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13563 Interagency Review. Subject to Further Policy Review.

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1 ENVIRONMENTAL PROTECTION AGENCY

2 40 CFR Part 50

3 [EPA-HQ-OAR-2007-1145]

4 RIN: 2060-AO72

5 Secondary National Ambient Air Quality Standards for Oxides of Nitrogen and Sulfur

6

7 AGENCY: Environmental Protection Agency (EPA).

8 ACTION: Proposed rule.

9 SUMMARY: Based on its review of the air quality criteria and the secondary national ambient 

10 air quality standards (NAAQS) for oxides of nitrogen and oxides of sulfur, EPA proposes to 

11 retain the current nitrogen dioxide (NO2) and sulfur dioxide (SO2) secondary standards to 

12 provide requisite protection for the direct effects on vegetation resulting from exposure to 

13 gaseous oxides of nitrogen and sulfur in the ambient air. With regard to protection from the 

14 deposition of oxides of nitrogen and sulfur to sensitive aquatic and terrestrial ecosystems, 

15 including acidification and nutrient enrichment effects, EPA is proposing to add secondary

16 standards identical to the NO2 and SO2 primary 1-hour standards and not to set a new multi17 pollutant secondary standard in this review. In addition, EPA has decided to undertake a field 

18 pilot program to gather and analyze additional relevant data so as to enhance the Agency’s 

19 understanding of the degree of protectiveness that a new multi-pollutant approach, defined in 

20 terms of an aquatic acidification index (AAI), would afford and to support development of an 

21 appropriate monitoring network for such a standard. The EPA solicits comment on the 

22 framework of such a standard and on the design of the field pilot program.
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1 DATES: Written comments on this proposed rule must be received by [insert date 60 days 

2 after date of publication in the Federal Register].

3 Public Hearings: The EPA intends to hold a public hearing around the end of August to 

4 early September and will announce in a separate Federal Register notice the date, time, and 

5 address of the public hearing on this proposed rule.

6 ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2007-

7 1145, by one of the following methods:

8  www.regulations.gov: Follow the on-line instructions for submitting comments.

9  Email: a-and-r-Docket@epa.gov.

10  Fax: 202-566-1741.

11  Mail: Docket No. EPA-HQ-OAR-2007-1145, Environmental Protection Agency, Mail 

12 code 6102T, 1200 Pennsylvania Ave., NW., Washington, DC 20460. Please include a 

13 total of two copies.

14  Hand Delivery: Docket No. EPA-HQ-OAR-2007-1145, Environmental Protection 

15 Agency, EPA West, Room 3334, 1301 Constitution Ave., NW, Washington, DC. Such 

16 deliveries are only accepted during the Docket’s normal hours of operation, and special 

17 arrangements should be made for deliveries of boxed information.

18 Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-2007-1145. The 

19 EPA’s policy is that all comments received will be included in the public docket without change 

20 and may be made available online at www.regulations.gov, including any personal information 

21 provided, unless the comment includes information claimed to be Confidential Business 

22 Information (CBI) or other information whose disclosure is restricted by statute. Do not submit 

23 information that you consider to be CBI or otherwise protected through www.regulations.gov or 
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1 email. The www.regulations.gov website is an “anonymous access” system, which means EPA 

2 will not know your identity or contact information unless you provide it in the body of your 

3 comment. If you send an email comment directly to EPA without going through 

4 www.regulations.gov, your email address will be automatically captured and included as part of 

5 the comment that is placed in the public docket and made available on the Internet. If you 

6 submit an electronic comment, EPA recommends that you include your name and other contact 

7 information in the body of your comment and with any disk or CD-ROM you submit. If EPA 

8 cannot read your comment due to technical difficulties and cannot contact you for clarification, 

9 EPA may not be able to consider your comment. Electronic files should avoid the use of special 

10 characters, any form of encryption, and be free of any defects or viruses. For additional 

11 information about EPA’s public docket, visit the EPA Docket Center homepage at 

12 http://www.epa.gov/epahome/dockets.htm. 

13 Docket: All documents in the docket are listed in the www.regulations.gov index. 

14 Although listed in the index, some information is not publicly available, e.g., CBI or other 

15 information whose disclosure is restricted by statute. Certain other material, such as copyrighted 

16 material, will be publicly available only in hard copy. Publicly available docket materials are 

17 available either electronically in www.regulations.gov or in hard copy at the Air and Radiation 

18 Docket and Information Center, EPA/DC, EPA West, Room 3334, 1301 Constitution Ave., NW, 

19 Washington, DC. The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday 

20 through Friday, excluding legal holidays. The telephone number for the Public Reading Room is 

21 (202) 566-1744 and the telephone number for the Air and Radiation Docket and Information 

22 Center is (202) 566-1742. 
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1 FOR FURTHER INFORMATION CONTACT: Dr. Richard Scheffe, Office of Air Quality

2 Planning and Standards, U.S. Environmental Protection Agency, Mail code C304-02, Research 

3 Triangle Park, NC 27711; telephone: 919-541-4650; fax: 919-541-2357; email: 

4 scheffe.rich@epa.gov. 

5 SUPPLEMENTARY INFORMATION:

6 General Information

7 What Should I Consider as I Prepare My Comments for EPA?

8 1. Submitting CBI. Do not submit this information to EPA through www.regulations.gov or 

9 email. Clearly mark the part or all of the information that you claim to be CBI. For CBI 

10 information in a disk or CD ROM that you mail to EPA, mark the outside of the disk or 

11 CD ROM as CBI and then identify electronically within the disk or CD ROM the specific 

12 information that is claimed as CBI. In addition to one complete version of the comment 

13 that includes information claimed as CBI, a copy of the comment that does not contain 

14 the information claimed as CBI must be submitted for inclusion in the public docket. 

15 Information so marked will not be disclosed except in accordance with procedures set 

16 forth in 40 CFR part 2.

17 2. Tips for Preparing Your Comments. When submitting comments, remember to:

18  Identify the rulemaking by docket number and other identifying information (subject 

19 heading, Federal Register date and page number).

20  Follow directions – The Agency may ask you to respond to specific questions or 

21 organize comments by referencing a Code of Federal Regulations (CFR) part or section 

22 number.
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1  Explain why you agree or disagree, suggest alternatives, and substitute language for your 

2 requested changes.

3  Describe any assumptions and provide any technical information and/or data that you 

4 used.

5  If you estimate potential costs or burdens, explain how you arrived at your estimate in 

6 sufficient detail to allow for it to be reproduced.

7  Provide specific examples to illustrate your concerns, and suggest alternatives.

8  Explain your views as clearly as possible.Make sure to submit your comments by the 

9 comment period deadline identified.

10 Availability of Related Information

11 A number of documents relevant to this rulemaking are available on EPA web sites. The 

12 Integrated Science Assessment for Oxides of Nitrogen and Sulfur - Ecological Criteria: FINAL 

13 REPORT (ISA) is available on EPAs National Center for Environmental Assessment web site. 

14 To obtain this document, go to http://www.epa.gov/ncea, and click on Air Quality then click on 

15 Oxides of Nitrogen and Sulfur. The Policy Assessment, Risk and Exposure Assessment (REA), 

16 and other related technical documents are available on EPA’s Office of Air Quality Planning and 

17 Standards (OAQPS) Technology Transfer Network (TTN) web site. The Policy Assessment is 

18 available at http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_pa.html, and the exposure and 

19 risk assessments and other related technical documents are available at 

20 http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_rea.html. These and other related 

21 documents are also available for inspection and copying in the EPA docket identified above.

22 Table of Contents

23 The following topics are discussed in this preamble:
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1 I. Background

2 A. Legislative Requirements

3 B. History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides

4 1. NAAQS for Oxides of Nitrogen

5 2. NAAQS for Oxides of Sulfur

6 C. History of Related Assessments and Agency Actions

7 D. History of the Current Review

8 E. Scope of the Current Review

9 II. Rationale for Proposed Decision on the Adequacy of the Current Secondary Standards 

10 A. Ecological Effects

11 1. Effects Associated with Gas-Phase Oxides of Nitrogen and Sulfur

12 a. Nature of ecosystem responses to gas-phase nitrogen and sulfur

13 b. Magnitude of ecosystem response to gas-phase nitrogen and sulfur

14 2. Acidification Effects Associated with Deposition of Oxides of Nitrogen 

15 and Sulfur

16 a. Nature of Acidification –related Ecosystem Responses

17 i. Aquatic ecosystems

18 ii. Terrestrial ecosystems

19 iii. Ecosystem sensitivity

20 b. Magnitude of Acidification-related Ecosystem Responses

21 i. Aquatic acidification

22 ii. Terrestrial acidification

23 c. Key Uncertainties Associated with Acidification

24 i. Aquatic acidification

25 ii. Terrestrial acidification

26 3. Nutrient Enrichment Effects Associated with Deposition of Oxides of 

27 Nitrogen 

28 a. Nature of Nutrient Enrichment-related Ecosystem Responses

29 i. Aquatic ecosystems

30 ii. Terrestrial ecosystems

31 iii. Ecosystem sensitivity to nutrient enrichment

32 b. Magnitude of Nutrient Enrichment-related Ecosystem Responses

33 i. Aquatic ecosystems

34 ii. Terrestrial ecosystems

35 c. Key Uncertainties Associated with Nutrient Enrichment

36 i. Aquatic ecosystems

37 ii. Terrestrial ecosystems

38 4. Other Ecological Effects

39 B. Risk and Exposure Assessment

40 1. Overview of Risk and Exposure Assessment

41 2. Key Findings

42 a. Air quality analyses

43 b. Deposition-related aquatic acidification

44 c. Deposition-related terrestrial acidification

45 d. Deposition-related aquatic nutrient enrichment

46 e. Deposition-related terrestrial nutrient enrichment 
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1 f. Additional effects

2 3. Conclusions on Effects

3 C. Adversity of Effects to Public Welfare

4 1. Ecosystem Services

5 2. Effects on Ecosystem Services

6 a. Aquatic Acidification

7 b. Terrestrial Acidification

8 c. Nutrient Enrichment

9 3. Summary

10 D. Adequacy of the Current Standards

11 1. Adequacy of the Current Standards for Direct Effects

12 2. Appropriateness and Adequacy of the Current Standards for Deposition13 related Effects

14 a. Appropriateness

15 b. Adequacy of Protection

16 i. Aquatic Acidification

17 ii. Terrestrial acidification

18 iii. Terrestrial nutrient enrichment

19 iv. Aquatic nutrient enrichment

20 v. Other effects

21 3. CASAC Views

22 4. Administrator’s Proposed Conclusions Concerning Adequacy of Current 

23 Standard

24 III. Rationale for Proposed Decision on Alternative Multi-pollutant Approach to Secondary

25 Standards for Aquatic Acidification

26 A. Ambient Air Indicators

27 1. Oxides of Sulfur

28 2. Oxides of Nitrogen

29 B. Form

30 1. Ecological Indicator

31 2. Linking ANC to Deposition

32 3. Linking Deposition to Ambient Air Indicators

33 4. Aquatic Acidification Index

34 5. Spatial Aggregation

35 a. Ecoregion Sensitivity

36 b. Representative Ecoregion-specific Factors

37 i. Factor F1

38 a) Acid-sensitive Ecoregions

39 b) Non-acid sensitive Ecoregions

40 ii. Factor F2

41 iii. Factors F3 and F4

42 c. Factors in Data-limited Ecoregions

43 d. Application to Hawaii, Alaska, and the U.S. Territories

44 6. Summary of the AAI Form

45 C. Averaging Time

46 D. Level
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1 1. Association Between pH Levels and Target ANC Levels 

2 2. ANC Levels Related to Effects on Aquatic Ecosystems 

3 3. Consideration of Episodic Acidity

4 4. Consideration of Ecosystem Response Time 

5 5. Prior Examples of Target ANC Levels 

6 6. Consideration of Public Welfare Benefits

7 7. Summary of Alternative Levels

8 E. Combined Alternative Levels and Forms

9 F. Characterization of Uncertainties

10 1. Overview of Uncertainty

11 2. Uncertainties Associated with Data Gaps

12 3. Uncertainties in Modeled Processes

13 G. CASAC Advice

14 H. Administrator’s Proposed Conclusions

15 IV. Field Pilot Program and Ambient Monitoring 

16 A. Field Pilot Program

17 a. Objectives

18 b. Overview of Field Pilot Program

19 c. Complementary Measurements

20 d. Complementary Research Efforts

21 e. Implementation Challenges

22 f. Final Monitoring Plan Development and Stakeholder Participation

23 B. Evaluation of Monitoring Methods

24 1. Potential FRMs for SO2 and p-SO4

25 2. Potential FRM for NOy

26 V. Statutory and Executive Order Reviews

27 A. Executive Order 12866: Regulatory Planning and Review

28 B. Paperwork Reduction Act

29 C. Regulatory Flexibility Act

30 D. Unfunded Mandates Reform Act

31 E. Executive Order 13132: Federalism

32 F. Executive Order 13175: Consultation and Coordination with Indian Tribal 

33 Governments

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

35 Safety Risks

36 H. Executive Order 13211: Actions that Significantly Affect Energy Supply, 

37 Distribution, or Use

38 I. National Technology Transfer and Advancement Act

39 J. Executive Order 12898: Federal Actions to Address Environmental Justice in 

40 Minority Populations and Low-Income Populations

41 References

42
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1 I. Background

2 A. Legislative Requirements

3 Two sections of the Clean Air Act (CAA) govern the establishment and revision of the 

4 NAAQS. Section 108 (42 U.S.C. section 7408) directs the Administrator to identify and list 

5 certain air pollutants and then to issue air quality criteria for those pollutants. The Administrator 

6 is to list those air pollutants that in her “judgment, cause or contribute to air pollution which may

7 reasonably be anticipated to endanger public health or welfare;” “the presence of which in the 

8 ambient air results from numerous or diverse mobile or stationary sources;” and “for which . . . 

9 [the Administrator] plans to issue air quality criteria...” Air quality criteria are intended to 

10 “accurately reflect the latest scientific knowledge useful in indicating the kind and extent of all 

11 identifiable effects on public health or welfare which may be expected from the presence of [a] 

12 pollutant in the ambient air . . .” 42 U.S.C. § 7408(b). Section 109 (42 U.S.C. 7409) directs the 

13 Administrator to propose and promulgate “primary” and “secondary” NAAQS for pollutants for 

14 which air quality criteria are issued. Section 109(b)(1) defines a primary standard as one “the 

15 attainment and maintenance of which in the judgment of the Administrator, based on such 

16 criteria and allowing an adequate margin of safety, are requisite to protect the public health.”1 A 

17 secondary standard, as defined in section 109(b)(2), must “specify a level of air quality the 

18 attainment and maintenance of which, in the judgment of the Administrator, based on such 

19 criteria, is requisite to protect the public welfare from any known or anticipated adverse effects 

20 associated with the presence of [the] pollutant in the ambient air.” Welfare effects as defined in 

21 section 302(h) (42 U.S.C. § 7602(h)) include, but are not limited to, “effects on soils, water, 

 

1 The legislative history of section 109 indicates that a primary standard is to be set at “the maximum permissible 

ambient air level . . . which will protect the health of any [sensitive] group of the population,” and that for this 

purpose “reference should be made to a representative sample of persons comprising the sensitive group rather than 

to a single person in such a group” S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
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1 crops, vegetation, man-made materials, animals, wildlife, weather, visibility and climate, damage 

2 to and deterioration of property, and hazards to transportation, as well as effects on economic 

3 values and on personal comfort and well-being.”

4 In setting standards that are “requisite” to protect public health and welfare, as provided 

5 in section 109(b), EPA’s task is to establish standards that are neither more nor less stringent 

6 than necessary for these purposes. In so doing, EPA may not consider the costs of implementing 

7 the standards. See generally, Whitman v. American Trucking Associations, 531 U.S. 457, 465-

8 472, 475-76 (2001). Likewise, “[a]ttainability and technological feasibility are not relevant 

9 considerations in the promulgation of national ambient air quality standards.” American 

10 Petroleum Institute v. Costle, 665 F. 2d at 1185. Section 109(d)(1) requires that “not later than 

11 December 31, 1980, and at 5-year intervals thereafter, the Administrator shall complete a 

12 thorough review of the criteria published under section 108 and the national ambient air quality

13 standards . . . and shall make such revisions in such criteria and standards and promulgate such 

14 new standards as may be appropriate . . . .” Section 109(d)(2) requires that an independent 

15 scientific review committee “shall complete a review of the criteria . . . and the national primary

16 and secondary ambient air quality standards . . . and shall recommend to the Administrator any

17 new . . . standards and revisions of existing criteria and standards as may be appropriate . . . .” 

18 Since the early 1980's, this independent review function has been performed by the Clean Air 

19 Scientific Advisory Committee (CASAC).

20 B. History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides

21 1. NAAQS for Oxides of Nitrogen

22 After reviewing the relevant science on the public health and welfare effects associated 

23 with oxides of nitrogen, EPA promulgated identical primary and secondary NAAQS for NO2 in 
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1 April 1971. These standards were set at a level of 0.053 parts per million (ppm) as an annual 

2 average (36 FR 8186). In 1982, EPA published Air Quality Criteria for Oxides of Nitrogen (US 

3 EPA, 1982), which updated the scientific criteria upon which the initial standards were based. In 

4 February 1984 EPA proposed to retain these standards (49 FR 6866). After taking into account 

5 public comments, EPA published the final decision to retain these standards in June 1985 (50 FR 

6 25532).

7 The EPA began the most recent previous review of the oxides of nitrogen secondary

8 standards in 1987. In November 1991 EPA released an updated draft air quality criteria 

9 document (AQCD) for CASAC and public review and comment (56 FR 59285), which provided 

10 a comprehensive assessment of the available scientific and technical information on health and 

11 welfare effects associated with NO2 and other oxides of nitrogen. The CASAC reviewed the 

12 draft document at a meeting held on July 1, 1993 and concluded in a closure letter to the 

13 Administrator that the document “provides a scientifically balanced and defensible summary of 

14 current knowledge of the effects of this pollutant and provides an adequate basis for EPA to 

15 make a decision as to the appropriate NAAQS for NO2” (Wolff, 1993). The Air Quality Criteria 

16 for Oxides of Nitrogen was then finalized (US EPA, 1995a). EPA’s OAQPS also prepared a 

17 Staff Paper that summarized and integrated the key studies and scientific evidence contained in 

18 the revised AQCD for oxides of nitrogen and identified the critical elements to be considered in 

19 the review of the NO2 NAAQS. CASAC reviewed two drafts of the Staff Paper and concluded 

20 in a closure letter to the Administrator that the document provided a “scientifically adequate 

21 basis for regulatory decisions on nitrogen dioxide” (Wolff, 1995).

22 In October 1995 the Administrator announced her proposed decision not to revise either 

23 the primary or secondary NAAQS for NO2 (60 FR 52874; October 11, 1995). A year later, the 
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1 Administrator made a final determination not to revise the NAAQS for NO2 after careful 

2 evaluation of the comments received on the proposal (61 FR 52852; October 8, 1996). While the 

3 primary NO2 standard was revised in January 2010 by supplementing the existing annual 

4 standard with the establishment of a new 1-hour standard (75 FR 6474), the secondary NAAQS 

5 for NO2 remains 0.053 ppm (100 micrograms per cubic meter [μg/m3] of air), annual arithmetic 

6 average, calculated as the arithmetic mean of the 1-hour NO2 concentrations.

7 2. NAAQS for Oxides of Sulfur

8 EPA promulgated primary and secondary NAAQS for SO2 in April 1971 (36 FR 8186). 

9 The secondary standards included a standard set at 0.02 ppm, annual arithmetic mean, and a 3-

10 hour average standard set at 0.5 ppm, not to be exceeded more than once per year. These 

11 secondary standards were established solely on the basis of evidence of adverse effects on 

12 vegetation. In 1973, revisions made to Chapter 5 (“Effects of Sulfur Oxide in the Atmosphere on 

13 Vegetation”) of Air Quality Criteria for Sulfur Oxides (US EPA, 1973) indicated that it could not 

14 properly be concluded that the vegetation injury reported resulted from the average SO2 

15 exposure over the growing season, rather than from short-term peak concentrations. Therefore, 

16 EPA proposed (38 FR 11355) and then finalized (38 FR 25678) a revocation of the annual mean 

17 secondary standard. At that time, EPA was aware that then-current concentrations of oxides of 

18 sulfur in the ambient air had other public welfare effects, including effects on materials, 

19 visibility, soils, and water. However, the available data were considered insufficient to establish 

20 a quantitative relationship between specific ambient concentrations of oxides of sulfur and such 

21 public welfare effects (38 FR 25679).

22 In 1979, EPA announced that it was revising the AQCD for oxides of sulfur concurrently 

23 with that for particulate matter (PM) and would produce a combined PM and oxides of sulfur 
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1 criteria document. Following its review of a draft revised criteria document in August 1980, 

2 CASAC concluded that acid deposition was a topic of extreme scientific complexity because of 

3 the difficulty in establishing firm quantitative relationships among (1) emissions of relevant 

4 pollutants (e.g., SO2 and oxides of nitrogen), (2) formation of acidic wet and dry deposition 

5 products, and (3) effects on terrestrial and aquatic ecosystems. CASAC also noted that acid 

6 deposition involves, at a minimum, several different criteria pollutants: oxides of sulfur, oxides 

7 of nitrogen, and the fine particulate fraction of suspended particles. CASAC felt that any

8 document on this subject should address both wet and dry deposition, since dry deposition was 

9 believed to account for a substantial portion of the total acid deposition problem.

10 For these reasons, CASAC recommended that a separate, comprehensive document on 

11 acid deposition be prepared prior to any consideration of using the NAAQS as a regulatory

12 mechanism for the control of acid deposition. CASAC also suggested that a discussion of acid 

13 deposition be included in the AQCDs for oxides of nitrogen and PM and oxides of sulfur. 

