviews 

Additional information about these 

statutes and Executive orders can be 

found at https://www.epa.gov/lawsregulations/laws-and-executive-orders. 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 14094: Modernizing Regulatory 

Review 

This action is a ‘‘significant regulatory 

action’’ as defined in Executive Order 

12866, as amended by Executive Order 

14094. Accordingly, EPA submitted this 

action to the Office of Management and 

Budget (OMB) for Executive Order 

12866 review. Documentation of any 

changes made in response to the 

Executive Order 12866 review is 

available in the docket. The EPA 

prepared an analysis to determine if 

additional emission reductions would 

be needed to meet the revised secondary 

SO2 NAAQS. This analysis is contained 

in the document ‘‘Air Quality Analyses 

Using Sulfur Dioxide (SO2) Air Quality 

Data, Updated’’ which is available in the 

docket for this NAAQS review (ID No. 

EPA–HQ–OAR–2014–0128). The 

analysis concluded that no additional 

emissions reductions beyond any 

needed to meet the current 1-hour 

primary SO2 NAAQS are expected to be 

necessary to meet the new annual 

secondary SO2 NAAQS of 10 ppb, 

averaged over three years. Thus, there 

are no pollution controls expected to be 

necessary, and accordingly no costs or 

monetized benefits associated with this 

NAAQS revision. Accordingly, no 

regulatory impact analysis has been 

prepared for this final rule. 

B. Paperwork Reduction Act (PRA) 

This action does not impose any new 

information collection burden under the 

PRA. The OMB has previously approved 

the information collection activities 

contained in the existing regulations 

and has assigned OMB control number 

2060–0084. The data collected through 

the information collection activities in 

the existing regulations consist of 

ambient air concentration 

measurements for the seven air 

pollutants with national ambient air 

quality standards (i.e., ozone, sulfur 

dioxide, nitrogen dioxide, lead, carbon 

monoxide, PM2.5 and PM10), ozone 

precursors, air toxics, meteorological 

variables at a select number of sites, and 

other supporting measurements. 

Accompanying the pollutant 

concentration data are quality 

assurance/quality control data and air 

monitoring network design information. 

The EPA and others (e.g., State and local 

air quality management agencies, Tribal 

entities, environmental organizations, 

academic institutions, and industrial 

groups) use the ambient air quality data 

for many purposes including informing 

the public and other interested parties 

of an area’s air quality, judging an area’s 

air quality in comparison with the 

established health or welfare standards, 

evaluating an air quality management 

agency’s progress in achieving or 

maintaining air pollutant levels below 

the national and local standards, 

developing and revising State 

Implementation Plans (SIPs), evaluating 

air pollutant control strategies, 

developing or revising national control 

policies, providing data for air quality 

model development and validation, 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105779 

supporting enforcement actions, 

documenting episodes and initiating 

episode controls, assessing air quality 

trends, and conducting air pollution 

research. 

C. Regulatory Flexibility Act (RFA) 

I certify that this action will not have 

a significant economic impact on a 

substantial number of small entities 

under the RFA. This action will not 

impose any requirements on small 

entities. Rather, this final rule 

establishes national standards for 

allowable annual average concentrations 

of SO2 in ambient air as required by 

section 109 of the CAA. See also 

American Trucking Associations v. 

EPA, 175 F.3d 1027, 1044–45 (D.C. Cir. 

1999) (NAAQS do not have significant 

impacts upon small entities because 

NAAQS themselves impose no 

regulations upon small entities), rev’d in 

part on other grounds, Whitman v. 

American Trucking Associations, 531 

U.S. 457 (2001). 

D. Unfunded Mandates Reform Act 

(UMRA) 

This action does not contain an 

unfunded mandate as described in 

UMRA, 2 U.S.C. 1531–1538, and does 

not significantly or uniquely affect small 

governments. Furthermore, as indicated 

previously, in setting a NAAQS the EPA 

cannot consider the economic or 

technological feasibility of attaining 

ambient air quality standards, although 

such factors may be considered to a 

degree in the development of state plans 

to implement the standards. See also 

American Trucking Associations v. 

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

because the EPA is precluded from 

considering costs of implementation in 

establishing NAAQS, preparation of the 

RIA pursuant to the Unfunded 

Mandates Reform Act would not furnish 

any information that the court could 

consider in reviewing the NAAQS). 

E. Executive Order 13132: Federalism 

This action does not have federalism 

implications. It will not have substantial 

direct effects on the states, on the 

relationship between the national 

government and the states, or on the 

distribution of power and 

responsibilities among the various 

levels of government. However, the EPA 

recognizes that states will have a 

substantial interest in this action and 

any future revisions to associated 

requirements. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

This action does not have Tribal 

implications, as specified in Executive 

Order 13175. It does not have a 

substantial direct effect on one or more 

Indian Tribes as Tribes are not obligated 

to adopt or implement any NAAQS. In 

addition, Tribes are not obligated to 

conduct ambient monitoring for SO2 or 

to adopt the ambient air monitoring 

requirements of 40 CFR part 58. Thus, 

Executive Order 13175 does not apply 

to this action. However, consistent with 

the EPA Policy on Consultation and 

Coordination with Indian Tribes, the 

EPA offered consultation to all 574 

Federally Recognized Tribes during the 

development of this action. Although no 

Tribes requested consultation, the EPA 

provided informational meetings and 

provided information on the monthly 

National Tribal Air Association calls, 

and during the public comment period 

we received comments on the proposed 

rule from this Tribal organization. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

EPA interprets Executive Order 13045 

as applying only to those regulatory 

actions that concern environmental 

health or safety risks that EPA has 

reason to believe may 

disproportionately affect children, per 

the definition of ‘‘covered regulatory 

action’’ in section 2–202 of the 

Executive order. 

Therefore, this action is not subject to 

Executive Order 13045 because it does 

not concern an environmental health 

risk or safety risk. Since this action does 

not concern human health, EPA’s Policy 

on Children’s Health also does not 

apply. 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution or Use 

This action is not a ‘‘significant 

energy action’’ because it is not likely to 

have a significant adverse effect on the 

supply, distribution, or use of energy. 

The purpose of this action is to revise 

the existing secondary SO2 standard, 

and also to retain the current secondary 

standards for NO2, PM2.5 and PM10. The 

action does not prescribe specific 

pollution control strategies by which 

these ambient air standards and 

monitoring revisions will be met. Such 

strategies will be developed by states on 

a case-by-case basis, and the EPA cannot 

predict whether the control options 

selected by states will include 

regulations on energy suppliers, 

distributors, or users. Thus, the EPA 

concludes that this action does not 

constitute a significant energy action as 

defined in Executive Order 13211. 

I. National Technology Transfer and 

Advancement Act (NTTAA) 

This action involves environmental 

monitoring or measurements. The EPA 

has decided to use the existing 

indicator, SO2, for measurements in 

support of this action and is not revising 

the SO2 FRMs or FEMs for measurement 

of this air pollutant. The EPA employs 

a Performance-Based Measurement 

System (PBMS) when designating 

monitoring methods as either FRM or 

FEM, which does not require the use of 

specific, prescribed analytic methods. 

This performance-based assessment of 

candidate methods is described in 40 

CFR part 50 and the reference and 

equivalency criteria described in 40 CFR 

part 53. The EPA does not preclude the 

use of other methods, whether it 

constitutes a voluntary consensus 

standard or not, as long as it meets the 

specified performance criteria defined 

in 40 CFR part 53 and is approved by 

EPA as an FRM or FEM. Our approach 

in the past has resulted in multiple 

brands of monitors being approved as 

FRM for SO2, and we expect this trend 

to continue. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations and Executive 

Order 14096: Revitalizing Our Nation’s 

Commitment to Environmental Justice 

for All 

The EPA believes that the human 

health and environmental conditions 

that exist prior to this action do not 

result in disproportionate and adverse 

effects on communities with 

Environmental Justice (EJ) concerns. As 

discussed in sections II.A.4. and II.B. 

above, and chapters 5 and 7 of the PA, 

the acid buffering capacity of 

waterbodies in key acid-sensitive 

ecoregions in recent years is estimated 

to meet protection targets in high 

percentages. As discussed in section 

II.A.3.b. above, impacts on acidsensitive waterbodies, if sufficiently 

severe, would have the potential to 

impact the public welfare through 

impacts to fisheries. Although recent 

conditions do not indicate such a level 

of severity, to the extent local 

communities relied on such fisheries 

disproportionately to their 

representation in the population, such 

effects of the past (e.g., effects associated 

with acidification risks of 20 or more 

years ago) would have had the potential 


khammond on DSK9W7S144PROD with RULES2
105780 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

for disproportionate impacts. Recent 

conditions do not indicate risk of 

aquatic acidification to such a level of 

severity, and the available information 

for recent acid buffering capacity levels 

does not include evidence of 

disproportionate and adverse impacts 

on communities with EJ concerns. As 

the action is to establish a new, more 

stringent standard to protect acidsensitive waterbodies to recent levels 

and protect against recurrence of 

acidification effects from the past, for 

which the potential for disproportionate 

and adverse effects on local 

communities is unknown, the EPA 

believes that this action is not likely to 

result in new disproportionate and 

adverse effects on communities with EJ 

concerns. The information supporting 

this Executive order review is contained 

in the PA for this review and sections 

II.A.3., II.A.4., II.B.1. and II.B.3. of this 

document. 

K. Congressional Review Act (CRA) 

This action is subject to the CRA, and 

the EPA will submit a rule report to 

each House of the Congress and to the 

Comptroller General of the United 

States. This action is not a ‘‘major rule’’ 

as defined by 5 U.S.C. 804(2). 

L. Judicial Review 

Under section 307(b)(1) of the CAA, 

this final action is ‘‘nationally 

applicable’’ and petitions for judicial 

review of this action must be filed in the 

U.S. Court of Appeals for the District of 

Columbia Circuit within 60 days from 

the date this final action is published in 

the Federal Register. Filing a petition 

for reconsideration by the Administrator 

of this final action does not affect the 

finality of the action for the purposes of 

judicial review, nor does it extend the 

time within which a petition for judicial 

review must be filed and shall not 

postpone the effectiveness of such 

action. 

