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1 ENVIRONMENTAL PROTECTION AGENCY 1 40 CFR Part 50 2 [EPA - HQ - OAR- 2007 - 1145] 3 RIN: 2060 - AO72 4 Secondary National Ambient Air Quality Standards for Oxides of Nitrogen and Sulfur 5 6 AGENCY: Environmental Protection Agency (EPA). 7 ACTION: Proposed rule. 8 SUMMARY: Based on its review of the air quality criteria and the secondary national ambient 9 air quality

standards (NAAQS) for oxides of nitrogen and oxides of sulfur, EPA proposes to 10 retain the current nitrogen dioxide (NO 2 ) and sulfur dioxide (SO 2 ) secondary stand ards to 11 provide requisite protection for the direct effects on vegetation resulting from exposure to 12 gaseous oxides of nitrogen and sulfur in the ambient air. With regard to protection from the 13 deposition of oxides of nitrogen and sulfur to sensitive aquatic and terrestrial ecosy stems, 14 including acidification and nutrient enrichment effects, EPA is proposing to add secondary 15 standards identical to the NO 2 and SO 2 primary 1

- hour standards and not set a new multi

16 pollutant secondary standard in this review

. The new 1

- hour secondary NO 2 standard is set at a 17 level of 100 ppb and the new 1

- hour secondary SO 2 standard is set at 75 ppb. The existing 18 secondary standards for NOx and SOx will also remain in effect. In addition , EPA has decided 19 to undertake a field pilot program to gather and analyze additional relevant data so as to enhance 20 the Agency ’s understanding of the degree of protectiveness that a new multi

- pollutant approach , 21 defined in terms of an aquatic acidification index (AAI), wo uld afford and to support 22

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2 development of an appropriate monitoring network for such a standard. The EPA solicits 1 comment on the framework of such a standard and on the design of the field pilot program. 2 DATES: Written comments on this proposed rule must be received b y [insert date 60 days 3 after date of publication in the Federal Register]

. 4 Public Hearings : The EPA intends to hold a public hearing around the end of August to 5 earl y September and will announce in a separate Federal Register notice the date , time, and 6 address of the public hearing on this proposed rule. 7 ADDRESSES: Submit your comments, identified by Docket ID No. EPA

- HQ

- OAR

- 2007

- 8 1145, by

one of the following methods: 9

 www.regulations.gov : Follow the o n - line instructions for submitting comments. 10

 Email: a - and

- r - Docket@epa.gov

. 11

 Fax: 202

- 566

- 1741. 12

 Mail: Docket No. EPA

- HQ

- OAR

- 2007

- 1145, Environmental Protection Agency , Mail 13 code 6102T, 1200 Pennsylvania Ave., NW., Washington, DC 20460. Please include a 14 total of two copies. 15

 Hand Delivery : Docket No. EPA

- HQ

- OAR

- 2007

- 1145, Environmental Protection 16 Agency , EPA West, Room 3334, 1301 Constitution Ave., NW, Washington, DC. Such 17 deliveries are onl y accepted during the Docket’s normal hours of operation, and special 18 arrangements should be made for deliveries of boxed information. 19 Instructions: Direct your comments to Docket ID No. EPA

- HQ

- OAR

- 2007

- 1145. The 20 EPA’s policy

is that all comments received will be included in the public docket without change 21 and may

be made available online at www.regulations.gov , including an y personal information 22 provided, unless the comment includes information claimed to be Confidential Business 23

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3 In formation (CBI) or other information whose disclosure is restricted b y statute. Do not submit 1 information that you consider to be CBI or otherwise protected through www.regulations.gov or 2 email. The www.regulations.gov website is an “anony mous access” sy stem, which means EPA 3 will not know your identity or contact information unless you provide it in the body of your 4 comment. If you send an email comment directl y to EPA wi thout going through 5 www.regulations.gov , your email address will be automatically captured and included as part of 6 the comment that is placed in the public docket and made available on the Internet. If you 7 submit an electronic comment, EPA recommends that you include your name and other contact 8 information in the body of your comment and with any disk or CD

- ROM you submit. I f EPA 9 cannot read your comment due to technical difficulties and cannot contact you for clarification, 10 EPA may

not be able to consider your comment. Electronic files should avoid the use of special 11 characters, an y form of encry ption, and be free of any

defects or viruses. For additional 12 information about EPA’s public docket, visit the EPA Docket Center homepage at 13 http://www.epa.gov/epahome/dockets.htm

. 14 Docket : All documents in the docket are listed in the www.regulations.gov index. 15 Although listed in the index, some information is not publicly available, e.g., CBI or other 16 information whose disclosure is restricted b y statute. Certain other material, such as cop yri ghted 17 material, will be publicly available onl y in hard copy . Publicly available doc ket materials are 18 available either electronically in www.regulations.gov or in hard cop y at the Air and Radiation 19 Docket and Information Center, EPA/DC, EPA West, Room 3334, 1301 Constitution Ave., NW, 20 Washington, D C. The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday 21 through Friday , excluding legal holiday s. The telephone number for the Public Reading Room is 22

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4 (202) 566

- 1744 and the telephone number for the Air and Radiation Docket and Information 1 Center is (202) 566

- 1742. 2 FOR FURTHER INFORMATION CONTACT: Dr. Richard Scheffe, Office of Air Quality 3 Planning and Standards, U.S. Environmental Protection Agency , Mail code C304

- 02, Research 4 Triangle Park, NC 27711; telephone: 919

- 541

- 4650; fax: 919

- 5 41

- 2357; email: 5 scheffe.rich@epa.gov. 6 SUPPLEMENTARY INFORMATION: 7 General Information 8 What Should IConsider as I Prepare M y Comments for EPA? 9 1. Submitting CBI. Do not submit this information to EPA through www.regulations.gov or 10 email. Clearl y mark the part or all of the information that you claim to be CBI. For CBI 11 information in a disk or CD ROM that you mail to EPA, mark the outside of the disk or 12 CD ROM as CBI and then identify electronically

within th e disk or CD ROM the specific 13 information that is claimed as CBI. In addition to one complete version of the comment 14 that includes information claimed as CBI, a cop y of the comment that does not contain 15 the information claimed as CBI must be submitted for inclusion in the public docket. 16 In formation so marked will not be disclosed except in accordance with procedures set 17 forth in 40 CFR part 2. 18 2. Tips for Preparing Your Comments . When submitting comments, remember to: 19

 Id entify

the rulemaking by

docket numbe r and other identifying information (subject 20 heading, Federal Register date and page number). 21

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5  Follow directions

– The Agency may ask you to respond to specific questions or 1 organize comments by referencing a Code of Federal Regulations (CFR) part or secti on 2 number. 3

 Explain why you agree or disagree, suggest alternatives, and substitute language for your 4 requested changes. 5

 Describe an y assumptions and provide an y technical information and/or data that you 6 used. 7

 If you estimate potential costs or burdens, explain how you arrived at your estimate in 8 sufficient detail to allow for it to be reproduced. 9

 Provide specific examples to illustrate your concerns, and suggest alternatives. 10

 Explain your views as clearl y as possible. Make sure to submit your comments b y the 11 comment period deadline identified. 12 Availability

of Related Information 13 A number of documents relevant to this rulemaking are available on EPA web sites. The 14 Integrated Science Assessment for Oxides of Nitrogen and Sulfur

- Ecological Criteria: FINAL 15 REPORT (I SA) is available on EPAs National Center for Environmental Assessment web site. 16 To obtain this document, go to http://www.epa.gov/ncea , and click on Air Quality then click on 17 Oxides of Nitrogen and Sulfur. The Policy Assessment, Risk and Exposure Assessment (REA) , 18 and other related technical documents are available on EPA’s Office of Air Quality Planning and 19 Standards (OAQPS) Technology Transfer Network (TTN) web site. The Policy Assessment is 20 available at http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_pa.html, and the exposure and 21 risk assessments and other related technical documents are available at 22

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6 http://www.epa.gov/ttn/na aqs/standards/no2so2sec/cr_rea.html

. These and other related 1 documents are also available for inspection and copying in the EPA docket identified above. 2 Table of Contents 3 The following topics are discussed in this preamble: 4 I. Background 5 A. Legislative R equirements 6 B. History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides 7 1. NAAQS for Oxides of Nitrogen 8 2. NAAQS for Oxides of Sulfur 9 C. History of Related Assessments and Agency Actions 10 D. History of the Current Review 11 E. Scope of the Current Review 12 II. Rationale for Proposed Decision on the Adequacy of the Current Secondary Standards 13 A. Ecological Effects 14 1. Effects Associated with Gas

- Phase Oxides of Nitrogen and Sulfur 15 a. Nature of ecos ys tem responses to gas
- phase nitrogen and sulfur 16 b. Magnitu de of ecos ys tem response to gas
- phase nitrogen and sulfur 17 2. Acidification Effects Associated with Deposition of Oxides of N itrogen 18 and Sulfur 19 a. Nature of Acidification

–related Ecosy stem Responses 20 i. Aquatic ecos ys tems 21 ii. Terrestrial ecos ys tems 22 iii. Ecosy stem sensitiv ity 23 b. Magnitude of Acidification

- related Ecos ys tem Responses 24 i. Aquatic acidification 25 ii. Terrestrial acidification 26 c. Key Uncertainties Associated with Acidification 27 i. Aquatic acidification 28 ii. Terrestrial acidification 29 3. Nutrient Enrichment Effects Associated with Deposition of Oxides of 30 Nitrogen 31 a. Nature of Nutrient Enrichment

- related Ecos ys tem Responses 32 i. Aquatic ecos ys tems 33 ii. Terrestrial ecos ys tems 34 iii. Ecosy stem sensitiv ity to nutrient enrichment 35 b. Magnitude of Nutrient Enrichment

- related Ecos ys tem Responses 36 i. Aquatic ecos ys tems 37 ii. Terrestrial ecos ys tems 38 c. Key Uncertainties Associated with Nutrient Enrichment 39 i. Aquatic ecos ys tems 40 ii. Terrestrial ecos ys tems 41 4. Other Ecological Effects 42

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7 B. Risk and Exposure Assessment 1 1. Overview of Risk and Exposure Assessment 2 2. Key Findings 3 a. Air quality anal ys es 4 b. Deposition

- related aquatic acidification 5 c. Deposition

- related terrestrial acidification 6 d. Deposition

- related aquatic nutrient enrichment 7 e. Deposition

- related terrestrial nutrient enrichment 8 f. Additional effects 9 3. Conclusions on Effects 10 C. Adversity of Effects to Public Welfare 11 1. Ecosy stem Services 12 2. Effects on Ecosy stem Services 13 a. Aquatic Acidification 14 b. Terrestrial Acidification 15 c. Nutrient Enrichment 16 3. Summary 17 D. Adequacy of the Current Standards 18 1. Adequacy of the Current Standards for Direct Effects 19 2. Appropriateness and Adequacy of the Current Standards for Deposition

20 related Effects 21 a. Appropriateness 22 b. Adequacy of Protection 23 i. Aquatic Acidification 24 ii. Terrestrial acidification 25 iii. Terrestrial nutrient enrichment 26 iv. Aquatic nutrient e nrichment 27 v. Other effects 28 3. CASAC Views 29 4. Administrator’s Proposed Conclusions Concerning Adequacy of Current 30 Standard 31 III. Rationale for Proposed Decision on Alternative Multi

- pollutant Approach to Secondary 32 Standard s for Aquatic Acidification 33 A. Ambient Air Indicators 34 1. Oxides of Sulfur 35 2. Oxides of Nitrogen 36 B. Form 37 1. Ecological Indicator 38 2. Linking ANC to Deposition 39 3. Linking Deposition to Ambient Air Indicators 40 4. Aquatic Acidification Index 41 5. Spatial Aggregation 42 a. Ecoregion Sensitivity 43 b. Representative Ecoregion

- specific Factors 44 i. Factor F1 45 a) Acid

- sensitive Ecoregions 46

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8 b) Non
- acid sensitive Ecoregions 1 ii. Factor F2 2 iii. Factors F3 and F4 3 c. Factors in Data

- limited Ecoregions 4 d. Application to Hawaii, Alaska, and the U.S. Territories 5 6. Summary of the AAI Form 6 C. Averaging Time 7 D. Level 8 1. Association Between pH Lev els and Target ANC Levels 9 2. ANC Levels Related to Effects on Aquatic Ecos ys tems 10 3. Consideration of Episodic Acidity 11 4. Consideration of Ecos ys tem Response Time 12 5. Prior Examples of Target ANC Levels 13 6. Consideration of Public Welfare Benefits 14 7. Summary of Alternative Levels 15 E. Combined Alternative Levels and Forms 16 F. Characterization of Uncertainties 17 1. Overview of Uncertainty 18 2. Uncertainties Associated with Data Gaps 19 3. Uncertainties in Modeled Processes 20 G. CASAC Advice 21 H. Administrator’s Proposed Conclusions 22 IV
. Field Pilot Program and Ambient Monitoring 23 A. Field Pilot Program 24 a. Objectives 25 b. Overview of Field Pilot Program 26 c. Complementary Measurements 27 d. Complementary Research Efforts 28 e. Implementation Challenges 29 f. Final Monitoring Plan Development and Stakeholder Participation 30 B. Evaluation of Monitoring Methods 31 1. Potential FRMs for SO 2 and p

- SO 4 32 2. Potential FRM for NO y 33 V. Statutory

and Executive Order Reviews 34 A. Executive Order 12866: Regulatory Planning and Review 35 B. Paperwork Reduction Act 36 C. Regulatory Flexibility

Act 37 D. Unfunded Mandates Reform Act 38 E. Executiv e Order 13132: Federalism 39 F. Executive Order 13175: Consultation and Coordination with Indian Tribal 40 Governments 41 G. Executive Order 13045: Protection of Children From Environmental Health and 42 Safety Risks 43 H. Executive Order 13211: Actions that Significantly Aff ect Energy

Suppl y, 44 Distribution, or Use 45 I. National Technology Transfer and Advancement Act 46

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9 J. Executive Order 12898: Federal Actions to Address Environmental Justice in 1 Minority Populations and L ow - Income Populations 2 References 3 4 I. Background 5 A. Legislative Requirements 6 Two sections of the Clean Air Act (CAA) govern the establishment and revision of the 7 NAAQS. Section 108 (42 U.S.C. section 7408) directs the Administrator to identify and list 8 certain air pollutants and then to issue air quality criteria for those pollutants. The Administrator 9 is to list those air pollutants that in her “judgment, cause or contribute to air pollution which may 10 reasonabl y be anticipated to endanger public health or welfare;” “the presence of which in the 11 ambient air results f rom numerous or diverse mobile or stationary sources;” and “for which . . . 12 [the Administrator] plans to issue air quality criteria...” Air quality criteria are intended to 13 “accuratel y reflect the latest scientific knowledge useful in indicating the kind an d extent of all 14 identifiable effects on public health or welfare which may be expected from the presence of [a] 15 pollutant in the ambient air . . .” 42 U.S.C. § 7408(b). Section 109 (42 U.S.C. 7409) directs the 16 Administrator to propose and promulgate “pri mary” and “secondary” NAAQS for pollutants for 17 which air quality criteria are issued. Section 109(b)(1) defines a primary standard as one “the 18 attainment and maintenance of which in the judgment of the Administrator, based on such 19 criteria and allowing an adequate margin of safety , are requisite to protect the public health.” 1 A 20 secondary

standard, as defined in section 109(b)(2), must “specify a level of air quality the 21 attainment and maintenance of which, in the judgment of the Administrator, based on such 22 criteria, is requisite to protect the public welfare from any known or anticipated adverse effects 23

1 The legislative history of section 109 ind icates that a primary standard is to be set at “the maximum permissible am bient 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 per sons 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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10 associated with the presence of [the] pollutant in the ambient air.” Welfare effects as defined in 1 section 302(h) (42 U.S.C. § 7602(h)) include, but are not limited to, “effects on soils, water, 2 crops, vegetation, man

- made materials, animals, wildlife, weather, visibility and climate, damage 3 to and deterioration of property , and hazards to transportation, as well as effects on economic 4 values and on person al comfort and well

- being.” 5 In setting standards that are “requisite” to protect public health and welfare, as provided 6 in section 109(b), EPA’s task is to establish standards that are neither more nor less stringent 7 than necessary

for these purposes. In so doing, EPA may not consider the costs of implementing 8 the standards. See generally , Whitman v. American Trucking Associations, 531 U.S. 457, 465

- 9 472, 475

- 76 (2001). Likewise, “[a]ttainability and technological feasibility are not relevant 10 considerati ons in the promulgation of national ambient air quality standards.” American 11 Petroleum Institute v. Costle, 665 F. 2d at 1185. Section 109(d)(1) requires that “not later than 12 December 31, 1980, and at 5

- ye ar intervals thereafter, the Administrator shall c omplete a 13 thorough review of the criteria published under section 108 and the national ambient air quality 14 standards . . . and shall make such revisions in such criteria and standards and promulgate such 15 new standards as may be appropriate . . . .” Sectio n 109(d)(2) requires that an independent 16 scientific review committee “shall complete a review of the criteria . . . and the national primary 17 and secondary

ambient air quality

standards . . . and shall recommend to the Administrator any 18 new . . . standards and revisions of existing criteria and standards as may be appropriate . . . .” 19 Since the earl y 1980's, this independent review function has been performed by the Clean Air 20 Scientific Advisory Committee (CASAC). 21 B. History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides 22 1. NAAQS for Oxides of Nitrogen 23

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11 After reviewing the relevant science on the public health and welfare effects associated 1 with oxides of nitrogen, EPA promulgated identical primary and secondary NAAQS for NO2 in 2 April 1971. These standards were set at a level of 0.053 parts per million (ppm) as an annual 3 average (36 FR 8186). In 1982, EPA published Air Quality Criteria for Oxides of Nitrogen (US 4 EPA, 1982), which updated the scientific criteria upon which the initial standards we re based. In 5 February

1984 EPA proposed to retain these standards (49 FR 6866). After taking into account 6 public comments, EPA published the final decision to retain these standards in June 1985 (50 FR 7 25532). 8 The EPA began the most recent previous revi ew of the oxides of nitrogen secondary 9 standards in 1987. In November 1991 EPA released an updated draft air quality criteria 10 document (AQCD ) for CASAC and public review and comment (56 FR 59285), which provided 11 a comprehensive assessment of the available scientific and technical information on health and 12 welfare effects associated with NO2 and other oxides of nitrogen. The CASAC reviewed the 13 draft document at a meeting held on July 1, 1993 and concluded in a closure letter to the 14 Administrator that the document “provides a scientifically balanced and defensible summary of 15 current knowledge of the effects of this pollutant and provides an adequ ate basis for EPA to 16 make a decision as to the appropriate NAAQS for NO2” (Wolff, 1993). The Air Quality Criteria 17 for Oxides of Nitrogen was then finalized (US EPA, 199 5 a). EPA’s OAQPS also prepared a 18 Staff Paper that summarized and integrated the key studies and scientific evidence contained in 19 the revised AQCD for oxides of nitrogen and identified the critical elements to be considered in 20 the review of the NO2 NAAQS. CASAC reviewed two drafts of the Staff Paper and concluded 21 in a closure letter to the Administrator that the document provided a “scientifically adequate 22 basis for regulatory decisions on nitrogen dioxide” (Wolff, 1995). 23

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12 In October 1995 the Administrator announced her proposed decision not to revise either 1 the primary

or secondary NAAQS fo r NO2 (60 FR 52874; October 11, 1995). A year later, the 2 Administrator made a final determination not to revise the NAAQS for NO2 after careful 3 evaluation of the comments received on the proposal (61 FR 52852; October 8, 1996). While the 4 primary

NO2 stan dard was revised in January 2010 by supplementing the existing annual 5 standard with the establishment of a new 1

- hour standard , set at a level of 100 ppb (75 FR 6474), 6 the secondary

NAAQS for NO2 remains 0.053 ppm (100 micrograms per cubic meter [μg/m3] of 7 air), annual arithmetic average, calculated as the arithmetic mean of the 1

- hour NO2 8 concentrations. 9 2. NAAQS for Oxides of Sulfur 10 EPA promulgated primary and secondary

NAAQS for SO2 in April 1971 (36 FR 8186). 11 The secondary

standards included a standard set at 0.02 ppm, annual arithmetic mean, and a 3

- 12 hour average standard set at 0.5 ppm, not to be exceeded more than once per year. These 13 secondary

standards were established solel y on the basis of evidence of adverse effects on 14 vegetation. In 1973, rev isions made to Chapter 5 (“Effects of Sulfur Oxide in the Atmosphere on 15 Vegetation”) of Air Quality Criteria for Sulfur Oxides (US EPA, 1973) indicated that it could not 16 properl y be concluded that the vegetation injury reported resulted from the average SO 2 17 exposure over the growing season, rather than from short

- term peak concentrations. Therefore, 18 EPA proposed (38 FR 11355) and then finalized (38 FR 25678) a revocation of the annual mean 19 secondary

standard. At that time, EPA was aware that then

- current concentrations of oxides of 20 sulfur in the ambient air had other public welfare effects, including effects on materials, 21 visibility , soils, and water. However, the available data were considered insufficient to establish 22

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13 a quantitative relationship between specific ambient concentrations of oxides of sulfur and such 1 public welfare effects (38 FR 25679). 2 In 1979, EPA announced that it was revising the AQCD for oxides of sulfur concurrentl y 3 with that for particulate matter (PM) and would produce a combined PM and oxides of sulfur 4 criteria document. Following its review of a draft revised criteria document in August 1980, 5 CASAC concluded that acid deposition was a topic of extreme scientific complexity because of 6 the difficulty

in establishing firm quantitativ e relationships among (1) emissions of relevant 7 pollutants (e.g., SO2 and oxides of nitrogen), (2) formation of acidic wet and dry deposition 8 products, and (3) effects on terrestrial and aquatic ecos ys tems. CASAC also noted that acid 9 deposition involves, at a minimum, several different criteria pollutants: oxides of sulfur, oxides 10 of nitrogen, and the fine particulate fraction of suspended particles. CASAC felt that any 11 document on this subject should address both wet and dry deposition, since dry

deposi tion was 12 believed to account for a substantial portion of the total acid deposition problem. 13 For these reasons, CASAC recommended that a separate, comprehensive document on 14 acid deposition be prepared prior to any consideration of using the NAAQS as a regulatory 15 mechanism for the control of acid deposition. CASAC also suggested that a discussion of acid 16 deposition be included in the AQCDs for oxides of nitrogen and PM and oxides of sulfur. 17 Following CASAC closure on the AQCD for oxides of sulfur in December 1981, EPA’s OAQPS 18 published a Staff Paper in November 1982, although the paper did not directly assess the issue of 19 acid deposition. Instead, EPA subsequently prepared the following documents to address acid 20 deposition: The Acidic Deposition Phenomen on and Its Effects: Critical Assessment Review 21 Papers, Volumes I and II (US EPA, 1984a, b) and The Acidic Deposition Phenomenon and Its 22 Effects: Critical Assessment Document (US EPA, 1985) (53 FR 14935