14 Following CASAC closure on the AQCD for oxides of sulfur in December 1981, EPA’s OAQPS 

15 published a Staff Paper in November 1982, although the paper did not directly assess the issue of 

16 acid deposition. Instead, EPA subsequently prepared the following documents to address acid 

17 deposition: The Acidic Deposition Phenomenon and Its Effects: Critical Assessment Review 

18 Papers, Volumes I and II (US EPA, 1984a, b) and The Acidic Deposition Phenomenon and Its 

19 Effects: Critical Assessment Document (US EPA, 1985) (53 FR 14935 -14936). These 

20 documents, though they were not considered criteria documents and did not undergo CASAC 

21 review, represented the most comprehensive summary of scientific information relevant to acid 

22 deposition completed by EPA at that point.
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1 In April 1988 (53 FR 14926), EPA proposed not to revise the existing primary and 

2 secondary standards for SO2. This proposed decision with regard to the secondary SO2 NAAQS 

3 was due to the Administrator’s conclusions that (1) based upon the then-current scientific 

4 understanding of the acid deposition problem, it would be premature and unwise to prescribe any

5 regulatory control program at that time and (2) when the fundamental scientific uncertainties had 

6 been decreased through ongoing research efforts, EPA would draft and support an appropriate set 

7 of control measures. Although EPA revised the primary SO2 standard in June 2010 by

8 establishing a new 1-hour standard and revoking the existing 24-hour and annual standards (75 

9 FR 35520), no further decisions on the secondary SO2 standard have been published. 

10 C. History of Related Assessments and Agency Actions

11 In 1980, the Congress created the National Acid Precipitation Assessment Program 

12 (NAPAP) in response to growing concern about acidic deposition. The NAPAP was given a 

13 broad 10-year mandate to examine the causes and effects of acidic deposition and to explore 

14 alternative control options to alleviate acidic deposition and its effects. During the course of the 

15 program, the NAPAP issued a series of publicly available interim reports prior to the completion 

16 of a final report in 1990 (NAPAP, 1990).

17 In spite of the complexities and significant remaining uncertainties associated with the 

18 acid deposition problem, it soon became clear that a program to address acid deposition was 

19 needed. The Clean Air Act Amendments of 1990 included numerous separate provisions related 

20 to the acid deposition problem. The primary and most important of the provisions, the 

21 amendments to Title IV of the Act, established the Acid Rain Program to reduce emissions of 

22 SO2 by 10 million tons and emissions of nitrogen oxides by 2 million tons from 1980 emission 

23 levels in order to achieve reductions over broad geographic regions. In this provision, Congress 
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1 included a statement of findings that led them to take action, concluding that (1) the presence of 

2 acid compounds and their precursors in the atmosphere and in deposition from the atmosphere 

3 represents a threat to natural resources, ecosystems, materials, visibility, and public health; (2) 

4 the problem of acid deposition is of national and international significance; and (3) current and 

5 future generations of Americans will be adversely affected by delaying measures to remedy the 

6 problem. 

7 Second, Congress authorized the continuation of the NAPAP in order to assure that the 

8 research and monitoring efforts already undertaken would continue to be coordinated and would 

9 provide the basis for an impartial assessment of the effectiveness of the Title IV program.

10 Third, Congress considered that further action might be necessary in the long term to 

11 address any problems remaining after implementation of the Title IV program and, reserving 

12 judgment on the form that action could take, included Section 404 of the 1990 Amendments 

13 (Clean Air Act Amendments of 1990, Pub. L. 101-549, § 404) requiring EPA to conduct a study

14 on the feasibility and effectiveness of an acid deposition standard or standards to protect 

15 “sensitive and critically sensitive aquatic and terrestrial resources.” At the conclusion of the 

16 study, EPA was to submit a report to Congress. Five years later, EPA submitted its report, 

17 entitled Acid Deposition Standard Feasibility Study: Report to Congress (US EPA, 1995b) in 

18 fulfillment of this requirement. That Report concluded that establishing acid deposition 

19 standards for sulfur and nitrogen deposition may at some point in the future be technically

20 feasible, although appropriate deposition loads for these acidifying chemicals could not be 

21 defined with reasonable certainty at that time. 

22 Fourth, the 1990 Amendments also added new language to sections of the CAA 

23 pertaining to the scope and application of the secondary NAAQS designed to protect the public 
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1 welfare. Specifically, the definition of “effects on welfare” in Section 302(h) was expanded to 

2 state that the welfare effects include effects “...whether caused by transformation, conversion, or 

3 combination with other air pollutants.” 

4 In 1999, seven Northeastern states cited this amended language in Section 302(h) in a 

5 petition asking EPA to use its authority under the NAAQS program to promulgate secondary

6 NAAQS for the criteria pollutants associated with the formation of acid rain. The petition stated 

7 that this language “clearly references the transformation of pollutants resulting in the inevitable 

8 formation of sulfate and nitrate aerosols and/or their ultimate environmental impacts as wet and 

9 dry deposition, clearly signaling Congressional intent that the welfare damage occasioned by 

10 sulfur and nitrogen oxides be addressed through the secondary standard provisions of Section 

11 109 of the Act.” The petition further stated that “recent federal studies, including the NAPAP 

12 Biennial Report to Congress: An Integrated Assessment, document the continued and increasing 

13 damage being inflicted by acid deposition to the lakes and forests of New York, New England 

14 and other parts of our nation, demonstrating that the Title IV program had proven insufficient.” 

15 The petition also listed other adverse welfare effects associated with the transformation of these 

16 criteria pollutants, including impaired visibility, eutrophication of coastal estuaries, global 

17 warming, and tropospheric ozone and stratospheric ozone depletion.

18 In a related matter, the Office of the Secretary of the U.S. Department of Interior (DOI) 

19 requested in 2000 that EPA initiate a rulemaking proceeding to enhance the air quality in 

20 national parks and wilderness areas in order to protect resources and values that are being 

21 adversely affected by air pollution. Included among the effects of concern identified in the 

22 request were the acidification of streams, surface waters, and/or soils; eutrophication of coastal 

23 waters; visibility impairment; and foliar injury from ozone.
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1 In a Federal Register notice in 2001, EPA announced receipt of these requests and asked 

2 for comment on the issues raised in them. EPA stated that it would consider any relevant 

3 comments and information submitted, along with the information provided by the petitioners and 

4 DOI, before making any decision concerning a response to these requests for rulemaking (65 FR 

5 48699).

6 The 2005 NAPAP report states that “... scientific studies indicate that the emission 

7 reductions achieved by Title IV are not sufficient to allow recovery of acid-sensitive ecosystems. 

8 Estimates from the literature of the scope of additional emission reductions that are necessary in 

9 order to protect acid-sensitive ecosystems range from approximately 40-80% beyond full 

10 implementation of Title IV....” The results of the modeling presented in this Report to Congress 

11 indicate that broader recovery is not predicted without additional emission reductions” (NAPAP, 

12 2005).

13 Given the state of the science as described in the ISA, REA, and in other recent reports, 

14 such as the NAPAP reports noted above, EPA has decided, in the context of evaluating the 

15 adequacy of the current NO2 and SO2 secondary standards in this review, to revisit the question 

16 of the appropriateness of setting secondary NAAQS to address remaining known or anticipated 

17 adverse public welfare effects resulting from the acidic and nutrient deposition of these criteria 

18 pollutants.

19 D. History of the Current Review

20 The EPA initiated this current review in December 2005 with a call for information (70 

21 FR 73236) for the development of a revised Integrated Science Assessment for Oxides of 

22 Nitrogen and Oxides of Sulfur- Ecological Criteria (henceforth the "ISA"). An Integrated 

23 Review Plan (IRP) was developed to provide the framework and schedule as well as the scope of 
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1 the review and to identify policy-relevant questions to be addressed in the components of the 

2 review. The IRP was released in 2007 (US EPA, 2007) for CASAC and public review. EPA 

3 held a workshop in July 2007 on the ISA to obtain broad input from the relevant scientific 

4 communities. This workshop helped to inform the preparation of the first draft ISA, which was 

5 released for CASAC and public review in December 2007; a CASAC meeting was held on April 

6 2-3, 2008 to review the first draft ISA. A second draft ISA was released for CASAC and public 

7 review in August 2008, and was discussed at a CASAC meeting held on October 1-2, 2008. The

8 final ISA (US EPA, 2008) was released in December 2008. 

9 Based on the science presented in the ISA, EPA developed a Risk and Exposure 

10 Assessment for Review of the Secondary National Ambient Air Quality Standards for Oxides of 

11 Nitrogen and Oxides of Sulfur (henceforth the “REA”) to further assess the national impact of 

12 the effects documented in the ISA. The Draft Scope and Methods Plan for Risk/Exposure 

13 Assessment: Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur outlining 

14 the scope and design of the future REA was prepared for CASAC and public in March 2008. A 

15 first draft REA was presented to CASAC and the public for review in August 2008 and a second 

16 draft was presented for review in June 2009. The final REA (US EPA 2009) was released in 

17 September 2009. A first draft Policy Assessment was released in March 2010 and reviewed by

18 CASAC on April 1-2, 2010. In a June 22, 2010 letter to the Administrator, CASAC provided 

19 advice and recommendations to the Agency concerning the first draft Policy Assessment (Russell 

20 and Samet, 2010a). A second draft Policy Assessment was released to CASAC and the public 

21 in September 2010 and reviewed by CASAC on October 6-7, 2010. CASAC provided advice 

22 and recommendations to the Agency regarding the second draft Policy Assessment in a 

23 December 9, 2010 letter (Russell and Samet 2010b). CASAC and public comments on the 
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1 second draft Policy Assessment were considered by EPA staff in developing a final Policy

2 Assessment (US EPA, 2011). CASAC requested an additional meeting to provide additional 

3 advice to the Administrator based on the final Policy Assessment on February 15–16, 2011. 

4 EPA released on January 14, 2011, the final Policy Assessment prior to final document 

5 production, to provide sufficient time for CASAC review of the document in advance of this 

6 meeting. A final Policy Assessment, incorporating final reference checks and document 

7 formatting, was released in February 2011. In a May 17, 2011 letter (Russell and Samet, 2011a),

8 CASAC offered additional advice to the Administrator with regard to recommendations and 

9 revisions to the secondary NAAQS for oxides of nitrogen and oxides of sulfur. 

10 On February 3, 2006, the Center for Biological Diversity and four other plaintiffs filed 

11 an amended complaint alleging that EPA had failed to complete the current review within the 

period provided by statute.212 The schedule for completion of this review is governed by a consent 

13 decree resolving that lawsuit and the subsequent extension granted to the Agency by the 

14 plaintiffs. The schedule presented in the original consent decree that governs this review, entered 

15 by the court on November 19, 2007, was revised on October 22, 2009 to allow for a 17 month 

16 extension of the schedule. The current decree provides that EPA sign for publication notices of 

17 proposed and final rulemaking concerning its review of the oxides of nitrogen and oxides of 

18 sulfur NAAQS no later than July 12, 2011 and March 20, 2012, respectively. 

19 This action presents the Administrator’s proposed decisions on the review of the current 

20 secondary oxides of nitrogen and oxides of sulfur standards. Throughout this preamble a number 

21 of conclusions, findings, and determinations proposed by the Administrator are noted. While 

22 they identify the reasoning that supports this proposal, they are not intended to be final or 

23 conclusive in nature. The EPA invites general, specific, and/or technical comments on all issues 

 

2 Center for Biological Diversity, et al. v. Johnson, No. 05–1814 (D.D.C.)
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1 involved with this proposal, including all such proposed judgments, conclusions, findings, and 

2 determinations.

3 E. Scope of the Current Review

4 In conducting this periodic review of the secondary NAAQS for oxides of nitrogen and 

5 oxides of sulfur, as discussed in the IRP and REA, EPA decided to assess the scientific 

6 information, associated risks, and standards relevant to protecting the public welfare from 

7 adverse effects associated jointly with oxides of nitrogen and sulfur. Although EPA has 

8 historically adopted separate secondary standards for oxides of nitrogen and oxides of sulfur, 

9 EPA is conducting a joint review of these standards because oxides of nitrogen and sulfur, and 

10 their associated transformation products are linked from an atmospheric chemistry perspective, 

11 as well as from an environmental effects perspective. The National Research Council (NRC) has 

12 recommended that EPA consider multiple pollutants, as appropriate, in forming the scientific 

13 basis for the NAAQS (NRC, 2004). As discussed in the ISA and REA, there is a strong basis for 

14 considering these pollutants together, building upon EPA’s past recognition of the interactions of 

15 these pollutants and on the growing body of scientific information that is now available related to 

16 these interactions and associated ecological effects.

17 In defining the scope of this review, it must be considered that EPA has set secondary

18 standards for two other criteria pollutants related to oxides of nitrogen and sulfur: ozone and 

19 particulate matter (PM). Oxides of nitrogen are precursors to the formation of ozone in the 

20 atmosphere, and under certain conditions, can combine with atmospheric ammonia to form 

21 ammonium nitrate, a component of fine PM. Oxides of sulfur are precursors to the formation of 

22 particulate sulfate, which is a significant component of fine PM in many parts of the U.S. There 

23 are a number of welfare effects directly associated with ozone and fine PM, including ozone-
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1 related damage to vegetation and PM-related visibility impairment. Protection against those 

2 effects is provided by the ozone and fine PM secondary standards. This review focuses on 

3 evaluation of the protection provided by secondary standards for oxides of nitrogen and sulfur 

4 for two general types of effects: (1) direct effects on vegetation associated with exposure to 

5 gaseous oxides of nitrogen and sulfur in the ambient air, which are the effects that the current 

6 NO2 and SO2 secondary standards protect against and (2) effects associated with the deposition 

7 of oxides of nitrogen and sulfur to sensitive aquatic and terrestrial ecosystems, including 

8 deposition in the form of particulate nitrate and particulate sulfate.

9 The ISA focuses on the ecological effects associated with deposition of ambient oxides of 

10 nitrogen and sulfur to natural sensitive ecosystems, as distinguished from commercially managed 

11 forests and agricultural lands. This focus reflects the fact that the majority of the scientific 

12 evidence regarding acidification and nutrient enrichment is based on studies in unmanaged 

13 ecosystems. Non-managed terrestrial ecosystems tend to have a higher fraction of N deposition 

14 resulting from atmospheric nitrogen (US EPA, 2008, section 3.3.2.5). In addition, the ISA notes 

15 that agricultural and commercial forest lands are routinely fertilized with amounts of nitrogen 

16 that exceed air pollutant inputs even in the most polluted areas (US EPA, 2008, section 3.3.9). 

17 This review recognizes that the effects of nitrogen deposition in managed areas are viewed 

18 differently from a public welfare perspective than are the effects of nitrogen deposition in 

19 natural, unmanaged ecosystems, largely due to the more homogeneous, controlled nature of 

20 species composition and development in managed ecosystems and the potential for benefits of 

21 increased productivity in those ecosystems.

22 In focusing on natural sensitive ecosystems, the Policy Assessment primarily considers 

23 the effects of ambient oxides of nitrogen and sulfur via deposition on multiple ecological 
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1 receptors. The ISA highlights effects including those associated with acidification and nitrogen 

2 nutrient enrichment. With a focus on these deposition-related effects, EPA’s objective is to 

3 develop a framework for oxides of nitrogen and sulfur standards that incorporates ecologically 

4 relevant factors and that recognizes the interactions between the two pollutants as they deposit to 

5 sensitive ecosystems. The overarching policy objective is to develop a secondary standard(s) 

6 that is based on the ecological criteria described in the ISA and the results of the assessments in 

7 the REA, and is consistent with the requirement of the CAA to set secondary standards that are 

8 requisite to protect the public welfare from any known or anticipated adverse effects associated 

9 with the presence of these air pollutants in the ambient air. Also consistent with the CAA, this 

10 policy objective necessarily includes consideration of “variable factors . . . which of themselves 

11 or in combination with other factors may alter the effects on public welfare” of the criteria air 

12 pollutants included in this review.

13 In addition, we have chosen to focus on the effects of ambient oxides of nitrogen and 

14 sulfur on ecological impacts on sensitive aquatic ecosystems associated with acidifying 

15 deposition of nitrogen and sulfur, which is a transformation product of ambient oxides of 

16 nitrogen and sulfur. Based on the information in the ISA, the assessments presented in the REA, 

17 and advice from CASAC on earlier drafts of this Policy Assessment (Russell and Samet, 2010a, 

18 2010b), and as discussed in detail in the Policy Assessment, we have the greatest confidence in 

19 the causal linkages between oxides of nitrogen and sulfur and aquatic acidification effects 

20 relative to other deposition-related effects, including terrestrial acidification and aquatic and 

21 terrestrial nutrient enrichment.

22
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1 II. Rationale for Proposed Decision on the Adequacy of the Current Secondary 

2 Standards

3 Decisions on retaining or revising the current secondary standards for oxides of nitrogen 

4 and sulfur are largely public welfare policy judgments based on the Administrator’s informed 

5 assessment of what constitutes requisite protection against adverse effects to public welfare. A 

6 public welfare policy decision should draw upon scientific information and analyses about 

7 welfare effects, exposure and risks, as well as judgments about the appropriate response to the 

8 range of uncertainties that are inherent in the scientific evidence and analyses. The ultimate 

9 determination as to what level of damage to ecosystems and the services provided by those 

10 ecosystems is adverse to public welfare is not wholly a scientific question, although it is 

11 informed by scientific studies linking ecosystem damage to losses in ecosystem services, and 

12 information on the value of those losses of ecosystem services. In reaching such decisions, the 

13 Administrator seeks to establish standards that are neither more nor less stringent than necessary

14 for this purpose.

15 This section presents the rationale for the Administrator’s proposed conclusions with 

16 regard to the adequacy of protection and ecological relevance of the current secondary standards 

17 for oxides of nitrogen and sulfur. As discussed more fully below, this rationale considered the 

18 latest scientific information on ecological effects associated with the presence of oxides of 

19 nitrogen and oxides of sulfur in the ambient air. This rationale also takes into account: (1) staff 

20 assessments of the most policy-relevant information in the ISA and staff analyses of air quality, 

21 exposure, and ecological risks, presented more fully in the REA and in the Policy Assessment,

22 upon which staff conclusions on revisions to the secondary oxides of nitrogen and oxides of 

23 sulfur standards are based; (2) CASAC advice and recommendations, as reflected in discussions 
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1 of drafts of the ISA, REA, and Policy Assessment at public meetings, in separate written 

2 comments, and in CASAC’s letters to the Administrator; and (3) public comments received 

3 during the development of these documents, either in connection with CASAC meetings or 

4 separately. 

5 In developing this rationale, EPA has drawn upon an integrative synthesis of the entire 

6 body of evidence, published through early 2008, on ecological effects associated with the 

7 deposition of oxides of nitrogen and oxides of sulfur in the ambient air (US EPA, 2008). As 

8 discussed below in section II.A, this body of evidence addresses a broad range of ecological 

9 endpoints associated with ambient levels of oxides of nitrogen and oxides of sulfur. In 

10 considering this evidence, EPA focuses on those ecological endpoints, such as aquatic 

11 acidification, for which the ISA judges associations with oxides of nitrogen and oxides of sulfur

12 to be causal, likely causal, or for which the evidence is highly suggestive that oxides of nitrogen 

13 and/or sulfur contribute to the reported effects.

14 Crucial to this review is the development of a form for an ecologically relevant standard 

15 that reflects both the geographically variable and deposition-dependent nature of the effects. 

16 The atmospheric levels of oxides of nitrogen and sulfur that afford a particular level of 

17 ecosystem protection are those levels that result in an amount of deposition that is less than the 

18 amount of deposition that a given ecosystem can accept without defined levels of degradation of 

19 the ecological indicator for a targeted effect.

20 Drawing from the framework developed in the REA, the framework we used to structure 

21 an ecologically meaningful secondary standard in the Policy Assessment and to further develop 

22 the indicator, form, level, and averaging time of such as standard in section III of this proposal is 
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1 depicted below and highlights the three key linkages that need to be considered in developing an 

2 ecologically relevant standard.

3

4

5 Figure II-1. Simplified conceptual design of the form of an aquatic acidification standard 

6 for oxides of nitrogen and sulfur.

7

8 The following discussion relies heavily on Chapters 2 and 3 of the Policy Assessment. 

9 The Policy Assessment includes staff’s evaluation of the policy implications of the scientific 

10 assessment of the evidence presented and assessed in the ISA and the results of quantitative 

11 assessments based on that information presented and assessed in the REA. Taken together, this 

12 information informs staff conclusions and the development of policy options in the Policy

13 Assessment for consideration in addressing public and welfare effects associated with the 

14 presence of oxides of nitrogen and oxides of sulfur in the ambient air. Of particular note, chapter 

15 2 of the Policy Assessment presents information not repeated here that characterizes emissions, 

16 air quality, deposition and water quality. It includes discussions of the sources of nitrogen and 

17 sulfur in the atmosphere as well as current ambient air quality monitoring networks and models. 

18 Additional information in this section includes ecological modeling and water quality data 

19 sources. 

20 Section II.A presents a discussion of the effects associated with oxides of nitrogen and 

21 sulfur in the ambient air. The discussion is organized around the types of effects being 
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1 considered, including direct effects of gaseous oxides of nitrogen and sulfur, deposition-related 

2 effects related to acidification and nutrient enrichment, and other effects such as materials 

3 damage, climate-related effects and mercury methylation.

4 Section II.B presents a summary and discussion of the risk and exposure assessment 

5 performed for each of the four major effects categories. The REA uses case studies representing 

6 the broad geographic variability of the impacts from oxides of nitrogen and sulfur to conclude 

7 that there are ongoing adverse effects in many ecosystems from deposition of oxides of nitrogen 

8 and sulfur and that under current emissions scenarios these effects are likely to continue.

9 Section II.C presents a discussion of adversity linking ecological effects to measures that 

10 can be used to characterize the extent to which such effects are reasonably considered to be 

11 adverse to public welfare. This involves consideration of how to characterize adversity from a 

12 public welfare perspective. In so doing, consideration is given to the concept of ecosystem 

13 services, the evidence of effects on ecosystem services, and how ecosystem services can be 

14 linked to ecological indicators.

15 Section II.D presents an assessment of the adequacy of the current oxides of nitrogen and 

16 oxides of sulfur secondary standards. Consideration is given to the adequacy of protection 

17 afforded by the current standards for both direct and deposition-related effects, as well as to the 

18 appropriateness of the fundamental structure and the basic elements of the current standards for 

19 providing protection from deposition-related effects. Considerations as to the extent to which 

20 deposition-related effects that could reasonably be judged to be adverse to public welfare are 

21 occurring under current conditions which are allowed by the current standards is also considered. 

22 Discussion of the structures and basic elements of the current NO2 and SO2 secondary standards 

23 and whether they are adequate to protect against such effects is presented.
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1 A. Ecological Effects

2 This section discusses the known or anticipated ecological effects associated with oxides 

3 of nitrogen and sulfur, including the direct effects of gas-phase exposure to oxides of nitrogen 

4 and sulfur (section II.A.1) and effects associated with deposition-related exposure (sections 

5 II.A.2 and 3). Section II.A. 2 addresses effects related to acidification of aquatic and terrestrial 

6 ecosystems and section II A.3 addresses effects related to nutrient enrichment of aquatic and 

7 terrestrial ecosystems. These sections also address questions about the nature and magnitude of 

8 ecosystem responses to reactive nitrogen and sulfur deposition, including responses related to 

9 acidification, nutrient depletion, and, in Section II.A 4 the mobilization of toxic metals in 

10 sensitive aquatic and terrestrial ecosystems. The uncertainties and limitations associated with the 

11 evidence of such effects are also discussed throughout this section. 