VII. References 

Altshuller, AP (1976). Regional transport and 

transformation of sulfur dioxide to 

sulfates in the United States. J Air Poll 

Contr Assoc 26: 318–324. 

Baker, LA, Herlihy, AT, Kaufmann, PR, and 

Eilers, JM (1991). Acidic lakes and 

streams in the United States: the role of 

acidic deposition. Science 252: 1151– 

1154. 

Baker JP; Schofield CL. (1985). Acidification 

impacts on fish populations: A review. 

In: Adams D; Page WP (Eds.), Acid 

deposition: environmental, economic, 

and policy issues (pp. 183–221). New 

York and London: Plenum Press. 

Banzhaf, S, Burtraw, D, Evans, D and 

Krupnick, A (2006). Valuation of Natural 

Resource Improvements in the 

Adirondacks. Land Econ 82: 445–464. 

Baron, JS, Driscoll, CT, Stoddard, JL and 

Richer, EE (2011). Empirical critical 

loads of atmospheric nitrogen deposition 

for nutrient enrichment and acidification 

of sensitive US lakes. Bioscience 61: 

602–613. 

Boyer, EW, Goodale, CL, Jaworski, NA and 

Howarth, RW (2002). Anthropogenic 

nitrogen sources and relationships to 

riverine nitrogen export in the 

northeastern USA. Biogeochemistry 57: 

137–169. 

Brown, CA and Ozretich, RJ (2009). Coupling 

between the coastal ocean and Yaquina 

Bay, Oregon: Importance of oceanic 

inputs relative to other nitrogen sources. 

Estuaries Coasts 32: 219–237. 

Bulger, AJ, Cosby, BJ, Dolloff, CA, Eshleman, 

KN, Webb, JR and Galloway, JN (1999). 

SNP:FISH. Shenandoah National Park: 

Fish in sensitive habitats. Project Final 

Report-Volume 1–4. Charlottesville, VA, 

University of Virginia. 

Bulger, AJ, Cosby, BJ and Webb, JR (2000). 

Current, reconstructed past, and 

projected future status of brook trout 

(Salvelinus fontinalis) streams in 

Virginia. Can J Fish Aquat Sci 57(7): 

1515–1523. 

Cosby, BJ, Hornberger, GM, Galloway, JN and 

Wright, RF (1985). Modeling the effects 

of acid deposition: Assessment of a 

lumped parameter model of soil water 

and streamwater chemistry. Water 

Resour Res 21(1): 51–63. 

Costanza, R, De Groot, R, Braat, L, 

Kubiszewski, I, Fioramonti, L, Sutton, P, 

Farber, S and Grasso, M (2017). Twenty 

years of ecosystem services: How far 

have we come and how far do we still 

need to go? Ecosyst Serv 28: 1–16. 

Cox, L, Kendall, R and Fernandez, I (2020a). 

Letter from Louis Cox, Chair, Clean Air 

Scientific Advisory Committee, Ronald 

Kendall, Chair, Secondary NAAQS 

Review Panel for Oxides of Nitrogen and 

Sulfur and Ivan Fernandez, Immediate 

Past Chair, Secondary NAAQS Review 

Panel for Oxides of Nitrogen and Sulfur 

to the Administrator Andrew R. Wheeler, 

Re: CASAC Review of the EPA’s 

Integrated Science Assessment for 

Oxides of Nitrogen, Oxides of Sulfur, 

and Particulate Matter—Ecological 

Criteria (Second External Review Draft— 

June 2018). May 5, 2020. EPA–CASAC– 

20–004. Office of the Administrator, 

Science Advisory Board, Washington, 

DC Available at: https://casac.epa.gov/ 

ords/sab/f?p=113:12:13429

72375271:::12. 

Cox, L, Kendall, R and Fernandez, I (2020b). 

Letter from Louis Cox, Chair, Clean Air 

Scientific Advisory Committee, Ronald 

Kendall, Chair, Secondary NAAQS 

Review Panel for Oxides of Nitrogen and 

Sulfur and Ivan Fernandez, Immediate 

Past Chair, Secondary NAAQS Review 

Panel for Oxides of Nitrogen and Sulfur 

to Administrator Andrew R. Wheeler, Re: 

Consultation on the EPA’s Review of the 

Secondary Standards for Ecological 

Effects of Oxides of Nitrogen, Oxides of 

Sulfur, and Particulate Matter: Risk and 

Exposure Assessment Planning 

Document (August—2018). May 5, 2020. 

EPA–CASAC–20–005. Office of the 

Administrator, Science Advisory Board, 

Washington, DC Available at: https://

casac.epa.gov/ords/sab/ 

f?p=113:12:1342972375271:::12. 

Cox, RD, Preston, KL, Johnson, RF, Minnich, 

RA and Allen, EB (2014). Influence of 

landscape scale variables on vegetation 

conversion to exotic annual grassland in 

southern California, USA. Glob Ecol 

Conserv 2: 190–203. 

Davis, TW, Bullerjahn, GS, Tuttle, T, Mckay, 

RM and Watson, SB (2015). Effects of 

increasing nitrogen and phosphorus 

concentrations on phytoplankton 

community growth and toxicity during 

planktothrix blooms in Sandusky Bay, 

Lake Erie. Environ Sci Technol 49: 7197– 

7207. 

Dietze, MC and Moorcroft, PR (2011). Tree 

mortality in the eastern and central 

United States:Patterns and drivers. Glob 

Change Biol 17(11): 3312–3326. 

Diez Roux, A and Fernandez, I (2016). Letter 

from Anna Diez Roux, Chair, Clean Air 

Scientific Advisory Committee and Ivan 

Fernandez, Chair, Secondary NAAQS 

Review Panel for Oxides of Nitrogen and 

Sulfur, to Administrator Gina McCarthy, 

Re: CASAC Review of the EPA’s Draft 

Integrated Review Plan for the National 

Ambient Air Quality Standards for 

Oxides of Nitrogen and Oxides of Sulfur. 

April 1, 2016. EPA–CASAC–16–001. 

Office of the Administrator, Science 

Advisory Board, Washington, DC 

Available at: https://casac.epa.gov/ords/ 

sab/f?p=113:12:1342972375271:::12. 

Diez Roux, A and Fernandez, I (2017). Letter 

from Anna Diez Roux, Chair, Clean Air 

Scientific Advisory Committee and Ivan 

Fernandez, Chair, Secondary NAAQS 

Review Panel for Oxides of Nitrogen and 

Sulfur, to the Honorable Gina McCarthy, 

Administrator, Re: CASAC Review of the 

EPA’s Integrated Science Assessment for 

Oxides of Nitrogen, Oxides of Sulfur, 

and Particulate Matter—Ecological 

Criteria (First External Review Draft— 

February 2017). September 28, 2017. 

EPA–CASAC–17–004. Office of the 

Administrator, Science Advisory Board, 

Washington, DC Available at: https://

nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=

P100X9FA.PDF. 

Driscoll, CT, Driscoll, KM, Fakhraei, H, 

Civerolo, K (2016). Long-term temporal 

trends and spatial patterns in the acidbase chemistry of lakes in the 

Adirondack region of New York in 

response to decreases in acidic 

deposition. Atm Chem 146: 5–14. 

Driscoll, CT, Lawrence, GB, Bulger, AJ, 

Butler, TJ, Cronan, CS, Eagar, C, Lambert, 

KF, Likens, GE, Stoddard, JL and 

Weathers, KC (2001). Acidic deposition 

in the northeastern United States: 

Sources and inputs, ecosystem effects, 

and management strategies. Bioscience 

51: 180–198. 

Driscoll, CT, Lehtinen, MD and Sullivan, TJ 

(1994). Modeling the acid-base chemistry 

of organic solutes in Adirondack, New 

York, lakes. Water Resour Res 30: 297– 

306. 

Duchesne, L and Ouimet, R (2009). Presentday expansion of American beech in 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105781 

northeastern hardwood forests: Does soil 

base status matter? Can J For Res 39: 

2273–2282. 

Dupont, J, Clair, TA, Gagnon, C, Jeffries, DS, 

Kahl, JS, Nelson, SJ and Peckenham, JM 

(2005). Estimation of critical loads of 

acidity for lakes in northeastern United 

States and eastern Canada. Environ 

Monit Assess 109(1): 275–291. 

Emmett, BA, Boxman, D, Bredemeier, M, 

Gunderson, P, Kjonaas, OJ, Moldan, F, 

Schleppi, P, Tietema, A and Wright, RF 

(1998). Predicting the effects of 

atmospheric nitrogen deposition in 

conifer stands: evidence from the 

NITREX ecosystem-scale experiments. 

Ecosystems 1: 352–360. 

Fakhraeri, H, Driscoll, CT, Selvendiran, P, 

DePinto, JV, Bloomfield, J, Quinn, S and 

Rowell, HC (2014). Development of a 

total maximum daily load (TMDL) for 

acid-impaired lakes in the Adirondack 

regon of New York. Atmos Environ 95: 

277–287. 

Fenn, ME, Allen, EB, Weiss, SB, Jovan, S, 

Geiser, LH, Tonnesen, GS, Johnson, RF, 

Rao, LE, Gimeno, BS, Yuan, F, Meixner, 

T and Bytnerowicz, A (2010). Nitrogen 

critical loads and management 

alternatives for N-impacted ecosystems 

in California. J Environ Manage 91: 

2404–2423. 

Friedlander, S (1982). Letter from Sheldon 

Friedlander, Chair, Clean Air Scientific 

Advisory Committee to Anne Gorsuch, 

Administrator, Re: CASAC Review and 

Closure of the Criteria Document for 

Sulfur Oxides/Particulate Matter. 

January 29, 1982. EPA–SAB–CASAC– 

82–003. Office of the Administrator, 

Science Advisory Board, Washington, 

DC Available at: https://casac.epa.gov/ 

ords/sab/f?p=113:12:13429

72375271:::12. 

Fuss, CB, Driscoll, CT and Campbell, JL 

(2015). Recovery from chronic and 

snowmelt acidification: Long-term trends 

in stream and soil water chemistry at the 

Hubbard Brook Experimental Forest, 

New Hampshire, USA. Jour Geo Res: 

Biog 120: 2360–2374. 