- 14936). These 23

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14 documents, though they

were not consid ered criteria documents and did not undergo CASAC 1 review, represented the most comprehensive summary of scientific information relevant to acid 2 deposition completed by EPA at that point. 3 In April 1988 (53 FR 14926), EPA proposed not to revise the existing primary and 4 secondary

standards for SO 2 . This proposed decision with regard to the secondary SO 2 NAAQS 5 was due to the Administrator’s conclusions that (1) based upon the then

- current scientific 6 understanding of the acid deposition problem, it would be premature and unwise to prescribe any 7 regulatory

control program at that time and (2) when the fundamental scientific uncertainties had 8 been decreased through ongoing research efforts, EPA would draft and support an appropriate set 9 of control measures. Alth ough EPA revised the primary SO 2 standard in June 2010 by 10 establishing a new 1

- hour standard at a level of 75 ppb and revoking the existing 24

- hour and 11 annual standards (75 FR 35520), no further decisions on the secondary SO 2 standard have been 12 published. 13 C. History of Related Assessments and Agency Actions 14 In 1980, the Congress created the National Acid Precipitation Assessment Program 15 (NAPAP) in response to growing concern about acidic deposition. The NAPAP was given a 16 broad 10

- ye ar mandate to examine the causes and effects of acidic deposition and to explore 17 alternative control options to alleviate acidic deposition and its effects. During the course of the 18 program, the NAPAP issued a series of publicl y available interim reports prior to the completi on 19 of a final report in 1990 (NAPAP, 1990). 20 In spite of the complexities and significant remaining uncertainties associated with the 21 acid deposition problem, it soon became clear that a program to address acid deposition was 22 needed. The Clean Air Act Amendments of 1990 included numerous separate provisions related 23

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15 to the acid deposition problem. The primary and most important of the provisions, the 1 amendments to Title IV of the Act, established the Acid Rain Program to reduce emissions of 2 SO 2 by

10 milli on tons and emissions of nitrogen oxides by 2 million tons from 1980 emission 3 levels in order to achieve reductions over broad geographic regions. In this provision, Congress 4 included a statement of findings that led them to take action, concluding that ( 1) the presence of 5 acid compounds and their precursors in the atmosphere and in deposition from the atmosphere 6 represents a threat to natural resources, ecos ys tems, materials, visibility , and public health; (2) 7 the problem of acid deposition is of national and international significance; and (3) current and 8 future generations of Americans will be adversely affected by

delaying measures to remedy the 9 problem. 10 Second, Congress authorized the continuation of the NAPAP in order to assure that the 11 research and monitoring efforts already undertaken would continue to be coordinated and would 12 provide the basis for an impartial assessment of the effectiveness of the Title IV program. 13 Third, Congress considered that further action might be necessary in the long ter

m to 14 address an y problems remaining after implementation of the Title IV program and, reserving 15 judgment on the form that action could take, included Section 404 of the 1990 Amendments 16 (Clean Air Act Amendments of 1990, Pub. L . 101

- 549, § 404) requiring EP A to conduct a study 17 on the feasibility

and effectiveness of an acid deposition standard or standards to protect 18 “sensitive and critically sensitive aquatic and terrestrial resources.” At the conclusion of the 19 study , EPA was to submit a report to Congress . Five years later, EPA submitted its report, 20 entitled Acid Deposition Standard Feasibility Study : Report to Congress (US EPA, 1995 b ) in 21 fulfillment of this requirement. That Report concluded that establishing acid deposition 22 standards for sulfur and nitrogen deposition may at some point in the future be technically 23

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16 feasible, although appropriate deposition loads for these acidifying chemicals could not be 1 defined with reasonable certainty at that time. 2 Fourth, the 1990 Amendments also added new languag e to sections of the CAA 3 pertaining to the scope and application of the secondary NAAQS designed to protect the public 4 welfare. Specifically , the definition of “effects on welfare” in Section 302(h) was expanded to 5 state that the welfare effects include e ffects “...whether caused by transformation, conversion, or 6 combination with other air pollutants.” 7 In 1999, seven Northeastern states cited this amended language in Section 302(h) in a 8 petition asking EPA to use its authority under the NAAQS program to promulgate secondary 9 NAAQS for the criteria pollutants associated with the formation of acid rain. The petition stated 10 that this language “clearly references the transformation of pollutants resulting in the inevitable 11 formation of sulfate and nitrate aeros ols and/or their ultimate environmental impacts as wet and 12 dry deposition, clearl y signaling Congressional intent that the welfare damage occasioned b y 13 sulfur and nitrogen oxides be addressed through the secondary standard provisions of Section 14 109 of the Act.” The petition further stated that “recent federal studies, including the NAPAP 15 Biennial Report to Congress: An Integrated Assessment, document the continued and increasing 16 damage being inflicted by acid deposition to the lakes and forests of New York , New England 17 and other parts of our nation, demonstrating that the Title IV program had proven insufficient.” 18 The petition also listed other adverse welfare effects associated with the transformation of these 19 criteria pollutants, including impaired visibi lity , eutrophication of coastal estuaries, global 20 warming, and tropospheric ozone and stratospheric ozone depletion. 21 In a related matter, the Office of the Secretary of the U.S. Department of Interior (DOI) 22 requested in 2000 that EPA initiate a rulemaking proceeding to enhance the air quality in 23

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17 national parks and wilderness areas in order to protect resources and values that are being 1 adversel y affected b y air pollution. Included among the effects of concern identified in the 2 request were the acidificatio n of streams, surface waters, and/or soils; eutrophication of coastal 3 waters; visibility

impairment; and foliar injury from ozone. 4 In a Federal Register notice in 2001, EPA announced receipt of these requests and asked 5 for comment on the issues raised in them. EPA stated that it would consider any relevant 6 comments and information submitted, along with the information provided by the petitioners and 7 DOI, before making an y decision concerning a response to these requests for rulemaking (65 FR 8 48699). 9 The 2005 NAPAP report states that “... scientific studies indicate that the emission 10 reductions achieved b y Title IV are not sufficient to allow recovery of acid

- sensitive ecos ys tems. 11 Estimates from the literature of the scope of additional emission reductions that are necessary in 12 order to protect acid

- sensitive ecosy stems range from approximately 40

- 80% beyond full 13 implementation of Title IV....” The results of the modeling presented in this Report to Congress 14 indicate that broader recovery is not predicted w ithout additional emission reductions (NAPAP , 15 2005 ). 16 Given the state of the science as described in the ISA, REA, and in other recent reports, 17 such as the NAPAP reports noted above, EPA has decided, in the context of evaluating the 18 adequacy

of the current NO2 and SO2 secondary standards in this review, to revisit the question 19 of the appropriateness of setting secondary NAAQS to address remaining known or anticipated 20 adverse public welfare effects resulting from the acidic and nutrient deposition of these c riteria 21 pollutants. 22 D. History of the Current Review 23

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18 The EPA initiated this current review in December 2005 with a call for information (70 1 FR 73236) for the development of a revised Integrated Science Assessment for Oxides of 2 Nitrogen and Oxides of Sulfur - Ecological Criteria (henceforth the "ISA"). An Integrated 3 Review Plan (IRP) was developed to provide the framework and schedule as well as the scope of 4 the review and to identify policy

- relevant questions to be addressed in the components of the 5 review . The IRP was released in 2007 (US EPA , 2007) for CASAC and public review. EPA 6 held a workshop in July 2007 on the I SA to obtain broad input from the relevant scientific 7 communities. This workshop helped to inform the preparation of the first draft ISA , which was 8 released for CASAC and public review in December 2007; a CASAC meeting was held on April 9 2

- 3, 2008 to review the first draft ISA. A second draft ISA was released for CASAC and public 10 review in August 2008, and was discussed at a CASAC meeting h eld on October 1

- 2, 2008. T he 11 final I SA (US EPA, 2008) was released in December 2008. 12 Based on the science presented in the I SA, EPA developed a Risk and Exposure 13 Assessment for Review of the Secondary National Ambient Air Quality Standards for Oxides of 14 Nitrogen and Oxides of Sulfur (henceforth the “REA”) to further assess the national impact of 15 the effects documented in the I SA. The Draft Scope and Methods Plan for Risk/Exposure 16 Assessment: Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur outlining 17 the scope and design of the future REA was prepared for CASAC and public in March 2008. A 18 first draft REA was presented to CASAC and the public for review in August 2008 and a second 19 draft was presented for review in June 2009. The final REA (US EPA 2009) was released in 20 September 2009. A first draft Policy Assessment was released in March 2010 and reviewed by 21 CASAC on April 1

- 2, 2010. In a June 22, 2010 letter to the Administrator, CASAC provided 22 advice and recommendations to the Agency concerning the first draft Policy Assessment (Russ ell 23

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19 and Samet, 2010 a). A second draft Policy Assessment was released to CASAC and the public 1 in September 2010 and reviewed b y CASAC on October 6

- 7, 2010. CASAC provided advice 2 and recommendations to the Agency regarding the second draft Policy Assessme nt in a 3 December 9, 2010 letter (Russell and Samet 2010 b ). CASAC and public comments on the 4 second draft Policy

Assessment were considered by EPA staff in developing a final Policy 5 Assessment (US EPA, 2011). CASAC requested an additional meeting to provi de additional 6 advice to the Administrator based on the final Policy Assessment on February 15

– 16, 2011. 7 EPA released on January 14, 2011, the final Policy Assessment prior to final document 8 production, to provide sufficient time for CASAC review of the do cument in advance of this 9 meeting. A final Policy Assessment, incorporating final reference checks and document 10 formatting, was released in February 2011. In a May

17, 2011 letter (Russel l and Samet, 2011 a), 11 CASAC offered additional advice to the Administrator with regard to recommendations and 12 revisions to the secondary NAAQS for oxides of nitrogen and oxides of sulfur. 13 On February 3, 2006, the Center for Biological Diversity and four other plaintiffs filed a 14 complaint alleging that EPA had failed to complete the current review within the period provided 15 by statute. 2 The schedule for completion of this review is governed b y a consent decree 16 resolving that lawsuit and the subsequent extension agreed to b y the parties . The schedule 17 presented in the orig inal consent decree that governs this review, entered by the court on 18 November 19, 2007, was revised on October 22, 2009 to allow for a 17 month extension of the 19 schedule. The current decree provides that EPA sign for publication notices of proposed and 20 f inal rulemaking concerning its review of the oxides of nitrogen and oxides of sulfur NAAQS no 21 later than July

12, 2011 and March 20, 2012, respectively

. 22

2 Center for Biological Diversity, et al. v. Johnson, No. 05

– 1814 (D.D.C.)

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20 This action presents the Administrator’s proposed decisions on the review of the current 1 secondary

o xides of nitrogen and oxides of sulfur standards. Throughout this preamble a number 2 of conclusions, findings, and determinations proposed by the Administrator are noted. While 3 they identify the reasoning that supports this proposal, they are onl y proposa ls and are not 4 intended to be final or conclusive in nature. The EPA invites general, specific, and/or technical 5 comments on all issues involved with this proposal, including all such proposed judgments, 6 conclusions, findings, and determinations. 7 E. Scope of the Current Review 8 In conducting this periodic review of the secondary NAAQS for oxides of nitrogen and 9 oxides of sulfur, as discussed in the IRP and REA, EPA decided to assess the scientific 10 information, associated risks, and standards relevant to protecting the public welfare from 11 adverse effects associated jointly with oxides of nitrogen and sulfur. Although EPA has 12 historically

adopted separate secondary standards for oxides of nitrogen and oxides of sulfur, 13 EPA is conducting a joint review of the se standards because oxides of nitrogen and sulfur, and 14 their associated transformation products are linked from an atmospheric chemistry perspective, 15 as well as from an environmental effects perspective. The National Research Council (NRC) has 16 recommende d that EPA consider multiple pollutants, as appropriate, in forming the scientific 17 basis for the NAAQS (NRC, 2004). As discussed in the I SA and REA, there is a strong basis for 18 considering these pollutants together, building upon EPA’s past recognition of the interactions of 19 these pollutants and on the growing body of scientific information that is now available related to 20 these interactions and associated ecological effects. 21 In defining the scope of this review, it must be considered that EPA has set sec ondary 22 standards for two other criteria pollutants related to oxides of nitrogen and sulfur: ozone and 23

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21 particulate matter (PM). Oxides of nitrogen are precursors to the formation of ozone in the 1 atmosphere, and under certain conditions, can combine with atmospheric ammonia to form 2 ammonium nitrate, a component of fine PM. Oxides of sulfur are precursors to the formation of 3 particulate sulfate, which is a significant component of fine PM in many parts of the U.S. There 4 are a number of welfare effects directl y associated with ozone and fine PM, including ozone

5 related damage to vegetation and PM

- related visibility impairment. Protection against those 6 effects is provided b y the ozone and fine PM secondary standards. This review focuses on 7 evaluation of th e protection provided b y secondary standards for oxides of nitrogen and sulfur 8 for two general types of effects: (1) direct effects on vegetation associated with exposure to 9 gaseous oxides of nitrogen and sulfur in the ambient air, which are the effects that the current 10 NO 2 and SO 2 secondary standards protect against and (2) effects associated with the deposition 11 of oxides of nitrogen and sulfur to sensitive aquatic and terrestrial ecos ys tems, including 12 deposition in the form of particulate nitrate and par ticulate sulfate. 13 The I SA focuses on the ecological effects associated with deposition of ambient oxides of 14 nitrogen and sulfur to natural sensitive ecos ys tems, as distinguished from commercially managed 15 forests and agricultural lands. This focus reflect s the fact that the majority of the scientific 16 evidence regarding acidification and nutrient enrichment is based on studies in unmanaged 17
ecos ys tems. Non
- managed terrestrial ecos ys tems tend to have a higher fraction of N deposition 18 resulting from atmospher ic nitrogen (US EPA , 2008, section 3.3.2.5). In addition, the I SA notes 19 that agricultural and commercial forest lands are routinely fertilized with amounts of nitrogen 20 that exceed air pollutant inputs even in the most polluted areas (US EPA , 2008, section 3.3.9). 21 This review recognizes that the effects of nitrogen deposition in managed areas are viewed 22 differentl y from a public welfare perspective than are the effects of nitrogen deposition in 23

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22 natural, unmanaged ecosystems, largely due to the more homogen eous, controlled nature of 1 species composition and development in managed ecos ys tems and the potential for benefits of 2 increased productivity in those ecos ys tems. 3 In focusing on natural sensitive ecosy stems, the Policy Assessment primarily

considers 4 the e ffects of ambient oxides of nitrogen and sulfur via deposition on multiple ecological 5 receptors. The ISA highlights effects including those associated with acidification and nitrogen 6 nutrient enrichment. With a focus on these deposition

- related effects, EPA’s objective is to 7 develop a framework for oxides of nitrogen and sulfur standards that incorporates ecologicall y 8 relevant factors and that recognizes the interactions between the two pollutants as they deposit to 9 sensitive ecos ys tems. The overarching policy objective is to develop a secondary standard(s) 10 that is based on the ecological criteria described in the I SA and the results of the assessments in 11 the REA, and is consistent with the requirement of the CAA to set secondary standards that are 12 requis ite to protect the public welfare from any known or anticipated adverse effects associated 13 with the presence of these air pollutants in the ambient air. C onsistent with the CAA, this policy 14 objective includes consideration of “variable factors . . . which of themselves or in combination 15 with other factors may alter the effects on public welfare” of the criteria air pollutants included in 16 this review. 17 In addition, we have chosen to focus on the effects of ambient oxides of nitrogen and 18 sulfur on ecological impacts on sensitive aquatic ecosy stems associated with acidifying 19 deposition of nitrogen and sulfur, which is a transformation product of ambient oxides of 20 nitrogen and sulfur. Based on the information in the I SA, the assessments presented in the REA, 21
and advice from CASAC on earlier drafts of this Policy Assessment (Russell and Samet, 2010a, 22 2010b), and as discussed in detail in the Policy Assessment , we have the greatest confidence in 23

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23 the causal linkages between oxides of nitrogen and sulfur and aquati c acidification effects 1 relative to other deposition

- related effects, including terrestrial acidification and aquatic and 2 terrestrial nutrient enrichment. 3 4 II. Rationale for Proposed Decision on the Adequacy of the Current Secondary 5 Standards 6 D ecisions on retaining or revising the current secondary standards for oxides of nitrogen 7 and sulfur are largely public welfare policy

judgments based on the Administrator’s informed 8 assessment of what constitutes requisite protection against adverse effect s to public welfare. A 9 public welfare policy decision should draw upon scientific information and anal ys es about 10 welfare effects, exposure and risks, as well as judgments about the appropriate response to the 11 range of uncertainties that are inherent in th e scientific evidence and analy ses. The ultimate 12 determination as to what level of damage to ecos ystems and the services provided by those 13
ecos ys tems is adverse to public welfare is not wholly a scientific question, although it is 14 informed b y scientific studies linking e cos ys tem damage to losses in ecosy stem services, and 15 information on the value of those losses of ecos ys tem services. In reaching such decisions, the 16 Administrator seeks to establish standards that are neither more nor less stringent than ne cessary 17 for this purpose. 18 This section presents the rationale for the Administrator’s proposed conclusions with 19 regard to the adequacy of protection and ecological relevance of the current secondary standards 20 for oxides of nitrogen and sulfur. As discussed more full y below, this rationale considered the 21 latest scientific information on ecological effects associated with the presence of oxides of 22 nitrogen and oxides of sulfur in the ambient air. This rationale also takes into account: (1) staff 23

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24 assessments of the most policy

- relevant information in the I SA and staff analy ses of air quality , 1 exposure, and ecological risks, presented more fully in the REA and in the Policy Assessment , 2 upon which staff conclusions on revisions to the secondary oxides of nitrog en and oxides of 3 sulfur standards are based; (2) CASAC advice and recommendations, as reflected in discussions 4 of drafts of the I SA, REA, and Policy Assessment at public meetings, in separate written 5 comments, and in CASAC’s letters to the Administrator; and (3) public comments received 6 during the development of these documents, either in connection with CASAC meetings or 7 separately

. 8 In developing this rationale, EPA has drawn upon an integrative s ynthesis of the entire 9 body of evidence, published through early 2008 , on ecological effects associated with the 10 deposition of oxides of nitrogen and oxides of sulfur in the ambient air (US EPA, 2008)

. As 11 discussed below in section II.A, this body of evidence addresses a broad range of ecological 12 endpoints asso ciated with ambient levels of oxides of nitrogen and oxides of sulfur

. In 13 considering this evidence, EPA focuses on those ecological endpoints, such as aquatic 14 acidification, for which the IS A judges associations with oxides of nitrogen and oxides of sulfur 15 to be causal, likel y causal, or for which the evidence is suggestive that oxides of nitrogen and/or 16 sulfur contribute to the reported effects. The categories of causality determinations have been 17 developed in the ISA (US EPA, 2008) and are discussed in Section 1.6 of the I SA. 18 Crucial to this review is the development of a form for an ecologicall y relevant standard 19 that reflects both the geographicall y variable and deposition

- dependent nature of the effects. 20 The atmospheric levels of oxides of nitrogen and sulfur that afford a particular level of 21
ecos ys tem protection are those levels that result in an amount of deposition that is less than the 22 amount of deposition that a given ecos ys tem can accept without defined levels of degradation. 23

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ramework developed in the REA, the framework we used to structure 1 an ecologically

meaningful secondary

standard in the 2 the indicator, form, level, and averaging time of such as standard in section III of this propos 3 depicted below and highlights the three key linkages that need to be considered in developing an 4 ecologicall y relevant standard. 5 6

7 Figure II - 1. Sim plified conceptual design of the form of an aquatic acidification standard 8 for oxides of nitrogen and sulfur. 9 10 The following discussion 11 The Policy

Assessment includes staff’s evaluation of the policy implications of the scientific 12 assessment of the evidence presented and assessed in the ISA and 13 assessments based on that information presented and assessed in the REA. Taken together, this 14 information informs staff conclusions and the development of policy options in the 15 Assessment for consideration in addressing p 16 presence of oxides of nitrogen and oxides of sulfur in the ambient air. Of particular note, chapter 17 2 of the Policy

Assessment presents information not repeated here that characterizes emissions, 18 air quality , deposition and water quality. It includes discussions of the sources of nitrogen and 19 sulfur in the atmosphere as well as current ambient air quality monitoring networks and models. 20 

### teragency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

25 ramework developed in the REA, the framework we used to structure an ecologically

meaningful secondary

standard in the Policy

Assessment and to further develop the indicator, form, level, and averaging time of such as standard in section III of this propos depicted below and highlights the three key linkages that need to be considered in developing an

Sim plified conceptual design of the form of an aquatic acidification standard

sulfur.
The following discussion relies heavil y on Chapters 2 and 3 of the Policy Assessment includes staff’s evaluation of the policy implications of the scientific assessment of the evidence presented and assessed in the ISA and the results of quantitative assessments based on that information presented and assessed in the REA. Taken together, this information informs staff conclusions and the development of policy options in the

for consideration in addressing p ublic and welfare effects associated with the presence of oxides of nitrogen and oxides of sulfur in the ambient air. Of particular note, chapter presents information not repeated here that characterizes emissions, deposition and water quality. It includes discussions of the sources of nitrogen and sulfur in the atmosphere as well as current ambient air quality monitoring networks and models. 

### teragency Working Comments on Draft Language under EO12866 and

ramework developed in the REA, the framework we used to structure and to further develop the indicator, form, level, and averaging time of such as standard in section III of this propos al is depicted below and highlights the three key linkages that need to be considered in developing an

Sim plified conceptual design of the form of an aquatic acidification standard

Policy Assessment

. includes staff’s evaluation of the policy implications of the scientific the results of quantitative assessments based on that information presented and assessed in the REA. Taken together, this information informs staff conclusions and the development of policy options in the Policy ublic and welfare effects associated with the presence of oxides of nitrogen and oxides of sulfur in the ambient air. Of particular note, chapter presents information not repeated here that characterizes emissions, deposition and water quality. It includes discussions of the sources of nitrogen and sulfur in the atmosphere as well as current ambient air quality monitoring networks and models.