12 1. Effects Associated with Gas-Phase Oxides of Nitrogen and Sulfur

13 Ecological effects on vegetation as discussed in earlier reviews as well as the ISA can be 

14 attributed to gas-phase oxides of nitrogen and sulfur. Acute and chronic exposures to gaseous 

15 pollutants such as sulfur dioxide (SO2), nitrogen dioxide (NO2), nitric oxide (NO), nitric acid 

16 (HNO3) and peroxyacetyl nitrite (PAN) are associated with negative impacts to vegetation. The 

17 current secondary NAAQS were set to protect against direct damage to vegetation by exposure to 

18 gas-phase oxides of nitrogen and sulfur, such as foliar injury, decreased photosynthesis, and 

19 decreased growth. The following summary is a concise overview of the known or anticipated 

20 effects to vegetation caused by gas phase nitrogen and sulfur. Most phototoxic effects associated 

21 with gas phase oxides of nitrogen and sulfur occur at levels well above ambient concentrations 

22 observed in the U.S. (US EPA, 2008, section 3.4.2.4).
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1 a. Nature of ecosystem responses to gas-phase nitrogen and sulfur

2 The 2008 ISA found that gas phase N and S are associated with direct phytotoxic effects 

3 (US EPA, 2008, section 4.4). The evidence is sufficient to infer a causal relationship between 

4 exposure to SO2 and injury to vegetation (US EPA, 2008, section 4.4.1 and 3.4.2.1). Acute foliar 

5 injury to vegetation from SO2 may occur at levels above the current secondary standard (3-h 

6 average of 0.50 ppm). Effects on growth, reduced photosynthesis and decreased yield of 

7 vegetation are also associated with increased SO2 exposure concentration and time of exposure.

8 The evidence is sufficient to infer a causal relationship between exposure to NO, NO2

9 and PAN and injury to vegetation (US EPA, 2008, section 4.4.2 and 3.4.2.2). At sufficient 

10 concentrations, NO, NO2 and PAN can decrease photosynthesis and induce visible foliar injury

11 to plants. Evidence is also sufficient to infer a causal relationship between exposure to HNO3

12 and changes to vegetation (US EPA, 2008, section 4.4.3 and 3.4.2.3). Phytotoxic effects of this 

13 pollutant include damage to the leaf cuticle in vascular plants and disappearance of some 

14 sensitive lichen species. 

15 b. Magnitude of ecosystem response to gas-phase nitrogen and sulfur

16 Vegetation in ecosystems near sources of gaseous oxides of nitrogen and sulfur or where 

17 SO2, NO, NO2, PAN and HNO3 are most concentrated are more likely to be impacted by these 

18 pollutants. Uptake of these pollutants in a plant canopy is a complex process involving 

19 adsorption to surfaces (leaves, stems and soil) and absorption into leaves (US EPA, 2008, section

20 3.4.2). The functional relationship between ambient concentrations of gas phase oxides of 

21 nitrogen and sulfur and specific plant response are impacted by internal factors such as rate of 

22 stomatal conductance and plant detoxification mechanisms, and external factors including plant 
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1 water status, light, temperature, humidity, and pollutant exposure regime (US EPA, 2008, section

2 3.4.2).

3 Entry of gases into a leaf is dependent upon physical and chemical processes of gas phase 

4 as well as to stomatal aperture. The aperture of the stomata is controlled largely by the 

5 prevailing environmental conditions, such as water availability, humidity, temperature, and light 

6 intensity. When the stomata are closed, resistance to gas uptake is high and the plant has a very

7 low degree of susceptibility to injury. Mosses and lichens do not have a protective cuticle barrier 

8 to gaseous pollutants or stomata and are generally more sensitive to gaseous sulfur and nitrogen 

9 than vascular plants (US EPA, 2008, section 3.4.2). 

10 The appearance of foliar injury can vary significantly across species and growth 

11 conditions affecting stomatal conductance in vascular plants (US EPA, 2009, section 6.4.1). For 

12 example, damage to lichens from SO2 exposure includes decreased photosynthesis and 

13 respiration, damage to the algal component of the lichen, leakage of electrolytes, inhibition of 

14 nitrogen fixation, decreased K+ absorption, and structural changes.

15 The phytotoxic effects of gas phase oxides of nitrogen and sulfur are dependent on the 

16 exposure concentration and duration and species sensitivity to these pollutants. Effects to 

17 vegetation associated with oxides of nitrogen and sulfur are therefore variable across the U.S. 

18 and tend to be higher near sources of photochemical smog. For example, SO2 is considered to be 

19 the primary factor contributing to the death of lichens in many urban and industrial areas. 

20 The ISA states there is very limited new research on phytotoxic effects of NO, NO2, PAN 

21 and HNO3 at concentrations currently observed in the United States with the exception of some 

22 lichen species (US EPA, 2008, section 4.4). Past and current HNO3 concentrations may be 

23 contributing to the decline in lichen species in the Los Angeles basin. Most phototoxic effects 
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1 associated with gas phase oxides of nitrogen and sulfur occur at levels well above ambient 

2 concentrations observed in the U.S. (US EPA, 2008, section 3.4.2.4).

3 2. Acidification Effects Associated with Deposition of Oxides of Nitrogen and Sulfur

4 Sulfur oxides and nitrogen oxides in the atmosphere undergo a complex mix of reactions 

5 in gaseous, liquid, and solid phases to form various acidic compounds. These acidic compounds 

6 are removed from the atmosphere through deposition: either wet (e.g., rain, snow), fog or cloud, 

7 or dry (e.g., gases, particles). Deposition of these acidic compounds to ecosystems can lead to 

8 effects on ecosystem structure and function. Following deposition, these compounds can, in 

some instances unless retained by soil or biota, leach out of the soils in the form of sulfate (SO4

2-

9

) and nitrate (NO3

-

10 ), leading to the acidification of surface waters. The effects on ecosystems 

11 depend on the magnitude and rate of deposition, as well as a host of biogeochemical processes 

12 occurring in the soils and waterbodies (US EPA, 2009, section 2.1). The chemical forms of 

13 nitrogen that may contribute to acidifying deposition include both oxidized and reduced chemical 

14 species, including NHx.

When sulfur or nitrogen leaches from soils to surface waters in the form of SO4

2-

or NO3

-

15 , 

16 an equivalent amount of positive cations, or countercharge, is also transported. This maintains 

electroneutrality. If the countercharge is provided by base cations, such as calcium (Ca2+ 17 ), 

magnesium (Mg2+), sodium (Na+), or potassium (K+), rather than hydrogen (H+18 ) and dissolved 

19 inorganic aluminum, the acidity of the soil water is neutralized, but the base saturation of the soil 

decreases. Continued SO4

2-

or NO3

-

20 leaching can deplete the available base cation pool in soil. 

As the base cations are removed, continued deposition and leaching of SO4

2-

and/or NO3

-

21 (with 

H

+

and Al3+ 22 ) leads to acidification of soil water, and by connection, surface water. Introduction 

23 of strong acid anions such as sulfate and nitrate to an already acidic soil, whether naturally or due 
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1 to anthropogenic activities, can lead to instantaneous acidification of waterbodies through direct 

2 runoff without any significant change in base cation saturation. The ability of a watershed to 

3 neutralize acidic deposition is determined by a variety of biogeophysical factors including 

4 weathering rates, bedrock composition, vegetation and microbial processes, physical and 

5 chemical characteristics of soils and hydrologic flowpaths. (US EPA, 2009, section 2.1) Some of 

6 these factors such as vegetation and soil depth are highly variable over small spatial scales such 

7 as meters, but can be aggregated to evaluate patterns over larger spatial scales. Acidifying 

8 deposition of oxides of nitrogen and sulfur and the chemical and biological responses associated 

9 with these inputs vary temporally. Chronic or long-term deposition processes in the time scale of 

10 years to decades result in increases in inputs of N and S to ecosystems and the associated 

11 ecological effects. Episodic or short term (i.e., hours or days) deposition refers to events in which 

12 the level of the acid neutralizing capacity (ANC) of a lake or stream is temporarily lowered. In 

13 aquatic ecosystems, short-term (i.e., hours or days) episodic changes in water chemistry can have 

14 significant biological effects. Episodic acidification refers to conditions during precipitation or 

15 snowmelt events when proportionately more drainage water is routed through upper soil horizons 

16 that tend to provide less acid neutralizing than is passing through deeper soil horizons (US EPA, 

17 2009, section 4.2). In addition, the accumulated sulfate and nitrate in snow packs can provide a 

18 surge of acidic inputs. Some streams and lakes may have chronic or base flow chemistry that is 

19 suitable for aquatic biota, but may be subject to occasional acidic episodes with deleterious 

20 consequences to sensitive biota.

21 The following summary is a concise overview of the known or anticipated effects caused 

22 by acidification to ecosystems within the United States. Acidification affects both terrestrial and 

23 freshwater aquatic ecosystems. 
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1 a. Nature of acidification-related ecosystem responses

2 The ISA concluded that deposition of oxides of nitrogen and sulfur and NHx leads to the 

3 varying degrees of acidification of ecosystems (US EPA, 2008). In the process of acidification, 

4 biogeochemical components of terrestrial and freshwater aquatic ecosystems are altered in a way

5 that leads to effects on biological organisms. Deposition to terrestrial ecosystems often moves 

6 through the soil and eventually leaches into adjacent water bodies.

7 i. Aquatic ecosystems

8 The scientific evidence is sufficient to infer a causal relationship between acidifying 

9 deposition and effects on biogeochemistry and biota in aquatic ecosystems (US EPA, 2008, 

10 section 4.2.2). The strongest evidence comes from studies of surface water chemistry in which 

11 acidic deposition is observed to alter sulfate and nitrate concentrations in surface waters, the sum 

12 of base cations, ANC, dissolved inorganic aluminum and pH (US EPA, 2008, section 3.2.3.2). 

13 ANC is a key indicator of acidification with relevance to both terrestrial and aquatic ecosystems. 

14 ANC is useful because it integrates the overall acid-base status of a lake or stream and reflects 

15 how aquatic ecosystems respond to acidic deposition over time. There is also a relationship 

16 between ANC and the surface water constituents that directly contribute to or ameliorate acidityrelated stress, in particular, concentrations of hydrogen ion (as pH), Ca2+ 17 , and aluminum. 

18 Moreover, low pH surface waters leach aluminum from soils, which is quite lethal to fish and 

19 other aquatic organisms. In aquatic systems, there is a direct relationship between ANC and fish 

20 and phyto-zooplankton diversity and abundance. 

21 Low ANC coincides with effects on aquatic systems (e.g., individual species fitness loss

22 or death, reduced species richness, altered community structure). At the community level,

23 species richness is positively correlated with pH and ANC because energy cost in maintaining 
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1 physiological homeostasis, growth, and reproduction is high at low ANC levels. For example, 

2 there is a logistic relationship between fish species richness and ANC class for Adirondack Case 

3 Study Area lakes that indicates the probability of occurrence of an organism for a given value of 

4 ANC. Biota are generally not harmed when ANC values are >100 microequivalents per liter

5 (μeq/L). The number of fish species also peaks at ANC values >100 μeq/L. Below 100 μeq/L

6 ANC, fish fitness and community diversity begin to decline (US EPA, section 4.2). At ANC 

7 levels between 100 and 50 μeq/L, the fitness of sensitive species (e.g., brook trout, zooplankton) 

8 also begins to decline. When ANC concentrations are <50 μeq/L, they are generally associated 

9 with death or loss of fitness of biota that are sensitive to acidification.

10 Consistent and coherent documentation from multiple studies on various species from all 

11 major trophic levels of aquatic systems shows that geochemical alteration caused by acidification 

12 can result in the loss of acid-sensitive biological species (US EPA, 2008, section 3.2.3.3). This is 

13 most often discussed with relation to pH. For example, in the Adirondacks, of the 53 fish species 

14 recorded in Adirondack lakes about half (26 species) were absent from lakes with pH below 6.0. 

15 Biological effects are linked to changes in water chemistry including decreases in ANC and pH 

16 and increases in inorganic Al concentration. The direct biological effects are caused by lowered 

17 pH which leads to increased inorganic Al concentrations (US EPA, 2011, Figures 3-1 and 3-2). 

18 While ANC level does not cause direct biological harm it is a good overall indicator of the risk 

19 of acidification (US EPA, 2011, section 3.1.3).

20 There are clear associations between ANC, pH, and aquatic species mortality and health 

21 which are summarized in section 3.1.1 of the Policy Assessment. Significant harm to sensitive 

22 aquatic species has been observed at pH levels below 6. Normal stream pH levels with little to 

23 no toxicity range from 6 to 7 (MacAvoy et al, 1995). Baker et al (1990) observed that “lakes 
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1 with pH less than approximately 6.0 contain significantly fewer species than lakes with pH levels 

2 above 6.0”. As noted in Chapter 3, typically at pH <4.5 and an ANC <0 μeq/L, complete to 

3 near-complete loss of many taxa of organisms occur, including fish and aquatic insect 

4 populations, whereas other taxa are reduced to only acidophilic species. 

5 Additional evidence can help refine the understanding of effects occurring at pH levels 

6 between 4.5 and 6. When pH levels are below 5.6, relatively lower trout survival rates were 

7 observed in the Shenandoah National Park. In field observations, when pH levels dropped to 5, 

8 mortality rates went to 100 percent. (Bulger et al, 2000). At pH levels ranging from 5.4 to 5.8, 

9 cumulative mortality continues to increase. Several studies have shown that trout exposed to 

10 water with varying pH levels and fish larvae showed increasing mortality as pH levels decrease. 

11 In one study almost 100 percent mortality was observed at a pH of 4.5 compared to almost 100 

12 percent survival at a pH of 6.5. Intermediate pH values (6.0, 5.5) in all cases showed reduced 

13 survival compared with the control (6.5), but not by statistically significant amounts (US EPA, 

14 2008, section 3.2.3.3). 

15 One important indicator of acid stress is increased fish mortality. The response of fish to 

16 pH is not uniform across species. A number of synoptic surveys indicated loss of species 

17 diversity and absence of several fish species in the pH range of 5.0 to 5.5. If pH is lower, there is 

18 a greater likelihood that more fish species could be lost without replacement, resulting in 

19 decreased richness and diversity. In general, populations of salmonids are not found at pH levels 

20 less than 5.0, and smallmouth bass (Micropterus dolomieu) populations are usually not found at 

21 pH values less than about 5.2 to 5.5. From Table 3-1, only one study showed significant 

22 mortality effects above a pH of 6, while a number of studies showed significant mortality when 

23 pH levels are at or below 5.5. 
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1 The highest pH level for any of the studies reported in the ISA, is 6.0, suggesting that pH 

2 above 6.0 is protective against mortality effects for most species. Most thresholds are in the 

3 range of pH of 5.0 to 6.0, which suggests that a target pH should be no lower than 5.0. 

4 Protection against mortality in some recreationally important species such as lake trout (pH 

5 threshold of 5.6) and crappie (pH threshold of 5.5), combined with the evidence of effects on 

6 larval and embryo survival suggests that pH levels greater than 5.5 should be targeted to provide 

7 protection against mortality effects throughout the life stages of fish.

8 Non-lethal effects have been observed at pH levels as high as 6. A study in the 

9 Shenandoah National Park found that the condition factor, a measure of fish health expressed as 

10 fish weight/length multiplied by a scaling constant, is positively correlated with stream pH 

11 levels, and that the condition factor is reduced in streams with a pH of 6.0 (US EPA, 2008, 

12 section 3.2.3.3).

13 Biodiversity is another indicator of aquatic ecosystem health. A key study in the 

14 Adirondacks found that lakes with a pH of 6.0 had only half the potential species of fish (27 of 

15 53 potential species). There is often a positive relationship between pH and number of fish 

16 species, at least for pH values between about 5.0 and 6.5, or ANC values between about 0 to 

17 100 μeq/L. Such observed relationships are complicated, however, by the tendency for smaller 

18 lakes and streams, having smaller watersheds, to also support fewer fish species, irrespective of 

19 acid-base chemistry. This pattern may be due to a decrease in the number of available niches as 

20 stream or lake size decreases. Nevertheless, fish species richness is relatively easily determined 

21 and is one of the most useful indicators of biological effects of surface water acidification. 

22 Changes in stream water pH and ANC also contribute to declines in taxonomic richness 

23 of zooplankton, and macroinvertebrates which are often sources of food for fish, birds and other 
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1 animal species in various ecosystems. These fish may also serve as a source of food and 

2 recreation for humans. Acidification of ecosystems has been shown to disrupt food web 

3 dynamics causing alteration to the diet, breeding distribution, and reproduction of certain species 

4 of birds (US EPA, 2008, section 4.2.2.2. and Table 3-9). For example, breeding distribution of 

5 the common goldeneye (Bucephala clangula), an insectivorous duck, may be affected by 

6 changes in acidifying deposition. Similarly, decreases in prey diversity and quantity have been 

7 observed to create feeding problems for nesting pairs of loons on low-pH lakes in the 

8 Adirondacks. 

9 ii. Terrestrial ecosystems

10 In terrestrial ecosystems, the evidence is sufficient to infer a causal relationship between 

11 acidifying deposition and changes in biogeochemistry (US EPA, 2008, section 4.2.1.1). The 

12 strongest evidence comes from studies of forested ecosystems, with supportive information on 

13 other plant taxa, including shrubs and lichens (US EPA, 2008, section 3.2.2.1.). Three useful 

14 indicators of chemical changes and acidification effects on terrestrial ecosystems, showing 

15 consistency and coherence among multiple studies are: soil base saturation, Al concentrations in 

16 soil water and soil C:N ratio (US EPA, 2008, section 3.2.2.2). 

17 As discussed in the ISA and REA, in soils with base saturation less than about 15 to 20%, 

18 exchange chemistry is dominated by Al. Under these conditions, responses to inputs of sulfuric 

19 acid and HNO3 largely involve the release and mobilization of dissolved inorganic Al. The 

20 effect can be neutralized by weathering from geologic parent material or base cation exchange. 

The Ca2+ 21 and Al concentrations in soil water are strongly influenced by soil acidification and 

22 both have been shown to have quantitative links to tree health, including Al interference with 

Ca2+ 23 uptake and Al toxicity to roots. Effects of nitrification and associated acidification and 
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1 cation leaching have been consistently shown to occur only in soils with a C:N ratio below about 

2 20 to 25.

3 Soil acidification caused by acidic deposition has been shown to cause decreased growth 

4 and increased susceptibility to disease and injury in sensitive tree species. Red spruce (Picea 

5 rubens) dieback or decline has been observed across high elevation areas in the Adirondack, 

6 Green and White mountains. The frequency of freezing injury to red spruce needles has 

7 increased over the past 40 years, a period that coincided with increased emissions of S and N 

8 oxides and increased acidifying deposition. Acidifying deposition can contribute to dieback in 

9 sugar maple (Acer saccharum) through depletion of cations from soil with low levels of available 

10 Ca. Grasslands are likely less sensitive to acidification than forests due to grassland soils being 

11 generally rich in base cations.

12 iii. Ecosystem sensitivity 

13 The intersection between current deposition loading, historic loading, and sensitivity

14 defines the ecological vulnerability to the effects of acidification. Freshwater aquatic and some 

15 terrestrial ecosystems, notably forests, are the ecosystem types which are most sensitive to 

16 acidification. The ISA reports that the principal factor governing the sensitivity of terrestrial and 

17 aquatic ecosystems to acidification from sulfur and nitrogen deposition is geology (particularly

18 surficial geology). Geologic formations having low base cation supply generally underlie the 

19 watersheds of acid-sensitive lakes and streams. Other factors that contribute to the sensitivity of 

20 soils and surface waters to acidifying deposition include topography, soil chemistry, land use, 

21 and hydrologic flowpaths. Episodic and chronic acidification tends to occur in areas that have 

22 base-poor bedrock, high relief, and shallow soils (US EPA, 2008, section 3.2.4.1).
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1 b. Magnitude of acidification-related ecosystem responses

2 Terrestrial and aquatic ecosystems differ in their response to acidifying deposition. 

3 Therefore the magnitude of ecosystem response is described separately for aquatic and terrestrial 

4 ecosystems in the following sections. The magnitude of response refers to both the severity of 

5 effects and the spatial extent of the U.S. which is affected.

6 i. Aquatic acidification

7 Freshwater ecosystem surveys and monitoring in the eastern United States have been 

8 conducted by many programs since the mid-1980s, including EPA’s Environmental Monitoring 

9 and Assessment Program (EMAP), National Surface Water Survey (NSWS), Temporally 

10 Integrated Monitoring of Ecosystems (TIME), and Long-term Monitoring (LTM) programs. 

11 Based on analyses of surface water data from these programs, New England, the Adirondack 

12 Mountains, the Appalachian Mountains (northern Appalachian Plateau and Ridge/Blue Ridge 

13 region), and the Upper Midwest contain the most sensitive lakes and streams (i.e., ANC less than 

14 about 50 μeq/L). Portions of northern Florida also contain many acidic and low-ANC lakes and 

15 streams, although the role of acidifying deposition in this region is less clear. The western U.S. 

16 contains many of the surface waters most sensitive to potential acidification effects, but with the 

17 exception of the Los Angeles Basin and surrounding areas, the levels of acidifying deposition are 

18 low in most areas. Therefore, acidification of surface waters by acidic deposition is not as 

19 prevalent in the western U.S., and the extent of chronic surface water acidification that has 

20 occurred in that region to date has likely been very limited relative to the Eastern U.S. (US EPA, 

21 2008, section 3.2.4.2 and US EPA, 2009, section 4.2.2).

22 There are a number of species including fish, aquatic insects, other invertebrates and 

23 algae that are sensitive to acidification and cannot survive, compete, or reproduce in acidic 
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1 waters (US EPA, 2008, section 3.2.3.3). Decreases in ANC and pH have been shown to 

2 contribute to declines in species richness and declines in abundance of zooplankton, 

3 macroinvertebrates, and fish. Reduced growth rates have been attributed to acid stress in a 

4 number of fish species including Atlantic salmon (Salmo salar), Chinook salmon (Oncorhynchus 

5 tshawytscha), lake trout (Salvelinus namaycush), rainbow trout (Oncorhynchis mykiss), brook 

6 trout (Salvelinus Fontinalis), and brown trout (Salmo trutta). In response to small to moderate 

7 changes in acidity, acid-sensitive species are often replaced by other more acid-tolerant species, 

8 resulting in changes in community composition and richness. The effects of acidification are 

9 continuous, with more species being affected at higher degrees of acidification. At a point, 

10 typically a pH <4.5 and an ANC <0 μeq/L, complete to near-complete loss of many taxa of 

11 organisms occur, including fish and aquatic insect populations, whereas other taxa are reduced to 

12 only acidophilic species. These changes in taxa composition are associated with the high energy

13 cost in maintaining physiological homeostasis, growth, and reproduction at low ANC levels (US 

14 EPA, 2008, section 3.2.3.3). Decreases in species richness related to acidification have been 

15 observed in the Adirondack Mountains and Catskill Mountains of New York, New England and 

16 Pennsylvania, and Virginia. From the sensitive areas identified by the ISA, further “case study” 

17 analyses on aquatic ecosystems in the Adirondack Mountains and Shenandoah National Park 

18 were conducted to better characterize ecological risk associated with acidification (US EPA, 

19 2009, section 4).