Geiser, LH, Nelson, PR, Jovan, SE, Root, HT 

and Clark, CM (2019). Assessing 

ecological risks from atmospheric 

deposition of nitrogen and sulfur to us 

forests using epiphytic macrolichens. 

Diversity 11(6): 87. 

Gobler, CJ, Burkholder, JM, Davis, TW, 

Harke, MJ, Johengen, T, Stow, CA and 

Van de Waal, DB (2016). The dual role 

of nitrogen supply in controlling the 

growth and toxicity of cyanobacterial 

blooms. Harmful Algae 54: 87–97. 

Hennigan, CJ, Sandholm, S, Kim, S, Stickel, 

RE, Huey, LG and Weber, RJ (2006). 

Influence of Ohio River valley emissions 

on fine particle sulfate measured from 

aircraft over large regions of the eastern 

United States and Canada during 

INTEX-NA. J Geophys Res Atmos 111: 

D24. 

Herlihy AT, Kaufman, PR and Mitch, ME 

(1991). Stream chemistry in the Eastern 

United States 2. Current sources of 

acidity in acidic and low acidneutralizing capacity streams. Water 

Resources Res 27(4): 629–642. 

Horn, KJ, Thomas, RQ, Clark, CM, Pardo, LH, 

Fenn, ME, Lawrence, GB, Perakis, SS, 

Smithwick, EA, Baldwin, D, Braun, S 

and Nordin, A (2018). Growth and 

survival relationships of 71 tree species 

with nitrogen and sulfur deposition 

across the conterminous U.S. PLoS ONE 

13(10): e0205296. 

Howarth, RW. (2008). Science for ecosystembased management: Narragansett Bay in 

the 21st century: Estimating atmospheric 

nitrogen deposition in the Northeastern 

United States: Relevance to Narragansett 

Bay. Springer. New York, NY. 

Isbell, F, Tilman, D, Polasky, S, Binder, S and 

Hawthorne, P (2013). Low biodiversity 

state persists two decades after cessation 

of nutrient enrichment. Ecol Lett 16: 

454–460. 

Janicki Environmental, Inc. (2013). Estimates 

of total nitrogen, total phosphorus, total 

suspended solids, and biochemical 

oxygen demand loadings to Tampa Bay, 

Florida: 2007–2011. St. Petersburg, FL: 

Tampa Bay Estuary Program. Available 

at: https://www.tbeptech.org/TBEP_

TECH_PUBS/2013/TBEP_03_13_FINAL_

TBEP_Loads_2007- 

2011%2019Mar2013.pdf. 

Kretser W; Gallagher J; Nicolette J. (1989). 

Adirondack Lakes Study, 1984–1987: An 

Evaluation of Fish Communities and 

Water Chemistry: Ray Brook, NY; 

prepared for: Adirondack Lakes Survey 

(ALS) Corporation. 

Latimer, JS and Charpentier, MA (2010). 

Nitrogen inputs to seventy-four southern 

New England estuaries: Application of a 

watershed nitrogen loading model. 

Estuar Coast Shelf Sci 89: 125–136. 

Latimer, JS and Rego, SA (2010). Empirical 

relationship between eelgrass extent and 

predicted watershed-derived nitrogen 

loading for shallow New England 

estuaries. Estuar Coast Shelf Sci 90: 231– 

240. 

Lawrence, GB, Hazlett, PW, Fernandez, IJ, 

Ouimet, R, Bailey, SW, Shortle, WC, 

Smith, KT and Antidormi, MR (2015). 

Declining acidic deposition begins 

reversal of forest-soil acidification in the 

northeastern US and eastern Canada. 

Environ Sci Technol 49: 13103–13111. 

Lebo, ME; Paerl, HW; Peierls, BL. (2012). 

Evaluation of progress in achieving 

TMDL mandated nitrogen reductions in 

the Neuse River Basin, North Carolina. 

Environ Manage 49: 253–266. 

Li, H and McNulty, SG (2007). Uncertainty 

analysis on simple mass balance model 

to calculate critical loads for soil acidity. 

Environ Pollut 149: 315–326. 

Lippman, M (1986). Letter from Morton 

Lippman, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Lee Thomas, Administrator, Re: Review 

of the 1986 Addendum to the 1982 Staff 

Paper on Particulate Matter (National 

Ambient Air Quality Standards for 

Particulate Matter: Assessment of 

Scientific and Technical Information). 

December 16, 1986. SAB–CASAC–87– 

010. Office of the Administrator, Science 

Advisory Board, Washington, DC 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P100RZZ1.PDF. 

Lippman, M (1987). Letter from Morton 

Lippman, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Lee Thomas, Administrator, Re: Review 

of the 1986 Addendum to the 1982 Staff 

Paper on Sulfur Oxides (Review of the 

National Ambient Air Quality Standards 

for Sulfur Oxides: Updated assessment 

of Scientific and Technical Information). 

February 19, 1987. SAB–CASAC–87– 

022. Office of the Administrator, Science 

Advisory Board, Washington, DC 

Available at: https://casac.epa.gov/ords/ 

sab/f?p=113:12:1342972375271:::12. 

Lynch, JA, Phelan, J, Pardo, LH, McDonnell, 

TC, Clark, CM, Bell, MD, Geiser, LH and 

Smith, RJ (2022). Detailed 

Documentation of the National Critical 

Load Database (NCLD) for U.S. Critical 

Loads of Sulfur and Nitrogen, version 

3.2.1 National Atmospheric Deposition 

Program, Wisconsin State Laboratory of 

Hygiene, Madison, WI. 

Magill, AH, Aber, JD, Currie, WS, 

Nadelhoffer, KJ, Martin, ME, McDowell, 

WH, Melillo, JM and Steudler, P (2004). 

Ecosystem response to 15 years of 

chronic nitrogen additions at the 

Harvard Forest LTER, Massachusetts, 

USA. For Ecol Manage 196: 7–28. 

McCrackin, ML, Harrison, JA and Compton, 

JE (2013). A comparison of NEWS and 

SPARROW models to understand 

sources of nitrogen delivered to US 

coastal areas. Biogeochemistry 114: 281– 

297. 

McDonnell, TC, Cosby, BJ and Sullivan, TJ 

(2012). Regionalization of soil base 

cation weathering for evaluating stream 

water acidification in the Appalachian 

Mountains, USA. Environ Pollut Control 

162: 338–344. 

McDonnell, TC, Sullivan, TJ, Hessburg, PF, 

Reynolds, KM, Povak, NA, Cosby, BJ, 

Jackson, W and Salter, RB (2014). 

Steady-state sulfur critical loads and 

exceedances for protection of aquatic 

ecosystems in the U.S. Southern 

Appalachian Mountains. J Environ 

Manage 146: 407–419. 

McNulty, SG, Boggs, J, Aber, JD, Rustad, L 

and Magill, A (2005). Red spruce 

ecosystem level changes following 14 

years of chronic N fertilization. For Ecol 

Manage 219: 279–291. 

Moore, RB, Johnston, CM, Smith, RA and 

Milstead, B (2011). Source and delivery 

of nutrients to receiving waters in the 

northeastern and mid-atlantic regions of 

the United States. J Am Water Resour 

Assoc 47: 965–990. 

NADP (National Atmospheric Deposition 

Program) (2021). National Atmospheric 

Deposition Program 2021 Annual 

Summary. Wisconsin State Laboratory of 

Hygiene, University of WisconsinMadison, WI. Available at: https://

nadp.slh.wisc.edu/wp-content/uploads/ 

2022/11/2021as.pdf. 

NAPAP (National Acid Precipitation 

Assessment Program) (1991). 1990 

Integrated Assessment Report. NAPAP 

Office of the Director, Washington, DC. 

Nash, TH, III and Sigal, LL (1999). Oxidant 

Air Pollution Impacts in the Montane 

Forests of Southern California: A Case 


khammond on DSK9W7S144PROD with RULES2
105782 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

Study of the San Bernadino Mountains: 

Epiphytic lichens in the San Bernadino 

Mountains in relation to oxidant 

gradients. Springer. New York, NY. 

NRC (National Research Council) (2004). Air 

quality management in the United States. 

National Academies Press. Washington, 

DC. 

Officer, CB, Biggs, RB, Taft, JL, Cronin, LE, 

Tyler, MA and Boynton, WR (1984). 

Chesapeake Bay anoxia: Origin, 

development, and significance. Science 

223: 22–27. 

Omernik, JM (1987). Ecoregions of the 

Conterminous United States. Ann Ass 

Am Geogr 77(1): 118–125. 

Omernik, JM and Griffith, GE (2014). 

Ecoregions of the Conterminous United 

States: Evolution of a Hierarchical 

Spatial Framework. Environ Manag 54: 

1249–1266. 

Padgett, PE, Parry, SD, Bytnerowicz, A and 

Heath, RL (2009). Image analysis of 

epicuticular damage to foliage caused by 

dry deposition of the air pollutant nitric 

acid. J Environ Monit 11: 63–74. 

Pardo, LH, Fenn, ME, Goodale, CL, Geiser, 

LH, Driscoll, CT, Allen, EB, Baron, JS, 

Bobbink, R, Bowman, WD, Clark, CM, 

Emmett, B, Gilliam, FS, Greaver, TL, 

Hall, SJ, Lilleskov, EA, Liu, L, Lynch, JA, 

Nadelhoffer, KJ, Perakis, SS, RobinAbbott, MJ, Stoddard, JL, Weathers, KC 

and Dennis, RL (2011). Effects of 

nitrogen deposition and empirical 

nitrogen critical loads for ecoregions of 

the United States. Ecol Appl 21: 3049– 

3082. 

Pavlovic, NR, Chang, SY, Huang, J, Craig, K, 

Clark, C, Horn, K and Driscoll, CT 

(2023). Empirical nitrogen and sulfur 

critical loads of U.S. tree species and 

their uncertainties with machine 

learning. Sci Total Environ 857: 159252. 

Phelan, J, Belyazid, S, Kurz, D, Guthrie, S, 

Cajka, J, Sverdrup, H and Waite, R 

(2014). Estimation of soil base cation 

weathering rates with the PROFILE 

model to determine critical loads of 

acidity for forested ecosystems in 

Pennsylvania, USA: Pilot application of 

a potential national methodology. Water 

Air Soil Pollut 225: 2109–2128. 