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26 Additional information in this section includes ecological modeling and wat er quality data 1
sources. 2 Section II.A presents a discussion of the effects associated with oxides of nitrogen and 3 sulfur in the ambient air. The discussion is organized around the ty pes of effects being 4 considered, including direct effects of gaseous oxides of nitrogen and sulfur , deposition

- related 5 effects related to acidification and nutrient enrichment , and other effects such as materials 6 damage, climate

- related effects and mercury methylation

. 7 Section II
. B presents a summary and discussion of the risk and exposure assessment 8 performed for each of the four major effects categories. The REA uses case studies representing 9 the broad geographic variability of the impacts from oxides of nitrogen and sulfur to co nclude 10 that there are ongoing adverse effects in many ecosy stems from deposition of oxides of nitrogen 11 and sulfur and that under current emissions scenarios these effects are likely to continue. 12 Section II
. C presents a discussion of adversity linking ecol ogical effects to measures that 13 can be used to characterize the extent to which such effects are reasonabl y considered to be 14 adverse to public welfare. This involves consideration of how to characterize adversity from a 15 public welfare perspective. In so doing, consideration is given to the concept of ecos ys tem 16 services, the evidence of effects on ecos ys tem services, and how ecos ys tem services can be 17 linked to ecological indicators. 18 Section II
. D presents an assessment of the adequacy of the current oxides of nitrogen and 19 oxides of sulfur secondary standards. Consideration is given to the adequacy of protection 20 afforded b y the current standards for both direct and deposition

- related effects, as well as to the 21 appropriateness of the fundamental structure an d the basic elements of the current standards for 22 providing protection from deposition

- related effects. Considerations as to the extent to which 23

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27 deposition

- related effects that could reasonably be judged to be adverse to public welfare are 1 occurring under current conditions which are allowed by the current standards is also considered

. 2 Discussion of the structures and basic elements of the current NO 2 and SO 2 secondary standards 3 and whether they are adequate to protect against such effects is presented

. 4 A. Ecological Effects 5 This section discusses the known or anticipated ecological effects associated with oxides 6 of nitrogen and sulfur, including the direct effects of gas

- phase exposure to oxides of nitrogen 7 and sulfur (section II.A.1) and effects associated with deposition

- related exposure (sections 8 II.A.2 and 3)

. Section II .A. 2 addresses effects related to acidification of aquatic and terrestrial 9
ecos ys tems and s ection II A.3 addresses effects related to nutrient enrichment of aquatic and 10 terrestrial ecos ys tems
. These sections also address questions about the nature and magnitude of 11
ecos ys tem responses to reactive nitrogen and sulfur deposition, including responses related to 12 acidification, nutrient depletion, and, in Section II.A 4 the mobilization of toxic metals in 13 sensitive aquatic and terrestrial ecos ys tems . T he uncertainties and limitations associated with the 14 evidence of such effects are also discussed throughout this section

. 15 1. Effects Associated with Gas

-Phase Oxides of Nitroge n and Sulfur 16 E cological effects on vegetation as discussed in earlier reviews as well as the I SA can be 17 attributed to gas

- phase oxides of nitrogen and sulfur. Acute and chronic exposures to gaseous 18 pollutants such as sulfur dioxide (SO 2 ), nitrogen dioxi de (NO 2 ), nitric oxide (NO), nitric acid 19 (HNO 3 ) and perox yacetyl nitrite (PAN) are associated with negative impacts to vegetation. The 20 current secondary

NAAQS were set to protect against direct damage to vegetation by exposure to 21 gas - phase oxides of nitrog en and sulfur , such as foliar injury , decreased photosy nthesis, and 22 decreased growth. The following summary is a concise overview of the known or anticipated 23

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28 effects to vegetation caused by gas phase nitrogen and sulfur

. Most phototoxic effects associate d 1 with gas phase oxides of nitrogen and sulfur occur at levels well above ambient concentrations 2 observed in the U.S. (US EPA, 2008, section 3.4.2.4). 3

a. Nature of ecosystem responses to gas

- phase nitrogen and sulfur 4 The 2008 I SA found that gas phase N and S are associated with direct ph yt otoxic effects 5 (US EPA, 2008, section 4.4). The evidence is sufficient to infer a causal relationship between 6 exposure to SO 2 and injury to vegetation (US EPA, 2008, section 4.4.1 and 3.4.2.1). Acute foliar 7 injury

to vegetation from SO 2 may occur at levels above the current secondary standard (3

- h 8 average of 0.50 ppm). Effects on growth, reduced photosy nthesis and decreased yi eld of 9 vegetation are also associated with increased SO 2 exposure concentration and time of exposure

. 10 The evidence is sufficient to infer a causal relationship between exposure to NO, NO 2 11 and PAN and injury to vegetation (US EPA, 2008, section 4.4.2 and 3.4.2.2). At sufficient 12 concentrations, NO, NO 2 and PAN can decrease photosy nthesis and induce visible foliar injury 13 to plants. Evidence is also sufficient to infer a causal relationship between exposure to HNO 3 14 and changes to vegetation (US EPA, 2008, section 4.4.3 and 3.4.2.3). Phytotoxic effects o f this 15 pollutant include damage to the leaf cuticle in vascular plants and disappearance of some 16 sensitive lichen species. 17 b. Magnitude of ecosystem response to gas

- phase nitrogen and sulfur 18 Vegetation in ecos ys tems near sources of gaseous oxides of nitroge n and sulfur or where 19 SO 2 , NO, NO 2 , PAN and HNO 3 are most concentrated are more likel y to be impacted b y these 20 pollutants. Uptake of these pollutants in a plant canopy is a complex process involving 21 adsorption to surfaces (leaves, stems and soil) and absor ption into leaves (US EPA, 2008, section 22 3.4.2). The functional relationship between ambient concentrations of gas phase oxides of 23

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29 nitrogen and sulfur and specific plant response are impacted b y internal factors such as rate of 1 stomatal conductance and pl ant detoxification mechanisms, and external factors including plant 2 water status, light, temperature, humidity , and pollutant exposure regime (US EPA, 2008, section 3 3.4.2). 4 Entry

of gases into a leaf is dependent upon phy sical and chemical processes of ga s phase 5 as well as to stomatal aperture. The aperture of the stomata is controlled largely by the 6 prevailing environmental conditions, such as water availability , humidity , temperature, and light 7 intensity . When the stomata are closed, resistance to gas uptake is high and the plant has a very 8 low degree of susceptibility to injury . Mosses and lichens do not have a protective cuticle barrier 9 to gaseous pollutants or stomata and are generall y more sensitive to gaseous sulfur and nitrogen 10 than vascular plant s (US EPA, 2008, section 3.4.2). 11 The appearance of foliar injury can vary significantly

across species and growth 12 conditions affecting stomatal conductance in vascular plants (US EPA, 200 9 , section 6.4.1). For 13 example, damage to lichens from SO 2 exposur e include s decrease d photosynthesis and 14 respiration, damage to the algal component of the lichen, leakage of electroly tes, inhibition of 15 nitrogen fixation, decreased K+ absorption, and structural changes. 16 The phy

totoxic effects of gas phase oxides of nitrogen and sulfur are dependent on the 17 exposure concentration and duration and species sensitivity to these pollutants. Effects to 18 vegetation associated with oxides of nitrogen and sulfur are therefore variable across the U.S. 19 and tend to be higher near sources of photochemical smog. For example, SO 2 is considered to be 20 the primary

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

. 21 The I SA states there is very limited new research on phy totoxic effects of NO, NO 2 , PAN 22 and HNO 3 at concentrations currentl y observed in the United States with the exception of some 23

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30 lichen species ( US EPA, 2008, section 4.4). Past and current HNO 3 concentrations may be 1 contributing to the decline in lichen s pecies in the Los Angeles basin . Most phy totoxic effects 2 associated with gas phase oxides of nitrogen and sulfur occur at levels well above ambient 3 concentrations observed in the U.S. (US EPA, 2008, section 3.4.2.4). 4 2. Acidification Effects Associated with Deposition of Oxides of Nitrogen and Sulfur 5 Sulfur oxides and nitrogen oxides in the atmosphere undergo a complex mix of reactions 6 in gaseous, liquid, and solid phases to form various acidic compounds. These acidic compounds 7 are removed from the atmosphere through deposition: either wet (e.g ., rain, snow), fog or cloud, 8 or dry

(e.g., gases, particles). Deposition of these acidic compounds to ecosy stems can lead to 9 effects on ecos ys tem structure and function. Following deposition, these compounds can, in 10 some instances unless retained b y soil or biota, leach out of the soils in the form of sulfate (SO 4 2

- 11 ) and nitrate (NO 3 - ), leading to the acidification of surface waters. The effects on ecosy stems 12 depend on the magnitude and rate of deposition, as well as a host of biogeochemical processes 13 occurring in the soils and waterbodies ( US EPA, 200 9 , section 2.1). The chemical forms of 14 nitrogen that may

contribute to acidifying deposition include both oxidized and reduced chemical 15 species , including NH x . 16 When sulfur or nitrogen leaches from soils t o surface waters in the form of SO 4 2

- or NO 3 - , 17 an equivalent amount of positive cations, or countercharge, is also transported. This maintains 18 electroneutrality . I f the countercharge is provided by base cations, such as calcium (Ca2+ ), 19 magnesium (Mg 2+ ), so dium (Na+ ), or potassium (K+ ), rather than h ydrogen (H + ) and dissolved 20 inorganic aluminum, the acidity of the soil water is neutralized, but the base saturation of the soil 21 decreases. Continued SO 4 2

- or NO 3 - leaching can deplete the available base cation p ool in soil. 22 As the base cations are removed, continued deposition and leaching of SO 4 2

- and/or NO 3 - (with 23

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31 H + and Al 3+ ) leads to acidification of soil water, and by connection, surface water. Introduction 1 of strong acid anions such as sulfate and nitrate t o an already acidic soil, whether naturally or due 2 to anthropogenic activities, can lead to instantaneous acidification of waterbodies through direct 3 runoff without an y significant change in base cation saturation. The ability of a watershed to 4 neutralize acidic deposition is determined by a variety of biogeoph ys ical factors including 5 weathering rates, bedrock composition, vegetation and microbial processes, phy sical and 6 chemical characteristics of soils and hydrologic flowpaths. (US EPA, 200 9 , section 2.1) Some of 7 these factors such as vegetation and soil depth are highl y variable over small spatial scales such 8 as meters, but can be aggregated to evaluate patterns over larger spatial scales. Acidifying 9 deposition of oxides of nitrogen and sulfur and the c hemical and biological responses associated 10 with these inputs vary temporally

. Chronic or long

- term deposition processes in the time scale of 11 ye ars to decades result in increases in inputs of N and S to ecos ys tems and the associated 12 ecological effects. Ep isodic or short term (i.e., hours or day s) deposition refers to events in which 13 the level of the acid neutralizing capacity (ANC) of a lake or stream is temporaril y lowered. In 14 aquatic ecosy stems, short

- term (i.e., hours or day s) episodic changes in water chemistry can have 15 significant biological effects. Episodic acidification refers to conditions during precipitation or 16 snowmelt events when proportionately more drainage water is routed through upper soil horizons 17 that tend to provide less acid neutraliz ing than is passing through deeper soil horizons (US EPA, 18 200 9 , section 4.2). In addition, the accumulated sulfate and nitrate in snow packs can provide a 19 surge of acidic inputs. Some streams and lakes may have chronic or base flow chemistry that is 20 suit able for aquatic biota, but may be subject to occasional acidic episodes with deleterious 21 consequences to sensitive biota. 22

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32 The following summary

is a concise overview of the known or anticipated effects caused 1 by acidification to ecos ys tems within the Uni ted States. Acidification affects both terrestrial and 2 freshwater aquatic ecosystems. 3

a. Nature of acidification

- related ecosystem responses 4 The I SA concluded that deposition of oxides of nitrogen and sulfur and NH x leads to the 5 varying degrees of acidifi cation of ecos ys tems (US EPA, 2008 ). In the process of acidification, 6 biogeochemical components of terrestrial and freshwater aquatic ecos ys tems are altered in a way 7 that leads to effects on biological organisms. Deposition to terrestrial ecosy stems ofte n moves 8 through the soil and eventually leaches into adjacent water bodies. 9 i. Aquatic ecosystems 10 The scientific evidence is sufficient to infer a causal relationship between acidifying 11 deposition and effects on biogeochemistry and biota in aquatic ecos ys tem s (US EPA, 2008, 12 section 4.2.2). The strongest evidence comes from studies of surface water chemistry in which 13 acidic deposition is observed to alter sulfate and nitrate concentrations in surface waters, the sum 14 of base cations, ANC, diss olved inorganic aluminum and pH (US EPA, 2008, section 3.2.3.2). 15 ANC is a key

indicator of acidification with relevance to both terrestrial and aquatic ecosy stems. 16 ANC is useful because it integrates the overall acid

- base status of a lake or stream and reflects 17 how aquat ic ecos ys tems respond to acidic deposition over time. There is also a relationship 18 between ANC and the surface water constituents that directl y contribute to or ameliorate acidity

19 related stress, in particular, concentrations of h ydrogen ion (as pH), Ca2+ , and aluminum. 20 Moreover, low pH surface waters leach aluminum from soils, which is quite lethal to fish and 21 other aquatic organisms. In aquatic s ys tems, there is a direct relationship between ANC and fish 22 and phy

to
- zooplankton diversity

and abundance

. 23

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33 L ow ANC coincides with effects on aquatic s ys tems (e.g., individual species fitness loss 1 or death, reduced species richness, altered community structure). At the community level, 2 species richness is positively correlated with pH and ANC because energy cost in maintaining 3 phy siological homeostasis, growth, and reproduction is high at low ANC levels. For example, 4 there is a logistic relationship between fish species richness and ANC class for Adirondack Case 5 Study

Area lakes that indicates the probability of o ccurrence of an organism for a given value of 6 ANC

. Biota are generally not harmed when ANC values are >100 microequivalents per liter 7 (μeq/L). The number of fish species also peaks at ANC values >100 μeq/L. Below 100 μeq/L 8 ANC , fish fitness and community diversity begin to decline (US EPA, section 4.2) . Specificall y 9
at ANC levels between 100 and 50 μeq/L , the fitness of sensitive species (e.g., brook trout, 10 zooplankton) begins to decline. When ANC concentrations are <50 μeq/L, they are generall y 11
assoc iated with death or loss of fitness of biota that are sensitive to acidification. 12 Consistent and coherent documentation from multiple studies on various species from all 13 major trophic levels of aquatic sy stems shows that geochemical alteration caused by acidification 14 can result in the loss of acid

- sensitive biological species (US EPA, 2008, section 3.2.3.3). This is 15 most often discussed with relation to pH. For example, in the Adirondacks, of the 53 fish species 16 recorded in Adirondack lakes about half ( 26 species) were absent from lakes with pH below 6.0. 17 Biological effects are linked to changes in water chemistry including decreases in ANC and pH 18 and increases in inorganic Al concentration. The direct biological effects are caused b y lowered 19 pH which leads to increased inorganic Al concentrations (US EPA, 20 11 , Figures 3

- 1 and 3

- 2 ). 20 While ANC level does not cause direct biological harm it is a good overall indicator of the risk 21 of acidification (US EPA, 20 11 , section 3.1.3). 22

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34 There are clear associatio ns between ANC, pH, and aquatic species mortality and health 1 which are summarized in section 3.1.1 of the Policy Assessment

. Significant harm to sensitive 2 aquatic species has been observed at pH levels below 6. Normal stream pH levels with little to 3 no t oxicity

range from 6 to 7 (MacAvo y et al, 1995). Baker et al (1990) observed that “lakes 4 with pH less than approximately 6.0 contain significantly

fewer species than lakes with pH levels 5 above 6.0”. As noted in Chapter 3, ty picall y at pH <4.5 and an ANC <0 μeq/L , complete to 6
near - complete loss of many taxa of organisms occur, including fish and aquatic insect 7 populations, whereas other taxa are reduced to only acidophilic species. A cid N eutralizing 8 C apacity is a measure of how much acid can be neutraliz ed in a specific surface water system. 9 An ANC value of 0 or below means that surface waters have no ability to neutralize any 10 additional acid inputs. 11 Additional evidence can help refine the understanding of effects occurring at pH levels 12 between 4.5 and 6. When pH levels are below 5.6, relativel y lower trout survival rates were 13 observed in the Shenandoah National Park. In field observations, when pH levels dropped to 5, 14 mortality

rates went to 100 percent. (Bulger et al, 2000). At pH levels ranging from 5.4 to 5.8, 15 cumulative mortality continues to increase. Several studies have shown that trout exposed to 16 water with varying pH levels and fish larvae showed increasing mortality as pH levels decrease. 17 In one study

almost 100 percent mortality was obse rved at a pH of 4.5 compared to almost 100 18 percent survival at a pH of 6.5. Intermediate pH values (6.0, 5.5) in all cases showed reduced 19 survival compared with the control (6.5), but not by statisticall y significant amounts (US EPA, 20 2008, section 3.2.3.3 ). 21 One important indicator of acid stress is increased fish mortality . The response of fish to 22 pH is not uniform across species. A number of s yn optic survey s indicated loss of species 23

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35 diversity and absence of several fish species in the pH range of 5. 0 to 5.5. If pH is lower, there is 1 a greater likelihood that more fish species could be lost without replacement, resulting in 2 decreased richness and diversity . In general, populations of salmonids are not found at pH levels 3 less than 5.0, and smallmouth bass (Micropterus dolomieu) populations are usually not found at 4 pH values less than about 5.2 to 5.5. From Table 3

- 1, only one study

showed significant 5 mortality

effects above a pH of 6, while a number of studies showed significant mortality when 6 pH leve ls are at or below 5.5. 7 The highest pH level for any of the studies reported in the I SA, is 6.0, suggesting that pH 8 above 6.0 is protective against mortality effects for most species. Most thresholds are in the 9 range of pH of 5.0 to 6.0, which suggests that a target pH should be no lower than 5.0. 10 Protection against mortality in some recreationally important species such as lake trout (pH 11 threshold of 5.6) and crappie (pH threshold of 5.5), combined with the evidence of effects on 12 larval and embryo sur vival suggests that pH levels greater than 5.5 should be targeted to provide 13 protection against mortality effects throughout the life stages of fish. 14 Non
- lethal effects have been observed at pH levels as high as 6. A study in the 15 Shenandoah National Park found that the condition factor, a measure of fish health expressed as 16 fish weight/length multiplied by a scaling constant, is positively correlated with stream pH 17 levels, and that the condition factor is reduced in streams with a pH of 6.0 ( US EPA, 2008, 18 section 3.2.3.3). 19 Biodiversity is another indicator of aquatic ecos ys tem health. A key study in the 20 Adirondacks found that lakes with a pH of 6.0 had only half the potential species of fish (27 of 21 53 potential species). There is often a positive relatio nship between pH and number of fish 22 species, at least for pH values between about 5.0 and 6.5, or ANC values between about 0 to 23

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36 100 μeq/L . Such observed relationships are complicated, however, by the tendency

for smaller 1 lakes and streams, having smaller w atersheds, to also support fewer fish species, irrespective of 2 acid

- base chemistry . This pattern may be due to a decrease in the number of available niches as 3 stream or lake size decreases. Nevertheless, fish species richness is relatively easily determine d 4 and is one of the most useful indicators of biological effects of surface water acidification. 5 Changes in stream water pH and ANC also contribute to declines in taxonomic richness 6 of zooplankton, and macroinvertebrates which are often sources of food f or fish, birds and other 7 animal species in various ecos ys tems. These fish may also serve as a source of food and 8 recreation for humans. Acidification of ecos ys tems has been shown to disrupt food web 9 dy namics causing alteration to the diet, breeding distribution , and reproduction of certain species 10 of birds (US EPA, 2008, section 4.2.2.2. and Table 3

- 9). For example, breeding distribution of 11 the common goldeneye (Bucephala clangula ) , an insectivorous duck , may be affected b y 12 changes in acidifying depositio n. Similarly , decreases in prey diversity and quantity

have been 13 observed to create feeding problems for nesting pairs of loons on low

- pH lakes in the 14 Adirondacks. 15 ii. Terrestrial ecosystems 16 In terrestrial ecos ys tems, the evidence is sufficient to infer a causal relationship between 17 acidifying deposition and changes in biogeochemistry ( US EPA, 2008, section 4.2.1.1). The 18 strongest evidence comes from studies of forested ecos ys tems, with supportive information on 19 other plant taxa, including shrubs and lichens (US EPA, 2008, section 3.2.2.1.). Three useful 20 indicators of chemical changes and acidification effects on terrestrial ecos ystems, showing 21 consistency

and coherence among multiple studies are: soil base saturation, Al concentrations in 22 soil water and s oil C:N ratio (US EPA, 2008, section 3.2.2.2). 23

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37 As discussed in the I SA and REA, i n soils with base saturation less than about 15 to 20%, 1 exchange chemistry

is dominated by

Al. Under these conditions, responses to inputs of sulfuric 2 acid and HNO 3 largel y involve the release and mobilization of dissolved inorganic Al. The 3 effect can be neutralized by weathering from geologic parent material or base cation exchange. 4 The Ca2+ and Al concentrations in soil water are strongl y influenced b y soil acidif ication and 5 both have been shown to have quantitative links to tree health, including Al interference with 6 Ca2+ uptake and Al toxicity to roots. Effects of nitrification and associated acidification and 7 cation leaching have been consistently shown to occur onl y in soils with a C:N ratio below about 8 20 to 25. 9 Soil acidification caused by acidic deposition has been shown to cause decreased growth 10 and increased susceptibility to disease and injury

in sensitive tree species. Red spruce ( Picea 11 rubens ) dieback or decline has been observed across high elevation areas in the Adirondack, 12 Green and White mountains. The frequency of freezing injury

to red spruce needles has 13 increased over the past 40 years, a period that coincided with increased emissions of S and N 14 oxides and increased acidifying deposition. Acidifying deposition can contribute to dieback in 15 sugar maple (

Acer saccharum ) through depletion of cations from soil with low levels of available 16 Ca. Grasslands are likel y less sensitive to acidification than forests due to grassland soils being 17 generall y rich in base cations. 18 iii. Ecosystem sensitivity 19 The intersection between current deposition loading, historic loading, and sensitivity 20 defines the ecological vulnerability to the effects of acidification. Freshwater aquatic and some 21 terrestrial ecos ys tems, notably forests, are the ecosy stem ty pes which are most sensitive to 22 acidification. The IS A reports that the principal factor governing the sensitivity of terrestrial and 23

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38 aquatic ecosy stems to acidification from sulfur and nitrogen deposition is geology (particularly 1 surficial geology). Geologic formations having low base cation supply generall y underlie the 2 watersheds of acid

- sensitive lakes and streams. Other factors that contribute to the sensitivity of 3 soils and surface waters to acidifying deposition include topography , soil chemistry , land use, 4
and hydrologic flowpaths. Episodic and chronic acidification tends to occur in areas that have 5 base

- poor bedrock, high relief, and shallow soils (US EPA, 2008, section 3.2.4.1). 6 b. Magnitude of acidification

- related ecosystem responses 7 Terrestrial and aquatic ecosy stems differ in their response to acidifying deposition. 8 Therefore the magnitude of ecos ys tem response is described separatel y for aquatic and terrestrial 9
ecos ys tems in the following sections. The magnitude of response refers to both the severity o f 10 effects and the spatial extent of the U.S. which is affected. 11 i. Aquatic acidification 12 Freshwater ecos ys tem survey s and monitoring in the eastern United States have been 13 conducted b y man y programs since the mid

- 1980s, including EPA’s Environmental Monitori ng 14 and Assessment Program (EMAP), National Surface Water Survey (NSWS), Temporall y 15 Integrated Monitoring of Ecosy stems (TIME), and L ong - term Monitoring (LTM) programs. 16 Based on anal ys es of surface water data from these programs, New England, the Adirondack 17 Mountains, the Appalachian Mountains (northern Appalachian Plateau and Ridge/Blue Ridge 18 region), and the Upper Midwest contain the most sensitive lakes and streams (i.e., ANC less than 19 about 50 μeq/L). Portions of northern Florida also contain many acidic and low

- ANC lakes and 20 streams, although the role of acidifying deposition in this region is less clear. The western U.S. 21 contains many

of the surface waters most sensitive to potential acidification effects, but with the 22 exception of the L os Angeles Basin and surrounding areas, the levels of acidifying deposition are 23