20 ANC is the most widely used indicator of acid sensitivity and has been found in various 

21 studies to be the best single indicator of the biological response and health of aquatic 

22 communities in acid-sensitive systems (Lien et al., 1992; Sullivan et al., 2006; US EPA, 2008). 

In the REA, surface water trends in SO4

2-

and NO3

-

23 concentrations and ANC levels were 
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1 analyzed to affirm the understanding that reductions in deposition could influence the risk of 

2 acidification. ANC values have been categorized according to their effects on biota, as shown in 

3 the table below. Monitoring data from TIME/LTM and EMAP programs were assessed for the 

4 years 1990 to 2006, and past, present, and future water quality levels were estimated by both 

5 steady-state and dynamic biogeochemical models.
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1

2 Table II-1. Ecological effects associated with alternative levels of acid neutralizing capacity 

3 (ANC). (source: USEPA, Acid Rain Program)

Category Label ANC Levels and Expected Ecological Effects

Acute 

Concern

<0 μeq/L Complete loss of fish populations is expected. Planktonic communities 

have extremely low diversity and are dominated by acidophilic taxa. The 

numbers of individuals in plankton species that are present are greatly 

reduced.

Severe 

Concern

0–20 μeq/L Highly sensitive to episodic acidification. During episodes of high 

acidifying deposition, brook trout populations may experience lethal 

effects. The diversity and distribution of zooplankton communities 

decline sharply. 

Elevated 

Concern

20–50 μeq/L Fish species richness is greatly reduced (i.e., more than half of expected 

species can be missing). On average, brook trout populations experience 

sublethal effects, including loss of health, ability to reproduce, and fitness. 

Diversity and distribution of zooplankton communities decline.

Moderate

Concern

50–100 

μeq/L

Fish species richness begins to decline (i.e., sensitive species are lost from 

lakes). Brook trout populations are sensitive and variable, with possible 

sublethal effects. Diversity and distribution of zooplankton communities 

also begin to decline as species that are sensitive to acidifying deposition 

are affected.

Low 

Concern

>100 μeq/L Fish species richness may be unaffected. Reproducing brook trout 

populations are expected where habitat is suitable. Zooplankton 

communities are unaffected and exhibit expected diversity and 

distribution.

4

5 Studies on fish species richness in the Adirondacks Case Study Area demonstrated the 

6 effect of acidification. Of the 53 fish species recorded in Adirondack Case Study Area lakes, 

7 only 27 species were found in lakes with a pH <6.0. The 26 species missing from lakes with a 

8 pH <6.0 include important recreational species, such as Atlantic salmon, tiger trout (Salmo trutta
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1 X Salvelinus fontinalis), redbreast sunfish (Lepomis auritus), bluegill (Lepomis macrochirus), 

2 tiger musky (Esox masquinongy X lucius), walleye (Sander vitreus), alewife (Alosa 

3 pseudoharengus), and kokanee (Oncorhynchus nerka), as well as ecologically important 

4 minnows that are commonly consumed by sport fish. A survey of 1,469 lakes in the late 1980s 

5 found 346 lakes to be devoid of fish. Among lakes with fish, there was a relationship between 

6 the number of fish species and lake pH, ranging from about one species per lake for lakes having 

7 a pH <4.5 to about six species per lake for lakes having a pH >6.5. In the Adirondacks, a 

8 positive relationship exists between the pH and ANC in lakes and the number of fish species 

9 present in those lakes (US EPA, 2008, section 3.2.3.4).

10 Since the mid-1990s, streams in the Shenandoah Case Study Area have shown slight 

declines in NO3

-

and SO4

2-

11 concentrations in surface waters. The 2006 concentrations are still 

12 above pre-acidification (1860) conditions. MAGIC modeling predicts surface water 

concentrations of NO3

-

and SO4

2-

13 are10- and 32-fold higher, respectively, in 2006 than in 1860. 

14 The estimated average ANC across 60 streams in the Shenandoah Case Study Area is 57.9 μeq/L 

15 (± 4.5 μeq/L). 55% of all monitored streams in the Shenandoah Case Study Area have a current 

16 risk of Elevated, Severe, or Acute. Of the 55%, 18% are chronically acidic today (US EPA, 

17 2009, section 4.2.4.3).

18 Based on a deposition scenario for this study area that maintains current emission levels 

19 from 2020 to 2050, the simulation forecast indicates that a large number of streams still have 

20 Elevated to Acute problems with acidity in 2050. In fact, from 2006 to 2050, the percentage of 

21 streams with Acute Concern is predicted to increase by 5%, while the percentage of streams in 

22 Moderate Concern decreases by 5%.
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1 Biological effects of increased acidification documented in the Shenandoah Case Study

2 Area include a decrease in the condition factor in blacknose dace and a decrease in fish 

3 biodiversity associated with decreasing stream ANC. On average, the fish species richness is 

4 lower by one fish species for every 21 μeq/L decrease in ANC in Shenandoah National Park 

5 streams (US EPA, 2008, section 3.2.3.4).

6 ii. Terrestrial acidification

7 The ISA identified a variety of indicators that can be used to measure the effects of 

8 acidification in soils. Most effects of terrestrial acidification are observed in sensitive forest 

9 ecosystem in the U.S. Tree health has been linked to the availability of base cations (Bc) in soil 

(such as Ca2+, Mg2+ 10 and potassium), as well as soil Al content. Tree species show a range of 

11 sensitivities to Ca/Al and Bc/Al soil molar ratios, therefore these are good chemical indicators 

12 because they directly relate to the biological effects. Critical Bc/Al molar ratios for a large 

13 variety of tree species ranged from 0.2 to 0.8. This range is similar to critical ratios of Ca/Al. 

14 Plant toxicity or nutrient antagonism was reported to occur at Ca/Al molar ratios ranging from 

15 0.2 to 2.5 (US EPA, 2009).

16 There has been no systematic national survey of terrestrial ecosystems to determine the 

17 extent and distribution of terrestrial ecosystem sensitivity to the effects of acidifying deposition. 

18 However, one preliminary national evaluation estimated that ~15% of forest ecosystems in the 

19 U.S. exceed the estimated critical load based on soil ANC leaching for S and N deposition by

20 >250 eq ha-1 yr-1 (McNulty et al., 2007). Forests of the Adirondack Mountains of New York, 

21 Green Mountains of Vermont, White Mountains of New Hampshire, the Allegheny Plateau of 

22 Pennsylvania, and high-elevation forest ecosystems in the southern Appalachians are the regions 

23 most sensitive to terrestrial acidification effects from acidifying deposition (US EPA, 2008, 
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1 section 3.2.4.2). While studies show some recovery of surface waters, there are widespread 

2 measurements of ongoing depletion of exchangeable base cations in forest soils in the 

3 northeastern U.S. despite recent decreases in acidifying deposition, indicating a slow recovery

4 time.

5 In the REA, a critical load analysis was performed for sugar maple and red spruce forests 

6 in the eastern United States by using Bc/Al ratio in acidified forest soils as an indicator to assess 

7 the impact of nitrogen and sulfur deposition on tree health. These are the two most commonly

8 studied tree species in North America for effects of acidification. At a Bc/Al ratio of 1.2, red 

9 spruce growth can be decreased by 20%. Sugar maple growth can be decreased by 20% at a 

10 Bc/Al ratio of 0.6 (US EPA, 2009, section 4.4). The REA analysis determined the health of at 

11 least a portion of the sugar maple and red spruce growing in the United States may have been 

12 compromised with acidifying total nitrogen and sulfur deposition. Specifically, total nitrogen and 

13 sulfur deposition levels exceeded three selected critical loads for tree growth in 3% to 75% of all 

14 sugar maple plots across 24 states--that is, it exceeded the highest (least stringent) of the three 

15 critical loads in 3% of plots, and the lowest (most stringent) in 75% of plots. For red spruce, total 

16 nitrogen and sulfur deposition levels exceeded three selected critical loads in 3% to 36% of all 

17 red spruce plots across eight states (US EPA, 2009, section 4.4). 

18 c. Key uncertainties associated with acidification

19 There are different levels of uncertainty associated with relationships between deposition, 

20 ecological effects and ecological indicators. In Chapter 7 of the REA, the case study analyses 

21 associated with each targeted effect area were synthesized by identifying the strengths, 

22 limitations, and uncertainties associated with the available data, modeling approach, and 

23 relationship between the selected ecological indicator and atmospheric deposition as described 
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1 by the ecological effect function (US EPA, 2009, Figure 1-1). A further discussion of 

2 uncertainty in aquatic and terrestrial ecosystems is presented below. The key uncertainties were 

3 characterized as follows to evaluate the strength of the scientific basis for setting a national 

4 standard to protect against a given effect (US EPA, 2009, section 7):

5 (1) Data Availability: high, medium or low quality. This criterion is based on the availability and 

6 robustness of data sets, monitoring networks, availability of data that allows for extrapolation 

7 to larger assessment areas, and input parameters for modeling and developing the ecological 

8 effect function. The scientific basis for the ecological indicator selected is also incorporated 

9 into this criterion.

10 (2) Modeling Approach: high, fairly high, intermediate, or low confidence. This value is based 

11 on the strengths and limitations of the models used in the analysis and how accepted they are 

12 by the scientific community for their application in this analysis.

13 (3) Ecological Effect Function: high, fairly high, intermediate, or low confidence. This ranking is 

14 based on how well the ecological effect function describes the relationship between 

15 atmospheric deposition and the ecological indicator of an effect.

16 i. Aquatic acidification

17 The REA concludes that the available data are robust and considered high quality. There 

18 is high confidence about the use of these data and their value for extrapolating to a larger 

19 regional population of lakes. The EPA TIME/LTM network represents a source of long-term, 

20 representative sampling. Data on sulfate concentrations, nitrate concentrations and ANC from 

21 1990 to 2006 used for this analysis as well as EPA EMAP and REMAP surveys, provide 

22 considerable data on surface water trends. 
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1 There is fairly high confidence associated with modeling and input parameters. 

2 Uncertainties in water quality estimates (i.e., ANC) from MAGIC were derived from multiple 

3 site calibrations. The 95% confidence interval for pre-acidification of lakes was an average of 15 

4 μeq/L difference in ANC concentrations or 10% and 8 μeq/L or 5% for streams (US EPA, 2009, 

5 section 7.1.2). The use of the critical load model to estimate aquatic critical loads is limited by

6 the uncertainties associated with runoff and surface water measurements and in estimating the 

7 catchment supply of base cations from the weathering of bedrock and soils (McNulty et al., 

8 2007). To quantify uncertainty in the model parameters, Monte Carlo methods were employed 

9 to develop an inverse function of exceedances. There is high confidence associated with the 

10 ecological effect function developed for aquatic acidification. In calculating the ANC function, 

11 the depositional load for N or S is fixed by the deposition of the other, so deposition for either 

12 will never be zero (US EPA, 2009, Figure 7.1-6).

13 ii. Terrestrial acidification 

14 The available data used to quantify the targeted effect of terrestrial acidification are 

15 robust and considered high quality. The USFS-Kane Experimental Forest and significant 

16 amounts of research work in the Allegheny Plateau have produced extensive, peer-reviewed data 

17 sets. A meta-analysis of laboratory studies showed that tree growth was decreased by 20% 

18 relative to controls for BC/Al ratios (US EPA, 2008, section 7.2.1 and Figure 7.2-1). Sugar 

19 maple and red spruce were the focus of the REA since they are demonstrated to be negatively 

affected by soil available Ca2+ 20 depletion and high concentrations of available Al, and occur in 

21 areas that receive high acidifying deposition, There is high confidence about the use of the REA 

22 terrestrial acidification data and their value for extrapolating to a larger regional population of 

23 forests. 
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1 There is high confidence associated with the models, input parameters, and assessment of 

2 uncertainty used in the case study for terrestrial acidification. The Simple Mass Balance (SMB) 

3 model, a commonly used and widely applied approach for estimating critical loads, was used in 

4 the REA analysis (US EPA, 2008, section 7.2.2). There is fairly high confidence associated with 

5 the ecological effect function developed for terrestrial acidification (US EPA, 2009, section

6 7.2.3).

7 3. Nutrient Enrichment Effects Associated with Deposition of Oxides of Nitrogen 

8 The following summary is a concise overview of the known or anticipated effects caused 

9 by nitrogen nutrient enrichment to ecosystems within the United States. Nutrient-enrichment 

10 affects terrestrial, freshwater and estuarine ecosystems. Nitrogen deposition is a major source of 

11 anthropogenic nitrogen. For many terrestrial and freshwater ecosystems other sources of 

12 nitrogen including fertilizer and waste treatment are greater than deposition. Nitrogen deposition 

13 often contributes to nitrogen-enrichment effects in estuaries, but does not drive the effects since 

14 other sources of N greatly exceed N deposition. Both oxides of nitrogen and reduced forms of 

15 nitrogen (NHx) contribute to nitrogen deposition. For the most part, nitrogen effects on 

16 ecosystems do not depend on whether the nitrogen is in oxidized or reduced form. Thus, this 

17 summary focuses on the effects of nitrogen deposition in total. 

18 a. Nature of nutrient enrichment-related ecosystem responses

19 The ISA found that deposition of nitrogen, including oxides of nitrogen and NHx, leads to 

20 the nitrogen enrichment of ecosystems (US EPA 2008). In the process of nitrogen enrichment, 

21 biogeochemical components of terrestrial and freshwater aquatic ecosystems are altered in a way

22 that leads to effects on biological organisms. 

23 i. Aquatic ecosystems
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1 In freshwater ecosystems, the evidence is sufficient to infer a causal relationship between 

2 N deposition and the alteration of biogeochemical cycling in freshwater aquatic ecosystems (US 

3 EPA, 2008, section 3.3.2.3). N deposition is the main source of N enrichment to headwater 

4 streams, lower order streams and high elevation lakes. The most common chemical indicators 

that were studied included NO3

−

5 and dissolved inorganic nitrogen (DIN) concentration in surface 

waters as well as Chl a:total P ratio. Elevated surface water NO3

−

6 concentrations occur in both 

7 the eastern and western U.S. Studies report a significant correlation between N deposition and

8 lake biogeochemistry by identifying a correlation between wet deposition and [DIN] and Chl a: 

9 Total P. Recent evidence provides examples of lakes and streams that are limited by N and show 

10 signs of eutrophication in response to N addition.

11 The evidence is sufficient to infer a causal relationship between N deposition and the 

12 alteration of species richness, species composition and biodiversity in freshwater aquatic 

13 ecosystems (US EPA, 2008, section 3.3.5.3). Increased N deposition can cause a shift in 

14 community composition and reduce algal biodiversity, especially in sensitive oligotrophic lakes.

15 In the ISA, the evidence is sufficient to infer a causal relationship between N deposition 

16 and the biogeochemical cycling of N and carbon (C) in estuaries (US EPA, 2008, section 4.3.4.1 

17 and 3.3.2.3). In general, estuaries tend to be nitrogen-limited, and many currently receive high 

18 levels of nitrogen input from human activities (US EPA, 2009, section 5.1.1). It is unknown if 

19 atmospheric deposition alone is sufficient to cause eutrophication; however, the contribution of 

20 atmospheric nitrogen deposition to total nitrogen load is calculated for some estuaries and can be 

21 >40% (US EPA, 2009, section 5.1.1).

22 The evidence is sufficient to infer a causal relationship between N deposition and the 

23 alteration of species richness, species composition and biodiversity in estuarine ecosystems (US 
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1 EPA, 2008, section 4.3.4.2 and 3.3.5.4). Atmospheric and non-atmospheric sources of N 

2 contribute to increased phytoplankton and algal productivity, leading to eutrophication. Shifts in 

3 community composition, reduced hypolimnetic DO, decreases in biodiversity, and mortality of 

4 submerged aquatic vegetation are associated with increased N deposition in estuarine systems. 

5 ii. Terrestrial Ecosystems

6 The evidence is sufficient to infer a causal relationship between N deposition and the 

7 alteration of biogeochemical cycling in terrestrial ecosystems (US EPA, 2008, section 4.3.1.1 

8 and 3.3.2.1). This is supported by numerous observational, deposition gradient and field addition 

9 experiments in sensitive ecosystems. The leaching of NO3- in soil drainage waters and the export 

10 of NO3- in stream water were identified as two of the primary indictors of N enrichment. Several 

11 N-addition studies indicate that NO3- leaching is induced by chronic additions of N. Studies 

12 identified in the ISA found that surface water NO3- concentrations exceeded 1 μeq/L in 

13 watersheds receiving about 9 to 13 kg N/ha/yr of atmospheric N deposition. N deposition 

14 disrupts the nutrient balance of ecosystems with numerous biogeochemical effects. The chemical 

indicators that are typically measured include NO3

−

15 leaching, soil C:N ratio, rates of N 

16 mineralization, nitrification, denitrification, foliar N concentration, and soil water NO3 − and 

NH4

+

17 concentrations. Note that N saturation (N leaching from ecosystems) does not need to 

18 occur to cause effects. Substantial leaching of NO3− from forest soils to stream water can acidify

19 downstream waters, leading to effects described in the previous section on aquatic acidification. 

20 Due to the complexity of interactions between the N and C cycling, the effects of N on C budgets 

21 (quantified input and output of C to the ecosystem) are variable. Regional trends in net 

22 ecosystem productivity (NEP) of forests (not managed for silviculture) have been estimated 

23 through models based on gradient studies and meta-analysis. Atmospheric N deposition has been 
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1 shown to cause increased litter accumulation and carbon storage in above-ground woody 

2 biomass. In the West, this has lead to increased susceptibility to more severe fires. Less is 

3 known regarding the effects of N deposition on C budgets of non-forest ecosystems.

4 The evidence is sufficient to infer a causal relationship between N deposition on the 

5 alteration of species richness, species composition and biodiversity in terrestrial ecosystems (US 

6 EPA, 2008, section 4.3.1.2). Some organisms and ecosystems are more sensitive to N deposition 

7 and effects of N deposition are not observed in all habitats. The most sensitive terrestrial taxa to 

8 N deposition are lichens. Empirical evidence indicates that lichens in the U.S. are affected by

9 deposition levels as low as 3 kg N/ha/yr. Alpine ecosystems are also sensitive to N deposition, 

10 changes in an individual species (Carex rupestris) were estimated to occur at deposition levels 

11 near 4 kg N /ha/yr and modeling indicates that deposition levels near 10 kg N/ha/yr alter plant 

12 community assemblages. In several grassland ecosystems, reduced species diversity and an 

13 increase in non-native, invasive species are associated with N deposition. 

14 iii. Ecosystem sensitivity to nutrient enrichment

15 The numerous ecosystem types that occur across the U.S. have a broad range of 

16 sensitivity to N deposition (US EPA, 2008, Table 4-4). Increased deposition to N-limited 

17 ecosystems can lead to production increases that may be either beneficial or adverse depending 

18 on the system and management goals. 

19 Organisms in their natural environment are commonly adapted to a specific regime of 

20 nutrient availability. Change in the availability of one important nutrient, such as N, may result 

21 in an imbalance in ecological stoichiometry, with effects on ecosystem processes, structure and 

22 function. In general, N deposition to terrestrial ecosystems causes accelerated growth rates in 

23 some species deemed desirable in commercial forests but may lead to altered competitive 
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1 interactions among species and nutrient imbalances, ultimately affecting biodiversity. The onset 

2 of these effects occurs with N deposition levels as low as 3 kg N/ha/yr in sensitive terrestrial 

3 ecosystems to N deposition. In aquatic ecosystems, N that is both leached from the soil and 

4 directly deposited to the water surface can pollute the surface water. This causes alteration of the 

5 diatom community at levels as low as 1.5 kg N/ha/yr in sensitive freshwater ecosystems. 

6 The degree of ecosystem effects lies at the intersection of N loading and N-sensitivity. 

7 N-sensitivity is predominately driven by the degree to which growth is limited by nitrogen 

8 availability. Grasslands in the western United States are typically N-limited ecosystems 

9 dominated by a diverse mix of perennial forbs and grass species. A meta-analysis discussed in 

10 the ISA (US EPA, 2008, section 3.3.3), indicated that N fertilization increased aboveground 

11 growth in all non-forest ecosystems except for deserts. In other words, almost all terrestrial 

12 ecosystems are N-limited and will be altered by the addition of anthropogenic nitrogen. 

13 Likewise, a freshwater lake or stream must be N-limited to be sensitive to N-mediated 

14 eutrophication. There are many examples of fresh waters that are N-limited or N and 

15 phosphorous (P) co-limited (US EPA, 2008, section 3.3.3.2). A large dataset meta-analysis 

16 discussed in the ISA (US EPA, 2008, section 3.3.3.2), found that N-limitation occurred as 

17 frequently as P-limitation in freshwater ecosystems. Additional factors that govern the 

18 sensitivity of ecosystems to nutrient enrichment from N deposition include rates and form of N 

19 deposition, elevation, climate, species composition, plant growth rate, length of growing season, 

20 and soil N retention capacity (US EPA, 2008, section 4.3). Less is known about the extent and 

21 distribution of the terrestrial ecosystems in the U.S. that are most sensitive to the effects of 

22 nutrient enrichment from atmospheric N deposition compared to acidification.
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1 Because the productivity of estuarine and near shore marine ecosystems is generally 

2 limited by the availability of N, they are susceptible to the eutrophication effect of N deposition 

3 (US EPA, 2008, section 4.3.4.1). A recent national assessment of eutrophic conditions in 

4 estuaries found the most eutrophic estuaries were generally those that had large watershed-to5 estuarine surface area, high human population density, high rainfall and runoff, low dilution, and 

6 low flushing rates. In the REA, the National Oceanic and Atmospheric Administration’s 

7 (NOAA) National Estuarine Eutrophication Assessment (NEEA) assessment tool, Assessment of 

8 Estuarine Tropic Status (ASSETS) categorical Eutrophication Index (EI) was used to evaluate 

9 eutrophication due to atmospheric loading of nitrogen. ASSETS EI is an estimation of the 

10 likelihood that an estuary is experiencing eutrophication or will experience eutrophication based 

11 on five ecological indicators: chlorophyll a, macroalgae, dissolved oxygen, nuisance/toxic algal 

12 blooms and submerged aquatic vegetation (SAV). 