Pregitzer, KS, Burton, AJ, Zak, DR and 

Talhelm, AF (2008). Simulated chronic 

nitrogen deposition increases carbon 

storage in Northern Temperate forests. 

Global Change Biol 14: 142–153. 

Riddell, J, Nash, TH, III and Padgett, P (2008). 

The effect of HNO3 gas on the lichen 

Ramalina menziesii. Flora 203: 47–54. 

Riddell, J, Padgett, PE and Nash, TH III 

(2012). Physiological responses of 

lichens to factorial fumigations with 

nitric acid and ozone. Environ Pollut 

170: 202–210. 

Robertson, DM amd Saad, DA (2013). 

SPARROW models used to understand 

nutrient sources in the Mississippi/ 

Atchafalaya river basin. J Environ Qual 

42: 1422–1440. 

Robinson, RB, Barnett, TW, Harwell, GR, 

Moore, SE, Kulp, M and Schwartz, JS 

(2008). pH and acid anion time trends in 

different elevation ranges in the Great 

Smoky Mountains National Park. J 

Environ Eng 134(9): 800–808. 

Russell, A (2007). Letter from Armistead 

Russel, Chair, Secondary NAAQS 

Review Panel for Oxides of Nitrogen and 

Sulfur, to the Honorable Stephen L. 

Johnson, Administrator, Re: Clean Air 

Scientific Advisory Committee’s 

(CASAC) NOX & SOX Secondary NAAQS 

Review Panel’s Consultation on EPA’s 

Draft Plan for Review of the Secondary 

NAAQS for Nitrogen Dioxide and Sulfur 

Dioxide (September 2007 Draft). 

November 29, 2007. EPA–CASAC–08– 

003. Office of the Administrator, Science 

Advisory Board, Washington, DC 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P1000QHW.PDF. 

Russell, A and Henderson, R (2008). Letter 

from Armistead Russel, Chair, Secondary 

NAAQS Review Panel for Oxides of 

Nitrogen and Sulfur and Rogene 

Henderson, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Stephen L. Johnson, Administrator, Re: 

Clean Air Scientific Advisory 

Committee’s (CASAC) Peer Review of 

EPA’s Integrated Science Assessment 

(ISA) for Oxides of Nitrogen and 

Sulfur—Environmental Criteria (First 

External Review Draft, December 2007). 

May 19, 2008. EPA–CASAC–08–012. 

Office of the Administrator, Science 

Advisory Board, Washington, DC 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P1000K05.PDF. 

Russell, A and Samet, JM (2008a). Letter from 

Armistead Russel, Chair, Secondary 

NAAQS Review Panel for Oxides of 

Nitrogen and Sulfur and Jonathan M. 

Samet, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Stephen L. Johnson, Administrator, Re: 

Peer Review of EPA’s Integrated Science 

Assessment (ISA) for Oxides of Nitrogen 

and Sulfur—Environmental Criteria 

(Second External Review Draft). 

November 18, 2008. EPA–CASAC–09– 

002. Office of the Administrator, Science 

Advisory Board, Washington, DC 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P1002E7G.PDF. 

Russell, A and Samet, JM (2008b). Letter from 

Armistead Russel, Chair, Secondary 

NAAQS Review Panel for Oxides of 

Nitrogen and Sulfur and Jonathan M. 

Samet, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Stephen L. Johnson, Administrator, Re: 

Peer Review of EPA’s Risk and Exposure 

Assessment to Support the Review of the 

Secondary National Ambient Air Quality 

Standard for Oxides of Nitrogen and 

Sulfur: First Draft. December 23, 2008. 

EPA–CASAC–09–004. Office of the 

Administrator, Science Advisory Board, 

Washington, DC Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P1002ZCZ.PDF. 

Russell, A and Samet, JM (2009). Letter from 

Armistead Russel, Chair, Secondary 

NAAQS Review Panel for Oxides of 

Nitrogen and Sulfur and Jonathan M. 

Samet, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Lisa P. Jackson, Administrator, Re: Peer 

Review of EPA’s Risk and Exposure 

Assessment to Support the Review of the 

Secondary National Ambient Air Quality 

Standard (NAAQS) for Oxides of 

Nitrogen and Sulfur: Second Draft. 

August 28, 2009. EPA–CASAC–09–013. 

Office of the Administrator, Science 

Advisory Board, Washington, DC 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P1005A11.PDF. 

Russell, A and Samet, JM (2010a). Letter from 

Armistead Russel, Chair, Secondary 

NAAQS Review Panel for Oxides of 

Nitrogen and Sulfur and Jonathan M. 

Samet, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Lisa P. Jackson, Administrator, Re: 

Review of the Policy Assessment for the 

Review of the Secondary National 

Ambient Air Quality Standard for NOX

and SOX Second Draft. December 9, 

2010. EPA–CASAC–11–003. Office of the 

Administrator, Science Advisory Board, 

Washington, DC Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=9101XP6G.PDF. 

Russell, A and Samet, JM (2010b). Letter from 

Armistead Russel, Chair, Secondary 

NAAQS Review Panel for Oxides of 

Nitrogen and Sulfur and Jonathan M. 

Samet, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Lisa P. Jackson, Administrator, Re: 

Review of the Policy Assessment for the 

Review of the Secondary National 

Ambient Air Quality Standard for NOX

and SOX First Draft (March 2010). June 

22, 2010. EPA–CASAC–10–014. Office of 

the Administrator, Science Advisory 

Board, Washington, DC Available at: 

https://casac.epa.gov/ords/sab/ 

f?p=113:12:1342972375271:::12. 

Russell, A and Samet, JM (2011). Letter from 

Armistead Russel, Chair, Secondary 

NAAQS Review Panel for Oxides of 

Nitrogen and Sulfur and Jonathan M. 

Samet, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Lisa P. Jackson, Administrator, Re: 

CASAC Comments on the Policy 

Assessment for the Review of the 

Secondary National Ambient Air Quality 

Standard for Oxides of Nitrogen and 

Oxides of Sulfur (February 2011). May 

17, 2011. EPA–CASAC–11–005. Office of 

the Administrator, Science Advisory 

Board, Washington, DC Available at: 

https://casac.epa.gov/ords/sab/ 

f?p=113:12:1342972375271:::12. 

Sales, R, Elizalde, C, Smith, T, Baublitz, C, 

and Evangelista, M (2024). Memorandum 

to Secondary NOX/SOX/PM NAAQS 

Review Docket (Docket ID No. EPA–HQ– 

OAR–2014–0128). Additional Technical 

Analyses of Air Quality and Deposition 

Estimates. September 24, 2024. Office of 

Air Quality Planning and Standards, 

Research Triangle Park, NC. Available at 

https://www.regulations.gov. Docket ID 

No. EPA–HQ–OAR–2014–0128. 

Schaberg, PG, Hawley, GJ, Rayback, SA, 

Halman, JM and Kosiba, AM (2014). 

Inconclusive evidence of Juniperus 

virginiana recovery following sulfur 

pollution reductions. Proc Natl Acad Sci 

111: E1. 

Scheffe, RD, Lynch, JA, Reff, A, Kelly, JT, 

Hubbell, B, Greaver, TL and Smith, JT 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105783 

(2014). The aquatic acidification index: 

A new regulatory metric linking 

atmospheric and biogeochemical models 

to assess potential aquatic ecosystem 

recovery. Water Air Soil Pollut 225: 1– 

15. 

Schwede, DB and Lear, GG (2014). A novel 

hybrid approach for estimating total 

deposition in the United States. Atmos 

Environ 92: 207–220. 

Shaw, GD, Cisneros, R, Schweizer, D, 

Sickman, JO and Fenn, ME (2014). 

Critical Loads of Acid Deposition for 

Wilderness Lakes in the Sierra Nevada 

(California) Estimated by the SteadyState Water Chemistry Model. Water Air 

Soil Pollut 225:1–15. 

Sheibley, RW, Enache, M, Swarzenski, PW, 

Moran, PW and Foreman, JR (2014). 

Nitrogen deposition effects on diatom 

communities in lakes from three national 

parks in Washington State. Water Air 

Soil Poll 225: 1–23. 

Sheppard, EA (2023). Letter from Elizabeth 

A. Sheppard, Chair, Clean Air Scientific 

Advisory Committee, to the Honorable 

Michael S. Regan, Administrator, Re: 

CASAC Review of the EPA’s Policy 

Assessment for the Review of the 

Secondary National Ambient Air Quality 

Standards for Oxides of Nitrogen, Oxides 

of Sulfur and Particulate Matter 

(External Review Draft—May 2023). 

September 27, 2023. EPA–CASAC–23– 

005. Office of the Administrator, Science 

Advisory Board, Washington, DC 

Available at: https://casac.epa.gov/ords/ 

sab/f?p=113:12:1342972375271:::12. 

Simkin, SM, Allen, EB, Bowman, WD, Clark, 

CM, Belnap, J, Brooks, ML, Cade, BS, 

Collins, SL, Geiser, LH, Gilliam, FS and 

Jovan, SE (2016). Conditional 

vulnerability of plant diversity to 

atmospheric nitrogen deposition across 

the United States. Proc Natl Acad Sci 

113(15): 4086–4091. 

Stein, AF, Draxler, RR, Rolph, GD, Stunder, 

BJB, Cohen, MD and Ngan, F (2015). 

NOAA’s HYSPLIT atmospheric transport 

and dispersion modeling system. Bull 

Amer Meteor Soc 96: 2059–2077. 

Stevens, CJ (2016). How long do ecosystems 

take to recover from atmospheric 

nitrogen deposition? Biol Conserv 200: 

160–167. 

Strengbom, J, Nordin, A, Na ̈sholm, T and 

Ericson, L (2001). Slow recovery of 

boreal forest ecosystem following 

decreased nitrogen input. Funct Ecol 15: 

451–457. 

Sullivan, TJ, Cosby, BJ, Driscoll, CT, 

McDonnell, TC, Herlihy, AT and Burns, 

DA (2012a). Target loads of atmospheric 

sulfur and nitrogen deposition for 

protection of acid sensitive aquatic 

resources in the Adirondack Mountains, 

New York. Water Resour Res 48(1): 

W01547. 