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39 low in most areas. Therefore, acidification of surface waters b y acidic deposition is not as 1 prevalent in the western U.S., and the extent of chronic surface water acidification that has 2
occurred in that region to date has likel y been very limited relative to the Eastern U.S. (US EPA, 3 2008, section 3.2.4.2 and US EPA, 200 9 , section 4.2.2). 4 There are a number of species including fish, aquatic insects, other invertebrates and 5 algae that are se nsitive to acidification and cannot survive, compete, or reproduce in acidic 6 waters ( US EPA, 2008, section 3.2.3.3). Decreases in ANC and pH have been shown to 7 contribute to declines in species richness and declines in abundance of zooplankton, 8 macroinvert ebrates, and fish. Reduced growth rates have been attributed to acid stress in a 9 number of fish species including Atlantic salmon (Salmo salar), Chinook salmon (Oncorhynchus 10 tshawy

tscha), lake trout (Salvelinus namaycush), rainbow trout (Oncorhy nchis my kis s), brook 11 trout (Salvelinus Fontinalis), and brown trout (Salmo trutta). In response to small to moderate 12 changes in acidity , acid

- sensitive species are often replaced by other more acid

- tolerant species, 13 resulting in changes in community composition and richness. The effects of acidification are 14 continuous, with more species being affected at higher degrees of acidification. At a point, 15 ty picall y a pH <4.5 and an ANC <0 μeq/L , complete to near

- complete loss of many taxa of 16 organisms occur, including fis h and aquatic insect populations, whereas other taxa are reduced to 17 only

acidophilic species. These changes in taxa composition are associated with the high energy 18 cost in maintaining ph ys iological homeostasis, growth, and reproduction at low ANC levels (U S 19 EPA, 2008, section 3.2.3.3) . Decreases in species richness related to acidification have been 20 observed in the Adirondack Mountains and Catskill Mountains of New York, New England and 21 Pennsylvania, and Virginia. From the sensitive areas identified by the ISA, further “case study” 22 anal ys es on aquatic ecosy stems in the Adirondack Mountains and Shenandoah National Park 23

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40 were conducted to better characterize ecological risk associated with acidification (US EPA, 1 200 9 , section 4). 2 ANC is the most widely used i ndicator of acid sensitivity and has been found in various 3 studies to be the best single indicator of the biological response and health of aquatic 4 communities in acid

- sensitive sy stems (Lien et al., 1992; Sullivan et al., 2006; US EPA, 2008). 5 In the REA, surface water trends in SO 4 2

- and NO 3 - concentrations and ANC levels were 6 anal yz ed to affirm the understanding that reductions in deposition could influence the risk of 7 acidification. ANC values have been categorized according to their effects on biota, as shown in 8 the table below . Monitoring data from TIME/LTM and EMAP programs were assessed for the 9 ye ars 1990 to 2006, and past, present, and future water quality levels were estimated by both 10 steady

- state and dyn amic biogeochemical models. 11

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41 Table II - 1. Ecological effects associated with alternative levels of acid neutralizing capacity 1 (ANC). (source: USEPA, Acid Rain Program) 2 Category

L abel ANC Levels and Expected Ecological Effects
Acute Concern <0 μeq/L Complete loss of fish populations is expected. Planktonic communities have extremel y low diversity and are dominated by

acidophilic taxa. The numbers of individuals in plankton species that are present are greatl y reduced. Severe Concern 0

–20 μeq/L Highl y 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 sharpl y. Elevated Concern 20

– 50 μeq/L Fish species richness is greatl y 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 zoo plankton 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. 3 Studies on fish species richness in the Adirondacks Case Study Area demonstrated the 4 effect of acidification. Of the 53 fish species recorded in Adirondack Case Study Area lakes, 5 only 27 species were found in lakes with a pH <6.0. The 26 species missing from lakes with a 6 pH <6.0 include important recreational species, such as Atlantic salmon, tiger trout (Salmo trutta 7 X Salvelinus fontinalis ), red breast sunfish (

Lepomis auritus ), bluegill ( Lepomis macrochirus ), 8

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42 tiger musky (

Esox masquinongy X lucius ), walleye (Sander vitreus ), alewife ( Alosa 1 pseudoharengus ), and kokanee ( Oncorhynchus nerka ), as well as ecologically important 2 minnows that are common ly consumed b y sport fish. A survey of 1,469 lakes in the late 1980s 3 found 346 lakes to be devoid of fish. Among lakes with fish, there was a relationship between 4 the number of fish species and lake pH, ranging from about one species per lake for lakes hav ing 5 a pH <4.5 to about six species per lake for lakes having a pH >6.5. In the Adirondacks, a 6 positive relationship exists between the pH and ANC in lakes and the number of fish species 7 present in those lakes ( US EPA, 2008, section 3.2.3.4). 8 Since the mi d

- 1990s, streams in the Shenandoah Case Study Area have shown slight 9 declines in NO 3 - and SO 4 2

- concentrations in surface waters. The 2006 concentrations are still 10 above pre

- acidification (1860) conditions. MAGIC modeling predicts surface water 11 concentrations of NO 3 - and SO 4 2

- are10

- and 32

- fold higher, respectively , in 2006 than in 1860. 12 The estimated average ANC across 60 streams in the Shenandoah Case Study Area is 57.9 μeq/L 13 (± 4.5 μeq/L). 55% of all monitored streams in the Shenandoah Case Study Area have a current 14 risk of Elevated, Severe, or Acute. Of the 55%, 18% are chronically acidic today ( US EPA, 15 200 9 , section 4.2.4.3). 16 Based on a deposition scenario for this study area that maintains current emission levels 17 from 2020 to 2050, the simulation forecast indicates that a large number of streams still have 18 Elevated to Acute problems with acidity in 2050 . In fact, from 2006 to 2050, the percentage of 19 streams with Acute Concern is predicted to increase by 5%, while the percentage of streams in 20 Moderate Concern decreases b y 5%. 21 Biological effects of increased acidification documented in the Shen andoah Case Study 22 Area include a decrease in the condition factor in blacknose dace and a decrease in fish 23

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43 biodiversity

associated with decreasing stream ANC. On average, the fish species richness is 1 lower b y one fish species for every 21 μeq/L decrease i n ANC in Shenandoah National Park 2 streams (US EPA, 2008, section 3.2.3.4). 3 ii. Terrestrial acidification 4 The I SA identified a variety of indicators that can be used to measure the effects of 5 acidification in soils. Most effects of terrestrial acidification are observed in sensitive forest 6
ecos ys tem in the U.S. Tree health has been linked to the availability of base cations (Bc) in soil 7 (such as Ca2+ , Mg 2+ and potassium), as well as soil aluminum (Al ) content. Tree species show a 8 range of sensitivities to Ca/Al and Bc/Al soil molar ratios, therefore these are good chemical 9 indicators because they directly relate to the biological effects. Critical Bc/Al molar ratios for a 10 large variety of tree species ranged from 0.2 to 0.8. This range is similar to critical ratios of 11 Ca/Al. Plant toxicity or nutrient antagonism was reported to occur at Ca/Al molar ratios ranging 12 from 0.2 to 2.5 (US EPA, 200 9 ). 13 There has been no s ys tematic national survey of terrestrial ecos ys tems to determine the 14 extent and distribution of terrestrial ecos ys tem sensitivity to the effects of acidifying deposition. 15 However, one preliminary national evaluation estimated that ~15% of forest ecosy stems in the 16 U.S. exceed the estimated critical load based on soil ANC leaching for S and N depositi on by 17 >250 eq/ha

/yr (McNulty

et al., 2007). Forests of the Adirondack Mountains of New York, Green 18 Mountains of Vermont, White Mountains of New Hampshire, the Allegheny Plateau of 19 Pennsylvania, and high

- elevation forest ecos ys tems in the southern Appalach ians are the regions 20 most sensitive to terrestrial acidification effects from acidifying deposition (US EPA, 2008, 21 section 3.2.4.2). While studies show some recovery of surface waters, there are widespread 22 measurements of ongoing depletion of exchangeable base cations in forest soils in the 23

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44 northeastern U.S. despite recent decreases in acidifying deposition, indicating a slow recovery 1 time. 2 In the REA, a critical load anal ys is was performed for sugar maple and red spruce forests 3 in the eastern United State s by using Bc/Al ratio in acidified forest soils as an indicator to assess 4 the impact of nitrogen and sulfur deposition on tree health. These are the two most commonly 5 studied tree species in North America for effects of acidification. At a Bc/Al ratio of 1.2, red 6 spruce growth can be decreased b y 20%. Sugar maple growth can be decreased by 20% at a 7 Bc/Al ratio of 0.6 (U S EPA, 2009 , section 4.4). The REA anal ys is determined the health of at 8 least a portion of the sugar maple and red spruce growing in the United States may have been 9 compromised with acidifyingtotal nitrogen and sulfur deposition. Specifically , total nitrogen and 10 sulfur deposition levels exceeded three selected critical loads for tree growth in 3% to 75% of all 11 sugar maple plots across 24 states
-- that is, it exceeded the highest (least stringent) of the three 12 critical loads in 3% of plots, and the lowest (most stringent) in 75% of plots . For red spruce, total 13 nitrogen and sulfur deposition levels exceeded three selected critical loads in 3% to 36% of all 14 red spruce plots across eight states ( US EPA, 200 9 , section 4.4). 15

c. Key uncertainties associated with acidification 16 There are different levels of uncertainty associated with relationships between deposition, 17 ecological effects and ecological indicators. In Chapter 7 of the REA, the case study analy ses 18 associated with each targeted effect area were s ynthesized by identifying the strengths, 19 limitations, and uncertainties associated with the available data, modeling approach, and 20 relationship b etween the selected ecological indicator and atmospheric deposition as described 21 by the ecological effect function (US EPA, 200 9 , Figure 1

- 1). A further discussion of 22 uncertainty in aquatic and terrestrial ecos ys tems is presented below. The key uncertaint ies were 23

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45 characterized as follows to evaluate the strength of the scientific basis for setting a national 1 standard to protect against a given effect (US EPA, 200 9 , section 7): 2 (1) Data Availability: high, medium or low quality . This criterion is based on the availability and 3 robustness of data sets, monitoring networks, availability of data that allows for extrapolation 4 to larger assessment areas, and input parameters for modeling and developing the ecological 5 effect function. The scientific basis for the ecolo gi cal indicator selected is also incorporated 6 into this criterion. 7 (2) Modeling Approach: high, fairl y high, intermediate, or low confidence. This value is based 8 on the strengths and limitations of the models used in the anal ys is and how accepted they are 9 by

the scientific community for their application in this analy sis. 10 (3) Ecological Effect Function: high, fairl y high, intermediate, or low confidence. This ranking is 11 based on how well the ecological effect function describes the relationship between 12 atmospheric deposition and the ecological indicator of an effect. 13 i. Aquatic acidification 14 The REA concludes that the available data are robust and considered high quality . There 15 is high confidence about the use of these data and their value for extrapolating to a larg er 16 regional population of lakes. The EPA TIME/LTM network represents a source of long

- term, 17 representative sampling. Data on sulfate concentrations, nitrate concentrations and ANC from 18 1990 to 2006 used for this analy sis as well as EPA EMAP and REMAP survey s, provide 19 considerable data on surface water trends. 20 There is fairly high confidence associated with modeling and input parameters. 21 Uncertainties in water quality estimates (i.e., ANC) from MAGIC were derived from multiple 22 site calibrations. Pre
- acid ification refers to retrospective modeling to estimate water quality 23

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46 conditions before man

- made contributions of acidifying inputs. The models are evaluated under 1 current conditions to determine how well they replicate observed ANC values. The 95% 2 confi dence interval for pre

- acidification of lakes was an average of 15 μeq/L difference in ANC 3 concentrations or 10% and 8 μeq/Lor 5% for streams (US EPA, 200 9 , section 7.1.2). The use of 4 the critical load model to estimate aquatic critical loads is limited by the uncertainties associated 5 with runoff and surface water measurements and in estimating the catchment supply of base 6 cations from the w eathering of bedrock and soils (McNulty et al., 2007). 7 ii. Terrestrial acidification 8 The available data used to quantify the targeted effect of terrestrial acidification are 9 robust and considered high quality. The USFS

- Kane Experimental Forest and significant 10 amounts of research work in the Allegheny Plateau have produced extensive, peer

- reviewed data 11 sets
. A meta

- analy sis of laboratory studies showed that tree growth (often measured b y changes 12 in fine root mass) was decreased b y 20% for BC/Al ratios below ??? (US EPA, 200 9 , section 13 7.2.1 and Figure 7.2

- 1). Sugar maple and red spruce were the focus of the REA since they are 14 demonstrated to be negatively affected by

soil available Ca2+ depletion and high concentrations 15 of available Al, and occur in areas that receive high acidifying deposition, There is high 16 confidence about the use of the REA terrestrial acidification data and their value for 17 extrapolating to a larger regional population of forests. 18 There is high confidence associated with the models, input parameters, and assessment of 19 uncertainty used i n the case study for terrestrial acidification. The Simple Mass Balance (SMB) 20 model, a commonly used and widel y applied approach for estimating critical loads, was used in 21 the REA anal ys is (US EPA, 2008, section 7.2.2). There is fairly high confidence ass ociated with 22

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47 the ecological effect function developed for terrestrial acidification (US EPA, 200 9 , section 1 7.2.3). 2 3. Nutrient Enrichment Effects Associated with Deposition of Oxides of Nitrogen 3 The following summary is a concise overview of the known or anticipated effects caused 4 by nitrogen nutrient enrichment to ecosy stems within the United States. Nutrient

- enrichment 5 affects terrestrial, freshwater and estuarine ecos ys tems. Nitrogen deposition is a major source of 6 anthropogenic nitrogen. For man y terrestrial and freshwater ecos ys tems other sources of 7 nitrogen including fertilizer and waste treatment are greater than deposition. Nitrogen deposition 8 often contributes to nitrogen

- enrichment effects in estuaries, but does not drive the effects since 9 other sources of N greatly exceed N deposition. Both oxides of nitrogen and reduced forms of 10 nitrogen (NH x ) contribute to nitrogen deposition. For the most part, nitrogen effects on 11
ecos ys tems do not depend on whether the nitrogen is in oxidized or reduced fo rm. Thus, this 12 summary

focuses on the effects of nitrogen deposition in total. 13

a. Nature of nutrient enrichment

- related ecosystem responses 14 The I SA found that deposition of nitrogen, including oxides of nitrogen and NH x , leads to 15 the nitrogen enrichment o f ecos ys tems (US EPA 2008). In the process of nitrogen enrichment, 16 biogeochemical components of terrestrial and freshwater aquatic ecos ys tems are altered in a way 17 that leads to effects on biological organisms. 18 i. Aquatic ecosystems 19 In freshwater ecos ys tem s, the evidence is sufficient to infer a causal relationship between 20 N deposition and the alteration of biogeochemical cycling in freshwater aquatic ecos ys tems (US 21 EPA, 2008, section 3.3.2.3 ). N deposition is the main source of N enrichment to headwater 22 streams, lower order streams and high elevation lakes. The most common chemical indicators 23

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48 that were studied included NO 3 − and dissolved inorganic nitrogen (DIN) concentration in surface 1 water s as well as Chl a:total P ratio. Elevated surface water NO 3 − concentrations occur in both 2 the eastern and western U.S. Studies report a significant correlation between N deposition and 3 lake biogeochemistry by identifying a correlation between wet depositi on and [DI N] and Chl a: 4 Total P. Recent evidence provides examples of lakes and streams that are limited by N and show 5 signs of eutrophication in response to N addition. 6 The evidence is sufficient to infer a causal relationship between N deposition and th e 7 alteration of species richness, species composition and biodiversity in freshwater aquatic 8
ecos ys tems (US EPA, 2008, section 3.3.5.3 ). Increased N deposition can cause a shift in 9 community

composition and reduce algal biodiversity , especially in sensitiv e oligotrophic lakes. 10 In the IS A, the evidence is sufficient to infer a causal relationship between N deposition 11 and the biogeochemical cycling of N and carbon (C) in estuaries (US EPA, 2008, section 4.3.4.1 12 and 3.3.2.3 ). In general, estuaries tend to be nitrogen

- limited, and many currentl y receive high 13 levels of nitrogen input from human activities (US EPA, 200 9 , section 5.1.1 ). It is unknown if 14 atmospheric deposition alone is sufficient to cause eutrophication; however, the contribution of 15 atmospheric ni trogen deposition to total nitrogen load is calculated for some estuaries and can be 16 >40% (US EPA, 200 9 , section 5.1.1 ). 17 The evidence is sufficient to infer a causal relationship between N deposition and the 18 alteration of species richness, species composi tion and biodiversity in estuarine ecos ys tems (US 19 EPA, 2008, section 4.3.4.2 and 3.3.5.4 ). Atmospheric and non

- atmospheric sources of N 20 contribute to increased phy toplankton and algal productivity , leading to eutrophication. Shifts in 21 community

compositio n, reduced h ypolimnetic DO, decreases in biodiversity , and mortality of 22 submerged aquatic vegetation are associated with increased N deposition in estuarine s ys tems. 23

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49 ii. Terrestrial Ecosystems 1 The evidence is sufficient to infer a causal relationship between N deposition and the 2 alteration of biogeochemical cycling in terrestrial ecos ys tems (US EPA, 2008, section 4.3.1.1 3 and 3.3.2.1 ). This is supported by numerous observational, deposition gradient and field addition 4 experiments in sensitive ecos ys tems. T he l eaching of NO 3 - in soil drainage waters and the export 5 of NO 3 - in str eam water w ere identified as two of the primary indictors of N enrichment. Several 6 N
- addition studies indicate that NO 3 - leaching is induced b y chronic additions of N

. Studies 7 identified in the I SA found that surface water NO 3 - concentrations exceeded 1 μeq/Lin 8 watersheds receiving about 9 to 13 kg N/ha/yr of atmospheric N deposition. N deposition 9 disrupts the nutrient balance of ecos ys tems with numerous biogeochemical effects. The chem ical 10 indicators that are typically measured include NO 3 − leaching, soil C:N ratio, rates of N 11 mineralization, nitrification, denitrification, foliar N concentration, and soil water NO 3 − and 12 NH 4 + concentrations. Note that N saturation (N leaching from ecos ys tems) does not need to 13 occur to cause effects. Substantial leaching of NO 3 − from forest soils to stream water can acidify 14 downstream waters, leading to effects described in the previous section on aquatic acidification. 15 Due to the complexity of interacti ons between the N and C cycling, the effects of N on C budgets 16 (quantified input and output of C to the ecosy stem) are variable. Regional trends in net 17
ecos ys tem productivity (NEP) of forests (not managed for silviculture) have been estimated 18 through model s based on gradient studies and meta

- anal ys is. Atmospheric N deposition has been 19 shown to cause increased litter accumulation and carbon storage in above

- ground woody 20 biomass. In the West, this has lead to increased susceptibility to more severe fires. Le ss is 21 known regarding the effects of N deposition on C budgets of non

- forest ecosy stems. 22

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50 The evidence is sufficient to infer a causal relationship between N deposition on the 1 alteration of species richness, species composition and biodiversity in terrestrial ecos ys tems (US 2 EPA, 2008, section 4.3.1.2 ). Some organisms and ecos ys tems are more sensitive to N deposition 3 and effects of N deposition are not observed in all habitats. The most sensitive terrestrial taxa to 4 N deposition are lichens. Empirical evidence indicates that lichens in the U.S. are affected by 5 deposition levels as low as 3 kg N/ha/yr. Alpine ecosystems are also sensitive to N deposition, 6 changes in an individual species ( Carex rupestris ) were estimated to occur at deposition levels 7 near 4 kg N /ha/y

r and modeling indicates that deposition levels near 10 kg N/ha/yralter plant 8 community

assemblages. In several grassland ecosy stems, reduced species diversity and an 9 increase in non

- native, invasive species are associated with N deposition. 10 iii. Ecos

ystem sensitivity to nutrient enrichment 11 The numerous ecos ys tem ty pes that occur across the U.S. have a broad range of 12 sensitivity

to N deposition (U S EPA, 2008, Table 4

- 4 ). Increased deposition to N

- limited 13
ecos ys tems can lead to production increases th at may be either beneficial or adverse depending 14 on the sy stem and management goals. 15 Organisms in their natural environment are commonly adapted to a specific regime of 16 nutrient availability . Change in the availability of one important nutrient, such a s N, may result 17 in an imbalance in ecological stoichiometry , with effects on ecos ys tem processes, structure and 18 function. In general, N deposition to terrestrial ecosy stems causes accelerated growth rates in 19 some species deemed desirable in commercial fore sts but may lead to altered competitive 20 interactions among species and nutrient imbalances, ultimately affecting biodiversity . The onset 21 of these effects occurs with N deposition levels as low as 3 kg N/ha/yr in sensitive terrestrial 22
ecos ys tems to N deposi tion. In aquatic ecos ys tems, N that is both leached from the soil and 23

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51 directly deposited to the water surface can pollute the surface water. This causes alteration of the 1 diatom community

at levels as low as 1.5 kg N/ha/y r in sensitive freshwater ecos ys tem s. 2 The degree of ecos ys tem effects lies at the intersection of N loading and N

- sensitivity

. 3 N
- sensitivity

is predominately

driven by the degree to which growth is limited by nitrogen 4 availability . Grasslands in the western United States are typically N

- limited ecos ys tems 5 dominated by

a diverse mix of perennial forbs and grass species. A meta

- analy sis discussed in 6 the I SA (US EPA, 2008, section 3.3.3), indicated that N fertilization increased aboveground 7 growth in all non

- forest ecosy stems except for deserts. In other words, almost all terrestrial 8
ecos ys tems are N
- limited and will be altered b y the addition of anthropogenic nitrogen. 9 Likewise, a freshwater lake or stream must be N

- limited to be sensitive to N

- mediated 10 eutrophication. There are many examples of fresh waters that are N

- limited or N and 11 phosphorous (P) co

- limited (US EPA, 2008, section 3.3.3.2 ). A large dataset meta

- analy sis 12 discussed in the I SA (US EPA, 2008, section 3.3.3.2), found that N

- limitation occurred as 13 frequentl y as P
- limitation in freshwater ecos ys tems. Additional factors that govern the 14 sensitivity

of ecos ys tems to nutrient enrichment from N deposition include rates and form of N 15 deposition, elevation, climate, species composition, plant growth rate, length of growing season, 16 and soil N retention capacity (US EPA, 2008, section 4.3 ). Less is known about the extent and 17 distribution of the terrestrial ecos ys tems in the U.S. that are most sensitive to the effects of 18 nutrient enrichment from atmospheric N deposition compared to ac idification. 19 Because the productivity of estuarine and near shore marine ecos ys tems is generall y 20 limited by

the availability

of N, they

are susceptible to the eutrophication effect of N deposition 21 (US EPA, 2008, section 4.3.4.1 ). A recent national assessm ent of eutrophic conditions in 22 estuaries found the most eutrophic estuaries were generall y those that had large watershed