13 In the REA, two regions were selected for case study analysis using ASSETS EI, the 

14 Chesapeake Bay and Pamlico Sound. Both regions received an ASSETS EI rating of Bad

15 indicating that the estuary had moderate to high pressure due to overall human influence and a 

16 moderate high to high eutrophic condition (US EPA, 2009, sections 5.2.4.1 and 5.2.4.2). These 

17 results were then considered with SPAtially Referenced Regression (SPARROW) modeling to 

18 develop a response curve to examine the role of atmospheric nitrogen deposition in achieving a 

19 desired decrease in load. To change the Neuse River Estuary’s EI score from Bad to Poor not 

20 only must 100% of the total atmospheric nitrogen deposition be eliminated, but considerably 

21 more nitrogen from other sources as well must be controlled (US EPA, 2009, section 5.2.7.2). In 

22 the Potomac River estuary, a 78% decrease of total nitrogen could move the EI score from Bad

23 to Poor (US EPA, 2009, section 5.2.7.1). The results of this analysis indicated decreases in 
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1 atmospheric deposition alone could not eliminate coastal eutrophication problems due to 

2 multiple non-atmospheric nitrogen inputs (US EPA, 2009, section 7.3.3). However, the 

3 somewhat arbitrary discreteness of the EI scale can mask the benefits of decreases in nitrogen 

4 between categories.

5 In general, estuaries tend to be N-limited, and many currently receive high levels of N 

6 input from human activities to cause eutrophication. As reported in the ISA (US EPA, 2008, 

7 section 3.2.2.2), atmospheric N loads to estuaries in the U.S. are estimated to range from 2-8% 

8 for Guadalupe Bay, TX on the lowest end to as high as 72% for St Catherines-Sapelo estuary, 

9 GA. The Chesapeake Bay is an example of a large, well-studied and severely eutrophic estuary

10 that is calculated to receive as much as 30% of its total N load from the atmosphere.

11 b. Magnitude of ecosystem responses

12 i. Aquatic ecosystems

13 The magnitude of ecosystem response may be thought of on two time scales, current 

14 conditions and how ecosystems have been altered since the onset of anthropogenic N deposition. 

15 As noted previously, studies found that N-limitation occurs as frequently as P-limitation in 

16 freshwater ecosystems (US EPA, 2008, section 3.3.3.2). Recently, a comprehensive study of 

17 available data from the northern hemisphere surveys of lakes along gradients of N deposition 

18 show increased inorganic N concentration and productivity to be correlated with atmospheric N 

19 deposition. The results are unequivocal evidence of N limitation in lakes with low ambient inputs 

20 of N, and increased N concentrations in lakes receiving N solely from atmospheric N deposition. 

21 It has been suggested that most lakes in the northern hemisphere may have originally been N22 limited, and that atmospheric N deposition has changed the balance of N and P in lakes.
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1 Available data suggest that the increases in total N deposition do not have to be large to 

2 elicit an ecological effect. For example, a hindcasting exercise determined that the change in 

3 Rocky Mountain National Park lake algae that occurred between 1850 and 1964 was associated 

4 with an increase in wet N deposition that was only about 1.5 kg N/ha. Similar changes inferred 

5 from lake sediment cores of the Beartooth Mountains of Wyoming also occurred at about 1.5 kg 

6 N/ha deposition. Pre-industrial inorganic N deposition is estimated to have been only 0.1 to 0.7 

7 kg N/ha based on measurements from remote parts of the world. In the western U.S., pre8 industrial, or background, inorganic N deposition was estimated by to range from 0.4 to 0.7 kg 

9 N/ha/yr.

10 Eutrophication effects from N deposition are most likely to be manifested in undisturbed, 

11 low nutrient surface waters such as those found in the higher elevation areas of the western U.S. 

12 The most severe eutrophication from N deposition effects is expected downwind of major urban 

and agricultural centers. High concentrations of lake or streamwater NO3

−

13 , indicative of 

14 ecosystem saturation, have been found at a variety of locations throughout the U.S., including the 

15 San Bernardino and San Gabriel Mountains within the Los Angeles Air Basin, the Front Range 

16 of Colorado, the Allegheny mountains of West Virginia, the Catskill Mountains of New York, 

17 the Adirondack Mountains of New York, and the Great Smoky Mountains in Tennessee (US 

18 EPA, 2008, section 3.3.8).

19 In contrast to terrestrial and freshwater systems, atmospheric N load to estuaries 

20 contributes to the total load but does not necessarily drive the effects since other combined 

21 sources of N often greatly exceed N deposition. In estuaries, N-loading from multiple 

22 anthropogenic and non-anthropogenic pathways leads to water quality deterioration, resulting in 

23 numerous effects including hypoxic zones, species mortality, changes in community composition 
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1 and harmful algal blooms that are indicative of eutrophication. The following summary is a 

2 concise overview of the known or anticipated effects of nitrogen enrichment on estuaries within 

3 the United States.

4 There is a scientific consensus (US EPA, 2008, section 4.3.4) that nitrogen-driven 

5 eutrophication in shallow estuaries has increased over the past several decades and that the 

6 environmental degradation of coastal ecosystems due to nitrogen, phosphorus, and other inputs is 

7 now a widespread occurrence. For example, the frequency of phytoplankton blooms and the 

8 extent and severity of hypoxia have increased in the Chesapeake Bay and Pamlico estuaries in 

9 North Carolina and along the continental shelf adjacent to the Mississippi and Atchafalaya 

10 rivers’ discharges to the Gulf of Mexico. 

11 A recent national assessment of eutrophic conditions in estuaries found that 65% of the 

12 assessed systems had moderate to high overall eutrophic conditions and generally received the 

13 greatest N loads from all sources, including atmospheric and land-based sources. Most eutrophic 

14 estuaries occurred in the mid-Atlantic region and the estuaries with the lowest degree of 

15 eutrophication were in the North Atlantic. Other regions had mixtures of low, moderate, and high 

16 degrees of eutrophication (US EPA, 2008, section 4.3.4.3).

17 The mid-Atlantic region is the most heavily impacted area in terms of moderate or high 

18 loss of submerged aquatic vegetation due to eutrophication (US EPA, 2008, section 4.3.4.2). 

19 Submerged aquatic vegetation is important to the quality of estuarine ecosystem habitats because 

20 it provides habitat for a variety of aquatic organisms, absorbs excess nutrients, and traps 

21 sediments (US EPA, 2008, section 4.3.4.2). It is partly because many estuaries and near-coastal 

22 marine waters are degraded by nutrient enrichment that they are highly sensitive to potential 

23 negative impacts from nitrogen addition from atmospheric deposition.
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1 ii. Terrestrial ecosystems

2 Little is known about the full extent and distribution of the terrestrial ecosystems in the 

3 U.S. that are most sensitive to impacts caused by nutrient enrichment from atmospheric N 

4 deposition. As previously stated, most terrestrial ecosystems are N-limited, therefore they are 

5 sensitive to perturbation caused by N additions (US EPA, 2008, section 4.3.1). Effects are most 

6 likely to occur where areas of relatively high atmospheric N deposition intersect with N-limited 

7 plant communities. The alpine ecosystems of the Colorado Front Range, chaparral watersheds of 

8 the Sierra Nevada, lichen and vascular plant communities in the San Bernardino Mountains and 

9 the Pacific Northwest, and the southern California coastal sage scrub (CSS) community are 

10 among the most sensitive terrestrial ecosystems. There is growing evidence (US EPA, 2008, 

11 section 4.3.1.2) that existing grassland ecosystems in the western United States are being altered 

12 by elevated levels of N inputs, including inputs from atmospheric deposition.

13 In the eastern U.S., the degree of N saturation of the terrestrial ecosystem is often 

assessed in terms of the degree of NO3

−

14 leaching from watershed soils into ground water or 

15 surface water. Studies have estimated the number of surface waters at different stages of 

16 saturation across several regions in the eastern U.S. Of the 85 northeastern watersheds examined 

17 60% were in Stage 1 or Stage 2 of N saturation on a scale of 0 (background or pretreatment) to 3 

18 (visible decline). Of the northeastern sites for which adequate data were available for assessment, 

19 those in Stage 1 or 2 were most prevalent in the Adirondack and Catskill Mountains. Effects on 

20 individual plant species have not been well studied in the U.S. More is known about the 

21 sensitivity of particular plant communities. Based largely on results obtained in more extensive 

22 studies conducted in Europe, it is expected that the more sensitive terrestrial ecosystems include 
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1 hardwood forests, alpine meadows, arid and semi-arid lands, and grassland ecosystems (US 

2 EPA, 2008, section 3.3.5).

3 The REA used published research results (US EPA, 2009, section 5.3.1 and US EPA, 

4 2008, Table 4.4) to identify meaningful ecological benchmarks associated with different levels of 

5 atmospheric nitrogen deposition. These are illustrated in Figure 3-4 of the Policy Assessment. 

6 The sensitive areas and ecological indicators identified by the ISA were analyzed further in the 

7 REA to create a national map that illustrates effects observed from ambient and experimental 

8 atmospheric nitrogen deposition loads in relation to Community Multi-scale Air Quality 

9 (CMAQ) 2002 modeling results and NADP monitoring data. This map, reproduced in Figure 3-

10 5 of the Policy Assessment, depicts the sites where empirical effects of terrestrial nutrient 

11 enrichment have been observed and site proximity to elevated atmospheric N deposition. 

12 Based on information in the ISA and initial analysis in the REA, further case study

13 analyses on terrestrial nutrient enrichment of ecosystems were developed for the CCS 

14 community and Mixed Conifer Forest (MCF) (US EPA, 2009). Geographic information systems 

15 (GIS) analysis supported a qualitative review of past field research to identify ecological 

16 benchmarks associated with CSS and mycorrhizal communities, as well as MCF nutrient17 sensitive acidophyte lichen communities, fine-root biomass in Ponderosa pine, and leached 

18 nitrate in receiving waters. 

19 The ecological benchmarks that were identified for the CSS and the MCF communities 

20 are included in the suite of benchmarks identified in the ISA (US EPA, 2008, section 3.3). There 

21 are sufficient data to confidently relate the ecological effect to a loading of atmospheric nitrogen. 

22 For the CSS community, the following ecological benchmarks were identified:
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1 (1) 3.3 kg N/ha/yr – the amount of nitrogen uptake by a vigorous stand of CSS; above this level, 

2 nitrogen may no longer be limiting

3 (2) 10 kg N/ha/yr – mycorrhizal community changes

4 For the MCF community, the following ecological benchmarks were identified:

5 (1) 3.1 kg N/ha/yr – shift from sensitive to tolerant lichen species

6 (2) 5.2 kg N/ha/yr – dominance of the tolerant lichen species

7 (3) 10.2 kg N/ha/yr – loss of sensitive lichen species

8 (4) 17 kg N/ha/yr – leaching of nitrate into streams.

9 These benchmarks, ranging from 3.1 to 17 kg N/ha/yr, were compared to 2002 

10 CMAQ/NADP data to discern any associations between atmospheric deposition and changing 

11 communities. Evidence supports the finding that nitrogen alters CSS and MCF communities. 

12 Key findings include the following: 2002 CMAQ/NADP nitrogen deposition data show that the 

13 3.3 kg N/ha/yr benchmark has been exceeded in more than 93% of CSS areas (654,048 ha). 

14 These deposition levels are a driving force in the degradation of CSS communities. Although 

15 CSS decline has been observed in the absence of fire, the contributions of deposition and fire to 

16 the CSS decline require further research. CSS is fragmented into many small parcels, and the 

17 2002 CMAQ/NADP 12-km grid data are not fine enough to fully validate the relationship 

18 between CSS distribution, nitrogen deposition, and fire. 2002 CMAQ/NADP nitrogen deposition 

19 data exceeds the 3.1 kg N/ha/yr benchmark in more than 38% (1,099,133 ha) of MCF areas, and 

20 nitrate leaching has been observed in surface waters. Ozone effects confound nitrogen effects on 

21 MCF acidophyte lichen, and the interrelationship between fire and nitrogen cycling requires 

22 additional research.
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1 c. Key uncertainties associated with nutrient enrichment

2 There are different levels of uncertainty associated with relationships between deposition, 

3 ecological effects and ecological indicators. The criteria used in the REA to evaluate the degree 

4 of confidence in the data, modeling and ecological effect function are detailed in Chapter 7 of the 

5 REA. Below is a discussion of uncertainty relating aquatic and terrestrial ecosystems to nutrient 

6 enrichment effects.

7 i. Aquatic ecosystems 

8 The approach for assessing atmospheric contributions to total nitrogen loading in the 

9 REA was to consider the main-stem river to an estuary (including the estuary) rather than an 

10 entire estuary system or bay. The biological indicators used in the NOAA ASSETS EI required 

11 the evaluation of many national databases including the US Geological Survey National Water 

12 Quality Assessment (NAWQA) files, EPA’s STORage and RETrieval (STORET) database, 

13 NOAA’s Estuarine Drainage Areas data, and EPA’s water quality standards nutrient criteria for 

14 rivers and lakes (US EPA, 2009, Appendix 6 and Table 1.2.-1). Both the SPARROW modeling 

15 for nitrogen loads and assessment of estuary conditions under NOAA ASSETS EI, have been 

16 applied on a national scale. The REA concludes that the available data are medium quality with 

17 intermediate confidence about the use of these data and their values for extrapolating to a larger 

18 regional area (US EPA, 2009, section 7.3.1). Intermediate confidence is associated with the 

19 modeling approach using ASSETS EI and SPARROW. The REA states there is low confidence 

20 with the ecological effect function due to the results of the analysis which indicated that 

21 reductions in atmospheric deposition alone could not solve coastal eutrophication problems due 

22 to multiple non-atmospheric nitrogen inputs (US EPA, 2009, section 7.3.3).
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1 ii. Terrestrial ecosystems

2 Ecological thresholds are identified for CSS and MCF areas and these data are considered 

3 to be of high quality, however, the ability to extrapolate these data to larger regional areas is 

4 limited (US EPA, 2009, section 7.4.1). No quantitative modeling was conducted or ecological 

5 effect function developed for terrestrial nutrient enrichment reflecting the uncertainties 

6 associated with these depositional effects. 

7 4. Other Ecological Effects 

8 It is stated in the ISA (US EPA, 2008, section 3.4.1 and 4.5) that mercury is a highly 

9 neurotoxic contaminant that enters the food web as a methylated compound, methylmercury. 

10 Mercury is principally methylated by sulfur-reducing bacteria and can be taken up by

11 microorganisms, zooplankton and macroinvertebrates. The contaminant is concentrated in higher 

12 trophic levels, including fish eaten by humans. Experimental evidence has established that only 

13 inconsequential amounts of methylmercury can be produced in the absence of sulfate. Once 

14 methylmercury is present, other variables influence how much accumulates in fish, but elevated 

15 mercury levels in fish can only occur where substantial amounts of methylmercury are present. 

16 Current evidence indicates that in watersheds where mercury is present, increased oxides of 

17 sulfur deposition very likely results in additional production of methylmercury which leads to 

18 greater accumulation of MeHg concentrations in fish. With respect to sulfur deposition and 

19 mercury methylation, the final ISA determined: The evidence is sufficient to infer a causal 

20 relationship between sulfur deposition and increased mercury methylation in wetlands and 

21 aquatic environments. 

22 The production of meaningful amounts of methylmercury (MeHg) requires the presence 

of SO4

2-

and mercury, and where mercury is present, increased availability of SO4

2-

23 results in 
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1 increased production of MeHg. There is increasing evidence on the relationship between sulfur 

2 deposition and increased methylation of mercury in aquatic environments; this effect occurs only 

3 where other factors are present at levels within a range to allow methylation. The production of 

4 methylmercury requires the presence of sulfate and mercury, but the amount of methylmercury

5 produced varies with oxygen content, temperature, pH, and supply of labile organic carbon (US 

6 EPA, 2008, section 3.4). In watersheds where changes in sulfate deposition did not produce an 

7 effect, one or several of those interacting factors were not in the range required for meaningful 

8 methylation to occur (US EPA, 2008, section 3.4). Watersheds with conditions known to be 

9 conducive to mercury methylation can be found in the northeastern United States and 

10 southeastern Canada. 

11 While the relationship between sulfur and methylmercury production was concluded to 

12 be causal in the ISA, the REA concluded that there was insufficient evidence to quantify the 

13 relationship between sulfur and methylmercury. Therefore only a qualitative assessment was 

14 included in Chapter 6 of the REA. The Policy Assessment was then unable to make a 

15 determination as to the adequacy of the existing SO2 standards in protecting against welfare 

16 effects associated with increased mercury methylation.

17 B. Risk and Exposure Assessment 

18 The risk and exposure assessment conducted for the current review was developed to 

19 describe potential risk from current and future deposition of oxides of nitrogen and sulfur to 

20 sensitive ecosystems. The case study analyses in the REA show that there is confidence that 

21 known or anticipated adverse ecological effects are occurring under current ambient loadings of 

22 nitrogen and sulfur in sensitive ecosystems across the United States. An overview of the 

23 material covered in the REA, a summary of the key findings from the air quality analyses, 
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1 acidification and nutrient enrichment case studies, and general conclusions from evaluating 

2 additional welfare effects, are presented below.

3 1. Overview of the Risk and Exposure Assessment

4 The REA evaluates the relationships between atmospheric concentrations, deposition, 

5 biologically relevant exposures, targeted ecosystem effects, and ecosystem services. To evaluate 

6 the nature and magnitude of adverse effects associated with deposition, the REA also examines 

7 various ways to quantify the relationships between air quality indicators, deposition of 

8 biologically available forms of nitrogen and sulfur, ecologically relevant indicators relating to 

9 deposition, exposure and effects on sensitive receptors, and related effects resulting in changes in 

10 ecosystem structure and services. The intent is to determine the exposure metrics that incorporate 

11 the temporal considerations (i.e., biologically relevant timescales), pathways, and ecologically 

12 relevant indicators necessary to determine the effects on these ecosystems. To the extent feasible, 

13 the REA evaluates the overall load to the system for nitrogen and sulfur, as well as the variability

14 in ecosystem responses to these pollutants. It also evaluates the contributions of atmospherically 

15 deposited nitrogen and sulfur individually relative to the combined atmospheric loadings of both 

16 elements together. Since oxidized nitrogen is the listed criteria pollutant (currently measured by 

17 the ambient air quality indicator NO2) for the atmospheric contribution to total nitrogen, the REA 

18 examines the contribution of nitrogen oxides to total reactive nitrogen in the atmosphere, relative 

19 to the contributions of reduced forms of nitrogen (e.g., ammonia, ammonium), to ultimately

20 assess how a meaningful secondary National Ambient Air Quality Standards (NAAQS) might be 

21 structured.

22 The REA focuses on ecosystem welfare effects that result from the deposition of total 

23 reactive nitrogen and sulfur. Because ecosystems are diverse in biota, climate, geochemistry, and 
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1 hydrology, response to pollutant exposures can vary greatly between ecosystems. In addition, 

2 these diverse ecosystems are not distributed evenly across the United States. To target nitrogen 

3 and sulfur acidification and nitrogen and sulfur enrichment, the REA addresses four main 

4 targeted ecosystem effects on terrestrial and aquatic systems identified by the ISA (US EPA, 

5 2008): aquatic acidification due to nitrogen and sulfur; terrestrial acidification due to nitrogen 

6 and sulfur; aquatic nutrient enrichment, including eutrophication; and terrestrial nutrient 

7 enrichment.

8 In addition to these four targeted ecosystem effects, the REA also qualitatively addresses 

9 the influence of sulfur oxides deposition on methylmercury production; nitrous oxide (N2O) 

10 effects on climate; nitrogen effects on primary productivity and biogenic greenhouse gas fluxes; 

11 and phytotoxic effects on plants. 

12 Because the targeted ecosystem effects outlined above are not evenly distributed across 

13 the United States, the REA identified case studies for each targeted effects based on ecosystems 

14 identified as sensitive to nitrogen and/or sulfur deposition effects. Eight case study areas and 

15 two supplemental study areas (Rocky Mountain National Park and Little Rock Lake, WI) are 

16 summarized in the REA based on ecosystem characteristics, indicators, and ecosystem service 

17 information. Case studies selected for aquatic acidification effects were the Adirondack 

18 Mountains and Shenandoah National Park. Kane Experimental Forest in Pennsylvania and 

19 Hubbard Brook Experimental Forest in New Hampshire were selected as case studies for 

20 terrestrial acidification. Aquatic nutrient enrichment case study locations were selected in the 

21 Potomac River Basin upstream of Chesapeake Bay and the Neuse River Basin upstream of the 

22 Pamlico Sound in North Carolina. The Coastal Sage Scrub Communities in southern California 

23 and the Mixed Confer Forest Communities in the San Bernardino and Sierra Nevada Mountains 
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1 of California were selected as case studies for terrestrial nutrient enrichment. Two supplemental 

2 areas were also chosen, one in Rocky Mountain National Park for terrestrial nutrient enrichment 

3 and one in Little Rock Lake, Wisconsin for aquatic nutrient enrichment.

4 2. Key findings

5 In summary, based on case study analyses, the REA concludes that known or anticipated 

6 adverse ecological effects are occurring under current conditions and further concludes that these 

7 adverse effects continue into the future. Key findings from the air quality analyses, acidification 

8 and nutrient enrichment case studies, as well as general conclusions from evaluating additional 

9 welfare effects, are summarized below.

10 a. Air quality analyses

11 The air quality analyses in the REA encompass the current emissions sources of nitrogen 

12 and sulfur, as well as atmospheric concentrations, estimates of deposition of total nitrogen, 

13 policy-relevant background, and non-ambient loadings of nitrogen and sulfur to ecosystems, both 

14 nationwide and in the case study areas. Spatial fields of deposition were created using wet 

15 deposition measurements from the National Atmospheric Deposition Program (NADP) National 

16 Trends Network and dry deposition predictions from the 2002 Community Multi-Scale Air 

17 Quality (CMAQ) model simulation. Some key conclusions from this analysis are:

18 (1) Total reactive nitrogen deposition and sulfur deposition are much greater in the East 

19 compared to most areas of the West.

20 (2) These regional differences in deposition correspond to the regional differences in oxides of 

21 nitrogen and SO2 concentrations and emissions, which are also higher in the East. Oxides of 

22 nitrogen emissions are much greater and generally more widespread than NH3 emissions 

23 nationwide; high NH3 emissions tend to be more local (e.g., eastern North Carolina) or sub-
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1 regional (e.g., the upper Midwest and Plains states). The relative amounts of oxidized versus 

2 reduced nitrogen deposition are consistent with the relative amounts of oxides of nitrogen

3 and NH3 emissions. Oxidized nitrogen deposition exceeds reduced nitrogen deposition in 

4 most of the case study areas; the major exception being the Neuse River/Neuse River Estuary

5 Case Study Area.

6 (3) Reduced nitrogen deposition exceeds oxidized nitrogen deposition in the vicinity of local 

7 sources of NH3.

8 (4) There can be relatively large spatial variations in both total reactive nitrogen deposition and

9 sulfur deposition within a case study area; this occurs particularly in those areas that contain 

10 or are near a high emissions source of oxides of nitrogen, NH3, and/or SO2.