Sullivan, TJ, Cosby, BJ, Jackson, WA, Snyder, 

K and Herlihy, AT (2011). Acidification 

and prognosis for future recovery of acidsensitive streams in the Southern Blue 

Ridge province. Water Air Soil Pollut 

219: 11–16. 

Sullivan, TJ, Cosby, BJ, McDonnell, TC, 

Porter, EM, Blett, T, Haeuber, R, Huber, 

CM and Lynch, J (2012b). Critical loads 

of acidity to protect and restore acidsensitive streams in Virginia and West 

Virginia. Water Air Soil Pollut 223: 

5759–5771. 

Sullivan, TJ, Driscoll, CT, Cosby, BJ, 

Fernandez, IJ, Herlihy, AT, Zhai, J, 

Stemberger, R, Snyder, KU, Sutherland, 

JW, Nierzwicki-Bauer, SA, Boylen, CW, 

McDonnell, TC and Nowicki, NA (2006). 

Assessment of the extent to which 

intensively-studied lakes are 

representative of the Adirondack 

Mountain region. Final Report 06–17. 

Corvallis, OR, E&S Environmental 

Chemistry, Inc. 

Thomas, RB, Spal, SE, Smith, KR and 

Nippert, JB (2013). Evidence of recovery 

of Juniperus virginiana trees from sulfur 

pollution after the Clean Air Act. Proc 

Natl Acad Sci 110: 15319–15324. 

Thomas, RQ, Canham, CD, Weathers, KC and 

Goodale, CL (2010). Increased tree 

carbon storage in response to nitrogen 

deposition in the US. Nat Geosci 3(1): 

13–17. 

Tillerson, C, Mintz, D and Hawes, T (2024). 

Memorandum to Secondary NOX/SOX/ 

PM NAAQS Review Docket (Docket ID 

No. EPA–HQ–OAR–2014–0128). 

Technical Analyses to Support 

Alternative Demonstration Approach for 

Proposed Secondary SO2 NAAQS under 

NSR/PSD Program. January 31, 2024. 

Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. 

Available at https://www.regulations.gov. 

Document ID EPA–HQ–OAR–2014– 

0128–0041 

U.S. DHEW (U.S. Department of Health, 

Education and Welfare) (1969a). Air 

quality criteria for sulfur oxides. 

National Air Pollution Control 

Administration. Washington, DC Pub. 

No. AP–50. January 1969. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=20013JXZ.PDF. 

U.S. DHEW (U.S. Department of Health, 

Education and Welfare). (1969b). Air 

quality criteria for particulate matter. 

National Air Pollution Control 

Administration. Washington, DC Pub. 

No. AP–49. January 1969. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=20013C3W.PDF. 

U.S. EPA (1971). Air Quality Criteria for 

Nitrogen Oxides. Air Pollution Control 

Office. Washington, DC. EPA 450–R–71– 

001. January 1971. Available at: https:// 

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=20013K3B.PDF. 

U.S. EPA (1973). ‘‘Effects of Sulfur Oxide in 

the Atmosphere on Vegetation’’. Revised 

Chapter 5 of Air Quality Criteria for 

Sulfur Oxides. Office of Research and 

Development. Research Triangle Park, 

N.C. EPA–R3–73–030. September 1973. 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=2000X8F8.PDF. 

U.S. EPA (1982a). Air Quality Criteria for 

Oxides of Nitrogen. Office of Research 

and Development. Research Triangle 

Park, N.C. EPA/600/8–82/026F. 

December 1982. Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=500021LI.PDF. 

U.S. EPA (1982b). Air Quality Criteria for 

Particulate Matter and Sulfur Oxides. 

Volume I–III. Office of Research and 

Development. Research Triangle Park, 

N.C. EPA/600/8–82/029. December 1982. 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=3000188Z.PDF. 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=300018EV.PDF. 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=300053KV.PDF. 

U.S. EPA (1982c). Review of the National 

Ambient Air Quality Standards for 

Sulfur Oxides: Assessment of Scientific 

and Technical Information. OAQPS Staff 

Paper. Office of Air Quality Planning and 

Standards. Research Triangle Park, NC. 

EPA–450/5–82–007. November 1982. 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=300068A0.PDF. 

U.S. EPA (1982d). Review of the National 

Ambient Air Quality Standards for 

Particulate Matter: Assessment of 

Scientific and Technical Information. 

OAQPS Staff Paper. Office of Air Quality 

Planning and Standards. Research 

Triangle Park, NC. EPA–450/5–82–001. 

January 1982. Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=2000NH6N.PDF. 

U.S. EPA (1984a). The Acidic Deposition 

Phenomenon and Its Effects: Critical 

Assessment Review Papers. Volume I 

Atmospheric Sciences. Office of 

Research and Development, Washington 

DC. EPA600/8–83–016AF. July 1984. 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=2000G4AJ.PDF. 

U.S. EPA (1984b). The Acidic Deposition 

Phenomenon and Its Effects: Critical 

Assessment Review Papers. Volume II 

Effects Sciences. Office of Research and 

Development, Washington DC. EPA–600/ 

8- 83–016BF. July 1984. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=2000G5FI.PDF. 

U.S. EPA (1985). The Acidic Deposition 

Phenomenon and Its Effects: Critical 

Assessment Document. Office of 

Research and Development, Washington, 

DC. EPA–600/8–85/001. August 1985. 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=2000AD53.PDF. 

U.S. EPA (1986). Review of the National 

Ambient Air Quality Standards for 

Particulate Matter: Updated Assessment 

of Scientific and Technical Information. 

Addendum to the 1982 OAQPS Staff 

Paper. Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. 

EPA–450/05–86–012. December 1986. 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=910113UH.PDF. 

U.S. EPA (1993). Air Quality Criteria for 

Oxides of Nitrogen. Volume I–III. U.S. 

Office of Research and Development, 

Research Triangle Park, NC. EPA/600/8– 

91/049aF-cF. August 1993. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=30001LZT.PDF. 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=300056QV.PDF. 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=30001NI2.PDF. 

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

Ambient Air Quality Standards for 


khammond on DSK9W7S144PROD with RULES2
105784 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

Nitrogen Dioxide: Assessment of 

Scientific and Technical Information, 

OAQPS Staff Paper. Office of Air Quality 

Planning and Standards, Research 

Triangle Park, NC. EPA–452/R–95–005. 

September 1995. Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=00002UBE.PDF. 

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

Feasibility Study: Report to Congress. 

Office of Air and Radiation, Acid Rain 

Division, Washington, DC. EPA–430–R– 

95–001a. October 1995. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=2000WTGY.PDF. 

U.S. EPA (1996). Review of the National 

Ambient Air Quality Standards for 

Particulate Matter: Policy Assessment of 

Scientific and Technical Information 

(OAQPS Staff Paper). Office of Air 

Quality Planning and Standards, 

Research Triangle Park, NC. EPA– 

452\R–96–013. July 1996. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=2000DLIE.PDF. 

U.S. EPA (2004a). Air Quality Criteria for 

Particulate Matter. (Vol I of II). Office of 

Research and Development, Research 

Triangle Park, NC. EPA–600/P–99– 

002aF. October 2004. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P100LFIQ.PDF. 

U.S. EPA (2004b). Air Quality Criteria for 

Particulate Matter. (Vol II of II). Office of 

Research and Development, Research 

Triangle Park, NC. EPA–600/P–99– 

002bF. October 2004. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P100LG7Q.PDF. 

U.S. EPA (2005). Review of the National 

Ambient Air Quality Standards for 

Particulate Matter: Policy Assessment of 

Scientific and Technical Information, 

OAQPS Staff Paper. Office of Air Quality 

Planning and Standards, Research 

Triangle Park, NC. EPA–452/R–05–005a. 

December 2005. Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P1009MZM.PDF. 

U.S. EPA (2007). Integrated Review Plan for 

the Secondary National Ambient Air 

Quality Standards for Nitrogen Dioxide 

and Sulfur Dioxide. Office of Research 

and Development, Research Triangle 

Park, NC, EPA–452/R–08–006. December 

2007. Available at: https://nepis.epa.gov/ 

Exe/ZyPDF.cgi?Dockey=P1001FDM.PDF. 

U.S. EPA (2008a). Integrated Science 

Assessment (ISA) for Oxides of Nitrogen 

and Sulfur Ecological Criteria. Office of 

Research and Development, Research 

Triangle Park, NC. EPA/600/R–08/082F. 

December 2008. Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P100R7MG.PDF. 

U.S. EPA (2008b). Integrated Review Plan for 

the National Ambient Air Quality 

Standards for Particulate Matter. Office 

of Air Quality Planning and Standards, 

Research Triangle Park, NC. EPA 452/R– 

08–004. March 2008. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P1001FB9.PDF. 

U.S. EPA (2009a). Risk and Exposure 

Assessment for Review of the Secondary 

National Ambient Air Quality Standards 

for Oxides of Nitrogen and Oxides of 

Sulfur (Main Content). Office of Air 

Quality Planning and Standards, 

Research Triangle Park, NC. EPA–452/R– 

09–008a. September 2009. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P100FNQV.PDF. 

U.S. EPA (2009b). Integrated Science 

Assessment for Particulate Matter. Office 

of Research and Development, Research 

Triangle Park, NC. EPA/600/R–08/139F. 

December 2009. Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P10060Z4.PDF. 

U.S. EPA (2009c). Particulate Matter National 

Ambient Air Quality Standards 

(NAAQS): Scope and Methods Plan for 

Urban Visibility Impact Assessment. 

Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. 

EPA–452/P–09–001. February 2009. 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P100FLUX.PDF. 

U.S. EPA (2010). Particulate Matter UrbanFocused Visibility Assessment—Final 

Document. Office of Air Quality 

Planning and Standards, Research 

Triangle Park, NC. EPA–452/R- 10–004. 

July 2010. Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P100FO5D.PDF. 

U.S. EPA (2011). Policy Assessment for the 

Review of the Secondary National 

Ambient Air Quality Standards for 

Oxides of Nitrogen and Oxides of Sulfur. 

Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. 

EPA–452/R–11–005a, b. February 2011. 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P1009R7U.PDF 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P1009RHY.PDF. 

U.S. EPA (2016). Integrated Review Plan for 

the National Ambient Air Quality 

Standards for Particulate Matter. Office 

of Air Quality Planning and Standards, 

Research Triangle Park, NC. EPA–452/R– 

16–005. December 2016. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P100R5VE.PDF. 