- to
- 23

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52 estuarine surface area, high human population density , high rainfall and runoff, low dilution, and 1 low flushing rates. In the REA, the National Oceanic and Atmospheric Administration’s 2 (NOAA) National Estuarine Eutrophication Assessment (NEEA) assessment tool, Assessment of 3 Estuarine Tropic Status (ASSETS) categorical Eutrophication Index (EI) was used to evaluate 4 eutrophication due to atmospheric loading of nitrogen. ASSETS EI is an estimation of the 5 likelihood that an estuary is experiencing eutrophication or will experience eutrophication based 6 on five ecological indicators: chlorophyll a , macroalgae, dissolved oxygen, nuisance/to xic algal 7 blooms and submerged aquatic vegetation (SAV). 8 In the REA, two regions were selected for case study anal ys is using ASSETS EI , the 9 Chesapeake Bay

and Pamlico Sound. Both regions received an ASSETS EIrating of Bad 10 indicating that the estuary ha d moderate to high pressure due to overall human influence and a 11 moderate high to high eutrophic condition (US EPA, 200 9 , section s 5.2.4.1 and 5.2.4.2 ). These 12 results were then considered with SPAtially Referenced Regression (SPARROW) modeling to 13 develop a response curve to examine the role of atmospheric nitrogen deposition in achieving a 14 desired decrease in load. To change the Neuse River Estuary ’s EI score from Bad to Poor not 15 only

must 100% of the total atmospheric nitrogen deposition be eliminated, b ut considerabl y 16 more nitrogen from other sources as well must be controlled (US EPA, 200 9 , section 5.2.7.2). In 17 the Potomac River estuary, a 78% decrease of total nitrogen could move the EI score from Bad 18 to Poor (US EPA, 200 9 , section 5.2.7.1 ). The resu lts of this analy sis indicated decreases in 19 atmospheric deposition alone could not eliminate coastal eutrophication problems due to 20 multiple non

- atmospheric nitrogen inputs (US EPA, 200 9 , section 7.3.3 ). However, the 21 somewhat arbitrary discreteness of the EIscale can mask the benefits of decreases in nitrogen 22 between categories. 23

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53 In general, estuaries tend to be N

- limited, and many currentl y receive high levels of N 1 input from human activities to cause eutrophication. As reported in the IS A (US EPA, 2008, 2 section 3.2.2.2), atmospheric N loads to estuaries in the U.S. are estimated to range from 2

- 8% 3 for Guadalupe Bay , TX on the lowest end to as high as 72% for St Catherines

- Sapelo estuary , 4 GA. The Chesapeake Bay is an example of a large, well

- studied and se verely eutrophic estuary 5 that is calculated to receive as much as 30% of its total N load from the atmosphere. 6 b. Magnitude of ecosystem responses 7 i. Aquatic ecosystems 8 The magnitude of ecos ys tem response may be thought of on two time scales, current 9 conditions and how ecosystems have been altered since the onset of anthropogenic N deposition. 10 As noted previousl y, studies found that N

- limitation occurs as frequently as P
- limitation in 11 freshwater ecos ys tems ( US EPA, 2008, section 3.3.3.2 ). Recentl y, a comprehensive study of 12 available data from the northern hemisphere surveys of lakes along gradients of N deposition 13 show increased inorganic N concentration and productivity to be correlated with atmospheric N 14 deposition. The results are unequivocal evidence of N limitation in lakes with low ambient inputs 15 of N, and increased N concentrations in lakes receiving N solel y from atmosphe ric N deposition. 16 It has been suggested that most lakes in the northern hemisphere may have originall y been N

17 limited, and that atmospheric N deposition has changed the balance of N and P in lakes. 18 Available data suggest that the increases in total N depo sition do not have to be large to 19 elicit an ecological effect. For example, a hindcasting exercise determined that the change in 20 Rocky

Mountain National Park lake algae that occurred between 1850 and 1964 was associated 21 with an increase in wet N deposition that was only about 1.5 kg N/ha. Similar changes inferred 22 from lake sediment cores of the Beartooth Mountains of Wyoming also occurred at about 1.5 kg 23

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54 N/ha deposition. Pre

- industrial inorganic N deposition is estimated to have been onl y 0.1 to 0.7 1 kg N/ha based on measurements from remote parts of the world. In the western U.S., pre

2 industrial, or background, inorganic N deposition was estimated b y to range from 0.4 to 0.7 kg 3 N/ha/y

r. 4 Eutrophication effects from N deposition are most likely to be manifest ed in undisturbed, 5 low nutrient surface waters such as those found in the higher elevation areas of the western U.S. 6 The most severe eutrophication from N deposition effects is expected downwind of major urban 7 and agricultural centers. High concentrations of lake or streamwater NO 3 − , indicative of 8
ecos ys tem saturation, have been found at a variety of locations throughout the U.S., including the 9 San Bernardino and San Gabriel Mountains within the L os Angeles Air Basin, the Front Range 10 of Colorado, the Allegh eny mountains of West Virginia, the Catskill Mountains of New York, 11 the Adirondack Mountains of New York, and the Great Smoky Mountains in Tennessee (US 12 EPA, 2008, section 3.3.8 ). 13 In contrast to terrestrial and freshwater s ys tems, atmospheric N load to es tuaries 14 contributes to the total load but does not necessarily drive the effects since other combined 15 sources of N often greatly exceed N deposition. In estuaries, N

- loading from multiple 16 anthropogenic and non

- anthropogenic pathway s leads to water quality deterioration, resulting in 17 numerous effects including h ypoxic zones, species mortality , changes in community composition 18 and harmful algal blooms that are indicative of eutrophication. The following summary is a 19 concise overview of the known or anticipated effects of nitrogen enrichment on estuaries within 20 the United States. 21 There is a scientific consensus (US EPA, 2008, section 4.3.4) that nitrogen

- driven 22 eutrophication in shallow estuaries has increased over the past several decades and that the 23

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55 environmental degradation of coastal ecos ys tems due to nitrogen, phosphorus, and other inputs is 1 now a widespread occurrence. For example, the frequency of phyt oplankton blooms and the 2 extent and severity

of hypoxia have increased in the Chesapeake Bay and Pam lico estuaries in 3 North Carolina and along the continental shelf adjacent to the Mississippi and Atchafalaya 4 rivers’ discharges to the Gulf of Mexico. 5 A recent national assessment of eutrophic conditions in estuaries found that 65% of the 6 assessed s ys tem s had moderate to high overall eutrophic conditions. Most eutrophic estuaries 7 occurred in the mid

- Atlantic region and the estuaries with the lowest degree of eutrophication 8 were in the North Atlantic. Other regions had mixtures of low, moderate, and high degrees of 9 eutrophication (US EPA, 2008, section 4.3.4.3 ). 10 The mid

- Atlantic region is the most heavily impacted area in terms of moderate or high 11 loss of submerged aquatic vegetation due to eutrophication (US EPA, 2008, section 4.3.4.2 ). 12 Submerged aquati c vegetation is important to the quality of estuarine ecosy stem habitats because 13 it provides habitat for a variety of aquatic organisms, absorbs excess nutrients, and traps 14 sediments (US EPA, 2008, section 4.3.4.2). It is partly because many estuaries and near - coastal 15 marine waters are degraded by nutrient enrichment that they are highl y sensitive to potential 16 negative impacts from nitrogen addition from atmospheric deposition. 17 ii. Terrestrial ecosystems 18 Little is known about the full extent and distribution of the terrestrial ecos ystems in the 19 U.S. that are most sensitive to impacts caused b y nutrient enrichment from atmospheric N 20 deposition. As previously stated, most terrestrial ecos ys tems are N
- limited, therefore they are 21 sensitive to perturbation caused b y N additions (US EPA, 2008, section 4.3.1) . Effects are most 22 likely

to occur where areas of relativel y high atmospheric N deposition intersect with N

- limited 23

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56 plant communities. The alpine ecos ys tems of the Colorado Front Range, chaparral watersheds of 1 th e Sierra Nevada, lichen and vascular plant communities in the San Bernardino Mountains and 2 the Pacific Northwest, and the southern California coastal sage scrub (CSS) community are 3 among the most sensitive terrestrial ecos ys tems. There is growing evidence (US EPA, 2008, 4 section 4.3.1.2) that existing grassland ecos ys tems in the western United States are being altered 5 by elevated levels of N inputs, including inputs from atmospheric deposition. 6 In the eastern U.S., the degree of N saturation of the terrestrial ecos ys tem is often 7 assessed in terms of the degree of NO 3 − leaching from watershed soils into ground water or 8 surface water. Studies have estimated the number of surface waters at different stages of 9 saturation across several regions in the eastern U.S . Of the 85 northeastern watersheds examined 10 60% were in Stage 1 or Stage 2 of N saturation on a scale of 0 (background or pretreatment) to 3 11 (visible decline). Of the northeastern sites for which adequate data were available for assessment, 12 those in Stage 1 or 2 were most prevalent in the Adirondack and Catskill Mountains. Effects on 13 individual plant species have not been well studied in the U.S. More is known about the 14 sensitivity

of particular plant communities. Based largel y on results obtained in more extensive 15 studies conducted in Europe, it is expected that the more sensitive terrestrial ecos ys tems include 16 hardwood forests, alpine meadows, arid and semi

- arid lands, and grassland ecos ys tems (US 17 EPA, 2008, section 3. 3.5)

. 18 The REA used published research results (US EPA, 200 9 , section 5.3.1 and US EPA, 19 2008, Table 4.4) to identify meaningful ecological benchmarks associated with different levels of 20 atmospheric nitrogen deposition. These are illustrated in Figure 3

- 4 of the Policy Assessment

. 21 The sensitive areas and ecological indicators identified by the IS A were analyzed further in the 22 REA to create a national map that illustrates effects observed from ambient and experimental 23

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57 atmospheric nitrogen deposition loads in relation to Community Multi

- s cale Air Quality 1 (CMAQ) 2002 modeling results and NADP monitoring data. This map, reproduced in Figure 3

- 2 5 of the Policy

Assessment , depicts the sites where empirical effects of terrestrial nutrient 3 enrichment have been observed and site proximity to elev ated atmospheric N deposition. 4 Based on information in the I SA and initial anal ys is in the REA, further case study 5 anal ys es on terrestrial nutrient enrichment of ecosy stems were developed for the CCS 6 community

and Mixed Conifer Forest (MCF) (US EPA, 200 9). Geographic information sy stems 7 (GI S) analy sis supported a qualitative review of past field research to identify ecological 8 benchmarks associated with CSS and mycorrhizal communities, as well as MCF nutrient

9 sensitive acidoph yt e lichen communities, fin e - root biomass in Ponderosa pine, and leached 10 nitrate in receiving waters. 11 The ecological benchmarks that were identified for the CSS and the MCF communities 12 are included in the suite of benchmarks identified in the I SA ( US EPA, 2008, section 3.3 ). There 13 are sufficient data to confidently relate the ecological effect to a loading of atmospheric nitrogen. 14 For the CSS community , the following ecological benchmarks were identified: 15 (1) 3.3 kg N/ha/yr
– the amount of nitrogen uptake b y a vigorous stand of CSS; ab ove this level, 16 nitrogen may

no longer be limiting 17 (2) 10 kg N/ha/yr
– mycorrhizal community changes 18 For the MCF community, the following ecological benchmarks were identified: 19 (1) 3.1 kg N/ha/yr
– shift from sensitive to tolerant lichen species 20 (2) 5.2 kg N/ha/yr
– d ominance of the tolerant lichen species 21 (3) 10.2 kg N/ha/yr
– loss of sensitive lichen species 22 (4) 17 kg N/ha/yr
– leaching of nitrate into streams. 23

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58 These benchmarks, ranging from 3.1 to 17 kg N/ha/y r, were compared to 2002 1 CMAQ/NADP data to discern an y associati ons between atmospheric deposition and changing 2 communities. Evidence supports the finding that nitrogen alters CSS and MCF communities

. 3 Key findings include the following: 2002 CMAQ/NADP nitrogen deposition data show that the 4 3.3 kg N/ha/yrbenchmark has been exceeded in more than 93% of CSS areas (654,048 ha). 5 These deposition levels are a driving force in the degradation of CSS communities. Although 6 CSS decline has been observed in the absence of fire, the contributions of deposition and fire to 7 the CSS decline require further research. CSS is fragmented into many small parcels, and the 8 2002 CMAQ/NADP 12

- km grid data are not fine enough to fully validate the relationship 9 between CSS distribution, nitrogen deposition, and fire. 2002 CMAQ/NADP nitrogen depo sition 10 data exceeds the 3.1 kg N/ha/y r benchmark in more than 38% (1,099,133 ha) of MCF areas, and 11 nitrate leaching has been observed in surface waters. Ozone effects confound nitrogen effects on 12 MCF acidoph yt e lichen, and the interrelationship between fire and nitrogen cycling requires 13 additional research. 14

c. Key uncertainties associated with nutrient enrichment 15 There are different levels of uncertainty associated with relationships between deposition, 16 ecological effects and ecological indicators. The criteria used in the REA to evaluate the degree 17 of confidence in the data, modeling and ecological effect function are detailed in Chapter 7 of the 18 REA . Below is a discussion of uncertainty relating aquatic and terrestrial ecosystems to nutrient 19 enrichment effects. 20 i. Aquatic ecosystems 21 The approach for assessing atmospheric contributions to total nitrogen loading in the 22 REA was to consider the main

- stem river to an estuary (including the estuary) rather than an 23

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59 entire estuary

s ys tem or bay . The biological indicators used in the NOAA ASSETS EIrequired 1 the evaluation of many national databases including the US Geological Survey National Water 2 Quality

Assessment (NAWQA) files, EPA’s STORage and RETrieval (STORET) database, 3 NOAA’s Estuarine Drainage Areas data, and EPA’s water quality standards nutrient criteria for 4 rivers and lakes (US EPA, 200 9 , Appendix 6 and Table 1.2.

- 1 ). Both the SPARROW modeling 5 for nitrogen loads and assessment of estuary conditions under NOAA ASSETS EI , have been 6 applied on a nation al scale. The REA concludes that the available data are medium quality with 7 intermediate confidence about the use of these data and their values for extrapolating to a larger 8 regional area ( US EPA, 200 9 , section 7.3.1 ). Intermediate confidence is associated with the 9 modeling approach using ASSETS EIand SPARROW. The REA states there is low confidence 10 with the ecological effect function due to the results of the anal ys is which indicated that 11 reductions in atmospheric deposition alone could not solve coast al eutrophication problems due 12 to multiple non

- atmospheric nitrogen inputs (US EPA, 200 9 , section 7.3.3 ). 13 ii. Terrestrial ecosystems 14 Ecological thresholds are identified for CSS and MCF areas and these data are considered 15 to be of high quality , however, the ability to extrapolate these data to larger regional areas is 16 limited (U S EPA, 2009 , section 7.4.1 ). No quantitative modeling was conducted or ecological 17 effect function developed for terrestrial nutrient enrichment reflecting the uncertainties 18 associated with these depositional effects. 19 4. Other Ecological Effects 20 It is stated in the ISA ( US EPA, 2008, section 3.4.1 and 4.5 ) that mercury is a highl y 21 neurotoxic contaminant that enters the food web as a methylated compound, methylmercury

. 22 Mercury

is prin cipally

methylated by

sulfur

- reducing bacteria and can be taken up by 23

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60 microorganisms, zooplankton and macroinvertebrates. The contaminant is concentrated in higher 1 trophic levels, including fish eaten b y humans. Experimental evidence has established that o nly 2 inconsequential amounts of methylmercury can be produced in the absence of sulfate. Once 3 methylmercury

is present, other variables influence how much accumulates in fish, but elevated 4 mercury

levels in fish can only

occur where substantial amounts of m ethylmercury are present. 5 Current evidence indicates that in watersheds where mercury is present, increased oxides of 6 sulfur deposition very likely results in additional production of methylmercury which leads to 7 greater accumulation of MeHg concentrations in fish. With respect to sulfur deposition and 8 mercury

methylation, the final I SA determined: The evidence is sufficient to infer a causal 9 relationship between sulfur deposition and increased mercury methylation in wetlands and 10 aquatic environments. 11 Th e production of meaningful amounts of meth yl mercury (MeHg) requires the presence 12 of SO 4 2

- and mercury , and where mercury is present, increased availability of SO 4 2

- results in 13 increased production of MeHg. There is increasing evidence on the relationship b etween sulfur 14 deposition and increased methylation of mercury in aquatic environments; this effect occurs onl y 15 where other factors are present at levels within a range to allow methylation. The production of 16 methylmercury

requires the presence of sulfate and mercury , but the amount of methylmercury 17 produced varies with ox ygen content, temperature, pH, and supply of labile organic carbon (US 18 EPA, 2008, section 3.4 ). In watersheds where changes in sulfate deposition did not produce an 19 effect, one or several o f those interacting factors were not in the range required for meaningful 20 methylation to occur (US EPA, 2008, section 3.4 ). Watersheds with conditions known to be 21 conducive to mercury methylation can be found in the northeastern United States and 22 southeast ern Canada. 23

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61 While the relationship between sulfur and methylmercury production was concluded to 1 be causal in the IS A, the REA concluded that there was insufficient evidence to quantify the 2 relationship between sulfur and meth yl mercury . Therefore onl y a qualitative assessment was 3 included in Chapter 6 of the REA

. The Policy Assessment was then unable to make a 4 determination as to the adequacy of the existing SO 2 standards in protecting against welfare 5 effects associated with increased mercury methylation. 6 B. Risk and Exposure Assessment 7 The risk and exposure assessment conducted for the current review was developed to 8 describe potential risk from current and future deposition of oxides of nitrogen and sulfur to 9 sensitive ecos ys tems. The case study anal ys es in the REA show that there is confidence that 10 known or anticipated adverse ecological effects are occurring under current ambient loadings of 11 nitrogen and sulfur in sensitive ecosy stems across the United States. An overview of the 12 material covered in the REA , a summary of the key

findings from the air quality analy ses, 13 acidification and nutrient enrichment case studies, and general conclusions from evaluating 14 additional welfare effects, are presented below. 15 1. Overview of the Risk and Exposure Assessmen t 16 The REA evaluates the relationships between atmospheric concentrations, deposition, 17 biologicall y relevant exposures, targeted ecos ys tem effects, and ecos ys tem services. To evaluate 18 the nature and magnitude of adverse effects associated with deposition , the REA also examines 19 various way s to quantify the relationships between air quality indicators, deposition of 20 biologicall y available forms of nitrogen and sulfur, ecologicall y relevant indicators relating to 21 deposition, exposure and effects on sensitive receptors, and related effects resulting in changes in 22
ecos ys tem structure and services. The intent is to determine the exposure metrics that incorporate 23

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62 the temporal considerations (i.e., biologically relevant timescales), pathways, and ecologicall y 1 relevant indicators necessary to determine the effects on these ecos ys tems. To the extent feasible, 2 the REA evaluates the overall load to the s ys tem for nitrogen and sulfur, as well as the variability 3 in ecosy stem responses to these pollutants . It also evaluate s the contributions of atmosphericall y 4 deposited nitrogen and sulfur individually relative to the combined atmospheric loadings of both 5 elements together . Since oxidized nitrogen is the listed criteria pollutant (currentl y measured b y 6 the ambient air quali ty indicator NO 2 ) for the atmospheric contribution to total nitrogen, the REA 7 examines the contribution of nitrogen oxides to total reactive nitrogen in the atmosphere, relative 8 to the contributions of reduced forms of nitrogen (e.g., ammonia, ammonium), t o ultimately 9 assess how a meaningful secondary National Ambient Air Quality Standards (NAAQS) might be 10 structured. 11 The REA focuses on ecosy stem welfare effects that result from the deposition of total 12 reactive nitrogen and sulfur. Because ecos ys tems are diverse in biota, climate, geochemistry , and 13 hydrology , response to pollutant exposures can vary greatl y between ecos ystems. In addition, 14 these diverse ecos ys tems are not distributed evenly across the United States. To target nitrogen 15 and sulfur acidificat ion and nitrogen and sulfur enrichment, the REA addresses four main 16 targeted ecos ys tem effects on terrestrial and aquatic sy stems identified by the IS A (US EPA, 17 2008 ): a quatic acidification due to nitrogen and sulfur ; t errestrial acidification due to nitro gen 18 and sulfur ; a quatic nutrient enrichment, including eutrophication ; and t errestrial nutrient 19 enrichment. 20 In addition to these four targeted ecos ys tem effects, the REA also qualitatively addresses 21 the influence of sulfur oxides deposition on methylmercury production; nitrous oxide (N 2 O) 22

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63 effects on climate; nitrogen effects on primary productivity and biogenic greenhouse gas fluxes; 1 and phy

totoxic effects on plants. 2 Because the targeted ecosy stem effects outlined above are not evenl y distributed across 3 the United States, the REA identified case studies for each targeted effects based on ecos ys tems 4 identified as sensitive to nitrogen and/or sulfur deposition effects. Eight case study areas and 5 two supplemental study areas (Rocky Mountain Natio nal Park and Little Rock Lake, WI) are 6 summarized in the REA based on ecos ys tem characteristics, indicators, and ecos ys tem service 7 information. Case studies selected for aquatic acidification effects were the Adirondack 8 Mountains and Shenandoah National P ark. Kane Experimental Forest in Pennsylvania and 9 Hubbard Brook Experimental Forest in New Hampshire were selected as case studies for 10 terrestrial acidification. Aquatic nutrient enrichment case study locations were selected in the 11 Potomac River Basin u pstream of Chesapeake Bay and the Neuse River Basin upstream of the 12 Pamlico Sound in North Carolina. The Coastal Sage Scrub Communities in southern California 13 and the Mixed Confer Forest Communities in the San Bernardino and Sierra Nevada Mountains 14 of Cal ifornia were selected as case studies for terrestrial nutrient enrichment. Two supplemental 15 areas were also chosen, one in Rocky Mountain National Park for terrestrial nutrient enrichment 16 and one in L ittle Rock Lake, Wisconsin for aquatic nutrient enrichm ent. 17 2. Key findings 18 In summary , based on case study anal ys es, the REA concludes that known or anticipated 19 adverse ecological effects are occurring under current conditions and further concludes that these 20 adverse effects continue into the future

. Key f indings from the air quality analyses, acidification 21 and nutrient enrichment case studies, as well as general conclusions from evaluating additional 22 welfare effects, are summarized below. 23

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64 a. Air quality analyses 1 The air quality analyses in the REA encompass the current emissions sources of nitrogen 2 and sulfur, as well as atmospheric concentrations, estimates of deposition of total nitrogen, 3 policy

- relevant background, and non

- atmospheric loadings of nitrogen and sulfur to ecos ys tems, 4 both nationwide and in the case study areas. Spatial fields of deposition were created using wet 5 deposition measurements from the National Atmospheric Deposition Program (NADP) National 6 Trends Network and dry deposition predictions from the 2002 Community Multi

- Scale Air 7 Quality (CMAQ) model simulation. Some key conclusions from this analysis are: 8 (1) Total reactive nitrogen deposition and sulfur deposition are much greater in the East 9 compared to most areas of the West. 10 (2) These regional differences in deposition correspond to the regional differences in oxides of 11 nitrogen and SO2 concentrations and emissions, which are also higher in the East. Oxides of 12 nitrogen emissions are much greater and generally more widespread than NH 3 emissions 13 nationwide; high NH 3 emissions tend to be more local (e.g., eastern North Carolina) or sub