11 (5) The seasonal patterns in deposition differ between the case study areas. For the case study 

12 areas in the East, the season with the greatest amounts of total reactive nitrogen deposition 

13 correspond to the season with the greatest amounts of sulfur deposition. Deposition peaks in 

14 spring in the Adirondack, Hubbard Brook Experimental Forest, and Kane Experimental 

15 Forest case study areas, and it peaks in summer in the Potomac River/Potomac Estuary, 

16 Shenandoah, and Neuse River/Neuse River Estuary case study areas. For the case study areas 

17 in the West, there is less consistency in the seasons with greatest total reactive nitrogen and 

18 sulfur deposition in a given area. In general, both nitrogen and/or sulfur deposition peaks in 

19 spring or summer. The exception to this is the Sierra Nevada Range portion of the Mixed 

20 Conifer Forest Case Study Area, in which sulfur deposition is greatest in winter.

21 b. Deposition-related aquatic acidification

22 The role of aquatic acidification in two eastern United States areas—northeastern New 

23 York’s Adirondack area and the Shenandoah area in Virginia—was analyzed in the REA to 
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assess surface water trends in SO4

2-

and NO3

-

1 concentrations and acid neutralizing capacity (ANC) 

2 levels and to affirm the understanding that reductions in deposition could influence the risk of 

3 acidification. Monitoring data from the EPA-administered Temporally Integrated Monitoring of 

4 Ecosystems (TIME)/Long-Term Monitoring (LTM) programs and the Environmental Monitoring 

5 and Assessment Program (EMAP) were assessed for the years 1990 to 2006, and past, present, 

6 and future water quality levels were estimated using both steady-state and dynamic 

7 biogeochemical models. 

8 Although wet deposition rates for SO2 and oxides of nitrogen in the Adirondack Case 

9 Study Area have reduced since the mid-1990s, current concentrations are still well above pre10 acidification (1860) conditions. Model of Acidification of Groundwater in Catchments (MAGIC) 

modeling predicts NO3

-

and SO4

2-

11 are 17- and 5-fold higher today, respectively. The estimated 

12 average ANC for 44 lakes in the Adirondack Case Study Area is 62.1 microequivalents per liter 

13 (μeq/L) (± 15.7 μeq/L); 78 % of all monitored lakes in the Adirondack Case Study Area have a 

14 current risk of Elevated, Severe, or Acute. Of the 78%, 31% experience episodic acidification, 

15 and 18% are chronically acidic today.

16 (1) Based on the steady-state critical load model for the year 2002, 18%, 28%, 44%, and 58% of 

17 169 modeled lakes received combined total sulfur and nitrogen deposition that exceeded

18 critical loads corresponding to ANC limits of 0, 20, 50, and 100 μeq/L respectively.

19 (2) Based on a deposition scenario that maintains current emission levels to 2020 and 2050, the 

20 simulation forecast indicates no improvement in water quality in the Adirondack Case Study 

21 Area. The percentage of lakes within the Elevated to Acute Concern classes remains the same 

22 in 2020 and 2050.
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1 (3) Since the mid-1990s, streams in the Shenandoah Case Study Area have shown slight declines 

in NO3 and SO4

2-

2 concentrations in surface waters. ANC levels increased from about 50 

3 μeq/L in the early 1990s to >75 μeq/L until 2002, when ANC levels declined back to 1991–

4 1992 levels. Current concentrations are still above pre-acidification (1860) conditions. 

MAGIC modeling predicts surface water concentrations of NO3 and SO4

2-

5 are 10- and 32-

6 fold higher today, respectively. The estimated average ANC for 60 streams in the 

7 Shenandoah Case Study Area is 57.9 μeq/L (± 4.5 μeq/L). 55% of all monitored streams in 

8 the Shenandoah Case Study Area have a current risk of Elevated, Severe, or Acute. Of the 

9 55%, 18% experience episodic acidification, and 18% are chronically acidic today.

10 (4) Based on the steady-state critical load model for the year 2002, 52%, 72%, 85%, and 93% of 

11 60 modeled streams received combined total sulfur and nitrogen deposition that exceeded 

12 critical loads corresponding to ANC limits of 0, 20, 50, and 100 μeq/L respectively.

13 (5) Based on a deposition scenario that maintains current emission levels to 2020 and 2050, the 

14 simulation forecast indicates that a large number of streams still have Elevated to Acute 

15 problems with acidity. In fact, from 2006 to 2050, the percentage of streams with Acute 

16 Concern increases by 5%, while the percentage of streams in Moderate Concern decreases 

17 by 5%.

18 c. Deposition-related terrestrial acidification

19 The role of terrestrial acidification was examined in the REA using a critical load 

20 analysis for sugar maple and red spruce forests in the eastern United States by using the base 

21 cation to aluminum (Bc/Al) ratio in acidified forest soils as an indicator to assess the impact of 

22 nitrogen and sulfur deposition on tree health. These are the two most commonly studied species 

23 in North America for impacts of acidification. At a Bc/Al ratio of 1.2, red spruce growth can be 
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1 reduced by 20%. Sugar maple growth can be reduced by 20% at a Bc/Al ratio of 0.6. Key 

2 findings of the case study are summarized below.

3 (1) Case study results suggest that the health of at least a portion of the sugar maple and red 

4 spruce growing in the United States may have been compromised with acidifying total 

5 nitrogen and sulfur deposition in 2002. 2002 CMAQ/NADP total nitrogen and sulfur 

6 deposition levels exceeded three selected critical loads in 3% to 75% of all sugar maple plots 

7 across 24 states. The three critical loads ranged from 107 to 6,008 eq/ha/yr for the Bc/Al 

8 ratios of 0.6, 1.2, and 10.0 (increasing levels of tree protection). The 2002 CMAQ/NADP 

9 total nitrogen and sulfur deposition levels exceeded three selected critical loads in 3% to 36% 

10 of all red spruce plots across 8 states. The three critical loads ranged from 180 to 4,278 

11 eq/ha/yr for the Bc/Al ratios of 0.6, 1.2, and 10.0 (increasing levels of tree protection).

12 (2) The Simple Mass Balance model assumptions made for base cation weathering (Bcw) and 

13 forest soil ANC input parameters are the main sources of uncertainty since these parameters 

14 are rarely measured and require researchers to use default values. Bcw contributed 49% to 

15 the total variability in the critical load estimates, and forest soil ANC contributed 46% to the 

16 total variability.

17 (3) The pattern of case study results suggests that nitrogen and sulfur acidifying deposition in the 

18 sugar maple and red spruce forest areas studied were very close to, if not greater than, the 

19 critical loads for those areas and both ecosystems are likely to be sensitive to any future 

20 changes in the levels of deposition.

21 d. Deposition-related aquatic nutrient enrichment

22 The role of nitrogen deposition in two main stem rivers feeding their respective estuaries 

23 was analyzed in the REA to determine if decreases in deposition could influence the risk of 
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1 eutrophication as predicted using the Assessment of Estuarine Trophic Status eutrophication 

2 index (ASSETS EI) scoring system in tandem with SPAtially Referenced Regression on 

3 Watershed Attributes (SPARROW) modeling. This modeling approach provides a transferrable, 

4 intermediate-level analysis of the linkages between atmospheric deposition and receiving waters, 

5 while providing results on which conclusions could be drawn. Future application of the methods 

6 to case study areas where atmospheric deposition plays a larger role in the nitrogen loading to an 

7 estuary will likely provide more tangible results. A summary of findings follows:

8 (1) 2002 CMAQ/NADP results showed that an estimated 40,770,000 kg of total nitrogen was 

9 deposited in the Potomac River watershed. SPARROW modeling predicted that 7,380,000 kg 

10 N/yr of the deposited nitrogen reached the estuary (20% of the total load to the estuary). The 

11 overall ASSETS EI for the Potomac River and Potomac Estuary was Bad (based on all 

12 sources of N). 

13 (2) To improve the Potomac River and Potomac Estuary ASSETS EI score from Bad to Poor, 

14 there is a slim chance that a decrease of at least 78% in the 2002 total nitrogen atmospheric 

15 deposition load to the watershed would be required.

16 (3) 2002 CMAQ/NADP results showed that an estimated 18,340,000 kg of total nitrogen was 

17 deposited in the Neuse River watershed. SPARROW modeling predicted that 1,150,000 kg 

18 N/yr of the deposited nitrogen reached the estuary (26% of the total load to the estuary). The 

19 overall ASSETS EI for the Neuse River/Neuse River Estuary was Bad.

20 (4) It was found that the Neuse River/Neuse River Estuary ASSETS EI score could not be 

21 improved from Bad to Poor with decreases only in the 2002 atmospheric deposition load to 

22 the watershed. Additional reductions would be required from other nitrogen sources within 

23 the watershed.
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1 The small effect of decreasing atmospheric deposition in the Neuse River watershed is 

2 because the other nitrogen sources within the watershed are more influential than atmospheric 

3 deposition to the total nitrogen loadings to the Neuse River Estuary as estimated with the 

4 SPARROW model. A waterbody’s response to nutrient loading depends on the magnitude (e.g., 

5 agricultural sources have a high influence in the Neuse), spatial distribution, and other 

6 characteristics of the sources within the watershed.

7 e. Deposition-related terrestrial nutrient enrichment

8 California Coastal Sage Scrub (CSS) and Mixed Conifer Forest (MCF) communities 

9 were the focus of the Terrestrial Nutrient Enrichment Case Studies of the REA. Geographic 

10 information systems (GIS) analysis supported a qualitative review of past field research to 

11 identify ecological benchmarks associated with CSS and mycorrhizal communities, as well as 

12 MCF’s nutrient-sensitive acidophyte lichen communities, fine-root biomass in Ponderosa pine, 

13 and leached nitrate in receiving waters. These benchmarks, ranging from 3.1 to 17 kg N/ha/yr, 

14 were compared to 2002 CMAQ/NADP data to discern any associations between atmospheric 

15 deposition and changing communities. Evidence supports the finding that nitrogen alters CSS 

16 and MCF. Key findings include the following:

17 (1) 2002 CMAQ/NADP nitrogen deposition data show that the 3.3 kg N/ha/yr benchmark has 

18 been exceeded in more than 93% of CSS areas (654,048 ha). These deposition levels are a 

19 driving force in the degradation of CSS communities. Although CSS decline has been 

20 observed in the absence of fire, the contributions of deposition and fire to the CSS decline 

21 require further research. CSS is fragmented into many small parcels, and the 2002 

22 CMAQ/NADP 12-km grid data are not fine enough to fully validate the relationship between 

23 CSS distribution, nitrogen deposition, and fire.
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1 (2) 2002 CMAQ/NADP nitrogen deposition data exceeds the 3.1 kg N/ha/yr benchmark in more 

2 than 38% (1,099,133 ha) of MCF areas, and nitrate leaching has been observed in surface 

3 waters. Ozone effects confound nitrogen effects on MCF acidophyte lichen, and the 

4 interrelationship between fire and nitrogen cycling requires additional research.

5 f. Additional effects

6 Ecological effects have also been documented across the United States where elevated 

7 nitrogen deposition has been observed, including the eastern slope of the Rocky Mountains 

8 where shifts in dominant algal species in alpine lakes have occurred where wet nitrogen 

9 deposition was only about 1.5 kg N/ha/yr. High alpine terrestrial communities have a low 

10 capacity to sequester nitrogen deposition, and monitored deposition exceeding 3 to 4 kg N/ha/yr 

11 could lead to community-level changes in plant species, lichens, and mycorrhizae.

12 Additional welfare effects are documented, but examined less extensively, in the REA. 

13 These effects include qualitative discussions related to visibility and materials damage, such as 

14 corrosion, erosion, and soiling of paint and buildings which are being addressed in the particulate 

15 matter (PM) NAAQS review currently underway. A discussion of the causal relationship 

between sulfur deposition (as sulfate, SO4

2-

16 ) and increased mercury methylation in wetlands and 

17 aquatic environments is also included in the REA. On this subject the REA concludes that 

18 decreases in sulfate deposition will likely result in decreases in methyl mercury concentration; 

19 however, spatial and biogeochemical variations nationally hinder establishing large scale dose20 response relationships. 

21 Several additional issues concerning oxides of nitrogen were addressed in the REA. 

22 Consideration was also given to nitrous oxide (N2O), a potent GHG. The REA concluded that it 

23 is most appropriate to analyze the role of N2O in the context of all of the GHGs rather than as 
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1 part of the REA for this review. The REA considered nitrogen deposition and its correlation 

2 with the rate of photosynthesis and net primary productivity. Nitrogen addition ranging from 

3 15.4 to 300 kg N/ha/yr is documented as increasing wetland N2O production by an average of 

4 207%. Nitrogen addition ranging from 30 to 240 kg N/ha/y increased CH4 emissions by 115%, 

5 averaged across all ecosystems, and methane uptake was reduced by 38% when nitrogen addition 

6 ranged from 10 to 560 kg N/ha/yr, but reductions were only significant for coniferous and 

7 deciduous forests. The heterogeneity of ecosystems across the United States, however, 

8 introduces variations into dose-response relationships.

9 The phytotoxic effects of oxides of nitrogen and sulfur on vegetation were also briefly 

10 discussed in the REA which concluded that since a unique secondary NAAQS exists for SO2, 

11 and concentrations of nitric oxide (NO), NO2, and PAN are rarely high enough to have 

12 phytotoxic effects on vegetation, further assessment was not warranted at this time. 

13 3. Conclusions on Effects 

14 For aquatic and terrestrial acidification effects, a similar conceptual approach was used 

15 (critical loads) to evaluate the impacts of multiple pollutants on an ecological endpoint, whereas 

16 the approaches used for aquatic and terrestrial nutrient enrichment were fundamentally distinct. 

17 Although the ecological indicators for aquatic and terrestrial acidification (i.e., ANC and Bc/Al) 

18 are very different, both ecological indicators are well-correlated with effects such as reduced 

19 biodiversity and growth. While aquatic acidification is clearly the targeted effect area with the 

20 highest level of confidence, the relationship between atmospheric deposition and an ecological 

21 indicator is also quite strong for terrestrial acidification. The main drawback with the 

22 understanding of terrestrial acidification is that the data are based on laboratory responses rather 
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1 than field measurements. Other stressors that are present in the field but that are not present in 

2 the laboratory may confound this relationship.

3 For nutrient enrichment effects, the REA utilized different types of indicators for aquatic 

4 and terrestrial effects to assess both the likelihood of adverse effects to ecosystems and the 

5 relationship between adverse effects and atmospheric sources of oxides of nitrogen. The 

6 ecological indicator chosen for aquatic nutrient enrichment, the ASSETS EI, seems to be 

7 inadequate to relate atmospheric deposition to the targeted ecological effect, likely due to the 

8 many other confounding factors. Further, there is far less confidence associated with the 

9 understanding of aquatic nutrient enrichment because of the large contributions from non10 atmospheric sources of nitrogen and the influence of both oxidized and reduced forms of 

11 nitrogen, particularly in large watersheds and coastal areas. However, a strong relationship exists 

12 between atmospheric deposition of nitrogen and ecological effects in high alpine lakes in the 

13 Rocky Mountains because atmospheric deposition is the only source of nitrogen to these 

14 systems. There is also a strong weight-of-evidence regarding the relationships between 

15 ecological effects attributable to terrestrial nitrogen nutrient enrichment; however, ozone and 

16 climate change may be confounding factors. In addition, the response for other species or species 

17 in other regions of the United States has not been quantified.

18 C. Adversity of Effects to Public Welfare

19 Characterizing a known or anticipated adverse effect to public welfare is an important 

20 component of developing any secondary NAAQS. According to the Clean Air Act, welfare 

21 effects include: “effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, 

22 weather, visibility, and climate, damage to and deterioration of property, and hazards to 

23 transportation, as well as effect on economic values and on personal comfort and well-being, 
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1 whether caused by transformation, conversion, or combination with other air pollutants” (CAA, 

2 Section 302(h)). While the text above lists a number of welfare effects, these effects do not 

3 define public welfare in and of themselves. 

4 Although there is no specific definition of adversity to public welfare, the paradigm of 

5 linking adversity to public welfare to disruptions in ecosystem structure and function has been 

6 used broadly by EPA to categorize effects of pollutants from the cellular to the ecosystem level. 

7 An evaluation of adversity to public welfare might consider the likelihood, type, magnitude, and 

8 spatial scale of the effect as well as the potential for recovery and any uncertainties relating to 

9 these considerations. 

10 Similar concepts were used in past reviews of secondary NAAQS for ozone and PM 

11 (relating to visibility), as well as in initial reviews of effects from lead deposition. Because 

12 oxides of nitrogen and sulfur are deposited from ambient sources into ecosystems where they

13 affect changes to organisms, populations and ecosystems, the concept of adversity to public 

14 welfare as a result of alterations in structure and function of ecosystems is an appropriate 

15 consideration for this review. 

16 Based on information provided in the Policy Assessment, the following section discusses 

17 how ecological effects from deposition of oxides of nitrogen and sulfur relate to adversity to 

18 public welfare. In the Policy Assessment, public welfare was discussed in terms of loss of 

19 ecosystem services (defined below), which in some cases can be monetized. Each of the four 

20 main effect areas (aquatic and terrestrial acidification and aquatic and terrestrial nutrient over21 enrichment) are discussed including current ecological effects and associated ecosystem services, 

22 including potential value of the services. The quantified indicators of value discussed below 
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1 relate to ecosystem services generally that are impacted by deposition related acidification, not to 

2 the specific effects of this action.

3 1. Ecosystem Services 

4 The Policy Assessment defines ecosystem services as the benefits individuals and 

5 organizations obtain from ecosystems. Ecosystem services can be classified as provisioning 

6 (food and water), regulating (control of climate and disease), cultural (recreational, existence, 

7 spiritual, educational), and supporting (nutrient cycling). Conceptually, changes in ecosystem 

8 services may be used to aid in characterizing a known or anticipated adverse effect to public 

9 welfare. In the REA and Policy Assessment ecosystem services are discussed as a method of 

10 assessing the magnitude and significance to the public of resources affected by ambient 

11 concentrations of oxides of nitrogen and sulfur and deposition in sensitive ecosystems. 

12 EPA has in previous NAAQS reviews defined ecological goods and services for the 

13 purposes of a Regulatory Impact Analysis as the “outputs of ecological functions or processes 

14 that directly or indirectly contribute to social welfare or have the potential to do so in the future. 

15 Some outputs may be bought and sold, but most are not marketed.” It is especially important to

16 acknowledge that it is difficult to measure and/or monetize the goods and services supplied by

17 ecosystems. It can be informative in characterizing adversity to public welfare to attempt to 

18 place an economic valuation on the set of goods and services that have been identified with 

19 respect to a change in policy however it must be noted that this valuation will be incomplete and 

20 illustrative only. 

21 Knowledge about the relationships linking ambient concentrations and ecosystem 

22 services is considered in the Policy Assessment as one method by which to inform a policy

23 judgment on a known or anticipated adverse public welfare effect. For example, a change in an 
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1 ecosystem structure and process, such as foliar injury, would be classified as an ecological effect, 

2 with the associated changes in ecosystem services, such as primary productivity, food 

3 availability, forest products, and aesthetics (e.g., scenic viewing), classified as public welfare 

4 effects. Additionally, changes in biodiversity would be classified as an ecological effect, and the 

5 associated changes in ecosystem services—productivity, existence (nonuse) value, recreational 

6 viewing and aesthetics—would also be classified as public welfare effects. 

7 As described in Chapters 4 and 5 of the REA, case study analyses were performed that 

8 link deposition in sensitive ecosystems to changes in a given ecological indicator (e.g., for 

9 aquatic acidification, to changes in acid neutralizing capacity [ANC]) and then to changes in 

10 ecosystems. Appendix 8 of the REA links the changes in ecosystems to the services they

11 provide (e.g., fish species richness and its influence on recreational fishing). To the extent 

12 possible for each targeted effect area, the REA linked ambient concentrations of nitrogen and 

13 sulfur (i.e., ambient air quality indicators) to deposition in sensitive ecosystems (i.e., exposure 

14 pathways), and then to system response as measured by a given ecological indicator (e.g., lake 

15 and stream acidification as measured by ANC). The ecological effect (e.g., changes in fish 

16 species richness) was then, where possible, associated with changes in ecosystem services and 

17 the corresponding public welfare effects (e.g., recreational fishing). 

18 2. Effects on Ecosystem Services

19 The process used to link ecological indicators to ecosystem services is discussed 

20 extensively in Appendix 8 of the REA. In brief, for each case study area assessed the ecological 

21 indicators are linked to an ecological response that is subsequently linked to associated services 

22 to the extent possible. For example in the case study for aquatic acidification the chosen 

23 ecological indicator is ANC which can be linked to the ecosystem service of recreational fishing. 
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1 Although recreational fishing losses are the only service effects that can be independently 

2 quantified or monetized at this time, there are numerous other ecosystem services that may be 

3 related to the ecological effects of acidification.

4 While aquatic acidification is the focus of this proposed standard, the other effect areas 

5 were also analyzed in the REA and these ecosystems are being harmed by nitrogen and sulfur 

6 deposition and will obtain some measure of protection with any decrease in that deposition 

7 regardless of the reason for the decrease. The following summarizes the current levels of specific 

8 ecosystem services for aquatic and terrestrial acidification, and aquatic and terrestrial nutrient 

9 over-enrichment and attempts to quantify and when possible monetize the harm to public 

10 welfare, as represented by ecosystem services, due to nitrogen and sulfur deposition. 

11 a. Aquatic Acidification

12 Acidification of aquatic ecosystems primarily affects the ecosystem services that are 

13 derived from the fish and other aquatic life found in surface waters. In the northeastern United 

14 States, the surface waters affected by acidification are not a major source of commercially raised 

15 or caught fish; however, they are a source of food for some recreational and subsistence fishers 

16 and for other consumers. Although data and models are available for examining the effects on 

17 recreational fishing, relatively little data are available for measuring the effects on subsistence 

18 and other consumers. Inland waters also provide aesthetic and educational services along with 

19 non-use services, such as existence value (protection and preservation with no expectation of 

20 direct use). In general, inland surface waters such as lakes, rivers, and streams also provide a 

21 number of regulating services, playing a role in hydrological regimes and climate regulation. 

22 There is little evidence that acidification of freshwaters in the northeastern United States has 

23 significantly degraded these specific services; however, freshwater ecosystems also provide 
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1 biological control services by providing environments that sustain delicate aquatic food chains. 

2 The toxic effects of acidification on fish and other aquatic life impair these services by disrupting 

3 the trophic structure of surface waters. Although it is difficult to quantify these services and how 

4 they are affected by acidification, it is worth noting that some of these services may be captured 

5 through measures of provisioning and cultural services. For example, these biological control 

6 services may serve as “intermediate” inputs that support the production of “final” recreational 

7 fishing and other cultural services.