U.S. EPA (2017). Integrated Review Plan for 

the Secondary NAAQS for Oxides of 

Nitrogen and Oxides of Sulfur and 

Particulate Matter—Final. Office of Air 

Quality Planning and Standards, 

Research Triangle Park, NC. EPA–452/R– 

17–002. January 2017. Available at: 

https://nepis.epa.gov/Exe/ 

ZyPDf.cgi?Dockey=P100R607.PDF. 

U.S. EPA (2018). Review of the Secondary 

Standards for Ecological Effects of 

Oxides of Nitrogen, Oxides of Sulfur, 

and Particulate Matter: Risk and 

Exposure Assessment Planning 

Document. Office of Air Quality 

Planning and Standards, Research 

Triangle Park, NC. EPA–452/D–18–001. 

August 2018. Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P100V7JA.PDF. 

U.S. EPA (2019). Integrated Science 

Assessment (ISA) for Particulate Matter 

(Final Report, Dec 2019). U.S. 

Environmental Protection Agency, 

Washington, DC, EPA/600/R–19/188, 

2019. 

U.S. EPA (2020a). Integrated Science 

Assessment (ISA) for Oxides of Nitrogen, 

Oxides of Sulfur and Particulate Matter 

Ecological Criteria (Final Report, 2020). 

Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. 

EPA/600/R–20/278. September 2020. 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P1010WR3.PDF. 

U.S. EPA (2023). Overview of Particulate 

Matter (PM) Air Quality in the United 

States. Available at: https://

www.epa.gov/air-quality-analysis/ 

particulate-matter-naaqs-reviewanalyses-and-data-sets. 

U.S. EPA (2024). Policy Assessment for the 

Review of the Secondary National 

Ambient Air Quality Standards for 

Oxides of Nitrogen, Oxides of Sulfur and 

Particulate Matter. Office of Air Quality 

Planning and Standards, Research 

Triangle Park, NC. EPA–452/R–24–003. 

January 2024. Available at: https://

www.epa.gov/system/files/documents/ 

2024-01/noxsoxpm-final.pdf. 

Wallace, ZP, Lovett, GM, Hart, JE and 

Machona, B (2007). Effects of nitrogen 

saturation on tree growth and death in a 

mixed-oak forest. For Ecol Manage 243: 

210–218. 

Waller, K, Driscoll, C, Lynch, J, Newcomb, D 

and Roy, K (2012). Long-term recovery of 

lakes in the Adirondack region of New 

York to decreases in acidic deposition. 

Atmos Environ 46: 56–64. 

Watkins, N, Boyette, L and Jager, D (2024). 

Memorandum to Secondary NOX/SOX/ 

PM NAAQS Review Docket (Docket ID 

No. EPA–HQ–OAR–2014–0128). 

Ambient Air SO2 Monitoring Network 

Review and Background (January 2024). 

January 18, 2024. Office of Air Quality 

Planning and Standards, Research 

Triangle Park, NC. Available at https://

www.regulations.gov. Document 

Identifier EPA–HQ–OAR–2014–0128– 

0040. 

Weaver, C (2024). Memorandum to 

Secondary NOX/SOX/PM NAAQS 

Review Docket (Docket ID No. EPA–HQ– 

OAR–2014–0128). List of Studies 

Identified by Public Commenters That 

Have Been Provisionally Considered in 

the Context of the Conclusions of the 

2020 Integrated Science Assessment for 

the Secondary National Ambient Air 

Quality Standard review of Oxides of 

Nitrogen, Oxides of Sulfur and 

Particulate Matter. October 16, 2024. 

Office of Research and Development, 

Research Triangle Park, NC. Available at 

https://www.regulations.gov. Docket ID 

No. EPA–HQ–OAR–2014–0128. 

WHO (World Health Organization) (2008). 

WHO/IPCS Harmonization Project 

Document No. 6. Part 1: Guidance 

Document on Characterizing and 

Communicating Uncertainty in Exposure 

Assessment. International Programme on 

Chemical Safety. World Health 

Organization. Geneva, Switzerland. 

Available at: https://www.who.int/ipcs/ 

methods/harmonization/areas/exposure/ 

en/. 

Williams, J and Labou, S (2017). A database 

of georeferenced nutrient chemistry data 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105785 

for mountain lakes of the Western United 

States. Sci Data 4: 170069. 

Wolff, GT (1993). Letter from George T. 

Wolff, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Carol M. Browner, Administrator, U.S. 

EPA. Re: Clean Air Scientific Advisory 

Committee Closure on the Air Quality 

Criteria Document for Oxides of 

Nitrogen. September 30, 1993. EPA– 

SAB–CASAC–LTR–93–015. Office of the 

Administrator, Science Advisory Board, 

Washington, DC. Available at: https://

casac.epa.gov/ords/sab/ 

f?p=113:12:1342972375271:::12. 

Wolff, GT (1995). Letter from George T. 

Wolff, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Carol M. Browner, Administrator, Re: 

CASAC Review of the Staff Paper for the 

Review of the National Ambient Air 

Quality Standards for Nitrogen Dioxide: 

Assessment of Scientific and Technical 

Information. August 22, 1995. EPA– 

SAB–CASAC–LTR–95–004. Office of the 

Administrator, Science Advisory Board, 

Washington, DC. Available at: https://

nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=P100FL6Q.PDF. 

Wolff, GT (1996). Letter from George T. 

Wolff, Chair, Clean Air Scientific 

Advisory Committee to the Honorable 

Carol M. Browner, Administrator, Re: 

Closure by the Clean Air Scientific 

Advisory Committee (CASAC) on the 

Staff Paper for Particulate Matter. June 

13, 1996. EPA–SAB–CASAC–LTR–96– 

008. Office of the Administrator, Science 

Advisory Board, Washington, DC. 

Available at: https://nepis.epa.gov/Exe/ 

ZyPDF.cgi?Dockey=9100TTBM.PDF. 

Zhou, Q, Driscoll, CT and Sullivan, TJ (2015). 

Responses of 20 lake-watersheds in the 

Adirondack region of New York to 

historical and potential future acidic 

deposition. Sci Total Environ 511: 186– 

194. 

List of Subjects in 40 CFR Part 50 

Environmental protection, Air 

pollution control, Nitrogen dioxide, 

Particulate matter, Sulfur oxides. 

Michael S. Regan, 

Administrator. 

For the reasons set forth in the 

preamble, the Environmental Protection 

Agency is amending chapter I of title 40 

of the Code of Federal Regulations as 

follows: 

PART 50—NATIONAL PRIMARY AND 

SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

■ 1. The authority citation for part 50 

continues to read as follows: 

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

■ 2. Add § 50.21 to read as follows: 

§ 50.21 National secondary ambient air 

quality standards for sulfur oxides (sulfur 

dioxide). 

(a) The level of the annual secondary 

national ambient air quality standard for 

oxides of sulfur is 10 parts per billion 

(ppb), measured in the ambient air as 

sulfur dioxide (SO2) by a reference 

method based on appendix A–1 and 

appendix A–2 of this part, or by a 

Federal Equivalent Method (FEM) 

designated in accordance with part 53 of 

this chapter. 

(b) The annual secondary standard is 

met when the 3-year average of the 

annual SO2 concentration is less than or 

equal to 10 ppb, as determined in 

accordance with appendix T of this part. 

■ 3. Revise appendix T to part 50 to read 

as follows: 

Appendix T to Part 50—Interpretation 

of the Primary and Secondary National 

Ambient Air Quality Standards for 

Oxides of Sulfur (Sulfur Dioxide) 

1. General 

(a) This appendix explains the data 

handling conventions and computations 

necessary for determining when the primary 

and secondary national ambient air quality 

standards for Oxides of Sulfur as measured 

by Sulfur Dioxide (‘‘SO2 NAAQS’’) specified 

in § 50.17 are met at an ambient air quality 

monitoring site. Sulfur dioxide (SO2) is 

measured in the ambient air by a Federal 

reference method (FRM) based on appendix 

A–1 or A–2 to this part or by a Federal 

equivalent method (FEM) designated in 

accordance with part 53 of this chapter. Data 

handling and computation procedures to be 

used in making comparisons between 

reported SO2 concentrations and the levels of 

the SO2 NAAQS are specified in the 

following sections. 

(b) Decisions to exclude, retain, or make 

adjustments to the data affected by 

exceptional events, including natural events, 

are made according to the requirements and 

process deadlines specified in §§ 50.1, 50.14 

and 51.930 of this chapter. 

(c) The terms used in this appendix are 

defined as follows: 

Annual mean refers to the annual average 

of all the daily mean values as defined in 

section 5.2 of this appendix. 

Daily maximum 1-hour values for SO2

refers to the maximum 1-hour SO2

concentration values measured from 

midnight to midnight (local standard time) 

that are used in NAAQS computations. 

Daily mean values for SO2 refers to the 24- 

hour average of 1-hour SO2 concentration 

values measured from midnight to midnight 

(local standard time) that are used in NAAQS 

computations. 

Design values are the metrics (i.e., 

statistics) that are compared to the NAAQS 

levels to determine compliance, calculated as 

specified in section 5 of this appendix. The 

design value for the primary 1-hour NAAQS 

is the 3-year average of annual 99th 

percentile daily maximum 1-hour values for 

a monitoring site (referred to as the ‘‘1-hour 

primary standard design value’’). The design 

value for the secondary annual NAAQS is the 

3-year average of the annual mean of daily 

mean values for a monitoring site (referred to 

as the ‘‘annual secondary standard’’). 

99th percentile daily maximum 1-hour 

value is the value below which nominally 99 

percent of all daily maximum 1-hour 

concentration values fall, using the ranking 

and selection method specified in section 5.1 

of this appendix. 

Pollutant Occurrence Code (POC) refers to 

a numerical code (1, 2, 3, etc.) used to 

distinguish the data from two or more 

monitors for the same parameter at a single 

monitoring site. 

Quarter refers to a calendar quarter. 

Year refers to a calendar year. 