14 regional (e.g., the upper Midwest and Plains states). The relative amounts of oxidized versus 15 reduced nitrogen deposition are consistent with the relative amounts of oxides of nitrogen 16 and NH 3 emissions. Oxidized nitrogen deposition exceeds reduced nitrogen deposition in 17 most of the case study areas; the major exception being the Neuse River/Neuse River Estuary 18 Case Study

Area. 19 (3) R educed nitrogen depositio n exceeds oxidized nitrogen deposition in the vicinity of local 20 sources of NH 3 . 21

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65 (4) There can be relativel y large spatial variations in both total reactive nitrogen deposition and 1 sulfur deposition within a case study area; this occurs particularl y in those areas that contain 2 or are near a high emissions source of oxides of nitrogen , NH 3 , and/or SO 2 . 3 (5) The seasonal patterns in deposition differ between the case study areas. For the case study 4 areas in the East, the season with the greatest amounts of total reacti ve nitrogen deposition 5 correspond to the season with the greatest amounts of sulfur deposition. Deposition peaks in 6 spring in the Adirondack, Hubbard Brook Experimental Forest, and Kane Experimental 7 Forest case study areas, and it peaks in summer in the Po tomac River/Potomac Estuary , 8 Shenandoah, and Neuse River/Neuse River Estuary case study

areas. For the case study areas 9 in the West, there is less consistency in the seasons with greatest total reactive nitrogen and 10 sulfur deposition in a given area. In ge neral, both nitrogen and/or sulfur deposition peaks in 11 spring or summer. The exception to this is the Sierra Nevada Range portion of the Mixed 12 Conifer Forest Case Study Area, in which sulfur deposition is greatest in winter. 13 b. Deposition

- related aquatic acidification 14 The role of aquatic acidification in two eastern United States areas

— northeastern New 15 York’s Adirondack area and the Shenandoah area in Virginia

— was anal yz ed in the REA to 16 assess surface water trends in SO 4 2

- and NO 3 - concentrations and acid neutralizing capacity (ANC) 17 levels and to affirm the understanding that reductions in deposition could influence the risk of 18 acidification. Monitoring data from the EPA

- administered Temporall y Integrated Monitoring of 19 Ecosy stems (T IME)/Long

- Term Monitoring (LTM) programs and the Environmental Monitoring 20 and Assessment Program (EMAP) were assessed for the years 1990 to 2006, and past, present, 21 and future water quality levels were estimated using both steady

- state and dy namic 22 biogeoch emical models. 23

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66 Although wet deposition rates for SO 2 and oxides of nitrogen in the Adirondack Case 1 Study

Area have reduced since the mid

- 1990s, current concentrations are still well above pre

2 acidification (1860) conditions. Model of Acidification of Gro undwater in Catchments (MAGIC) 3 modeling predicts NO 3 - and SO 4 2

- are 17

- and 5

- fold higher today , respectively . The estimated 4 average ANC for 44 lakes in the Adirondack Case Study Area is 62.1 microequivalents per liter 5 (μeq/L) (± 15.7 μeq/L); 78 % of all m onitored lakes in the Adirondack Case Study Area have a 6 current risk of Elevated, Severe , or Acute . Of the 78%, 31% experience episodic acidification, 7 and 18% are chronicall y acidic today

. 8 (1) Based on the steady- state critical load model for the year 2002, 18 %, 28%, 44%, and 58% of 9 169 modeled lakes received combined total sulfur and nitrogen deposition that exceeded 10 critical load s corresponding to ANC limits of 0, 20, 50, and 100 μeq/Lrespectivel y. 11 (2) Based on a deposition scenario that maintains current emissi on levels to 2020 and 2050, the 12 simulation forecast indicates no improvement in water quality in the Adirondack Case Study 13 Area. The percentage of lakes within the Elevated to Acute Concern classes remains the same 14 in 2020 and 2050. 15 (3) Since the mid

- 1990s, streams in the Shenandoah Case Study Area have shown slight declines 16 in NO 3 and SO 4 2

- concentrations in surface waters. ANC levels increased from about 50 17 μeq/Lin the earl y 1990 s to >75 μeq/Luntil 2002, when ANC levels declined back to 1991

– 18 1992 levels. Current concentrations are still above pre

- acidification (1860) conditions. 19 MAGI C modeling predicts surface water concentrations of NO 3 and SO 4 2

- are 10

- and 32

- 20 fold higher today , respectively . The estimated average ANC for 60 streams in the 21 Shenandoah Case Study Area is 57.9 μeq/L (± 4.5 μeq/L). 55% of all monitored streams in 22

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67 the Shenandoah Case Study Area have a current risk of Elevated, Severe , or Acute . Of the 1 55%, 18% experience episodic acidification, and 18% are chronically acidic today

. 2 (4) Based on the steady- state critical load model for the year 2002, 52%, 72%, 85%, and 93% of 3 60 modeled streams received combined total sulfur and nitrogen deposition that exceeded 4 critical load s corresponding to ANC limits of 0, 20, 50, and 100 μeq/Lrespectivel y. 5 (5) Based on a deposition scenario that maintains current emission levels to 2020 and 2050, the 6 simulation forecast indicates that a large number of streams would still have Elevated to 7 Acute problems with acidity

. In fact, from 2006 to 2050, the percentage of streams in the 8 range of Moderate to Acute Concern ...[add in the percentage increase or decrease for this 9 range as a whole]. The percentage with Acute Concern increases b y 5%, while the 10 percentage of streams in M oderate Concern decreases by 5%. 11
c. Deposition

- related terrestrial acidification 12 The role of terrestrial acidification was examined in the REA using a critical load 13 anal ys is for sugar maple and red spruce forests in the eastern United States by using the base 14 cation to aluminum (Bc/Al) ratio in acidified forest soils as an indicator to assess the impact of 15 nitrogen and sulfur deposition on tree health. These are the two most commonly studied species 16 in North America for impacts of acidification. At a Bc/Al ratio of 1.2, red spruce growth can be 17 reduced b y 20%. Sugar maple growth can be reduced b y 20% at a Bc/Al ratio of 0.6. Key 18 findings of the case study are summarized below. 19 (1) Case study

results suggest that the health of at least a portion of the sugar ma ple and red 20 spruce growing in the United States may have been compromised with acidifying total 21 nitrogen and sulfur deposition in 2002

. 2002 CMAQ/NADP total nitrogen and sulfur 22 deposition levels exceeded three selected critical loads in 3% to 75% of all s ugar maple plots 23

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68 across 24 states. The three critical loads ranged from 6,008 to 107 eq/ha/y r for the Bc/Al 1 ratios of 0.6, 1.2, and 10.0 (increasing levels of tree protection). The 2002 CMAQ/NADP 2 total nitrogen and sulfur deposition levels exceeded three s elected critical loads in 3% to 36% 3 of all red spruce plots across 8 states. The three critical loads ranged from 4,278 to 180 4 eq/ha

- yr for the Bc/Al ratios of 0.6, 1 .2, and 10.0 (increasing levels of tree protection). 5 (2) The Simple Mass Balance model assumpt ions made for base cation weathering (Bcw) and 6 forest soil ANC input parameters are the main sources of uncertainty since these parameters 7 are rarel y measured and require researchers to use default values. 8 (3) The pattern of case study results suggests that n itrogen and sulfur acidifying deposition in the 9 sugar maple and red spruce forest areas studied were similar in magnitude to the critical 10 loads for those areas and both ecos ys tems are likely to be sensitive to any

future changes in 11 the levels of deposition. 12 d. Deposition

- related aquatic nutrient enrichment 13 The role of nitrogen deposition in two main stem rivers feeding their respective estuaries 14 was anal yz ed in the REA to determine if decreases in deposition could influence the risk of 15 eutrophication as predicted using the Assessment of Estuarine Trophic Status eutrophication 16 index (ASSETS EI) scoring s ys tem in tandem with SPAtially Referenced Regression on 17 Watershed Attributes (SPARROW) modeling. This modeling approach provides a transferrable, 18 intermediate

- level anal ys is of the linkages between atmospheric deposition and receiving waters, 19 while providing results on which conc lusions could be drawn. A summary of findings follows: 20 (1) 2002 CMAQ/NADP results showed that an estimated 40,770,000 kg of total nitrogen was 21 deposited in the Potomac River watershed. SPARROW modeling predicted that 7,380,000 kg 22 N/y

r of the deposited nitrogen reached the estuary (20% of the total load to the estuary). The 23

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69 overall ASSETS EIfor the Potomac River and Potomac Estuary was Bad (based on all 1 sources of N)

. 2 (2) To improve the Potomac River and Potomac Estuary ASSETS EIscore from Bad to Poor , a 3 decrease of at least 78% in the 2002 total nitrogen atmospheric deposition load to the 4 watershed would be required. 5 (3) 2002 CMAQ/NADP results showed that an estimated 18,340,000 kg of total nitrogen was 6 deposited in the Neuse River watershed. SPARROW modeling predict ed that 1,150,000 kg 7 N/y

r of the deposited nitrogen reached the estuary (26% of the total load to the estuary). The 8 overall ASSETS EIfor the Neuse River/Neuse River Estuary was Bad

. 9 (4) It was found that the Neuse River/Neuse River Estuary ASSETS EIscore cou ld not be 10 improved from Bad to Poor with decreases onl y in the 2002 atmospheric deposition load to 11 the watershed. Additional reductions would be required from other nitrogen sources within 12 the watershed. 13 The small effect of decreasing atmospheric depositi on in the Neuse River watershed is 14 because the other nitrogen sources within the watershed are more influential than atmospheric 15 deposition in affecting the total nitrogen loadings to the Neuse River Estuary , as estimated with 16 the SPARROW model . A waterbody’ s response to nutrient loading depends on the magnitude 17 (e.g., agricultural sources have a high er influence in the Neuse than in the Potomac ), spatial 18 distribution, and other characteristics of the sources within the watershed ; therefore a reduc tion 19 in nitrogen deposition does not alway s produce a linear response in reduced load to the estuary , 20 as demonstrated b y these two case studies. 21

e. Deposition

- related terrestrial nutrient enrichment 22

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70 California C oastal S age S crub (CSS) and M ixed C onifer F orest (MCF) communities 1 were the focus of the Terrestrial Nutrient Enrichment Case Studies of the REA. Geographic 2 information sy stems (GIS) analy sis supported a qualitative review of past field research to 3 identify

ecological benchmarks associated with CSS and mycorrhizal communities, as well as 4 MCF’s nutrient

- sensitive acidophy te lichen communities, fine

- root biomass in Ponderosa pine, 5 and leached nitrate in receiving waters. These benchmarks, ranging from 3.1 to 17 kg N/ha/yr, 6 were compared to 2002 CMAQ/NADP data to discern an y associations between atmospheric 7 deposition and changing communities. Evidence supports the finding that nitrogen alters CSS 8 and MCF. Key

findings include the following: 9 (1) 2002 CMAQ/NADP nitrogen deposition data show that the 3.3 kg N/ha/yr benchmark has 10 been exceeded in more than 93% of CSS areas (654,048 ha). This suggests that such 11 deposition is a driving force in the degradation of CSS communities. One potentially 12 confounding factor is the role of fire. Although CSS decline has been observed in the 13 absence of fire, the contributions of deposition and fire to the CSS decline require further 14 research. CSS is fragmented into many small parcels, and the 2002 CMAQ/NADP 12

- km 15 grid data are not fine enough to full y validate the relationship between CSS distribution, 16 nitrogen deposition, and fire. 17 (2) 2002 CMAQ/NADP nitrogen deposition data exceeds the 3.1 kg N/ha/yr benchmark in more 18 than 38% (1,099,133 ha) of MCF areas, and nitrate leaching has been observed in surface 19 waters. Ozone effects confound nitrogen effects on MCF acidoph yt e lichen, and the 20 interrelationship between fire and nitrogen cycling requires additional research. 21 f. Additional effects 22

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71 Ecological effects have also been documented ac ross the United States where elevated 1 nitrogen deposition has been observed, including the eastern slope of the Rocky Mountains 2 where shifts in dominant algal species in alpine lakes have occurred where wet nitrogen 3 deposition was only

about 1.5 kg N/ha/yr . High alpine terrestrial communities have a low 4 capacity

to sequester nitrogen deposition, and monitored deposition exceeding 3 to 4 kg N/ha/ yr 5 could lead to community

- level changes in plant species, lichens, and mycorrhizae. 6 Additional welfare effects are documented, but examined less extensively , in the REA. 7 These effects include qualitative discussions related to visibility and materials damage, such as 8 corrosion, erosion, and soiling of paint and buildings which are being addressed in the particulate 9 matter (PM) NAAQS review currently underway . A discussion of the causal relationship 10 between sulfur deposition (as sulfate, SO 4 2

- ) and increased mercury methylation in wetlands and 11 aquatic environments is also included in the REA. On this subject the REA concludes that 12 decreases in sulfate deposition will likely result in decreases in methyl mercury concentration; 13 however, spatial and biogeochemical variations nationally hinder establishing large scale dose

14 response relationships. 15 Several additional is sues concerning oxides of nitrogen were addressed in the REA. 16 Consideration was also given to nitrous oxide (N 2 O), a potent GHG. The REA concluded that it 17 is most appropriate to analyze the role of N 2 O in the context of all of the GHGs rather than as 18 part of the REA for this review. The REA considered n itrogen deposition and its correlation 19 with the rate of photos ynthesis and net primary productivity

. Nitrogen addition ranging from 20 15.4 to 300 kg N/ha/yr is documented as increasing wetland N 2 O production by an average of 21 207% across all ecos ys tems . Nitrogen addition ranging from 30 to 240 kg N/ha/y r increased CH 4 22 emissions by

115%, averaged across all ecos ys tems, and methane uptake was reduced b y 38% 23

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72 averaged across all ecosystems when nitrogen addition ranged from 10 to 560 kg N/ha/yr , but 1 reductions were onl y significant for coniferous and deciduous forests. The heterogeneity of 2
ecos ys tems across the United States, however, introduces variations into dose

- response 3 relationships. 4 The p hy

totoxic effects of oxides of nitrogen and sulfur on vegetation were also briefl y 5 discussed in the REA which concluded that since a unique secondary NAAQS exists for SO 2 , 6 and concentrations of nitric oxide (NO ) , NO 2 , and PAN are rarely high enough to have 7 phy totoxic effects on vegetation , further assessment was not warranted at this time. 8 3. Conclusions on E ffects 9 For aquatic and terrestrial acidification effects, a similar conceptual approach was used 10 (critical loads) to evaluate the impacts of multiple pollutants on an ecological e ndpoint, whereas 11 the approaches used for aquatic and terrestrial nutrient enrichment were fundamentally distinct. 12 Although the ecological indicators for aquatic and terrestrial acidification (i.e., ANC and Bc/Al) 13 are very

different, both ecological indicators are well

- correlated with effects such as reduced 14 biodiversity

and growth. While aquatic acidification is clearl y the targeted effect area with the 15 highest level of confidence, the relationship between atmospheric deposi tion and an ecological 16 indicator is also quite strong for terrestrial acidification. The main drawback with the 17 understanding of terrestrial acidification is that the data are based on laboratory responses rather 18 than field measurements. Other stressors th at are present in the field but that are not present in 19 the laboratory

may confound this relationship. 20 For nutrient enrichment effects, the REA utilized different types of indicators for aquatic 21 and terrestrial effects to assess both the likelihood of adv erse effects to ecosy stems and the 22 relationship between adverse effects and atmospheric sources of oxides of nitrogen. The 23

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73 ecological indicator chosen for aquatic nutrient enrichment, the ASSETS EI, seems to be 1 inadequate to relate atmospheric deposition to the targeted ecological effect, likely due to the 2 many

other confounding factors. Further, there is far less confidence associated with the 3 understanding of aquatic nutrient enrichment because of the large contributions from non

4 atmospheric sources of n itrogen and the influence of both oxidized and reduced forms of 5 nitrogen, particularl y in large watersheds and coastal areas. However, a strong relationship exists 6 between atmospheric deposition of nitrogen and ecological effects in high alpine lakes in th e 7 Rocky

Mountains because atmospheric deposition is the only source of nitrogen to these 8 sy stems. There is also a strong weight

- of

- evidence regarding the relationships between 9 ecological effects attributable to terrestrial nitrogen nutrient enrichment; how ever, ozone and 10 climate change may be confounding factors. In addition, the response for other species or species 11 in other regions of the United States has not been quantified. 12 C. Adversity of Effects to Public Welfare 13 Characterizing a known or anticipated adverse effect to public welfare is an important 14 component of developing any secondary

NAAQS. According to the Clean Air Act, welfare 15 effects include: “ effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, 16 weather, visibility , and climate, damage to and deterioration of property, and hazards to 17 transportation, as well as effect on economic values and on personal comfort and well

- being, 18 whether caused b y transformation, conversion, or combination with other air pollutants ” (CAA, 19 Section 302(h)). While the text above lists a number of welfare effects, these effects do not 20 define public welfare in and of themselves. 21 Although there is no specific definition of adversity to public welfare, the paradigm of 22 linking adversity to public welfare to disruptions in ecos ys tem structure and function has been 23

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74 used broadl y b y EPA to categorize effects of pollutants from the cellular to the ecos ys tem level. 1 An evaluation of adversity to public welfare might consider the likelihood, ty pe, magnitude, and 2 spatial scale of the effect as well as the potential for recovery and any uncertainties relating to 3 these considerations. 4 Similar concepts were used in past reviews of secondary NAAQS for ozone and PM 5 (relating to visibility ), as well as in ini tial reviews of effects from lead deposition. Because 6 oxides of nitrogen and sulfur are deposited from ambient sources into ecosy stems where they 7 affect changes to organisms, populations and ecosystems, the concept of adversity to public 8 welfare as a resu lt of alterations in structure and function of ecos ys tems is an appropriate 9 consideration for this review. 10 Based on information provided in the Policy Assessment , the following section discusses 11 how ecological effects from deposition of oxides of nitrog en and sulfur relate to adversity to 12 public welfare

. In the Policy Assessment , p ublic welfare was discussed in terms of loss of 13
ecos ys tem services (defined below) , which in some cases can be monetized. Each of the four 14 main effect areas (aquatic and terr estrial acidification and aquatic and terrestrial nutrient over

15 enrichment) are discussed including current ecological effects and associated ecos ys tem services

. 16 1. Ecosystem Services 17 The Policy

Assessment defines ecosy stem services as the benefits indiv iduals and 18 organizations obtain from ecosy stems. Ecos ys tem services can be classified as provisioning 19 (food and water), regulating (control of climate and disease), cultural (recreational, existence, 20 spiritual, educational), and supporting (nutrient cycli ng). Conceptuall y, changes in ecos ys tem 21 services may

be used to aid in characterizing a known or anticipated adverse effect to public 22 welfare. In the REA and Policy Assessment ecosy stem services are discussed as a method of 23

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75 assessing the magnitude and si gnificance to the public of resource s affected by ambient 1 concentrations of oxides of nitrogen and sulfur and deposition in sensitive ecos ys tems
. 2 EPA has in previous NAAQS reviews defined ecological goods and services for the 3 purposes of a Regulatory Imp act Anal ys is as the “outputs of ecological functions or processes 4 that directly

or indirectl y contribute to social welfare or have the potential to do so in the future. 5 Some outputs may

be bought and sold, but most are not marketed.” It is e specially important to 6 acknowledge that it is difficult to measure and/or monetize the goods and services supplied by 7
ecos ys tems. It can be informative in characterizing adversity to public welfare to attempt to 8 place an economic valuation on the set of goods and ser vices that have been identified with 9 respect to a change in policy however it must be noted that this valuation will be incomplete and 10 illustrative only

. 11 Knowledge about the relationships linking ambient concentrations and ecosy stem 12 services is considered in the Policy Assessment as one method by which to inform a policy 13 judgment on a known or anticipated adverse public welfare effect. For example, a change in an 14
ecos ys tem structure and process, such as foliar injury , would be classified as an ecological effect, 15 with the associated changes in ecos ys tem services, such as primary productivity , food 16 availability , forest products, and aesthetics (e.g., scenic viewing), classified as public welfare 17 effects. Additionally , changes in biodiversity wo uld be classified as an ecological effect, and the 18 associated changes in ecosy stem services

— productivity , existence (nonuse) value, recreational 19 viewing and aesthetics

—would also be classified as public welfare effects. 20 As described in Chapters 4 and 5 of the REA , case study analy ses were performed that 21 link deposition in sensitive ecos ys tems to changes in a given ecological indicator (e.g., for 22 aquatic acidification, to changes in acid neutralizing capacity [ANC]) and then to changes in 23

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76 ecos ys tems. App endix 8 of the REA links the changes in ecos ys tems to the services they 1 provide (e.g., fish species richness and its influence on recreational fishing). To the extent 2 possible for each targeted effect area, the REA linked ambient concentrations of nitrogen and 3 sulfur (i.e., ambient air quality indicators) to deposition in sensitive ecosy stems (i.e., exposure 4 pathway s), and then to system response as measured by a given ecological indicator (e.g., lake 5 and stream acidification as measured b y ANC). The ecolog ical effect (e.g., changes in fish 6 species richness) was then, where possible, associated with changes in ecosy stem services and 7 the corresponding public welfare effects (e.g., recreational fishing). 8 2. Effects on Ecosystem Services 9 The process used to l ink ecological indicators to ecosy stem services is discussed 10 extensively

in Appendix 8 of the REA. In brief, for each case study area assessed the ecological 11 indicators are linked to an ecological response that is subsequently linked to associated service s 12 to the extent possible. For example in the case study for aquatic acidification the chosen 13 ecological indicator is ANC which can be linked to the ecos ys tem service of recreational fishing. 14 Although recreational fishing losses are the onl y s ervice effec ts that can be independentl y 15 quantified or monetized at this time, there are numerous other ecos ys tem services that may be 16 related to the ecological effects of acidification. 17 While aquatic acidification is the focus of this proposed standard, the other ef fect areas 18 were also anal yz ed in the REA and these ecos ys tems are being harmed by nitrogen and sulfur 19 deposition and will obtain some measure of protection with any decrease in that deposition 20 regardless of the reason for the decrease. The following summarizes the current levels of specific 21
ecos ys tem services for aquatic and terrestrial acidification, and aquatic and terrestrial nutrient 22

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77 over - enrichment and attempts to quantify and when possible monetize the harm to public 1 welfare, as represented by ecos ys t em services, due to nitrogen and sulfur deposition. 2
a. Aquatic Acidification 3 Acidification of aquatic ecosy stems primarily affects the ecos ys tem services that are 4 derived from the fish and other aquatic life found in surface waters. In the northeastern Uni ted 5 States, the surface waters affected b y acidification are not a major source of commerciall y raised 6 or caught fish; however, they are a source of food for some recreational and subsistence fishers 7 and for other consumers. Although data and models are av ailable for examining the effects on 8 recreational fishing, relatively little data are available for measuring the effects on subsistence 9 and other consumers. Inland waters also provide aesthetic and educational services along with 10
non - use services, such a s existence value (protection and preservation with no expectation of 11 direct use). In general, inland surface waters such as lakes, rivers, and streams also provide a 12 number of regulating services, playing a role in hydrological regimes and climate regulation. 13 There is little evidence that acidification of freshwaters in the northeastern United States has 14 significantl y degraded these specific services; however, freshwater ecos ys tems also provide 15 biological control services by providing environments that su stain delicate aquatic food chains. 16 The toxic effects of acidification on fish and other aquatic life impair these services b y disrupting 17 the trophic structure of surface waters. Although it is difficult to quantify these services and how 18 they

are affect ed b y acidification, it is worth noting that some of these services may be captured 19 through measures of provisioning and cultural services. For example, these biological control 20 services may

serve as “intermediate” inputs that support the production of “fi nal” recreational 21 fishing and other cultural services. 22