8 As summarized in Chapter 4 of the Policy Assessment, recent studies indicate that 

9 acidification of lakes and streams can result in significant loss in economic value. For example, 

10 data indicate that more than 9% of adults in the northeastern part of the country participate 

11 annually in freshwater fishing yielding 140 million freshwater fishing days. Each fishing day

12 has an estimated average value per day of $35. Therefore, the implied total annual value of 

13 freshwater fishing in the northeastern United States was $5 billion in 2006. Embedded in these 

14 numbers is a degree of harm to recreational fishing services due to acidification that has occurred 

15 over time. These harms have not been quantified on a regional scale; however, a case study was 

16 conducted in the Adirondacks area (US EPA, 2011, section 4.4.2). 

17 In the Adirondacks case study, estimates of changes in recreational fishing services were 

18 determined, as well as changes more broadly in “cultural” ecosystem services (including 

19 recreational, aesthetic, and nonuse services). First, the MAGIC model (US EPA, 2009,

20 Appendix 8 and section 2.2) was applied to 44 lakes to predict what ANC levels would be under 

21 both “business as usual” conditions (i.e., allowing for some decline in deposition due to existing 

22 regulations) and pre-emission (i.e., background) conditions. Second, to estimate the recreational 

23 fishing impacts of aquatic acidification in these lakes, an existing model of recreational fishing 
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1 demand and site choice was applied. This model predicts how recreational fishing patterns in the 

2 Adirondacks would differ and how much higher the average annual value of recreational fishing 

3 services would be for New York residents if lake ANC levels corresponded to background 

4 (rather than business as usual) conditions. To estimate impacts on a broader category of cultural 

5 (and some provisioning) ecosystem services, results from the Banzhaf et al (2006) valuation 

6 survey of New York residents were adapted and applied to this context. The survey used a 

7 contingent valuation approach to estimate the average annual household WTP for future 

8 reductions in the percent of Adirondack lakes impaired by acidification. The focus of the survey 

9 was on impacts on aquatic resources. Pretesting of the survey indicated that respondents 

10 nonetheless tended to assume that benefits would occur in the condition of birds and forests as 

11 well as in recreational fishing. 

12 By extrapolating the 44 lake Adiriondack case study to all 3,000 Adirondack lakes and by

13 applying the willingness to pay survey results to all New York residents, the study estimated 

14 aggregated benefits between $300 and $800 million annually for the equivalent of improving 

15 lakes in the Adirondacks region to an ANC level of 50 μeq/L. The REA estimated 44% of the 

16 Adirondack lakes currently fall below an ANC of 50 μeq/L. Several states have set goals for 

17 improving the acid status of lakes and streams, generally targeting ANC in the range of 50 to 60 

18 μeq/L, and have engaged in costly activities to decrease acidification. 

19 These results imply significant value to the public in addition to those derived from 

20 recreational fishing services. Note that the results are only applicable to improvements in the 

21 Adirondacks valued by residents of New York. If similar benefits exist in other acid-impacted 

22 areas, benefits for the nation as a whole could be substantial. The analysis provides results on 
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1 only a subset of the impacts of acidification on ecosystem services and suggests that the overall 

2 impact on these services is likely to be substantial.

3 b. Terrestrial Acidification

4 Chapter 4.4.3 and 4.4.4 of the Policy Assessment review several economic studies of 

5 areas sensitive to terrestrial acidification. Forests in the northeastern United States provide 

6 several important and valuable provisioning ecosystem services, which are reflected in the 

7 production and sales of tree products. Sugar maples are a particularly important commercial 

8 hardwood tree species in the United States, producing timber and maple syrup that provide 

9 hundreds of millions of dollars in economic value annually. Red spruce is also used in a variety 

10 of wood products and provides up to $100 million in economic value annually. Although the 

11 data do not exist to directly link acidification damages to economic values of lost recreational 

12 ecosystem services in forests, these resources are valuable to the public. A recent study, 

13 reviewed in the Policy Assessment, suggests that the total annual value of recreational off-road 

14 driving was more than $9 billion and the value of hunting and wildlife viewing was more than $4 

15 billion each in the northeastern United States. EPA is not able to quantify at this time the 

16 specific effects on these values of acid deposition, or of any specific reductions in deposition,

17 relative to the effects of many other factors that may affect them.

18 c. Nutrient Enrichment

19 Chapter 4.4.5 and 4.4.6 of the Policy Assessment summarize economic studies of east 

20 coast estuaries affected by nutrient over-enrichment or eutrophication. Estuaries in the eastern 

21 United States are important for fish and shellfish production. The estuaries are capable of 

22 supporting large stocks of resident commercial species, and they serve as the breeding grounds 

23 and interim habitat for several migratory species. To provide an indication of the magnitude of 
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1 provisioning services associated with coastal fisheries, from 2005 to 2007, the average value of 

2 total catch was $1.5 billion per year in 15 East Coast states. Estuaries also provide an important 

3 and substantial variety of cultural ecosystem services, including water-based recreational and 

4 aesthetic services. For example, data indicate that 4.8% of the population in coastal states from 

5 North Carolina to Massachusetts participated in saltwater fishing, with a total of 26 million 

6 saltwater fishing days in 2006. Based on estimates in the Policy Assessment, total recreational 

7 value from these saltwater fishing days was approximately $1.3 billion. Recreational 

8 participation estimates for 1999–2000 showed almost 6 million individuals participated in 

9 motorboating in coastal states from North Carolina to Massachusetts. The aggregate value of 

10 these coastal motorboating outings was $2 billion per year. EPA is not able to quantify at this 

11 time the specific effects on these values of acid deposition, or of any specific reductions in 

12 deposition, relative to the effects of many other factors that may affect them.

13 Terrestrial ecosystems can also suffer from nutrient over-enrichment. Each ecosystem is 

14 different in its composition of species and nutrient requirements. Changes to individual 

15 ecosystems from changes in nitrogen deposition can be hard to assess economically. Relative 

16 recreational values are often determined by public use information. Chapter 4.4.7 of the Policy

17 Assessment reviewed studies related to park use in California. Data from California State Parks 

18 indicate that in 2002, 68.7% of adult residents participated in trail hiking for an average of 24.1 

19 days per year. The analyses in the Policy Assessment indicate that the aggregate annual benefit 

20 for California residents from trail hiking in 2007 was $11.59 billion. EPA is not able to quantify 

21 at this time the specific effects on these values of acid deposition, or of any specific reductions in 

22 deposition, relative to the effects of many other factors that may affect them.
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1 The Policy Assessment also identified fire regulation as a service that could be affected 

2 by nutrient over-enrichment of the Coastal Sage Scrub and Mixed Conifer Forest ecosystems by

3 encouraging growth of more flammable grasses, increasing fuel loads, and altering the fire cycle. 

4 Over the 5-year period from 2004 to 2008, Southern California experienced, on average, over 

5 4,000 fires per year, burning, on average, over 400,000 acres per year. It is not possible at this 

6 time to quantify the contribution of nitrogen deposition, among many other factors, to increased 

7 fire risk.

8 3. Summary

9 Adversity to public welfare can be understood by looking at how deposition of oxides of 

10 nitrogen and sulfur affect the ecological functions of an ecosystem (see II.A.), and then

11 understanding the ecosystem services that are degraded. The monetized value of the ecosystem 

12 services provided by ecosystems that are sensitive to deposition of oxides of nitrogen and sulfur 

13 are in the billions of dollars each year, though it is not possible to quantify or monetize at this 

14 time the effects on these values of acid deposition or of any changes in deposition that may result 

15 from new secondary standards. Many lakes and streams are known to be degraded by acidic 

16 deposition which affects recreational fishing and tourism. Forest growth is likely suffering from 

17 acidic deposition in sensitive areas affecting red spruce and sugar maple timber production, sugar 

18 maple syrup production, hiking, aesthetic enjoyment and tourism. Nitrogen deposition 

19 contributes significantly to eutrophication in many estuaries affecting fish production, 

20 swimming, boating, aesthetic enjoyment, and tourism. Ecosystem services are likely affected by

21 nutrient enrichment in many natural and scenic terrestrial areas, affecting biodiversity, including 

22 habitat for rare and endangered species, fire control, hiking, aesthetic enjoyment, and tourism. 

23 D. Adequacy of the Current Standards
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1 An important issue to be addressed in the current review of the secondary standards for 

2 oxides and nitrogen and sulfur standard is whether, in view of the scientific evidence reflected in 

3 the ISA, additional information on exposure and risk discussed in the REA, and conclusions 

4 drawn from the Policy Assessment, the existing standards provide adequate protection. The 

5 Administrator therefore, has considered the extent to which the current standards are adequate 

6 for the protection of public welfare. Having reached the general conclusion that aquatic and 

7 terrestrial ecosystems can be degrade by deposition of oxides of nitrogen and sulfur, it is then 

8 necessary to evaluate the adequacy of the current secondary standards for oxides of nitrogen and 

9 sulfur to provide requisite protection by considering to what degree risks to sensitive ecosystems 

10 would be expected to occur in areas that meet the current standards. Conclusions regarding the 

11 adequacy of the current standards are based on the available ecological effects, exposure and 

12 risk-based evidence. In evaluating the strength of this information, EPA has taken into account 

13 the uncertainties and limitations in the scientific evidence. This section addresses the adequacy

14 of the current standards to protect against direct exposure effects on plants from oxides of 

15 nitrogen and sulfur, the appropriateness of the current structure of the standards to address 

16 deposition-related effects of oxides of nitrogen and sulfur on sensitive ecosystems and finally, 

17 the adequacy of such standards to protect against adverse effects related to the deposition of 

18 oxides of nitrogen and sulfur. 

19 1. Adequacy of the Current Standards for Direct Effects

20 The current secondary oxides of nitrogen and sulfur standards are intended to protect 

21 against adverse effects to public welfare. For oxides of nitrogen, the current secondary standard 
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was set identical to the primary standard31 , e.g. an annual standard set for NO2 to protect against 

2 adverse effects on vegetation from direct exposure to ambient oxides of nitrogen. For oxides of 

3 sulfur, the current secondary standard is a 3-hour standard intended to provide protection for 

4 plants from the direct foliar damage associated with atmospheric concentrations of SO2. It is 

5 appropriate to consider whether the current standards are adequate to protect against the direct 

6 effects on vegetation resulting from ambient NO2 and SO2 which were the basis for the current 

7 secondary standards. The ISA concluded that there was sufficient evidence to infer a causal 

8 relationship between exposure to SO2, NO, NO2 and PAN and injury to vegetation. Additional 

9 research on acute foliar injury has been limited and there is no evidence to suggest foliar injury

10 below the levels of the current secondary standards for oxides of nitrogen and sulfur. There is 

11 sufficient evidence to suggest that the levels of the current standards are likely adequate to 

12 protect against direct phytotoxic effects. 

13 2. Appropriateness and Adequacy of the Current Standards for Deposition-related Effects

14 This section addresses two concepts necessary to evaluate the current standards in the 

15 context of deposition related effects. First, appropriateness of the current standards is considered 

16 with regard to indicator, form, level and averaging time. This discussion centers around the 

17 ability of the current standards to evaluate and provide protection against deposition related 

18 effects that vary spatially and temporally. It includes particular emphasis on the indicators and 

19 forms of the current standards and the degree to which they are ecologically relevant with regard 

20 to deposition related effects. Second, this section evaluates the current standardsin terms of 

21 adequacy of protection. 

22 a. Appropriateness

 

3 The current primary NO2 standard has recently been changed to the 3 year average of the 98th percentile of the 

annual distribution of the 1 hour daily maximum of the concentration of NO2. The current secondary standard 

remains as it was set in 1971.
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1 The ISA has established that the major effects of concern for this review of the oxides of 

2 nitrogen and sulfur standards are associated with deposition of N and S caused by atmospheric 

3 concentrations of oxides of nitrogen and sulfur. The current standards are not directed toward 

4 depositional effects, and none of the elements of the current NAAQS – indicator, form, 

5 averaging time, and level – are suited for addressing the effects of N and S deposition. 

6 Four issues arise that call into question the ecological relevance of the structure of the 

7 current secondary standards for oxides of nitrogen and sulfur. 

8 (1) The current SO2 secondary standard (0.5 ppm SO2 over a 3-hour average) does not utilize an 

9 exposure period that is relevant for ecosystem impacts. The majority of deposition related 

10 impacts are associated with depositional loads that occur over periods of months to years. 

11 This differs significantly from exposures associated with hourly concentrations of SO2 as 

12 measured by the current secondary standard. By addressing short-term concentrations, the 

13 current SO2 secondary standard, while protective against direct foliar effects from gaseous 

14 oxides of sulfur, does not take into account the findings of effects in the ISA, which notes the 

15 relationship between annual deposition of S and acidification effects which are likely to be 

16 more severe and widespread than phytotoxic effects under current ambient conditions, and 

17 include effects from long term deposition as well as short term. Acidification is a process 

18 which occurs over time, as the ability of an aquatic system to counteract acidic inputs is 

19 reduced as natural buffers are used more rapidly than they can be replaced through geologic 

20 weathering. The relevant period of exposure for ecosystems is therefore not the exposures 

21 captured in the short averaging time of the current SO2 secondary standard. The current 

22 secondary standard for oxides of nitrogen is an annual standard (0.053 ppm averaged over 1 

23 year) and as such is more ecologically relevant. Current standards do not utilize appropriate 
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1 atmospheric indicators. NO2 and SO2 are used as the component of oxides of nitrogen and 

2 sulfur that are measured, but they do not provide a complete link to the direct effects on 

3 ecosystems from deposition of oxides of nitrogen and sulfur as they do not capture all 

4 relevant chemical species of oxidized nitrogen and oxidized sulfur that contribute to 

5 deposition. The ISA provides evidence that deposition related effects are linked with total 

6 nitrogen and total sulfur deposition, and thus all forms of oxidized nitrogen and oxidized 

7 sulfur that are deposited will contribute to effects on ecosystems. Thus, by using 

8 atmospheric NO2 and SO2 concentrations as indicators, the current standards address only a 

9 fraction of total atmospheric oxides of nitrogen and sulfur, and do not take into account the 

10 effects from deposition of total atmospheric oxides of nitrogen and sulfur. This suggests that 

11 more comprehensive atmospheric indicators should be considered in designing ecologically 

12 relevant standards. 

13 (2) Current standards reflect separate assessments of the two individual pollutants, NO2 and SO2, 

14 rather than assessing the joint impacts of deposition to ecosystems. Recognizing the role that 

15 each pollutant plays in jointly affecting ecosystem indicators, functions, and services is vital 

16 to developing a meaningful standard. The clearest example of this interaction is in 

17 assessment of the impacts of acidifying deposition on aquatic ecosystems. Acidification in 

18 an aquatic ecosystem depends on the total acidifying potential of the deposition of both N 

19 and S from both atmospheric deposition of oxides of nitrogen and sulfur as well as the inputs 

20 from other sources of N and S such as reduced nitrogen and non-atmospheric sources. It is 

21 the joint impact of the two pollutants that determines the ultimate effect on organisms within 

22 the ecosystem, and critical ecosystem functions such as habitat provision and biodiversity. 

23 Standards that are set independently are less able to account for the contribution of the other 
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1 pollutant. This suggests that interactions between oxides of nitrogen and oxides of sulfur 

2 should be a critical element of the conceptual framework for ecologically relevant standards. 

3 There are also important interactions between oxides of nitrogen and sulfur and reduced 

4 forms of nitrogen, which also contribute to acidification and nutrient enrichment. It is 

5 important that the structure of the standards address the role of reduced nitrogen in 

6 determining the ecological effects resulting from deposition of atmospheric oxides of 

7 nitrogen and sulfur. Consideration will also have to be given to total loadings as ecosystems 

8 respond to all sources of N and S.

9 (3) Current standards do not take into account variability in ecosystem sensitivity. Ecosystems 

10 are not uniformly distributed either spatially or temporally in their sensitivity to oxides of 

11 nitrogen and sulfur. Therefore, failure to account for the major determinants of variability, 

12 including geological and soil characteristics related to the sensitivity to acidification or 

13 nutrient enrichment as well as atmospheric and landscape characteristics that govern rates of 

14 deposition, may lead to standards that do not provide requisite levels of protection across 

15 ecosystems. The current structures of the standards do not address the complexities in the 

16 responses of ecosystems to deposition of oxides of nitrogen and sulfur. Ecosystems contain 

17 complex groupings of organisms that respond in various ways to the alterations of soil and 

18 water that result from deposition of nitrogen and sulfur compounds. Different ecosystems 

19 therefore respond in different ways depending on a multitude of factors that control how 

20 deposition is integrated into the system. For example, the same levels of deposition falling 

21 on limestone dominated soils have a very different effect from those falling on shallow 

22 glaciated soils underlain with granite. One system may over time display no obvious 

23 detriment while the other may experience a catastrophic loss in fish communities. This 
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1 degree of sensitivity is a function of many atmospheric factors which control rates of 

2 deposition as well as ecological factors which control how an ecosystem responds to that 

3 deposition. The current standards do not take into account spatial and seasonal variations not 

4 only in depositional loadings but also in sensitivity of ecosystems exposed to those loadings. 

5 Based on the discussion summarized above, the Policy Assessment concludes that the current 

6 secondary standards for oxides of nitrogen and oxides of sulfur are not ecologically relevant 

7 in terms of averaging time, form, level or indicator.

8 b. Adequacy of Protection

9 As described in the Policy Assessment, 2005 ambient conditions indicate that the current 

10 SO2 and NO2 secondary standards are not exceeded (US EPA, 2011, Figures 6-1 and 6-2) in 

11 locations where ecological effects have been observed. In many locations, SO2 and NO2 

12 concentrations are substantially below the levels of the secondary standards. This pattern 

13 suggests that levels of deposition and any effects on ecosystems due to deposition of oxides of 

14 nitrogen and sulfur under recent conditions are occurring even though areas meet or are below 

15 current standards. In addition, these levels based on conclusions in the REA will not decline in 

16 the future to levels below which it is reasonable to anticipate effects.

17 In determining the adequacy of the current secondary standards for oxides of nitrogen 

18 and sulfur the Policy Assessment considered the extent to which ambient deposition contributes 

19 to loadings in ecosystems. Since the last review of the secondary standard for oxides of nitrogen, 

20 a great deal of information on the contribution of atmospheric deposition associated with 

21 ambient oxides of nitrogen has become available. The REA presents a thorough assessment of 

22 the contribution of oxidized nitrogen to nitrogen deposition throughout the U.S., and the relative 

23 contributions of ambient oxidized and reduced forms of nitrogen. The REA concludes that based 
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1 on that analysis, ambient oxides of nitrogen are a significant component of atmospheric nitrogen 

2 deposition, even in areas with relatively high rates of deposition of reduced nitrogen. In 

3 addition, atmospheric deposition of oxidized nitrogen contributes significantly to total nitrogen 

4 loadings in nitrogen sensitive ecosystems. 

5 The ISA summarizes the available studies of relative nitrogen contribution and finds that 

6 in much of the U.S., oxides of nitrogen contribute from 50 to 75 percent of total atmospheric 

7 deposition relative to total reactive nitrogen that includes oxidized and reduced nitrogen species 

8 (US EPA, 2008, section 2.8.4). Although the proportion of total nitrogen loadings associated 

9 with atmospheric deposition of nitrogen varies across locations (N deposition in the eastern U.S. 

10 includes locations with greater than 9 kg N/ha-yr, and in the central U.S. high deposition 

11 locations with values on the order of 6 to 7 kg N/ha-yr), the ISA indicates that atmospheric N 

12 deposition is the main source of new anthropogenic N to most headwater streams, high elevation 

13 lakes, and low-order streams. Atmospheric N deposition contributes to the total N load in 

14 terrestrial, wetland, freshwater, and estuarine ecosystems that receive N through multiple 

15 pathways. In several large estuarine systems, including the Chesapeake Bay, atmospheric 

16 deposition accounts for between 10 and 40 percent of total nitrogen loadings (US EPA, 2008). 

17 Atmospheric concentrations of oxides of sulfur account for nearly all S deposition in the 

18 US. For the period 2004–2006, mean S deposition in the U.S. was greatest east of the 

19 Mississippi River with the highest deposition amount, 21.3 kg S/ha-yr, in the Ohio River Valley 

20 where most recording stations reported 3 year averages >10 kg S/ha-yr. Numerous other stations 

21 in the East reported S deposition >5 kg S/ha-yr. Total S deposition in the U.S. west of the 100th 

22 meridian was relatively low, with all recording stations reporting <2 kg S/ha-yr and many 

reporting <1 kg S/ha-yr. S was primarily deposited in the form of wet SO4

2−

23 followed in 
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1 decreasing order by a smaller proportion of dry SO2 and a much smaller proportion of deposition 

as dry SO4

2−

2 . 

3 As discussed throughout the REA (US EPA, 2009 and section IIB above), there are 

4 several key areas of risk that are associated with ambient concentrations of oxides of nitrogen 

5 and sulfur. As noted earlier, in previous reviews of the secondary standards for oxides of 

6 nitrogen and sulfur, the standards were designed to protect against direct exposure of plants to 

7 ambient concentrations of the pollutants. A significant shift in understanding of the effects of 

8 oxides of nitrogen and sulfur has occurred since the last reviews, reflecting the large amount of 

9 research that has been conducted on the effects of deposition of nitrogen and sulfur to 

10 ecosystems. The most significant current risks of adverse effects to public welfare are those 

11 related to deposition of oxides of nitrogen and sulfur to both terrestrial and aquatic ecosystems. 

12 These risks fall into two categories, acidification and nutrient enrichment, which were 

13 emphasized in the REA as most relevant to evaluating the adequacy of the existing standards in 

14 protecting public welfare from adverse ecological effects.

15 i. Aquatic acidification

16 The focus of the REA case studies was on determining whether deposition of sulfur and 

17 oxidized nitrogen in locations where ambient oxides of nitrogen and sulfur were at or below the 

18 current standards was resulting in acidification and related effects, including episodic 

19 acidification and mercury methylation. Based on the case studies conducted for lakes in the 

20 Adirondacks and streams in Shenandoah National Park (case studies are discussed more fully in 

21 section IIB and US EPA, 2009), there is significant risk to acid sensitive aquatic ecosystems at 

22 atmospheric concentrations of oxides of nitrogen and sulfur at or below the current standards. 

23 The REA also supports strongly a relationship between atmospheric deposition of oxides of 
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1 nitrogen and sulfur and loss of ANC in sensitive ecosystems and indicates that ANC is an 

2 excellent indicator of aquatic acidification. The REA also concludes that at levels of deposition 

3 associated with oxides of nitrogen and sulfur concentrations at or below the current standards, 

4 ANC levels are expected to be below benchmark values that are associated with significant 

5 losses in fish species richness.