2. Requirements for Data Used for 

Comparisons With the SO2 NAAQS and Data 

Reporting Considerations 

(a) All valid FRM/FEM SO2 hourly data 

required to be submitted to EPA’s Air Quality 

System (AQS), or otherwise available to EPA, 

meeting the requirements of part 58 of this 

chapter including appendices A, C, and E 

shall be used in design value calculations. 

Multi-hour average concentration values 

collected by wet chemistry methods shall not 

be used. 

(b) Data from two or more monitors from 

the same year at the same site reported to 

EPA under distinct Pollutant Occurrence 

Codes shall not be combined in an attempt 

to meet data completeness requirements. The 

Administrator will combine annual 99th 

percentile daily maximum concentration 

values from different monitors in different 

years, selected as described here, for the 

purpose of developing a valid 1-hour primary 

standard design value. If more than one of 

the monitors meets the completeness 

requirement for all four quarters of a year, the 

steps specified in section 5.1(a) of this 

appendix shall be applied to the data from 

the monitor with the highest average of the 

four quarterly completeness values to derive 

a valid annual 99th percentile daily 

maximum concentration. If no monitor is 

complete for all four quarters in a year, the 

steps specified in sections 3.1(c) and 5.1(a) of 

this appendix shall be applied to the data 

from the monitor with the highest average of 

the four quarterly completeness values in an 

attempt to derive a valid annual 99th 

percentile daily maximum concentration. 

Similarly, the Administrator will combine 

annual means from different monitors in 

different years, selected as described here, for 

the purpose of developing a valid annual 

secondary standard design value. If more 

than one of the monitors meets the 

completeness requirement for all four 

quarters of a year, the steps specified in 

section 5.2(a) of this appendix shall be 

applied to the data from the monitor with the 

highest average of the four quarterly 

completeness values to derive a valid annual 

mean. If no monitor is complete for all four 

quarters in a year, the steps specified in 

sections 3.2(c) and 5.2(a) of this appendix 

shall be applied to the data from the monitor 

with the highest average of the four quarterly 

completeness values in an attempt to derive 

a valid annual mean. This paragraph does not 


khammond on DSK9W7S144PROD with RULES2
105786 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

prohibit a monitoring agency from making a 

local designation of one physical monitor as 

the primary monitor for a Pollutant 

Occurrence Code and substituting the 1-hour 

data from a second physical monitor 

whenever a valid concentration value is not 

obtained from the primary monitor; if a 

monitoring agency substitutes data in this 

manner, each substituted value must be 

accompanied by an AQS qualifier code 

indicating that substitution with a value from 

a second physical monitor has taken place. 

(c) Hourly SO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

3. Comparisons With the NAAQS 

3.1 Comparisons With the 1-Hour Primary 

SO2 NAAQS 

(a) The 1-hour primary SO2 NAAQS is met 

at an ambient air quality monitoring site 

when the valid 1-hour primary standard 

design value is less than or equal to 75 parts 

per billion (ppb). 

(b) An SO2 1-hour primary standard design 

value is valid if it encompasses three 

consecutive calendar years of complete data. 

A year meets data completeness requirements 

when all four quarters are complete. A 

quarter is complete when at least 75 percent 

of the sampling days for each quarter have 

complete data. A sampling day has complete 

data if 75 percent of the hourly concentration 

values, including State-flagged data affected 

by exceptional events which have been 

approved for exclusion by the Administrator, 

are reported. 

(c) In the case of one, two, or three years 

that do not meet the completeness 

requirements of section 3.1(b) of this 

appendix and thus would normally not be 

useable for the calculation of a valid 3-year 

1-hour primary standard design value, the 3- 

year 1-hour primary standard design value 

shall nevertheless be considered valid if one 

of the following conditions is true. 

(i) At least 75 percent of the days in each 

quarter of each of three consecutive years 

have at least one reported hourly value, and 

the design value calculated according to the 

procedures specified in section 5.1 is above 

the level of the primary 1-hour standard. 

(ii)(A) A 1-hour primary standard design 

value that is equal to or below the level of 

the NAAQS can be validated if the 

substitution test in section 3.1(c)(ii)(B) of this 

appendix results in a ‘‘test design value’’ that 

is below the level of the NAAQS. The test 

substitutes actual ‘‘high’’ reported daily 

maximum 1-hour values from the same site 

at about the same time of the year 

(specifically, in the same calendar quarter) 

for unknown values that were not 

successfully measured. Note that the test is 

merely diagnostic in nature, intended to 

confirm that there is a very high likelihood 

that the original design value (the one with 

less than 75 percent data capture of hours by 

day and of days by quarter) reflects the true 

under-NAAQS-level status for that 3-year 

period; the result of this data substitution test 

(the ‘‘test design value,’’ as defined in section 

3.1(c)(ii)(B) of this appendix) is not 

considered the actual design value. For this 

test, substitution is permitted only if there 

are at least 200 days across the three 

matching quarters of the three years under 

consideration (which is about 75 percent of 

all possible daily values in those three 

quarters) for which 75 percent of the hours 

in the day, including State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator, have reported concentrations. 

However, maximum 1-hour values from days 

with less than 75 percent of the hours 

reported shall also be considered in 

identifying the high value to be used for 

substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture but at least 50 percent data capture, 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator; 

if any quarter has less than 50 percent data 

capture then this substitution test cannot be 

used. Identify for each quarter (e.g., January– 

March) the highest reported daily maximum 

1-hour value for that quarter, excluding Stateflagged data affected by exceptional events 

which have been approved for exclusion by 

the Administrator, looking across those three 

months of all three years under 

consideration. All daily maximum 1-hour 

values from all days in the quarter period 

shall be considered when identifying this 

highest value, including days with less than 

75 percent data capture. If after substituting 

the highest reported daily maximum 1-hour 

value for a quarter for as much of the missing 

daily data in the matching deficient 

quarter(s) as is needed to make them 100 

percent complete, the procedure in section 5 

yields a recalculated 3-year 1-hour standard 

‘‘test design value’’ less than or equal to the 

level of the standard, then the 1-hour primary 

standard design value is deemed to have 

passed the diagnostic test and is valid, and 

the level of the standard is deemed to have 

been met in that 3-year period. As noted in 

section 3.1(c)(i) of this appendix, in such a 

case, the 3-year design value based on the 

data actually reported, not the ‘‘test design 

value,’’ shall be used as the valid design 

value. 

(iii)(A) A 1-hour primary standard design 

value that is above the level of the NAAQS 

can be validated if the substitution test in 

section 3.1(c)(iii)(B) of this appendix results 

in a ‘‘test design value’’ that is above the 

level of the NAAQS. The test substitutes 

actual ‘‘low’’ reported daily maximum 1-hour 

values from the same site at about the same 

time of the year (specifically, in the same 

three months of the calendar) for unknown 

hourly values that were not successfully 

measured. Note that the test is merely 

diagnostic in nature, intended to confirm that 

there is a very high likelihood that the 

original design value (the one with less than 

75 percent data capture of hours by day and 

of days by quarter) reflects the true aboveNAAQS-level status for that 3-year period; 

the result of this data substitution test (the 

‘‘test design value,’’ as defined in section 

3.1(c)(iii)(B) of this appendix) is not 

considered the actual design value. For this 

test, substitution is permitted only if there 

are a minimum number of available daily 

data points from which to identify the low 

quarter-specific daily maximum 1-hour 

values, specifically if there are at least 200 

days across the three matching quarters of the 

three years under consideration (which is 

about 75 percent of all possible daily values 

in those three quarters) for which 75 percent 

of the hours in the day have reported 

concentrations. Only days with at least 75 

percent of the hours reported shall be 

considered in identifying the low value to be 

used for substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture. Identify for each quarter (e.g., 

January–March) the lowest reported daily 

maximum 1-hour value for that quarter, 

looking across those three months of all three 

years under consideration. All daily 

maximum 1-hour values from all days with 

at least 75 percent capture in the quarter 

period shall be considered when identifying 

this lowest value. If after substituting the 

lowest reported daily maximum 1-hour value 

for a quarter for as much of the missing daily 

data in the matching deficient quarter(s) as is 

needed to make them 75 percent complete, 

the procedure in section 5.1 of this appendix 

yields a recalculated 3-year 1-hour standard 

‘‘test design value’’ above the level of the 

standard, then the 1-hour primary standard 

design value is deemed to have passed the 

diagnostic test and is valid, and the level of 

the standard is deemed to have been 

exceeded in that 3-year period. As noted in 

section 3.1(c)(i) of this appendix, in such a 

case, the 3-year design value based on the 

data actually reported, not the ‘‘test design 

value’’, shall be used as the valid design 

value. 

(d) A 1-hour primary standard design value 

based on data that do not meet the 

completeness criteria stated in section 3.1(b) 

of this appendix and also do not satisfy 

section 3.1(c) of this appendix, may also be 

considered valid with the approval of, or at 

the initiative of, the Administrator, who may 

consider factors such as monitoring site 

closures/moves, monitoring diligence, the 

consistency and levels of the valid 

concentration measurements that are 

available, and nearby concentrations in 

determining whether to use such data. 

(e) The procedures for calculating the 1- 

hour primary standard design values are 

given in section 5.1 of this appendix. 

3.2 Comparisons With the Annual 

Secondary SO2 NAAQS 

(a) The annual secondary SO2 NAAQS is 

met at an ambient air quality monitoring site 

when the valid annual secondary standard 

design value is less than or equal to 10 parts 

per billion (ppb). 

(b) An SO2 annual secondary standard 

design value is valid if it encompasses three 

consecutive calendar years of complete data. 

A year meets data completeness requirements 

when all four quarters are complete. A 

quarter is complete when at least 75 percent 

of the sampling days for each quarter have 

complete data. A sampling day has complete 

data if 75 percent of the hourly concentration 

values, including State-flagged data affected 


khammond on DSK9W7S144PROD with RULES2
Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 105787 

by exceptional events which have been 

approved for exclusion by the Administrator, 

are reported. 

(c) In the case of one, two, or three years 

that do not meet the completeness 

requirements of section 3.2(b) of this 

appendix and thus would normally not be 

useable for the calculation of a valid 3-year 

annual secondary standard design value, the 

3-year annual secondary standard design 

value shall nevertheless be considered valid 

if one of the following conditions is true. 