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78 As summarized in Chapter 4 of the Policy Assessment , recent studies indicate that 1 acidification of lakes and streams can result in significant loss in economic value. For example, 2 data indicate that more than 9% of adults in the northeastern part of the country participate 3 annually

in freshwater fishing yielding 140 million freshwater fishing day s. Each fishing day 4 has an estimated average value per day of $35. Therefore, the implied total annual value of 5 freshwater fishing in the northeastern United States was $5 billion in 2006. Embedded in these 6 numbers is a degree of harm to recreational fishing services due to acidification that has occurred 7 over time. These harms have not been quantified on a regional scale; however, a case study was 8 conducted in the Adirondacks area ( US EPA, 20 11 , section 4.4.2). 9 In the Adirondacks case study , estimates of changes in recreational fishing services were 10 determined, as well as changes more broadl y in “cultural ” ecos ys tem services (including 11 recreational, aesthetic, and nonuse services). First, the MAGIC model (U S EPA, 2009, 12 Appendix 8 and s ec tion 2.2) was applied to 44 lakes to predict what ANC levels would be under 13 both “business as usual” conditions (i.e., allowing for some decline in deposition due to existing 14 regulations) and pre

- emission (i.e., background) conditions. Second, to estimate the recreational 15 fishing impacts of aquatic acidification in these lakes, an existing model of recreational fishing 16 de mand and site choice was applied. This model predicts how recreational fishing patterns in the 17 Adirondacks would differ and how much higher the average annual value of recreational fishing 18 services would be for New York residents if lake ANC levels corres ponded to background 19 (rather than business as usual) conditions. To estimate impacts on a broader category of cultural 20 (and some provisioning) ecos ys tem services, results from the Banzhaf et al (2006) valuation 21 survey

of New York residents were adapted and applied to this context. The survey used a 22 contingent valuation approach to estimate the average annual household WTP for future 23

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79 reductions in the percent of Adirondack lakes impaired b y acidification. The focus of the survey 1 was on impacts on aquatic resources. Pretesting of the survey indicated that respondents 2 nonetheless tended to assume that benefits would occur in the condition of birds and forests as 3 well as in recreational fishing. 4 By

extrapolating the 44 lake Adiriondack case study to all 3, 000 Adirondack lakes and b y 5 applying the willingness to pay survey results to all New York residents, the study estimated 6 aggregated benefits between $300 and $800 million annually for the equivalent of improving 7 lakes in the Adirondacks region to an ANC l evel of 50 μeq/L . The REA estimated 44% of the 8 Adirondack lakes currently fall below an ANC of 50 μeq/L . Several states have set goals for 9 improving the acid status of lakes and streams, generall y targeting ANC in the range of 50 to 60 10 μeq/L , and have en gaged in costly activities to decrease acidification. 11 These results imply significant value to the public in addition to those derived from 12 recreational fishing services. Note that the results are onl y applicable to improvements in the 13 Adirondacks valued b y residents of New York. If similar benefits exist in other acid

- impacted 14 areas, benefits for the nation as a whole could be substantial. The anal ys is provides results on 15 only

a subset of the impacts of acidification on ecosy stem services and suggests that the overall 16 impact on these services is likely to be substantial. 17 b. Terrestrial Acidification 18 Chapter 4.4.3 and 4.4.4 of the Policy Assessment review several economic studies of 19 areas sensitive to terrestrial acidification. Forests in the northeastern United States provide 20 several important and valuable pro visioning ecos ystem services, which are reflected in the 21 production and sales of tree products. Sugar maples are a particularly important commercial 22 hardwood tree species in the United States, producing timber and maple s yrup that provide 23

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80 hundreds of milli ons of dollars in economic value annually . Red spruce is also used in a variety 1 of wood products and provides up to $100 million in economic value annually . Although the 2 data do not exist to directly link acidification damages to economic values of lost recreational 3
ecos ys tem services in forests, these resources are valuable to the public. A recent study, 4 reviewed in the Policy Assessment, suggests that the total annual value of recreational off

- road 5 driving was more than $9 billion and the value of hunting and wildlife viewing was more than $4 6 billion each in the northeastern United States. EPA is not able to quantify at this time the 7 specific effects on these values of acid deposition, or of any specific reductions in deposition, 8 relative to the effe cts of many other factors that may

affect them. 9

c. Nutrient Enrichment 10 Chapter 4.4.5 and 4.4.6 of the Policy Assessment summarize economic studies of east 11 coast estuaries affected by nutrient over

- enrichment or eutrophication. Estuaries in the eastern 12 Unite d States are important for fish and shellfish production. The estuaries are capable of 13 supporting large stocks of resident commercial species, and they serve as the breeding grounds 14 and interim habitat for several migratory species. To provide an indicatio n of the magnitude of 15 provisioning services associated with coastal fisheries, from 2005 to 2007, the average value of 16 total catch was $1.5 billion per year in 15 East Coast states. Estuaries also provide an important 17 and substantial variety of cultural e cos ys tem services, including water

- based recreational and 18 aesthetic services. For example, data indicate that 4.8% of the population in coastal states from 19 North Carolina to Massachusetts participated in saltwater fishing, with a total of 26 million 20 saltwater fishing day s in 2006. Based on estimates in the Policy Assessment , total recreational 21 value from these saltwater fishing days was approximately $1.3 billion. Recreational 22 participation estimates for 1999

–2000 showed almost 6 million individuals parti cipated in 23

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81 motorboating in coastal states from North Carolina to Massachusetts. The aggregate value of 1 these coastal motorboating outings was $2 billion per year. EPA is not able to quantify at this 2 time the specific effects on these values of nitrogen deposition, or of an y s pecific reductions in 3 deposition, relative to the effects of man y other factors that may affect them. 4 Terrestrial ecos ys tems can also suffer from nutrient over

- enrichment. Each ecos ys tem is 5 different in its composition of species and nutrient requirements. Changes to individual 6
ecos ys tems from changes in nitrogen deposition can be hard to assess economically . Relative 7 recreational values are often determined b y public use information. Chapter 4.4.7 of the Policy 8 Assessment reviewed studies related to park use in California. Data from California State Parks 9 indicate that in 2002, 68.7% of adult residents participated in trail hiking for an average of 24.1 10 day s per year. The anal ys es in the Policy Assessment indicate that the aggregate annual benefit 11 for California residents from trail hiking in 2007 was $11.59 billion. EPA is not able to quantify 12 at this time the specific effects on these values of nitrogen deposition, or of any specific 13 reductions in deposition, relative to the eff ects of many

other factors that may

affect them. 14 The Policy

Assessment also identified fire regulation as a service that could be affected 15 by nutrient over

- enrichment of the Coastal Sage Scrub and Mixed Conifer Forest ecos ys tems by 16 encouraging growth of m ore flammable grasses, increasing fuel loads, and altering the fire cycle. 17 Over the 5

- ye ar period from 2004 to 2008, Southern California experienced, on average, over 18 4,000 fires per ye ar, burning, on average, over 400,000 acres per ye ar. It is not possib le at this 19 time to quantify

the contribution of nitrogen deposition, among many other factors, to increased 20 fire risk. 21

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82 3. Summary 1 Adversity

to public welfare can be understood by looking at how deposition of oxides of 2 nitrogen and sulfur affect the ecological functions of an ecos ys tem (see II.A.), and then 3 understanding the ecosy stem services that are degraded. The monetized value of the ecos ys tem 4 services provided b y ecosystems that are sensitive to deposition of oxides of nitrogen and sulfur 5 are in the bil lions of dollars each ye ar , though it is not possible to quantify or monetize at this 6 time the effects on these values of nitrogen and sulfur deposition or of an y changes in deposition 7 that may

result from new secondary standards . Many

lakes and streams are known to be 8 degraded b y acidic deposition which affects recreational fishing and tourism. Forest growth is 9 likely

suffering from acidic deposition in sensitive areas affecting red spruce and sugar maple 10 timber production, sugar maple sy rup production, hiking, aesthetic enjo ym ent and tourism. 11 Nitrogen deposition contributes significantly to eutrophication in many estuaries affecting fish 12 production, swimming, boating, aesthetic enjo ym ent, and tourism. Ecosy stem services are 13 likely

affected by

nutrient enrichment in many natural and scenic terrestrial areas , affecting 14 biodiversity , including habitat for rare and endangered species, fire control, hiking, aesthetic 15 enjoy

ment, and tourism. 16 D. Adequacy of the Current Standards 17 An important issue to be ad dressed in the current review of the secondary standards for 18 oxides and nitrogen and sulfur standard is whether, in view of the scientific evidence reflected in 19 the I SA, additional information on exposure and risk discussed in the REA, and conclusions 20 drawn from the Policy Assessment , the existing standards provide adequate protection. The 21 Administrator therefore, has considered the extent to which the current standards are adequate 22 for the protection of public welfare. Having reached th e general conclusion that aquatic and 23

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83 terrestrial ecos ys tems can be degrade d by deposition of oxides of nitrogen and sulfur , it is then 1 necessary

to first evaluate the appropriateness (in terms of form and structure) of the current 2 standards to address the ecological effects of oxides of nitrogen and sulfur as well as the 3 adequacy

of the current secondary standards for oxides of nitrogen and sulfur to provide requisite 4 protection by

considering to what degree risks to sensitive ecos ys tems would be expecte d to 5 occur in areas that meet the current standards. Conclusions regarding the adequacy of the current 6 standards are based on the available ecological effects, exposure and risk

- based evidence. In 7 evaluating the strength of this information, EPA has taken into account the uncertainties and 8 limitations in the scientific evidence. This section addresses the adequacy of the current 9 standards to protect against direct exposure effects on plants from oxides of nitrogen and sulfur, 10 the appropriateness of the current structure of the standards to address deposition

- related effects 11 of oxides of nitrogen and sulfur on sensitive ecos ystems and finally, the adequacy of such 12 standards to protect against adverse effects related to the deposition of oxides of nitrogen and 13 sulfur. 14 1. Adequacy of the Current Standards for Direct Effects 15 The current secondary oxides of nitrogen and sulfur standards are intended to protect 16 against adverse effects to public welfare. For oxides of nitrogen, the current secondary standard 17 was set identical to the primary standard 3 , e.g. an annual standard set for NO 2 to protect against 18 adverse effects on vegetation from direct exposure to ambient oxides of nitrogen. For oxides of 19 sulfur, the current secondary standard is a 3

- hour standard intended to provide protection for 20 plants from the direct foliar damage associated with atmospheric concentrations of SO 2 . It is 21 appropriate to consider whether the current standards are adequate to protect against the direct 22

3 The current primary NO 2 standard has recently been changed to the 3 year average of the 98 th percentile of the annual distribution of the 1 hour daily maximum of the concentration of NO 2 . The current secondary standard remains as it was set in 1971.

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84 effects on vegetation result ing from ambient NO 2 and SO 2 which were the basis for the current 1 secondary

standards. The I SA concluded that there was sufficient evidence to infer a causal 2 relationship between exposure to SO 2 , NO, NO 2 and PAN and injury to vegetation. Additional 3 research on acute foliar injury has been limited and there is no evidence to suggest foliar injury 4 below the levels of the current secondary standards for oxides of nitrogen and sulfur. There is 5 sufficient ev idence to suggest that the levels of the current standards are likely adequate to 6 protect against direct ph ytotoxic effects. 7 2. Appropriateness and Adequacy of the Current Standards for Deposition

- related Effects 8 This section addresses two concepts necessary to evaluate the current standards in the 9 context of deposition related effects. First, appropriateness of the current standards is considered 10 with regard to indicator, form, level and averaging time. This discussion centers around the 11 ability

of the c urrent standards to evaluate and provide protection against deposition related 12 effects that vary

spatiall y and temporall y. It includes particular emphasis on the indicators and 13 forms of the current standards and the degree to which they are ecologically

r elevant with regard 14 to deposition related effects. Second, this section evaluates the current standards in terms of 15 adequacy

of protection. 16
a. Appropriateness 17 The I SA has established that the major effects of concern for this review of the oxides of 18 nitrog en and sulfur standards are associated with deposition of N and S caused by atmospheric 19 concentrations of oxides of nitrogen and sulfur. The current standards are not directed toward 20 depositional effects, and none of the elements of the current NAAQS

– in dicator, form, 21 averaging time, and level

– are suited for addressing the effects of N and S deposition. 22

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85 F ive issues arise that call into question the ecological relevance of the structure of the 1 current secondary

standards for oxides of nitrogen and sulfur. 2 (1) The current SO 2 secondary standard (0.5 ppm SO 2 over a 3

- hour average) does not utilize an 3 exposure period that is relevant for ecos ys tem impacts. The majority of deposition related 4 impacts are asso ciated with depositional loads that occur over periods of months to years. 5 This differs significantl y from exposures associated with hourly concentrations of SO 2 as 6 measured b y the current secondary standard. By addressing short

- term concentrations, the 7 current SO 2 secondary standard, while protective against direct foliar effects from gaseous 8 oxides of sulfur, does not take into account the findings of effects in the I SA, which notes the 9 relationship between annual deposition of S and acidification effec ts which are likel y to be 10 more severe and widespread than ph yt otoxic effects under current ambient conditions, and 11 include effects from long term deposition as well as short term. Acidification is a process 12 which occurs over time, as the ability of an aqu atic sy stem to counteract acidic inputs is 13 reduced as natural buffers are used more rapidly than they can be replaced through geologic 14 weathering. The relevant period of exposure for ecosy stems is therefore not the exposures 15 captured in the short averaging time of the current SO 2 secondary standard. The current 16 secondary

standard for oxides of nitrogen is an annual standard (0.053 ppm averaged over 1 17 ye ar) and as such is more ecologicall y relevant. 18 (2) Current standards do not utilize appropriate atmospheric indicators. NO 2 and SO 2 are used 19 as the component of oxides of nitrogen and sulfur that are measured, but they do not provide 20 a complete link to the direct effects on ecos ys tems from deposition of oxides of nitrogen and 21 sulfur as they

do not capture all re levant chemical species of oxidized nitrogen and oxidized 22 sulfur that contribute to deposition. The I SA provides evidence that deposition related 23

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86 effects are linked with total nitrogen and total sulfur deposition, and thus all forms of 1 oxidized nitrogen and oxidized sulfur that are deposited will contribute to effects on 2
ecos ys tems. Thus, by using atmospheric NO 2 and SO 2 concentrations as indicators, the 3 current standards address only a fraction of total atmospheric oxides of nitrogen and sulfur, 4 and do n ot take into account the effects from deposition of total atmospheric oxides of 5 nitrogen and sulfur. This suggests that more comprehensive atmospheric indicators should 6 be considered in designing ecologicall y relevant standards. 7 (3) Current standards reflec t separate assessments of the two individual pollutants, NO 2 and SO 2 , 8 rather than assessing the joint impacts of deposition to ecos ys tems. Recognizing the role that 9 each pollutant play s in jointly affecting ecos ys tem indicators, functions, and services is vital 10 to developing a meaningful standard. The clearest example of this interaction is in 11 assessment of the impacts of acidifying deposition on aquatic ecosy stems. Acidification in 12 an aquatic ecos ys tem depends on the total acidifying potential of the deposition of both N 13 and S from both atmospheric deposition of oxides of nitrogen and sulfur as well as the inputs 14 from other sources of N and S such as reduced nitrogen and non

- atmospheric sources. It is 15 the joint impact of the two pollutants that determine s the ultimate effect on organisms within 16 the ecos ys tem, and critical ecos ys tem functions such as habitat provision and biodiversity

. 17 Standards that are set independentl y are less able to account for the contribution of the other 18 pollutant. This suggests that interactions between oxides of nitrogen and oxides of sulfur 19 should be a critical element of the conceptual framework for ecologically relevant standards. 20 There are also important interactions between oxides of nitrogen and sulfur and reduced 21 forms of nitrogen, which also contribute to acidification and nutrient enrichment. It is 22 important that the structure of the standards address the role of reduced nitrogen in 23

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87 determining the ecological effects resulting from deposition of atmospheric oxides of 1 nitrogen and sulfur. Consideration will also have to be given to total loadings as ecos ys tems 2 respond to all sources of N and S. 3 (4) Current standards do not take into account variability in ecos ys tem sensitivity . Ecos ys tems 4 are not uniforml y distributed either spatiall y or temporally in their sensitivity to oxides of 5 nitrogen and sulfur . Therefore, failure to account for the major determinants of variability , 6 including geological and soil characteristics related to the sensitivity to acidification or 7 nutrient enrichment as well as atmospheric and landscape characteristics that govern rates of 8 deposition, may

lead to standards that do not provide requisite levels of protection across 9
ecos ys tems. The current structures of the standards do not address the co mplexities in the 10 responses of ecos ys tems to deposition of oxides of nitrogen and sulfur. Ecosy stems contain 11 complex groupings of organisms that respond in various way s to the alterations of soil and 12 water that result from deposition of nitrogen and sulfur compounds. Different ecos ys tems 13 therefore respond in different way s depending on a multitude of factors that control how 14 deposition is integrated into the sy stem. For example, the same levels of deposition falling 15 on limestone dominated soils have a very different effect from those falling on shallow 16 glaciated soils underlain with granite. One s ys tem may over time display

no obvious 17 detriment while the other may experience a catastrophic loss in fish communities. This 18 degree of sensitivity is a funct ion of man y atmospheric factors which control rates of 19 deposition as well as ecological factors which control how an ecosy stem responds to that 20 deposition. The current standards do not take into account spatial and seasonal variations not 21 only

in depositio nal loadings but also in sensitivity of ecos ys tems exposed to those loadings. 22 Based on the discuss ion summarized above, the Policy Assessment concludes that the current 23

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88 secondary standards for oxides of nitrogen and oxides of sulfur are not ecologically r elevant 1 in terms of averaging time, form, level or indicator. 2 b. Adequacy of Protection 3 As described in the Policy Assessment , 2005 ambient conditions indicate that the current 4 SO 2 and NO 2 secondary standards are not exceeded (US EPA, 20 11 , Figures 6

- 1 and 6

- 2) in 5 locations where ecological effects have been observed. In many locations, SO 2 and NO 2 6 concentrations are substantially below the levels of the secondary standards. This pattern 7 suggests that levels of deposition and any effects on ecos ys tems due to deposition of oxides of 8 nitrogen and sulfur under recent conditions are occurring even though areas meet or are below 9 current standards. In addition, these levels based on conclusions in the REA will not decline in 10 the future to levels below which it is reasonable to anticipate effects. 11 Indetermining the adequacy of the current secondary standards for oxides of nitrogen 12 and sulfur the Policy Assessment considered the extent to which ambient deposition contributes 13 to loadings in ecos ys tems. Since the la st review of the secondary standard for oxides of nitrogen, 14 a great deal of information on the contribution of atmospheric deposition associated with 15 ambient oxides of nitrogen has become available. The REA presents a thorough assessment of 16 the contributi on of oxidized nitrogen to nitrogen deposition throughout the U.S., and the relative 17 contributions of ambient oxidized and reduced forms of nitrogen. The REA concludes that based 18 on that anal ys is, ambient oxides of nitrogen are a significant component of atmospheric nitrogen 19 deposition, even in areas with relatively high rates of deposition of reduced nitrogen. In 20 addition, atmospheric deposition of oxidized nitrogen contributes significantly to total nitrogen 21 loadings in nitrogen sensitive ecosy stems. 22

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89 The I SA summarizes the available studies of relative nitrogen contribution and finds that 1 in much of the U.S., oxides of nitrogen contribute from 50 to 75 percent of total atmospheric 2 deposition relative to total reactive nitrogen that includes oxidized an d reduced nitrogen species 3 (US EPA, 2008, section 2.8.4 ). Although the proportion of total nitrogen loadings associated 4 with atmospheric deposition of nitrogen varies across locations the IS A indicates that 5 atmospheric N deposition is the main source of n ew anthropogenic N to most headwater streams, 6 high elevation lakes, and low

- order streams. Atmospheric N deposition contributes to the total N 7 load in terrestrial, wetland, freshwater, and estuarine ecos ys tems that receive N through multiple 8 pathway s. In several large estuarine s ys tems, including the Chesapeake Bay , atmospheric 9 deposition accounts for between 10 and 40 percent of total nitrogen loadings (U S EPA, 2008 ). 10 Atmospheric concentrations of oxides of sulfur account for nearl y all S deposition in the 11 US. For the period 2004

–2006, mean S deposition in the U.S. was greatest east of the 12 Mississippi River with the highest deposition amount, 21.3 kg S/ha

- yr, in the Ohio River Valley 13 where most recording stations reported 3 year averages >10 kg S/ha

- yr . Numerous other stations 14 in the East reported S deposition >5 kg S/ha

- yr. Total S deposition in the U.S. west of the 100th 15 meridian was relativel y low, with all recording stations reporting <2 kg S/ha

- yr and man y 16 reporting <1 kg S/ha

- yr. S was primarily d eposited in the form of wet SO 4 2

− followed in 17 decreasing order b y a smaller proportion of dry SO 2 and a much smaller proportion of deposition 18 as dry

SO 4 2

−

. 19 As discussed throughout the REA (US EPA, 200 9 and s ection IIB above), there are 20 several key

areas of risk that are associated with ambient concentrations of oxides of nitrogen 21 and sulfur. As noted earlier, in previous reviews of the secondary standards for oxides of 22 nitrogen and sulfur, the standards were designed to protect against direct exposure of plants to 23

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90 ambient concentrations of the pollutants. A significant shift in understanding of the effects of 1 oxides of nitrogen and sulfur has occurred since the last reviews, reflecting the large amount of 2
re search that has been conducted on the effects of deposition of nitrogen and sulfur to 3
ecos ys tems. The most significant current risks of adverse effects to public welfare are those 4 related to deposition of oxides of nitrogen and sulfur to both terrestrial a nd aquatic ecosy stems. 5 These risks fall into two categories, acidification and nutrient enrichment, which were 6 emphasized in the REA as most relevant to evaluating the adequacy of the existing standards in 7 protecting public welfare from adverse ecological effects. 8 i. Aquatic acidification 9 The focus of the REA case studies was on determining whether deposition of sulfur and 10 oxidized nitrogen in locations where ambient oxides of nitrogen and sulfur were at or below the 11 current standards was resulting in acidif ication and related effects, including episodic 12 acidification and mercury meth yl ation. Based on the case studies conducted for lakes in the 13 Adirondacks and streams in Shenandoah National Park (case studies are discussed more full y in 14
s ection IIB and US EP A, 2009 ), there is significant risk to acid sensitive aquatic ecos ys tems at 15 atmospheric concentrations of oxides of nitrogen and sulfur at or below the current standards. 16 The REA also supports strongl y a relationship between atmospheric deposition of oxid es of 17 nitrogen and sulfur and loss of ANC in sensitive ecosystems and indicates that ANC is an 18 excellent indicator of aquatic acidification. The REA also concludes that at levels of deposition 19 associated with oxides of nitrogen and sulfur concentrations at or below the current standards, 20 ANC levels are expected to be below benchmark values that are associated with significant 21 losses in fish species richness. 22