6 Significant portions of the U.S. are acid sensitive, and current deposition levels exceed 

7 those that would allow recovery of the most acid sensitive lakes in the Adirondacks (US EPA, 

8 2008, Executive Summary). In addition, because of past loadings, areas of the Shenandoah are 

9 sensitive to current deposition levels (US EPA, 2008, Executive Summary). Parts of the West 

10 are naturally less sensitive to acidification and subjected to lower deposition (particularly SOx) 

11 levels relative to the eastern United States, and as such, less focus in the ISA is placed on the 

12 adequacy of the existing standards in these areas, with the exception of the mountainous areas of 

13 the West, which experience episodic acidification due to deposition. 

14 In describing the effects of acidification in the two case study areas the REA uses the 

15 approach of describing benchmarks in terms of ANC values. Many locations in sensitive areas of 

16 the U.S. have ANC levels below benchmark levels for ANC classified as severe, elevated, or 

17 moderate concern (US EPA, 2011, Figure 2-1). The average current ANC levels across 44 lakes 

18 in the Adirondack case study area is 62.1 μeq/L (moderate concern). However, 44 percent of 

19 lakes had deposition levels exceeding the critical load for an ANC of 50 μeq/L, and 28 percent of 

20 lakes had deposition levels exceeding the (higher) critical load for an ANC of 20 μeq/L (US 

21 EPA, 2009, section 4.2.4.2). This information indicates that almost half of the 44 lakes in the 

22 Adirondacks case study area are at an elevated concern level, and almost a third are at a severe 

23 concern level. These levels are associated with greatly diminished fish species diversity, and 
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1 losses in the health and reproductive capacity of remaining populations. Based on assessments of 

2 the relationship between number of fish species and ANC level in both the Adirondacks and 

3 Shenandoah areas, the number of fish species is decreased by over half at an ANC level of 20 

4 μeq/L relative to an ANC level at 100 μeq/L (US EPA, 2009, Figure 4.2-1). When extrapolated 

5 to the full population of lakes in the Adirondacks area using weights based on the EMAP 

6 probability survey (US EPA, 2009, section 4.2.6.1), 36 percent of lakes exceeded the critical load 

7 for an ANC of 50 μeq/L and 13 percent of lakes exceeded the critical load for an ANC of 20 

8 μeq/L.

9 Many streams in the Shenandoah case study area also have levels of deposition that are 

10 associated with ANC levels classified as severe, elevated, or moderate concern. The average

11 ANC under recent conditions for the 60 streams evaluated in the Shenandoah case study area is 

12 57.9 μeq/L, indicating moderate concern. However, 85 percent of these streams had recent 

13 deposition exceeding the critical load for an ANC of 50 μeq/L, and 72 percent exceeded the 

14 critical load for an ANC of 20 μeq/L. As with the Adirondacks area, this information suggests 

15 that significant numbers of sensitive streams in the Shenandoah area are at risk of adverse 

16 impacts on fish populations under recent conditions. Many other streams in the Shenandoah area 

17 are also likely to experience conditions of elevated to severe concern based on the prevalence in 

18 the area of bedrock geology associated with increased sensitivity to acidification suggesting that 

19 effects due to stream acidification could be widespread in the Shenandoah area (US EPA, 2009, 

20 section 4.2.6.2).

21 In addition to these chronic acidification effects, the ISA notes that “consideration of 

22 episodic acidification greatly increases the extent and degree of estimated effects for acidifying 

23 deposition on surface waters.” (US EPA, 2008, section 3.2.1.6) Some studies show that the 
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1 number of lakes that could be classified as acid-impacted based on episodic acidification is 2 to 3 

2 times the number of lakes classified as acid-impacted based on chronic ANC. These episodic 

3 acidification events can have long term effects on fish populations (US EPA, 2008, section 

4 3.2.1.6). Under recent conditions, episodic acidification has been observed in locations in the 

5 eastern U.S. and in the mountainous western U.S. (US EPA, 2008, section 3.2.1.6). 

6 The ISA, REA and Policy Assessment all conclude that the current standards are not 

7 adequate to protect against the adverse impacts of aquatic acidification on sensitive ecosystems. 

8 In the ISA it is noted that significant portions of the U.S. are acid sensitive, and that current 

9 deposition levels exceed those that would allow recovery of the most acid sensitive lakes in the 

10 Adirondacks (US EPA, 2008, Executive Summary). In addition, because of past loadings, areas 

11 of the Shenandoah are sensitive to current deposition levels (US EPA, 2008, Executive 

12 Summary). A recent survey, as reported in the ISA, found sensitive streams in many locations in 

13 the U.S., including the Appalachian Mountains, the Coastal Plain, and the Mountainous West 

14 (US EPA, 2008, section 4.2.2.3). In these sensitive areas, between 1 and 6 percent of stream 

15 kilometers are chronically acidified. The REA further concludes that both the Adirondack and 

16 Shenandoah case study areas are currently receiving deposition from ambient oxides of nitrogen 

17 and sulfur in excess of their ability to neutralize such inputs. In addition based on the current 

18 emission scenarios, forecast modeling out to the year 2020 as well as 2050 indicates a large 

19 number of streams in these areas will still be adversely impacted (section IIB). Based on these 

20 considerations, the Policy Assessment concludes that the current secondary NAAQS for oxides 

21 of nitrogen and sulfur do not provide adequate protection of sensitive ecosystems with regard to 

22 aquatic acidification. 

23 ii. Terrestrial acidification
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1 Based on the terrestrial acidification case studies (Kane Experimental Forest in 

2 Pennsylvania and Hubbard Brook Experimental Forest described in section IIB) on sugar maple 

3 and red spruce habitat, the REA concludes that there is significant risk to sensitive terrestrial 

4 ecosystems from acidification at atmospheric concentrations of NOx and SOX at or below the 

5 current standards. The ecological indicator selected for terrestrial acidification is the base 

6 cation to aluminum ratio (BC:Al), which has been linked to tree health and growth. The results 

7 of the REA strongly support a relationship between atmospheric deposition of oxides of nitrogen 

8 and sulfur and BC:Al, and that BC:Al is a good indicator of terrestrial acidification. At levels of 

9 deposition associated with oxides of nitrogen and sulfur concentrations at or below the current 

10 standards, BC:Al levels are expected to be below benchmark values that are associated with 

11 significant effects on tree health and growth. Such degradation of terrestrial ecosystems could 

12 affect ecosystem services such as habitat provisioning, endangered species, goods production 

13 (timber, syrup, etc.) among others. 

14 Many locations in sensitive areas of the U.S. have BC:Al levels below benchmark levels 

15 classified as providing low to intermediate levels of protection to tree health. At a BC:Al ratio of 

16 1.2 (intermediate level of protection), red spruce growth can be reduced by 20 percent. At a 

17 BC:Al ratio of 0.6 (low level of protection), sugar maple growth can be decreased by 20 percent. 

18 The REA did not evaluate broad sensitive regions. However, in the sugar maple case study area 

19 (Kane Experimental Forest), recent deposition levels are associated with a BC:Al ratio below 

20 1.2, indicating between intermediate and low level of protection, which would indicate the 

21 potential for a greater than 20 percent reduction in growth. In the red spruce case study area 

22 (Hubbard Brook Experimental Forest), recent deposition levels are associated with a BC:Al ratio 
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1 slightly above 1.2, indicating slightly better than an intermediate level of protection (US EPA, 

2 2009, section 4.3.5.1). 

3 Over the full range of sugar maple, 12 percent of evaluated forest plots exceeded the 

4 critical loads for a BC:Al ratio of 1.2, and 3 percent exceeded the critical load for a BC:Al ratio 

5 of 0.6. However, there was large variability across states. In New Jersey, 67 percent of plots 

6 exceeded the critical load for a BC:Al ratio of 1.2, while in several states on the outskirts of the 

7 range for sugar maple (e.g. Arkansas, Illinois) no plots exceeded the critical load for a BC:Al 

8 ratio of 1.2. For red spruce, overall 5 percent of plots exceeded the critical load for a BC:Al ratio 

9 of 1.2, and 3 percent exceeded the critical load for a BC:Al ratio of 0.6. In the major red spruce 

10 producing states (Maine, New Hampshire, and Vermont), critical loads for a BC:Al ratio of 1.2 

11 were exceeded in 0.5, 38, and 6 percent of plots.

12 The ISA, REA and Policy Assessment all conclude that the current standards are not 

13 adequate to protect against the adverse impacts of terrestrial acidification on sensitive 

14 ecosystems. The ISA reported one study that estimated 15 percent of U.S. forest ecosystems 

15 exceeded the critical loads for acidity for N and S deposition by >250 eq/ha/year under current 

16 conditions (US EPA, 2008, section 4.2.1.3). The main drawback, as stated in the REA and 

17 Policy Assessment, with the understanding of terrestrial acidification lies in the availability of 

18 data by which we can predict critical loads and that the data are based on laboratory responses 

19 rather than field measurements. Other stressors that are present in the field but that are not 

20 present in the laboratory may confound this relationship. The REA does however, conclude that 

21 the case study results suggest that nitrogen and sulfur acidifying deposition in the sugar maple 

22 and red spruce forest areas studied were very close to, if not greater than, the critical loads for 
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1 those areas, and both ecosystems are likely to be sensitive to any future changes in the levels of 

2 deposition.

3 iii. Terrestrial nutrient enrichment

4 Nutrient enrichment effects are due to nitrogen loadings from both atmospheric and non5 atmospheric sources. Evaluation of nutrient enrichment effects requires an understanding that 

6 nutrient inputs are essential to ecosystem health and that specific long term levels of nutrients in 

7 a system affect the types of species that occur over long periods of time. Short term additions of 

8 nutrients can affect species competition, and even small additions of nitrogen in areas that are 

9 traditionally nutrient poor can have significant impacts on productivity as well as species 

10 composition. Most ecosystems in the United States are nitrogen-limited, so regional decreases 

11 in emissions and deposition of airborne nitrogen compounds could lead to some decrease in 

12 growth of the vegetation that surrounds the targeted aquatic system but as discussed below 

13 evidence for this is mixed. Whether these changes in plant growth are seen as beneficial or 

14 adverse will depend on the nature of the ecosystem being assessed. 

15 Information on the effects of changes in nitrogen deposition on forestlands and other 

16 terrestrial ecosystems is very limited. The multiplicity of factors affecting forests, including other 

17 potential stressors such as ozone, and limiting factors such as moisture and other nutrients, 

18 confound assessments of marginal changes in any one stressor or nutrient in forest ecosystems. 

19 The ISA notes that only a fraction of the deposited nitrogen is taken up by the forests, most of 

20 the nitrogen is retained in the soils (US EPA, 2008, section 3.3.2.1). In addition, the ISA 

21 indicates that forest management practices can significantly affect the nitrogen cycling within a 

22 forest ecosystem, and as such, the response of managed forests to NOx deposition will be 

23 variable depending on the forest management practices employed in a given forest ecosystem 
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1 (US EPA, 2008, Annex C C.6.3). Increases in the availability of nitrogen in N-limited forests 

2 via atmospheric deposition could increase forest production over large non-managed areas, but 

3 the evidence is mixed, with some studies showing increased production and other showing little 

4 effect on wood production (US EPA, 2008, section 3.3.9). Because leaching of nitrate can 

5 promote cation losses, which in some cases create nutrient imbalances, slower growth and 

6 lessened disease and freezing tolerances for forest trees, the net effect of increased N on forests 

7 in the U.S. is uncertain (US EPA, 2008, section 3.3.9).

8 The scientific literature has many examples of the deleterious effects caused by excessive 

9 nitrogen loadings to terrestrial systems. Several studies have set benchmark values for levels of 

10 N deposition at which scientifically adverse effects are known to occur. Large areas of the 

11 country appear to be experiencing deposition above these benchmarks. The ISA indicates that 

12 studies have found that at 3.1 kg N/ha-yr, the community of lichens begins to change from 

13 acidophytic to tolerant species; at 5.2 kg N/ha-yr, the typical dominance by acidophytic species 

14 no longer occurs; and at 10.2 kg N/ha-yr, acidophytic lichens are totally lost from the 

15 community. Additional studies in the Colorado Front Range of the Rocky Mountain National 

16 Park support these findings. These three values (3.1, 5.2, and 10.2 kg/ha-yr) are one set of 

17 ecologically meaningful benchmarks for the mixed conifer forest (MCF) of the pacific coast 

18 regions. Nearly all of the known sensitive communities receive total nitrogen deposition levels 

19 above the 3.1 N kg/ha-yr ecological benchmark according to the12 km, 2002 CMAQ/NADP 

20 data, with the exception of the easternmost Sierra Nevadas. MCFs in the southern portion of the 

21 Sierra Nevada forests and nearly all MCF communities in the San Bernardino forests receive 

22 total nitrogen deposition levels above the 5.2 N kg/ha-yr ecological benchmark. 
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1 Coastal Sage Scrub communities (CSS) are also known to be sensitive to community

2 shifts caused by excess nitrogen loadings. Studies have investigated the amount of nitrogen 

3 utilized by healthy and degraded CSS systems. In healthy stands, the authors estimated that 3.3 

4 kg N/ha-yr was used for CSS plant growth. It is assumed that 3.3 kg N/ha-yr is near the point 

5 where nitrogen is no longer limiting in the CSS community. Therefore, this amount can be 

6 considered an ecological benchmark for the CSS community. The majority of the known CSS 

7 range is currently receiving deposition in excess of this benchmark. Thus, the REA concludes 

8 that recent conditions where oxides of nitrogen ambient concentrations are at or below the 

9 current oxides of nitrogen secondary standards are not adequate to protect against anticipated 

10 adverse impacts from N nutrient enrichment in sensitive ecosystems. 

11 iv. Aquatic nutrient enrichment

12 The REA aquatic nutrient enrichment case studies focused on coastal estuaries and 

13 revealed that while current ambient loadings of atmospheric oxides of nitrogen are contributing 

14 to the overall depositional loading of coastal estuaries, other non-atmospheric sources are 

15 contributing in far greater amounts in total, although atmospheric contributions are as large as 

16 some other individual source types. The ability of current data and models to characterize the 

17 incremental adverse impacts of nitrogen deposition is limited, both by the available ecological 

18 indicators, and by the inability to attribute specific effects to atmospheric sources of nitrogen. 

19 The REA case studies used as the ecological indicator for aquatic nutrient enrichment an index 

20 of eutrophication known as the Assessment of Estuarine Trophic Status Eutrophication Index 

21 (ASSETS EI). This index is a six level index characterizing overall eutrophication risk in a 

22 waterbody. This indictor is not sensitive to changes in nitrogen deposition within a single level 
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1 of the index. In addition, this type of indicator does not reflect the impact of nitrogen deposition 

2 in conjunction with other sources of nitrogen. 

3 Based on the above considerations, the REA concludes that the ASSETS EI is not an 

4 appropriate ecological indicator for estuarine aquatic eutrophication and that additional analysis 

5 is required to develop an appropriate indicator for determining the appropriate levels of 

6 protection from N nutrient enrichment effects in estuaries related to deposition of oxides of 

7 nitrogen. As a result, EPA is unable to make a determination as to the adequacy of the existing 

8 secondary oxides of nitrogen standard in protecting public welfare from N nutrient enrichment 

9 effects in estuarine aquatic ecosystems.

10 Additionally, nitrogen deposition can alter species composition and cause eutrophication 

11 in freshwater systems. In the Rocky Mountains, for example, deposition loads of 1.5 to 2 kg/ha12 yr which are well within current ambient levels are known to cause changes in species 

13 composition in diatom communities indicating impaired water quality (US EPA, 2008, section 

14 3.3.5.3). This suggests that the existing secondary standard for oxides of nitrogen may not 

15 protect such ecosystems and their resulting services from impairment.

16 v. Other effects

17 An important consideration in looking at the effects of deposition of oxides of sulfur in 

18 aquatic ecosystems is the potential for production of methylmercury, a neurotoxic contaminant. 

The production of meaningful amounts of methylmercury (MeHg) requires the presence of SO4

2-

19

and mercury, and where mercury is present, increased availability of SO4

2-

20 results in increased 

21 production of MeHg. There is increasing evidence on the relationship between sulfur deposition 

22 and increased methylation of mercury in aquatic environments; this effect occurs only where 

23 other factors are present at levels within a range to allow methylation. The production of 
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1 methylmercury requires the presence of sulfate and mercury, but the amount of methylmercury

2 produced varies with oxygen content, temperature, pH, and supply of labile organic carbon (US 

3 EPA, 2008, section 3.4). In watersheds where changes in sulfate deposition did not produce an 

4 effect, one or several of those interacting factors were not in the range required for meaningful 

5 methylation to occur (US EPA, 2008, section 3.4). Watersheds with conditions known to be 

6 conducive to mercury methylation can be found in the northeastern United States and 

7 southeastern Canada (US EPA, 2009, section 6). 

8 With respect to sulfur deposition and mercury methylation, the final ISA determined: The 

9 evidence is sufficient to infer a causal relationship between sulfur deposition and increased 

10 mercury methylation in wetlands and aquatic environments. However, EPA did not conduct a 

11 quantitative assessment of the risks associated with increased mercury methylation under current 

12 conditions. As such, EPA is unable to make a determination as to the adequacy of the existing 

13 SO2 secondary standards in protecting against welfare effects associated with increased mercury

14 methylation.

15 vi. Summary of Adequacy Considerations

16 In summary, the Policy Assessment concludes that currently available scientific evidence 

17 and assessments clearly call into question the adequacy of the current standards with regard to 

18 deposition-related effects on sensitive aquatic and terrestrial ecosystems, including acidification 

19 and nutrient enrichment. Further, the Policy Assessment recognizes that the elements of the 

20 current standards -- indicator, averaging time, level and form – are not ecologically relevant, and 

21 are thus not appropriate for standards designed to provide such protection. Thus, the Policy

22 Assessment concludes that consideration should be given to establishing a new ecologically

23 relevant multi-pollutant, multimedia standard to provide appropriate protection from deposition-
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1 related ecological effects of oxides of nitrogen and sulfur on sensitive ecosystems, with a focus 

2 on protecting against adverse effects associated with acidifying deposition in sensitive aquatic 

3 ecosystems.

4 3. CASAC Views

5 In a letter to the Administrator (Russell and Samet 2011a), the CASAC Oxides of 

6 Nitrogen and Oxides of Sulfur Panel, with full endorsement of the chartered CASAC, 

7 unanimously concluded that:

8 EPA staff has demonstrated through the Integrated Science Assessment (ISA), 

9 Risk and Exposure Characterization (REA) and the draft PA that ambient NOx 

10 and SOx can have, and are having, adverse environmental impacts. The Panel 

11 views that the current NOx and SOx secondary standards should be retained to 

12 protect against direct adverse impacts to vegetation from exposure to gas phase 

13 exposures of these two families of air pollutants. Further, the ISA, REA and draft 

14 PA demonstrate that adverse impacts to aquatic ecosystems are also occurring due 

15 to deposition of NOx and SOx. Those impacts include acidification and 

16 undesirable levels of nutrient enrichment in some aquatic ecosystems. The levels 

17 of the current NOx and SOx secondary NAAQS are not sufficient, nor the forms 

18 of those standards appropriate, to protect against adverse depositional effects; thus 

19 a revised NAAQS is warranted.

20

21 In addition, with regard to the joint consideration of both oxides of nitrogen and oxides 

22 of sulfur as well as the consideration of deposition related effects, CASAC concluded that the 

23 Policy Assessment had developed a credible methodology for considering such effects. The 

24 Panel stated that “the Policy Assessment develops a framework for a multi-pollutant, multimedia 

25 standard that is ecologically relevant and reflects the combined impacts of these two pollutants as 

26 they deposit to sensitive aquatic ecosystems.”

27 4. Administrator’s Proposed Conclusions Concerning Adequacy of Current Standard

28 Based on the above considerations and taking into account CASAC advice, the 

29 Administrator recognizes that the purpose of the secondary standard is to protect against 

30 “adverse” effects resulting from exposure to oxides of nitrogen and sulfur, discussed above in 
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1 section IIA. The Administrator also recognizes the need for conclusions as to the adequacy of the 

2 current standards for both direct and deposition related effects as well as conclusions as to the 

3 appropriateness and ecological relevance of the current standards. 

4 In considering what constitutes an ecological effect that is also adverse to the public 

5 welfare, the Administrator took into account the ISA conclusions regarding the nature and 

6 strength of the effects evidence, the risk and exposure assessment results, the degree to which the 

7 associated uncertainties should be considered in interpreting the results, the conclusions 

8 presented in the Policy Assessment, and the views of CASAC and members of the public. On 

9 these bases, the Administrator concludes that the current secondary standards are adequate to 

10 protect against direct phytotoxic effects on vegetation. Thus, the Administrator proposes to 

retain the current secondary standard for oxides of nitrogen at 53 ppb,411 annual average 

12 concentration, measured in the ambient air as NO2, and the current secondary standard for oxides 

13 of sulfur at 0.5 ppm, 3-hour average concentration, measured in the ambient air as SO2. 

14 With regard to deposition-related effects, the Administrator has first to consider the 

15 appropriateness of the structure of the current standards to address ecological effects of concern. 

16 Based on the evidence as well as considering the advice given by CASAC on this matter, the 

17 Administrator concludes that the elements of the current standards are not ecologically relevant 

18 and thus are not appropriate to provide protection of ecosystems. On the subject of adequacy of 

19 protection with regard to deposition-related effects, the Administrator considered the full nature 

20 of ecological effects related to the deposition of ambient oxides of nitrogen and sulfur into 

21 sensitive ecosystems across the country. Her conclusions are based on the evidence presented in 

22 the ISA with regard to acidification and nutrient enrichment effects, the findings of the REA with 

 

4 The annual secondary standard for oxides of nitrogen is being specified in units of ppb to conform to the current 

version of the annual primary standard, as specified in the final rule for the most recent review of the NO2 primary 

NAAQS (75 FR 6531; February 9, 2010).
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1 regard to scope and severity of the current and likely future effects of deposition, the synthesis of 

2 both the scientific evidence and risk and exposure results in the Policy Assessment as to the 

3 adequacy of the current standards, and the advice of both CASAC and the public. After such 

4 consideration, the Administrator concludes that current levels of oxides of nitrogen and sulfur are 

5 sufficient to cause acidification of both aquatic and terrestrial ecosystems, nutrient enrichment of 

6 terrestrial ecosystems and contribute to nutrient enrichment effects in estuaries that could be 

7 considered adverse, and the current secondary standards do not provide adequate protection from 

8 such effects. 

9 Having reached these conclusions, the Administrator determines that it is appropriate to 

10 consider alternative standards that are ecologically relevant. These considerations support the 

11 conclusion that the current secondary standards is neither appropriate nor adequate to protect 

12 against deposition related effects. The Administrator’s consideration of such alternative 

13 standards is discussed below in Section III.