(i) At least 75 percent of the days in each 

quarter of each of three consecutive years 

have at least one reported hourly value, and 

the design value calculated according to the 

procedures specified in section 5.2 of this 

appendix is above the level of the secondary 

annual standard. 

(ii)(A) An annual secondary standard 

design value that is equal to or below the 

level of the NAAQS can be validated if the 

substitution test in section 3.2(c)(ii)(B) of this 

appendix results in a ‘‘test design value’’ that 

is below the level of the NAAQS. The test 

substitutes actual ‘‘high’’ reported daily mean 

values from the same site at about the same 

time of the year (specifically, in the same 

calendar quarter) for unknown or incomplete 

(less than 75 percent of hours reported) daily 

mean values. Note that the test is merely 

diagnostic in nature, intended to confirm that 

there is a very high likelihood that the 

original design value (the one with less than 

75 percent data capture of hours by day and 

of days by quarter) reflects the true underNAAQS-level status for that 3-year period; 

the result of this data substitution test (the 

‘‘test design value,’’ as defined in section 

3.2(c)(ii)(B)) of this appendix is not 

considered the actual design value. For this 

test, substitution is permitted only if there 

are at least 200 days across the three 

matching quarters of the three years under 

consideration (which is about 75 percent of 

all possible daily values in those three 

quarters) for which 75 percent of the hours 

in the day, including State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator, have reported concentrations. 

However, daily mean values from days with 

less than 75 percent of the hours reported 

shall also be considered in identifying the 

high daily mean value to be used for 

substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture but at least 50 percent data capture, 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator; 

if any quarter has less than 50 percent data 

capture then this substitution test cannot be 

used. Identify for each quarter (e.g., January– 

March) the highest reported daily mean value 

for that quarter, excluding State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator, looking across those three 

months of all three years under 

consideration. All daily mean values from all 

days in the quarter period shall be 

considered when identifying this highest 

value, including days with less than 75 

percent data capture. If after substituting the 

highest daily mean value for a quarter for as 

much of the missing daily data in the 

matching deficient quarter(s) as is needed to 

make them 100 percent complete, the 

procedure in section 5 of this appendix 

yields a recalculated 3-year annual standard 

‘‘test design value’’ less than or equal to the 

level of the standard, then the annual 

secondary standard design value is deemed 

to have passed the diagnostic test and is 

valid, and the level of the standard is deemed 

to have been met in that 3-year period. As 

noted in section 3.2(c)(i) of this appendix, in 

such a case, the 3-year design value based on 

the data actually reported, not the ‘‘test 

design value,’’ shall be used as the valid 

design value. 

(iii)(A) An annual secondary standard 

design value that is above the level of the 

NAAQS can be validated if the substitution 

test in section 3.2(c)(iii)(B) of this appendix 

results in a ‘‘test design value’’ that is above 

the level of the NAAQS. The test substitutes 

actual ‘‘low’’ reported daily mean values 

from the same site at about the same time of 

the year (specifically, in the same three 

months of the calendar) for unknown or 

incomplete (less than 75 percent of hours 

reported) daily mean values. Note that the 

test is merely diagnostic in nature, intended 

to confirm that there is a very high likelihood 

that the original design value (the one with 

less than 75 percent data capture of hours by 

day and of days by quarter) reflects the true 

above-NAAQS-level status for that 3-year 

period; the result of this data substitution test 

(the ‘‘test design value,’’ as defined in section 

3.2(c)(iii)(B) of this appendix) is not 

considered the actual design value. For this 

test, substitution is permitted only if there 

are a minimum number of valid daily mean 

values from which to identify the low 

quarter-specific daily mean values, 

specifically if there are at least 200 days 

across the three matching quarters of the 

three years under consideration (which is 

about 75 percent of all possible daily values 

in those three quarters) for which 75 percent 

of the hours in the day have reported 

concentrations. Only days with at least 75 

percent of the hours reported shall be 

considered in identifying the low daily mean 

value to be used for substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture. Identify for each quarter (e.g., 

January–March) the lowest reported daily 

mean value for that quarter, looking across 

those three months of all three years under 

consideration. All daily mean values from all 

days with at least 75 percent capture in the 

quarter period shall be considered when 

identifying this lowest value. If after 

substituting the lowest reported daily mean 

value for a quarter for as much of the missing 

daily data in the matching deficient 

quarter(s) as is needed to make them 75 

percent complete, the procedure in section 

5.2 of this appendix yields a recalculated 3- 

year annual standard ‘‘test design value’’ 

above the level of the standard, then the 

annual secondary standard design value is 

deemed to have passed the diagnostic test 

and is valid, and the level of the standard is 

deemed to have been exceeded in that 3-year 

period. As noted in section 3.2(c)(i) of this 

appendix, in such a case, the 3-year design 

value based on the data actually reported, not 

the ‘‘test design value,’’ shall be used as the 

valid design value. 

(d) An annual secondary standard design 

value based on data that do not meet the 

completeness criteria stated in section 3.2(b) 

of this appendix and also do not satisfy 

section 3.2(c) of this appendix, may also be 

considered valid with the approval of, or at 

the initiative of, the Administrator, who may 

consider factors such as monitoring site 

closures/moves, monitoring diligence, the 

consistency and levels of the valid 

concentration measurements that are 

available, and nearby concentrations in 

determining whether to use such data. 

(e) The procedures for calculating the 

annual secondary standard design values are 

given in section 5.2 of this appendix. 

4. Rounding Conventions 

4.1 Rounding Conventions for the 1-Hour 

Primary SO2 NAAQS 

(a) Hourly SO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

(b) Daily maximum 1-hour values and, 

therefore, the annual 99th percentile of those 

daily values are not rounded. 

(c) The 1-hour primary standard design 

value is calculated pursuant to section 5.1 of 

this appendix and then rounded to the 

nearest whole number or 1 ppb (decimals 0.5 

and greater are rounded up to the nearest 

whole number, and any decimal lower than 

0.5 is rounded down to the nearest whole 

number). 

4.2 Rounding Conventions for the Annual 

Secondary SO2 NAAQS 

(a) Hourly SO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

(b) Daily mean values and the annual mean 

of those daily values are not rounded. 

(c) The annual secondary standard design 

value is calculated pursuant to section 5.2 of 

this appendix and then rounded to the 

nearest whole number or 1 ppb (decimals 0.5 

and greater are rounded up to the nearest 

whole number, and any decimal lower than 

0.5 is rounded down to the nearest whole 

number). 

5. Calculation Procedures 

5.1 Calculation Procedures for the 1-Hour 

Primary SO2 NAAQS 

(a) Procedure for identifying annual 99th 

percentile values. When the data for a 

particular ambient air quality monitoring site 

and year meet the data completeness 

requirements in section 3.1(b) of this 

appendix, or if one of the conditions of 

section 3.1(c) of this appendix is met, or if 

the Administrator exercises the discretionary 

authority in section 3.1(d) of this appendix, 

identification of annual 99th percentile value 

is accomplished as follows. 


khammond on DSK9W7S144PROD with RULES2
105788 Federal Register / Vol. 89, No. 248 / Friday, December 27, 2024 / Rules and Regulations 

(i) The annual 99th percentile value for a 

year is the higher of the two values resulting 

from the following two procedures. 

(A) Procedure 1. For the year, determine 

the number of days with at least 75 percent 

of the hourly values reported. 

(1) For the year, determine the number of 

days with at least 75 percent of the hourly 

values reported including State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator. 

(2) For the year, from only the days with 

at least 75 percent of the hourly values 

reported, select from each day the maximum 

hourly value excluding State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator. 

(3) Sort all these daily maximum hourly 

values from a particular site and year by 

descending value. (For example: (x[1], x[2], 

x[3], . . . x[n]). In this case, x[1] is the largest 

number and x[n] is the smallest value.) The 

99th percentile is determined from this 

sorted series of daily values which is ordered 

from the highest to the lowest number. Using 

the left column of table 1, determine the 

appropriate range (i.e., row) for the annual 

number of days with valid data for year y 

(cny). The corresponding ‘‘n’’ value in the 

right column identifies the rank of the annual 

99th percentile value in the descending 

sorted list of daily site values for year y. 

Thus, P0.99, y = the nth largest value. 

(B) Procedure 2. For the year, determine 

the number of days with at least one hourly 

value reported. 

(1) For the year, determine the number of 

days with at least one hourly value reported 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator. 

(2) For the year, from all the days with at 

least one hourly value reported, select from 

each day the maximum hourly value 

excluding State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator. 

(3) Sort all these daily maximum values 

from a particular site and year by descending 

value. (For example: (x[1], x[2], x[3], . . . 

x[n]). In this case, x[1] is the largest number 

and x[n] is the smallest value.) The 99th 

percentile is determined from this sorted 

series of daily values which is ordered from 

the highest to the lowest number. Using the 

left column of table 1, determine the 

appropriate range (i.e., row) for the annual 

number of days with valid data for year y 

(cny). The corresponding ‘‘n’’ value in the 

right column identifies the rank of the annual 

99th percentile value in the descending 

sorted list of daily site values for year y. 

Thus, P0.99,y = the nth largest value. 

(b) The 1-hour primary standard design 

value for an ambient air quality monitoring 

site is mean of the three annual 99th 

percentile values, rounded according to the 

conventions in section 4.1 of this appendix. 

TABLE 1 

Annual number of 

days with valid 

data for year ‘‘y’’ 

(cny) 

P0.99,y is the nth 

maximum value of 

the year, where n 

is the listed number 

1–100 .................... 1 

101–200 ................ 2 

201–300 ................ 3 

301–366 ................ 4 

5.2 Calculation Procedures for the Annual 

Secondary SO2 NAAQS 

(a) When the data for a site and year meet 

the data completeness requirements in 

section 3.2(b) of this appendix, or if the 

Administrator exercises the discretionary 

authority in section 3.2(c), the annual mean 

is simply the arithmetic average of all the 

daily mean values. 

(b) The annual secondary standard design 

value for an ambient air quality monitoring 

site is the mean of the annual means for three 

consecutive years, rounded according to the 

conventions in section 4.2 of this appendix. 

[FR Doc. 2024–29463 Filed 12–26–24; 8:45 am] 

BILLING CODE 6560–50–P 


khammond on DSK9W7S144PROD with RULES2