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91 Significant portions of the U.S. are acid sensitive, and current deposition levels exceed 1 those th at would allow recovery of the most acid sensitive lakes in the Adirondacks (US EPA, 2 2008, Executive Summary). In addition, because of past loadings, areas of the Shenandoah are 3 sensitive to current deposition levels (US EPA, 2008, Executive Summary). Parts of the West 4 are naturall y less sensitive to acidification and subjected to lower deposition (particularl y SO x ) 5 levels relative to the eastern United States, and as such, less focus in the ISA is placed on the 6 adequacy

of the existing standards in these areas, with the exception of the mountainous areas of 7 the West, which experience episodic acidification due to deposition. 8 In describing the effects of acidification in the two case study areas the REA uses the 9 approach of describing benchmarks in terms of ANC values. Many locations in sensitive areas of 10 the U.S. have ANC levels below benchmark levels for ANC classified as severe, elevated, or 11 moderate concern (US EPA, 20 11 , Figure 2

- 1). The average current ANC levels across 44 lakes 12 in the Adirondack ca se study area is 62.1 μeq/L (moderate concern). However, 44 percent of 13 lakes had deposition levels exceeding the critical load for an ANC of 50 μeq/L , and 28 percent of 14 lakes had deposition levels exceeding the (higher) critical load for an ANC of 20 μeq/L (US 15 EPA, 200 9 , section 4.2.4.2). This information indicates that almost half of the 44 lakes in the 16 Adirondacks case study area are at an elevated concern level, and almost a third are at a severe 17 concern level. These levels are associated with greatly di minished fish species diversity , and 18 losses in the health and reproductive capacity of remaining populations. Based on assessments of 19 the relationship between number of fish species and ANC level in both the Adirondacks and 20 Shenandoah areas, the number of fish species is decreased by over half at an ANC level of 20 21 μeq/Lrelative to an ANC level at 100 μeq/L (US EPA, 200 9, Figure 4.2

- 1). When extrapolated 22 to the full population of lakes in the Adirondacks area using weights based on the EMAP 23

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92 probability

survey (US EPA, 200 9 , section 4.2.6.1 ) , 36 percent of lakes exceeded the critical load 1 for an ANC of 50 μeq/L and 13 percent of lakes exceeded the critical load for an ANC of 20 2 μeq/L

. 3 Many

streams in the Shenandoah case study area also have levels of depos ition that are 4 associated with ANC levels classified as severe, elevated, or moderate concern. The average 5 ANC under recent conditions for the 60 streams evaluated in the Shenandoah case study area is 6 57.9 μeq/L , indicating moderate concern. However, 85 pe rcent of these streams had recent 7 deposition exceeding the critical load for an ANC of 50 μeq/L , and 72 percent exceeded the 8 critical load for an ANC of 20 μeq/L . As with the Adirondacks area, this information suggests 9 that ANC levels may decline in the future and significant numbers of sensitive streams in the 10 Shenandoah area are at risk of adverse impacts on fish populations if recent conditions persist

. 11 Many other streams in the Shenandoah area are also likely to experience conditions of elevated 12 to severe concern based on the prevalence in the area of bedrock geology associated with 13 increased sensitivity to acidification suggesting that effects due to stream acidification could be 14 widespread in the Shenandoah area (US EPA, 200 9 , section 4 .2.6.2). 15 In addition to these chronic acidification effects, the I SA notes that “consideration of 16 episodic acidification greatly increases the extent and degree of estimated effects for acidifying 17 deposition on surface waters.” (US EPA, 2008, section 3.2. 1.6 ) Some studies show that the 18 number of lakes that could be classified as acid

- impacted based on episodic acidification is 2 to 3 19 times the number of lakes classified as acid

- impacted based on chronic ANC. These episodic 20 acidification events can have l ong term effects on fish populations (US EPA, 2008, section 21 3.2.1.6 ). Under recent conditions, episodic acidification has been observed in locations in the 22 eastern U.S. and in the mountainous western U.S. (US EPA, 2008, section 3.2.1.6 ). 23

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93 The I SA, REA an d Policy Assessment all conclude that the current standards are not 1 adequate to protect against the adverse impacts of aquatic acidification on sensitive ecos ys tems. 2 In the IS A it is noted that significant portions of the U.S. are acid sensitive, and that current 3 deposition levels exceed those that would allow recovery of the most acid sensitive lakes in the 4 Adirondacks ( US EPA, 2008, Executive Summary). In addition, because of past loadings, areas 5 of the Shenandoah are sensitive to current deposition leve ls (US EPA, 2008, Executive 6 Summary). A recent survey , as reported in the ISA, found sensitive streams in many locations in 7 the U.S., including the Appalachian Mountains, the Coastal Plain, and the Mountainous West 8 (US EPA, 2008, section 4.2.2.3). In these sensitive areas, between 1 and 6 percent of stream 9 kilometers are chronically acidified. The REA further concludes that both the Adirondack and 10 Shenandoah case study areas are currentl y receiving deposition from ambient oxides of nitrogen 11 and sulfur in ex cess of their ability to neutralize such inputs. In addition based on the current 12 emission scenarios, forecast modeling out to the year 2020 as well as 2050 indicates a large 13 number of streams in these areas will still be adversely impacted ( s ection IIB). Based on these 14 considerations, the Policy Assessment concludes that the current secondary NAAQS for oxides 15 of nitrogen and sulfur do not provide adequate protection of sensitive ecos ystems with regard to 16 aquatic acidification

. 17 ii. Terrestrial acidification 18 Based on the terrestrial acidification case studies (Kane Experimental Forest in 19 Pennsylvania and Hubbard Brook Experimental Forest described in section IIB) on sugar maple 20 and red spruce habitat, the REA concludes that there is significant risk to sensi tive terrestrial 21
ecos ys tems from acidification at atmospheric concentrations of NO x and SO X at or below the 22 current standards. The ecological indicator selected for terrestrial acidification is the base 23

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94 cation to aluminum ratio (BC:Al), which has been l inked to tree health and growth. The results 1 of the REA strongly

support a relationship between atmospheric deposition of oxides of nitrogen 2 and sulfur and BC:Al, and that BC:Al is a good indicator of terrestrial acidification. At levels of 3 deposition associated with oxides of nitrogen and sulfur concentrations at or below the current 4 standards, BC:Al levels are expected to be below benchmark values that are associated with 5 significant effects on tree health and growth. Such degradation of terrestrial ecos ys tems could 6 affect ecos ys tem services such as habitat provisioning, endangered species, goods production 7 (timber, sy

rup, etc.) among others. 8 Many locations in sensitive areas of the U.S. have BC:Al levels below benchmark levels 9 classified as providing low to intermediate levels of protection to tree health. At a BC:Al ratio of 10 1.2 (intermediate level of protection), red spruce growth can be reduced by 20 percent. At a 11 BC:Al ratio of 0.6 (low level of protection), sugar maple growth can be decreased b y 20 percent. 12 The REA did not evaluate broad sensitive regions. However, in the sugar maple case study area 13 (Kane Experimental Forest), recent depos ition levels are associated with a BC:Al ratio below 14 1.2, indicating between intermediate and low level of protection, which would indicate the 15 potential for a greater than 20 percent reduction in growth. In the red spruce case study area 16 (Hubbard Brook E xperimental Forest), recent deposition levels are associated with a BC:Al ratio 17 slightly

above 1.2, indicating slightl y better than an intermediate level of protection (US EPA, 18 200 9 , section 4.3.5.1 ). 19 Over the full range of sugar maple, 12 percent of evaluated forest plots exceeded the 20 critical loads for a BC:Al ratio of 1.2, and 3 percent exceeded the critical load for a BC:Al ratio 21 of 0.6. However, there was large variability across states. In New J ersey , 67 percent of plots 22 exceeded the critical load for a BC:Al ratio of 1.2, while in several states on the outskirts of the 23

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95 range for sugar maple (e.g. Arkansas, Illinois) no plots exceeded the critical load for a BC:Al 1 ratio of 1.2. For red spruce, o verall 5 percent of plots exceeded the critical load for a BC:Al ratio 2 of 1.2, and 3 percent exceeded the critical load for a BC:Al ratio of 0.6. In the major red spruce 3 producing states (Maine, New Hampshire, and Vermont), critical loads for a BC:Al rati o of 1.2 4 were exceeded in 0.5, 38, and 6 percent of plots. 5 The I SA, REA and Policy Assessment all conclude that the current standards are not 6 adequate to protect against the adverse impacts of terrestrial acidification on sensitive 7
ecos ys tems. The main drawback, as stated in the REA and Policy Assessment , with the 8 understanding of terrestrial acidification lies in the availability of data by

which we can predict 9 critical loads and that the data are based on laboratory responses rather than field measureme nts. 10 Other stressors that are present in the field but that are not present in the laboratory may 11 confound this relationship. The REA does however, conclude that the case study results , when 12 extended to a 27 state region, show that nitrogen and sulfur acidifying deposition in the sugar 13 maple and red spruce forest areas caused the calculated Bc/Al ratio to fall below 1.2 (the 14 intermediate level of protection) in 12% of the sugar maple plots and 5% of the red spruce plots; 15 however, results from individual states ranged from 0 to 67% of the plots for sugar maple and 0 16 to 100% of the plots for red spruce . 17 iii. Terrestrial nutrient e nrichment 18 Nutrient enrichment effects are due to nitrogen loadings from both atmospheric and non

19
atmospheric sources. Evaluation of nutrient enrichment effects requires an understanding that 20 nutrient inputs are essential to ecosy stem health and that specific long term levels of nutrients in 21 a sy stem affect the types of species that occur over long peri ods of time. Short term additions of 22 nutrients can affect species competition, and even small additions of nitrogen in areas that are 23

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96 traditionally

nutrient poor can have significant impacts on productivity as well as species 1 composition. Most ecosy ste ms in the United States are nitrogen

- limited, so regional decreases 2 in emissions and deposition of airborne nitrogen compounds could lead to some decrease in 3 growth of the vegetation that surrounds the targeted aquatic s ys tem but as discussed below 4 evidenc e for this is mixed. Whether these changes in plant growth are seen as beneficial or 5 adverse will depend on the nature of the ecosy stem being assessed. 6 In formation on the effects of changes in nitrogen deposition on forestlands and other 7 terrestrial ecos ys tems is very limited. The multiplicity of factors affecting forests, including other 8 potential stressors such as ozone, and limiting factors such as moisture and other nutrients, 9 confound assessments of marginal changes in any one stressor or nutrient in forest ecos ys tems. 10 The I SA notes that only a fraction of the deposited nitrogen is taken up by the forests, most of 11 the nitrogen is retained in the soils (US EPA, 2008, section 3.3.2.1 ). In addition, the I SA 12 indicates that forest management practices can significantly affect the nitrogen cycling within a 13 forest ecos ys tem, and as such, the response of managed forests to NO x deposition will be 14 variable depending on the forest management practices emplo yed in a given forest ecos ys tem 15 (US EPA, 2008, Annex C C .6.3 ). Increases in the availability of nitrogen in N

- limited forests 16 via atmospheric deposition could increase forest production over large non

- managed areas, but 17 the evidence is mixed, with some studies showing increased production and other showing little 18 effect on wood production (US EPA, 2008, section 3.3.9) . Because leaching of nitrate can 19 promote cation losses, which in some cases create nutrient imbalances, slower growth and 20 lessened disease and freezing tolerances for forest trees, the net effect of increased N on forests 21 in the U.S. is uncertain ( US EPA, 2008, section 3.3.9). 22

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97 The scientific literature has many examples of the deleterious effects caused by excessive 1 nitrogen loadings to terrestrial sy stems. Several studies have set benchmark values for levels of 2 N deposition at which scientifically adverse effects are known to occur. Large areas of the 3 country

appear to be experiencing deposition above these benchmarks. The I SA indicates that 4 studies have found that at 3.1 kg N/ha /yr , the co mmunity

of lichens begins to change from 5 acidoph yt ic to tolerant species; at 5.2 kg N/ha /yr, the typical dominance b y acidoph yt ic species 6 no longer occurs; and at 10.2 kg N/ha /yr , acidophy

tic lichens are totall y lost from the 7 community . Additional studies in the Colorado Front Range of the Rocky Mountain National 8 Park support these findings. These three values (3.1, 5.2, and 10.2 kg/ha/y r) are one set of 9 ecologicall y meaningful benchmarks for the mixed conifer forest (MCF) of the pacific coast 10 regions. Nearly

all of the known sensitive communities receive total nitrogen deposition levels 11 above the 3.1 N kg/ha/yr ecological benchmark according to the12 km, 2002 CMAQ/NADP 12 data, with the exception of the easternmost Sierra Nevadas. MCFs in the southern portion of the 13 Sierra Nevada forests and nearl y all MCF communities in the San Bernardino forests receive 14 total nitrogen deposition levels above the 5.2 N kg/ha/y r ecological benchmark. 15 Coastal Sage Scrub communities (CSS) are also known to be sensitive to community 16 shifts caused b y excess nitrogen loadings. Studies have investigated the amount of nitrogen 17 utilized by

healthy and degraded CSS sy stems. In healthy stands, the authors estimated that 3.3 18 kg N/ha/yr was used for CSS plant growth. It is assumed that 3 .3 kg N/ha/yr is near the point 19 where nitrogen is no longer limiting in the CSS community and above which level community 20 changes occur, including dominance b y invasive species and loss of coastal sage scrub

. 21 Therefore, this amount can be considered an ecological benchmark for the CSS community . The 22 majority

of the known CSS range is currentl y receiving deposition in excess of this benchmark. 23

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98 Thus, the REA concludes that recent conditions where oxides of nitrogen ambient concentrations 1 are at or below the current oxides of nitrogen secondary standards are not adequate to protect 2 against anticipated adverse impacts from N nutrient enrichment in sensitive ecos ys tems. 3 iv. Aquatic nutrient e nrichment 4 The REA aquatic nutrient enrichment case studies focused on coastal estuaries and 5 revealed that while current ambient loadings of atmospheric oxides of nitrogen are contributing 6 to the overall depositional loading of coastal estuaries, other non

- atmospheric sources are 7 contributing in far greater amounts in total, although atmospheric contributions are as large as 8 some other individual source types. The ability of current data and models to characterize the 9 incremental adverse impacts of nitrogen deposition is limited, both by the available ecological 10 indicators, and b y the inability to attribute specific effects to atmospheric sources of nitrogen. 11 The REA case studies used as the ecological indicator for aquatic nutrient enrichment an index 12 of eutrophication known as the Assessment of Estuarine Trophic Status Eutrophication Index 13 (ASSETS EI). This index is a six level index characterizing overall eutrophication risk in a 14 waterbody. This indictor is not sensitive to changes in nitrogen deposition within a single level 15 of the index . In addition, this ty pe of indi cator does not reflect the impact of nitrogen deposition 16 in conjunction with other sources of nitrogen. 17 Based on the above considerations, the REA concludes that the ASSETS EI is not an 18 appropriate ecological indicator for estuarine aquatic eutrophicati on and that additional analy sis 19 is required to develop an appropriate indicator for determining the appropriate levels of 20 protection from N nutrient enrichment effects in estuaries related to deposition of oxides of 21 nitrogen . As a result, EPA is unable to make a determination as to the adequacy of the existing 22

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99 secondary oxides of nitrogen standard in protecting public welfare from N nutrient enrichment 1 effects in estuarine aquatic ecos ys tems. 2 Additionally , nitrogen deposition can alter species composition and cause eutrophication 3 in freshwater s ys tems. In the Rocky Mountains, for example, deposition loads of 1.5 to 2 4 kg/ha

/yr which are well within current ambient levels are known to cause changes in species 5 composition in diatom communities indicating im paired water quality (US EPA, 2008, section 6 3.3.5.3 ). This suggests that the existing secondary standard for oxides of nitrogen does not 7 protect such ecos ys tems and their resulting services from impairment. 8

v. Other effects 9 An important consideration in looking at the effects of deposition of oxides of sulfur in 10 aquatic ecosy stems is the potential for production of methylmercury , a neurotoxic contaminant

. 11 The production of meaningful amounts of meth yl mercury (MeHg) requires the presence of SO 4 2

- 12 and merc ury , and where mercury is present, increased availability of SO 4 2

- results in increased 13 production of MeHg. There is increasing evidence on the relationship between sulfur deposition 14 and increased meth yl ation of mercury in aquatic environments; this effect occurs onl y where 15 other factors are present at levels within a range to allow methylation. The production of 16 methylmercury

requires the presence of sulfate and mercury , but the amount of methylmercury 17 produced varies with ox ygen content, temperature, pH, and supply of labile organic carbon (US 18 EPA, 2008, section 3.4). In watersheds where changes in sulfate deposition did not produce an 19 effect, one or several of those interacting factors were not in the range required for meaningful 20 methylation to occur (US EPA, 2008, section 3.4). Watersheds with conditions known to be 21 conducive to mercury methylation can be found in the northeastern United States and 22 southeastern Canada (US EPA, 200 9 , section 6). 23

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100 With respect to sulfur deposition and mercury methylation, the final ISA determined: The 1 evidence is sufficient to infer a causal relationship between sulfur deposition and increased 2 mercury

methylation in wetlands and aquatic environments. However, EPA did not conduct a 3 quantitative assessment of the risks assoc iated with increased mercury methylation under current 4 conditions. As such, EPA is unable to make a determination as to the adequacy of the existing 5 SO 2 secondary standards in protecting against welfare effects associated with increased mercury 6 methylation

. 7 vi. Summary of Adequacy Considerations 8 In summary , the Policy Assessment concludes that currently available scientific evidence 9 and assessments clearl y call into question the adequacy of the current standards with regard to 10 deposition

- related effects on sensitive aquatic and terrestrial ecos ys tems, including acidification 11 and nutrient enrichment. Further, the Policy Assessment recognizes that the elements of the 12 current standards

-- indicator, averaging time, level and form

– are not ecologically relevant, and 13 are thus not appropriate for standards designed to provide such protection. Thus, the Policy 14 Assessment concludes that consideration should be given to establishing a new ecologically 15 relevant multi

- pollutant, multimedia standard to provide appropriate protection from deposition

16 related ecological effects of oxides of nitrogen and sulfur on sensitive ecosys tems, with a focus 17 on protecting against adverse effects associated with acidifying deposition in sensitive aquatic 18
ecos ys tems. 19 3. CASAC Views 20 In a letter to the Administrator (Russell and Samet 2011 a), the CASAC Oxides of 21 Nitrogen and Oxides of Sulfur Panel , with full endorsement of the chartered CASAC, 22 unanimously

concluded that : 23

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101 EPA staff has demonstrated through the Integrated Science Assessment (I SA), 1 Risk and Exposure Characterization (REA) and the draft PA that ambient NOx 2 and SOx can have, and are having, adverse environmental impacts. The Panel 3 views that the current NOx and SOx secondary standards should be retained to 4 protect against direct adverse impacts to vegetation from exposure to gas phase 5 exposures of these two families of air pollutants. Further, the ISA, REA and draft 6 PA demonstrate that adverse impacts to aquatic ecosy stems are also occurring due 7 to deposition of NOx and SOx. Those impacts include acidification and 8 undesirable levels of nutrient enrichment in some aquatic ecosy stems. The levels 9 of the current NOx and SOx secondary NAAQS are not sufficient, nor the forms 10 of those standards appropriate, to protect against adverse depositional effects; thus 11 a revised NAAQS is warranted. 12 13 Inaddition, with regard to the joint consideration of both oxides of nitrogen and oxides 14 of sulfur as well as the consideration of deposition related effects, CASAC concluded that the 15 Policy

Assessment had developed a credible methodology for considering such effects. The 16 Panel stated that “the Policy Ass essment develops a framework for a multi

- pollutant, multimedia 17 standard that is ecologically relevant and reflects the combined impacts of these two pollutants as 18 they

deposit to sensitive aquatic ecosy stems.” 19 4. Administrator’s Proposed Conclusions Concerning Adequacy of Current Standard 20 Based on the above considerations and taking into account CASAC advice, the 21 Administrator recognizes that the purpose of the secondary standard is to protect against 22 “adverse” effects resulting from exposure to oxid es of nitrogen and sulfur, discussed above in 23 section IIA. The Administrator also recognizes the need for conclusions as to the adequacy of the 24 current standards for both direct and deposition related effects as well as conclusions as to the 25 appropriatenes s and ecological relevance of the current standards. 26 In considering what constitutes an ecological effect that is also adverse to the public 27 welfare, the Administrator took into account the ISA conclusions regarding the nature and 28 strength of the effect s evidence, the risk and exposure assessment results, the degree to which the 29 associated uncertainties should be considered in interpreting the results, the conclusions 30

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13563 Interagency Review. Subject to Further Policy Review.

102 presented in the Policy

Assessment , and the views of CASAC and members of the public. On 1 these bases, the Administrator concludes that the current secondary standard s are adequate to 2 protect against direct ph ytotoxic effects on vegetation. Thus , the Administrator proposes to 3 retain the current secondary standard for oxides of nitrogen at 5 3 ppb, 4 annual average 4 concentration, measured in the ambient air as NO 2 , and the current secondary standard for oxides 5 of sulfur at 0.5 ppm , 3

- hour average concentration, measured in the ambient air as SO 2 . 6 With regard to deposition

- related effects, the Administrator has first to consider the 7 appropriateness of the structure of the current standards to address ecological effects of concern. 8 Based on the evidence as well as considering the advice given b y CASAC on this matter, the 9 Administrator conclud es that the elements of the current standards are not ecologically relevant 10 and thus are not appropriate to provide protection of ecos ys tems
. On the subject of adequacy of 11 protection with regard to deposition

- related effects, the Administrator considered the full nature 12 of ecological effects related to the deposition of ambient oxides of nitrogen and sulfur into 13 sensitive ecos ys tems across the country . Her conclusions are based on the evidence presented in 14 the IS A with regard to acidification and nutrient enrichment effects, the findings of the REA with 15 regard to scope and severity of the current and likely

future effects of deposition, the sy nthesis of 16 both the scientific evidence and risk and exposure results in the Policy Assessment as to the 17 adequacy

of the current standards, and the advice of both CASAC and the public. After such 18 consideration, the Administrator concludes that current levels of oxides of nitrogen and sulfur are 19 sufficient to cause acidifi cation of both aquatic and terrestrial ecos ys tems, nutrient enrichment of 20 terrestrial ecos ys tems and contribute to nutrient enrichment effects in estuaries that could be 21

4 The annual secondary stand ard 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 NO 2 primary NAAQS (75 FR 6531; February 9, 2010).

### In teragency Working Comments on Draft Language under EO12866 and

13563 Interagency Review. Subject to Further Policy Review.

103 considered adverse, and the current secondary standards do not provide adequate protec tion from 1 such effects. 2 Having reached these conclusions, the Administrator determines that it is appropriate to 3 consider alternative standards that are ecologically relevant. These considerations support the 4 conclusion that the current secondary stand ard s is neither appropriate nor adequate to protect 5 against deposition related effects

. The Administrator’s consideration of such alternative 6 standards is discussed below in Section III. 7